# 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 mechanism proposed followed the scheme described above where protons are reduced by electrons cumulating into Pt particles, while the chemisorbed or physisorbed ethanol is stepwise oxidized by holes [153]. The stepwise oxidation of the substrate gives predominantly acetaldehyde, acetic acid and CO2, but also compounds with larger number of carbon atoms, such as acetone and 2-butenal. 2-butenal (croonaldehyde) can be formed by the aldol condensation of two acetaldehyde molecules or through the formation of the 'CH3 CHO radicals. In a similar way a possible path for the production of acetone is the reaction of acetaldehyde (or acetic acid) with 'CH3 radicals [154]. The accumulation of byproducts can give a partial poisoning of the catalyst with its progressive deactivation [155]. 2. Glycerol, which is the main byproduct of the biodiesel industry, is alternatively produced through the sorbitol hydrogenolysis or the fermentation of glucose [156; 157]. Considering that (_i_) the total reforming of one molecule of glycerol gives 7 molecules of H2 and (_ii_) the raw glycerol obtained as byproduct of the biodiesel industry is a complex mixture--containing methanol, water, inorganic salts, free fatty acids, triglycerides, and methyl esters--that requires expensive treatments to be purified, the energetic valorization of glycerol is a very promising path. The mechanism of transformation of glycerol on bare TiO2 was investigated by [PERSON] et al. [140] Glyceraldehyde (GAD), dihydroxyacetone (DHA), formaldehyde (FORM) and glycolaldehyde (GLY) were identified as the main byproducts obtained through two different reaction paths. GAD and DHA are the products formed from the OH radical mediated oxidation, while FORM and GLY are produced through a direct electron transfer (hole transfer from TiO2 to the chemisorbed glycerol). Interestingly, the ratio between the produced byproducts is strongly affected by the surface properties of the catalyst. The direct hole transfer is favored on catalysts with more defective surfaces able to strongly adsorb the substrate on the more defective (and more reactive) sites. 3. Glucose, and other simple or complex sugar, which are mainly obtained from the degradation of cellulose materials [158; 159]. The photoreforming of sugars, starches and cellulose was demonstrated for the first time on the ternary photocatalyst RuO2/TiO2/Pt [155]. As reported above, these substrates are ideal candidates to promote an efficient photoreforming because of the presence in the sugar molecules of numerous hydroxyl groups with hydrogen atom in \(\alpha\) position. The more complex the carbohydrates, the lower is the efficiency. [PERSON] and coworkers investigated systematically the glucose photoreforming under irradiated TiO2 decorated with different metal cocatalysts in the absence of oxygen, and observed the following reaction rate: Pd \(>\) Pt \(>\) Au \(\approx\) Rh \(>\) Ag \(\approx\) Ru. The presence of O\({}_{2}\)--as expected--and the decrement of the pH inhibited the hydrogen production [159]. Recently, [PERSON] et al. compared the hydrogen production with C2-C5 polyols, cyclic alcohols and mono and di-saccharides under irradiated Pd-TiO\({}_{2}\). For the compounds in the first class the hydrogen evolution rate is directly related to the number of OH groups and the availability of hydrogen atoms in \(\alpha\) position to the hydroxyl. This rule is not followed by the cyclic alcohols and especially sugar (glucose, fructose and sucrose). For the reforming of cyclic alcohols it was also observed the dehydrogenation and decarbonylation of \(\alpha\) CC bond [107]. Furthermore, other organic compounds were investigated such as diols, superior alcohols and polysaccharides. [PERSON] and coworkers focused on the photoreforming of cellulose on a TiO\({}_{2}\) catalyst loaded with both Pt and Ni with UV irradiation. In the presence of both cocatalysts the photoreforming occurred with significant hydrogen production.