Alkali(ne earth) metal promotion in Ni-catalyzed thermal decomposition of methane

Abstract

To move towards a more sustainable form of energy production, COx emissions need to be drastically reduced. As renewable energy sources cannot yet provide for the growing demand, the thermal catalytic decomposition of methane could be a promising intermediate. Apart from energy-rich hydrogen, this reaction also adds value to the solid carbon by-product in the form of carbon nanostructures. Promotion with alkali and alkaline earth metals of Ni-based catalysts was investigated by following the carbon growth in-situ using a thermogravimetric analyzer. Nickel catalysts with Li, Na, K, and Mg as promoters were tested. It was found that under similar reaction conditions, all the promoters show different effects on the catalytic properties of the catalysts, with promotion by Na and Mg showing a large increase in both activity and lifetime. K, on the other hand, shows a reduction in both properties. For the Na-promoted catalysts a concentration dependency was found, suggesting that lower promotion concentrations are needed when going down the alkali metals. On the other hand, Mg-promotion showed a slight increase in catalytic performance when increasing the promoter concentration, this might suggest that the promoter effect of Mg is different from that of the alkali metals. Measurements performed at different temperatures found activation energies of 77, 83, and 102 kJ/mol for the tested Ni reference, Na-promoted, and Mg-promoted catalysts, respectively. Only for the Mg-promoted catalysts an optimum in the catalytic lifetime was found at around 450 °C. While the other two catalysts showed increasing lifetimes and total carbon yields with decreasing temperatures, where this was not expected. The formed carbon structures seem mostly unaffected by the introduction of the different promoters compared to the reference catalyst, suggesting limited changes in the carbon growth mechanism upon promotion.