Different modification effects of carbidic and graphitic carbon on Ni surfaces

The dehydrogenation of cyclohexene was used as a probe reaction to examine the effect of carbide and graphite modifications on Ni surfaces. The carbon-modified surfaces were first characterized with Auger electron spectroscopy (AES) and near edge X-ray absorption fine structure (NEXAFS), where the transformation from carbidic to graphitic carbon was detected after heating the carbon overlayer to 900 K. The reaction pathways of cyclohexene on the modified Ni surfaces were then compared with those on the unmodified Ni surface using temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS). Thermal desorption studies showed that on the clean Ni surface, 39% of the adsorbed cyclohexene underwent dehydrogenation to benzene, while the remaining 61% completely decomposed to surface carbon and gas-phase hydrogen. The formation of carbide significantly modified the surface chemistry of Ni. For example, upon carbide modification, the selectivity toward benzene increased to 79%. After converting the carbide to graphite, the overall surface activity was reduced by a factor of 2.2, but the selectivity toward benzene remained at 79%. Vibrational studies further revealed that cyclohexene was converted to benzene by 200 K on the modified and unmodified Ni surfaces, and that the degree of interaction was different between benzene and the three surfaces.