Journal of Physical Chemistry and Electrochemistry
Journal homepage: http://journals.miau.ac.ir/jpe
Journal of Physical Chemistry and Electrochemistry Vol.2 No.1(2013) 31-37
Islamic Azad University
Marvdasht Branch
*Correspondig Author
E-mail address: hossein.jalali1354@gmail.com
An EPR Study of Heterogeneous Reaction of CH3O2 Radicals with Organic Compounds:
Effect of Organic Compound and Surface Nature
Hossein Jalali Jahromi
Department of Chemistry, Mahshahr Branch, Islamic Azad University, Mahshahr, Iran
Abstract
The reaction of CH3O2 radicals with acetaldehyde on the solid surface of NH4NO3 has been studied in a flow capillary reactor at low pressure and room temperature. The experiments were performed using EPR spectrometer combined with the kinetic method of radicals freezing. Peroxy radicals revealed an oscillation complex behavior during the process regarding to the phenomenon of radicals multiplication. The kinetic peculiarities of process depend intensively on the concentration and the nature of organic compound and aerosol surface. The increase of mass percentage of oxygen of the aerosol substances can be responsible for increasing the amplitude of radicals oscillation behavior.
Keywords: EPR; Peroxy Radical; Organic Compound; NH4NO3; Heterogeneous Reaction
1. Introduction
The aerosol particles play a key role in the atmosphere, not only affecting the atmospheric heterogeneous processes but also impacting local and global climate [1, 2]. Now, it is well known that peroxy radicals (HO2, RO2 with R being an organic group, e.g. CH3) are the key reactive intermediates/chain propagators.
The investigation of the photochemical degradation of volatile organic compounds (VOCs) in the smog chamber in the presence of aerosols [3] revealed that the reaction between any environmentally important trace gases such as CH4, CO with OH and HO2 (RO2) radicals can proceed not only in the gas phase [4] but also on the surface of aerosols. The reactions of CH3CO3 and CH3O2 radicals with CH3CHO and C2H5CHO on the surface of SiO2 and KCl/SiO2 were studied by EPR and IR spectroscopy methods [5, 6].
The study of methane oxidation in a reactor treated with boric acid suggested that methyl hydroperoxide (CH3OOH) is formed during the interaction of CH3O2 radicals with methane on the surface [7].
The participation of radicals in the multistep transformations of organic compounds, in particular ketones, proposed that the free radical pathways involving both organoperoxy and superoxide radicals can be responsible for the thermal and photodecomposition of ketones over TiO2 [8].
In a similar work, the formation of stable surface peroxyacyl radicals was observed
during the co-adsorption of aldehydes with O2 and subsequent UV irradiation on the reduced TiO2 surface at 100 K [9]. Also, it was proposed that the TiO2 surface can participate in both the initiation and propagation stages of aldehyde photo-oxidation.
An EPR study of the reaction of СН3О2 radicals with CH4 and NO2 on the surfaces of the alkali metal salts [10, 11] with a composition similar to that of atmospheric particles and aerosols revealed a considerable consumption of peroxy radicals while the behavior of radicals was entirely different and complex on the TiO2 surface regarding to occur the wonderful phenomenon of radicals multiplication [12, 13].
These results were used in a heterogeneous model describing the peculiarities of reaction mechanism of peroxy radicals with organic compounds [14, 15].
This paper reports and describes the results of the reaction between CH3O2 radicals and organic compounds (methane and acetaldehyde) on the different surface such as
NH4NO3, TiO2, NaCl, KCl.
2. Experimental
The heterogeneous interaction of peroxy radicals CH3O2 with selected organic compound (methane) on the surface of NH4NO3 representing solid aerosol has been studied. For this purpose a new complex method was developed. All of the experiments were carried out using EPR spectroscopy combined with the radicals freezing kinetic method in a flow capillary reactor coated by the different substances. The experiments were carried out by the laboratory apparatus, shown schematically in Fig. 1.
The СН3О2 radicals were obtained by the heterogeneous radical decomposition of peroxyacetic acid in the presence of trace O2 in a quartz flask (1) via;
CH3CO3H → CH3 + CO2 + OH (1)
CH3 + O2 (trace) → CH3O2 (2)
The injection of CH3CO3H vapor was carried out with the help of the liquid peracid vessel (2) and dosed with a manometer (3). Under the experimental conditions the quantity of peracid was between 2 – 6 torr in the gas phase. CH3CO3H was adsorbed at T = 78 K on the inner surface of the quartz flask (1) with d = 3.8 cm, previously treated with a saturated water solution of cobalt oxide.
СН3О2 radicals were generated into the gas phase by heating the adsorbed peracid on a water bath with heating intervals 1 – 1.5 min. The important difference between the present and previous set-up is the modification of CH3O2 radicals source (1). The radical decay of CH3CO3H was carried out on the surface treated not by boric acid as before [10] but by cobalt oxide. This allows to decrease the temperature of radicals generation source from 513 – 543 K to 340 – 370 K. As a result
Fig. 1. Principle scheme of the set up (1) quartz flask for the adsorption of peracid, (2) vessel of peracid, (3) manometer, (4) reactor, (5) freezing unit, (6) vessel for gas reagents (CH4,CH3CHO), (7) Magnet, (8) resonator of the EPR spectrometer, (9) quartz Dewar vessel, (10) trap for analysis of gas products, (11), (12) bypass, (13) electrical furnace with power supply unit.
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the greater activity of the treated surface in the
reaction of radical decay of peracetic acid and replacing the oil bath with the more harmless water bath which promoted the safety of the work achieved.
The change of the radical yield (Δ[RO2]%) from the heterogeneous peracid decomposition on the cobalt oxide was plotted as a function of the bath temperature in Fig. 2. The optimal temperature for the source of CH3O2 radicals is 333 K.
0510152030405060708090100t, °CΔ[RO2], %
Fig.2.Temperature dependence of radical yield of peracid decomposed on the surface of CoO.
For minimizing the homogeneous interaction the experiments were performed in a flow pyrex capillary reactor (1= 2 cm, d= 0.15 cm) at low partial pressures of reagents (10-1 – 10-2 torr). The geometrical surface of the capillary reactor was approximately 0.94 cm2.
The flow of radicals was forwarded via the reactor (4) into freezing unit (5) placed in the resonator of the EPR spectrometer. The quantity of radicals accumulated in the detection freezing unit was of order of 1015 particles.
In addition, the temperature in the capillary reactor zone (20 °C) was usually lower than the temperature of the radical generation zone. Therefore, the role of secondary processes in the reactor was minimized. The reactor was washed previously with the acid and the distilled water and then its inner surface was covered by the thin film of substance such as TiO2 and NH4NO3 by treating with a 10% aqueous solution. The reagents were injected from the vessel (6) both through the reactor entrance and bypass (11) for controlling the homogeneous reaction. Methane and acetaldehyde purity were 99.99%. The purity of the TiO2 and NH4NO3 powder (rutile) and NH4NO3 were marked as “pure for analysis”.
The procedure of experiments consists of three steps; firstly, it was registered the initial quantity of peroxy radicals fed into the reactor, secondly it was fed the mixture of CH3O2 radicals and organic compound into the reactor and finally it was only fed the CH3O2 radicals into reactor for controlling the reproducibility of results. The interaction of radicals with various reactants was quantified by measuring the consumption of RO2 radicals in relation to their initial quantity. The reproducibility of experimental results was satisfactory.
3. Results and discussion
The absence of the interaction of peroxy radicals with reagents in the capillary reactor was established by the following calculations and the special experiments which the reagents forwarded directly into a bypass (11). These experiments showed that the radicals were not consumed when methane was introduced in the stream after the reactor. The principle condition for the reaction (3) is τ < τ1.
CH3O2 + RH → CH3O2H + R (3)
Where τ is the residence time of reacting mixtures, τ1 is the time of the homogeneous interaction in the reactor and RH is methane or acetaldehyde.
The average pressure of reagents mixture at the input and output of reactor was calculated 6.3 torr and measured 8 ×10-2 torr respectively. The times τ and τ1 is defined by the following expression;
τ = Pr.Vr/W.Po (1)
τ1 = 1/khom [reagent] (2)
Where Pr is the average pressure in the
H. Jalali Jahromi / Journal of Physical Chemistry and Electrochemistry Vol.2 No.1 (2013) 31-37 33
reactor, Vr = 3.5×10-2 cm3 is the volume of the reactor, Po is the average pressure at the entrance of reactor, W = 1cm3s-1 is the flow velocity through the reactor. Pr = 0.7 torr was calculated assuming that the value of Po in the reactor is decreased by the exponential law. Thus, using equation (1)
τ = 3.9 ×10-3 s
The calculations based on the mathematical approach of the vacuum technique [16] showed that the process proceeds in molecular – viscosity regime and the value of τ obtained consequently;
τ = 2.5×10-4 s
The time of τ1 was calculated using the rate constants for methane and acetaldehyde homogeneous reaction [17] at room temperature according to
khom(CH4) = 10-27.8 cm3.molecule-1.s-1
khom(CH3CHO) = 2 × 10-20.3 cm3.molecule-1.s-1
The average concentration of CH4 or CH3CHO equals to 1.1 × 1015 molecules.cm-3 in the reactor. Thus, using equation (2)
τ1(CH4) = 9 × 1011.8 s
τ1(CH3CHO) = 4.5 × 105.3 s
As regard the average experimental value of [CH3O2] was 1.1 ×1013 particles cm-3 the value of the corresponding rate constant for reaction (4) is
CH3O2 + CH3O2 → CH3O + CH3O + O2, (4)
k = 1.66 × 10-13 cm3.molecule-1.s-1 [112] and hence τ1(CH3O2) = 5.5 × 10-1 s. It is evident under experimental condition the time of τ is less than τ1 for both of the reagents as the radical or the molecular nature. Therefore, it is natural to conclude that the reaction proceeds only on the solid surface.
3.1. Effect of organic compound (RH) nature
The influence of RH nature on the reaction with the adsorbed CH3O2 radicals on the TiO2 surface has been studied. The change in the quantity of radicals (Δ[RO2]) has been plotted as a function of the initial quantity of RH (CH4 and CH3CHO) in Fig. 3. The “X” axis shows also the level of the initial quantity of CH3O2 radicals. The quantity of molecular reagent was changed from 4×1015 till 6 × 1017 molecules. The radicals’ concentration is given in arbitrary units.
Fig. 3. Dependence of the change in the quantity of radicals (Δ[RO2]) on the initial quantity
of CH3CHO (curve 1) and CH4 (curve 2) (N × 1016 molecules) on the TiO2 surface,
293 K, [СН3О2]o = 25 arbitrary units.
At small amounts of acetaldehyde the remarkable consumption of radicals was observed, indicating the heterogeneous interaction of peroxy radicals with organic compound on the TiO2 surface. However, the further rise in the acetaldehyde quantity leads to increase the radicals quantity exceeding their initial one. For example at quantity of CH3CHO equals to 8×1016 molecules, Δ[RO2] is 32% approximately. The further increase of molecular reagent quantity leads to the decrease of the radical quantity. Curve 2 (Fig. 3) also shows the behavior of radicals quantity as a function of the quantity of methane [13]. Comparing the curves it is evident that their qualitative picture is the same, indicating the similar complex oscillation behavior of radicals in the case of other class of organic compound.
However, the quantitative picture is different. Not only the decrease of Δ[RO2] in the case of
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acetaldehyde (46%) is higher than methane (32%) but also its degree of the radicals multiplication (32%) is also more than the case of methane (18%). one of the reasons can be a better acetaldehyde adsorption on the surface compared to methane.
The second one is the higher rate constant of the heterogeneous interaction of peroxy radicals with acetaldehyde than methane. It results not only in the highest rate of the initiation stage but also the further chain branching consumption of organic compound.
The ratio of possible values of the effective rate constants of reaction between RO2 radicals and organic compounds in the point of maximal consumption of radicals can be estimated by the next expression;
Δ[RO2] /[RO2]RH = kRH × Δt × [RH] (3)
Hence
kCH3CHO / kCH4 = 5.75
3.2. Effect of surface nature
The influence of the nature of solid surface on the reaction of peroxy radical with organic compound was studied on the example of methane. In Fig. 4 the dependence of the change in the quantity of radicals (Δ[RO2]) on
-80-60-40-200200102030405060708090100Nx 10-16, molecules D[RO2], %(1)(2)(3)
Fig. 4. Dependences of the change in the quantity of radicals (Δ[RO2]) on the initial quantity of CH4 (N × 1016 molecules) on the (1) NaCl, (2) KCl and (3) TiO2 surfaces, 293 K, [СН3О2]o = 25 arbitrary unit.
the initial quantity of methane is shown at 293 K on the NaCl, KCl [10,11] and TiO2 surface.
As seen the shape of curves (1) and (2) differ than curve (3) both quantitatively and qualitatively. In the case of oxygen free surfaces it is observed only the consumption of radicals which is much more in the case of NaCl.
However, in the case of TiO2 alongside with the consumption of radicals due to reaction (3) at small amounts of RH it is observed that the phenomenon of radicals multiplication. It is also observed that the consumption rate of radicals on the TiO2 surface is more than on the oxygen free surfaces. For example, in the fed methane quantity equals to 3.2 ×1016 molecules the consumption of CH3O2 radicals is at least 30 % while on the NaCl surface is about 7 %. Obviously, it can be conclude the rate constant of process on the TiO2 surface is more than the oxygen free surfaces.
The ratio of the possible effective values of the rate constants due to the reaction of RO2 radicals with methane on the different salt surfaces can be estimated by the following expression;
Δ[RO2] /[RO2]salt = ksalt × Δt × [CH4]salt (4)
The calculations have been performed on the base of corresponding data of radical consumption quantity (Fig. 4). For the same concentration of methane and time (3.9 ×10-3 s) it obtains;
k KCl ; k NaCl ; k TiO2 = 0.03 ; 0.06 ; 0.3
It means the maximum heterogeneous consumption of peroxy radicals in the presence of the above mentioned solid surfaces was higher by factors of 1 (KCl); 2 (NaCl); 10 (TiO2).
The influence of the nature of oxygen containing surface on the process was studied on the example of NH4NO3. The change in the quantity of radicals (Δ[RO2]) has been plotted as a function of the initial quantity of methane
H. Jalali Jahromi / Journal of Physical Chemistry and Electrochemistry Vol.2 No.1 (2013) 31-38 35
on the NH4NO3 (curve 1) and TiO2 (curve 2) surfaces in Fig. 5.
-80-60-40-2002040600246810121416N x 10-16, mleculesΔ[RO2] %
(1)(2)
Fig. 5. Dependence of the change in the quantity of radicals (Δ[RO2]) on the initial quantity of CH4 (N × 1016 molecules), on the surface of (1) NH4NO3 and (2) TiO2, 293 K.
Comparing these curves it is obvious that the behavior of peroxy radicals in the case of NH4NO3 differs in relation to the free oxygen surfaces such as NaCl and KCl (Fig. 4). It is similar to the data received from the case of TiO2. However, the amplitude of oscillations of RO2 radicals for the case of NH4NO3 and the degree of phenomenon of radicals multiplication (curve 1) is more than the case of TiO2 (curve 2). In addition, the radicals consumption is more on the NH4NO3 surface than on the TiO2 surface. These results indicate the possible participation of the lattice surface oxygen during the interaction of peroxy radicals with organic compound. One can be estimated the impact of oxygen on the process by considering the mass percentage of oxygen in the compounds used as a surface. For example this ratio for NH4NO3 is 1.5 times more than for TiO2. Thus, it is possible to arrange the sequence of the activity of substances as;
NH4NO3 > TiO2 > NaCl > KCl.
4. Conclusion
The concentration and nature of organic compound play a critical role on the amplitude of radicals’ oscillations and peculiarities of the heterogeneous reaction between peroxy radicals and organic compounds such as the phenomenon of the radicals’ multiplication. In addition, the surfaces nature of aerosol substances can significantly affect on the quantities of active sites and intermediates (ROOH) formed on the surface. When they increases, the radicals generated as a result of the decomposition of ROOH react with methane in the presence of trace oxygen or surface oxygen and cause RH consumption by the short chain branching mechanism. In other words, the solid surface can participate in all stages of mechanism. The increase of mass percentage of oxygen of the aerosol substances can be also responsible for increasing the amplitude of radicals’ oscillations.
Acknowledgment
The author gratefully thanks the Islamic Azad University, Mahshahr Branch for supporting the research project entitled: Interaction of methyl peroxy radicals with organic compounds on the surface of atmospheric aerosol substances.
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