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LSAC: Laboratory for the Science and Application of Catalysis

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.

Understanding Consequences of Diffusion in Bifunctional Catalysis [to top]

Bifunctional catalysts simultaneously promote reactions over two different types of catalytic sites, separated by distances larger than molecular dimensions. Designing such systems traditionally involved optimizing the ratio of one function to another, ignoring mass transfer limitations. This research investigates systems where diffusional effects are non-negligible and can be harnessed via the tunable diffusivity of zeolite solid acids, allowing the development of detailed reaction-transport models to predict reactivity and selectivity. Insights into diffusional effects will be applied to the design of catalytic architectures to control diffusion-enhanced secondary acid-catalyzed reactions. Furthermore, the implications of this research are widely applicable to other diffusion-enhanced catalytic phenomena, including hydrogen transfer events and the use of co-catalysts.

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.

Kinetics Studies and Experimental Evaluation of Novel Materials for NO Oxidation [to top]

Though combustion engines become more efficient at high air-to-fuel ratios, this leads to higher nitrogen oxides (NOx) levels which cause stratospheric ozone depletion, acid rain, and photochemical smog. However, nitrogen oxides can be converted to dinitrogen when reacted with ammonia and oxygen through selective catalytic reduction (SCR) processes (NO + NO2 + 2NH3 → 2N2 + 3H2O). In order to increase the SCR reaction, it is necessary to have a presence of equimolar NO-NO2 mixtures; thus performing NO oxidation in the effluent streams to create NO2 is required for efficient performance of NH3-NOx processes. Active sites and reaction mechanisms of metals such as Pt and their oxides (ex. PdO, RhO2, etc.) have been extensively studied to be effective in NO oxidation but also effective to NH3 oxidation, thus degrading the speed of SCR reactions.  However, catalysts dispersed on ceria (CeO2) exhibit high SCR activity as well as N2 selectivity thus increasing the speed of SCR reactions. Current research focuses on NO oxidation turnover rates on catalyst systems dispersed on ceria to assess the kinetic relevance of the elementary steps involved. Future studies will involve kinetics studies and experimental evaluations of other novel catalyst systems for NO oxidation.

 

Current Research- Postdoctoral Fellows

Coupling Dimerization and Metathesis Reactions [to top]

This project consists of understanding the role of metal and acid-sites on coupling dimerization and metathesis reactions to produce propene from ethene. The effect of the support and ratio of metal-sites to acid-sites will be studied to determine the contribution via dimerization and isomerization to butenes. The internal butenes will be later introduced into the metathesis pathway to produce propene from ethene.

CO reactions with H2 on supported Co clusters: kinetic measurements combined with spectroscopic studies [to top]

The catalytic methanation reaction has attracted considerable interest due to the fact that this reaction is essential in various industrial processes, including the manufacture of gas from coal, the removal of oxo-compounds in the feed gas for the ammonia synthesis, final purification step of reformate gas to reduce concentration of CO to the extremely low levels in PEM fuel cell electrodes and it is a starting point in Fischer-Tropsch synthesis for long-chain hydrocarbons formation. Many researchers have focused on evaluating and establishing the thermodynamic and kinetic behavior of methanation of carbon monoxide over a number of catalysts with emphasis on Ni, Rh, Co and Ru. However, the methanation reaction pathways, site requirements and active surface species are still contradictive topics that need to be adressed. CO activation in methanation is still a matter of debate. The objective of this work is to investigate the kinetics of carbon monoxide methanation over a non-deactivating Co catalysts, derive a suitable kinetic equation, and to obtain new indications about the reaction mechanism by doing kinetic measurements combined with spectroscopic studies.

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.

Kinetic Assessment of Alkanol Dehydrogenation on Metal Catalysts [to top]

Oxygen removal is essential for biomass-derived molecules to be used as fuels and chemicals. Deoxygenation includes C-O hydrogenolysis and C-C hydrogenolysis reactions, which involve either alkanols or alkanones as the reactant. Experimental studies show alkanols and alkanones are at equilibrium as the approach to equilibrium (η) value for the dehydrogenation of alkanols to alkanones approaches 1. However, DFT predicts that the dehydrogenation of alkanols is not equilibrated because the O-H activation in alkanols is difficult. Therefore, this projects looks at the dehydrogenation of alkanols to alkanones using dispersed Cu clusters, performing a kinetic study in order to determine the kinetically relevant steps of non-equilibrium alkanol-alkanone.

 

Current Research - Visiting Scholars

The synthesis, characterization and mechanistic evaluation of microporous materials for catalytic applications [to top]

Copper-containing CHA is an important class of catalysts because of its thermal stability and hydrocarbon resistance due to its small apertures. However, post-synthetic confinement of metal precursors is seldom possible for CHA because its apertures preclude the diffusion of the solvated metal precursor. Furthermore, the location and configuration (i.e., isolated Cu cations or Cu dimers) of Cu species, are also important for catalytic application. Interzeolite transformation methods have been used to successfully encapsulate metal clusters within different zeolites of frameworks where encapsulation proved infeasible through involving direct hydrothermal synthesis with ligand-stabilized metal precursors or post-synthesis exchange. In the present study, interzeolite transformation protocols for synthesizing copper-containing CHA from copper-containing FAU will be developed. Specific synthesis parameters to be investigated include MOH/SiO2 ratio (M = K, Na), synthesis temperature and time duration, Si/Al ratio, Cu/Al ratio. This systematic study will provide insight into the mechanism of interzeolite transformation when parent and target structure contain isolated metal cations. Ultimately, these protocols will be applied to different systems (i.e., isolated Ni or Fe cations and/or dimers within zeolites) that preclude direct synthesis through more traditional means.

DFT Simulation of N2O decomposition on a Co3O4 catalyst [to top]

As one of the most harmful greenhouse gases and ozone depleting agents, nitrous oxide (N2O) plays a significant role in global warming. The science community seeks to improve catalysts for N2O decomposition into harmless N2 and O2. My project deals with the DFT simulation of the N2O decomposition on a Co3O4 catalyst, specifically the N-O bond cleavage upon N2O interaction with the surface, which is proposed to be the kinetically-relevant step for this reaction. The aim is to identify and optimize the transition state structure to establish the energy diagram along the reaction coordinate, revealing the site requirement as well as kinetic barrier for N2O decomposition on the Co3O4 surface. The conclusions reached upon completion of this project can be used to identify active sites on Co3O4 surface for N2O decomposition and provide insight on the site requirement for N2O decomposition on metal oxide in general.

The synthesis, characterization and mechanistic evaluation of microporous materials for catalytic applications [to top]

Computational study to establish the site requirements for NOx decomposition and reduction, with the objective to guide the design of improved catalyst structures.

 


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