Combinatorial Synthesis and Reactivity Screening of Electro-Oxidation Catalyst Gradients
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COMBINATORIAL SYNTHESIS AND REACTIVITY SCREENING OF ELECTRO-OXIDATION CATALYST GRADIENTS Shrisudersan Jayaraman and Andrew C. Hillier* Department of Chemical Engineering, Department of Chemistry and Institute of Combinatorial Discovery Iowa State University, Ames, IA 50011 ABSTRACT Combinatorial methods represent an appealing experimental method for the discovery of heterogeneous catalysts. One can efficiently identify candidate materials or sample vast regions of composition space using a combination of dense catalyst libraries and high-throughput reactivity screening techniques. This is particularly appealing for the discovery of novel catalysts for low temperature fuel cells where multi-component systems have shown improved performance. For example, the poison tolerance of typical anode catalysts can be improved by the addition of oxophilic components such as ruthenium, molybdenum, tin or osmium. Consequently, a vast composition space must be sampled in order to identify catalyst compositions or regions of composition space with greater activity. Combinatorial methods represent a practical means to speed-up the catalyst discovery process. In this manuscript, we demonstrate a novel method for combinatorial catalyst discovery based upon the synthesis and reactivity mapping of catalyst composition gradients. Samples consisting of uniform variations in surface composition of metals catalysts (Pt-M1 and Pt-M1-M2, where M1, M2 = Ru, Mo, Sn or Os) are fabricated using a gel-transfer technique. A concentration gradient of source metal ions is produced in a swollen polymer gel and then transferred onto a surface by electrodeposition to create a continuous composition gradient. An in situ reactivity-mapping tool based on the scanning electrochemical microscope is used to interrogate these catalyst gradients for the hydrogen oxidation reaction in the presence of adsorbed carbon monoxide. INTRODUCTION Combinatorial techniques are becoming increasingly important as a method of materials discovery. The discovery of improved catalysts for low-temperature fuel cells is a particularly promising application of combinatorial materials discovery.6 Presently, one of the major barriers for development of technologically viable fuel cells is poor activity of anode and cathode catalysts. At the anode, carbon monoxide (CO) poisoning represents a major problem because CO is often present in the anode feed as a result of up-stream reforming or is produced as an intermediate during the direct oxidation of liquid fuels.7,8 The addition of elements with oxophilic character, such as ruthenium (Ru), molybdenum (Mo), osmium (Os) and tin (Sn), to platinum catalysts can be used to improve poison tolerance.8 These additives typically form surface oxides more readily than Pt and, therefore, oxidize CO from the catalyst surface to liberate active sites.9 Given the large number of candidate materials and the vast composition space to be sampled to thoroughly investigate the binary, ternary and quaternary catalyst candidates, combinatorial me
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