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Combinatorial Methods Successful in Solid-State Catalyst Discovery
Scientists discover important new catalysts by adapting combinatorial methods
by Gail KaretDespite the popularity of combinatorial chemistry as a drug-discovery technique and recent favorable publicity concerning its applicability to solid-state chemistry, many solid-state chemists remain skeptical that it will find widespread use in their field. Although academic and industrial researchers have shown that solids can be made by combinatorial methods, few useful compounds have actually been synthesized until now.
Research groups led by Tom Mallouk, professor of chemistry at Pennsylvania State Univ.,University Park, and Eugene Smotkin, associate professor of chemical engineering at the Illinois Institute of Technology, Chicago, have adapted combinatorial methods to the task of screening solid catalysts for direct-methanol fuel cells. The use of combinatorial chemistry allowed them to screen many more compounds than they otherwise could and led to the recent discovery of new alloys that catalyze methanol oxidation. Key to this discovery was the ability to study many of the possible combinations for four or more different metals and to try alloys with low metal concentrations.
The screening of these alloys for catalytic activity was a problem especially well-suited for combinatorial methods. Although a synthetic strategy was developed by Smotkin's group, actually finding the catalysts would have been tedious and time-consuming without combinatorial methods.
Their synthetic strategy was based on a knowledge of the reaction mechanism. For the reaction to occur quickly, two metals were needed--platinum to bind carbon and another metal to activate water. The metal that activates water is not very soluble in platinum. Addition of a third or fourth metal solves this solubility problem, but making and testing all of the possible stoichiometries one by one would have required an unreasonable amount of time. The use of combinatorial methods sped up the implementation of their strategy.
A limitation for the application of combinatorial chemistry to solid-state systems is that there were not many ways to screen compounds rapidly. Mallouk's group attacked this problem by developing a screening technique using a fluorescent dye in solution. They irradiated the sample with ultraviolet light and allowed the electrochemical potential to vary. The pH of the solution near the most active catalytic spots dropped first, due to the production of protons. The dyes emit visible light when the pH of the solution decreases, causing the most active catalytic spots to glow first. "He can screen hundreds of catalysts in a shot," says Smotkin.
To automate the process of making catalysts, Mallouk's group took an inkjet printer and modified the ink cartridges so they could fill them with solutions of metal salts in water. They use this modified printer to rapidly deposit catalytic spots of controlled composition on a carbon support. The metal salts in the arrays are chemically reduced to make the alloys.
One catalyst they found, Pt44Ru41Os10Ir5, contains only 5% iridium. It has a current density about 40% higher at 400 mV than the best commercially available Pt/Ru anode catalyst. This quarternary alloy was an improvement on a ternary compound, Pt65Ru25Os10, found by Kevin Ley, a researcher in Smotkin's group, by non-combinatorial methods.
Pennsylvania State Univ.
427 Thomas Building
University Park, PA 16802
Phone: 814-863-4682
E-mail: science@psu.eduResouce: http://research.chem.psu.edu/mallouk/eriknews/rd.html
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