Effect of Formaldehyde on the Heterogeneous Reaction of Nitrogen

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The Effect of Formaldehyde on the Heterogeneous Reaction of Nitrogen Dioxide on Gamma-Alumina Zhenyu Sun, Lingdong Kong, Xi Zhao, Xiaoxiao Ding, Hongbo Fu, Tiantao Cheng, Xin Yang, and Jianmin Chen J. Phys. Chem. A, Just Accepted Manuscript • Publication Date (Web): 18 Aug 2015 Downloaded from http://pubs.acs.org on August 19, 2015

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The Effect of Formaldehyde on the Heterogeneous Reaction of Nitrogen Dioxide on Gamma-Alumina Zhenyu Sun,† Lingdong Kong,*,†,‡ Xi Zhao,† Xiaoxiao Ding,† Hongbo Fu,† Tiantao Cheng,† Xin Yang,† Jianmin Chen*,† †

Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention,

Department of Environmental Science & Engineering, Fudan University, Shanghai 200433, China. ‡

Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution

Control, School of Environmental Science and Engineering, Nanjing University of Information Science & Technology, Nanjing 210044, China.

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ABSTRACT: Heterogeneous reactions of NO2 on various mineral aerosol particles have been investigated in many previous studies, but a fundamental understanding of how the adsorption of formaldehyde influences the heterogeneous reactions of NO2 remains unclear. In this work, the effect of formaldehyde preadsorption on heterogeneous reaction of NO2 on the surface of γ-Al2O3 at 298K and ambient pressure was investigated by using diffuse reflectance infrared Fourier transform spectrometry (DRIFTS). It was found that the preadsorption of HCHO on γ-Al2O3 could suppress the formation of nitrate, and the rate of nitrate formation decreased with increasing amount of preadsorbed HCHO, while the following heterogeneous uptake of NO2 could suppress the hydration reaction of HCHO and promote the production of HCOO- during the reaction. Surface nitrite was formed and identified to be an intermediate product and gradually disappeared as the reaction proceeded. The amount of the formed nitrite decreased when the amount of HCHO increased. Uptake coefficients of heterogeneous reactions were calculated and found to be sensitive to the adsorption of HCHO. A possible mechanism for the influence of HCHO adsorption on the heterogeneous conversion of NO2 on γ-Al2O3 was proposed, and atmospheric implications based on these results were discussed.

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1. INTRODUCTION: NO2 is a major pollutant gas in the atmosphere, and its chemical reactivity is of great importance in atmospheric chemistry.1 It is closely related to the formation of nitrate aerosol, acid rain and tropospheric ozone.2−5 The heterogeneous reactions of NO2 on mineral aerosols will not only change the concentration of NO2 but also alter the surface properties of mineral aerosols, which have considerable effects on the lifetime and cloud condensed nuclei (CCN) ability of dust aerosols,6 and hence the heterogeneous reactions of NO2 have been received increasing attention in the past decades. Up to now, numerous laboratory studies have been performed on the heterogeneous reactions of NO2 on various atmospherically relevant particles, such as soot, sea salt aerosols and mineral oxides,7-21 and previous laboratory studies have shown that a synergistic effect exists in the heterogeneous reaction between NO2 and SO2 on different mineral oxides.22 However, little attention was paid to the effect of pre-adsorption of the organic pollutants on the uptake and reaction processes of NO2 on the mineral aerosols. Formaldehyde (HCHO), as the simplest and most abundant volatile carbonyl compound in the atmosphere,23 is emitted directly from a variety of domestic and industrial sources and produced by photochemical oxidation of hydrocarbons including alkanes and alkenes. It is an important chemical for the global economy, widely used in construction, wood processing, textiles, carpeting, and in the chemical industry. However, it has been classified as a human carcinogen that causes nasopharyngeal cancer and probably leukemia.24 In addition, it is regarded as an 3



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important precursor in the photochemical formation of tropospheric ozone. The main removal processes of HCHO in the atmosphere are currently thought to be its photolysis. HCHO photolysis will form HO2 radicals, which react with NO during the morning hours, and then rapidly convert to OH radicals. In some cities, HCHO photolysis is the dominant radical source sustaining photochemical smog formation throughout the day. Heterogeneous reactions of HCHO on oxide particles have also been regarded as an important sink in the atmosphere. Previous studies have shown that HCHO molecules can undergo heterogeneous reactions to yield several irreversibly adsorbed species on the surfaces, such as formate and methoxide,25−28 and thus the heterogeneous processes of HCHO on mineral dust aerosols will change the surface properties of those aerosols, which would impact on the following uptake behaviors of these particles for other gases in the atmosphere.The heterogeneous reactions of NO2 and HCHO have been individually studied in many conditions. The heterogeneous reaction kinetic and mechnism of NO2 and HCHO have been discussed well separately.5, 7, 9, 29 However, less is known about the impact of formaldehyde on the heterogeneous reactions of NO2 on mineral aerosol. Alumina is an important component of mineral dust in the atmosphere. There are two forms of alumina, α-Al2O3, and γ-Al2O3. Although α-Al2O3 is the most common phase of alumina in the troposphere, γ-Al2O3 is widely used as a model of mineral aerosol for its higher Brunauer-Emmett-Teller (BET) surface area and better quality of spectra information, which serves to obtain useful information about the mechanism of atmospheric heterogeneous reactions.10, 11 4


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In this study, the influence of formaldehyde on the heterogeneous reactions of NO2 on γ-Al2O3 was investigated. With the use of DRIFTS, a series of reactive uptake coefficients for heterogeneous reactions of NO2 on the surface of γ-Al2O3 particles with the introduction of different amounts of HCHO were obtained. The mechanism of the effect of HCHO on the nitrite and nitrate formation was also discussed. The results of this study contribute to a better understanding of the heterogeneous reaction of NO2 on mineral aerosol particles in the troposphere, and it will also provide significant information for atmospheric chemistry studies.

2. EXPERIMENTAL SECTION 2.1. Chemicals. Commercially available γ-Al2O3 particles (99.997% purity, Surface Area: 50 m2/g) purchased from Alfa Aesar were used for the spectroscopic measurements. Some γ-Al2O3 particles were kept in a desiccator at 68% RH for 48 h after the particles were heated at 100 ºC for 24 h, and this treatment made some adsorbed water molecule layers to be present on the particles. Standard gases NO2 (200 ppm, NO2/N2) and HCHO (100 ppm, HCHO/N2) (Shanghai Qingkuan Chemical Co., Ltd) were used as reactant gases. O2 (99.999% purity, Shanghai Qingkuan Chemical Co., Ltd) and Ar (99.999% purity, Shanghai Qingkuan Chemical Co., Ltd) were introduced into reaction chamber through gas dryers before use. 2.2. In situ DRIFTS Experiment. In this study the heterogeneous reaction of NO2 with γ-Al2O3 has been studied by using diffuse reflectance infrared Fourier transform spectroscopic (DRIFTS) technique. The DRIFTS spectra were recorded on the 5



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Nicolet Avatar 360 FTIR spectrometer, which equipped with a Spectra-Tech diffuse reflectance accessory and a high-sensitivity mercury cadmium telluride (MCT) detector cooled by liquid N2. About 14 mg (±0.02 mg) γ-Al2O3 sample was placed into the ceramic crucible in the chamber, and the temperature in the chamber was kept at 298 K by using an automatic temperature controller. After the reaction chamber was purged with synthetic air (21%O2 and 79%Ar) at a total flow rate of 100 mL/min for 60 min, HCHO (79 mL/min) and O2 (21 mL/min) were introduced into the chamber to adsorb onto γ-Al2O3 for 0 min, 10 min, 30 min and 60 min respectively, then synthetic air was used to purge the chamber for 60 min again, and then a background spectrum was recorded. After collecting the background spectrum, a mixture of gases [NO2 (1.12× 1015 molecules cm−3), O2 (21% v/v) and Ar (59% v/v)] were introduced into the chamber at a total flow rate of 100 mL/min for 120 min. When the mixture of gases introduced, the IR spectra were recorded automatically every 10 minutes. All the IR spectra were recorded with a resolution of 4 cm-1 for 100 scans.

3. RESULTS AND DISCUSSION 3.1 Heterogeneous Reaction of HCHO on γ-Al2O3. In this study, heterogeneous reactions of HCHO on dry and humid γ-Al2O3 were firstly investigated with in situ DRIFTS at 298K. It was found that HCOO- species appeared on the surface of dry γ-Al2O3 particles, but was not detected on the surface of humid γ-Al2O3 particles, while methylene glycol (HOCH2OH, a product of the hydration of HCHO) formed on the humid γ-Al2O3 particles. No bands around 1700 cm-1 were observed, suggesting 6


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that no adsorbed HCHO molecules were present, and hence inferring that HCHO molecules were quickly transformed to methylene glycol on the surface of humid γ-Al2O3 particles. These results were consistent with previous studies.26, 28 In this study, negative bands at 3737, 3730 and 3717 cm-1 were also observed, and these negative bands increased with the increase in exposure time. These bands were mainly assigned to the surface isolated hydroxyl groups, indicating the loss of surface hydroxyl groups bonded to the surface of γ-Al2O3, which suggested that the OH groups were the reaction active sites for the heterogeneous uptake of HCHO with γ-Al2O3. The explanation may be that with the introduction of HCHO onto the humid γ-Al2O3 particles, the O atom of C=O group of HCHO, with highly polar, is trap in a hydrogen atom of the surface isolated hydroxyl group to form adsorbed HCHO, which makes the carbonyl carbon more electrophilic and easier to be attracted by water molecular, resulting in quick formation of methylene glycol and consumption of the surface hydroxyls on the surface of humid γ-Al2O3 particles.26, 28 In addition, water molecules preferentially take up Al3+ active sites (i.e. Lewis acid sites) on the humid surface of γ-Al2O3, which would suppress the interaction of HCHO with Al3+ and thus inhibit the subsequent heterogeneous oxidation of the adsorbed formaldehyde (combining with unsaturated Al3+) by active oxygen species,26 and therefore no formate, methoxy and polyoxymethylene on the humid γ-Al2O3 were observed. Considering that HCHO can be continuously converted into surface products such as HCOO- and methylene glycol on γ-Al2O3 particles,26, 28 quantitative analysis of surface coverage of adsorbed HCHO on the particle surfaces would be 7



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difficult to obtain. And also, in order to investigate the effects of different stages of the heterogeneous reaction of HCHO on γ-Al2O3 particles on the following reaction of NO2, the different preset times for the introduction of HCHO have been adopted in this study. 3.2 Observed Products of Heterogeneous Uptake of NO2 on HCHO Preadsorbed γ-Al2O3. In uptake experiments, the DRIFTS spectrum of the unreacted sample has been used as a background spectrum. And then, reaction products formed during the uptake can be observed as positive absorption bands, whereas negative bands indicate the loss of the corresponding species. The alumina sample was exposed to NO2 (1.12×1015 molecules cm−3) balanced with synthesized air in a total flow of 100 mL/min at 298 K. The in situ DRIFTS spectra as a function of time are shown in Figure 1:

Figure1. DRIFTS spectra of surface products during the reaction of humid γ-Al2O3 8


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with NO2 as a function of reaction time, the particles had been preadsorbed by HCHO (2.21× 1015 molecules cm−3) for 60 min. When humid γ-Al2O3 particles preadsorbed HCHO for 60 min were exposed to NO2 (1.12× 1015 molecules cm−3), several new absorption bands at 1242, 1343, 1401, 1504, 1540, 1586, 1619, 1662, 1744, 3705 and 3730 cm−1 were observed (Figure 1). The band at 1242 cm−1 grew in the early stage of the reaction, and then gradually disappeared, implying that this surface species undergoes secondary chemistry on the sample surface. The band at 1242 cm−1 was assigned to nitrite species (NO2−) based on previous studies.15,

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In general, nitrite species had several geometries on the

surface, such as monodentate nitro, linear nitrite, and bidentate nitrite. In our study, the former two species can be ruled out because of no bands at 1440 and 1340 cm−1 for monodentate nitro species and no band at 1435 cm−1 for linear nitrite species.20, 29 Bidentate nitrite species show asymmetric and symmetric stretching vibrations at 1300-1320 and ~1230 cm−1, respectively.15, 19 The asymmetric stretching feature was overlapped by the broad bands of nitrate in our study. In the previous study, the band at 1245 cm−1 was assigned to bidentate nitrite by Guan et al.,12 and thus it is reasonable to assign the band at 1242 cm−1 to bidentate nitrite. Therefore, the nitrite was still an intermediate product of the heterogeneous reaction of NO2 on the surface of humid γ-Al2O3 particles preadsorbed HCHO, which is consistent with the result in the heterogeneous reactions of NO2 on pure humid γ-Al2O3 particles. According to previous studies, the absorption bands in the region from 1250 to 1670 cm−1 are mainly assigned to the degenerate ν3 mode of nitrate ion coordinated to 9



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the surface.11, 13, 15, 16 Usually, there would be only one asymmetric stretching of free NO3−,30 but in our study, there were several other bands at 1343, 1401, 1504, 1540, and 1619 cm−1 in the region of 1250 to 1670 cm−1, which suggests that there were some other adsorbed nitrates besides free NO3− on the particle surfaces, for nitrate can be adsorbed on the surface in different ways, such as monodentate, bidentate, and bridging.31 The band at 1540 cm−1 were assigned to the monodentate nitrate species. The bands at 1343 and 1401 cm−1 reflected the formation of adsorbed water-solvated nitrate species. The two absorption bands at 1619 and 1504 cm−1 were assigned to the bridging nitrate species. These results agree well with previous studies.11,

12

In

addition, the band at 1586 cm-1 was observed growing as function of the exposed time. This band reflected the formation of HCOO- species,26 indicating that the preadsorbed HCHO can be oxidized during the introduction of NO2. No methylene glycol formation was observed on the humid γ-Al2O3 particles in the end, which is different from that only the HCHO was introduced on the humid γ-Al2O3 particles, suggesting that the following heterogeneous reaction of NO2 also suppress the formation of methylene glycol. One possible explanation is that the production of HCOO- during the reaction shifts the equilibrium from methylene glycol toward surface HCHO, and thus suppresses the formation of methylene glycol. Compared with the heterogeneous reaction of HCHO on humid γ-Al2O3 as discussed above, this result suggests that the preadsorbed HCHO can participate in the following heterogeneous process of NO2, and may influence its reaction pathways. As shown in Figure 1, the weak band around 1744 cm-1 was assigned to N2O4.18, 23 10


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The broad absorption band extending from 3680 to 3470 cm−1 slowly increases in intensity with the increase in reaction time. This broad peak is primarily associated with O-H vibration of hydrogen-bonded OH groups of acid.23, 32 The negative band at 1662 cm−1 was assigned to the bending vibration of water, the two negative absorption bands at 3705 and 3730 cm−1 were assigned to the bending vibration of O-H groups, and these O-H groups were coordinated with two aluminum atoms.12, 16 The detailed assignments of bands formed during the reaction are summarized in Table 1. Table 1. Main Absorption Bands Observed during the Reaction wavenumber/cm−1 1242 1540 1343, 1401 1504, 1619 1586 1662 1744 3470-3680 3705, 3717, 3730, 3737,

vibration type/functional groups bidentate nitrites11, 12 monodentate nitrates11 water-solvated nitrates11 bridging nitrates11 Formate26, Water12 N2O418, 23 H-O-H11, 23, 32 O-H groups11, 12, 16

3.3 Formaldehyde Effects. The effects of formaldehyde on the heterogeneous reaction of NO2 on humid γ-Al2O3 were studied, including the effects of HCHO on the formation of nitrate, on the rate of nitrite formation and on the uptake coefficient. 3.3.1 Effect of Formaldehyde on the Formation of Nitrate. In the experiments, the formation of nitrate under the same concentration of NO2 was investigated as a function of the different preadsorbed time of HCHO. As shown in figure 2, preadsorbed HCHO can clearly suppress the formation of nitrate, and the more 11



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HCHO is preadsorbed, the more intense suppression is. Previous studies have shown that hydroxyl groups are the active sites for the heterogeneous reaction of NO2 on the surface of γ-Al2O3 particles.9, 12 However, some of the hydroxyl groups were also consumed by the preadsorbed HCHO, and the more HCHO was introduced, the more hydroxyl groups were consumed, which resulting in the decreasing of the active sites for the heterogeneous reaction of NO2, and thus, the formation of nitrate was suppressed.12 In addition, methylene glycol formed through the preadsorption of HCHO and the formate and nitrate generated during the following heterogeneous reaction of NO2 may also occupy the active sites for the reaction of NO2, which may be another reason for the suppression of nitrate formation.

Figure 2. Integrated absorbance areas of the nitrate absorption band (1250-1645 cm−1) for different HCHO preadsorbed times. In order to determine the role of the formed nitrate in the oxidation process of 12


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HCHO to HCOO-, more experiments were performed. Higher concentration of NO2 (4.48× 1015 molecules cm−3) was introduced into the reaction chamber after the synthesized air was introduced for 60 min. After the reaction of NO2 for 60 min, the synthesized air was used to purge again for 60 min. Then, HCHO (2.21× 1015 molecules cm−3) was introduced synthesized air into the reaction chamber for another 60 min. The introduction of a higher NO2 concentration was to make sure that only nitrate species exist on the surface of particles when HCHO was introduced.

Figure 3. In situ DRIFTS spectra of surface products after exposing to NO2 (4.48 × 1015 molecules cm−3) for 60 min (a), and the further introduction of HCHO (2.21× 1015 molecules cm−3) for 60 min (b). Figure 3a shows the DRIFT spectra of heterogeneous uptake of NO2 (4.48 × 1015 molecules cm−3) on γ-Al2O3 particles. Positive bands at 1343, 1401, 1504, 1540 and 1619 cm-1 were observed growing as a function of increasing exposure time. As

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discussed above, these bands were assigned to nitrates. The negative band at 1662 cm−1 was assigned to the bending vibration of water, the two negative absorption bands at 3705 and 3730 cm−1 were assigned to the bending vibration of O-H groups. The band around 1744 cm-1 was observed to grow as the particle surface was exposed to NO2. The band was assigned to N2O4, indicating the formation of adsorbed N2O4.18, 23

The band at 1242 cm-1 was ascribed to the formation of nitrite, and it disappeared

after 20 min, which means that the nitrite would not be involved in the following of heterogeneous reaction of formaldehyde (see Figure 3b). In contrast to the spectra collected from the uptake of the lower concentration of NO2, nitrite disappeared more quickly, indicating that the formation rate of nitrite was sensitive to NO2. This result is consistent with the previous study.12 It was observed that saturation phenomenon of nitrate formation gradually appeared after the reaction proceeding for 40 min. The spectra of heterogeneous uptake of HCHO on γ-Al2O3 particles after pretreatment with NO2 (4.48 × 1015 molecules cm−3) are shown in Figure 3b, and each spectrum was obtained by referencing to the coating γ-Al2O3 spectrum after preadsorption of NO2. As can be seen from Figure 3b, the observed two weak broad bands from 1360 to 1460 cm-1 and from 1650 to 1690 cm-1 were mainly assigned to water, and the two weak broad bands were also overlapped by some very weak bands of nitrate (e.g. 1343 and 1401 cm−1), indicating the formation of small amount of water and adsorbed nitrate. The observed weak negative band at 1540 cm-1 was assigned to surface monodentate nitrate, indicating its loss after the following introduction of HCHO. No HCOO-, HCOOH and HCHO species were observed on the surface of γ-Al2O3, 14


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indicating that HCHO cannot be oxidized to HCOO- or HCOOH by the formed NO3-, and no adsorbed HCHO molecules were present when the saturation phenomenon of nitrate formation happened. Therefore, some surface monodentate nitrate (1540 cm−1) may be involved in the following reaction (R1) and water was one of the products of the reaction. The presence of water may favor the transformation of adsorption mode from other adsorbed nitrate to water-solvated nitrate. The further heterogeneous reactions of the formed NO2 on γ-Al2O3 particles would also produce some adsorbed nitrate species.33 HCHO + 4NO3- + 4H+ → CO2 + 4NO2 + 3H2O

(R1)

3.3.2. Effect of HCHO on the Rate of Nitrite Formation. In this part, we will discuss the formaldehyde effect on the rate of nitrite formation through the main band at 1242 cm-1 of nitrite species. The integrated absorbance areas of the formed nitrite for the introduction of different amounts of HCHO and the formation of formate with preadsorbed HCHO for 60 min as a function of time are shown in Figure 4.

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Figure 4. The integrated absorbance areas of nitrite absorption band (1180−1250 cm−1) for different preadsorbed times of HCHO; (a) 0 min; (b) 10 min; (c) 30 min; (d) 60 min. And the integrated absorbance areas of formate absorption band (1570-1600 cm−1) with preadsorbed HCHO for 60 min (e). Figure 4 shows the integrated absorbance areas of nitrite absorption band for different preadsorbed times of HCHO. The integrated areas of the formed formate absorption band for the preadsorbed HCHO for 60 min are also shown in Figure 4. As can be seen from Figure 4, the amounts of NO2- species firstly increased, then decreased and finally disappeared with the increase of reaction time. During the reaction, the presence of HCHO neither promoted the formation of NO2- nor accelerated the decrease of NO2- compared with the case without preadsorption of HCHO, and we found that the preadsorbed HCHO suppressed the formation of nitrite. At the same time, it was found that HCOO- species was formed and increased linearly 16


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within 80 min (R2=0.996). After 80 min, nitrite disappeared, but more HCOO- species was still formed and it also presented a linear increase (R2=0.988). The formation rate of HCOO- species at this stage had a slight increase compared with that before 80 min. These results indicate that nitrite had a contribution to the suppression of the HCOOspecies formation during the heterogeneous reaction. Another control experiment was performed to further confirm the role of nitrite during the formation of HCOOspecies. Firstly, the synthesized air was introduced into the reaction chamber for 60 min; secondly, HCHO was preadsorbed for 60 min; thirdly, purged the chamber with the synthesized air for 60 min again; fourthly, NO2 was introduced with the synthesized air for 30 min and then stopped, and acquired the FTIR spectra every 10 min; at last, purged the chamber with the synthesized air for 90 min, also acquired the FTIR spectra every 10 min. It was found that both nitrite and formate formed at stage 4, and the amounts of nitrite and formate did not changed at stage 5, which strongly support the conclusion discussed above, that is, HCHO cannot be oxidized to HCOOby the formed nitrite. The reason for the decrease of the HCOO- formation rate affected by nitrite will be discussed in the following section. As discussed above, HCHO cannot be oxidized into HCOO- species by nitrite and nitrate, and therefore the oxidation of HCHO should be attributed to the participation of NO2. To further support the deduction, two experiments were performed by using two sets of FTIR instruments (including Nicolet Avatar 360 and 380). The steps of one experiment: γ-Al2O3 particles in the chambers using Nicolet Avatar 360 and 380 were purged with the synthesized air for 60 min, respectively, and two backgrounds 17



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were obtained. And then NO2 was introduced into the reaction chamber using Nicolet Avatar 360 for 60 min, while the inlet gases of the chamber using Nicolet Avatar 380 was the exhaust from the chamber using Nicolet Avatar 360, during the introduction of NO2, the DRIFTS spectra were acquired every 10 min. The steps of the other experiment: γ-Al2O3 particles were purged with the synthesized air for 60 min; HCHO (2.80×1015 molecules cm−3) was introduced into the chamber using Nicolet Avatar 360 with the synthesized air for 30 min; then repeated the steps of the former experiment.

Figure 5. The integrated absorbance areas of the nitrate absorption bands acquired by Nicolet Avatar 360 and 380, a: without HCHO preadsorption; b: with HCHO preadsorption for 30 min. Figure 5 shows the integrated absorbance areas of the nitrate absorption bands acquired by Nicolet Avatar 360 and 380 with and without preadsorbed HCHO. It was 18


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found that nitrate species formed with no preadsorbed HCHO is more than that with HCHO preadsorbed for 30 min detected by Nicolet Avatar 360, and also detected by Nicolet Avatar 380. The results from Nicolet Avatar 380 clearly indicate that more NO2 was consumed on the particles with preadsorbed HCHO than that without preadsorbed HCHO in the chamber using Nicolet Avatar 360, however, the results from Nicolet Avatar 360 show that less nitrate species formed with preadsorbed HCHO than without. It can be inferred that in the case of using Nicolet Avatar 360, a portion of NO2 would react with the preadsorbed HCHO, and HCOO- species was the possible product of the reaction between NO2 and HCHO in our experiment. For the heterogeneous reaction of NO2 on γ-Al2O3 particles preadsorbed by HCHO, with the increase of reaction time, NO2 would decreasingly undergo heterogeneous reaction to form adsorbed products such as surface nitrate and nitrite because that reactive sites were gradually consumed or covered by the adsorbed products, and part of the NO2 was decreasingly consumed by its reaction with the formed nitrite.12 Therefore, there would be more and more NO2 reacting with HCHO, resulting in the formation rate of HCOO- species changed as shown in Figure 4. Previous studies indicated that NO was observed during the heterogeneous reaction of NO2 on the surface of mineral oxide particles and the products of the reaction between nitrite and NO2 would be nitrate and NO,12, 15 however, it was not easily observed in our experiments because it was oxidized to NO2 quickly in the presence of excess O2,11 and the vibration band at ∼1465 cm−1 of NO might be covered by surface nitrates.12 19



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In the process of NO2 reacting with preadsorbed HCHO, HCOO- species was detected as an oxidation product, but there should be some reduction products. Previous theoretical study has shown that the presence of SiO2 could accelerate the atmospheric reaction of HCHO with NO2 to form HONO in the troposphere,34 and hence an additional experiment was performed to detect the possible gaseous products formed from the heterogeneous reaction of NO2 with HCHO by using White cell-FTIR (White cell reactor, model 19-V, Infrared Analysis Inc.). The infrared data showed that a small amount of N2O was observed with the decreasing of NO2 concentration during the whole reaction. However, previous study indicated that the predominant gas-phase product NO and a small amount of detectable N2O were observed during the reaction of NO2 with γ-Al2O3, and gas-phase N2O was considered to be the result of the reaction of the product NO with the surface of γ-Al2O3, also, it is difficult to quantify the amount of the two gases from the infrared data.15 Therefore, we cannot judge whether N2O is the reduction product of NO2 reduced by the preadsorbed HCHO. In addition, NO2 standard gas used in our experiment is prepared by mixing NO2 with N2, and N2 isn’t an infrared active molecule, and hence we also cannot determine whether N2 is the reduction product during the reaction of NO2 with HCHO. But anyway, the preadsorbed HCHO provides an additional heterogeneous conversion pathway of NO2 on the surface of γ-Al2O3. This aspect still needs further study. 3.3.3 Effect of Formaldehyde on Reaction Mechanism for the Heterogeneous Reaction of NO2 with γ-Al2O3. Based on the experimental observations described 20


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above, a possible reaction mechanism for the heterogeneous reaction of NO2 on γ-Al2O3 preadsorbed HCHO was proposed. During the preadsorption of HCHO, the gas-phase HCHO was adsorbed onto the surface of γ-Al2O3, and this process would make some of OH groups be consumed:26

HCHOg ⇔ HCHOa

(R2)

And then, the adsorbed HCHO would quickly react with the surface-adsorbed water to form methylene glycol:25

HCHOa + H Oa ⇔ CH OH a

(R3)

When NO2 was introduced, on the one hand, the following introduction of NO2 can suppress the formation of methylene glycol through the consumption of surface water and the oxidation conversion of surface HCHO into HCOO- during the reaction. On the other hand, the gas-phase NO2 would be adsorbed rapidly onto the surface of γ-Al2O3 to form adsorbed NO2 (a):9

NO g ⇔ NO a

(R4)

And a disproportionation of two NO2 (a) molecules led to the formation of surface coordinated nitrates, nitrites, and water as expressed in R5:12

2OH − NO a → NO a + NO a + H O

(R5)

Previous study have demonstrated that there had been a disproportionation reaction between NO2 and surface-adsorbed water,35 and some of the nitrite and nitrate species formed on the particle surface was attributed to the reaction of surface-adsorbed water with NO2.9 In this study, the used particles were humid, and thus there were some surface-adsorbed water which could react with NO2 to form water-solvated nitrates: 21



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2NO a + H Oa → 2H + NO a + NO a

(R6)

As the reaction proceeded, nitrites were oxidized by excess NO2, resulting in the loss of nitrite and the production of bidentate nitrates:12 NO

a + NO g → NO a + NOg

(R7)

NO was not observed in this study as it was oxidized to NO2 quickly in the presence of excess O2:11

2NOg + O g → 2NO g

(R8)

Besides nitrate and nitrite species, a very small amount of N2O4 (a) was also observed in this study, which is formed from NO2:9, 36

2NO a ⇔ N O a

(R9)

Then N2O4 (a) was converted into nitrite and nitrate species by the disproportion reaction.9, 20 In this study, once HCHO was adsorbed onto the surface of γ-Al2O3 particles, it would make the OH groups be consumed or be occupied by some products, while some of the OH groups were also the active sites for the heterogeneous reaction of NO2. This would suppress the formation of nitrate. In addition, we confirmed that NO2 could react with preadsorbed HCHO, and the oxidation product of this reaction was HCOO-, the reduced product may be one or both of N2 and N2O. This reaction would be a new conversion pathway of NO2 on the surface of γ-Al2O3 particles in the presence of HCHO. 3.3.4. Effect of Formaldehyde Adsorption on the Uptake Coefficient for NO2 to Nitrate. Based on the rate of nitrate formation, the initial uptake coefficients 22


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measured for the heterogeneous reactions of NO2 on various mineral oxide particles have been reported in many studies.11, 37, 38 However, no information about the uptake coefficient of NO2 on γ-Al2O3 under the influence of preadsorbed HCHO has been found. Therefore, we also calculated the uptake coefficients for NO2 reaction on the HCHO-preadsorbed γ-Al2O3, and these values are given for comparison purposes only. The amount of the formed nitrate on the γ-Al2O3 grew with increasing reaction time in all the cases studied. To quantify the rate of nitrate formation, the amount of nitrate ions formed during the reaction on the γ-Al2O3 was determined by the DRIFTS calibration curve, which was made by measuring the total number of nitrate ions on the sample after the reaction by IC (ion chromatography). {NO3−} = f × (integrated absorbance area) The conversion factor f was independent of reaction time proposed in the previous study. 11 In this study, f was determined to be 1.13(±0.012) × 1015 ions g−1 ABU−1. The reactive uptake coefficient (γ) is defined as the ratio of the reactive gas−surface collision rate to the total gas−surface collision rate. γ=

/Z

Z represents the total NO2-Al2O3 surface collision frequency as determined by the kinetic theory of gases. +.-

ANO 8RT Z= # * 4 πM)

A is the total surface area, and M) is the molar mass for the reactant gas. R is the gas constant, and T is the temperature.37 23



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Table 2. Reactive uptake coefficients in the initial region for the reaction of NO2 on γ-Al2O3 with a series of HCHO preadsorption time. The preadsorbed time of HCHO(min) 0 10 30 60

γBET(×107)

γgeometric(×104)

3.07±0.66 2.72±0.39 1.85±0.32 1.44±0.07

1.95±0.42 1.73±0.25 1.18±0.21 0.91±0.05

Table 2 shows the calculated reactive uptake coefficients under the conditions with different amounts of HCHO preadsorption. As can be seen from Table 2, the uptake coefficients decreased with increasing preadsorbed time of HCHO from 0 to 60 min, indicating that the reactive uptake coefficients are sensitive to the amounts of HCHO preadsorption. Moreover, the uptake coefficient with no preadsorbed HCHO was about twice as great as that with preadsorbed HCHO for 60 min. This difference would directly affect the formation of secondary nitrate in the atmosphere.

4. CONCLUSIONS AND ATMOSPHERIC IMPLICATIONS In this study, the heterogeneous reaction of NO2 on γ-Al2O3 preadsorbed HCHO was examined by DRIFTS in order to elucidate the effect of formaldehyde on the nitrite and nitrate formation during the heterogeneous uptake of NO2. The kinetics and mechanism of the heterogeneous reaction of NO2 on HCHO-preadsorbed γ-Al2O3 particles were explored by monitoring FTIR spectra. The results indicate that formaldehyde could suppress the formation of nitrite and nitrate species, and influence the reactive uptake coefficients of NO2. While the following heterogeneous reaction of NO2 also suppress the formation of methylene 24


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glycol and promote the production of HCOO- during the reaction. Preadsorbed HCHO can increase the pathways of the heterogeneous reactions of NO2 on the surface of γ-Al2O3. The initial uptake coefficient with no HCHO preadsorption was about two times as great as that with HCHO preadsorption for 60 min. All of these observations indicate that formaldehyde preadsorption can affect the heterogeneous reaction of NO2 on mineral aerosol, especially for secondary nitrate formation. Furthermore, IR spectra show that nitrite was an intermediate product, which grew at the beginning of the reaction, and then reached the maximum in a short time, and the formation of nitrite was sensitive to the amounts of preadsorbed HCHO and NO2. Results from this study have important atmospheric implications. The results suggest that a significant impact of HCHO on the heterogeneous conversion of NO2 and the formation of nitrite and nitrate in the atmosphere, and it further emphasize the complexity of the reaction of NO2 on the surface of mineral particles. Formaldehyde is ubiquitous in the atmosphere. This chemistry may occur on surfaces of airborne dust particles that are known to be transported and play a role in the chemistry of the troposphere, which would affect the level of particulate nitrate in the troposphere and the estimation of the amount of global atmospheric nitrate. In addition, the results presented here may also imply that such a heterogeneous conversion pathway of NO2 that affected by HCHO adsorption on dust particles may be suitable for other aldehydes in the troposphere.

■ AUTHOR INFORMATION 25



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Corresponding Authors *E-mail: [email protected]. Tel: +86-21-65642521, fax: +86-21-6564-2080. *E-mail: [email protected]. Tel: +86-21-65642521, fax: +86-21-6564-2080. Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 41475110, 21277028, 21190053 and 41275126) and the open fund by Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control (KHK1311).

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Effect of Formaldehyde on the Heterogeneous Reaction of Nitrogen

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