Quantum chemistry study on the nature of the CO and H 2 activation over CuO-ZnO catalysts for methanol synthesis

Abstract

Self-consistent charge-discrete variation-X α (SCC-DV-X α) quantum chemical calculations were used to study the nature of the activation of CO and H 2 on CuO-ZnO catalysts for methanol synthesis. Based on X-ray photoelectron spectroscopy, electron spin resonance and temperature programmed desorption mass spectrometry investigations (L.F. Chen and K.H. Huang, J. Catal. (in Chinese), 10 (1989) 14; W.J. Lai and K.H. Huang, J. Chem. Phys. (in Chinese), 3 (2) (1990) 86.) into CO and H 2 activation adsorption, a model of CO adsorption on the CuO-ZnO cluster and a model of H 2 adsorption on the (101̄0) surface cluster of ZnO were proposed. The results show that CO is terminally bonded to the Cu atom of the CuO-ZnO cluster and its activation is caused through transfer of 0.30 electrons from the σ molecular orbitais of CO to the Cu and back-donation of 0.23 electrons from Cu to the lowest unfilled 2π ∗ orbital of the adsorbed CO molecule. Moreover, DV-X α calculations for the Zn 8O 8 cluster model on the (101̄0) surface of ZnO for the proposed adsorption of the hydrogen species show that the density of states of Zn 8O 8 is in good agreement with known ultraviolet photoelectron spectroscopy results. The lowest unoccupied molecular orbital (LUMO) of cluster model is mainly composed of surface states Zn(4s + 4p), and consists of a linear combination of the sp 3 hybridized orbitals centred on the surface Zn ions and directed from the surface along the tetrahedral direction. The highest occupied molecular orbital (HOMO) is mainly made up of a 2p x + 2p y + 2p z surface orbital on O 2p. Both frontier orbitals play a role in H − or H 0 adsorption on Zn 2+ and H 0 or H + adsorption on O 2−. The calculations also show that H 2 adsorbs heterolytically on ZnO, yielding Zn 2+-H − and O 2−-H +, which are consistent with the IR spectra.