High-Pressure CO2 Photoreduction, FSP and Z-scheme: a Promising Synergy - Keynote lecture

INTRODUCTION CO2 emissions are responsible for more than 60% of global warming, with annual emissions exceeding 30 gigatons, primarily from fossil fuels. The conversion of CO2 into value-added chemicals is particularly appealing as it also offers the potential to decrease fossil fuel consumption and one of the most intriguing approaches is artificial photosynthesis. In recent years, a variety of materials have been adopted as efficient and environmentally sustainable photocatalysts for CO2 photoreduction, including TiO2, WO3, ZnO, CdS, CuO and g-C3N4. In Z-scheme photocatalysis, two different semiconductors capable of absorbing light are utilized. When light irradiates the semiconductor surfaces, each generate an excited electron that transitions from the valence band to the conduction band. Consequently, two excited electrons and holes are created and electrons from the lower conduction band can recombine with the hole in the highest energy band, following the Z-scheme and making the potential difference between hole and electron higher than each of the materials alone. For an efficient Z-scheme action, an intimate contact between the two oxides is needed. This can be efficiently achieved through the Flame Spray Pyrolysis (FSP) synthesis which is a high temperature flame method which allows the formation of dense nanoparticles through rapid residence time in the hot zone of the flame. EXPERIMENTAL/THEORETICAL STUDY In this work, three catalysts, TiO2, WO3 and TiO2/WO3 have been synthesized through flame spray pyrolysis synthesis (FSP) and have been tested for CO2 photoreduction. The catalysts were fully characterized by XRD, DRS UV-Vis, N2 physisorption and SEM. Experimental tests have been performed in a one-of-a-kind high-pressure reactor at 18 bar. TiO2 P25 was used as benchmark to compare the productivities of the newly synthetized catalysts. RESULTS AND DISCUSSION The two single oxides showed comparable productivities, both slightly lower than the P25 reference value (ca. 17 mol/kgcat h). The mixed oxide TiO2/WO3 showed instead an impressive productivity of formic acid with 36 mol/kgcat h which is around 2.5 times higher than both the single oxides alone. The formation of a type II – heterojunction have been confirmed through DRS analysis. The remarkable productivity demonstrates how FSP synthesis can be a crucial tool to produce Z-scheme catalysts. This approach has already been successfully scaled up for the industrial production of various catalysts, showcasing its versatility and efficiency. CONCLUSION Three semiconductors have been synthetized using FSP tested for CO2 photoreduction. The results showed that the two single oxides, TiO2 and WO3, despite the significant difference in the bandgap energy (3.10 eV vs 2.73 eV) exhibited comparable productivities in CO2 photoreduction, each slightly underperforming the P25 benchmark, which had a productivity of 16.9 mol/kgcat h. Remarkably, the mixed oxide TiO2/WO3 demonstrated instead a significantly higher productivity in formic acid production. The superior performance of the TiO2/WO3 catalyst can be attributed to the formation of a type II – heterojunction, as confirmed by DRS analysis. The bandgap of this latter catalysts, with a 2.70 eV value, is even lower than the one of pure WO3. This heterojunction facilitates efficient charge separation and transfer, enhancing the photoreduction process. By achieving an outstanding productivity of 36 mol/kg cat h, this method leverages the synergy between Flame Spray Pyrolysis (FSP), high-pressure photoreduction, and the Z-scheme mechanism. ACKNOWLEDGMENTS This research was funded by Fondazione Cariplo (Italy) through the grant “2021-0855 SCORE – Solar energy for circular CO2 photoconversion and chemicals regeneration”, in the frame of the 2021 call on circular economy and by Next Generation EU - PIANO NAZIONALE DI RIPRESA E RESILIENZA (PNRR), Missione 4 “Istruzione e Ricerca” - Componente C2 Investimento 1.1, “Fondo per il Programma Nazionale di Ricerca e Progetti di Rilevante Interesse Nazionale (PRIN2022PNRR)” through the grant “P20227LB45 - SCORE2 - Solar-driven COnveRsion of CO2 with HP-HT 2156 photoReactor”.