Origin of Gold's Catalytic Activity

FROM THE ACS MEETING

In the bulk, gold is essentially inert. But in nanostructured form, the noble metal can be a surprisingly good catalyst. Researchers may have just uncovered the secret of the element's high catalytic activity.

In a talk at the ACS national meeting in Philadelphia, chemistry professor D. Wayne Goodman of Texas A&M University described experiments that identify a gold structure that is two atomic layers thick as the form of the metal responsible for gold's ability to catalyze the oxidation of carbon monoxide to an unprecedented degree. The study addresses questions regarding catalytic reaction mechanisms and eventually may lead to more active commercial catalysts.

Scientists have been interested in gold's potential for catalyzing CO and propylene oxidation and other reactions since the unexpected discovery by Japanese researchers several years ago that nanometer-sized gold clusters supported on metal oxides such as TiO₂ exhibit high catalytic activity for certain reactions.

Several research teams have tried to uncover the origin of the nanostructured metal's unique reactivity. The results have prompted debate in the catalysis community, with researchers questioning the role of particle thickness and shape, metal oxidation state, and interactions with the oxide support material. Now, some of those questions have been answered.

Using vapor deposition methods, Goodman and postdoc Ming-Shu Chen prepared model catalysts and used them to study CO oxidation, a reaction relevant to fuel-cell processes and similar to propylene oxidation. The catalysts, which feature structures that were challenging to prepare and verify experimentally, consist of atomically ordered monolayer and bilayer films of gold that cover a TiO₂ support. The precise arrangement of gold atoms in the model systems eliminates particle shape and direct interactions with TiO₂ as factors in the catalytic reaction mechanism.

Goodman reported that the bilayer gold catalyst is more than 10 times more active for CO oxidation than the monolayer form and that reactivity decreases when gold coverage exceeds two atomic layers. In addition, the reaction proceeds roughly 50 times faster on the bilayer catalyst than it does on Au/TiO₂ catalysts prepared via conventional synthesis methods [Science, published online Aug. 26, http://dx.doi.org/10.1126/science.1102420].

"These results say that titania is activating gold through an electronic effect--by making gold slightly electron rich--rather than by titania interacting with [CO and oxygen] directly at the gold-titanium interface," Goodman asserted. The two-layer structure readily lends itself to examination by computational methods, he added. "The theoreticians have already begun calculating."