## 3 Results and Discussion  
Two copper oxide-based catalysts supported on ZnO were prepared and compared to CuO/TiO\({}_{2}\), a photocatalyst widely recognized for its superior photocatalytic efficiency [36]. However, concerns over the potential health risks related to TiO\({}_{2}\)[10] have driven the exploration of alternative materials [37], arousing interest in developing less hazardous photocatalyst supports, such as ZnO. CuO/TiO\({}_{2}\) was synthesized by the impregnation method, as reported in the literature [31], yielding the copper-doped titania catalyst. The CuO/ZnO catalyst, instead, was synthesized in two steps; first, ZnO NPs were prepared using sodium lignocellulosic, a by-product of the paper industry, as a template for NPs; then, these NPs were doped with copper (II) salt and calcined. The resulting catalyst was characterized using XRD and TEM. XRD revealed a pattern with characteristic signals attributable to the hexagonal wurtzite structure of ZnO, which were not influenced by the small amount of copper oxide present, likely undetectable due to the sensitivity limits of the technique (Figure S1A) [27]. TEM images showed the approximately spherical shape of the CuO/ZnO NPs with dimensions of 25 \(\pm\) 11 nm (Figure 1A). Another catalyst, composed of Cu\({}_{2}\)O and ZnO, was prepared using a natural shell of PDA synthesized from dopamine hydrochloride in a basic solution (Cu\({}_{2}\)O/ZnO-PDA). The synthesis of Cu\({}_{2}\)O-doped ZnO NPs embedded in a PDA shell was easily carried out in 15 min in a water solution using a probe sonicator. This solution containing the photocatalyst was characterized by XRD and TEM [27]; the diffractogram showed broad peaks attributable to ZnO; the signals of copper oxide were not visible due to the low Cu\({}_{2}\)O amount (Figure S1B). To investigate the oxidation state of copper oxide, the same synthesis was performed without the addition of Zn (using dopamine hydrochloride and CuSO\({}_{4}\cdot\)5H\({}_{2}\)O as starting materials) [27] since the reduction of CuSO\({}_{4}\) is not influenced by the presence of Zn [38], as it is driven by the reducing properties of PDA [39]. The XRD analysis revealed the characteristic signals of Cu\({}_{2}\)O (broad peaks at 20 values of 37\({}^{\circ}\), 42\({}^{\circ}\), and 61\({}^{\circ}\)) [40] (Figure S1C). TEM images showed Cu\({}_{2}\)O/ZnO-PDA NPs as a uniform distribution of spherical particles with an average diameter of 14 \(\pm\) 2 nm, mostly encapsulated within PDA shells measuring 230 nm in size (Figure 1B). Initial tests were conducted by irradiating aqueous solutions of MA with 254 nm light in the presence of the photocatalysts. The best results were observed with CuO/TiO\({}_{2}\) at 0.5% molar ratio Cu/ reagent, achieving a 32% yield of SA after 24 h of reaction time with 32% selectivity at full reagent conversion; it is assumed that other products, undetectable by NMR, have been formed. The results were not improved by longer reaction times as the catalyst loses its activity over extended periods, as observed in MA reduction reactions carried out over 24 h under the same conditions reported above; further pH modifications of the reaction mixture were tested, but they did not influence the results of the reaction. Instead, the presence of air in the reactor inhibited the reaction, leading to only FA as the product (32% yield); so, it is crucial to conduct the process under an inert N\({}_{2}\) atmosphere. The CuO/ZnO catalyst was active in the reduction of MA to SA, producing 10% of yield after 24 h (47% yield of FA with 57% of MA conversion), thus exhibiting a lower catalytic efficiency compared to CuO/TiO\({}_{2}\), which led to 32% of SA yield with a total MA conversion (FA was not observed). CuO/TiO\({}_{2}\) was superior in selectivity, reaching 32%, while CuO/ZnO only achieved 17%. To enhance the catalytic performance of CuO/ZnO, the reaction time should be extended.