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 H2 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% H2 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 H2/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 O2 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.