A novel heterogeneous reaction for generating

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The short-lived reactive specimen nitrous acid HONO was generated in the gas phase by the hetero- geneous reaction of gaseous HCl with AgNO2 which can ...
Chinese Science Bulletin © 2007

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A novel heterogeneous reaction for generating gaseous nitrous acid WANG WeiGang1, GE MaoFa1†, YAO Li1, ZENG XiaoQing1 & WANG ZiFa2 1

Beijing National Laboratory for Molecular Sciences, and State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China; 2 State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China

The short-lived reactive specimen nitrous acid HONO was generated in the gas phase by the heterogeneous reaction of gaseous HCl with AgNO2 which can generate higher concentration of HONO than other methods. We investigated the process from generation to dissociation in the gas phase under different controlled temperatures, and discussed the ionization and reaction on the solid surface by combination of the photoelectron spectroscopy and photoionization mass spectroscopy (PES-PIMS) and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). HONO, PES-PIMS, DRIFTS

OH is a key reactive species in the troposphere, playing a primary role in the formation of ozone and other secondary atmospheric pollutants[1]. Photodissociation of nitrous acid (HONO) is thought to be a significant source of OH radicals in remote and polar regions[2,3] and also in the polluted troposphere. For example, Winer and Biermann[4] reported that HONO photolysis accounted for almost all the OH produced during the first few hours after sunrise on a typical day in Long Beach, CA. In addition to being a source of OH, HONO has been shown to form nitrous oxide in surface reaction of which the mechanism is not yet well defined[5]. This is potentially significant on a global scale, since there are uncertainties in the budget of N2O[6]. The mechanisms of HONO formation in the atmosphere are not very clear at present. Evidence of heterogeneous HONO production through the surface reaction of NOx with water or soot was experimentally observed in smog chamber[7] and flow tube studies[8]. This kind of heterogeneous reaction is used to explain the high concentrations measured in the atmosphere. The electronic and molecular structures of molecule are related to their activity. We have a long-standing inwww.scichina.com

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terest in the generation, spectroscopy and structure of small stable and unstable species[9,10]. Herein we present a novel method to generate nitrous acid in the gas phase, and the processes of generation and dissociation studied by photoelectron spectroscopy (PE), photoionization mass spectroscopy (PIMS) and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) are also discussed. The measured ionization energy has been assigned with the aid of OVGF calculations.

1 Experiment section Nitrous acid is generated by passing HCl vapor over finely powdered AgNO2. In situ photoelectron spectrum and mass spectrum were recorded simultaneously. The reaction may be represented by eq. (1) at 100℃.

HCl + AgNO 2 → HONO + AgCl

(1)

Received February 25, 2007; accepted April 27, 2007 doi: 10.1007/s11434-007-0474-8 † Corresponding author (email: [email protected]) Supported by the Knowledge Innovation Program of the Chinese Academy of Sciences (Grant No. KZCX2-YW-205) and Hundred Talents Fund, the 973 Program of Ministry of Science and Technology of China (Grant No. 2006CB403701), the National Natural Science Foundation of China (Grant Nos. 20577052, 20673123, 20473094, and 20503035)

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2 Results and discussion 2.1 Molecule structures

There are many experiments and theoretical calculations about the structures of nitrous acid[15,16]. As pointed out by Cox et al.[17], the two cis and trans isomers of nitrous acid are planar and have Cs symmetry. The calculation results of Bauerfeldt et al.[16] proved that the MP2 results are satisfactory, with low relative deviations for bond distances and angles. So in this article, we use the MP2 method to optimize the structures of nitrous acid and corresponding fragments. Jones et al.[18] studied the infrared spectrum and made vibrational assignments of cis and trans nitrous acid. They located four in-plane vibrational fundamental bands at roughly 3500, 1700, 1250 and 850 cm−1. The two out-of-plane modes tautomers are at lower frequencies. From the study of the IR spectrum at different temperatures, Jones et al. concluded that the trans-isomer is lower in energy than cis-isomer[18]. The

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structural parameters calculated at MP2/6-311G(3df, 2p) level are depicted in Table 1, the trans-isomer is more stable than cis-isomer and the energy gap between them is calculated to be 0.45 kcal·mol−1 with zero-point of energy corrections, which is consistent with Varma et al.’s results[19]. These structures are also used to predict the vertical ionization energy of HONO and ionization processes in the next section. Table 1 Ab initio data (MP2/6-311G(3df, 2p) level) for trans-HONO and cis-HONO trans-HONO cis-HONO −205.3834624

−205.3827391

0

0.45

Cs

Cs

O—H

0.968

0.978

N—O

1.414

1.376

N== O

1.176

1.190

HON

102.1

105.2

ONO

110.9

113.4

E(au) Erel (kcal·mol−1) Symmetry

2.2 Spectroscopic analysis

HeI photoelectron spectra of the reaction products of gaseous HCl with AgNO2 at different temperatures are shown in Figures 1 and 2, respectively. At ambient temperature, the bands at 12.55 and 12.75 eV which are characteristic bands for HCl are the main component in the PE spectrum[20]. There are almost no other peaks in the photoelectron spectrum. So the reaction is normally difficult at room temperature. Figure 1 illustrates the PE spectrum of the reaction at 100℃. With the rise of reaction temperature, the intensity of bands which is characteristic bands for HCl becomes weak, but the bands at 11.0 and 13.0 eV which are distinguishable from reactants become the main components in the PE spectrum. According to calculated vertical ionization energy using ROVGF/6-311G(3df, 2p) method, the experimental results match well with the ionization potentials for nitrous acid. The results of mass spectroscopy also approved it, which would be discussed later. Since the two conformers with comparable energy are possible in some cases, first we should investigate whether the spectra can originate from a mixture of all conformers and whether some of them dominate. However, in comparison to the calculated orbital energy, we focus our discussion on the analysis of the slightly more stable trans-conformer. Firstly, we compare the PE spectrum of HONO with the spectra of related molecules NO2[20] and CH3ONO[21],

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The PE spectrum was recorded on a double-chamber UPS-II machine which was built specifically to detect transient species at a resolution of about 35 meV as indicated by the Ar+(2P2/3) photoelectron band[11]. Experimental vertical ionization energy (Ev in eV) is calibrated by simultaneous addition of a small amount of argon and methyl iodide to the sample. Mass analysis of ions is achieved with a time-of-flight mass analyzer described in detail elsewhere[10]. In situ DRIFTS spectrum was recorded on a Bruker Tensor FT-IR, equipped with an in situ diffuse reflection chamber and a high-sensitivity MCT detector cooled by liquid N2. The sample for the in situ DRIFTS studies was finely ground and placed into a ceramic crucible in the in situ chamber. Mass flow controllers and a sample temperature controller were used to control the reaction conditions. All spectra reported here were recorded at a resolution of 4 cm−1 for 128 scans[12]. The calculations were performed using program package Gaussian 03[13]. Full optimizations were done applying ab initio (HF and MP2) methods. Local minimum was confirmed with the vibrational analysis. The vertical ionization energy (Ev) for cis and trans isomers was calculated at the ab initio level according to Cederbaum’s[14] outer valence Green’s function (OVGF) method at 6-311G(3df, 2p) basis set based on the MP2/6-311G(3df, 2p) optimized geometry.

detected by our equipment. So we generate nitrous acid by a novel heterogeneous reaction of gaseous HCl with AgNO2, which can get higher concentration of HONO that makes detection by PE and PIMS is easy to achieve.

Figure 1

Full HeI photoelectron spectrum of reaction 1 at 100℃.

and the first band for HONO is attributed to the long pair nNO-. The most recent experimental adiabatic ionization potential for HONO was determined to be 10.97±0.03 eV using synchrotron photoionization mass spectrometry[22], which coincides with the value of PE spectrum (11.10 eV) of our results. This result is more consistent with the calculated value (11.11 eV) for trans-isomer than cis-isomer (11.49 eV). The second band at 12.90 eV derives from the ionization of 11a″ orbital, coinciding with the calculated value, 12.89 eV for trans-isomer, this band is mainly ascribed to the non-bonding out-of-plane (perpendicular to the molecular plane) π (no) orbital, and the ionization of electrons from similar orbitals also appeared in CH3ONO (12.6 eV) and CH3NO (14.9 eV)[21]. Figure 2 depicts the recorded spectrum of the reaction at 130℃. It is clear that along with the rise of temperature, the intensity of the broad band at 11.10 eV becomes weak. Those peaks marked with solid square are characteristic bands for H2O, while those marked with dots represent NO2 and asterisks stand for characteristic bands for NO which become strong[20]. When the temperature is above 150℃, there are not distinct bands belonging to HONO. All compounds could derive from the decomposition of HONO. It confirms the reaction mechanism proposed for the formation of HONO in the initial step at relatively low temperature. Then this compound is easy to decompose when the temperature becomes high. The similar reaction between gaseous hydrochloric acid and solid sodium nitrate can generate highly pure nitrous acid, but the concentration of nitrous acid is relatively low (5―2000 ppb)[2] which cannot be 3058

Figure 2

Full HeI photoelectron spectrum of reaction 1 at 130℃.

The PIMS of the reaction at 100℃ is shown in Figure 3. Combining with the PES and the theoretical results, the analysis of the PIMS results becomes easier. The peak (m/z = 43) is taken as parent ion. This result is consistent with the results of Taatjes et al.[22]. Then the peak at m/z = 30 (NO+) represents a loss of 17 amu (OH).

Figure 3 HeI photoionization mass spectrum of HONO.

The ionization and dissociation processes are given in Figure 4. In the neutral state, trans-HONO lies lower in

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at 100℃, and the spectrum of AgNO2 without HCl exposure is set as baseline. The bands centered at 1876 and 1618 cm−1 are contributed by NO and NO2, respectively[23]. Except those bands, two bands centered at 1283 and 848 cm−1 are observed as shown in Figure 5. These bands agree well with the ν3 of trans-HONO and ν4 of cis-HONO[24]. From the results of FTIR, we can see that HONO generates on the surface of AgNO2. At the same time, a few nitrous acid molecules decompose to NO, NO2 and H2O. These results are consistent with the observation from the gas phase by photoelectron spectroscopy.

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energy than cis-isomer by about 0.45 kcal·mol−1. The barrier from the trans-isomer is 10.89 kcal·mol−1 at MP2/ 6-311G(3df, 2p) level of theory, which makes the transition between trans and cis isomer very easy to occur at the ambient temperature. This result is proved by the single point calculation at QCISD(T)/6-311 + G(3df,2p) level of theory (12.02 kcal·mol−1), which is also consistent with the experimental torsion barrier for the trans→cis isomerization (11.6 ±0.2 kcal·mol−1)[16]. In the ionization state, trans-HONO+ is also more stable than cis-isomer by about 5.54 kcal·mol−1 and the transition energy is calculated to be 13.50 kcal·mol−1. So the transition is relatively more difficult than that of the neutral molecule. When the neutral molecule is ionized, it is very easy to dissociate to NO+ (2.96 kcal·mol−1). The other dissociation pathways need more energy (96.79 kcal·mol−1) for producing OH+. So NO+ would be dominant in the mass spectrum of HONO theoretically. As can be seen from Figure 3, the NO+ is the most abundant ion in the mass spectrum, compared to HONO+ and OH+.

Figure 5 In situ DRIFTS spectrum of HCl contact with AgNO2 surface at 100℃.

Figure 4 Dissociation and ionization pathways for HONO calculated at the MP2/ 6-311G(3df, 2p) level of theory (energy in eV).

To characterize the chemical composition of AgNO2 with HCl reaction and to further investigate the mechanism of HCl reaction with AgNO2 , we performed DRIFTS spectroscopic measurements for this process. Figure 5 shows the in situ DRIFTS spectrum of reaction 1 2

Finlayson P B J, Pitts J N. Chemistry of the Upper and Lower At-

The short-lived reactive specimen nitrous acid HONO is generated in the gas phase by the heterogeneous reaction of gaseous HCl with AgNO2. The electronic structure is depicted by in situ photoelectron spectroscopy (PE), combined with MP2 calculations. With controlling different temperatures, we investigated the process from generation to dissociation in the gas phase. Combined with the PIMS and DRIFTS, we discussed the ionization and reaction on the surface in more detail. The authors would like to thank Prof. Zhengping Hao in RCEES, CAS for providing support to DRIFTS measurement.

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