When Coverage is Key: The Lowest (Free) Energy Reaction Pathway Does Not Always Properly Predict the Evolution of Heterogeneously Catalyzed Reactions

The design of novel catalysts requires a deep understanding of the events occurring at the microscopic scale: adsorption and desorption of molecules, diffusion, and bond breaking and bond forming events. These elementary processes that occur at the interface between the catalyst and the reaction fluid can be studied to a great level of detail, most often, by means of density functional theory (DFT) calculations. However, the temporal evolution at the catalyst surface is a result of an intricate interplay between all elementary events and the interaction between different intermediate species and the surrounding environment. Kinetic modeling techniques, and in particular kinetic Monte Carlo (kMC) simulations that can capture most of the effects and complexity of real catalysts, unravel the dominant reaction mechanisms and provide fundamental understanding towards the rational design of novel catalysts. In this work, we combine DFT calculations with kMC simulations to study the reverse water-gas shift (RWGS) reaction on Ni/TiC, a bifunctional catalyst, as a case example where the predictions from DFT (free) energy profiles are in complete disagreement with the outcome of kMC simulations, evidencing that the coverage of surface species and site blocking of actives sites plays a crucial role on the actual catalytic activity and dominant reaction mechanism. The kMC simulations also unveil the origin of the synergic effects in Ni/TiC, which is two orders of magnitude more active than the clean TiC surface. Finally, our simulation results are in remarkable agreement with the experimental data, proving that the complex chemistry of the RWGS reaction on a bifunctional catalyst can be correctly captured by the kMC approach.