Investigation on the temperature-dependent mechanism shift of CO oxidation over RuO2/TiO2 catalysts

Under technologically relevant oxygen-rich conditions, the reaction mechanism for CO oxidation over platinum group metals (PGMs) is often complicated by oxide formation. Ru, as a special PGM exhibiting "pressure gap" phenomenon, its optimal active state has been proved to be an ultrathin Ru oxide, rather than pure metal like other PGMs. Therefore, it is plausible to suspect that the sharp rise of CO oxidation rate within narrow temperature range for Ru-based catalysts is not solely controlled by the O 2 activation rate. In this study, we investigated this temperature-dependent sharp rise in the CO oxidation rate through experimental and theoretical calculations. In the rate mutation region, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) detected a rapid increase in CO 2 production as the adsorbed CO species were annihilated, accompanied by a shift in the reaction orders of CO and O 2 from -0.29 and +0.59 (∼205 °C) to +0.94 and +0.15 (∼215 °C), respectively. The thermodynamic phase diagram indicates that the RuO 2 (110) surface consistently accommodates O* species, but fails to form a stable CO* phase when the temperature exceeds 580 K. Density functional theory (DFT) calculations suggest that initiating the Mars-van Krevelen ( M −vK) mechanism at low temperatures requires overcoming a relatively high activation energy (Ea 1 = 1.28 eV), hence CO prefers the Langmuir-Hinshelwood ( L − H ) mechanism on the Ru/TiO 2 surface. However, as the temperature rises, the M −vK mechanism, once initiated, the activation and reaction of O 2 assisted by oxygen vacancies are thermodynamically(△ H = -2.03 eV) and kinetically (Ea 2 = 0.37 eV) more favorable, respectively. This shift from the L − H to the M −vK mechanism significantly enhances the CO oxidation rate on RuO 2 /TiO 2 catalysts.