- Email: [email protected]
Selective photocatalytic destruction of airborne VOCs
Solar Energy Vol. 56, No. 5, pp. 445-453, 1996
Pergamon
PIh S0038-092X (96)00031-X
Copyright © 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038-092X/96 $15.00 + 0.00
SELECTIVE P H O T O C A T A L Y T I C D E S T R U C T I O N O F A I R B O R N E VOCs N A Z I M Z. M U R A D O V , * A L I T - R A I S S I , * t D O N A L D M U Z Z E Y , * C H A R L E S R. P A I N T E R * * a n d M I C H A E L R. K E M M E * * * *University of Central Florida, Florida Solar Energy Center, 1679 Clearlake Road, Cocoa, FL 32922-5703, U.S.A.; **Naval Surface Warfare Center, Indian Head, MD 20640-5035, U.S.A. and ***U.S. Army Construction Engineering Research Laboratories, Champaign, IL 61826-9005, U.S.A. Abstract--This article describes a photocatalytic method for selective oxidation of the airborne nitroglycerine (NG), in the presence of ethanol and acetone vapors at high concentrations (exceeding 1% by volume). Process selectivities toward NG photooxidation were examined for various photoreactor configurations and UV lamps and techniques used for the photocatalyst preparation. In addition, we have studied the effect of temperature, residence time, various additives (e.g. ozone, water vapor, nitrogen), and initial concentration of the solvents in air. Our data indicate that modifying TiO2 with silico-tungstic acid (STA) catalyst results in selective NG oxidation without affecting ethanol and acetone significantly. Platinization of TiO2 showed an adverse effect on NG destruction selectivity. In general, low residence times and initial concentration of oxidants and high temperatures and initial concentration of solvents favor selectivity toward NG destruction. In the case of temperature, our observation can be explained by the temperature dependent gas diffusion and surface processes. In most cases, the yield of ethanol oxidation was generally higher than that of acetone. Results from the bench-scale experiments using artificial UV light sources were used to build and test a solar photocatalytic oxidation reactor for selective NG treatment in the presence of ethanol and acetone. Copyright © 1996 Elsevier Science Ltd.
1. I N T R O D U C T I O N O r d n a n c e m a n u f a c t u r i n g p l a n t s for the p r o d u c t i o n of energetic m a t e r i a l s (e.g. nitroglycerine, N G ) often use volatile o r g a n i c c o m p o u n d s ( V O C s ) such as e t h a n o l a n d acetone. These solvents are r e m o v e d at v a r i o u s stages of the process. Thus, m a n y significant p o i n t - s o u r c e s of air p o l l u t i o n are p r o d u c e d that are subject to the p r o v i s i o n s of the Clean Air Act of 1990. A n y strategy for the c o n t r o l of these emissions m u s t a c c o u n t for small a m o u n t s of N G c o - e m i t t e d with e t h a n o l a n d acetone. T h e toxicity of N G a n d o t h e r nitrate esters is well k n o w n ( U r b a n s k i , 1965). A c e t o n e and, especially, ethanol are n o t as toxic as N G . In fact, the U.S. E n v i r o n m e n t a l P r o t e c t i o n Agency ( E P A ) has recently g r a n t e d a p e t i t i o n deleting acetone from the list of h a z a r d o u s chemicals on the Toxics Release I n v e n t o r y (C&E News, 1995). A c c o r d i n g to the EPA, acetone has such a low p h o t o c h e m i cal activity that it presents no concern for f o r m a t i o n of t r o p o s p h e r i c o z o n e or o t h e r air pollutants. There are m a n y a p p l i c a t i o n s where the recovery of the solvents m a y present a m o r e a t t r a c t i v e o p t i o n t h a n their a b a t e m e n t along with the a i r b o r n e N G . If solvent v a p o r s are present in the air at high levels, they m a y be recovered, cost effectively, by any n u m b e r of c o n v e n t i o n a l techniques such as c a r b o n a d s o r p tion, c o n d e n s a t i o n , etc. H o w e v e r , because of tAuthor to whom correspondence should be addressed.
safety c o n s i d e r a t i o n s a n d p r o d u c t purity, recovery of the solvents m u s t p r o c e e d by selective a n d q u a n t i t a t i v e destruction of nitroglycerine. In s u m m a r y , the concept of selective d e s t r u c t i o n of a i r b o r n e h a z a r d o u s o r g a n i c c o m p o u n d s ( H O C s ) m a y be plausible when in the multic o m p o n e n t wastestream: (1) C o n c e n t r a t i o n of V O C s c o - e m i t t e d with N G are high e n o u g h to w a r r a n t their recovery. (2) C o n c e n t r a t i o n of V O C s c o - e m i t t e d with N G are low b u t some or all of the solvents can be r e g a r d e d as n o n - t o x i c and, thus, only the low-level toxic c o m p o n e n t of the flow stream needs to be treated. The situation described a b o v e provides a unique o p p o r t u n i t y for the a p p l i c a t i o n of p h o t o catalysis. This is so because the c o n v e n t i o n a l chemical t r e a t m e n t m e t h o d s such as incineration a n d t h e r m o c a t a l y t i c o x i d a t i o n convert all of the organic constituents of the w a s t e s t r e a m indiscriminately. By selective t r e a t m e n t of the target c o m p o u n d s , p o t e n t i a l l y large savings in the energy a n d capital a n d o p e r a t i n g costs of the process m a y be realized. In m u l t i - c o m p o n e n t systems, the i n t e r p l a y between the n o n - t a r g e t e d c o m p o n e n t s such as e t h a n o l a n d acetone and t a r g e t e d H O C s (e.g. N G ) m u s t be considered carefully. The use of n e a r - U V r a d i a t i o n a n d p h o t o c a t a lysts for d e s t r u c t i o n of a i r b o r n e V O C s is well known, as D i b b l e a n d R a u p p (1992), Peral a n d 445
446
N.Z. Muradov et al.
Ollis (1992), T-Raissi and Muradov (1993), and Turchi et al. (1994) have noted. However, researchers Muradov and T-Raissi (1993), A1-Ekabi et al. (1993), and Weedon (1994) report in most cases complete mineralization of all HOCs using a variety of photoreactor designs that contain immobilized titania and low pressure mercury lamps (LPML). With few exceptions [e.g. T-Raissi and Muradov (1993)], most of the reported work deals with singlecomponent wastestreams or multi-component systems with constituents all belonging to the same class of chemicals, e.g. aromatic hydrocarbons (BTEX), chlorinated or oxygenated hydrocarbons, etc. A vast majority of toxic air emissions from industrial sources are multicomponent wastestreams containing HOCs of wildly different physical and chemical properties. Multi-component emission at the ordnance manufacturing facilities is but one example of such situations. The fact that the physical and chemical nature of HOCs in a given wastestream may be vastly different tends to complicate the engineering design of a selective photocatalytic treatment system. For example, the energy efficiency of the photocatalytic process is affected to a large extent by the destruction and removal efficiency (DRE) of the most resilient targeted compound of the wastestream. In general, the destruction selectivities for a multi-component system depend on the following parameters: reaction temperature; light intensity; wavelength of radiation; type of photocatalyst and co-catalyst used; other gas-phase constituents/ additives (e.g. ozone, water vapor, etc.). Studies involving selective photocatalysis of multi-component wastestreams containing HOCs with wildly different physico-chemical properties are scarce. With few exceptions, e.g. Muradov and T-Raissi (1994), no selective photocatalytic detoxification study involving multicomponent wastestreams has been reported in the literature. The main objective of this work was then to conduct a systematic study to determine the feasibility of selective photocatalytic oxidation of a multi-component wastestream in air, containing ethanol, acetone and NG. In particular, we were interested in utilizing solar energy for selective destruction of airborne N G using simple photoreactor designs with TiO2 directly immobilized onto the reactor wall. 2. EXPERIMENTAL Main features of the system are briefly discussed below. A more detailed description of
our experimental setup can be found in T-Raissi and Muradov (1993), Muradov and T-Raissi (1993), and Painter et al. (1993). The experimental setup consisted of a reagent delivery system (RDS), flow photoreactor and analytical instrumentation. The reagents feed system was designed to provide a uniform airstream containing VOCs and N G at all concentration levels. NG, ethanol and acetone were introduced at desired flow rates into the airstream, passing through a heated Pyrex manifold, using a Sage model 341B syringe pump. The airborne NG/VOCs were carried into the photoreactor maintained at a pre-set temperature (between 80-100°C). The concentrations of NG, ethanol and acetone in air varied in the following ranges: 1-10ppmv, 740-8700ppmv and 370-4350ppmv, respectively. N G was introduced into the mixing manifold in 0.33-0.45% (by weight) blended solutions of ethanol/acetone mixture. Residence times within the photocatalytic reactors have been corrected to account for elevated temperatures. Several photoreactor modifications were used. Some of the reactors were illuminated from within (internal radiation) and others were lit from outside (external radiation), in both photocatalytic and photo-thermocatalytic experiments. The volume of photoreactors varied from 20 mL to approximately 1 L. The wall temperatures for the photoreactors and mixing manifold were carefully controlled and maintained in a temperature range of 80-100°C. We used Pyrex ® glass, optical grade quartz and aluminum in the fabrication of the photoreactors. In photothermocatalytic experiments, the UV lamp and quartz photoreactor were placed within a temperature controlled jacket with highly reflective walls. This arrangement allowed a more uniform illumination of the photoreactor while keeping the temperatures within the cavity at a desired range, 80-200°C. To eliminate the temperature effects on the intensity of UV light produced by the lowpressure mercury lamp (LPML), the lamp's cold-spot temperature was maintained at 25°C at all times by keeping one end of the lamp always at ambient temperature. The advantage of this light source-photoreactor arrangement was that it mimicked the manner in which our solar-driven system was constructed, i.e. the UV transparent photoreactor was placed within a parabolic trough. Therm0catalytic experiments were carried out in three annular reactors, i.e. Pyrex, quartz and
Selectivephotocatalyticdestruction of airborne VOCs aluminum tubes, 6 mm ID and reaction volumes that varied from 15 to 19 mL. In all three reactors, the photocatalyst was deposited onto the inner surface of the outer wall of the reactor. Beside neat titania, we experimented with two modifications of the as-received powder, i.e. doping with platinum and silico-tungstic acid H4SiWllO40.nH20 (STA). The catalyst loading densities on the photoreactor wall varied between 0.1 and 1.0 mg cm -2. Doping of TiO2 with 0.1-0.5 wt% platinum was accomplished by reduction of chloroplatinic acid (H2PtC16) solution with hydrogen at 60°C and in the presence of 10 wt% TiO/suspension. STA/TiO2 photocatalysts were prepared at 1/10 to 1/1 weight ratios by depositing TiO2 onto the inner wall of the photoreactor followed by treatment of the surface with 0.1 M STA solution and then drying the surface by passing a stream of hot air through the tube. In some experiments, 0.1 wt% HEPtCI6 solution was used on the aluminum reactor wall so that a uniform coverage of the surface with metallic Pt was achieved. We used TiO2 (Degussa P25, BET surface area of 50 m z g- 1, average particle size of 30 nm, primarily anatase crystalline form) for all catalyst preparations. We also used Fisher, Optimagrade acetone, food-grade ethanol (Florida Distillers), Fisher brand SiO2.12WO3.26HzO, and ACS-certified HzPtC16 from Aldrich Chemicals. Spirits of 5 wt% N G in ethanol were shipped directly from Indian Head Division, Naval Surface Warfare Center, MD, and used as such without further treatment or purification. A low pressure Hg lamp, germicidal with nominal power consumption of 40 W, was used as the light source in all of the photocatalytic experiments. The light intensities were measured using a potassium ferrioxalate actinometer. Solar experiments were conducted in a quartz photoreactor placed within a reinforced aluminum foil trough in a non-tracking configuration and achieved a concentration factor of approximately 2. The temperature inside the solar photoreactor reached as high as 60°C. Clearly, introducing a simple parabolic trough arrangement into the design of a solar-assisted photocatalytic detoxification system improves its operating performance significantly. A detailed description of the GC-MS method used for the analysis of nitrate esters is given in T-Raissi and Muradov (1994). Briefly, the analytical system consisted of a capillary GC, connected to Varian Saturn II ion-trap MS system. The GC column was a J&W fused silica capil-
447
lary column, 15 m, ¼ mm ID with 1 #m coating of DB-1. Fixed gases and volatile organics were analyzed on a packed column (9.14 m, 3.2 mm O D Hayesep Ds) using a Varian GC 3400 equipped with FID and TCD. NO2 and N O were analyzed using a T E C O model 46 chemiluminescence analyzer. The chemiluminescence analyzer was capable of NO/NO2 detection at concentration levels as low as 0.5 ppbv. Instrument responses for many compounds including CO, CO2, C H 4, C 2 H 6, C H 3 C H O , CzHsOH, CH3COCH3, CH3COOH, NO and NOa were calibrated using certified master gases provided by Scott Speciality Gases. Reaction products were cryogenically trapped and manually injected into the GC column for analysis. We used 0.432mg Nitrostat ® pills (produced by Parke-Davis) in 20 mL Fisher Optima acetone for quantifying NG. The Naval Surface Warfare Center, Indian Head Division supplied 4.998% glycerol 1,3-dinitrate (I&3-GDN), 4.997% glycerol 1,2-dinitrate ( I & 2 - G D N ) and 5.000% glycerol 1-mononitrate (1-GMN) in ethanol standards. Optical spectra of NG, STA and other compounds as well as the quantitative analysis of nitrate ions were conducted using a Milton Roy model 601 Spectrophotometer. 3. RESULTS AND DISCUSSION 3.1. Effect of Residence Time Since reaction kinetics of the photogenerated active species (presumably, O H radicals) with ethanol and acetone differ considerably [kEtOH = 1.9 x 10 9 L mol-1 s - l , kacetone= 1.1 x 108 L mo1-1 s -1, from Buxton et al. (1988)], one can expect that the residence time should affect the photocatalytic process selectivity. Note that no kNa values are available for nitroglycerine. However, if the process is mass transfer or adsorption rate controlled, reactants transport to the catalyst surface and adsorption kinetics of each individual component affect the overall process selectivity significantly. We conducted several experiments to assess competitive adsorption rates of NG, ethanol and acetone on TiO2 immobilized onto fiberglass mesh. Airborne N G (14 ppmv), ethanol and acetone (both 600 ppmv) were introduced at room temperature into a continuous flow reactor with TiO2/fiberglass mesh (residence time 48s). Breakthrough half-lives for acetone and ethanol (that is, the time at which the concentrations of the acetone and ethanol in the effluent gas reached 300 ppmv) were found to be 25 and 75
448
N . Z . M u r a d o v et al.
min, respectively. No N G was detected in the effluent air streams even after 90 min through the experiment. These results demonstrate the following order of adsorption rate of the components: N G > ethanol > acetone. Similar results were obtained by Peral and Ollis (1992) for competitive adsorption of acetone and 1-butanol onto TiO2 surface. The researchers report an adsorption rate of 1-butanol that was one order of magnitude higher than that observed for acetone. Again, this shows strong adsorption of alcohols on metal oxide surfaces.
100
Ethanol
80
o~ E O
60
,= c
40
8 20
0
0
,
tion of airborne NG, ethanol and acetone was carried out using a tubular reactor made of both glass (Pyrex) and aluminum. The main goals of these experiments were: (i) to assess the relative importance of thermocatalytic (at the temperature range of 80-100°C) versus photocatalytic oxidation and destruction and (ii) to determine the effect of low to moderate temperatures (125-200°C) on the kinetics and selectivity of oxidation of a multi-component waste stream. Figure l(a) and (b) depicts the yields of oxidation/destruction of three-component mixture as a function of residence time (at 150°C) and temperature (at residence time of 5.8 s) using Pt on TiO2 catalyst. In the presence of 0.1 wt% Pt/TiO2, at 150°C and residence time of 1.7 s, we observed 96.4% N G destruction but no acetone conversion and only 27.1% of ethanol oxidized. G o o d selectivity toward N G conversion could be achieved at lower temperatures and higher residence times using 0.5 wt% Pt on TiO2 catalyst. For example, at 100°C and residence time of 5.8 s we observed 96.6%, 19.6% and 0% oxidation yields for NG, ethanol and acetone, respectively. The yield of NG/ethanol/ acetone thermocatalytic oxidation in the presence of TiO2 as a function of temperature is given by Fig. 2. TiO2 demonstrated better selectivity towards N G destruction in the presence of ethanol and acetone at high residence times than Pt/TiO2 catalyst. At 125°C and residence time 29 s TiO2 revealed the highest selectivity towards N G destruction: the oxidation yields were found to be 99.1%, 6.1% and 1.4% for NG, ethanol and acetone, respectively. Platinized aluminum tube showed the lowest selectivity amongst all tested catalysts (see Fig. 3). We conducted a few thermocatalytic experiments at high solvent concentration using
o~
80
co
60
~
,o
~
30
time, s
Residence
lo0
,
20
10
(a)
3.2. Effect of temperature 3.2.1. Effect of temperature on thermocatalytic oxidation of NG/VOCs. Thermocatalytic oxida-
.t
NG
,
~ . .
NG /
Ethanol
20
0
,
so
J=
,
LA,
i
A
i
,
i
J
,
i
lOO 120 140 160 180 2o0 22c
(b)
Temperature. °C
Fig. 1. Thermocatalytic oxidation of NG/ethanol/acetone using Pt on TiO 2 catalyst in tubular Pyrex reactor, volume: 14.7 mL, [ N G ] = 8 . 7 p p m v , [etbanol]=818ppmv, [acetone]=409 ppmv. (a) at 150°C temperature (0.1 wt% Pt); (b) residence time of 5.8 s (0.5 wt% Pt). 100
A
A
_=
80 o~ d 60 .9 E
Ethanol
40
8 20 0 ' w 80 100
Acetone
120
140
160
180
200
i
220
Temperature, °C Fig. 2. T e m p e r a t u r e d e p e n d e n c e of NG/ethanol/acetone catalytic o x i d a t i o n in the presence of TiO2, t u b u l a r Pyrex reactor, volume: 14.7 mL, [ N G ] = 7 . 7 ppmv, [ e t h a n o l ] = 724 ppmv, [acetone] = 362 ppmv, residence time = 29.4 s.
Pt(0.5 wt%)/TiO2 and STA/TiO2 deposited onto the inner wall of aluminum tube and Pyrex glass tube. The input concentrations of NG, ethanol and acetone in air were 8.3, 7585 and 3792 ppmv, respectively. At a residence time of
Selective photocatalytic destruction of airborne VOCs
449
N/
~oo I I
.no,
N
60
o
g
o
g o
4O
Acetone
i
20
0
i 0
i 50
0
100
150
200
250
2
3
4
5
7 8 10
Fig. 3. Temperature dependent NG/ethanol/acetone oxidation using platinized tubular aluminum reactor, volume: 15.8 mL, [NG] =8.7 ppmv, [ethanol] =818 ppmv, [acetone]= 409 ppmv, residence time = 6.3 s.
6 s and I50°C, acetone remained practically unconverted while the yield of ethanol oxidation was 37%, which represents a significant drop in the oxidation yield in c o m p a r i s o n with experiments using one order of magnitude lower concentrations of ethanol and acetone. At the same experimental conditions (residence time 6s, 150°C) and the same concentrations of solvents practically no conversion of both ethanol and acetone was observed using STA/TiO2 catalyst.
3.2.2. Effect of temperature on photothermocatalytic oxidation of airborne NG/ethanol/acetone. We used a tubular quartz photoreactor, w r a p p e d with a L P M L inside a reflective and temperature controlled jacket. Photocatalysts (i.e. TiO2 and P t - d o p e d TiO2) were deposited o n t o the inner wall of the p h o t o r e a c t o r and irradiated from outside. Temperatures varied in the range of 80-200°C. O u r data indicate that temperature has a minimal effect on the rate of N G photodestruction. In the range of residence times examined, i.e. 2.3 to 39 s, nitroglycerine underwent almost quantitative conversion with both TiO2 and Pt/TiO2 photocatalysts. Figure 4 depicts the effect of temperature on the yield of ethanol and acetone p h o t o o x i d a t i o n using Pt/TiO2 catalyst. Increasing reaction temperature had a profound effect on the rates of both ethanol and acetone oxidation. This effect was more p r o n o u n c e d for acetone. For example, under U V (254 nm) exposure at a residence time of 2.3 s and 150°C, acetone leaves the p h o t o r e a c t o r essentially intact. P h o t o - t h e r m a l oxidation of N G / e t h a n o l /
20
30
40 50
70
Residence time, s
Temperature, °C
Fig. 4. Oxidation of NG/ethanol/acetone in the presence of 0.1 wt% Pt on TiO2, [NO]=8.5-8.Tppmv, [ethanol]= 799 818 ppmv, [acetone] =399 409 ppmv, tubular reactor irradiated from outside using a germicidal UV lamp, volume: 19.5 mL, (a) photocatalytic at reactor temperature: 87'~C, (b) photothermocatalytic at reactor temperature: 150'C.
acetone in the presence of TiOz also showed an inhibiting effect caused by reactor temperature. Figure 5 shows the temperature dependency of ethanol and acetone oxidation in the presence of T i O / at residence time 2.3 s. Clearly, the reaction temperature has a much more pronounced effect on the rate of acetone oxidation than that of ethanol. Since the lamp's cold spot temperature for both photocatalytic and photo-thermocatalytic experiments was at r o o m temperature, the observed effect can not be explained by the variations in the lamp's emissive power due to temperature fluctuations (which is normally the case when the entire lamp is subjected to ele100
11
II
It
80
E ._o
60
---
t
~
NG
•
Ethanol
/
•
Acetone
/
I 120
U--~-140
i,° 2O
0
I 80
,
I
100
,
/
/
Temperature, °C
Fig. 5. Temperature dependence of TiO2 photothermocatalytic oxidation of NG, ethanol and acetone, ENG] = 8.4 ppmv, [ethanol ] = 789 ppmv, [acetone] = 395 ppmv, residence time=2.3 s, tubular reactor irradiated from outside using a germicidal UV lamp, volume: 19.5 mL.
450
N . Z . M u r a d o v et al.
vated temperatures). This observation can be explained by the temperature dependent gas diffusion and surface processes. It can be expected that temperature affects the rate of adsorption-desorption more strongly than the rate of diffusion from the bulk gas to the catalyst surface. Since adsorption is an exothermic process, an increase in the surface temperature tends to suppress the photocatalytic oxidation rate. This effect is more pronounced in the case of acetone as it is less strongly adsorbed on TiO2 surface than N G or ethanol.
100
|
80
Acet°nli o~ 60, 8 >=
~
/
40
20
3.3. Selective destruction of NG using STA/TiO2 catalyst We studied the effect of catalyst additives such as STA on the selectivity of TiO2 photocatalyzed destruction of NG, ethanol and acetone vapors. The rationale for the modification of TiO2 by STA, known as multi-electron photoredox agent (Papaconstantinou et al., 1986), was to affect yield of the active species formed on the titania surface via interaction of STA molecules with photogenerated electrons and holes. It was found that the deposition of STA on the titania surface hinders ethanol and acetone oxidation but has no effect on the catalyst activity toward N G destruction. Figure6(a) and (b) depicts DREs vs residence time for NG, ethanol and acetone, in the presence of neat TiO2 and STA-treated titania. It can be seen that, at high residence times, all reactants are fully mineralized over TiO2 catalyst. At somewhat lower residence times, only a partial destruction of N G and VOCs is achieved. It is important to note that, at low residence times, NG, ethanol and acetone do not photooxidize to the same extent. Acetone appears considerably less reactive than N G and ethanol. For example, at residence time of 25 s, N G and ethanol conversion exceeds 90%, whereas acetone conversion is well below 10%. Our results indicate that NO2 is the primary N2-containing product of N G decomposition. G C - M S analysis of the effluent did not show any partial N G destruction products such as mono- and dinitroglycerols. Carbon dioxide was the only carbonaceous product of ethanol and acetone oxidation obtained at high residence times (greater than 60 s). For example, at a residence time of 5.5 s, the selectivity (in mol%) to the products of ethanol partial oxidation is: acetaldehyde, 68.1; CO, 0.9; CH4, 0.03; and CO2, 31.0. Presence of STA/TiO2 photocatalyst affects the process selectivity toward NG, ethanol and
•
0' 4
m, • I 5 6 7 8 910
(a)
20 30 Residencetime, s
40 50 60 70
100
6o
8 O8
40 Ethanol 20
\
• 4 (b)
5 6 7 8 910
20
Aceton~.~ 30
40 50 6070
Residencetime, s
Fig. 6. Photocatalytic oxidation of airborne NG, ethanol and acetone in the presence of (a) TiOz and (b) STA/TiO2 catalysts, [ N G ] = 13.1 ppmv, [ e t h a n o l ] = 892 ppmv, [acetone] = 446 ppmv, reactor volume: 530 mL, reaction temperature = 80°C.
acetone destruction significantly. While the extent of N G decomposition remains high for residence times shorter than 100 s, ethanol conversion was much less than that obtained over neat TiOz at comparable residence times. In the case of acetone, in the presence of STA/TiO2 photocatalyst, no appreciable conversion was observed. Figure 6(b) depicts that, at a residence time of 10.6 s, practically no acetone or ethanol is converted, whereas N G conversion exceeds 91%. Our findings indicate that selective decomposition of NG, in the presence of ethanol and acetone, occurs over STA/TiO2 photocatalyst. Thus, by proper reaction engineering, it is possible to convert N G selectively while leaving acetone and large portions of ethanol unaffected. Unlike in experiments with neat TiO2, nitro-
Selective photocatalyticdestruction of airborne VOCs glycerine conversion does occur over STA/TiO2 photocatalyst in the dark (no UV radiation) at room temperature. Under dark conditions, it took 5 min for approximately 75% of N G deposited on the STA/TiO2 surface to react. The detailed mechanism by which the inhibition of ethanol and acetone photooxidation on STAmodified titania occurs is not yet fully understood. Apparently, STA blocks the active sites on the TiO2 surface, filtering all radiation below 400 nm from reaching the titania surface. The strong catalytic activity of STA towards N G decomposition may be explained by the combination of STA's hydrolyzing (as a strong acid) and photooxidizing properties. 3.4. Effect of added oxidants and water vapor We conducted a series of photocatalytic experiments in the presence of ozone and in inert atmosphere of nitrogen to infer the role of oxidants on the selectivity of NG, ethanol and acetone photooxidation. We found that addition of 0.6 vol% ozone in air decreased the reaction selectivity toward NG. However, by completely starving the reactor from oxidants, a considerable improvement in the process selectivity for N G destruction was achieved. For example, we observed high N G DREs (99%) and practically no conversion of ethanol or acetone at a residence time of 15 s, in nitrogen atmosphere and LPML/TiO2 photocatalytic system, N O and a minute amount of NO2 were the only detectable products of N G photocatalytic decomposition in nitrogen and together accounted for more than 80% of the nitrogen input NG. It was not possible to analyze for all possible nitrogencontaining N G transformation by-products. For example, molecular nitrogen may be formed as a result of N G photocatalysis. Detection and quantitation for the by-product N 2 from N G in a reaction environment consisting of mostly nitrogen gas from input air proved to be too difficult to overcome. Therefore, we were not able to fully account for all input nitrogen originated from N G and the nitrogen mass balance in a typical experiment rarely closed to better than 80%. Simple calculations show that in these experiments the amount of N G introduced into the photoreactor was sufficient to cover the TiO2 surface with 8-9 monolayers of NG, providing a sustained catalytic process. These findings are consistent with our earlier results obtained using a photogravimetric analyzer (Muradov et al., 1995). We have reported that the apparent quantum yield of N G photo-
451
catalytic decomposition, obtained from PGA measurements, was highest (10.2%) in the inert (N2) environment. One explanation for this interesting observation may be that in the absence of electron scavengers such as O2, adsorbed N G molecules react with both electrons and holes (or O H radicals). Our data on electrochemical reduction of N G in aqueous and organic solutions are also in agreement with the foregoing hypothesis (Linkous et al., 1994). Thus, reducing the concentration of oxidant in gaseous streams can be very beneficial in increasing the selectivity of the photosystem towards N G destruction. With respect to water, our data indicate that the addition of water vapor into the input air stream noticeably increases the oxidation yields of all three: acetone, ethanol and NG. For example, in the presence of 1.5 vol% water vapor, N G decomposition yield in the photocatalytic (TiO2) reactor at a residence time of 9.7 s increased from 98.7% to 99.4%. We observed the same effect during ethanol-acetone photocatalytic oxidation under identical conditions. Thus, in the presence of humid air, photooxidation yields of ethanol and acetone increased as much as 13.3% and 5.6%, respectively, over that obtained when dry air was used. Apparently, the effect of water vapor in promoting hydroxyl radical formation outweighs its inhibiting effect caused by its competition for the available active sites on the TiO2 surface. 3.5. Effect of ethanol and acetone at high concentrations In these experiments, the concentration of the solvents (i.e. ethanol and acetone) were increased ten fold and the effect of this increase on the yields and selectivities of NG, ethanol and acetone photooxidation was examined. We used a photoreactor which was externally radiated as described before. Degussa P25 titania was used as photocatalyst and the initial concentration of NG, ethanol and acetone varied in the range of 8.3-9.1, 7585-8325 and 3792-4162 ppmv in air, respectively. Our data indicate that increasing solvent concentration does not affect the yield of N G destruction in either photocatalytic or photo-thermocatalytic (heated to 150°C) reactors significantly. However, the rate of acetone oxidation was strongly affected by increased input concentration of these solvents. At residence times of 1.9-7.8 s, practically no appreciable acetone conversion was observed in either photocata-
452
N.Z. Muradov et al.
lytic or photo-thermocatalytic (150°C) reactor runs. As far as ethanol is concerned, we again observed a significant drop in its oxidation yield, especially at lower residence times. For example, the extent of ethanol photocatalytic oxidation at 7.8, 3.9 and 1.9 s was 80.6%, 60.7% and 22.5%, respectively. Increasing reactor temperature further decreased the yields of ethanol oxidation. In the photo-thermocatalytic (150°C) runs, the ethanol conversion fell from 80.6% to 73.4% and from 60.7% to 45.9% at residence times of 7.8 and 3.9 s, respectively. This effect can also be explained by the species competition for the available adsorption sites on the photocatalyst surface. The heavier N G molecules are more strongly adsorbed onto the titania surface than ethanol and acetone. Further, since N G concentration is two orders of magnitude lower than that for solvents, even a ten fold increase in the ethanol and acetone concentration in air did not appear to significantly affect the rate of N G photodestruction. Ethanol, as we discussed earlier, has more affinity toward titanium oxide surface (as most of the hydroxyl group-containing organic compounds do) than acetone has and more readily occupies the active sites on the catalyst surface. Since in our experiments UV flux remained essentially constant even after a ten fold increase in the solvent concentration, effect of species competition for the catalyst active sites becomes even more apparent. Our actinometric measurements indicate a photon-limited situation prevailing at high solvent concentrations. Finally, increasing reactor temperatures hinders solvent adsorption onto the catalyst surface, reducing oxidation yields and increasing the process selectivity toward N G destruction. 3.6. Solar detoxification of NG in gaseous phase
Experiments involving solar detoxification of NG-contaminated, aqueous and gaseous streams were conducted in mid-summer in Florida (average solar insolation of 700900 W m -2 with a maximum near 1000 W m -2, the estimated UV fraction of solar insolation varied between 4.2 and 4.7% of the total radiation). We used a continuous flow photoreactor made of quartz with TiO2 immobilized onto its inner wall surface (TiO2 loading density of 0.2 mg cm-2). The photoreactor was placed in a parabolic aluminum trough and subjected to solar irradiation. NG, ethanol and acetone were airborne in the usual manner using a specially made mixing manifold described before. The
concentrations of NG, ethanol and acetone in air were 10.9, 900 and 450 ppmv, respectively. Residence times were varied in the range of 4-12 s. After each experiment, the inner surface of the photoreactor was carefully washed using diethyl ether and subjected to N G analysis. In a typical experiment, at a residence time of 6 s, the extent of N G decomposition was found to be 95% with N O 2 being essentially the only detectable reaction product of N G destruction (with traces of N O and HNO3). Under these conditions, acetone left the solar photoreactor unconverted and less than half of the ethanol input was photooxidized to CO2. 4. CONCLUSIONS The possibility of selective detoxification of HOCs with simultaneous recovery of the nontargeted organic components in air has been described. Several techniques were discussed that are equally effective in accomplishing selective photocatalytic destruction of N G in the presence of organic solvents (i.e. ethanol and acetone). The concentrations of airborne NG, ethanol and acetone considered varied in a wide range: 1-14, 740-8700 and 370-4350ppmv, respectively. Different UV light sources (black light and germicidal L P M L s and solar radiation) and photoreactor designs (internally and externally irradiated) have been used in this study. TiO2 (Degussa P25) was used as the base photocatalyst which was immobilized directly onto the photoreactor walls (at loading densities that varied in the range of 0.1-1.0mgcm-2). TiO2 surface was modified by Pt and silicotungstic acid. Pt (0.1-0.5 wt%) on TiO/catalyst was deposited onto the glass, quartz and aluminum tubes and used as such in the photocatalytic experiments. Also, platinized aluminum tubes were used as catalysts in the thermocatalytic oxidation experiments. The effects caused by the residence time, temperature, solvent concentration at the reactor inlet, photocatalyst modification, light source and intensity, added oxidants and water vapor on the process selectivity toward N G destruction, in the presence of ethanol and acetone vapors, were studied. A summary of our experimental findings from the photocatalytic, photo-thermocatalytic and thermocatalytic oxidation of NG, ethanol and acetone in air follows. In the photocatalytic experiments, the process selectivity toward N G decomposition increased as temperature increased, solvent concentrations
Selective photocatalytic destruction of airborne VOCs at the reactor inlet increased, residence times decreased, U V light (254 nm) intensity decreased a n d oxygen c o n c e n t r a t i o n decreased with the use of STA-modified TiO2. Process selectivity t o w a r d N G p h o t o d e s t r u c t i o n deteriorated by a d d i t i o n of ozone, water v a p o r a n d / o r the use of Pt-modified TiO2. I n the thermocatalytic experiments, N G destruction selectivity was found to be adversely affected by the platinization of the titania, a n d increasing residence times. The effect of t e m p e r a t u r e on the selectivity of N G , e t h a n o l a n d acetone oxidation can be explained by the t e m p e r a t u r e d e p e n d e n t gas diffusion a n d surface processes. It can be expected that t e m p e r a t u r e affects the rate of a d s o r p t i o n ~ l e s o r p t i o n more readily t h a n the rate of diffusion from the bulk gas to the catalyst surface. I n all experiments (photocatalytic a n d thermocatalytic), the yield of e t h a n o l oxidation was always higher t h a n that of acetone. A l t h o u g h thermocatalytic systems at certain c o n d i t i o n s d e m o n s t r a t e fairly good selectivity toward N G decomposition, in general, these processes require either elevated temperatures (200°C a n d higher) or m u c h larger reactor volumes in order to achieve q u a n t i t a t i v e N G destruction at the same t h r o u g h p u t as that of photocatalytic or p h o t o - t h e r m o c a t a l y t i c reactors. I n the case of N G - c o n t a m i n a t e d airstreams, elevated temperatures present safety concerns. Increasing the reactor volumes results in m u c h higher capital a n d operating costs for the t h e r m o c a t a l y t i c process c o m p a r e d with photocatalytic systems. Finally, the concept of selective destruction of N G in the presence of added solvents was d e m o n s t r a t e d for a solardriven system. Acknowledgments--The financial support for this work was
provided by the U.S. Department of Navy, Indian Head Division Naval Surface Warfare Center, Maryland, Office of Naval Research and the U.S. Army Corps of Engineers, Construction Engineering Research Laboratories, Illinois. REFERENCES Acetone removed from toxics release inventory, C&E News, 26 June 1995. AI-Ekabi H., Butters B., Delany D., Holden W., Powell T. and Story J. (1993) The photocatalytic destruction of gaseous trichloroethylene and tetrachloroethylene over
453
immobilized titanium dioxide. In Photocatalytic Purification and Treatment of Water and Air, D.F. Ollis and H. AI-Ekabi (Editors). Elsevier, Amsterdam. Buxton G., Greenstock C., Helman W. and Ross A. (1988) Critical review of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals in aqueous solutions. J. Phys. Chem. Ref. Data, 17, 513. Dibble L. and Raupp G. (1992) Fluidized-bedphotocatalytic oxidation of tri-chloroethylene in contaminated airstreams. Environ. Sci. Technol., 26, 492-495. Linkous C. A., Muradov N. Z. and T-Raissi A. (1994) A perspective on photo- and electrochemicaldetoxification of aqueous nitrate esters. In Proc. 185th Electrochemical Soc. Meeting, San Francisco. Muradov N. Z. and T-Raissi A. (1993) Titania-catalyzed photooxidative detoxification of nitroglycerinecontaminated airborne VOCs. Proc. Third Int. Symp. on Chemical Oxidation: "'Technology for the Nineties", Nashville, TN. Muradov N. Z. and T-Raissi A. (1994) Photooxidative destruction of nitroglycerine over UV-excited heteropolyacids. ACS Sixth Annual Syrup on Emerging Technologies in Hazardous Waste Management, Atlanta, GA. Muradov N. Z., T-Raissi A., Jaganathan S. and Painter C. R. (1995) PGA and FTIR study of nitroglycerine photodecomposition on titania surface. Proc. Air & Waste Management Association, 88th Annual Meeting & Exhibition, San Antonio, TX.
Painter C. R., Muradov N. Z. and T-Raissi A. (1993) Photocatalyzed decomposition of nitroglycerine-contaminated air streams. CPIA Publication 600, Proc. J A N N A F Safety and Environmental Protection Subcommittee Meeting, Las Cruces, New Mexico.
Papaconstantinou E., Argitis P., Dimoticali D., Hiskia A. and loannidis A. (1986) Photocatalytic oxidation of organic compounds with heteropolyelectrolytes. In Homogeneous and Heterogeneous Photocatalysis, E. Pelizzetti and N. Serpone (Editors). Reidel Publishers (1986). Peral J. and Ollis D. (1992) Heterogeneous photocatalytic oxidation of gas-phase organics for air purification: acetone, 1-butanol, butyraldehyde, formaldehyde and m-xylene oxidation. J. Catal., 136, 554-565. T-Raissi A. and Muradov N. Z. (1993) Flow reactor studies of TiOz photocatalytic treatment of airborne nitroglycerin. In Photocatalytic Purification and Treatment of Water and Air, D.F. Ollis and H. AI-Ekabi (Editors). Elsevier, Amsterdam. T-Raissi A. and Muradov N. Z. (1994) GC mass spectrometric study of nitroglycerinephotoreaction products in air. ACS Sixth Annual Syrup. on Emerging Technologies in Hazardous Waste Management, Atlanta, GA.
Turchi C., Wolfrum E. and Miller R. (1994) Offgas treatment by photocatalytic oxidation: concepts and economy. In Proc. First Int. Conf. Advanced Oxidation Technologies for Water and Air Remediation, London, ON, Canada,
pp. 125 126. Urbanski T. (1965) Chemistry and Technology t f Explosives. Pergamon Press, Oxford. Weedon A. (1994) Photocatalytic oxidation of organic vapors in air using a titanium dioxide catalyst. In Proc. First Int. Conf. on Advanced Oxidation Technologies .for Water and Air Remediation, London, ON, Canada,
p. 127.
Pergamon
PIh S0038-092X (96)00031-X
Copyright © 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038-092X/96 $15.00 + 0.00
SELECTIVE P H O T O C A T A L Y T I C D E S T R U C T I O N O F A I R B O R N E VOCs N A Z I M Z. M U R A D O V , * A L I T - R A I S S I , * t D O N A L D M U Z Z E Y , * C H A R L E S R. P A I N T E R * * a n d M I C H A E L R. K E M M E * * * *University of Central Florida, Florida Solar Energy Center, 1679 Clearlake Road, Cocoa, FL 32922-5703, U.S.A.; **Naval Surface Warfare Center, Indian Head, MD 20640-5035, U.S.A. and ***U.S. Army Construction Engineering Research Laboratories, Champaign, IL 61826-9005, U.S.A. Abstract--This article describes a photocatalytic method for selective oxidation of the airborne nitroglycerine (NG), in the presence of ethanol and acetone vapors at high concentrations (exceeding 1% by volume). Process selectivities toward NG photooxidation were examined for various photoreactor configurations and UV lamps and techniques used for the photocatalyst preparation. In addition, we have studied the effect of temperature, residence time, various additives (e.g. ozone, water vapor, nitrogen), and initial concentration of the solvents in air. Our data indicate that modifying TiO2 with silico-tungstic acid (STA) catalyst results in selective NG oxidation without affecting ethanol and acetone significantly. Platinization of TiO2 showed an adverse effect on NG destruction selectivity. In general, low residence times and initial concentration of oxidants and high temperatures and initial concentration of solvents favor selectivity toward NG destruction. In the case of temperature, our observation can be explained by the temperature dependent gas diffusion and surface processes. In most cases, the yield of ethanol oxidation was generally higher than that of acetone. Results from the bench-scale experiments using artificial UV light sources were used to build and test a solar photocatalytic oxidation reactor for selective NG treatment in the presence of ethanol and acetone. Copyright © 1996 Elsevier Science Ltd.
1. I N T R O D U C T I O N O r d n a n c e m a n u f a c t u r i n g p l a n t s for the p r o d u c t i o n of energetic m a t e r i a l s (e.g. nitroglycerine, N G ) often use volatile o r g a n i c c o m p o u n d s ( V O C s ) such as e t h a n o l a n d acetone. These solvents are r e m o v e d at v a r i o u s stages of the process. Thus, m a n y significant p o i n t - s o u r c e s of air p o l l u t i o n are p r o d u c e d that are subject to the p r o v i s i o n s of the Clean Air Act of 1990. A n y strategy for the c o n t r o l of these emissions m u s t a c c o u n t for small a m o u n t s of N G c o - e m i t t e d with e t h a n o l a n d acetone. T h e toxicity of N G a n d o t h e r nitrate esters is well k n o w n ( U r b a n s k i , 1965). A c e t o n e and, especially, ethanol are n o t as toxic as N G . In fact, the U.S. E n v i r o n m e n t a l P r o t e c t i o n Agency ( E P A ) has recently g r a n t e d a p e t i t i o n deleting acetone from the list of h a z a r d o u s chemicals on the Toxics Release I n v e n t o r y (C&E News, 1995). A c c o r d i n g to the EPA, acetone has such a low p h o t o c h e m i cal activity that it presents no concern for f o r m a t i o n of t r o p o s p h e r i c o z o n e or o t h e r air pollutants. There are m a n y a p p l i c a t i o n s where the recovery of the solvents m a y present a m o r e a t t r a c t i v e o p t i o n t h a n their a b a t e m e n t along with the a i r b o r n e N G . If solvent v a p o r s are present in the air at high levels, they m a y be recovered, cost effectively, by any n u m b e r of c o n v e n t i o n a l techniques such as c a r b o n a d s o r p tion, c o n d e n s a t i o n , etc. H o w e v e r , because of tAuthor to whom correspondence should be addressed.
safety c o n s i d e r a t i o n s a n d p r o d u c t purity, recovery of the solvents m u s t p r o c e e d by selective a n d q u a n t i t a t i v e destruction of nitroglycerine. In s u m m a r y , the concept of selective d e s t r u c t i o n of a i r b o r n e h a z a r d o u s o r g a n i c c o m p o u n d s ( H O C s ) m a y be plausible when in the multic o m p o n e n t wastestream: (1) C o n c e n t r a t i o n of V O C s c o - e m i t t e d with N G are high e n o u g h to w a r r a n t their recovery. (2) C o n c e n t r a t i o n of V O C s c o - e m i t t e d with N G are low b u t some or all of the solvents can be r e g a r d e d as n o n - t o x i c and, thus, only the low-level toxic c o m p o n e n t of the flow stream needs to be treated. The situation described a b o v e provides a unique o p p o r t u n i t y for the a p p l i c a t i o n of p h o t o catalysis. This is so because the c o n v e n t i o n a l chemical t r e a t m e n t m e t h o d s such as incineration a n d t h e r m o c a t a l y t i c o x i d a t i o n convert all of the organic constituents of the w a s t e s t r e a m indiscriminately. By selective t r e a t m e n t of the target c o m p o u n d s , p o t e n t i a l l y large savings in the energy a n d capital a n d o p e r a t i n g costs of the process m a y be realized. In m u l t i - c o m p o n e n t systems, the i n t e r p l a y between the n o n - t a r g e t e d c o m p o n e n t s such as e t h a n o l a n d acetone and t a r g e t e d H O C s (e.g. N G ) m u s t be considered carefully. The use of n e a r - U V r a d i a t i o n a n d p h o t o c a t a lysts for d e s t r u c t i o n of a i r b o r n e V O C s is well known, as D i b b l e a n d R a u p p (1992), Peral a n d 445
446
N.Z. Muradov et al.
Ollis (1992), T-Raissi and Muradov (1993), and Turchi et al. (1994) have noted. However, researchers Muradov and T-Raissi (1993), A1-Ekabi et al. (1993), and Weedon (1994) report in most cases complete mineralization of all HOCs using a variety of photoreactor designs that contain immobilized titania and low pressure mercury lamps (LPML). With few exceptions [e.g. T-Raissi and Muradov (1993)], most of the reported work deals with singlecomponent wastestreams or multi-component systems with constituents all belonging to the same class of chemicals, e.g. aromatic hydrocarbons (BTEX), chlorinated or oxygenated hydrocarbons, etc. A vast majority of toxic air emissions from industrial sources are multicomponent wastestreams containing HOCs of wildly different physical and chemical properties. Multi-component emission at the ordnance manufacturing facilities is but one example of such situations. The fact that the physical and chemical nature of HOCs in a given wastestream may be vastly different tends to complicate the engineering design of a selective photocatalytic treatment system. For example, the energy efficiency of the photocatalytic process is affected to a large extent by the destruction and removal efficiency (DRE) of the most resilient targeted compound of the wastestream. In general, the destruction selectivities for a multi-component system depend on the following parameters: reaction temperature; light intensity; wavelength of radiation; type of photocatalyst and co-catalyst used; other gas-phase constituents/ additives (e.g. ozone, water vapor, etc.). Studies involving selective photocatalysis of multi-component wastestreams containing HOCs with wildly different physico-chemical properties are scarce. With few exceptions, e.g. Muradov and T-Raissi (1994), no selective photocatalytic detoxification study involving multicomponent wastestreams has been reported in the literature. The main objective of this work was then to conduct a systematic study to determine the feasibility of selective photocatalytic oxidation of a multi-component wastestream in air, containing ethanol, acetone and NG. In particular, we were interested in utilizing solar energy for selective destruction of airborne N G using simple photoreactor designs with TiO2 directly immobilized onto the reactor wall. 2. EXPERIMENTAL Main features of the system are briefly discussed below. A more detailed description of
our experimental setup can be found in T-Raissi and Muradov (1993), Muradov and T-Raissi (1993), and Painter et al. (1993). The experimental setup consisted of a reagent delivery system (RDS), flow photoreactor and analytical instrumentation. The reagents feed system was designed to provide a uniform airstream containing VOCs and N G at all concentration levels. NG, ethanol and acetone were introduced at desired flow rates into the airstream, passing through a heated Pyrex manifold, using a Sage model 341B syringe pump. The airborne NG/VOCs were carried into the photoreactor maintained at a pre-set temperature (between 80-100°C). The concentrations of NG, ethanol and acetone in air varied in the following ranges: 1-10ppmv, 740-8700ppmv and 370-4350ppmv, respectively. N G was introduced into the mixing manifold in 0.33-0.45% (by weight) blended solutions of ethanol/acetone mixture. Residence times within the photocatalytic reactors have been corrected to account for elevated temperatures. Several photoreactor modifications were used. Some of the reactors were illuminated from within (internal radiation) and others were lit from outside (external radiation), in both photocatalytic and photo-thermocatalytic experiments. The volume of photoreactors varied from 20 mL to approximately 1 L. The wall temperatures for the photoreactors and mixing manifold were carefully controlled and maintained in a temperature range of 80-100°C. We used Pyrex ® glass, optical grade quartz and aluminum in the fabrication of the photoreactors. In photothermocatalytic experiments, the UV lamp and quartz photoreactor were placed within a temperature controlled jacket with highly reflective walls. This arrangement allowed a more uniform illumination of the photoreactor while keeping the temperatures within the cavity at a desired range, 80-200°C. To eliminate the temperature effects on the intensity of UV light produced by the lowpressure mercury lamp (LPML), the lamp's cold-spot temperature was maintained at 25°C at all times by keeping one end of the lamp always at ambient temperature. The advantage of this light source-photoreactor arrangement was that it mimicked the manner in which our solar-driven system was constructed, i.e. the UV transparent photoreactor was placed within a parabolic trough. Therm0catalytic experiments were carried out in three annular reactors, i.e. Pyrex, quartz and
Selectivephotocatalyticdestruction of airborne VOCs aluminum tubes, 6 mm ID and reaction volumes that varied from 15 to 19 mL. In all three reactors, the photocatalyst was deposited onto the inner surface of the outer wall of the reactor. Beside neat titania, we experimented with two modifications of the as-received powder, i.e. doping with platinum and silico-tungstic acid H4SiWllO40.nH20 (STA). The catalyst loading densities on the photoreactor wall varied between 0.1 and 1.0 mg cm -2. Doping of TiO2 with 0.1-0.5 wt% platinum was accomplished by reduction of chloroplatinic acid (H2PtC16) solution with hydrogen at 60°C and in the presence of 10 wt% TiO/suspension. STA/TiO2 photocatalysts were prepared at 1/10 to 1/1 weight ratios by depositing TiO2 onto the inner wall of the photoreactor followed by treatment of the surface with 0.1 M STA solution and then drying the surface by passing a stream of hot air through the tube. In some experiments, 0.1 wt% HEPtCI6 solution was used on the aluminum reactor wall so that a uniform coverage of the surface with metallic Pt was achieved. We used TiO2 (Degussa P25, BET surface area of 50 m z g- 1, average particle size of 30 nm, primarily anatase crystalline form) for all catalyst preparations. We also used Fisher, Optimagrade acetone, food-grade ethanol (Florida Distillers), Fisher brand SiO2.12WO3.26HzO, and ACS-certified HzPtC16 from Aldrich Chemicals. Spirits of 5 wt% N G in ethanol were shipped directly from Indian Head Division, Naval Surface Warfare Center, MD, and used as such without further treatment or purification. A low pressure Hg lamp, germicidal with nominal power consumption of 40 W, was used as the light source in all of the photocatalytic experiments. The light intensities were measured using a potassium ferrioxalate actinometer. Solar experiments were conducted in a quartz photoreactor placed within a reinforced aluminum foil trough in a non-tracking configuration and achieved a concentration factor of approximately 2. The temperature inside the solar photoreactor reached as high as 60°C. Clearly, introducing a simple parabolic trough arrangement into the design of a solar-assisted photocatalytic detoxification system improves its operating performance significantly. A detailed description of the GC-MS method used for the analysis of nitrate esters is given in T-Raissi and Muradov (1994). Briefly, the analytical system consisted of a capillary GC, connected to Varian Saturn II ion-trap MS system. The GC column was a J&W fused silica capil-
447
lary column, 15 m, ¼ mm ID with 1 #m coating of DB-1. Fixed gases and volatile organics were analyzed on a packed column (9.14 m, 3.2 mm O D Hayesep Ds) using a Varian GC 3400 equipped with FID and TCD. NO2 and N O were analyzed using a T E C O model 46 chemiluminescence analyzer. The chemiluminescence analyzer was capable of NO/NO2 detection at concentration levels as low as 0.5 ppbv. Instrument responses for many compounds including CO, CO2, C H 4, C 2 H 6, C H 3 C H O , CzHsOH, CH3COCH3, CH3COOH, NO and NOa were calibrated using certified master gases provided by Scott Speciality Gases. Reaction products were cryogenically trapped and manually injected into the GC column for analysis. We used 0.432mg Nitrostat ® pills (produced by Parke-Davis) in 20 mL Fisher Optima acetone for quantifying NG. The Naval Surface Warfare Center, Indian Head Division supplied 4.998% glycerol 1,3-dinitrate (I&3-GDN), 4.997% glycerol 1,2-dinitrate ( I & 2 - G D N ) and 5.000% glycerol 1-mononitrate (1-GMN) in ethanol standards. Optical spectra of NG, STA and other compounds as well as the quantitative analysis of nitrate ions were conducted using a Milton Roy model 601 Spectrophotometer. 3. RESULTS AND DISCUSSION 3.1. Effect of Residence Time Since reaction kinetics of the photogenerated active species (presumably, O H radicals) with ethanol and acetone differ considerably [kEtOH = 1.9 x 10 9 L mol-1 s - l , kacetone= 1.1 x 108 L mo1-1 s -1, from Buxton et al. (1988)], one can expect that the residence time should affect the photocatalytic process selectivity. Note that no kNa values are available for nitroglycerine. However, if the process is mass transfer or adsorption rate controlled, reactants transport to the catalyst surface and adsorption kinetics of each individual component affect the overall process selectivity significantly. We conducted several experiments to assess competitive adsorption rates of NG, ethanol and acetone on TiO2 immobilized onto fiberglass mesh. Airborne N G (14 ppmv), ethanol and acetone (both 600 ppmv) were introduced at room temperature into a continuous flow reactor with TiO2/fiberglass mesh (residence time 48s). Breakthrough half-lives for acetone and ethanol (that is, the time at which the concentrations of the acetone and ethanol in the effluent gas reached 300 ppmv) were found to be 25 and 75
448
N . Z . M u r a d o v et al.
min, respectively. No N G was detected in the effluent air streams even after 90 min through the experiment. These results demonstrate the following order of adsorption rate of the components: N G > ethanol > acetone. Similar results were obtained by Peral and Ollis (1992) for competitive adsorption of acetone and 1-butanol onto TiO2 surface. The researchers report an adsorption rate of 1-butanol that was one order of magnitude higher than that observed for acetone. Again, this shows strong adsorption of alcohols on metal oxide surfaces.
100
Ethanol
80
o~ E O
60
,= c
40
8 20
0
0
,
tion of airborne NG, ethanol and acetone was carried out using a tubular reactor made of both glass (Pyrex) and aluminum. The main goals of these experiments were: (i) to assess the relative importance of thermocatalytic (at the temperature range of 80-100°C) versus photocatalytic oxidation and destruction and (ii) to determine the effect of low to moderate temperatures (125-200°C) on the kinetics and selectivity of oxidation of a multi-component waste stream. Figure l(a) and (b) depicts the yields of oxidation/destruction of three-component mixture as a function of residence time (at 150°C) and temperature (at residence time of 5.8 s) using Pt on TiO2 catalyst. In the presence of 0.1 wt% Pt/TiO2, at 150°C and residence time of 1.7 s, we observed 96.4% N G destruction but no acetone conversion and only 27.1% of ethanol oxidized. G o o d selectivity toward N G conversion could be achieved at lower temperatures and higher residence times using 0.5 wt% Pt on TiO2 catalyst. For example, at 100°C and residence time of 5.8 s we observed 96.6%, 19.6% and 0% oxidation yields for NG, ethanol and acetone, respectively. The yield of NG/ethanol/ acetone thermocatalytic oxidation in the presence of TiO2 as a function of temperature is given by Fig. 2. TiO2 demonstrated better selectivity towards N G destruction in the presence of ethanol and acetone at high residence times than Pt/TiO2 catalyst. At 125°C and residence time 29 s TiO2 revealed the highest selectivity towards N G destruction: the oxidation yields were found to be 99.1%, 6.1% and 1.4% for NG, ethanol and acetone, respectively. Platinized aluminum tube showed the lowest selectivity amongst all tested catalysts (see Fig. 3). We conducted a few thermocatalytic experiments at high solvent concentration using
o~
80
co
60
~
,o
~
30
time, s
Residence
lo0
,
20
10
(a)
3.2. Effect of temperature 3.2.1. Effect of temperature on thermocatalytic oxidation of NG/VOCs. Thermocatalytic oxida-
.t
NG
,
~ . .
NG /
Ethanol
20
0
,
so
J=
,
LA,
i
A
i
,
i
J
,
i
lOO 120 140 160 180 2o0 22c
(b)
Temperature. °C
Fig. 1. Thermocatalytic oxidation of NG/ethanol/acetone using Pt on TiO 2 catalyst in tubular Pyrex reactor, volume: 14.7 mL, [ N G ] = 8 . 7 p p m v , [etbanol]=818ppmv, [acetone]=409 ppmv. (a) at 150°C temperature (0.1 wt% Pt); (b) residence time of 5.8 s (0.5 wt% Pt). 100
A
A
_=
80 o~ d 60 .9 E
Ethanol
40
8 20 0 ' w 80 100
Acetone
120
140
160
180
200
i
220
Temperature, °C Fig. 2. T e m p e r a t u r e d e p e n d e n c e of NG/ethanol/acetone catalytic o x i d a t i o n in the presence of TiO2, t u b u l a r Pyrex reactor, volume: 14.7 mL, [ N G ] = 7 . 7 ppmv, [ e t h a n o l ] = 724 ppmv, [acetone] = 362 ppmv, residence time = 29.4 s.
Pt(0.5 wt%)/TiO2 and STA/TiO2 deposited onto the inner wall of aluminum tube and Pyrex glass tube. The input concentrations of NG, ethanol and acetone in air were 8.3, 7585 and 3792 ppmv, respectively. At a residence time of
Selective photocatalytic destruction of airborne VOCs
449
N/
~oo I I
.no,
N
60
o
g
o
g o
4O
Acetone
i
20
0
i 0
i 50
0
100
150
200
250
2
3
4
5
7 8 10
Fig. 3. Temperature dependent NG/ethanol/acetone oxidation using platinized tubular aluminum reactor, volume: 15.8 mL, [NG] =8.7 ppmv, [ethanol] =818 ppmv, [acetone]= 409 ppmv, residence time = 6.3 s.
6 s and I50°C, acetone remained practically unconverted while the yield of ethanol oxidation was 37%, which represents a significant drop in the oxidation yield in c o m p a r i s o n with experiments using one order of magnitude lower concentrations of ethanol and acetone. At the same experimental conditions (residence time 6s, 150°C) and the same concentrations of solvents practically no conversion of both ethanol and acetone was observed using STA/TiO2 catalyst.
3.2.2. Effect of temperature on photothermocatalytic oxidation of airborne NG/ethanol/acetone. We used a tubular quartz photoreactor, w r a p p e d with a L P M L inside a reflective and temperature controlled jacket. Photocatalysts (i.e. TiO2 and P t - d o p e d TiO2) were deposited o n t o the inner wall of the p h o t o r e a c t o r and irradiated from outside. Temperatures varied in the range of 80-200°C. O u r data indicate that temperature has a minimal effect on the rate of N G photodestruction. In the range of residence times examined, i.e. 2.3 to 39 s, nitroglycerine underwent almost quantitative conversion with both TiO2 and Pt/TiO2 photocatalysts. Figure 4 depicts the effect of temperature on the yield of ethanol and acetone p h o t o o x i d a t i o n using Pt/TiO2 catalyst. Increasing reaction temperature had a profound effect on the rates of both ethanol and acetone oxidation. This effect was more p r o n o u n c e d for acetone. For example, under U V (254 nm) exposure at a residence time of 2.3 s and 150°C, acetone leaves the p h o t o r e a c t o r essentially intact. P h o t o - t h e r m a l oxidation of N G / e t h a n o l /
20
30
40 50
70
Residence time, s
Temperature, °C
Fig. 4. Oxidation of NG/ethanol/acetone in the presence of 0.1 wt% Pt on TiO2, [NO]=8.5-8.Tppmv, [ethanol]= 799 818 ppmv, [acetone] =399 409 ppmv, tubular reactor irradiated from outside using a germicidal UV lamp, volume: 19.5 mL, (a) photocatalytic at reactor temperature: 87'~C, (b) photothermocatalytic at reactor temperature: 150'C.
acetone in the presence of TiOz also showed an inhibiting effect caused by reactor temperature. Figure 5 shows the temperature dependency of ethanol and acetone oxidation in the presence of T i O / at residence time 2.3 s. Clearly, the reaction temperature has a much more pronounced effect on the rate of acetone oxidation than that of ethanol. Since the lamp's cold spot temperature for both photocatalytic and photo-thermocatalytic experiments was at r o o m temperature, the observed effect can not be explained by the variations in the lamp's emissive power due to temperature fluctuations (which is normally the case when the entire lamp is subjected to ele100
11
II
It
80
E ._o
60
---
t
~
NG
•
Ethanol
/
•
Acetone
/
I 120
U--~-140
i,° 2O
0
I 80
,
I
100
,
/
/
Temperature, °C
Fig. 5. Temperature dependence of TiO2 photothermocatalytic oxidation of NG, ethanol and acetone, ENG] = 8.4 ppmv, [ethanol ] = 789 ppmv, [acetone] = 395 ppmv, residence time=2.3 s, tubular reactor irradiated from outside using a germicidal UV lamp, volume: 19.5 mL.
450
N . Z . M u r a d o v et al.
vated temperatures). This observation can be explained by the temperature dependent gas diffusion and surface processes. It can be expected that temperature affects the rate of adsorption-desorption more strongly than the rate of diffusion from the bulk gas to the catalyst surface. Since adsorption is an exothermic process, an increase in the surface temperature tends to suppress the photocatalytic oxidation rate. This effect is more pronounced in the case of acetone as it is less strongly adsorbed on TiO2 surface than N G or ethanol.
100
|
80
Acet°nli o~ 60, 8 >=
~
/
40
20
3.3. Selective destruction of NG using STA/TiO2 catalyst We studied the effect of catalyst additives such as STA on the selectivity of TiO2 photocatalyzed destruction of NG, ethanol and acetone vapors. The rationale for the modification of TiO2 by STA, known as multi-electron photoredox agent (Papaconstantinou et al., 1986), was to affect yield of the active species formed on the titania surface via interaction of STA molecules with photogenerated electrons and holes. It was found that the deposition of STA on the titania surface hinders ethanol and acetone oxidation but has no effect on the catalyst activity toward N G destruction. Figure6(a) and (b) depicts DREs vs residence time for NG, ethanol and acetone, in the presence of neat TiO2 and STA-treated titania. It can be seen that, at high residence times, all reactants are fully mineralized over TiO2 catalyst. At somewhat lower residence times, only a partial destruction of N G and VOCs is achieved. It is important to note that, at low residence times, NG, ethanol and acetone do not photooxidize to the same extent. Acetone appears considerably less reactive than N G and ethanol. For example, at residence time of 25 s, N G and ethanol conversion exceeds 90%, whereas acetone conversion is well below 10%. Our results indicate that NO2 is the primary N2-containing product of N G decomposition. G C - M S analysis of the effluent did not show any partial N G destruction products such as mono- and dinitroglycerols. Carbon dioxide was the only carbonaceous product of ethanol and acetone oxidation obtained at high residence times (greater than 60 s). For example, at a residence time of 5.5 s, the selectivity (in mol%) to the products of ethanol partial oxidation is: acetaldehyde, 68.1; CO, 0.9; CH4, 0.03; and CO2, 31.0. Presence of STA/TiO2 photocatalyst affects the process selectivity toward NG, ethanol and
•
0' 4
m, • I 5 6 7 8 910
(a)
20 30 Residencetime, s
40 50 60 70
100
6o
8 O8
40 Ethanol 20
\
• 4 (b)
5 6 7 8 910
20
Aceton~.~ 30
40 50 6070
Residencetime, s
Fig. 6. Photocatalytic oxidation of airborne NG, ethanol and acetone in the presence of (a) TiOz and (b) STA/TiO2 catalysts, [ N G ] = 13.1 ppmv, [ e t h a n o l ] = 892 ppmv, [acetone] = 446 ppmv, reactor volume: 530 mL, reaction temperature = 80°C.
acetone destruction significantly. While the extent of N G decomposition remains high for residence times shorter than 100 s, ethanol conversion was much less than that obtained over neat TiOz at comparable residence times. In the case of acetone, in the presence of STA/TiO2 photocatalyst, no appreciable conversion was observed. Figure 6(b) depicts that, at a residence time of 10.6 s, practically no acetone or ethanol is converted, whereas N G conversion exceeds 91%. Our findings indicate that selective decomposition of NG, in the presence of ethanol and acetone, occurs over STA/TiO2 photocatalyst. Thus, by proper reaction engineering, it is possible to convert N G selectively while leaving acetone and large portions of ethanol unaffected. Unlike in experiments with neat TiO2, nitro-
Selective photocatalyticdestruction of airborne VOCs glycerine conversion does occur over STA/TiO2 photocatalyst in the dark (no UV radiation) at room temperature. Under dark conditions, it took 5 min for approximately 75% of N G deposited on the STA/TiO2 surface to react. The detailed mechanism by which the inhibition of ethanol and acetone photooxidation on STAmodified titania occurs is not yet fully understood. Apparently, STA blocks the active sites on the TiO2 surface, filtering all radiation below 400 nm from reaching the titania surface. The strong catalytic activity of STA towards N G decomposition may be explained by the combination of STA's hydrolyzing (as a strong acid) and photooxidizing properties. 3.4. Effect of added oxidants and water vapor We conducted a series of photocatalytic experiments in the presence of ozone and in inert atmosphere of nitrogen to infer the role of oxidants on the selectivity of NG, ethanol and acetone photooxidation. We found that addition of 0.6 vol% ozone in air decreased the reaction selectivity toward NG. However, by completely starving the reactor from oxidants, a considerable improvement in the process selectivity for N G destruction was achieved. For example, we observed high N G DREs (99%) and practically no conversion of ethanol or acetone at a residence time of 15 s, in nitrogen atmosphere and LPML/TiO2 photocatalytic system, N O and a minute amount of NO2 were the only detectable products of N G photocatalytic decomposition in nitrogen and together accounted for more than 80% of the nitrogen input NG. It was not possible to analyze for all possible nitrogencontaining N G transformation by-products. For example, molecular nitrogen may be formed as a result of N G photocatalysis. Detection and quantitation for the by-product N 2 from N G in a reaction environment consisting of mostly nitrogen gas from input air proved to be too difficult to overcome. Therefore, we were not able to fully account for all input nitrogen originated from N G and the nitrogen mass balance in a typical experiment rarely closed to better than 80%. Simple calculations show that in these experiments the amount of N G introduced into the photoreactor was sufficient to cover the TiO2 surface with 8-9 monolayers of NG, providing a sustained catalytic process. These findings are consistent with our earlier results obtained using a photogravimetric analyzer (Muradov et al., 1995). We have reported that the apparent quantum yield of N G photo-
451
catalytic decomposition, obtained from PGA measurements, was highest (10.2%) in the inert (N2) environment. One explanation for this interesting observation may be that in the absence of electron scavengers such as O2, adsorbed N G molecules react with both electrons and holes (or O H radicals). Our data on electrochemical reduction of N G in aqueous and organic solutions are also in agreement with the foregoing hypothesis (Linkous et al., 1994). Thus, reducing the concentration of oxidant in gaseous streams can be very beneficial in increasing the selectivity of the photosystem towards N G destruction. With respect to water, our data indicate that the addition of water vapor into the input air stream noticeably increases the oxidation yields of all three: acetone, ethanol and NG. For example, in the presence of 1.5 vol% water vapor, N G decomposition yield in the photocatalytic (TiO2) reactor at a residence time of 9.7 s increased from 98.7% to 99.4%. We observed the same effect during ethanol-acetone photocatalytic oxidation under identical conditions. Thus, in the presence of humid air, photooxidation yields of ethanol and acetone increased as much as 13.3% and 5.6%, respectively, over that obtained when dry air was used. Apparently, the effect of water vapor in promoting hydroxyl radical formation outweighs its inhibiting effect caused by its competition for the available active sites on the TiO2 surface. 3.5. Effect of ethanol and acetone at high concentrations In these experiments, the concentration of the solvents (i.e. ethanol and acetone) were increased ten fold and the effect of this increase on the yields and selectivities of NG, ethanol and acetone photooxidation was examined. We used a photoreactor which was externally radiated as described before. Degussa P25 titania was used as photocatalyst and the initial concentration of NG, ethanol and acetone varied in the range of 8.3-9.1, 7585-8325 and 3792-4162 ppmv in air, respectively. Our data indicate that increasing solvent concentration does not affect the yield of N G destruction in either photocatalytic or photo-thermocatalytic (heated to 150°C) reactors significantly. However, the rate of acetone oxidation was strongly affected by increased input concentration of these solvents. At residence times of 1.9-7.8 s, practically no appreciable acetone conversion was observed in either photocata-
452
N.Z. Muradov et al.
lytic or photo-thermocatalytic (150°C) reactor runs. As far as ethanol is concerned, we again observed a significant drop in its oxidation yield, especially at lower residence times. For example, the extent of ethanol photocatalytic oxidation at 7.8, 3.9 and 1.9 s was 80.6%, 60.7% and 22.5%, respectively. Increasing reactor temperature further decreased the yields of ethanol oxidation. In the photo-thermocatalytic (150°C) runs, the ethanol conversion fell from 80.6% to 73.4% and from 60.7% to 45.9% at residence times of 7.8 and 3.9 s, respectively. This effect can also be explained by the species competition for the available adsorption sites on the photocatalyst surface. The heavier N G molecules are more strongly adsorbed onto the titania surface than ethanol and acetone. Further, since N G concentration is two orders of magnitude lower than that for solvents, even a ten fold increase in the ethanol and acetone concentration in air did not appear to significantly affect the rate of N G photodestruction. Ethanol, as we discussed earlier, has more affinity toward titanium oxide surface (as most of the hydroxyl group-containing organic compounds do) than acetone has and more readily occupies the active sites on the catalyst surface. Since in our experiments UV flux remained essentially constant even after a ten fold increase in the solvent concentration, effect of species competition for the catalyst active sites becomes even more apparent. Our actinometric measurements indicate a photon-limited situation prevailing at high solvent concentrations. Finally, increasing reactor temperatures hinders solvent adsorption onto the catalyst surface, reducing oxidation yields and increasing the process selectivity toward N G destruction. 3.6. Solar detoxification of NG in gaseous phase
Experiments involving solar detoxification of NG-contaminated, aqueous and gaseous streams were conducted in mid-summer in Florida (average solar insolation of 700900 W m -2 with a maximum near 1000 W m -2, the estimated UV fraction of solar insolation varied between 4.2 and 4.7% of the total radiation). We used a continuous flow photoreactor made of quartz with TiO2 immobilized onto its inner wall surface (TiO2 loading density of 0.2 mg cm-2). The photoreactor was placed in a parabolic aluminum trough and subjected to solar irradiation. NG, ethanol and acetone were airborne in the usual manner using a specially made mixing manifold described before. The
concentrations of NG, ethanol and acetone in air were 10.9, 900 and 450 ppmv, respectively. Residence times were varied in the range of 4-12 s. After each experiment, the inner surface of the photoreactor was carefully washed using diethyl ether and subjected to N G analysis. In a typical experiment, at a residence time of 6 s, the extent of N G decomposition was found to be 95% with N O 2 being essentially the only detectable reaction product of N G destruction (with traces of N O and HNO3). Under these conditions, acetone left the solar photoreactor unconverted and less than half of the ethanol input was photooxidized to CO2. 4. CONCLUSIONS The possibility of selective detoxification of HOCs with simultaneous recovery of the nontargeted organic components in air has been described. Several techniques were discussed that are equally effective in accomplishing selective photocatalytic destruction of N G in the presence of organic solvents (i.e. ethanol and acetone). The concentrations of airborne NG, ethanol and acetone considered varied in a wide range: 1-14, 740-8700 and 370-4350ppmv, respectively. Different UV light sources (black light and germicidal L P M L s and solar radiation) and photoreactor designs (internally and externally irradiated) have been used in this study. TiO2 (Degussa P25) was used as the base photocatalyst which was immobilized directly onto the photoreactor walls (at loading densities that varied in the range of 0.1-1.0mgcm-2). TiO2 surface was modified by Pt and silicotungstic acid. Pt (0.1-0.5 wt%) on TiO/catalyst was deposited onto the glass, quartz and aluminum tubes and used as such in the photocatalytic experiments. Also, platinized aluminum tubes were used as catalysts in the thermocatalytic oxidation experiments. The effects caused by the residence time, temperature, solvent concentration at the reactor inlet, photocatalyst modification, light source and intensity, added oxidants and water vapor on the process selectivity toward N G destruction, in the presence of ethanol and acetone vapors, were studied. A summary of our experimental findings from the photocatalytic, photo-thermocatalytic and thermocatalytic oxidation of NG, ethanol and acetone in air follows. In the photocatalytic experiments, the process selectivity toward N G decomposition increased as temperature increased, solvent concentrations
Selective photocatalytic destruction of airborne VOCs at the reactor inlet increased, residence times decreased, U V light (254 nm) intensity decreased a n d oxygen c o n c e n t r a t i o n decreased with the use of STA-modified TiO2. Process selectivity t o w a r d N G p h o t o d e s t r u c t i o n deteriorated by a d d i t i o n of ozone, water v a p o r a n d / o r the use of Pt-modified TiO2. I n the thermocatalytic experiments, N G destruction selectivity was found to be adversely affected by the platinization of the titania, a n d increasing residence times. The effect of t e m p e r a t u r e on the selectivity of N G , e t h a n o l a n d acetone oxidation can be explained by the t e m p e r a t u r e d e p e n d e n t gas diffusion a n d surface processes. It can be expected that t e m p e r a t u r e affects the rate of a d s o r p t i o n ~ l e s o r p t i o n more readily t h a n the rate of diffusion from the bulk gas to the catalyst surface. I n all experiments (photocatalytic a n d thermocatalytic), the yield of e t h a n o l oxidation was always higher t h a n that of acetone. A l t h o u g h thermocatalytic systems at certain c o n d i t i o n s d e m o n s t r a t e fairly good selectivity toward N G decomposition, in general, these processes require either elevated temperatures (200°C a n d higher) or m u c h larger reactor volumes in order to achieve q u a n t i t a t i v e N G destruction at the same t h r o u g h p u t as that of photocatalytic or p h o t o - t h e r m o c a t a l y t i c reactors. I n the case of N G - c o n t a m i n a t e d airstreams, elevated temperatures present safety concerns. Increasing the reactor volumes results in m u c h higher capital a n d operating costs for the t h e r m o c a t a l y t i c process c o m p a r e d with photocatalytic systems. Finally, the concept of selective destruction of N G in the presence of added solvents was d e m o n s t r a t e d for a solardriven system. Acknowledgments--The financial support for this work was
provided by the U.S. Department of Navy, Indian Head Division Naval Surface Warfare Center, Maryland, Office of Naval Research and the U.S. Army Corps of Engineers, Construction Engineering Research Laboratories, Illinois. REFERENCES Acetone removed from toxics release inventory, C&E News, 26 June 1995. AI-Ekabi H., Butters B., Delany D., Holden W., Powell T. and Story J. (1993) The photocatalytic destruction of gaseous trichloroethylene and tetrachloroethylene over
453
immobilized titanium dioxide. In Photocatalytic Purification and Treatment of Water and Air, D.F. Ollis and H. AI-Ekabi (Editors). Elsevier, Amsterdam. Buxton G., Greenstock C., Helman W. and Ross A. (1988) Critical review of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals in aqueous solutions. J. Phys. Chem. Ref. Data, 17, 513. Dibble L. and Raupp G. (1992) Fluidized-bedphotocatalytic oxidation of tri-chloroethylene in contaminated airstreams. Environ. Sci. Technol., 26, 492-495. Linkous C. A., Muradov N. Z. and T-Raissi A. (1994) A perspective on photo- and electrochemicaldetoxification of aqueous nitrate esters. In Proc. 185th Electrochemical Soc. Meeting, San Francisco. Muradov N. Z. and T-Raissi A. (1993) Titania-catalyzed photooxidative detoxification of nitroglycerinecontaminated airborne VOCs. Proc. Third Int. Symp. on Chemical Oxidation: "'Technology for the Nineties", Nashville, TN. Muradov N. Z. and T-Raissi A. (1994) Photooxidative destruction of nitroglycerine over UV-excited heteropolyacids. ACS Sixth Annual Syrup on Emerging Technologies in Hazardous Waste Management, Atlanta, GA. Muradov N. Z., T-Raissi A., Jaganathan S. and Painter C. R. (1995) PGA and FTIR study of nitroglycerine photodecomposition on titania surface. Proc. Air & Waste Management Association, 88th Annual Meeting & Exhibition, San Antonio, TX.
Painter C. R., Muradov N. Z. and T-Raissi A. (1993) Photocatalyzed decomposition of nitroglycerine-contaminated air streams. CPIA Publication 600, Proc. J A N N A F Safety and Environmental Protection Subcommittee Meeting, Las Cruces, New Mexico.
Papaconstantinou E., Argitis P., Dimoticali D., Hiskia A. and loannidis A. (1986) Photocatalytic oxidation of organic compounds with heteropolyelectrolytes. In Homogeneous and Heterogeneous Photocatalysis, E. Pelizzetti and N. Serpone (Editors). Reidel Publishers (1986). Peral J. and Ollis D. (1992) Heterogeneous photocatalytic oxidation of gas-phase organics for air purification: acetone, 1-butanol, butyraldehyde, formaldehyde and m-xylene oxidation. J. Catal., 136, 554-565. T-Raissi A. and Muradov N. Z. (1993) Flow reactor studies of TiOz photocatalytic treatment of airborne nitroglycerin. In Photocatalytic Purification and Treatment of Water and Air, D.F. Ollis and H. AI-Ekabi (Editors). Elsevier, Amsterdam. T-Raissi A. and Muradov N. Z. (1994) GC mass spectrometric study of nitroglycerinephotoreaction products in air. ACS Sixth Annual Syrup. on Emerging Technologies in Hazardous Waste Management, Atlanta, GA.
Turchi C., Wolfrum E. and Miller R. (1994) Offgas treatment by photocatalytic oxidation: concepts and economy. In Proc. First Int. Conf. Advanced Oxidation Technologies for Water and Air Remediation, London, ON, Canada,
pp. 125 126. Urbanski T. (1965) Chemistry and Technology t f Explosives. Pergamon Press, Oxford. Weedon A. (1994) Photocatalytic oxidation of organic vapors in air using a titanium dioxide catalyst. In Proc. First Int. Conf. on Advanced Oxidation Technologies .for Water and Air Remediation, London, ON, Canada,
p. 127.