Softened C-H modes of adsorbed methyl and their implications for dehydrogenation: An ab initio study

To investigate the softening of CH vibrational frequencies and their implic ations for dehydrogenation of adsorbed hydrocarbons, an issue of scientific and technological importance, density functional theory calculations have been performed on the chemisorption and dehydrogenation of CH3 on Cu(111) a nd Pt(111) surfaces. By comparing these results with those of Ni(111) we fi nd that the CH bonds of the adsorbate, when close enough, interact with met al atoms of the surface. It is this interaction and its associated lengthen ing and weakening of CH bonds that is the physical origin of mode softening . We rule out the possibility of a relationship between the mere presence o f mode softening and dehydrogenation. We do show, however, that there is a clear relationship between the extent to which a surface can induce mode so ftening and the activation energy to dehydrogenation. In addition, periodic trends concerning the extent of mode softening are reproduced. (C) 2001 Am erican Institute of Physics.