Probing the Photocatalytic Pathways of Iron (III) Protoporphyrin IX-Functionalized TiO2: A Study on Oxo-Iron(IV) Porphyrin π-Cation Radical Formation
Authors: Romero Maria , Hernández-Anzaldo Samuel , Vázquez-Lima Hugo , Reyes-Ortega Yasmi 
Published: August 2024

The presence of phenolic compounds in aquatic environments, resulting from industrial activities such as coal processing and petroleum refining, poses significant concerns due to their adverse effects on water safety, even at minimal concentrations. These compounds present challenges during remediation using traditional techniques such as solvent extraction, biological treatments or chemical oxidations. In response to these challenges, Advanced Oxidation Processes have emerged as a promising solution, with methods like heterogeneous photocatalysis, ozonation, sonification and so on, showing potential for degrading persistent organic molecules[1]
[2].

Titanium dioxide (TiO2) has gained importance in environmental remediation due to its ease of synthesis, cost-effectiveness, and versatility. Its unique properties, including exceptional chemical stability and non-toxic nature, make it suitable for water purification among other applications. Several studies aim to extend responsiveness of TiO2 beyond UV-A radiation, seeking to harness the wider and more prevalent visible light spectrum. This endeavor is driven by the desire to optimize the utilization of solar energy in various applications. To enhance TiO2 absorption of visible light, alternative chromophores, particularly porphyrins, are employed. Porphyrins, known for their extended lifespan in the excited state and efficient photoabsorption in the visible region, serve as excellent sensitizers when combined with TiO2, enabling effective charge separation[3].

This study explores the activity enhancement of TiO2 catalyst by incorporating iron (III) protoporphyrin IX (FePPIX). The objective is to understand how hydrogen peroxide (H2O2), generated through photocatalysis on the TiO2 surface, influences the formation of oxo-iron(IV) porphyrin π-cation radicals. These radicals specifically form through the interaction with iron porphyrin attached to TiO2, offering a unique pathway for efficiently degrading phenolic compounds, as is proposed in Figure 1. The investigation underscores the significance of TiO2 surface sensitization and the simultaneous emergence of porphyrin radicals in shaping the dynamics of the photocatalytic process.

We fabricated TiO2 nanotube sheets (TiO2-NTs) by anodizing metallic titanium sheets and then functionalized them with FePPIX. The process involved degreasing, chemical cleaning, anodization, exfoliation, and thermal treatment for anatase phase formation. Additionally, we utilized anatase-TiO2 nanoparticles. We carried out functionalization, which included ozone exposure, UV-C light treatment, and immersion in an acidic FePPIX solution in methanol, reacting with the hydroxylated surface induced by ozone treatment. After functionalization, we carefully washed away the excess iron protoporphyrin with methanol, and we air-dried the materials at room temperature. We comprehensively characterized the nanomaterials using diverse analytical techniques, including Scanning Electron Microscopy (SEM), Fourier Transform Infrared (FTIR), Raman and UV-Vis spectroscopies, and ESR analyses. We further investigated the photogeneration of hydroxyl radicals (•OH) on FePPIX-functionalized and non-functionalized TiO2 nanotube arrays through the photobleaching of N-N-dimethyl-4-nitrosoaniline. Additionally, we studied phenol degradation under UV irradiation. Our findings provide detailed insights into the physical and chemical properties, morphology, and composition of the fabricated nanomaterials.

The SEM images reveal well-defined nanotube arrays, underscoring the efficacy of the fabrication technique. Energy-Dispersive X-ray Spectroscopy mapping further solidifies this observation, confirming a uniform distribution of iron in functionalized nanoparticles and supporting the success of functionalization. FTIR and Raman spectroscopy provide detailed insights into the presence of functional groups and vibrational modes, establishing the success of the functionalization process as well. Tauc plots show a narrowed energy gap in functionalized nanotubes, hinting at their potential for visible light activity.

Photocatalytic assessments uncover diminished behavior, with lower •OH generation rates observed under UV-A light for functionalized nanotubes but notable activity under visible light, aligning with the reduced band gap energy. Furthermore, enhanced phenol degradation on functionalized nanotubes under UV-A light, potentially associated with compound I, introduces complexity to the photocatalytic pathways. The application of ESR study confirms the presence of paramagnetic species, such as [PPFeIV=O]+•, thereby highlighting the influence of protoporphyrin on the TiO2 surface and its integral role in photocatalysis.

In summary, the study confirms successful TiO2 nanomaterial functionalization with FePPIX, promising applications in environmental remediation. Analyses validate binding with hydroxyl groups, Tauc plots indicate enhanced photocatalytic activity under visible light, and ESR analysis suggests the presence of paramagnetic species that could improved phenol degradation. Further investigation is needed for a comprehensive understanding. Overall, these findings hold potential for efficient and sustainable wastewater treatment.

[1] S. Rasalingam, R. Peng, R. T. Koodali, J. Nanomater.
2014, 617405.

[2] Y. Deng, R. Zhao, Curr Pollut. Rep
2015, 1, 167.

[3] C. C. Huang, P.S. Parasuraman, H. C. Tsai, J. J. Jhu, T. Imae, RSC Adv.
2014, 4, 6540.

Figure 1