## 1 Introduction
For example, Pt/TiO\({}_{2}\) samples, under visible light and in the presence of glycerol (50 wt % initial concentration) and methanol, have produced about 7500 umol/L of H\({}_{2}\) after 5 h of irradiation time, while in presence of only TiO\({}_{2}\), the hydrogen production was equal to only 500 umol/L [30]. However, practical applications of photocatalytic hydrogen production based on a noble metal cocatalyst are restricted due to their high cost. So, as an alternative, it has been proposed to use different metals that are less expensive than noble metals but with interesting properties. This is the case with copper, which, as demonstrated in the literature [23; 31; 32], is able to enhance the photocatalytic properties of semiconductors such as TiO\({}_{2}\) and ZnO, promoting hydrogen production even in the presence of visible light. It has been reported that Cu nanoparticles, loaded on the TiO\({}_{2}\) surface by the photodeposition method, enhanced visible light absorption due to the plasmon resonance effect and also acted as cocatalysts to separate photogenerated charges in TiO\({}_{2}\)[33]. As a result, Cu/TiO\({}_{2}\) photocatalysts exhibited enhanced photocatalytic hydrogen production under visible light from a glycerol aqueous solution [33]. In this case, the maximum hydrogen production rate was equal to 0.24 mmol h\({}^{-1}\) g\({}^{-1}\). However, an excessive addition of Cu decreased the hydrogen production rate, suggesting that copper nanoparticles on the TiO\({}_{2}\) surfaces hinder photon absorption [33]. Moreover, the role of copper as a cocatalyst or a doping element for semiconducting materials has been clarified in a recent review paper [34]. In particular, the review highlighted several papers concerning different species of copper used as cocatalysts for titanium dioxide [34]. The author showed that, generally, copper is present on the semiconductor surface, which in turn is coupled to another semiconductor such as graphene and alumina [35]. ZnO has recently generated much interest within the scientific community. In fact, this semiconductor is a promising material due to its environmental stability, low cost compared with other metal oxides [31], and good photocatalytic property [36; 37]. ZnO has a band gap energy similar to TiO\({}_{2}\), being equal to 3.2 eV. As a consequence, it is active mainly in the presence of UV light [38]. In order to make it active under visible light irradiation, different strategies have been proposed in the literature for ZnO doping. Among these, the introduction of different types of metal dopant (e.g., Co, Mn, and Ni) into a ZnO semiconductor has been reported [39]. Different papers report the use of Cu-doped ZnO as an effective photocatalytic material [31; 40]. However, this photocatalyst was studied mainly for the degradation of organic pollutants from water and wastewater [40; 41; 42; 43]. In the recent years, the effect of Cu ions in ZnO nanorod arrays for photoelectrochemical water splitting under visible light was investigated [44]. In this work, the authors reported a considerable photocurrent density equal to 18 uA/cm\({}^{2}\) at 0.8 V during the water splitting reaction, which was about 11 times higher than that of undoped ZnO nanorod arrays. However, no data about the effective hydrogen production was reported. On the other hand, the use of Cu-doped ZnO as a photocatalyst for H\({}_{2}\) production from a glycerol aqueous solution has not been extensively investigated yet. For this reason, the aim of this work was to evaluate the influence of operating conditions on photocatalytic hydrogen production from glycerol solutions in the presence of a visible-light-active Cu-doped ZnO photocatalyst, previously optimized towards the photocatalytic degradation of methylene blue and oxidation of arsenite into arsenate [37].