Authors: Wang, QY; Yeung, KL; Banares, MA

J. Catal.. vol: 364. page: 0021-9517.
Date: AUG. 2018.
Doi: 10.1016/j.jcat.2018.05.001.

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 (T-50(*)= 260 and 290 degrees C) were more active and selective toward the complete oxidation of toluene than VOx/CeO2 catalyst (T*(50) = 370 degrees C), where T*(50) refers to the temperature for 50% of CO2 yield. The eta(2)-peroxide O-2ads(2-) ads was detected on Au/CeO2 and CeO2 catalysts, where toluene molecules preferentially adsorbed parallel to the surface via pi-bonding. Au/CeO2 gave complete combustion producing mainly CO2. On the other hand, eta(1)-superoxide O-2(-) ads was found on VOx/CeO2 catalyst and the toluene molecule adsorbed via sigma-bonding forming carbenium ions on the vanadia Bronsted 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 important roles in the catalyst activity and selectivity leading to a large contrast in the catalytic behavior of the CeO2, VOx/CeO2 and Au/CeO2 catalysts. (C) 2018 Elsevier Inc. All rights reserved..