## 3 Results and Discussion
However, our experimental data have shown that the catalytic activity of CuO/ZnO is limited to short reaction times, as CuO is susceptible to photodegradation. So, synthesizing a strong CuO-based catalyst that can endure prolonged UV irradiation is crucial for its successful application in photochemical processes. Maintaining its structural integrity and active sites while achieving high conversion and selectivity is crucial for a durable catalyst. Figure 1: TEM images of (**A**) CuO/ZnO NPs and (**B**) Cu\({}_{2}\)O/ZnO-PDA NPs embedded in PDA shell. Scale bar: 200 nm. To deal with this, a protective PDA shell was applied to the CuO/ZnO catalyst. PDA, derived from the natural oxidation of dopamine, was selected because of its many advantages: it is eco-friendly, exhibits strong adhesion to various substrates, is easy to synthesize under mild conditions, and provides chemical stability [41; 42]. The synthesis of the Cu\({}_{2}\)O/ZnO-PDA catalyst was sustainably performed in water using an ultrasonic processor with an immersion probe, taking only 15 min. The presence of PDA with its catecholamine moiety [43], during synthesis, led to the reduction of copper (II) to copper (I) (obtaining Cu\({}_{2}\)O), as demonstrated in the previous article [27]. Even Cu\({}_{2}\)O/ZnO-PDA was used in the photoreduction of MA to SA, yielding 9% of SA with a selectivity of 15% after 24 h of reaction time (the secondary product was FA with a yield of 24% and a selectivity of 40%), 17% of SA with a selectivity of 24% after 48 h (FA 19% with a selectivity of 27%), and 25% of SA with a selectivity of 27% after 72 h (FA 7% with a selectivity of 8%). These results were comparable to those obtained in the presence of CuO/TiO\({}_{2}\) (32% of SA yield with 32% of SA selectivity after 24 h) (Table 1 and Figure 2). Despite light exposure, the PDA-coated catalyst remained efficient over 72 h, consistently yielding 25% SA. Sodium ascorbate, a sustainable and biocompatible reducing agent [44], was added to the reaction system to further enhance reaction outcomes. Sodium ascorbate offers several advantages: it is non-toxic, water-soluble, and compatible with green chemistry principles. When added in equimolar amount relative to the substrate, significant improvements were observed; after 72 h, the reaction achieved 78% yield of SA with excellent selectivity of 78%. A test was conducted in deuterated water to determine whether the ascorbate was the direct hydrogen donor for the substrate or if the hydrogen came from the water, as initially intended. In the D\({}_{2}\)O experiment, the conversion of MA (20%) was observed by the \({}^{1}\)H NMR spectrum, but the signal corresponding to SA was found with an integration value equal to half of that observed when the experiment was carried out in water (Figure S11); \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Reaction Time (h)**} & **\% SA Yield** & **\% FA Yield** & **\% MA Conversion** \\  & **(mol/mol)** & **(mol/mol)** & **(mol/mol)** \\ \hline
24 & 9 & 24 & 60 \\ \hline
48 & 17 & 19 & 70 \\ \hline
72 & 25 & 7 & 92 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Conversion of MA and yield in SA during the time (24, 48, and 72 h) of aqueous MA photoreduction reaction, employing Cu\({}_{2}\)O/ZnO-PDA as photocatalyst under 254 nm. Figure 2: Conversion of MA and selectivity to SA during the time (24, 48, and 72 h) of aqueous MA photoreduction reaction, employing Cu\({}_{2}\)O/ZnO-PDA as photocatalyst under 254 nm. this suggested the formation of the deuterated SA (yield 18%), confirmed by GC-MS analysis, which identified a molecular mass signal corresponding to that of SA + 2. These results imply that sodium ascorbate is not the direct hydrogen donor but acts as a radical initiator, promoting electron transfer in the reaction; in fact, the photolysis of ascorbic acid in an inert atmosphere and water can result in the formation of radicals, like ascorbate radicals and superoxide radicals [45; 46]. More research is needed to define the actual mechanism of the reaction.