NiAlxFe2−xO4 mixed oxide catalysts for methane reforming with CO2: Effect of Al vs Fe contents and precursor salts

NiAlxFe2−xO4 (0 ≤x ≤ 2) was synthesized, characterized and evaluated in methane reforming with CO2 without any pretreatment. It was shown that the nature of the precursor has a significant effect on the structural, textural and reactivity properties. The cell parameters decrease with increasing Al-content which can be explained by the partial substitution of Fe3+ ions (∼0.63 Å in tetrahedral site and ∼0.78 Å and high spin in octahedral site) by Al3+ ions (∼0.53 Å) either in the tetrahedral or the octahedral sites. If, on the one hand, the catalytic activity of the aluminum compound exhibits high CH4 and CO2 conversions as well as excellent syngas selectivity, on the other hand, a poor activity is observed for NiFe2O4 whatever the precursors. For low temperatures (T < 750 °C), powders prepared with chlorides are more active than powders prepared with nitrates. However, this behavior mitigates at high temperature. NiAl2O4 issued from nitrate precursors shows the best catalytic performances. Negligible contributions to WGS Reaction are illustrated by H2/CO ratios close to stoichiometry. The intermediate FeAl2O4 spinel phase plays an important role as catalytic precursor in the in-situ production of Ni° nanoparticles, highly dispersed and less prone to coke formation in spite of the severe reaction conditions. A lack of affinity of Al-species to combine with Ni° to form detrimental Ni-Al alloy is proposed to explain the good performances. Strong interactions developed between metallic active sites and mixed oxides substrates allow the stabilization and improvement of catalytic performances in dry reforming of methane.