Current Research - Graduate Students Understanding Support Effects in Hydrogen Spillover-Mediated Catalysis [to top] Hydrogen spillover is a catalytic phenomenon involving an activated hydrogen species migrating from a metal cluster to a conventionally inert support. Over the past several decades, catalytic systems that are hydrogen spillover active have been repeatedly shown to increase kinetic rates. To date, the mechanism for hydrogen spillover is still heavily disputed. My project seeks to answer the unresolved questions of hydrogen spillover including the identity of the activated hydrogen species, the active sites present on the supports, and the change in reaction mechanism on the catalyst with a hydrogen spillover active support. Hydrogenation reactions are used as model reactions to study the mobility of hydrogen species. This work will shed light on the effects of catalyst and support interactions and kinetic rates and reactivity. Exploiting Diffusional Constraints in Small Pore Zeolites for C1-C2 Reactions [to top] Zeolites have the capability to alter product distributions and reaction rates through shape selectivity and molecular sieving of chemical species. Small pore zeolites, in particular, offer a unique opportunity to selectively impose intraparticle concentration gradients on methane. Combining this with recent advances in encapsulating metal clusters developed within our group, we have the capability to control local reaction stoichiometry inside the zeolite crystal and enhance the performance of catalysts in certain processes. My project focuses on exploring the types of systems for which this control would be beneficial and tuning the catalyst to the reaction. For example, methane reforming over conventional Ni supported catalysts is known to deactivate due to coking unless a large excess of steam or carbon dioxide is supplied. Imposing a concentration gradient in methane would allow for a stoichiometric ratio in the bulk phase, while retaining an excess of co-reactant at the metal sites. This research will give insights into diffusional effects in small pore zeolites, and the implications of the work can help lower the cost of a variety of systems where an excess of reactant is currently needed. Furthermore, the diffusional effects may be leveraged to couple certain reactions in C1 processes and lower energy requirements. Dehydrogenation of Light Alkanes [to top] The selective catalytic conversion of light alkanes to alkenes and arenes offers a viable starting point for the production of many high-value chemicals. Dehydrogenation rates are often limited by thermodynamics but undesired side reactions are not as limited, necessitating that conversions be kept low in order to maintain sufficient selectivity. The thermodynamics of the dehydrogenation reaction can be improved by oxidizing the hydrogen formed, but this needs to be done selectively so that the hydrocarbons are not oxidized as well. My work involves the use of encapsulated metal clusters within small-pore zeolites to impose diffusional constraints for bulky hydrocarbons, while allowing the hydrogen and oxygen molecules access to the active sites, in order to facilitate the selective oxidation of hydrogen. The synthesis and tunability of these encapsulated metal clusters within zeolites will be explored so that their ability to impose intracrystalline diffusional constraints may be exploited for other reactions as well. Encapsulation of Bimetallic Clusters in Zeolites [to top] Transition metal catalysts are of key importance to the production of many commercially ubiquitous materials, and also for displacing traditional methods of fine chemical production, which often result in waste products that are harmful and challenging to dispose of. My project concerns bimetallic catalysts in particular, which can exhibit synergy compared to their monometallic counterparts via ligand or ensemble effects that act to increase the activity of one metal through electronic modification or dilution by another. As is the case for single component catalysts, the synthesis of metal alloy catalysts is complicated by the need for high dispersion, which makes efficient use of metal species and results in the ideal coordination environment for active metal atoms but also compromises their stability. This is especially true of bimetallic nanoparticles, which exhibit thermodynamics that are dominated by surface tension effects and are apt to undergo phase separation and changes in surface composition as a consequence. Encapsulation of these materials into the pores of zeolites has the potential to impart greater particle stability in a fashion analogous to that of monometallic particles, namely the preclusion of particle agglomeration due to a reduction in their mobility. Furthermore, encapsulation can combine the catalytic advantages that result from confinement, including size selectivity of reactants and transition states plus protection from poisons, with the enhanced reactivity that is characteristic of certain metal alloys. Understanding Capillary Condensation in Solid Brønsted Acid Catalysts through High Pressure Ethanol Dehydration [to top] Demand for renewable carbon sources has surged requiring an increase of ethanol, ethylene, and diethyl ether production through dehydration on solid Brønsted acids. This project aims to utilize Tungsten polyoxometalate (POM) clusters with Keggin structures and charge-balancing protons (H8-nXn+W12O40) which are Brønsted acids that exhibit uniform and well-defined cluster size and atomic connectivity, diverse chemical composition, and relatively high stability. Keggin POM clusters with two different central atoms (Xn+ = P5+, Si4+) will be supported on amorphous SiO2 and MCM-41 to study high pressure capillary condensation. MCM-41 consists of a regular arrangement mesoporous material and has very narrow pore size distribution making it a suitable choice for studies. Furthermore, this research will provide insights on the effects of capillary condensation for alkanol dehydration on solid Brønsted acid catalysts. Current Research- Postdoctoral Fellows Composition-Function Relations for Oxidation Catalysis on Polyoxometalate Clusters [to top] Selective oxidation of alcohols is an important chemical reaction to produce fine chemicals. Our major goal in this project is to develop a rigorous relationship between compositions of metal oxides and their catalytic properties. In particular, polyoxometalates (POMs) are used as our model catalysts due to their well-defined structures, diverse compositions, as well as suitable redox properties. By combining DFT calculations, kinetic measurements and in-situ UV-vis studies, we first investigate properties of active sites, kinetics, and the reaction mechanism to provide mechanistic details of the elementary steps in selective alcohol oxidations over POMs. We are then moving further to compare POMs with various compositions to understand composition effect on the catalytic activities. The result of this research will help us to understand structure-function relationship in heterogeneous catalysis. NOx Conversion over Metal Oxide Catalysts [to top] NOx (NO, NO2 and N2O) is a class of air pollutant commonly found in mobile and stationary emission sources. Mechanistic understanding of the conversion of NOx, e.g, NO oxidation and N2O decomposition, is of vital importance in design of high-performance catalyst for NOx abatement. This project is aimed to establish the site requirements for NOx conversion over metal oxide catalysts and ultimately a guideline for future catalyst design. Various metal oxides, including monometallic metal oxides and substituted metal oxides, are used in this project and a combined experimental (kinetic and spectroscopic study) and theoretical (DFT) investigation on the catalysis of these metal oxides in NOx conversion reactions is dedicated to reveal the structure-performance relationship in terms of catalytic activity as well as resistance towards impurities such as H2O and O2. The site requirements established in this project will guide the future development of NOx abatement catalyst as well as provide in-depth understanding in NOx chemistry over metal oxide surface. The Effect of Water on the Mechanism and Kinetics of Fischer-Tropsch over Bimetallic Co-based Catalysts: Theoretical and Experimental Studies [to top] Fischer-Tropsch (FT) synthesis is an industrial catalytic reaction for the production of cleaner diesel and gasoline transportation fuels. Notably, the unstable prices and diminishing resources of crude oil have led to actions towards a more sustainable energy system. FT synthesis converts synthesis gas (CO/H2 gas mixture) into long-chain hydrocarbons. The synthesis gas can be produced from natural gas, coal and biomass. The main objective of this project is to use advanced theoretical and experimental methods seeking to provide a useful contribution beyond the state-of-the-art in the mechanistic role of water on the rate and selectivity of FT synthesis for the production of long-chain hydrocarbon on bimetallic Co-based catalytic systems. Current Research - Visiting Scholars Non-Oxidative Catalytic Dehydrogenation of Light Alkanes on Dispersed Oxide Nanostructures [to top] Catalytic dehydrogenation of light alkane is of great importance for the chemical and petrochemical industries. The most common catalysts for alkane dehydrigenation are Cr-based systems. This project proposes to explore the properties of catalysts based on W oxides for the dehydrogenation of light alkanes, specifically propane and isobutane, as materials that can potentially circumvent the use of toxic Cr-based catalysts and the cost and sensitivity to feed impurities of Pt-based materials in current use. In replacing Cr-based systems with those based on earth-abundant elements, WO3 systems are chosen here because of their ability to form stable and non-volatile sub-oxides, carbides and oxycarbides at the conditions of alkane dehydrogenation catalysis and because of their dispersion stability at the conditions required for their oxidative regeneration. Such properties render these materials amenable to cycling between the aggressive reductive and carburizing conditions of catalysis and regeneration without loss of the structures formed through specific synthetic protocols designedto form certain WO3 catalyst precursor species. W carbides and also sub-oxides likely to convert into oxycarbide structures during catalysis are known to catalyze alkane dehydrogenation reactions.