Evonik P25 photoactivation in the visible range by surface grafting of modified porphyrins for p-nitrophenol elimination in water

An Evonik P25 TiO₂ material is modified using a porphyrin containing Si-(OR)₃ extremities to extend its absorption spectrum in the visible range. Two different loadings of porphyrin are grafted at the surface of P25. The results show that the crystallinity and the texture of the P25 are not modified with the porphyrin grafting and the presence of the latter is confirmed by Fourier-transform infrared spectroscopy (FTIR) measurements. All three samples are composed of anatase/rutile titania nanoparticles around 20 nm in size with a spherical shape. The absorption spectra of the porphyrin modified samples show visible absorption alongside the characteristic Soret and Q bands of porphyrin, despite slightly shifted peak values. The ²⁹Si solid state nuclear magnetic resonance (NMR) spectra show that the porphyrin is linked with Ti–O–C and Ti–O–Si bonds with the Evonik P25, allowing for a direct electron transfer between the two materials. Finally, the photoactivity of the materials is assessed on the degradation of a model pollutant—p-nitrophenol (PNP)—in water. The degradation is substantially enhanced when the porphyrin is grafted at its surface, whereas a very low activity is evidenced for P25. Indeed, with the best sample, the activity increases from 9% to 38% under visible light illumination. This improvement is due to the activation of the porphyrin under visible light that produces electrons, which are then transferred to the TiO₂ to generate radicals able to degrade organic pollutants. The observed degradation is confirmed to be a mineralization of the PNP. Recycling experiments show a constant PNP degradation after 5 cycles of photocatalysis of 24 h each.