Engineering surface and subsurface oxygen vacancies of Ce_xZr_1-xO₂ solid solution for enhanced total toluene oxidation

Most studies have shown that oxygen vacancies on Ce_xZr_1-xO₂ solid solution are important for enhancing the catalytic oxidation performance. However, a handful of studies investigated the different roles of surface and subsurface oxygen vacancies on the performance and mechanisms of catalysts. Herein, a series of zirconium doping on CeO₂ samples (CeO₂, Ce_0.95Zr_0.05O₂, and Ce_0.85Zr_0.15O₂) with various surface-to-subsurface oxygen vacancies ratios have been synthesized and applied in toluene catalytic oxidation. The obtained Ce_0.95Zr_0.05O₂ exhibits an excellent catalytic performance with a 90 % toluene conversion at 295 °C, which is 68 °C lower than that of CeO₂. Additionally, the obtained Ce_0.95Zr_0.05O₂ catalyst also exhibited good catalytic stability and water resistance. The XRD and HRTEM results show that Zr ions are incorporated into CeO₂ lattice, forming Ce_xZr_1-xO₂ solid solution. Temperature-programmed experiments reveal that Ce_0.95Zr_0.05O₂ shows excellent low-temperature reducibility and abundant surface oxygen species. In-situ DRIFTS tests were used to probe the reaction mechanism, and the function of Zr doping in promoting the activation of oxygen was further determined. Density functional theory (DFT) calculations indicate that the vacancy formation energy and O₂ adsorption energy are both lower on Ce_0.95Zr_0.05O₂, confirming the reason for its superior catalytic performance.