The mechanism of the hydrogenation of olefins catalyzed by metal surfaces was probed by using isotope labeling in conjunction with a high-flux effusive molecular beam setup capable of sustaining steady-state conversion under well-controlled ultrahigh vacuum (UHV). The unique conditions afforded by this instrument, namely, a single collision regime and impinging frequencies equivalent to pressures in the mTorr range, led to the clear identification of two competing pathways: a multiple H-D isotope exchange channel explained by the well-known Horiuti-Polanyi mechanism but with an unusually high probability for β-hydride elimination from the alkyl surface intermediate (versus its reductive elimination to the alkane), and a direct addition route that produces dideuterated alkanes selectively. The latter may follow an Eley-Rideal mechanism involving an adsorbate (either the olefin or the hydrogen/deuterium atoms resulting from dissociative adsorption of H2/D2) and a gas-phase molecule (the other reactant), or, alternatively, it could reflect the limited diffusion of the hydrogen atoms on the surface under catalytic conditions because of site blocking by the islands of strongly bonded carbonaceous (alkylidyne) layers present during catalysis. Regardless, our data clearly show that the distribution of alkane isotopologues obtained from the conversion of olefins with deuterium can deviate significantly from statistical expectations.
The effect of carbonaceous deposits on the performance of Pt(111) surfaces as catalysts for the hydrogenation of ethylene was tested by decoupling their preparation, which was done beforehand in an ultrahigh vacuum (UHV) environment, from the catalytic runs, which were carried out in an enclosed "high pressure" cell. The time evolution of the gas composition during reaction was followed continuously by mass spectrometry, and the nature of the surface species was determined by in situ reflection−absorption infrared absorption spectroscopy (RAIRS). Reaction rates were seen to vary by up to approximately 40% depending on the type of molecules preadsorbed at room temperature, with the maximum activity seen with a propylidyne layer, the result of propylene adsorption. The rate with ethylidyne-covered Pt( 111) is reduced by ∼20%, with butylidyne by ∼35%, and with benzyl moieties by ∼40%. These changes are reversible: the surface regains the activity expected when starting with the clean substrate after one or two catalytic runs to full conversion. RAIRS data show that this is because the initial species are slowly replaced by a new layer of the adsorbate that forms with the olefin in the reaction mixture (ethylidyne for ethylene hydrogenation). More significant changes are seen if the adsorbed layers are dehydrogenated at higher temperatures: the turnover frequencies for ethylene hydrogenation are reduced by more than an order of magnitude upon the conversion of propylidyne to C n H(ads) species, by annealing at temperatures between 500 and 650 K. Some activity is regained upon annealing to even higher temperatures, presumably because of the formation of graphitic layers, which have a smaller footprint on the surface. It was concluded that although the main role of carbonaceous deposits in catalytic hydrogenations is to block surface sites, their influence is also affected by the reversibility of their bonding to the surface in a hydrogenation atmosphere and by the structure of their carbon skeleton.
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