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 O2 on a highly active TiO2-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, O2 temperature-programmed desorption (O2 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 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. O2 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. O2 adsorbed on the oxygen vacancies to form superoxide O−2 with g1=2.020, g2=2.010, and g3=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 Ti4+ 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 νCO peaks on Au/Ti(OH)*4 occurred upon O2 adsorption, suggesting that O2 was not directly bound to the Au particles on which CO adsorbed. Annealing of Au/Ti(OH)*4 under O2 atmosphere significantly suppressed the O2 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 O2-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.