Addressing Key Unanswered Questions Regarding the Mechanism of Ethylene Hydrogenation on Pt Single Crystals.

摘要

Since its first proposal by Horiuti and Polanyi, the mechanism of the catalytic hydrogenation of ethylene over Pt has been the subject of much debate, and there are some pending questions still regarding the transformations that take place on the surface of the working catalysts. Model studies on ideal transition metal surfaces under ultra-high vacuum (UHV) conditions have allotted very useful information regarding the possible pathways for this reaction. However, past findings from studies with model systems do not always correlate with results obtained under more realistic catalytic conditions.;This work has focused on implementing a recently developed operando setup installed in a UHV chamber to follow the evolution of the gas-phase and surface species simultaneously during the catalytic hydrogenation of ethylene at atmospheric pressures (~760 Torr) by means of mass spectrometry (MS) and reflection absorption infrared spectroscopy (RAIRS). Corroborating past reports, our results have shown that the ethylidyne moieties product of ethylene adsorption on the Pt (111) crystal during reaction are hydrogenated at a much slower rate than gas phase ethylene, ruling them out as a possible intermediate. In addition, it was also found that at low enough ethylene pressures the formation of ethylidyne could be significantly decreased, suggesting that the catalytic reaction takes place over a seemingly clean Pt (111) surface.;Additional surprising results have been obtained during the probing of the dissociative adsorption of hydrogen as the possible rate-limiting step in the catalytic hydrogenation of ethylene, which is being studied simultaneously following the hydrogen-deuterium (HD) exchange conversion during reactions with H2+D2+C2H4 mixtures. Results show that the rate of HD exchange does not follow linear kinetics, it displays a strong dependence with the pressure of gas phase ethylene and it seems to be minimally affected by changes in ethylidyne surface coverage.