Coordination chemistry of metal surfaces. 3. Benzene and toluene interactions with nickel surfaces

Ni surfaces were the low Miller index planes (111), (110), and (100) and the stepped and stepped-kinked surfaces 9(111) x (111) and 7(111) x (310). Effects of surface C and O were established for low surface coverages. Benzene chemisorption was fully associative (molecular) on Ni(111) up to 100/sup 0/C; no H-D exchange occurred between C/sub 6/H/sub 6/ and C/sub 6/D/sub 6/ up to 100/sup 0/C. Up to 100/sup 0/C, benzene was displaced as C/sub 6/H/sub 6/(g) by P(CH/sub 3/)/sub 3/. Benzene reversible thermal desorption occurred at 115 to 125/sup 0/C, accompanied by extensive irreversible decomposition to H/sub 2/(g) + Ni(111)-C. Analogous behavior was observed for benzene chemisorbed on the stepped and stepped-kinked surface. On the (110) surface, the benzene thermal desorption and benzene chemical displacement by P(CH/sub 3/)/sub 3/ was comparable to the stepped-kinked surface. Chemisorption of benzene on Ni(100) was differentiated from the other surfaces: the degree of thermal reversibility was higher than and the temperature for the thermal desorption maximum was 100/sup 0/C higher than that for Ni(111). H-D exchange did not occur with D/sub 2/ + C/sub 6/H/sub 6/ and with C/sub 6/H/sub 6/ + C/sub 6/D/sub 6/ on Ni(100). Toluene chemisorption on all the nickel surfaces was thermally irreversible and toluene was not displaced from these surfaces by strong field ligands like P(CH/sub 3/)/sub 3/ and CH/sub 3/NC. Heating the Ni-toluene surfaces led to hydrogen evolution. For the decomposition of C/sub 6/D/sub 5/CD/sub 3/ on Ni(111) there were two D/sub 2/ thermal desorption maxima, 130 and 190/sup 0/C. For the analogous decomposition of C/sub 6/H/sub 5/CD/sub 3/ and C/sub 6/D/sub 5/CH/sub 3/ respectively, there were single D/sub 2/ desorption curves with maxima at 130 and 185/sup 0/C, respectively. Thus, aliphatic C-H bonds are broken first and no aromatic C-H bond is broken until near 160/sup 0/C. Identical behavior was observed for the stepped and stepped-kinked surfaces, and an analogous behavior was observed for the (100) surface.