Computational Study of the Evolution of Ni-Based Catalysts during the Dry Reforming of Methane

Abstract

We evaluated the Ni-based catalyst surface properties during possible transformation pathways between its metallic, oxide, and carbide phases, causing catalytic deactivation. The study uses density functional theory (DFT) calculations to determine thermodynamics and reaction mechanisms of elementary reactions, and the ratings concept is introduced previously as an evaluation tool for the dry reforming reaction of methane (DRR) catalyst. The results for carbon atom adsorption strength and activation energy of higher coke formation (2C* ⇄ C–C* + *) suggest that on metallic surfaces, coke formation would be easy on the (111) facet but suppressed on the (100). Likewise, the carbide surface exposing metal atoms strongly binds to carbon and easily forms higher coke. In contrast, the oxide of Ni exhibits coke-resistant properties as it weakly adsorbs carbon. Finally, a ternary contour plot featuring metallic/oxide/carbide phases of Ni on the (111) facet was employed for identifying surface compositions, yielding highly reactive and stable DRR catalysts through a microkinetics model. It is found that, to become coke-resistant, the surface should contain less than 10% of carbide, whereas more than 75% of metallic surface is needed for the catalyst to be out of the coke formation zone, and to enter the coke removal zone, up to 80% of the metallic surface is required.