Chemisorption and Reactivity of CHx (x = 0−4) on Fe−Co Alloy Surfaces

Abstract

The present work reports results from density functional theory studies of the methanation on the FeCo(110) surface for the surface coverage below 0.25 ML. Except for CH3 which favors the adsorption at the long-bridge Fe sites, all CHx (x = 0−2) species are chemisorbed preferentially at the long-bridge Co and 3-fold Fe sites. On the other hand, CH4 is only physisorbed on FeCo(110) with the tripod configuration and does not exhibit an apparent site preference. The effects of surface coverage on the adsorption energies are not significant when the surface concentration of CHx increases from 0.125 to 0.250 ML; but geometrically, the less compact p(√2 × √2)R45 is more favorable than the p(1 × 2) configuration. Among all possible C1 species, CH is identified to be the most dominant on the surface, and the resulting kinetic potential energy surface, which includes the lateral interactions between coadsorbed species, shows that the formation of CH4 is exothermic. These observations are in good agreement with the experimental observations that FeCo catalysts, which are immune to carbon deposition in the Fischer−Tropsch (FT) synthesis, possess similar FT catalytic properties to the conventional Co catalysts.