Active oxygen species and reaction mechanism for low-temperature CO oxidation on an Fe2O3-supported Au catalyst prepared from Au(PPh3)(NO3) and as-precipitated iron hydroxide

Abstract

Active oxygen species and the reaction mechanism for catalytic CO oxidation with O2 on a highly active Fe2O3-supported Au catalyst (denoted as Au/Fe(OH)3*), which was prepared by supporting Au(PPh3)(NO3) on as-precipitated wet iron hydroxide followed by calcination at 673 K, have been studied by means of oxygen isotope exchange, O2-temperature programmed desorption (TPD) and FT-IR. Surface lattice oxygen atoms on the Au/Fe(OH)3* catalyst were inactive for oxygen exchange with O2 and CO, and also for CO oxidation at room temperature. The surface lattice oxygen atoms were exchanged only with the oxygen atoms of CO2 probably via carbonates. There is no evidence that O2 dissociates to atomic oxygen on the catalyst. TPD spectra following adsorption of 36O2 or a mixture of 32O2+36O2 showed no oxygen exchange, where the adsorbed oxygen on Au/Fe(OH)3* desorbed below 500 K. Upon CO exposure, all the adsorbed oxygen species disappeared. FT-IR spectra revealed that CO reversibly adsorbed on Au particles and irreversibly adsorbed on Fe3+ sites on the Au/Fe(OH)3* surface. Only CO molecules adsorbed on the Au particles were active for low-temperature CO oxidation. No band for adsorbed CO was observed on Fe2O3* prepared by calcination of the as-precipitated wet Fe(OH)3* at 673 K, which indicates that the presence of Au particles causes a profound effect on the surface state of Fe-oxide. Annealing of Au/Fe(OH)3* under an O2 atmosphere did not suppress the catalytic CO oxidation, unlike a remarkable suppression observed with Au/Ti(OH)4*. The presence of water vapor did not significantly decrease the CO oxidation rate due to the facile water gas shift reaction on Au/Fe(OH)3*, also unlike the case of Au/Ti(OH)4*. From the systematic oxygen isotope exchange experiments along with O2-TPD and FT-IR, it is most likely that CO adsorbed on Au metallic particles and O2 adsorbed on oxygen vacancies at the oxide surface adjacent to the Au particles contribute to the low-temperature catalytic CO oxidation on Au/Fe(OH)3*. The mechanism for the catalytic CO oxidation on the active Au/Fe(OH)3* catalyst is discussed in detail and compared with those reported previously.