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 w et titanium hydroxide followed by calcination at 673 K, have been studied b y means of oxygen isotope exchange, O-2 temperature-programmed desorption ( O-2 TPD), electron spin resonance (ESR), and Fourier-transformed infrared s pectroscopy (FT-IR). Surface lattice oxygen atoms on the Au/Ti(OH)(4)* cata lyst were inactive for oxygen exchange with O-2 and CO and also for CO oxid ation at room temperature. The surface lattice oxygen atoms were exchanged only with the oxygen atoms of CO2, probably via carbonates. O-2 did not dis sociate to atomic oxygen on the catalyst. The catalyst showed a paramagneti c 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 supero xide O-2(-) with g(1) = 2.020, g(2) = 2.010, and g(3) = 2.005, which are ch aracteristic of O-2(-) with an angular arrangement. Upon CO exposure, all t he 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 wer e 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 o f Au particles has a profound effect on the surface state of Ti oxide. No s hifts of vco peaks on Au/Ti(OH)(4)* occurred upon O-2 adsorption, suggestin g 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 t he O-2 adsorption and the CO oxidation due to a decrease in the amount of o xygen vacancies, while CO adsorption was not affected by annealing. From th e systematic oxygen isotope exchange experiments along with O-2-TPD, ESR, a nd 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 mec hanism 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.