Ni-Co-Zn-Al Catalysts From Hydrotalcite-Like Precursors for Hydrogen Production by Ethanol Steam Reforming

A series of well crystallized Ni-Co-Zn-Al LDHs materials has been prepared by the urea hydrolysis method as precursors of mixed oxide catalysts for the Ethanol Steam Reforming (ESR) reaction. The calcination of the layered precursors gives rise to high surface area mixed oxides, mainly a mixture of rock-salt phase (NiO), wurtzite phase (ZnO) and spinel phase. Both precursors and mixed oxides have been throughtfully characterized and the steam reforming of ethanol has been investigated over the calcined catalysts in flow reactor and in-situ FT-IR experiments. The data here reported provide evidence of the good catalytic activity of Co-Zn-Al and Co-Ni-Zn-Al catalysts prepared from hydrotalcite-like LHD precursors for ethanol steam reforming. At 823 K the most active Co/Ni catalyst containains a predominant spinel phase with composition near Zn_0.58Ni_0.42[Al_0.44Co_0.56]₂O₄ and small amounts of NiO and ZnO. On the other side, at 873 K the selectivity to hydrogen increases with cobalt content. In particular, the presence of cobalt increases selectivity to H₂ and CO₂ and decreases selectivity to methane in the low temperature range 720–870 K. The most selective catalyst is the Ni-free Co-Zn-Al mixed oxide essentially constituted by a single spinel type phase Zn_0.55Co_0.45[Al_0.45Co_0.55]₂O₄. Cobalt catalysts appear consequently to behave better than nickel based catalysts in this temperature range. The key feature for high selectivity to hydrogen is proposed to be associated to a stability of a relatively high oxidation state at the catalyst surface, the most relevant selectivity determining step being constituted by the evolution of surface acetate species. In fact, over oxidized catalyst surface the acetate species evolve producing carbon dioxide and hydrogen while over a more reduced surface they evolve giving rise to methane and CO_x. Water is supposed to have the main role of allowing surface sites to stay in an unreduced state at least in the temperature range 720–870 K.