Interaction Energies and Spectroscopic Effects in the Adsorption of Formic Acid on Mineral Aerosol Surface Models

Heterogeneous reactions of atmospheric volatile organic compounds (VOCs) on aerosol particles may play an important role in atmospheric chemistry. Silicate particles are present in airborne mineral dust in aerosols, and the atmospheric chemistry in general can be modified by their presence. In this work, the adsorption of a single formic acid molecule on different silicate surface models has been studied using quantum-mechanical methods. Both molecular clusters and a periodic crystal model of the (001) pyrophyllite surface have been employed, and all possible adsorption geometries have been considered. We find that silanol groups are always the most reactive formic acid adsorption sites. In the case of a periodic system, silanol groups at the crystal edges are favored. However, OH groups on the phyllosilicate octahedral sheet are also reactive sites through the tetrahedral cavities. The effect of formic acid adsorption on the spectroscopic properties of the whole system is also analyzed. Significant frequency shifts are detected in the vibration modes of both adsorbate and surface models. These results can be a useful tool for experimental adsorption investigations using vibration spectroscopy.