Environ. Sci. Techno/. 1995, 29,1223-1231
Ti02 Photocatalysis for Indoor Air Applications: Effects of Humidity and Trace Contaminant Levels on the Oxidation Rates of Formaldehyde, Toluene, and 1,%Butadiene TIMOTHY N. OBEE* AND ROBERT T. BROWN United Technologies Research Center, Silver Lune, East Hartford, Connecticut 06108
Persistent indoor air contaminants, those originating from emissions by interior furnishings, occupants, and materials of construction, typically exist in concentrations below 100 parts per billion (ppb) on an individual basis. The total of distinct contaminants may number in the hundreds with an equivalent accumulated concentration of one part per million. This study investigated the effects of humidity and trace (sub-ppmv) contaminant levels on the oxidation rates of formaldehyde, toluene, and 13-butadiene. The evaluation also included variations in UV intensity and flow residence time. UV intensities from inexpensive mercury fluorescent lamps, those which are expected to be employed in a practical photocatalytic purifier, are in the mW/cm2 range. For this reason, the study included UV intensities in that range. The reactor element used in the study was a low pressure drop alumina reticulate, wash-coated with Degussa P25 titania. The data indicated that the reaction was first-order for the three reactants at the subppmv level. An important finding was that competitive adsorption between water and trace (sub-ppmv) contaminants has a significant effect on the oxidation rate. The dependencies of humidity and contaminant concentrations on the oxidation rates are explained as being the results of competitive adsorption on available hydroxyl adsorption sites and of changes in hydroxyl radical population levels.
0013-936~95/0929-1223$09.00/00 1995 American Chemical Society
Introduction Titanium dioxide (titania), as a photooxidative catalyst, exhibits a number of attractive characteristics: (a) it is inexpensive, (b) it promotes room temperature oxidation of the major classes of indoor air pollutants (11, and (c) it produces the highest oxidation rates of the many photoactive metal oxides investigated (2). Many titania reactor designs have been investigated and are reviewed by Ollis (3). As discussed therein (31, monolith reactors, like the one investigated here, have inherent attractive characteristics such as potentially high conversion rates and low pressure drop. The latter characteristic makes the monolith design suitable for the high duct velocities and low pressure drop tolerance of the building application. Furthermore, the monolith type support is commercially available. A disadvantage of the monolith is that the light distribution is not known (3)and is not considered a controllable design parameter. The primary purpose of this studywas to investigate the effect ofwater vapor on the photocatalytic activity of titania against several contaminates-not to establish the competitive position of the monolith used in this study. The monolith was chosen as the support structure because it provides a convenient support for the titania catalyst to study the effects of humidity on the oxidation rates, it is commercially available, and it is a potentially attractive design for the intended applications. The influence of water vapor on the performance of a titania photooxidation reactor is unclear for applications such as purification of air in residential and commercial buildings,transportationvehicles, and commercial aircraft. Humidity levels vary widely between these applications. Widely differing effects of water vapor have been reported ( I , 2, 4-9). In these studies, the investigators employed pollutant levels well above 1 ppmv. In contrast, investigations (10-13) of gaseous contaminant levels in buildings, including those exhibiting the ‘sick building syndrome’, have established that concentrations of individual species lie below 0.100 ppmv and that the equivalent accumulated or total VOC (volatile organic compound) concentrations are generally in the range of 0.5-2.0 ppmv. In this paper, the effect of water vapor on the photooxidation of three gases (formaldehyde, toluene and 1,3butadiene) is presented. These gases are found in buildings, transportation vehicles, and commercial aircraft (10-13). Formaldehyde, an aldehyde, is a major indoor air contaminant and presents a significant health effect. Toluene, representative of the aromatic contaminant class, is ubiquitous and is generally a member of the VOCs having the highest concentrations found in the indoor environment (10-13). These two Contaminants are being considered as surrogates for two of the six major classes (aromatic, aldehyde, alkane, ketone, alcohol, and chlorocarbon) of indoor air contaminants in a proposed ASHRAE (American Society of Heating, Refrigerating, and Air-conditioning Engineers) test standard for air filtration equipment, VanOsdell (14). 1,3-Butadiene, an unsaturated hydrocarbon, is used as a plasticizer and is emitted from plastic based furnishings. Extrapolation of oxidation performance data collected at concentrations much higher than the intended applica-
VOL. 29, NO. 5, 1995 /ENVIRONMENTAL SCIENCE &TECHNOLOGY
1223
UV
attenuator
\ Celiwindow element Shutter
FIGURE 1. Schematic of the experimental apparatus.
tionmaynotbevalid. Thisiscertainlytruewhenthephysics ofthe oxidation process is complexand poorlyunderstood. as is the case for oxidation on titania, and is certainly true when the physics of the oxidation process is critically dependent on the contaminant concentration. Indeed, it is shown that the effect of water vapor on the oxidation of the three selected gases is critically dependent on the humidity and contaminant concentrations. For these reasons. sub-ppmv concentrations close to known application concentrations are included in this study. In a practical air purifier, commercial germicidal W lamps deliveringintensitiesin the lowmWlcm2range would be used. In keepingwith anoverallobjectiveof investigating the oxidation process under realistic conditions, mWlcm2 W intensities are included.
Experimental Section Apparatus. Figure 1 is a schematic of the apparatus used. W illumination is generated by a 1000-W, high-pressure Hg-Xe lamp (Oriel Corp.). The action of the water filter and optical train results in a uniform-intensity, 3-W, W continuum fromabout250 to35Onmat thecatalystsurface. Neutral-density optical filters are used to adjust the delivered W from the maximum 3-W to the desired level. High-purity nitrogen gas passes through a water bubbler. The desired humidity level is set via the bypass valve. The pollutant is generatedeitherfrom compressed gas cylinders (toluene and 1.3-butadiene) or by vaporizing paraformaldehyde (formaldebyde). Oxygen gas flowjoins with the nitrogen and pollutant flows to produce an air-equivalent mixture (i.e., 20% oxygen, 80%nitrogen). Thermocouples are used to measure the temperature of the ambient air, inlet, and exit gas streams and reactor front surface and perimeter. For measuring gas concentrations, two detectors are used The Briiel& Kjrer 1302 multigas monitor measures the concentration of water vapor, carbon dioxide, carbon monoxide, toluene, formaldehyde, and L3-butadiene. The gas chromatograph (GC) measures the Concentration of 1224 rn ENVIRONMENTAL SCIENCE &TECHNOLOGY IVOL. 29.
NO. 5.1995
TABLE 1
Measurement Uncertainty of Measured Gas Concentrations gas
water vapor carbon dioxide carbon monoxide formaldehyde toluene 1.3.butadiene
t3m Monitor Ippmvl
gas chmmatwraph
(ppmv)
f25.0 +0.010 +0.100 +0.020 +0.100
+0.200
0.005 0.020
toluene usingapacked column-10% carbowax2OM Chrom WAW 80-100M-and of 1,3-butadiene using a packed column-PorapakQS80-lOOM. Experimentaluncertainty estimates of gas concentrations are given in Table 1 for these detectors. The resultant design has produced aversatile apparatus having a number of important characteristics: W and pollutant fluxes are uniform across the reactor face. The apparatus can hold the operating conditions steady for any time period. The apparatus is used primarily for open loop (single pass) operation, but can be employed for closed loop studies. With this apparatus, the design parameter space (Wintensities, contaminant and humidity concentrations,flow rates. temperature, reactor element thickness) can be explored. Consequently, the performance data so generated may be used to scale the reactor element to any sue.
Reactor Element The reactor element is an alumina [awith 0.9 m21gBETsurfacearea)reticulate (HiTechCorp.), 10 poreslin., and wash-coated (proprietary process) with Degussa Titania P25 (2-3% by weight-manufacturer's claim). DegussaTitaniaP25 catalyst has a primary particle diameter of 300 A, a surface area of 50 m21g, and a crystal distribution of 70% anatase and 30% rutile. The reactor element is cylindrical with a thickness of 2 cm and face area (or flow area) of 25.5 cm2, The active depth is about 1 cm as determined by W light absorption (opticaldepth) and toluene oxidation tests; this is consistent with the W
TABLE 2
Photodissociation of Formaldehyde for a Fixed Inlet level of 3.3 ppmv (20.9 Reaction Time) UV (W/crn*)
0.007 0.025 0.047 0.098
AHCHO (pprnv)
0.00 0.07 0.14
0.35
AC02(pprnv)
0.0 0.0 0.0 0.0
ACO (pprnv)
0.00 0.10 0.19
0.39
light absorption results by Hale and Bohn (15). Auger electron spectroscopy of the reticulate indicates a nonuniform titania coating thickness which varies from 15 to 70pm. The titaniacoatingis complete; no exposed alumina was found. Controls. In a typical test, the following procedure is followed: First, the gas flows and humidity levels are set. After the reactor inletloutlet humidity levels reach equilibrium, the pollutant is then introduced. Atransient period ensues as the pollutant adsorbs on the titania. This represents the adsorption phase, defines an adsorption time interval (i.e., the time interval before proceeding to the next phase), and provides a qualitative indication of the adsorption affinity between the titania catalyst and the pollutant. After the adsorption process has reached equilibrium as indicated by equality between the inletloutlet pollutant concentrations, the UV is turned on. With the W lamp on, a transient period precedes a steady-state phase that is established when the outlet pollutant concentration has reached a steady state. It is the steady-state result that is reported here. In all tests, the pollutant (Le., toluene, 1,3-butadiene, or formaldehyde), water vapor, carbon monoxide, and carbon dioxide concentrations are all measured. Control tests using an uncoated (no titania) alumina reticulate and a Wintensity of 0.125 Wlcm2 (highest level used) had no effect on the pollutant concentration and did not generate any carbon monoxide or carbon dioxide. Control tests for possible gas-phase reactions performed in the absence of the reactor element revealed no reactions with toluene or 1,3-butadiene at a W intensity of 0.125 W/cm2, but formaldehyde did show photodissociation. Several results for the formaldehyde test are given in Table 2. The last three columns show the change in concentration of formaldehyde (column two), carbon dioxide, and carbon monoxide occurring between the inlet and the outlet of the apparatus as a function of UV intensity (column one) for a fNed 3.3 ppmv inlet formaldehyde level. Photodissociation of formaldehyde into carbon monoxide is seen and is a well-established phenomenon Okabe (16). For the UV intensity of the last row ofTable 2, the absorption coefficient for formaldehyde (16)predicts a carbon monoxide rise of 0.3 0.1 ppmv. This is in agreement with the carbon monoxide rise seen in Table 2. Furthermore, the change in carbon monoxide concentration increases linearly with UVflux as seen in Table 2 and as predicted (16). To reduce the influence of gas-phase photodissociation to an inconsequential level, all formaldehyde tests use a UVflux below 0.010 Wlcm2. As a final check, in each experiment the reactor element is removed and a check for gas-phase photodissociation is made. In all cases, no reduction in the contaminant nor generation of any carbon monoxide or carbon dioxide is observed.
*
Normalized oxidation rate 1.01 -
I
0.8
4
:I
P f -
I
0.6
I
0
Formaldehvde
E
I
,
,
IS
20
0.0
0
5
10
Face Velocity ( c d s ) FIGURE 2. Oxidation rate dependence on face velocity: toluene, 0.29 ppmv inlet, 11 OOOppmv humidity,O.W W / c d UV;formaldehyde, 6.0 ppmv inlet, 11 OOO pprnv humidity, 0.009 W / c d UV.
Results Oxidation Rate Parameter. In all the data to follow, the oxidation rate is used as the dependent parameter and is defined as
where Cin and Gout are the inlet and outlet contaminant concentrations, respectively; Q is the volumetric flow rate; A is the flow area; and q is the single pass efficiency. The rate term is normalized by the flow area, as the illuminated or active area of the catalytic element is not preciselyknown. The ratio QIA is the face velocity, Le., the flow velocity entering the reactor. The clean air delivery rate (CADR) defined by qQ is used by the Association of Home Appliance Manufacturers as a standard for rating particle and gaseous filtration devices (17, 18). This parameter is also being proposed for use in rating gaseous filtration in the W A C (heating, ventilation, and air conditioning) industry (19, 20). The CADR is a measure of the contaminant removal rate, Le., the clean air flow rate causing the same removal rate. Thus, the oxidation rate defined above is directly related to the CADR. Kinetic Flow Region. Kinetic oxidation rates free of the influence of :nass transfer are desirable because this allows a clearer interpretation of the measured data. In Figure 2, the oxidation rates for toluene at 0.29 ppmv and for formaldehyde at 6 ppmv are shown as a function of face velocity (QIA).As the face velocity is increased, the effects of mass transfer from the bulk to the near-surface (reactor) diminishes and becomes small above ca. 10 cmls. In the region below 10 cmls the effects of mass transfer are important, whereas above 10 cmls the effects of mass transfer are unimportant (negligible). Equivalently, the contaminant concentration gradient (mean) in the bulk is near zero for face velocities above ca. 10 cmls but differs from zero at face velocities below 10 cmls. At high face velocities, the difference between Ci, and Cou, becomes progressively smaller and difficult to accurately measure; this is indicated by the increase in the experimental uncertainty bars. Thus, a compromise is made between the desire to generate kinetic data and the difficulty of generating accurate measurements. Most of the data presented here are obtained at 12 cmls face velocity where the influence of mass transfer is small, and yet accurate data is generated. The UV intensity also VOL. 29. NO. 5,1995 /ENVIRONMENTAL SCIENCE &TECHNOLOGY
1
1225
Oxidation rate (p-mole/cmL-hr)
Oxidation rate (p-mole/cm*-hr)
0
0 . 5 O p p n v d
Langmuir-Hi&lwod
0.6
!
0.01 0.1
1
1
1
10
I 100
"."
, 0
5000
10000
Toluene inlet concentration (ppmv)
affects the location of the kinetic flow region. For UV intensities below those given in Figure 2, the contaminant concentration gradient in the bulk diminishes. Conversely, for UV intensities above those given in Figure 2, the contaminant concentration gradient in the bulk increases. Thus, for the 12 cm/s face velocities and UV intensities below 0.033 W/cm2, the influence of mass transfer on toluene oxidation rates is unimportant (negligible). Similarly, for the 12 cmls face velocitiesand UVintensities below 0.009 W/cmz, the influence of mass transfer on formaldehyde oxidation rates is unimportant (negligible). Kinetic Equation. A Langmuir-Hinshelwood (L-HI rate form for bimolecular surface reactions (21)was found to provide a good correlation to the oxidation rate data: = k,FpFw
+ klCp+ k,C,) = k4Cw/(1+ k3Cp+ k4Cw)
(2)
Fp = k,Cp/(l
(3)
F,
(4)
where r &mol/cm-2 h-l) is the oxidation rate, ko &mol/ cm-2 h-l) is the constant of proportionality; k l , kz, k3, and k4 (each of unit ppmv-') are the Langmuir adsorption equilibrium constants (ratio of adsorption to desorption rates); and Cp (ppmv) and C, (ppmv) are the gas-phase concentrations of the pollutant and water vapor, respectively. The factors Fp and Fw represent competitive adsorption between the pollutant and water for the same adsorption site (21). This form represents a generalization of the bimolecular form; the true bimolecular is found by setting k3 = kl and k4 = kz. The unimolecular L-H form is found by setting k4 = m (i.e., Fw= 1). Toluene Oxidation Rates. The oxidation rate for toluene is shown in Figure 3 as a function of toluene inlet concentration. This data is for a humidity level of 10 900 ppmv or ca. 40% relative humidity (tests were performed at room temperature, 72-76 OF). The shape of this curve is characteristic of bimolecular adsorption (22). The effect of humidity on the oxidation rate is shown in Figure 4 for two toluene levels. In generating this data, the humidity level was incrementally increased from low to high to cover the range shown, and then the process was reversed. In each instance, the oxidation rate at a given humidity level repeated (within experimental uncertainty), i.e., the direction of approach did not matter. Thus, the drop in oxidation from ca. 5000 to 1500 ppmv humidity along the 2.13 ppmv toluene data curve is repeatable and, 1226
1
I
20000
25000
Humidity (ppmv)
FIGURE 3. Oxidation dependence on toluene concentration: 10 900 ppmv humidity, 0.033 W / c d UV, 12 cmls face velocity.
I
15000
ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 5, 1995
FIGURE 4. Toluene oxidation dependence on humidity: 0.033 W/cm' UV, 12 cm/s face velocity.
TABLE 3
Langmmir- Hinshelwood Correlation C d c i e n t s gas
ko
ki
kz
ks
k3
2.02 0.000727 2.02 toluene 3.84 0.868 0.000729 0.0281 formaldehyde 16.2 0.00818 0 1,3-butadiene 0.796 12.1
0.000727 0.000719 m
importantly, completely reversible;i.e., no irreversibleloss in activity occurs. The same functional dependence seen in Figure 4 for the 2.13 ppmv data curve was also observed by Peral and Ollis (1)for another aromatic m-xylene over a limited humidity range of 0-7500 ppmv (the m-xylene level was not found in the paper). Along the 2.13 ppmv toluene data curve, the oxidation rate exhibits a negative slope for humidity levels above ca. 5000 ppmv and a positive slope for humidity levels below ca. 5000 ppmv. Along the 0.50 ppmv toluene data curve, the oxidation rate exhibits a negative slope for the humidity range investigated. The bimolecular L-H rate coefficientsfor the correlation are given in Table 3. Since the correlation is nonlinear, a least squares optimization procedure was used to defrne the coefficients. These coefficientswere defined using the data from Figure 3 and the 2.13 ppmv data from Figure 4. The resulting correspondence between the L-H correlation and these data is good. The correspondence between the L-H correlation and the 0.50 ppmv data curve from Figure 4, which was not used to establish the L-H coefficients, provides an independent validation test of the L-H correlation. Here, again the resulting correspondence between the L-H correlation and the data is good. The effect of humidity on the oxidation rate for lower UV intensities and lower toluene levels is shown in Figure 5. Again the resulting correspondence between the L-H correlation and the data is good and provides a further independent validation test of the L-H Correlation. The functional dependence of the oxidation rate for U V variations is given below. The important observation is the increased sensitivity to humidity, Le., a steeper negative slope. The oxidation rate decreases by about a factor of 3 over the given humidity range. The relevant humidityrange (23)for HVAC applications is roughly 40-60% relative humidity (this defines the ASHR4E comfort range) or 11 000-16 000 ppmv (room temperature), and for aircraft it is ca. 15-30% relative humidity or 4000-8000 ppmv. For these humidity levels,
Oxidation rate (p-mole/cm2-hr)
Oxidation rate (p-mole/cm2-hr)
1 Measured Laa~~U-Hinshelwood
0.2 1 1° 1
O.O
1 I
I
I
1
I
0
5000
10000
15000
20000
I
Humidity (ppmv) FIGURE 5. Toluene oxidation dependence on humidity: 0.29 ppmv toluene inlet, 0.016 W/cmZ UV, 12 cm/s face velocity.
0.14 0.1
........ . . . . . I
'..I
. . . . . . . . . . ......I 1
10 100 Formaldehyde inlet concentration (ppmv) 1
1000
FIGURE 8. Oxidation dependence on formaldehyde concentration: 12 cm/s face velocity, 10 600 ppmv humidity, 0.0093 W / c d UV.
Oxidation rate (p-mole/cm2-hr) 10
Oxidation rate (p-mole/cm2-hr) I."
I I
0.8 -
0.60.4 -
,,,,,/m ____----
+.""'. . . . (..
.(',I
:I
I
'.._..._ ... ....
I
, I
0.2 -
0.0 v
I
8-
---__
_ - _ - 3.0 Toluene 8.0 Inlet ppmv Level
. I
I
----
....... 0.5 ppmv .......... .................. I
I
I
I
0 5000 10000 15000 20000 FIGURE 6. Oxidation dependence on humidity and toluene inlet concentration for Langmuir-Hinshelwood correlation: 0.033 W / c d UV, 12 cm/s face velocity.
Oxidation rate (p--mole/cm*-hr)
04 0
1
I
1
5000
10000
15000
20 100
Humidity (ppmv) FIGURE9. Formaldehydeoxidation dependence on humidity: 0.0093 W/cmZ UV, 12 cmls face velocity.
required nor expected. Low velocities were used because the best signal to noise is found at these low velocities. Also, since pressure drop will probably prevent operating at the higher velocities where the oxidation rate is maximized (kinetic region), some mass transfer influence will likely be experienced by a titania-photocatalytic device designed for the intended applications. The important trend is the progression of the oxidation dependence from a primarily negative slope at low toluene levels (0.5 ppmv, Figure 6) to a primarily positive slope at high toluene levels (8 ppmv, Figure 6) (exclusive of behavior at the very low humidity levels, Le., humidity levels below ca. 2000 ppmv). When toluene reaches sufficiently high levels, the other terms in the denominator of the L-H correlation may be neglected, and the oxidation rate becomes first-order in humiditylevel. For humiditylevels below 16 000 ppmv (or 60% relative humidity), this first-order dependence would occur for toluene levels above ca. 60 ppmv. Just such a first-order dependence was found by Ibusuki and Takeuchi (2)for toluene at 80 ppmv (based on the generation rate of carbon dioxide). Also, enhanced oxidation rates for several organic gaseous contaminants including toluene were observed by Weedon (9)for concentrations above 50 PPmv. Formaldehyde Oxidation Rates. The oxidation rate for formaldehyde is shown in Figure 8 as a function of formaldehyde inlet concentration and in Figure 9 as function of humidity for two formaldehyde levels. The same trends seen for the toluene data are observed here for formaldehyde. One exception is that the drop in oxidation at high
olzzzYk! 0.3 4-0.07 4-
0.01 0
5000
10000
15000
20000
25000
30000
Humidity (ppmv)
FIGURE 7. Oxidation dependence on humidity and toluene inlet concentration: 0.125 W/cmZ UV, 2.6 cm/s face velocity.
the L-H correlation indicates that the toluene oxidation rate is first-order for toluene levels below ca. 1 ppmv. As noted in the introduction, toluene levels found in the intended applications are also below 1ppmv. Thus, reactor performance would show a first-order dependence on toluene levels for the intended applications. The dependence of the oxidation rate on toluene and humidity levels is shown in Figure 6 for the bimolecular L-H correlation (Table 3). The functional dependencies displayed in Figure 6 are in qualitative agreement with the data of Figure 7. The data of Figure 7 include significant mass transfer influence as evidencedby the low face velocity and the high U V level (per the discussion above under Kinetic Flow Region). Thus, quantitative agreement is not
VOL. 29, NO. 5,1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY a 1227
4
Oxidation rate (p-mole/cm'-hr)
Oxidation rate (pmole/cmL-hr) 1,
xWa formaldehyde level -A- 6.1 p p v -01.5 ppmv
x
= -
1
0.01 0.1 1 10 100 FIGURE 11. Oxidation dependence on lJ-butadiene concentration: 11 OOO ppmv humidity, 0.009 W / c d UV, 9 cm/s face velocity.
Oxidation rate ()t-mole/crn2-hr) 0.30
0.251 \
0.20
T
0.154
".""
!
I
I
1
1
I
0
5000
10000
15000
20000
25000
Humidity (ppmv) FIGURE 12. 1,tButadiene oxidation dependence on humidity: 0.7 ppmv 1.3-butadiene inlet, 0.009 W / c d UV, 9 cm/s face velocity.
The reactivity of formaldehyde compared to toluene (compare Figures 4 and 8) is much higher (an adjustment for the U V difference would amplify the difference still further). This result is consistent with results of Peral and Ollis (1). 1,3-ButadieneOxidation Rates. The oxidation rate for 1,3-butadiene is shown in Figure 11 as a function of the 1,3-butadiene inlet concentration and in Figure 12 as function of humidity level. The data are not as extensive as shown for formaldehyde and toluene. The drop off in oxidation rate found for toluene and formaldehyde is not seen in either figure. This may be because the concentration was not high enough in Figure 11 and because the 1,3butadiene level was too low in Figure 12. Only a unimolecular L-H fit was attempted because of the limited data. The L-H rate coefficients for the correlation are given in Table 3. These coefficients were defined using the data from Figures 11 and 12. For Figure 11, the resulting correspondence between the L-H correlation and the data is good. Since the data were generated at a face velocity below 10 cmls, there is likely some mass transfer influence. The elimination of this effect would likely shift the data curve slightly to the left and upwards in Figure 11. With this caveat, the data in Figure 11indicate the oxidation rate is first-order below 1 ppmv. W IntensityInfluence. The functional dependence of the oxidation rate on UV intensity is well established (1, 24). For illumination levels appreciably above one sun equivalent, the oxidation rate increases with the square root of the intensity. While for UV intensities below one sun, the oxidationrate increases linearlywith the intensity. Although one sun is not quantitatively defined by refs 1 1228 1 ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 5,1995
log,, ( Oxidation rate (p-mole/cm2-hr))
Oxidation rate (pmole/cm*-hr) 1.61
-0.6 7
-1 Humidity (ppmv)
1
-0.7 -0.8
-
-0.9
-
/rate = uv O ”
I I
-2.0
-1.9
I
I
I
I
-1.8
-1.7
-1.6
-1.5
1
1.413 1.2 1 .o
-
f,5 I
J
0.8
1
I
1
-1.4
loglo (UVintensity (watts/cm*)) FIGURE 13. Toluene oxidation dependence on UV (0.01-0.04 W/cmZ): 0.29 ppmv toluene inlet, 12 cm/s face velocity.
Oxidation rate (pmole/cm2-hr)
0.061
1
I 0.04
0.02
0.00
0.6
0.4
I
1
I
Oxidation rate (p-mole/cm2-hr)
OF, and the return air from a hot air furnace is about 140 OF. Both toluene and 1,3-butadiene show a positive slope, which increases with humidity. Formaldehyde shows a negative slope, which is weakly dependent on humidity. The face velocity for these data is less than 10 cm/s; thus, there is mass transfer influence. Since a pressure drop constraint usually requires operating under mass transfer influence as pointed out above (Toluene Oxidation Rates), the trends shown in these figures are likely to be realized. Carbon Balance. For formaldehyde, complete mineralization to carbon dioxide is observed for W intensities below 0.010 W/cm2. This result held true for all humidity levels (0-20 000 ppmv) and for the face velocities investigated (2- 12 cmls). For toluene, complete mineralization to carbon dioxide is observed for low toluene levels (below ca. 1 ppmv). For high toluene levels (above 2 ppmv), a carbon balance is realized when both carbon dioxide and carbon monoxide are included. In these cases, the ratio of carbon monoxide to carbon dioxide varied between 0.05 and 0.15 f 0.02. A dependence with humidity and face velocity was investigated, but no evidence of a dependence was found. 1,3-Butadiene displayed the same pattern as toluene.
fl
Discussion
Humidity @pmv)
0.2
v
12000
I7000
0.0 50
75
100
125
150
Temperature (F) FIGURE 15. 1.3-Butadiene oxidation dependence on temperature: 2.5 pprnv inlet, 0.02 W/cm2 UV, 2.6 cm/s face velocity.
and 24,the sun emits about 1-2 mW/cm2for wavelengths below 350 and 400 nm, respectively (25). For toluene, the oxidation rate dependence on W intensity followed a power law with an exponent of 0.55 f 0.03, as shown in Figure 13. For formaldehyde and 1,3butadiene, the same exponent was found. Since this exponent was established from data generated for U V intensities above one sun, the result is in agreement with Peral and Ollis (1) and Ollis et al. (24). Temperature Influence. The effect of temperature is shown in Figures 14-16 for the three gases. The selected temperature range is based on the temperature range developed by a typical W A C unit during an annual operation; cooling coils of an air conditioner are about 55
The surface of titania is hydroxylated when exposed to moisture as shown by G r e g and Sing (26) and Turchi and Ollis (271. The hydroxyls are formed as a result of dissociative chemisorption of water onto the Ti4+sites (2628). Water can be physisorbed on the surface hydroxyl groups via hydrogen bonding as shown by Gregg and Sing (26) and Raupp and Dumesic (29). Toluene is shown by Nagao and Suda (30)to be adsorbed (on rutile) through the OH. vc electron type interaction on the surface hydroxyl groups. Formaldehyde is more polar than water and like water is probably physisorbed on the surface hydroxyl groups via hydrogen bonding. The nature of the adsorbed 1,3-butadienehas not been investigated. In all likelihood, the bonding is similar to toluene, i.e., a OH..n electron type complex. Since water, formaldehyde, toluene, and 1,3-butadieneare all apparently bonding to the same surface hydroxyl group or adsorption site, a competitive interaction is expected. The degree of this interaction depends on the relative bonding energies (1,26). The hydrogen bonding of formaldehyde and water is expected to be stronger than the OH...x electron type complex. The possibility of competitive adsorption is qualitatively suggested by Figures 17 and 18. These figures show the VOL. 29, NO. 5,1995 / ENVIRONMENTAL SCIENCE &TECHNOLOGY
1
1229
Toluene (ppmv)
Humidity (ppmv) ,20000
40 1
-
-Toluene Humidity
30 -
'"1
20 -
't
- 15000
- 10000
- 5000
10h
L
r
I
6oii 40
1J W U W
-Formaldehyde
Humidity
Jil
~
. .. . .
. .
,
. . ... .
.. ........ ........
. . ..
,,
. . . .. .
20000
4 IUOO 1
20
t
tl,,O
10000
. ... . .. ..
0
0 0
2
1
3
Time (hrs) FIGURE 18. Formaldehydeand humidity sorption dynamics: 6.5 ppmv formaldehyde inlet, 8 cm/s face velocity.
sorption dynamics of toluene and formaldehyde on the titania reactor element (in the dark). For these tests, the titania surface is first brought into adsorption equilibrium with both the contaminant and water (as measured by the equality of the inlet and outlet concentrations). Then, as shown in the figures, the humidity level is incrementally increased and decreased. In Figures 17 and 18,the effluent concentration is shown. At each step-increase in humidity level, a concomitant desorption pulse in effluent contaminant level is observed. Conversely, at each step-decrease in humiditylevel,a concomitant adsorption pulse in effluent contaminant level is seen. Since the contaminant inlet levels are the same for toluene and formaldehyde, a qualitatively comparison can be made of their relative adsorption strengths. As discussed above (Reactor Element), the alumina is a with a small BET surface area. This, coupled with the thickness continuity of the coating, suggests that the alumina does not influence the sorption process. The duration and amplitude of the formaldehyde pulses are evidently much longer and more extensive than those of toluene. Since the more strongly bound contaminant would have more molecules adsorbed on the surface and would take longer to displace, an integrated desorption pulse would be of a greater magnitude for the more strongly bound contaminant (both at the same inlet concentrations). These data (Figures 17 and 18)imply that formaldehyde is more strongly adsorbed than toluene. As mentioned above, this is in agreement with the expectation that the hydrogen bonding of formaldehydeis stronger than the OH. * an electron type complex bonding for toluene. Two types of surface hydroxyl groups have been found as described by Peral and Ollis ( I ) and shown by Primet et al. (28). If the less strongly-bound hydroxyl,which is easily 1230
removed at modest temperatures, is removed, the photoactivity is not affected ( 1 ) . However, the removal of the strongly-boundhydroxyl resulted in complete photoactivity loss (I). Thus, surface hydroxyl radical groups are generally accepted as the primary oxidant of organics ( 1 ) . In the oxidation process, the hydroxyl radical is consumed (27), Le., dehydroxylation of the surface. However,the presence of water allows for rehydroxylation of the surface. During the oxidation process, the current picture of the surface chemistry would consist of the following: dehydroxylation by hydroxyl radical attack of the organic and adsorbed intermediates, rehydroxylation through dissociative chemisorption of water, hydroxyl radical formation through trapping of a U V photogenerated hole by the surface adsorbed hydroxyl (reconstitution of the hydroxyl radical population), and competition for adsorption sites (adsorption on the surface hydroxyl groups) between the contaminant and water. Activity loss also occurs through recombination of the photogenerated electron and hole pairs; the hydroxyl radical reverts back to an adsorbed surface hydroxyl. A dynamic equilibrium is eventually reached. By applying the above description, the results are explained as follows: In general, at very low humidity levels (belowca. 2000 ppmv) and high contaminant levels, a drop in oxidation rate with decreasing humidity levels [Figures 4 (2.13 ppmv toluene) and 9 (12.1ppmv formaldehyde)] is seen. The drop in oxidation rate is likely a result of a decrease in activity through a reduction in the hydroxyl radical population (adsorption sites-bonding to the hydroxyl). This same argument was used to explain the decrease in oxidation rate of rn-xylene (I) under very low humidity levels (below 1500 ppmv). Similarly, at high humidity levels and high contaminant levels, an increase in the oxidation rate with increasing humidity [Figures 6 (8 ppmv toluene) and 7 (11 ppmv toluene)] is seen. Just such an increase in oxidation rate was also observed by Ibusuki and Takeuchi (2) in the oxidation of 80 ppmv toluene, by Murabayashi et al. ( 7 ) in the oxidation of 927 ppmv trichloroethylene,and by Weedon (9)in the oxidation of several organics for concentrations above 50 ppmv. These two situations are explained by changes in hydroxyl radical populations (adsorption sites-bonding to the hydroxyls) and are not fundamentally different. At moderate to high humidity levels (above ca. 5000 ppmv) and low contaminant levels, a decrease in the oxidation rate with increasinghumidity [Figures4 (0.5 ppmv toluene), 5, 9 (2.67 ppmv formaldehyde), 10 (0.5 and 1.5 ppmv formaldehyde), and 121 is seen. This oxidation rate dependence on humidity was also seen for m-xylene by Peral and Ollis (11,for 6 ppmv trichloroethylene by Dibble and Raupp (5,69, and for below 30 ppmv trichloroethylene by Kutsuna et al. (4). This oxidation rate dependence on humidity is likely the result of competitive adsorption between water and the contaminant. This and the previous description were used to explain the initial increase in oxidation of 2-propanol (over Pt/titania) with increasing humidity at low humidity levels and the subsequent decrease in oxidation with increasing humidity at moderate humidity levels (31). In summary, there appears to be two distinguishable oxidation regions: (1) a region is defined in which the hydroxyl radical population (adsorption sites population) determines the oxidation rate's functional dependence and (2) a region is defined in which competitive adsorption
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(coadsorption) determines the oxidation rate’s functional dependence. The location of these two oxidation regions depends on the relative adsorption affinity of titania for the contaminant and water and on the mechanism of the hydroxyl radical attack (thisdepends on the type of organic) For some other combinations of contaminant and humidity levels, both effects, hydroxyl radical population and competitive adsorption, would be important; for example, the two oxidation regions overlap.
Conclusions A commercial photocatalytic reactor element was used to investigate the effect of humidity on the oxidation rate of formaldehyde, 1,3-butadiene,and toluene. Concentrations (sub-ppmv) as found in actual buildings, transportation vehicles, and commercial aircraft (the intended applications) were included in this study. The problem levels for formaldehyde in buildings, which range from sub-ppmv to several ppmv, were included in this evaluation. W intensities (mW) as expected from current commercially available W lamps were used. The effect of humidity on the oxidation rate was found to depend critically on the concentration of the contaminant. However, at sub-ppmv concentrations, two important results were found to hold for the three gases studied: (1)the oxidation rate increases with decreasing humidity for humiditylevels above ca. 1000 ppmv, and (2) the reaction rate is first-order. The L-H rate form was successfully used to correlate the rate data. The bimolecular L-H rate form was particularly successful in correlating the toluene data. The dependencies of humidity and contaminant concentrationson the oxidation rates were explained as being a result of competitive adsorption on available hydroxyl adsorption sites and of changes in hydroxyl radical population (adsorption sites population) levels.
Acknowledgments Support for this study was provided by UTRC. The authors thank Dr. S. 0. Hay and Dr. B. R. Weinberger of United Technologies Research Center for their helpful criticism pertaining interpretation of the data and thank the publication reviewers for their helpful criticism.
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Received for review July 19, 1994. Revised manuscript received January 26, 1995. Accepted January 30, 1995.@
ES9404431 @
Abstract published in Advance ACS Abstracts, March 1, 1995.
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