**1. Introduction**

Fossil fuel depletion and environmental concerns have resulted in the search for clean energies, with one alternative being hydrogen [1]. Its use has two main advantages [2,3]: i) a high chemical energy per mass (120 KJ/g), superior to that of many fossil fuels, and ii) its combustion only results in water; therefore, it does not emit any toxic substance or greenhouse gas into the atmosphere.

Nevertheless, hydrogen does not exist in nature in its molecular H2 form but combined to other elements; thus, it requires dedicated methods for its production. Therefore, whether or not the use of hydrogen as an energy vector can be termed as "fully green" is dependent on its method of production.

Currently, the most widespread hydrogen production methodologies are hydrocarbon reforming with water vapor and water electrolysis. Hydrocarbon reforming has the disadvantages of being based on raw materials which are taken from non-renewable fossil sources and, therefore, the co-generated CO2 directly impacts the environment by the greenhouse effect. An additional drawback of hydrocarbon reforming with water vapor is its high operating temperature. On the other hand, regarding the production of hydrogen through water electrolysis, its main associated problem is the high consumption of electrical energy to carry out the process. Solar thermal energy can be used as an alternative for water electrolysis but, in this case, large and expensive facilities are needed.

Recently, in addition to the mentioned technologies, innovative techniques have been being developed and could be complementary to those already existing in the medium-term future. Some of these hydrogen production techniques are plasma technology [4], biological production methods such as dark fermentation [5,6] or the photocatalytic reforming of oxygenated organic compounds [7,8]. The photocatalytic reforming of oxygenated organic compounds consists in the treatment of these compounds with light radiation in the presence of water, at room temperature and under anaerobic conditions, to generate gaseous hydrogen and carbon dioxide. The potential of hydrogen production through photocatalytic reforming is fulfilled when biomass residues (bio-glycerol or glucose, among others) are used as oxygenated organic compounds since, in this case, the generated CO2 was previously consumed by the biomass during its growth, so there is no net emission of CO2 but a closing of the carbon cycle [9]. In the process, light is used to activate a semiconductor, promoting electrons from the valence to the conduction band. The oxygenated organic compound is used as a sacrificial agen<sup>t</sup> to favor the elimination of the positively-charged holes, whereas electrons are used to reduce protons and generate H2. As for the sacrificial agents, glycerol is an excellent candidate since it is a by-product of biodiesel production [10].

One of the keys to the success of this emergen<sup>t</sup> technology is the development of suitable catalysts (i.e., semiconductors) which are able to maximize light harvesting and therefore the hydrogen production [11]. TiO2 is the most widely used semiconductor as a result of its high photocatalytic activity and due to the fact that it is inexpensive, not toxic and biologically and chemically inert [12]. Its main drawback is its band gap value (ca. 3.2 eV), which means that only ca. 5% of solar irradiation is absorbed. Furthermore, it also exhibits a high electron–hole recombination rate, which is detrimental to the photocatalytic activity [13].

One alternative to overcome these problems is the incorporation of metals to the semiconductor [14] (Figure 1), which can shift the absorption to the visible light and also act as electron traps, thus preventing electron–hole recombination. Noble metals such as silver [15], gold [16], platinum [17] or palladium [18] have been found to be particularly e ffective, although there is a need to implement some more cost-e ffective transition metals such as iron [19], nickel [20,21] and copper [22,23].

**Figure 1.** Activation of titania using a metal as a co-catalyst.

In the present piece of research, nickel structural (particle size) and chemical properties (oxidation state) in Ni-modified titania photocatalysts has been addressed, and their influence on hydrogen production from glycerol photo-reforming studied. Two catalyst synthetic methods (impregnation vs. deposition–precipitation) and catalyst pre-reduction treatment were analyzed and their influence on the amount of hydrogen photo-produced revealed under both UV and solar radiation.
