Active Oxygen Species and Mechanism for Low-Temperature CO Oxidation Reaction on a TiO2-Supported Au Catalyst Prepared from Au(PPh3)(NO3) and As-Precipitated Titanium Hydroxide

Abstract The active oxygen species and mechanism for catalytic CO oxidation with O 2 on a highly active TiO 2 -supported Au catalyst (denoted as Au/Ti(OH) * 4 ), which was prepared by supporting a Au–phophine complex on as-precipitated wet titanium hydroxide followed by calcination at 673 K, have been studied by means of oxygen isotope exchange, O 2 temperature-programmed desorption (O 2 TPD), electron spin resonance (ESR), and Fourier-transformed infrared spectroscopy (FT-IR). Surface lattice oxygen atoms on the Au/Ti(OH) * 4 catalyst were inactive for oxygen exchange with O 2 and CO and also for CO oxidation at room temperature. The surface lattice oxygen atoms were exchanged only with the oxygen atoms of CO 2 , probably via carbonates. O 2 did not dissociate to atomic oxygen on the catalyst. The catalyst showed a paramagnetic signal at g =2.002 due to unpaired electrons trapped at oxygen vacancies mainly at the surface. O 2 adsorbed on the oxygen vacancies to form superoxide O − 2 with g 1 =2.020, g 2 =2.010, and g 3 =2.005, which are characteristic of O − 2 with an angular arrangement. Upon CO exposure, all the adsorbed oxygen species disappeared. The adsorbed oxygen on Au/Ti(OH) * 4 desorbed below 550 K. O − 2 species were also observed on TiO * 2 prepared by calcination of as-precipitated wet titanium hydroxide at 673 K, but were unreactive with CO. FT-IR spectra revealed that CO reversibly adsorbed on both Au particles and Ti 4+ sites on the Au/Ti(OH) * 4 surface. No band for adsorbed CO was observed on the TiO * 2 , which indicates that the presence of Au particles has a profound effect on the surface state of Ti oxide. No shifts of ν CO peaks on Au/Ti(OH) * 4 occurred upon O 2 adsorption, suggesting that O 2 was not directly bound to the Au particles on which CO adsorbed. Annealing of Au/Ti(OH) * 4 under O 2 atmosphere significantly suppressed the O 2 adsorption and the CO oxidation due to a decrease in the amount of oxygen vacancies, while CO adsorption was not affected by annealing. From the systematic oxygen isotope exchange experiments along with O 2 -TPD, ESR, and FT-IR, it is most likely that CO adsorbed on Au metallic particles and O − 2 adsorbed on oxygen vacancies at the oxide surface adjacent to the Au particles contribute to the low-temperature catalytic CO oxidation. The mechanism for the catalytic CO oxidation on the active Au/Ti(OH) * 4 catalyst is discussed in detail and compared with mechanisms reported previously.

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