Competition between Displacement and Dissociation of a Strong Acid

May 25, 2016 - The adsorption of nitric (HNO3) and formic (HCOOH) acids on silica particle surfaces and the effect of adsorbed water have been investi...
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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

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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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Department of Chemistry, University of Iowa, Iowa City, IA 52242, USA

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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