Catalytic Activities of Low-Coordinated Ce for Methane Conversion

In heterogeneous catalysis, the catalytic activity of an active center strongly correlates with its coordination number. In this work, density functional theory calculations corrected by on-site Coulomb interactions were conducted to investigate the catalytic performances of the CeO4-terminated reconstructed CeO2(100) (CeO4-t) and the Ca-doped CeO4-t surfaces. The undoped CeO4-t surface contains single 4-fold coordinated Ce (Ce-4c) sites on its top surface layer, and the Ca-doped CeO4-t surface can expose even lower-coordinated 3-fold Ce (Ce-3c) sites. We found that the empty 4f states of the single Ce-3c sites on the Ca-doped CeO4-t surface have much lower energies than that of the single Ce-4c sites on the undoped CeO4-t surface, which can effectively boost the reduction of Ce4+ to Ce3+. We further studied the selective oxidation of CH4 to CH3OH on the two surfaces and found that the isolated nature combined with the strong electron-reservoir character of the single Ce-3c site makes it highly desirable for the selective activation of the first C-H bond of CH4, while the subsequent C-H bond cleavage step is less affected by such low-coordinated Ce and the CH3OH formation becomes more facile on the Ca-doped CeO4-t surface, giving rise to the high activity and selectivity of the Ca-doped CeO4-t surface toward the selective oxidation of CH4 to CH3OH.

Automated summary

In this study, density functional theory calculations were used to investigate the catalytic performances of CeO4-terminated and Ca-doped surfaces. It was found that the Ca-doped surface exhibits lower-coordinated Ce sites which facilitate reduction of Ce4+ to Ce3+ and enhance the selective oxidation of CH4 to CH3OH. The isolated nature and strong electron-reservoir character of the low-coordinated Ce sites on the Ca-doped surface play a crucial role in the high activity and selectivity towards CH4 oxidation.