Hydrogen production by methane decomposition over Co-Al mixed oxides derived from hydrotalcites: Effect of the catalyst activation with H2 or CH4

This study examines the influence of catalyst activation for methane decomposition over Co-Al mixed oxides derived from hydrotalcites. Samples were prepared by coprecipitation and characterized by surface area measurements and temperature-programmed reduction. Spent catalysts and carbon produced in the reaction were characterized by X-ray diffractometry, temperature-programmed oxidation and scanning electron microscopy. Activity runs using previously reduced samples with H₂ or activated under CH₄ flow were carried out in a fixed-bed reactor between 500 and 750 °C using in-line gas chromatography analysis. The specific surface area decreases as the Co/Al ratio increases, which is related to the increased Co₃O₄ phase rather than Co-Al mixed oxides. The TPR results indicate the reduction of four types of Co species: Co³⁺ and Co²⁺ species from Co₃O₄, and Co from inverse spinel (Co₂AlO₄) and normal spinel (CoAl₂O₄). Reduction with hydrogen at 750 °C was very severe. Samples reduced with H₂ showed large Co° crystallites, which increased with the Co/Al ratio. Co-Al catalyst activation under methane flow leads to lower crystallite size and higher thermal stability for hydrogen production by methane decomposition.