Hydrogen from steam reforming of ethanol over cobalt nanoparticles: Effect of boron impurities

Cobalt nanoparticles have been prepared by reduction of cobalt chloride by sodium borohydride. They are very active catalysts for ethanol steam reforming at moderate temperature. The best catalytic materials show full ethanol conversion and hydrogen yields of the order of 83% and stable activity for 8 h when tested at 773 K, GHSV 324,000 h-1 with diluted 1:6 ethanol:water feed in He 41.6%. The amount of coproduced methane is very low (yield <0.05%). Experiments performed at different space velocities provide evidence of the intermediate formation of acetaldehyde in the ethanol steam reforming path. The fresh Co nanopowders are amorphous and are actually contaminated mainly by Boron species and traces of Na, Cl and C. During reaction, Cobalt nanoparticles are transformed into cubic metallic Co crystallites. The surface properties of these materials are very sensitive to: (I) the separation procedure from the reaction medium at the end of the preparation step, and (II) the conditions of ageing and storage in the laboratory. An increasing time of ageing seems to favor the formation of borates species and the observed oxidation of metallic cobalt to cobalt oxides. These phenomena could be associated to the loss in the catalytic activity that results in a lower ethanol conversion, lower hydrogen yield and sometimes, higher selectivity to acetaldehyde.