Dynamic Simulation on Surface Hydration and Dehydration of Monoclinic Zirconia

The commonly used oxide-supported metal catalysts are usually prepared in aqueous phase, which then often need to undergo calcination before usage. Therefore, the surface hydration and dehydration of oxide supports are critical for the realistic modeling of supported metal catalysts. In this work, by ab initio molecular dynamics (AIMD) simulations, the initial anhydrous monoclinic ZrO_2(111) surfaces are evaluated within explicit solvents in aqueous phase at mild temperatures. During the simulations, all the two-fold-coordinated O sites will soon be protonated to form the acidic hydroxyls (HO_\rmL), remaining the basic hydroxyls (HO^*) on Zr. The basic hydroxyls (HO^*) can easily diffuse on surfaces via the active proton exchange with the undissociated adsorption water (H_2O^*). Within the temperatures ranging from 273 K to 373 K, in aqueous phase a certain representative equilibrium hydrated m-ZrO_2(111) surface is obtained with the coverage (\theta) of 0.75 on surface Zr atoms. Later, free energies on the stepwise surface water desorption are calculated by density functional theory to mimic the surface dehydration under the mild calcination temperatures lower than 800 K. By obtaining the phase diagrams of surface dehydration, the representative partially hydrated m-ZrO_2(111) surfaces (0.25\leq\theta < 0.75) at various calcination temperatures are illustrated. These hydrated m-ZrO_2(111) surfaces can be crucial and readily applied for more realistic modeling of ZrO_2 catalysts and ZrO_2-supported metal catalysts.