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Transient Kinetic Study of the Oxidation and Hydrogenation of Carbon Species Formed during CH4/He, CO2/He, and CH4/CO2Reactions over Rh/Al2O3Catalyst

Abstract The dissociation of CH 4 and CO 2 on 0.5 wt% Rh/Al 2 O 3 catalyst has been investigated at 650°C using transient techniques with on-line mass spectrometry. The dissociation of CH 4 results in the formation of large amounts of gaseous H 2 and carbonaceous species (C x H y , y  ≅ 0) on the surface after 10 min of reaction. Oxidation of these carbon species to CO 2 proceeds with an intrinsic activation energy of 63 kJ mol −1 , while hydrogenation to CH 4 with an intrinsic activation energy of the order of 240 kJ mol −1 . On the other hand, dissociation of CO 2 results in the formation of much lower quantities of carbon species under the same reaction conditions. In this case, two kinds of carbon species were identified. Hydrogenation of the main carbon species proceeds with an activation energy of 96 kJ mol −1 , while its oxidation proceeds with significantly different kinetics than the carbon derived from CH 4 dissociation. Characterization of carbon accumulated on the catalyst surface during reforming reaction of CH 4 with CO 2 has also been performed. It was found that this carbon mostly originates from the CO 2 molecule and it is significantly more reactive than the carbon derived from CH 4 decomposition, but of similar reactivity as the carbon derived from CO 2 dissociation. Temperature-programmed hydrogenation (TPH) experiments of the carbonaceous species formed during reforming reaction at 650°C revealed three different kinds of carbon species. The carbon species with the largest quantity hydrogenated to CH 4 , according to the TPH response, is found to be associated with an intrinsic activation energy of 125 kJ mol −1 for its hydrogenation process.