Oxygen-free conversion of methane to higher alkanes through an isothermal two-step reaction on platinum (EUROPT-1) .1. Chemisorption of methane

When submitted to CH4 at atmospheric pressure, EUROPT-1 (a 6.3 wt% Pt/SiO2 catalyst) is able to chemisorb CH4 with parallel evolution of H-2 at T greater than or equal to 150 degrees C. C2H6 is also produced but at a rate approximately 10 times lower, so that H-deficient hydrocarbonaceous fragments build up on the metal surface. Temperature-programed desorption spectra obtained just after chemisorption show that CH4 can be removed from the surface mainly in two peaks, one at approximate to 60 degrees C and the second at a temperature varying between 180 and 250 degrees C. Formation of CH4, accompanied by small amounts of C2H6 and traces of C3H8, is observed during subsequent temperature-programed surface reaction with H-2 of the adspecies remaining after each TPD interrupted at 300 degrees C. The total amount of CH4 initially chemisorbed can be easily derived from those experiments and the WC ratio of the surface species formed during chemisorption can also be estimated. It has been shown that the amount of chemisorbed CH4 increased and the WC ratio strongly decreased when the temperature or the duration of exposure was increased. When EUROPT-1 was submitted to (Ar, CH4) mixtures instead of pure CH4, the increase of the CH4 content had an effect similar to that of the increase of either the temperature or the duration of exposure. Moreover the TPD and TPSR spectra are very sensitive to the conditions of the exposure, so that they constitute some fingerprint of the adspecies at the end of the adsorption step. (C) 1996 Academic Press, Inc.