Proton activity on oxide surface is paid considerable attention not only from the view point of basic science but also the applications to catalysts and electrolytes. To reveal the intrinsic protonic activity, hydration reactions on oxide surfaces are studied by ab initio molecular dynamic (AIMD) simulations. In these studies, it is concluded that surface acidity is governed by deprotonation reactions of chemisorbed H2O molecules on metal ions (M-OH2 → M-OH− + H+, pKa1) and polydentate hydroxyl groups formed on oxide ions (&gt;OH+→ &gt;O + H+ pKa0). Here, pH at point of zero charge condition (pHPZC), an indicator of surface acidity based on the experiment, is determined as an average of acid constants, pHPZC= 1/2(pKa1+pKa0). Therefore, since catalytic activity is not governed by pHPZC but by the strength of acidity or basicity of reaction sites, pKas of hydroxyl groups on the surface are also estimated in several AIMD simulations. However, the effect of acid-base molecules on surface acidity has not been studied yet. In catalytic reactions such as methanol synthesis, intermediate species on the surface like carbonates and formates could be reduced by proton and electron migrations. Therefore, the efficiency of these processes is highly related to the acid-base property on the surface. Here, we report AIMD simulations for hydration reactions on carbonate, formate, and methanol adsorbed surfaces to study the effect of acid and base modification on oxide surface. To obtain these surfaces, a few ps AIMD simulations for CO2, HCOOH, and CH3OH adsorption on bare cubic ZrO2 (110) surface are performed. After the adsorption, all organic molecules stably exist on each surface throughout several-ps hydration reactions by introducing H2O molecules. Bond overlap population analysis shows that carbonates and formates are inert to the acid-base reactions, whereas methoxy groups tend to form strong hydrogen bonds with surrounding H2O molecules. In addition, the bond length and strength of Zr-OH2 and Zr-OH− are constant regardless of organic molecule adsorption, which suggests pKa1 is maintained constant. On the other hand, thermodynamic discussion suggests that the basicity of surface oxide ions, pKa0, decreases as the coverage of organic molecules increases, which results in pHPZC reduction. Finally, according to these results, the relation between acid-base property of molecules and the change in acidity on the surface is discussed in terms of surface equilibrium.

Ab initio molecular dynamic study about effect of acid-base molecule adsorption on acidity on ZrO2 surface

255th National Meeting (2018)

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