Kinetic Analysis of Methane Pyrolysis over Carbon Catalysts: Distinction between Ordered and Amorphous Carbon Catalysts

The detailed mechanistic “surface chemistry” in kinetic models for hydrocarbon pyrolysis, unlike the simpler HACA mechanism, has been developed mainly for ordered carbon catalysts, such as carbon fibers, assuming parallel graphene sheets. However, these models fail to describe pyrolysis over generally more active amorphous carbon catalysts. In this work, we optimize the kinetic parameters of an existing surface reaction model to improve predictions for methane pyrolysis over so-called low-order carbon catalysts. Enhanced parameters for C–H bond cleavage on armchair and zigzag sites enable a clear distinction between the reactivities of active sites on ordered and amorphous carbon, with activation energies on amorphous carbon estimated at 382 kJ/mol (zigzag) and 337.3 kJ/mol (armchair) compared to 470 kJ/mol for ordered carbon reported in the literature. The dominant methane activation routes were identified as (i) reaction with gas-phase H radicals originating from surface C–H bond cleavage on high-surface-area catalysts and (ii) reaction on vacant sites on low-surface-area catalysts.