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

Abstract

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 surfacesmore » 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.« less