Effect of hard-template residues of the nanocasted mesoporous LaFeO3 with extremely high surface areas on catalytic behaviors for methyl chloride oxidation

Abstract

Mesoporous LaFeO₃ having extremely high surface area was prepared by means of one-step infiltration hard-template (HT) nanocasting method using SBA-15 as a template, which could be partially leached out by NaOH solution. Although this lanthanum ferrite is not an exact replica of the corresponding HT, its specific surface area could reach 158 m² g⁻¹, which is much higher than that of the sample prepared by a traditional sol–gel method (19 m² g⁻¹). Subsequently, LaFeO₃/SBA-15 hybrid samples containing the different amounts of Si residue were obtained by carefully controlling the leaching time under 2 M NaOH. These as-prepared samples were characterized by XRD, TEM, XPS, XRF, H₂-TPR, N₂ physisorption techniques, NH₃-TPD, CH₃Cl-TPD, as well as in situ NH₃ DRIFTS, which were further evaluated for the catalytic combustion of methyl chloride (CH₃Cl). Thereafter, the effect of silica residues of HT on the texture, morphology, acidity/basicity, and the catalytic activities of the prepared nanocasted samples was systematically studied. The more silica HT was left in samples, the lower surface area associated with the less adsorbed oxygen and the worse catalytic activity was achieved. The acidity of the prepared LaFeO₃ samples decreased along with reduction in the residual Si content; however, the opposite occurred for the related basicity which was found to play an important role on CH₃Cl catalytic oxidation. The total collapse of mesoporous structure and the generation of some new crystal phases were observed simultaneously with an obvious drop in its BET surface area (down to 69 m² g⁻¹), as the sample was treated by a highly concentrated NaOH (10 M) in order to entirely remove HT silica.