Facile Cr3+-Doping Strategy Dramatically Promoting Ru/CeO2 for Low-Temperature CO2 Methanation: Unraveling the Roles of Surface Oxygen Vacancies and Hydroxyl Groups

Cr cation doping in the support of the Ru/CeO2 catalyst with a Cr/Ce molar ratio of 1:9 dramatically improved the CO2 methanation activity at low temperatures, with the turnover frequency value on Ru/Ce0.9Cr0.1Ox at 150 °C being 5.3 times higher than that on Ru/CeO2. X-ray diffraction and Raman spectroscopy results confirmed the Cr3+ doping in the lattice of the CeO2 support. Thus, more reactive surface oxygen formed on the Ce0.9Cr0.1Ox support, and the Ru/Ce0.9Cr0.1Ox catalyst contained more oxygen vacancies and hydroxyl groups during the reduction process than the Ru/CeO2 catalyst. In situ Fourier transform infrared spectroscopy and temperature-programed surface reaction revealed that CO2 methanation on both Ru/Ce0.9Cr0.1Ox and Ru/CeO2 catalysts followed the formate and CO* pathways, with the former being dominant at low temperatures. The formate pathway was identified, in which CO2 interacted with surface hydroxyl groups to produce adsorbed bicarbonates; then, the bicarbonates were further converted to formates, followed by the formation of CH4*. Cr3+ doping increased the number of surface oxygen vacancies and hydroxyl groups, thus increasing the amount of bicarbonates and formates. Consequently, Cr doping strongly promoted the formate pathway, greatly improving the activity of the Ru/Ce0.9Cr0.1Ox catalyst at low temperatures.