| 内容提要: |
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 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. |