The surface of atmospheric aerosols is known to adsorb tropospheric gases, leading to various coordination modes between surface and adsorbate. Upon adsorption, symmetry-breaking of adsorbed molecules leads to changes in electronic and vibrational transitions, with important implications in the optical and chemical properties of aerosols in the atmosphere. For example, the optical properties of chemisorbed nitrates change with coordination symmetry, impacting the ion’s photochemical fate and climate effects. The addition of co-adsorbed water molecules further affects the coordination and reactivity of the nitric acid adsorbed onto TiO2. In this work, several coordination structures of nitrate on TiO2 were modeled and the effect of coadsorbed water was determined. In addition, all calculations were correlated to experimental data. The energy minimization and vibrational frequency calculations were performed on a binuclear cluster including a TiO2 active site and dry, partially solvated, or fully solvated nitrate ions. Our optimizations of chemisorbed nitrates yielded four distinctive structures in which nitrate loses its D3h symmerty in favor of a C2v symmetry. By adding co-adsorbed water to simulate the deliquescent layer of an aerosol particle, the nitrate symmetry-breaking is reduced. Vibrational frequency calculations correlated the minimized nitrate geometries with a deconvoluted experimental vibrational spectrum of nitric acid chemisorbed onto TiO2 under dry and humidified conditions.

Quantum mechanical calculations of the effects of humidity on HNO3 chemisorbed onto TiO2

253rd National Meeting (2017)

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