First-Principles Investigation of Surface pK_a and the Behavior of Acids at Aqueous-Metal Interfaces

Several computational methods were reported for the accurate determination of pK_a values in a solvent medium, but the research on surfaces and interfaces is limited. This study reports a new method for accurately determining the surface pK_a (*pK_a) of acids on surfaces. The *pK_a is defined as the function of the adsorption energies of neutral acids and their deprotonated form. In the suggested method, the estimated proton solvation-free energy value is not required, a fact that increases the accuracy of the results. The *pK_a values of various organic and inorganic acids on the (111) surfaces of Ag, Au, and Pt were evaluated. The results are validated with available experimental results on various surface coverage ratios. The results point out that weak acids adsorbed on the metal-aqueous interface are orders of magnitude stronger acids than those in homogeneous solutions. The shift of the pK_as is largest on platinum. These results are of major importance in electrochemistry and heterogeneous catalysis.