Experimental Study of the Competitive Adsorption of HNO3 and H2O

Mar 24, 2014 - The competitive adsorption of HNO3 and H2O from the gas phase onto fused silica surfaces is investigated. Brewster angle cavity ring-do...
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Experimental Study of the Competitive Adsorption of HNO3 and H2O on Surfaces by Using Brewster Angle Cavity Ring-Down Spectroscopy in the 295−345 nm Region Juan Du,† Robert G. Keesee,*,‡ and Lei Zhu*,†,§ †

Wadsworth Center, New York State Department of Health, Albany, New York 12201-0509, United States Department of Atmospheric and Environmental Sciences, University at Albany, State University of New York, Albany, New York 12222, United States § Department of Environmental Health Sciences, State University of New York, Albany, New York 12201-0509, United States ‡

ABSTRACT: The competitive adsorption of HNO3 and H2O from the gas phase onto fused silica surfaces is investigated. Brewster angle cavity ring-down spectroscopy is used to measure absorption of a laser probe beam by the HNO3/H2O coadsorbed on fused silica surfaces as a function of the mixture pressure. The laser absorption measurements were made in the 295−345 nm region. Langmuir adsorption constants for nitric acid and water were found to be 107 ± 17 and 562 ± 21 Torr−1, respectively. A method has been developed for calculating absorption by HNO3 and H2O codeposited on the surface as a function of the HNO3/H2O mixture pressure using multicomponent Langmuir adsorption isotherms and absorption cross-sections at a given wavelength for surface-adsorbed HNO3 and H2O. The validity of this treatment has been evaluated both as a function of wavelength and as a function of mixing ratio.

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INTRODUCTION Nitric acid (HNO3) is an important member of the reactive atmospheric nitrogen species (NOY). After formation in the ambient environment through oxidation of NOx (NOx = NO + NO2), nitric acid is removed from the atmosphere mainly by dry and wet deposition. Deposition of nitric acid on surfaces of the environment was long thought to be a permanent sink for NOx until a recent field study1 reported that the photolysis rate of HNO3 deposited on surfaces to form HONO and NOx was 1−2 orders of magnitude faster than that in the gas phase. By employing Brewster angle cavity ring-down spectroscopy,2 a variant of the cavity ring-down technique,3,4 the absorption cross-sections of HNO3 adsorbed on fused silica surfaces in the 290−365 nm wavelength region were indeed found to be at least 2 orders of magnitude larger than the gas phase HNO3 cross-sections.5,6 Water vapor is ubiquitous in the atmosphere. Surfaceadsorbed water is pervasive on environmental surfaces. Recent work has shown that surface-adsorbed water is an absorber of solar radiation.7 Surface-adsorbed water can enhance the uptake of nitric acid on solid films, crystal grains, and particle surfaces of oxides, mineral dust, and NaCl.8−13 The adsorption of HNO3 on surfaces containing water vapor has been studied by using infrared spectroscopy.14−16 When a mixture of HNO3/ H2O is deposited on surfaces to form a monolayer of adsorbed molecules, HNO3 and H2O can compete for surface adsorption sites. The adsorption of nitric acid and water vapor individually on fused silica has been previously examined. In the present study, © 2014 American Chemical Society

we investigate the coadsorption of nitric acid and water vapor on fused silica. By using Brewster angle cavity ring-down spectroscopy, we measure the absorption of a laser probe beam in the 295 to 345 nm wavelength region due to adsorbed HNO3 and H2O, which is codeposited from gas phase mixtures of the vapors. By using relationships for a multicomponent Langmuir adsorption isotherm, we estimate saturation monolayer coverage as a function of gas phase mole fraction.

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EXPERIMENTAL TECHNIQUE The competitive adsorption of HNO3 and H2O on fused silica surfaces is determined by measuring probe laser absorption by HNO3 and H2O codeposited on surfaces. The adsorbed mixture absorption in the 295−345 nm region is obtained by introducing the linearly polarized probe laser beam into the ring-down cavity containing a pair of mutually compensating fused silica Brewster window(s) (1″ diameter and 1 mm thickness; Brewster angle for the vacuum/fused silica interface is ∼56°) placed in the path of the main optical axis inside the cavity. When p-polarized light is incident on an optical window oriented at Brewster’s angle, the reflection loss is zero, and almost all of the light is transmitted.17 Experimental details can be found elsewhere.5,6 The essential features are briefly Special Issue: A. W. Castleman, Jr. Festschrift Received: January 26, 2014 Revised: March 21, 2014 Published: March 24, 2014 8177

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water bath) into a trap cooled to 195 K (dry ice/ethanol bath). Five or more repeated distillations are carried out to remove NO2 impurities. To further reduce impurity before each experiment, we purge out NO2 from the liquid HNO3 bubbler by flowing N2 carrier gas through the bubbler for 30 min,20 followed by pumping the bubbler for 10 min in the absence of N2 flow. The NO2 impurity in the highly purified HNO3 vapor is less than 0.05%, as determined by measuring NO 2 absorption21,22 at 446.1 nm. The water sample is purified by pumping deionized water inside a bubbler for 30 min to remove dissolved air. Prior to introduction into the cavity cell, a specified gaseous mixture of nitric acid and water is prepared in a stainless steel gas mixer. Mixtures (with ratios of 1:0.2, 1:0.6, or 1:1 HNO3 to H2O) are made by first introducing the HNO3 vapor into a mixer and measuring the pressure of the HNO3 vapor inside the mixer with an MKS Baratron capacitance manometer (10 Torr full scale; measurement uncertainty is ≤0.25% of the pressure reading), then water vapor is introduced into the mixer and the total pressure of the mixture is measured. The HNO3/H2O mixture is then introduced into the cavity. Absorption of the probe beam by coadsorbed HNO3 and H2O is determined under static conditions. All stainless steel components inside the cell, on the gas transport lines, and inside the gas-mixer are precoated with halocarbon wax or grease (Halocarbon Series 1500 wax or 25−2S grease; Halocarbon Products Corp.) to prevent HNO3 dissociation on surfaces. All measurements are made at an ambient temperature of 294 ± 1 K.

described here and depicted in Figure 1. A pair of highreflectance cavity mirrors vacuum-seal both ends of the cell.

Figure 1. Schematic of the experimental setup.

Several pairs of cavity mirrors with reflectivity of 99.7% to 99.9% are used to cover the 295−345 nm range. The tunable probe laser beam in the 295−345 nm region is provided by the fundamental or the second harmonic output of a dye laser pumped by a 308 nm excimer laser. Laser dyes used were Rhodamine 590, Rhodamine 610, Rhodamine 640, DCM, and p-terphenyl. The probe beam is subsequently p-polarized and attenuated to be less than 0.25 mJ/pulse before it enters the ring-down cavity. The photon intensity decay inside the cavity is monitored with a photomultiplier tube (PMT) placed after the rear cavity mirror. The PMT output is amplified, digitized, and transferred to a computer. The decay curve is fitted to a single-exponential decay function, from which the ring-down time constant (τ) and the total loss (Γ) per round-trip pass are extracted. When Brewster angle cavity ring-down spectroscopy is used to measure probe laser absorption by HNO3 and H2O coadsorbed on fused silica surfaces, the probe beam inside the cavity experiences mirror transmission loss, absorption of the probe beam by HNO3 and H2O in the gas phase and by HNO3 and H2O coadsorbed on the front and back surfaces of each fused silica window, as well as absorption and transmission loss through the fused silica windows. Round-trip absorption of the probe beam by coadsorbed HNO3 and H2O is acquired by subtracting mirror transmission loss, absorption and transmission loss through fused silica windows, and round-trip absorption by HNO3 and H2O in the gas phase, from the total cavity losses. Mirror transmission loss and absorption and transmission loss through fused silica windows are obtained by measurement of cavity losses in the absence of HNO3 and H2O inside the ring-down cavity containing the fused silica Brewster windows. Gas phase HNO3 and H2O absorption is calculated for a given total pressure and HNO3/H2O mixture inside the cavity based on the gas phase absorption cross-sections of HNO3 and H2O in the near UV region that have been determined previously.5,6,18 Since each round trip in the cavity passes through a silica surface four times, the total loss (Γ) per round-trip pass is divided by four to determine the single surface pass absorption. Highly purified HNO3 is prepared by vacuum distillating19 a 2:3 mixture of nitric acid (70%; Mallinckrodt Baker) and sulfuric acid (98%; Mallinckrodt Baker) held at 273 K (ice−

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ANALYTICAL APPROACH When a mixture of HNO3 and H2O is codeposited on surfaces to form a single layer of adsorbed molecules, HNO3 and H2O may compete for surface adsorption sites. Assuming the adsorption of HNO3 and H2O on fused silica surfaces fits the multicomponent Langmuir adsorption isotherm, the fractional coverage of HNO3 (θHNO3) and H2O (θH2O) on surfaces can be determined from their partial pressures (PHNO3 and PH2O) and their Langmuir adsorption constants (bHNO3 and bH2O) by the expression:23,24 θi = biPi /(1 +

∑ biPi)

(1)

where the summation over components i denotes HNO3 and H2O for this particular study. The surface coverage Ci for each component i can be related to the total number of surface sites per unit area (Ctot) via eq 2: C i = C totθi = C totbiPi /(1 +

∑ biPi)

(2)

Since the absorption per pass is small, the total absorption Asurf,tot of the probe beam at a given wavelength due to coadsorbed HNO3 and H2O on the window surfaces can be written as the sum of the individual contributions: A surf,tot =

∑ Asurf,i = ∑ (σsurf,iCi)

(3)

where σsurf,HNO3 and σsurf,H2O denote absorption cross-sections of adsorbed HNO3 and H2O at a given wavelength. Substituting eq 2 into eq 3 and knowing the partial pressure Pi of component i is related to its mole fraction (Xi) for a given total pressure Ptotal (Pi = Xi × Ptotal), we obtained the relationship 8178

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A surf,tot = C totPtotal∑ (σsurf,ibiX i)/(1 + Ptotal ∑ biX i)

(4)

Inverting both sides of the eq 4, we obtain 1/A surf,tot = (C tot∑ (σsurf,ibiX i))−1(1/Ptotal) + (∑ biX i)/(C tot ∑ (σsurf,ibiX i))

(5)

From eq 5, we see that a plot of 1/Asurf,tot versus 1/Ptotal should be linear with slope m = (C tot ×

∑ (σsurf,ibiX i))−1

(6)

and zero-intercept y0 = (∑ biX i)/(C tot ∑ (σsurf,ibiX i))

(7)

and the ratio of the zero-intercept (y0) to the slope (m) is equal to ∑biXi. Absorption cross-sections for surface-adsorbed HNO3 and H2O in the 295−345 nm region have been determined in previous studies5−7 and are listed in Table 1. Consequently, by

Figure 2. Plot of 1/Asurf,tot versus 1/Ptotal for a 1:0.2 HNO3/H2O mixture (XHNO3 = 0.833) at a probe wavelength of 340 nm. Also shown is a linear least-squares fit of the experimental data.

The Langmuir adsorption constants bHNO3 and bH2O for adsorption on fused silica were determined from previous studies on the single components with a bHNO3 value5,6 of 93 ± 47 Torr−1 and bH2O value7 of 553 ± 277 Torr−1. Figure 3 shows

Table 1. Surface Absorption Cross-Sections HNO3 and H2O in the 295-345 nm Region λ (nm) 295 300 305 310 315 320 325 330 335 340 345

σHNO3 (cm2/molecule)a (1.59 ± 0.18) (1.48 ± 0.15) (1.09 ± 0.17) (1.31 ± 0.05) (1.30 ± 0.12) (1.00 ± 0.23) (4.7 ± 0.5) × (4.5 ± 0.5) × (3.1 ± 0.5) × (2.7 ± 0.3) × (2.0 ± 0.3) ×

× 10−18 × 10−18 × 10−18 × 10−18 × 10−18 × 10−18 10−19 10−19 10−19 10−19 10−19

σH2O (cm2/molecule)b (4.66 (4.08 (3.87 (3.76 (2.66 (1.60 (2.69 (2.23 (1.31 (6.34 (5.91

± ± ± ± ± ± ± ± ± ± ±

0.83) 0.65) 0.19) 0.19) 0.52) 0.21) 0.48) 1.01) 0.33) 0.80) 1.13)

× × × × × × × × × × ×

10−20 10−20 10−20 10−20 10−20 10−20 10−20 10−20 10−20 10−21 10−21

Adapted from refs 5 and 6. The HNO3 surface coverage of 1.1 × 1014 molecule/cm2 was used in deriving HNO3 surface cross-section values. b Adapted from ref 7. The H2O surface coverage of 1.6 × 1015 molecule/cm2 was used in deriving H2O surface cross-section values. a

varying total pressure and measuring the total absorption by the surface for different HNO3/H2O gas phase mixtures at given wavelengths, one can determine the b values and the Ctot from the slope and zero-intercept of a plot of 1/Asurf,tot versus 1/Ptotal. One should note that the absolute values for the absorption cross-sections of adsorbed HNO3 and H2O on fused silica were determined using monolayer surface concentrations that were calculated based on their van der Waals radii.

Figure 3. Plot of intercept/slope (y0/m, equal to bHNO3XHNO3 + bH2OXH2O) versus nitric acid gas phase mole fraction (XHNO3). Circles are based on previously reported values for the Langmuir adsorption constants (refs 5−7). Filled squares are mean values from the present study. The error boxes show the range for the central half, and the lines show the range of the experimental measurements for each mixture. The solid line is the least-squares fit of all the points. The dashed line is the least-squares fit of the data from the present study only.

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RESULTS AND DISCUSSION Single surface pass absorption by nitric acid and water adsorbed on fused silica has been determined at 5 nm wavelength intervals from 295 to 345 nm as a function of the total pressure for gas phase mixtures with HNO3 to H2O ratios of 1:0.2, 1:0.6, and 1:1. Figure 2 shows a plot of experimentally obtained 1/ Asurf,tot against 1/Ptotal for a 1:0.2 HNO3/H2O mixture at 340 nm. The plot is linearly consistent with eq 5, suggesting that the assumptions of equivalent single occupancy sites and negligible interactions between adsorbates on adjacent sites in the Langmuir formulizm are acceptable in this case. In most cases, measurements were obtained for three runs at each wavelength providing over 30 results for each mixture.

a plot of the intercept to the slope ratio (y0/m) versus the HNO3 gas phase mole fraction (XHNO3) for the HNO3/H2O mixtures. We have used the median value of the determined y0/ m ratios of the all runs for a given mixture since the distribution of the experimental ratios is skewed toward larger values by random errors in the intercept and slope, as is apparent in the error bars shown in Figure 3. A least-squares fit that includes the previous single component values yields a bHNO3 value of 8179

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107 ± 17 Torr−1 and bH2O value of 562 ± 21 Torr−1. A leastsquares fit of only the median values from the present study yields Langmuir adsorption constants of 107 ± 36 and 583 ± 36 Torr−1, respectively, demonstrating the consistency of the results. A fit using the central half of the experimental measurements (within the error boxes in Figure 3) yields 122 ± 18 and 572 ± 32 Torr−1, respectively. With the determined b values, the saturation monolayer coverages Ctot for different HNO3/H2O mixtures are calculated by using eqs 6 and 7 with the cross-sections in Table 1. The average values obtained for each mixture are presented in Table 2. Table 2. Saturation Monolayer Coverage (Ctot) for HNO3/ H2O Mixtures Coadsorbed on Fused Silica Surfaces HNO3 mole fraction

Ctot (molecule/cm2)a

0.833 0.625 0.500

(2.32 ± 0.80) × 1014 (3.10 ± 0.81) × 1014 (4.09 ± 0.96) × 1014

Figure 5. Saturation monolayer coverage Ctot as a function of XHNO3 for nitric acid−water vapor mixtures (XH2O = 1 − XHNO3). The line is the relationship Ctot = (14.9XH2O2 + 1.1) × 1014 molecules/cm2.

a

Error is standard deviation of all the calculated values for a given mixture.

molecules/cm2. With this expression for Ctot, the Langmuir adsorption constants, and absorption cross-sections reported herein, eq 4 may be used to estimate the light absorption expected by a saturated monolayer of coadsorbed HNO3 and H2O on fused silica under an arbitrary nitric acid−water vapor gas phase mixture.

The results averaged for each probe wavelength are presented in Figure 4. Within the experimental uncertainties of the data, the results demonstrate independence of the probe wavelength.

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CONCLUSIONS In this article, we report direct measurements of probe laser absorption by HNO3 and H2O codeposited on fused silica surfaces as a function of the HNO3/H2O mixture pressure by using Brewster angle cavity ring-down spectroscopy. From the experimental data, we extracted saturation monolayer surface coverage for various HNO3/H2O mixtures. We have developed a method of calculating HNO3/H2O mixture surface absorption based upon the multicomponent Langmuir adsorption isotherm, the total surface sites per unit area for a HNO3/ H2O mixture, and HNO3 and H2O surface absorption cross section at a given wavelength. We found the treatment provides adequate description of the mixture surface absorption profiles for various HNO3/H2O mixtures over a wide wavelength region. The method we have developed to treat absorption by coadsorbed HNO3 and H2O is expected to be useful for researchers who are interested in modeling or estimating adsorbed HNO3/H2O mixture photophysical and photochemical properties.

Figure 4. Total saturation monolayer coverage Ctot as a function of wavelength for various HNO3 gas phase mole fractions XHNO3. XHNO3 = 0.5 (filled circles), 0.625 (open squares), and 0.833 (filled triangles).

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Figure 5 presents the dependence of the saturation monolayer coverage Ctot on the composition of gas phase nitric acid−water vapor mixture. The shape of the curve is dependent on the relative magnitudes of Ctot from the single component systems. The relative values of Ctot in turn are dependent on the relative sizes of H2O and HNO3. The van der Waals radius of nitric acid taken to be 3.8 times larger than that of water leading to a saturation monolayer coverage that is 14.5 times smaller.5−7 The trend in the saturation monolayer coverage for coadsorbed nitric acid−water vapor is well represented by a relationship depending on the square of the water vapor mole fraction, Ctot = (14.9XH2O2 + 1.1) × 1014

AUTHOR INFORMATION

Corresponding Authors

*(R.G.K.) Tel: 518-442-4566. Fax: 518-442-5825. E-mail: [email protected]. *(L.Z.) Tel: 518-474-6846. Fax: 518-473-2895. E-mail: zhul@ wadsworth.org. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the support provided by the National Science Foundation under grant #AGS-0969985. 8180

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