Steam reforming of CH₄over supported Ni catalysts prepared from a 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 as the precursors and tested for steam reforming of CH4 into synthesis gas. The precursors based on [Mg(II)1−xAl(III)x(OH)2]x+(CO3−)x·mH2O, in which a part of Mg(II) ions were replaced by Ni(II) ions, were prepared by co-precipitation method, thermally decomposed and reduced to form spc-Ni/MgAl catalyst. Surface areas of spc-Ni/MgAl catalysts were always high at around 150 m2 g-cat−1. Ni(II) ions first substituted a part of the Mg(II) 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 formation of highly dispersed Ni metal particles after the reduction. The activity of spc-Ni/MgAl catalyst was high when Ni/Mg was larger than 0.2, and the most suitable ratio of Mg/Al was 1/3. When the catalysts were tested in the steam reforming of CH4, spc-Ni0.5/Mg2.5Al showed high CH4 conversion following thermodynamic equilibrium even at a high space velocity of 9 × 105 ml h−1 g-cat−1, followed by imp-Ni/Mg3Al-aq prepared by impregnation of Mg–Al mixed oxide in aqueous solution. It is considered that, during the preparation of imp-Ni/Mg3Al-aq, surface reconstitution of the Mg–Al hydrotalcite layered structure took place by a "Memory Effect" of the hydrotalcite, resulting in the high dispersion of Ni metal particles. When spc-Ni0.5/Mg2.5Al was tested in the steam reforming of CH4, no deterioration in the catalytic activity was observed for 600 h of reaction time even under a low steam to carbon ratio of 1.6, while a commercial Ni/α-Al2O3 catalyst showed a clear decline in the activity. The CH4 conversion as well as the distribution of products followed thermodynamic equilibrium during the reaction for 600 h. It is concluded that spc-Ni0.5/Mg2.5Al is a hopeful candidate as a catalyst for the production of H2 for polymer electrolyte fuel cells.