Unique catalytic mechanisms of methanol dehydrogenation at Pd-doped ceria: A DFT+U study

Pd-doped ceria is highly active in promoting oxidative dehydrogenation (ODH) reactions and also a model single atom catalyst (SAC). By performing density functional theory calculations corrected by on-site Coulomb interactions, we systematically studied the physicochemical properties of the Pd-doped CeO₂(111) surface and the catalytic methanol to formaldehyde reaction on the surface. Two different configurations were located for the Pd dopant, and the calculated results showed that doping of Pd will make the surface more active with lower oxygen vacancy formation energies than the pristine CeO₂(111). Moreover, two different pathways for the dehydrogenation of CH₃OH to HCHO on the Pd-doped CeO₂(111) were determined, one of which is the conventional two-step process (stepwise pathway) with the O-H bond of CH₃OH being broken first followed by the C-H bond cleavage, while the other is a novel one-step process (concerted pathway) involving the two H being dissociated from CH₃OH simultaneously even with a lower energy barrier than the stepwise one. With electronic and structural analyses, we showed that the direct reduction of Pd⁴⁺ to Pd²⁺ through the transfer of two electrons can outperform the separated Ce⁴⁺ to Ce³⁺ processes with the help of configurational evolution at the Pd site, which is responsible for the existence of such one-step dehydrogenation process. This novel mechanism may provide an inspiration for constructing ceria-based SAC with unique ODH activities.