Elucidation of the higher coking resistance of small versus large nickel nanoparticles in methane dry reforming via computational modeling

Abstract

Dry reforming of methane (DRM) is a promising utilization process of greenhouse gases, namely CO₂ and CH₄. Nickel-based catalysts are the most popular ones for DRM, because they are inexpensive and relatively active but deactivate rapidly mainly due to carbon formation. Carbon gasification by either partial or complete oxidation is considered to be the main route for carbon-free catalysis. To clarify the competition between carbon deposition and carbon gasification in this computational study, we modeled the formation of C–C and C–O bonds on large and small Ni-nanoparticles (∼1 nm). We found that C prefers to penetrate into the subsurface, whereas O prefers to adsorb on the surface of both large and small nickel particles. The formation of CO at low concentrations is significantly more exothermic than C₂ formation but the C₂ moiety is formed faster than CO. At low carbon concentrations, the formation of the C–C bond is not favorable with respect to the two C species located in the subsurface region. However, at high C concentrations, on the large metal particles, multicarbon C_n species are formed; those species are potential precursors of carbon deposits such as graphene or coke. On the other hand, the flexibility of the small nickel nanoparticles allows separation of monoatomic C to remain stable as subsurface species. Thus, C_n species, considered to be precursors of carbon deposits leading to catalyst deactivation, are found to preferably form on large rather than on small Ni nanoparticles.