Hydrogen production by ethanol steam reforming over Ni catalysts derived from hydrotalcite-like precursors: Catalyst characterization, catalytic activity and reaction path

摘要

The urea hydrolysis method has been applied to prepare Ni-Zn-Al and Ni-Mg-Al layered double hydroxides (LDHs) to be used as precursors of mixed oxide catalysts for the ethanol steam reforming (ESR) reaction. Well crystallized hydrotalcite-like LHDs have been prepared for both systems. IR spectroscopy provides evidence of the carbonateitrate copresence and of the additional presence, in the case of the NiMgAl system, of a Mg-free Ni hydroxide phase. The calcinations of the layered precursors give rise to high surface area mixed oxides which essentially retain the lamellar morphology of the precursors. However, the mixed oxides obtained from Ni-Zn-Al LDHs are definitely polyphasic, being actually a mixture of a rock salt phase (NiO), a wurtzite phase (ZnO) and a spinel phase (likely mostly ZnAl2O4). On the contrary, the mixed oxides obtained from Ni-Mg-Al LDHs are essentially monophasic, being mostly constituted by a rock salt NiO-MgO solid solution. IR data show the incorporation of tetrahedrally coordinated Al ions in such a rock salt phase. The steam reforming of ethanol has been investigated over these catalysts with flow reactor and IR experiments. All these catalysts are active for ESR with slight differences with Mg/Zn incorporation and Ni loading. It has been found that, above 750-800 K it is only possible to have the products H2, CO2, CO and CH4. The formation of CO and methane limits the yield to hydrogen to no more than 90% in the best conditions. In fact, working at high temperature water gas shift equilibrium does not allow such a yield to increase, while at lower temperature the yield is limited also by the methane steam reforming equilibrium. IR spectroscopy suggests that the decomposition of acetate ions is the main source of methane. A mechanism via adsorbed oxygenate species is proposed for ethanol steam reforming.