Oxygen-Free Conversion of Methane to Higher Alkanes through an Isothermal Two-Step Reaction on Platinum (EUROPT-1): I. Chemisorption of Methane

When submitted to CH4at atmospheric pressure, EUROPT-1 (a 6.3 wt% Pt/SiO2catalyst) is able to chemisorb CH4with parallel evolution of H2atT≥ 150°C. C2H6is 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 CH4can be removed from the surface mainly in two peaks, one at ≈60°C and the second at a temperature varying between 180 and 250°C. Formation of CH4, accompanied by small amounts of C2H6and traces of C3H8, is observed during subsequent temperature-programed surface reaction with H2of the adspecies remaining after each TPD interrupted at 300°C. The total amount of CH4initially chemisorbed can be easily derived from those experiments and the H/C ratio of the surface species formed during chemisorption can also be estimated. It has been shown that the amount of chemisorbed CH4increased and the H/C 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 CH4content 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.