Symposium ES05—Cooperative Catalysis for Energy and Environmental Applications

Heterogeneous olefin metathesis catalysts are made by dispersing a mid-transition metal oxide on an "inert" support such as silica-alumina or alumina. However, the support is an essential part of the catalyst; no metathesis is observed in its absence. Moreover, the activity and stability of the catalyst depends strongly on the choice of support, including its crystallinity and Lewis/Brønsted acidity. The origin of the cooperativity between support and active site was investigated by spectroscopic characterization of dispersed Re(VII) sites, including advanced NMR characterization methods. The number of active sites is limited by the availability of certain strong Lewis acid sites which are identified as five-coordinate Al centers. The lifetime of these sites is a function of their distance from hydroxyl groups, both terminal and bridging. A rapid deactivation process is attributed to the presence of local hydroxyls, while a slower deactivation process is associated with mobility of the active sites which brings them into proximity with hydroxyl groups. These insights allow us to design more effective metathesis catalysts with greatly improved activity and productivity.

Single metal atoms and metal clusters have attracted much attention thanks to their possibility as heterogeneous catalysts.[1,2] However, the generation of stable single atoms and clusters on a solid support is still challenging.[2] Recently, we report a new strategy for the generation of single Pt atoms and Pt clusters with exceptionally high thermal stability, formed within purely siliceous MCM-22 during the growth of a two-dimensional precursor of the zeolite into three-dimensional structure.[4] These subnanometric Pt species are stabilized in the zeolite, even after treatment in air up to 540 ^oC. Furthermore, we have studied the dynamic structural transformation of those subnanometric Pt species during reduction-oxidation treatments and under reaction conditions (CO+O₂, water-gas shift, NO+CO and NO+H₂) by environmental transmission electron microscopy (ETEM).[5]
The Pt@MCM-22 was prepared by the transformation of 2D into 3D zeolite, using purely siliceous ITQ-1 as precursor. In situ electron microscopy experiments were performed using a Titan 80-300 Environmental Transmission Electron Microscope at the Centre for Functional Nanomaterials (CFN), Brookhaven National Laboratory.
Subnanometric Pt species are finely dispersed in MCM-22 crystallites. With the help of aberration-corrected electron microscopy, we are able to directly measure the size of those Pt species and figure out their position in the zeolite. Some of the Pt atoms and clusters are located in the surface “cups” of MCM-22 and some of them are anchored to the zeolite framework. Nevertheless, a large part of subnanometric Pt species are located in the internal space of the structure, which is confirmed by the size-selective hydrogenation of propene and isobutene. The exceptional high stability of the encapsulated subnanometric Pt species has also been reflected in the propane dehydrogenation reaction to propylene, showing higher activity and stability than Pt nanoparticles prepared by conventional impregnation method.
Furthermore, using Pt@MCM-22 material as a model system, we have studied the evolution of Pt single atoms and clusters during reduction-oxidation treatments and under reaction conditions by ETEM. Pt nanoparticles and clusters will disintegrate into smaller clusters or even single atoms after calcination in O₂. And single atoms and clusters will agglomerate into small particles after reduction by H₂. Besides, it can be a general phenomenon that subnanometric metal species will undergo dynamic structural evolution under reaction conditions. For the same reaction, the states of metal species are dependent on the temperature and atmosphere.
Our synthesis strategy can also be applied for preparation of Au@MCM-22 and Pd@MCM-22 materials, with subnanometric Au and Pd species encapsulated in MCM-22 zeolite. Those subnanometric metal clusters show unique catalytic behavior for selective aerobic oxidation of cyclohexane to KA-oil and low-temperature combustion of CH₄, respectively.
Subnanometric metal species (single atoms and clusters with a few atoms) with exceptional high stability can be generated and stabilized in MCM-22 zeolite. Those subnanometric metal species show dynamic structural transformation during reduction-oxidation treatments and under reaction conditions and also show unique catalytic properties for various reactions.
References
[1] M. Boronat et al. Acc. Chem. Res. 47, 834-844 (2014).
[2] L. Liu et al. Chem. Rev. 118, 4981-5079 (2018).
[3] A. Corma et al. Nat. Chem. 5, 775-781 (2014).
[4] L. Liu et al. Nat. Mater. 16, 132–138 (2017).
[5] L. Liu et al. Nat. Commun. 9, 574 (2018).

We stabilize plasmonic Cu nanoparticles by establishing extensive interfacial contact between photodeposited Cu and an anatase TiO₂ aerogel support. The three-dimensional (3D) interfacial contact between sub-5-nm Cu nanoparticles and the covalently networked ~10-nm TiO₂ particulates that create the aerogel framework stabilizes Cu against oxidation to an extent that preserves the plasmonic behavior of the nanoparticles, even after long-term exposure to ambient air. The Cu nanoparticles entrained in the TiO₂ aerogel comprise a metallic Cu core with a shell featuring high fractions of Cu⁰ and Cu¹⁺ species, with a small fraction Cu²⁺.

Both the plasmonic character and low oxidation–state speciation of Cu prove critical to several thermal and photochemical oxidation and hydrolytic degradation pathways. The Cu/TiO₂ aerogels catalyze oxidation of carbon monoxide (CO) at temperatures as low as ~75 °C at rates comparable to those observed at nanostructured Au/TiO₂ materials. Varying the relative amounts of Cu⁰, Cu¹⁺, and Cu²⁺ in the supported Cu nanoparticles determines the activation energy (E_a) for CO oxidation, with the lower oxidation states driving more energetically facile pathways with faster kinetics. In contrast, depositing Cu nanoparticles on commercial anatase powders results in Cu oxide nanoparticles with no detectable Cu⁰, and much higher fractions of Cu²⁺: Cu¹⁺ than observed when supporting Cu on TiO₂ aerogels, resulting in poorer CO oxidation rates.

The Cu/TiO₂ composite aerogels also fully hydrolyze the chemical warfare (CW) simulant dimethyl methylphosphonate (DMMP) under aerobic and anaerobic conditions. In-situ diffuse-reflectance infrared spectroscopy (DRIFTS) shows that all initial reactants and final hydrolysis products identified are bound to TiO₂, however, aerogel-expressed anatase TiO₂ alone does not hydrolyze DMMP. Excess surface concentration of OH species at Cu/TiO₂ aerogels created near Cu||TiO₂ junctions drives DMMP hydrolysis.

Finally, we describe the thermal and hydrolytic and oxidative degradation of the organophosphorous chemical warfare (CW) agents Sarin (GB) and Soman (GD) and the enhancement of their degradation by visible light–driven surface plasmon resonance (SPR)-induced pathways. We previously demonstrated SPR-driven photoelectrochemical oxidation of methanol u nder visible-light illumination, which in contrast is not observed for Cu photodeposited at non–networked anatase TiO₂ nanopowders [1]. The Cu/TiO₂ aerogels enable us to apply a suite of thermal and photochemical degradation pathways for CW agent decontamination in one composite multifunctional material.

The exceptional activity for Cu-catalyzed oxidations and hydrolysis at a 3D Cu||oxide interface characteristic of aerogel supports highlights the importance of the interfacial design motif on both preserving plasmonic Cu and on driving oxidation and hydrolysis reactions in the dark—conditions requiring most of the promoting effects of the active support oxide to occur in very close proximity to the Cu||oxide interface.

[1] P. A. DeSario, J. J. Pietron, T. H. Brintlinger, M. McEntee, J. F. Parker, O. Baturina, R. M. Stroud, D. R. Rolison, Nanoscale, 2017, 9, 11720.

Supported nanoparticles are broadly employed in industrial catalytic processes, where the active sites can be tuned by metal-support interactions (MSIs). Although it is well accepted that supports can modify the chemistry of metal nanoparticles, systematic utilization of MSIs for achieving desired catalytic performance is still challenging. The developments of supports with appropriate chemical properties and identification of the resulting active sites are the main barriers. Here, we develop two-dimensional transition metal carbides (MXenes) supported platinum as efficient catalysts for light alkane dehydrogenations. Ordered Pt₃Ti and surface Pt₃Nb intermetallic compound nanoparticles are formed via reactive metal-support interactions on Pt/Ti₃C₂T_x and Pt/Nb₂CT_x catalysts, respectively. MXene supports modulate the nature of the active sites, making them highly selective toward C–H activation. Such exploitation of the MSIs makes MXenes promising platforms with versatile chemical reactivity and tunability for facile design of supported intermetallic nanoparticles over a wide range of compositions and structures. This work is recently published online at Nature Communications on December 10, 2018.

Fischer-Tropsch to olefins (FTO) synthesis has drawn significant attention due to the high demand of light olefins as building blocks for the chemical industry, the desire to reduce the dependence on petroleum cracking for these chemicals, and the need for improving environmental sustainability. Iron-based catalysts have emerged as promising FTO catalysts because of their low cost and excellent catalytic performance. We have designed a promising nanocatalyst of porous interconnected carbon nanosheets supported iron oxide nanoparticles for FTO synthesis. The catalysts demonstrate an extremely high iron time yield (FTY) of 1882 μmol_CO/g_Fes with 41% selectivity for light olefins and excellent stability (> 100 h time on stream). Our FTY value is found to be one of the highest FTYs and in particular, is 50 to ~1300 times higher compared with those catalysts exhibiting similar light olefins selectivity reported in literature. Mössbauer characterization demonstrates the presence of catalytically active iron carbide species after FTO reaction. Ex situ XAFS measurements reveal that the carbon nanosheets support stabilizes metallic iron during the initial catalyst reduction step, which leads to more carbon uptake under FTO reaction conditions to more efficiently form highly active iron carbide phase. In contrast, a control sample with carbon nanotubes as the catalyst support exhibits oxidized iron in the catalyst after removal from H₂ reduction pretreatment, suggesting a substantially reduced stabilizing effect of carbon nanotubes towards supported metallic iron nanoparticles. This results in inefficient formation of iron carbide species under FTO conditions. Additional EXAFS results further confirm the buildup of iron carbide phase as a function of time on stream in the carbon sheets supported catalysts, whereas a much smaller portion and negligible growth of iron carbide phase was observed in the carbon nanotube supported catalysts under the same reaction conditions.

Pt catalysts are widely studied for the oxygen reduction reaction (ORR), but their cost and susceptibility to poisoning limit use. A strategy to address both problems is to incorporate a second transition metal to form a bimetallic alloy; however, the durability of such catalysts can be hampered by leaching of non-noble metal components. Here, we show that random alloyed surfaces can be stabilized to achieve high performance and durability by depositing the phase on top of intermetallic seeds. Specifically, random alloyed PtCu shells were deposited on PdCu seeds that were either the atomically random face-centered cubic phase (FCC A1) or the atomically ordered phase (CsCl-like B2). Precise control over crystallite size, particle shape, and composition allowed for comparison of these two core@shell PdCu@PtCu catalysts and the effects of the core phase on electrocatalytic performance. Indeed, the nanocatalysts with intermetallic cores saw only an 18% decrease in activity after stability testing, whereas the random alloy core catalyst saw a 58% decrease. Although many random alloy and intermetallic nanocatalysts have been evaluated, this study is the first to directly compare random alloy and intermetallic cores for electrocatalysis and will facilitate the design of electrocatalysts with high activity and durability.

Electrolytic water splitting with concurrent hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), is an attractive way to produce hydrogen in large scale. However, in traditional electrolytic cells, the OER and HER reactions are generally coupled in the same cell compartment separated by a membrane, and the energy efficiency is largely compromised by gas crossover as well as sluggish OER reaction. Herein, we report an effective means to spatially decouple the OER and HER reactions from the cell to separate gas production tanks by using dissolved redox mediators. Meanwhile, a cost-effective OER catalyst NiOOH is synthesized by a facile electrodeposition method which exhibits excellent activity and stability in alkaline solution. Upon operation, oxygen is produced on the NiOOH catalyst kept in tank when a regenerative redox electrolyte circulates through the system, which transports electrons between the electrode in the cell and catalyst in the tank, thus obviating complex gas electrode design and preventing gas mixing. The oxidized redox mediator is regenerated when it flows back to the electrode. Such a decoupled OER system presents relatively low overpotential, high Faradaic efficiency and long-term durability for water splitting. The redox targeting-based catalytic process are scrutinized, and the mechanisms are unequivocally manifested with various electrochemical and spectroscopic measurements. We anticipate the decoupled OER and HER reactions based on the redox targeting concept allow greater flexibility and practicality for the development of electrolytic and even photoelectrolytic water splitting technologies.

The ability to directly produce fuels by consumption of carbon dioxide (CO₂) is one of the promising solutions that would have a transformational impact on both global energy and environmental sustainability. The success of this groundbreaking technology depends on products that forms from consumption of CO₂, and hence it is important to develop effective catalytic materials for facilitating the CO₂ conversion process. However, existing catalysts do not exhibit high efficiency and good selectivity, which is leading to multiple products. Major reported CO₂ reduction catalysts are limited with formation of CO and formate that is two-electron reduced products. The formation of hydrocarbon products, which involve multiple proton and electron transfers still remain one of the major scientific challenges that needs to be addressed. Presently, there is no known catalyst that can facilitate CO₂ conversion to hydrocarbon fuels with both good efficiency and high selectivity mainly because of the little mechanistic understanding. Here, we will discuss tuning the hybrid structure of metal-oxide nanoparticles and two-dimensional (2D) layered materials as an efficient and selective CO₂ reduction electrocatalyst with the density functional theory calculations, which reveals the unusual electrocatalytic properties of hybrid structure creating intrinsic chemical and electronic coupling and assist to understand the mechanism beyond the process.

Direct ethanol fuel cell (DEFC) is a promising electrochemical energy conversion device. Its application, however, is hindered by lack of efficient catalysts for complete oxidation of ethanol into carbon dioxide. Here we report the study of ethanol electro-oxidation on Pt3Sn alloy nanoparticles. Electrochemical studies and product analysis were conducted comparatively on Pt3Sn/C, commercial Pt/C as well as Pt3Sn/C with surface tin species removed via KOH treatment. Our investigation reveal the dual role of Sn in Pt3Sn/C for ethanol electro-oxidation: surface SnOx species facilitate oxidation of *CHx to *CO while subsurface Sn species enhances oxidative removal of *CO.

Platinum group metal (PGM) catalysts are the current standard for control of pollutants in automotive exhaust streams. In spite of their high cost, PGM catalysts exhibit low catalytic activities for CO oxidation at low temperatures (<200 °C) because of inhibition by hydrocarbons in exhaust streams. Here we present nanostructured ternary and high entropy oxide catalysts that potentially outperform commercial PGM catalysts for CO oxidation in simulated exhaust streams and other energy-related oxidative catalysis reactions. The synthesis strategies toward various nanostructured multi-component oxide catalysts as well as that using the entropy maximization will be discussed. In conclusion, these nanostructured multi-component oxide catalysts show great potential as a low-cost component for various low-temperature oxidative processes.

Electrochemical spliting water into hydrogen and oxygen provides alternative way to reduce the current reliance on fossil fuels and thereby mitigate net carbon dioxide emissions.^[1] The oxygen evolution reaction (OER) in water spliting process typically requires noble-metal-based catalysts to reach the desired current density of 10 mA cm^-2 with a substantial overpotential.^[2] Unfortunately, the high overpotential to drive the reaction significantly reduces the conversion efficiency of water splitting. Furthermore, it is difficult to fabricate these precious metal catalysts in mass production owing to their high cost and low elemental abundance. To solve these problems, considerable efforts have been devoted to explore low-overpotential, stable and earth-abundant OER catalysts.
Given that transition metal compounds, including oxides, sulfides, phosphides, and nitrides, inevitably transform into their corresponding oxides after OER,^[3] the transition metal oxides are the most promising alternatives owing to considerable stability and theoretical activity. In particular, cobalt oxide (Co₃O₄) holds great promise to substitute for rare precious metal based catalysts due to its low cost and earth abundance. However, the utilization of Co₃O₄ as an OER catalyst is limited by poor mass-transfer ability. CeO₂, with the flexible switch between Ce³⁺ and Ce⁴⁺ oxidation states, shows reversible surface oxygen ion exchange, high oxygen-storage capacities, and good electronic/ionic conductivity,^[4] which offers the opportunity to generate strong electron interactions with other materials. Therefore, CeO₂ is expected to modulate the electronic structure and adsorption property of Co₃O₄ effectively.
We develop heterolayered Co₃O₄/CeO₂ nanotubes catalysts by using Cu₂O nanowires as templates. The Co₃O₄/CeO₂ nanotubes exhibit superior activity for oxygen evolution in comparison with pristine CeO₂ nanotubes. Our density functional theory (DFT) calculations show that CeO₂ can systematically modulate charge density of Co₃O₄, which provides near-optimal adsorption energies at Co catalytic sites for various OER intermediates simultaneously, and substantially enhances OER performance of Co₃O₄. This work underscores the importance of modulating the electron density of catalytic sites, and provides the guidance to implement more targeted strategies for developing highly efficient OER catalysts.

[1] Z. W. Seh, J. Kibsgaard, C. F. Dickens, I. Chorkendorff, J. K. Nørskov, T. F. Jaramillo, Science 2017, 355, eaad4998.
[2] N.-T. Suen, S.-F. Hung, Q. Quan, N. Zhang, Y.-J. Xu, H. M. Chen, Chem. Soc. Rev. 2017, 46, 337.
[3] S. Jin, ACS Energy Lett. 2017, 2, 1937.
[4] H. Xu, J. Cao, C. Shan, B. Wang, P. Xi, W. Liu, Y. Tang, Angew. Chem. 2018, 130, 8790.

Given the crucial role of catalysis science in our society, understanding how to design catalyst structures for desired catalytic activity and selectivity becomes a grand challenge. Core-shell nanostructured materials hold the promise of being active, cost-effective, and stable catalysts. I will demonstrate how we understand the structure-performance correlation at the atomic-scale level through a set of computational approaches and apply it to address the catalyst challenge in the fields of energy conversion and storages. Computationally, we designed heterostructured perovskites for electrochemical water splitting, which were experimentally synthesized, characterized and validated(Figure 1).¹ This sets the stage for the future design of core-shell oxide nanoparticles and their use for chemical reactions under harsh chemical environments. I will provide an understanding of the activation mechanism of the heterostructures based on first principle simulations (Figure 2).² Finally, I will bridge core-shell architectured oxide and transition metal systems and discuss our vision of potential design strategies.


1. Akbashev, A.R.; Zhang, L.; Chueh, W.C.; Vojvodic, A. “Activation of SrTiO3 with Subsurface SrRuO3for Oxygen Evolution Reaction”, 2018, Energy Environ. Sci. in press, DOI: 10.1039/C8EE00210J

2. Zhang, L.; Raman, S. A.; Vojvodic, A. “Subsurface Engineering of SrTiO3 for Electrochemical Water Splitting”, 2018, submitted.

Many researchers are eager to achieve novel systems for the synthesis of fuels or organic substances from CO₂ under solar irradiation in an effort to help alleviate global warming and fossil fuel shortages. If CO₂ can be reduced by utilizing water as an electron donor, then this would constitute an ideal reaction that would mimic photosynthesis in plants. We have previously reported CO₂ reduction with very low overpotential and high reaction selectivity in an aqueous medium using abundant manganese complex polymer ([Mn-MeCN]) catalysts.[1] Moreover, as a catalyst for oxygen evolution reaction, we have developed akaganeite-type iron oxyhydroxide surface-modified with amorphous nickel hydride (β-FeOOH:Ni/a-Ni(OH)₂).[2-3] In this presentation, we demonstrate photoelectrochemical CO₂ reduction to CO with a solar-to-chemical conversion efficiency (η_CO) of 3.7% by combining the earth-abundant catalysts wired with a cheap poly-crystalline silicon photovoltaic cell (poly-Si cell) as a light absorber. Over 80% of current efficiency for CO production was observed in a one-compartment reactor, even in the presence of both oxygen gas and protons. We have also successfully conducted the reaction in neutral aqueous solution by formation of a Ni(OH)₂/NiOOH redox system by pre-treatment of the β-FeOOH:Ni/a-Ni(OH)₂. A photocathode composed of [Mn-MeCN] and triple junction amorphous silicon connected with a β-FeOOH:Ni/a-Ni(OH)₂ anode also achieved η_CO of 1.0%, which suggests a future possibility to realize an ultimately-simplified monolithic artificial photosynthesis device to produce only gaseous products through the use of earth-abundant catalysts and a photoabsorber.[4]

References
[1] Sato, S.; Saita, K.; Sekizawa, K.; Maeda, S.; Morikawa, T., Low-energy electrocatalytic CO₂ reduction in water over Mn-complex catalyst electrode aided by a nanocarbon support and K⁺ cations. ACS Catal. 2018, 8, 4452–4458.
[2] Suzuki, T. M.; Nonaka, T.; Suda, A.; Suzuki, N.; Matsuoka, Y.; Arai, T.; Sato, S.; Morikawa, T., Highly crystalline b-FeOOH(Cl) nanorod catalysts doped with transition metals for efficient water oxidation. Sustainable Energy & Fuels 2017, 1, 636-643.
[3] Suzuki, T. M.; Nonaka, T.; Kitazumi, K.; Takahashi, N.; Kosaka, S.; Matsuoka, Y.; Sekizawa, K.; Suda, A.; Morikawa, T., Highly enhanced electrochemical water oxidation reaction over hyperfine β-FeOOH(Cl):Ni nanorod electrode by modification with amorphous Ni(OH)₂. Bull. Chem. Soc. Jpn. 2018, 91, 778-786.
[4] Arai, T.; Sato, S.; Sekizawa, K.; Suzuki, T. M.; Morikawa, T., 2018, submitted.

The world’s oceans contain about 100 mg of dissolved CO₂ L^-1. On a volumetric basis, this is 140 times more concentrated than atmospheric levels, and thus represents an untapped reservoir of naturally sequestered CO₂. Under appropriate hydrogenation conditions, it has been estimated that 1 L of liquid fuel can be produced from the CO₂ dissolved in only 89 L of sea water.¹ Given that hydrogen gas can be isolated via the electrolysis of sea water, one can imagine that with the appropriate infrastructure and energy input (renewable or otherwise), liquid fuels could be synthesized on demand, using seawater as the lone chemical precursor. The U.S. Naval Research Laboratory has recently developed a patented 2-step approach to realize the synthesis of fungible fuels from seawater. The first step includes isolation of CO₂ and H₂ gas using an electrochemical cell. In a subsequent step, the gasses can be thermochemically transformed into various products, depending on the reaction conditions and catalyst system employed.

In this poster, we describe recent advances from our group as it pertains to the synthesis of hydrocarbons from seawater. Specifically, we detail the discovery and characterization of supported transition metal carbide catalysts for the reverse water gas shift reaction (CO₂ + H₂ -> H₂O + CO). This reaction represents the first step in the transformation of CO₂ to liquid fuels, and thus is critical to the overall yield. These carbide systems are stable, earth abundant, and comparable to state-of-the-art RWGS catalysts in terms of both activity and selectivity.


References:
Willauer et al. “The feasibility and current estimated capital costs of producing jet fuel at sea using carbon dioxide and hydrogen”. Journal of Renewable and Sustainable Energy, 4, 2012, 033111.

Electrocatalysts for the oxygen reduction reaction (ORR) are essential elements for the redox reactions in fuel cells and metal–air batteries. Pt-based ORR catalytic systems show excellent catalytic activities but their high costs and low durability are inherent critical drawbacks. Pt-free systems, other metal systems, carbon-based metal-free materials, and nonprecious-metal–carbon hybrids have been suggested as alternative candidates for this purpose. Organometallic complexes with transition metal coordination are an important class of molecular catalysts in various applications owing to their highly tunable properties, such as coordination numbers, oxidation states of metal centers, and binding structures between metals and ligands. Consequently, the coordination of such organometallic active centers to nanomaterials, which have huge steric bulk and variable electrical properties, can influence their catalytic performances in a dramatic way. In particular, electrocatalysts composed of low-cost transition metals (Co or Fe), nitrogen, and carbon materials (M-N-C) have shown promising catalytic performances for the ORR. However, most of the M-N-C catalysts require high-temperature treatment for a high ORR activity. This energy- and time-consuming processing step makes it hard to prepare such catalysts in rational, predictive, and cost-effective ways. Owing to the high abundance and good catalytic activity of cobalt, cobalt-based composite materials have been investigated widely as electrocatalysts for the ORR. Chemically modified graphenes, carbon nanotubes (CNTs), and other carbonaceous materials were often used as supports in the composites. Cobalt or cobalt oxide nanoparticles dispersed on the surfaces of carbon-based materials and Co-N-C species embedded into carbon-based networks are efficient active species for the ORR. In this work, we developed a hybrid ORR catalyst through the coordination of Co-based organometallic molecules to N-doped multiwalled carbon nanotubes (MWCNTs). Notably, our method offers a room-temperature reaction pathway for the formation of a stable catalytic complex through the exploitation of N-doped structures. The hybrid ORR catalyst shows excellent catalytic performance with an onset potential of 0.95 V (RHE), superior durability, and good methanol tolerance. Chemical and structural characterization after many reaction cycles reveals that the Co-based organometallic species maintained the original structure of cobalt(II) acetylacetonate with coordination to the carbon-based materials. Also chemical and morphological characterization demonstrate after many reaction cycles reveals that a molecular well-dispersed active species at the surfaces of the carbon-based materials.

Hollow nanostructures with ultrathin walls provide readily accessible, high specific surface area for heterogeneous catalysis and sensing applications [1]. We report here the development of a room temperature synthesis process that produces tubular Cu₂O nanostructures with ultrathin wall (<10 nm). The synthesis process consists of using pre-formed Cu nanowires as templates, rapid annealing to oxidize the outside layers of the Cu nanowires, and a final well-controlled electrochemical corrosion process to obtain the final tubular structures. Characterization of the fabricated tubular structures by XRD and electron microscopy revealed that the morphology of the tubular structure conforms to the template Cu nanowires and that the crystalline tubular structure consists of Cu₂O single phase. The synthesized Cu₂O nanotubes have an average inner radius of 29 nm, outer radius of 35 nm and wall thickness ranging from 4 nm to 9 nm. After conditioning of the fabricated Cu₂O nanotube catalyst during the first cycle of low temperature CO oxidation it demonstrated high activity with 100% CO conversion at 150°C in the following cycles, probably due to in situ reconstruction of the nanotube surface structures. The activation energy, stability and the structural evolution of the Cu₂O nanotube catalyst during low temperature CO oxidation will be discussed [2].

[1] Wang, X.; Feng, J.; Bai, Y.; Zhang, Q.; Yin, Y. Synthesis, Properties, and Applications of Hollow Micro-/Nanostructures Chem. Rev. 2016
[2] Acknowledgment is made to the Donors of the American Chemical Society Petroleum Research Fund for support of this research. The authors acknowledge the use of facilities within the John M. Cowley Center for High Resolution Electron Microscopy at Arizona State University

Water electrolysis is a type of oxygen electrochemistry that has been studied as a sustainable and effective energy conversion and storage device. The reaction occurring at the cathode electrode of the water electrolysis is referred to as HER (hydrogen evolution reaction), and the reaction occurring at the anode electrode is referred to as OER (oxygen evolution reaction). The OER reaction is slow because of the four-electron reaction, which is a major cause of the efficiency degradation. There is a need to develop new catalysts that allow the OER reaction to occur rapidly.
Precious metal oxides such as IrO₂ and RuO₂, exhibit great electrocatalytic activity for OER in acidic and alkaline conditions. However, because of high cost and scarcity of the precious metal elements, cost-effective and Earth-abundant metals, such as Fe, Co, and Ni. Among them, Ni-containing materials, show excellent electrocatalytic performances in alkaline medium.
Carbon materials are applied to various fields such as catalytic reaction, adsorption reaction, battery, and energy storage material. It has been known that bonding of nitrogen to such carbon materials can improve the properties of carbon materials, and much research has been conducted. Among them, carbon nitride (C₃N₄) is what we made in this experiment. In the case of C₃N, the synthesis method is simple and not toxic, and has a band gap of about 2.7 eV, which is applied in various fields. Particularly, in the case of the most stable form of C₃N₄, graphitic carbon nitride (g-C₃N₄) is attracting attention as a catalyst which does not use metal as a material having a two-dimensional structure.

Hybrid catalytic materials containing single atoms or molecule-based active species fixed on nano-materials have been suggested as advanced catalyst for various reactions. In this study,
We compared the OER performance of Ni-CN-x (x = 10, 30, 50, 70, 100, 200) according to the composition of g-C₃N₄ and nickel. Novel hybrid system consisting of molecularly dispersed Ni-based materials on a g-C₃N₄ framework show excellent electrocatalytic performance for the OER.

We concentrated on Ni-CN-200 because it shows the best catalytic performance for the OER. Chemical analysis revealed that Ni-CN-200 contained Ni-based molecular species that were not aggregated in the porous g- C₃N₄ network and did not interfere with the network structure of C₃N₄. As calculated using BET data, the pore size of Ni-CN-200 powder was 70 nm of diameter and surface area of Ni-CN-200 was found to be 25.7 m²/g. EXAFS analysis investigated the local structure around Ni, Ni-CN-200 shows the main peak at 1.6 A indicating the presence of Ni-N bonds.
In addition, electrochemical characterization results suggest dispersed Ni-containing molecular entities are active species for the OER. The Ni-CN-200 sample exhibits the best electrocatalytic activity with an onset potential of 1.54 V (vs. RHE), which is less positive by 80 mV relative to g-C₃N₄. Not only the reduced onset potential, but also the kinetics, of the OER is enhanced by the attaching of the Ni-containing species on C₃N₄.The corresponding tafel slopes of IrOx, Ni-CN-200, Ni-CN-100, g-C₃N₄, and pristine GCE are 86, 60, 88, 230, and 470 mV/s, respectively. In test duration, the CV curves after 500 and 1000 cycles were almost like the initial curve and the chronoamperometry (CA) shows Ni-CN-200 exhibited excellent durability toward the OER. Our approach using hybridized molecular nanomaterials would be highly valuable for developing new electrocatalytic systems.

With the development of industrial technology, the amount of organic pollutants is constantly growing. In particular, water pollution produced by printing and textile factories has seriously affected the environment and caused severe consequences, such as aesthetic pollution, toxicity, and perturbation of aquatic life. Recently, with growing demand for clean and comfortable environment, purification technologies with high efficiency and low cost to reduce the pollutants of wastewater are urgently needed. Heterogeneous Fe3O4@TiO2@Au,Ag">Fe₃O₄@TiO_2@Au,Ag core-shell microspheres, a facile and highly efficient catalyst have been fabricated by a simple surface modification. The fabrication process involved the coating of TiO₂ nanoshell onto the magnetic core using by sol-gel process, and then the anchoring of noble metal(Au, Ag) nanoparticles onto the surface of the Fe₃O₄@TiO₂ microspheres through the wet chemical reaction of 3-aminopropyltriethoxysilane (APTES). The as-synthesized Fe₃O₄@TiO₂ microspheres exhibited a narrow size distribution, with a typical size of 350 nm and shell thickness of 25 nm. The Fe₃O₄@TiO_2@Au,Ag microspheres can be easily collected by applying external magnetic field due to the magnetic property of core Fe₃O₄ particles. The noble metal(Au,Ag) coated Fe₃O₄@TiO₂ photocatalyst exhibited high photocatalytic activity in the degradation of rhodamine B (Rh.B) and methylene blue (MB) under visible light. The Fe3O4@TiO2@Au,Ag">Fe₃O₄@TiO₂@Au,Ag microspheres possesses a relatively large specific surface area induced by noble metal(Au,Ag) nanoparticles, magnetic behavior for easy reuse, and enhanced photocatalytic efficiency caused by metal-semiconductor charge transfer or energy transfer. The increased photocatalytic activity of the Fe₃O₄@TiO₂@Au,Ag microspheres is related to the increase in the electron–hole separation, caused by the interparticle charge transfer or energy transfer between TiO₂ and noble metal(Au, Ag). The photodynamic process of TiO₂ rapidly generates reactive oxygen species (ROS) as for instance superoxide anion radical (O₂⁻), hydrogen peroxide (H₂O₂), singlet oxygen (¹O₂), and hydroxyl radical (OH). Thus, detection methods and generation mechanisms of the intrinsic reactive oxygen species (ROS), in photocatalysis, were surveyed comprehensively. The renewable photocatalytic activity of the photocatalyst was also examined.

To date, great progress has been made in the shape-controlled synthesis of noble-metal nanocrystals. However, there still exists a major gap between academic studies and industrial applications due to the inability to produce nanocrystals in large quantities while retaining their uniformity. To help fill this gap, herein, we provide a new route to scale up and accelerate the production of non-layered palladium nanosheets (Pd NSs) by incorporating etching while retaining effective capping during the synthesis. The key to this rapid synthesis is the etching induced by selected etchants (e.g., Fe³⁺/Fe²⁺, Cl^-/O₂, Br^-/O₂, and I^-/O₂). Specifically, this synthesis can be accomplished within 3 min, reaching a yield as high as 7.2 g L^-1 h^-1. The thickness of Pd NSs can be tuned to 1.6, 2.0, 2.3, and 3.5 nm by controlling the etching and reducing rates via choosing different type of etchants. Moreover, these non-layered Pd NSs are fabricated in an aqueous solution without the addition of any organic compounds; therefore, the surface of these NSs is extremely clean. When used as a catalyst for the formic acid oxidation reaction, the as-prepared non-layered Pd NSs exhibit a mass activity as high as 1350 mA mg^-1, which is 3.7 times higher than that of commercial Pd/C, due to their much larger electrochemical surface area (66.2 m² g^-1, which is 2.7 times higher than that of commercial Pd/C).

Noble metal nanocrystals with planar defects (e.g., twins, dislocations, stacking faults, and grain boundaries) symbolize a rising family in heterogeneous catalysts. It is aware of surface terminations of planar defects of noble metal nanocrystals play the significant role in their catalytic activity. However, there are still great challenges in preparation for noble metal nanocrystals with controllable high-density defects.

Here, we demonstrate a unique approach to the creation of Ag nanowires with high-density surface terminations of planar defects through controllable nanoparticles coalescence in one-dimensional pores of mesoporous silica. The density of defects can be easily adjusted by tuning the annealing temperature during synthetic process. The high-density defects promote the adsorption and activation of more reactants on the surface of Ag nanowires during catalytic reactions. As a result, the as-prepared Ag nanowires exhibit enhanced activities in catalyzing dehydrogenative coupling reaction of silane in terms of apparent activation energy and turnover frequency (TOF). We show further that the silane conversion rate can be enhanced by maximizing the defect density and thus the number of active sites on the Ag nanowires, reaching a remarkable TOF of 8288 h^-1, which represents the highest TOF that has been achieved by far on Ag catalysts. The adsorption tests have further demonstrated the enhancement of the catalytic activity can be ascribed to the improved adsorption of reactants. This work not only proves the important role of surface terminations of planar defects in catalysis but also provides a new and general strategy for constructing high-density defects in metal catalysts.

In reactions involving gas-solid interactions on nanostructured supports, the distinction between catalytically significant atomic structures and unreactive spectators is obscured by the variety of surface states that exist under reaction conditions. Studies of catalysts with well-defined support morphologies could assist in limiting the scope of structural configurations under consideration. Cerium dioxide (CeO₂), a technologically important redox catalyst owing to its aliovalent behavior, can be synthesized into nanocrystalline cubes and rods; cubes surface-terminate with {100} planes, while rods surface-terminate with {110} and {100} planes [1]. Density functional theory (DFT) calculations indicate that CeO₂ {100} surfaces are less stable than {110} surfaces, which suggests that cubes should be more reactive than rods [2]. However, for the CO oxidation reaction, rods are reported to be more active than cubes [1, 3], which is consistent with our results.

No atomic-resolution, in situ information exists for the surface structures or metal-support interfaces that occur on CeO₂ rod/cube-supported catalysts, so we may only speculate upon the origin of the rods’ superior activity. This study employs ex situ and in situ environmental transmission electron microscopy (ETEM) to uncover the cooperative interactions that give CeO₂ rod-supported catalysts enhanced catalytic activity. The CO oxidation reaction (CO + ½ O₂ --> CO₂) will be employed as a probe for catalytic activity, due to its chemical simplicity and relevance to clean energy conversion. Nanostructured CeO₂ cubes and rods will be synthesized through established hydrothermal methods [1], and Pt nanoparticles will be loaded onto them through conventional impregnation methods. A quartz tube reactor coupled to a gas chromatograph will be used to determine turn-over frequencies and activation energies for the catalyst, informing the reaction space to be explored during environmental transmission electron microscopy (ETEM) experiments. We expect that a Mars van Krevelen-type of mechanism regulates activity in these catalysts, which would suggest that highly dynamic interfacial structures are likely to be catalytically significant. To this end, aberration-corrected ETEM (AC-ETEM) will be used to visualize the dynamic atomic structures that form in reaction conditions. The structures at the metal-support interface - and the dynamics that occur there - will be compared amongst the different types of interfaces that are observed (for example, Pt/CeO₂ interfaces formed from Pt supported on CeO₂ [100] vs. CeO₂ [110] planes) [4].

[1] Mai, H. et al., Journal of Physical Chemistry B 109, (2005), p. 24380 – 24385.
[2] Jiang, Y. et al., Journal of Chemical Physics 123 (2005), p. 064701-1 – 9.
[3] Wu, Z. et al., Journal of Catalysis 285 (2012), p. 61 – 73.
[4] We gratefully acknowledge support of NSF grant CBET-1604971 and the use of facilities at Arizona State University’s John M. Cowley Center for High Resolution Electron Microscopy.

Human-engineered systems capable of generating fuels from sustainable energy sources provide an approach to satiating modern societies' energy demands, with minimal environmental impact. Strategies to address this challenge for science and the imagination often draw inspiration from the biological process of photosynthesis that powers our biosphere and supplied the fossil fuels global economies rely on. In this context, the active sites of enzymes have inspired researchers to develop molecular complexes that capture key structural and functional principles of nature's catalysts. However, not all aspects of biological energy transducing systems are or should be targets of chemical mimicry in designing an artificial photosynthesis, and some of the more favorable properties associated with solid-state heterogeneous catalysts have motivated molecular based surface-modification strategies. In this presentation, I will discuss efforts from our research group to develop heterogeneous–homogeneous architectures that combine the form factors of their underpinning solid-state supports with molecular coatings, enabling cooperative control and tunability of physical properties.^1-6

[1] B. L. Wadsworth, A. M. Beiler, D. Khusnutdinova, S. I. Jacob, G. F. Moore, ACS Catal., 6, 8048–8057 (2016); DOI: 10.1021/acscatal.6b02194.

[2] A. M. Beiler, D. Khusnutdinova, S. I. Jacob, G. F. Moore, ACS Appl. Mater. Interfaces, 8, 10038–10043 (2016); DOI: 10.1021/acsami.6b01557.

[3] D. Khusnutdinova, A. M. Beiler, B. L. Wadsworth, S. I. Jacob, G. F. Moore, Chem. Sci., 8, 253–259 (2017); DOI: 10.1039/c6sc02664h.

[4] A. M. Beiler, D. Khusnutdinova, B. L. Wadsworth, G. F. Moore, Inorg. Chem., 56, 12178–12185 (2017); DOI:10.1021/acs.inorgchem.7b01509.

[5] B. L. Wadsworth, D. Khusnutdinova, G. F. Moore, J. Matter. Chem. A., 6, 21654–21665 (2018); DOI:10.1039/C8TA05805A.

[6] D. Khusnutdinova, B. L. Wadsworth, M. Flores, A. M. Beiler, E. A. Reyes Cruz, Y. Zenkov, G. F. Moore, ACS Catal., 8, 9888–9898 (2018); DOI: 10.1021/acscatal.8b01776.

We present our measurements of the oxygen electro-adsorption and oxygen evolution reaction (OER) on well-defined Ir_1-xTi_xO₂ and Ru_1-xTi_xO₂ surfaces. The slow OER kinetics contributes the largest source of overpotential (inefficiency) to water-splitting devices. An OER catalyst can reduce this overpotential by stabilizing the OER intermediates via surface electro-adsorption. We study the impact of the Ti addition on the OER kinetics of IrO₂ and RuO₂, two of the most active OER catalysts, in this work. To understand the influence of Ti, we also test how Ti impacts the electro-adsorption by quantifying the oxygen electro-adsorption on Ir_1-xTi_xO₂ and Ru_1-xTi_xO₂ as a function of Ti. Using the relationship between the Ti content, the oxygen electro-adsorption, and the OER kinetics, we present a physical model on how the Ti addition affects the OER kinetics in the context of the classical volcano plot. Finally, we discuss the mechanism on why the Ti addition can be beneficial and why sometimes not, and how to design a material with distinct cooperative sites to achieve high activity as a route to design future catalysts.

Bismuth-based materials can promote the electrocatalytic reduction of CO₂ to a variety of high energy density products depending on the conditions employed. For example, Bi cathodes promote the reduction of CO₂ to CO with fast kinetics and high efficiencies in the presence of imidazolium ([Im]⁺) based cations. The Bi/[Im]⁺ systems can drive the electroreduction of CO₂ to CO with Faradaic efficiencies and current densities of FE_CO ~90 % and j_CO ~5–25 mA/cm², respectively at applied overpotentials of ~300 mV. By contrast, when in the presence of [DBU–H]⁺ based electrolytes, Bi cathodes promote the conversion of CO₂ to formate (FE_FA ~80% at j_FA ~45 mA/cm²) via an orthogonal 2e^– reduction process. These studies reveal that variation of the electrolyte cation is a viable strategy to tune CO₂RR with bismuth cathodes. We have undertaken a multipronged effort to understand how the cooperative interactions between polarized bismuth electrodes and electrolyte cations dictate the products of CO₂ reduction. Through a combination of electroanalytical and computational methods along with advanced analyses that include XANES, EXAFS and X-Ray reflectivity experiments, we demonstrate that the Bi/[Im]⁺ and Bi/[DBU–H]⁺ interfaces are not static. Under conditions at which CO₂ electrolysis takes place, the bismuth materials undergo significant compositional and structural changes that are all believed to be important to the activity and selectivity of these systems for CO₂RR. The mechanistic pathway(s) by which bismuth electrodes undergo potential-dependent chemical and structural transformations is dependent on cooperative effects between the electrode/electrolyte and manifest in the catalytic plasticity that is displayed by these Bi/[cation] assemblies. Implications for future development of catalyst/electrolyte combinations that can efficiently promote CO₂ conversion will also be discussed.

Eelectrochemical reduction of CO₂ (CO₂RR) to value-added chemicals and liquid fuels using off-peak electricity or renewable energy is a green approach for energy storage and carbon cycling. The Faradaic efficiency and selectivity of CO₂ reduction remain very low due to the very high energy barrier for CO₂ activation. However, co-production of H₂ and CO from the CO₂RR in aqueous electrolytes is appealing because H₂ and CO with appropriate ratios as the synthesis gas can be used for methanol synthesis or Fischer-Tropsch reactions. Pd has been found to generate H₂ and CO with a ratio between 1:1 and 2:1, which is in contradiction to the fact that Pd is prone for CO poisoning. In situ X-ray absorption spectroscopy and X-ray diffraction spectroscopy studies reveal that Pd has transformed into β-PdH under the electrochemical reduction conditions of CO₂RR. Density functional theory calculations suggest that in situ formed β-PdH is not subjected to CO poisoning, and therefore the active species for the CO₂RR.

Inspired by nature’s use of multiple enzymes to produce multi-carbon products from CO₂ and sunlight [1], it is attractive to consider analogous cascade catalysis schemes in electrochemical CO₂ reduction to achieve higher selectivity than what is possible now with conventional metal electrocatalysts [2]. Cascade catalytic processes perform multi-step chemical transformations without isolating the intermediates. Here, we demonstrate a sequential electrocatalytic cascade to convert CO₂ to C₂₊ hydrocarbons and oxygenates using CO as the intermediate [3]. CO₂ to CO conversion is performed by using Ag and further conversion of CO to C-C coupled products is performed with Cu. Temporal separation between the two reaction steps is accomplished by situating the Ag electrode upstream of the Cu electrode in a laminar flow cell. Convection-diffusion simulations and evaluation of the electrodes individually were performed to identify optimal conditions for cell operation. With the upstream Ag electrode poised at -1 V vs RHE in a flow of CO₂-saturated water in aqueous carbonate buffer, over 50% of the CO is further converted to C-C coupled products (ethylene, ethanol, propanol) on the downstream Cu electrode held at -1.2 V vs RHE. By independent control of the electrodes, control over hydrocarbon and oxygenates selectivity is achieved, with increasing CO activity at the Cu electrode increasing the FE to C₂₊ oxygenates. Additionally, by supplying CO as well as CO₂ to downstream electrode, mass transfer of reactants is increased, which allows for higher CO₂ reduction currents compared to single electrode controls.

[1] Shi, J.; Jiang, Y.; Jiang, Z.; Wang, X.; Wang, X.; Zhang, S.; Han, P.; Yang, C. Chem. Soc. Rev. 2015, 44, 5981–6000.
[2] Yang, K. D.; Lee, C. W.; Jin, K.; Im, S. W.; Nam, K. T. J. Phys. Chem. Lett. 2017, 8, 538–545.
[3] Lum, Y.; Ager, J. W. Energy Environ. Sci. 2018, 10, 2935-2944.

Carbon dioxide utilization will play a critical role as the community aims to reduce the levels of atmospheric carbon dioxide. While 10% of current CO₂ emissions can be converted directly into fuels, the transformation of CO₂ into useful chemical feedstocks can account for another 7%. One central example of CO₂ utilization is the addition of CO₂ to cyclic epoxides, generating cyclic carbonates, a polymer precursor. This reaction is characterized by the attack of a Lewis acidic metal on the epoxide to activate it toward CO₂ insertion. This reactivity has been demonstrated by many metal organic framework scaffolds. The Morris group recently synthesized a new class of metal organic frameworks termed VPI- 100- M. The MOFs incorporate a tetra-azamacrocyclic and are competent catalysts for the aforementioned reactivity. Herein, a detailed mechanistic study of the interactions between the VPI-100 analogs (Zr and Hf) and substrates of interest is presented. The implications of these results on the mechanism of catalytic action are discussed and placed in the context of the MOF catalytic field. The node appears to play a minor role during catalysis, as the dominant interacting center was determined to be the metallocyclam via both quartz crystal microbalance studies and calculations. In contrast, the node appears to dominate the CO₂ sorption properties of the MOF. Therefore, there is a synergistic effect between the linker and the node imparting enhanced reactivity.

Electrochemical recycling of CO₂ to high-valued carbon-based products such as hydrocarbons and alcohols offers not only mitigation of greenhouse gas emissions but also the opportunity to use CO₂ as a medium to store energy. Copper is a promising electrocatalyst for CO₂ reduction, capable of producing such products with reasonable activity. However, poor product selectivity results in low energetic efficiency – a critical challenge. In this study, we apply tensile strain to planar Cu catalysts and demonstrate the ability to direct the aqueous CO₂ reduction reaction toward particular products, indicating that we manipulate the binding energy of key intermediates via strain.
We observe that the magnitude of the tensile strain applied to the planar Cu catalysts strongly governs the CO₂ reduction selectivity. At overpotentials where higher reduction products are favored, the strongest tradeoff is between formation of CO and CH₄. This tradeoff indicates a change in CO binding energy, one of the key descriptors of CO₂ reduction to hydrocarbons and oxygenates. These findings provide evidence for strain engineering as a tunable approach to modifying the surface binding energy of CO₂ reduction intermediates and offer a practical experimental approach to identifying favorable strain states. We will describe our latest understanding of the changes in surface chemistry that strain introduces, offering insights to tailor the design of high surface-area electrocatalysts with enhanced selectivity for CO₂ reduction.

Recent advances in catalysis by supported single metal atoms have demonstrated that single-atom catalysts (SACs) can be highly active and selective for a variety of important catalytic reactions. The key challenge to practical applications of SACs is, however, the stabilization of high number density of single metal atoms dispersed onto high-surface-area supports during the desired catalytic reactions. Unless they are strongly anchored by defect sites supported metal atoms are thermodynamically unstable, especially at elevated temperatures and under reducing environment. The use of nanoglues, commonly practiced in industry to anchor metal nanoparticles, provides an alternative approach to confining the movement of supported single metal atoms during catalytic reactions. We recently demonstrated a novel strategy to anchor supported Pt single metal atoms by using CeO_x clusters (1-2 nm) as nanoglues. The synthesis processes consist of dispersing CeO_x clusters onto inexpensive and high-surface-area SiO₂ supports and then depositing Pt atoms onto the CeO_x clusters. We used CO oxidation as a probe reaction to evaluate the stability of the Pt₁/CeO_x/SiO₂ SAC. The results demonstrate that not only the CeO_x clusters stabilized the Pt₁ single atoms during the CO oxidation but they also significantly enhanced the activity, presumably due to the redox capability of CeO_x clusters that facilitate CO oxidation. Our strategy is general and can be extended to other types of functional nanoglues to develop robust SACs for a variety of catalytic transformation of important molecules*.

*This research was funded by the National Science Foundation under CHE-1465057. We gratefully acknowledge the use of facilities in the John M. Cowley Center for High Resolution Electron Microscopy at Arizona State University.

The photocatalytic conversion of the greenhouse gas CO₂ to chemical fuels such as hydrocarbons and alcohols continues to be a promising technology for renewable generation of energy. Major advancements have been made in improving the efficiencies and product selectiveness of currently known CO₂ reduction electrocatalysts, nonetheless, materials discovery is needed to enable economically viable, industrial-scale CO₂ reduction. We report here the largest CO₂ photocathode search to date, starting with 68860 candidate materials, using a rational first-principles computation-based screening strategy to evaluate synthesizability, corrosion resistance, visible-light absorption, and compatibility of the electronic structure with fuel synthesis. The results confirm the observation of the literature that few materials meet the stringent CO₂ photocathode requirements, with only 52 materials meeting all requirements. The results are well validated with respect to the literature, with 9 of these materials having been studied for CO₂ reduction, and the remaining 43 materials are discoveries from our screening that merit further investigation.

The rapid advances in catalysis by supported single metal atoms are clearly manifested by the explosive growth of journal publications on this research frontier in the last few years. Single-atom catalysts (SACs), defined as those catalysts that consist of only isolated metal atoms dispersed onto an appropriate support, have demonstrated unparalleled selectivity and/or significantly enhanced activity for a variety of important catalytic reactions including gas- and liquid-phase, electrocatalytic, and photocatalytic reactions. From the materials science perspective, we can view the active sites of SACs as surface dopants or anchored impurities which modify the surface electronic structure of the support material. From the homogeneous catalysis perspective, we can view the active sites of SACs as metal centers coordinated by a rigid ligand of the solid support; the specific coordination environment depends on the anchoring site of the metal single atom. The catalytic performance as well as the stability of SACs strongly depend on the support material and the electronic atom-support interactions. Strong metal atom-support interactions usually involve charge transfer or formation of covalent bonds which can significantly modify the physiochemical properties of the metal atoms and consequently their adsorption properties for reactant molecules. Since we do not have much freedom to tune the properties of single atoms support (surface) engineering or functionalization becomes the central research theme in the field of catalysis by supported metal atoms. In this presentation, we will discuss the recent strategies in anchoring metal atoms to high-surface-area supports, key catalytic reactions that demonstrate the excellent selectivity/activity on SACs, challenges to commercial applications of SACs, and perspectives for developing robust SACs for energy and environmental applications*.

*This research was funded by the National Science Foundation under CHE-1465057. We gratefully acknowledge the use of facilities in the John M. Cowley Center for High Resolution Electron Microscopy at Arizona State University.

The behavior of electron and hole charge carriers at surfaces of titanium dioxide (TiO₂) controls performance for important applications including photocatalysts and solar cells. While anatase TiO₂ exhibits high electron mobility in the bulk a commonly held belief is that strong coupling between electrons and phonons can lead to electron trapping at anatase surfaces. However, direct evidence is scarce and the nature of the trapping sites and electronic properties remains unclear. To address this question we investigate the trapping of electrons and holes at low and high index surfaces of anatase TiO₂ using an accurate hybrid density functional theory approach. We find that, as in the bulk, electrons do not trap on the low index planes (001, 100, 101, 110, 112) of anatase crystals. For the higher index planes (103, 105, 107) that contain structural step defects, we find that electrons do trap at the low coordinated Ti cations present on the steps. The trapping of holes at the surfaces of anatase TiO₂ is a more complicated picture, as the distribution of hole traps is facet dependent. 001 and 100 surfaces, as well as 105 and 107 surfaces which have 001 type terraces, have the strongest affinity to trap holes. Hole trapping for the 101, 110, 112 and 103 surfaces is found to be favoured in the sub surface layers and not at the surface facet. These results provide crucial insights into the behavior of electrons and holes in TiO₂ relevant for applications in photocatalysis and challenge the common perception that electrons trap at low index surfaces of anatase TiO₂.

Composite catalytic aerogels provide a highly flexible design motif for solar-fuels photocatalysis. This multifunctional platform enabled us to physically connect surface plasmon resonant (SPR)–driven visible-light sensitization, electron and ion transport, and oxidation and reduction catalysis within Au–TiO₂ aerogels—a single, hierarchical composite catalyst nanoarchitecture [1].

Although Au–TiO₂ aerogels exhibit attractive properties for photocatalytic H₂ generation [1] and photochemical water splitting [2], quantum efficiencies at the globular ~5-nm Au guests remain low. Here, we focus on the challenge of enhancing SPR-driven visible light water oxidation in our composite aerogels. We introduce iridium oxide (IrO₂)–based co-catalyst nanoparticles into the plasmonic aerogels to enhance SPR-generated electron–hole pairs. We also describe a new synthetic strategy to vary the size of SPR-active Au nanoparticles in the Au–TiO₂ aerogel architecture to increase the efficiency of the SPR sensitization process. Ultimately, we wish to retain the connectivity of the sensitization, charge separation, and catalytic steps we previously demonstrated in our composite plasmonic photocatalytic aerogels and add in the high (as high as 20–50%) SPR-sensitized charge separation efficiencies reported elsewhere [3] to create a robust composite aerogel-based solar fuels photocatalyst delivering state-of-the-art photocatalytic efficiency.

[1] P. A. DeSario, J. J. Pietron, A. Dunkelburger, T. H. Brintlinger, O. Baturina, R. M. Stroud, J. C. Owrutsky, D. R. Rolison, Langmuir, 2017, 33, 9444.
[2] P.A. Desario, J.J. Pietron, D.E. DeVantier, T.H. Brintlinger, R.M. Stroud, and D.R. Rolison, Nanoscale 2013, 5, 8073.
[3] D. C. Ratchford, A. D. Dunkelberger, I. Vurgaftman, J. C. Owrutsky, P. E. Pehrsson, Nano. Lett. 2017, 17, 6047.

Strategies for closing the anthropogenic carbon cycle using the chemical conversion of CO₂ into useful molecules have become a scientific and technological priority. Of all chemical ways to convert CO₂ into CO, oxygenates or hydrocarbons, the direct electrochemical coupling of water electrolysis and CO₂ reduction appears attractive, as it is a one-pot process step, occurs at ambient temperature and pressures, involves solely water and CO₂ as reactants, and may involve renewable electricity from wind, solar or hydro power plants.

The CO₂ electroreduction is by and large carried out on the surface of metal catalysts. At industrially relevant current densities and electrode potentials, however, these catalysts typically suffer from low faradaic C₁ product selectivities due to the competing, very fast reduction of water, i.e. the hydrogen evolution reaction. New catalyst concepts with tunable selectivity for hydrogen evolution versus CO₂ reduction are needed.

In this contribution, we share recent comparative experimental and computational mechanistic insight in the electroreduction of CO₂ on metallic multi-site versus molecularly inspired metal ion-N modified high surface area carbon single-site catalysts. We provide new insight into CO selectivity trends and put particular emphasis on the competing reaction pathways to methane and methanol. We show that carbon-based single site catalysts are an intriguing cost-effective alternative to their metallic rivals at industrial current densities.

Supported single-atom catalysts provide an ideal system to study reaction mechanisms on a molecular level and to bridge heterogeneous and homogeneous catalysis. Single atoms have shown remarkably different activity and selectivity for several reactions, however for low temperature CO oxidation the reaction mechanism is not well understood and there is debate whether single atoms have higher intrinsic activity than nanoparticles. In this talk, the mechanism of CO oxidation on atomically dispersed Ir/MgAl₂O₄ catalysts will be presented and compared to that on nanoparticles. Using in-situ and in-operando DRIFTS and HERFD-XANES complemented by DFT calculations, we identified the active Ir single-atom complex, and resting state of the catalyst for low temperature CO oxidation. We provide evidence that strong CO adsorption on single atoms leads to an Eley-Rideal mechanism where Ir(CO) is the active complex (i.e. a CO ligand is present during the catalytic cycle). The results show that due to the ability of single atoms to coordinate with more than one ligand, strong adsorption by CO does not necessarily lead to catalyst poisoning. Additionally, we demonstrate that the unusual kinetics on single atoms can be used as a facile surface sensitive characterization tool for quantifying the number of single atom sites in catalysts containing a mixture of single atoms and nanoparticles. The results provide a new perspective for understanding reaction mechanisms and interpreting spectroscopic and catalytic activity of supported metal single atoms.

New semiconductor metal oxides capable of driving water-splitting reactions by solar irradiation alone are required for sustainable hydrogen production. Whereas most metal oxides only marginally deliver the photochemical energy to split water molecules, uranium oxides are efficient photoelectrocatalysts due to their absorption properties (E_g ~ 2.0 - 2.6 eV) and easy valence switching among uranium centers that additionally augment the photocatalytic efficiency. Although considered a scarce resource, the abundance of uranium compounds in the environment is manifested in the huge quantities of stored UF₆ gas, produced as waste streams in the nuclear fuel enrichment process. Here we demonstrate that thin films of depleted uranium oxide (U₃O₈) and their bilayers with hematite (a-Fe₂O₃) are high activity water oxidation catalysts due to electronically coupled phase boundaries. The electronic structure of uranium oxides showed an optimal band edge alignment in U₃O₈//Fe₂O₃ bilayers (DFT calculations) resulting in improved charge-transfer at the heterojunction as supported by TAS and XAS measurements. The enhanced photocurrent density of the heterostructures with respect to well-known hematite offers unexplored potential of uranium oxide in artificial photosynthesis.

The abundance of methane and its low cost makes it an optimal raw material for chemical precursors and energy-dense fuels. Conventional synthesis of methanol involve a multistep process, which requires high energy input and high cost [1]. Alternatively, electrochemical methane oxidation using a catalyst is a promising single-step approach to achieve a direct conversion of methane to methanol at lower temperatures and cost [2].

An efficient catalyst needs to be developed for this energetically challenging process. The catalyst should be active for C-H bond activation, thereby enabling oxidative hydroxylation of methane, and simultaneously inhibit further methanol oxidation. To date, a variety of the catalysts, including supported metals (Pd, Ru, Au, Ag) and metal oxides (V₂O₅, Fe₂O₃, CoO, Mn₂O₃, MoO₃, CrO), have been tested in electrochemical cells and shown promise for this approach [2]. However, further systematic studies are essential for understanding the mechanism for methane oxidation and thereby enabling the rational design of the catalysts. In this regard, porous metal oxides and supported single-atom catalysts (SAC) are particularly interesting [3]. These catalysts show potential for high methane conversion and are ideal model catalysts to identify active sites and facilitate understanding of the reaction process at a molecular level. In spite of great interest; however, it still remains challenging to achieve atomically dispersed metals in high loadings for efficient catalysis.

In this study, a design of first-row transition metal oxides and MOF-derived supported single-atom catalysts will be presented. The catalysts are synthesized with wet-chemistry, and subsequently, characterized for electrochemical methane-to-methanol oxidation. Structural and chemical analyses of catalysts using a combination of various microscopic and spectroscopy techniques are used to establish structure/property relationship. These insights provide valuable basis for a scientific-guided approach toward new metal oxide and supported single-atom catalysts for this challenging process.


1. Ravi, M., et al, Angew. Chem. Int. Ed., 2017, 56, 26464.
2. Tomita, A., et al, Angew. Chem. Int. Ed., 2008, 47, 1462.
3. Starokon, E. V., Phys Chem. C., 2011, 115, 2155.

Graphene, an atomically thin two dimensional carbon allotrope with hexagonal lattice structures, has been intensively studied in recent years owing to its outstanding electrical, mechanical, and chemical properties. In particular, graphene has received much attention as a promising barrier material not only because of its densely packed structure that does not allow the transmission of gases or liquids but also because of outstanding optical transparency and mechanical flexibility. This is particularly important for flexible organic devices as the performance of organic materials considerably degrades in the presence of water or oxygen molecules from ambient air. Inorganic materials such as silicone oxides (SiOx) and aluminum oxides (Al₂O₃) have been predominantly employed as the gas barrier films. However, the complicated fabrication processes as well as the poor mechanical flexibility of these materials have hindered the practical application to flexible electronic devices. In this regard, the excellent gas impermeability, flexibility, and transmittance of graphene films are expected to be useful for more reliable and durable operation of organic devices. Nevertheless, the defects in CVD graphene are unavoidable due to imperfect synthesis, etching, and transfer processes, which considerably degrades the barrier performance of graphene films. Thus, we report the method of staking multilayered graphene-organic hybrid films to minimize the transmission of gas molecules in large area by patching and healing the defects on graphene. The water vapor transmission rate (WVTR) of alternatively stacked 4-layer graphene and self-assembled monolayers of organic molecules is measure to be ~10^-4 g/m²day, which is believed to the lowest WVTR reported so far. The hybrid graphene films transferred to a stretchable substrate show 10~30% stretchability, which would enable perfect protection of various flexible organic devices from oxygen and moisture environment for practical use.

Presented herein is an investigation of a promising Chevrel-phase metal sulfide catalyst that is capable of electrochemically converting CO₂ to liquid and gas fuels such as methanol and hydrogen. When promoted by copper, extended structures of Chevrel-phase Mo₆S₈ clusters are capable of reducing CO₂ and CO to methanol in aqueous electrolyte with an overpotential of -0.4 V vs RHE. H₂ gas is simultaneously evolved during this process, contributing to total current densities as high as 35 mA/cm². Methanol and formate are the only CO₂ reduction products formed from electrolysis, although utilizing CO as a feedstock has been shown to mitigate parasitic current density toward formate production. Also presented is the formulation of an interesting electronic structure-function correlation founded on the basis of ex-situ and operando X-ray absorption spectroscopic analyses and corroborated by results of electroanalytical evaluation as well as theoretical calculation.

Clean hydrogen energy produced from water-alkali electrolyzers has shown great potential to replace current fossil fuels due to its carbon-neutral and sustainable nature.^[1] Although robust hydrogen evolution (HER) electrocatalytic water splitting performance has been achieved, the overall water splitting performance is still hindered by oxygen evolution reaction (OER) due to the sluggish four-proton-coupled electron transfer process.^[2] In addition, the OER process is also essential for CO₂ reduction reactions and plays dominant role in rechargeable metal-air batteries.^[3] Thus, it is of great significance to develop effective electrodes for large-scale oxygen evolution at the minimum cost of electricity. From the perspective of practical water-alkali electrolysis, commercial water splitting requires an overpotential below 300 mV to realize high current density above 500 mA/cm², superior long-term stability and cost-effective OER electrocatalysts.^[4] Therefore, designing highly efficient OER electrodes based on non-noble metals is highly desirable for practical water splitting.
In this manuscript, we successfully fabricate self-supported Fe-doped Ni₂P electrocatalyst on 304-type stainless steel (SS) mesh and realize first-class OER performance. The as-prepared OER electrode exhibits an overpotential below 300 mV (about 255 mV) to achieve high current densities of 500 mA/cm², which meets the commercial water electrolyzer requirements and outperforms most of other state-of-the-art OER electrodes. Besides, extremely low Tafel slope and superior long-term stability are also achieved due to the highly active catalytic sites, great conductivity and strong adhesion between electrocatalyst and current collector. DFT calculation was then introduced to verify the robust OER performance of Fe-doped Ni₂P. According to the theoretical results, Fe-doping brings about reduced adsorption energy of O-containing intermediates and improve the charge transfer from catalytic surface to intermediates, which is beneficial for intermediates activation and O₂ evolution. More importantly, Fe doping can greatly improve the OH^- adsorption on oxidized Ni-Fe-P surface, leading to activated OH^- species for enhanced OER performance. The combination of experimental and theoretical results deepens our understanding to the synergistic effect of Ni- and Fe-sites during OER process and provides significative guidance to design OER electrodes for practical water electrolysis.

Reference:
[1] Z. W. Seh, J. Kibsgaard, C. F. Dickens, I. Chorkendorff, J. K. Nørskov, T. F. Jaramillo, Science 2017, 355, eaad4998.
[2] G. Maayan, N. Gluz, G. Christou, Nat. Catal. 2017, 1, 48.
[3] X. Xu, F. Song, X. Hu, Nat. Commun. 2016, 7, 12324.
[4] H. Zhou, F. Yu, J. Sun, R. He, S. Chen, C.-W. Chu, Z. Ren, Proc. Natl. Acad. Sci. USA 2017, 114, 5607.

Tungsten oxide (WO_x) has been widely studied for versatile applications based on its photocatalytic, intrinsic catalytic, and electrocatalytic properties. Among the several nanostructures, we focused on the flower-like structures to increase the catalytic efficiency on the interface with both increased substrate interaction capacities due to their large surface area and efficient electron transportation. There are three keys to obtain the improved flower-shaped WO_x (WONFs) with a large surface area. First is acidic environment (pH 1.6), seconds is the synthesis was conducted at a low temperature (to provide a low reaction rate), and last is high precursor concentration. Like hexagonal zinc oxide nanorods, hexagonal WO_x crystals have been considered polar crystals with ± (0001) polar planes. These polar crystals are prone to growing along their polar directions (c-axis) at a low growth rate. The difference in crystal growth rate between the polar plane and nonpolar plane would produce anisotropic crystal growth. Another factor influencing the crystal morphology is the capping agent, which can selectively adsorb onto the preferred crystal planes and regulate the crystal growth rate. In this reaction system, NaCl can act as a capping reagent by adsorbing onto the crystal plane parallel to the c-axis of the WO₃ crystal nucleus. Moreover, Cs₂WO₄-based experiments were performed to investigate the influence of the Na⁺ cation on the overall structure of WO_x; however, the resulting nanoparticles, Cs_0.3(WO₃), were not flower-shaped and denoted as CsWONPs. The WONFs were compared with CsWONPs in terms of structural and catalytic properties. Therefore, improved WO_x nanoflowers (WONFs) with large surface areas were developed through a simple hydrothermal method using sodium tungstate and hydrogen chloride solution at low temperature, without any additional surfactant, capping agent, or reducing agent. Structural determination and electrochemical analyses revealed that the WONFs have hexagonal Na_0.17WO_3.085 0.17H₂O structure. To verify the catalytic efficiency enhancement by the unique shape and structure of the WONFs, they were compared with calcinated WONFs, cesium WO_x nanoparticles, and other peroxidase-like nanomaterials. The WONFs were characterized using a combination of experimental techniques, and it was found that WONFs have the largest surface area (51.936 m²/g) despite having a particle size larger than that of CsWONPs. Furthermore, WONFs have intrinsic peroxidase-like activity and electrocatalytic properties. For colorimetric detection, we investigate their peroxidase-like activity which is turning from colorless to blue by catalyzing the oxidation of a peroxidase substrate, such as 3,3′,5,5′-tetramethylbenzidine, in the presence of H₂O₂ and the LOD was 138 μM within a linear range of 0–100 mM. The results indicated that the WONFs showed a low Michaelis-Menten constant (k_m), high maximal reaction velocity (v_max), and large surface area. Additionally, a WONF-modified glassy carbon electrode was adopted to monitor the electrocatalytic reduction of H₂O₂. When using electrochemical methods, a modified GCE was used, and the LOD was 56.0 nM within a linear range of 0–280 μM. The LOD obtained when using WONFs in electrochemical sensing is lower than that obtained when using WONFs in colorimetric detection. Additionally, the characteristics and catalytic activity of cWONFs and CsWONPs were studied. However, neither material exhibited both intrinsic peroxidase-like activity and good electrocatalytic activity.

In heterogeneous catalysis, metal-support interactions can lead to enhanced catalytic performance. Cu and Pt nanoparticles supported on CeO₂, for example, are more active for CO oxidation than those supported on non-reducible metal oxides (such as SiO₂ [1]). The enhancement arises from CeO₂’s ability to locally donate oxygen at the metal-support interface, through a Mars van Krevelen-like mechanism. The ease with which oxygen may be removed from the CeO₂ lattice has been shown theoretically to depend on the atomic structure of the metal-support interface [2]. At present, though, there is little experimental data on the atomic structures that comprise the metal-support interface during catalysis. Correspondingly, many important questions remain as to what interfacial or proximal surface structures participate in or enhance interfacial oxygen transfer.

In the present study, we seek to elucidate the cooperative atomic-scale dynamics that occur at the metal-support interface during catalysis. The CO oxidation reaction (CO + ½ O₂ -> CO₂) will be employed as a probe for catalytic activity, due to its chemical simplicity and relevance to clean energy conversion. Nanostructured CeO₂ will be used as a model support, and Pt nanoparticles will be loaded onto them through conventional impregnation methods. A quartz tube reactor coupled to a gas chromatograph will be used to determine turn-over frequencies and activation energies for the catalyst, informing the reaction space to be explored during environmental transmission electron microscopy (ETEM) experiments. Aberration-corrected ETEM (AC-ETEM) will be used to visualize the atomic structures that form in reaction conditions. Operando electron energy-loss spectroscopy will be implemented to track the gas composition during catalysis, allowing catalytic activity to be correlated directly with the observed surface and interfacial structures.

The results of these experiments will provide operando information on the interfacial and surface structures that correlate with activity for an important clean energy conversion reaction. These relationships are of fundamental interest to the heterogeneous catalysis community, and they may facilitate the engineering of highly active Pt/CeO₂ catalysts for the energy and environmental remediation applications where they are indispensably used [3].

[1] Jia, A.-P., et al; Journal of Physical Chemistry C 114, 21605-10 (2010).
[2] Vayssilov et al; Nature Materials 10, 310–315 (2011).
[3] We gratefully acknowledge support of NSF grant CBET-1604971 and use of facilities at Arizona State University’s John M. Cowley Center for High Resolution Electron Microscopy.

Electrospinning technique is simple method to polymer based nanofiber film fabrication. Electrospun (ES) film is fabricated continuously stacked nanofiber that has porous structure. The porous characteristic of ES films permit their application in many fields such as textiles, filters, biomedicine, drug delivery, energy, and sensors. Also, the electrospinning technique is readily adapted for the addition of micro and nanoparticles to provide specific functions in the mixing of the polymer solution. For further applications, the fiber diameter should be easily controlled by the electric field in the electrospinning, especially ES film based filter application needs to have adequate film characteristics in uniformity, porosity and permeability. Traditional electrospinning using flat electrode for fiber collection has disadvantages to produce a ES film having a homogeneous thickness distribution. Also traditional electrospinning has difficulties in nanofiber film acquisition due to the deformation film layer in film detaching process. In this study, we investigate the electrospinning method using circular electrode for freestanding state nanofiber film fabrication. Freestanding ES film is possible to transfer layer by layer without film deformation through the repetition process. For the optimized freestanding ES film fabrication, numerical simulations are performed to analyze the electric field, equipotential line, electrical potential, and electrostatic pressure on the surfaces of the circle electrode collector in the electrospinning area. The electrostatic results explain the focusing phenomenon of freestanding films via ES fibers stacked on the surface of the 20 mm inner diameter circle electrode collector. Under the conditions with a 70 mm TCD and 12.5 kV, ES film is successfully fabricated on the top surface of the circular collector. After the optimization of ES film fabrication, PVDF/zeolite multilayer film fabricated by electrospinning with the circle electrode collector and repeated film transfer processes for the humidity removal test. Through the transferring process, the ES layers are laminated onto other film surfaces to fabricate a multilayer film. An ES multilayer film is composed with three-layer sandwich structure having a 2:5:2 volume ratio. The outer layers of filters I and II are ES layers of 15 wt% PVDF. The middle layers of filters I and II are 15 wt% PVDF and 18 wt%PVDF/zeolite (4:1) ES layers, respectively. The quantity of zeolite contained in the filter II multilayer is 17.2 μg. Filter I removes only 0.5% of the moisture with the film’s porous structure. In contrast, filter II absorbs 13.8% of the moisture from the humid air. The zeolite particles embedded in the ES PVDF film capture moisture, which is absorbed to 15.1% of the weight of zeolite. As a result, freestanding ES film using circular electrode enables to functional multilayer for dust filtering, air ventilation and humidity removal.

The synthesis of mesoporous materials has attracted great interest due to their wide range of applications in drug delivery, catalysis, sensors, photovoltaic cells, and fuel cells.^1,2 Low-molecular weight surfactants³ have normally been used to synthesize mesoporous materials. In this study, a new strategy has been proposed to synthesize large size mesoporous metal oxides. The use of asymmetric triblock copolymers with core-shell-corona nanoarchitecture as a template and structure directing agent gives rise a wide variety of mesoporous materials with large mesopores (10-50 nm).⁴ The catalytic activity of large size mesoporous metal oxides towards oxidation of CO will be discussed.⁵

1. W. Li, J. Liu, D. Zhao Nat. Rev. Mater. 2016, 16023.
2. Y. Li, B. P. Bastakoti, V. Malgras, C. Li, J. Tang, Y. Yamauchi, Angew. Chem. Int. Ed. 2015, 54, 11073.
3. B. P. Bastakoti, H. Oveisi, C.-C. Hu, K. C.-W. Wu, N. Suzuki, K. Takai, Y. Kamachi, Y. Yamauchi, Eur J. Inorg. Chem., 2013, 1109.
4. B. P. Bastakoti, S. Ishihara, S.-Y. Leo, K. Ariga, K. C.-W. Wu, Y. Yamauchi, Langmuir, 2014, 30, 651.
5. B. P. Bastakoti, Y. Li, N. Miyamoto, N. M. Sanchez-Ballester, H. Abe, J. Ye, P. Srinivasu, Y. Yamauchi, Chem. Commun., 2014, 50, 9101.

The theoretical prediction of new catalysts requires precise knowledge of structural information on which to base accurate quantum chemical studies such as density functional theory (DFT) investigations of activation energies and reaction energies. Often, structural information is difficult to obtain, for instance when surfaces and nanoparticles undergo structural transformation under pretreatment and reaction conditions, exposing novel reactive sites responsible for the experimentally observed catalytic activity. A theoretical study based on ideal surfaces or particles created from bulk will entirely miss these important features, resulting in a theory-experiment disconnect.

The density-functional tight-binding (DFTB) method [1-5] is an approximation to DFT that reduces the computational effort by 2-3 orders of magnitude, and was used for the prediction of the fullerene formation mechanism [6], the Haeckelite formation mechanism on metal surfaces [7], the existence of a “sweet spot” for the catalyst oxygen content of carbon nanotube formation [8], and many other phenomena in materials sciences requiring highly complex quantum chemical modeling. The tremendous speed of the method originates from the careful parameterization of the Hamiltonian [9], eliminating all expensive integral evaluations. The method can deliver accuracy in structure and energetics comparable to DFT [4,5,10], and has recently been successfully employed in the pre-screening of reaction and activation energies for heterogeneous catalysis [11] and in the study the topology of binary core-shell nanoparticles [12]. In this presentation we will highlight recent accomplishments in the parameterization effort related to the DFTB-based study of heterogeneous catalysis and illustrate the predictive capabities in the context of graphene CVD synthesis on metal surfaces [13].

References:
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[2] Elstner, M.; Porezag, D.; Jungnickel, G.; Elsner, J.; Haugk, M.; Frauenheim, T.; Suhai, S.; Seifert, G. Phys. Rev. B 1998, 58, 7260-7268.
[3] Gaus, M.; Cui, Q.; Elstner, M. J. Chem. Theory Comput. 2011, 7, 931-948.
[4] Kubillus, M.; Kubar, T.; Gaus, M.; Rezac, J.; Elstner, M. J. Chem. Theory Comput. 2015, 11, 332-342.
[5] Cui, Q.; Elstner, M. Phys. Chem. Chem. Phys. 2014, 16, 14368-14377.
[6] a) Irle, S.; Zheng, G.; Wang, Z.; Morokuma, K. J. Phys. Chem. B 2006, 110, 14531-14545; b) Saha, B.; Irle, S.; Morokuma, K. J. Phys. Chem. C 2011, 115, 22707-22716.
[7] Wang, Y.; Page, A. J.; Nishimoto, Y.; Qian, H.-J.; Morokuma, K.; Irle, S. J. Am. Chem. Soc. 2011, 133, 18837-18842.
[8] Wang, Y.; Song, W.; Jiao, M.; Wu, Z.; Irle, S. Carbon 2017, 121, 292-300.
[9] Chou, C.-P.; Nishimura, Y.; Fan, C.-C.; Mazur, G.; Irle, S.; Witek, H. A. J. Chem. Theory Comput. 2016, 12, 53-64.
[10] Gruden, M.; Andjeklovic, L.; Jissy, A. K.; Stepanovic, S.; Zlatar, M.; Cui, Q.; Elstner, M. J. Comput. Chem. 2017, 38, 2171-2185.
[11] Ulissi, Z. W.; Medford, A. J.; Bligaard, Th.; Nørskov, J. K. Nat. Commun. 2017, 8, 14621/1-7.
[12] Shi, H.; Koskinen, P.; Ramasubramaniam, J. Phys. Chem. A 2017, 121, 2497-2502.
[13] Page, A. F.l; Ding, F.; Irle, S.; Morokuma, K. Rep. Prog. Phys. 2015, 78, 036501/1-38.

The ability to perform high-resolution mapping of adsorbate molecules or layers and correlate this with atomic structure would provide a transformative new tool for investigating the surface chemistry taking place on nanoparticles. Such a tool can be used to detect dissociative and non-dissociative sites on catalyst surfaces and study different bonding arrangements between the adsorbate and the catalyst. Electron energy-loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM) has been used for studying vibrational modes in materials with high spatial resolution and sensitivity [1, 2]. Radiation damage may be substantial when a STEM electron probe is placed directly on the adsorbate layer of interest. We adopt the aloof beam EELS technique that makes use of the long-range Coulomb interaction between electrons and adsorbate molecules to minimize damage when the probe is placed just outside the adsorbate layer. The delocalized vibrational signal in the aloof beam mode has allowed for probing variations in the amine content of graphitic carbon nitride photocatalysts [3]. A theoretical treatment of surface-enhanced EELS has also been given which exploits the electromagnetic coupling between the adsorbate and catalyst to demonstrate enhancement of the vibrational signal from the adsorbate [4]. Here, we explore three classes of adsorbate/catalyst systems because of their simplicity and overall scientific importance and their suitability for developing the aloof beam vibrational EELS technique, viz. PVP ligand shell on Au nanoparticles, CO on Pt nanoparticles supported on CeO₂ nanocubes, and CO₂ on MgO nanocubes.
All our results were obtained using a NION UltraSTEM 100 aberration-corrected microscope operated at 60 kV and equipped with a monochromator. Aloof beam EELS from the PVP ligand shell/Au nanoparticle system suffers from a low signal-to-noise ratio (SNR) and consists of two weak and broad vibrational signals centered at 160 meV and 205 meV. These signals correspond to peaks associated with the C-N stretch, CH₂ scissor, and wag, and C=O stretch modes as observed in the Fourier-transform infrared (FTIR) spectrum from pure PVP, convolved with the experimental energy-resolution [5]. Preliminary aloof beam EELS results from CO on Pt (111) particles demonstrate that the “on-top” vibrational mode of CO at 260 meV can be detected from a monolayer of adsorbate, although the signal suffers from poor SNR. This is in qualitative agreement with the diffuse reflectance infrared Fourier-transform spectrum (DRIFTS) from the CO on Pt/CeO₂ system. Aloof-beam EELS from the CO₂ on MgO system showed that the MgO surface was hydrated before chemisorption of CO₂ which led to the formation of a bicarbonate species on the surface. Further vibrational EELS results to investigate the bonding arrangements at different crystallographic surfaces in all three systems will be presented.
References:
[1] O.L. Krivanek et al., Nature, 514 (2014), p. 209.
[2] K. Venkatraman et al., Microscopy 67 (suppl_1) (2018), p. i14.
[3] D. Haiber et al., ACS Nano 12 (2018), p. 5463.
[4] A. Konečná et al., ACS Nano 12 (2018), p. 4775.
[5] Y. Borodko et al., J. Phys. Chem. B, 110 (2006), p. 23052.
[6] The support from National Science Foundation CHE-1508667 and the use of (S)TEM at John M. Cowley Center at Arizona State University is gratefully acknowledged.

Selective hydrogenation of unsaturated aldehydes to unsaturated alcohols is extremely valuable for synthesis of fine chemicals but is a major challenge. We here report that single Rh atoms anchored on the edges of 2D MoS₂(Rh₁/MoS₂) can efficiently convert unsaturated crotanaldehyde to crotyl alcohol with 100% selectivity via steric confinement effect. The Rh₁/MoS₂single-atom catalyst (SAC) is not only 100% selective but also active at moderate reactions conditions with a turnover frequency (TOF) of crotonaldehyde of 26.4/h at 80 ^oC (H₂pressure is 5 bar). To our knowledge, the TOF of Rh₁/MoS₂SAC is more than 50 times higher than that of the other reported heterogeneous catalysts that provides 100% selectivity for hydrogenation of crotonaldehyde to crotyl alcohol. Atomic resolution electron microscopy and CO-DRIFTS (diffuse reflectance infrared Fourier transform spectroscopy) results suggest that the Rh₁atoms strongly anchored to the edges of the MoS₂sheets creates, resulting in unique electronic properties. The DFT calculations suggest that the edge anchored Rh₁can facilely dissociate H₂molecules to H atoms which spill over to form OH groups on the oxidized Mo edge sites of the MoS₂sheets. The OH group and the associated Mo atom on each side of anchored Rh₁atom form a pocket-like active site with the configuration of HO-Mo-Rh₁-Mo-OH, in close resemblance of pocket sites of enzymes. Such specifically configured active sites impose severe steric hindrance that limits the possible adsorption configurations of the crotonaldehyde molecules and thus guide the chemical reaction to 100% selectivity. The strategy we used to construct pocket-like active site configurations is general and can be broadened to develop highly selective catalysts for a variety of important catalytic transformation of molecules [1].

[1]This work was supported by the National Science Foundation under CHE-1465057. The authors acknowledge the use of facilities within the John M. Cowley Center for High Resolution Electron Microscopy at Arizona State University. This research was funded in part by the International Energy Joint R & D Program (No. 20168510011350) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Ministry of Knowledge Economy, Korean government.

Hydrogen appears as a next-generation clean energy source to replace fossil fuels. One of the most promising ways to produce hydrogen is photoelectrochemical (PEC) water splitting in which sunlight-illuminated photoelectrodes generate electrical current to transform water into useful hydrogen. However, the existing photoelectrodes such as Si with noble catalysts still suffer from low efficiency and poor stability. Moreover, the extremely high cost of the noble metal catalysts limits the wide use of water splitting photoelectrodes. Recently 2-dimensional transition metal disulfides (2D TMDs) have been attracted much attention as promising candidates to replace Pt, because 2D TMDs such as molybdenum disulfide (MoS₂) and tungsten disulfide (WS₂) have inherently large surface-to-volume ratios and possess high densities of catalytically active sites for hydrogen evolution reaction (HER). Herein, we demonstrate that large-area (12 cm x 12 cm) and transferable MoS2 thin-film catalysts with vertically aligned layers, which maximally expose the edges on the thin film surface. The vertically aligned domains in the transferable MoS₂ thin films not only facilitate the charge transport toward reaction interfaces, but maximize the exposure of active edge sites of MoS₂. Furthermore, the Fe-doped Ni₃S₂ (Fe:Ni₃S₂) nanoparticles (NPs) grown on the vertically aligned MoS₂ (v-MoS₂) thin films are obtained by in situ growth during the synthesis of v-MoS₂. The synthesized Fe:Ni₃S₂/v-MoS₂ thin films exhibits only 105 mV at 10 mA/cm² for HER with exceptionally low Tafel slope of 66.6 mV/dec among the thin film type catalysts. Moreover, the Fe:Ni₃S₂/v-MoS₂/p-Si heterostructure photocathode can provide the high photocurrent density of 25 mA/cm² at 0 V versus a reversible hydrogen electrode. Density functional theory (DFT) calculations further reveal that the constructed coupling interface between v-MoS₂ and Fe:Ni₃S₂ facilitates the absorption of protons (H⁺), consequently enhancing the catalytic activity of the Fe:Ni₃S₂/v-MoS₂ thin film catalyst. Such outstanding performances indicate that the unique structure suggests a novel approach for designing advanced 2D materials-based electrocatalysts.

The functionalities associated with many thermochemical and photochemical catalysts rely on a variety of different materials properties which regulate and control ionic, electron and hole transfer across surfaces, interfaces and three-phase boundaries. Developing a fundamental understanding of these processes requires an atomic level picture of the structure and bonding during reactions. The interfaces between different components of the catalytic system can be explored with aberration corrected imaging and spectroscopy in a transmission electron microscope (TEM). The critical surface motifs that convert reactants into products may form only under reaction conditions and must be investigated with in situ or operando approaches (1, 2). Here we show how advanced imaging and spectroscopy can be employed to elucidate the functionally important features for a variety of catalysts. One strategy for addressing the problems associated with storage of solar energy is to generate solar fuels. Photocatalytic splitting of water to generate hydrogen is extremely challenging systems must be able to harvest light and transfer electrons/holes to suitable reaction sites at the surface. Carbon nitrides have bandgaps in the visible and are promising materials for hydrogen evolution (3). The domain size and location of the Pt co-catalyst nanoparticles on the surface plays a critical role in interfacial charge transfer and the overall H₂ production efficiency. Interfacial sites may also play an important role in light harvesting. For example, TiO₂, which usually only harvests UV light, can be functionalize to absorb in the visible by depositing thin layers of CeO₂ onto the surface. Cooperative mechanisms are also in play in many oxidation reactions performed over reducible oxides where oxygen transfer occurs via a Mars van Krevelen mechanism. For example, in CeO₂, in situ electron microscopy shows that strained terrace sites are very active for oxygen exchange(4). Placing a metal nanoparticle such as Pt or Ni onto the surface of CeO₂ will introduce strain into the surface layer of CeO₂. This strain is presumably why interfacial oxygen exchange is easy at the three-phase boundaries for these systems. Operando experiments for CO oxidation over Pt/CeO₂ are being employed to explore the behavior of the interfacial sites under reactions conditions. The spatial separation between the active sites can strongly affect the degree to which a cooperation effect will dominate a reaction. The benefit of cooperative catalysts can only be realized if the reaction intermediates are able to sense the presence of both sites within the time of the reaction. We used in situ electron microscopy to illustrate this for the case of carbon deposition onto Ni particles supported on a CeO₂ support (5). For ethene, rapid dehydrogenation takes place on contact with the Ni surface resulting in the formation of graphite. For ethane, dehydrogenation is sluggish in Ni but preferred at the interfacial sites resulting in no carbon deposition. References: 1. Crozier PA & Hansen TW (2015) MRS Bull. 40(01):38-45. 2. Chenna S & Crozier PA (2012) ACS Catalysis 2:2395-2402. 3. Haiber D & Crozier PA (2018) ACS Nano DOI 10.1021/acsnano.8b00884. 4. Lawrence EL, Levin BDA, Boland T, Chang SLY, & Crozier PA (2018) Nature Materials (under review). 5. Lawrence EL & Crozier PA (2018) ACS Applied Nano Materials 1(3):1360-1369. 6.The support from the National Science Foundation (NSF-CBET 1134464, DMR-1308085 CHE-1508667), US Department of Energy (DE-SC0004954) and the use of TEMs at the John M. Cowley Center for High Resolution Microscopy at Arizona State University are gratefully acknowledged.

Transition metal dichalcogenides (TMDs) such as MoS₂ have technological application from chemical catalysis to emerging microelectronic integration. At the same time gold nanoparticles have demonstrated catalytic properties that differ from bulk gold depending on the substrate and particle size. We have indication that there may be catalytic states on gold nanoparticles supported by MoS₂ that allow for the room temperature binding of CO. We have shown with XPS the retention of CO and subsequent its removal with O₂. This may open up the potential to convert CO to CO₂ and methanol to higher order hydrocarbons such as ethanol. The MoS₂ is grown on a SiO₂ layer on a silicon wafer. We then deposit the gold nanoparticles by using e-beam deposition onto our MoS₂.

According to Environmental Protection Agency, over 55% of NO_x production in the US is caused by automobile exhaust, also resulting in the formation of CO₂ and hydrocarbon residues. Transition metal doped CeO₂-ZrO₂ (CZO) nanoparticles (NPs) are extensively employed in three-way catalysis due to their ability to adsorb and release oxygen helping to prevent coking. These CZO NPs have shown a 5.3 mW/mg catalytic activity comparable to state of the art catalysts utilized in dry reforming of methane. However, the activity of these structures under reaction conditions, which is function of the position of the active site (Ni), has been shown to reduce the oxygen storage capacity and increase coking over time. To address this, the current work aims to understand the structural and electronic properties of the active site of CZO NPs, allowing for an in-depth study of the formation of oxygen vacancies in the lattice due to the position of the active site.

In this work, a two-step process, co-precipitation/molten salt synthesis (MSS), has been explored for a CZO:Ni catalyst, comparing its catalytic activity and oxygen vacancy concentration to a standard prepared by sol-gel technique. Transmission electron microscopy and X-ray diffraction identify that the as-synthesized NPs are monodisperse spheres, with 10 nm diameter, exhibiting fluorite phase. The concentration of oxygen vacancies in these NPs was studied using Raman spectroscopy, which shows a higher intensity defect-induced (D) band at 600 cm^-1 for the as-synthesized catalyst than the reported standard. Furthermore, the information regarding the site occupancy of the active site (Ni) in these catalysts was obtained by calculating the change in FWHM of the F_2g band. To further correlate the concentration of oxygen vacancies to the position of the active site (Ni) in the lattice, surface sensitive electronic characterizations such as X-ray photoelectron (XPS) and soft X-ray absorption spectroscopy (XAS) were performed. The oxidation state of Ni was identified as 2+, indicating that the metallic Ni clusters are not present. Furthermore, the local bond structure and geometry of the active site was accurately probed from Ni L edge XAS measurements in addition to the site occupancy measurements from Raman spectroscopy. These preliminary results on CZO:Ni catalysts demonstrate that controlling the oxygen vacancies in the lattice by varying the position of the active site (Ni) is the first step to understand the diffusion of the active site during a reaction and deactivation pattern of the catalyst.