Comprehensive study about the heterogeneous conversion of NH₃ on nanoscale TiO₂ particles: insight into the mechanism and atmospheric implications

Abstract

The heterogeneous reactions of NH₃ on TiO₂ were comprehensively investigated using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and a flow tube reactor. The effect of SO₂ and illumination on the formation of surface and gaseous products was explored. The results reveal that surface hydroxyls and H₂O play important roles in the adsorption of NH₃ while adsorbed sulfur species like sulfite and sulfate can significantly promote the conversion of NH₃ to NH₄⁺ under dark conditions. The reaction mechanism under light conditions is quite different from that under dark conditions. NH₃ can be directly converted into gaseous NO and NO₂ and particulate nitrate on the surface of TiO₂ by photooxidation. Surface sulfate formed from the photooxidation of SO₂ can combine with NH₃ to form ammonium sulfate, which inhibits the oxidation of NH₃ and alters the production ratios of NO/NO₂. The uptake coefficients of NH₃ on TiO₂ are in the range of 1.5–2.5 × 10⁻⁵ regardless of illumination while SO₂ shows an inhibition effect on the initial uptake of NH₃ due to competition in adsorption on surface hydroxyls. These results demonstrate that the heterogeneous reaction of NH₃ on TiO₂ can be a significant sink of NH₃ while its transformation pathway greatly depends on the environmental factors in the atmosphere.

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Environmental significance

As the main alkaline constituent in the atmosphere, NH₃ has significant impacts on the formation of secondary aerosol as well as nucleation events. However, the heterogeneous reactions of NH₃ on particles are not fully understood and still not considered in the air quality model, which greatly restricted the formulation of ammonia emission control strategies. In this study, a systematic laboratory study of the heterogeneous reaction of NH₃ on nanoscale TiO₂ under air pollution complex conditions is conducted. It was found that the heterogeneous transformation mechanism of ammonia and its effect on secondary pollution formation greatly depends on environmental conditions such as irradiation and coexisting SO₂. These results are helpful for understanding NH₃ migration and transformation in the atmosphere.

1. Introduction

Atmospheric particulate matter (APM) has been a prominent air pollution problem, especially in south Asia and East Asia.^1,2 Exposure to fine particulate matter (PM_2.5, particulate matter with an aerodynamic diameter less than 2.5 μm) has a serious health risk by causing respiratory and cardiovascular diseases. A large number of field observations have indicated that secondary inorganic aerosol (SIA, e.g., sulfate, nitrate, ammonium) accounts for 25–60% of PM_2.5 in China.^3–6 Thus, lots of attention have been paid to the transformation of gaseous precursors to secondary inorganic aerosol in laboratory research in recent decades. The formation pathways of sulfate and nitrate have been widely investigated. For example, the well-known conversion pathways of SO₂ to sulfate include gas-phase oxidation by the OH radical or stabilized Criegee intermediate to form sulfuric acid,^7,8 aqueous-phase oxidation in cloud and fog droplets,^9,10 and heterogeneous reactions on surfaces of atmospheric particles.^11,12 For the formation pathways of nitrates, it includes the homogeneous oxidation of NO₂ by OH radicals to form HNO₃ and by O₃ to form N₂O₅ (ref. 13) which then transforms to nitrate on the surface of particles,¹⁴ as well as the direct transformation of NO₂ to nitrates through heterogeneous reactions.¹⁵ Compared with sulfate and nitrate, there is still a lack of understanding of the formation pathway of ammonium aerosol.

NH₃ has significant impacts on atmospheric chemistry since it is the main alkaline constituent in the atmosphere. It is reported that global ammonia emission has increased more than 2–5 times since preindustrial times.¹⁶ The main emission sources of NH₃ include livestock and human excreta, fertilizer application, agricultural activities, nitrogen-fixing plants, the chemical industry, traffic, and so on.^17,18 The average urban concentration of NH₃ in Beijing was about 36 ppb while the maximum value reached 800 ppb in 2013.¹⁹ NH₃ has been considered a key species for neutralizing H₂SO₄ and HNO₃ in the atmosphere and forming (NH₄)₂SO₄/NH₄HSO₄ and NH₄NO₃, respectively.^20,21 Several recent model assessment studies showed that the large atmospheric NH₃ emissions in northern China can reduce the effectiveness of existing PM_2.5 control strategies through SO₂ and NOx emission reductions,^22,23 while ammonia emission reduction can be considered as an effective measure to reduce PM_2.5.²⁴ Field observation and laboratory simulation indicated that NH₃ can promote the formation of secondary pollutants including sulfate, nitrate, and HONO.^25–30 In addition, NH₃ has been found to react with carbonyl compounds in secondary organic aerosol (SOA) which results in the formation of nitrogen-containing organic compounds (NOC) and significantly affects regional air quality.^31–37 These results demonstrate that besides acid–base neutralization reactions in the gas phase, heterogeneous and multiphase reactions of ammonia should also be investigated systematically.

Mineral aerosols, which originate from arid and semi-arid regions with an annual flux of about 1600–2000 Tg, represent one of the largest mass fractions of global aerosols.³⁸ Mineral dust particles can transport over thousands of kilometers and alter the chemical balance of the atmosphere by providing a reactive substrate or medium for gaseous pollutants. These reactions can not only change atmospheric trace gas distributions but also modify the chemical composition and mixing state of particles. TiO₂ is an important component of mineral dust particles, with a content of 1–10 wt%, and has strong photocatalytic activity.³⁹ Kebede et al.⁴⁰ found that heterogeneous photooxidation of NH₃ over TiO₂ under atmospherically relevant concentrations leads to the formation of NOx. In addition, NH₄⁺ in NH₄NO₃ coated on TiO₂ can be converted to NOx upon irradiation and can be significantly affected under complex air pollution conditions.⁴¹ Therefore, the heterogeneous reactions process of NH₃ on TiO₂ under complex air pollution conditions need further study.

In this study, we comprehensively studied the heterogeneous reaction process of NH₃ on TiO₂ using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and a wall-coated flow tube. It mainly focused on the role of irradiation and the interaction between SO₂ and NH₃ in heterogeneous reactions. The relevant research results are helpful to further understand the atmospheric migration and transformation of NH₃ and the environmental impacts of mineral dust.

2. Experimental section

In situ DRIFTS

The heterogeneous reactions of NH₃ on TiO₂ particles were measured using a DRIFTS instrument (NEXUS 670, Thermo Nicolet Instrument Corporation), equipped with an in situ reaction chamber and a high-sensitivity mercury cadmium telluride (MCT) detector cooled with liquid N₂. The TiO₂ sample (about 15 mg) was finely ground and placed into a ceramic crucible in the in situ chamber. The total flow rate was 100 mL min⁻¹ in all flow systems. The reference spectrum was measured after the pretreated sample was cooled to 298 K in a synthesized air stream. The infrared spectra were collected and analyzed using a data acquisition computer with OMNIC 6.0 software (Nicolet Corp.). All spectra reported here were recorded at a resolution of 4 cm⁻¹ for 100 scans in the spectral range of 4000 to 600 cm⁻¹. The spectra are presented in the Kubelka–Munk (K–M) scale, which can reduce or eliminate the mirror effect and give a better linear relation with concentration.⁴² Before DRIFTS measurement, the TiO₂ sample was pretreated in an in situ infrared cell by heating in 100 mL min⁻¹ of synthesized air (20% O₂ + 80% N₂) at 473 K for 2 h. UV light was introduced into the DRIFTS reaction cell via a UV fiber (Φ = 5 mm). The light acquired with a mercury lamp (λ > 200 nm, CHF-XM-500 W, the Trusttech. Co. Ltd., Beijing) was used as the light resource. The spectrum of the light in the DRIFTS experiments was measured with a fiber optic spectrometer (BLUE-Wave-UVNb, Stellar Net Inc., USA).

Coated-wall flow tube reactor

The uptake of NH₃ on TiO₂ was studied in a horizontal cylindrical coated-wall flow tube reactor, which has been described in detail elsewhere.^43–45 The experiments were performed at ambient pressure and maintained at 298 K by a circulating water bath through the outer jacket of the flow tube reactor. The tube with the deposited TiO₂ sample was introduced into the main reactor along its axis. Synthetic air was used as the carrier gas, which was introduced into the flow tube reactor in a total flow rate of 1800 mL min⁻¹, ensuring a laminar regime at ambient pressure. NH₃ was introduced into the gas flow by a movable injector with a 0.3 cm radius. The reactor was surrounded by 6 UV lamps (365 nm, T5, UVA, 8 W) with a broad UV emission spectrum between 330 and 420 nm. The UV lamps were installed into a stainless light-tight box covering the main reactor tube. The irradiance intensity in the reactor could be regulated by switching off some lamps. When all lamps were switched on, the irradiance intensity in the reactor was 1.27 W m⁻² at 370 nm. All reactions were conducted under dry conditions with relative humidity (RH) lower than 5%.

Uptake coefficient

The kinetic behavior can be well described by assuming a pseudo first-order reaction with respect to the gas-phase reactant concentration. The first-order rate constant (k) is related to the geometric uptake coefficient (γ_geo) using eqn (1):⁴³where r_tube, t and 〈c〉 are the flow tube radius, the exposure time, and the reactant average molecular velocity, respectively. C₀ and C_i are the reactant concentration at t = 0 and t = i, respectively.

If the loss of reactant at the TiO₂ surface is too rapid to be recovered with the reactant supply, a radial concentration gradient in the gas phase will be formed, which may cause diffusion limitations. Therefore, a correction for diffusion in the gas phase should be taken into account and the Cooney–Kim–Davis (CKD) method was used to correct uptake coefficients.^46,47

Materials

TiO₂ particles were purchased from commercial sources (P25: 80% anatase, 20% rutile, Degussa). A total mass of 1.0 g of TiO₂ powder was dissolved in 20.0 mL of water. This suspension was dripped into a quartz tube (20.0 cm length, 1.1 cm i.d.) and dried overnight in an oven at 373 K. The resulting homogeneous film covered the entire inner area of the tube. The Brunauer–Emmett–Teller (BET) area of the sample is 54.6 m² g⁻¹. NH₃ (N₂ balance, Beijing Huayuan), SO₂ (N₂ balance, Beijing Huayuan), O₂ (99.999%, Beijing Huayuan) and N₂ (99.999%, Beijing Huayuan) were used as-received. Gas analyzers (THERMO 42i, 43) were used to monitor the concentrations of NOx and SO₂, respectively. NH₃ was monitored by a Quantum Cascade Tunable Infrared Laser Differential Absorption Spectrometer (QC-TILDAS, Aerodyne Research).

3. Results and discussion

3.1 DRIFTS study the heterogeneous reaction of NH₃ on TiO₂

The heterogeneous reactions of NH₃ on TiO₂ were studied using in situ DRIFTS. Fig. 1 shows the DRIFTS spectra of TiO₂ exposed to NH₃ with or without SO₂ and UV irradiation. In the case of ammonia individual dark reaction (Fig. 1A), a dominant peak at 1185 cm⁻¹ and a weak peak at 1440 cm⁻¹ were observed. These two peaks can be assigned to NH_3(ads) adsorbed on Lewis acid sites and NH₄⁺ adsorbed on Brønsted acid sites, respectively.³⁰ These results indicate that the adsorption of NH₃ on TiO₂ under dark conditions leads to the formation of adsorbed NH₃ species as the major products and a small amount of NH₄⁺ species. Meanwhile, several negative peaks decreasing in intensity at 1620, 3300, 3490, 3563, and 3693 cm⁻¹ were observed. The peak at 3693 cm⁻¹ is attributed to surface OH groups,⁴⁸ suggesting that surface OH groups were involved in the adsorption of NH₃ on TiO₂. Other peaks at 1620, 3300, 3490, and 3563 cm⁻¹ are attributed to surface water. It has been reported that NH₃ would react with water to produce NH₄⁺viareaction (2) or bond to water molecules through a hydrogen bond between the H atom of H₂O and the N atom of NH₃.^40,49,50

NH₃ + H₂O(ads) → NH₄⁺(ads) + OH⁻(ads) (2)

Thus, the adsorption of NH₃ on the TiO₂ surface not only generates NH₄⁺ but also increases the surface alkalinity by reacting with surface water.

Fig. 1 In situ DRIFTS spectra of TiO₂ exposed to various gases as a function of time (0, 10, 30, 60, 90, 120, and 180 min). (A) 5 ppm NH₃ under dark conditions; (B) 5 ppm NH₃ under UV irradiation; (C) 5 ppm NH₃ + 5 ppm SO₂ under dark conditions; (D) 5 ppm NH₃ + 5 ppm SO₂ under UV irradiation.

Compared to the dark reaction, the adsorption of NH₃ on TiO₂ upon UV irradiation showed different results. As seen in Fig. 1B, several peaks at 1253, 1295, 1488, 1585, and 1608 cm⁻¹ increased in intensity with the reaction proceeding. These peaks are assigned to surface nitrate,⁵¹ which indicate the conversion of NH₃ to nitrate on TiO₂ upon UV irradiation. The decrease in the intensity of the peak at 3693 cm⁻¹ in the photoreaction is much weaker than that in the dark reaction. This suggested that surface OH groups were not the main sites for the photooxidation reaction of NH₃. Moreover, the negative peaks attributed to surface water are also not observed. Yamazoe et al.⁵² proposed that the reaction between NH₃ with a photoinduced hole on TiO₂ can result in the formation of the amino radical, ˙NH₂:

NH₃ + h⁺ → ˙NH₂(ads) + H⁺ (3)

The H⁺ generated in this reaction can neutralize the hydroxyl ions generated in reaction (2), which results in no net consumption of water. Kebede et al.⁴⁰ suggested another possible reaction where ˙OH radicals generated on the TiO₂ surface can react with NH₃ by abstracting a hydrogen atom to form the amino radical:

NH₃ + ˙OH → ˙NH₂(ads) + H₂O(ads) (4)

The formed H₂O in this reaction can also compensate for the consumption of H₂O in reaction (2). The formed ˙NH₂ on the surface can be oxidized by O₂ to produce gaseous NO and NO₂ and surface nitrate, which will be discussed later.

When SO₂ was introduced simultaneously with NH₃, the reaction pathway of NH₃ was changed. In the SO₂ + NH₃ dark reaction (Fig. 1C), both peaks at 1185 and 1448 cm⁻¹ were also observed with greater intensity compared to those in NH₃ individual dark reaction (Fig. 1A), indicating the promotion effect of SO₂ on the adsorption of NH₃ on TiO₂. A weak peak at 1068 cm⁻¹ attributed to sulfite species and no peak attributed to sulfate was observed. This suggests that NH₃ has little effect on the heterogeneous oxidation of SO₂ on TiO₂.⁵³ Two negative peaks at 3693 and 3628 cm⁻¹ attributed to surface OH groups were also observed. Meanwhile, the decreasing intensity of the peak at 1620 cm⁻¹ assigned to surface water is stronger than that in the NH₃ reaction alone. This could be due to the involvement of H₂O in the heterogeneous reaction of SO₂ on TiO₂. Nanayakkara et al.⁵⁴ proposed that the complex between SO₂ and H₂O is in equilibrium with bisulfite on TiO₂ according to the reaction:

SO₂·H₂O(ads) ↔ HSO₃⁻H⁺(ads) (5)

The H⁺ in HSO₃⁻H⁺ may further interact with NH₃ to generate ammonium ions, which is consistent with the intensity enhancement of the infrared peak at 1448 cm⁻¹ attributed to NH₄⁺ adsorbed on Brønsted acid sites (Fig. 1C).

In the case of the NH₃ + SO₂ reaction upon UV irradiation (Fig. 1D), several peaks at 1174, 1250, and 1289 cm⁻¹ were observed with increasing intensity with reaction time. These peaks are assigned to surface sulfate species, indicating the promotion effect of UV irradiation on the conversion of SO₂ to sulfate or H₂SO₄ on TiO₂:⁵³

SO₂ + ˙OH + O₂ → HO₂˙ + SO₃ (6)

SO₃ + H₂O(ads) → H₂SO₄(ads) (7)

Meanwhile, a dominate peak at 1436 cm⁻¹ and several peaks at 2830, 3050, and 3207 cm⁻¹ with remarkable intensity were observed. These peaks can be assigned to NH₄⁺ in (NH₄)₂SO₄ due to the reaction between H₂SO₄ and NH₃:⁵⁵

H₂SO₄(ads) + 2NH₃ → (NH₄)₂SO₄(ads) (8)

The peaks of surface nitrate observed in Fig. 1B were not observed in the presence of SO₂ which indicates the inhibition effect of coexisting SO₂ on the photooxidation of NH₃ to nitrate.

The surface products in different reactions are compared by integrating the IR peak area and are shown in Fig. 2. The infrared peak integral area of the surface product became more obvious after 40 min of the reaction, which may be due to the low infrared sensitivity of ammonia species adsorbed on the TiO₂ surface in the beginning. Fig. 2A shows the comparison of the integrated area of peaks due to NH_3(ads) on Lewis acid sites. Because these species were not observed in the reaction upon UV irradiation, here we only compared the heterogeneous reactions of NH₃ on TiO₂ under dark conditions. These reactions were analyzed including individual NH₃ adsorption on fresh TiO₂ particles and SO₂-aged TiO₂ particles, and mixing NH₃ and SO₂ adsorption on TiO₂ particles. In the initial 120 min, the amount of NH_3(ads) on Lewis acid sites showed little difference among these three reactions. This may be due to the formation of NH_3(ads) mainly through the interaction of NH₃ with surface OH or H₂O. After 120 min, the increase rate of NH_3(ads) in both NH₃ + SO₂ aged TiO₂ and NH₃ + SO₂ systems is greater than that in the individual NH₃ system. These results suggest that the coexistence of SO₂ or SO₂ aging treatment can slightly promote the adsorption of NH₃ on TiO₂.

Fig. 2 Dynamic change of surface products formed in heterogeneous reactions of NH₃ on TiO₂ with/without SO₂ and UV irradiation. (A) NH_3(ads) on Lewis acid sites; (B) NH₄⁺ on Brønsted acid sites; (C) surface nitrate species; (D) surface S species.

As for NH₄⁺ species adsorbed on Brønsted acid sites, coexisting SO₂ also promotes the formation of NH₄⁺ species under dark conditions compared to individual NH₃ adsorption (seen in Fig. 2B). This is because the sulfite/bisulfite species formed during SO₂ adsorption on the surface of TiO₂ could also serve as Brønsted acid sites for NH₄⁺ adsorption. Moreover, the formation of NH₄⁺ species was greatly enhanced when NH₃ was adsorbed on SO₂ photochemically aged TiO₂ or coexisting SO₂ on TiO₂ in the presence of UV irradiation. The increase in the formation of NH₄⁺ species was attributed to the presence of surface sulfate or H₂SO₄ which can readily combine with NH₃. It should be noted that individual NH₃ adsorption on fresh TiO₂ does not produce surface NH₄⁺ species and only surface nitrate was formed upon UV irradiation. Thus, in the absence of SO₂, the reaction mechanism of NH₃ on TiO₂ upon UV irradiation is photooxidation, but it goes through an acid–base neutralization reaction in the presence of SO₂. Fig. 2C shows the formation of nitrate in different photochemical reactions. The nitrate species shows a continuous increase in the photooxidation of NH₃ on fresh TiO₂ while no nitrate was observed in the adsorption of NH₃ on SO₂ photochemically aged TiO₂. When SO₂ was coexisting with NH₃, the formation of nitrate increased in the initial step of reaction and decreased in the later step. This is not unexpected. In the photochemical reaction of coexisting SO₂ and NH₃ on TiO₂, there are enough surface sites for oxidation of both SO₂ and NH₃. Then nitrate can form in the initial stage. With the continuous increase of surface sulfate, the generated nitrate is gradually replaced or decomposed.⁴⁰ Therefore, in the photoreaction where SO₂ and NH₃ coexist, the surface nitrate tends to increase first and then decrease.

NH₃ shows little effect on the formation of surface sulfur species in both dark reactions and photoreactions (Fig. 2D). In a previous study, Yang et al.³⁰ found that NH₃ can promote the formation of sulfate on the TiO₂ surface under dark conditions, in which much higher concentrations of SO₂ (200 ppm) and NH₃ (100 ppm) were used compared to 5 ppm in the present study. The adsorption of SO₂ on the oxide surface is mainly through the interaction with surface OH or H₂O. When the concentration of SO₂ and NH₃ is relatively low, enough OH on the surface makes the competition between them not obvious. As for the photoreaction, the oxidation of SO₂ is attributed to surface reactive oxygen species (ROS) like ˙OH while NH₃ mainly participates in the subsequent neutralization reaction.

3.2 Flow tube reactor study for the uptake of NH₃ on TiO₂

The uptake of NH₃ on TiO₂ particles was further studied with a flow tube reactor. Fig. 3 shows the results of NH₃ uptake on TiO₂ particles under various conditions. Under dark conditions (Fig. 3A and B), whether SO₂ is present or not, an abrupt decrease in NH₃ concentration was observed when TiO₂ was exposed to NH₃, indicating strong interaction between NH₃ and TiO₂ particles. It should be noted that the concentration of NH₃ reached the minimum point after half an hour, and then gradually increased. The lowest concentration of NH₃ in the presence of SO₂ is higher than that in the absence of SO₂. This could be due to the competition effect between SO₂ and NH₃ since surface OH groups are the main adsorption sites for both NH₃ and SO₂. For the products, no NOx formed under dark conditions, suggesting that the oxidation of NH₃ on the TiO₂ surface hardly occurred under dark conditions.

Fig. 3 Uptake of NH₃ on TiO₂ under various conditions. (A) dark condition; (B) dark condition with 200 ppb SO₂ coexisting; (C) UV irradiation condition; (D) UV irradiation with 200 ppb SO₂ coexisting. Sample masses: 5.1–5.4 mg.

When the uptake of NH₃ took place in the presence of UV irradiation (Fig. 3C and D), an abrupt decrease in NH₃ concentration was also observed at the beginning. However, the concentration of NH₃ had not achieved the minimum value and gradually reached a plateau after about 90 min. The formation of NO and NO₂ was accompanied by the uptake of NH₃. The concentrations of NO and NO₂ also reached a plateau simultaneously with NH₃. These results suggested that NH₃ was converted to NO and NO₂ through the heterogeneous photocatalytic oxidation reaction on the TiO₂ surface. Kebede et al.⁴⁰ proposed a net photooxidation reaction mechanism of NH₃ on TiO₂ as follows:

˙NH₂ + O₂ → NO + H₂O (9)

Ti⁴⁺–O⁻ + NO → Ti⁴⁺–ONO⁻ → Ti³⁺ + NO₂ (10)

The intermediate nitrite was not observed in the present study which may be due to a higher concentration (5 ppm) than that (0.74 ppm) used in Kebede et al. In addition, surface nitrate species were the dominant surface products in the present study, which could be due to the further photooxidation of NOx to nitrate on TiO₂:

2NO₂ + O₂ + 2e⁻ → 2NO₃⁻(ads) (11)

NO + O₂ + e⁻ → NO₃⁻(ads) (12)

It is interesting to note that coexisting SO₂ can greatly change the distribution of products. Compared to NH₃ individual uptake, the presence of 100 ppb SO₂ caused the concentration of NO to be higher than that of NO₂ (Fig. 3C and D). In order to reveal the effect of SO₂ on the photooxidation of NH₃ on the TiO₂ surface, the yields of NO and NO₂ from NH₃ conversion at different coexisting SO₂ concentrations are compared, as shown in Fig. 4. These values were based on the plateau concentrations in uptake experiments. The aged samples were prepared by exposing the TiO₂ samples to 5 ppm SO₂ for 4 h upon UV irradiation. The total yields of NO and NO₂ were about 90% in the absence of SO₂. Combined with the DRIFTS result (Fig. 1B), it suggests that partial NOx has been converted to surface nitrate. It should be noted that the nitrate on the surface of TiO₂ can produce NO and NO₂ through photolysis.⁴⁰ Hence, there could be an equilibrium between surface nitrate and gaseous NOx (NO + NO₂). When the concentration of coexisting SO₂ increased, the total yields of NO and NO₂ from NH₃ conversion decreased. The photooxidation of SO₂ on TiO₂ leads to the formation of H₂SO₄/sulfate which can combine with NH₃ to form ammonium sulfate, as seen in Fig. 1D. Therefore, NH₃ is trapped on the surface as ammonium, instead of being converted to NOx as well as nitrate. On the other hand, the photooxidation of SO₂ to H₂SO₄/sulfate will consume ROS on the surface. This could decrease the oxidation of NH₃ to NO and the consequent oxidation of NO to NO₂.

Fig. 4 The yields of NO and NO₂ from NH₃ conversion at different SO₂ concentrations.

3.3 The uptake coefficients of NH₃ on TiO₂

Fig. 5 shows the uptake process of NH₃ on TiO₂ with various masses under dark conditions. The time for NH₃ concentration to reach the minimum value under dark conditions is prolonged with increasing sample mass. When the sample mass exceeds 5 mg, the time to reach the minimum concentration exceeds 30 minutes. It is possible that the diffusion of NH₃ to the underlayers is much slower than the NH₃ reaction on surface sites. In other words, the diffusion of NH₃ may be the rate determining step for uptake on TiO₂. To calculate the uptake coefficient in the dark reaction, the minimum concentration of NH₃ is also selected as a convention which represents the initial uptake coefficient. In the presence of UV irradiation, the concentration of NH₃ had not achieved the minimum value and gradually reached a plateau (Fig. 3C and D). As for the photoreaction, the plateau concentration of NH₃ is selected for the calculation of the uptake coefficient which represents the steady-state uptake coefficient. The measured uptake coefficients (γ_geo) of NH₃ on TiO₂ under various conditions are in the range of 2.0–2.5 × 10⁻⁵ while the linear mass dependence of γ_geo was not observed. Coexisting SO₂ exhibits an inhibition effect on the γ_geo of NH₃ on TiO₂ because of the competitive effect between SO₂ and NH₃ on the surface hydroxyl groups. Upon UV irradiation, the measured steady-state uptake coefficients of NH₃ on TiO₂ are also in the range of 2.0–2.5 × 10⁻⁵ with no linear mass dependence.

Fig. 5 Uptake of NH₃ on TiO₂ with various masses under dark conditions.

4. Conclusion and environmental implications

The heterogeneous reactions of NH₃ on TiO₂ with or without irradiation were investigated using DRIFTS and a flow tube reactor. Under dark conditions, adsorbed NH₃ on Lewis acid sites was found to be the dominant surface species regardless of the coexisting SO₂. The adsorption of SO₂ produces sulfite and bisulfite species which can act as Brønsted acid sites to enhance the formation of NH₄⁺ on the surface. Upon irradiation, NH₃ can be converted to NO, NO₂, and nitrate on TiO₂ without the formation of surface adsorbed NH₃ and NH₄⁺ species. However, the presence of SO₂ can promote the formation of NH₄⁺ due to the combination of H₂SO₄/sulfate with NH₃ to form (NH₄)₂SO₄, which inhibits the photochemical oxidation of NH₃. The main heterogeneous reaction pathways of NH₃ on TiO₂ are summarized in Scheme 1.

Scheme 1 The heterogeneous reaction pathways of NH₃ on TiO₂.

The initial uptake coefficients of NH₃ on TiO₂ are in the range of 2.0–2.5 × 10⁻⁵ and independent of the sample mass in the range of 1–10 mg. SO₂ showed an inhibition effect on the uptake coefficients of NH₃ under both dark and photoreaction conditions, because of the competition effect between SO₂ and NH₃ adsorption. These results show that the conversion of NH₃ on the surface of TiO₂ is an important sink. It also demonstrates that the importance of the synergistic effect of coexisting pollutants in the heterogeneous reaction is worthy of further study.

As an important alkaline gas in the atmosphere, NH₃ is the main precursor of ammonium aerosol, which has an important impact on the formation of secondary aerosol.^3,26 However, at present, there is little understanding of the atmospheric chemical process of NH₃, and model research always applies the thermodynamic equilibrium parameter of NH₃ with main acidic substances such as H₂SO₄ and HNO₃, resulting in inaccurate evaluation of the contribution of NH₃. China has been facing a serious problem of atmospheric fine particle and NH₃ emissions from various sources are huge, resulting in very high NH₃ concentration in ambient atmosphere.^17,19,21 The results of this study show that the heterogeneous process participated by NH₃ can contribute to not only ammonium aerosol but also nitrate and NOx, which is also closely related to the formation process of sulfate. Therefore, much attention should be paid to the atmospheric chemical processes involved in ammonia in future atmospheric chemistry and models. At the same time, it also reminds us that in the control strategy of atmospheric fine particles, we should not only control SO₂ and NOx, but also control NH₃ emission.²² In addition, TiO₂ is widely used as a photocatalyst material in the environment for the removal of ammonia.^52,56,57 These catalysts showed good catalytic removal activity to NH₃ in experimental tests. However, the synergistic effect of coexisting pollutants should be fully considered when they are further applied to the actual atmospheric environment.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was financially supported by the National Natural Science Foundation of China (21922610, 22188102, and 22276205). The authors appreciate the Ozone Formation Mechanism and Control Strategies Project of the Research Center for Eco-Environmental Sciences, CAS (RCEES-CYZX-2020), and the Young Talent Project of the Center for Excellence in Regional Atmospheric Environment, CAS (CERAE201801).

References

1. WHO, Ambient air pollution: a global assessment of exposure and burden of disease, World Health Organization, Geneva, 2016.
2. C. L. Reddington, L. Conibear, C. Knote, B. J. Silver, Y. J. Li, C. K. Chan, S. R. Arnold and D. V. Spracklen, Exploring the impacts of anthropogenic emission sectors on PM_2.5 and human health in South and East Asia, Atmos. Chem. Phys., 2019, 19, 11887–11910.
3. R. K. Pathak, W. S. Wu and T. Wang, Summertime PM_2.5 ionic species in four major cities of China: nitrate formation in an ammonia-deficient atmosphere, Atmos. Chem. Phys., 2009, 9, 1711–1722.
4. F. Yang, J. Tan, Q. Zhao, Z. Du, K. He, Y. Ma, F. Duan, G. Chen and Q. Zhao, Characteristics of PM_2.5 speciation in representative megacities and across China, Atmos. Chem. Phys., 2011, 11, 5207–5219.
5. X. Zhang, Y. Wang, T. Niu, X. Zhang, S. Gong, Y. Zhang and J. Sun, Atmospheric aerosol compositions in China: spatial/temporal variability, chemical signature, regional haze distribution and comparisons with global aerosols, Atmos. Chem. Phys., 2012, 12, 779–799.
6. R.-J. Huang, Y. Zhang, C. Bozzetti, K.-F. Ho, J.-J. Cao, Y. Han, K. R. Daellenbach, J. G. Slowik, S. M. Platt, F. Canonaco, P. Zotter, R. Wolf, S. M. Pieber, E. A. Bruns, M. Crippa, G. Ciarelli, A. Piazzalunga, M. Schwikowski, G. Abbaszade, J. Schnelle-Kreis, R. Zimmermann, Z. An, S. Szidat, U. Baltensperger, I. E. Haddad and A. S. H. Prévôt, High secondary aerosol contribution to particulate pollution during haze events in China, Nature, 2014, 514, 218–222.
7. M. Kulmala, L. Pirjola and J. Mäkelä, Stable sulphate clusters as a source of new atmospheric particles, Nature, 2000, 404, 66–69.
8. M. Kulmala, How particles nucleate and grow, Science, 2003, 302, 1000.
9. E. Harris, B. Sinha, D. van Pinxteren, A. Tilgner, K. W. Fomba, J. Schneider, A. Roth, T. Gnauk, B. Fahlbusch and S. Mertes, Enhanced role of transition metal ion catalysis during in-cloud oxidation of SO₂, Science, 2013, 340, 727–730.
10. J. Xue, Z. Yuan, S. M. Griffith, X. Yu, A. K. Lau and J. Z. Yu, Sulfate formation enhanced by a cocktail of high NOx, SO₂, particulate matter and droplet pH during haze-fog events in megacities in China: An Observation-Based Modeling investigation, Environ. Sci. Technol., 2016, 50, 7325–7334.
11. C. R. Usher, A. E. Michel and V. H. Grassian, Reactions on mineral dust, Chem. Rev., 2003, 103, 4883–4939.
12. J. N. Crowley, M. Ammann, R. A. Cox, R. G. Hynes, M. E. Jenkin, A. Mellouki, M. J. Rossi, J. Troe and T. J. Wallington, Evaluated kinetic and photochemical data for atmospheric chemistry: Volume V–heterogeneous reactions on solid substrates, Atmos. Chem. Phys., 2010, 10, 9059–9223.
13. R. Atkinson, D. L. Baulch, R. A. Cox, J. N. Crowley, R. F. Hampson, R. G. Hynes, M. E. Jenkin, M. J. Rossi and J. Troe, Evaluated kinetic and photochemical data for atmospheric chemistry: Volume I – gas phase reactions of Ox, HOx, NOx and SOx species, Atmos. Chem. Phys., 2004, 4, 1461–1738.
14. W. L. Chang, P. V. Bhave, S. S. Brown, N. Riemer, J. Stutz and D. Dabdub, Heterogeneous Atmospheric Chemistry, Ambient Measurements, and Model Calculations of N₂O₅: A Review, Aerosol Sci. Technol., 2011, 45, 665–695.
15. V. H. Grassian, Chemical reactions of nitrogen oxides on the surface of oxide, carbonate, soot, and mineral dust particles: Implications for the chemical balance of the troposphere, J. Phys. Chem. A, 2002, 106, 860–877.
16. S. Reis, R. W. Pinder, M. Zhang, G. Lijie and M. A. Sutton, Reactive nitrogen in atmospheric emission inventories, Atmos. Chem. Phys., 2009, 9, 7657–7677.
17. X. Huang, Y. Song, M. Li, J. Li, Q. Huo, X. Cai, T. Zhu, M. Hu and H. Zhang, A high-resolution ammonia emission inventory in China, Global Biogeochem. Cycles, 2012, 26, GB1030.
18. S. N. Behera, M. Sharma, V. P. Aneja and R. Balasubramanian, Ammonia in the atmosphere: a review on emission sources, atmospheric chemistry and deposition on terrestrial bodies, Environ. Sci. Pollut. Res., 2013, 20, 8092–8131.
19. Z. Meng, X. Xu, W. Lin, B. Ge, Y. Xie, B. Song, S. Jia, R. Zhang, W. Peng, Y. Wang, H. Cheng, W. Yang and H. Zhao, Role of ambient ammonia in particulate ammonium formation at a rural site in the North China Plain, Atmos. Chem. Phys., 2018, 18, 167–184.
20. F. J. Dentener and P. J. Crutzen, A three-dimensional model of the global ammonia cycle, J. Atmos. Chem., 1994, 19, 331–369.
21. Z. Y. Meng, W. L. Lin, X. M. Jiang, P. Yan, Y. Wang, Y. M. Zhang, X. F. Jia and X. L. Yu, Characteristics of atmospheric ammonia over Beijing, China, Atmos. Chem. Phys., 2011, 11, 6139–6151.
22. X. Fu, S. X. Wang, J. Xing, X. Y. Zhang, T. Wang and J. M. Hao, Increasing Ammonia Concentrations Reduce the Effectiveness of Particle Pollution Control Achieved via SO₂ and NOx Emissions Reduction in East China, Environ. Sci. Technol. Lett., 2017, 4, 221–227.
23. Y. Wang, Q. Q. Zhang, K. He, Q. Zhang and L. Chai, Sulfate-nitrate-ammonium aerosols over China: response to 2000-2015 emission changes of sulfur dioxide, nitrogen oxides, and ammonia, Atmos. Chem. Phys., 2013, 13, 2635–2652.
24. M. Liu, X. Huang, Y. Song, J. Tang, J. Cao, X. Zhang, Q. Zhang, S. Wang, T. Xu, L. Kang, X. Cai, H. Zhang, F. Yang, H. Wang, J. Z. Yu, A. K. H. Lau, L. He, X. Huang, L. Duan, A. Ding, L. Xue, J. Gao, B. Liu and T. Zhu, Ammonia emission control in China would mitigate haze pollution and nitrogen deposition, but worsen acid rain, Proc. Natl. Acad. Sci. U. S. A., 2019, 116, 7760–7765.
25. W. Yang, Q. Ma, Y. Liu, J. Ma, B. Chu, L. Wang and H. He, Role of NH₃ in the Heterogeneous Formation of Secondary Inorganic Aerosols on Mineral Oxides, J. Phys. Chem. A, 2018, 122, 6311–6320.
26. R.-J. Huang, J. Duan, Y. Li, Q. Chen, Y. Chen, M. Tang, L. Yang, H. Ni, C. Lin, W. Xu, Y. Liu, C. Chen, Z. Yan, J. Ovadnevaite, D. Ceburnis, U. Dusek, J. Cao, T. Hoffmann and C. D. O'Dowd, Effects of NH₃ and alkaline metals on the formation of particulate sulfate and nitrate in wintertime Beijing, Sci. Total Environ., 2020, 717, 137190.
27. W. Xu, Y. Kuang, C. Zhao, J. Tao, G. Zhao, Y. Bian, W. Yang, Y. Yu, C. Shen, L. Liang, G. Zhang, W. Lin and X. Xu, NH₃-promoted hydrolysis of NO₂ induces explosive growth in HONO, Atmos. Chem. Phys., 2019, 19, 10557–10570.
28. S. Ge, G. Wang, S. Zhang, D. Li, Y. Xie, C. Wu, Q. Yuan, J. Chen and H. Zhang, Abundant NH₃ in China Enhances Atmospheric HONO Production by Promoting the Heterogeneous Reaction of SO₂ with NO₂, Environ. Sci. Technol., 2019, 53, 14339–14347.
29. L. Li, Z. Duan, H. Li, C. Zhu, G. Henkelman, J. S. Francisco and X. C. Zeng, Formation of HONO from the NH₃-promoted hydrolysis of NO₂ dimers in the atmosphere, Proc. Natl. Acad. Sci. U. S. A., 2018, 115, 7236–7241.
30. W. Yang, H. He, Q. Ma, J. Ma, Y. Liu, P. Liu and Y. Mu, Synergistic formation of sulfate and ammonium resulting from reaction between SO₂ and NH₃ on typical mineral dust, Phys. Chem. Chem. Phys., 2016, 18, 956–964.
31. Y. Liu, J. Liggio, R. Staebler and S. M. Li, Reactive uptake of ammonia to secondary organic aerosols: kinetics of organonitrogen formation, Atmos. Chem. Phys., 2015, 15, 13569–13584.
32. C. M. Stangl and M. V. Johnston, Aqueous Reaction of Dicarbonyls with Ammonia as a Potential Source of Organic Nitrogen in Airborne Nanoparticles, J. Phys. Chem. A, 2017, 121, 3720–3727.
33. G. Zhang, X. Lian, Y. Fu, Q. Lin, L. Li, W. Song, Z. Wang, M. Tang, D. Chen, X. Bi, X. Wang and G. Sheng, High secondary formation of nitrogen-containing organics (NOCs) and its possible link to oxidized organics and ammonium, Atmos. Chem. Phys., 2020, 20, 1469–1481.
34. J. R. Horne, S. Zhu, J. Montoya-Aguilera, M. L. Hinks, L. M. Wingen, S. A. Nizkorodov and D. Dabdub, Reactive uptake of ammonia by secondary organic aerosols: Implications for air quality, Atmos. Environ., 2018, 189, 1–8.
35. K. Wu, S. Zhu, Y. Liu, H. Wang, X. Yang, L. Liu, D. Dabdub and C. D. Cappa, Modeling Ammonia and Its Uptake by Secondary Organic Aerosol Over China, J. Geophys. Res., 2021, 126, e2020JD034109.
36. Z. Feng, F. Zheng, C. Yan, P. Fu, Y. Zhang, Z. Lin, J. Cai, W. Du, Y. Wang, J. Kangasluoma, F. Bianchi, T. Petäjä, Y. Wang, M. Kulmala and Y. Liu, The impact of ammonium on the distillation of organic carbon in PM_2.5, Sci. Total Environ., 2022, 803, 150012.
37. T. Chen, Y. Liu, Q. Ma, B. Chu, P. Zhang, C. Liu, J. Liu and H. He, Significant source of secondary aerosol: formation from gasoline evaporative emissions in the presence of SO₂ and NH₃, Atmos. Chem. Phys., 2019, 19, 8063–8081.
38. R. Gieré and X. Querol, Solid Particulate Matter in the Atmosphere, Elements, 2010, 6, 215–222.
39. H. Chen, C. E. Nanayakkara and V. H. Grassian, Titanium dioxide photocatalysis in atmospheric chemistry, Chem. Rev., 2012, 112, 5919–5948.
40. M. A. Kebede, M. E. Varner, N. K. Scharko, R. B. Gerber and J. D. Raff, Photooxidation of Ammonia on TiO₂ as a Source of NO and NO₂ under Atmospheric Conditions, J. Am. Chem. Soc., 2013, 135, 8606–8615.
41. Q. Ma, C. Zhong, J. Ma, C. Ye, Y. Zhao, Y. Liu, P. Zhang, T. Chen, C. Liu, B. Chu and H. He, Comprehensive Study about the Photolysis of Nitrates on Mineral Oxides, Environ. Sci. Technol., 2021, 55, 8604–8612.
42. T. Armaroli, T. Becue and S. Gautier, Diffuse reflection infrared spectroscopy (DRIFTS): Application to the in situ analysis of catalysts, Oil Gas Sci. Technol., 2004, 59, 215–237.
43. Y. Liu, C. Han, J. Ma, X. Bao and H. He, Influence of relative humidity on heterogeneous kinetics of NO₂ on kaolin and hematite, Phys. Chem. Chem. Phys., 2015, 17, 19424–19431.
44. C. Han, Y. Liu and H. He, Role of organic carbon in heterogeneous reaction of NO₂ with soot, Environ. Sci. Technol., 2013, 47, 3174–3181.
45. Q. Ma, T. Wang, C. Liu, H. He, Z. Wang, W. Wang and Y. Liang, SO₂ initiates the efficient conversion of NO₂ to HONO on MgO surface, Environ. Sci. Technol., 2017, 51, 3767–3775.
46. D. M. Murphy and D. W. Fahey, Mathematical treatment of the wall loss of a trace species in denuder and catalytic converter tubes, Anal. Chem., 1987, 59, 2753–2759.
47. D. O. Cooney, S.-S. Kim and E. J. Davis, Analyses of mass transfer in hemodialyzers for laminar blood flow and homogeneous dialysate, Chem. Eng. Sci., 1974, 29, 1731–1738.
48. C. Liu, Q. Ma, H. He, G. He, J. Ma, Y. Liu and Y. Wu, Structure–activity relationship of surface hydroxyl groups during NO₂ adsorption and transformation on TiO₂ nanoparticles, Environ. Sci.: Nano, 2017, 2388–2394.
49. C. Lee, G. Fitzgerald, M. Planas and J. J. Novoa, Ionization of Bases in Water: Structure and Stability of the NH₄⁺···OH^- Ionic Forms in Ammonia−Water Clusters, J. Phys. Chem., 1996, 100, 7398–7404.
50. D. J. Donaldson, Adsorption of Atmospheric Gases at the Air−Water Interface. I. NH₃, J. Phys. Chem. A, 1999, 103, 62–70.
51. G. M. Underwood, T. M. Miller and V. H. Grassian, Transmission FT-IR and Knudsen cell study of the heterogeneous reactivity of gaseous nitrogen dioxide on mineral oxide particles, J. Phys. Chem. A, 1999, 103, 6184–6190.
52. S. Yamazoe, T. Okumura, Y. Hitomi, T. Shishido and T. Tanaka, Mechanism of Photo-Oxidation of NH₃ over TiO₂: Fourier Transform Infrared Study of the Intermediate Species, J. Phys. Chem. C, 2007, 111, 11077–11085.
53. Q. Ma, L. Wang, B. Chu, J. Ma and H. He, Contrary Role of H₂O and O₂ in the Kinetics of Heterogeneous Photochemical Reactions of SO₂ on TiO₂, J. Phys. Chem. A, 2019, 123, 1311–1318.
54. C. E. Nanayakkara, J. Pettibone and V. H. Grassian, Sulfur dioxide adsorption and photooxidation on isotopically-labeled titanium dioxide nanoparticle surfaces: roles of surface hydroxyl groups and adsorbed water in the formation and stability of adsorbed sulfite and sulfate, Phys. Chem. Chem. Phys., 2012, 14, 6957–6966.
55. Z. Zhang, L. Chen, Z. Li, P. Li, F. Yuan, X. Niu and Y. Zhu, Activity and SO₂ resistance of amorphous CeaTiOx catalysts for the selective catalytic reduction of NO with NH₃: in situ DRIFT studies, Catal. Sci. Technol., 2016, 6, 7151–7162.
56. S. Yamazoe, T. Okumura and T. Tanaka, Photo-oxidation of NH₃ over various TiO₂, Catal. Today, 2007, 120, 220–225.
57. M. Chen, J. Ma, B. Zhang, F. Wang, Y. Li, C. Zhang and H. He, Facet-dependent performance of anatase TiO₂ for photocatalytic oxidation of gaseous ammonia, Appl. Catal., B, 2018, 223, 209–215.

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