- Email: [email protected]
gas reaction rates by microparticle Raman spectroscopy
J. Aerosol Sci., Vol. 23, Suppl. I, pp. $429-$432, 1992
0021-8502/92 $5.00 + 0.00 Pergamon Press Ltd
Printed in Great Britain.
M e a s u r e m e n t of Aerosol/Gas Reaction Rates by Microparticle R a m a n Spectroscopy E. J. DAVIS, S. D. RASSAT and W. FOSS Department of Chemical Engineering, BF-10, University of Washington Seattle, Washington 98195, USA
ABSTRACT Chemical reactions between a reactive gas and single solid and liquid aerosol particles have been studied using an electrodynamic balance coupled to a Raman spectrometer. Electrodynamic levitation was used to suspend a single charged microparticle in a polarized laser beam, which was used as the light source for spontaneous Raman emission. The highly exothermic reaction between bromine vapor and an olefin microdroplet, 1-octadecene, and the reaction between a sorbent particle, calcium oxide, and a humid air stream containing sulfur dioxide have been exploredby following the formation of a Raman-active chemical bond. KEYWORDS Aerosol reactions; light scattering; microparticle chemistry; Raman spectroscopy. INTRODUCTION Although the physics of aerosols has been studied very extensively, the literature on aerosol chemistry is relatively sparse, particularly if one excludes gas-phase reactions which form aerosols. Chemical reactions between reactive gases and pre-existing aerocolloidal particles occur in the atmosphere and in industrial processes. For example,HNO3 is formed in the atmosphere by reaction of N205 with water vapor on sulfate aerosols, and SO2 can be removed from stack gases either by means of Ca(OH)2 slun'y sprays or by dry-scrubbing the SO2 with CaO in a fluidized bed coal combustor. This paper deals with such gas/liquid and gas/solid reactions. Aerosol science can also be used to study reactions that can be very difficult to study in bulk. Examples are the reaction between linear alkyl benzene and SO3, which is used to produce detergents, and the reaction between an olefin and bromine, which is used to determine the extent ot unsaturation of a hydrocarbon. Both reactions are highly exothermic and readily overheat when carried out in bulk. The large surface-to-volume ratio of an aerosol droplet mates it possible to explore such reactions isothermally, for heat is readily transferred to the surrounding gas from a microdroplet. Within the past decade techniques have been developed to exolore the chemistry of microparticles with masses as low at 10-9 g. Thurn and Kiefer (19~84a, b) were the first to obtain Raman spectra for levitated droplets using the optical levitation principle introduced by Ashkin and Dziedzic (1971), and since that time numerous investigators have obtained Raman spectra for static systems. The spectra published by Thum and Kiefer showed that the Raman scattering from a microdroplet is much more complicated than comparable measurements obtained conventionally using bulk samples in standard cells. The complexity is due to morphological resonances, which correspond to high internal electric fields generated by the interaction of the laser light source with the droplet. These morphological resonances lead to enhancement of the Raman signal, and they are particularly sensiuve to size and refractive index changes. Solid microparticles do not exhibit resonances.
$429
$430
E.J. DAVISet
al.
It was the objective of this research to appl~¢ Raman scattering techniques to the study of .chemical reactions involving microdroplets andmicroparticles. The reactions of interest are the bromination of 1-octadecene and the reaction of a calcium oxide particle with water vapor and SO2 in a stream of either nitrogen or air. APPARATUS Electrodynamic balances were coupled to a Raman spectrometer for this research, and Fig.1 is an overhead view of the equipment. A linear photodiode array was mounted on the balance chamber to record phase funcUons (scattered intensity as a function of angle) of microdroplets, and morphological resonance spectra were recorded with a photomultiplier tube (PMT) mounted at a right angle to the laser beam. The particle was illuminated from below with an argon-ion laser operala~ngat a wavelength of 488 nm. The Raman-scattered light was collected with the lens system shown (labeled optics) and focused on the slit of a double monochromator. The Raman signal was detected with either a photomultiplier tube (PMT) with photon-counting electronics or an optical multichannel analyzer (OMA). VIDEO MONITOR
MIRROR
t COMPUTER
I
CCD CAMERA
I
PMT
DOUBLE MONOCHROMATOR
SOURCE
OPTICS ::L_._
\
acSOURCE J PHOTODIODE ARRAY
ELECTRODYNAMIC BALANCE
Fig. 1. An overview of the apparatus used for the experiments. The double ring electrodynamic balance used for the calcium oxide experiments had the ac and de potentials needed for levitation superposed on the two rings. A second balance described by Buehler (1991) was used for the bromination experiments. In his chamber the ac potential was applied to two rings, and the de potential was applied to stainless steel mesh electrodes through wliich the gas flowed. A solid particle or microdroplet was introduced into the balance chamber through an opening in the top of the balance, the balance was then sealed, and gas was passed upwardthrough the chamber. In most of the experiments the carrier gas was mtrogen, but for some ot the calcium oxide experiments humid air was used. BROMINATION RESULTS For four reasons the OCT/Br2 reaction is a good model reaction with which to develop experimental techniques: (i) there is a large mass change associated with the reaction, which m ~ e s gravimetric an~y.sis possble (the molecular weights of OCT and DBO are 252.49 and 412.30, respectively), 0i) there is a significant refractive index difference between OCT (N = 1.4448) and DBO (N = 1.4800), (iii) infrared spectroscopy can be used to characterize the reactant and product if a sufficient amount of material is available, and (iv) the C----Cbond and the C-Br bond are Raman-active so that Raman spectroscopy can be applied for chemical analysis. In the bromination exoeriments the monochromator was set to folt6w the formation of the ~-Br bond by recording the'Raman intensity at a wavenumber shift of 650 cm-1.
Measurement of aerosol/gas reaction rates
$431
McRae etal. (1975, 1978) used a laminar flow aerosol generator to study the reaction between bromine vapor and an aerosol of the olefin, 1-octadecene (OCT), which forms 1,2dibromooctadecane (DBO) according to the overall reaction
CH2=CH(CH2)15CH 3 + Br2 ~
CH2Br2CH(CH2)I5CH 3.
(1)
McRae and his associates used infrared spectroscopy to analyze samples of the aerosol collected at the exit of the reactor, and by modeling the process they found that the results were consistent with a reaction tirst order in OCT and thirdorder in dissolved bromine. Taflin and Davis (1990) studied the same reaction using electrodynamic levitation of a single microdroplet together with optical resonance measurements and simultaneous mass measurements to determine the droplet composition as a function of time. Recently, Buehler (1991) re-examined the reaction using Raman spectroscopy and mass measurements to follow the reaction between a levitated OCT droplet and a gas stream containing bromine vapor. We have analyzed these results to model the kinetics of the reaction and to examine the consistency of the results. Typical data of Buehler are presented in Fig. 2 as conversion versus time of reaction. The Raman data of Buehler are more scattered than the resonance data of Taflin and Davis because of the effects of morphological resonances on the Raman scattering, but both sets of data show the same sigmoid shape. Also shown on the figure is the conversion predicted using a model based on first order in OCT and third order in bromine concentration, the model suggested by McRae and his coworkers. The data of Taflin and Davis and of Buehler are consistent with such a model. I
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RAMAN DATA
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-20
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20
'
'
40
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Fig. 2. Comparison of the OCT conversion data of Buehler with predicted conversions. DESULFURIZATION RESULTS At elevated temperatures the reaction between CaO, SO2 and 02 involves the formation of CaSO4, but at room temperature the hemi-hydrate, CaSO3.1/2H20, is produced by reaction in humid air (Rassat and Davis, 1992). Neither of these reactions goes to completion because the product plugs the pores of the porous CaOparticles. We have explored the room-temperature reaction viaRaman spectroscopy and particle mass measurements to determine the extent of the reaction. The room temperature reaction involves the formation of Ca(OH)2 and then reaction of the hydroxide with SO2. Figure 3 displays a sequence of Raman spectra which show the development of the OH bond in the hydroxide and the appearance of the SO3 bond in the hemihydrate as the reaction proceeds.
$432
E.J. DAVaSet al. =SO. 400 300
8 2o0
lOO_ u)
1000
1500
2000
2500
RAMAN SHIFT, c m
3000
3500
.1
Fig. 4. Raman spectra for the reaction between CaO, H;zO and SO2 obtained during the course of the reacuon. REFERENCES Ashkin, A. and J. M. Dziedzic (1971). Optical levitation by radiation pressure, Appl. Phys.
Lett. , 19, 283-285. Buehler, M. F. (1991). Raman spectroscopy of levitated microparticles, Ph.D. Dissertation, University of Washington. McRae, D. D., E. Matijevic and E. J. Davis (1975). Chemical reactions in aerosols I. bromination of octadecene droplets, J. Colloid Interface Sci., 53, 411-421. McRae, D. D., E. Matijevic and E. J. Davis (1978). Chemical reactions in aerosols II. The effects of various parameters on the bromination of 1-octadecene droplets, J. Colloid
Interface Sci., 67,526-537. Rassat, S. D. and E. J. Davis (1992). Chemical reaction of sulfur dioxide with a calcium oxide aerosol particle, J. Aerosol Sci., 23, 165-180. Taflin, D. C. and E. J. Davis (1990). A study of aerosol chemical reactions by optical resonance spectroscopy, J. Aerosol Sci., 21, 73-86. Thurn, R. and W. Kiefer (1984a). Observations of st~alctural resonances in the Raman spectra of optically levitated dielectric microspheres, J. Raman Spectrosc. ,15 411-413. Thum, R and W. Kiefer (1984b). Raman-microsampling technique applying optical levitation by radiation pressure, AppL Spectrosc., 38, 78-83.
0021-8502/92 $5.00 + 0.00 Pergamon Press Ltd
Printed in Great Britain.
M e a s u r e m e n t of Aerosol/Gas Reaction Rates by Microparticle R a m a n Spectroscopy E. J. DAVIS, S. D. RASSAT and W. FOSS Department of Chemical Engineering, BF-10, University of Washington Seattle, Washington 98195, USA
ABSTRACT Chemical reactions between a reactive gas and single solid and liquid aerosol particles have been studied using an electrodynamic balance coupled to a Raman spectrometer. Electrodynamic levitation was used to suspend a single charged microparticle in a polarized laser beam, which was used as the light source for spontaneous Raman emission. The highly exothermic reaction between bromine vapor and an olefin microdroplet, 1-octadecene, and the reaction between a sorbent particle, calcium oxide, and a humid air stream containing sulfur dioxide have been exploredby following the formation of a Raman-active chemical bond. KEYWORDS Aerosol reactions; light scattering; microparticle chemistry; Raman spectroscopy. INTRODUCTION Although the physics of aerosols has been studied very extensively, the literature on aerosol chemistry is relatively sparse, particularly if one excludes gas-phase reactions which form aerosols. Chemical reactions between reactive gases and pre-existing aerocolloidal particles occur in the atmosphere and in industrial processes. For example,HNO3 is formed in the atmosphere by reaction of N205 with water vapor on sulfate aerosols, and SO2 can be removed from stack gases either by means of Ca(OH)2 slun'y sprays or by dry-scrubbing the SO2 with CaO in a fluidized bed coal combustor. This paper deals with such gas/liquid and gas/solid reactions. Aerosol science can also be used to study reactions that can be very difficult to study in bulk. Examples are the reaction between linear alkyl benzene and SO3, which is used to produce detergents, and the reaction between an olefin and bromine, which is used to determine the extent ot unsaturation of a hydrocarbon. Both reactions are highly exothermic and readily overheat when carried out in bulk. The large surface-to-volume ratio of an aerosol droplet mates it possible to explore such reactions isothermally, for heat is readily transferred to the surrounding gas from a microdroplet. Within the past decade techniques have been developed to exolore the chemistry of microparticles with masses as low at 10-9 g. Thurn and Kiefer (19~84a, b) were the first to obtain Raman spectra for levitated droplets using the optical levitation principle introduced by Ashkin and Dziedzic (1971), and since that time numerous investigators have obtained Raman spectra for static systems. The spectra published by Thum and Kiefer showed that the Raman scattering from a microdroplet is much more complicated than comparable measurements obtained conventionally using bulk samples in standard cells. The complexity is due to morphological resonances, which correspond to high internal electric fields generated by the interaction of the laser light source with the droplet. These morphological resonances lead to enhancement of the Raman signal, and they are particularly sensiuve to size and refractive index changes. Solid microparticles do not exhibit resonances.
$429
$430
E.J. DAVISet
al.
It was the objective of this research to appl~¢ Raman scattering techniques to the study of .chemical reactions involving microdroplets andmicroparticles. The reactions of interest are the bromination of 1-octadecene and the reaction of a calcium oxide particle with water vapor and SO2 in a stream of either nitrogen or air. APPARATUS Electrodynamic balances were coupled to a Raman spectrometer for this research, and Fig.1 is an overhead view of the equipment. A linear photodiode array was mounted on the balance chamber to record phase funcUons (scattered intensity as a function of angle) of microdroplets, and morphological resonance spectra were recorded with a photomultiplier tube (PMT) mounted at a right angle to the laser beam. The particle was illuminated from below with an argon-ion laser operala~ngat a wavelength of 488 nm. The Raman-scattered light was collected with the lens system shown (labeled optics) and focused on the slit of a double monochromator. The Raman signal was detected with either a photomultiplier tube (PMT) with photon-counting electronics or an optical multichannel analyzer (OMA). VIDEO MONITOR
MIRROR
t COMPUTER
I
CCD CAMERA
I
PMT
DOUBLE MONOCHROMATOR
SOURCE
OPTICS ::L_._
\
acSOURCE J PHOTODIODE ARRAY
ELECTRODYNAMIC BALANCE
Fig. 1. An overview of the apparatus used for the experiments. The double ring electrodynamic balance used for the calcium oxide experiments had the ac and de potentials needed for levitation superposed on the two rings. A second balance described by Buehler (1991) was used for the bromination experiments. In his chamber the ac potential was applied to two rings, and the de potential was applied to stainless steel mesh electrodes through wliich the gas flowed. A solid particle or microdroplet was introduced into the balance chamber through an opening in the top of the balance, the balance was then sealed, and gas was passed upwardthrough the chamber. In most of the experiments the carrier gas was mtrogen, but for some ot the calcium oxide experiments humid air was used. BROMINATION RESULTS For four reasons the OCT/Br2 reaction is a good model reaction with which to develop experimental techniques: (i) there is a large mass change associated with the reaction, which m ~ e s gravimetric an~y.sis possble (the molecular weights of OCT and DBO are 252.49 and 412.30, respectively), 0i) there is a significant refractive index difference between OCT (N = 1.4448) and DBO (N = 1.4800), (iii) infrared spectroscopy can be used to characterize the reactant and product if a sufficient amount of material is available, and (iv) the C----Cbond and the C-Br bond are Raman-active so that Raman spectroscopy can be applied for chemical analysis. In the bromination exoeriments the monochromator was set to folt6w the formation of the ~-Br bond by recording the'Raman intensity at a wavenumber shift of 650 cm-1.
Measurement of aerosol/gas reaction rates
$431
McRae etal. (1975, 1978) used a laminar flow aerosol generator to study the reaction between bromine vapor and an aerosol of the olefin, 1-octadecene (OCT), which forms 1,2dibromooctadecane (DBO) according to the overall reaction
CH2=CH(CH2)15CH 3 + Br2 ~
CH2Br2CH(CH2)I5CH 3.
(1)
McRae and his associates used infrared spectroscopy to analyze samples of the aerosol collected at the exit of the reactor, and by modeling the process they found that the results were consistent with a reaction tirst order in OCT and thirdorder in dissolved bromine. Taflin and Davis (1990) studied the same reaction using electrodynamic levitation of a single microdroplet together with optical resonance measurements and simultaneous mass measurements to determine the droplet composition as a function of time. Recently, Buehler (1991) re-examined the reaction using Raman spectroscopy and mass measurements to follow the reaction between a levitated OCT droplet and a gas stream containing bromine vapor. We have analyzed these results to model the kinetics of the reaction and to examine the consistency of the results. Typical data of Buehler are presented in Fig. 2 as conversion versus time of reaction. The Raman data of Buehler are more scattered than the resonance data of Taflin and Davis because of the effects of morphological resonances on the Raman scattering, but both sets of data show the same sigmoid shape. Also shown on the figure is the conversion predicted using a model based on first order in OCT and third order in bromine concentration, the model suggested by McRae and his coworkers. The data of Taflin and Davis and of Buehler are consistent with such a model. I
~
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i
I
I
i
1.0
0.8
Z O O
I
"8
•
WEIGHTDATA
o
RAMAN DATA
m
m
tlJ
i
O
0.6 -
I
MODEL
7
I/
/oO 0.4'
o
-20
*
0
20
'
'
40
'
I
60
I
80
TIM E, s
Fig. 2. Comparison of the OCT conversion data of Buehler with predicted conversions. DESULFURIZATION RESULTS At elevated temperatures the reaction between CaO, SO2 and 02 involves the formation of CaSO4, but at room temperature the hemi-hydrate, CaSO3.1/2H20, is produced by reaction in humid air (Rassat and Davis, 1992). Neither of these reactions goes to completion because the product plugs the pores of the porous CaOparticles. We have explored the room-temperature reaction viaRaman spectroscopy and particle mass measurements to determine the extent of the reaction. The room temperature reaction involves the formation of Ca(OH)2 and then reaction of the hydroxide with SO2. Figure 3 displays a sequence of Raman spectra which show the development of the OH bond in the hydroxide and the appearance of the SO3 bond in the hemihydrate as the reaction proceeds.
$432
E.J. DAVaSet al. =SO. 400 300
8 2o0
lOO_ u)
1000
1500
2000
2500
RAMAN SHIFT, c m
3000
3500
.1
Fig. 4. Raman spectra for the reaction between CaO, H;zO and SO2 obtained during the course of the reacuon. REFERENCES Ashkin, A. and J. M. Dziedzic (1971). Optical levitation by radiation pressure, Appl. Phys.
Lett. , 19, 283-285. Buehler, M. F. (1991). Raman spectroscopy of levitated microparticles, Ph.D. Dissertation, University of Washington. McRae, D. D., E. Matijevic and E. J. Davis (1975). Chemical reactions in aerosols I. bromination of octadecene droplets, J. Colloid Interface Sci., 53, 411-421. McRae, D. D., E. Matijevic and E. J. Davis (1978). Chemical reactions in aerosols II. The effects of various parameters on the bromination of 1-octadecene droplets, J. Colloid
Interface Sci., 67,526-537. Rassat, S. D. and E. J. Davis (1992). Chemical reaction of sulfur dioxide with a calcium oxide aerosol particle, J. Aerosol Sci., 23, 165-180. Taflin, D. C. and E. J. Davis (1990). A study of aerosol chemical reactions by optical resonance spectroscopy, J. Aerosol Sci., 21, 73-86. Thurn, R. and W. Kiefer (1984a). Observations of st~alctural resonances in the Raman spectra of optically levitated dielectric microspheres, J. Raman Spectrosc. ,15 411-413. Thum, R and W. Kiefer (1984b). Raman-microsampling technique applying optical levitation by radiation pressure, AppL Spectrosc., 38, 78-83.