Ultrafast Dynamics of the SO2(H2O)n Cluster System

Enhanced heterogenous hydration of SO2 through immobilization of pyridinic-N on carbon materials

Carbon materials doped with nitrogen have long been used for SO2 removal from flue gases for the benefits of the environment. The role of water is generally regarded as hydration of SO3 which is formed through the oxidization of SO2. However, the hydration of SO2, especially on the surface of N-doped carbon materials, was almost ignored. In this study, the hydration of SO2 was investigated in detail on the pyridinic nitrogen (PyN)-doped graphene (GP) surfaces. It is found that, compared with the homogeneous hydration of SO2 assisted with NH3 in gas phase, the heterogeneous hydration is much more thermodynamically and kinetically favourable. Specifically, when a single H2O molecule is involved, the energy barrier for SO2 hydration is as low as 0.15 eV, with 0.59 eV released, indicating the hydration of SO2 can occur at rather low water concentration and temperature. Thermodynamic integration molecular dynamics results show the feasibility of the hydrogenated substrate recovery and the immobilized N acting as a catalytic site for SO2 hydration. Our findings show that the heterogeneous hydration of SO2 should be universal and potentially uncover the puzzling reaction mechanism for SO2 catalytic oxidation at low temperature by N-doped carbon materials.

A New Mechanism of Acid Rain Generation from HOSO at the Air-Water Interface

The photochemistry of SO₂ at the air-water interface of water droplets leads to the formation of HOSO radicals. Using first-principles simulations, we show that HOSO displays an unforeseen strong acidity (pK_a = -1) comparable with that of nitric acid and is fully dissociated at the air-water interface. Accordingly, this radical might play an important role in acid rain formation. Potential implications are discussed.

Single photon ionization of van der Waals clusters with a soft x-ray laser: (SO2)n and (SO2)n(H2O)m

van der Waals cluster (SO2)n is investigated by using single photon ionization of a 26.5 eV soft x-ray laser. During the ionization process, neutral clusters suffer a small fragmentation because almost all energy is taken away by the photoelectron and a small part of the photon energy is deposited into the (SO2)n cluster. The distribution of (SO2)n clusters decreases roughly exponentially with increasing cluster size. The photoionization dissociation fraction of I[(SO2)(n-1)SO+] / I[(SO2)n+] decreases with increasing cluster size due to the formation of cluster. The metastable dissociation rate constants of (SO2)n+ are measured in the range of (0.6-1.5) x 10(4) s(-1) for cluster sizes 5< or =n< or =16. Mixed SO2-H2O clusters are studied at different experimental conditions. At the condition of high SO2 concentration (20% SO2 partial pressure), (SO2)n+ cluster ions dominate the mass spectrum, and the unprotonated mixed cluster ions (SO2)nH2O+ (1< or =n< or =5) are observed. At the condition of low SO2 concentration (5% SO2 partial pressure) (H2O)nH+ cluster ions are the dominant signals, and protonated cluster ions (SO2)(H2O)nH+ are observed. The mixed clusters, containing only one SO2 or H2O molecule, SO2(H2O)nH+ and (SO2)nH2O+ are observed, respectively.