Titania’s Prowess

Gold nanoparticles, a relatively new class of surprisingly active supported catalysts, may owe key aspects of their catalytic prowess to the titania (TiO₂) support on which they are commonly dispersed, according to scientists in New Orleans (J. Am. Chem. Soc., DOI: 10.1021/ja907865t). The mechanistic details uncovered by the investigation may lead to strategies for designing improved industrial catalysts.

The unexpected discovery several years ago that gold—generally considered an inert metal—can function as an active catalyst when prepared in nanoparticulate form touched off a wave of research into the precious metal’s catalytic capabilities.

For example, in 2006, researchers in Valencia, Spain, reported that nanoparticles of gold supported on titania selectively convert nitro groups to amino groups in multifunctional organic molecules. Those researchers later found that the same catalyst can also be used in a two-step reaction for hydrogenation of nitrobenzene to aniline followed by oxidation to azobenzene. Various explanations have been proposed to account for the catalytic system’s good performance, with much of the attention focused on gold.

Now, Tulane University physicists Shao-Chun Li and Ulrike Diebold report that titania alone facilitates the key steps in interconversion reactions between aniline and azobenzene and that the role of gold, at least in those reactions, may simply be to activate oxygen or hydrogen.

The team explains that depositing azobenzene on titania at sufficiently high concentration causes the molecules to adopt a configuration that leads to cleavage of the N=N double bond. That process forms surface-bound phenyl imide (C₆H₅N) intermediates, which can react with hydrogen to form aniline. The same intermediate, or a very similar one, forms on titania when aniline is dehydrogenated and converted to azobenzene, the team says.

This collection of mechanistic details—the N=N bond scission, hydrogenation of the phenyl imide intermediate, and the catalyst’s proclivity to facilitate the reverse reaction “is a very significant first observation of the complex catalytic function exhibited by titania surfaces,” according to D. Wayne Goodman, a catalysis specialist and professor of chemistry at Texas A&M University.

Goodman adds that these properties of titania are likely applicable to a broad range of catalytic reactions. As such, he says, future studies should focus on the active role of titania itself in the overall catalytic chemistry mediated by titania and titania-supported metals such as gold in hydrogenation and dehydrogenation reactions.