Methane formation from successive hydrogenation of C over stepped Ni and Ni3Fe surfaces: Effect of surface composition

As an integral part in methanation, CH₄ formation from successive hydrogenation of atomic C was systematically studied over stepped Ni(211) and Ni₃Fe(211) surfaces via periodic density functional theory calculations. The effect of surface composition was explicitly examined by taking into account of two termination structures of Ni₃Fe(211) (the surfaces with NiNi- and NiFe-type step are denoted as Ni₃Fe(211)-AA and Ni₃Fe(211)-AB, respectively). Both alloyed surfaces are found to benefit the initial C hydrogenation through enhancing H adsorption as well as weakening the binding strength of C. Because CH₂ generation and subsequent CH₂ hydrogenation favor different sites over such stepped surfaces, the step of CH₂ migration was proposed and incorporated into the mechanism. The hydrogenation reactions manifest significant structural sensitivity since Ni₃Fe(211)-AB is superior to Ni₃Fe(211)-AA in lowering the overall potential energy surfaces towards CH₄ formation. In combination with our previous work focusing on CO activation and dissociation, the present results confirmed the improved activity of Ni₃Fe alloy catalyst for CO methanation suggested experimentally and the Ni₃Fe(211)-AB termination is further identified to dominate the promoting effect. A newly proposed Brønsted–Evans–Polanyi relationship is also found to hold for the successive hydrogenation steps herein.