We performed semi-empirical AM1 molecular orbital calculations to study the dissociation of ammonia near reconstructed clean and stepped Si(111)-2 x 1 surface models. We also investigated the NH3→ NH2 + H process in the gas phase in the absence and presence of external electrostatic fields by ab initio calculations including electron correlation. It was found that the reaction mechanism near the surface essentially differs from that of the dissociation in the gas phase because in the former case both the leaving hydrogen atom and the remaining NH2 fragment bind to adjacent silicon atoms of the surface thereby making the structure of the transition-state complex different. The presence of a step on the reconstructed surface leads to a reduction of the activation energy by 41 kJ/mol. Comparative calculations indicate that this is due to the electrostatic effects, the enhanced field provided by the presence of the reconstructed step stabilizes the transition state of the reaction that is much more polar than the initial state. Our study provides evidence for electrostatic catalysis in this specific case which means that, like in case of enzymes and some other condensed systems, the strong electrostatic field of the catalyst stabilizes the transition- state complex that is much more polar than the initial state. (C) 2000 Elsevier Science B.V.
ASJC Scopus subject areas
- Condensed Matter Physics
- Physical and Theoretical Chemistry