Kinetic studies of crotonaldehyde hydrogenation on model Pt–CeO2 and Pt–TiO2 catalysts have been carried out to investigate the mechanism of selective partial hydrogenation on these bifunctional oxide–metal surfaces. Previous reports have shown that selective C=O bond hydrogenation in unsaturated aldehydes requires an electronically active metal–support interface. However, the catalytic mechanism and the precise active site in these bifunctional catalysts has not yet been determined. By systematically controlling the coverage of TiO2 and CeO2 on a Pt catalyst, we demonstrate that the kinetics of selective C=O bond hydrogenation are rate limited by the Pt surface extending well beyond the actual oxide–metal interface. Passivating Pt with TiO2 at coverages approaching unity is shown to decrease the rate of C=O bond hydrogenation in exact proportion to the rate of C=C bond hydrogenation, indicating that both pathways become rate limited by the accessible Pt surface area at high oxide coverage. A similar kinetic result is demonstrated for CeO2 nanocubes deposited on a Pt film. Specifically, we show that within a region of uniformly packed CeO2 nanocubes on a Pt surface, little or no C=O bond hydrogenation occurs, while at the interface between rafts of nanocubes, the rate of C=O bond hydrogenation is quite high. These results indicate that production of unsaturated alcohols extends well beyond the 3-phase boundary defined by the oxide–metal interface. Efforts to quantify this length scale indicate that C=O bond hydrogenation occurs on Pt at lengths ≥10 nm away from the oxide–metal interface. We consider the possibility that this length scale corresponds to the mean free path of a reaction intermediate that must diffuse either to or across the active oxide–metal interface in order to promote C=O bond hydrogenation.

Determining the active site for selective C=O bond hydrogenation: Understanding the role of the Pt–support interface

253rd National Meeting (2017)

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