###### Abstract  
In this work, three catalysts, TiO\({}_{2}\), WO\({}_{3}\) and TiO\({}_{2}\)/WO\({}_{3}\), have been synthesized through flame spray pyrolysis synthesis (FSP) and have been tested for CO\({}_{2}\) photoreduction. The catalysts were fully characterized by XRD, DRS UV-Vis, N\({}_{2}\) physisorption and SEM. Experimental tests were performed in a one-of-a-kind high-pressure reactor at 18 bar. TiO\({}_{2}\) P25 was used as a benchmark to compare the productivities of the newly synthetized catalysts. The two single oxides showed comparable productivities, both slightly lower than the P25 reference value (ca. 17 mol/kgcat*h). The mixed oxide, TiO\({}_{2}\)/WO\({}_{3}\), instead showed an impressive productivity of formic acid with 36 mol/kgcat*h, which is around 2.5 times higher than both of the single oxides alone. The formation of a type-II heterojunction has been confirmed through DRS analysis. The remarkable productivity demonstrates how FSP synthesis can be a crucial tool to obtain highly active and stable photocatalysts. This approach has already been successfully scaled up for the industrial production of various catalysts, showcasing its versatility and efficiency. photoreduction of CO\({}_{2}\); heterogeneous catalysts; flame spray pyrolysis; high-pressure photoreceptors; carbon dioxide; solar fuels 202515383  
21 February 20253 April 202510 April 2025  
However, each of these technologies faces some challenges, including requirements for high temperatures or electrical voltages to break the stable CO\({}_{2}\) molecule, limitations in raw materials, high operational costs and issues related to sustainability [4]. Given these factors, one of the most promising approaches for the reutilization of CO\({}_{2}\) is artificial photosynthesis. Despite the intermittency and variability in its intensity, sunlight is an inexhaustible energy source that is freely accessible. Natural photosynthesis is a process that transforms atmospheric CO\({}_{2}\) into sugar-based biomass. Artificial photosynthesis emulates this process as a model to attempt the storing of solar energy in chemical form. The generation of solar fuels under sunlight can be achieved through photocatalysis. In heterogeneous photosynthetic processes, solid semiconductors are employed, activated by the absorption of photons with energy exceeding the bandgap [5]. When irradiation reaches the semiconductor surface, an electron-hole pair is generated; the excited electron moves from the valence band to the conduction band, leaving a positive hole that facilitates oxidation and reduction with the redox pairs present in the reaction medium. The efficiency of the reaction is negatively impacted by the possibility of electron-hole recombination. In recent years, a variety of materials have been adopted as efficient and environmentally sustainable photocatalysts for CO\({}_{2}\) photoreduction, including TiO\({}_{2}\), WO\({}_{3}\), ZnO, CdS, CuO and g-C\({}_{3}\)N\({}_{4}\). Among these, TiO\({}_{2}\) stands out as one of the most promising due to its abundance, high chemical stability, low price and robust catalytic properties [6; 7; 8; 9; 10; 11]. However, it has a limitation due to its wide bandgap of approximately 3.2 eV, making it responsive only to ultraviolet (UV) light, which constitutes approximately 3% of the solar spectrum [12; 13]. Over recent decades, numerous studies have been published aiming to reduce the bandgap, enhance the light absorption and decrease the charge transfer and recombination rates of TiO\({}_{2}\) by coupling it with other semiconductors that have narrower bandgap values, such as WO\({}_{3}\), CdS, MoO\({}_{2}\)[8; 14; 15; 16; 17; 18]. WO\({}_{3}\) has received considerable attention due to its relatively narrow bandgap of approximately 2.6-2.8 eV, non-toxicity, resistance to photo-corrosion, chemical stability and capability to exist in a wide range of oxidation states (2+, 3+, 4+, 5+ and 6+), which facilitates the storage of photogenerated electrons [19].