The hydrogenation and dehydrogenation of C<sub>2</sub>-C<sub>4</sub> hydrocarbons on Pt(111) monitored in situ over 13 orders of magnitude in pressure with infrared-visible sum frequency generation

Abstract

The hydrogenation and dehydrogenation of ethylene, propylene, and isobutene were monitored in situ during heterogeneous catalysis over Pt(111) between 10-10 Torr and 1000 Torr with infrared-visible sum frequency generation (SFG). SFG is a surface specific vibrational spectroscopy capable of achieving submonolayer sensitivity under reaction conditions in the presence of hundreds of Toff of reactants and products. Olefin dehydrogenation experiments were carried out with SFG under ultra high vacuum (UHV) conditions on the (111) crystal face of platinum Ethylene chemisorbed on Pt(111) below 230 K in the di-σ bonded conformation (Pt-CH2CH2-Pt). Upon annealing the system to form the dehydrogenation product, ethylidyne (M=CCH3), evidence was found for an ethylidene intermediate (M=CHCH3) from its characteristic vas(CH3) near 2960 cm-1. Hydrogenation of ethylene was carried out between 1 Toff and 700 Torr of H2 while the vibrational spectrum of surface species was monitored with SFG. Simultaneously, gas chromatography was used to obtain the turnover rate for the catalytic reaction, which could be correlated with the adsorbed intermediate concentration to determine the reaction rate per surface intermediate. Di-σ bonded ethylene, π-bonded ethylene, ethyl groups and ethylidyne resided on the surface during reaction. The mechanistic pathway for ethylene hydrogenation involved the stepwise hydrogenation of π-bonded ethylene through an ethyl intermediate to ethane. The hydrogenation of propylene was carried out under the same conditions as ethylene. It was found that propylene hydrogenates from π-bonded propylene through a 2-propyl intermediate to propane on Pt(111). The rate of reaction was approximately 50% slower than that of ethylene hydrogenation. Isobutene, however, was found to hydrogenate almost two order of magnitude slower than propylene on Pt(111).