Hydrogen gasification of HNO3-oxidized carbons

The oxidation of carbon prior to hydrogen gasification as a means of enhancing methane formation rate is investigated. Carbon is oxidized by HNO3 and gasified in hydrogen in a high-pressure differential reactor. Samples are analyzed by x-ray photoelectron spectroscopy (XPS) for surface oxygen content before and after pretreatment and reaction. Results for uncatalyzed gasification show a correlation between initial surface oxygen content and hydrogasification rate, but XPS results reveal that essentially no oxygen is present on the carbon surface during hydrogen gasification. This indicates that desorption of oxygen groups from the carbon surface generates reactive sites at which hydrogen gasification occurs. These nascent sites arise from acidic oxygen groups both fixed during oxidation and from oxygen in bulk carbon. In potassium carbonate-catalyzed hydrogen gasification, oxidation enhances the catalyzed rate as much as threefold. The catalyst interacts with basic oxygen groups on carbon to form reactive sites which are formed and regenerated continuously during gasification. Analysis by XPS shows that substantial oxygen and potassium are present on the carbon surface during hydrogen gasification; at high catalyst loadings and 725°C the Cls peak shows both carbonate groups and singly bound oxygen-carbon groups tentatively assigned as M—O—C.