Reaction Behavior of NO Molecule on Surface of Mn Particle (M = Ru, Rh, Pd, and Ag; n = 13 and 55): Theoretical Study of Its Dependence on Transition-Metal Element.

Abstract

Reaction of NO molecule on M13 and M55 clusters (M = Ru, Rh, Pd, and Ag) was theoretically investigated to elucidate the reason why its reaction behavior depends on the position of metal element in the periodic table. DFT computations show that NO dissociative adsorption occurs on M = Ru and Rh, NO molecular adsorption occurs on M = Pd, and NO dimerization occurs on M = Ag, which agree with experimental findings. The d-band center and d-band top become lower in energy following the order Ru > Rh > Pd > Ag; this is one of the characteristic features of the periodic table. In Ag cluster, the valence band-top consists of Ag 5s orbital and its energy is higher than the d-band top of Pd. For NO dissociative adsorption, the M-N and M-O bond formations are crucially important at the transition state and the product, to which the metal d orbital contributes very much. Ru and Rh clusters have high energy d-band center and d-valence band top, leading to the formation of strong M-N and M-O bonds. Pd and Ag clusters have low energy d-band center and d-band top, leading to the formation of weak M-N and M-O bonds. Because Ag cluster has high energy 5s valence band which can overlap well with the \uf070*+\uf020\uf070* MO of ONNO (NO dimer) moiety due to the same symmetry, charge-transfer (CT) occurs from Ag cluster to the \uf070*+\uf020\uf070* MO, which is indispensable for NO dimerization. The 4d-valence band top of Ru and Rh clusters does not fit to the \uf070*+\uf020\uf070* MO because of the different symmetry. Though the d-valence band top of Pd cluster can overlap with the \uf070*+\uf020\uf070* MO, its energy is low, which is not good for the CT. Thus, the reactivity of metal cluster for NO is determined by the energy and type (4d or 5s) of the valence band top, which both depend on the position of element in the periodic table; accordingly, Ru and Rh clusters are reactive for NO dissociative adsorption, Ag cluster is reactive for NO dimerization, but Pd cluster is not reactive for both and only NO molecular adsorption is possible.