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Competition Between Displacement and Dissociation of a Strong Acid Compared to a Weak Acid Adsorbed on Silica Particle Surfaces: The Role of Adsorbed Water Yuan Fang, Mingjin Tang, and Vicki H Grassian J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b02262 • Publication Date (Web): 25 May 2016 Downloaded from http://pubs.acs.org on June 5, 2016
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The Journal of Physical Chemistry A is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
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The Journal of Physical Chemistry
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Competition between Displacement and Dissociation of a Strong Acid
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Compared to a Weak Acid Adsorbed on Silica Particle Surfaces: The Role of
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Adsorbed Water
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Yuan Fang,1,2 Mingjin Tang,1 Vicki H. Grassian1,2*
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1
Department of Chemistry, University of Iowa, Iowa City, IA 52242, USA
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2
Department of Chemistry and Biochemistry, University of California San Diego, La Jolla, CA
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92093, USA
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Correspondence: Vicki H. Grassian (Tel: (858)534-2499; email:
[email protected])
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Abstract
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The adsorption of nitric (HNO3) and formic (HCOOH) acid on silica particle surfaces and
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the effect of adsorbed water have been investigated at 296 K using transmission FTIR
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spectroscopy. Under dry conditions, both nitric and formic acids adsorb reversibly on silica.
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Additionally, the FTIR spectra show that both of these molecules remain in the protonated form.
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At elevated relative humidities (RH), adsorbed water competes both for surface adsorption sites
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with these acids as well as promotes their dissociation to hydronium ions and corresponding
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anions. Compared to HNO3, the extent of dissociation is much smaller for HCOOH, very likely
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due to its weaker acidity. This study provides valuable insights into the interaction of HNO3 and
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HCOOH with silica surface on the molecular level, and further reveals the complex roles of
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surface-adsorbed water in atmospheric heterogeneous chemistry of mineral dust particles – many
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of these containing silica.
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The Journal of Physical Chemistry
1 Introduction
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Mineral dust aerosol is ubiquitous in the atmosphere,1 and reactions on the surface of
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mineral dust particles play an important role in atmospheric chemistry.2,3 Heterogeneous
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reactions of mineral dust particles can impact the concentrations of important trace gases and
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radicals (such as NOx, O3, and HOx) 4,5 and modify aerosol particle composition,6,7 which in turn
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leads to changes in the ability of dust particles to serve as cloud condensation nuclei (CCN) 8-12
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and ice nuclei (IN).12-15 Nitric acid (HNO3) and formic acid (HCOOH) are among the most
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abundant acidic gases in the troposphere,16,17 and their heterogeneous reactions are of great
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interest. As such, a number of studies have investigated the heterogeneous reactions of mineral
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dust particles with HNO3 18-23 and HCOOH.24-29
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Silica (SiO2) is a major component of mineral dust particles in the atmosphere.2,30,31 It is a 18
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neutral oxide with abundant surface hydroxyl groups
and several studies have investigated
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heterogeneous reaction of silica with acidic atmospheric gases. For example, using a Knudsen
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cell reactor, the uptake coefficient of HNO3, γ(HNO3), was reported to be (2.9±2)×10-5 for silica
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particles.20 Vlasenko et al.23 investigated the uptake of HNO3 onto silica aerosol particles at room
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temperature using an aerosol flow tube, and γ(HNO3) was found to be smaller than 5×10-4 at 33%
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RH. Goodman et al.18 used transmission FTIR to study the heterogeneous reaction of HNO3 with
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silica, and found that HNO3 is molecularly and reversibly adsorbed on silica surface although the
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effect of RH was not thoroughly investigated. However, previous studies suggested that exposure
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to HNO3 could enhance the water uptake by silica particles.32,33
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The uptake of HCOOH by CaCO3,24 clay minerals,25,28 TiO2,29 Al2O3,26-28 and silica28 has
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been examined previously. Using diffuse reflectance infrared Fourier transform spectroscopy
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(DRIFTS), Tong et al.26 studied the uptake of HCOOH by α-Al2O3 at room temperature, and
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found that the uptake coefficients increase with RH for RH below 20%, and then decrease with
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RH. The effect of temperature on the uptake of HCOOH by α-Al2O3 was also explored.27
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Attenuated Total Reflection-Fourier Transmission Infrared Spectroscopy (ATR-FTIR) and
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Quartz Crystal Microbalance (QCM) were used to investigate the heterogeneous uptake of
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HCOOH by silica and γ-Al2O3.28 It was found that under dry conditions (RH
