α-Fe2O3/SiO2 composites for the enhanced photocatalytic NO oxidation

Highlights: • New α-Fe₂O₃/SiO₂ composites act as enhanced photocatalyst in De-NO_X processes. • Thermal procedure determine the crystalline growth of α-Fe₂O₃. • Higher NO conversion value for hematite is obtained. • The maximum selectivity is associated with the presence of large crystals. • Crystalline particles with {001} facets favor the NO₂ molecules adsorption. α-Fe₂O₃ catalyst supported on rice husk ash (RHA) with enhanced photochemical NO oxidation is reported. The Fe₂O₃/SiO₂ composites are prepared through the calcination of RH and FeCl₃ at temperatures of 600 and 900 °C, during 1 and 4 h. Fe₂O₃ crystalizes covering the cob-shaped silica skeleton. The thermal procedure determines the mineralogical composition, crystalline growth and surface area of samples. Samples obtained at 600 °C are α-Fe₂O₃ (hematite) with low levels of Fe₂SiO₄ and exhibit a microstructure constituted of meso/macropores, with surface area values of 19.6 and 16.1 m² g⁻¹. At 900 °C only pure hematite is obtained but the microstructure is denser with very small surface area. Increasing the time of calcination to 4 h, hexagonal plates and truncated pseudocubes large crystals are obtained. All the samples exhibit light absorption in the 300 to 900 nm UV–vis range. The photocatalytic oxidation of NO molecules is related to the surface area of the samples. In the best case an NO conversion value of 24% is found, the highest reported for hematite photocatalysts. Moreover, the selectivity of NO → NO₂ → NO₃⁻ oxidation process reaches values as high as 72% with the presence of large hexagonal plate hematite crystals. The existence of particles, which exhibit {001} facets, favors the adsorption of NO₂ molecules and, therefore, the achievement of the photochemical oxidation process.