Electrostatic effects on the catalytic cleavage of ammonia at the stepped Si(111)-2 x 1 surface

We performed semiempirical 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 i nitio calculations including electron correlation. It was found that the re action mechanism near the surface essentially differs from that of the diss ociation in the gas phase because in the former case both the leaving hydro gen atom and the remaining NH2 fragment bind to adjacent silicon atoms of t he surface thereby making the structure of the transition-state complex dif ferent. The presence of a step on the reconstructed surface leads to a redu ction of the activation energy by 41 kJ/mol. Comparative calculations indic ate that this is due to the electrostatic effects, the enhanced field provi ded by the presence of the reconstructed step stabilizes the transition sta te of the reaction that is much more polar than the initial state. Our stud y 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 S cience B.V. All rights reserved.