1997 MRS Fall Meeting & Exhibit

Symposium Z—Recent Advances in Catalytic Materials

Chairs

Jan Hrbek, Brookhaven National Laboratory
Nelly Rodriguez, Northeastern Univ
Stuart Soled, Corp Research Lab

* Invited paper

SESSION Z1: OXIDE CATALYSTS I 
Chair: Enrique Iglesia 
Tuesday Morning, December 2, 1997 
Hampton (S)

8:45 AM *Z1.1 
TEXTURAL AND ACIDIC PROPERTIES OF MIXED ALUMINA-SILICA OXIDES PREPARED WITH COMMERCIALLY AVAILABLE SOLS. Edmond I. Ko, Steven J. Monaco, Carnegie Mellon Univ, Dept of Chemical Engineering, Pittsburgh, PA.

One challenge in the sol-gel preparation of mixed oxides is to control the distribution, or the extent of mixing, of the individual components. With alkoxide-based chemistry, mixing is to a large extent governed by the relative reactivity of the precursors. In fact, with the careful manipulation of sol-gel parameters, materials with different homogeneity can be obtained at a single composition. In this study we present an alternative approach that uses commercially available preformed sols as molecular building blocks. As a class of precursors, preformed sols are less well studied and manipulatable than alkoxides, but they offer the advantages of low cost and ease of handling. The specific sols used are two colloidal aluminas from Dispal and three colloidal silicas from Nyacol. First, the particle size distributions of the sols and the pore characteristics of the derived single oxides were characterized. Sols of different particle sizes were then used to prepare mixed oxides, leading to the combinations of small alumina-small silica, small alumina-large silica, and large alumina-small silica. With each combination the atomic ratio of Si to Al was varied from 1:9 to 1:1 to 9:1. This set of samples allowed a systematic study of the effects of particle packing and composition on the textural and acidic properties of alumina-silica. In general, our results showed that at Si:Al ratios of 1:9 and 9:1, the pore characteristics of the mixed oxides reflected those of the major component. The minor component, however, retarded the sintering process so that the crystallization of the major component and the loss of surface area was delayed to higher calcination temperatures. At an atomic ratio of 1:1, the two sols led to a product that had intermediate pore characteristics and showed evidence of chemical interaction in the form of new acid sites.

9:15 AM Z1.2 
SOL-GEL PRECURSORS AND THE OXYGEN STORAGE CAPACITY OF PrOy-ZrO2 MATERIALS. C.K. Narula, K. Taylor, Chemistry Department, Ford Motor Co., Dearborn, MI; L. F. Allard, HTML, Oak Ridge National Laboratory, Oak Ridge, TN; M. Yu. Sinev, M. Shelef, R.W. McCabe, G.W. Graham, Chemical Engineering Department, Ford Motor Co., Dearborn, MI.

Cerium oxide, an oxygen storage material, is essential for the near stoichiometric operation of automotive catalysts. The impetus to find superior alternatives comes from efforts to reduce slow but persistent decrease in automotive catalyst performance during prolonged use at high temperatures. Praseodymium oxide materials are under investigation as alternatives because they undergo oxygen exchange at a lower temperature compared to CeO2 without significant loss in capacity after high temperature sintering. Due to the difficulties in preparing PrO2 materials in the phases which allow Pr2O3 - PrO2 interconversion, they are not yet commercialized. In this presentation, we will describe the preparation of PrOy dispersed in the zirconia, alumina-zirconia, and ceria-zirconia matrices by sol-gel processes employing a mixture of alkoxide precursors or a single source alkoxide, Pr2Zr6(-O)2(-OAc)6(-O-i-Pr)10(O-i-Pr)10. We will show that the selection of precursors and processing methods determine the distribution of various phases of PrOy-matrix materials and, consequently, the oxygen storage capacity as determined by TPR. The observations on structural changes on thermal treatment of materials by XRD and HREM will also be presented.

9:30 AM Z1.3 
STRUCTURE-PROPERTY RELATIONSHIPS FOR BaCeO PEROVSKITES FOR THE OXIDATIVE DEHYRODENATION OF ALKANES. Tina M. Nenoff, Nancy B. Jackson, James E. Miller, Allen G. Sault, Daniel Trudell, Sandia National Laboratories, Catalysis and Chemical Technologies Department, Albuquerque, NM.

The oxidative dehydrogenation (OD) reactions for the formation of two important organic feedstocks ethylene and propylene, respectively, are of great interest because of the potential in capital and energy savings associated with these reactions. Theoretically, OD can achieve high conversions of the starting materials (ethane and propane) at lower temperatures than conventional dehydrogenation catalysis. The important focus in our studies of OD catalysts is the development of a structure-property relationship for catalyst with respect to selectivity, so as to avoid the more favorable combustion reaction. Catalysts for this reaction generally consist of mixed metal oxides. Since lattice oxygen is known to participate in the reaction, catalysts are sought with surface oxygen atoms that are labile enough to perform dehydrogenation, but not so plentiful as to promote complete combustion. Also, catalysts must be able to replenish surface oxygen by transport from the bulk. Perovskite based catalysts appear to fulfill these requirements. We are studying BaCeO3 perovskites doped with elements such as Y, La, Nd, Ca, Sr, and Mg. During the OD of the paraffins, the perovskite structure is not retained and a mixture of carbonates and oxides is formed, as revealed by XRD. While both the Ca and Y doped materials showed enhanced total combustion activity below 873 K, only the Ca doped material displays enhanced olefin production at 973 K. Structural and surface changes, as monitored by including powder X-ray diffraction, FTIR, and X-ray photoelectron spectroscopy, are being correlated with activity in order to understand the factors affecting catalyst performance, and to modify catalyst formulations to improve conversion and selectivity.

9:45 AM Z1.4 
PHOTOCATALYSIS OVER NANOCRYSTALLINE TITANIA-BASED MATERIALS. Chenchi Wang*, Zhibo Zhang, and Jackie Y. Ying*, *Department of Chemical Engineering and Department of Physics, Massachusetts Institute of Technology, Cambridge, MA.

TiO2-based photocatalysts represent an attractive approach in dealing with a variety of environmental pollution. The materials design was accomplished through controlling the grain size, introducing the dopant and the Pt metal deposition to enhance the photocatalytic activity. Nanocrystalline TiO2 systems with well-controlled size (5, 10, 20 nm), dopant concentration (Fe3+, Nb5+ = 0.05-2 at.%) were synthesized through chemical precipitation and sol-gel method. It was shown that for different particle sizes of TiO2, the photocatalytic activity could be significantly enhanced by appropriate dopant species, dopant concentrations and Pt loading. Photocatalytic decomposition of liquid-phase chloroform and gas-phase trichloroethylene has been investigated in a projection-type photoreactor. Materials synthesis, characterization, as well as catalytic activities of the modified titania systems will be presented. 10:30 AM *Z1.5 
DENSE OXIDE CERAMIC MEMBRANES FOR OXYGEN SEPARATION AND METHANE CONVERSION. A. J. Jacobson, S. Kim, Y. L. Yang and B. Abeles, University of Houston, Houston, TX.

Methane conversion to syn gas in membrane reactors that use dense mixed electronic-ionic conducting oxide membranes for oxygen separation have received much recent attention. The oxygen flux achievable in these reactors depends on a combination of the bulk diffusion rate for oxygen transport and the surface reaction rates for oxygen activation and either recombination or reaction with methane. We will report bulk diffusion coefficients and surface reaction rates obtained by analysis of oxygen permeation data through perovskite oxide membranes with different compositions. The effects of modification of the membrane surfaces on the surface reaction rates will be discussed. The practical application of oxide membranes requires materials that have chemical and mechanical stability at high temperature in large oxygen partial pressure gradients. Aspects of perovskite oxide phase stability and preferential segregation of specific components will also be discussed.

11:00 AM Z1.6 
CATALYTIC PROPERTIES OF NOBLE METALS ON NANOCRYSTALLINE OXIDES. Heiko Th. Hesemann, Horst Hahn, Darmstadt Univ of Technology, Dept of Materials Science, Thin Films Div, Darmstadt, GERMANY; Gar B. Hoflund, Univ of Florida, Dept of Chemical Engineering, Gainesville, FL.

Methane is about 25 times more potent than CO2 with respect to the green house effect and the most difficult hydrocarbon to oxidize. A new class of catalysts is introduced for this purpose. Noble metals on poly- and nanocrystalline reducible metal oxides (NMRO) in comparison to the pure poly- and nanocrystalline metal oxides were tested for the catalytic oxidation of methane as a function of reactor temperature. Serveral combinations of noble metals (Pd, Ag, Au) and supporting oxides (ZrO2, CeO2, TiO2, MnO2, FeO2, Al2O3) were examined. It is found that nanostructured supporting materials achieve higher activities at a given temperature or the same activity at much lower temperatures. ZrO2 as supporting oxide yields the best results. Pd-containing catalysts show higher activities than Ag-containing catalysts. Additionally, the catalytic activity depends on the pretreatment parameters, such as calcination temperature. The nanocrystalline material was prepared by the inert gas condensation technique.

11:15 AM Z1.7 
ELECTRONIC PROPERTIES AND PHASE TRANSFORMATIONS IN CoMoO4 AND NiMoO4 CATALYSTS: XANES AND TIME RESOLVED SYNCHROTRON XRD STUDIES. Sanjay Chaturvedi, Jose A. Rodriguez, Jon C. Hanson, Brookhaven National Laboratory, Department of Chemistry, Upton, NY; Joaquin L. Brito, Alberto Albornoz, Instituto Venezolano de Investigaciones Cietnificas (IVIC), Centro de Quimica, Caracas, VENEZUELA.

The catalytic properties of cobalt and nickel molybdates have attracted a lot of attention in recent years. These novel materials exhibit interesting relationships between their structural and catalytic properties. At atmospheric presure, two phases of NiMoO4 and CoMoO4 are known to exist, designated as the alfa and the beta phase. The main difference between them is that coordination of Mo6+ ions is octahedral in the alfa phase and tetrahedral in the beta phase. In both phases, Co2+ or Ni2+ ions occupy octahedral sites. The catalytic properties of NiMoO4 are closely related to its structure. The beta phase is almost twice more selective for the dehydrogenation of propane to propene than the alfa phase. In a similar way, sulfided beta-NiMoO4 was found to be a much better catalyst for the HDS of thiophene than the sulfided alfa isomorph. In order to fully understand these catalytic processes, it is essential to understand both, the structural as well as the electronic properties of these molybdates. We carried out a detailed study of these properties using time resolved synchrotron X-ray diffraction and X-ray absorption near edge spectroscopy (XANES). Investigations at Brookhaven Laboratory have recently established the feasibility of conducting sub-minute, time resolved XRD experiments under a wide variety of sample conditions ( C < T < C, P < 45 atm). This important advance results from combining the high intensity of synchrotron radiation with rapid new parallel data-collection devices. Using this approach, we monitored the changes that occur in AMoO4.xH2O and alfa-AMoO4 systems (A=Co/Ni) as a function of temperature. The hydrated CoMoO4 and NiMoO4 compounds lose water by 350 and C , respectively to form the pure beta phases. On heating to C, alfa-NiMoO4 starts to transform into the beta isomorph and the transformation is complete by C. The alfa to beta transformation in CoMoO4 occurs around C. The time resolved XRD patterns provide detailed information about the kinetic parameters of these transformations. The electronic properties of pure and sulfided molybdate catalysts were probed through XANES measurements (K-edges of Co, Ni, O and S; L2,3 edges of Co, Ni and Mo). The results of these studies will be presented and correlations involving structural, electronic and catalytic properties of molybdate catalysts will be discussed.

11:30 AM *Z1.8 
SYNTHESIS AND CHARATERIZATION OF TAILOR MADE PURE AND DOPED METAL OXIDE MATERIALS. J.Z. Larese,W. Kunnmann, Chemistry Department, Brookhaven National Laboratory, Upton, NY.

We will describe the materials produced and the characterization of tailor made, pure and doped metal oxide materials. This will include a description of adsorptive, structural and chemical properties of materials like MgO and ZnO doped with measured quantities of such metals as Cu, Li, Ni, Fe. These materials have also been examined with time resolved x-ray scattering techniques in order to probe simple chemical reactions in-situ and under realistic conditions. This work has been supported by the U. S. Dept. of Energy, Materials Science Division, under Contract No. DE-AC02-76CH00016.

SESSION Z2: METAL CATALYSTS 
Chair: Jan Hrbek 
Tuesday Afternoon, December 2, 1997 
Hampton (S)

1:30 PM *Z2.1 
THE FABRICATION OF HIGH TECHNOLOGY CATALYSTS. Gabor A. Somorjai, Department of Chemistry, University of California, Berkeley, CA, and Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA.

Electron beam lithography using the ``nanowriter'' in the Lawrence Berkeley National Laboratory has been applied to deposit platinum and silver nanoparticle arrays over a 1cm2 area of SiO2, Al2O3 and TiOx flat oxide supports. The metal particle size and spacing could be varied in the 2.5 nm to 100 nm range while the clusters were all the same size for a given sample. The nanoparticle systems were characterized by electron microscopy, atomic force microscopy, scanning auger spectroscopy and temperature programmed desorption. The metal particles could be cleaned by low energy ion bombardment. Chemisorption of CO, H2 and C2 H4 have been explored as a function of particle size. Ethylene hydrogenation and partial oxidation, butane isomerization and CO oxidation have been studied and compared with results obtained on single crystal surfaces. The thermal stability and the chemical stability of the nanoparticles under reducing and oxidizing conditions have been investigated and will also be discussed.

2:00 PM Z2.2 
SYNTHESIS AND CHARACTERIZATION OF SHAPE-CONTROLLED PLATINUM NANOCRYSTALS. Z.L. Wang, T.S. Ahmadi, J.M. Petroski and M.A. El-Sayed, *School of Material Science and Engineering, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA.

The selectivity and activities of platinum (Pt) particles strongly depend on their sizes and shapes. A technique is reported for controlling the shapes and sizes of Pt particles [1]. Pt particles were prepared by bubbling Ar gas through the solution of K2PtCl4, and the Pt ions were reduced by flowing H2 gas through the solution. The shape control was performed by changing the ratio of the concentration of the capping polymer material to that of the platinum cations used in the reductive synthesis of colloidal particles in solution at room temperature [2]. High percentage of cubic, tetrahedral and octahedral particles have been prepared at room temperature, making it possible for studying the chemical activities of particles with different shapes and facets. The microstructures of the as-synthesized cubic, tetrahedral and truncated octahedral Pt particles are studies by high-resolution transmission electron microscopy (HRTEM) [3]. The truncated shapes of these particles are mainly defined by the {100}, {111} and {110} facets, on which numerous atom-high surface steps, ledges and kinks have been observed. These atomic scale fine structures of the particle surfaces are expected to play a critical role in their catalytic activity and selectivity.

2:15 PM Z2.3 
ENCAPSULATION OF METAL NANOCLUSTER CATALYSTS IN XEROGEL AND AEROGEL MATERIALS VIA AN INVERSE MICELLE/SOL-GEL SYNTHESIS. Anthony Martino, Allen Sault, Jeffrey Kawola, Elaine Boespflug, Catalysis and Chemical Technologies Department; Jeffrey Brinker, Direct Fabrication Department, Sandia National Laboratories, Albuquerque, NM.

We have synthesized novel gel supported nanocluster catalysts by encapsulating nanometer sized metal particles in the micropores of xerogels and aerogels. The ëship in a bottleí approach offers potential advantages over traditional metal-gel matrixed materials made through impregnation techniques or with chelating agents. The nanoclusters are trapped in gel pores of comparable size limiting the modes of particle sintering. In addition, the inverse micelle mediated synthesis provides in a single methodology ultra-small, highly dispersed particles of one size, particle size control, and composition variability (i.e. various metals, mixtures of different metals, alloys, layered particles, metal oxides, and metal sulfides). We have studied Au and Pt nanoclusters encapsulated in both SiO2 and Al2O3 xerogels and aerogels. The synthesis involves the reduction of a metal salt followed by sol-gel processing in an inverse micelle solution. In a second type synthesis, we prepare gels in traditional sol-gel solvents (ethanol) and allow nanoclusters separately synthesized in inverse micelles to diffuse into the monolith. Porosity control is achieved by chemical surface modification of the silica monolith to control structural collapse under ambient drying conditions. Encapsulation occurs when particle and pore size are comparable. The effects of material (inverse micelle vs. traditional solvents, xerogels vs. aerogels), support type (SiO2 vs. Al2O3), and various post-synthesis heat treatments on particle stability, support surface area, and catalyst activity in propane dehydrogenation will be presented. Characterization was completed with TGA, atomic absorption, chemisorption, BET, and transmission electron microscopy.

2:30 PM Z2.4 
MOCVD PALLADIUM AND PLATINUM SUPPORTED ON ALUMINA CATALYSTS: PREPARATION AND CHARACTERIZATION. Zulema K. Lopez, Miguel A. Valenzuela, Instituto Politecnico Nacional, Dept. of Chemical Engineering, Mexico D.F., MEXICO; J. Roberto Vargas, Instituto Politecnico Nacional, Dept. of Metallurgical Engineering, Mexico D.F., MEXICO.

Metalorganic chemical vapor deposition (MOCVD) and conventional impregnation methods were used to prepare Pd and Pt supported on alumina catalysts. Powdered alumina ranging from 140 to 190 micrometers in size was used as support. The impregnation process was carried out at 80C using chloro-platinic acid and palladium nitrate (II) as precursors. Supported catalysts were heated after impregnation up to 500C in static air. Pd and Pt were deposited on the support by MOCVD at 400C from metal-acetylacetonate precursors. Temperature Programmed Reduction analysis indicated that Pt and Pd are in their oxidized states on the surface of the support in the catalyst prepared by the impregnation method. Transmission electron microscope observation and Auger spectroscopy showed that MOCVD Pd and Pt deposits are aggregated metallic particles about several nanometers in diameter. Catalysts prepared by both MOCVD and impregnation will be tested in a reaction of dehydrogenation of alkanes to compare their catalytic properties.

2:45 PM Z2.5 
MECHANICALLY ALLOYED NICKEL-ZIRCONIUM POWDER AS A HETEROGENEOUS CATALYST AND A CATALYST PRECURSOR. William E. Brower, Jr., Arvin J. Montes, Kevin A. Prudlow, Marquette University, Dept of Mechanical and Industrial Engineering, Milwaukee, WI; Hans Bakker, Albertus C. Moleman, and Hua Yang, University of Amsterdam, Van der Waals/Zeeman Laboratory, Amsterdam, NETHERLANDS.

Mechanical milling provides a processing route to atomically mix catalytically active elements with cations which form catalyst oxide supports by forming glasses or extended solid solutions. Amorphous NiZr alloy was produced by milling for about a week in less than one microtorr vacuum. High temperature in situ XRD aging studies in air showed only the formation of ZrO2 followed by fcc nickel from the initially glassy phase. Thermogravimetric analysis in oxygen resulted in a weight gain consistent with the complete conversion of the Zr to ZrO2. High temperature in situ XRD studies in less than one microtorr vacuum resulted in the formation of a crystalline NiZr phase, with no XRD indication of ZrO2 or NiO. The as milled NiZr powder was tested as a catalyst for the decomposition of nitric oxide and compared to both a commercial supported Ni catalyst and to pure Ni powder. The NiZr powder exhibited a much higher turn over fraquency (TOF) than the supported Ni catalyst and the Ni powder. However the rate of NO decomposition, expressed as a rate per gram of catalyst was far greater on the supported Ni commercial catalyst, due to its high dispersion. The surface area of the NiZr powder increased during the reaction from 0.1 to 3.2 sq m/g. Chemisorption of CO showed the surface metal atom concentration to increase only by a factor of 3 during the reaction, far less than the factor of 32 for the surface area. Most of the new surface area appears to be ZrO2. The activation energy for the NO decomposition reaction over the NiZr was 38 kcal/mole. The ball milled NiZr powder appears to be a very active catalyst for the direct reduction of nitric oxide.

3:15 PM *Z2.6 
THE PHYSICAL AND CHEMICAL OF METAL CLUSTERS SUPPORTED ON PLANAR OXIDE SURFACES. D. Wayne Goodman, Department of Chemistry, Texas A&M University, College Station, TX.

A new surface science approach to the study of supported-metal catalysts will be described. Thin oxide films ( 5 nm) of SiO2, Al2O3, TiO2 or MgO supported on a refractory metal substrate (e.g., Mo, Ta, Re, or W) have been prepared by depositing the oxide metal precursor in a background of oxygen (ca. 1 10-5. Metal (e.g., Cu, Pd, Ni, Au) clusters with varying sizes have been formed on these oxide films and the properties of the metal/oxide system studied with an array of surface techniques, including scanning tunneling microscopy (STM) and spectroscopy (STS). By carefully defining the amount of metal deposited, clusters of varying sizes from the sub-nanometer range to tens of nanometers have been synthesized. These studies have provided detailed information with respect to the physical properties (size, morphology, sublimation energies, electronic structure) of metal clusters supported on oxides. Kinetic studies of CO oxidation, CO methanation, alkane hydrogenolysis, and acetylene trimerization have illustrated the correspondence between cluster size, shape, and catalytic activity/selectivity.

3:45 PM *Z2.7 
CHEMICAL CONTROL OF NOBLE METAL CATALYSIS BY MAIN GROUP ELEMENTS. Kiyotaka Asakura and Yasuhiro Iwasawa, Research Center for Spectrochemistry and Department of Chemistry, Graduate School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo, JAPAN.

Main group elements usually make a strong chemical bonding with metal catalysts to modify their electronic structure, which may poison the catalysis. However, we can create noble active site composed of metal and main group element by controlling the interaction if we carefully choose the preparation conditions. Pt/SbOx shows low activity for iso-butane partial oxidation reaction when it is reduced with H2. In this case we found PtSb alloy formation and the structure change of SbOx supports. On the other hand, on the Pt/SbOx treated at 773 K under oxygen, methacrolein was formed from iso-butane in a high selectivity. XRD, EXAFS, RAMAN, adsorption measurement and TEM indicated the decoration of Pt surface by Sb oxide and changed its catalytic activity. Such a moderate modification of Sb oxide suppresses the total oxidation reaction but keeps the dehydrogenation activity of Pt surface. In the other example of Rh/one-atomic layer GeO2/SiO2, we found synergistic catalyst between metal particle and main group element oxide. When we deposited Rh6(CO)16 on the one-atomic layer GeO2/SiO2 and reduced it at low temperature, we found the high activity for ester hydrogenolysis reaction to alcohol at 1 atm. On the other hand, no reaction occurred on the Rh particle deposited on GeO2 bulk. In the latter case, we found the RhGe alloy formation while in the Rh/one-atomic GeO2/SiO2, Rh particle was present. FTIR studies showed that the hydrogen activated on the Rh particle spilled over to GeO2 one-atomic layer surface and reacted with the ester that had adsorbed on the GeO2 surface. Through these examples we show the possibility to control catalysis of noble metal by the moderate interaction of main group elements.

4:15 PM Z2.8 
THE RAFT-LIKE STRUCTURE OF SUPPORTED PtRu5. Judith Yang, Steve Bradley, Ajay Singhal, Michael Nashner, Ralph Nuzzo, J. Murray Gibson, Univ. of Illinois at Urbana-Champaign, Materials Research Lab, Urbana, IL; UOP, Des Plaines, IL.

Multicomponent metal nanoclusters can show superior catalytic behavior than single component nanoclusters. One example is PtRu5 on carbon which demonstrates better methanol electrooxidation in fuel cells as compared to only Pt on Carbon [1]. To understand why this bimetallic catalyst is better than Pt requires knowledge of the shape of the clusters on the carbon support. We have examined supported PtRu5 specimens by a variety of electron microscopy techniques, including high resolution and analytical electron microscopy. We have also applied a novel mass-spectroscopic technque to this system. We have demonstrated that very high angle annular dark-field images in a scanning transmission electron microscope can be used to quantitatively measure the number of atoms of individual nano-sized clusters on a support material [2]. Analytical electron microscopy results showed that the relative atomic concentration of Pt to Ru for each PtRu5 particle is 1 to 5. The average diameter of the clusters was 16‰, and the average number of atoms was measured to be 24 atoms per cluster. The combination of these techniques demonstrate that the PtRu5 clusters are oblate or raft-like on the carbon support. We are presently examining other supported nanosized metal clusters in order to elucidate their structure on the support material.

4:30 PM Z2.9 
MICROSTRUCTURE AND GAS SENSING PROPERTIES OF PLATINUM-DISPERSED TiO2 THIN FILM DERIVED FROM PRECURSOR CONTAINING Ti AND Pt. Issei Hayakawa, Fine Ceramics Res. Association, Synergy Ceramics Lab., Nagoya, JAPAN; Ko-ichi Kikuta, Nagoya Univ, Dept of Crystalline Materials Science; Shin-ichi Hirano, Dept of Applied Chemistry, Nagoya Univ.

Precursor solution was synthesized using Ti alkoxide derivative, amino acid, platinum salt and methanol as a solvent. The solution was dip- or spin-coated on glass substrate and dried. The precursor on substrate changed from amorphous to anatase at about 400C, and part of anatase to rutile at about 600C. The prepared thin film with fine Pt particles(1-2nm) was dense and not sensitive to reductive gas such as H2, CO and CH4 gases. However, gas sensing property was promoted by coating the precursor solution containing proper amount of TiO2 sol. The thin film revealed high sensitivity and selectivity to air containing H2 at 200C. Also, effects of dopants on gas sensitivity and selectivity were examined.

4:45 PM Z2.10 
ON EXTENDED X-RAY ABSORPTION FINE STRUCTURE CHARACTERISATION OF IRON PLATINUM CLUSTERS. Gregorio D'Agostino, ENEA, Inn. Dept, Roma, ITALY; Adriano Filipponi, ESRF, Grenoble, FRANCE; Alessandra Verrazzani, Alessandro Fortunelli, Giovanni Vitulli, CNR, Chem. Inst. Pisa, ITALY.

Present contribution is aimed at reporting on Extended X-ray Absorption Fine Structure (EXAFS) characterisation of iron-platinum nano-clusters (2-3 nm). The technique allows to achieve information on the short range structural environment of both iron and platinum atoms. The structural information, in turn, provide insights on the enhanced catalytic effect of platinum clusters upon iron addiction. Samples have been prepared by evaporation in a saturated solute (typically arene) atmosphere. Two different concentration of iron, namely FePt4 and FePt2, have been prepared and characterised exhibiting similar structural behaviour. Samples supported on silica substrate have also been prepared and charcterized by the florescence technique. All samples do exhibit enhanced activity and selectivity in the hydrosilylation of acetylene's and nitrile's.

SESSION Z3: POSTER SESSION: 
CATALYTIC MATERIALS 
Chair: Edmond I. Ko 
Tuesday Evening, December 2, 1997 
8:00 P.M. 
Grand Ballroom (S)

Z3.1 
STABILITY AND SURFACE ACIDITY OF ALUMINIUM OXIDE GRAFTED ON SILICA GEL. Lilian L.L. Prado, Pedro A.P. Nascente, CCDM/DEMa/UFSCar, Sao Carlos, SP, BRAZIL; Sandra G.C. de Castro, IFGW/UNICAMP, Campinas, SP, BRAZIL; Yoshitaka Gushiken, IQ/UNICAMP, Campinas, SP, BRAZIL.

The synthesis and characterization of metallic oxides grafted on a support surface with high surface area have presented increasing interest in recent years, since these materials exhibit high catalytic activity in several reactions. The synthesis of aluminium oxide grafted on silica gel surface was carried out by the reaction of a suitable aluminium precursor with the surface hidrolysis of the oxide support. The chemical and physical properties of the attached oxide, Al2O3/SiO2, can be quite different than those found for bulk Al2O3. The advantage of this preparation method, compared to the conventional ones (impregnation, precipitation, and calcination), is that the oxide is highly dispersed on the surface (monolayer or submonolayer). The object of this work is the determination of the surface oxides treated at the temperature range of 423 to 1573 K. We employed X-ray photoelectron spectroscopy (XPS), solid state nuclear magnetic resonance spectroscopy (NMR), and diffuse reflectance spectroscopy (DRS) to characterize the Al2O3/SiO2 surfaces. XPS was used to identify the oxidation states and atomic ratios. Al27 NMR detected two species for samples heated up to 1173 K, and another one above this temperature. DRS, using pyridine as a molecular probe, showed that both Lewis and Bronsted acid sites are stable up to 1173 K. We concluded that the aluminium oxide is highly dispersed on the silica gel surface and it remains stable up to 1173 K.

Z3.2 
SURFACE CHARACTERIZATION OF W/Ni/Al2O3 CATALYSTS. Maura H. Jordao, Jose M. Assaf, DEQ/UFSCar, Sao Carlos, BRAZIL; Pedro A.P. Nascente, CCDM/DEMa/UFSCar, Sao Carlos, BRAZIL.

Catalysts containing tungsten and nickel oxides are important in hydrodessulfurization (HDS), hydrogenation (HY), and steam reforming of hydrocarbons. A series of W/Ni/Al2O3 catalysts were prepared by two different methods; (1) coprecipitation of nickel and aluminum from their nitrates, followed by impregnation of tungsten; (2) precipitation of aluminum from its nitrate, followed by impregnations of nickel, firstly, and tungsten. The nickel content was kept constant, while the amount of tungsten varied from 2,5 to 15,5 wt %. The resulting oxides were characterized by inductively coupled plasma spectroscopy (ICP), atomic absorption spectroscopy (AAS), X-ray diffraction (XRD), temperature programmed reduction (TPR), and X-ray photoelectron spectroscopy (XPS). ICP and AAS were used to determine the W, Ni, and Al concentrations. XRD detected two phases: NiO and NiAl2O4 (no phase containing tungsten was detected). Increasing the amount of W, the quantity of NiAl2O4 rised, the quantity of NiO decreased, and the particle size of NiO enlarged. The TPR profiles presented three peaks: one above 1220 K, associated to a very stable phase; for the samples prepared by coprecipitation, the other two peaks corresponded to free NiO and a nonstoichiometric aluminate. For the samples prepared by impregnation, those two peaks corresponded to NiO and NiAl2O4. XPS identified Al2O3, NiAl2O4, and Al2(WO4)3 for both preparation methods. Increasing the amount of tungsten in the impregnated samples, NiWO4 was also observed.

Z3.3 
ADVANCED METAL/CERAMIC CATALYSTS FOR HYDROGEN GENERATION BY STEAM REFORMING OF HYDROCARBONS. S.F. Tikhov, Vladislav A. Sadykov, A.N. Salanov, S.N. Pavlova, Yu.V. Palesskaya, G.N. Kustova, G.S. Litvak, S.V. Tsybulya, Boreskov Inst. of Catalysis RAS, Novosibirsk, RUSSIA.

Steam reforming of methane is currently the main industrial process of the hydrogen generation. It requires intense heating of the catalysts due to endothermic nature of the reaction. Existing catalysts do not meet demands for high thermal conductivity. New types of advanced catalysts comprised of thick metalloceramic layers strongly adhering to the internal and/or external surfaces of tubes were elaborated using proprietary hydrothermal technique. Main features of the metalloceramic layer formation from the mixture of aluminum powder and active component were elucidated. Pore structure, specific surface and crushing strength were shown to be mainly determined by the process of the anisotropic growth of boemite particles during aluminum oxidation by water vapors and their subsequent dehydration at the air calcination stage. Nickel-based composites including those with a perovskite structure demonstrated high performance in the steam reforming of methane. Apart from the high thermal conduction, these catalysts are also promising for the microwave heating. Tube catalysts with different active components at the internal and external side can be used to efficiently conjugate the exothermic reaction of fuels combustion and endothermic reaction of the methane steam reforming.

Z3.4 
NEW TYPE OF ADVANCED CATALYTIC MATERIALS BASED UPON ALUMINA EPITAXIALLY GROWN ONTO THIN ALUMINUM FOIL. S.F. Tikhov, G.V. Chernykh, Vladislav A. Sadykov, A.N. Salanov, S.V. Tsybulya, Boreskov Inst. of Catalysis RAS, Novosibirsk, RUSSIA; V.F.Lysov, Institute of Applied Physics,Novosibirsk, RUSSIA.

Highly efficient, flexible and wasteless technology of aluminum foil anodic spark oxidation in the water-based electrolites has been applied to form strongly adhering protective alumina layer on the surface. Basic stages of the oxidation were studied by SEM and XPD. Oxohydroxide islands were shown to randomly nucleate at the early stage followed by their subsequent lateral growth. After islands overlapping, cylindrical macropores remain. Fine tuning of the anodic oxidation parameters allowed to optimize the layer thickness and its microstructure. Dehydration stage converts oxohydroxide into the microporous defect alumina spinel without changing morphology of the coating layer, though some cracks appear. Promoters and modifiers including rare- earth oxides were used to stabilize spinel structure and fill macropores and cracks. As a result, composite materials were elaborated which endure overheating up to 1000 C without evident aluminum oxidation or leakage of a molten metal confined within the oxidic walls. Such materials were assembled as thin wall honeycomb supports including those with a criss-crossing channels pattern. Procedures of the secondary supports (stabilized alumina, zeolites etc) and/or active components washcoating/impregnation were elaborated. These catalysts were tested in the reactions of CO and CHx oxidation, NOx selective reduction by hydrocarbons and demonstrated high performance at high space rates. These systems are promising for applications requiring high thermal conductivity, low pressure drop etc.

Z3.5 
PHOTOCATALYTIC WATER DECOMPOSITION IN Pt LOADED Ti/B AQUEOUS SUSPENSION SYSTEM. Sang-Chul Moon, Katsumi Okada, Hiroaki Mametsuka, Yoshiko Nakahara, RITE, Catalysis Science Lab, Kyoto, JAPAN.

The photocatalytic decomposition of water has received attentions to be extremely useful for the production of clean energy and reduction of C02. Since the K. Honda et. al.'s report, it has been actively investigated to apply the principle of water decomposition over the photoelectrochemical cells to heterogeneous photocatalysis system using powdered semiconductors because of their some general advantages as compared to photoelectrochemical cells, i.e., low cost to construct, chemical stability on the light, and their large surface areas. The Pt loaded TiO2 has been most used for the photocatalytic decomposition of water. However, most of the studies have concluded that in aqueous suspension system Pt-TiO2 could not stoichiometrically decompose water, and until now, exception of some, there have been no investigations on the photocatalyses to decompose water stoichiometrically in aqueous suspension system under all conditions. Here we present results of the stoichiometric water decomposition over Pt loaded Ti/B binary oxides. We have recently found that Ti/P binary oxide prepared by sol-gel method exhibited a special photocatalytic activity for the water decomposition in aqueous suspension systems. The Pt-Ti/B photocatalyses decomposed water stoichiometrically with no addition of carbonates, and their photocatalytic activities were not dependent on the direction of irradiation. In the present work, we show the role of boron in the reaction of stoichiometric water decomposition over Pt-Ti/B photocatalyses and the amazing yields with the addition of Na2CO3 into the Pt-Ti/B suspension.

Z3.6 
THEORETICAL STUDY OF THE ELECTRONIC STATES OF OXOCATION SITES IN CU EXCHANGED ZEOLITES. B. R. Goodman, R. Ramprasad, University of Illinois, Urbana IL; W. F. Schneider, K. C. Hass, Ford Research Labs, Dearborn, MI; J. B. Adams, Arizona State University, Tempe AZ.

Some Cu exchanged zeolites are known to effectively catalyze the decomposition of NO under oxidizing conditions and are being studied for use in lean burn and diesel engine applications. While the mechanism of NO decomposition is not known, various pathways and active sites have been postulated. One such proposal is that NO decomposition involves pairs of Cu sites in the form of an oxocation [Cu-O-Cu]2+ within the zeolite. Density functional theory (DFT) calculations are used to study the electronic structure of the oxocation and the modifications introduced by the supporting zeolite. Water ligands and other small models are used as surrogates for the zeolite framework. In particular, we explore the existence of a broken-symmetry singlet state. The calculated thermodynamic stability of the oxocation sites is compared to that of mono-Cu sites.

Z3.7 
A NOVEL NANOPARTICULATE TITANIA FILM CATALYST -PREPARATION, PROPERTIES AND RESEARCH ON ITS PHOTOCATALYTIC OXIDIZED ACTIVITY. Yaan Cao, Liqing Chong, Xintong Zhang, Xiangdong Cai, Zhiyun Wu, Yubai Bai, Tiejin Li, Jilin Univ., Department of Chemistry, Changchun, P.R.CHINA.

A new kind of nanoparticulate titania film catalyst was prepared by Plasma-enhanced chemical vapor deposition (PCVD) method. X-ray photoelectron spectra (XPS) and surface photovoltaic spectra (SPS) showed that this kind of titania film had abundant surface states. It was found that the film had high activity on the photocatalytic degradation of phenol aqueous solution and its specific photocatalytic activity was close to P25 (Degussa). Further,the influences of surfacial properties and thickness of the titania films on the photocatalytic degradation of phenol were also discussed.

Z3.8 
IN-SITU NMR CHARACTERIZATION OF TEMPLATED MICROPOROUS ALUMINO-PHOSPHATES. Corine Gerardin, Mohamed Haouas, Francis Taulelle, NMR and Solid State NMR, UMR 50 CNRS, Universite Louis Pasteur, Strasbourg, FRANCE; Thierry Loiseau and Gerard Ferey, Institut LAVOISIER UMR 173 CNRS, Universite de Versailles, Versailles, FRANCE.

Elucidation of the initial mechanism of microporous solid necessitate an in-situ investigation. In-situ NMR under autogeneous pressure of aluminophosphate allows to measure the pH variation during synthesis and to follow P-31, Al-27, F-19 and N-14 nuclei. Environement changes are thus followed while synthesis proceeds. NMR observation and quantitation of different nuclei allows for determination of the sequential reactions : dissolution of reactants and their kinetics, formation of competing species, redissolution of all but the targetted phase, and its growth. Aluminum to fluorine chemical bond has been studied and its lifetime determined. An exchange occurs, it goes from slow at ambient temperature to fast in hydrothermal conditions at 150 C. For the first time it has been possible to evidence a change of coordination of aluminum in solution when going from room temperature to hydrothermal condition. In addition to this coordination change of the Primary Building Unit, observation of a signal which is believed to belong to the Secondary Building Unit responsible of solid growth. A detailed liquid to solid mechanism at a molecular level is therefore possible and proposed.

Z3.9 
CHARACTERIZATION OF ULM AND MIL ALUMINOPHOSPHATES BY SOLID STATE NMR MAS AT VERY HIGH SPEED, MQMAS, RFDR AND DOUBLE QUANTUM NMR. Francis Taulelle, Mohamed Haouas, Corine Gerardin, NMR and Solid State NMR, Universite Louis Pasteur, Strasbourg Cedex, FRANCE. Thierry Loiseau and Gerard Ferey, Institut LAVOISIER, Universite de Versailles,Versailles Cedex, FRANCE.

ULM and MIL compounds contains fluorinated aluminophosphates or gallophosphates. On this family of compounds several NMR methods are used to complement the X-ray diffraction structure determination. Inequivalents sites are determined on powders by using MAS at high or very high speed (15 to 35 kHz) and MQMAS. Topological connectivities in the structures are directly proved by using RFDR. Homoatomic connectivities between equivalents sites may be demonstrated by Double quantum experiments. Both RFDR or Double quantum experiments may lead to a direct proof of non centrosymmetry of the compounds. Mixtures of amorphous products can be quantitatively measured by combined usage of X-ray diffraction, RFDR and MAS characterization. Most of these techniques will be exemplified on ULM-3 , ULM-4, ULM-18. Fluorines spectra at very high spinning speed will demonstrate the usefulness of such a characterization to elucidate the fluorine localization directly on the powders issued from hydrothermal synthesis. Further insights on synthesis mechanisms are given by combining in-situ NMR during synthesis and ex-situ high resolution solid state NMR.

Z3.10 
PREPARATION AND STRUCTURAL CHARACTERIZATION OF ULM-18, A NEW LAYERED FLUORINATED GALLIUM PHOSPHATE. Thierry Loiseau and Gerard Ferey, Institut LAVOISIER UMR CNRS 173, Universite de Versailles, Versailles, FRANCE. Francis Taulelle, NMR and Solid State NMR Laboratory, Universite Louis Pasteur,Strasbourg Cedex, FRANCE.

Ga4(PO4)4(HPO4)F, 1.5 N2C6H18, H2O or ULM-18, is a new layered oxyfluorinated gallium phosphate obtained by hydrothermal synthesis using N,N,N',N'-tetramethylethylendiamine as a template. The structure is described in the triclinic space group P-1 (n2) with a = 8.5264(7) , b = 9.2512(7) , c= 17.870(2) , = 101.742(7), = 99.137(7), = 87.020(6), V = 1362.3(2) 3, Z = 2. The structure was determined by means of single-crystal X-ray diffraction. The 2D framework which is built up from sheets containing double four-ring units (D4R) connected together and with HPO4 tetrahedra. The D4R unit consists of four PO4 tetrahedra, three GaO4 tetrahedra and one GaO5 trigonal bipyramid. A fluorine atom is trapped within the D4R cage and is linked preferentially to the three gallium atoms in tetrahedral coordination. The connection of the D4R and the HPO4 units gives rise to 8-ring channels along [010] and [011]. The cohesion of the structure is ensured by hydrogen bonds between the inorganic network and the intercalated N,N,N',N'-tetramethylethylenediamine and water molecules. Another organic template is also found to be encapsulated within the channels of the layer along [011].

Z3.11 
CATALYTIC PROPERTIES OF ULTRA-HIGH SURFACE AREA BARIUM HEXAALUMINATE FOR CATALYTIC COMBUSTION. Andrey Zarur, Henry H. Hwu, and Jackie Y. Ying, Massachusetts Institute of Technology, Dept. of Chemical Engineering, Cambridge, MA.

Ultra-high surface area barium hexaaluminate was prepared through controlled hydrolysis and polycondensation of alkoxide precursors in reversed emulsion nanoreactors. The materials prepared through this technique exhibit enhanced thermal stability, preserving surface areas in excess of 100 m2/g after calcination to 1300oC. X-ray diffraction analysis was used to verify the presence of the correct barium hexaaluminate phases active for catalytic combustion. The samples were used to convert methane in a flow reactor at different temperatures and space velocities for a range of air-to-fuel ratios. The results show enhanced catalytic activity and hydrothermal stability over conventional barium hexaaluminate.

Z3.12 
SYNTHESIS OF Ni-Mg-Al HYDROTALCITE-LIKE COMPOUNDS USING MICROWAVE IRRADIATION. G. Fetter, M.A. Valenzuela, Instituto Politécnico Nacional, ESIQIE-SEPI, Mexico D.F., MEXICO; P. Bosch, Univ. Autónoma Metropolitana, Depto. de Química, Mexico D.F., MEXICO.

Hydrotalcites are minerals whose lamellar structure is brucite like where magnesium is octahedrally coordinated and these magnesium ions may be substituted by aluminium. The substitution degree is variable. Hence some anions have to be incorporated between the layers to ensure neutral charge. Due to the basic properties of these microporous materials, they find application mainly as catalysts and adsorbers. Hydrotalcite-like compouns have the hydrotalcite structure where the divalent and trivalent cations can be others than magnesium and aluminium They are synthetic materials and are conventionally prepared by coprecipitation whose main disadvantages are the time required to crystallize the hydrotalcite (about one day), high pressure and temperature. Hence new synthesis methods have been explored. In a previous work we successfully reported the use of microwave irradiation on the synthesis of Mg-AI hydrotalcites. In this work, Ni-Mg-AI hydrotalcite-like compouns were prepared using microwave irradiation during the hydrothermal step. The synthesis was carried out with a Al/(Ni+Mg)+AI molar ratio of 0.25 and varying the Ni/Mg molar ratio from 0.0 to 3.0. The properties of these samples were compared with those obtained by the coprecipitation method. Samples were characterized by X-ray diffaction, DTA, TGA, TPR and N2 physisorption. It was found that all samples prepared by microwave irradiation presented a hydrotalcitelike structure and no other Ni-phases were detected. The crystallinity of the samples decreased with increasing the nickel content. The coprecipitated samples showed the hydrotalcite-like structure and other phases not identified.

Z3.13 
CHARACTERIZATION AND REACTIVITY OF LOW-SILICA M41S MATERIALS. Joseph G. Moore, Dean Y. Shahriari, and Christopher C. Landry, Dept of Chemistry, Univ of Vermont, Burlington, VT; Gregory M. Hendricks, Dept of Anatomy and Neurobiology, Univ of Vermont, Burlington, VT.

Recent investigations have shown that the ordered mesoporous aluminosilicates MCM-41 and MCM-48 can be synthesized with Si/Al ratios as low as 1/1 without destruction of the ordered arrangements of the materials. The high aluminum concentrations may impart stronger acidity to these materials, in contrast to the weaker acidity shown by samples with Si/Al ratios greater than 16. In this report, we examine several reaction parameters such as the identity of the aluminum source, reaction pH, and reaction temperature. Powder X-ray diffraction and 27Al NMR are used to determine the optimum reaction conditions. In addition, changes in the morphology of the samples are examined by scanning electron and transmission electron microscopy. Finally, the desorption of Lewis bases from the samples are examined by thermogravimetric analysis, in order to draw conclusions regarding the acidity of these materials.

Z3.14 
VIBRATIONAL-STATE-SELECTED STUDIES OF METHANE ACTIVATION ON Ni(111). Paul R. McCabe, Ludo B. F. Juurlink, Richard R. Smith, Arthur L. Utz, Department of Chemistry and W. M. Keck Foundation Laboratory of Materials Chemistry, Tufts University, Medford, MA.

The dissociative chemisorption of methane into a surface-bound methyl group and hydrogen atom on Ni(111) is known to be an activated process and is likely the rate-limiting step in the heterogeneously catalyzed conversion of methane over Raney Nickel catalysts. Previous studies have shown that translational excitation of methane promotes reactivity as does vibrational excitation, but the molecular-level mechanism for this activation remains controversial. Our experiments combine a supersonic molecular beam source with infrared laser excitation of methane molecules in the beam and UHV surface spectroscopies to quantify the reactivity of methane excited to single vibrational quantum states and reveal the molecular motions responsible for methane activation on Ni(111). Preliminary studies focus on the reactivity of methane with one quantum of excitation in the antisymmetric C-H stretching mode. These studies reveal the efficacy of a C-H stretching motion in promoting the dissociative chemisorption of methane on a clean Ni(111) surface.

Z3.15 
FORMATION OF PLATINUM AND PALLADIUM BIMETALLIC NANOPARTICLES BY ULTRASONIC IRRADIATION.Toshiyuki Fujimoto, Shinya Terauchi, Hiroyuki Umehara, Isao Kojima, National Insutitute of Materials and Chemical Research, Tsukuba, JAPAN; William Henderson, University of Waikato, Hamilton, NEW ZEALAND.

Electron beam lithography using the ``nanowriter'' in the Lawrence Berkeley National Laboratory has been applied to deposit platinum and silver nanoparticle arrays over a 1cm2 area of SiO2, Al2O3 and TiOx flat oxide supports. The metal particle size and spacing could be varied in the 2.5 nm to 100 nm range while the clusters were all the same size for a given sample. The nanoparticle systems were characterized by electron microscopy, atomic force microscopy, scanning auger spectroscopy and temperature programmed desorption. The metal particles could be cleaned by low energy ion bombardment. Chemisorption of CO, H2 and C2 H4 have been explored as a function of particle size. Ethylene hydrogenation and partial oxidation, butane isomerization and CO oxidation have been studied and compared with results obtained on single crystal surfaces. The thermal stability and the chemical stability of the nanoparticles under reducing and oxidizing conditions have been investigated and will also be discussed.

SESSION Z4: OXIDE CATALYSTS II 
Chair: Allan J. Jacobson 
Wednesday Morning, December 3, 1997 
Hampton (S)

8:30 AM *Z4.1 
CATALYSIS ON OXIDES: MONOMERIC CATIONS, CLUSTERS, AND BULK OXIDES. Enrique Iglesia, Department of Chemical Engineering, University of California at Berkeley, Berkeley, CA.

The reduction-oxidation properties of metal oxides and isolated cations depend on cluster size and on interactions with anchoring exchange sites. These properties influence the behavior of such materials in catalytic oxidation reactions. Our recent studies show that domain size also influences the rate of non-oxidative dehydrogenation and acid catalysis on metal oxides. The size of supported WOx clusters prepared by controlled sintering of dispersed precursors leads to marked changes in the isomerization rate and selectivity of n-alkanes and alkylaromatics. Clusters of intermediate size and distorted octahedral structure catalyze such reactions more effectively than larger or smaller clusters. Cluster structure and electronic properties were probed by X-ray absorption and UV-visible spectroscopies and by site titration methods. Dihydrogen is involved in the formation and maintenance of Bronsted acid sites and the effect of cluster size appears to be related to the formation of reduced W centers during catalysis. A compromise between the ease of reduction and the loss of acidic protons by migration into the bulk of WO3 crystallites or by desorption as water leads to optimum catalytic behavior for clusters of intermediate size. The difference in reduction-oxidation properties between bulk oxides and cations isolated at exchange sites for Zn, Ga, and Mo centers also leads to marked changes in their catalytic properties for rate-determining hydrogen desorption steps during alkane dehydrogenation reactions. Attachment of these multi-valent cations to two or more exchange sites in pentasil zeolites with high Al content stabilizes them against reduction during reactions at high temperature (773-973 K) and prevent the formation of mobile intermediates leading to sintering or loss by elution during catalysis. Yet, such stabilized cations can undergo momentary reduction-oxidation cycles required for adsorption-desorption of hydrogen. As a result, Zn, Ga, and Mo exchanged cations, in contrast with the corresponding bulk oxides, become stable dehydrogenation and aromatization catalysts at reaction temperatures required for favorable thermodynamics.

9:00 AM Z4.2 
RELATION BETWEEN CATALYTIC ACTIVITY AND ELECTRICAL CONDUCTIVITY IN THE OXIDES NixMn3-xO. Christel Laberty, Dept of Chemical Engineering and Materials Science, University of California at Davis, Davis CA; Pierre Alphonse, Abel Rousset, Laboratoire de Chimie des Materiaux Inorganiques, Toulouse, FRANCE.

The NixMn3-xO4 spinels have been widely studied in view of their application as thermistor ceramics. They are usually prepared by thermal decomposition of mixed manganese-nickel oxalate at high temperature in air. Recently, the study of the thermal decomposition of these oxalates at low temperature has shown the formation of cation deficient spinels NixMn3-xO. Because of the high level of vacancies, these compounds have a high activity for CO oxidation. The CO/CO2 conversion starts at -20C. Like the stoichiometric spinels, these nonstoichiometric spinels have good electrical conductivity due to the possible existence of Ni_2+^Ni3+ and Mn3+^Mn4+ couples on the same crystallographic sites. The aim of this study was to measure how the adsorption of molecules O2 affects the electrical conductivity. Electrical conductivities of these compounds were measured under O2 and Ar. The conductivity varies with nickel content and has a maximum at x = 0.6. The electrical conductivity and catalytic activity variations of these oxides are correlated.

9:15 AM Z4.3 
THE CHARACTERIZATION AND CONTROL OF MoO3 SURFACE STRUCTURES FOR THE PARTIAL OXIDATION OF ALCOHOLS. Richard L. Smith and Gregory S. Rohrer, Carnegie Mellon University, Department of Materials Science and Engineering, Pittsburgh, PA

MoO3 is a well studied model for the more complex partial and selective oxidation catalysts used industrially. We have used atomic force microscopy (AFM) and X-ray diffraction (XRD) to characterize the structural evolution of the MoO3 (010) surface during gas phase reduction in H2, N2, H2O, CO, propene, and a selection of alcohols (methanol, ethanol, propanol and butanol) and dung reoxidation in air. Through this study, we have identified specific surface structural features that can be influenced by processing and that can alter the reactivity of the (010) surface. High temperature pre-treatments in controlled atmospheres can be used to alter the morphology of the (010) surface. For example, thermal treatments can be used to introduce different types of voids on the basal surface. One type is bounded predominately by <101> oriented step edges (which contain 4 coordinate Mo) and the other by <001> step edges (which contain 5 coordinate Mo). When alcohols are oxidized by MoO3, it is known that O loss is compensated by the formation of point and extended defects. In this study we have found direct evidence for a second and fundamentally different mechanism by which the oxide is reduced during reactions with alcohols; in addition to losing oxygen, MoO3 intercalates H removed from the alcohol during the initial dissociative chemisorption of the alcohol. Based on AFM observations, we will describe the mechanism of alcohol oxidation on MoO3 and the influence of different types of surface sites (step edges) on the reaction.

9:30 AM Z4.4 
CATALYTIC DECOMPOSITION OF PERFLUOROCOMPOUNDS. Shuichi Kanno, Shinzou Ikeda, Ken Yasuda, Hisao Yamashita, Shigeru Azuhata, Hitachi, Ltd., Hitachi Research Laboratory, Ibaraki, JAPAN; Kazuyoshi Irie, Shin Tamata, Hitachi, Ltd., Hitachi Works, Ibaraki, JAPAN.

It is becoming increasingly important to decompose Perfluorocompounds (PFCs; CF4, C2F6, CHF3, etc.) which cause greenhouse effect. Catalytic decomposition process is an effective method which can be operated at lower temperatures if a catalyst of high activity and durability is developed. PFCs decomposition activities by metal oxides, a system which can decompose PFCs at lower temperature and the durabilities of these catalysts were investigated. By the catalyst we have developed, CF4 C2F6, CHF3 were decomposed with conversion above 99% at 700 C . And by controlling a crystal structure change of metal oxide in the catalyst, the durability of the catalyst was improved remarkably.

9:45 AM Z4.5 
REDOX BEHAVIOR BELOW 1000K OF Pt-IMPREGNATED CeO2-ZrO2 SOLID SOLUTIONS: AN IN-SITU NEUTRON DIFFRACTION STUDY. C.-K. Loong1, M. Ozawa2, S. M. Short1, S. Suzuki2, 1Argonne National Laboratory, Argonne, IL.2Nagoya Institute of Technology, Gifu, JAPAN.

The Ce3+ Ce4+ redox process in automotive three-way catalysts such as Ce-ZrO2/Pt provides an essential mechanism to oxygen storage/release under dynamic air:fuel ratio cycling. Such a function requires a metal-support interaction which is not completely understood. We have carried out neutron powder diffraction to monitor the crystal structure (a mixture of a major tetragonal and a minor monoclinic phase) of 10mol% Ce-doped ZrO2 with and without Pt (1wt%) impregnation under oxidizing and reducing conditions over the temperature range of 300-1000 K. The samples were heated first in flowing 2%O2/Ar from room temperature to 673 K and then in 1%CO/Ar to about 1000K. A discontinued increase of the tetragonal unit-cell volume, a decrease of tetragonality (c/a), and a change of color from light yellow to gray when changing from oxidizing to reducing atmosphere were observed only in the sample containing Pt. This result supports the model which assumes the formation of oxygen vacancies near the Pt atoms as some Ce ions are reduced from 4+ to 3+ oxidation states. Work performed at Argonne National Laboratory is supported by the U. S. DOE-BES under the contract No. W-31-109-ENG-38.

SESSION Z5: CARBON BASED CATALYSTS 
Chair: Gabor A. Somorjai 
Wednesday Morning, December 3, 1997 
Hampton (S)

10:30 AM *Z5.1 
CATALYTIC PROPERTIES OF FULLERENE MATERIALS. Ripudaman Malhotra, Albert S. Hirschon, Donald F. McMillen, SRI International, Menlo Park, CA; W.L. Bell, TDA Research, Wheat Ridge, CO.

Fullerenes have been found to catalyze coupling and transalkylation reactions of mesitylene, engage in transfer hydrogenations with dihydroaromatics, and cleave strong bonds such as those in diarylmethanes. In all these reactions, fullerenes show a remarkable ability to accept and to transfer hydrogen atoms. The key structural feature that endows fullerenes with many of its characteristics is the presence of a pentagon surrounded by hexagons. We suspected that the fullerene soot, unlike graphitic carbon, will likely contain pentagons in a hexagonal lattice, and these sites will impart the soot with the desired chemical attributes of strong electrophilic nature and an ability to stabilize radicals. Indeed, in subsequent studies we have shown fullerene soot to be very effective in catalyzing various H-transfer reactions, including conversion of methane into higher hydrocarbons. When compared with other carbons, such as activated carbons and acetylene black, the fullerene soots are much reactive for oligomerization and hydrodealkylation of alkylbenzenes. Because this activity remains even in chemically extracted and partially oxidized soots, the observed catalysis is not a result of residual soluble fullerenes.

11:00 AM Z5.2 
THE IMPACT OF THE GRAPHITE NANOFIBER STRUCTURE ON THE CATALYTIC BEHAVIOR OF SUPPORTED NICKEL. C. Park and R.T.K. Baker, Department of Chemistry, Northeastern University, Boston, MA.

In the current investigation we have used the hydrogenation of simple alkenes and dienes including, ethylene, 1-butene and 1,3-butadiene as probe reactions in an attempt to monitor any possible changes in catalytic behavior induced by supporting nickel on different types of graphite nanofiber support material. This study is designed to compare the catalytic behavior of the metal particles when dispersed on three types of nanofibers, where the orientation of the graphite platelets within the structures is significantly different in each case. The metal crystallites are located in such a manner that the majority of particles are in direct contact with graphite edge regions. It should be emphasized, however, that there are subtle differences in the spacing between adjacent exposed carbon atoms in the various nanofiber structures. As a consequence, it is highly probable that the atomic arrangement of the surfaces of nickel particles that nucleate on these different graphite edges will be dictated to a large degree by the interaction with the atoms in nanofiber supports. Under such circumstances one might reasonably expect that different crystallographic faces of nickel will be exposed to the reactant gas depending on which type of nanofiber structure is used as the supporting medium. The ramifications of this remarkable example of a metal-support interaction is that it opens up the possibility of a tailoring the morphological characteristics of metal particles in such a fashion so to achieve a desired catalytic performance. For comparison purposes, the same set of hydrogenation reactions were carried out under similar conditions over -Al2O3 supported nickel particles.

11:15 AM Z5.3 
ROOM TEMPERATURE ELECTRON SPIN RESONSANCE OF THE PURIFIED CARBON NANOTUBES PRODUCED IN DIFFERENT HELIUM PRESSURES. Haiyan Zhang, Shaoqi Peng, Dept. of Physics, Zhongshan Univ., Guangzhou, CHINA; S.P. Wong and Ning Ke, Dept. of Electronic Engg., Chinese Univ. of Hong Kong, HONG KONG.

Electronic and magnetic properties of carbon nanotubes have attracted more and more attention since theoretical studies have predicted that these tubes with nanometer-scale diameters will exhibit conducting properties ranging from metals to moderate bandgap semiconductors depending on their diameter and degree of helicity. In this paper, we present the results of room temperature ESR study of purified carbon nanotubes produced under different helium pressures ranging from 150 torr to 600 torr In arc dlscharge. Effects of He pressures on ESR g value, linewidth and relative spin densities of purified nanotubes are found and compared with that of graphite. The electronic properties of purified nanotubes varying wlth He pressure are discussed. The result may be useful for producing some nanotubes which conductivity can be controlled by controling He pressure.

11:30 AM *Z5.4 
CARBON BASED MATERIALS. M. Dresselhaus, MIT, Cambridge, MA.

Abstract Not Available

SESSION Z6: PILLARED, LAYERED AND POROUS CATALYSTS 
Chair: Stacey I. Zones 
Wednesday Afternoon, December 3, 1997 
Hampton (S)

2:00 PM Z6.2 
DESIGNING COORDINATELY UNSATURATED METAL SITES IN POROUS METAL-ORGANIC SOLIDS. O.M. Yaghi, Hailian Li, and Charles Davis, Arizona State University, Department of Chemistry and Biochemistry, Tempe, AZ.

We have recently developed strategies aimed at synthesizing porous coordination solids having large channels that are decorated with metal ions having labile ligands. For example, copolymerizing Zn(II) with 1,3,5-benzenetricarboxylate in ethanol yields an extended cubic framework having 15 channels. Pointing toward the center of the channels are ethanol ligands that can be easily removed at room temperature without destruction of the framework thus allowing the introduction of other molecules into the voids. We have found that such inclusions occur with high electronic selectivity. The synthesis, structure and inclusion properties of extended metal-1,3,5- benzenetricarboxylate and -1,4-benzenedicarboxylate solids will be presented. In addition, the extension of this idea to designing coordinately unsaturated Co(II) centers in zinc phosphate porous solids will be discussed.

2:15 PM Z6.3 
ORDERED MACROPOROUS MATERIALS BY EMULSION TEMPLATING. Arnout Imhof, David J. Pine, Univ of California at Santa Barbara, Dept. of Chemical Engineering, Santa Barbara, CA.

A method was developed for the production of macroporous oxide materials by using the droplets of a nonaqueous emulsion as the templates around which material is deposited through a sol-gel process. The liquid droplets are then removed by drying. After a heat treatment the material contains spherical pores in the range from 0.05-5 micrometers left behind by the emulsion droplets. The deformability of the droplets limits cracking during drying and allows very high droplet volume fractions. We demonstrated the process for titania, silica, and zirconia. These macroporous materials may find applications as catalytic supports or adsorbents. Furthermore, by starting with an emulsion of equally-sized droplets pores with a uniform and controllable size have been obtained. Self-assembly of these droplets into a colloidal crystal leads to materials which contain ordered arrays of pores. This makes them potentially useful as materials with special optical properties such as optical filters and photonic band gaps.

2:30 PM Z6.4 
HETEROGENEOUS CATALYSIS WITH METAL GRAFTED MESOPOROUS MATERIALS. Christian P. Mehnert, David W. Weaver and Jackie Y. Ying, Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA.

Recently discovered hexagonally-packed mesoporous materials, designated MCM-41, opened a new area in heterogeneous catalysis. Due to their high surface area and uniform pore size, these new materials are exciting as a support matrix. Through the introduction of active species into the pore structure, highly active heterogeneous catalysts can be prepared. Such molecular sieves have been used as the porous framework in the preparation of a heterogeneous catalyst for carbon-carbon coupling reactions (Heck reaction). The palladium precusor is deposited via vapor grafting onto the inner walls of the mesoporous material generating a highly dispersed metallic surface. The prepared catalyst, designated Pd-TMS11 is used in carbon-carbon coupling reactions (Heck reaction) and demonstrates outstanding activity. Exceptional stability, easy accessibility and remarkable activity of the synthesized Pd-TMS11 materials provide an excellent example for a new generation of heterogeneous Heck catalysts.

2:45 PM Z6.5 
THE HYDROTHERMAL SYNTHESIS AND CHARACTERIZATION OF NEW ORGANICALLY TEMPLATED LAYERED VANADIUM OXIDES BY METHYLAMINE. Rongji Chen, Peter Zavalij, and M. Stanley Whittingham, Chemistry Department and Materials Research Center, State University of New York at Binghamton, Binghamton, NY.

A wide variety of vanadium oxides can be synthesized hydrothermally. The reaction of vanadium pentoxide with methylamine leads to a series of new layered vanadium oxides by using different acids to adjust the pH. The structure of these layered vanadium oxides was established by X-ray single crystal and powder diffraction. The new phases were also characterized by TGA, SEM and FTIR. The oxidation state of vanadium was determined. These materials are readily reduced by the intercalation of lithium ions both chemically and electrochemically, and are expected to exhibit catalytic activity. Supported by NSF-DMR.

3:30 PM Z6.6 
SOFT CHEMICAL SYNTHESIS OF MIXED METAL MOLYBDATE OXIDATION CATALYSTS AND THEIR STRUCTURAL RELATIONSHIP TO HYDROTALCITE. Stuart Soled, Sal Miseo, Exxon Research and Engineering Co., Annandale, NJ; Doron Levin, Jackie Ying, Dept. of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA.

We report on a low temperature solution syntheses route to prepare metastable ammonium mixed-metal molybdates that have a hydrotalcite-related superstructure and which are useful precursors for high surface area mixed metal oxides. The novel low temperature solution route involves reacting an aqueous suspension containing a mixed oxide, prepared by calcining a Zn-Al hydrotalcite with ammonium heptamolybdate. The solid reacts to form a new structure related to the parent Zn-Al hydrotalcite, but with molybdate anions bonded directly to the hydroxyl sheets and ammonium ions inserted in the gallery. A full structure determination was done with synchrotron powder diffraction data to confirm the relationship to hydrotalcite. MoO4 tetrahedra cap hydroxy layers of divalent cations (in this case Zn). Ammonium cations (as opposed to the carbonate anions in the starting hydrotalcite) are present in the interlayers. The ordering of molybdate anions provides for a doubling of the a and b axes of the parent hydrotalcite and the bonding of the molybdate anions to the hydroxyl sheets creates a slight shrinking of the c axis relative to the parent hydrotalcite. A wide range of stoichiometry exists that occurs by changes in proton content such that the (divalent cation/Mo) ratio can vary from 0.75 to 1.5. The structure accommodates divalent cations such as Ni, Co, Cu, Mg, and Zn and hexavalent cations such a Mo and Cr to provide a rich family of atomically-mixed oxide precursors. The particular low temperature reaction described above occurs readily for the Zn-Al but not for Mg-Al hydrotalcites. The presence of Al in tetrahedral sites in the Zn-Al hydrotalcite precursor appears to be a necessary condition to induce the metastability in the resulting oxide that forms after 500ƒC calcination. However, direct solution routes do exist to prepare phases that do not react through the hydrotalcite precursor.

3:45 PM Z6.7 
SURFACTANT-MEDIATED SYNTHESIS AND CHARACTERIZATION OF A MESOPOROUS MIXED OXIDE ACID CATALYST. Michael S. Wong and Jackie Y. Ying, Massachusetts Institute of Technology, Department of Chemical Engineering, Cambridge, MA.

Zirconia-based materials have been extensively studied for industrially-important acid catalytic applications. In particular, catalysts composed of significant amounts of zirconia and silica are known to be among the most active of binary mixed oxide materials. There has been much work done in doping MCM-41 molecular sieves with small amounts of secondary metal dopants (<10 mol%) into the mesoporous silicate framework. Through a number surfactant-mediated synthesis techniques, we have synthesized a material with much higher loadings of the minor component, such that a mixed zirconium-silicon oxide material is formed. The supramolecular templating chemistry will be discussed, along with a description of the physicochemical properties of this novel acid catalyst. Results from nitrogen adsorption, x-ray diffractometry, transmission electron microscopy, and Fourier-transform infrared spectroscopy will be discussed along with catalytic results from 1-butene isomerization studies.

4:00 PM Z6.8 
TEMPERATURE-INDUCED PHASE TRANSFORMATIONS OF MESOPOROUS SILICATES. Karl W. Gallis, Jennifer I. Epperlein, and Christopher C. Landry, Dept of Chemistry, Univ of Vermont, Burlington, VT.

The mesoporous silicates MCM-41 and MCM-48 have recently received attention as potential catalysts and adsorbents. MCM-48 is a more attractive candidate than MCM-41 for these applications, since its highly interwoven and branched pore structure prevents problems with clogging which may occur in the straight channels of MCM-41. These materials are formed by a self-assembly process in which surfactants are used to organize silicate oligomers in solution. We have examined the effects of heating mesoporous silicate samples to intermediate temperatures (100-200C) while the inorganic phase is incompletely polymerized, thus inducing rapid polymerization and altering the surfactant-silicate interactions enough to cause conversion from one phase to another. We present a new method of easily and rapidly (less than 6 hours) producing MCM-48 from MCM-41, and draw conclusions regarding the reasons for the selective nature of the transformation.

4:15 PM Z6.9 
CATALYTIC ASYMMETRIC EPOXIDATION OVER CHIRAL METALLOPORPHYRIN ENCAPSULATED IN SILICA-BASED MESOPOROUS MATRIX. Lei Zhang, Jackie Y. Ying, Massachusetts Institute of Technology, Dept. of Chemical Engineering, Cambridge, MA.

Mesoporous materials with a well-defined pore structure and high surface area have recently attracted a great deal of research attention due to their potential for catalytic application. In this study, silica-based mesoporous materials were used for the synthesis of a heterogeneous chiral metalloporphyrin catalyst by a rational design strategy. A series of metal-doped hexagonally-packed mesoporous silica has been successfully synthesized by varying silica precursor sources and dopant concentration levels. The formation of covalent bonding between the metal dopants and the functional groups on the metalloporphyrin is crucial for immobilizing the iron complex within the mesoporous matrix. The fixation mechanism is different from the Coulombic forces and hydrogen bonding interaction involved in the conventional supported catalytic systems. It effectively isolates the porphyrin molecules and guarantees continued usage of the heterogeneous catalyst system. The well-defined spacious mesoporous channels further allow for free diffusion of reactants and products. The catalytic activity and selectivity for the epoxidations of prochiral olefins have been investigated. This novel rational design strategy provides a significant improvement on the fixation mechanism for achieving a superior heterogeneous catalyst. By manipulating the structural characteristics, such as the surface area, pore size, nature of the dopant and dopant concentration of the support material, as well as metal centers of porphyrin, the catalytic behavior could be further controlled systematically via synthesis parameters.

4:30 PM Z6.10 
NOVEL PROCESSING FOR MESOPOROUS FILMS WITH ONE DIMENSIONAL THROUGH CHANNELS. Shinji Kondoh, Yuji Iwamoto, FIne Ceramics Research Association, Synergy Ceramics Laboratory, Nagoya, JAPAN; Shin-ichi Hirano, Nagoya University, Nagoya, JAPAN.

Mesoporous ceramic films with one dimensional through channels, whose elongation direction is perpendicular to the film surface, are considered to be ideal candidates for high performance catalytic films and gas separation membranes for high-temperature use. whereas such membranes have never been synthesized. Recently, we successfully synthesized mesoporous silica film with one dimensional through channels whose elongation direction is perpendicular to the film surface by using a novel three step processing method which is a combination of eutectic decomposition of amorphous film with a transition metal and subsequent chemical etching. At first, Fe-Si-O amorphous precursor films were prepared by sputtering, then, annealed in oxidizing condition to synthesize regular array of needlelike iron oxide and amorphous silica matrix. Finally, iron oxides were removed by chemical etching. Using this method, mesoporous SiO2 film with one dimensional through channels (channel diameter 4nm) was fabricated. The pore size distribution of the films was measured by isothermal N2 gas adsorption and desorption measurement. A sharp desorption peak was observed at pore diameter of 4nm, which coincided well with the value measured by TEM observation. An estimated specific surface area of the film was as high as 1000m2/g.

4:45 PM Z6.11
NANOPOROUS SEMICONDUCTORS: THEORY AND EXPERIMENT. Alexander A. Demkova,*, Otto F. Sankeya, J. Gryko**, R. Marzkea, J. Diefenbachera, and P. McMillan***, aDepartment of Physics, Arizona State University, Tempe, AZ; *Semiconductor Products Sector, Motorola Inc, Mesa, AZ; **College of Eastern Utah, Price, UT. ***Department of Chemistry, Arizona State University, Tempe AZ.

Nanoporous materials such as zeolites have many important technological applications. Traditionally, with an important exception of porous (anodically etched) Si, nanoporous materials are chemically complex (more than one atomic species are involved). Recently, we have suggested that a variety of nanoporous structures of group IV semiconductors such as Si ( and also C, Ge, and Sn) can be synthesized. Silisils (nanoporous phases of Si) are structurally related to zeolites, and may be viewed as zeolites without oxygen. When compared to fullerene solids, silisil structures are bonafide covalently bonded 4-connected nets. Two silisils, Si(46) and Si(136) clathrates (which in zeolite nomenclature are MEP and MTN) were first synthesized in the 1960's, and have been recently studied at several institutions. A relatively high density of silisils (1.8-2.45 g/cm3) results in a high density of exchanged atoms, and makes them potentially attractive to catalysis. We have extended the concept of nanoporous semiconductors to binary compounds such as AlN and GaAs, structurally related to AlPO molecular sieves. In this talk we will give an overview of our work on silisils, their Ge and C analogs, nanoporous AlN and GaAs phases, both from the electronic structure theory and experimental points of view.Demkov

SESSION Z7: SULFIDES AND CARBIDES 
Chair: Nelly M. Rodriguez 
Thursday Morning, December 4, 1997 
Hampton (S)

9:00 AM Z7.1 
EARLY TRANSITION METAL NITRIDE AND CARBIDE CATALYSTS: A COMPARISON OF THEIR BUTANE ACTIVATION RATES. Hoeck-Hoi Kwon, Saemin Choi, Kendrick E. Curry, Levi T. Thompson, Department of Chemical Engineering, University of Michigan, Ann Arbor, MI.

Nitrogen and carbon incorporation have been demonstrated to confer precious metal like catalytic properties to inexpensive early transition metals. In this paper, we compare the properties of Group V and VI metal nitrides and carbides to those of platinum group metals. The nitrides and carbides were synthesized via temperature programmed reaction of the appropriate oxide with ammonia or methane. Their surface areas were varied by varying the space velocity, heating rates and final temperatures. The resulting materials were characterized using bulk and surface methods, and their alkane activation activities and selectivities were evaluated. Except for the niobium compounds, all the materials were active for butane dehydrogenation and/or hydrogenolysis. In some cases the catalytic properties were similar to those of a commercial Pt-Sn/Al2O3 catalyst. Activities and selectivities of the nitrides and carbides were strong functions of the compound type and surface area or particle size. The vanadium nitrides and carbides were excellent dehydrogenation catalysts with selectivities greater than 98% to olefins. The reaction rate increased linearly with the surface area suggesting that the reaction was structure-insensitive. The tungsten and molybdenum nitrides and carbides demonstrated activities up to 75 times higher than that of the Pt-Sn catalyst; however, their selectivities were poor. In most cases hydrogenolysis products were favored over dehydrogenation products. The hydrogenolysis activities increased with increasing particle size while the dehydrogenation activity was essentially invariant. This result implied that hydrogenolysis was structure-sensitive over these materials. Structure-sensitivity also manifested itself in differences between the tungsten carbides. While dehydrogenation activities for the WC (hex), W2C (hcp) and WC1-x (fcc) catalysts were within experimental error of each other, the WC1-x catalysts displayed extraordinarily high hydrogenolysis activities. The implications of these results will be discussed and suggestions will be made for tailoring the catalytic properties of nitrides and carbides.

9:15 AM Z7.2 
EFFECTS OF SYNTHESIS CONDITIONS ON THE STRUCTURAL AND CATALYTIC PROPERTIES OF BULK MOLYBDENUM CARBIDES. Michael K. Neylon and Levi T. Thompson, Univ. of Michigan, Dept. of Chemical Engineering, Ann Arbor, MI.

Early transition metal carbides and nitrides have proven to be excellent catalysts for hydrotreating, alkane activation, and isomerization reactions. The optimization of these materials requires an understanding of the effects of synthesis conditions on the structural and catalytic properties. In this paper, we describe the synthesis of bulk molybdenum carbides from the reaction of methane with molybdenum trioxide. These materials were analyzed for a variety of properties, including bulk crystal structure, elemental content, and activity towards the hydrodenitrogenation (HDN) of pyridine. The syntheses were duplicated to examine the reproducibility of the structural and catalytic properties. The temperature profile and gas flow rate had significant effects on the microstructural properties. XRD analysis revealed that the materials were composed mostly of -Mo2C, although some evidence of molybdenum dioxide was present in certain samples. Surface areas approaching 100 m2/g have been achieved. Pore size distribution analysis revealed a bimodal size distribution, with pores of approximately 12 and 24 Åin diameter, supporting the idea of a ëwishboneí pore structure. Pyridine HDN rates were similar to those of commercial supported Ni-Mo catalysts, and showed a selectivity towards n-pentane with some cyclopentane. It is unknown whether the molybdenum dioxide present in some samples had a quantifiable effect on the HDN activity, however, these samples had lower surface areas and different pore size distributions than the fully carbided materials. We have concluded that the microstructure of the catalyst is created primarily during the carburization of molybdenum dioxide.

9:30 AM Z7.3 
NOVEL USES OF NICKEL SULFIDE CATALYSTS: SYNTHESIS AND CHARACTERIZATION. A. Olivas*, M. Avalos, J. Cruz-Reyes, V. Petranovskii, S. Fuentes, Instituto de Fisica-UNAM, Laboratorio de Ensenada, Ensenada, MEXICO; *Also at Programa de Posgrado en Fisica de Materiales, CICESE, Ensenada, MEXICO.

Nickel sulfide catalysts have been widely used in the oil refining industry. However, there are a few applications in non-traditional areas, like fine chemistry. In particular, they are commonly used in hydrogenation reactions. Nickel sulfide catalysts were synthesized by the homogeneous sulfide precipitation (HSP) method by varying the homogenization time, and followed by the usual sulfide treatment by changing the sulfiding temperature. Several nickel sulfide phases were found by X-ray diffration (XRD) which depend on the preparation conditions. These phases are NiS (millerite), NiS1.03, Ni3S2, Ni7S6, Ni9S8. Besides XRD, they were also analyzed by other techniques like transmission electron microscopy, scanning electron microscopy, and nitrogen adsorption. The catalytic behavior of the samples are studied on the hydrogenation reaction of cyclohexanone used as the model of ketones. This is done in a reactor of continuous flow at atmospheric pressure.

SESSION Z8: ZEOLITES AND RELATED MATERIALS 
Chair: Abraham Clearfield 
Thursday Morning, December 4, 1997 
Hampton (S)

10:15 AM *Z8.1 
INTRAZEOLITE CHELATE COMPLEXES AS HIGHLY SELECTIVE EPOXIDATION CATALYSTS. Thomas Bein, Dirk E. DeVos, Steve Jurgens, Julia Meinershagen, Steven B. Ogunwumi, Purdue University, Department of Chemistry, West Lafayette, IN.

We will give an overview on our recent studies of intrazeolite metal chelate complexes and their activity in hydrocarbon oxidation reactions. The encapsulation of manganese complexes of N-substituted 1,4,7-triazacyclononane (TACN) in the cages and channels of zeolites and mesoporous hosts will be discussed. These systems are of interest as catalysts for the selective epoxidation of olefins with the clean oxidant hydrogen peroxide. Both alkyl- and hydroxyalkyl-substituted TACN ligands were studied; the latter result in hexadentate coordination. Zeolite encapsulated manganese polyamine complexes were synthesized by sorption of the alkyl-substituted ligand into Mn-exchanged zeolite NaY. The encapsulation often improves the selectivity of the epoxidation compared to reactions in solution. Furthermore, we will discuss the covalent attachment of similar TACN-based catalysts to the channel walls of mesoporous MCM-41 type hosts. These systems permit access of large substrates to the heterogeneous active epoxidation sites. The above hybrid catalysts will be contrasted with Mn-salen-type asymmetric epoxidation catalysts assembled, in a multi-step synthesis, in the cages of zeolite EMT, the hexagonal form of faujasite. This novel heterogeneous system produces high enantiomeric excess in the epoxidation of aromatic olefins with aqueous sodium hypochlorite. Size-exclusion experiments show that the activity is located inside the zeolite cage system.

11:00 AM *Z8.3 
EFFICIENT STABLE CATALYSTS FOR LOW TEMPERATURE CARBON MONOXIDE OXIDATION. Yuan-Gen Yin, Jin Yun Wang, Guan-Guang Xia, Dept. of Chemistry, University of Connecticut, Storrs, CT; William S. Willis, Department of Chemistry and Institute of Materials Science, University of Connecticut, Storrs, CT; Steven L. Suib, Departments of Chemistry and Chemical Engineering, and Institute of Materials Science, University of Connecticut, Storrs, CT.

Octahedral molecular sieves (OMS), doped with Ag+, Co2+, and Cu2+, have been tested for their catalytic activity for carbon monoxide oxidation at low temperatures for long times on stream. M-OMS are highly active for this catalytic reaction, and compare favorably with other catalysts such as hopcalite-like CuMn2O4 catalysts, supported Ag catalysts, and supported noble-metal catalysts, especially with respect to resistance to deactivation in a long run. CO-doped OMS-2 has been tested for selective oxidation in the presence of a large surplus of hydrogen in the feed gas. This catalyst shows nearly exclusive oxidation of CO versus hydrogen with oxygen present in stoichiometric amounts with carbon monoxide. Its stability against reduction in CO or H2 containing gas is demonstrated from comparisons of X-ray diffraction patterns and X-ray photoelectron spectra before and after exposure to these gases. Average oxidation numbers and populations of Mn valence states were determined for these catalysts. A good correlation between the rate of CO oxidation and the average oxidation number of manganese was established supporting a redox mechanism. The structure of the active sites and the mechanism of the reaction are proposed.

11:30 AM Z8.4 
THE NATURE OF ZEOLITE EXTERNAL SURFACE AS INVESTIGATED BY MOLECULAR SIMULATION. Yasunori Oumi, Hiromitsu Takaba, Momoji Kubo, Abhijit Chatterjee, Akira Miyamoto, Tohoku Univ., Graduate School of Eng., Sendai, JAPAN.

Recently much attention has been given for the utilization of zeolite membrane for separation and its use in reactions. In this context, it is needed to sutdy both the zeolite external surface and bulk to understand the physicochemical properties in terms of its catalytic, adsorption and diffusion performances. In the present study, molecular dynamics (MD), grand canonical Monte Carlo (MC) methods and atomic force microscopy (AFM) simulation were applied to understand the nature of zeolite external surface and the difference between various possible adsorption sites on it were studied. We have modeled two types of ideal (010) external surface of ZSM-5 type silicalite; (a) with uniform 5 member ring on the external surface, (b) no 5 member ring on the top of the 10 member ring. It in both the cases the hydroxyl group arrangement was ordered. Due to more terminal 5 member rings (a) is more stable than (b) as observed form MD results. We studied the difference between adsorpton of p-xylene in bulk(a) and external surface(b). We compare the results of the number of possible configurations during the MC calculation as a function of the adsorption energy of p-xylene for both (a) and (b). This results confirm the existence of adsorption site on the external surface (b) which is dominant over that of the bulk (a).

11:45 AM Z8.5 
HYBRID CATALYSTS: INTERNAL OR EXTERNAL CONFIGURATIONS FOR BETTER CATALYTIC PERFORMANCES? R. Le Van Mao, M.A. Saberi, S. Xiao and G. Denes, Dept of Chemistry and Laboratories for Inorganic Materials, Concordia University, Montreal, CANADA.

Hybrid catalysts containing zeolite particles (i.e. ZSM-5) and a solid cocatalyst component (i.e. alumina or quartz) embedded in a clay matrix are shown to be very performing in some high-temperature reactions such as the aromatization of low paraffins and olefins (450-500C). Such an ``external hybrid'' configuration (Hybext ) which provides the maximum surface of contact between the particles of zeolite and cocatalyst, allows the rapid transfer of the products from one surface to another. However, for reactions occurring at lower temperatures such as the isomerization of n-heptane (200C), the Hybext does not result in significantly higher catalytic performances, when compared to the parent zeolite. This is probably due to a more difficult transfer of products from the Y zeolite pore network to the alumina cocatalyst surface. With the ``internal hybrid'' configuration (Hybint), the cocatalyst precursor is directly incorporated into the zeolite pores. Such a hybrid configuration shows much better performances in the n-heptane isomerization owing to much shorter distances between the zeolite acid sites and the cocatalyst ones. The limited pore size of the Y zeolite imposes a certain restriction in the loading of the cocatalyst precursor. The strong influence of the nature and the activation temperature of the incorporated species are indicative of interactions with the zeolite acid sites.

SESSION Z9: ACIDS AND BASES 
Chair: Stuart L. Soled 
Thursday Afternoon, December 4, 1997 
Hampton (S)

1:30 PM Z9.1 
MIXED METAL PHOSPHO-SULFATES FOR ACID CATALYSIS. Steven G. Thoma, Nancy B. Jackson, Tina M. Nenoff, Sandia National Laboratories; William L. Earl, Los Alamos National Laboratories.

We are studying a new class of non-layered mixed-metal phosphates and phospho-sulfates materials for use as acid catalysts. In-depth studies into their structure-property relationship show an unusually slow deactivation during acid catalyzed hydrocarbon reactions when compared to equally or more acidic oxide materials. Furthermore, some of these materials exhibit substantially greater levels of Bronsted acidity than sulfated zirconia as well as significantly lower rates and magnitudes of deactivation. The deposition of carbon on Lewis acid sites during the course of the isomerization reaction does not appear to affect greatly the activity of the mixed-metal phosphates. Acidity was measured using two model reactions, isomerization of 2-methyl-2-pentene and xylene isomerization. 31P MAS NMR has been used to examine the role of phosphorous in contributing to the Bronsted acidity and deactivation behavior of these materials. The catalysts were also analyzed using direct current plasma for elemental analysis; BET for surface area; powder X-ray diffraction for crystal phase identification; and FTIR for investigation of surface hydroxyls, sulfates and phosphates.

1:45 PM Z9.2 
SOLID ACID/BASE CATALYSTS FOR AN ENVIRONMENTALLY CLEAN SUBSTITUTION OF PYRIDINE. U. Kameswari, Department of Chemistry, Indian Institute of Technology, Madras, INDIA.

The need for more economical production processes and the need for the replacement of conventional but hazardous liquid acid/base catalysts in the chemical industry opens up opportunity for innovations in catalysis. New solid acid/base catalysts like hydrotalcites, montmorilonite clays, zeolite materials have exhibited superiority over the existing catalysts in various organic transformations. Alkylpyridines are important intermediates in the synthesis of a variety of organic fine chemicals. If one could manipulate the substitutions and transformations of existing substituents in pyridine ring with equal ease as in a benzene ring then it would lead to a breakthrough in industrial economics. Nucleophilic substitutions in pyridines leading to 2- and/or 4-alkylpyridines are known. However, direct alkylation of pyridine at the 3- position could not be achieved over any catalysts. The results on the selective methylation of pyridine over different types of solid acids like molecular sieves, transition metal exchanged Y-zeolites and clays and on the vinylation of pyridine over various types of hydrotalcites will be reported in the present paper. Methylation of pyridine over HY, NaY, ZSM-5, Mordenite, SAPO-5, MgAlPO-11, Zeolite Beta, montmorillonite and transition metal exchanged Y zeolites was studied under gas phase now reaction conditions. Picolines were the main products along with other higher alkylpyridines. FT-IR studies on the catalysts showed no quantitative correlation between acidity and the activity/selectivity pattern The exchanged zeolites showed considerable activity at much lower temperatures. The strength of the acidic site, the metal function and the access of the base molecule to the active site were important for the catalytic behaviour. The fine tuning of the acidic catalysts suitably could lead to the selective synthesis of 3- and/or 5- substituted alkylpyridines. Similarly, the vinylation of pyridine over various types of hydrotalcites could be achieved under much milder conditions by tailor making the solid base catalysts.

2:00 PM Z9.3 
AMORPHOUS COMPLEX PHOSPHATES AS TAILORED ACID CATALYSTS:SYNTHESIS AND PROPERTIES. Vladislav A. Sadykov, D.I. Kochubei, S.P.Degtyarev, E.A.Paukshtis, E.B. Burgina, Boreskov Inst. of Catalysis RAS, Novosibirsk, RUSSIA; V.F.Tretyakov, Topchiev Institute of Petrochemical Synthesis RAS, Moscow, RUSSIA; Dinesh Agrawal, Rustum Roy, Penn State Univ, Materials Research Lab, University Park, PA.

This work considers relation between chemical composition, bulk structure, surface properties and catalytic activity of highly dispersed complex zirconium phosphates of the Me(x)Zr(4)P(6)O(24) composition (Me = Ca, Sr, La, Cu, Co, Fe; x<1). These systems are X-ray amorphous precursors of the phosphates with NZP structure known for its structural flexibility and promising as tailored acid catalysts. Samples were synthesized by sol-gel method using nitrates and phosphoric acid as starting compounds; after calcination at 500 C their surface area was ca. 100 m2/g. EXAFS and IR spectroscopy of lattice data revealed that local structure of all samples differs from that of the NZP and can be described as formed by layers or bands of oxidic Zr- O- Zr fragments. Coordination sphere of zirconium is rather symmetric with Zr- O coordination number close to 8. Between such layers, bidentate phosphate groups (at least two per one Zr atom) are located. In most samples, bulk hydrogen -bonded hydroxyls ensuring charge compensation were detected. Modifying cations are mainly located in octahedra weakly affecting Zr coordination. Rather strong interaction was observed only for the case of Fe -substituted sample. IR spectroscopy of adsorbed CO and surface hydroxyls revealed that surface is mainly covered by non-reactive P-OH groups. Modifying cations (save La) generate Lewis acid centers -coordinatively unsaturated cations of Zr, Cu (+), Fe (2+, 3+), Co(2+), while La incorporation increases concentration of surface Broensted acid centers -Zr-OH groups. Several samples were found to have excellent low-temperature activity in the reaction of NOx reduction by methane and propane in the excess of oxygen in mixtures containing water and sulfur dioxide. Activity was found to correlate with the density of Lewis and/or Broensted acid centers responsible for hydrocarbons activation.

SESSION Z10: SURFACE MODIFICATIONS 
Chair: Steven L. Suib 
Thursday Afternoon, December 4, 1997 
Hampton (S)

2:45 PM *Z10.1 
DESIGN OF CATALYSTS WITH ARTIFICIALLY CONTROLLABLE FUNCTIONS USING SURFACE ACOUSTIC WAVES AND RESONANCE OSCILLATIONS GENERATED ON FERROELECTRIC MATERIALS. Y. Inoue, Department of Chemistry, Nagaoka University of Technology, Nagaoka, JAPAN.

In the development of a heterogeneous catalyst with high activity and selectivity, it is strongly desirable to artificially change the structure of surfaces and electron density of surface atoms. In an attempt to control these two factors, surface acoustic waves (SAWs) and resonance oscillations(ROs) generated by radio frequency electric power on a ferroelectric crystal were applied to thin film catalysts deposited on the ferroelectric substrate. The effects of the SAW on the catalyst activation were examined with respect to the thickness and surface states of a thin film catalyst. For ethanol oxidation on Pd, activity enhancement by Rayleigh SAW generated on a ferroelectric LiNbO3 single crystal became significantly larger with increasing thickness in case that the Pd surface was oxidized. Laser Doppler measurements showed that lattice displacement by the SAW increased with the presence of thick metal layers but remarkably decreased when the layers were oxidized. From propagation loss and changes in the electric conductivity of the thin film catalyst, it was demonstrated that significantly large SAW effects in the presence of oxide surfaces are mainly due to interactions with electrons. The effects of ROs on ethanol oxidation over Pd caused an increase in the activity by a factor of 1900 together with a dramatic decrease in activation energy of the reaction. Activity enhancement by the ROs was different between the (+) and (-) surfaces of the ferroelectric substrate which were differentiated by the direction of spontaneous polarization. The kinetic results showed that the adsorption of oxygen became stronger with the (+) surface by the ROs, but remained nearly unchanged with the (-) polar surface. Surface potential measurements demonstrated that the ROs increased surface electron density for the (+) polar surface but decreased it for the (-) polar surface. These changes were explained in terms of phonon drag-like interactions between the electrons and sonic wave generated by large forced lattice displacement and were correlated with the behavior of oxygen adsorption. The ROs caused changes in selectivity: ethanol dehydration was promoted on oxide catalysts, compared to dehydrogenation. SAWs and ROs are concluded to be effective for the development of catalysts with artificially controllable functions.

3:15 PM Z10.2 
INFLUENCE OF ACOUSTIC WAVE EXCITATION ON CATALYTIC CO OXIDATION OVER PLATINUM SINGLE CRYSTALS. S. Kelling, T. Mitrelias, Y. Matsumoto, D. A. King, Department of Chemistry, University of Cambridge, Cambridge, UNITED KINGDOM; and Y. Watanabe, Y. Inoue, Department of Chemistry, Nagaoka University of Technology, Niigata, JAPAN.

A novel ultra high vacuum compatible experimental system capable of exciting a platinum single crystal sample with narrowband acoustic waves has been designed and commissioned. The excitation system is based on a Surface Acoustic Wave delay line and consists of a thin single crystal bonded onto the electroded surface of a lithium niobate InterDigital piezo-electric Transducer (IDT). This experimental set-up enables us to provide clear evidence for the dramatic influence of acoustic excitation on catalytic reactions. The oxidation of carbon monoxide over platinum thin film single crystals has been chosen as a model reaction. Using the IDT excitation system experiments were performed under ultra high vacuum (uhv), high vacuum, and high pressure condition. In all experiments an increase in the rate of carbon monoxide oxidation reaction was found, when exciting the catalyst with Rayleigh waves. This increase is certainly not attributable to artefacts, such as electromagnetic interference, since special measures to eliminate the later have been taken and control experiments to measure it have been performed. The observed increase in reaction rate cannot be associated with a pure thermal effect arising from sample heating while applying the RF power. In this paper results obtained under the three different pressure conditions will be presented, compared and discussed. Furthermore, pressure and temperature dependence data, measured under uhv conditions, will be presented which enable us for the first time to gain some insight into the physical mechanism by which acoustic waves enhance catalytic reactions on solid surfaces. Possible mechanisms will be proposed and discussed. In further examinations we intend to explore the use of acoustic excitation as a potentially efficient route to control the rate and the selectivity of heterogeneous catalytic processes.

3:30 PM Z10.3 
HUMMIDITY EFFECTS ON THE SURFACE PROPERTIES OF EPITAXIAL RUTILE THIN FILMS. D R Burgess, Univ of Florida, Dept of Chemical Engineering; P A Morris Hotsenpiller, DuPont Co CR&D; T J Anderson, Univ of Florida, Dept of Chemical Engineering; G S Rohrer, J B Lowekamp, Carnegie Mellon Univ, Dept of Materials Science and Engineering.

TiO2 is widely studied for use in chemical sensing applications where the presence of a substance is detected by measuring changes in the surface conductivity. Among these studies, significant work can be found on the interaction of the TiO2 surface with a variety of substances under UHV conditions, but this work is difficult to translate to operating conditions. This study is an effort to understand surface behavior during operating conditions by measuring the conductivity changes of epitaxial (100), (001) and (101) rutile in response to humidity. These films were grown by MOCVD with some having an n- or p-type dopant, Nb or Ga, to vary the surface properties. Results indicate that the specific dopant, film orientation, and in-plane film direction affect the conductivity of epitaxial rutile surfaces in the presence of humidity.

3:45 PM Z10.4 
DIRECT SPACE INVESTIGATION OF OXYGEN POINT DEFECTS AND OF CHLORINE ADSORPTION ON A RUTILE TITANIA (110) SURFACE. Ulrike Diebold, Lanping Zhang, Dept of Physics, Tulane UniversitY, New Orleans, LA; Georg Leonardelli, Wilhelm Hebenstreit, Michael Schmid, Peter Varga, Inst. f. Allgemeine Physik, TU Vienna, AUSTRIA.

Surface defects should play a dominant role in the catalytic reactivity of metal oxides, but little data exist on their influence on surface reaction mechanisms. Scanning Tunneling Microscopy (STM) with atomic resolution allows direct-space observations of defect-related surface chemistry with microscopic detail. We combine STM and surface spectroscopies to study adsorption of chlorine on TiO2(110). After sputtering and annealing the TiO2 sample to 850oC, the (1x1) terminated surface contains terraces with a width of 80-150Å, and steps oriented along [001], [], and []. The annealing procedure produces point defects in the bridging oxygen rows running along the [001] direction, with a concentration of 0.05 - 0.1 defects per unit cell. These defects show a strong interaction with the STM tip. Chlorine was dosed from a calibrated electrolytic AgCl cell, and was found to adsorb dissociatively on TiO2 at room temperature. The saturation coverage is 0.3 Cl/unit cell, with an initial sticking coefficient close to unity. Pairs of chlorine atoms are observed at low coverages. The initial mobility of Cl is highest along [001], and results in a typical Cl-Cl distance of 6 /pm 3 unit cells. No long-range or pronounced short-range order is observed at higher coverages. Point defects do not appear to play a major role in the adsorption process. A depletion region of Cl is observed close to the upper step edges of terraces. The adsorption is strong at room temperature, with little mobility observed by STM. An STM sample bias voltage of +4V is necessary to remove the Cl atoms from the surface. Strong electron-stimulated desorption effects dependent on the reduction state of the substrate are found with Auger electron spectroscopy.

4:00 PM *Z10.5 
MOLECULAR MODELING OF SELECTIVE ADSORPTION FROM MIXTURES. Teresa J. Bandosza, Keith E. Gubbinsb, Felipe Jimenezc, Christine L. McCallumb, Susanne L. Sowersb and Lourdes F. Vegac. aDepartment of Chemistry, City College of New York, New York, NY; bSchool of Chemical Engineering, Cornell University, Ithaca, NY; cDepartemento de Ingenieria Quimica, Universitat Rovira i Virgili, Tarragona, SPAIN.

Molecular simulation methods provide a means for carrying out systematic studies of the factors affecting adsorption phenomena. The most useful of these methods will be briefly reviewed, followed by a discussion of applications to selective adsorption. Selectivity is strongly affected by energy of interaction with the pore walls, molecular size and shape, site specific interactions, entropic effects, differences in diffusion rates, and networking effects. Following a brief discussion of these different effects, three recent studies of site specific selectivity will be described. The first is an investigation of the effect of oxygenated surface sites on the adsorption of water vapor on activated carbons. Hydrogen-bonding sites are modeled using off-center square well interactions for both water and wall sites, and wall sites are placed at the edges of the graphite microcrystals. New experimental results for water adsorption at low pressures on carefully characterized activated carbons are reported, and are found to be in good agreement with the simulations. In the second application, the same model is used to investigate the influence of water vapor on adsorption of carbon tetrachloride from mixtures of carbon tetrachloride and nitrogen, with the CCl4 being present in only trace amounts. The pore width, relative humidity and surface site density are all found to have a major effect on both adsorption capacity and selectivity. In the third and final application, we consider the separation of alkene/alkane mixtures using aluminas whose surfaces have been doped with metal ions. Pi-complexation between these metal ions and the alkenes can produce a highly selective separation. The simulations are found to be in good agreement with the available experimental data, and have been used to predict separations for other conditions not yet studied in the laboratory.

4:30 PM Z10.6 
CYCLIC OZONE IN MAGNESIUM OXIDE (111) NATIVE SURFACE RECONSTRUCTIONS. Marija Gajdardziska-Josifovska, Richard Plass, Kenneth Egan, Department of Physics, University of Wisconsin-Milwaukee, Milwaukee, WI; Chris Collazo-Davila, Daniel Grozea, Eric Landree, Laurence D. Marks, Department of Materials Science and Engineering, Northwestern University, Evanston, IL.

MgO(100) substrates have long been used in the study of catalytic systems, less well studied is the MgO(111) surface. In high temperature annealing studies of this surface we have come across an unusual form of ozone. The existence of Œ¹cyclic¹¹ ozone, three oxygen atoms forming an equilateral triangle, has been theoretically speculated ever since the triatomic nature of ozone was established. Several theoretical studies have predicted a metastable state of cyclic ozone compared to the more well know Œ¹bent¹¹ structure of atmospheric ozone. We present the first experimental evidence for cyclic ozone, which appears to be stabilized and chemically bonded to lone magnesium atoms in three different high temperature annealed MgO(111) reconstructed surfaces. Bulk MgO(111) transmission electron microscope (TEM) samples were prepared by mechanical polishing, hot nitric acid etching, and ion beam milling. These samples were then annealed in a MgO(l00) lined, resistively heated, vacuum furnace to temperatures between 1500C and 1 800C. The samples were air transferred to a Hitachi H9000 NAR TEM for transmission electron diffraction (TED). Within the annealing temperature range three different surface periodicities were identified on different samples: (3x 3)R30, (2x2), and (2 3x2 3)R30, in order of increasing annealing temperature. Using direct phasing techniques and X2 refinements the MgO(111)-(3x 3)R30 surface is found to consist cyclic ozone with an approximate 1.3 Å spacing. Similarly, the MgO(111)-(2x2) and MgO(111 )-(2 3x2 3)R30 surfaces consist of periodic arrangements of cyclic ozone and single oxygen atoms. From the structure of MgO(111)-(2 3x2 3)R30 as well as from ionic ligand theory we conclude that the trimer rings must be centered over Mg atoms. All three surface reconstructions are stable in air. Possible electronic configurations of this new 03 species, its possible charge states, as well as potential catalytic properties of this surface bonded form of ozone will be discussed.