Hydrogen production by methane decomposition: A comparative study of supported and bulk ex-hydrotalcite mixed oxide catalysts with Ni, Mg and Al

A catalytic comparative study of COx-free hydrogen production by methane decomposition was carried out. Catalytic performances of bulk Ni-mixed oxides derived from Ni/Mg/Al-hydrotalcites (ex-HTs-Ni) were compared with those obtained with Ni supported on mixed oxides derived from Mg/Al-hydrotalcites (Ni/ex-HTs), or on commercial supports (γ-Al2O3, MgO and MgO-modified γ-Al2O3). Catalyst characterization and their catalytic performance showed both ex-HTs-Ni and Ni/ex-HTs appear to be a similar regardless of their method of preparation. Ni/γ-Al2O3 was the best supported catalyst, although the catalytic performances of the ex-HTs catalysts were better. Higher NiMg interaction in ex-HTs provides higher resistance to deactivation. Characterization by TG, Raman spectroscopy and TEM of spent catalysts in the reaction suggest the degree of ordering of the graphitic layers of the carbon deposit onto the catalyst surface is the key factor in the catalyst deactivation. The higher degree of ordering or graphitization of the carbon produced with the higher concentration of sp2 carbons on the surface of the Ni/γ-Al2O3 favours its faster deactivation by Ni-coverage than the bulk catalyst (ex-HT-Ni), in which the MWNT type carbon is mainly obtained.