Active Oxygen Species and Mechanism for Low-Temperature CO Oxidation Reaction on a TiO_2-Supported Au Catalyst Prepared from Au(PPh_3)(NO_3) and As-Precipitated Titanium Hydroxide

摘要

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 TiO oxide. No shifts of v_(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.