Paper SS+AS+EN-MoM8
C₂ Hydrogenation at Ambient Pressure on Pt(111)

Monday, November 10, 2014, 10:40 am, Room 309

Carbon has been shown to be the decomposition product from catalytic reactions involving hydrocarbons adsorbed on metal catalysts. Its presence reduces the amount of active surface sites available during a reaction. The decomposition products from adsorbed acetylene and ethylene on Pt(111) are C₂ and C₁ species, respectively. A previous UHV study showed that C₂H₂ adsorbed on Pt(111) at 750 K immediately decomposes to mostly C₂ species. H₂ was then coadsorbed with C₂ at 85 K and annealed to 400 K, which produced ethylidyne (CCH₃), ethynyl (CCH), and methylidyne (CH) species. None of the species were hydrogenated to ethylene or ethane, and after annealing to 750 K, a percentage of the carbon on the surface could be rehydrogenated after cooling the crystal to 300 K and coadsorbing H₂.

In this study, the hydrogenation of C₂ species in 1×10^-2 to 1 Torr of H₂ was monitored with RAIRS. The species was created on Pt(111) with C₂H₂ adsorption at 750 K as done previously and the crystal was cooled to 300 K. The crystal was then annealed in an ambient pressure of H₂. The C₂ species are hydrogenated to ethylidyne at 400 K and then to ethane at approximately 450-500 K. This reaction is shown to be dependent on the pressure of H₂. The results show that ethylidyne will be hydrogenated at 450 and 500 K at 1.0 and 1×10^-2 Torr H₂, respectively. To show that the C₂ species are fully hydrogenated and desorbed as ethane, which indicates that the surface is clean, CO was leaked into the cell with H₂. We observe after the 500 K anneal that the peak assigned to the CO species is similar in intensity to one from CO adsorbed on a clean surface. This indicates that there are no C₂ species remaining on the surface. Also, the peak positions of the terminal and bridge sites are shifted, which indicates that there is a high coverage of H atoms adsorbed on the surface.