Understanding of the oxygen activation on ceria- and ceria/alumina-supported gold catalysts: a study combining 18O/16O isotopic exchange and EPR spectroscopy

摘要

Gold supported on ceria or ceria–alumina mixed oxides are very active catalysts for total oxidation of a variety of molecules. The key step of the oxygen activation on such catalysts is still a matter of debate. Gold–ceria (Au/CeO2) and gold–ceria–alumina (Au/CeO2/Al2O3) catalysts were prepared by deposition–precipitation of gold precursor with urea as in former works where their efficiency to catalyze the oxidation of propene and propan-2-ol was demonstrated. To understand the phenomenon of oxygen activation over this class of catalysts, efficient techniques generally used to characterize the interaction between oxygen and cerium-based oxides were applied; the oxygen storage capacity (OSC) measurement, the 18O2/16O2 isotopic exchange study (OIE), as well as characterizations by in situ Raman and electron paramagnetic resonance (EPR) spectroscopies. Each of the techniques allowed showing the impact of the gold nanoparticles on the activation of dioxygen, on the kinetic governing the gas-phase/solid oxygen atom exchange, and on the nature and the location of the adsorbed oxygen species. Gold nanoparticles were shown to increase drastically the OSC values and the rate of oxygen exchange. OIE study demonstrated the absence of pure equilibration reaction (16O2(g) + 18O2(g) ↔ 2 16O18O(g)), indicating that gold did not promote the dissociation of dioxygen. Peroxo adspecies were observed by Raman spectroscopy only in the presence of gold. On the contrary, EPR spectroscopy indicated that the concentration of superoxo adspecies was lower for oxide-supported gold samples than for bare oxides. The combination of techniques allowed reinforcing the hypothesis that the gold nanoparticules promote the activation of dioxygen by generating extremely mobile diatomic-oxygenated species at the gold/ceria interfacial perimeter. This specific gold–ceria interaction, which leads to the increase in oxygen mobility, is probably also responsible for the higher catalytic performance of Au/CeO2 and Au/CeO2/Al2O3 in oxidation reaction compared to bare supports.