## 1 Introduction  
Hydrogen production from water under sunlight is, nowadays, one of the most ecological and rational alternatives for obtaining an energy carrier. Hydrogen is a zero-carbon-emission fuel and it is expected to be an important energy source in the near future [1]. Actually, hydrogen is mainly produced by natural gas through the steam methane reforming process [2]. Unfortunately, this approach involves the use of fossil fuels and CO\({}_{2}\) production, and for this reason, it is not considered sustainable. So, attention is increasingly being paid to eco-sustainable processes that can be carried out in mild conditions (at room temperature and pressure) and without the use of fossil fuels. In recent years, the photocatalytic process for hydrogen production from water has attracted considerable interest [3; 4; 5]. In particular, by means of heterogeneous photocatalysts, hydrogen can be produced mainly by two processes: the direct splitting of water into H\({}_{2}\) and O\({}_{2}\)[6; 7] or the photoreforming of organic compounds [8]. This latter process represents a very attractive method for the removal of organic pollutants in wastewaters with the simultaneous vaporization of these substances [9; 10; 11]. Photocatalytic H\({}_{2}\) production from water using organic substances as sacrificial agents has been studied since the 1980s [12]. For this purpose, different organic compounds such as methanol [13], ethanol [14], sugar [13], glycerol [15], and lactic acid [16] have been used. In particular, the use of glycerol as a sacrificial agent for H\({}_{2}\) production via the photoreforming process has aroused great interest [17; 18]. The reason for this is the huge production of glycerol, the content of which in by-product streams from the biodiesel industry is about 10 wt %, while glycerol itself still has limited demand in the market [19]. In the literature, there are several papers on the efficiency of photocatalysis for hydrogen production starting from a glycerol aqueous solution [18; 20; 21]. In particular, different photocatalysts have been tested and the most investigated ones were TiO\({}_{2}\)-based materials [12; 22]. However, the fast recombination of photogenerated electron-hole pairs and the large band gap of semiconductors may hinder their photocatalytic performances. Forming composites with another semiconductor or adding metal nanoparticles represents an interesting strategy that has been developed to solve this problem. For example, photocatalytic systems, such as TiO\({}_{2}\)-CuO\({}_{\mathrm{x}}\), TiO\({}_{2}\)-Pt, TiO\({}_{2}\)-Pd, and NiO-TiO\({}_{2}\), have been studied for photocatalytic hydrogen production from a glycerol aqueous solution [21; 23]. These synthesized photocatalysts proved to be much more active with respect TiO\({}_{2}\) alone, since the separation of the electron-hole pairs was improved. In addition, with the ZnO-ZnS/graphene composite, using a glycerol solution at 10 wt % initial concentration, the hydrogen production was equal to 289 umol/L after 3 h of UV irradiation time [24]. Interesting results have also been reported for the Ag\({}_{2}\)O/TiO\({}_{2}\) photocatalyst that, in the presence of glycerol and after 4 h of UV irradiation time, was able to produce 2100 umol/L [25]. However, in order to use solar energy, the development of visible-light-driven photocatalytic systems is highly required. For this purpose, the use of a noble metal, such as Au, Ag, and Pt, as a cocatalyst element or for photocatalyst doping has been reported [26; 27; 28]. Generally, the effect of the presence of the noble metal is the plasmon resonance absorption property in the visible light region [29]. Specifically, the noble metal's roles are the enhancement of visible light absorption of the photocatalyst and the separation of photogenerated charges in semiconductors, such as TiO\({}_{2}\) or ZnO.