## 2 Results and Discussion
### Photoreduction of CO\({}_{2}\)  
Tests were performed at a pH of 14, 18 bar and 6 h of reaction time, with 1.66 g/L of sodium sulfite as a hole scavenger (HS) and 31 mg/L of catalyst concentration on the prepared catalysts, as previously optimized. The productivity results and HS conversions are reported in Figure 6. Starting from the productivity, the total stored energy was calculated considering the (Lower Heating Value) values of the primary reduction products [32], using Equations (1) and (2). Both the productivity and the calculated total stored energy are reported in Table 2.  
\[n_{i}(\mathrm{mol})=\mathrm{productivity}_{i}\bigg{(}\frac{\mathrm{mol}}{\mathrm{ kg}_{\mathrm{cat}}\times\mathrm{h}}\bigg{)}\times t(\mathrm{h})\times m_{cat}( \mathrm{kg}) \tag{1}\]  
The performance of our benchmark, namely, a commercial TiO\({}_{2}\) P25 produced by Evonik (formerly Degussa) and supplied by EIGENMANN & VERONELLI S.p.A, was ca. 15% better than that of the titania sample prepared by FSP. This can be attributed to the combined effects of the higher anatase content and higher surface area of the P25 sample. Figure 5: (**Left**) Diffuse reflectance (DR) UV–Vis spectra of FSP catalysts; (**right**) Tauc plots of FSP catalysts. Figure 6: Productivity and HS conversion of tested catalysts. The performances of the two bare oxides prepared by FSP were comparable, with the FSP titania showing a slightly lower productivity of HCOOH (13.9 mol/kgcat h) compared to that of the FSP WO\({}_{3}\) (14.7 mol/kgcat h). This result is attributed to the lower bandgap of the WO\({}_{3}\), which suggests a better utilization capacity of the absorbed light when provided the use of the same UV lamp. The WO\({}_{3}\)/TiO\({}_{2}\) 40/60 showed a two-fold increase in HCOOH productivity compared to the P25 with an astonishing value of 36.5 mol/kgcat h, among the highest reported in the literature, as reported in Table 3.  
The hole scavenger conversion followed the productivity of the different catalysts, with the mixed oxide reporting a conversion of 77% compared to the 43% of the WO\({}_{3}\) and the 40% of the TiO\({}_{2}\). The greater HS conversion is in line with the higher productivity recorded. It should be underlined that HS conversion should be maintained well below 90 percent to avoid the photoreforming of the obtained products [33]. As shown by the DRS measurements, the TiO\({}_{2}\) FSP sample showed a slightly lower bandgap compared to the P25, which can be attributed to the higher rutile content. When TiO\({}_{2}\) and WO\({}_{3}\) are put in intimate contact, such as through FSP synthesis, a type-II heterojunction is formed, as confirmed by the DRS measurements. The latter showed a bandgap slightly inferior even to that of the pure WO\({}_{3}\). In type-II heterojunctions, the charge recombination is reduced by spatially separating the photogenerated electrons and holes, with the electrons migrating to the lower conduction band (WO\({}_{3}\)) and the holes to the higher valence band (TiO\({}_{2}\)). In this case, a small reduction in the measured bandgap was observed, as reported in Figure 1. Some authors have also reported that the greater acidic character of WO\({}_{3}\) should interact more with the carbonate ions present in the solution, due to its greater acidity [34]. As previously mentioned, CO\({}_{2}\) photoreduction is now a well-known and growing field. The photocatalytic reduction of CO\({}_{2}\) to formic acid represents a small fraction of the research carried out, despite the high added value of the formic acid produced. Recently, an interesting review was published on this topic [35]. A quick overview of the relevant productivities recorded in the literature is shown in Table 3.