Optimization of photocatalytic activity in transition metal-modified mesoporous silicas: fine-tuning properties to elucidate radical reaction pathways by EPR†

Ti/Z-based (Z = Co, Cu or Mn) SBA-15-type mesoporous photocatalysts were synthesized via wet impregnation. The materials were then characterized to study their morphology by TEM and photochemical properties through electron paramagnetic resonance (EPR) using the 5,5-dimethyl-1-pyrroline N-oxide (DMPO) spin trapping agent in aqueous dimethyl sulfoxide (DMSO) solution at room temperature. Tests with radical scavengers were carried out, which allowed identification of radical recombination processes and assessment of the radical pathways promoted by the materials. Both Cu and Mn were effective in enhancing the photoactivity of Ti-based materials, with Cu intensifying the hydroxyl path and Mn showing affinity towards superoxide. Meanwhile, Co did not increase the photoactivity compared to Ti-modified mesoporous photocatalysts. As the Ti/Mn-modified mesoporous photocatalyst was the most active photocatalytic material, further modifications with different calcination heating rates or Mn loads were tested. Materials synthesized with a heating rate of 8 °C min⁻¹ presented the largest photoactivity, reaching a maximum with a 10 wt% nominal Mn load, which also shows high affinity for the superoxide radical reaction pathway. Instead, a 2.5 wt% nominal Mn load with a heating rate of 4 °C min⁻¹ enhanced the generation of hydroxyl radicals. Thus, the photogeneration of different radical species from mesoporous photocatalysts can be driven by the nature and load of metals, as well as the heating rate used for the material modification, leading to different active species on the surface.