Coadsorption of methyl radicals and oxygen on Rh(1 1 1)

The chemisorption of CH3 on Rh(1 1 1) is studied to understand the origin of the weakened symmetric stretch mode. A few different explanations for this weakened mode have been suggested in previous studies. These include C–H bond depletion and donation into C–H anti-bond orbitals either in an upright or tilted geometry. We investigate these possibilities by performing first-principles density functional calculations. Our results show strong adsorption at all high-symmetry sites with methyl in two possible orientations. A thorough analysis of the adsorption geometry shows that C3v symmetry is preferred over a tilted species, ruling out tilting as a mechanism for C–H mode softening. Evidence of a multi-center bond between methyl and the surface rhodium atoms (similar to the kind shown recently by Michaelides and Hu for methyl on Ni(1 1 1)) is presented, showing that C–H bond depletion is the cause of mode-softening for methyl on Rh(1 1 1). Experimental results have shown that mode-softening diminishes when an electronegative species is coadsorbed, suggesting that donation into C–H anti-bonding orbitals is the mechanism for mode-softening. We therefore examine the coadsorption of oxygen and methyl on Rh(1 1 1). Our results suggest a new model for the effect of O on CH3. Analysis of charge density differences shows that the dominant initial effects of O coadsorption are the removal of charge from the C-surface bond and the transfer of charge to the C–H bond. Subsequent increase of the H–Rh distance further reduces mode softening.