Littlest catalysts get a lot of support

Industry has long used reactive metal clusters stuck to larger, inert particles as chemical catalysts. Today, developers of catalysts are making those clusters as small as possible to maximize exposed metal and thus speed up reactions.

New experiments reveal a surprise: With small clusters, the distinction between the active metal and the presumably inert carrier begins to blur, says Bruce C. Gates of the University of California, Davis.

That’s noteworthy because as the carrier becomes a player in the reactions, it can alter the relative amounts of byproducts coming from intermediate steps in the reactions, Gates and his colleagues report in the Feb. 7 Nature.

Moreover, such a modification of a chemical process can also change its rate, Gates notes.

In the study, the team observed four-atom clusters of iridium metal catalyzing reactions such as the conversion of mixed propylene and hydrogen into propane gas. The iridium clusters were attached to either aluminum oxide or magnesium oxide particles.

The scientists determined that the intermediate stages of the reaction differed between the two oxides, as did the geometry between the cluster and the oxide surface supporting it. Also, the propane-producing reaction was 10 times faster on aluminum oxide than on magnesium oxide, another indication that these carriers aren’t so inert.

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