The Surface Chemistry of Carbon-Nitrogen Bonds on Pt(111)
Michael
Trenary
Department of Chemistry,
The formation of carbon-nitrogen bonds and the hydrogenation and dehydrogenation reactions of molecules containing CN bonds on platinum surfaces are relevant to a variety of catalytic processes such as the synthesis of hydrogen cyanide from ammonia and methane. We have used reflection absorption infrared spectroscopy (RAIRS) to investigate several aspects of CN surface chemistry on Pt(111). It is found that HCN itself will undergo a disproportionation reaction to form the aminocarbyne species, CNH2. The CNH2 species is a stable intermediate that forms from a variety of CN containing molecules and is produced when adsorbed CN is exposed to hydrogen. The appearance of the strong and characteristic CNH2 RAIR spectrum upon H2 exposure is a sensitive indicator of the presence of surface CN, which is not directly detectable with RAIRS. This method was used to study the coupling of carbon and nitrogen atoms to form CN on Pt(111) where the carbon was produced from the dissociation of methyl iodide and the surface nitrogen was produced from the electron stimulated dissociation of ammonia. In the course of these studies the CH and NH intermediates were also characterized with RAIRS. The stability of the simplest aminocarbyne, CNH2, suggests that other aminocarbynes might also be stable on Pt(111) and RAIRS has been used to identify and characterize methyl aminocarbyne (CNHCH3) and dimethyl aminocarbyne (CN(CH3)2). Methyl aminocarbyne is produced by two distinct routes: the N-protonation of chemisorbed methyl isocyanide and the partial dehydrogenation of dimethyl amine. Dimethyl aminocarbyne is produced from the partial dehydrogenation of trimethyl amine. To increase the reliability of the spectral assignments, the experimental spectra are compared with spectra calculated using density functional theory based on models consisting of the given adsorbate attached to metal clusters consisting of one to nine Pt atoms. In recent work, the determination of the adsorption site for methyl isocyanide on Pt(111) based on vibrational frequencies is compared to observations of individual methyl isocyanide molecules on Pt(111) at 4.2 K with scanning tunneling microscopy.