Temperature Dependence of Heterogeneous Uptake of Hydrogen

Jul 11, 2012 - The heterogeneous kinetic processes of hydrogen peroxide on silicon dioxide (SiO2) and calcium carbonate (CaCO3) have been studied over...
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Temperature Dependence of Heterogeneous Uptake of Hydrogen Peroxide on Silicon Dioxide and Calcium Carbonate Li Zhou, Wei-Gang Wang,* and Mao-Fa Ge* Beijing National Laboratory for Molecular Science (BNLMS), State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, People's Republic of China ABSTRACT: The heterogeneous kinetic processes of hydrogen peroxide on silicon dioxide (SiO2) and calcium carbonate (CaCO3) have been studied over the temperature range from 253 to 313 K using a Knudsen cell reactor, and the functions of temperature were obtained. The kinetic study indicates that the initial uptake coefficients increase evidently with a temperature decrease. They can be calculated by the equations γBET(SiO2) = [exp(934.5/T − 12.7)]/[1 + exp(934.5/T − 12.7)] and γBET(CaCO3) = [exp(1193.0/T − 11.9)]/[1 + exp(1193.0/T − 11.9)]. On the basis of the temperature dependence of uptake coefficients, the enthalpy (ΔHobs) and entropy (ΔSobs) of uptake progresses were determined to be −(7.77 ± 1.55) KJ mol−1 and −(105.8 ± 21.2) J K mol−1 for SiO2 and −(9.92 ± 1.98) KJ mol−1 and −(98.6 ± 19.7) J K mol−1 for CaCO3. The activation energies for desorption (Edes) of H2O2 on CaCO3 and SiO2 were calculated to be (5.9 ± 0.9) KJ mol−1 and (9.15 ± 1.1) KJ mol−1. The results suggest that hydrogen peroxide could mainly be adsorbed on SiO2 and CaCO3 reversibly in this temperature region, and the quick uptake on these mineral aerosols, especially at low temperature, provides an active surface for further complex reactions.

1. INTRODUCTION Mineral dust is one of the most important aerosols in the troposphere, which is emitted into the atmosphere from arid and semiarid regions such as the Saharan Desert and Central Asian. About 1000−3000 Tg of mineral dust is injected into the atmosphere every year.1 Because of the long atmospheric lifetime and transport distances, mineral dust can react with various trace gases, and the condensed phase influences the chemical balance of the atmospheric species. Meanwhile, the lifetimes of the various trace gases may change via the heterogeneous interaction with dust. After a series of reactions, the thermodynamic and optical properties of mineral dust will also be altered.2 Mineral aerosol is a general expression for fine particles of crustal origins, which consist mostly of silica and silicate minerals. Silicon dioxide is a major oxide constituent of mineral dust found in the atmosphere with an abundance of ∼60% by weight.1 Calcium carbonate is also an important and ubiquitous mineral in biological and geochemical systems.3 As one reactive component of mineral aerosol in the troposphere, calcium carbonate can alter the chemical balance of the atmosphere by heterogeneous reactions with trace gases,3−7 further influencing the global CO2 budget.8 Accordingly, mineral dust is now recognized as one of the major aerosols in the atmosphere for its potential impact on climate changes. In the atmosphere, hydrogen peroxide is normally thought as a secondary photochemical product, primarily arising from the bimolecular recombination of hydroperoxy (HO2) radicals.9 This reaction is in a particular high speed in polluted areas where the steady-state concentration of hydroperoxyl radicals is considerably higher than that in clean air. As sources of oddoxygen radicals, hydrogen peroxide and organic hydroperoxide can reflect the radical levels of the troposphere and effectively © 2012 American Chemical Society

remove the contaminants in the atmosphere. Because of the high solubility and oxidizing property, hydrogen peroxide acts as an oxidant in the droplets, which is particularly important in the oxidation of sulfur dioxide to produce sulfuric acid in the aqueous phase. Nowadays, the catalytic decomposition of hydrogen peroxide using metal oxides in the aqueous phase has been widely investigated.10,11 It is clear that the removal of hydroperoxide by wet or dry deposition will lower the oxidizing capacity of the atmosphere.12 However, in the gas phase, hydrogen peroxide is relatively stable, due to its low photolysis rate and relatively slow reaction rate with other contaminants. Because of the significant effect of hydrogen peroxide on atmospheric chemistry, various reactions of hydrogen peroxide, such as aqueous phase reaction and dissociation, have been widely investigated.13−17 Since the first measurement of hydrogen peroxide, hydroperoxide has been observed by many field measurements.18,19 Recently, Pradhan et al. reported the uptake coefficient of gaseous hydrogen peroxide on TiO2 aerosol and mineral dust using an entrained aerosol flowtube (AFT) coupled with a chemical ionization mass spectrometer (CIMS).20,21 Zhao et al. investigated the heterogeneous reactions of hydrogen peroxide on SiO2 and α-Al2O3 particles, using transmission Fourier transform infrared (T-FTIR) spectroscopy and high-performance liquid chromatography (HPLC).22 In our previous study, the heterogeneous uptakes of hydrogen peroxide on a variety of mineral oxide particles, which represent the most abundant elements in the atmosphere, were investigated using a Knudsen cell reactor.23 Received: May 8, 2012 Revised: July 5, 2012 Published: July 11, 2012 7959

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performed by containing the similar weight of the mineral aerosols and entering the same H2O2 concentration. The effective area of the escape aperture was measured in each independent experiment according to the attenuation of the N2 signal from one steady state to another.28 It was about 0.1613 mm2 in our experiments. The geometric area of the sample holder (As) was 5.3 cm2. The powdered samples were prepared in Teflon-coated metal sample holders by dispersing evenly with deionized water and then heating a hydrosol of the powder until a dry coating of the sample remained in the bottom surface of the holder. Typically, the samples and chamber were evacuated for 18 h to a base pressure of approximately 10−7 Torr prior to exposure to hydrogen peroxide. 2.3. Temperature Dependence Studies. To control various temperatures within the Knudsen chamber range from 253 to 313 K, the entire external surface of the chamber containing the samples was either heated or cooled by a DHJF4005 refrigerated circulator, which passed ethanol through a cooling coil. For the sake of establishing an equilibrium temperature and avoiding temperature differences of the sample holders with the solid sample, the system was allowed to thermalize or refrigerate for at least 1 h before each experiment. During the experiment, the temperature was stable for ±1 K.

However, most of the previous work was performed at room temperature. The laboratory investigation of the reaction of hydrogen peroxide on ice surfaces in a wide temperature range has been studied, but few direct studies of the heterogeneous oxidation of hydrogen peroxide on mineral dust were undertaken.24−26 The temperature in the atmosphere varies with latitude, longitude, and altitude above the earth's surface, as well as with season and time of day.27 The experimental determination of rate constants for important atmospheric reactions and how these rate constants vary with temperature remain a crucially important part of atmosphere science. The temperature dependence for the uptake coefficients of hydrogen peroxide on SiO2 and CaCO3 was investigated over the temperature region of 253−313 K, which is quite an ubiquitous scope near the Earth's surface in the atmospheric boundary layer, and can indicate the common effects of temperature on these reactions in ambient. The present studies provide information to understand the mechanism of uptake and decomposition processes of hydrogen peroxide on these particles, which is a critical property for evaluating aerosol's environmental and climate impacts.

2. EXPERIMENTAL SECTION 2.1. Materials. Silicon dioxide (SiO2, with a stated purity of 99.5%, diameter 2 μm) and calcium carbonate (CaCO3, with a stated purity of 99.5%, diameter 5 μm) samples were purchased from Alfa Aesar. These powdered chemicals were used without further purification. Hydrogen peroxide (H2O2, 30 wt %) was provided by Beijing Chemical works. Aqueous solutions of H2O2 (30 wt %) were first prepared by concentrating to 75% by distillation under 2100−3200 Pa, which were then concentrated by bubbling dry nitrogen through the sample for approximately 5 days. Density measurements indicated that the weight percentage was greater than 95 wt % The BET area of each powder sample was measured using a Quantachrome Autosob-1-C instrument. It was shown that the SiO2 sample had a total BET area of 6.419 m2 g−1, and the specific surface area of CaCO3 sample was 1.066 m2 g−1. 2.2. Experimental Methods. A Knudsen cell reactor coupled with a quadrupole mass spectrometer (Hiden, HAL 3F 501) was used to measure the uptake coefficient of hydrogen peroxide on SiO2 and CaCO3. This experimental apparatus has been described in detail elsewhere.23 Briefly, the Knudsen cell reactor with a volume of 461 cm3 was made up of a chamber with four isolated sample compartments and a small escape aperture. The quadrupole mass spectrometer (Hiden, HAL 3F 501) was used to detect reactant and product gas flow through the escape aperture. In the Knudsen cell reactor, to ensure that the mean free path of the molecules exceeded the dimensions of the cell, the pressure used was quite low (10−3 Pa). The pressure of the mass spectrometer chamber was about 5 × 10−5 Pa. The uptake of hydrogen peroxide on mineral oxide was monitored by mass 34 channel. For minimizing wall reactions during the experiments, the reactor walls were passivated with a coating of Teflon. In a blank experiment, the background signal intensity budget when turning on the H2O2 supply was lower than 5% of the original MS signal of H2O2 at m/z = 34, revealing that there was no remarkable uptake due to the sample holder and the sample cover. In addition, the parallel experiment using the four sample cells to ensure the repetitiousness of the kinetic results was

3. RESULTS AND DISCUSSION 3.1. Uptake Kinetics of Hydrogen Peroxide on SiO2 and CaCO3 at 298 K. The heterogeneous reaction kinetics measurement using this Knudsen cell reactor is comparable to others.28,29 For all of the kinetic experiments, the temperature of chamber was kept at 298 K first. The system was passivated with hydrogen peroxide for an hour until the signal of hydrogen peroxide reached a steady state, while the samples were kept isolated from the gas reagent before each experiment. Then, one sample was exposed to gas hydrogen peroxide by quickly opening the holder cover, and at the same time, the signal of H2O2 parent ion monitored at m/z = 34 dropped below its original value suddenly. An observed uptake coefficient, γobs, can be derived from the Knudsen cell eq 1: γobs =

A h ⎛ I0 − I ⎞ ⎜ ⎟ As ⎝ I ⎠

(1)

where Ah is the effective area of the escape hole, As is the geometric area of the sample holder, and I0 and I are the QMS intensity detected when sample holder covered and exposed, respectively. The values obtained from this equation are the initial uptake coefficients, γinit,obs, representing the uptake coefficient observed at the initial stages of the reactions. Figure 1 shows a typical QMS signal response of CaCO3 exposed to hydrogen peroxide. This experiment was performed at 298 K with about 15 mg CaCO3. The average H2O2 pressure in the Knudsen cell reactor was kept approximately at 0.0247 mTorr. As can be seen from Figure 1, the mass spectral intensity of hydrogen peroxide (m/z = 34) decreased immediately when the sample holder was opened, and then, the signal intensity recovered to the initial intensity. The uptake curve for the SiO2 was similar to that of CaCO3. Besides hydrogen peroxide, other mass channels, including O2 (m/z = 32) and CO2 (m/z = 44), were also monitored synchronously during the uptake experiment. However, the production O2 and CO2 signals were not observed evidently. 7960

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times the sample mass.30 The results for each experiment done in the linear regime are given in Table 1. The first column of each oxide gave the average value of the BET area-corrected initial uptake coefficients determined from different samples. Table 1. Uptake Coefficients and Desorption Rate Constants of SiO2 and CaCO3 at Different Temperatures SiO2 temp (K)

Figure 1. Uptake curves of hydrogen peroxide on 15 mg of CaCO3 at 298 K.

253 268 283 298 313

The uptake coefficients of hydrogen peroxide depend on the sample mass, correlating to the particle layers and total surface area. The uptake coefficient γobs is determined using the geometric surface area of the sample, as eq 1 is derived assuming that the total number of the gas-surface collisions is only with the top layer. The observed mass dependence involves the diffusion of the reactant gas to underlying layers, resulting in an increase in the number of collisions with the total surface area. Thus, it is an important factor in determining the uptake coefficient. It can be seen in Figure 2 that the

CaCO3

γBET/10−5a

kdes/10−4a

γBET/10−4a

± ± ± ± ±

5.63 ± 0.7 7.18 ± 0.9 8.93 ± 1.2

7.11 6.51 5.41 3.71 3.00

12.6 9.25 7.84 6.89 6.08

2.52 1.85 1.57 1.38 1.22

± ± ± ± ±

1.42 1.30 1.10 0.74 0.60

kdes/10−3a 2.67 3.53 3.79 4.47 4.63

± ± ± ± ±

0.07 0.03 0.09 0.12 0.18

a

Each value is the average of at least three measurements, and the error corresponds to one standard deviation (σ).

The influence of the hydrogen peroxide concentration to the uptake coefficient was also considered over the range from 3.7 × 1011 to 3.7 × 1012 molecules cm−3. By varying the initial concentration of hydrogen peroxide and its residence time, the uptake experiments of hydrogen peroxide on SiO2 and CaCO3 showed a similar phenomena. There was no obvious dependence of initial uptake coefficient on different H2O2 concentration ranging between 3.7 × 1011 and 3.7 × 1012 molecules cm−3. After a long time exposure, the uptake of hydrogen peroxide on SiO2 and CaCO3 surface showed saturation and became inactive to further H2O2 decomposition, and the hydrogen peroxide signal intensity could return to the initial baseline. There was no obviously steady state uptake, which is correlated to the heterogeneous surface reaction. These phenomena suggest that the adsorption of hydrogen peroxide on these mineral aerosols was the main process of the system. As compared with our previous study,23 some other metal oxide powders exhibit catalytic behavior toward the decomposition of hydrogen peroxide. Nevertheless, according to the results obtained from this experimental measurement, the heterogeneous uptake on SiO2 and CaCO3 surface was assumed to pass through the following scheme: H 2O2 + (SiO2 )* ⇄ (SiO2 )*H 2O2

H 2O2 + (CaCO3)* ⇄ (CaCO3)*H 2O2

The absorption and desorption of the hydrogen peroxide on SiO2 and CaCO3 surface were reversible. 3.2. Effects of Temperature on Uptake Kinetics of Hydrogen Peroxide on SiO2 and CaCO3. The temperature dependence for the uptake coefficients of heterogeneous reactions on SiO2 and CaCO3 was further investigated over the temperature region of 253−313 K, which is really a common scope in the environments, and can reflect the real effects of temperature about these reactions near the Earth's surface. As shown in Table 1, the initial uptake coefficients of SiO2 were in the range of 1.26 × 10−4−6.08 × 10−5, and the ones of CaCO3 were in the range of 7.11 × 10−4−3.00 × 10−4. It was obvious that the uptake coefficients of hydrogen peroxide on the mineral aerosols decreased with an increase in temperature. This trend of the uptake coefficients with temperature agrees well with the physical adsorption process. Now that the uptake coefficients showed negative temperature dependence, the changes of observed enthalpy (ΔHobs)

Figure 2. Linear mass-dependent regions of the observed uptake coefficient for hydrogen peroxide on (a) SiO2 and (b) CaCO3.

observed uptake values are dependent on the mass of the samples. The plot in Figure 2 shows the region where γobs is linearly dependent on the mass of samples. From the plot, a mass independent uptake coefficient can be derived as γBET =

As A h ⎛ I0 − I ⎞ ⎜ ⎟ = γ obs ⎝ ⎠ ABET I ABET

(2)

where ABET is the surface area of the sample, taken as the BET area, which is equal to the specific BET area of the powder 7961

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sample mass of SiO2 was about 8 mg, and the mass of CaCO3 was approximately 15 mg, which were in the mass liner ranges. The desorption rate constants can be calculated using the following equations,33,34 based on the flux balance equation according to the uptake profiles as shown in Figure 1.

and entropy (ΔSobs) for H2O2 adsorption on CaCO3 and SiO2 can be obtained by the following equation31,32 ⎛ γ ⎞ ΔHobs ΔSobs + ln⎜⎜ BET ⎟⎟ = − RT R ⎝ 1 − γBET ⎠

(3)

consequently, from the plot the left side of eq 3 versus inverse temperature, as show in Figure 3, the enthalpy (ΔHobs) and

⎛ k ⎞ Fdes(t ) = F(t )⎜1 + ini ⎟ − F0 kesc ⎠ ⎝

(6)

Fdes(t ) = kdesNads(t )

(7)

where Fdes(t) represents the desorption flux of H2O2 from the surface at time t (molecules s−1), kini is the initial rate constant and equal to kads (s−1), kesc is the escape rate constant of the Knudsen cell (s−1), and kesc = ω Ah, where ω is the collision frequency of H2O2 with mineral oxide. kdes is the desorption rate constant (s−1), F(t) is the flux out of the reactor at time t (molecules s−1), F0 is the flux into the reactor (molecules s−1), and Nads(t) is the number of adsorbed molecules on the surface. The number of molecules adsorbed on the surface at time t, Nads(t), can be determined by integrating the QMS signal between t = 0 and the desired time t. The temperature dependence of kdes is shown in Figure 4. With the temperature

Figure 3. Plot to determine the enthalpy (ΔHobs) and entropy (ΔSobs) of hydrogen peroxide on SiO2 and CaCO3 using the initial uptake coefficient.

entropy (ΔSobs) were determined to be −(7.77 ± 1.55) KJ mol−1 and −(105.8 ± 21.2) J K mol−1 for SiO2 and −(9.92 ± 1.98) KJ mol−1 and −(98.6 ± 19.7) J K mol−1 for CaCO3, respectively. In Figure 3, the average initial uptake coefficients and desorption rate constants in Table 1 were used. The small ΔHobs value can also demonstrate that H2O2 only weakly adsorbs on SiO2 and CaCO3. The empirical formula between γBET of hydrogen peroxide on CaCO3 and SiO2 with temperature was calculated as exp(934.5/T − 12.7) γBET(SiO2 ) = 1 + exp(934.5/T − 12.7)

γBET(CaCO3) =

exp(1193.0/T − 11.9) 1 + exp(1193.0/T − 11.9)

Figure 4. Plot to determine the dependence of desorption rate (Kdes) constant on temperature.

increasing, the calculated kdes values are also shown in Table 1. According to the slope of this plot, the activation energy for desorption (Edes) of H2O2 on SiO2 and CaCO3 was calculated to be (9.15 ± 0.11) and (5.9 ± 0.9) KJ mol−1, respectively. Because of the small value of the uptake coefficient on SiO2, which would cause larger error to the calculation of Edes at high temperatures, so only the data obtained at low temperatures were used here. The initial uptake coefficients of hydrogen peroxide on SiO2 and CaCO3 resembled the other mineral oxides, which can represent an important sink of hydrogen peroxide, especially at low temperature. At 253 K, the initial uptake coefficients of SiO2 and CaCO3 were approximately two

(4)

(5)

where T is temperature (K). Thus, the γBET at the other temperature can be obtained using this equation. Table 1 showed the uptake coefficient and desorption rate constants (Kdes) measured at different temperatures. The concentration of H2O2 in the reactor was about 30 ppb. The 7962

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10−5 cm2 cm−3. Our measured uptake coefficients are about 7.11 × 10−4 and 12.6 × 10−5 at 253 K for CaCO3 and SiO2, respectively, which lead to the corresponding atmospheric lifetimes with respect to processing by CaCO3 and SiO2 of 2.0 h to 2.5 days and 11.1 h to 14.3 days, respectively. The lifetimes of H2O2 via uptake onto mineral aerosols at different temperature are listed in Table 2. The uptake coefficient

times larger than those at 298 K. However, the heterogeneous loss on aging SiO2 and CaCO3 cannot consume more hydrogen peroxide because of the easy saturation on these mineral surfaces. The relatively small Edes value also confirms the weak adsorption of hydrogen peroxide on SiO2 and CaCO3.

4. CONCLUSIONS AND ATMOSPHERIC IMPLICATIONS In this work, the uptake coefficients of hydrogen peroxide on SiO2 and CaCO3 were investigated over a temperature range from 253 to 313 K using a Knudsen cell reactor. The initial uptake coefficients decreased with an increase in temperature for the two mineral aerosols. While the uptake coefficients of SiO2 and CaCO3 showed no observable dependence on the concentration of hydrogen peroxide. The initial uptake coefficients varied significantly with temperature. At 253 K, the initial uptake coefficients of SiO2 and CaCO3 were approximately two times larger than those at 298 K, and it can be expressed as eqs 4 and 5. On the basis of the temperature dependence of uptake coefficients, the enthalpy (ΔHobs) and entropy (ΔSobs) were determined to be −(7.77 ± 1.55) KJ mol−1 and −(105.8 ± 21.2) J K mol−1 for SiO2 and −(9.92 ± 1.98) KJ mol−1 and −(98.6 ± 19.7) J K mol−1 for CaCO3, respectively. The activation energies for desorption (Edes) of H2O2 on SiO2 and CaCO3 were calculated to be (9.15 ± 0.11) and (5.9 ± 0.9) KJ mol−1, which were quite small and confirmed the weak adsorption of hydrogen peroxide on SiO2 and CaCO3. Hydrogen peroxide could mainly be adsorbed on SiO2 and CaCO3 reversibly in this temperature region, and the initial uptake coefficients varied evidently with temperature. For these two mineral aerosols, only fresh ones can represent a significant sink of hydrogen peroxide, while the heterogeneous loss ability on aging SiO2 and CaCO3 was limited, because of the easy saturation on these mineral surfaces. However, the initial uptake can provide an oxide surface for the further complex reactions, especially at low temperatures. The values of uptake coefficients for hydrogen peroxide were assumed in the range of 2.0 × 10−4−1.0 × 10−5 in Zhang et al. box model study.35 They found that the presence of the dust resulted in decreases in the concentration of HxOy by 11−59%. As the uptake coefficient was based on the projected, geometric surface area of the dust sample, it represents an upper limit of the uptake progress.36 Zhao et al. studied the interaction between hydrogen peroxide and two major components of mineral dust aerosol SiO2 and α-Al2O3, using T-FTIR spectroscopy and HPLC.22 The uptake coefficients that they observed represented the steady state rather than the initial state. Because of the diversity of measure methods, the Knudsen cell reactor can detect a lower concentration of oxidative gas approaching 30 ppb, which is comparable to the real one in the atmosphere.9 The rate of removal of hydrogen peroxide by uptake onto mineral aerosol can be estimated in a simple model. The lifetime of hydrogen peroxide due to uptake onto SiO2 and CaCO3 can be estimated by τ=

4 γ c ̅A

Table 2. Atmospheric Lifetime of Hydrogen Peroxide by Uptake onto Mineral Aerosols hydrogen peroxide atmospheric lifetimes temp (K)

SiO2

253 268 283 298 313

11.1 15.1 17.9 20.3 23.0

h h h h h

to to to to to

14.3 19.5 23.0 26.1 29.6

CaCO3 days days days days days

2.0 2.2 2.6 3.8 4.7

h h h h h

to to to to to

2.5 2.8 3.3 4.9 6.0

days days days days days

calculated from this experiment was based on BET surface area of the dust sample, which represents a lower limit. As compared with the lifetime of 1 day for photolysis of hydrogen peroxide, the interaction between mineral aerosol and hydrogen peroxide can still significantly influence the concentration of hydrogen peroxide in the atmosphere, especially at low temperature.

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AUTHOR INFORMATION

Corresponding Author

*Tel: 86-10-62558682. Fax: 86-10-62559373. E-mail: [email protected] (W.-G.W.). Tel: 86-10-62554518. Fax: 86-10-62559373. E-mail: [email protected] (M.-F.G.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was supported by the National Basic Research Program of China (973 Program, No. 2011CB403401) of Ministry of Science and Tech nol ogy of China, Knowledge Innovation Program (Grant No. KJCX2-EW-H01) of the Chinese Academy of Sciences, and the National Natural Science Foundation of China (Contract Nos. 40925016, 40830101, 21077109, 41005070, 41173112, and 2190052).

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REFERENCES

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(8)

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