Single Ni Sites Supported on CeO2(111) Reveal Cooperative Effects in the Water-Gas Shift Reaction

Density functional theory (DFT) calculations have been performed to study the water-gas shift (WGS) reaction mechanism via the formation of a carboxyl (COOH) intermediate over a low-loaded Ni/CeO2(111) model catalyst for which adatoms and tiny clusters of nickel are in direct contact with the ceria support. Strong metal-support interactions stabilize Ni2+ adatom species which favor the dissociation of O-H bonds. In this system, the rate-limiting step is the formation of COOH species. We found that if CO and water are adsorbed on neighboring Ni adatoms COOH species can form, overcoming an energy barrier below 0.5 eV, which is about 0.2 eV lower than that for an isolated single Ni site. We further show that high hydration/hydroxylation of the system significantly lowers the activation barrier for the deprotonation of the chemisorbed carboxyl species and facilitates the evolution of the WGS reaction. The results show cooperative effects between single Ni sites that render the reaction more efficient.