Carbon dioxide is the main gas responsible for the greenhouse effect, currently considered one of the most serious global environmental problems [1]. This gas in high concentrations in the atmosphere has caused several climate changes that directly affecting the survival of some ecosystems and the quality of life of humanity. To minimize these problems, the photoelectrocatalytic (PEC) processes are widely used, since they can reduce the CO₂ molecules to high value-added compounds such as CO, CH₄, CH₃OH, HCHO and HCOOH, among others [2]. For this, the development of new semiconductors able to generate charge carriers (e^-/h⁺) when properly irradiated is of fundamental importance. Studies have found significant improvements in the CO₂ reduction performance using TiO₂ electrodes modified with MOFs [1,2]. This can probably be explained by synergy between the photocatalyst and the MOFs, since the electrons photogenerated in the semiconductor under photoexcitation are rapidly transferred to CO₂ molecules trapped within the pores of the MOF on the surface of the electrode, avoiding the recombination of e^-/h⁺ photogenerated [1]. Thus, the aims this work were investigated the performance of TiO₂ nanotubes electrode (TiO₂NT) modified with UiO-Zr-NH₂ MOF to CO₂ reduction by PEC process. Initially, TiO₂NT electrode was prepared by anodization [1] and modified with UiO-Zr-NH₂ MOF. The MOF was synthesized directly onto the TiO₂NT in solution of 60 mL of diphenylformanide, 0.240 g of ZrCl₄ and 0.186 g of 2-aminoterephthalic acid at 90 °C for 24 h under magnetic stirring. The activation TiO₂NT/UiO-Zr-NH₂ electrode was performed by heating at 160 °C for 24 h in a vacuum oven [3]. TiO₂NT/UiO-Zr-NH₂ electrode was characterized by scanning electron microscopy (FEG-SEM) and energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), diffuse reflectance spectroscopy (DRS) and linear sweep voltammetry (LSV). CO₂ reduction was performed in 150 mL of 0.1 mol L^-1 Na₂SO₄ saturated with CO₂ using a photoelectrochemical reactor of two compartments, three electrodes (TiO₂NT or TiO₂NT/UiO-Zr-NH₂ work, Ag/AgCl (4 mol L^-1 KCl) reference and DSA^â counter) with a bias potential of -0.5 V and irradiated using a 300 W Xe arc lamp for 3 h. The methanol generated was quantification by gas chromatography with flame ionization detection (GC-FID using the solid phase microextraction technique (SPME) performed with a DVB/PDMS fiber [1]. The FEG-SEM micrographs of the electrode showed homogeneous TiO₂ tubes, well distributed and UiO-Zr-NH₂ particles with 37 nm of average size dispersed on the entire surface of TiO₂NT. EDS elemental mapping analysis showed the presence of the Ti, O, Zr, N and C elements. XRD diffractograms presented the characteristics peaks of anatase crystalline phase and peaks characteristics of the UiO-Zr-NH₂ MOF. XPS spectrum confirmed the presence of C 1s, O 1s, N 1s and Zr 3d assigned to Zr 3d5/2 and Zr 3d3/2 located around 182 and 185 eV respectively. Bad gap was estimated by Kubelka-Munk equation in a Tauc plot using the DRS measurements, and the values found were of 3.2 and 2.9 eV to TiO₂NT and TiO₂NT/UiO-Zr-NH₂ respectively. The linear scanning voltammograms showed an increase in current under light incidence and presence of CO₂ and a peak at around -0.5 V which can be attributed to CO₂ reduction. The photoelectroreduction of CO₂ onto TiO₂NT resulted in a methanol concentration below the quantification limit. On the other hand, when TiO₂NT was modified with UiO-Zr-NH₂ generated 73, 153, 280 µmol L^-1 of methanol per hour of experiment, respectively. The methanol concentrations showed an almost linear temporal trend, indicating good stability of the electrode. Therefore, the better performance of the Ti/TiO₂NT modified with UiO-Zr-NH₂ MOF could be explained by greater preconcentration of CO₂ on the electrode surface and application of a bias potential of -0.5 V, which minimized the rate of charge recombination, increasing the efficiency of the process and the selectivity of the product formed.

References

[1] K. Irikura, J.A.L. Perini, J.B.S. Flor, R.C.G. Frem, M.V.B. Zanoni, J. CO₂ Util. 43 (2021) 101364.

[2] J.C. Cardoso, S. Stulp, J.F. de Brito, J.B.S. Flor, R.C.G. Frem, M.V.B. Zanoni, Appl. Catal. B Environ. 225 (2018) 563–573.

[3] M. Kandiah, S. Usseglio, S. Svelle, U. Olsbye, K.P. Lillerud, M. Tilset, J. Mater. Chem. 20 (2010) 9848–9851.