CO Methanation over Ni-Fe Alloy Catalysts: An Inverse Design Problem

We propose an approach to solve the inverse design problem in heterogeneous catalysis in which the goal is to find a material composition and structure that will maximize the active surface site under reaction conditions with knowledge of the active site motif being given. Taking CO methanation over Ni-Fe bimetallic alloys as our basis catalyst systems, we aim to identify a Ni-Fe bulk/surface composition that can provide the highest activity under reaction conditions. First, the stability of various (211) surfaces with different surface and bulk compositions is studied, especially if the CO adsorption could induce surface segregation has been well studied since CO is found to dominantly cover the surface during CO methanation. Due to a similar binding strength of CO over Ni and Fe, we did not observe surface segregation induced by CO adsorption. Reaction kinetics on the corresponding stable surfaces are obtained through coverage- and surface-consistent MKM. The 4-Fe Ni₃Fe(211) surface site, which corresponds to 4 Fe atoms on the surface, is the most active site among all the stable surfaces. This high activity is attributed to the presence of a pure Ni step site and an adjacent Fe site, which are particularly active for CO activation (CO + H → COH) and dissociation (COH → C + OH), respectively. Additional calculations on reaction barriers for these two rate-controlling steps on similar Ni₃Fe(211) surfaces confirmed that the 1-Fe Ni₃Fe(211) surface, despite being less stable, shows lower reaction barriers, suggesting the potential for further activity enhancement. Consequently, we propose that optimizing Ni_1-xFe_x catalysts for CO methanation may involve synthesizing Ni₃Fe catalysts with a focus on stabilizing the active site motif identified under the reaction conditions. The proposed approach offers a strategic pathway for researchers aiming to solve the inverse design problem for catalysts in other reaction systems.