J. Phys. Chem. 1995, 99, 15814-15821
15814
Experimental and Theoretical Study of the Reaction Mechanism of the Photoassisted Catalytic Degradation of Trichloroethylene in the Gas Phase Suzuko Yamazaki-Nishida,*3t Salvador Cervera-March$ Kazue J. Nagano? Marc A. Anderson? and Kenzi Horil Department of Chemistry, Faculty of Liberal Arts, Yamaguchi University, Yoshida, Yamaguchi 753, Japan; Department of Chemical Engineering, University of Barcelona, Marti i Franques, 1. Barcelona 08028, Spain; Water Science and Engineering Laboratory, Water Chemistry Program, University of Wisconsin, 660 North Park Street, Madison, Wisconsin 53706; and Institute for Fundamental Research of Organic Chemistry, Kyushu University, Hakozaki, Higashi-ku, Fukuoka 812, Japan Received: March I , 1995; In Final Form: July 13, 1995" The photoassisted catalytic degradation of trichloroethylene (TCE) in the gas phase was studied using a packed bed reactor with Ti02 pellets prepared by sol-gel techniques. A primary product of the reaction was monochloroacetic acid at 23 "C while TCE was completely mineralized to C02 and HC1 at 64 "C. A b inifio molecular orbital calculations served to elucidate the mechanism of the TCE degradation. According to the frontier molecular orbitals of TCE, on the catalyst surface the OH radical attacks preferentially at the CC12 side of TCE, path A. The formation of the radical intermediate CHClCCL(0H) was estimated to be exothermic by -41.0 kcal mol-'. The large stabilization energy is in agreement with the experimental results that TCE is degraded very rapidly and the reaction was not dependent on the temperature of the reaction. A C1 radical elimination from the intermediate produces 1,2-dichloroethenol, which changes its form to monochloroacetyl chloride. This is the so-called keto-enol tautomerism with the large activation energy of 53.3 kcal mol-'. The water assists by lowering the activation barrier of the tautomerism by 21.5 kcal mol-'. Hydrolysis of monochloroacetyl chloride or its reaction with a OH radical produces monochloroacetic acid, the primary product of our catalytic system. The addition of OH radical to the CHCl side favorably occurs in the gas phase, path B, and forms a radical intermediate CHCl(0H)CCh. Then, it releases a C1 radical to form 2,2dichloroethenol followed by the tautomerism to produce dichloroacetaldehyde. The activation energy of the reaction turned out to be as large as 64.2 kcal mol-'. Such a high activation barrier is responsible for the previous observation that only 2,2-dichloroethenol was detected in the homogeneous gas phase degradation of TCE.
Introduction Volatile chlorinated organic compounds (VOCs) such as trichloroethylene (TCE) and tetrachloroethylene (PCE) have been widely used as industrial solvents for degreasing of metals and for dry cleaning.' Many soils and groundwater supplies have become contaminated as a result of leaks from underground storage tanks and improper disposable practices.2 Soil vapor extraction (SVE) and groundwater air stripping are the methods now in use to remediate VOC-contaminated groundwater. These methods do not decompose these pollutants but shift them to the atmosphere. Recently, much attention has been focused on their concentrations in the atmosphere since many of these compounds are carcinogenic and/or toxic. Therefore, it is urgent to develop effective and inexpensive remediation techniques for the contaminants. Many authors have described heterogeneous photocatalytic techniques using semiconductor particles which are expected to be capable of completely decomposing the organic pollut a n t ~ . ~Most - ~ of the works used powder suspensions of Ti02 in aqueous solution^.^-'^ Ollis and co-workers reported that TCE was completely mineralized to C02 and HCl in a dilute aqueous s ~ l u t i o n . ~ - ' ~ A combination of S V E units with gas-phase reactors could be one of the alternative techniques for decontaminating VOCs. ~
______~
Yamaguchi University University of Barcelona 5 University of Wisconsin Kyushu University Abstract published in Advance ACS Abstmcts, October 1, 1995
*
+
@
Dibble et al. first reported the photocatalytic oxidation of TCE using Ti02 powder in a gas phase system without identifying the reaction product^.'^-'^ Recently, Nimlos et al. also adopted the Ti02 powder catalyst to photocatalytically oxidize TCE and PCE in the gas phase. This system produced dichloroacetyl chloride and phosgene as the products so that the degradation is still incomplete.I8 We used the synthesized Ti02 pellets for the TCE photodegradation and found that monochloroacetic acid (MCAA) was formed as the primary products at room temperature while TCE was completely mineralized to COz and HC1 at 64 0C.19920 Hydroxyl radicals on the Ti02 surface initiate the photocatalytic degradation of the chlorinated compound^.'^^^'^^^ The radicals attack the CC12 side (path A) of TCE (1) to form a radical intermediate 2 as shown in Scheme 1. As will be discussed later, after several steps of reactions, 2 is led to monochloroacetyl chloride (3) which reacts with H20 to form monochloroacetic acid (MCAA, 4), the product of our catalytic system. There is an alternative path that the OH radical adds to the carbon atom of the CHCl side and form a radical 5 (path B). Dichloroacetaldehyde (6), which is oxidized to dichloroacetic acid (DCAA, 7), forms through the reactions similar to those for 2. In the homogeneous oxidation of TCE with OH radicals, Kirchner et al. identified 2,2-dichloroetheno11 which is the product of path B. They used the reaction of NO2 and H2 in a discharge-flow apparatus in order to produce OH radicals in the gas phase. The reaction involving radical species contains some fast elementary reactions, the kinetics and structures of intermediates
0022-3654/95/2099-15814$09.00/0 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 43, 1995 15815
Catalytic Degradation of Trichloroethylene
SCHEME 1
- 0.2 80 -
0
100
PathA (TQ suff.cs1
0
0
. o
C,I
,Cl
3
,c=c, CI
ti
Ha
MCAA
C
4
'$
0
e 8 5 Path B (in the gas phase)
6
60-
o
40
-
20
-= I
DCAA
"
7
of which are unable to be detected experimentally. Ab initio molecular orbital (MO) calculations allow us to optimize geometries of the unstable intermediates and to estimate the energetic relation among reactants, intermediates and products. In the present paper, therefore, the theoretical method served to investigate the reaction mechanism of the TCE degradation. We described the experimental results and proposed the reaction mechanisms, which can explain well the experimental results obtained, for the heterogeneous and homogeneous degradation of TCE. The proposed mechanisms were discussed in terms of both the experimental and the theoretical results.
Results and Discussion Photodegradation Experiments. The space time,,' which is estimated by dividing the amount of catalyst by the inlet TCE molar flow rate, WTCE", largely relates to the degradation of TCE. Given in Figure 1A is the effect of space time on the conversion efficiency, the ratio of and the TCE degraded, WTCE' - WTCE,where WTCEmeans the molar flow rate in the outlet gas. The figure shows that TCE was efficiently degraded on Ti02 pellets; the TCE conversion goes up sharply and becomes 100% at the space time of 1.7 x lo7 g s mol-'. The curve for the conversion at 23 "C almost overlaps with that at 64 "C so that the temperature is not an important factor for degrading TCE. Although TCE similarly disappears at both the temperatures, the product distribution is also dependent on the space time. The distribution is well described by the stoichiometry ratio
2
2.0
4
6
a
10
0
0
B w
c
8 r-
1.o
0 0 0.5
0
Experimental Section The photodegradation experiments were carried out in a packed bed tubular photoreactor in a non-circulating mode. Four 4 W fluorescence black light bulbs (GE F4T5-BLB) surrounded the tubular reactor. The apparatus and analytical method were described p r e v i o u ~ l y . ' ~The - ~ ~Ti02 pellets were prepared by the sol-gel method and fired at 300 OC. The diameter of the pellets was 1 mm. Analysis of Brunauer-Emmett-Teller (BET) adsorption isotherm showed that the pellets have porosities of 5 0 4 6 % and specific surface areas of 160-194 m2 g-'. The catalysts were anatase as indicated by powder X-ray diffraction analysis. The ab initio MO calculations were canied out on the GAUSSIAN86 program23at the Institute for Molecular Science and the GAUSSIAN92 program24for the Fujitsu S4/10 (SUN SPARCstation 10) computer. The energy gradient method was served for optimization of molecular geometries including transition states (TS). All the optimized geometries were checked with vibration frequency calculations. The intrinsic reaction coordinatesZ5(IRC) were calculated in order to check and obtain energy profiles of the proposed reactions. We used the 6-31G basis26set for all calculation. For the better energy description, we estimated MP2/6-3 lG*//HF/6-31G energies.
0
2
4
6
8
10
Space Time (10' g s mol")
Figure 1. Effect of space time on TCE conversion (A) and stoichiometry (B) at 23 "C (9)and 64 "C (0).The molar fractions of TCE, 0 2 , and water vapor were 4.5 x 0.02, and 3 x respectively. ~,,TcE, the
ratio of the molar flow rate of a product Wj in the outlet gas and that of the TCE degraded, WTCE"- WTCE,where j represents a component of the products. For example, the ratio for CO2 is expressed by YCO~,TCE = WC02/(WTCEo - WTCE) and the complete mineralization of TCE appears at the value of YCO*,TCE = 2.0 since TCE includes two C atoms. As shown in Figure 1B which depicts the dependence of the stoichiometry ratio on the space time, the larger space time has the YCO~,TCE value increased. Even at the space time of 8.0 x lo7 g s mol-', however, YCO~,TCEwas ca. 0.8 at 23 "C since the other part of TCE is degraded to monochloroacetic acid (4) identified as the primary product of our reaction system at the lower temperature. On the other hand, the space time of 6 x lo7 g s mol-' was enough to completely mineralize TCE at 64 OC. Although YCO~RCE, which does not alter in the range of the space time between 8 x lo5 and 5 x lo6 g s mol-', is around 0.35 at 23 "C, the TCE conversion increases from 13 to 58%. It follows that the major part of TCE is degraded to 4 which is accumulated on the Ti02 surface. In order to explain this behavior of the stoichiometry ratio, we have to consider the following parallel paths,
+ 3/202 -2C0, + 3HC1 Cl,C-CHCl+ 2H20 - ClCH,COOH + 2HC1 ClCH,COOH + 3/202 -2C0, + HC1-k H,O Cl,C=CHCl+ H,O
(1) (2) (3)
Equation 1 represents the path that TCE directly decomposes to the final products, C02 and HC1. The formation of the intermediate 4 (eq 2) is followed by its oxidation to C02, HCl, and H20 (eq 3) especially for the reaction at the higher temperature. If the decomposition of 4 is slow enough to neglect at the lower temperature, the reaction can be discussed without taking eq 3 into account.
15816 J. Phys. Chem., Vol. 99, No. 43, 1995
Yamazaki-Nishida et al.
SCHEME 2
a\ Hol,;c-c; a
P
A2b %;c-cIiH P * Q n
H
a
"
-cr
@ C' a-c-c,
II O
P ,la
0 0 (a) TS 11
H
(b) TS 12
a c
Q
O H
7
7
According to eqs 1 and 2, Wj is expressed as follows,
1
4
6
(c) TS 13
b o , = 2x1
WMCAA=x2 where XI and x2 are the contributions of eqs 1 and 2 to the TCE degradation, respectively. Combining eqs 4,5, and 6 gives the following equation:*
(e) TS 15
(f) TS 16
6
6
or
5
x*
7
The stoichiometry ratio Y H C ~ C E= WHCI/(WTCE"- W ~ E ww ) observed to be 2.1 f0.2 at 23 oC.20 This ratio and the obtained value of Y C O ~ ~ C= E WC~J(WTCE' - WTCE) = 0.35 meet eq 8. Furthermore, combining eqs 4, 5 , and 6, the following expressions can be derived:
By using the stoichiometry data above, one obtains XI M (2/9)X2 so that at 23 "C, 18.2% of TCE is degraded via the path represented by eq 1 and 8 1.8% by eq 2. Most of the TCE was degraded via the formation of monochloroacetic acid. Frontier Molecular Orbitals of TCE. The frontier molecular orbitals (FMO's), the highest occupied MO (HOMO) and the lowest unoccupied MO (LUMO)of TCE, largely relate to the initial step of degradation of TCE on Ti02 surface. Therefore, a preliminary calculation of TCE was performed using the PM3 Hamiltonian in the MOPAC Ver. 6 The orbital coefficients of C(2), the carbon atom at the CC12 side, were 0.411 and -0.670 in HOMO and LUMO,respectively, while those of C(3) at the CHCl side were 0.463 and 0.686. It is noteworthy that both the coefficients of C(3) are larger than those of C(2). In the TCE degradation on the Ti02 surface, the reaction involves species adsorbed on the catalyst surface in that the Langmuir-Hinshelwood-Hougen-Watson (LHHW) formula explained the observed kinetic data.I5 It is known that the n-electron donation from the n-orbital, the HOMO of ethylene,
1
Figure 2. Optimized structures for the transition states (a) 11, (b) 12, (c) 13, (d) 14, (e) 15, (f) 16, (8) 21, and (h) 22.
to the Ti d-orbital is responsible for the chemisorption on TiC4.30-32 According to the FMO theory,33the carbon with a larger coefficient interacts stronger with the transition metal. Therefore, TCE uses its p orbital of the C(3) atom in chemisorbing on the catalysis surface and then OH radicals can only attack the other carbon atom C(2) of TCE. It follows that the reaction on the Ti02 surface proceeds via path A. In the gas phase reactions observed by Kirchner et al., OH radicals directly attack TCE; the OH radical donates its electron density to the LUMO and accepts it from the HOMO, respectively.33 nail
0.463
-0.670
0.686
C
CI
C
H
Therefore, both the orbitals of TCE play important roles in determining which carbon of TCE accepts the radical. Since the orbital coefficients of C(3) in HOMO and LUMO are larger than those of C(2), the addition of OH radical favorably occurs at C(3). The homogeneous gas phase reaction, therefore, proceeds via the radical intermediate 5 (path B). Reaction Mechanism on the Ti02 Surface (Path A). Scheme 2 shows three feasible reaction paths which lead to
J. Phys. Chem., Vol. 99, No. 43, 1995 15817
Catalytic Degradation of Trichloroethylene 1
a\ OH'
0-H
CI\
F'
CI'
+ cl/c=c'H
,, .c1 --c I , , H 'CI
TS
11
I
c-c
'\\
p
CI,
O H :
pi i
/ CI
C,/C-C\H 12
+ CI'
2 10
3
Figure 3. Energy-level diagram of TCE degradatic3n with OH radical (path A). TABLE 1: Total Energies, Optimized Bond Lengths, and Bond and Dihedral Angles: for 8, TS 13, TS., 14, and 3 for Path A 8 TS 13 TS, 14 3 E(6-3 1G) E(MPY6-3 1G*) C(2)-C(3) H(6)-C(3) c(2)-0(5) 0(5)-H(6) LH(7)-C(3)-C(2) LH(7) -C (3)-Cl(4) LC(3)-C(2)-0(5) LC(3)-C(2)-Cl(l) LC(2)-0(5)-H(6) LH(6)-C(3)-H(7) t[H(7)-C(3)-C(2)-C1(4)] t[C(3)-C(2)-0(5)-Cl( l)] t[O(5)-C(2)-C(3)-Cl(4)]
- 1070.56816 -1071.36885 1.309 3.171 1.347 0.95 1 123.5 114.5 124.8 120.0 116.6 141.3 180.0 180.0
0.0
-1070.44033 - 107 1.28387 1.425 1.547 1.271 1.251 112.8 110.0 106.9 131.7 82.8 93.9 132.5 -177.9 150.4
-1146.50881 -1147.53102 1.386 1.239 1.682 122.4 110.7 128.7 113.4 103.1 -141.1 -167.8 -35.9
-1070.59701 -1071.41214 1.493 1.076 1.180 3.100 110.1 107.4 130.2 109.6 27.1 109.8 119.4 180.0
0.0
Total energy in hartrees, bond lengths (X-Y) in A, and bond angles (LXYZ) and dihedral angle (t[WXYZ]) in degrees. W, X, Y, and Z refer to atoms as in Figure 2. formation of acid chloride intermediate 3 from the radical intermediate 2 via the intermediates 8,9, and 10. A C1 radical elimination brings 2 to 1,2-dichloroethenol(8) (path Al) which reacts with a water molecule to form 3. The second path (path A2), which produces an intermediate 9, includes the migration of the H atom of the OH fragment to C(3). 9 loses a C1 radical from the CChO fragment and produces 3. The last path (path A3), which requires the HC1 elimination followed by the addition of an H radical, passes through an intermediate 10. The transition state geometries for paths A2 and A3 are depicted in Figure 2, a and b, respectively. The energy-level diagram for paths Al-A3 are depicted in Figure 3. The formation of 2 tumed out to be exothermic by -41.0 kcal mol-'. Such a large stabilization energy should be responsible for the observation of the rapid degradation of TCE and its temperature independence. The activation energies of the H migration and HC1 elimination were estimated to be 38.7 and 26.5 kcal mol-', respectively. Path A1 includes a homolytic C-C1 bond cleavage, which is considered not to have a higher activation energy than the energy difference (16.9 kcal mol-') between 2 and 8 C1. This energy is smaller by 21.8 and 9.6 kcal mol-' than the activation energies of paths A2 and A3, respectively. It follows that path A1 is the most probable to reduce the concentration of 2.
+
It is well-known that vinyl alcohol is unstable and easily tautomerizes to acetaldehyde in solution, i.e., the keto-enol t a ~ t o m e r i s m .There ~ ~ should be a similar equilibrium between chloro-substituted vinyl alcohol (8) and 3. Figure 2c shows the TS structure of the mechanism that the H atom directly moves from the OH fragment to the CHCl moiety. The geometrical parameters of 8, TS 13, TS,, 14, and 3 are summarized in Table 1. The z[H(7)-C(3)-C(2)-C1(4)] angle in the TS was calculated to be 132.5", and C(3) has already changed its hybridization from sp2 to sp3 in order to accept an H atom from the OH fragment. The LC(3)-C(2)-0(5) angle, which is calculated to be 106.9", deviates very much from that in 8. Such geometrical features of the TS cause the activation energy as high as 53.3 kcal mol-', as shown in Figure 4A. It follows that no H atom transfer occurs via this direct mechanism. It was reported that a solvent water molecule participates in lowering the activation barrier for the H-transfer reaction.35As the gas with moisture was introduced into the photoreactor with the Ti02 catalyst, there is an alternative mechanism that a water molecule assists the H migration; the H20 molecule accepts a proton from 8 and donates other proton to the CHCl fragment via the TS shown in Figure 2d. Figure 5 displays the energy profile along the IRC of the water-assisted mechanism. The energy rises gradually until s
Yamazaki-Nishida et al.
15818 J. Phys. Chem., Vol. 99, No. 43, 1995
B
A
TS 21 m I
TS 13
(I 64.2
\
I
53.3 I
\
/
\
I
TS, I It
'\\
\I \
A -
\
\
\
\
\
\ /I (31.8)
I
\
'\
I I
I
I
I
It
0.0 I
. 11.3 H\
I
\
/
0.0
\
3
\ \
I
I
CI I
! \
I
I \-
\
I
I
\\
I
14
22
28.7
II
20.5
I
H
\
TS,,
\
/
/CI
\
I
\
/
HO\
\
I
\
I
0
\
I I
'\\
I
I I I II
CI-c-c.
\\
6
\
\
;
7
/H t
4
\
'\
- 11.9
I I I I I 1
d
H'
0-H
'
\ \
I
\ \
1
I II
-
\
I
CI
I
"\c-0 H,
\
I I I
H\
0-H
- 23.7
-14.6
I
I'
CI
c=o I
C=O
H
I
CI
Figure 4. Energy-level diagram of the keto-enol tautomerism between 8 and 3 (A),or 17 and 6 (B).TS,, indicates the transition state including a H20 molecule (see text). The values in the figure are the energies of each relative to the position of alcohols 8 and 17.Those in parentheses are the activation energies for the water assisted tautomerism.
__
,
-6
.
,
-4
.
I
-2
.
6
.
I
2
.
,
.
4
IRC (amu"' Bohr)
Figure 5. Potential energy profile for the keto-enol between 8 and 3.
tautomerism
= -2.0 amu'/' bohr and goes up sharply until the TS.25c The potential curve drops rapidly from the TS to s = 3.0 bohr. The decrement of the energy between s = 3.0 and 6.0 amu112bohr is less than 10 kcal mol-'. Therefore, the drastic change in the energy was seen between s = -2.0 and 3.0 amu1I2 bohr and largely relates to the geometrical change along the IRC. The structures of the several points on the IRC are depicted in Figure 6. The H(6) in the OH fragment moves gradually toward C(3); H(6) is ready to move toward O(9) at s = -1.59 amu112bohr: it locates the middle of O(5) and 0(9) at s = -0.99 amu112bohr: the TS, where H(8) of the H30' moiety begins to migrate to C(3), has no H(6)-O(5) bond and includes an oxonium ion: there is a weak bond between H(8) and C(3) at s = 1.00 amu112 bohr: the 0(9)-H(8) bond disappears and the H(8)-C(3) bond has already formed in the geometry of s = 2.19 amu112bohr. The activation energy via the TS,,14 turned out to be 31.8 kcal mol-'. The banier of
the mechanism is smaller by 21.5 kcal mol-' than the direct H atom transfer mechanism. Ti02 may has an another effect to reduce more the activation energy of the keto-enol tautomerism. On the catalyst surface 0 2 cannot react with 2 produced so that 3 forms via path A1 shown in Scheme 3. However, some portion of the radical 2 on the surface loses the C-Ti bond and they diffuse into the gas phase. The carbon-centered radicals immediately react with 0 2 to form the corresponding peroxy radical^.^^,^^ Indeed, this process was estimated to be exothermic by - 14.1 kcal mol-'. This reaction should be responsible for the constant stoichiometry ratio of ca. 0.35 in the low space time regions. This is the direct path represented by eq 1. Recently, Glaze and co-workers described the reductive degradation of TCE by electrons photogenerated in the conduction band.38339This reaction leads to the formation of dichlorinated byproducts 6 and 7. They performed the experiments in aqueous solutions where the concentration of oxygen dissolved is limited. Under our reaction conditions in the gas phase, the molar fraction of 0 2 is excess relative to that of TCE. Therefore, the fate of the conduction band electron is to reduce oxygen to superoxide and the reductive degradation of TCE is negligible. The superoxide ion 0 2 - contributes to additional formation of OH radicals.40 Reaction Mechanism in the Gas Phase (Path B). The FMO's suggest that the OH radical attacks the C(3) of TCE and initiates the degradation in the gas phase as discussed above. It is possible to similarly considei three candidates as the reaction paths for formation of dichloroacetaldehyde (6) from the radical intermediate 5. The first is path B1 that 5 initially releases a C1 radical to form 2,2-dichloroethenol (17). The activation energy is considered to be less than 20.0 kcal mol-' which is the energy difference between 5 and 17 C1 (see Figure 7). As discussed below, this unsaturated alcohol tautomerizes to its keto form 6.
+
J. Phys. Chem., Vol. 99, No. 43, 1995 15819
Catalytic Degradation of Trichloroethylene 10
A
6
7
5 b 1
s = - 5.37
s = 0.00 (TS)
s = - 3.39
s = - 1.59
s = - 0.99
s = 1.00
s = 2.19
s = 3.19
s = 5.16
Figure 6, Change of the geometry along the IRC in Figure 5. 1
CI\
20
/CI
CI/'='\H
a\ /a a,c=c\oH
+OH'
40.5
40.3 7
+H'
38.5
0
17
+ CI'
16
19
Figure 7. Energy-level diagram of TCE degradation with OH radical (path B).
SCHEME 3 2
1
-Ti-
.
I
-Ti-
Path B2 requires the migration of the H atom of the OH fragment to the CC12 side of 5. This reaction produces a radical 18 and has an activation energy of 40.5 kcal mol-'. An HCl is eliminated from 5 to form a radical 19 in path B3, which possesses the activation barrier of 3 1 .O kcal mol-'. The obtained TS geometries for paths B2 and B3 are displayed in Figure 2, e and f, respectively. The activation banier for path B1 is smaller by 20.5 and 11.O kcal mol-' than those for paths B2
-Ti-
I
-Ti-
and B3, respectively. Therefore, 17 forms via path B1. 5 decomposes via another route shown in Figure 7; the radical releases an H radical to produce trichloroethenol (20) which tautomerizes to dichloroacetyl chloride. However, this reaction is more endothermic than path B1 by 18.5 kcal mol-'. There are two tautomerism paths, which are similar to those for 8. The geometrical parameters of 17, TS 21, TS, 22, and 6 are summarized in Table 2. Figure 2g shows the TS structure
15820 .I Phys. . Chem., Vol. 99, No. 43, 1995
Yamazaki-Nishida et al.
TABLE 2: Total Energies, Optimized Bond Length, and Bond and Dihedral A n g l e for 17, TS 21, TS,, 22, and 6 for Path B 17 TS 21 TS, 22 6 E(6-31G) E(MP2/6-31G*) w-c(3) H(6)-C(2) C(3)-0(5) 0(5)-H(6) LC1( 1)-C(2)-C1(4) LC(2)-C(3)-0(5) LC(2)-C(3)-H(7) LC(3)-0(5)-H(6)
t[Cl(l)-C(2)-C(3)-CI(4)] t[C(2)-C(3)-0(5)-H(7)]
- 1070.56136 - 1071.36281 1.312 3.151 1.361 0.949 115.4 121.6 120.2 115.3 180.0 180.0
Total energy in hartrees, bond lengths (X-Y) in 2.
to atoms as in'igure
- 1070.43460 - 107 1.26046 1.498 1.635 1.252 1.164 109.9 103.7 134.2 90.1 119.9 180.0
-1 146.49248 - 1147.51202 1.410 1.262 1.552 111.6 122.9 117.5 106.1 -135.3 -169.9
A, and bond angles - (LXYZ) and dihedral angle (t[WXYZ]) in degrees.
which connects 17 and the product 6 . The z[Cl(l)-C(2)C(3)-C1(4)] angle (1 19.9') in the TS deviates from the sp2value in 17 since C(2) has almost sp3 hybridization. The change in the dihedral angle around C(2) in TS 21, z[Cl(l)-C(2)-C(3)C1(4)], is smaller by 12.6' than the corresponding angle of TS 13, z[H(7)-C(3)-C(2)-Cl(4)], in path A. The activation energy given in Figure 4B tumed out to be 64.2 kcal mol-' which is larger by 10.9 kcal mol-' than the corresponding value of path A. The assistance of a water molecule decreases by 23.6 kcal mol-' the energy barrier of the keto-enol tautomerism. In the reaction conditions of Kirchner et al.,' the reactant gas stream contains neither 0 2 nor H20 and they obtained 17 as a product. The reaction of the carbon-centered radical 5 with 0 2 does not proceed. There is no H20 assistance to reduce the activation energy in the keto-enol tautomerism, either. The energy barrier as high as 64.2 kcal mol-' prevents 17 from transferring the H atom to form dichloroacetaldehyde. The OH radical-initiated reaction stops at the formation of 17 and no further reactions proceed in the gas phase. Therefore, the results of the present calculations are well consistent with the experimental results.
Conclusions The FMO's of TCE suggest which side, the CC12 or the CHCl side, is preferable for the OH radical attack; the analysis of the orbitals indicates that the OH radical preferentially attacks the CHCl side in the gas phase. On the other hand, in the heterogeneous system, TCE adsorbs on the surface of Ti02 whose d orbitals can effectively interact with the p-orbital of the carbon at the CHCl side. Therefore, the addition of the OH radical is possible only at the CC12 side. The mechanism shown in Scheme 1 explained well the formation of monochloroacetic acid on the Ti02 catalyst or 2,2-dichloroethenol in the gas phase. In order to produce OH radical the presence of water vapor is essential in the heterogeneous TCE degradation over Ti02 catalyst. The present study also indicates that water molecule has an another role for the reaction mechanism; it assists the tautomerism of 1,2-dichloroethenol by lowering the activation energy by 21.5 kcal mol-'. Nimlos et al. described that the C1 radical-initiated mechanism is a primary pathway for the heterogeneous TCE photodegradation using commercial Ti02 catalyst.'* In our systems using the porous Ti02 catalyst prepared, the main pathway is the OH radical-initiated mechanism. The difference of the reaction mechanism in both catalytic systems will be discussed in details in another publication.
Acknowledgment. This work was supported in part by Grant-in-Aid for Scientific Research provided by the Ministry
- 1070.57645 -1071.38605 1SO6 1.074 1.203 3.046 111.3 124.2 113.9 33.0 124.0 178.5 W, X, Y, and Z refer
of Education, Japan (No. 06740533). The authors thank the Computer Center, Institute for Molecular Science at the Okazaki National Research Institutes, for the use of the HITAC M 200 H Computer.
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