Lateral interaction of CO and H 2 molecules on ZnO surfaces: an AM1 study  J.B.L. Martins a, * , C.A. Taft b , S.K. Lie c , E. Longo d a Universidade de Brası ´lia, Instituto de Quı ´mica, Caixa Postal 04478, CEP 70919-970, Brası ´lia, DF, Brazil b Centro Brasileiro de Pesquisas Fı ´sicas, Departamento de Mate ´ria Condensada e Fı ´sica Estatı ´stica, R. Xavier Sigaud, 150, Rio de Janeiro, R.J., CEP 22290-180, Brazil c Universidade Federal Fluminense, Instituto de Fı ´sica, Nitero ´i, R.J., Brazil d Universidade Federal de Sa ˜o Carlos, Departmento de Quı ´mica, CP 676, Sa ˜o Carlos, SP, CEP 13560-905, Brazil Received 8 January 1999; received in revised form 14 October 1999; accepted 17 November 1999 Abstract We have studied the effects of lateral interactions for CO and H 2 adsorbed on large (ZnO) 60 cluster models. The calculations were performed with the AM1 semi-empirical method. The geometric parameters of the adsorbed molecules were fully optimized. CO interacts with the zinc cation located at the site having the lowest coordination at the edge sites between the (0001) and 10  10 surfaces. The binding energy is increased as we increase the number of adsorbed CO molecules on the ZnO surface. For H 2 molecular interaction, the calculated energy gaps and ionization potentials are modified relative to the bare cluster. We have analyzed the optimized geometric parameters, charge transfer as well as the density of states and compared our results with available experimental data such as density of states, vibrational frequencies, adsorption energies and surface charge.  2000 Elsevier Science B.V. All rights reserved. Keywords: ZnO surface; (ZnO) 60 clusters; CO and H 2 adsorption; AM1 semi-empirical model 1. Introduction ZnO is a wurtzite type semiconductor, which is widely used as a catalyst. The adsorption of CO and H 2 on ZnO surfaces play an important role in the synthesis of methanol [1]. The lateral interactions of CO and H 2 molecules on ZnO surfaces have been studied by Temperature Programmed Desorption (TPD) and Infrared (IR) techniques [2,3]. Our theore- tical studies of H 2 and CO interaction with ZnO surfaces include ab initio and semi-empirical MNDO, AM1 and PM3 methods [4–10]. Spectral studies of interaction between co-adsorbed molecules on solids are not only of great importance for understanding the mechanisms of catalysis, but also provide unique information about the structure and properties of polycrystalline surfaces. Adsorption of diatomic molecules may result in discrete shifts of IR bands of pre-adsorbed surface species, due to successive occupation of the surrounding surface sites [11]. Boccuzi [12,13] has shown that adding carbon monoxide to pre-adsorbed hydrogen on the ZnO surfaces results in a shift and splitting of the Zn–H vibration band into three peaks which was assigned to structures having unequivalent number Journal of Molecular Structure (Theochem) 528 (2000) 161–170 0166-1280/00/$ - see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S0166-1280(99)00498-4 www.elsevier.nl/locate/theochem  Presented at the 9th Brazilian Symposium (SBQT), Caxambu, Brazil. * Corresponding author. Fax: +55-61-273-4149. E-mail address: lopes@unb.br (J.B.L. Martins).