Activity and coke formation of nickel and nickel carbide in dry reforming: A deactivation scheme from density functional theory

Dry reforming is a promising reaction to utilise the greenhouse gases CO₂ and CH₄. Nickel-based catalysts are the most popular catalysts for the reaction, and the coke formation on the catalysts is the main obstacle to the commercialisation of dry reforming. In this study, the whole reaction network of dry reformation on both flat and stepped nickel catalysts (Ni(1 1 1) and Ni(2 1 1)) as well as nickel carbide (flat: Ni₃C(0 0 1); stepped: Ni₃C(1 1 1)) is investigated using density functional theory calculations. The overall reaction energy profiles in the free energy landscape are obtained, and kinetic analyses are utilised to evaluate the activity of the four surfaces. By careful examination of our results, we find the following regarding the activity: (i) flat surfaces are more active than stepped surfaces for the dry reforming and (ii) metallic nickel catalysts are more active than those of nickel carbide, and therefore, the phase transformation from nickel to nickel carbide will reduce the activity. With respect to the coke formation, the following is found: (i) the coke formation probability can be measured by the rate ratio of CH oxidation pathway to C oxidation pathway (r_CH/r_C) and the barrier of CO dissociation, (ii) on Ni(1 1 1), the coke is unlikely to form, and (iii) the coke formations on the stepped surfaces of both nickel and nickel carbide can readily occur. A deactivation scheme, using which experimental results can be rationalised, is proposed.