A DFT investigation of the adsorption of methyl on Rh(1 1 1)

The adsorption of methyl on the Rh(1 1 1) surface at the coverage of 0.25 ML has been investigated by using ab initio total energy calculations. It was found that the fcc hollow site (with H near top) is preferred over the less stable top site by only about 10 meV. An energy barrier of 0.461 eV was identified for methyl diffusion from fcc hollow to top sites. Our calculated results are similar to those reported by Walter and Rappe [Surf. Sci. 549 (2004) 265] at the coverage of 0.33 ML except for the second most stable site and variations in mode softening. It was indicated that the diffusion pathway is coverage-dependent. The calculated C–H stretching frequencies revealed that significant mode softening occurs for hollow site adsorption with H near top. It was turned out that the C–H symmetric stretching frequencies decreased significantly as the coverage increases from 0.25 to 0.33 ML. The DOS analysis has shown that the 2a1, 1e and 3a1 orbitals of CH3 are involved in bonding with the metal and the most significant contribution to CH3–Rh bonding comes from the 3a1 orbital.