EFFECT OF HYDROGEN ON METHANE CONVERSION TO HYDROCARBONS IN ONE-STEP REACTION UNDER NONOXIDATIVE 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 un der non-oxidative conditions at low temperature was here first introdu ced and investigated over Co-Pd/SiO2 catalysts at 250 degrees C as a f unction of hydrogen concentration in helium and of catalyst compositio n. A maximum in the production of C2+ hydrocarbons including aromatics (benzene and toluene) was observed at 1.3 vol% H-2/He mixture in whic h one pulse of methane was introduced. Additional hydrogenation with t he same H2/He mixture at 400 degrees C was efficient to remove the lar ger hydrocarbon fragments already existing on the surface. On pure Pd/ SiO2 the one-step process is not so efficient as on cobalt-rich sample s, but in the latter case the hydrocarbon removal is the most efficien t during high-temperature hydrogenation. It was found that methane con version in the one-step process is at least 2.5 times greater than tha t measured in the ''two-step'' process and, in some cases, 80% of the methane introduced is converted to larger hydrocarbons. The results ar e discussed in terms of the hydrogen coverage ensuring the optimum hyd rogen content in the surface CHx species leading to chain growth.