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.