Methane decomposition kinetics on unfunctionalized alumina surfaces

The pyrolytic conversion of methane for the production of hydrogen and carbon was investigated over nonporous α-Al2O3 surfaces in the range of 900–1300 °C. Two devices were used: i) a single particle reactor to determine the carbon deposition rate at various temperatures and ii) a fixed bed in which both methane conversion and carbon deposition were measured. It was observed that at 1000 °C and below, the selectivity towards carbon (and hydrogen) was initially low over fresh α-Al2O3 (e.g. 38% at 250 s reaction time), increasing to 100% over time. Methane conversion was constant at 20% during this period. These observations point towards the presence of an activation process for the formation of carbon and hydrogen from the intermediates products (e.g. benzene) of methane pyrolysis. A temperature dependent maximum in carbon loading was observed. When this maximum carbon loading was reached, methane conversion also stopped completely, indicating 100% selectivity towards carbon and hydrogen. Two kinetic models for carbon deposition were derived and applied. After parameterization of these models using single particle data, they were able to predict carbon growth and CH4 conversion as function of temperature, specific bed area, carbon loading and gas composition in the new data set from the fixed bed.