Hydrogen production by methane decomposition: Origin of the catalytic activity of carbon materials

A wide variety of carbon materials (ordered mesoporous carbons, carbon blacks, activated carbon, carbon nanotubes, coke and graphite) have been investigated as catalysts for hydrogen production by methane decomposition, with the aims of identifying the carbon properties which control in a greater extension the catalytic activity and determine the nature of the active sites involved in the reaction. The catalytic activity of the different carbon materials was determined and compared using temperature-programmed experiments in a thermobalance. The initial activity was followed through the threshold temperature, defined as the temperature at which hydrogen production starts being detected, whereas the average reaction rate was also calculated and compared. The lowest threshold temperature was observed with ordered mesoporous carbons (CMK materials), followed by activated carbon and carbon blacks. On the other hand, at long reaction times activated carbon was quickly deactivated yielding a relatively low average reaction rate. The deactivation process seems to be greatly linked to the presence of micropores while the long-term activity is retained in those materials with ordered mesoporosity (CMKs) or formed by nanoparticles (carbon blacks), which make them more resistant to deactivation by the formation of carbonaceous deposits. Whereas no clear dependence is observed between the threshold temperature and the surface area neither with the presence of polar groups in the carbon catalysts, characterization of these materials by XPS shows that a direct relationship exists with the amount of defects present on the graphene layers. This fact strongly supports that these defects are the main active sites for methane decomposition over carbon catalysts.