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

The active oxygen species and mechanism for catalytic CO oxidation with O-2 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, 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 CO2, 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 TiO2* 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 TiO2*, which indicates that the presence of Au particles has a profound effect on the surface state of Ti oxide. No shifts of vco peaks on Au/Ti(OH)(4)* occurred upon O-2 adsorption, suggesting that O-2 was not directly bound to the An 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. (C) 1999 Academic Press.