CO₂ transformation to higher value-added compounds is a topic of great importance nowadays due to the problems generated by the increase of the CO₂ emission in the atmosphere and consequently the global warming. So, the present work aims to prepare, characterize and check TiO₂ nanotubes (TNT) catalyst modified with Au nanoparticles (NP) during the CO₂ photocatalytic reduction. The deposition of Au NPs onto TNT catalyst was performed by reduction of a solution of chloroauric acid with citric acid from lime juice, adapted from the method of Turkevich et al. [1]. The reduction of CO₂ was conducted in a stainless steel reactor, filled with 150 mL of Na₂SO₃ solution containing dissolved CO₂ by bubbling this gas and ionic liquid (IL) BMIM-BF4 up to 4% v/v. After that, the reactor was sealed and pressurized with CO₂, up to 5 atm. The surface of the catalyst was irradiated using a solar simulator using an AM 1.5G filter and a 300 W Xe arc lamp. The methanol generation was monitored by GC-FID after extraction using a solid-phase microextraction fiber and methane was monitored by GC-TCD using a gastight syringe. The results obtained showed that the generation of methanol and methane using TNT catalyst was negligible. On the other hand, in the presence of TNT catalyst modified with gold nanoparticles, the methanol generation was higher, achieving 57.6 μM after 180 min of reaction. The incorporation of the gold NPs on the TNT catalyst surface can improve the photoexcitation of the catalyst since there was greater absorption of the radiation from the light source in the visible light region. Besides that, the work function of gold is higher than TiO₂, thus, when Au NPs and TiO₂ are in contact, Schottky barriers is formed, that provides an effective electron trap for the photogenerated electrons, minimizing the electron-hole recombination. So, increasing the life-time of the electron and finally enhancing the generation of CO₂ reduction products. Regarding the pressure, experiments at 1 atm resulted in the generation of 57.6 µM of methanol. When the pressure was increased to 3 and 5 atm, the generation of methanol increased to 94.8 and 151.2 µM, respectively, due to the increase dissolution of CO₂ in water. The effect of BMIM-BF4 IL was evaluated increasing the IL percentage from 0 to 4% v/v at 5 atm pressure. The generation of methanol and methane using 4% v/v of IL was 320.8 μM and 159.2 μM after 180 min of reaction, respectively. These values mean an increase of 112.2% (methanol) and 142.7% (methane) compared to experiments carried out in the absence of BMIM-BF4. The quantum yield for CO₂ photo-conversion calculated during the best catalytic condition (5 atm and 4% v/v IL) showed a quantum yield of 0.60% and 0.52% for methanol and methane, respectively. Isotope-labeled studies proved that ¹³CO₂ is the only source for photoproduction of ¹³CH₃OH. In conclusion, these findings indicate that the combination of the Au co-catalyst size in nanometric size, high pressure, and BMIN-BF4 IL can provide efficient modulation of CO₂ conversion.

Acknowledgment:

FAPESP and CNPq.

References:

[1] M J. Turkevich, P.C. Stevenson, J. Hillier, Discuss. Faraday Soc. 11 (1951) 55–75.