Inner-Sphere Sorption at (110) Facet of α-Al₂O₃: Insights from Density Functional Theory

Inner-sphere complexation of AsO43– and CrO42– at the hydroxyl-functionalized (110) cleaved surface of α-Al2O3 was theoretically investigated aiming at interrogating the effect of surface termination variety on the adsorption process. Four different terminations of the above surface were considered, viz., Perfect-H1, Perfect-H2, Defect-A-H1, and Defect-A-H2. An ab initio thermodynamics approach was briefly used to calculate the surface free energies for these terminations and to rank them based on their stability in ambient conditions. A DFT + thermodynamics approach was used to calculate the energetics of the adsorption process. This approach accounts for experimental conditions, such as temperature and concentration. The thermodynamics cycle associated with each adsorption process includes experimentally obtained data, effectively adding no computational overhead while extending beyond the current capability of purely DFT-based calculations. A bidentate adsorption mode was found energetically favorable for both AsO43– and CrO42– at Perfect and Defect terminations, with the latter termination having a significantly greater tendency to adsorb species compared to the former. A vibrational analysis was also performed for the most stable surface complexes and was compared to other findings for similar systems when available. The findings of this analysis can inform future experiments on mode assignments. The results of this work lay the foundation to examine surface adsorption behavior as it relates to surface termination and reactivity.