*2.5. Photocatalytic Activity*

Therefore, among all materials prepared in this work, only CP-Cu2O/In2O3 was tested in CO2–H2O coprocessing, according to procedure described in Methods section, as reputed the only one able to carry out the redox process. Three different reactors were used for running the reaction, all under Xe-lamp irradiation (Figure 8).

**Figure 8.** Reactors used for running the photochemical reaction. (**a**) Bulk nano-sized catalyst in a batch-reactor. (**b**) Flow-reactor loaded with bulk nano-catalyst. (**c**) Nano-sized catalyst finely dispersed on the wall of the reactor, simulating a nano-film.

When reactor (a) or (c) was used, the reaction gas was sampled with a gas-syringe, and with reactor (b) the gas was directly injected into the GC-column. Hydrogen and reduced species of CO2 were monitored at regular intervals of time up to 6 h. (see Figure S2 in Supplementary Materials) Table 6 gives the results for the reactors (a)–(c). Using bulk nano-catalysts, after 3 h of irradiation of a H2O-saturated CO2 stream at 298 K no reduction products were observed. Conversely, when

the nano-catalyst was finely dispersed on the surface of the reactor the same mixture gave positive formation of H2 and traces of reduced CO2 species.


**Table 6.** Photocatalytic reaction for three different reactors.

Results in Table 6 shift attention towards the fundamental role played by reactor design and reaction setup in carrying out a process difficult as the solar fuel production. It is clearly visible how the catalyst film distribution (reactor c), allowing for a better and more homogeneous light penetration to the photo-active centers with respect to the massive-powdered-material (reactors a,b), allows the reaction to go. In practice, a film-like distribution of the photomaterial is preferred to its bulk packing as the larger surface of the photo-catalyst allows more photons to be active. Most likely, if the illumination system is changed with respect to the one we have used (Xe-lamp), the reactor (b) will be working too. The lesson learned with such synthesis of photo-materials and photocatalytic experiments, prompts us to a more focused approach to photocatalyst development coupled to reactor design and engineering for a more active conversion of CO2 and water into energy products.
