Black liquor is an alkaline liquid residue from paper industry, containing similar to 80 wt% of water and similar to 20 wt% of organics and minerals. Hydrothermal conversion of black liquor in a batch reactor is particularly interesting towards hydrogen production, as the carbon dioxide is highly solubilized in the basic aqueous media. Thus hydrogen with higher purity than other processes is obtained. Cerium oxide nanocatalyst (cubic CeO2) is used to improve the amount of hydrogen produced and to decrease the coke formation, based on the water splitting into active species. Experiments have been carried out at sub and supercritical conditions 350 degrees C or 450 degrees C, 25 MPa) in a batch reactor for 15 min or 60 min. CeO2 nanocatalyst improved the conversion of black liquor at sub and supercritical conditions as expected. The amount of carbonaceous solid recovered was decreased due to the catalytic effect. Recovered hydrogen was not significantly affected by the catalyst as it was involved in secondary reactions such as hydrogenation. Oxidations reactions, as well as O-2 production, were also improved using catalyst and supercritical media. Supercritical water media combined with catalyst clearly affects the fragmentation of dissolved lignin compared to subcritical conditions. Then it was assumed that water splitting occurs and produces active hydrogen and oxygen species. Thus these actives species produced using catalyst are able to improve hydrogenation and oxidation reactions, as well as recombination into H-2 and O-2. (C) 2015 Elsevier B.V. All rights reserved.