Innovative systems for CO2 reduction: production of solar fuels through the photocatalytic reduction of CO2 on TiO2/CuInS2 using Na2SO3 as hole scavenger.

The widespread use of fossil fuels tied to industrial development over the last century and the growth of the world population have led to the release of significant amounts of carbon dioxide (CO2) into the atmosphere, contributing to damage the ecosystem. One of the solutions that has attracted researchers in recent years is the photocatalytic reduction of CO2, a process that converts CO2 into value-added chemicals and solar fuels (such as CO, HCOOH, CH3OH, CH4). This process is particularly interesting when solar light is used to activate the photocatalyst. The latter is usually a semiconductor, whereby absorbing light of proper energy electrons are promoted from the valence band to the conduction band leaving holes in the valence band. For this process to happen, a source of light of proper energy is required, typically in the UV range. Unfortunately, only a minor fraction of solar photon has such high energy, being most of the solar radiation in the visible and NIR range. To cope with this issues, different solutions have been studied in the literature. In this regard, semiconductor quantum dots (QDs) have been recently identified as one of the most promising materials to harvest light and to boost an efficient artificial due to their facile synthesis, multiple exciton generation, feasible charge-carrier regulation, and abundant surface sites. TiO2 instead is a well-known photocatalyst, able to efficiently exploit the UV part of the solar spectrum. In this thesis work, the photocatalysts used are two types of titania (TiO2): commercial P25 and a synthesized TiO2 powder called AR, due to the presence of anatase and rutile, similar to P25 in composition. Both powders are coupled with CuInS2 semiconductor quantum dots through a thermal treatment at 150°C in inert atmosphere. Coupling with QDs make TiO2 active in the visible light spectrum. The synthesized powders were characterized using XRD (X-Ray Diffraction), UV-Vis, N2 adsorption/desorption isotherms at -196°C, and Z-potential measurements. The use of a sacrificial agent (hole-scavenger) is recommended to increase the efficiency of the reduction process; in this case, Na2SO3 (sodium sulfite) was chosen. Photocatalytic reduction tests on CO2 were conducted by irradiating a reactor containing an aqueous solution of the photocatalyst (TiO2+QDs) with a lamp simulating sunlight (1 SUN). Tests were carried out with and without the hole scavenger, and in the range of visible wavelengths and the full spectrum (UV+Visible). The samples were analyzed using a micro-gas chromatograph directly during the experiments to detect the formation of hydrogen and other gaseous compounds, whereas the solutions after 1 hour were analyzed by HPLC (High-performance liquid chromatography) to detect liquid compounds such as acids and alcohols. In particular, the focus was on the production of acetic acid (CH3COOH), formic acid (HCOOH), ethanol (C2H6O), and methanol (CH3OH). The results obtained with the laboratory-synthesized powders were compared with those achieved with the commercial P25 powder. The HPLC results indicated that the peaks of all four compounds and the scavenger are very close to each other in terms of retention time, and a high concentration of scavenger significantly affects compound recognition. Consequently, from the analysis of the absorption spectra, it was decided to lower the concentration of the scavenger in the solution.