## 3 Results
### Photoreduction of CO\({}_{2}\) over Deposited Photocatalysts  
The consecutive/parallel reactions that took place are summarized in Scheme S1 (Supplementary Information). A test was carried out on the immobilized titania modified via surface metallization with silver NPs (recipe #16; 0.01 M AgNO\({}_{3}\)). The test was conducted at 80 \({}^{\circ}\)C with the help of a 3D support printed in polypropylene (PP) mixed with glass fibers, which was able to resist at the chosen temperature. Twelve glass tiles were used for the test with a cumulative loading corresponding to 50 mg/L (compared to the test with powder P25, which used 31 mg/L). A total of 1.67 g/L of sodium sulfite was used as HS in every case. The tests were performed under 18 bar of CO\({}_{2}\) for 24 h at pH 14. The glass tiles used for this test had an estimated Ag loading of 3.6% \(w/w\). The results obtained from the tests are reported in Figure 8.  
The productivity of formic acid obtained with the Ag/P25 deposited catalyst was about three times the one obtained with the dispersed P25 powder, with values of ca. 20 mol/kg\({}_{\text{cat}}\) h and 6 mol/kg\({}_{\text{cat}}\) h, respectively. The deposited catalyst also showed a  
Figure 8: CO2 photoreduction carried out with P25 in the form of powder and Ag/P25 immobilized. 80 \({}^{\circ}\)C, 1.14 L, 24 h, 18 bar CO2, 1.67 g/L Na2 SO3, 38 mg P25 or 50 mg Ag/P25. productivity of hydrogen ca. that was one sixth of the one obtained in the test with the suspended P25 powder (ca. 0.7 mol/kgcat h and 4 mol/kgcat h, respectively). The production of H\({}_{2}\) is attributed to the consecutive photoreforming of the produced HCOOH after consumption of the inorganic hole scavenger (Scheme S1, Supplementary Information). Its lower productivity with the Ag-promoted sample testifies its lower activity in this reaction. The productivities of methane and carbon monoxide were negligible during the test with the commercial P25 powder, while for the deposited Ag/P25, the productivity obtained was low but not nil. In particular, 15 mmol/kgcat h and 37 mmol/kgcat h were obtained for CH\({}_{4}\) and CO, respectively. The conversion of the hole scavenger due to the long reaction time was over 95% in both cases. Additionally, the slides were collected after the test, washed with distilled water, and dried overnight at 105 \({}^{\circ}\)C, then weighed to verify any loss of catalyst during the test, which was found to be negligible. Therefore, it is demonstrated that, to counteract the loss of activity due to surface coating, it is possible to tune the formulation of the catalyst, achieving an improvement of formic acid productivity with respect to bare P25 just by the addition of a simple metal co-catalyst. The effects of silver deposition on titanium dioxide are indeed well known; this metal presents an LSPR band, thus enhancing visible light exploitation, though the catalyst was characterized by a wider band gap, and it improves the separation of the photogenerated charges by acting as an electron sink [50]. The lower hydrogen productivity of the Ag/P25-deposited catalyst suggests that the latter was less active in photoreforming the obtained organic products [51], thus preventing the consecutive consumption of formic acid. Second, the deposited silver likely played a less active role in photoreforming the obtained species compared to the CO\({}_{2}\) photoreduction. In any case, the selectivity of the deposited catalyst towards liquid products should be further investigated. Many authors have reported an increase in photocatalytic activity after functionalization [52; 53; 54]; however, being able to directly modify the immobilized titania through a simple dipping process and perform CO\({}_{2}\) photoreduction with such a system represents a great advancement for the technique.