Systematics of the binding energy of oxygen and hydrogen on transition-metal surfaces. I.

The chemisorption energy of simple gases on transition metals follows some remarkable systematic trends. We construct a simple theory which leads to an understanding of the observed trends and relates the chemisorption energy to the essential parameters characterizing the transition metals, viz., the mean energy of their density of states, the bandwidth, and the number of $d$ electrons. The theory is applied to the adsorption of hydrogen and oxygen on $3d$ and $4d$ transition metals. The positions of the atomic levels of hydrogen and oxygen (and nitrogen) with respect to the $d$ bands of the transition metals is such that the primary binding comes from transfer of $d$ electrons from surface metal atoms to the adatoms and to low-energy bonding resonances induced in the metal atoms. However, there is large enough hybridization of the adatom orbitals with the surface metal-atom orbitals that the local density of states of the metal atoms is significantly altered throughout the band. Correspondingly the metal parameters that determine the binding energy are the average position of the $d$ band with respect to the adatom orbital energy and the overall width of the $d$ band.