Autothermal reforming of CH 4 over supported Ni catalysts prepared from Mg–Al hydrotalcite-like anionic clay

Abstract

spc-Ni/MgAl (spc: solid-phase crystallization method) catalysts have been prepared from Mg–Al hydrotalcite-like compounds containing Ni at the Mg site as the precursors and tested for partial oxidation of CH 4 into synthesis gas. The precursors based on [Mg 2+ 1− x Al 3+ x (OH) 2] x+ (CO 3 − x ) mH 2O, in which a ratio of Mg/Al varied and a part of the Mg 2+ ions were replaced by Ni 2+ ions, were prepared by a coprecipitation method, thermally decomposed, and reduced to form spc-Ni/MgAl catalyst. Surface areas of spc-Ni/MgAl catalysts were around 150 m 2 g cat −1. Ni 2+ ions first substituted a part of the Mg 2+ sites in the Mg–Al hydrotalcite-like compounds and then incorporated in the rock-salt-type Mg–Ni–O solid solutions in the mixed oxide after the decomposition. The dispersion of Ni was thus repeatedly enhanced during the spc preparation, resulting in the highly dispersed Ni metal particles after the reduction. The activity of the spc-Ni/MgAl catalyst was the highest at the ratio of Mg/Al of 1/3. When the catalysts were tested in the partial oxidation of CH 4, spc-Ni 0.5/Mg 2.5Al afforded enough high CH 4 conversion even at the high space velocity (9×10 5 ml h −1 g cat −1), exceeding the value obtained over 1 wt% Rh/MgO. Ni species on spc-Ni 0.5/Mg 2.5Al catalysts were stable even under the presence of O 2, while Ni catalysts prepared by the conventional impregnation quickly lost activity due to the surface oxidation of Ni particles. Moreover, the total heat produced by the reaction was the lowest over the spc-Ni 0.5/Mg 2.5Al catalyst among the catalysts tested. This strongly suggests that the heat of exothermic CH 4 combustion to H 2O and CO 2 could be quickly consumed by the following endothermic CH 4 reforming by H 2O and CO 2 over spc-Ni 0.5/Mg 2.5Al. Thus, the spc-Ni 0.5/Mg 2.5Al catalyst is a hopeful candidate for the autothermal reforming of CH 4 which can be carried out under the copresence of both H 2O and O 2 to feed H 2 to the fuel cell economically. Actually the autothermal reforming of CH 4 has been successfully carried out over spc-Ni 0.5/Mg 2.5Al catalysts.