Fotocatalizzatori a base di g-C₃N₄, attivati da luce visibile, per applicazioni sostenibili: conversione della CO₂, valorizzazione della biomassa e degradazione di inquinanti.

The massive burning of fossil fuels results in the excessive emission of carbon dioxide (CO2), disrupting the carbon balance of the nature and then leading to a range of environmental consequences. Utilizing photocatalytic technology to reduce CO2 to solar fuels and to oxidize organics to obtain high-value chemicals, is a sustainable way to increase, respectively, the supply of energy and chemical feedstocks, while reducing greenhouse gas emissions, and decreasing environmental pollution. In particularly, a semiconductor with appropriate band gap can realize effective photocatalysis using clean and sustainable sunlight as the energy input. As a sustainable 2D nanomaterial, graphitic carbon nitride (g-C3N4) with the narrow band gap (2.7 eV) responds to visible light and then presents a great potential for photocatalytic conversions. In addition, due to its simple elemental composition and easy structure adjustment, general semiconductor modification strategies can be realized on g-C3N4. Especially, the construction of heterojunction between g-C3N4 and metal oxides with suitable properties can promote charge transfer and improve the photocatalytic performance. Tungsten oxide (WO3) is among the promising materials that can form a heterojunction with g-C3N4. Furthermore, the holes generated in the valence band of WO3 exhibit strong oxidation ability and can be used for the oxidation of organics and pollutants degradation. This dissertation focuses on visible-light-driven g-C3N4-based photocatalysts for sustainable applications, including CO2 conversion, biomass upgrading and pollutant degradation. (1) In the process of photocatalytic reduction of CO2, the reducing half-reaction is usually on the focus. However, the oxidation half-reaction is also an important factor to determine the overall photocatalytic efficiency, Therefore, the sacrificial agents such as triethanolamine are commonly used as the electron donors to trap holes and further promote the CO2 reduction. Although this improves the photocatalytic efficiency, it also leads to the waste for the energy of holes. We proposed a dual-functional reaction system to realize photocatalytic CO2 reduction coupled with a reaction of interest in the field of biomass conversion. This approach enables full utilization of photogenerated carriers and produces high value-added chemicals. In this part, two kinds of g-C3N4 were prepared, starting from melamine and urea, respectively. They are then combined with WO3 to build a Z-scheme heterojunction that enhances the separation of electrons and holes. This system was exploited for CO2 reduction coupled with 5-hydroxymethylfurfural (HMF) oxidation. Under visible light irradiation, CO2 and HMF are converted to CO and 2, 5-diformylfuran (DFF), respectively, with high selectivity. The formation of Z-scheme heterojunction can effectively promote the separation of photogenerated carriers, while retaining the higher redox potential, which enhance the photocatalytic activity. The dual-function cooperative photo-redox system integrates carbon neutrality and high value utilization of biomass, which opens a sustainable way to address greenhouse gas emission and energy crisis. (2) The biochar prepared from some biomass waste can be used to adsorb the pollutants in water. Therefore, we decided to load photocatalyst onto the biochar to realize the photodegradation of the adsorbed Rhodamine B (RB). The composite of WO3 and Biochar shows a strong adsorption capacity for RB; moreover, biochar with a certain degree of graphitization has good conductivity and can accept photogenerated electrons from WO3 and then promote the separation of photogenerated carriers. Coupling WO3/Biochar with g-C3N4 not only promotes charge separation, but also generates more reactive oxygen species, leading to the highest photodegradation efficiency, which provides an efficient and sustainable approach for the degradation of pollutants and the utilization of biomass waste.