Towards understanding the carbon catalyzed CO2 reforming of methane to syngas

In this paper, a fixed-bed reactor is used to study the influence of different conditions on carbon catalyzed CO2–CH4 reforming. The surface structure and functional groups of carbonaceous material have been characterized through SEM, XPS, XRD, BET and chemical titration before and after the reaction. Studies have revealed that under non-catalytic condition, methane pyrolysis happens first, followed by the gasification reaction between CO2 and carbon deposit produced from the pyrolysis. While with carbonaceous material, CO2 gasification, methane pyrolysis and CO2–CH4 reforming can take place at the same time, with the reforming as the main reaction, CO2 gasification and methane pyrolysis as the side reaction. Catalytic activity varies from one carbonaceous material to another, but their reaction trend is the same on the whole. Those high specific surface area carbonaceous materials show higher catalytic activity. The increase in reaction temperature and residence time of the reforming can improve the conversion of reactant gas. Adjusting the partial pressure of methane can control carbon–hydrogen ratio of the synthesis gas. XPS and XRD characterizations demonstrate that the structural ordering of carbonaceous materials becomes a little messier after the reforming reaction, and the number and content of oxygen functional groups decrease. That means these oxygen functional groups on the surface of carbonaceous materials are involved in the reforming and these groups along with pore structure on the surface are the major factors influencing the catalytic properties. Different oxygen species make the nature of electrical energy on the surface different; the catalytic activity depends on the polarity of oxygen from different species. Those whose polarity is strong have strong activity. The dipole force can be associated with methane in the form of hydrogen bond, so that the material can display strong activity. Those whose polarity is weak have weak activity, the catalytic activity is weak too. The results of chemical titration and XPS characterization show that the oxygen in the anhydride and lactone structures on the surface of carbonaceous materials are active oxygen, and which is the main active component, it can reduce the activation energy of methane dehydrogenation.