摘要：

We have investigated the elementary steps of the cyclization reaction of acetylene to benzene on Pd(111) by means of ab initio cluster model wave functions. The geometry of reactants, intermediates and final products has been fully optimized by total energy calculations. A vibrational analysis of some of the adsorbed species has been performed. We have identified the structure of a stable surface intermediate, C 4 H 4 , which results from the combination of two acetylene molecules. This surface species, which has already been characterized by several surface techniques, is reminiscent of metallopentacycles in organometallic chemistry; in C 4 H 4 the two C atoms directly bonded to the surface are sp 3 hybridized, while the other two C atoms assume an sp 2 -like coordination; the C 4 H 4 is tilted by ≅ 40 degrees from the surface normal. By the addition of one acetylene molecule to C 4 H 4 one obtains benzene; once formed, C 6 H 6 easily desorbs from the surface. An alternative route for benzene formation has also been considered. It implies the reaction of two C 4 H 4 units to form C 8 H 8 , cyclo-octatetraene, which easily splits off to give benzene plus an acetylene molecule. On the basis of the present calculations, however, the two routes appear to be associated to a similar thermodynamic stabilization.

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