On the reactivity of carbonate species on a Pt/CeO2 catalyst under various reaction atmospheres:: Application of the isotopic exchange technique

The reactivity and stability of the carbonate species formed at the surface of a Pt/CeO2 catalyst during the reverse was-gas-shift (RWGS) reaction was investigated by in situ diffuse reflectance (DRIFT) FT-IR spectroscopy at 523 K. Like most oxide materials presenting some basic character, the surface of the sample (not yet exposed to the RWGS stream) exhibited a significant concentration of carbonate species, mostly of the polydentate-type. A fraction of these carbonates was reduced by H, at 573 K, leading to the formation of Pt-bound surface carbonyls. Additional carbonates (mostly bidentate species) were formed under RWGS conditions (1% CO2 + 4% H-2 in Ar at 523 K), along with formate species and more carbonyl groups. These additional carbonates, like the polydentate species, were essentially stable under Ar and, surprisingly, under H-2/Ar. On the contrary, all carbonate species were readily reacted and/or exchanged under the RWGS feed, as evidenced by steady-state isotopic transient kinetic analysis (SSITKA), and under CO2 and O-2 mixtures. It is concluded that the bonding strength of the carbonates and, as a consequence, the reactivity of those depends on the nature of the feed. The possibility of ceria over-reduction is discussed as being a factor that would increase carbonate stability and result in surface poisoning. The possibility of adsorption-assisted desorption is also presented. The data reported here clearly show the need to use operando or steady-state techniques (such as SSITKA) to determine the reactivity of surface species in actual catalysis conditions, as misleading conclusions can otherwise be drawn on reaction mechanisms. (c) 2005 Elsevier B.V. All rights reserved.