Interactions between Heterogeneous Uptake and Adsorption of Sulfur

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The Interactions between Heterogeneous Uptake and Adsorption of Sulfur Dioxide and Acetaldehyde on Hematite Xi Zhao, Lingdong Kong, Zhenyu Sun, Xiaoxiao Ding, Tiantao Cheng, Xin Yang, and Jianmin Chen J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.5b01359 • Publication Date (Web): 07 Apr 2015 Downloaded from http://pubs.acs.org on April 15, 2015

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The Journal of Physical Chemistry

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The Interactions between Heterogeneous Uptake and

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Adsorption of Sulfur Dioxide and Acetaldehyde on

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Hematite

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Xi Zhao, Lingdong Kong*, Zhenyu Sun, Xiaoxiao Ding, Tiantao Cheng, Xin Yang, Jianmin

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Chen*

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Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention, Department of

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Environmental Science & Engineering, Fudan University, Shanghai 200433, China.

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ACS Paragon Plus Environment

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ABSTRACT: Sulfur dioxide and organic aldehydes in the atmosphere are ubiquitous and often

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correlated with mineral dust aerosols. Heterogeneous uptake and adsorption of one of these

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species on mineral aerosols can potentially change the properties of the particles, and further

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affect the subsequent heterogeneous reactions of the other species on the coating particles. In this

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study, the interactions between heterogeneous uptake and adsorption of sulfur dioxide and

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acetaldehyde on hematite are investigated by using in situ diffuse-reflectance infrared Fourier-

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transform spectroscopy (DRIFTS) at room temperature. It is found that the pre-adsorption of SO2

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on α-Fe2O3 can significantly hinder the subsequent heterogeneous oxidation of CH3CHO to

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acetate, while the pre-adsorption of CH3CHO significantly suppresses the heterogeneous reaction

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of large amount of SO2 on the surface of α-Fe2O3, and has a little influence on the uptake of

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small amount of SO2. The heterogeneous reactions of SO2 on α-Fe2O3 pre-adsorbed by CH3CHO

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change the existing acetate on the particle surface into chemisorbed acetic acid, for the

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enhancement of surface acidity after the uptake of SO2. During these processes, different surface

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hydroxyl groups showed different reactivities. Atmospheric implications of this study are

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discussed.

29 30 31 32 33 34 35 36


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1. INTRODUCTION

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Mineral aerosols, which account for about 30-60% of global aerosols, are important types of

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particles in the atmosphere and have strong effects on climate and public health.1,2 It is estimated

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that 1000 to 3000 Tg of such aerosols are emitted annually into the atmosphere,3 and their

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existence in the Earth’s atmosphere provides reactive surfaces for heterogeneous chemistry.4

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Therefore the heterogeneous reactions of atmospheric trace gases on mineral aerosols have been

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receiving increasing attention in the past decade. Past studies on heterogeneous reactions with

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aerosol particles have focused mainly on individual trace gas or aerosol particles with single

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composition. However, heterogeneous uptake of individual trace gas such as SO2, NO2 and

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aldehydes on the mineral aerosols can significantly change the physical and chemical properties

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of the particles due to the surface formed water-soluble ions like sulfate, nitrate and

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carboxylates,5-7 which may impact the following heterogeneous reactions of other pollutant gas

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on the coating aerosols. However, little attention was paid to the effect of pre-adsorption of the

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pollutants on the adsorption and reaction processes on the mineral aerosols. Previous laboratory

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studies of both inorganic gas pollutants have shown that a synergistic effect exists in the

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heterogeneous reaction between SO2 and NO2 on different mineral oxides,8-10 while in the case

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of inorganic and organic gas pollutants, HCOOH is found hindered significantly by coexisting

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SO2,2 and organic amines pre-adsorbed on hematite significantly enhanced the reactivity of

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COS.11 However, the research on this aspect is still limited. It is then interested to study on the

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surface of mineral aerosols how the adsorption of one trace gas would impact the following

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uptake of the other trace gas.

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Hematite (α-Fe2O3) is one of the typical components of mineral aerosols, which contributes to

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~ 6 wt% to it in the atmosphere.12As an essential nutrient for all organisms, soluble iron supply


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plays a limiting factor on phytoplankton growth over vast areas of the modern ocean and has

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effects on the oceanic CO2 uptake, which affects ocean biogeochemistry and therefore having

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feedback effects on global climate.13 The long-range transport of aeolian dust is the dominant

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iron external input to the open ocean surface, during which atmospheric chemical processing of

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iron containing dust aerosols can affect the amount of soluble iron.14,15 Therefore, interest has

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been put on the atmospheric chemistry of α-Fe2O3, and it is here chosen as model compound.

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Sulfur dioxide (SO2) is a principal sulfur-containing anthropogenic pollutant in the

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atmosphere,12 and it is the major precursor of sulfuric acid and sulfate aerosols. It is well known

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that sulfuric acid plays a significant role in the formation of atmospheric new particles,15 and

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sulfate aerosols are known to cause detrimental health effects

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solar radiation, resulting in a net cooling effect, as well as acting as cloud condensation nuclei

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and thereby indirectly affecting climate.12 Hence,

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atmosphere is of great concern, and three oxidation pathways have been revealed, including gas

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phase oxidation, aqueous phase oxidation in cloud or fog droplets, and heterogeneous reactions

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on the surfaces of aerosol particles.17 Many models have been applied to investigate the

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formation of sulfate aerosols on a global scale. The results show that atmospheric concentrations

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of SO2 are typically overestimated while sulfate tend to be underestimated,18,19 and the two

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pathways including gaseous oxidation and aqueous oxidation are insufficient to bridge the gap

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between field and modeling studies,20 indicating remaining uncertainties of the atmospheric

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sulfate formation mechanisms and implying that the heterogeneous conversion of SO2 to sulfate

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on aerosols may make an important contribution to atmospheric sulfate abundance. Increasing

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attention has been paid on the heterogeneous oxidation of SO2 to sulfate on aerosols for its

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importance and complexities in recent years.2,8,10,11,21

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and affect climate by scattering

the chemical conversion of SO2 in the


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Aldehydes are ubiquitous in the atmosphere. They are emitted directly from combustion

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sources and produced by photochemical oxidation of hydrocarbons including alkanes and

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alkenes. Acetaldehyde (CH3CHO), as one of the most abundant aldehydes in the atmosphere,22

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originates from a variety of domestic and industrial sources.23 The widespread use of aldehyde-

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based resins and construction materials

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oxygenates result in the emission of CH3CHO.25,26 By itself, low-level exposure to CH3CHO has

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been shown to induce adverse health effects.27 Indirectly, it can act as a precursor in the

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photochemical formation of tropospheric ozone to compromise domestic air quality.28

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Heterogeneous reactions of CH3CHO on oxide particles have been studied,22,23,29,30 and the

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results show that through hydrogen bonding interaction between the carbonyl compound and the

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surface hydroxyl groups, CH3CHO molecules are weakly and reversibly physisorbed on the

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surface of the SiO2 particles, while on the surface of other oxides (α-Al2O3, CaO and TiO2)

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CH3CHO molecules can undergo heterogeneous reactions to yield irreversibly adsorbed higher

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molecular weight unsaturated carbonyl compounds. However, there is little reaction on the

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surface of α-Fe2O3 as suggested by Li et al.29 It is expected that under atmospheric conditions

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CH3CHO molecules can adsorb on mineral dust aerosols,29 and may change the surface

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properties of those aerosols, which would impact on the uptake behaviors of these particles.

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together with the incomplete combustion of fuel

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Based on the heterogeneous reactions on α-Fe2O3 particles of individual gas-phase CH3CHO

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and SO2, in the current work we focus on the impact these reactions have on trace gas uptake

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behaviors of the particles, that is the interactions between the heterogeneous uptake and

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adsorption of SO2 and CH3CHO on hematite, using in situ diffuse reflectance infrared Fourier

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spectroscopy (in situ DRIFTS). The results of this study contribute to a better understanding of

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how CH3CHO and SO2 affect each other on the heterogeneous reactions with atmospheric


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aerosols and, from this, a better understanding of the complexities of atmospheric heterogeneous

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reactions.

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2. EXPERIMENTAL SECTION

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2.1 Materials. α-Fe2O3 powder was synthesized according to the procedure in previous work,21

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and was kept in a desiccator at 68% relative humidity (RH) for 48 h before in situ DRIFTS

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experiments. The sample was still loose fine powders after the equilibration, and this treatment

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caused some adsorbed water molecule layers to be present on the samples. Powder X-ray

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diffraction (Rigaku D/MAX-II X-ray diffractometer with Cu Kα) was used to verify the purity of

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the synthesized sample (see Figure S1), only hematite peaks were observed (JCPDS, Card

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No.33-0664).The synthesized particles are fairly uniformed, in size (ca.100 µm), and diamond

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shaped (see Figure S2),which is consistent with previous study.21

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Ar (99.999% purity, Shanghai Yunguang Specialty Gases Inc.) and O2 (99.999% purity,

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Shanghai Yunguang Specialty Gases Inc.) were introduced through an air dryer of silica gel and

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molecular sieve before use. SO2 (100 ppm, SO2/N2), CH3CHO (100 ppm, CH3CHO/N2) were

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used as reactant gases.

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2.2 In situ DRIFTS Experiments. Heterogeneous reaction studies were performed by using

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in situ DRIFTS, similar to our previous works.11,21,31 Infrared spectra were recorded in the

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spectral range from 4000 to 650 cm-1 using a Nicolet Avatar 360 FTIR spectrometer equipped

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with a MCT detector and a diffuse reflectance accessory. IR spectra were recorded at a resolution

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of 4 cm-1, and 100 scans were averaged for each spectrum resulting in a time resolution of 1 min.

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A 40 mg (± 0.02 mg) sample was placed in ceramic crucible into the chamber, with the

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temperature of which kept at 298 K by using a temperature controller.


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The sample was first purged with Ar (100 mL/min) for 1 h, and followed by a sample

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background spectrum used for subsequent spectra. And then the reactant gas along with synthetic

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air (21% O2 and 79% N2) was introduced into the chamber at a total flow rate of 100 mL/min for

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2 h, during which the IR spectrum in the flow reaction system was recorded automatically every

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10 min. All the IR spectra were recorded at a resolution of 4 cm-1 for 100 scans.

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3. RESULTS AND DISCUSSION

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To investigate the impact of pre-adsorption of CH3CHO on the heterogeneous reaction of SO2

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and vice versa, the following aspects were studied: 1) Investigation of heterogeneous reaction of

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CH3CHO and SO2 on pure α-Fe2O3 individually. 2) Investigation of heterogeneous reaction of

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CH3CHO (or SO2) on α-Fe2O3 pre-treated with SO2 (or CH3CHO).

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3.1 Heterogeneous reaction of CH3CHO on α-Fe2O3. The heterogeneous uptake of

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acetaldehyde onto the surface of α-Fe2O3 particles at 298 K was investigated at first. DRIFTS

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spectra following the exposure of CH3CHO (80 ppm) in synthetic air on α-Fe2O3 particles at a

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flow of 100 mL/min, as a function of time for a period of 2 h are shown in Figure 1. These

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DRIFTS spectra provide valuable information of different surface species adsorbed on the α-

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Fe2O3 particles by the vibrational modes. Bands around 1700 cm-1 (ν C=O) are observed only at

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the very beginning (see Figure S3), the signal of which is very weak and disappears quickly,

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corresponding to the limited molecular CH3CHO (physisorbed and chemisorbed) on the surface

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of α-Fe2O3.32 It can be inferred that CH3CHO molecules are quickly transformed to other species

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on the surface of α-Fe2O3. At low exposure (1 min, shown in Figure S3), weak bands at 2977 (νas

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CH3), 2930 (ν CH), 2860 (νs CH3), 1460 (δas CH3), 1370 (δs CH3), 1180 (ν C-O-C), and 1100 (νs

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C-O) cm-1 are observed, corresponding to the formation of predominant product gem-diol

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(CH3CH(OH)2) and a small amount of dimer (CH3CH(OH)OCH(OH)CH3).33,34 No consumption


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of surface hydroxyl groups at this stage implies the role of Lewis acid sites. The Lewis acid sites

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can interact with carbonyl oxygen of CH3CHO to form adsorbed CH3CHO, and may catalyze the

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hydration of CH3CHO to produce gem-diol, and further catalyze the subsequent polymerization

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reaction between the hydroxyl groups of gem-diol and adsorbed CH3CHO33 or dehydration

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between gem-diol molecules to produce the dimer.34 With increasing exposure, intensive bands

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at 1558 (νas COO), 1454 (δas CH3), 1410 (νs COO), 1338 (δs CH3) cm-1 and two weak shoulder

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bands at 1432 cm-1(νas COO) and 1388 (δs CH3) gradually show up, 35-36 corresponding to acetate

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(already observed on the same surface after the adsorption of acetic acid).32 This indicates that

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CH3CHO can be oxidized into acetate on the surface of α-Fe2O3, which is in good agreement

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with the previous report.32 Weak bands at 1130 (ν C-O-C) and 1053 (νs C-O) cm-1 can also be

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observed for the increasing formation of gem-diol and dimer. When the exposure time of

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CH3CHO is increased to 1 h, bands at 1053 and 1100 cm-1 begin to decrease with other bands

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increasing continuously in intensity (see Figure S4), indicating the occurrence of secondary

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reactions of surface-formed gem-diol. One possible explanation is that the oxidation of adsorbed

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CH3CHO to acetate suppresses the formation of gem-diol molecules, while the existed gem-diol

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continues to transform to dimer.

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Overall, CH3CHO can transform into other species when adsorbed onto α-Fe2O3. As discussed

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above, within the 2 h reaction time, at least three products are observed on the surface of α-

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Fe2O3. Acetate is identified as the dominant surface product while gem-diol and dimer as the

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minor surface products, because the intensives of bands in the region of 1600-1250 cm-1 is much

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higher than those in the region of 1250-1000 cm-1. The weaker bands in the region of 1250-1000

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cm-1 may also imply the low coverage of Lewis acid sites on humid α-Fe2O3.


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Some studies have reported that adol condensation occurred when CH3CHO or acetone was

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adsorbed on some oxide particles,22,29,37 while others have reported that the formation of

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crotonaldehyde cannot occur on α-Fe2O329,32 and oxidized anatase38 after the exposure of

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CH3CHO. In our study, except that the ν(C=O) mode of adsorbed CH3CHO is observed only at

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the very beginning, neither bands around 1700 cm-1 (ν C=O) nor bands near 1600 cm-1 (ν C=C)

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are observed with increasing exposure. This rules out the formation of crotonaldehyde

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(CH3CH=CHCHO), indicating no occurrence of adol condensation.

1410

3634 3621 1432

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3654 3671 3664

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Time

1456 0.04

1338 1558

2972 2930 2873

3000

1750

1180 1100 1130 1053

1500

1250

1000

-1

Wavenumber (cm )

180 181

Figure 1. In situ DRIFTS spectra of individual CH3CHO (80 ppm) adsorption on α-Fe2O3 as a

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function of time, with background recorded just before the introduction of CH3CHO. The inset is

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the enlarged spectral region from 3700 to 3600 cm-1.

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In addition, after the initial stage, three negative bands at 3671, 3664 and 3654 cm-1 and two

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weak negative bands at 3634 and 3621 cm-1 are observed, and these negative bands increase with

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the increase of the exposure time, indicating the loss of isolated hydroxyl groups bonded to the


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surface of α-Fe2O3,39 which suggests that the OH groups are the reaction active sites for the

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heterogeneous reaction of CH3CHO with α-Fe2O3.

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It is worth noting that the intensities of all the bands are rather weak when the concentration

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of CH3CHO is set to be 3 ppm, indicating little reaction on the surface of α-Fe2O3, which is

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consistent with that previously reported by Li et al.29 However, when the concentration of

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CH3CHO is changed up to 80 ppm, a number of bands emerged with the increase of exposure

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time, and the surface of α-Fe2O3 is almost saturated after adsorption of 80 ppm CH3CHO for 2 h.

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Therefore, all the experiments in this study, the concentration of CH3CHO was set to be 80 ppm.

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The detailed assignments of bands formed during the reaction are summarized in Table 1. The

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proposed mechanism of the heterogeneous uptake of CH3CHO on α-Fe2O3 is depicted as follows

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(Scheme 1):

198 199

Scheme 1. Mechanism of the heterogeneous uptake of CH3CHO on α-Fe2O3

200 201 202


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Table 1. Infrared vibrational assignments for adsorbed species on α-Fe2O3 modes

frequencies (cm-1)

ν (CH)

2977, 2930, 2873, 2860

δ (CH)

1460, 1454, 1407, 1370,1338

ν (C=O)

1716 (physisorbed CH3CHO) 1682 (chemisorbed CH3COOH) 1652 (chemisorbed CH3CHO)

νas (COO)

1558

νs (COO)

1432, 1410

ν (C-O-C)

1180, 1130

νs (C-O)

1100, 1053

νs (S=O)

1263, 1260, 1447

νas (S=O)

1163, 1158, 1154

ν (OH)

3700-3600 (isolated surface -OH) 3600-3000 (perturbed surface-OH)

δ (OH)

1300

205 206

3.2 Heterogeneous reaction of SO2 on α-Fe2O3. Figure 2 shows the infrared spectra recorded

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during the heterogeneous reaction of SO2 (3 ppm) on α-Fe2O3 particles as a function of SO2

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exposure time. Three main bands at 1261, 1158, and 1010 cm−1 and three weak shoulder bands at

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1361, 1337 and 1050 cm-1 are observed in the spectra, and the intensities of these bands increase

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as the reaction proceeds. These bands can be assigned to the adsorbed sulfate and/or bisulfate,

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which is consistent with previous studies.4,31,37,40


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Five major negative bands at 3671, 3664, 3652, 3634 and 3621 cm-1 were observed during the

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reaction. These 5 bands can be assigned to stretching vibration modes of isolated surface

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hydroxyl groups bonded to the surface iron ions of octahedral site and tetrahedral sites,37 which

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is in good agreement with the previous studies,4,31,37 indicating the consumption of hydroxyl

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groups on the surface of α-Fe2O3 during the reaction. Another broad band from 3600 to 2800 cm-

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1

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formation of acidic species such as HSO4-.

is shown as the increasing intensity of hydrogen-bonded hydroxyl region,40 indicating the

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Additionally, similar spectra were obtained when increasing the concentration of SO2 from 3

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ppm to 80 ppm (see Figure S5), indicating the formation of the same surface products. In

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contrast to the spectra collected from the uptake of low SO2 concentration (3 ppm), the intensities

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of the surface products significantly increase at first and then grow more and more slowly as the

223

reaction proceeds, which suggests that saturation phenomenon on sulfate formation gradually

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appears. Therefore, in this study, the concentration of SO2 is chosen to be 3 ppm and 80 ppm to

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represent the small amount and enough amount of SO2 respectively.

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3671 3634 3664

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3621 1263

3652

Time

0.03

1337 1361

3600 3200 2800

1300

10501010

1200

1100

1000

-1

Wavenumber (cm )

226


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Figure 2. In situ DRIFTS spectra of individual SO2 (3 ppm) adsorption on α-Fe2O3 as a function

228

of time within 2 h, with background recorded just before the introduction of SO2. The inset is the

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enlarged spectral region from 3700 to 3600 cm-1.

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3.3 Interactions between heterogeneous uptake and adsorption of SO2 and CH3CHO on α-

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Fe2O3. To understand the effect of pre-adsorption of CH3CHO on heterogeneous reaction of SO2

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on α-Fe2O3, and vice versa, two exposure experiments were performed. There were 5 steps for

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each exposure experiment: 1) α-Fe2O3 particles were purged with Ar for 1 h at 298 K; 2)

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CH3CHO (or SO2) was introduced with synthetic air for 2 h; 3) particles were purged with Ar for

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1 h for the second time; 4) SO2 (or CH3CHO) was introduced with synthetic air for 2 h; 5)

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particles were purged with Ar for 1 h for the third time. In the end of step 1 and 3, backgrounds

237

were recorded for subsequent spectrum collection. IR spectra in the flow reaction system were

238

recorded automatically every 10 min during step 2 and 4, and in the end (1 h) of step 3 and 5.

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3.3.1 Heterogeneous uptake of SO2 on CH3CHO pre-adsorbed α-Fe2O3. FTIR spectra of

240

heterogeneous uptake of SO2 on α-Fe2O3 particles after pre-treatment with CH3CHO (80 ppm)

241

are shown in Figure 3. Each spectrum was obtained by referencing to the coating α-Fe2O3

242

spectrum after pre-adsorption of CH3CHO. Positive bands at 1682, 1300, 1260, 1154 cm-1 and

243

two negative bands at 1558 and 1410 cm-1 are observed growing as a function of increasing

244

exposure time. As discussed above, the bands at 1260 and 1154 cm-1 are assigned to

245

bisulfate/sulfate. New bands at 1682 cm-1 (ν C=O) together with 1300 cm-1 (ν C-OH) are

246

ascribed to the formation of chemisorbed acetic acid, bonded coordinatively through the

247

carbonyl oxygen to Lewis acid sites,32,41 which is not affected by subsequent Ar purging.

248

Negative bands at 1558 and 1409 cm-1 indicates the loss of COO-. In addition, the isolated

249

surface hydroxyl groups (3632, and 3621 cm-1) are also consumed as the reaction proceeds.


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1154 1260 3632 3621

1300

1682

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Time

1558 0.03 1410

1800

1650

1500

1350

1200

1050

-1

Wavenumber(cm )

250 251

Figure 3. In situ DRIFTS spectra of SO2 (3 ppm) adsorption after the pre-adsorption of

252

CH3CHO (80 ppm, 2 h) on α-Fe2O3, with background recorded just before the introduction of

253

SO2. The inset is the enlarged spectral region from 3700 to 3600 cm-1.


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1154

(a)

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1260

1682 (1) (2)

1558

0.04 1410

1800

1650

1500

1350

1200

1050

-1

Wavenumber (cm )

254

(b)

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(1)

(2)

0.08

1800

1650

1500

1350

1200

1050

-1

Wavenumber (cm )

255 256

Figure 4. In situ DRIFTS spectra of surface products on α-Fe2O3, with background recorded just

257

before the introduction of SO2. Comparison between different conditions: (1) after sequentially

258

pre-adsorption of CH3CHO (80 ppm, 2 h), Ar purging (1 h) and exposure of SO2 (2 h); (2) after

259

individual adsorption of SO2 (2 h) on pure α-Fe2O3. The concentration of SO2 is different: (a) 3

260

ppm and (b) 80 ppm.


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Page 16 of 41

261

The explanation might be that with the introduction of SO2 onto the α-Fe2O3 particles, surface

262

acidity increases as bisulfate/sulfate forms, and thus surface coordinated acetate ions are

263

transformed into chemisorbed acetic acid, which results in the consumption of COO- and the

264

formation of –COOH, as well as the possible transformation of coordination modes of sulfates.42

265

It also can be supported by the evidence that the intensities of the broad band from 3600 to 2800

266

cm-1 is weaker in the case of α-Fe2O3 pre-adsorbed CH3CHO than that of pure α-Fe2O3 (Figure

267

4a). Also, this evidence indicates that the products formed by the pre-adsorption of CH3CHO on

268

α-Fe2O3 disturb the formation of hydrogen-bonds based on the acidic species produced from the

269

heterogeneous reaction of SO2.

270

It is worth noting that when the concentration of introduced SO2 is 3 ppm, the pre-adsorption

271

of CH3CHO does not significantly affect the bisulfate/sulfate formation as shown in Figure 4a.

272

However, when the concentration of introduced SO2 increases up to 80 ppm, the formation of

273

bisulfate/sulfate is hindered significantly within the 2 h (Figure 4b). In other words, the pre-

274

adsorption of CH3CHO significantly suppresses the heterogeneous reaction of large amount of

275

SO2 on the surface of α-Fe2O3, while it has a little influence on the uptake of small amount of

276

SO2.

277

Earlier studies of SO2 and CH3CHO adsorption on α-Fe2O3 have shown that hydroxyl groups

278

and adsorbed water play an important role in the surface chemistry, and can be consumed during

279

the uptake of SO2 and CH3CHO.23,31 The disappearance of adsorbed water will hinder the

280

regeneration of hydroxyl groups,31 and hence the total amount of hydroxyl groups during the

281

whole reaction will be limited. In this study, the suppression may be attributed to these limited

282

surface hydroxyl groups. The pre-adsorption of CH3CHO mainly consumes the hydroxyl groups

283

with vibrational frequencies in the region from 3700-3647 cm-1. However, SO2 has no preference


16

Page 17 of 41

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284

for the hydroxyl groups with vibrational frequencies in the region from 3700-3600 cm-1 when

285

heterogeneous reaction occurs. Therefore, after the pre-adsorption of CH3CHO for 2 h, the left

286

hydroxyl groups are sufficient for the uptake of small amount of SO2 (3 ppm, 2 h), but

287

inadequate for large amount of SO2 (80 ppm, 2 h). The details will be further discussed in section

288

3.3.3.

289

3.3.2 Heterogeneous reaction of CH3CHO on SO2 pre-adsorbed α-Fe2O3. DRIFT spectra

290

of heterogeneous reaction of CH3CHO on α-Fe2O3 particles after pre-treatment with SO2 (3 ppm)

291

are shown in Figure 5. Predominant bands at 1716, 1682, 1652, 1558 and 1410 cm-1 are observed

292

increasing as a function of time. The two bands at 1558 and 1410 cm-1 are assigned to acetate as

293

discussed above, while the three bands at 1716, 1682 and 1652 cm-1 are in the region of C=O

294

stretching vibrations. To show the difference of surface products with uptake of CH3CHO (2 h)

295

in various conditions, Figure 6 is presented. Compared with the adsorption of CH3CHO on pure

296

α-Fe2O3, several new bands (1716, 1682 and 1652 cm-1) appear in the case of heterogeneous

297

reaction of CH3CHO on SO2 pre-treated α-Fe2O3. The band at 1716 cm-1 disappears after the

298

purging, while the other two bands at 1682 and 1652 cm-1 are more resistant to the purging,

299

which suggests that the band at 1716 cm-1 corresponds to physisorbed CH3CHO while the bands

300

at 1682 and 1652 cm-1 correspond to chemisorbed acetic acid and CH3CHO respectively.30,32

301

Apart from the appearance of new bands, intensities of the bands for acetate (1558 and 1410 cm-1)

302

are much weaker on the surface of SO2 pre-treated α-Fe2O3 than pure one, suggesting that the

303

pre-adsorption of SO2 can significantly hinder the heterogeneous reaction between CH3CHO and

304

α-Fe2O3 particles. When the pre-adsorption concentration of SO2 increases from 3 ppm to 80

305

ppm, molecular CH3CHO and CH3COOH become the dominant products on the particle surfaces,

306

and acetate and gem-diol species are hard to detect. No negative band shows up in the range of


17


307

1300-1000 cm-1, suggesting that the surface sulfates have not been affected by the heterogeneous

308

reaction of CH3CHO on α-Fe2O3. The negative effect of pre-adsorption of SO2 on heterogeneous

309

reaction of CH3CHO on α-Fe2O3 may be attributed to that the heterogeneous reaction between

310

SO2 and α-Fe2O3 particles can consume the surface isolated hydroxyl groups and form steric

311

hindrance of the surface catalysis,37 resulting in a weaker conversion from CH3CHO to acetate.

312

Additionally, the pre-adsorption of SO2 on α-Fe2O3 increases the acidity of particle surface by

313

the formation of acidic species, which leads to the formation of CH3COOH rather than CH3COO-

314

, as the acidity of HSO4- (pKa=1.99) is stronger than that of CH3COOH (pKa=4.75).

1410 3634 3621

1432

3654 3671

1388

3664

1716

K-M

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 41

1682

1456

Time

0.004 1652 1558 2980

1338

2930 2880

3000

1800

1600

1400

1095 1038 1247 1163

1200

1000

-1

Wavenumber (cm )

315 316

Figure 5. In situ DRIFTS spectra of CH3CHO adsorption (80 ppm) after the pre-adsorption of

317

SO2 (3 ppm, 2 h) on α-Fe2O3, with background recorded just before the introduction of

318

CH3CHO. The inset is the enlarged spectral region from 3700 to 3600 cm-1.

319 320


18

Page 19 of 41

1410

(1) (2) (3) (4)

K-M

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.05 1682 1558 1716 1652

1800

1650

1500

1350

1200

1050

-1

Wavenumber (cm )

321 322

Figure 6. In situ DRIFTS spectra of surface products on α-Fe2O3, with background recorded just

323

before the introduction of CH3CHO (80 ppm). Comparison among different conditions: (1) after

324

individual adsorption of CH3CHO (80 ppm, 2 h) on pure α-Fe2O3; (2) after sequentially pre-

325

adsorption of SO2 (3 ppm, 2 h), Ar purging (1 h) and exposure of CH3CHO (80 ppm,2 h); (3)

326

after sequentially pre-adsorption of SO2 (3 ppm, 2 h), Ar purging (1 h), exposure of CH3CHO

327

(80 ppm, 2 h) and Ar purging (1 h); (4) after sequentially pre-adsorption of SO2 (80 ppm, 2 h),

328

Ar purging (1 h) and exposure of CH3CHO (80 ppm, 2 h).

329 330

3.3.3 Consumption Characteristics of Isolated Surface Hydroxyl Groups. In the current

331

study, the adsorption of SO2 and CH3CHO on α-Fe2O3 particles would consume hydroxyl

332

groups, resulting in negative bands from 3600 to 3700 cm-1. As discussed above, α-Fe2O3

333

particles are almost saturated after the introduction of 80 ppm CH3CHO or SO2 for 2 h, which

334

may suggest that after 2 h adsorption of individual CH3CHO (or SO2), the surface hydroxyl


19


335

groups can be further used for the heterogeneous reaction of CH3CHO (or SO2) on α-Fe2O3

336

particles are nearly exhausted.

(I) 3700-3647

(II) 3647-3600 (d)

(e) (f)

(c) (b)

K-M

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Page 20 of 41

0.008

3700

(a)

3650

3600 -1

Wavenumber (cm )

337 338

Figure 7. Spectra of α-Fe2O3 after (a) individual introduction of CH3CHO (80 ppm, 2 h); (b)

339

individual introduction of SO2 (80 ppm, 2 h); (c) sequentially introduction of CH3CHO (80 ppm,

340

2 h), Ar (1 h) and SO2 (80 ppm, 2 h); (d) sequentially introduction of SO2 (80 ppm, 2 h), Ar(1 h)

341

and CH3CHO (80 ppm, 2 h); (e) individual introduction of SO2 (3 ppm, 2 h); (f) sequentially

342

introduction of SO2 (3 ppm, 2 h), Ar(1 h) and CH3CHO (80 ppm, 2 h). The background was

343

recorded just before the introduction of the last reactant gas.

344

The surface hydroxyl groups on α-Fe2O3 particles in our study can be classified into 3 types

345

according to their vibration frequencies: (I) bands between 3700-3647 cm-1, (II) bands between

346

3647-3600 cm-1 and (III) bands less than 3600 cm-1. Hydroxyl groups of type I and II are

347

assigned to isolated surface hydroxyl groups, while those of type III are assigned to perturbed

348

surface hydroxyl groups,39 which are perturbed by the hydrogen bond interactions with adjacent


20

Page 21 of 41

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349

hydroxyl groups, and only isolated surface hydroxyl groups are consumed in all of our

350

experiments. As shown in Figure 7a, a large amount of surface hydroxyl groups of type I and a

351

small part of type II are consumed when large amount of CH3CHO (80 ppm, 2 h) is introduced

352

onto α-Fe2O3 sample, while in the case of SO2 (80 ppm, 2 h), surface hydroxyl groups of type I

353

and II are evenly consumed (see Figure 7b). It can be inferred that the heterogeneous reaction of

354

80 ppm CH3CHO on α-Fe2O3 for 2 h mainly exhausts surface hydroxyl groups of type I, while

355

SO2 exhausts hydroxyl groups of type I and II.

356

When α-Fe2O3 particles are pre-treated with CH3CHO (80 ppm, 2 h), the surface hydroxyl

357

groups of type I are already exhausted and part of type II are left, and thus only some left surface

358

hydroxyl groups of type II can be used as active sites for the following uptake of SO2. For the

359

following conversion of small amount of SO2 (3 ppm, 2 h), the results show that the left surface

360

hydroxyl groups of type II are sufficient, and the amount of the formed sulfate/bisulfate is the

361

same as that of the individual uptake of SO2 (3 ppm, 2 h) on the pure particles. However, when

362

the SO2 (80 ppm, 2 h) is introduced onto the coated particles (see Figure 7c), no appearance of

363

negative bands in the region of the hydroxyl groups of type I indicates that no hydroxyl groups

364

of type I can be consumed. The left hydroxyl groups of type II compared to the pure particles

365

with enough hydroxyl groups of type I and II are insufficient to transform the same amount of

366

SO2 to bisulfate/sulfate as on pure α-Fe2O3 particles, and results in the formation of less amount

367

of sulfate/bisulfate compared with the case without pre-adsorption of CH3CHO, suggesting the

368

suppression for the heterogeneous reaction of high concentration SO2.

369

Inversely, when α-Fe2O3 particles are pre-treated with SO2 (80 ppm, 2 h), the surface hydroxyl

370

groups of type II are already exhausted and only some surface hydroxyl groups of type I are left.

371

However, when the CH3CHO (80 ppm, 2 h) is introduced onto this kind of coated particles (see


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 41

372

Figure 7d), no further consumption of the left hydroxyl groups of type I is observed, indicating

373

the indispensable role of the surface hydroxyl groups of type II and the influence of the

374

heterogeneous uptake of SO2 with high concentration. Instead of reacting with the left surface

375

hydroxyl groups of type I, surface adsorbed CH3CHO together with some acetic acid is still

376

observed (also see Figure 6). The possible explanation may be that the other active sites such as

377

Lewis acid sites and oxygen vacancies may be responsible for the oxidation of CH3CHO and the

378

formation of acetic acid in the presence of O2. When α-Fe2O3 particles are pre-treated with

379

insufficient SO2 (3 ppm, 2 h), the surface hydroxyl groups of type I and type II are not exhausted

380

(see Figure 7e), and the left type I and II surface hydroxyl groups can be further used as active

381

sites. When the CH3CHO (80 ppm, 2 h) is introduced onto the coated particles (see Figure 7f), it

382

would use up the left surface hydroxyl groups of type II, which is supported by the following fact,

383

that is, in the range of type II in Figure 7e, 7f and 7a, the sum of spectrum of 2 h individual SO2

384

(3 ppm) and spectrum of 2 h CH3CHO (80 ppm) on SO2 (3 ppm, 2 h) coating particles coincides

385

with the spectrum of 2 h individual CH3CHO (80 ppm).

386

4. CONCLUSIONS AND ATMOSPHERIC IMPLICATIONS.

387

In this study, the effects of pre-adsorption of CH3CHO (or SO2) and the role of hydroxyl groups

388

on uptake of SO2 (or CH3CHO), two ubiquitous atmospheric gases, on α-Fe2O3 were

389

investigated. According to the experimental observations reported in this study, isolated

390

hydroxyl groups rather than perturbed surface hydroxyl groups play a significant role in the

391

heterogeneous oxidations of CH3CHO to acetate and SO2 to sulfate/bisulfate on α-Fe2O3, and

392

isolated hydroxyl groups have different reactivities in the individual uptake of CH3CHO and SO2

393

due to different surface sites. It is found that the pre-adsorption of SO2 on α-Fe2O3 significantly

394

hinder the following heterogeneous oxidation of CH3CHO to acetate, change the adsorption into


22

Page 23 of 41

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


395

physisorption and chemisorption of CH3CHO, as fewer isolated hydroxyl groups can be obtained

396

as active sites after the uptake of SO2. In the other case, the pre-adsorption of CH3CHO

397

significantly suppresses the heterogeneous reaction of large amount of SO2 on the surface of α-

398

Fe2O3, while it has a little influence on the uptake of small amount of SO2. Additionally,

399

heterogeneous reactions of SO2 on α-Fe2O3 pre-adsorbed by CH3CHO change the existing

400

acetate on the particle surface into chemisorbed acetic acid, for the enhancement of surface

401

acidity after the uptake of SO2.

402

Previously, much attention was paid on the mechanism and kinetics of heterogeneous reactions

403

of SO2 and CH3CHO, individually, on mineral aerosols.4,22,30 However, in the atmosphere,

404

during the transportation of airborne mineral aerosols, heterogeneous reaction between SO2, or

405

CH3CHO with those aerosols would occur. Then, the physical and chemical properties of those

406

aerosols could be changed, providing secondary aerosols, and the hereafter heterogeneous

407

reactions would not be the same as on the pure one. According to the results of this study, uptake

408

of CH3CHO on α-Fe2O3 particles is affected by the pre-adsorption of SO2, and vice versa. This

409

shows the complexity of the heterogeneous reaction of SO2 and CH3CHO on airborne mineral

410

aerosols, and could provide more information of the sulfate and acetate formation in the real

411

atmosphere, by considering the pre-adsoprtion of CH3CHO and SO2.

412 413

ASSOCIATED CONTENT

414

Supporting Information.

415

(1)X-ray diffraction pattern of the synthesized hematite; (2) Electron micrographs of the

416

synthesized hematite:(a)SEM.(b)TEM (3) In situ DRIFTS spectra of individual CH3CHO (80

417

ppm) adsorption on α-Fe2O3 with contact time for 1 min; (4) Enlarged spectral region from 1200


23


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Page 24 of 41

418

to 900 cm-1 of Figure 1. (5) (a) In situ DRIFTS spectra of individual SO2 (80 ppm) adsorption on

419

α-Fe2O3 as a function of time for 120 min. (b) Dynamic changes in the integrated absorbance

420

area of sulfate (1450-950 cm-1) for α-Fe2O3 particles exposed to SO2 (80 ppm, 120 min) (6)

421

Complete ref 19. This material is available free of charge via the Internet at http://pubs.acs.org.

422 423

■ AUTHOR INFORMATION

424

Corresponding Author

425

Tel: +86-21-65642521, fax: +86-21-6564-2080, E-mail: [email protected] (Lingdong Kong)

426

Tel: +86-21-65642298, fax: +86-21-6564-2080, E-mail: [email protected] (Jianmin Chen)

427

Notes

428

The authors declare no competing financial interest.

429

■ ACKNOWLEDGEMENTS

430

The authors thank the National Natural Science Foundation of China (Grant Nos. 41475110,

431

21277028, 21190053, 41275126, and 41475109) for supporting this research.

432

■ References:

433 434 435 436

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34. Iraci, L. T.; Tolbert, M. A. Heterogeneous Interaction of Formaldehyde with Cold

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Sulfuric Acid: Implications for the Upper Troposphere and Lower Stratosphere. J. Geophys.

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Res.: Atmos. 1997, 102, 16099-16107.

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35. Idriss, H.; Diagne, C.; Hindermann, J. P.; Kiennemann, A.; Barteau, M. A. Reactions of Acetaldehyde on CeO2 and CeO2-Supported Catalysts. J. Catal. 1995, 155, 219-237.

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36. Rachmady, W.; Vannice, M. A. Acetic Acid Reduction to Acetaldehyde Over Iron

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Catalysts II. Characterization by Mossbauer Spectroscopy, DRIFTS, TPD, and TPR. J. Catal.

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2002, 208, 170-179.

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37. Watanabe, H.; Gutleben, C. D.; Seto, J. Sulfate-Ions on the Surface of Maghemite and Hematite. Solid State Ionics 1994, 69, 29-35.

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38. Rekoske, J. E.; Barteau, M. A. Competition between Acetaldehyde and Crotonaldehyde

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During Adsorption and Reaction on Anatase and Rutile Titanium Dioxide. Langmuir 1999, 15,

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2061-2070.38. Rekoske, J. E.; Barteau, M. A., Competition Between Acetaldehyde and

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Crotonaldehyde During Adsorption and Reaction On Anatase and Rutile Titanium Dioxide.

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39. Watanabe, H.; Seto, J. Specific Acidities of the Surface Hydroxyl-Groups On Maghemite. Bull. Chem. Soc. Jpn. 1993, 66, 395-399.

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40. Nanayakkara, C. E.; Pettibone, J.; Grassian, V. H. Sulfur Dioxide Adsorption and

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41. Busca, G.; Lorenzelli, V. Infrared Study of Methanol, Formaldehyde, and Formic-Acid Adsorbed On Hematite. J. Catal. 1980, 66, 155-161. 42. Sugimoto, T.; Wang, Y. S. Mechanism of the Shape and Structure Control of Monodispersed α-Fe2O3 Particles by Sulfate Ions. J. Colloid Interface Sci. 1998, 207, 137-149.

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Figure 1. In situ DRIFTS spectra of individual CH3CHO (80 ppm) adsorption on α-Fe2O3 as a function of time, with background recorded just before the introduction of CH3CHO. The inset is the enlarged spectral region from 3700 to 3600 cm-1. 431x303mm (300 x 300 DPI)



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Figure 2. In situ DRIFTS spectra of individual SO2 (3 ppm) adsorption on α-Fe2O3 as a function of time within 2 h, with background recorded just before the introduction of SO2. The inset is the enlarged spectral region from 3700 to 3600 cm-1. 431x303mm (300 x 300 DPI)


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Figure 3. In situ DRIFTS spectra of SO2 (3 ppm) adsorption after the pre-adsorption of CH3CHO (80 ppm, 2 h) on α-Fe2O3, with background recorded just before the introduction of SO2. The inset is the enlarged spectral region from 3700 to 3600 cm-1. 431x303mm (300 x 300 DPI)



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Figure 4. In situ DRIFTS spectra of surface products on α-Fe2O3, with background recorded just before the introduction of SO2. Comparison between different conditions: (1) after sequentially pre-adsorption of CH3CHO (80 ppm, 2 h), Ar purging (1 h) and exposure of SO2 (2 h); (2) after individual adsorption of SO2 (2 h) on pure α-Fe2O3. The concentration of SO2 is different: (a) 3 ppm and (b) 80 ppm. 431x303mm (300 x 300 DPI)


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Figure 4. In situ DRIFTS spectra of surface products on α-Fe2O3, with background recorded just before the introduction of SO2. Comparison between different conditions: (1) after sequentially pre-adsorption of CH3CHO (80 ppm, 2 h), Ar purging (1 h) and exposure of SO2 (2 h); (2) after individual adsorption of SO2 (2 h) on pure α-Fe2O3. The concentration of SO2 is different: (a) 3 ppm and (b) 80 ppm. 431x303mm (300 x 300 DPI)



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Figure 5. In situ DRIFTS spectra of CH3CHO adsorption (80 ppm) after the pre-adsorption of SO2 (3 ppm, 2 h) on α-Fe2O3, with background recorded just before the introduction of CH3CHO. The inset is the enlarged spectral region from 3700 to 3600 cm-1. 431x303mm (300 x 300 DPI)


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graph 6. In situ DRIFTS spectra of surface products on α-Fe2O3, with background recorded just before the introduction of CH3CHO (80 ppm). Comparison among different conditions: (1) after individual adsorption of CH3CHO (80 ppm, 2 h) on pure α-Fe2O3; (2) after sequentially pre-adsorption of SO2 (3 ppm, 2 h), Ar purging (1 h) and exposure of CH3CHO (80 ppm,2 h); (3) after sequentially pre-adsorption of SO2 (3 ppm, 2 h), Ar purging (1 h), exposure of CH3CHO (80 ppm, 2 h) and Ar purging (1 h); (4) after sequentially preadsorption of SO2 (80 ppm, 2 h), Ar purging (1 h) and exposure of CH3CHO (80 ppm, 2 h). 431x303mm (300 x 300 DPI)



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graph 7. Spectra of α-Fe2O3 after (a) individual introduction of CH3CHO (80 ppm, 120 min); (b) individual introduction of SO2 (80 ppm, 120 min); (c) sequentially introduction of CH3CHO (80 ppm, 120 min), Ar (60 min) and SO2 (80 ppm, 120 min); (d) sequentially introduction of SO2 (80 ppm, 120 min), Ar(60 min) and CH3CHO (80 ppm, 120 min); (e) individual introduction of SO2 (3 ppm, 120 min); (f) sequentially introduction of SO2 (3 ppm, 120 min), Ar(60 min) and CH3CHO (80 ppm, 120 min). The background was recorded just before the introduction of the last reactant gas. 431x301mm (300 x 300 DPI)


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Mechanism of the heterogeneous uptake of CH3CHO on α-Fe2O3 50x22mm (600 x 600 DPI)



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Table 1. Infrared vibrational assignments for adsorbed species on α-Fe2O3 11x10mm (600 x 600 DPI)


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53x21mm (600 x 600 DPI)


Interactions between Heterogeneous Uptake and Adsorption of Sulfur

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