Effect of hydrogen on methane conversion to hydrocarbons in "one-step" reaction under non-oxidative conditions at low temperature over Pd-Co/SiO2 catalysts prepared by the sol/gel method

Methane conversion to higher hydrocarbons in a "one-step" process under non-oxidative conditions at low temperature was here first introduced and investigated over Co-Pd/SiO2 catalysts at 250 degrees C as a function of hydrogen concentration in helium and of catalyst composition. A maximum in the production of C2+ hydrocarbons including aromatics (benzene and toluene) was observed at 1.3 vol% H-2/He mixture in which one pulse of methane was introduced. Additional hydrogenation with the same H2/He mixture at 400 degrees C was efficient to remove the larger hydrocarbon fragments already existing on the surface. On pure Pd/SiO2 the one-step process is not so efficient as on cobalt-rich samples, but in the latter case the hydrocarbon removal is the most efficient during high-temperature hydrogenation. It was found that methane conversion in the one-step process is at least 2.5 times greater than that measured in the "two-step" process and, in some cases, 80% of the methane introduced is converted to larger hydrocarbons. The results are discussed in terms of the hydrogen coverage ensuring the optimum hydrogen content in the surface CHx species leading to chain growth.