# The Role of Surface Texture on the Photocatalytic H\({}_{2}\) Production on TiO\({}_{2}\)
## 3 Photocatalytic Hydrogen Production from Anatase TiO\({}_{2}\)
### Effect of the Sacrificial Agent  
The increment of the H\({}_{2}\) photoproduction rate can be obtained choosing properly the hole scavenger. Despite of the huge works on the use of methanol as a reductant, its implementation cannot at this stage be considered a green technology, because no green process for the production of methanol is really operative and its production is mainly carried out through the catalytic conversion of synthesis gas to methanol in fixed bed reactor [141]. However, methanol is an ideal substrate for the investigation of the process and this justifies the efforts carried out to clarify the operative mechanism. Particularly, the oxidation of gaseous methanol to CO\({}_{2}\) involves successive oxidative steps comprising both 'OH mediated oxidation and direct hole transfer to the adsorbed methanol with the production of formaldehyde, formic acid and CO\({}_{2}\)[45]. [PERSON] and coworkers investigated the photocatalytic reforming of a variety of oxygenates compounds under UV irradiated Pd/TiO\({}_{2}\) providing insight into the effect of molecular structure on the reaction kinetics [108]. The rate of H\({}_{2}\) production with the different hole scavengers is reported in Figure 12. From these experimental evidences some general rules were proposed to achieve high H\({}_{2}\) production rate from the reforming of alcohols over Pd -TiO\({}_{2}\) catalysts: the alcohol must have a hydrogen in the adjacent position (\(\alpha\)) with respect to the oxygen atom (few H\({}_{2}\) production was observed in the presence of acetic acid or acetone); the primary alcohols release CO\({}_{2}\) for decarbonylation and the related alkyl groups yields the corresponding alkanes (with the clear exception of methanol) and the carbon in the methylene groups finally undergoes complete oxidation to yield CO\({}_{2}\). [PERSON] et al. [142] underlined the essential role that the hydroxyl groups in the organic reductant have to achieve effective photoreforming, being chemicals such as alkanes and alkenes inactive. Figure 12: Hydrogen evolution rates under irradiated Pd/TiO\({}_{2}\) catalyst in the presence of different alcohols. Reprinted with permission from [102]. Copyright Elsevier, 2011. The mechanism of reaction of organic substrates in photocatalytic condition has been in-depth investigated since from the seminal works on photocatalysis [143; 144; 145; 146; 147; 148]. The fundamentals of semiconductor photocatalysis are now well understood and the presence of a complex network of oxidative paths involving both direct and mediated electron transfer at different reactive sites well accepted. Moreover, catalysts with different surface properties can promote transformation in which the relative weight of the different paths is different, with the consequent production of different byproducts [149; 150]. Furthermore, the photocatalytic reactivity is dramatically influenced by both the intrinsic (crystallographic phase, surface defects, band potentials, doping, etc.) and extrinsic (pH, composition of the solution, presence complexing ions, etc.) properties of the photocatalyst in a manner often difficult to be rationalized. Even if rationalized with some difficulties, also the optical properties (absorption vs scattering) of the photocatalyst affects dramatically the ability of a semiconductor to harvest the radiation and consequently to promote the photocatalytic process (e.g., the photocatalytic reforming of organic compounds) [151; 48]. As underlined above, photoreforming can be really considered a green technology only if the organic compounds used as sacrificial agents are wastes or they are produced through CO2 neutral processes. In this light the best candidates and the most studied substrates are:  
1. Ethanol, which is produced in a large number of fermentation processes of second-generation biomasses, as well as by the transformation of cellulose and lignocellulosic [152]. The mechanism of reaction for the ethanol photoreforming was investigated on Pt/TiO2 catalyst.