Insight into the Nanoparticle Growth in Supported Ni Catalysts during the Early Stage of CO Hydrogenation Reaction: The Important Role of Adsorbed CO Molecules

In heterogeneous catalysis, reactive gases often accelerate the growth of catalytically active metal nanoparticles (NPs) by the formation of volatile metal-molecule intermediates, leading to undesired catalyst deactivation. This gas-enhanced particle growth is usually described by Ostwald ripening, in which the volatile metal-molecule intermediates are assumed to be transported from small metal NPs to large metal NPs. In this contribution, we demonstrated the strong steric hindrance effect of the adsorbed CO molecules on the transport of Ni(CO) 4 molecules between the Ni NPs during the early stage of CO hydrogenation reaction. Extensive analysis of the growth behaviors of different-sized Ni NPs revealed a critical concentration for the Ni(CO) 4 decomposition on the surface of Ni NPs, which was confirmed by Born–Oppenheimer molecular dynamics (BOMD) simulations. By considering the existence of the critical concentration, the modified Ostwald ripening model successfully described the main features of particle growth observed experimentally. In addition, the Ni(CO) 4 decomposition rate was found to be accelerated by the formation of Ni 3 C phase, because of the weak steric hindrance effect of CO molecules on these surfaces. The role of the adsorbed molecules found herein may provide an important prospective for understanding the growth behaviors of highly dispersed metal NPs in the presence of reactant gases.