# The Role of Surface Texture on the Photocatalytic H\({}_{2}\) Production on TiO\({}_{2}\)
## 1 Introduction
### Semantics and Thermodynamics
For example, formic acid and glycerol are characterized by a H\({}_{2}\)O to CO\({}_{2}\) ratio of 0 and 1, respectively, and in both cases their reforming is a downhill reaction:  
\[\mathrm{HCOOH}\rightarrow\mathrm{H_{2}}+\mathrm{CO_{2}} \mathrm{\Delta G^{0}}=-83\;\mathrm{kJ}\;\mathrm{mol^{-1}} \tag{7}\]  
\[\mathrm{C_{3}H_{8}O_{3}+3\;H_{2}O\to 7\;H_{2}+3\;CO_{2}} \Delta\mathrm{G^{0}}=-300\;\mathrm{kJ}\;\mathrm{mol^{-1}} \tag{8}\]  
whereas in the case of ethanol and propan-2-ol, where the H\({}_{2}\)O to CO\({}_{2}\) ratio is larger than 1, the Gibbs free energy of their reforming is positive:  
\[\mathrm{C_{3}H_{8}O+5\;H_{2}O\to 9\;H_{2}+3\;CO_{2}} \Delta\mathrm{G^{0}}=183\;\mathrm{kJ}\;\mathrm{mol^{-1}} \tag{9}\]  
\[2\;\mathrm{H}_{2}\mathrm{O}_{\mathrm{(g)}}\to 2\;\mathrm{H}_{2\mathrm{(g)}}+ \mathrm{O}_{2\mathrm{(g)}}\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\; \Delta\mathrm{G}^{0}=229\;\mathrm{kJ}\;\mathrm{mol}^{-1} \tag{2}\]0  
Therefore, when the previous reforming reaction are promoted by light we can be in the presence of either photosynthetic or photocatalytic processes, depending on the \(\Delta\mathrm{G}\). Nevertheless, we feel that the claim of artificial photosynthesis in the cases of reactions (6), (9) and (10) would be exaggerated, even though their positive \(\Delta\mathrm{G}\) would make such a claim technically correct. In the case of organic reforming the oxidation potentials are less positive, and the kinetics of reaction less complicated compared with water oxidation, even though also in these cases multielectron and multiproton transfers are involved (Figure 1). Accordingly, also the minimum photon energy required will be lower. Focusing our attention to TiO\({}_{2}\), its band gap is larger than 3 eV; therefore, its excitation requires UV photons, or in terms of energy 300 kJ mol\({}^{-1}\), which is larger than the \(\Delta\)G\({}^{0}\) of all the reactions considered until now (Figure 2a). Considering that reactions 1-10 involve all multiple electron transfers, the use of TiO\({}_{2}\) in hydrogen photo production implies a significant waste of photon energy [9]. Returning to the energy diagram of Figure 1 we can argue that the energy loss is mainly due to the valence band, which is considerably more oxidizing compared with the reduction potential of the organic compounds considered until now. Conversely, the conduction band potential is very favorable, because it allows water reduction with a reasonable overpotential, which ensures a sufficient drive to carry out the reaction, without important losses. To minimize energy losses of photocatalytic reforming on TiO\({}_{2}\), hydrogen production should be coupled with the oxidation of recalcitrant compounds, when this reaction is kinetically limited or other strategies would not work, such as in the case of toxic compounds, aromatics, and antibiotics (scarcely removed in traditional wastewater treatment plants). Alternatively, the valorization of biomasses in remote regions could be another valuable and promising option, also because in that case H\({}_{2}\)/O\({}_{2}\) separation could be ruled out [15].