Ab initio cluster model approach to the chemisorption of NH3 on Pt(111)

The chemisorption of NH, on Pt(111) has been studied using several Pt,, clu sters that model different adsorption sites of the Pt(lll) surface. Ab init io methods have been used to obtain a theoretical estimate of several spect roscopic features that can be directly compared to experiment. The comparis on includes the variation of the difference between the 3a(1) and le levels , the vibrational frequency shifts and the order of stability on different surface sites. Chemisorption at the on-top site is predicted to be favoured , the calculated interaction energy appears to be quite close to the experi mental estimate, and it is suggested that NH3 chemisorbs molecularly, in an N-down orientation, with an equilibrium geometry representing a small dist ortion from the gas-phase molecular geometry and no azimuthal preference, i n good agreement with ESDIAD experiments. Constrained space orbital variati on (CSOV) analysis of the interaction has also been performed using a Hartr ee-Fock wave function. This analysis shows that the leading bonding mechani sms are substrate polarisation and charge transfer from ammonia to the surf ace. (C) 1999 Elsevier Science B.V. All rights reserved.