Unsupported versus alumina-supported Ni nanoparticles as catalysts for steam/ethanol conversion and CO2 methanation

Nickel nanoparticles (NPs, particle radius <8 nm) were prepared via reduction method in aqueous solution and were structurally and morphologically characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). A DC-SQUID magnetometer was also used to determine the particle size. As a reference, a high loading (125% Ni wt(Ni)/wt(support)) Ni/Al2O3, prepared through wet impregnation technique, was also studied. The results of the catalytic tests in ethanol conversion in the presence of steam on unsupported Ni NPs at 523 K show already an ethanol conversion higher than 90% together with high yields in methane, CO and H-2. These NPs actually behave as very active ethanol decomposition catalysts, in these conditions. Increasing the reaction temperature (673 K) they convert into good steam reforming catalysts. On the other hand, the conventional alumina-supported Ni catalyst is poorly active in dehydrogenation to acetaldehyde below 673 K, but behaves as a good steam reforming catalyst at this temperature. In CO2 methanation at 773 K the best catalytic activity was obtained on 125% Ni/Al2O3, while Ni NPs result poorly active in catalysing this reaction. The characterization of the exhaust materials shows: (i) hexagonal Ni particles on both catalysts after ethanol conversion in the presence of steam, (ii) cubic nickel particles on both samples after methanation of CO2. Mechanistic implications of these results are discussed. (C) 2013 Elsevier B.V. All rights reserved.