Insights into the oxidation and decomposition of CO on Au/α-Fe2O3 and on α-Fe2O3 by coupled TG-FTIR

CO oxidation and decomposition behaviors over nanosized 3% Au/alpha-Fe2O3 catalyst and over the alpha-Fe2O3 support were studied in situ via thermogravimetry coupled to on-line FTIR spectroscopy (TG-FTIR), which was used to obtain temperature-programmed reduction (TPR) curves and evolved gas analysis. The catalyst was prepared by a sonication-assisted Au colloid based method and had a Au particle size in the range of 2-5 nm. Carburization studies of H-2-prereduced samples were also made in CO gas. According to gravimetry, for the 3% Au/alpha-Fe2O3 catalyst, there were three distinct stages of CO interaction with the Au catalyst but only two stages for the catalyst support. At low temperatures (<= 100 degrees C), only the Au catalyst had a rapid weight loss, which confirmed that CO reacted with highly active absorbed oxygen species and/or OH species which were associated with and promoted by the Au nanoparticles. Around 300 degrees C, both the catalyst and support samples experienced the reduction of Fe2O3 to Fe3O4, while above 400 degrees C further reduction to FeO and Fe metal took place. An played no part in the kinetics of Fe3O4 formation because lattice 0 mobility was rate-limiting. At higher temperature where Fe3O4 was further reduced to FeO and Fe-0, the initially formed metallic Fe-0 nuclei could decompose CO molecules and release O species. Both this coproduced 0 species and the lattice oxygen could react with CO molecules. Thus, the CO oxidation was not limited by the mobility of lattice oxygen, and the catalytic function of Au was revealed again. Carburization of metallic Fe, created by prereduction in H, revealed a distinct weight gain at 350 degrees C corresponding to Fe3C formation, as subsequently confirmed by X-ray diffraction (XRD). Sustained carbon deposition ensued at 450 degrees C. In the cases of the 3% Au/gamma-Al2O3 and Au/ZrO2 catalysts prepared by the same method, however, after exposure to CO in the same temperature range, no carbon deposit was observed, indicating that although Au nanoparticles could activate the absorbed oxygen molecules at low temperatures, they were not able to activate the lattice oxygen in the three catalyst supports or to dissociate the CO molecules directly.