Surface water-assisted preferential CO oxidation on Pt/CeO2 catalyst

The production of clean hydrogen is a key requirement for a future hydrogen economy, in general, and, specifically, for the application of proton exchange membrane fuel cells (PEMFC). Here, we focus on one of the essential purification methods, the so-called "PROX" reaction, the preferential oxidation of traces of CO in a large hydrogen excess. Small platinum particles on a reducible support like ceria are effective to remove CO from hydrogen feed. The paper specifically addresses the mechanism of the PROX reaction on a Pt/CeO2 catalyst using in situ experimentation with time-resolved and temperature-programmed diffuse reflectance infrared spectroscopy. Surface species (carbonates, formates, carbonyls, hydroxyls, and adsorbed water) present under reaction conditions are identified, and correlations of their abundance with catalytic performance allow the discrimination between mechanistically relevant species (intermediates) and spectator species. The following scenario is proposed: hydrogen initially adsorbed on platinum spills over to the support, leading to ordered vacancy formation in the ceria bulk as well as hydroxylation and hydration of the surface. CO is mainly adsorbed in on-top orientation on metallic platinum. The linear relationship between the amount of adsorbed water (H2Oads) and the CO2 production indicates that the hydrated ceria supplies an oxidizing agent at the metal/support interface reacting with the nearby surface carbonyls on the Pt particles yielding CO2. Moreover, adsorbed water also blocks hydrogen oxidation because of desorption hindrance. From the correlations in the presented results, an intelligent PROX catalyst can be formulated, providing a guideline for future developments.