(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, VO_x/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 290°C) were more active and selective toward the complete oxidation of toluene than VO_x/CeO₂ catalyst (T₅₀^*=370°C), where T₅₀^* refers to the temperature for 50% of CO₂ yield. The η²-peroxide O₂^2-_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, η¹-superoxide O₂^-_ads was found on VO_x/CeO₂ catalyst and the toluene molecule adsorbed via σ-bonding forming carbenium ions on the vanadia Brønsted 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, VO_x/CeO₂ and Au/CeO₂ catalysts.