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-Ni_0.5/Mg_2.5Al 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-Ni_0.5/Mg_2.5Al 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-Ni_0.5/Mg_2.5Al is a hopeful candidate as a catalyst for the production of H₂ for polymer electrolyte fuel cells.