Cobalt, nitrogen-doped carbons as catalysts for sodium borohydride hydrolysis: role of surface chemistry

The development of an effective method for modifying a carbon surface in order to improve its activity and electronic properties for the electrocatalysts creating is a priority task of numerous studies, as well as the current one. This study describes an attempt to modify the surface of traditional porous carbon support in several ways, for the further increment of cobalt, in order to create a highly active and stable catalyst for the hydrolysis of sodium borohydride. Two series of cobalt- and/or nitrogen-doped catalysts were prepared by the impregnating of pre-oxidized industrial active carbon. These were: two catalysts with 10 and 20% of cobalt content based on the oxidized active carbon, as well as two catalysts with 10 and 20% of cobalt content based on the nitrided oxidized active carbon. The presence of cobalt in all the doped samples was confirmed by both X-ray phase and X-ray structural analysis. It was found that the oxidation, nitriding, and doping with cobalt lead to the consistent decrease in the surface area and the total porosity; however, it did not affect their catalytic activity. The largest H2 volumes at temperatures of 60, 70, and 80 °C and hydrogen generation rates of 1667, 2857, and 5000 mL/(gcat·min), respectively, were found in the presence of the catalyst doped with nitrogen and cobalt, with the content of the latter of 10%; the activation energy constituted 41 kJ/mole. The study of the surface chemistry of the catalysts by the two methods (pH-potentiometric titration, and adsorption of Hammett indicators from an aqueous medium) made it possible to explain the mechanism of cobalt attachment and to determine the possible forms of its existence on the surface of the oxidized and nitrided oxidized activated carbon matrix.