Energetics of methane dissociative adsorption on Rh{111} from DFT calculations

We present DFT calculations of methane activation and CHx (image) adsorption at a Rh{111} surface and compare our results with data for other elements. The activation mechanism has similar energetics as found for other transition metal surfaces, where the first and last steps are the most difficult. On Rh{111}, the CH dehydrogenation barrier is the highest. The CH radical is also the most stable fragment. The barrier that we find for the first methane activation step on Rh{111} is relatively low, lower than expected from comparing it with that step on Ru{0001} [I.M. Ciobîcă, F. Frechard, R.A. van Santen, A.W. Kleyn, J.P.J. Hafner, J. Phys. Chem. B 104 (14) (2000) 3364–3369] or Ni{111} [R.M. Watwe, H.S. Bengaard, J.R. Rostrup-Nielsen, J.A. Dumesic, J.K. Nørkov, J. Catal. 189 (1) (2000) 16–30] in terms of the Brønsted–Evans–Polanyi relation. Therefore, methane is likely more easily activated on Rh{111} than on Ru{0001} or Ni{111}. Adsorption on Rh{111} in general favors hollow sites, but the energy differences between sites are often image.