Synthesis of Benzene from CH4 over a Ni(111) Catalyst

The ability to bypass the high pressure requirement has allowed us to carry out a high pressure reaction at low pressure: the synthesis of C6H6 from CH4. In addition, because this reaction is carried out at low pressure, we have been able to identify the adsorbed intermediates by high resolution electron energy loss spectroscopy and to determine the mechanism of this reaction. The synthesis is effected by exposing a monolayer of CH4 physisorbed on Ni(111) at 47 K to a beam of Kr atoms. The collision of the incident Kr with the physisorbed CH4 distorts the CH4 from its tetrahedral configuration, thereby lowering the barrier to dissociation into an adsorbed methyl radical and an adsorbed hydrogen atom. As the surface temperature is raised to 230 K, all the adsorbed CH3 dissociates to CH and the CH recombines to form adsorbed C2H2. Some of the C2H2 trimerizes to adsorbed C6H6 and at 410 K and 425 K, respectively, the hydrogen desorbs as H2 and some of the chemisorbed C6H6 desorbs. This study, described in J. Am. Chem. Soc. 111, 8748 (1989), shows conclusively that C6H6 is produced from CH4 on a Ni(111) surface under low pressure conditions. These data also provide mechanistic information useful to the possible extrapolation of this synthesis from molecular beam-UHV environments to more practical conditions.