Near-Barrierless Ammonium Bisulfate
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Communication pubs.acs.org/JACS Near-Barrierless Ammonium Bisulfate Formation via a Loop- Structure Promoted Proton-Transfer Mechanism on the Surface of Water † † † † † † ‡ Lei Li, Manoj Kumar, Chongqin Zhu, Jie Zhong, Joseph S. Francisco,*, and Xiao Cheng Zeng*, , † Department of Chemistry, University of Nebraska-Lincoln, Lincoln, Nebraska 68588, United States ‡ Collaborative Innovation Center of Chemistry for Energy Materials, University of Science and Technology of China, Hefei, Anhui 230026, China *S Supporting Information can be significantly influenced by the participation of four or ABSTRACT: In the atmosphere, a well-known and more water molecules or other atmospheric molecules such as 19,20 conventional pathway toward the formation of ammonium the HO2 radical and formic acid. In a highly polluted μ sulfate is through the neutralization of sulfuric acid with atmosphere, the concentration of NH3 may rise up to 10 g −3 ∼ × 17 · −3 ammonia (NH3) in water droplets. Here, we present direct m ( 3.5 10 m ), much higher than that of the HO2 ab initio molecular dynamics simulation evidence of the radical and formic acid and may even be comparable with that of 17 −3 formation of ammonium bisulfate (NH4HSO4) from the water monomers (∼7.73 × 10 molecules · m at 100 % relative 21 hydrated NH3 and SO3 molecules in a water trimer as well humidity and 298.15 K). This suggests that ammonia as on the surface of a water droplet. This reaction suggests molecules can play a more direct role in the formation of a new mechanism for the formation of ammonium sulfate ammonium sulfate. An early study has shown that ammonia can in the atmosphere, especially when the concentration of act as a catalyst to promote the formation of sulfuric acid by ∼ μ −3 NH3 is high (e.g., 10 gm ) in the air. Contrary to the lowering the energy barrier to ∼3.80 kcal/mol, but without the water monomer and dimer, the water trimer enables near- + 22 23 formation of NH4 . Another possible mechanism considered barrierless proton transfer via the formation of a unique involves the barrierless formation of a donor−acceptor NH ·SO 3 − 3 loop structure around the reaction center. The formation complex, which has a binding energy of ∼20 kcal/mol,24 27 of the loop structure promotes the splitting of a water · ∼ much higher than that of the H2O SO3 complex ( 7.9 kcal/ molecule in the proton-transfer center, resulting in the 16 · + − mol). Though the NH3 SO3 complex can also lead to sulfamic generation a NH4 /HSO4 ion pair. The loop-structure acid,26,28 the reaction is endothermic with a high barrier of promoted proton-transfer mechanism is expected to be ∼16.13 kcal/mol.22 Although recent experimental studies have ubiquitous on the surface of cloud droplets with adsorbed shown the promoting role of NH3 in the nulceation of aerosol NH3 and SO3 molecules and, thus, may play an important particles, a detailed chemical mechanism for the formation of role in the nucleation of aerosol particles (e.g., fine ammonium sulfate from NH3 and SO3 is still largely unknown. particles PM2.5) in water droplets. Here, we show direct ab initio molecular dynamics (AIMD) simulation evidence of the near-barrierless formation of ammonium bisulfate (NH4HSO4) from the hydrated NH3 and tmospheric aerosols have become a central topic in the SO3 molecules. In this new chemical mechanism, a loop structure environmental and atmospheric science due to their fi A − is identi ed to play a key role in promoting the water-mediated fl 1 2 6 7,8 in uence on the biosphere, climate, and public health. proton transfer that leads to the formation of NH4HSO4. Many experimental studies have shown that sulfate salts Observation of the near-barrierless formation of ammonium (ammonium sulfate in particular) are one of the main bisulfate (NH HSO ) in the water trimer as well as on surface of fi 4 4 constituents in aerosols (e.g., ne particles PM2.5) and play a a water droplet suggests an alternative dominant channel toward key role in the formation of sulfur-containing aerosols by acting the formation of ammonium sulfate, a major component of − as condensation nuclei.9 14 Investigation of the formation aerosol particles in the water droplets. mechanism of ammonium sulfates in the atmosphere is both Born−Oppenheimer AIMD simulation is performed on the timely and important to the understanding of the nucleation and basis of density functional theory (DFT) methods as growth of aerosol particles, thereby may lead to better science- implemented in the CP2K code.29 The exchange and correlation based solutions in resolving the severe haze problems (such as interactions of electrons are treated with the Becke−Lee−Yang− those occurred recently in the winter season of eastern China). Parr (BLYP) functional,30,31 and the Grimme’s dispersion In the atmosphere, ammonium sulfate is generally believed to correction method is applied to account for the weak dispersion 32 ζ be formed via the neutralization of sulfuric acid (H2SO4) with interaction. A double- Gaussian basis set combined with an 15 33 − − ammonia (NH3) molecules. Although the sulfuric acid auxiliary basis set and the Goedecker Teter Hutter (GTH) formation from the bimolecular reaction between SO3 and H O entails a high activation energy barrier (28−32 kcal/ Received: December 14, 2015 2 − mol),16 18 recent studies have shown that the reaction energetics © XXXX American Chemical Society A DOI: 10.1021/jacs.5b13048 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX Journal of the American Chemical Society Communication norm-conserved pseudopotentials34,35 are adopted to treat the toward NH , leading to the formation of NH +, while the 3 − 4 − valence electrons and the core electrons, respectively. An energy remaining OH ion binds with the SO3 to form HSO4 . Overall ff cuto of 280 Ry is set for the plane wave basis set and 40 Ry for the water molecule near NH3 acts as a proton transporter by the Gaussian basis set. A supercell (20 × 20 × 20 Å3) with accepting the proton from one source and delivering it to another periodic boundary conditions is selected for the (NH3) molecule. Such a reaction mechanism can be viewed as a water- − 38 (SO3)(H2O)n (n =1 3) systems, which is large enough to mediated proton-transfer mechanism as previously reported, + − neglect interaction between the neighboring replica. For the and this mechanism enables the formation of NH4 /HSO4 ion water droplet system, a relative large supercell with size (35 × 35 pair in the dimer system with a slightly lower barrier of ∼3.30 × 35 Å3) is used. The water droplet with 192 water molecules is kcal/mol (3.80 kcal/mol at the MP2/6-311++G(d,p) level). carved out from the supercell of pre-equilibrated bulk water at However, during the AIMD simulations of both water monomer ∼ + − 300 K and re-equilibrated at 300 K for 6 ps. Upon the and dimer systems, neither the NH4 /HSO4 ion pair nor NH3/ adsorption of a NH3 and a SO3 molecule onto the surface of the H2SO3 complex is observed due largely to the notable energy water droplet, the structure is further optimized at DFT level of barriers for the formation reactions (see Figures S1 and S2, and theory before the AIMD simulations. All the AIMD simulations Movies S1 and S2). are performed in the constant volume and temperature ensemble In the atmosphere, although the population of the water with the temperature controlled at 300 K. The climbing image trimers is about 1 order of magnitude less than that of the water nudged-elastic-band (CI-NEB) method is used to locate the dimers,39,40 the water trimer may still play a key role in certain transition state.36,37 In addition, the geometries of reactant and chemical reactions if the hydration effect becomes increasingly product states are also optimized at the MP2/6-311++G(d,p) important with increasing the size of water clusters. This is level of theory, while the transition state is further confirmed via indeed the case for the formation of ammonium bisulfate. Unlike frequency analysis at the MP2 level of theory, using the Gaussian the water monomer and dimer systems, the formation of 09 software. These separated MP2 computations allow NH4HSO4 can be directly observed during the AIMD simulation confirmation of the activation energy barrier for each reaction. with the water trimer system (Movie S3). As shown in Figure 2a, First, we compute the energy barrier to the formation of NH4HSO4 from NH3 and SO3 associated with one or two water molecules, using the CI-NEB method. For the water monomer system, as shown in Figure 1, the splitting of the water molecule Figure 1. A schematic illustration of the energy profiles for the reaction of NH3 and SO3 molecules with a water monomer (in gray), water dimer Figure 2. (a) Snapshot structures taken from the AIMD simulation of (in blue), and water trimer (in red). The horizontal bar denotes the the water trimer system with the NH3 and SO3 molecules. The gray reactant or the product state. The larger solid circles represent the spheres mark the transferred protons during the reaction. (b) The time − − − − − − transition states and the smaller solid circles correspond to the replicas evolution of the N H1,S O2,O1 H2,O1 H1,O2 H2, and O3 H3 in the CI-NEB methods. The white, blue, red and yellow spheres lengths in the course of the AIMD simulation. The inset illustrates the represent H, N, O and S atoms, respectively. loop structure formed prior to the formation of the NH4/HSO4 ion pair.