Adsorption and decomposition of acetylene on vanadium (100), (110), and (111) surfaces; The effect of large d orbitals

Abstract

Four physical variables affect C≡C bond activation on transition metal surface: metal atom arrangement, d band position, d band occupation and d orbital size. The large d orbitals of vanadium are found in an atom superposition and electron delocalization molecular orbital theory study to be responsible for the high activity of vanadium surfaces compared to iron, platinum and nickel. The large d orbitals lead to large vanadium d-carbon p overlap integrals. Acetylene is predicted to dissociate with no barrier into CH fragments at all sites on all large surface cluster models studied. On the μ π sites of V(111) interaction is found between acetylene and a third-layer atom of this open surface. Interaction with the third layer cause the CH fragments, which form when acetylene dissociates, to bond tilted to the surface in a three-fold site involving first-, second- and third-layer atoms. Barriers to CH dissociation on the surfaces are also relatively small because of high orbital overlap between the hydrogen 1s and large vanadium d orbitals. On small 6- and 7-atom clusters of the (111) surface a small barrier to C≡C dissociation is found in a μ π site and for undissociated acetylene in this site the tilting of the molecule brings a CH close to a top-layer metal atom. A σ donation interaction from CH to the surface atom results in a significant bond stretch.