## 3 Results
### Set Up and Optimization of the Catalyst Deposition Process  
The supports employed in this work were glass slides typically used in microscopy with the dimensions of 26 \(\times\) 76 \(\times\) 1 mm\({}^{3}\) because they are inexpensive and easily available in the market. The deposition was performed through a homemade dip coating apparatus, and a suspension of P25 powder in distilled water was used as a base for the impregnation of the glasses. Different coating procedures have been reported in the literature for obtaining structured catalysts or microchannel reactors. They have been extensively reviewed in [28] for the most frequent applications. Surface pretreatment is often suggested to increase the roughness, which is achieved via a thermal pretreatment (e.g., oxidation at high temperatures), anodization (suitable only for metals, e.g., Al), and an acid treatment (used here, as it is the only one applicable to glass substrate). The suspension method allows for the covering of the substrate with a pre-synthesized catalyst, and it is, therefore, possible to learn and control the main features of this catalyst via basic characterization. On the contrary, a sol-gel technique can be adapted to form the catalyst (oxide) directly over the surface that will be coated; however, using this method, the properties of the material are  
Figure 2: (**a**) Final version of the support printed and set up in ABS; (**b**) Detail of the clip connection to the shaft. less easily predictable and controllable. Both these methods and some hybrid variants have been applied to the coating of TiO\({}_{2}\). Electrophoretic and electrochemical plating have usually been applied for coating over-conducting substrates and for the deposition of metal layers. Impregnation can be used to support metals or oxide precursors over porous supports, but it does not fit glass supports unless a porous material has been previously added. For instance, it can be used for the addition of metal promoters over an oxide semiconductor coating. Chemical or physical vapor deposition (CVD or PVD) may be suitable solventless deposition techniques when volatile precursors are available. A more complex and dedicated apparatus is needed with respect to the previously described methods. Overall, the suspension technique allows researchers to obtain, in an easy and rapid way, relatively thick coatings (ten-hundreds of microns) of oxides over any kind of surface, making it a quite simple and scalable technique. The homemade dip coating apparatus, equipped with a 3D-printed adapter, allows for the processing of up to six samples at the same time. A typical immobilization experiment consists of the immersion of glass tile(s) at a selected constant speed by means of a step motor equipped with reducing gears. The composition of the mother suspension was optimized during the study. The first coatings were applied to the untreated tiles through several subsequent dipping cycles, as reported in Table S1 (Supporting Information File). In the blank experiment, the glasses dipped into distilled water and dried at 500 \({}^{\circ}\)C under air did not show any weight variation. On the other hand, after the first cycle of dipping with P25/water suspension at a 5% \(w/w\), an average loading of 0.7 mg of powder per tile was obtained with a \(\pm\)0.1 mg discrepancy after tens of repetitions. This P25 suspension was selected as it is fairly stable once the mechanical stirring is halted, allowing us to carry out the dipping while avoiding a massive precipitation of the titania, which could impact the reliability of the deposition itself. The tiles were clamped on the top, so the resulting powder was distributed over approximately 80% of the surface. Two cycles only led to a slight increase in the deposited P25, up to 1.2 mg, with a mean value of 0.9 mg; thus, the deposition after each cycle was ca. 0.5 mg. It is noteworthy to point out that the deposited layer was not homogeneous.