Surface Energies, Al-Substitution Effects, and Water Adsorption on Ferrihydrite: Insights from Density Functional Theory Calculations

Abstract

Ferrihydrite (Fe 5 O 7 (OH), Fh) plays a crucial role in various environmental processes involving iron oxyhydroxides. Its poorly crystalline nature, high surface area, and numerous reactive sites make it significant in the formation of different oxyhydroxides, as well as in the adsorption and reactive processes. To understand its surface properties, we performed density functional/plane wave (density functional theory/PW) calculations to estimate the cleavage surface energies along directions with low Miller indices. We investigated 25 cleavage surfaces in five different directions. Among these, the (110) direction with the MO termination is the most favorable, with a cleavage energy of 2.61 J m −2 , followed by the (100)-MO and (010)-MO cleavages, with energies of 2.84 and 2.96 J m −2 , respectively. The surface energy of the (110) surface was estimated to be 1.65 J m −2 . We also examined the effect of Al-substitution at the Fe sites. Calculations for a ferrihydrite with 20% Al content (Fh20) indicated an average increase in the cleavage energy by 0.10 J m −2 . The surface energy of the Fh20 (110) surface was estimated to be 1.99 J m −2 . Water adsorption on these surfaces was found to occur preferentially in a dissociative manner, with an adsorption energy of 122.7 kJ mol −1 . Adsorption sites for Al on the Fh20 surface exhibited a slightly higher adsorption energy of 124.6 kJ mol −1 . Using the Wulff theorem, we determined the mineral habit of Fh, which revealed a trade-off: the predominance of the most stable (110) surface implies a larger, less stable (001) surface. These findings suggest that the (001) surface is likely the most reactive, playing a significant role in many chemical processes on Fh surfaces.