Our research probes fundamental aspects of catalysis and provides exciting means for the discovery of potent, new catalytic materials. The power of CombiSci methods is exploited to address questions and issues such as:

1) determining how catalyst composition and structure control activity and selectivity and

2) developing a large structure-reactivity database from which mechanistic paradigms can be generated and subsequently refined. CombiSci tools provide important ways to distinguish desired reactivity from ineffective pathways.

Reactivity Mapping using the SAC M. (A) Schematic of tip-surface measurement to quantity the rate constant k. (B) Reactivity map of PtxRuy gradient versus electrode potential.

Combinatorial Discovery of Fuel Cell Catalysts (Hillier, Hebert, Houk): One of the key problems in low temperature fuel cells is the poor performance of catalysts for anode and cathode reactions. To address this issue, we are developing methods for the synthesis and rapid screening of catalyst libraries for both types of processes. The most efficient catalysts are multicomponent systems that span a vast composition space of binary, ternary, and quaternary transition metal phases. We have constructed dense, composition gradients on an electrode surface. Microfluidic delivery systems also provide a unique route for the creation of composition gradients. Scanning electrochemical and scanning mass spectrometric methods are used for in situ reactivity mapping of individual library addresses to directly determine reaction kinetics and delineate reaction pathways. A range of key anode and cathode reactions for low temperature fuel cells are characterized, including those based on the oxidation of simple liquid fuels such as methanol and the reduction of oxygen and peroxide. We also exploit in situ surface plasmon resonance and ellipsometry imaging as rapid, non-invasive methods for mapping adsorbed reactive intermediates and surface poisons. This work will not only lead to the discovery of more active electrocatalysts, but also provide important fundamental insights into composition-reactivity relationships in complex systems.

Combinatorial Strategies for Synthesis, Characterization, and Micro-reactor Engineering of Catalysts (Yeung, Miller, Shanks, Houk): We have established a Combinatorial catalysis program focused on the synthesis, spectroscopic analysis, and materials modeling of libraries of mixed metal oxides used for selective hydrocarbon oxidation and ozone generation for environmental remediation. CombiSci approaches are particularly useful since highly active and selective materials typically consist of multi-component phases or mixtures involving 5-10 metal oxides. We have utilized sputtering to produce ternary metal oxide component arrays that can successfully convert 1,3-butadiene to furan (a key industrial intermediate) at much lower temperatures and at much higher selectivities than previously reported. We have also studied the catalytic activity of mixed metal oxides in the oxidation of naphthalene to naphthoquinone (NQ) using high-throughput screening based on laser induced fluorescence imaging.

A 3 x 5 library of ternary metal oxide library was constructed with V, Sn, and Mo. The relationship between catalytic activity (proportional to NQ fluorescence intensity) and catalyst composition is shown in the image to the right. An oxide composition of 45% V, 45% Sn, and 10% Mo yielded a 70% increase in activity for this conversion compared to conventional industrial catalysts (V2O5). These results are strong testimonies to the value of CombiSci in rapidly identifying hot spots in problems with a large search space. We plan to enhance this capability by cross-correlating fluorescence imaging with infrared thermography.

Screening of naphthalene oxalate catalysts by fluorescence imaging.

In Vitro Evolution of Catalysts (Woo, Nilsen-Hamilton, Pohl): This effort develops and applies an in vitro evolution process (SELEX, systematic enhancement of ligands by exponential enrichment) in the design of transition metal catalysts. Our initial focus in on the development of DNA-based catalysts for cyclopropanation (Scheme 1). This reaction requires a transition metal complex to couple a diazo reagent and olefin to produce the cyclopropane. The olefin is attached to single stranded DNA with a flexible poly(ethylene glycol) linker. The first generation fixed-length DNA library contains 10^15 unique strands, each with a random section of up to 80 deoxynucleotides. This library is treated with a transition metal ion such as Co(II) or Cu(II) and a diazo reagent that is attached to an affinity tag. DNA strands that fold to form an active pocket self-catalyze the diazo coupling to its appended olefin. Active DNA strands are covalently tagged with an affinity label that provides a high throughput means of isolating effective catalysts using an affinity column. This sub-population is then amplified using PCR. The cycle is repeated by manipulating variables such as reactant and metal ion concentrations, reaction time, and temperature to increase the selection pressure and to produce more effective forms. Once the population achieves the target efficiency, the DNA is cloned, sequenced, and characterized. Determination of tertiary structures will provide insights on how active site shape and composition controls activity and selectivity. This CombiSci approach will significantly impact how transition metal catalysts are designed and optimized. Moreover, our in vitro evolution strategy can be adapted to many organic coupling reactions, and can establish new benchmarks in developing efficient catalysts.

In vitro Evolution of Catalysts. The challenges include to Improve selectivity and yield of catalysts. The research strategy is to exploit molecular biology and inorganic chemistry in homogeneous catalysis development, and the impact is a breakthrough Green manufacturing for fine chemicals

 

High-Throughput Catalyst Screening with Multiplexed Capillary Electrophoresis (Larock, Kraus, Yeung): Combinatorial screening of homogeneous catalysts based on multiplexing has been developed. Recently, we coupled non-aqueous capillary electrophoresis to microreactors as a highly effective approach to meet the throughput needs of the massive number of samples generated by this approach to reaction optimization. With this new multiplexed method, catalytic activity, selectivity, and kinetics can be quickly determined. It is possible, for example, to simultaneously screen 88 different catalyst CombiSci nations to find the most efficient palladium catalyst for the synthesis of carbolines. Other applications include screening for asymmetric catalysts and library synthesis.

Results of a multiplexed CE separation from 88 micro reactors.