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First and Second Deprotonation of H2SO4 on Wet Hydroxylated (0001) A-Quartz† Cite This: Phys

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First and Second Deprotonation of H2SO4 on Wet Hydroxylated (0001) A-Quartz† Cite This: Phys UC Irvine UC Irvine Previously Published Works Title First and second deprotonation of H₂SO₄ on wet hydroxylated (0001) α-quartz. Permalink https://escholarship.org/uc/item/67d3d9h5 Journal Physical chemistry chemical physics : PCCP, 16(40) ISSN 1463-9076 Authors Murdachaew, Garold Gaigeot, Marie-Pierre Halonen, Lauri et al. Publication Date 2014-10-01 DOI 10.1039/c4cp02752c License https://creativecommons.org/licenses/by/4.0/ 4.0 Peer reviewed eScholarship.org Powered by the California Digital Library University of California PCCP View Article Online PAPER View Journal | View Issue First and second deprotonation of H2SO4 on wet hydroxylated (0001) a-quartz† Cite this: Phys. Chem. Chem. Phys., 2014, 16, 22287 Garold Murdachaew,*a Marie-Pierre Gaigeot,bc Lauri Halonena and R. Benny Gerberade We present an ab initio molecular dynamics study of deprotonation of sulfuric acid on wet quartz, a topic of atmospheric interest. The process is preferred, with 65% of our trajectories at 250 K showing deprotonation. The time distribution of the deprotonation events shows an exponential behavior and predicts an average deprotonation time of a few picoseconds. The process is exoergic, with most of the temperature increase being due to formation of hydrogen bonds prior to deprotonation. In agreement with existing studies of H2SO4 in water clusters, in liquid water, and at the air–water interface, the main determinant of deprotonation is the degree of solvation of H2SO4 by neighboring water molecules. However, we find that if both hydrogens of H2SO4 are simultaneously donated to water oxygens, deprotonation is disfavored. Predicted spectroscopic signatures showing the presence of solvated hydronium and bisulfate are presented. Increasing the tempera- Received 23rd June 2014, ture up to 330 K accelerates the process but does not change the main features of the deprotonation Accepted 8th September 2014 mechanisms or the spectroscopic signatures. The second deprotonation of H2SO4, studied only at 250 K, DOI: 10.1039/c4cp02752c occurs provided there is sufficient solvation of the bisulfate by additional water molecules. In comparison to HCl deprotonation on the identical surface examined in our previous work, the first deprotonation of H2SO4 www.rsc.org/pccp occurs more readily and releases more energy. 1 Introduction sulfur-containing ice particles may contribute to increased rain fall and ozone depletion in polar stratospheric clouds, respectively. Sulfuric acid and related sulfur-containing molecules play important The net negative radiative forcing due to increased albedo of roles both technologically and in the natural environment1,2 and the sulfur-containing aerosols will lead to cooling.4 This effect participate in the biogeochemical sulfur cycle. Molecules such as has motivated geoengineering proposals to counter a warming 5,6 Published on 09 September 2014. Downloaded by University of California - Irvine 01/08/2017 18:52:28. OCS and SO2 can be produced by natural processes (biological climate. In addition to atmospheric chemistry, the uptake and activity in marine organisms and volcanism, respectively) or possible reaction of acids on environmental interfaces is of anthropogenically (SO2 from combustion of sulfur-containing interest to fields such as heterogeneous catalysis, sensor science, fossil fuels). In the troposphere, in the presence of oxidants and/ corrosion science, and cultural heritage science.7 or humidity, sulfate species are quickly oxidized and/or hydrolyzed H2SO4 is a strong diprotic acid and readily gives up a single À into H2SO4, a major component of acid rain and also a critical proton, forming bisulfate, HSO4 . Further solvation can allow 3 2À element in new particle formation. Such particles may in turn act full deprotonation resulting in sulfate, SO4 . A bulk water as cloud condensation nuclei (CCN), and the resulting clouds and solution at 298 K has acid dissociation constants of pKa1 = À3 and pKa2 = 1.99. Only at high concentration, greater than 14 M, 8 a Laboratory of Physical Chemistry, Department of Chemistry, will H2SO4 remain in neutral form. H2SO4 dissociation has University of Helsinki, P.O. Box 55, FI-00014 Helsinki, Finland. been extensively studied both experimentally and theoretically E-mail: [email protected]; Tel: +358 29 415 0291 in water clusters, in bulk liquid water, and at the air–water b LAMBE, CNRS UMR 8587, Universite´ d’Evry val d’Essonne, interface. Some examples of experimental and theoretical studies Boulevard François Mitterrand, Baˆtiment Maupertuis, 91025 Evry, France c Institut Universitaire de France, 103 Boulevard Saint-Michel, 75005 Paris, France follow. Raman spectroscopy has been used to study the dissociation 9 d Institute of Chemistry and the Fritz Haber Research Center, in bulk liquid water. At the air–liquid interface and for surface The Hebrew University, 91904 Jerusalem, Israel studies, vibrational surface-sensitive spectroscopies such as VSFG e Department of Chemistry, University of California, Irvine, California 92697, USA (vibrational sum-frequency generation) have been employed both † Electronic supplementary information (ESI) available: Details of the quartz experimentally10–18 and through theoretical modeling.13–15 Finally, surface model, validation of the theory level used, trajectories, temperature effects (on deprotonation rate, mechanisms, and spectroscopic signatures), the process has also been studied at the air–water interface with 8 disordering of the water layer, and low energy configurations and bonding motifs photoelectron spectroscopy. Research questions include the of H2SO4 on wet quartz. See DOI: 10.1039/c4cp02752c structure of the air–liquid interface and determining the balance This journal is © the Owner Societies 2014 Phys. Chem. Chem. Phys., 2014, 16, 22287--22298 | 22287 View Article Online Paper PCCP between (partially) dissociated and undissociated sulfuric acid as multiple layers. Under normal to dry conditions of about 20% a function of its concentration and the temperature.8 humidity and 296 K, a full monolayer may form.34 The water layers H2SO4 may be produced on ice (or water) surfaces and also closest to the silica surface are highly ordered and form a tessellation on mineral dust surfaces19 through adsorption and hydrolysis ice-type structure (ice XI,35 a form of proton-ordered ice) on the 35–40 of gas phase SO2 or SO3. Regarding the latter species, the surface, and such ices are of interest in their own right. inverse reaction, the dissociation H2SO4 - SO3 +H2O due to Thedissociationofanacidmoleculeonanoxidesurfaceisan an overtone OH stretch excitation of H2SO4, has been studied example of an atmospherically-relevant, fundamental heterogeneous theoretically.20 However, while the association is exothermic, the chemical reaction. Such reactions, occurring in the presence of a dissociation is strongly endothermic, and we did not observe ubiquitous water or ice layer,37,41 determine surface pH, and may this dissociation mechanism in our work. Recent experimental also play a role in enabling other reactions, such as those which 43 work has shown that the adsorption of acids, including H2SO4 lead to ozone depletion (see, e.g., ref. 42). In previous work, we and HCl, on atmospheric mineral dust particles acts to inhibit used ab initio molecular dynamics to examine the dissociation of ice nucleation at moderate humidities.21 In our work, we hope to HCl on the hydroxylated (0001) a-quartz surface. Acid dissociation shed some light on this issue. and subsequent proton hopping on the surface was observed when Recent computational studies of H2SO4 dissociation include the surface water coverage was at least 40%. In addition, the special those of Hammerich et al. on this process in both liquid water22 properties of the ordered water monolayer present on quartz and at the air–water interface.23 As reviewed in ref. 23, simulations of (in particular, the lattice mismatch between the water layer and asingleH2SO4 molecule in water clusters show that only 3–4 waters quartz substrate) enhanced the initial ionization and subsequent are required for the first deprotonation (see ref. 24 and 25 and proton migration. additional references in ref. 23 and also ref. 26–29), resulting in the In the present study, we explore computationally the À + contact ion pair (CIP) HSO4 ÁÁÁH3O . The second deprotonation mechanisms and time scale of the deprotonation of H2SO4 on requires further solvation, a minimum of 6 waters, resulting in the the (0001) a-quartz surface covered by a water monolayer. The + 2À + dicontact ion pair of H3O ÁÁÁSO4 ÁÁÁH3O , although this structure is monolayer regime is of interest for low to moderate humidity about 8 kcal molÀ1 higher in energy than the most stable structure condition present in the atmosphere. In addition, it allows the À which is a solvent-separated ion pair (SSIP) involving HSO4 and quartz surface to exert effects on the ionization process. For the + H3O , separated by at least one H2O, and this is the trend for H2SO4– mostpart,wehavenotconsideredmultiplelayersofwatersinceat water clusters with up to 9 waters, at which point the doubly these temperatures the subsequent layers quickly begin to resemble À1 30 44,45 deprotonated structure is only about 1 kcal mol higher in energy. the liquid–vapor interface, and H2SO4 at the vapor–liquid 8,10–18,23 In that study of H2SO4 and 6–9 waters, in the cases where the doubly interface has been studied previously. We employ ab initio 2À deprotonated structures were observed, SO4 was found to molecular dynamics (AIMD), using density functional theory participate in about 6 H-bonds with the water molecules.30 (DFT),46,47 for computing the forces ‘‘on the fly’’.48,49 AIMD simula- 22 Hammerich et al. studied H2SO4 in liquid water and found tions can provide an unbiased, atomic-level view of the detailed that as expected experimentally, about 30% of the time double mechanisms and time scales of chemical reactions. deprotonation occurred. This requires full solvation of the resulting sulfate by surrounding waters and/or hydroniums, Published on 09 September 2014. Downloaded by University of California - Irvine 01/08/2017 18:52:28.
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  • Bronsted-Lowry
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