Surface chemistry of C2 molecules on the Pt(111) surface

COLL 112

Rongping Deng, rdeng@uic.edu and Michael Trenary, mtrenary@uic.edu. Department of Chemistry, University of Illinois at Chicago, 845 West Taylor Street, Chicago, IL 60607
Surface carbon is ubiquitous in transition metal catalysis yet identification of the reactive forms of elemental carbon on surfaces is difficult. We have found that exposure of Pt(111) to acetylene at a surface temperature of 750 K produces structurally stable yet chemically reactive C2 molecules. The exposure temperature of 750 K is high enough to ensure complete dehydrogenation of acetylene, and of its decomposition products, yet is low enough to avoid aggregation of the carbon into unreactive graphitic carbon. The presence of C2 molecules is established through their reaction with hydrogen to form the ethylidyne (CCH3) and ethynyl species (CCH), which are readily identified with reflection absorption infrared spectroscopy (RAIRS). Quantification of the reactive carbon coverage indicates that essentially all of the acetylene-derived carbon is in the from of reactive C2 molecules. Deposition of the same coverage of carbon through exposure of the surface at 750 K to ethylene yields surface carbon that is largely unreactive. In addition to the reaction of C2 molecules with hydrogen, we have also found C2 to readily react with ammonia to form CN bonds. The reaction of C2 with NH3 is observed after adsorbing ammonia onto a C2-covered surface and then heating to elevated temperatures. Temperature programmed reaction spectroscopy reveals the desorption of HCN at 570 K and of C2N2 at 650 K. The carbon-nitrogen coupling reaction is clearly observed with X-ray photoelectron spectroscopy when the C2/NH3 layer is annealed from 85 to 200 K by the appearance of a chemically shifted C 1s peak at 286.6 eV, which is well resolved from the unreacted C 1s peak at 284.9 eV. The ability of surface carbon to activate the NH bonds of ammonia is relevant to catalytic processes such as the synthesis of HCN over platinum catalysts.