(560do) Investigation of Ceria, Vanadia/Ceria and Gold/Ceria Catalysts By Operando Raman-Online FTIR during Toluene Oxidation
Toluene oxidation on three ceria catalysts, CeO2, VOx/CeO2 and Au/CeO2, was investigated by an operando Raman-online FTIR reactor cell. The reactive surface oxygen sites were preferred sites for vanadium and gold deposition. The deposited vanadium existed as V5+, while most of the gold was in Au+ state and roughly a third of ceria was in the reduced Ce3+ state. The Au/CeO2 and CeO2 catalysts (T50*=260 and 290Â°C) were more active and selective toward the complete oxidation of toluene than VOx/CeO2 catalyst (T50*=370Â°C), where T50* refers to the temperature for 50% of CO2 yield. The Î·2-peroxide O22-adswas detected on Au/CeO2 and CeO2 catalysts, where toluene molecules preferentially adsorbed parallel to the surface via Ï-bonding. Au/CeO2 gave complete combustion producing mainly CO2. On the other hand, Î·1-superoxide O2-ads was found on VOx/CeO2 catalyst and the toluene molecule adsorbed via Ï-bonding forming carbenium ions on the vanadia BrÃ¸nsted acid sites. This catalyst produced significant amount of benzaldehyde as partial oxidation byproduct and CO (< 25%). The nature of the active sites, configurational adsorption of toluene and the reactive oxygen species play an important role in the catalyst activity and selectivity leading to a large contrast in the catalytic behavior of the CeO2, VOx/CeO2 and Au/CeO2 catalysts.