A theoretical investigation of methane dissociation on Rh(111)

Methane dissociation (CH₄ → CH_x + (4 - x) H → C + 4H) over a cluster model of the Rh(111) surface has been investigated using a quasi-relativistic density functional method. The main aim is to evaluate the dissociation energies which may determine catalytic activity of the metal in the partial oxidation of methane to syngas. Dissociative adsorption of methane can be initiated on the Rh surface. However, the abstraction of H from CH₄ to produce gas-phase CH_3,g radical is energetically unfavorable. This would explain the absence of C₂ products. The presence of adsorbed oxygen at an on-top site promotes methane dehydrogenation while that at a hollow site exerts the opposite effect. All calculated dissociation energies of CH_x,s are compatible with those obtained with the bond-order conservation Morse-potential approach. The combination and desorption of surface species, which are critical to the formation of syngas, are also discussed.