Trends in adsorption energies as a function of transition metal position in the periodic system may vary with adsorption site. When the adsorption energy is compared on a transition metal from right to left along a row of the periodic system, for a particular adsorption site and adsorbate two adsorption energy patterns, that are sometimes intermixed, can be distinguished: a) there is a maximum in adsorption energy b) the adsorption energy uniformly increases The balance of σ and π symmetric electronic interactions that constitute the chemisorptive bond determines these differences largely. Partial density of states (PDOS) and bond order overlap population densities (BOOP) based on state of the art DFT electronic structure calculations are used to determine the relative weight of the σ and π electronic orbital contributions to the chemical bond. We will present results of adsorption of H, C ,N, O atoms and related adsorbates CHx, NHx , OH and CO for the dense transition metal surfaces of the 4th and 5th row. The ratio’s of electron occupied and unoccupied bonding versus antibonding surface electron density regimes usually correlate. However for systems with Fermi level close to the non-bonding valence electron density regime, differences in transition metal electron affinity become especially important. They may even invert trends based d-valence electron occupation. Comparison of the surface adsorption energies with the formation energies of the surface-molecule complex gives a grip on the reason of adsorption energy differences that deviate from scaling rule trends. The relative importance of variation in surface molecule embedding energy that relates to the strength of the metal atom bonds as well as that of the redistribution of electrons over s- and d-valence electrons in the molecular complex can be determined. We will correlate these with trends in the activation energies of methane and CH3 activation.

Site preference of chemisorption

251st National Meeting (2016)

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