Crotonaldehyde hydrogenation on platinum–titanium oxide and platinum–cerium oxide catalysts: selective CO bond hydrogen requires platinum sites beyond the oxide–metal interface

Abstract

We have investigated a series of Pt–TiO₂ and Pt–CeO₂ catalysts for crotonaldehyde hydrogenation with the goal of better understanding the kinetics of CO bond hydrogenation. It has been established that in these systems, Pt is the active phase for CC bond hydrogenation, while the interface between Pt and an active metal oxide is required for selective CO bond hydrogenation. In this work we demonstrate that in addition to the presence of an active oxide–metal interface, there is also a need for accessible Pt sites beyond this interface to facilitate CO bond hydrogenation. Passivating Pt with TiO₂ at coverages approaching unity is shown to decrease the rate of CO bond hydrogenation in exact proportion to the rate of CC bond hydrogenation, suggesting 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, where the rate of CO bond hydrogenation initially increases with CeO₂ nanocube coverage and then decreases again at high CeO₂ coverage, even though the nanocubes remain discrete. Because the density of interface sites scales linearly with the coverage of CeO₂ nanocubes on the Pt surface, this decrease in activity shows that accessible Pt sites rather than oxide–metal interface sites become rate limiting for CO bond hydrogenation at high oxide coverage. Finally, in bilayer catalysts of CeO₂ and Pt nanocubes, we find that both Pt-on-CeO₂ and CeO₂-on-Pt are active for CC bond hydrogenation, while only Pt-on-CeO₂ is active for CO bond hydrogenation. These results are discussed in terms of a spillover-mediated reaction mechanism in which a reaction intermediate must diffuse either to or across the active oxide–metal interface in order to promote CO bond hydrogenation.