Effects of Fe Species on the Surface of TiO₂ Nanostructures and Their Influence on Photocatalytic CO₂ Reduction

In the last decades, global energy demand for fossil fuels has resulted in increased CO₂ emission in the atmosphere, leading to several negative impacts on the environment^1-3. Among the various methods studied to overcome this problem, the photocatalytic conversion of CO₂ into solar fuels (CH₄, HCO₂H, CH₂O and CH₃OH) using semiconductors has been considered a promising technology^1-3. Titanium dioxide (TiO₂), which has several advantages when applied in heterogeneous photocatalysis, have been extensively studied for the photocatalytic conversion of CO₂^1-4. As a matter of fact, different modification techniques have been employed to improve the efficiency of TiO₂ in this process, such as surface modifications, doping of metals and non-metals, and so on^1-4. Thus, the aim of this work is to evaluate the effects of Fe on the photocatalytic CO₂ reduction of different TiO₂ morphologies. The different TiO₂ morphologies were obtained by the synthesis route proposed by Mendonça et al. through a hydrothermal reaction by changing the pH of the reaction medium⁵. TiO₂ decoration was performed using an impregnation method adapted from Xie et al.⁴ Different proportions of Fe were evaluated (0.2, 0.8 and 1.6 % of mass). For comparison purposes, the same process was performed using commercial TiO₂ anatase. XRD measurements revealed that all samples exhibited TiO₂ anatase crystalline phase, while EDS results confirmed the presence of Fe and the absence of contaminants in the samples. DRS UV-Vis showed a tendency of band gap energy (Eg) decrease related to the increase in the amount of Fe in all studied samples. Electron microscopy revealed that the as-synthesized samples had a faceted - TiO₂ morphology regardless of the presence of Fe. XPS results confirmed the presence of different Fe species (FeOOH and Fe₂O₃) in the samples, with a strong interaction between Ti and loaded Fe. The photocatalytic CO₂conversion results showed that the studied samples successfully converted CO₂ into CO and CH₄. The higher amount of Fe (1.6 % of Fe) on TiO₂nanocrystals led to a better performance in the production of CH₄. The improved photocatalytic performance can be attributed to faceted TiO₂nanocrystals and the efficient electron transfer due to the presence of Fe species on their surface.<br/>References:<br/>1 - Xiong, Z.; Lei,Z.; Youzi, L.; Liangchen, D.; Zhao, Y.; Junying, Z. A review on modification of facet-engineered TiO₂ for photocatalytic<br/>CO₂ reduction. Journal of Photochemistry and Photobiology C: Photochemistry Reviews 36, 24–47, 2018<br/>2 - Low, J.; Cheng, B.; Yu, J. Surface modification and enhanced photocatalytic CO₂ reduction performance of TiO₂: a review. Applied Surface Science, 392, 658–686, 2017.<br/>3 - Shehzad, N.; Muhammad, T.;Johari, K; Murugesan, T.; Hussain, M. A critical review on TiO₂ based photocatalytic CO₂ reduction system: Strategies to improve efficiency. <i>Journal of CO₂ Utilization</i>, 26, 98-122, 2018.<br/>4 - Xie, Jijia et al. Highly selective oxidation of methane to methanol at ambient conditions by titanium dioxide-supported iron species. <i>Nature Catalysis</i>, 1(11), 889–896, 2018.<br/>5 - De Mendonça, V. R.; Ribeiro, C. Influence of TiO₂ morphological parameters in dye photodegradation: a comparative study in peroxo-based synthesis. <i>Applied Catalysis B: Environmental</i>, 105(3–4), 298–305, 2011.