Microcalorimetry, Adsorption, and Reaction Studies of CO, O2, and CO+O2 over Au/Fe2O3, Fe2O3, and Polycrystalline Gold Catalysts

Abstract

To understand the effect of catalytic activity of Au/Fe2O3 at low temperatures on a CO oxidation reaction, adsorption and changes in enthalpy were determined for the interaction of CO, O2, or CO+O2 (2:1) pulses over Au (5 at.%)/Fe2O3, Fe2O3, and polycrystalline gold catalysts between 300 and 470 K. The results demonstrate that the oxidation of CO on both Fe2O3 and Au/Fe2O3 occur by means of similar redox mechanisms involving the removal and replenishment of lattice oxygen, where the presence of gold promotes these processes. The FTIR data reveal that gold facilitates the chemisorption of CO on Au/Fe2O3, leading predominantly to the formation of Au0–CO species. The carbonate-like species, formed on both Fe2O3 and Au/Fe2O3 during the adsorption of CO or CO+O2, are stable below 375 K and are regarded to be mere by-products that do not play a major role in the CO oxidation process, particularly at low reaction temperatures (<400 K). The larger gold particles inhibited the formation of COad species during exposure of Au/Fe2O3 to CO+O2; this was accompanied by a decrease in the adsorption of both CO and O2 and a decrease in the formation of CO2. The promotional effect of gold is attributed to the presence of small (nanosize) Au crystallites that facilitate the chemisorption of CO molecules because of their inherent defective structural sites. It is suggested that the energy that evolves during the chemisorption of CO molecules is responsible for the surge in temperature at the Au–Fe2O3 interfaces; these eventually serve as sites for the accelerated reaction between CO and the support.