Alloying is known to change the catalytic properties of metal catalysts significantly. This is widely exploited to optimize their activity and selectivity with regard to a desired chemical reaction. While combinatorial screening methods may relatively quickly help to find the best alloy composition this approach does not lead to a detailed understanding of the microscopic parameters which ultimately determine the reaction paths of possibly competing elementary reaction steps. This latter information, which may lead to a generalized strategy for a catalyst design, can only be obtained by the so called "surface science approach", i.e. by using structurally and chemically well defined alloy surfaces, and starting with relatively simple reactants. – Since molecules with –C=C– double bonds play a major role as intermediates in the chemical industry, in this study we have investigated the interaction of the simplest representative, namely ethene, with ordered Pt3Sn/Pt(111) and Pt2Sn/Pt(111) surface alloys as well as, for reference purposes, the Pt(111) surface. The experimental results from Temperature Programmed Desorption (TPD) spectroscopy and High Resolution Electron Energy Loss Spectroscopy (HREELS) are interpreted by the help of detailed theoretical DFT calculations using the VASP code, and lead to a very detailed and new understanding of the involved surface chemistry.
While the most favourable adsorption geometry of ethene is a di-ó conformation on all three surfaces the desorption and decomposition behaviour of ethene shows significant differences: The desorption temperature decreases with increasing Sn-concentration in the surface. Furthermore, on the Pt(111) surface a conversion to ethylidyne occurs by dehydrogenation, while this is not the case on the Sn containing surfaces. From these latter ones ethene desorbs fully intact. The most exciting result, however, is that a minority ð-bonded ethene species is found to coexist with the majority di-ó-species on all three surfaces. The equilibrium between both species shifts towards more ð-bonded ethene as the Sn content of the surface increases. This can be explained in terms of a decreasing difference in interaction energy of both species with the respective surface.
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