Spectroscopy of Sonoluminescence and Sonochemistry in Water

Article pubs.acs.org/JPCB

Spectroscopy of Sonoluminescence and Sonochemistry in Water Saturated with N2−Ar Mixtures Temim Ouerhani, Rachel Pflieger,* Warda Ben Messaoud, and Sergey I. Nikitenko Institut de Chimie Séparative de Marcoule, UMR5257, UM-CEA-CNRS, Centre de Marcoule, BP 17171, 30207 Bagnols-sur-Cèze cedex, France S Supporting Information *

ABSTRACT: Sonoluminescence spectra in relation with sonochemical activity of water sparged with Ar/N2 gas mixtures were systematically studied at two ultrasonic frequencies (20 and 359 kHz). At 20 kHz, solely the molecular emission of OH (A2Σ+−X2Πi) is observed in addition to a broad continuum typical for multibubble sonoluminescence. On the contrary, at high frequency a second emission band is present around 336 nm which is assigned to the NH (A3Π−X3Σ−) system. In addition, the sonolysis of a 0.2 M NH3·H2O solution at 359 kHz in the presence of pure Ar yields the emission bands of NH (A3Π − X3Σ−) (336 nm) and NH (C1Π−A1Δ) (322 nm) systems confirming the sonochemical production of NH radicals. The N2 (C3Πu−B3Πg) emission band is absent at both frequencies. This uncommon phenomenon can be explained by the quenching of the N2 (C3Πu) excited state with water molecules inside the bubbles. The sonoluminescence of NH radicals at 359 kHz indicates more effective intrabubble dissociation of N2 molecules at high ultrasonic frequency compared to low-frequency (20 kHz) ultrasound. Its absence at 20 kHz may also be related to strong quenching, e.g., by water molecules. The kinetic study of the formation of principal sonochemical products (H2, H2O2, HNO3, HNO2) confirmed the more drastic conditions produced during bubble collapse at higher ultrasonic frequency. O2−H2O system.8 The NO production is supposed to occur by Zeldovich mechanism:9

1. INTRODUCTION Sonochemistry, or in other words the chemical effects of ultrasound, originates from acoustic cavitation: the nucleation, growth, and implosive collapse of gas bubbles in liquids submitted to an ultrasonic field.1 Recent spectroscopic studies of multibubble sonoluminescence (MBSL) in water saturated with noble gases revealed the formation of a nonequilibrium plasma during bubble collapse.2,3 In principle, MBSL spectroscopy is quite universal: a thorough analysis of the MBSL spectra allows researchers to probe the intrabubble conditions and to identify the chemically reactive species generated inside the cavitation bubbles.2−4 However, the application of MBSL to better understand the reaction mechanisms occurring under acoustic cavitation is only beginning to emerge. This paper focuses on the study of MBSL and sonochemical reactivity in water saturated with N2−Ar gaseous mixtures. The sonochemistry of nitrogen in aqueous solutions was pioneered in 1936 by Schultes and Gohr.5 They reported the formation of NO2− and NO3− under the effect of 900 kHz ultrasound in water sparged with a N2−O2 mixture. Much later Misik and Riesz6 suggested that H2O2 and NO2− were the primary products of water sonolysis in the presence of air and that NO3− ion resulted from the secondary oxidation of nitrite ion by hydrogen peroxide. According to Wakeford et al.,7 the highly reactive oxygen required for NOx formation from molecular nitrogen would come from the dissociation of oxygen molecules. The occurrence of the latter reaction was confirmed by ultrasonically driven isotopic exchange in the © 2015 American Chemical Society

O2 − ))) → 2O

(1)

N2 + O → NO + N

(2)

N + O2 → NO + O

(3)

N + OH• → NO + H

(4)

where the symbol “)))” indicates a reaction initiated by the cavitation event. Then, further oxidation takes place induced by OH• radicals (originated from H2O molecules homolytic dissociation) or by O2 molecules:10 H 2O−))) → H + OH•

(5)

NO + OH• → HNO2

(6)

2NO + O2 → 2NO2

(7)

NO2 + OH• → HNO3

(8)

By contrast, the sonochemistry of nitrogen in the absence of oxygen has been much less studied. It was reported that the sonolysis of a H2−N2 mixture in water at 38011 and 900 kHz12 led to NH3 formation suggesting the dissociation of both H2 and N2 molecules inside the cavitation bubbles: Received: October 19, 2015 Revised: December 2, 2015 Published: December 8, 2015 15885

DOI: 10.1021/acs.jpcb.5b10221 J. Phys. Chem. B 2015, 119, 15885−15891

Article

The Journal of Physical Chemistry B

H 2−))) → H + H

(9)

N2−))) → N + N

(10)

N + H 2 → NH + H

(11)

N + H → NH

(12)

NH + H 2 → NH3

(13)

min before sonication and during the ultrasonic treatment with a total flow rate of 212 mL/min. The temperature was measured by a thermocouple immersed approximately 2 cm below the surface of solution and kept constant using a cryostat (Lauda RE210). Gas flow rates were measured with a volumetric flowmeter with stainless steel float (Aalborg). The calibration chart provided by the manufacturer was validated against a numerical mass flowmeter (Aalborg GFM17). The light emission spectra were collected through a quartz window using parabolic Al-coated mirrors and recorded in the spectral range from 250 up to 400 nm using a SP 2356i Roper Scientific spectrometer (grating 300blz300, slit width 0.09 mm) coupled to a CCD camera with UV coating (SPEC10-100BR Roper Scientific) cooled by liquid nitrogen. Spectral calibration was performed using a Hg(Ar) pen-ray lamp (model LSP035, LOTOriel). The spectra acquisition was started after reaching a steady-state temperature. For each experiment, at least three 300 s spectra were averaged and corrected for background noise and for the quantum efficiencies of grating and CCD. The SL spectra were collected at the focusing point providing the highest light emission intensity. The formation rate of hydrogen was measured online by mass spectrometry (model VG Prolab Benchtop). The kinetics of hydrogen peroxide formation was monitored by UV−vis absorption spectrometry as described in the literature.17 Samples of the sonicated solution were taken and added to a solution of TiOSO4 2 × 10−2 M in H2SO4 0.5 M to form an orange Ti(IV) peroxide complex. The maximum of absorbance was measured at 400 nm (ε = 714 cm−1 mol−1 L) using a UV− vis spectrophotometer. The formation rate of nitrite was determined by the Griess method at 530 nm (ε = 41 724 cm−1 mol−1 L).17 The formation rate of nitrate was determined by ion chromatography (Dionex-ion Pac AS15, 0.4 mm × 250 mm).18

In addition, nitrogen isotopic exchange studies of sonicated water saturated with pure N2 (at 1 MHz)13 and N2−Ar mixture (at 300 kHz)14 clearly showed the dissociation of N2 molecules during bubble collapse. However, the proposed mechanisms of this process are controversial. Margulis et al.13 suggested direct N2 dissociation, while Hart et al.14 presumed nondirect dissociation due to the reaction of N2 with OH• radicals: N2 + OH• → N2O + H

(14)

N2O → N2 + O

(15)

O + N2 → NO + N

(16)

An indirect dissociation of N2 by reaction with H atoms is also conceivable:15 N2 + H → NH + N

(17)

The direct dissociation of N2 requires prior excitation of the molecule. This is why the emission of the N2 (C3Πu−B3Πg) system is usually observed in conditions where N2 dissociates and traditionally applied for diagnostics of N2-containing plasma.16 To the best of our knowledge, the MBSL spectra of nitrogen in sonicated water have never been reported. Moreover, contrary to high-frequency ultrasound, the sonochemistry of N2−Ar mixtures in aqueous solutions has been poorly studied at low ultrasonic frequency. In particular, there is no certitude whether the dissociation of N2 molecules can take place in the cavitation bubbles at low frequency. The present work aims at probing the hypothesis of N2 intrabubble dissociation by means of MBSL study of water saturated with N2−Ar gaseous mixtures and a comparative study of N2 sonochemical activity at high and low frequencies.

3. RESULTS AND DISCUSSION 3.1. High-Frequency Ultrasound. 3.1.1. Sonoluminescence Study. At 359 kHz the MBSL spectra collected in the 250−400 nm spectral range in pure water saturated with N2− Ar gas mixtures show the emission of the OH (A2Σ+−X2Πi) system and a broad continuum typical for MBSL of pure water saturated with noble gases (Figure 1).19 Surprisingly, the emission of the N2 (C3Πu−B3Πg) system usually present in the emission spectra of N2−Ar plasmas and composed of three main peaks at 315 (1−0), 336 (0−0), and 357 (0−1) nm16 is absent in MBSL spectra. Instead, an intense peak is observed around 336 nm that can be assigned to the NH (A3Π−X3Σ−) system.20−22 It might appear surprising to observe NH emission in the presence of OH radicals but no NO nor NO2 emission. This absence may be accounted for by the further reaction of these molecules prior to emission. It is known that excited NH radicals are formed in Ar/NH323 or Ar/(N2 + H2)24 plasmas. Therefore, in order to verify the hypothesis of NH species formation inside the cavitation bubble MBSL spectra were collected in 0.1 M NH3·H2O solution in the presence of pure Ar. Figure 1 clearly shows the same emission band at 336 nm as in water saturated by the N2−Ar mixture. In addition, another band at 322 nm can be attributed to the NH (C1Π−A1Δ) system21 confirming NH radical formation during the sonochemical degradation of NH3 molecules. Usually, the dissociation of the N2 molecule required for NH radical formation is stimulated by electron and vibrational excitation.24 In this view, the absence of a N2 (C3Πu−B3Πg) emission band

2. EXPERIMENTAL METHODS 2.1. Materials. Reagents used in this study were all of analytical grade and were purchased from Merck, SigmaAldrich, and Alfa Aesar. To prepare aqueous solutions for experiments, purified ultrapure water having a resistivity higher than 18.2 MΩ cm at 25 °C was used. The gases Ar and N2 (purity >99.999%) were provided by Air Liquide. The gas mixtures were prepared in situ by adjusting the flow rates of Ar and N2. 2.2. Procedures. The high-frequency device described recently2−4 consisted of a thermostated glass-made batch reactor equipped with a piezoelectric transducer (ELAC Nautik, 25 cm2) providing 359 kHz (Pac = 50 W, measured by the thermal probe method) ultrasound. The transducer was fitted at the bottom of the reactor and connected to a multifrequency generator (T&C Power Conversion, Inc.). A second part of the experiments was done with 20 kHz (Pac = 25 W) ultrasound using a 1 cm2 titanium probe (750 W, Sonics generator). The probe was placed reproducibly on top of the reactor opposite the high-frequency transducer using a tight Teflon ring. For all experiments, 250 mL of water was sparged with gas (pure Ar or different mixtures of N2 and Ar) about 30 15886


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the presence of nitrogen at least up to 28% N2. This might be explained by faster population of the vibrational levels of the OH (A2Σ+) state than their consumption in the chemical processes. In contrast to the hOH band, the intensity of continuum emission decreases regularly with an increase in N2 content. The continuum emission is usually attributed to the superposition of H + OH• recombination, bremsstrahlung, water molecule de-excitation, and OH (B2Σ+−A2Σ+) emission band.19 Consequently, the I(380 nm) value should be influenced by several parameters like the number and size of SL-emitting bubbles 27 (which depend on the gas nature), 28 the sonochemical reactivity of species, the radiative or nonradiative de-excitation efficiencies, etc. Thus, the decrease in continuum intensity may be related to a drop in the number and/or size of sonoluminescing bubbles as well as to the quenching of sonoluminescing species. Finally, hNH intensity increases with N2 content up to a maximum between 24% and 36% of N2 in Ar and then decreases. However, the emission of NH (A3Π− X3Σ−) normalized by N2 content shows a trend similar to that of the continuum intensity. This result may be explained by the double role of nitrogen in the studied system. On the one hand, nitrogen is required for the generation of NH species (reactions 11 and 12), and on the other hand NH (A3Π−X3Σ−) emission can be quenched by N atoms29 or by N2 molecules30 according to the following reactions:

Figure 1. MBSL spectra of 0.1 M NH3·H2O under Ar, water under Ar, and water under 72% Ar−28% N2 or 92% Ar−8% N2 mixtures, f = 359 kHz, Pac = 50 W, and T = 14 °C. The MBSL spectra were obtained after normalization to 308 nm (OH (A2Σ+−X2Πi) 0−0 transition).

would be attributed to two nonradiative processes involving excited N2 molecules: quenching by water molecules known to be very efficient25 and nonradiative dissociation leading to NH radical formation. The evolution of the continuum intensity at 380 nm (I (380 nm)) and of the heights of NH (A3Π−X3Σ−) and OH (A2Σ+− X2Πi) emission peaks, hNH and hOH, respectively, with N2 content in Ar is shown in Figure 2. The hOH increases until

NH* + N → N2 + H

(18)

NH* + N2 → N3H

(19)

The superposition of all these processes yields an apparent maximum in NH (A3Π−X3Σ−) emission at about 30% of N2 in Ar. 3.1.2. Sonochemical Activity. At the same conditions (359 kHz, Pac = 50 W), the yields of sonolytic products (H2, H2O2, HNO2, and HNO3) were followed during the sonolysis of water sparged with various Ar/N2 gas mixtures. Results are shown in Figure 3 (typical linear kinetic curves of those products are shown in Figures 1SI and 2SI). In general, the sonochemical data fit well the MBSL studies. The GH2 and the GH2O2 values linearly decrease when the N2 content increases. The decrease in H2 yield can be explained by

Figure 2. Evolution of the height of the OH (A2Σ+−X2Πi) emission peak, hOH (measured at 315 nm after subtraction of a linear baseline in the range 250−540 nm), the height of the NH (A3Π− X3Σ−) emission peak, hNH (measured at 336 nm after subtraction of a linear baseline in the range 326−375 nm), hNH/N2, and the continuum intensity I (380 nm) with N2 content in Ar at 359 kHz, Pac = 50 W, T = 14 °C.

there is 17% of N2 in Ar and then decreases. This decrease may be explained by a quenching of OH (A2Σ+−X2Πi) emission with N2 molecules,26 especially at high N2 content. At low N2 concentration this effect is probably counterbalanced by a lesser recombination of OH• + H radicals due to scavenging of H by N and N2 (eqs 12 and 17), hence the higher OH• emission. On the other hand, the shape of the OH (A2Σ+−X2Πi) emission band (Figure 1), or in another words, the vibrational temperature of the OH (A2Σ+) state,19 is not influenced by

Figure 3. Yields of sonolytic products (in μmol/kJ) as a function of N2 content in Ar, 359 kHz, Pac = 50 W, T = 14 °C. 15887


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The Journal of Physical Chemistry B hydrogen consumption to form NH and NH3 species (reactions 11−13). On the other hand, the oxidation of nitrogen species by OH• radicals leads to a decrease in H2O2 yield and to the formation of nitrous and nitric acids, according to the following sequences of reactions:31,32 The formation of NOH radicals is shown here N + OH• → NOH

(20)

For the oxidation of NOH radicals, the first sequence is NOH + OH• → NO + H 2O

(21)

NO + OH• + M → HNO2 + M

(22)

H 2O2 + HNO2 → HNO3 + H 2O

(23)

The second sequence is NOH + OH• → NO2 + H 2 •

(24)

OH + M + NO2 → HNO3 + M

(25)

2NO2 + H 2O → HNO2 + HNO3

(26)

Figure 5. Water SL spectra under Ar, Ar-10% N2 gas mixture, and Ar36% N2 gas mixture, 20 kHz, Pac = 25 W, T = 14 °C. The emission spectra were collected in close vicinity to the ultrasonic tip.

The yield of nitrous acid is proportional to N2 percentage in Ar up to 45% of N2 and is then approximately constant between 45% and 65% of N2 content in Ar. This inflection is attributed to the decreasing amount of OH• radicals present. On the other hand, the formation rate of HNO3 is approximately constant whatever the N2 content, which means that the rate of HNO3 formation is not limited by the amount of reagent present, but rather by the kinetics of reaction 23 and/or 26. A comparison of NH (A3Π−X3Σ−) emission intensity in SL with the formation yield of HNO2 as a function of N2 content in Ar is shown in Figure 4. The sonoluminescence intensity of

under Ar, Ar-10% N2, and Ar-36% N2 gas mixture. The SL intensity decreases when the N2 rate in Ar increases. Similarly to the high-frequency ultrasound case, no difference can be observed in the shape of OH (A2Σ+−X2Πi) emission band. This means that the relative populations of OH (A2Σ+) vibrational levels are not impacted by the presence of N2. However, contrary to the high-frequency case, no NH (A3Π−X3Σ−) emission is observed at 20 kHz under Ar/10% N2 at Pac = 25 W. 3.2.2. Sonochemical Activity. The yields of sonolysis products of water sparged with various Ar/N2 gas mixtures and submitted to 20 kHz ultrasound are given in Table 1. Figure 6 summarizes the sonochemical yields of H2 and HNO2 for both studied ultrasonic frequencies at different values of acoustic power. The follow-up of these products reveals the existence of sonochemical activity at 20 kHz in the presence of N2. Like for the high-frequency case, H2O2 and H2 formation rates decrease with an increase in N2 content, with this decrease being much stronger (10 times) at 20 kHz. In addition, HNO 2 sonochemical yield appears to be much lower at low frequency compared to 359 kHz at Pac = 50 W (13−17 times less for 20 kHz, Pac = 18 W and 20−42 times less for 20 kHz, Pac = 33 W). 3.3. Comparison of High- and Low-Frequency Ultrasound. The dramatic effect of ultrasonic frequency on NH (A3Π−X3Σ−) MBSL first questions the formation mechanism of the NH (A3Π) state. The NH (A3Π−X3Σ−) emission is usually observed in low-pressure plasma discharges of nitrogen and argon mixtures, where the formation of the excited specie NH (A3Π) is initiated by electron impact that dissociates N2 molecules.33 The so-formed N atoms then recombine with H2 molecule or H atoms. In the presence of an ammonia molecule, the NH radical can also be formed by collisions of NH3 molecules with metastable argon atoms Ar*m (3P2, 3P0) which have higher energy (11.55 and 11.72 eV, respectively) than the threshold dissociation energy of NH3 (8.9 eV).23 In cavitation bubbles, the pressure is much higher than in typical plasma discharge experiments. At such high pressures the vibrational excitation of N2 molecules is known to occur directly through VV pumping mechanism.34 Collisional dissociation with metastable argon atoms Ar*m (3P2, 3P0) is also probable:

Figure 4. Comparison of the formation yield of HNO2 and of the height of the NH (A3Π−X3Σ−) emission peak, hNH with N2 percentage in Ar, 359 kHz, Pac = 50W, T = 14 °C.

NH (A3Π− X3Σ−) starts to decrease while HNO2 formation yield is still increasing: GHNO2 is proportional to N2 content in Ar up to 55% of N2 while hNH increases until there is 30% of N2 in Ar and then rapidly decreases. As discussed in section 3.1.2, this different trend is attributed to the quenching of NH excited state by N2 molecules. 3.2. Low-Frequency Ultrasound. 3.2.1. Sonoluminescence Study. Figure 5 presents water SL spectra at 20 kHz 15888


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The Journal of Physical Chemistry B Table 1. Yields of Sonolysis Products at 20 kHz under Ar and N2/Ar Gas Mixture US frequency and power

N2 content in Ar/%

GH2/μmol/kJ

GH2O2/μmol/kJa

20 kHz, Pac = 18 W

0 36 54 36 54

0.14 0.01 0.015 0.005 0.005

0.14 0.012 0.022

20 kHz, Pac = 33 W a

GHNO2/μmol/kJ 0.015 0.013 0.005 0.004

H2O2 formation rate was measured in a smaller volume (50 mL) since its concentration was too low for quantification in 250 mL.

50W) under 10% N2 in Ar at bulk temperatures between 6 and 30 °C (see Figure 3 SI). Indeed, as the bulk temperature increases, the water vapor pressure Pvap (H2O) and, in turn, the number of water molecules in bubbles increase. The height of NH emission peak, hNH, is plotted against the corresponding water vapor pressure in Figure 7 as black

Figure 7. Evolution of the height of NH emission peak, hNH, normalized at 16 °C, hNH divided by the corresponding N2 solubility normalized at 16 °C, height of OH emission divided by the corresponding water vapor and of the continuum intensity as a function of water vapor pressure, 359 kHz, Pac = 50 W.

squares. To take into account the decrease in N2 solubility in water with temperature, this emission was corrected by dividing hNH by N2 solubility for each bulk temperature (blue open triangles). The rapid decrease of this corrected NH emission may be modeled by quenching of NH excited state by water molecules:

Figure 6. Comparison of the sonochemical yields of (a) H2 and (b) HNO2 at high- and low-frequency ultrasound.

Ar*m (3P2 , 3P0) + N2 → 2N + Ar

NH* → NH + hv

(27)

NH* + H 2O → NH + H 2O*

3 −

At 20 kHz NH (A Π−X Σ ) radical emission is not seen, which might be attributed to a lower extent of N2 direct dissociation because of the less extreme conditions reached at collapse (possibly due to a lower shape stability leading to softer collapse35) and thus lower electron temperature.19 In principle, this hypothesis is in agreement with the very low yield of HNO2 at low-frequency ultrasound that can originate from nondirect dissociation of N2 via reactions 14−16. Another important difference between low- and high-frequency cavitation bubbles is their contents, with low-frequency bubbles containing much more water vapor.36,37 Therefore, we suggest that water molecules (possibly also N2 molecules) may quench the NH (A3Π) excited state which can be formed by the reaction of N2 molecules with H (reaction 17) as discussed in section 3.2.2 rather than via direct dissociation of N2. To probe this hypothesis, SL spectra were measured at 359 kHz (Pac 3

(k′)

hNHαk′/(k′ + k″[H 2O])

(28)

(k ″ )

(29) (30)

From these equations, hNH should be proportional to (1 + BPvap(H2O))−1 where B is a constant. Indeed, a nice fit can be obtained using this formula (plotted as a dash-dotted blue line in Figure 7; B = 173.4), confirming the quenching hypothesis. For the sonochemical activity, when N2 is added to Ar in the saturating gas, H2O2 and H2 formation rates decrease with an increase in N2 content at both high and low frequencies. However, the decrease is much stronger at 20 kHz, which may be attributed to higher scavenging of H and OH• radicals by N2 (reactions 14, 17) at low frequency. Indeed, at 20 kHz the bubbles are bigger,35,38 and the expansion period is longer, which favor gas diffusion into the bubbles.36 Moreover, reached conditions are less extreme at 20 kHz, as indicated by both spectroscopic19 and chemical39 approaches, that found higher 15889



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“temperatures” at high frequency (vibrational temperature of excited species in the first case, chemical bubble temperature in the second case). Therefore, a higher N2 concentration relative to the number of the OH• radicals is expected, leading to a more pronounced scavenging. This higher relative concentration in N2 molecules in low-frequency bubbles is confirmed by a very strong quenching of the overall SL (Figure 5). The strong scavenging of H and OH• radicals by N2 molecules then limits the concentration of OH• radicals available for oxidation of NO to HNO2. This higher concentration in N2 in 20-kHz cavitation bubbles could explain the fact that the HNO2 maximum yield is reached at lower N2 percentage in Ar at 20 kHz compared to high-frequency ultrasound.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.5b10221.

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REFERENCES

(1) Lorimer, J.; Mason, T. Applied Sonochemistry: Uses of Power Ultrasound in Chemistry and Processing; Wiley-VCH Verlag: Weinheim, Germany, 2002. (2) Ndiaye, A. A.; Pflieger, R.; Siboulet, B.; Nikitenko, S. I. The Origin of Isotope Effects in Sonoluminescence Spectra of Heavy and Light Water. Angew. Chem., Int. Ed. 2013, 52, 2478−2481. (3) Pflieger, R.; Ndiaye, A. A.; Chave, T.; Nikitenko, S. I. Influence of Ultrasonic Frequency on Swan Band Sonoluminescence and Sonochemical Activity in Aqueous Tert-Butyl Alcohol Solutions. J. Phys. Chem. B 2015, 119, 284−290. (4) Navarro, N. M.; Pflieger, R.; Nikitenko, S. I. Multibubble Sonoluminescence as a Tool to Study the Mechanism of Formic Acid Sonolysis. Ultrason. Sonochem. 2014, 21, 1026−1029. (5) Schultes, H.; Gohr, H.; Ü ber Chemische. Wirkungen der Ultraschallwellen. Angew. Chem. 1936, 49, 420−423. (6) Mišík, V.; Riesz, P. Detection of Primary Free Radical Species in Aqueous Sonochemistry by EPR Spectroscopy. In Sonochemistry and Sonoluminescence; Crum, L., Mason, T., Reisse, J., Suslick, K., Eds.; Springer: Amsterdam, The Netherlands, 1999; Vol. 524; pp 225−236. (7) Wakeford, C. A.; Blackburn, R.; Lickiss, P. D. Effect of Ionic Strength on the Acoustic Generation of Nitrite, Nitrate and Hydrogen Peroxide. Ultrason. Sonochem. 1999, 6, 141−148. (8) Fischer, C. H.; Hart, E. J.; Henglein, A. Ultrasonic Irradiation of Water in the Presence of 18, O18: Isotope Exchange and Isotopic Distribution of H2O2. J. Phys. Chem. 1986, 90, 1954−1956. (9) Zeldovich, Y.; Frank-Kamenetskii, D.; Sadovnikov, P. Oxidation of Nitrogen in Combustion; Publishing House of the Academy of Sciences of USSR: Moscow, 1947. (10) Neta, P.; Huie, R. E.; Ross, A. B. Rate Constants for Reactions of Inorganic Radicals in Aqueous-Solution. J. Phys. Chem. Ref. Data 1988, 17, 1027−1284. (11) Ė l′piner, I. E. Ultrasound; Physical, Chemical, and Biological Effects; Consultants Bureau: New York, 1964. (12) Supeno; Kruus, P. Fixation of Nitrogen with Cavitation. Ultrason. Sonochem. 2002, 9, 53−59. (13) Margulis, M. A.; Didenko, Y. T.; Gorbarenko, S. A. Determination of the Velocity of the Nitrogen Atom Recombination in Ultrasonic Fields with the Help of N15. Zh. Fiz. Khim. 1985, 59, 2026−2030. (14) Hart, E. J.; Fischer, C. H.; Henglein, A. Isotopic Exchange in the Sonolysis of Aqueous Solutions Containing 14,14N2 and 15,15N2. J. Phys. Chem. 1986, 90, 5989−5991. (15) Bezgin, L.; Kopchenov, V.; Titova, N.; Starik, A. Mechanisms of Pollutant Formation in the H2 Fuelled Combustor of High Velocity AirBreathing Engine: Modeling Study. In 29th Congress of the International Council of the Aeronautical Sciences: St. Petersburg, Russia, 2014. (16) Zhu, X.-M.; Pu, Y.-K. Optical Emission Spectroscopy in LowTemperature Plasmas Containing Argon and Nitrogen: Determination of the Electron Temperature and Density by the Line-Ratio Method. J. Phys. D: Appl. Phys. 2010, 43, 403001. (17) Venault, L.; Moisy, P.; Blanc, P.; Madic, C. Kinetics of Hydrazinium Nitrate Decomposition in Nitric Acid Solutions Under the Effect of Power Ultrasound. Ultrason. Sonochem. 2001, 8, 359−366. (18) Ito, K.; Takayama, Y.; Makabe, N.; Mitsui, R.; Hirokawa, T. Ion Chromatography for Determination of Nitrite and Nitrate in Seawater Using Monolithic ODS Columns. J.Chromatogr.A 2005, 1083, 63−67. (19) Ndiaye, A. A.; Pflieger, R.; Siboulet, B.; Molina, J.; Dufrêche, J.F.; Nikitenko, S. I. Nonequilibrium Vibrational Excitation of OH Radicals Generated During Multibubble Cavitation in Water. J. Phys. Chem. A 2012, 116, 4860−4867. (20) Kusano, Y.; Leipold, F.; Fateev, A.; Stenum, B.; Bindslev, H. Production of Ammonia-Derived Radicals in a Dielectric Barrier Discharge and their Injection for Denitrification. Surf. Coat. Technol. 2005, 200, 846−849. (21) Cvejanovic, D.; Adams, A.; King, G. C. Radiative Lifetime Measurements of NH And CH Using the Electron-Photon Delayed Coincidence Method. J. Phys. B: At. Mol. Phys. 1978, 11, 1653.

4. CONCLUSIONS In summary, this paper reports for the first time the sonoluminescence of NH (A3Π) radicals during the sonolysis of water saturated with N2−Ar gas mixtures. The NH (A3Π− X3Σ−) emission band can be easily detected at 359 kHz ultrasound while it is not observed at all at 20 kHz in the entire range of studied N2 concentrations. Such a striking difference can be attributed to more drastic conditions inside the cavitation bubbles at higher ultrasonic frequency providing more efficient dissociation of nitrogen molecules and to higher concentration of N2 and H2O molecules inside bubbles at 20 kHz leading to strong quenching. The MBSL studies correlate with higher sonochemical activity of high-frequency ultrasound leading to enhanced yields of HNO2 compared to 20 kHz ultrasound. Two other interesting phenomena were observed during this study. The first is that the N2 (C3Πu−B3Πg) emission band usually accompanying NH emission in N2−Ar plasmas is not observed in water MBSL spectra whatever the ultrasonic frequency or nitrogen concentration. It may be explained by the quenching of the N2 (C3Πu) state with water molecules. The second striking feature is that the distribution of OH (A2Σ+) vibrational states observed simultaneously with NH (A3Πν−X3Σ−) emission band is not influenced by the presence of nitrogen at least up to 30% N2 for both high- and lowfrequency ultrasound. This phenomenon can be understood presuming that the population of OH (A2Σ+) vibrational states occurs more rapidly than the OH• radical consumption in the reactions with nitrogen species. More generally, this work confirmed the key role of N2 and H2O molecules intrabubble splitting in both the MBSL and the sonochemistry of water saturated with N2/Ar gaseous mixtures. On the other hand, these molecules strongly contribute to the quenching of excited and chemically reactive species generated by acoustic bubble collapse.

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Kinetic curves and SL spectra (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: rachel.pfl[email protected]. Notes

The authors declare no competing financial interest. 15890


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Spectroscopy of Sonoluminescence and Sonochemistry in Water

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