Periodic trends of oxygen vacancy formation and C-H bond activation over transition metal-doped CeO ₂ (1 1 1) surfaces

Substitutional transition metal dopants in the cerium oxide surface can alter the surface reducibility and catalytic activity for hydrocarbon conversion. Density functional theory (DFT + U) methods are used to examine the electronic and structural effects of transition metal dopants (groups IV-XII) in the CeO ₂ (1 1 1) surface. Surface reducibility (oxygen vacancy formation) and dissociative adsorption of methane (forming H ^* and CH ₃ ^*) are considered. Both the methane dissociative adsorption energy and activation barriers correlate linearly with the surface oxygen vacancy formation energy. Charge analysis is used to determine the role of dopant metal in serving as a reduction center or altering the Ce reducibility. Dopants in groups IV and V alter the reducibility of the surface and dopants in groups X-XII become the reduction center. The dopant plays the same role in both oxygen vacancy formation and methane adsorption. A Bronsted-Evans-Polanyi relationship is established between the methane activation barrier, through H-abstraction, and the dissociative adsorption energy. The sensitivity of quantitative and qualitative trends to the inclusion of U terms for the dopant transition metal d-states is considered. The optimal M/CeO ₂ dopant for methane conversion to CO or CO ₂ follows a volcano relationship with oxygen vacancy formation: Highly reducible surfaces will be limited by re-oxidation, whereas surfaces difficult to reduce will show high barriers for C-H bond activation. Transition metal dopants near the peak region of the volcano are Pd, Co, Ni, and Mn.