Polarization modulation- reflection absorption infrared spectroscopy (PM-RAIRS) bridges ultra high vacuum (UHV) studies and industrial applications by enabling the simultaneous observation of surface and gas-phase species formed during heterogeneous catalytic reactions performed at ambient pressures.  In this study, surface species and gas-phase products of acetylene (C2H2) hydrogenation on Pt (111) were monitored between 120 and 370 K using a static pressure of C2H2 and H2 in an ambient pressure IR cell connected to a UHV chamber.  When C2H2 and H2 are coadsorbed on Pt (111) at 88 K in UHV conditions, acetylene is only partially converted to ethylene at 285 K.  By using a static pressure of 1x10-2 Torr H2, a monolayer of acetylene adsorbed at 120 Kis converted completely to ethylene through a vinyl (CHCH2) surface species at 250 K.  To study the effects of gas phase acetylene hydrogenation on Pt (111), a partial pressure of 1.0�10-2 Torr of acetylene was leaked into the cell at 300 K.  At this temperature, acetylene is adsorbed as vinylidene (CCH2).  Upon adding 1.0 Torr of H2, three surface intermediates are observed: ethylidyne (CCH3), di-s-bonded ethylene (s-C2H4), and ethylidene (CHCH3).  After annealing to higher temperatures, the concentration of these species remain constant.  Besides these surface species, excess gas-phase acetylene is completely hydrogenated into gas-phase ethylene and ethane, which are observed at 350 and 370 K, respectively.  By monitoring this reaction with PM-RAIRS, surface intermediates are distinguished from gas-phase products, allowing complete characterization of a catalytic reaction not observable at UHV conditions.

Acetylene hydrogenation on Pt (111) at ambient pressure using PM-RAIRS

246th National Meeting (2013)

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