## 4 Conclusions  
The present study demonstrates that FSP is a highly effective synthesis method for producing photocatalysts with well-defined properties to enhance photocatalytic performance. Three semiconductors were synthetized: TiO\({}_{2}\), WO\({}_{3}\) and a type-II heterojunction, WO\({}_{3}\)/TiO\({}_{2}\) (40/60 weight ratio). These three semiconductors were subsequently tested for their efficacy in CO\({}_{2}\) photoreduction. TiO\({}_{2}\) is one of the most common photocatalysts used in the world. Of all commercial products, the TiO\({}_{2}\) (P25) produced by Evonik (formerly Degussa) is not only one of the best known in the world, but is also produced industrially in a flame with a similar technique to FSP, using titanium tetrachloride (TiCl\({}_{4}\)) in an oxygen-rich flame. For these reasons, TiO\({}_{2}\) P25 (Evonik) was therefore used as a reference catalyst for benchmarking purposes. All catalysts tested were characterized by X-ray diffraction (XRD), nitrogen physisorption, scanning electron microscopy (SEM) and diffuse reflectance spectroscopy (DRS) UV-Vis. The latter returned values that align with those of the literature. The slightly lower bandgap of the TiO\({}_{2}\) FSP compared to P25 can be attributed to the higher rutile content, as confirmed by the XRD analysis. The experimental tests were conducted using a unique high-pressure reactor operating at 18 bar. The results showed that the two single oxides, TiO\({}_{2}\) and WO\({}_{3}\), despite their significant difference in bandgap energy (3.10 eV vs. 2.73 eV), exhibited comparable productivities in CO\({}_{2}\) photoreduction, with each slightly underperforming relative to the P25 benchmark, which had a productivity of 16.9 mol/kg\({}_{\text{cat}}\cdot\)h.  
A key advantage of FSP emerged in the synthesis of the mixed oxide WO\({}_{3}\)/TiO\({}_{2}\), where intimate contact between the phases was obtained, leading to the formation of a type-II heterojunction. This heterojunction is evidenced by the slight reduction in the bandgap compared to the pure WO\({}_{3}\), as observed in the DRS measurements. As shown in Figure 1, the type-II alignment facilitated charge separation by directing the photogenerated electrons to the conduction band of the WO\({}_{3}\) and the holes to the valence band of the TiO\({}_{2}\), suppressing recombination and improving the photocatalytic efficiency. The WO\({}_{3}\)/TiO\({}_{2}\) (40/60 weight ratio) catalyst exhibited a remarkable two-fold increase in formic acid productivity compared to P25, reaching 36.5 mol/kg\({}_{\text{cat}}\cdot\)h, which is among the highest values reported in the literature. To further reinforce the advantages of FSP, a comparative benchmark with sol-gel or other synthesis techniques would be beneficial. However, the results presented herein already indicate that FSP enables intimate contact between semiconductor phases, leading to superior photocatalytic performance. By achieving an outstanding productivity of 36 mol/kgcat-h, this method leveraged the synergy between flame spray pyrolysis (FSP), high-pressure photoreduction and the type-II heterojunction. FSP proved to be an efficient and scalable synthesis technique for the synthesis of type-II heterojunctions, where phase interaction plays a crucial role in enhancing activity. The integration of these techniques not only allows us to optimize catalytic performance, but also paves the way for innovative applications in industrial catalysis. Conceptualization, I.R. ; methodology, M.T. and A.G.; validation, F.G., I.R. and M.T. ; formal analysis, S.N.D. ; investigation, A.G. and M.T. ; resources, I.R. ; writing--original draft preparation, M.T. ; writing--review and editing, M.T., I.R. and F.G.; visualization, M.T. and S.N.D. ; supervision, I.R. ; project administration, I.R. ; funding acquisition, I.R. All authors have read and agreed to the published version of the manuscript.