Efforts to utilize metal-organic frameworks, a new class of materials exhibiting high surface areas, tunable pore dimensions, and tailorable surface functionality, for CO2 capture will be presented. Open metal coordination sites on the framework surface can deliver a high CO2 loading capacity at low pressures. However, additional criteria, such as water stability and the selective binding of CO2 over N2, must also be considered. Towards that end, we have targeted air- and water-stable frameworks bearing surfaces coated with alkylamine groups. Use of 1,3,5-benzenetristriazolate (H3BTTri) as a bridging ligand has led to sodalite-type frameworks of the type M3[(M4Cl)3(BTTri)8]2, possessing open M2+ coordination sites and exhibiting good chemical and thermal stability. Attachment of ethylenediamine to the M2+ sites within this structure can generate a material that selectively binds CO2 over N2. In addition, the application of frameworks with redox-active transition metal sites for the capture of O2 from air will be discussed.

Metal-organic frameworks (MOFs), hybrid lattices of metal ions and organic electron donors, have received considerable attention for the storage of hydrogen and other light gases due to their high surface areas and tailorable structures. The synthetic flexibility of these compounds makes them ideal candidates for systematically probing the influence of specific structural attributes, such as chemical functionality, surface area, and pore size and shape, on gas adsorption. Thus far, however, the influence of pore polarizability in tuning gas-MOF interactions has not been extensively explored. To this end, we prepared a complete series of halogenated MOFs with the isoreticular (IRMOF) topology. Derivatives of the IRMOF-1 structure in which one hydrogen is substituted with â?"F, -Cl, -Br, and â?"I were synthesized, and adsorption isotherms for these materials for a variety of weakly interacting gases, including hydrogen were measured. We find that, as the polarizability of the linker increases with the size of the halogen, uptake of light gases increases. Activation procedures were also investigated in detail and were shown to strongly influence gas sorption. The best activation procedures yield pore volumes and gas uptake consistent with those predicted by atomistic modeling.

A porous metal organic framework (MOF) is a promising material for gas capturing and storage. [1-3] For instance, MOF can be used for CO₂ sequestration [2, 4], i.e. for capturing, storage and conversion of CO₂ to an environmentally more acceptable substance. There are many uptake experiments performed on various types of MOFs but very few of them study the uptake sites of the gas molecules on the MOF structure in a realistic ambient environment. This knowledge is essential for optimizing the functionality of MOFs, e.g. by knowing the exact CO₂ binding sites on MOF, we can design the uptake sites to release the captured molecules. By identifying the physical and chemical stability of the MOF system, we can manipulate these factors to our benefi t such as to harvest the captured gas from the framework. We have synthesized a MOF that is deposited on a Si wafer to enable characterization with surface science techniques. Our first objective is to study CO₂ uptake in the MOF at ambient pressure by photoelectron spectroscopies. As a proof of concept, we hope to capture CO₂ with the MOF. Our preliminary photoemission spectroscopic (PES) results show a drastic but systematic change between the C 1s spectra taken from a MOF before, during and after CO₂ uptake. The changes may represent a profound CO₂ uptake in the MOF. In addition, the MOF C 1s pro file takes a long time to recover to its original state as in before CO₂ uptake. This implies that the captured CO₂ in the framework is stable. Most interestingly, the uptake occurs at room temperature and without any chemical assistance. References [1] O. M. Yaghi, M. O'Keeffe, N. W. Ockwig, H. K. Chee, M. Eddaoudi, J. Kim, Nature, 2003, Vol. 423, 705. [2] D. Britt, D. Tranchemontagne, O. M. Yaghi, PNAS, 2008, vol. 105, no. 33, 11623. [3] U. Mueller, M. Schubert, F. Teich, H. Puetter, J. Schierle-Arndt, J. Pastré, J. Mater. Chem., 2006, 16, 626. [4] A. R. Millward, O. M. Yaghi, J. Am. Chem. Soc., 2005, 127, 17998.

Programmable polymers are under development to promote the reversible capture and release of CO2 from aqueous solutions. The mechanism for reversible CO2 sequestration in water involves the conversion of relatively insoluble CO2 into soluble bicarbonates for capture and the conversion of bicarbonates back to CO2 for release. Such conversions can be stimulated by switching the solution pH, by use of a programmable polymer alone or such a polymer in the presence of enzymes such as carbonic anhydrase. The programmable polymers that we are investigating involve either poly(n-isopropylacrylamide) (PNIPAM) or the polypeptide elastin that are modified to contain acid groups such as carboxylic acids. Programming is achieved by changing the solution temperature to promote a reversible phase transition. Below the transition, the acid groups find themselves in a hydrophilic environment that promotes acid dissociation and the generation of an acidic solution pH. Above the transition, the acid groups find themselves in a hydrophobic environment that leads to protonation of the polymer and the generation of a basic solution pH. We have found that with a copolymer of PNIPAM and polyacrylic acid (PAA), we can change the pKa of the polymer by over 5 units, which is sufficient for both CO2 loading and unloading processes. This paper describes the range of variations we have explored in polymer composition to control: the transition temperature, the magnitude of the pKa change, the pH range over which programming occurs, the use of electric fields to stimulate programming, and tethering of the polymer to a support that would be suitable for deployment in a large scale process. We find that optimization of polymer performance is controlled by tuning the relative concentrations of hydrophobic and hydrophilic groups on the polymer chains. Implication of the results on large-scale CO2 sequestration processes will be discussed. This work was supported by the Laboratory Directed Research and Development program at Sandia National Laboratories. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energyâ?Ts National Nuclear Security Administration under contract DE-AC04-94AL85000.

The field of hybrid materials has recently seen the emergence of a new class of nanomaterials referred to as Nanoparticle Organic Hybrid Materials (NOHMs). These inorganic-organic materials consist of polymer chains (canopy) grafted onto an inorganic nanoparticle (core), and can exhibit liquid-like behavior in the absence of solvent. Owing to their negligible vapor pressure and great thermal stability, which represent significant advantages compared to the widely used amine solvents, NOHMs appear to be promising candidates for CO2 capture. These materials can absorb CO2 via an enthalpic effect related to the tunable chemistry of the canopy with proper selection of task-specific groups (e.g., amine), as well as an entropic effect arising from the â?ofrustratedâ? structural arrangement of the polymer chains. In this study, silica nanoparticle-based NOHMs with various canopy configurations were synthesized and characterized before and after exposure to CO2 using NMR, Raman, and ATR FT-IR spectroscopies in order to investigate the effect of the grafting densities and polymer chain configurations (linear vs branched) on the NOHM canopy structure as well as CO2 capture behavior. Moreover, the impact of N2, SO2 and H2O, three species also present in the flue gas, on CO2 capture was analyzed using a high pressure reactor. The results indicate that the canopy species can be â?ofrustratedâ? which translates into a greater ordering and alignment of the polymer chains compared to the bulk polymer. This effect allows the formation of â?ounoccupiedâ? space or favorable â?opathwaysâ? for CO2 to be absorbed. It also enables a reduction in CO2-induced swelling as well as a distinct CO2-packing behavior compared to the unbound polymer. All these phenomena can be tuned through the variation of the grafting density and/or the polymer chains configuration. In addition, the materials benefit from a very good selectivity towards CO2 relative to N2. On the other hand, the impact of H2O and SO2 on the CO2 capture capacity is dependent on the type of NOHMs.

Traditional technologies for capturing CO2 from flue gas streams involve the use of the aqueous amine technology, which is an energy intensive process due to the high heat capacity of water and need for heating to regenerate the solvent (CO2 desorption). As alternative technologies, several solvent-free approaches based on solid sorbents have been proposed to capture CO2 emissions. In particular, amine-modified silica materials have great potential as temperature swing CO2 sorbents from postcombustion flue gas because of their inherent ability to selectively adsorb CO2 over N2 at atmospheric pressures. Within this class of materials, hyperbranched aminosilicas (HAS) â?" in which the aminopolymers are polymerized in situ within the mesoporous silica support â?" have recently gained attention due to their high amine loading, large CO2 capacity, and demonstrated multicycle stability. However, experimental insights on the nature of the amine-CO2 interaction at molecular level remain largely unexplored. Understanding the molecular level interactions is critical for further improving the material design. We have used near edge X-ray absorption fine structure (NEXAFS) spectroscopy to probe the surface state of adsorbed CO2 on an HAS material. An overview of the NEXAFS technique will be presented highlighting the utility of the environmental chamber externally attached to the NEXAFS endstation to allow for sample heating, cooling, and CO2 dosing immediately prior to NEXAFS analysis. We will present the NEXAFS analysis of an HAS material before and after sample dosing with CO2 based on data recorded in the spectral region close to the C, N, and O K absorption edges (from. 250 to 590 eV).

CO2 capture on solid sorbents is an important technique in the context of greenhouse gas emission reduction as well as adsorption enhanced reaction processes. Traditional CO2 adsorbents such as hydrotalcites, alumina and zeolite have been extensively investigated. This work will focus on the study of CO2 adsorption on sol-gel derived MgO aerogels, which could have a great potential because of their large surface area (~ 500 m2 g-1). In this study amorphous MgO aerogels were compared with Alumina, crystalline (low surface area) MgO, and amorphous MgO xerogels. A thermogravimetric analyzer (TGA) was employed where the weight gain of the sample due to CO2 adsorption was measured. CO2 was introduced at different partial pressures starting from 0.0204 atm and going on to 0.0417 atm, 0.0638 atm, 0.087 atm, 0.111 atm, 0.5 atm, and finally 1 atm. Adsorption isotherms and the adsorption capacity of each material were determined. Data showed that the adsorption capacity of the MgO aerogel at 1 atm partial pressure of CO2 was 0.314 mmol CO2/g adsorbent. This was more than twice the CO2 capacity of the next highest adsorbent. It was also observed that MgO aerogel has faster CO2 adsorption kinetics and is less prone to N2 adsorption as compared to Alumina. The CO2 capacity was correlated to surface area, porosity, and structure.

The manganese oxide OMS-2 (Octahedral Molecular Sieve) material, also known as α-MnO2, is of great interest for carbon storage applications. Our experiments show that OMS-2 exhibits CO2 sorption hysteresis at pressures larger than 7 bar. The hysteretic behavior is strongly dependent on time, temperature and pressure. To better understand the atomic structures and sorption mechanism we have performed first-principles total energy calculations. OMS-2 materials have (2Ã-2)+(1Ã-1) tunnel structures, built from chains of edge-sharing MnO6 octahedra. The small (1Ã-1) tunnel is empty while the large (2Ã-2) tunnel contains additional species, such as K+ and H2O, to stabilize the structure. Our first-principles calculations indicate that the binding energy of H2O in the OMS-2 is 0.39eV, smaller than that of K+ by 4eV, and that therefore H2O can be easily removed from the OMS-2, but K+ remains. We find that K+ behaves as a â?ogate keeperâ?, which blocks the diffusion of CO2 in the OMS-2 due to high energy barrier (5.44eV/CO2 molecule). Increasing the concentration of CO2 can dramatically lower the energy barrier for CO2 to bypass K+. The gate-keeping behavior accounts for the observed sorption hysteresis. Based on the experimental and computational results, a possible CO2 sorption mechanism by OMS-2 will be discussed.

Since carbon dioxide (CO2) is recognized as greenhouse gas, advanced technologies for removing CO2 from the flue gas stream of power plants have been intensively developed. Among several different approaches to capture CO2, adsorption using solids is one of the most promising technologies to achieve lower energy demands or energy penalties. It is well known that amorphous Mg-Al mixed oxides have a larger surface area and good stability at high temperature. Heteroelement or anion including P, S, O and F plays an important role of creation of basic active site (basicity) or increase of polarity leading to stronger attraction of CO2 on the surface. In the study, development of phosphorus modified-MgAlOx for CO2 adsorption was clearly described with the synthesis method and the direct observation results by characterization study. Quantum calculation by using density function theory (DFT) was introduced to understand CO2 adsorption mechanism. MgAlOx and P-MgAlOx materials have been prepared by co-precipitation and ion-exchange methods. Co-precipitation method for preparation of hydrotalcite-like compounds (HTCs) reported in the literature. As-prepared HTCs have been ion-exchanged with aqueous solution of phosphorus salt at room temperature for 1day. The resultants were washed and dried. Finally all of the resultants were calcined at 400 oC for 5h to derive the corresponding mixed oxides. The characterization of synthesized sorbents was analyzed by ICP-AES, XRD, TEM, and N2 adsorption and desorption. In addition, CO2 adsorption was estimated as both total CO2 capacity and 90% breakthrough CO2 capacity. The CO2 activity has been examined over a packed-bed flow reactor system employed sorbent of 25mg under adsorption condition of 10 vol.% CO2 in balance N2. Each test run was carried out at 200 to 400 oC at atmospheric pressure. The density function theory (DFT) calculations with GGA-PBE functional were performed to determine adsorption energies and electronic properties including density of states and Mulliken population analysis of the interaction between the adsorbed CO2 molecules and the surface of sorbents in the three dimensional periodic-slab models. Geometry optimizations were reliably carried out using DMol3. The XRD patterns of as-prepared HTCs revealed the layered structure with the present of nitrate and phosphate in the interlayer. After calcinations, XRD patterns and TEM image for P-MgAlOx indicated the typical amorphous mixed oxide structure which results in a larger surface area and good thermal stability. From the results, prepared P-MgAlOx showed superior CO2 adsorption capacity. It is due to formation of amorphous structure and exchanged high P contents with basic active sites for CO2 adsorption. In addition, the other characterizations to determine material properties, CO2 adsorption and cyclic study, and Quantum calculations with density function theory (DFT) will be also evaluated and discussed in the present study.

Even though there has been extensive research on gas adsorption properties of various carbon materials based on activated carbon, nanotubes, and graphite/graphene, there has been little work done on the gas adsorption properties of graphite oxide (GO). In this study, we show that one-and-a-half-century-old graphite oxide can be easily turned into a potentially useful gas storage material. In order to create high-surface nanoporous materials from GO, we used two different approaches. In the first approach, we have successfully synthesized graphene-oxide framework materials (GOFs) by interlinking GO layers by diboronic acids. The resulting GOF materials have well defined pore size and BET surface area up to 500 m2/g with twice larger heat of adsorption of H2 and CO2 than those found in other physisorption materials such as MOF5. In the second approach, we synthesized a range of high surface area GO derived activated carbons (AGs) and studied their applications toward high pressure CO2 and CH4 gas storage. The AGs, with wide range of pore structure, have been prepared by chemical activation with potassium hydroxide (KOH). We obtain largely increased surface areas up to nearly 1900 m2/g for AG samples from 10 m2/2 for initial GO. Increase in activation temperature and KOH concentration leads to an increase in pore volume and pore width. A detailed experimental study of high pressure excess sorption isotherms on AGs reveal an increase in both CO2 and CH4 storage capacities compared to other high surface area activated carbons. Finally, we compared the gas sorption properties of our GO-based matarials with other physisorption materials such as MOFs, ZIFs, and COFs.

Carbon dioxide capture is fast becoming a necessity. Only two thirds of CO2 is produced at large stationary sources. To this end, capture from the atmosphere and concentration by cyclic adsorption-desorption processes are studied for the first time. Zeolites of type X are compared with different cations, and the effect of the alumina to silica ration is explored. Breakthrough performance showed low silica type X (LSX) capable of gas hourly space velocities of at least 63,000 hr-1. When compared with amine based adsorbents we find that the uptake rate is as important as the cyclic capacity. Li-LSX was found to have double the capacity of zeolite NaX at atmospheric conditions, which is higher than all other reported zeolites. Using a combined temperature and vacuum swing cycle, the CO2 concentration in the desorption product reaches >90%. This is the first report of a single cycle atmospheric CO2 capture system achieving a sequestration ready concentration of CO2.

Todayâ?Ts impressive progress in the capacity of hydrogen storage materials and the capabilities of hydrogen storage systems is the result of world-wide interest and significant research funding over the last decade. In that time, a hydrogen and fuel cell economy has made some first steps forward, primarily in stationary and indoor applications. Despite this progress, world governments have elected to switch their focus to battery powered or battery assisted vehicles. That change reflects both political needs and technical judgments. A look back at what has been accomplished in hydrogen storage and what is left to do provides a reference frame for further commercialization and the research that will enable it.

We present a review of the experimental and theoretical methods used in the discovery of new metal-hydrogen materials systems for hydrogen storage applications. The focus is on a specific subset of successful methods utilizing theoretical crystal structure prediction methods, computational approaches for screening large numbers of compounds, and medium-throughput experimental methods for the preparation of such materials. Monte Carlo techniques paired with a simplified empirical Hamiltonian provide crystal structure candidates that are refined using Density Functional Theory. First-principle methods using high-quality structural candidates are further screened for an estimate of reaction energetics, decomposition enthalpies, and determination of reaction pathways. Experimental synthesis utilizes a compacted-pellet sintering technique under high-pressure hydrogen at elevated temperatures. Crystal structure determination follows from a combination of Rietveld refinements of diffraction patterns and first-principles computation of total energies and dynamical stability of competing structures. The methods presented within are general and applicable to a wide class of materials for energy storage.

Recently we are focusing on â?otransitionâ? between bonding states of hydrogen in hydrides. An example was found in CaPd hydride, that is, the hydride showed the multimode reversible hydriding/dehydriding reactions over a wide temperature range [1]. At lower temperatures below 523 K, hydrogen solutes into a complex hydride CaPdH2.0 with [PdH2]2- complex anion to form a perovskite-type hydride CaPdH2.4, and then dissolutes from CaPdH2.4 to reform CaPdH2.0. At higher temperatures over 523 K, on the other hand, the complex hydride CaPdH2.0 decomposes into CaPd2 and CaH2, which recombines into CaPdH2.0. Another example was found in YMn2 hydride, that is, the hydride showed the continuous transformation from a metal hydride YMn2H4.5 to a complex hydride YMn2H6 with [MnH6]5- complex anion, even under 5 MPa of hydrogen at 423 K [2]. The related studies will provide us a new guideline for designing advanced hydrogen storage materials. [1] K. Ikeda, N. Okuda, K. Ohoyama, H.-W Li, H.T. Takeshita, T. Otomo S. Orimo, Chem. Commun. 46, 8380 (2010). [2] M. Matsuo, K. Miwa, S. Semboshi, H.-W. Li, M. Kano, and S. Orimo, Appl. Phys. Lett. 98, 221908 (2011).

High capacity solid-state storage of hydrogen suitable for transportation applications remains elusive despite more than a decade of intense renewed interest. However, efforts towards hydrogen/oxygen fuel cell vehicles have continued to progress using designs based on high-pressure (~350 to 700 bar) compressed hydrogen tanks. To augment the capacity of compressed tanks and achieve a solid-state storage material that is readily integrated with developing vehicle designs, we have focused on room temperature high-pressure molecular hydrogen storage materials. Here we present results for storage in zeolite-templated carbon (ZTC) at pressures up to 200 bar. We will describe the synthesis and characterization of ZTC based on Zeolite Y and compare its hydrogen storage properties with conventional activated carbon materials, such as MSC-30. High fidelity templating yields ZTCs with high surface area (>3500 m2/g), high micropore volume (>1.5 cm3/g), and a well-ordered pore structure (sharp diffraction feature at 6 degrees two theta). In addition, room temperature high-pressure storage capacities can exceed those for MSC-30 (~1.2 wt% compared to 1.0 wt% at 215 bar). Importantly, the isotherms for our best ZTC materials are straighter than those for MSC-30, potentially indicating much greater saturation capacities at higher pressures. This difference from MSC-30 and other activated carbons may be a manifestation of the unique ordered pore structure for ZTC materials.

Complex hydrides such as alkali amides, borohydrides, and alanates have been considered as potential materials for hydrogen storage due to their high hydrogen content. Yet these materials usually exhibit poor thermodynamics, kinetics, and/or reversibility. For the purpose of optimizing their hydrogen storage and release capacity, it is essential to understand the fundamental mechanisms behind the decomposition and dehydrogenation of the materials, identifying the rate-limiting processes involved in the hydrogen desorption. First-principles calculations based on density-functional theory is a powerful tool for addressing the issues. In this talk, we present results for the structure, energetics, and migration of native point defects and defect complexes in LiNH₂, and propose specific mechanisms for its decomposition, providing an explanation for the effects of ball milling and the dehydrogenation of (LiNH₂+LiH) mixtures. We also discuss the relationship between the structure of hydrogen-related defects and the end products in the decomposition reaction, which can be extended to the various complex hydrides.

Metal borohydrides [M(BH₄)_n] are receiving a lot of attention in hydrogen storage research due to their high gravimetric capacities of hydrogen. In particular, the most widely-studied compounds in this class, Mg(BH₄)₂ (14.9 wt.%), and Ca(BH₄)₂ (11.6 wt.%), exhibit theoretical hydrogen contents above the DOE system-level target for passenger vehicles. Unfortunately, the temperatures for hydrogen release are too high and hydrogen cannot be extracted using the waste heat from proton exchange membrane (PEM) fuel cells (operating at approximately 80 C). Therefore, recent research has concentrated on clarifying the physical and chemical factors that determine the decomposition temperatures of borohydrides. Density-functional theory (DFT) has been used to accurately study a wide range of thermodynamic and kinetic properties of many H₂ storage materials. The DFT+Monte-Carlo based method to predict low energy crystal structures, the prototype electrostatic ground state (PEGS) method, has successfully predicted crystal structures of complex hydrides with anion groups like [BH₄]^- and [B₁₂H₁₂]^2-. Here we present an overview of our recent PEGS+DFT work on crystal structures, hydrogen decomposition pathways, and decomposition intermediates of metal borohydrides: Ca(BH₄)₂, Mg(BH₄)₂ and AlB₄H₁₁.

Realizing the widespread commercialization of hydrogen-fueled vehicles still requires overcoming technical barriers such as the development of an on-board hydrogen storage system. The research community has been actively trying to develop advanced hydrogen solid-state storage alternatives to compressed gas which is the present benchmark technology. In this study we report the research conducted at General Motors Global R&D Center on adsorption based systems. An extended set of microporous adsorbents has been under evaluation for on-board application either at low pressures/cryogenic conditions or high pressures/ambient temperature. The materials hydrogen storage properties (gravimetric and volumetric uptakes, kinetics and thermodynamics) were assessed through measurements and analysis. The objective was to understand the sorption storage mechanism and its relationship to key materials properties for the specific (P, T) operating conditions. Useful insights about performance and limitations of such materials as well as potential optimization paths to reach the on-board hydrogen storage targets will be presented.

Hydrogen is an appealing energy carrier for clean energy use. However, hydrogen storage is still one of the main bottleneck for an energy economy based on hydrogen. Many materials have been synthesized to store a sufficient amount of hydrogen for practical use. However, no material yet meets the US DOE recommendations for reversible hydrogen storage under near ambient conditions. We have started a systematic investigation of promising hydrogen storage candidate materials to meet this challenge. We consider â?otraditionalâ?o carbon materials on the one hand, on the other we focus on open framework zeolitic materials like MOFs, COFs and recently ZIFs. MOFs and ZIFs [1,2] are mechanically versatile and easy to synthesise. On combining topological enumeration, molecular dynamics and structure optimization we are poropsing a catalogue with useful hydrogen uptakes. Upon chemical subtitutions we are able to enhance the specificity for hydrogen, for instance by introducing polarizing centers in the framework. On the carbon front [3], we propose matrices of carbon nanotubes with different orientations and entanglements as a highly promising medium surpassing all carbon materials proposed so far, with its 5.5% H2 adsorption at 300 K. The talk will have a review character and will span over different material classes and computational methodologies. [1] I. A. Baburin, B. Assfour, G. Seifert and S. Leoni, Dalton Trans. DOI: 10.1039/C0DT01441A. [2] B. Assfour, S. Leoni and G. Seifert, JPC C DOI: 10.1021/jp101958p. [3] B. Assfour, S. Leoni, G. Seifert, I. A. Baburin, Adv. Mat. DOI: 10.1002/adma.201003669

Reversibility is the key for any hydrogen storage material to be developed. Borohydrides such as LiBH4 have been studied or proposed as candidates for hydrogen storage because of their high hydrogen contents (18.4 wt% for LiBH4). Limited success has been made in reducing the dehydrogenation temperature. However, full rehydrogenation has not been realized. It is found that the dehydrogenation mechanism of metal borohydrides differs signicantly from the well-known metal hydrides such as LaNi5H6 and MgH2 that release hydrogen in a single chemical decomposion step. Some of the steps in the multiple step decomposition processes of metal borohydrides are not reversible. Furthermore, the decomposition also produces stable intermediate compounds that can not be rehydrided easily. Although our experiments show the partial reversibility of the doped LiBH4, it was not sustainable during dehydriding-rehydriding cycles because of the accumulation of hydrogen inert species. Doping with additives reduces the stability of LiBH4, but it also makes LiBH4 less reversible. It raises reasonable doubt on the feasibility of making metal borohydrides suitable for reversible hydrogen storage. We will present our experimental data on doped LiBH4 and bimetallic borohydride synthesized in our labs.

Reducing anthropogenic global CO2 emissions is a complex issue. The scale of the problem, the costs, its interdependence with energy production, and the intrinsic uncertainties in making long- term predictions about something as complex as the climate are a few of the factors contributing to one of the biggest challenges of our time. Despite advances in alternative energy, most, if not all, future energy scenarios include continuing growth in the absolute use of fossil energy. Carbon dioxide capture and storage (CCS) deployed at an industrial scale, is one of the few viable technologies that mitigate anthropogenic CO2 emissions. For power plants, post combustion CCS involves the separation of CO2 from flue gas, followed by its compression and then sequestration in geological formations. CCS is very energy intensive, and capture dominate both the energy consumption and the cost. One of the main bottlenecks to deploying large-scale CCS in power plants is the energy required to separate the CO2 from flue gas. For example, near-term CCS technology applied to coal-fired power plants is projected to reduce the net output of the plant by some 30% and to increase the cost of electricity by 60-80%. Developing capture materials and processes that reduce the parasitic energy imposed by CCS is therefore an important area of research. We have developed a computational approach to rank adsorbents for their performance in CCS. Using this analysis, we have screened hundreds of thousands of zeolite and ZIF structures and identified many different structures that have the potential to reduce the parasitic energy of CCS by 30-40% compared to near-term technologies.

The Carbon Capture Simulation Initiative is a Department of Energy-wide project aimed at reducing the development time for carbon capture technology through advanced simulation. The goal of the project is to link scientifically sophisticated models at the scale of chemical reactions through to the scale of plant operations with the aid of rigorous statistical analysis. As a demonstration of the simulation methodology, the project is focusing on solid sorbent technology. The particular sorbent of interest is a mesoporous silica supported amine. Thermodynamic, kinetic and microstructural models at various levels of detail have been produced for this sorbent; the presentation will contain an overview of ongoing modeling efforts, including a discussion of insights gained from comparison of models with experimental data, and the role of uncertainty quantification in model integration.

Zeolitic imidazolate frameworks (ZIFs) are promising materials for carbon capture and other CO_2 separation applications because of superior chemical and thermal stability, and reasonable adsorption and selectivity. In these processes, the kinetics of adsorption and selectivity is as important as the equilibrium adsorption and selectivity. We report on classical MD simulations studying the effect of different functionalizations of the imidazole linkers on the diffusion of CO_2 and other gases in ZIFs with identical zeolite-ρ topologies. The chemical differences in the functional groups can have a large impact on transport rates. For example, the tracer diffusion constant for CO_2 varies by a factor of 20 over the ZIFs studied. This research is supported by the Energy Frontier Research Center: Molecularly Engineered Energy Materials, funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001342.

Zeolitic Imidizolate Frameworks (ZIFs) are nanoporous materials that have considerable potential in carbon capture and separation applications due to high CO₂ adsorbance and selectivity. In previous work [J. Amer. Chem. Soc. 132, 1008 (2010)], we examined the role of functionality on CO₂ uptake using a series of ZIFs with identical topology, but varying functionalization. In this current work, we use direct experimental measurements and computer simulation to study the effect of topology by examining the adsoption and selectivity of CO₂, N₂ and CH₄ in ZIFs with fixed imidazolate functionalization but varying topology.

With the current prevalence of hydrocarbon-based energy sources, carbon capture and sequestration are essential technologies for minimizing the emission of carbon dioxide and the resulting increased atmospheric concentration of CO2. Current technologies based on absorption require high temperature regeneration of the solvent, ultimately leading to significantly decreased efficiency and increased cost. Development of an adsorption-based technology, based on physical adsorption in optimized porous media, would greatly reduce the regeneration costs. Here we discuss the carbon capture performance of a range of metal-organic frameworks (MOFs), with particular focus on their applicability to swing-sorption techniques. In particular, we demonstrate that MOFs can capture significant amounts of CO2 and that the CO2 can be readily removed from the MOF using standard pressure/vacuum swing techniques. The role of flue gas impurities on the capture capacity is also discussed.

Zeolitic Imidazolate Frameworks (ZIFs) possess high surface area, selectivity, and stability which make them excellent candidates as carbon capture and natural gas separation materials. Due to the complexity and variety of these structures, a fundamental understanding of the interactions between gas and framework molecules that govern CO2 and CH4 uptake is useful to guide the design of materials with optimal capture and selectivity related properties. To do this we performed a detailed first principles computational analysis of multiple binding sites on a set of ZIFs with different chemical functionalizations. We found a variety of binding behaviors depending on the functional groups near to the binding site and this effect varied in different sites and with different gas species. Since a large contribution of the attraction comes from van der Waals (vdW) forces, we report binding energies obtained from different flavors of the vdW-DF [2,3,4], a still evolving density functional theory (DFT) method which includes a non-local correlation energy. Binding sites are identified with inexpensive classical simulations and comparisons are made between calculations and experimentally measured values of the heat of adsorption and adsorption isotherms [1]. This research is supported by the Energy Frontier Research Center â?oMolecularly Engineered Energy Materials,â? funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001342. [1] W. Morris, B. Leung, H. Furukawa, O. K. Yaghi, N. He, H. Hayashi, Y. Houndonougbo, M. Asta, B. B. Laird, O. M. Yaghi, J. AM. CHEM. SOC. 2010, 132, 11006-11008 [2] M. Dion, H. Rydberg, E. Schroder, D. C. Langreth, B. I. Lundqvist, Phys. Rev. Let. 92, 246401 (2004) [3] K. Lee, E. D. Murray, L. Kong, B. I. Lundqvist, and D. C. Langreth, Phys. Rev. B 82, 081101 (2010). [4] J. Klimes, D. R. Bowler, and A. Michaelides, J. Phys. Condens. Matter 22, 022201 (2010).

The unusual stepped uptake behavior in isotherms and the high selective adsorption of CO₂ over N₂ and methane observed in a flexible framework, Zn₂(bpdc)₂(bpee) where bpdc= 4,4â?T-biphenyl dicarboxylate, bpee=1,2-bis(4-pyridyl)ethylene are examined using Raman scattering, infrared (IR) absorption spectroscopy and van der Waals-Density Functional Theory (vdW-DFT) calculations. Clear evidence for structural changes in the framework occurring as a result of CO₂ adsorption is observed. The high quadrupole moment of CO₂ and its specific interaction through its oxygen induces a twisting of the two benzene rings in the bpdc linker. This twisting induces a small rotation of the C-O bond around the metal center. This phenomena is derived from a red shift in the C-C stretch mode, a decrease in Raman intensity of the C=O bond and a blue shift in the unidentate C-O bond stretch mode of the bpdc linker. As a response to the twisting of the bpdc linker, a blue shift of the C=C bond absorption band of the bpee linker is also observed. These observations indicate changes in the frameworkâ?Ts structure that opens the initially distorted parallelogram into a more rectangular structure, thereby allowing more CO₂ to be adsorbed. IR absorption spectroscopy measurements performed at lower pressures indicate that the CO₂ interaction with the C-C inter-ring bond is through the CO₂ oxygen. vdW-DFT calculations show that there are 16 CO₂/unit cell at full loading, with more energetically favorable adsorption positions for the CO₂ molecules closer to C-C bond of the bpdc linker and the C=C bond of the bpee linker instead of the benzene and pyridine rings of these linkers. At higher loading, the initial positions of the CO₂ molecules change due to CO₂-CO₂ interactions, causing a slightly weakening of the CO₂ interaction with the bpee linker. This theoretical prediction is observed experimentally through a red shift in the C=C bond stretch mode as more CO₂ is adsorbed. In summary, this work points to the main structural transformations resulting from the frameworkâ?Ts flexibility and identifies the connectivity of the bpdc linker to the metal at one end in a monodentate configuration and the flexibility of the bpee pillars themselves as the reasons for the frameworkâ?Ts flexibility. The unique CO₂ interaction with the C-C inter-ring of the bpdc linker and the resulting twisting of the bpdc linker (gate opening mechanism) accounts for the high selectivity towards CO₂.

As a unique class of green solvents, ionic liquids (ILs) are nonvolatile and nonflammable with high thermal stability, and thus might potentially substitute traditional solvents. Since the first observation of high CO2 dissolution in ILs, a large number of experimental and theoretical studies have been devoted to the potential use of ILs for CO2 capture. However, there are two major issues in the use of ILs for CO2 capture, i.e., the cost and high viscosity of ILs. As a new strategy, supported ILs (SILs) have been recently proposed in which ILs impregnate the pores of a solid porous support. Most studies to date have used either organic polymers or inorganic zeolites as supports to prepare SILs. In the last decade, a new family of porous materials, namely metal-organic frameworks (MOFs) have emerged. The vast choice of organic linkers and metal oxides allow unlimited number of MOF structures to be tuned in a rational way. Consequently, MOFs possess a wide range of surface areas (up to 6500 m2/g) and pore sizes (up to 4.5 nm), thus could act as versatile porous supports for SILs. Although numerous experimental and simulation studies have been reported separately for ILs and MOFs, we are not aware of any study for MOF-supported ILs. In this study, for the first time, we propose a MOF-supported IL (or IL/MOF composite) and demonstrate its application for post-combustion CO2 capture by a computational approach. The composite selectively adsorbs CO2 from CO2/N2 mixture, and the selectivity increases with increasing IL ratio in the composite. At a weight ratio IL/MOF = 1.5, the selectivity is about 70 at ambient conditions (300 K and 1 bar), which is higher than that in pure MOF, pure IL, and many other supported ILs.

Tunnel-containing manganese dioxide (MnO₂)-based materials are of great interest for carbon storage applications. First-principles density functional theory calculations are used to investigate α-MnO2, a structure containing a framework of corner and edge sharing MnO₆ octahedra with tunnels in between. Calculations show that placing additional species, such as K⁺ ions, into the tunnels can stabilize α-MnO₂ with respect to the rutile-structure β-MnO₂ phase, in agreement with experiment. The determined magnetic structure has antiferromagnetic Mn-Mn interactions between corner-sharing octahedra and ferromagnetic Mn-Mn interactions between edge-sharing octahedra, but long-range magnetic order is only expected at cryogenic temperatures. Pure α-MnO₂ is found to be a semiconductor with an indirect band gap of 1.3 eV. The main effect of placing ions such as K⁺ inside the tunnel is to shift the Fermi level. Water and related hydrides (OH^-; H₃O⁺) can also be accommodated in the tunnels. When both K+ and an oxygen hydride are present in the tunnel, the K-O distance increases with increasing oxygen hydride charge, as expected from electrostatics.

Amine-based aqueous liquids used for carbon dioxide capture remain the benchmark for large-scale management of carbon dioxide emissions, but suffer from a variety of problems including high cost, energy demand, and potential environmental impacts. These problems might be mitigated by using aqueous buffer salts such as potassium bicarbonate/potassium carbonate as the carbon dioxide sorption solvents. However, while non-amine based buffer systems have high capacities, they tend to suffer from poor kinetics of carbon dioxide hydration. In order to compete with the amine based solvents their hydration rates need to be improved. Small molecule mimics of the carbonic anhydrase active site facilitate enhanced CO2 hydration at near neutral pH, with a concomitant reduction in the energy penalty for solvent regeneration, therefore have the potential to dramatically improve the post-combustion capture of carbon dioxide in environmentally benign solvents. We have used computational design to develop catalysts that mimic the active site of carbonic anhydrase with the goal of surpassing the kinetics of the amine-based solvents and have shown that our computational models are predictive. In order to use the resulting optimized catalysts and solvents more effectively, we have developed new materials which will improve carbon dioxide mass transfer from the gas to the liquid phase. These new materials include catalytic surfactants that increase carbon dioxide hydration rates at the air/water interface while reducing the amount of catalyst needed in the overall process. We will describe the synthesis, catalytic rates, and surfactant properties of this new class of functional surfactant. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

Nanoconfinement of energy storage materials such as metal hydrides and battery materials has been studied in a number of cases to modify physical and chemical properties of these materials.1 Hydrogen storage materials exhibit faster kinetics when they are infiltrated in mesoporous scaffolds. However, thermodynamic effects can only be observed if the particle size is below a critical value of 1.5-2 nm which has been shown by several theoretical studies. Particles with sizes lower than this value may change their thermodynamic properties leading to increased or decreased equilibrium pressures. Examples will be shown of NaAlH4 and MgH2 which was nanoconfined in microporous carbon hosts.^2,3. The surprising effects with nanoconfined NaAlH₄ (kinetic destabilization but thermodynamic stabilization) seems to have structural origins and can be explained by both theoretical calculations and recent measurements with inelastic neutron spectroscopy. Although strong kinetic effects have been observed, nanoconfinement of so far irreversible systems such as Mg(BH₄)₂ did not improve the reversibility even though a template directed approach was tried. The concept has also been applied in other energy storage materials and the properties of nanocompositic electrode materials will be discussed with several examples of insertion materials and conversion materials. In most cases, the cyclic stability and rate capability could be improved in these cases which points at kinetic effects and a stabilization of the particular microstructure of the system. Due to the direct coupling of the free energy with the electro motoric force a modification of the latter can be expected in the order of several 100 mV.4 Ref.: [1] M. Fichtner, Conversion materials for hydrogen storage and electrochemical applications - concepts and similarities, J. Alloys Compd. 509S (2011) S529 â?" S534 [2] W. Lohstroh, A. Roth, H. Hahn, and M. Fichtner, Thermodynamic effects in nanoconfined NaAlH4, Chem. Phys. Chem. 11 (2010) 789-792 [3] Zh. Zhao-Karger, JJ. Hu, A. Roth, D. Wang, Ch. Kübel, W. Lohstroh and M. Fichtner, Altered Thermodynamic and Kinetic Properties of MgH2 Infiltrated in Microporous Scaffold, Chem. Comm. 46 (2010) 8353-8355. [4] M. Fichtner, Nanoconfinement effects in energy storage materials, Phys. Chem. Chem. Phys. (2011) Perspective Article, DOI: 10.1039/c1cp22547b

Porous carbons are well known as one of the best adsorbents for gases. This property is due to the ability of this material to exhibit very different morphologies with highly porous structure and the existence of particular interactions between the carbon atoms and gas molecules. In this study, the fabricated mesoporous carbon nanofibers (MCNF) were tested for their capacities of hydrogen storage. The MCNF were synthesized by using a template of mesoporous silicate nanofibers within anodic aluminum oxide (AAO) film and furfuryl alcohol for the carbon source at the carbonization temperatures (600, 900, 1200 °C). Due to the easy controllability of pore size and thickness of AAO nanochannels, the diameter and length of MCNF could be custom synthesized for a specific application. The morphology differences of the MCNFs carbonized at 600, 900, and 1200 °C were studied based on the factors of the I(D)/I(G) ratio, the size of graphene layer (La), the BET surface area, and the pore volume. The hydrogen uptake capacity which sensitively depended on the above factors was measured at 77 K and ambient pressure of 0.1 MPa. The MCNF-1200 with the largest BET area and pore volume provided the highest hydrogen strorage of 0.74 wt.% among the MCNFs investigated in this study. Such higher hydrogen adsorption with the MCNFs carbonized at high temperature is expected to be from the larger surface area which is caused by bigger volume contraction and the change of the structure. This study successfully demonstrates the improved hydrogen uptake efficiency of MCNF induced by the carbonization temperature for the first time. Furthermore, the characteristics of the MCNF can be temperature tuned for a specific application purpose.

Large scale production of conical carbon nanostructures (so-called carbon cones) [1] that are fundamentally different from the other nanocarbon materials, such as buckyballs and nanotubes, can be made using the so-called Kvaerner Carbon Black & Hydrogen Process [2]. This involves pyrolysis of hydrocarbons using a torch plasma process. The carbon cones that occur appear in five distinctly different forms. Earlier reports indicated that these structures exhibit unusually high H2 uptake and release at room temperature not known in other forms of carbon [3]. Here we report about the recent progress of the experimental research and theoretical modelling of these cluster particles with emphasis on H2 storage. References: [1] Helgesen et al, Mater.Res.Soc.Symp.Proc. Vol. 1057, 2008 Materials Research Society. [2] Kvaerner ASA, patent No. PCT/NO98/00093 for production and micro domain particles by use of plasma process. [3] Norwegian patent No. 307986 (2000), US patent no.6,290, 753 (2001), EPO Patent No. 1051539 (2004), Hydrogen storage in carbon materials

Microporous polymer networks (MPNs) with their rigid, crosslinked architecture can show high S_BET surface areas. The resulting high gas storage capacities make such MPNs promising candidates for an use as gas storage materials. One important synthetic challenge is the synthesis of such MPNs under mild reaction conditions including moderate processing temperatures, inexpensive catalysts and a preferably metal-free regime. The BET isotherms of a series of novel MPNs generated in an acid catalyzed Friedel-Craft-type self-condensation of A₂B₂- and A₂B₄- type fluorenone monomers have been recorded. The monomers form a cross-linked network in a single reaction step. The observed high intrinsic porosity indicates an open network structure. The condensation products show S_BET areas of up to 1400 m² g^-1. An additional increase of the S_BET value to up to 1800 m² g^-1 could be achieved by treating the MPNs with supercritical CO₂.^[1] [1] E. Preis, C. Widling, U. Scherf, S. Patil, G. Brunklaus, J. Schmidt, A. Thomas, Polym. Chem. 2011, 2, 2186.

Instability of most prototypical metal organic frameworks (MOFs) in the presence of moisture is always a limitation to commercial scale development. In this work, we combine in situ IR adsorption spectroscopy, ex situ Raman spectroscopy and Powder X-ray Diffraction to monitor the structural decomposition of paddle wheel metal cluster building units in a well-known class of metal organic frameworks M(bdc)(ted)0.5 [H2bdc = 1,4-benzenedicarboxylic acid; ted = triethylenediamine; M=Cu, Ni, Zn, Co] as a function of humidity and temperature. For Cu(bdc)(ted)0.5, the stability of this pillared framework in water vapor is found to related with water vapor condensation inside the pores. A hydrolysis reaction of water molecules with Cu-O-C group is observed by in-situ IR spectroscopy at pressures sufficient to induce condensation, namely the appearance of M-OH modes and the loss of M-O- bonds. In these conditions, water dissociates at the metal oxide sites, i.e. interaction among water molecules upon condensation weakens the H-O-H bond and therefore lower the barrier for the dissociation reaction of water molecules with metal-oxygen bonds. A series of tests have been performed to determine the reversibility and regeneration of the frameworks. Other M(bdc)(ted)0.5 series(M=Zn, Ni, Co) have been measured in humid environments to obtain an in-depth understanding of the effects of different central metal ions on the stability of the structure. The initial decomposition pathway of paddle wheel building units and the overall stability in vapor are examined in the context of intrinsic parameters in these isotypical M[dicarboxylate][diamine]0.5 compounds (Bond strength or bond dissociation energy of metal oxygen or metal nitrogen, Inertness of central metal ions, Irvine-William series for transition metals). The findings of this work provide information that is necessary for determining operating conditions of this class of MOFs with a paddle wheel secondary building unit in harsh environments and to guide developments of more robust units.

Hydrogen absorption in metal organic frameworks with partly exposed metal ions has attracted significant interest in view of the potentially larger interaction strength of the metal with the adsorbed hydrogen molecule, resulting in higher operating temperatures. Likewise, highly porous structures are of interest as a template for nanosizing light metal hydrides, altering their sorption properties. Here we report that in the metal organic framework Cu3(BTC)2, containing open Cu2+ sites, at temperatures between 323 â?" 423 K and 2 bar H2 pressure, H2 dissociates and chemically binds with a total irreversible hydrogen uptake of 1.1 wt.%. FT-IR and NMR experiments confirm that the BTC linkers are converted into the carboxylic acid starting material, while XRD reveals the presence of Cu0 metal particles and decomposition of the framework. The exposed Cu2+ sites thus show strong enough chemical reactivity to split H2. When designing MOFs for storage applications and as template for light metal hydrides one has to take into account the potentially destructive reactivity of the metal centers.

Understanding the energetics of hydrogen induced phase transformations at solid-gas interfaces is of particular importance as it provides a route to tune the thermodynamic and kinetic limitations of metal hydrides. Nanocrystal size-effects play a key role in controlling the thermodynamics and kinetics of nucleation events associated with phase transformations. Currently, experimental understanding of how nanoparticle size controls the rates and energies of metal-hydride formation and decomposition is limited, due to challenges both in directly probing these events at the nanoscale and preparing uniform samples over a series of sizes. Here, by monitoring the luminescence of metal nanocrystals in-situ, we determine for the first time the thermodynamics and kinetics of hydride nucleation and decomposition within a size-series of monodisperse palladium (Pd) nanocrystals. The unique combination of a rapid and sensitive optical technique applied to size- and shape-controlled nanocrystals reveals clearly how nanocrystal surfaces impact thermodynamic and kinetic barriers to hydride nucleation, unencumbered by substrate or antenna-coupling effects. Furthermore, by partnering our experimental data with a statistical mechanical model, it strongly suggests that hydride nucleation in palladium nanocrystals is driven by thermal fluctuations, enabling prediction of hydride nucleation barriers as a function of temperature and nanocrystal size. Such understanding is critically important for the rational design of nanocrystals not only for hydrogen storage, but also for gas sequestration, gas sensing, and catalysis.

Anion substitution is a possible strategy to tune the thermodynamic properties of hydrides with high gravimetric density [1]. Hydrogen can be partially substituted by anions with different size and electronegativity, changing the H-bonding strength and thereby modifying the thermodynamics of hydrogen sorption. If the forming solid solution has a positive enthalpy of mixing, a decrease of the enthalpy of dehydrogenation for the pure hydride is expected. As a consequence, the decomposition temperature of very stable hydrides can be reduced. On the contrary, if a negative enthalpy of mixing is observed, the enhanced enthalpy of dehydrogenation of the solid solution might increase the decomposition temperature of hydrides with low stability. Fluorine is a good candidate for H-substitution in hydrides, because of a size similar to H, high electronegativity and low weight. So, even for a minor degree of substitutions, relevant changes in the thermodynamics of hydrogen sorption are expected, without significant reduction of gravimetric density. Anion substitutions in CaH₂ have been observed during hydrogen sorption reactions of Ca(BH₄)₂ destabilized with fluorine additions [2]. The formation of a Ca(H,F)₂ solution with the cubic structure of the fluoride was experimentally evidenced [3]. A recent theoretical study [4] suggests a relevant solubility of F in the orthorhombic structure of the hydride. So, in the present work, fluorine substitution in calcium hydride has been investigated, combining experimental and theoretical studies. CaH₂-CaF₂ mixtures with various molar ratios have been prepared by ball milling. Partial H-F substitution has been evidenced after ball milling. In situ X-ray Diffraction analysis has been carried out as a function of temperature by synchrotron radiation experiments. An increase of mixing has been observed during heating, with the formation of a single solid solution for 1:1 mixture. Ab-initio DFT calculations were performed to estimate the thermodynamic properties of solid solutions. Both orthorhombic and cubic solid solutions were modelled. In order to compare experimental data with calculations, a thermodynamic assessment within the CALPHAD framework has been performed and the pseudo binary CaH₂-CaF₂ phase diagram has been calculated. The formation of orthorhombic and cubic terminal solid solutions in the CaH2-CaF2 system is predicted, in good agreement with experimental findings. [1] H. W. Brinks, A. Fossdal, B. C. Hauback, J. Phys. Chem. C, 112 (2008) 5658 [2] R. Gosalawit-Utke, K.Suarez, J. M. Bellosta Von Colbe, U. Bösenberg, T. R. Jensen, Y. Cerenius, C. Bonatto Minella, C. Pistidda, G. Barkhordarian, M. Schulze, T. Klassen, R. Bormann, M. Dornheim, J. Phys. Chem. C 115 (2011) 3762 [3] J. -F. Brice, A. Courtois, J. Aubry, J. Sol. St. Chem. 24 (1978) 381 [4] J.Y. Lee, Y.-S. Lee, J.-Y. Suh, J.-H. Shim, Y. W. Cho, J. All. and Comp. 506 (2010) 721

We will first discuss our recent alloy design efforts for both â?obulkâ? thin films and for thin film multilayer nanocomposites. The research culminates in the creation of several classes of catalysts that enable for relatively rapid room temperature volumetric absorption over multiple cycles at hydrogen pressures as low as 2 atmospheres. The same catalysts allow for ultra-rapid 250+ cycles elevated temperature absorption/desorption at hydrogen pressures between 1 and 3 atm. TEM analysis is utilized to elucidate the key microstructural features that allow for such exquisite kinetics. Second we will discuss our most recent cryogenic stage transmission electron microscopy (TEM) â?" based findings on the MgH2 to Mg (and vise versa) phase transformation. We show that both reactions proceed unevenly via sporadic nucleation, rather than by any â?ocontracting volumeâ? type of mechanism, and identify the dominant metal - hydride orientation relationships. Deformation twins in the hydride are discovered and are discussed in relation to the kinetics of hydrogen diffusion.

High specific surface area (SSA) makes carbon nanotubes (CNTs) a potential nanomaterial for CO2 capture. Lately such chemical modification methods as element doping, polymer grafting and nitrogen groups impregnation have been investigated to further improve CO2 adsorbing performance of Carbon Nanotubes (CNTs). Chemical activation is also a promising approach. Herein a close look has been taken at the effect of both acid and base activations on multiwall CNTs by studying its adsorbing capacity of CO2. For base-activation, KOH is reported to be able to develop different sizes of pores on CNT structures. Effects of activation time and temperature are investigated in order to find out the optimized condition of activation. By analyzing the pore size distribution the relationship between pore size and adsorbing capacity can be determined. For acid-activation, various mixtures of sulfuric acid and nitric acid were used to introduce oxidized groups before nitrogen groups were grafted onto the surface of CNTs via chemical modification. TEM and XPS were employed to study the surface morphology and functional groups on the surface of CNTs. For both approaches, we use thermal gravimetric analysis to determine the CO2 adsorbing capacity. The results show that base-activated CNTs could improve the adsorbing capacity by more than 200%, while acid-activated CNTs can be further modified with amino groups and achieve equally/even better performance.

Metallic membranes can be utilized to separate hydrogen from a mixture of different gas molecules. By the solid state diffusion of atomic hydrogen through the interstitial sites of a metallic lattice, the solid membrane provides an efficient way to separate hydrogen with high hydrogen selectivity against other gases. Although palladium has been widely used for this application, recent interest in the hydrogen energy prompted the search for an alternative to the precious metal. Although Body-Centered Cubic (BCC) metals, such as V, Nb, and Ta, are most promising in terms of hydrogen flux, they are mechanically unstable when hydrogen is introduced and they lack catalytic ability for the dissociation and recombination of the hydrogen molecules. Another alternative group has been amorphous metal since the introduction of Ni64Zr36 reported in the year of 2000. Although it has been reported that some amorphous alloys with high zirconium content exhibit values for the hydrogen flux as high as that of palladium membranes, not much information are available regarding the diffusion characteristics and structural stabilities. This study evaluated the hydrogen flux and diffusivity through Ni-based metallic glass membranes composed by the elemental groups of Ni, Nb, and Zr as major constituents and of Fe, Co, and Ta as minor addition. The hydrogen diffusivity was measured by the Time-lag method which analyzes the initial transient behavior of the hydrogen diffusion until it reaches a steady state. The hydrogen diffusivity of the amorphous metallic membranes turned out to be about one order of magnitude smaller than that of palladium membrane. On the other hand, the amorphous membranes have higher hydrogen solubility to result in hydrogen flux comparable to that of palladium membrane. Furthermore, our data indicated that an increase in the zirconium content led to an increase in the hydrogen diffusivity as well as in the hydrogen solubility. However, the degradation of the hydrogen flux was found to occur more rapidly in amorphous membranes with high zirconium content. The mechanism of this degradation is discussed in terms of the hydrogen diffusivity and solubility.

Adsorption process using the solid adsorbent has been known as the most promising technique for removal of CO₂ from flue gases. Alkaline earth metal oxides, especially magnesium oxide and calcium oxide, have been utilized as the CO₂ adsorbents. Magnesium oxide has advantage over calcium oxide due to its favorable CO₂ adsorption-desorption kinetics at relatively low temperature (<400 ^oC). The surface to bulk ratio of magnesium oxide in the adsorbent is critical for the practical application because CO₂ can only be adsorbed on the surface of magnesium oxide. Therefore, it is expected that adsorption performance of magnesium oxide can be maximized through the formation of magnesia-alumina composite structure with high mesoporosity. The objective of this work is to investigate the performance of mesoporous magnesia-alumina composite adsorbents in the CO₂ adsorption, and to understand their adsorption mechanism. A series of mesoporous magnesia-alumina composites with different Mg/Al atomic ratio was prepared by a single-step sol-gel method. The prepared mesoporous magnesia-alumina composite adsorbents were denoted as XMgAl (X = 0.13, 0.25, 0.5, 1, and 3), where X represents the Mg/Al atomic ratio in the adsorbent. CO₂ adsorption was carried out in a continuous flow fixed-bed reactor at atmospheric pressure. Each calcined adsorbent (25 mg) was charged into a tubular quartz reactor. CO₂ concentration in N₂ balance was fixed at 10 vol.%, and the total feed rate with respect to the adsorbent was maintained at 48,000 ml h^{â^'1}/g. The CO₂ adsorption was carried out at 200 to 400 ^oC. Outlet gases were continuously analyzed using an on-line gas analyzer equipped with infra-red detectors. From the XRD and TEM results, it was revealed that crystalline structure of XMgAl was in the transition state between magnesium oxide and magnesium aluminate, indicating the successful formation of composite structure. In the CO₂ adsorption, prepared XMgAl adsorbents exhibited superior capacity, selectivity for CO₂, and cyclic performance compared to commercial CO₂ adsorbent. It might be due to that many new active sites for CO₂ adsorption were generated by synergistic effect of magnesium oxide and magnesium aluminate in the XMgAl. The physics and chemistry behind for above results are currently under investigation. Detailed CO₂ adsorption mechanism on the XMgAl composite adsorbents will also be evaluated and discussed in this work. Mesoporous magnesia-alumina composite adsorbents were prepared by a single-step sol-gel method, and it was applied to the CO₂ adsorption. The prepared mesoporous magnesia-alumina composite exhibited superior capacity, selectivity for CO₂, and cyclic performance compared to the commercial CO₂ adsorbent.

The development of economically viable and/or energy-efficient pathways/approaches for carbon dioxide (CO2) capture/adsorption is critically important to mitigate climate change and preserve the eco system. Furthermore, CO2 is a renewable feedstock which can serve both as a reagent in chemical synthesis and as an environmentally benign solvent system. At present, given the depleting petroleum feedstocks that we heavily depend on to produce industrially important chemicals, it is of utmost importance that we develop new and novel uses for CO2 in chemical synthesis and purification in order to curb global petroleum consumption. Hence, for a sustainable energy future, new materials and processes need to be investigated and developed that, in addition to capturing CO2, would also convert it into useful chemicals/products including fuels. The major challenge, however, lies in activating stable CO2. We show a novel approach of both CO2 capture and then converting it to commercially important chemicals using porous polymer nanocomposite materials. These multicomponent nanocomposites are made of combination of different metal oxide nanoparticles and catalysts in a porous polymer matrix. In contrast to conventional single-component polymer membrane systems, our nanocomposites exhibit unique functional capabilities. This is due to the fact that this material system provides numerous localized catalytically active hot reaction spots generated by the dispersed multifunctional oxide nanoparticles. This dramatically enhances CO2 capture and conversion properties. The nanocomposite material system contains reactive metal catalysts which on reaction with in situ generated reducing gas forms a reactive species for further conversion of the captured CO2 into some useful products. The integration of polymer materials with catalytically active nanomaterials shows a promising strategy for the enhanced CO2 capture and conversion towards a sustainable energy future.

Palladium and its alloys are well-studied media for use in hydrogen storage. Introduction of porosity and compositional modulation at the nanoscale affects both the thermodynamics and kinetics of hydrogen storage with important consequences for applications in catalysis, electrochemistry, and hydrogen isotope storage and separation. Presented herein are two strategies for scalable synthesis of palladium and palladium alloys having porosity on the scale of a few to about 100 nanometers. The first approach makes use of partial consolidation of nanoparticles to form a porous network. It has the advantages of excellent compositional control, extremely high surface area, and pore stability to temperatures that exceed 400°C. The second approach uses polymer templates that yield material exhibiting regular arrays of uniform pores and provides pore size control. Pore uniformity is a potential means to achieve uniform, optimized kinetic properties. Details on the synthetic methods employed and the physical properties, hydrogen storage capacity, and thermal stability of the Pd/alloys will be presented and compared. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energyâ?Ts National Nuclear Security Administration under contract DE-AC04-94AL85000.

Metal-Organic Frameworks (MOFs) are crystalline nanoporous materials that are composed of metals ions, or clusters, linked to organic ligands. They have generated a great deal of attention in the scientific community for their multiple potential applications. Primarily they represent a significant prospect for gas storage, particularly hydrogen. In addition they can be used in catalysis, gas processing, molecular separations, non-linear optics, electronics and ion-exchange. This paper is an initial study into the thermal stability of MOFs, including their thermal decomposition pathways. Our interest lies in the resulting products and the decomposition temperatures of the samples, in order to understand their characteristics better, so that their robustness can be improved in the future. First, well known MOFs such as MOF-5, HKUST-1 and ZIF-8 will be studied. Their thermal stability will be observed in an inert argon atmosphere and then in more interesting atmospheres such as hydrogen, methane and carbon dioxide which will also lead to look at their ability to uptake these gases. Secondly, this paper will discuss a novel type of promising MOFs containing silicon based connecting units (1). They are more synthetically accessible compared to their carbon equivalents and, furthermore, their structural and chemical properties are easily modifiable. Bibliography: (1) Robert P. Davies*, Rob Less, Paul D. Lickiss, Karen Robertson, and Andrew J. P. White. CRYST GROWTH DES 2010, 10, 4571-4581

50 nm-thick nanocomposite layers containing antimony (Sb) nanocrystals (NCs) embedded in SiO₂ matrix were deposited on Si substrate by the target-attachment sputtering method. Afterward, the nanocomposite samples were annealed at various temperatures in air ambient and their hydrogen (H₂) gas sensing behaviors were investigated. The gas sensing responses of the samples were evaluated by transient current measurement in air and N₂/H₂ ambient (H₂ content = 1000 ppm), respectively. For the sample subjected to the annealing at 400^oC, it exhibited a significant p-type response (S defined as Î"R/Rgas) of 67.74% at 100^oC operational temperature with a fast response time = 45 sec and a recovery time = 20 sec. As to the 500^oC-annealed sample, the n-type response (S defined as Î"R/Rair) of 37.12% at the same operational condition was also observed. Experimental analyses indicated the sensing capability is originated from the interfacial properties of nanocomposite layer. The H₂ gas sensing is correlated to the formation of antimony oxides due to the oxidation reactions on the surface of Sb NCs and the relatively high sensing response is in fact induced by the high specific surface area (SSA) feature provided by numerous Sb NCs embedded in the nanocomposite. Grazing-incidence x-ray diffraction and x-ray photoelectron spectroscopy revealed the dominant antimony oxide type in nanocomposite layer varies with the change of annealing temperature. In 400^oC-annealed sample, Sb₂O₃ is the prevailing oxide phase and is responsible to the p-type sensing mechanism. In contrast, the Sb₂O₅ became the dominant oxide type in 500^oC-annealed sample and, consequently, resulted in the n-type gas sensing behavior. Moreover, post annealing temperature was found to affect the morphology of Sb NCs and thus the gas sensitivity of sample. The sizes of NCs were enlarged when escalating the annealing temperature and, accordingly, reduced the SSA of nanocomposite. This suppressed the sites for the progress of sensing reactions and hence a lower sensing response for the 500^oC-annealed sample. In addition to demonstrating the feasibility of Sb-SiO₂ nanocomposite layer to the H₂ gas sensor fabrications with satisfied sensing ability, analytical results presented above illustrate such a nanocomposite thin film is a versatile gas sensing material since its sensing mechanism can be modulated by simply changing the heating temperature.

A pure amine-functionalized metal-organic framework (MOF) MIL-101(Cr) was successfully synthesized using mixed linker. This material is very attractive because it integrates the compositional and structural features of amino group and MIL-101(Cr). Here 2-aminoterephthlic acid was incorporated into MIL-101(Cr) type structure by in situ synthesis. The content of 2-aminoterephthlic acid in the resulting framework can be readily controlled by modifying the reaction stoichiometry. We show that the as-prepared amine-functionalized MIL-101(Cr) had a large Brunauer-Emmett-Teller (BET) surface area and high thermal stability, with a decomposition temperature above 593K in air. More importantly, it exhibited a high CO2 storage capacity up to 18.5 mmol/g at a pressure of 40 bars and a temperature of 303K, as well as high CO2/N2 ideal selectivity up to 16. We believe that the as-prepared material is a very promising candidate for CO2 capture and the synthesis approach can facilitate the development of functionalized MOFs.

CO2 capture and sequestration (CCS) has gained a lot of attention in the recent years because of an increasing need to deal with green gas emissions resulting from them global warming. Calorimetric techniques are the analytical tools especially suitable to assess energetics of processes involved in the CCS applications. When coupled with a high pressure gas dosing system a Calvet calorimeter is capable to measure such properties as adsorption capacity, heats of adsorption and kinetics for CCS materials under high pressure and supercritical conditions. The results on coupled experiments for some metal organic framework materials (MOFs) i.e. Cu-BTC are presented. The thermodynamic properties (both isosteric and directly measured enthalpies of adsorption) are presented and discussed. A combined gas sorption and calorimetric experiment has been shown to be particularly useful for materials characterization in CCS and MOFs particularly. A Calvet calorimeter provided an accurate measure of heats of adsorption of CO2 into the MOFs studied.

Reactive nanoparticles are of great interest for applications ranging from catalysis to energy storage. However, broad pore size distributions and poorly characterized pore environments in many microporous templates hamper efforts to relate cluster size to thermodynamic stability and chemical reactivity. Metal hydrides are a clear example of this problem: theory suggests that reducing particle size to the nanoscale can destabilize the hydride in some cases, and there is experimental evidence for accelerated kinetics. Quantitative measurements establishing the influence of size on thermodynamics are lacking, however. Here, we employ a suite of nanoporous templates to quantify the effects of confinement on hydrogen desorption from NaAlH₄ and LiBH₄. Metal-organic frameworks (MOFs) serve as tunable â?onanoreactorâ? that allows detailed, quantitative probing of the thermodynamics and kinetics of H₂ desorption from hydride nanoparticles in the 1 â?" 3 nm size range, while highly ordered hard-carbon templates provide tunable pore sizes in the 2 â?" 20 nm range. We find that 1-nm NaAlH₄ within a MOF undergoes a single-step dehydrogenation reaction without Na₃AlH₆ intermediate formation, and that 90% of the accessible hydrogen is desorbed by 150 °C without a catalyst. These nanoclusters are thermodynamically slightly stabilized by confinement, but are kinetically very destabilized, with the activation energy for H₂ desorption > 60 kJ/(mol-H₂) lower than bulk. We also find that co-infiltrating with TiCl₄ enables the hydride to be substantially regenerated following desorption. When combined with previous calculations of cluster thermodynamics, these experiments suggest that, although interactions with the pore walls play a role in stabilizing these particles, size exerts the greater influence on the thermodynamics and reaction rates.

Recently much controversy has arisen over the nature of H₂ adsorption within the metal-organic framework referred to as MOF-74 and CPO-27. Initial neutron diffraction data showed simple sequential filling in which the open metal site fills first followed in turn by the less energetically favored oxygen and benzene sites [1]. However, this interpretation was contested in a series of infrared based papers [2]. A new mechanism was proposed in which H₂ molecules first fill as pairs on adjacent metal and oxygen sites. These claims indicate, that the often neglected H₂ â?¦ H₂ interactions are in fact dominant in the adsorption process. Consequently, the use of variable temperature IR spectroscopy to determine site specific binding energies was called into question, and a dramatic reassessment of H₂ vibrational frequency shifts would be required. In our talk we will present IR spectra for the isostructural MOF-74 series (Mg, Mn, Co, Ni, and Zn) obtained over a wide temperature range (35 to 298 K). These show complete agreement with the original sequential filling model and no evidence for a pairing mechanism. Spectra collected at H₂ concentrations ranging from 0.1 to 3.0 H₂ per metal cation reveal that strongly redshifted vibrational modes arise from isolated H₂ bound to the available metal coordination site. The redshift of the bands associated with this site correlate with reported isosteric enthalpies of adsorption (at small surface coverage), which in turn depend on the identity of the metal. Most importantly, the room temperature IR spectra that were central to the pairing mechanism model could only be reproduced after deliberately contaminating the adsorbent with ambient air. 1. Liu et al., Langmuir2008, 24, 4772. 2. Nijem et al., J. Am. Chem. Soc.2010 132, 14834.

NaAlH4 (sodium alanate) is widely studied as a hydrogen storage material and can be considered an archetype representative of complex metal hydrides. This class of materials has inherent issues with the thermodynamics and kinetics of hydrogen release. In the recent years, â?~nano-sizingâ?T has evolved as a popular approach to address this issue. In this paper, quasi-elastic neutron scattering study of NaAlH4 confined within the pores of Fe-BTC (a MOF variant) is reported. The window of accessible timescales for this study has been broadened by the use of a high resolution backscattering spectrometer and a time of flight spectrometer capable of probing a larger energy range. Two distinct hydrogen jump diffusion processes were observed â?" a fast localized diffusion and a slower long range diffusion. The nano-confined NaAlH4 showed a remarkable increase in overall dynamics (76% of mobile hydrogen) even at temperatures of 800C as compared with bulk NaAlH4 (<10% mobile H). However, the dynamics observed were mostly due to the localized H-jumps, rather than long range. Prior studies on bulk NaAlH4 had showed the exact opposite trend where there was ample scope for long range diffusion owing to lower activation barriers. It was also observed that the nano-confined samples showed additional phonon states at lower energies indicating that the thermodynamic destabilization may be a more important outcome of nano-confinement rather than kinetic enhancements. The hydrogen jump lengths in nano-confined NaAlH4 (2Ã. to 2.8Ã.) evaluated by QENS were in very good agreement with the bulk NaAlH4 and the bare scaffolds showed no quasi-elastic signal â?" both confirming that the scattering observed were from the infiltrated alanate. In-situ XRD was used to provide conclusive evidence both for the stability of the scaffold and the confinement of the alanate within the pores.

NaBH₄ is a promising material for on-board hydrogen storage in cars due to its high gravimetric hydrogen content (10.8 wt%). However the elevated temperatures (above 530 °C) [1] required to release the hydrogen and non reversibility of the hydrogen release render it unfit for practical application. Nanoconfinement in porous materials has been shown to be an effective strategy to improve the hydrogen sorption kinetics of light metal hydrides [2]. In this study, we investigate the thermal hydrogen release from NaBH₄, and the effect of nanoconfinement in porous carbon material on the kinetics, reversibility and thermodynamics of hydrogen release from NaBH₄. NaBH₄/C nanocomposites were successfully prepared via pore volume impregnation of nanoporous carbon material with an aqueous alkaline NaBH₄ solution, and also by melt infiltration under hydrogen pressure[3]. XRD, solid-state NMR and hydrogen release measurements revealed that NaBH₄ decomposes into elemental Na and Na₂B₁₂H₁₂ when heated to 600 °C under Ar, with a release of 8.1 wt% H₂. Nanosizing and confinement of NaBH₄ in a nanoporous carbon material resulted in much faster hydrogen desorption kinetics. The onset of hydrogen release was reduced from 470 °C for the bulk to below 250 °C for the nanoconfined NaBH₄. Remarkably, reversible formation of NaBH₄ from its dehydrogenation products was demonstrated for the first time; the dehydrogenated NaBH₄/C nanocomposites released about 3.4 wt% H₂ after rehydrogenation at relatively mild condition (325 °C and 60 bar H₂), while negligible amount of hydrogen was released from the dehydrogenated bulk after rehydrogenation under similar condition. Reversibility of the system is limited by loss of Na at high temperatures during hydrogen release. Compensation of Na loss by extra Na resulted in almost full rehydrogenation of the dehydrogenated products to NaBH₄. The equilibrium decomposition temperature of NaBH₄ is ~540 °C in 1 bar H₂ [1], however for the nanoconfined NaBH₄ hydrogen release started around 280 °C under 1.1 bar H₂. References [1] Martelli, et al. J. Phys. Chem. C (2010), 114, 7173 [2] Gross et al. J.Phys. Chem. C. (2008), 112, 5651 [3] Ngene, de Jongh et al., Energy Environ. Sci, (2011), 4, 4108

Complex hydrides are attractive candidates for on-vehicle hydrogen storage because they offer high gravimetric hydrogen density (a consequence of binding multiple hydrogens in NH₂^-, BH₄^-, or AlH₄^- anion complexes for each cation charge) along with excellent volumetric density (65-110 g H₂/liter). Practical implementation for fuel cell vehicles, however, will require development of complex hydrides with thermodynamics suitable for operation below 85 °C in 3-8 bar of H₂ gas. Alkali metal-transition metal borohydrides are promising materials to investigate in this regard. At such near-ambient temperature, obtaining sufficiently fast hydrogen release and reabsorption kinetics is equally challenging, and must be solved concurrently. Recently, nanoconfinement in nanoporous scaffolds has been explored to improve kinetics, using melt or solvent infusion to embed the hydride into nanoscale pores within a solid. By melt infusing NaAlH₄ into nanoporous carbon aerogel we have shown enhanced kinetic performance and reversibility, even in the absence of a catalyst. Hydrogen cycling of uncatalyzed NaAlH₄ in aerogel approaches that of TiCl₃-catalyzed NaAlH₄. The pore volume of the nanoporous framework, however, limits the NaAlH₄ loading in the scaffold, and furthermore co-incorporation of a catalyst with the NaAlH₄ is difficult. By simply combining the NaAlH₄ with nanoporous carbon aerogel or activated carbon by ball milling, we have demonstrated kinetic enhancement similar to that achieved by melt infusion into carbon aerogel at the same NaAlH₄ loading (~50%). In contrast to melt infusion, however, the NaAlH₄ loading can be increased to ~90%, far above the loading that would entirely fill the scaffold pore volume, while still achieving substantial kinetic enhancement. Curiously, there is little evidence for a two-step desorption, as would be expected for a mixture of infused and external (bulk-like) NaAlH₄, showing that carbon is responsible for kinetic enhancement beyond that of providing the nanoscaffold alone. Additionally, TiCl₃ catalyst is easily co-incorporated by milling, resulting in a small but significant improvement over TiCl₃-catalyzed NaAlH₄ without carbon. Kinetic enhancement is preserved during cycling using 20 bar rehydrogenation, at least for the second-stage NaAlH₄ decomposition reaction, Na₃AlH₆ â?" 3 NaH + Al + 3/2 H₂. Cycling experiments at 100 bar demonstrate that the first stage reaction 3NaAlH₄ â?" Na₃AlH₆ + 2Al + 3H₂ is also kinetically enhanced by ball milling with nanoporous carbon, although the improvement is not as dramatic.

An overview is given of the development of advanced nanoporous carbons as storage materials for natural gas (methane) and molecular hydrogen in on-board fuel tanks for next-generation clean automobiles. High specific surface areas, porosities, and sub-nm/supra-nm pore volumes are quantitatively selected by controlling the degree of carbon consumption and metallic potassium intercalation into the carbon lattice during the activation process. Tunable bimodal pore-size distributions of sub-nm and supra-nm pores are established by subcritical nitrogen adsorption, small-angle x-ray scattering, and transmission electron microscopy. Optimal pore structures for gravimetric and volumetric gas storage, respectively, are presented. We present an experimental study of the prediction that substitution of surface carbon with boron (â?oboron dopingâ?) increases the binding energy for hydrogen and, therefore, enhances hydrogen adsorption (electron donation from H2 to electron-deficient boron). Microscopic Fourier transform infrared spectroscopy establishes the presence of B-C bonds. A sample with 8 wt% boron exhibits a 30% increase in excess adsorption per surface area at 303 K and 200 bar, relative to the undoped parent material. Methane and hydrogen adsorption isotherms up to 250 bar on monolithic and powdered activated carbons are reported and validated, using several gravimetric and volumetric instruments. Current best gravimetric and volumetric storage capacities are: 256 g CH4/kg carbon and 132 g CH4/liter carbon at 293 K and 35 bar; 26, 44, and 107 g H2/kg carbon at 303, 194, and 77 K respectively and 100 bar. Adsorbed film density, specific surface area, and binding energy are analyzed separately using the Clausius-Clapeyron equation, Langmuir model, and lattice gas models.

The low thermal conductivity of porous adsorbents such as MOF-5 (e.g. <0.1 W/m*K for powders) can limit the performance of hydrogen storage systems, where hydrogen uptake and release involves the liberation (adsorption) and consumption (desorption) of heat . To improve the thermal properties of these materials we have synthesized a series of high-density MOF-5 composites containing up to 10 wt% expanded natural graphite (ENG), which serves as a thermal conduction enhancer. For this series, we have identified processing strategies that preserve the high surface area (and consequently hydrogen capacity) of MOF-5 while maximizing thermal conduction. For example, we find that the addition of 10 wt% ENG to MOF-5 and compaction to 0.5 g/cm3 produces a five-fold increase in thermal conductivity relative to neat MOF-5 of the same density. Further, these composites have an excess volumetric H2 uptake that is almost three times larger than that of the powdered MOF-5. Trends in the hydrogen uptake and thermal conductivity for the complete set of MOF-5/ENG composites at 77 K are reported. Anisotropy of the thermal conductivity in the axial and radial directions is also described.

Among many known hydrogen storage materials family, ammonia borane (NH3BH3; AB) has considered as a promising candidate for chemical hydrogen storage applications because of its high gravimetric hydrogen content of 19.6 wt.% [1]. However, to be useful for practical applications one has to aim for extracting faster dehydrogenation kinetics well below 100 oC. Generally, reducing the hydride particles to nanoscale, is an effective and fascinating strategy for enhancing both the kinetic and thermodynamic properties. Herein we demonstrate the effect of nanoconfinement on the thermal dehydrogenation behavior of AB. The nanoconfinement is obtained by AB infiltration into the porous media by solution blending method. The nanoconfinement results in reduced dehydrogenation temperature than the bulk AB. The nanoconfinement also leads to the much improved dehydrogenation capacity and very fast dehydrogenation kinetics well below 100 oC. We conclude that the dehydrogenation behavior is dependent on the pore size and type of metal-center in the porous media. Our systematic investigation shows that the dehydrogenation properties of the confined AB dependent on the level of AB loading, indicating the significant role of AB-AB interactions on the dehydrogenation kinetics [2]. [1]. A. Staubitz, A. P. M. Robertson, I. Manners, Chem. Rev. 110 (2010) 4079. [2]. Gadipelli Srinivas, Jamie Ford, Wei Zhou, Hui Wu, Terrence J. Udovic and Taner Yildirim, Nanoconfinement and Catalytic Dehydrogenation of Ammonia Borane by Magnesium-Metal-Organic Framework-74, Chemistry A European Journal, 17 (2011) 6043.

In NaAlH4, the rehydriding reaction requires that metal atoms be transported; this has been recognized as a key bottleneck step in the hydrogen reactions of this and other complex (covalent-ionic) hydrides. Using in-situ 27-Al NMR, we have discovered and identified a mobile species that is a vacancy-ridden form of NaAlH4. The T1 of H in PdHx proceeds by two parallel mechanisms. The intrinsic relaxation inside the hydride is supplemented by exchange with the surrounding and rapidly relaxing gas phase. We report results on PdHx/H2 and PdDx/D2, with the exchange rate varying by three decades in the latter system. The activation energy for hydride-gas exchange will be reported.

Carbonaceous nanomaterials have been widely reported [1-4] to enhance both the thermodynamics and kinetics of several metal hydrides to make them more viable for hydrogen storage applications. With very few exceptions [4, 5], the interactions of these hydrides with the carbon materials have not been directly evaluated. Due to its high accessibility in probing local chemical bonds and geometries, solid-state NMR has been playing a key role in assessing potential hydrogen storage systems. By implementing Magic Angle Spinning (MAS) and Cross Polarization Magic Angle Spinning (CPMAS), NMR spectroscopy has clearly identified various interactions occurring within sodium alanate and fullerene (C₆₀) composites following preparation and after hydrogen desorption/absorption cycling. High-resolution spectra of ¹H, ²D, ¹³C, ²³Na, and ²⁷Al nuclei were obtained from as-prepared [3] composites of NaAlH₄ and NaAlD₄ with C₆₀ and after dehydrogenation and rehydrogenation processing. Two distinct hydrogen environments were seen from the ¹H and ²D spectra of the as-prepared composites although the ²⁷Al spectra showed that only the original NaAlH(D)₄ phase being present. However, the ¹³C peak at 144 ppm for pure C₆₀ had disappeared resulting in a least three new peaks that indicated phase transformations, which caused changes in the carbon bonding, had occurred during processing. These differences are larger than previously found in LiBH₄/C₆₀ composites [5]. Desorption at 553 K led to a 77% decrease in hydrogen content along with formation of Al metal, NaH, and a changed carbon material. Hydrogen absorption at 423 K caused reformation of NaAlH₄ along with creating Na₃AlH₆ but still retaining much residual Al metal. These results will be compared to behavior reported for several nano-encapsulated hydrides [1-5]. 1. P Adelhelm, K. P. de Jong, P. E. de Jongh, Chem. Commun. 2009, 6261. 2. J. J. Vajo, Current Opinion Sol. St. Mater. Sci. 15, 52 (2011). 3. J. A. Teprovich, Jr. et al., J. Alloys Compds. 509S, S562 (2011). 4. P. Adelhelm and P. E. de Jongh, J. of Mater. Chem. 21, 2417 (2011). 5. D. T. Shane, et al., J. Phys. Chem. C 114, 19862 (2010).

Hydrogenation thermodynamics and kinetics of Mg-4Fe at% thin films capped with Pd at temperatures ranging from 363 K to 423 K were studied by a number of different methods: in-situ infrared imaging, volumetric PCI measurements, and ex situ X-ray diffraction and transmission electron microscopy (TEM). Hydride growth rate was determined by utilizing wedge-shaped films and IR imaging; with an assumption of a continuous hydride layer the growth rate was found to range from ~3.8 nm/s at lower temperature to ~36.7 nm/s at higher temperature. Apparent activation energy of the thermally activated hydrogenation kinetics was measured as 56 kJ/mole; the value suggests that hydrogen diffusion along grain boundaries of MgH2 at low temperatures is the mechanism controlling the growth of a hydride layer. Reproducible pressure-composition isothermal (PCI) measurements of 600 nm-thick uniform films showed a pressure plateau and large hysteresis; from these measurements enthalpy was estimated as 66.9 kJ/mole, which is slightly less than what was is known for pure magnesium (either films or bulk). These results show remarkable kinetics of Mg-Fe, with possible practical suggesting that the powder having a metastable Mg(4 at% Fe) solid-solution of 1-2 μm size can be fully charged with hydrogen during 1 min for temperatures close to 150 C.

Metal Organic Frameworks (MOFs) are materials with an exceptionally high surface area, low density and adjustable pore size and functionality. Among these merits, functionality is attractive because it allows the user to vary not only the organic ligands but also metal ions (clusters) or introduce functional groups into the pores aimed at specific applications such as catalysis, gas sorption, etc. With respect to hydrogen storage, introducing a functional moiety into the MOF framework opens the possibility to tune the parent pore structure and change the chemical microenvironment of the pores. In this paper we will present the hydrogen storage capability of a new series of MOFs and compare the data with computational models. Mono-substituted MOF-5 derivatives have been synthesized using the pre-modification approach. Introducing different functional groups (-CH3, -OCH3, -Cl and -Br) affect both the coordination ability of ligands with Zn2+ and the electron density of the aromatic ring. This results in a change of the MOF-5 structure, the thermostability and the hydrogen storage capability. MOF-5 modified with -CH3, -Cl and -Br display crystal structures comparable to MOF-5 but in some cases the crystal quality becomes compromised. MOF-5 modified with -OCH3 forms the same crystal structure as MOF-5 but only when terephthalic acid is incorporated as a co-ligand. We found that the thermostability of the MOF-5 framework is reduced when -OCH3, -Cl and -Br were introduced. The modification also has a negative impact on the hydrogen uptake capability of MOF-5. Clearly, the -Br and -Cl modifications reduce the hydrogen uptake capacity of MOF-5 at 77 K below 1 bar. In contrast, the -CH3 and -OCH3 modifications maintain a hydrogen uptake capacity comparable to that of MOF-5 (Table 1). Computational studies demonstrate that the heat of adsorption of hydrogen in MOF-5 is not impacted by the chemical modifications. Also, the functional groups do not seem to affect the fast hydrogen adsorption kinetics in MOF-5. An interesting finding was that the stability of MOF-5 towards moisture was improved dramatically when one or more -CH3 units are introduced. The H2 uptake capacities of methyl modified MOF-5 can recover after 4 days exposure to ambient air (relative humidity of 32-37%) and even after 8 days exposure the hydrogen uptake capacity was only reduced by 35%. In contrast, MOF-5 loses the hydrogen uptake capability completely after 1 day under the identical exposure conditions.

Borohydride-based materials have garnered much interest in recent years within the hydrogen-storage research community. The exceptionally large neutron scattering cross section for hydrogen renders neutron-based probes invaluable for elucidating the location, spectroscopy, and dynamics of hydrogen in these materials, both in their neat and nanosequestered states. In particular, neutron diffraction, neutron vibrational spectroscopy, and quasielastic neutron scattering techniques in conjunction with first-principles calculations can help unravel key crystallographic details, bonding interactions, and reorientational mechanisms associated with hydrogen-storage-related materials. In this talk, we summarize the results of our recent neutron scattering investigations at the NIST Center for Neutron Research for a variety of these materials, including alkali-metal tetrahydroborates, dodecahydro-closo-dodecaborates, and other more novel compounds.

We report our thermodynamic, neutron scattering (INS and QENS)and computer modeling investigations of H2 adsorbed on bare and Pd decorated metal oxide (MO) surfaces, specifically ZnO, SBA-15 silica, and alumina. Guided by our volumetric adsorption measurements, we used INS and QENS to probe the dynamics of the adsorbed hydrogen molecules. These measurements provide insight into how the microscopic behavior of hydrogen is changed when it is confined at interfaces or interacts with a Pd catalyst. Using INS, the motion of the adsorbed hydrogen are examined as a function of surface adsorbate composition. For rotational motion we use the ortho-to-para transition as a guide and find that the rotational barrier for H2 adsorbed on some of these MO surfaces shift to lower energy (relative to bulk H2). For comparison, the hydrogen adsorption and microscopic behavior when the MO are decorated with 1{\%} Pd metal will be discussed. Evidence for the presence of adsorbed H2, Zn hydroxide and the potential role of spillover will be discussed. This work was partially supported by the U.S. DOE, BES under contract No. DE-AC05-00OR22725 with ORNL managed and operated by UT-Battelle, LLC, the NSF under grant DMR-0412231 and a grant from the University of Tennessee, JINS.

MgH₂ remains an attractive hydrogen storage material due to its high storage capacity, albeit hampered by high release temperatures and slow desorption kinetics. While an improved kinetics of nanosized MgH₂ has been reported,¹ the expected thermodynamical destabilization compared to its bulk equivalent² has proven difficult to show experimentally. A destabilization is expected due to a larger surface energy of MgH₂ compared to Mg, which affects the thermodynamics of hydrogenation only at the nanoscale, when surface-to-volume ratios are large. Although Mg nanoparticles can be prepared, particle coagulation causes a large decrease in the effective surface area, thus diminishing any a surface energy effect.³ Therefore, nanosized Mg will have to be confined by for instance a protective coating, thus exchanging the surface energy effect by an analogue interface energy effect. We show that MgH₂ becomes destabilized when it is confined by surrounding TiH2 layers using thin films as a model system.⁴ The accurate control over the layer thickness allows for measuring the equilibrium hydrogen pressure as a function of the layer thickness, and thus of the surface-to-volume ratio. From this the interface energy difference between MgH₂ and Mg, both interfaced with TiH₂, is extracted. We find an interface energy difference of 0.3 J/m², a value in good agreement with calculations based on first principles. Although indeed MgH₂ is destabilized due to the interface with TiH₂, the possibility that other materials cause a larger destabilization is explored. Current efforts are aimed at Fe as a material with a higher potential of destabilizing the magnesium hydride phase. References1. [ Jeon et al. Nat. Mater.2011, 10, 286-290 ] 2. [Kim et al. Nanotechnology2009, 20, 204001 ] 3. [ Berube et al. Int. J. Hydrogen Energ.2008, 33, 4122-4131 ] 4. [ Mooij et al. Adv. Energy Mater.2011, 1, 754-758 ]

Mesoporous palladium is an attractive material for use in high value hydrogen storage applications. In the case of tritium storage, it has been hypothesized that the pores should mitigate nucleation of helium-3 bubbles arising from β-decay. Scalable approaches to preparation of palladium with pore diameters of 2-3 nm have been reported, but these pores collapse at temperatures as low as 150°. Additionally, these small pores may be suboptimal for gas flow, highlighting the need for a versatile approach to soft templating of such materials. We have produced 100 mg-scale batches of mesoporous palladium with regular arrays of pores of larger diameter (~10 - 15 nm) by chemical reduction of palladium(II) salts within polystyrene-b-polyethylene oxide block copolymer templates. Control of the pore size is effected by varying the molecular weight of the polymer blocks. Analyses of pore geometry by microscopic and bulk techniques correlate well. The materials retain the favorable hydrogen storage properties of bulk palladium and larger pores show greatly increased thermal stability. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energyâ?Ts National Nuclear Security Administration under contract DE-AC04-94AL85000.

With the tremendous interest in spillover-type materials for hydrogen storage, NREL and DOE have dedicated considerable resources to demonstrate the reproducible synthesis of specific materials and to develop, validate and perform the requisite capacity measurements. Despite the fact that processes involving spiltover-hydrogen have been known for over 40 years in the catalysis community, the nature of this phenomenon has not been fully investigated or clarified. Without direct spectroscopic evidence, the debate regarding the chemical nature of the activated hydrogen moieties, their diffusion mechanism and ultimate hydrogen sorption capacities is still ongoing. Our approach to resolve some of these critical issues with spillover for hydrogen storage applications include the validation of syntheses and the rigorous characterization of a specifically selected set of materials that have an apparent enhanced hydrogen sorption capacity beyond what is expected for physisorption of dihydrogen, i.e., possibly metal catalyzed hydrogen spillover. After volumetric instruments, we evaluate the materials spectroscopically with diffuse reflectance infrared fourier transform spectroscopy (DRIFTS), neutron scattering and nuclear magnetic resonance (NMR) to identify specific reversible interactions between the receptor material and the hydrogen atoms. A summary of our collaborative results will be presented.

Sodium alanate (NaAlH4) is the most investigated metal complex hydride for its hydrogen sorption properties. Its composites doped with transition or rare earth metal compounds can absorb and release hydrogen in the low and medium temperature range with excellent reversibility and stability in cycling.1, 2 The NaAlH4 composites with CeCl3 as dopant differentiate from other dopants by the fast sorption kinetics and stable capacity.2, 3 In this paper, we applied various precursors of Ce by ball milling to investigate their effects on the sorption performance of this hydrogen storage material. It is found that the re-hydrogenation could complete in ca. 10 minutes. Although all the Ce precursors used result in reversible hydrogen storage materials, the desorption kinetics is enhanced upon formation of cerium aluminide (CeAl4) in the composites. While the use of CeAl4 instead of CeCl3 can increase the hydrogen capacity by bypassing the formation of the ineffective NaCl, the highest capacity of 4.9 wt%, close to the calculated value, was obtained from NaAlH4 doped directly with metallic cerium. Furthermore, dehydriding under back pressures was also investigated to evaluate the H2 deliverability under practical conditions. At 3 bar H2 pressure, the second desorption step of NaAlH4 was fully suppressed and only 2.5 wt% H was released at 150°C, whereas the capacity (about 4.5 wt%) was not much affected at 180°C, which is interesting for high temperature PEM fuel cell. Microscopic observation reveals that micropores ranging from a few dozens to hundreds of nanometers were developing in this material in the cycling process, which may be an important factor for the fast kinetics.

Metal borohydrides have attracted much attention in recent years as hydrogen storage materials for mobile applications due to their high gravimetric and volumetric hydrogen densities. Among metal borohydrides, Ca(BH4)2 releases 9.6 wt% hydrogen according to the following overall reaction: Ca(BH4)2 â?' 2/3 CaH2 + 1/3 CaB6 + 10/3 H2 (1) However, the actual dehydrogenation reaction involves at least two intermediate phases as along as the hydrogen back pressure is below a critical value. At 1 bar H2, Ca(BH4)2 decomposes into CaB2Hx or CaB12H12, indicating that there is a subtle competition between two different dehydrogenation routes. At higher PH2, the formation of the intermediate CaB2Hx is suppressed and direct decomposition into CaH2 and CaB6 is promoted. We also found that the intermediate CaB2Hx is reversible without any catalytic additives.

Lithium borohydride (LiBH4) is one of promising solid-state hydrogen storage materials owing to its high hydrogen capacity (13.9 wt%). However, its impractical hydrogen sorption conditions have been regarded as a major drawback to on-board applications. Accordingly, many efforts to improve its sorption conditions have been attempted by making its composites with other metal hydrides. The 4LiBH4+YH3 composite is one of their typical examples. We report a back pressure effect on the dehydrogenation reaction of the 4LiBH4+YH3 composite. In this study, we found out that the increase of hydrogen back pressure enhances the overall dehydrogenation reaction rate of the 4LiBH4+YH3 composite significantly, which is in contrast with Le Chatelierâ?Ts principle. More interestingly, the argon back pressure, which does not participate in the dehydrogenation reaction, plays a similar role to hydrogen during the dehydrogenation of the composite. Gas back pressure seems to be effective in suppressing the release of diborane gas (B2H6) and subsequent formation of Li2B12H12, which is known to form by the reaction between B2H6 and LiBH4 and to hinder the dehydrogenation reaction of the composite. Raman spectroscopy reveals the formation of Li2B12H12 in the samples dehydrogenated under static vacuum and 1 bar of Ar.

In this talk we will discuss recent advances from our group involving transition metal hydrazides in Kubas-type hydrogen storage. The majority of the presentation will focus on chromium hydrazide hydride gels, synthesized from bis(trimethylsilylmethyl) chromium and hydrazine. These materials use low-coordinate Cr hydride centers linked together via a network of hydrazine ligands as the principal H2 binding sites. The material with the highest capacity has an excess reversible storage of 3.23 wt.% at 298 K and 170 bar without saturation, corresponding to 40.8 kg H2/m3, comparable to the 2015 DOE system goal for volumetric density (40 kg/m3) at an operating pressure lower than the 200 bar standard for hydrogen transportation. These materials are distinct from hydrides and physisorption materials in their physical properties, possessing linear isotherms and enthalpies that rise on coverage, while also retaining up to 100 % of their adsorption capacities on warming from 77 K to 298 K with no kinetic barrier to adsorption or desorption. The talk will also cover more recent work on Ti and Mn hydrazide gels in hydrogen storage.

Metal ammine salts are interesting for both energy storage [1] and selective catalytic reduction of NO_x gases [2]. Metal ammines generally have a high ammonia density and exhibit fast ab- and desorption kinetics [3]. As Sr(NH₃)₈Cl₂ stores 8.2 wt%H it is a one of the metal ammine salts that has the highest gravimetric density of hydrogen, despite this the ab-/desorption mechanisms are not nearly as well understood as the other top candidates e.g. Mg(NH₃)₆Cl₂ or Ca(NH₃)₈Cl₂. We have measured ammonia desorption isotherms that reveal a stable diammine, Sr(NH₃)₂Cl₂, phase, often disregarded in the literature. The atomic structure of the diammine phase has subsequently been determined with synchrotron x-ray powder diffraction and the stability of the phase has been confirmed with density functional theory (DFT). By including van der Waals forces in the exchange correlation functional [4], DFT calculations can give desorption enthalpies for these systems that lie within a few kJ/mol of experimental values. We can then show that the desorption pathway of Sr(NH₃)₈Cl₂ goes through a diammine phase, but if the desorption is taking place above room temperature, degassing will continue to the monoammine phase. Furthermore, absorption experiments reveal two distinct ammonia equilibrium pressure regimes indicating the presence of a stable surface state, never before reported. These surface states have been determined using DFT as NH₃ molecules stabilised by two long hydrogen bonds to Cl atoms in the surface. The adsorption energies have again been determined to within a few kJ/mol of the experimental values. This presentation is focused on the theoretical aspects of the investigation. References [1] R. Z. Sørensen, J. S. Hummelshøj, A. Klerke, J. B. Reves, T. Vegge, J. K. Nørskov and C. H. Christensen, J. Am. Chem. Soc. 2009, 130, 8660. [2] T. D. Elmøe, R. Z. Sørensen, U. Quaade, C. H. Christensen, J. K. Nørskov and T. Johannesen, Chemical Engineering Science, 2006, 61, 2618. [3] A. Tekin, J. S. Hummelshøj, D. Sveinbjörnsson, H.S. Jacobsen, D. Blanchard, J. K. Nørskov and T. Vegge, Energy&Environmental Science, 2010, 3, 448-456, doi: 10.1039/b921442a. [4] M. Dion, H. Rydberg, E. Schröder, D.C. Langreth and B.I. Lundqvist, Phys. Rev. Lett., 2004, 92 (24), 246401

Hydrogen storage using magnesium hydride is attractive for its high reversible hydrogen capacity, which at 7.6 wt% outweighs most other hydrides. However, slow kinetics and low equilibrium hydrogen pressure at room temperature remain as drawbacks for practical applications. In this work, nanostructured Mg/Pd thin films are sputtered by magnetron sputtering. An optical transmission technique is used to measure equilibrium pressure of the sample. A remarkable increase of equilibrium hydrogen pressure (400 times over bulk Mg at the same temperature) caused by intermixing of Mg with Pd is observed. Transmission electron microscopy (TEM) and depth profiling x-ray photoemission spectroscopy are used as a direct measure of intermixing at the interface. It is also observed that the equilibrium pressure is further increased by annealing, which increases the intermixing region at the interface. In-situ stress measurements show that thereâ?Ts no stress in the hydride layer and we conclude that the increase of equilibrium hydrogen pressure is caused by the chemical energy involved during alloy formation, not strain energy induced by volume expansion upon hydrogenation. A simple thermodynamic model is present to show the effect of alloying on equilibrium pressure and this model is consistent with our experiment results. In addition to nanostructured Mg-based thin films, Mg-Pd core-shell and Mg-Pd alloy nanoparticles can also have enhanced kinetics and thermodynamics. Nanoparticles are produced by a â?oterminated gas condensationâ? method under high argon pressure in an ultra-high vacuum chamber. An in-situ quadruple mass spectroscopy allows us to generate size-selected nanoparticles, ranging from 1 nm to 20 nm with spherical morphology. These particles are soft-landed on the substrate which eliminates the substrate effects. An optical transmission technique is also used to measure equilibrium hydrogen pressure of these nanoparticles.

Many kinetically stabilized hydrides (e.g., AlH₃, LiAlH₄, MgAlH₄) exhibit high gravimetric and volumetric hydrogen densities along with rapid hydrogen evolution rates at low temperature, making them well suited for mobile PEM fuel cell applications (e.g., portable electronics and vehicles). However, a critical challenge exists to regenerate or recycle these hydrides from the spent fuel and H₂ gas in a low cost process. Previously we proposed a two-step regeneration process, which involves initially forming a stabilized hydride adduct using amines or ethers, followed by adduct separation and hydride recovery. Based on this simple concept, a low energy regeneration procedure was developed for LiAlH₄, which involved the formation of LiAlH₄-nTHF from catalyzed Al, LiH and THF, followed by desolvation and recovery of pure LiAlH₄. Here we propose a low energy route to regenerate AlH₃ that involves the formation of a stabilized AlH₃ adduct, followed by adduct separation and hydride recovery. The first step of AlH₃ regeneration involves the low-pressure hydrogenation of catalyzed Al and an amine (NR₃) in a liquid medium to form an amine alane (AlH₃-NR₃). We found a number of different tertiary amines were suitable including trimethylamine, triethylenediamine, dimethylethylamine, quinuclidine and hexamine. However, the more stable alane amines that form readily at low pressures tend to decompose at high temperatures where AlH₃ is unstable. Therefore, the direct separation and recovery of AlH₃ has proven to be difficult. A transamination step is necessary to exchange the amine with one that forms a less stable adduct (e.g., TEA = triethylamine). AlH₃-TEA is a liquid at room temperature and can be separated into AlH₃ and TEA by heating to 75°C under a nitrogen sweep. The full regeneration procedure for AlH₃ is shown below: Hydrogenation: Al + NR₃ + 3/2H₂ â?' AlH₃-NR₃ Transamination: AlH₃-NR₃ + TEA â?' AlH₃-TEA + NR₃â?' â?' AlH₃ + TEAâ?' Separation: AlH₃-TEA â?' AlH₃ + TEAâ?' We note that this simple regeneration procedure may be broadly applicable to other kinetically stabilized hydrides (e.g., Mg(AlH₄)₂).

Synthesis of Mg(BH4)2 by hydrogenation of layer compound MgB2 has been proved to be a feasible method [1]. However, it is still missing that how hydrogen was inserted into the interlayer consisted of Mg and B layers. In this study, we investigated the hydrogenation properties of layered compound MgB2 mechanically milled under a hydrogen pressure of 1.0 MPa. X-ray diffraction profiles showed that the interlayer distance was not changed substantially with increasing milling durations, which indicates that no hydrogen was inserted into MgB2 interlayer. The total hydrogen concentration of approximately 1.0 mass% after milling for 600 min suggests that mechanical milling generated the nanostructured MgB2 with grain boundary defects in which Bâ?"H covalent bond and/or Mgâ?"H bond were assumed to be formed. The nanostructure with grain boundary defects significantly promoted the subsequent high pressure (40.0 MPa) hydrogenation of MgB2 for the synthesis of Mg(BH4)2 comprising [BH4]â^' anions with Bâ?"H covalent bonds [2]. [1] G. Severa, E. Rönnebro, C.M. Jensen Chem. Commun. 2010, 46, 421; [2] H.-W. Li et al., J. Alloys Compd. 2010, 505, 654.

Metal Organic frameworks are being considered for hydrogen storage because of their excellent hydrogen adsorption and release rates, their tailorable structures and high surface area. Unlike materials that chemically incorporate hydrogen via dissociation and chemical bonding, and require heating for H₂ release, MOFs utilize physisorption as the adsorption mechanism. They are crystalline so that hydrogen adsorbs at specific adsorption sites. This work explores H₂-H₂ interactions between adsorbed H₂ in different binding sites in MOF-74 (M₂(dhtp),dhtp=2,5-dihydroxyterephthalate) that is composed of unsaturated metal centers. Combining vdW-DFT calculations and IR spectroscopic techniques we are able to characterize these interactions by monitoring the IR shifts (from the unperturbed H₂ stretch frequency) and the dipole moments of adsorbed H₂. In MOF-74, these adsorption sites are, in order of decreasing binding energy, the metal, oxygen, benzene and center sites, as determined by neutron scattering by Liu et al. Our study shows that the IR shifts and dipole moment of adsorbed H₂ are greatly affected by H₂-H₂ interactions. The frequency of adsorbed H₂ at the metal site suffers an additional ~-30 cm^-1 red shift (for Mg and Zn) and ~-84 cm^-1 (for Co) when the neighboring oxygen site is occupied due to H₂-H₂ interaction of a â?opairâ? of hydrogen atoms. The dipole moment of adsorbed H₂ at the metal site also decreases when the oxygen site is occupied. The larger shift observed for the cobalt case can be attributed to the proximity of the H₂ molecules to the structure. IR measurements performed at 77 K show the appearance of the IR band corresponding to such a pair (i.e. occupation of both the metal and oxygen sites) even at low loading (when all metal sites are not occupied). This observation suggests that the formation of a pair is favorable and the existence of a diffusion barrier to occupy the next available metal site. For MOF-74-Co, the frequency of the H₂ adsorbed at the metal site is affected by the H₂ adsorbed at the benzene sites due to the proximity of the hydrogen to the framework. Performing measurements at low temperatures (77 and 30K) enables us to study H₂-H₂ interactions as a function of loading. At these low temperatures (30K), kinetics limit the diffusion and the loading is therefore larger at the peripheries of the sample. With increased local loading and occupation of the benzene sites, the dipole moment of adsorbed H₂ at the metal site decreases without modification of its frequency shift. However, when the benzene sites in the unit cell are completely occupied the frequency of H₂ adsorbed at the metal sites undergo an additional ~-10 cm^-1 red shift. All these changes strongly suggest the existence of H₂-H₂ interactions in MOF-74 system and these interactions extend to the lower binding energy sites with the proximity of H₂ to the framework.

Within the widespread activity in the development of innovative hydrogen storage materials, rising attention has been recently addressed, by the scientific community, to the sorption properties of nanosized materials confined into the mesoporous networks of matrixes with different chemical nature. Improvements of the thermodynamic properties and kinetics of hydrogen release and uptake are expected through the reduction to nanometer scale and the control of grain size of hydrides particles. As a further beneficial effect, the undesired grain growth and particle agglomeration would be avoided [1]. While most of the papers recently published deal with C based porous materials [2], less attention has been devoted to chemically different scaffolds which could offer interesting opportunities in tuning the sorption properties of hydrogen rich phases. Within this context, in this work we evaluate the possibility to infiltrate single and multicomponent chemical hydrides into Si- and transition metal-based mesoporous structures, previously prepared by sol-gel methods. Different mixtures of NaBH4 and MgH2 were embedded in high surface matrixes by wet chemical impregnation and by melting infiltration. Structural characterization was performed by X Ray diffraction, small angle neutron scattering and transmission electron microscopy in order to estimate the efficiency of impregnation techniques. The thermodynamics and kinetics of the absorption and desorption processes, and the chemical nature of the released gas were analyzed by high-pressure differential scanning calorimetric device (HP-DSC), Sievert apparatus (PCT-pro2000) and mass spectroscopy (MS). [1] M. Fichtner, Nanotechnology, 20, (2009) 204009. [2] P. Adelhelm, J. Gao, M.H.W. Verkuijlen, C. Rongeat, M. Herrich, P.J.M. van Bentum, O. Gutfleisch, A.P.M. Kentgens, K.P. de Jong, P.E. de Jongh, Chemistry of Materials 22 (2010) 2233-2238.

Due to its high surface area and correspondingly rapid kinetics, nanoporous palladium is a promising material for chromatographic separation and storage of hydrogen isotopes. In the case of tritium storage, it has been hypothesized that pores should mitigate nucleation of helium-3 bubbles arising from tritium decay. A technique for preparing nanoporous palladium alloys based on partial consolidation of dendrimer encapsulated nanoparticles (DEN) will be presented. Destabilization of a colloidal suspension of DEN and purification yields a high surface area material with pore diameters between 20 and 200 nm. Compared to other aqueous chemical reduction methods, this approach has the advantage of compositional uniformity that is tunable based on the composition of the DEN. Nanoporous Pd/Rh-10% alloys with uniform composition or with Rh enrichment at pore walls and grain boundaries have been prepared and characterized. Their hydrogen and deuterium storage properties were evaluated and compared to nonporous Pd/Rh-10% powders. In addition, the materials have been demonstrated to be thermally stable by microscopic and bulk techniques, with high surface area and pore structure retained to 300-400°C. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energyâ?Ts National Nuclear Security Administration under contract DE-AC04-94AL85000.

Hydrogen is recognized as a potential and extremely interesting energy carrier system, but a remaining major challenge in a future â?~hydrogen economyâ?T is the development of a safe and efficient means of hydrogen storage [1]. Here we report our recent research results within nano-confinement of different metal hydrides and the discovery of a porous magnesium borohydride, γ-Mg(BH4)2 [2-6]. Magnesium hydride (MgH2) nanoparticles was directly synthesized in monoliths of nanoporous carbon aerogel scaffolds and lithium borohydride (LiBH4) was melt infiltrated. The reactive hydride composite MgH2 and LiBH4 reacts during release of hydrogen and form MgB2 [3,4]. The hydrogen desorption kinetics is significantly improved compared to bulk conditions, and the nanoconfined system has a high degree of reversible stability and possibly also improved thermodynamic properties. Recently, we successfully nanoconfined NaAlH4 in a nanoporous scaffold functionalized with TiCl3 nanoparticles. Hydrogen desorption kinetics was significantly improved as compared to nanoconfined NaAlH4 without the catalysts and also to bulk ball-milled samples of NaAlH4-TiCl3. The system has high reversible stability and was cycled four times. We also present systematic studies of changes in hydrogen storage properties as a function of pore size and scaffold surface area. A fascinating structural chemistry is discovered within metal borohydrides, e.g. zeolite-type structures with up to 30% â?~emptyâ?T space in the porous structures [6]. A new polymorphs of magnesium borohydrides form the most open-structured hydride, which stores both chemically bonded hydrogen (14.9 wt%) and additionally physisorped molecular hydrogen, a total of 17.4 wt% H2 [6]. Another polymorph is among the most dense hydrides with a volumetrical hydrogen density of 147 g H2/L for δ-Mg(BH4)2. [1] L. Rude, et al, Physica Status Solidi 2011, 208(8) 1754. [2] T. K. Nielsen, et al, Nanoscale, 2011, 3, 2086. [3] T. K. Nielsen, et al, ACS Nano, 2009, 3, 3521. [4] T. K. Nielsen, et al, ACS Nano, 2010, 4, 3903. [5] Nielsen, et al, ACS Nano, 2011, 5, 4056. [6] Y. Filinchuk, et al, Angew. Chem. Int. Ed. 2011, DOI: 10.1002/anie.201100675.