Comprehensive Density Functional and Kinetic Monte Carlo Study of CO₂ Hydrogenation on a Well-Defined Ni/CeO₂ Model...

A detailed multiscale study of the mechanism of CO₂ hydrogenation on a well-defined Ni/CeO₂ model catalyst is reported that couples periodic density functional theory (DFT) calculations with kinetic Monte Carlo (kMC) simulations. The study includes an analysis of the role of Eley-Rideal elementary steps for the water formation step, which are usually neglected on the overall picture of the mechanism, catalytic activity, and selectivity. The DFT calculations for the chosen model consisting of a Ni₄ cluster supported on CeO₂ (111) show large enough adsorption energies along with low energy barriers that suggest this catalyst to be a good option for high selective CO₂ methanation. The kMC simulations results show a synergic effect between the two 3-fold hollow sites of the supported Ni₄ cluster with some elementary reactions dominant in one site, while other reactions prefer the another, nearly equivalent site. This effect is even more evident for the simulations explicitly including Eley-Rideal steps. The kMC simulations reveal that CO is formed via the dissociative pathway of the reverse water-gas shift reaction, while methane is formed via a CO₂ → CO → HCO → CH → CH₂ → CH₃ → CH₄ mechanism. Overall, our results show the importance of including the Eley-Rideal reactions and point to small Ni clusters supported on the CeO₂ (111) surface as potential good catalysts for high selective CO₂ methanation under mild conditions, while very active and selective toward CO formation at higher temperatures.