Effect of TiO2 morphology on the benzyl alcohol oxidation activity of Fe2O3–TiO2 nanomaterials

Abstract

Three series of Fe loaded TiO₂ anatase (1, 3, 5 and 7 mol% Fe) nanomaterials with different morphologies: nanoparticles (NP), nanotubes (NT) and nanofibers (NF) were synthesized and calcined at 400 °C. The physico-chemical properties of the catalysts were studied by using elemental analysis, XRD, UV-vis, N₂-physisorption, SEM, TEM, XPS, pyridine adsorption using FTIR and H₂-TPR techniques. It was observed that iron oxide was highly dispersed on the TiO₂-NP support due to its strong interaction. The catalytic activity of the catalysts was tested in the oxidation of benzyl alcohol with hydrogen peroxide at mild reaction temperatures (70–100 °C) and atmospheric pressure. The highest activity was observed with 3 mol% Fe supported on TiO₂-NP at 100 °C. It seems that TiO₂-NP is unique in stabilizing small Fe₂O₃ nanoparticles. A greater number of surface hydroxyl groups in TiO₂-NT and NF tend to increase the density of adsorption sites and/or the affinity of the surfaces with the Fe₂O₃ precursor. This appears to favor agglomeration, which results in a higher density of larger iron oxide particles. Fe-TiO₂-NP catalysts display high activities due to a detrimental morphology effect, high surface area of the TiO₂ support, dispersion of Fe₂O₃, more Lewis acid sites and easy reducibility of total catalysts. It was also observed that the Fe-TiO₂ nanomaterials possessed two types of Fe³⁺ ions on the support surface; one dispersed in which Fe³⁺ ions interact with the TiO₂ surface and another in which Fe₂O₃ crystals are located on the surface of the catalyst. The catalyst which possessed the former species exhibited the best performance in the oxidation of benzyl alcohol. Metal oxide leaching studies prove the true heterogeneous nature of the reaction. The catalysts are found to be reusable and resistant to rapid deactivation.