# Water on Actinide Dioxide Surfaces: A Review of Recent Progress
## 3 Water on AnO\({}_{2}\) Surfaces
### Theoretical Calculations
#### 3.2.1 Adsorption and dissociation of H\({}_{2}\)O on AnO\({}_{2}\) surfaces
_Ab-initio_ molecular dynamics (AIMD) simulations have also been carried out to explore the mechanism of adsorption and dissociation of H\({}_{2}\)O on AnO\({}_{2}\) surfaces. It was found that the dissociative adsorption of H\({}_{2}\)O is feasible for a large range of temperature and water partial pressures on UO\({}_{2}\) (111) surface [69]. The dissociation of H\({}_{2}\)O on PuO\({}_{2}\) (110) surface is a two-step hydroxylation process both for single H\({}_{2}\)O molecule and H\({}_{2}\)O clusters [70]. The dissociation is initiated by the dehydrogenation of water molecules to form a hydroxyl group with surface oxygen atom, followed by successive surface hydroxylation on plutonium with the remaining hydroxide ion of the dissociating molecule. The H\({}_{2}\)O molecule dissociates as the consequence of hybridizations between the molecular orbitals of water and  
Figure 8: H\({}_{2}\)O on ThO\({}_{2}\) (111) surface [38]. Top and side views of molecular adsorption (**a**,**b**) and dissociative adsorption (**c**,**d**). \begin{table}
\begin{tabular}{c c c c} \hline \hline
**In Units of eV** & **(111)** & **(110)** & **(100)** \\ \hline [PERSON] et al. [22; 54] & 0.61 & 0.62 & 1.02 \\  & _0.68_ & **1.27** & **1.71** \\ \hline [PERSON] et al. [67] & 0.53 & 0.93 & 0.97 \\  & _0.50_ & **1.39** & **1.55** \\ \hline [PERSON] et al. [55] & 0.62 & - & - \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Adsorption energies of one H\({}_{2}\)O molecule on perfect UO\({}_{2}\) surfaces. The bold italic numbers correspond to adsorption energies in the dissociative form. The rest are adsorption energies in the molecular form. the electronic state of the surface with a reaction energy barrier of 0.18 eV. In contrast, the dissociation of H\({}_{2}\)O clusters is exothermic by \(-\)0.42 eV with no energy barrier [70]. On reduced AnO\({}_{2}\) surfaces with oxygen vacancies, H\({}_{2}\)O has much stronger adsorption energies, as shown in Table 6. On perfect ThO\({}_{2}\), UO\({}_{2}\), and PuO\({}_{2}\) (111) surfaces, the adsorption energy of water is smaller than 1.1 eV from DFT calculations [38; 54; 64; 67; 55], while the adsorption energies are larger than 1.8 eV on surfaces with oxygen vacancies [53; 54; 55; 64]. These DFT calculations are in agreement with experimental results by [PERSON] et al. that the adsorption of H\({}_{2}\)O on UO\({}_{2}\) (111) surface is weak and reversible (80%) at 300 K [58]. DFT simulations also found that the dissociation of H\({}_{2}\)O on reduced AnO\({}_{2}\) surfaces can occur spontaneously at room temperature, as there is a very small energy barrier for the dissociation [54; 55]. After H\({}_{2}\)O dissociation, the hydroxylate group moves to the oxygen vacancy site, and the proton binds with one surface oxygen atom as shown in Figure 9. The excess electrons are distributed differently on reduced AnO\({}_{2}\) surfaces upon the adsorption of H\({}_{2}\)O. On reduced ThO\({}_{2}\) (111) surface, the excess electrons still stay at the vacancy site after H\({}_{2}\)O dissociation. One proton from the H\({}_{2}\)O molecule is attracted to the vacancy site and forming H\({}^{\cdot}\). However, on both reduced surfaces, UO\({}_{2}\) and PuO\({}_{2}\) (111), the OH\({}^{\cdot}\) group from H\({}_{2}\)O tends to fill the vacancy site. When OH\({}^{\cdot}\) moves to the vacancy site for UO\({}_{2}\) surface, it will push the excess electron to a neighboring uranium site and reduce it to U\({}^{3+}\).