Investigation of the mechanism of methane formation from carbon impurities in zirconium–cobalt alloys

Zirconium-cobalt alloys have a wide range of applications for large-scale hydrogen isotope gas storage, but trace carbon impurities are inevitably introduced during metal smelting. Carbon impurities may react with hydrogen to form methane in the ZrCo hydrogen storage cycle, which is often overlooked in practical testing, especially when there have been relatively few studies about the direct formation of methane from carbon and hydrogen. In this study, we reveal the mechanism of C impurities in Zr–Co alloys reacting with hydrogen to form methane. By analyzing the gas composition released during the initial several hydrogen reaction cycles at different dehydrogenation temperatures of 573 K, 673 K and 773 K, it is found that the amount of methane produced tends to increase with increasing temperature. Sample analysis indicates that there is a process of carbon migration on a macroscopic scale that is associated with a reduction in apparent enthalpy change, and lattice restructuring during the high-temperature hydrogen release of Zr–Co metals appears to be the key factor influencing this process. In addition, by combining computational chemistry with experimental data, we have revealed a possible pathway for the methanation reaction on the crystal surface. The carbon atom is more prone to methanation than graphite because the formation of methyl radicals on the Zr–Co surface promotes methane production.