1436
Znd. Eng. Chem. Res. 1994,33, 1436-1443
KINETICS, CATALYSIS, AND REACTION ENGINEERING Photodecolorization of Methyl Orange Using Silver Ion Modified Ti02 as Photocatalyst LungChyuan Chen and Tse-Chuan Chou' Department of Chemical Engineering, National Cheng Kung University, Tainan, Taiwan, R.O.C. 701
The photodecolorization of methyl orange catalyzed by the Ag+ ion modified Ti02 suspended solution was both theoretically and experimentally studied. A semitheoretical kinetic equation was obtained, Ri = (((6.32 X 10-3)(0.11)IMO~l)/(l 0.11[MOb]))[Ti02]0.24[Ag+b]0.46p0.46, which correlated the experimental results well with an average deviation of 2.2 % The results revealed that Ag+ ion could trap photogenerated electrons to avoid the recombination of electrons and holes and significantly improve the photodecolorization of methyl orange. The experimental results also indicated that Ag+ ion was more favorable for promoking this reaction than Cu2+,Co2+,Fe3+,and Ce4+ions. The counterion NO3- was better than s04'- ion for photodecolorization of methyl orange. The activity of rutile form Ti02 was comparable t o the anatase form in the presence of Ag+ ion. T h e effect of bubbling oxygen and nitrogen in photodecolorization of methyl orange was insignificant in the presence of Ag+ ion. On the other hand, the effect of bubbling oxygen was significant in the absence of Ag+ ion. The decolorization rate increased with pH and reached a maximum value a t p H 8.75, and then decreased in higher pH due to the precipitation of Ag+ cation with OH- anion. The calcination temperature of Ti02 gave an insignificant effect on the photodecolorization of methyl orange in the presence of Ag+ compared t o that in the absence of Ag+ ion. Increasing loading of Ag+ ion significantly increased the apparent primary quantum yield.
+
Introduction Heterogeneous photocatalytic reactions using Ti02 as catalyst have been paid much attention recently (Bahnemann et al., 1991; Barbeni et al., 1984; Bideau et al., 1980, Chen and Chou, 1993a; Frank and Bard, 1977; Harvey et al., 1983; Hidaka et al., 1988; Hsiao et al., 1983; Matthews, 1991, 1987; Mozzanega et al., 1979; Nguyen and Ollis, 1984; Okamoto et al., 1985; Ollis et al., 1984; Pruden and Ollis, 1983; Sabate et al., 1991; Wei and Wan, 1991). Many photoreactions can be achieved in this way, such as energy storage (Fujishima and Honda, 19721, decomposition of organic pollutants (Bahnemann et al., 1991; Chen and Chou, 1993b), and organic synthesis (Fujihira, 1982). It is a potential technique in various industries. However, the quantum efficiency is low by using bare Ti02 as catalyst due to the fast recombination of photogenerated electrons and holes (Ward and Bard, 1982). Some reports tried to overcome the disadvantages of recombination by using metalized Ti02 as catalyst. Kiwi and Gratzel (1984) applied Pt/TiOz for photochemical water cleavage; Kogo et al. (1980) employed Pt/TiOz for the photocatalytic oxidation of cyanide. Both reports indicated that platinization of Ti02 could increase the photoefficiency. However, the platinized procedure introduces difficulty in the application of this method; furthermore, platinum is a very expensive noble metal. The reaction mechanism of photocatalytic oxidation using metalized Ti02 as catalyst is also unclear (Truchi and Ollis, 1990). Okamoto et al. (1985) charged Cu2+ ion to the Ti02 suspended solution for the photooxidation of phenol and
* Author to whom correspondence should be addressed. 0888-5885/94/2633-1436$04.50/0
.
pointed out that the photooxidation rate increased significantly. On the other hand, Cu2+was also reduced to Cu metal and deposited on Ti02 in this process. This technique can be applied to recover the noble metal ions and to treat organic pollutants simultaneously; hence, it is particularly suitable for the treatment of wastewater which usually contains both heavy metal ion and organic pollutants. However, few or no reports concerned the kinetics of photoreaction catalyzed by Ti02 in the presence of noble metal ions. Especially the role of metal ions in the photoreaction is unclear. Photodecolorization of wastewater containing azo dyes using Ti02 as catalyst has been reported in our previous reports (Chen and Chou, 1993a,b). Methyl orange was chosen as a model compound of azo dyes. It is interesting to study the photodecolorization of methyl orange using Ag+ ion modified Ti02 as catalyst.
Experimental Section Materials. Laboratory-grade titanium dioxide powder (Merck) with 7.5 m2/g surface area was used as photocatalyst. The lattice of titanium dioxide was proved to be predominantly anatase by X-ray diffraction (Siemens D500). Silver nitrate, sodium hydroxide, and hydrochloric acid were supplied by Wako Pure Chemical Industries. All chemicals were extra pure grade and were used as received, without further purification. The reactant solution was prepared by mixing the desired amounts of methyl orange and distilled water (Millipore Q). Photoreactor. A 250-mL quartz reactor of 6.56 cm in diameter and 8.11 cm in height was used as the photoreactor. A 1000-W ORIEL super high pressure mercury arc lamp built in a light house with rear reflectors was 0 1994 American Chemical Society
Ind. Eng. Chem. Res., Vol. 33, No. 6, 1994 1437 used as the light source which was located at 30 cm in front of the reactor. The output intensity of the light source could be adjusted and was proven to be proportional to the intensity of the power supply (Chen and Chou, 1993a). Accordingly, the intensity of power supply was used to represent the illumination power. A magnetic stirrer operated at 500 rpm was used to provide a good mixing of the suspension solution. Procedures. At the beginning of a run, a desired amount of methyl orange solution was fed into the reactor. Then, the desired amounts of both photocatalyst, titanium dioxide, and modifier, silver nitrate, were added. Nitrogen was bubbled through a gas disperser into the reactor and vented through the condenser at a flow rate of 200 mL/ min, unless otherwise described. The pH of the reaction solution was adjusted by using NaOH and HCl solutions. The mercury arc lamp was operated at 800 W by adjusting the intensity of the power supply. The reaction time was recorded as the light was started. Samples were taken periodically from the reactor by using a pipet and were analyzed by a Jasco UV-vis spectrophotometer at 465-nm wavelength with a calibration curve after centrifugation and filtration by a 0.2-pm syringe filter made of poly(vinylidene fluoride). The pH values were determined by a pH meter (Activon, A211). The UV intensity was measured by a UVX radiometer (UVP, Inc.).
Table 1. Results of the Preliminary Tests. ~~
[Mob], r M [Ag'b], p M 392 1150 1950 2730 392 392 1950 392 392
0
[TiOt], g/L 0.0 0.0 0.0 0.0 0.0 6.7 6.7 6.7 6.7 0.0
illumination -b
-
+c -
+
0 rnin
30 min
90.1 90.7 88.6 89.8 90.8 91.4 91.3 61.2 30.6 91.3
90.1 90.6 87.9 47.5 81.6d 90.5 90.2 60.2 30.5 91.3
a Temperature = 27 f 1 "C; stirring rate = 500 rpm; pH = 6 f 0.2. Without illumination. With illumination. With a rate of 0.3rM/ min.
On the other hand, the reactive species HO' may terminate on the inactive surfaces which include the wall of the reactor, the inactive surface of the catalyst, and the inactive media in the reactor, such as HzO. HO.,
+ S(inert surface)
ks
inactive species
(8)
The rate equation was obtained as shown in eq 9 (the detailed derivation is shown in the Appendix).
Theoretical Analysis Reaction Mechanism. Based on the proposedreaction mechanism of photodecolorization of methyl orange without Ag+ cation (Chen and Chou, 1993a) and the experimental influence of Cu2+ion on the photooxidation of phenol (Okamoto et al., 1985),the reaction mechanism of photodecolorization of methyl orange in the presence of Ag+ ion was proposed. The electrons and holes were generated when Ti02 was exposed to ultraviolet illumination and then the electron was trapped by Ag+ ion or recombined with holes and evolved heat as shown in eqs 1-3. The Ag metal deposited on the surface of Ti02 can
ki
TiO,
X Ag2S04 > Cu(N0312 > Co(N03)2 = Fe(N03)~ > CuSO4 = Ce(S04)~.The metal ions might be adsorbed on Ti02 surface and complexizeor precipitate with methyl orange (Chiu and Wei, 1975). A 30-min premixing is enough to reach the equilibrium concentration, which was defined as the remaining concentration of
- -
1438
Ind. Eng. Chem. Res., Vol. 33, No. 6, 1994
Table 2. Effect of Various Metal Salts on the Photodecolorization of Methyl Orange* metal salt equilib concn, pM C~(N03h 51.1 CO(N03)z 29.4 AgW 52.3 AgzS04 51.7 Fe(N03h 41.3 cuso4 50.8 Ce(S04h 3.1 a
init pH 5.13 5.15 4.87 5.23 5.07 4.89
init rate, pM/min 0.15 0.08 1.99 1.12 0.06 0.02 0.01
Concentration of initialmethylorange = 53 i 1pM; temperature
= 28 f 1 "C; concentration of metal ion = 392 pM; stirring rate =
500 rpm; illumination power = 800 W; Nz flow rate = 200 mL/min.
methyl orange after 30 min of premixing of the solution of salt compound and suspended Ti02 without illumination. The equilibrium concentrations of methyl orange were 52.3, 51.7, 51.1, 50.8, 41.3, 29.4, and 3.1 pM when AgNO3, Ag2S04, Cu(N012, CuSO4, Fe(N0313, Co(N03)2 and Ce(S04)2were added, respectively, as shown in Table 2. The redox potentials of Ag+/Ag, Cu2+/Cu+,Fe3+/Fe2+, C03+/C02+,and Ce4+/Ce3+are 0.799,0.521,0.771,1.82, and 1.61 V vs normal hydrogen electrode (Bard et al., 19851, respectively. On the basis of these data, all these metal ions could trap the photoelectron generated by Ti02 under illumination. However, only Ag+ ion showed a promotive effect on this reaction system. Other metal ions revealed either inhibitive or unfavorable influence on this reaction. The decolorization rate correlated well with the equilibrium concentration of methyl orange, except for Fe(N03)3 and Co(NO&. The results indicated that more positive charge of the metal ion leaded to a lower equilibrium concentration of methyl orange and a lower decolorization rate. Since Fe3+ ion easily reacted with OH- to become hydroxyl compounds or reduced to Fe2+ion, the precipitation or complexation of Fe3+ ion with methyl orange decreased and developed a higher methyl orange equilibrium concentrationthan that of Co2+ion. Furthermore, the redox potential of Co3+/Co2+was larger than that of Fe3+/Fe2+; consequently, the decolorization rate in the presence of Co3+/Co2+was larger than that of Fe3+/Fe2+. The presence of sulfate ion 504%was more poisonous than NO3- to the surface of Ti02 photocatalyst. A lower reaction rate was obtained by using Ag2S04 instead of AgN03. However, the equilibrium concentration of methyl orange was not affected by changing NO3- to sod2-.This indicates that the decrease of methyl orange concentration by adding metal ions was almost due to the precipitation of metal ions with methyl orange molecules in the bulk solution, and did not result from the adsorption of methyl orange on Ti02 surface. The redox potential of Cu2+/Cu+ was also good enough to react with photogenerated electron, and the equilibrium concentration of methyl orange was almost equal to that of Ag+ ion. However, a negative influence on photodecolorizationof methyl orange was obtained. This result was different from the results reported by Okamoto (1985) and Fujiha (19821, who studied the photooxidation of phenol and toluene, respectively. Both of them pointed out that Cu2+showed a promotive effect on their reaction systems. The inhibitive effect of this study may result from the stable complexation of Cu2+with methyl orange molecules, and in general, the reactivity of this complex was smaller than that of the free methyl orange molecules. On the other hand, the complexation of Ag+ with methyl orange was smaller and less stable than that of Cu2+, hence the decolorization rate was the fastest in the presence of Ag+ ion.
100
90
80
-gE
60
v
50
40
30
0
10
20 Time, min
30
Figure 1. Effect of bubbling Nz and 02 on the photodecolorization of methyl orange. Concentration of methyl orange = 91 i 1 pM; temperature = 27 f 1"C; concentration of Ag+ion = 392 pM; stirring rate = 500 rpm; volume of solution = 150 mL; loading of Ti02 = 6.67 g/L; illumination power = 800 W; gas flow rate = 200 mL/min; pH = 7.0 i 0.2.
Effect of Oxygen/Nitrogen. The initial decolorization rate and the percentage of decolorization of methyl orange of a 30-min run in the presence of Ag+ ion with bubbling of N2, air, and pure 0 2 were 3.03, 3.15, and 3.34 pM/min and 56.2,57.9, and 60.2%,respectively as shown in Figure 1. In our previous report (Chen and Chou, 1993b),Nzand 0 2 showed great inhibitive and promotive effects, respectively, on the photodecolorization of methyl orange in the absence of Ag+ ion. However, the difference between bubbling N2 and 02 in this study was insignificant in the presence of Ag+ ion. The results revealed that Ag+ could take the place of 0 2 , which trapped the photogenerated electrons to avoid the recombination of electrons and holes according to eq 10. Alternatively, if the loading of Ag+ is
+ + -
0, + e-cb 02*- H+ HO,'
HO,'
H202
02'-
(10)
HO,'
(11)
H202+ 0,
2HO'
(12) (13)
not sufficient to trap all the photogenerated electrons and makes 0 2 trap few photogenerated electrons, it results in an increase of the reaction rate when methyl orange is bubbled through oxygen. The insignificant difference of decolorization of methyl orange between bubbling N2 and 0 2 could be attributable to reactions 10-13 which describe the formation of HO* from the reaction of 0 2 and electron (Okamotoet al., 1985). Consequently, this reaction mechanism depends on pH significantly. Effect of Loading of Ag+ Ion. Increasing Ag+ ion loading from 0 to 1550 p M increased the initial decolorization rate and percentage of decolorization of methyl orange at a 30-min run from 0.06 to 5.57 pM/min and 3 to 90.7 5% ,respectively. The plot of the double logarithms of the initial rate against the Ag+ ion loading gave a straight line with slope of 0.46 and intercept of -1.68, respectively, as shown in Figure 2. This infers that the relationship
Ind. Eng. Chem. Res., Vol. 33, No. 6, 1994 1439 1.3
c 1.6
1
igo
,
I
- 58
1.2 -
- 6 \ e 0)
M
i
l /
-48
1.1 -
8e
o
7
5.8
6.1
6.4
'
' 6.7
h 1
60
: i i E i o r i z a r i o n percentage "
0
-I'
I1 %/// '
8
/
O
1.2
1.0'
/
3
'
7.0
'
' 7.3
.:decolorization
"50
1.0 1.5 2.0 2.5 Ln[TiOZ] Figure 3. Effect of loading of Ti02 on the photodecolorization of methyl orange. Concentration of methyl orange = 89 f 1 pM; temperature = 27 f 1OC; loading of Ag+ ion = 392 pM;stirring rate = 500 rpm; volume of solution = 150mL; N2 flow rate = 200 mL/min; illumination power = 800 W, initial pH = 7.0 f 0.2. O"b.0
Ln [Ag+l
Figure 2. Effect of loading of Ag+ ion on the photodecolorization of methyl orange. Concentration of methyl orange = 89 f 1 pM; temperature = 27 f 1 OC; volume of solution = 150 mL; loading of Ti02 = 6.67 g/L; illumination power = 800 W N2 flow rate = 200 mL/min; stirring rate = 500 rpm; pH = 7.0 f 0.2.
percentage
0.5
between the initial rate and Ag+ ion loading can be expressed as
Ri = 0.186[Ag+b]0'46
(14)
Equation 14 correlated with the theoretical analysis one well when compared with eq 9. However, eq 14 did not hold when the loading of Ag+ ion exceeds 1550 pM; the experimental result of the reaction rate was lower than the calculated one. Increasing the Ag+ ion loading increased the trapping rate of photoelectron and then increased the decolorization rate. However, when the loading of Ag+ ion exceeded a critical value, the complexation effect was getting important. Furthermore, the illumination area decreased with Ag+ ion loading. The decolorization rate decreased with Ag+ concentration when Ag+ ion concentration was higher than the critical value. The reduced Ag metal will compete with OH- for holes. Both of these two phenomena slowed down the reaction rate of decolorization. Effect of loading of TiOz. Increasing the loading of Ti02 from 1.67 to 13.3 g/L increases the initial decolorization rate and percentage of decolorization of methyl orange in a 30-min run from 1.96 to 3.37 pM/min and 46 to 5876, respectively. The plot of the double logarithms of rate against Ti02 loading gave a straight line with slope and intercept of 0.24 and 0.63, respectively, as shown in Figure 3. This points out that the relationship between decolorization rate and Ti02 loading could be expressed as
Ri = 1.88[Ti0210.24
6.5 6.7 Ln (power) Figure 4. Effect of illumination power on the photodecolorization of methyl orange. Concentration of methyl orange = 89 f 1 pM; temperature = 27 f 1 O C ; loading of Ag+ ion = 392 pM;stirring rate = 500 rpm; volume of solution = 150mL; loading of Ti02 = 6.67 g/L; initial pH = 7.0 f 0.2; N2 flow rate = 200 mL/min. 0.8%. 1
6.3
rate against intensity of power supply yielded a straight line with a slope of 0.46 and an intercept of -1.98 as shown in Figure 4. This indicates that the initial rate was proportional to 0.46 order of intensity of power supply, which could be expressed as
(15)
The order 0.24 shown in eq 15 was smaller than 0.5, which was obtained from the theoretical analysis as shown in eq 9. Increasing the Ti02 loading decreased the amount of Ag+ ion adsorbed on TiO2, and a lower reaction order was accomplished. Furthermore, increasing Ti02 loading increased the shield and collision as reported in our previous study (Chen and Chou, 1993a). Effect of Power Intensity. Increasing the intensity of power supply from 500 to 800 W increased the reaction rate from 2.45 to 3.03 pM/min. A logarithm plot of reaction
Ri = 0.14PM
(16)
Accordingly, eq 9 was revised to
Effect of Initial Methyl Orange Concentration. Increasing the initial methyl orange concentration from 21.4 to 87.2 pM increased the initial decolorization rate
1440 Ind. Eng. Chem. Res., Vol. 33, No. 6, 1994 4.0 I
0.45
1
0.42
3.0
-
d
k0.39
'i
'i
22.0 3
\
r:
30.36
2
> 1.0
0.33
:decolorization percentage
1
0.0
Figure 5. Effect of initial concentration of methyl orange on the photodecolorization of methyl orange. Loading of Ag+ ion = 392 pM;temperature = 27 & 1"C; volume of solution = 150 mL; loading of Ti02 = 6.67 g/L; illumination power = 800 W; stirring rate = 500 rpm; initial pH = 7.0 f 0.2; Nz flow rate = 200 mL/min.
1'
I
I
I
i
3
6
9
12
4:i;:tceolorization
0
percentage
1
' 0
PH Figure 6. Effect of initial pH on the photodecolorization of methyl orange. Concentration of methyl orange = 89 f 1 pM;volume of solution = 150 mL; illumination power = 800 W, stirring rate = 500 rpm; temperature = 27 f 1 "C; loading of Ti02 = 6.67 g/L; loading of Ag+ ion = 392 p M , N2 flow rate = 200 mL/min.
from 2.30 to 3.00 pM/min; however, it decreased the percentage of decolorization of methyl orange from 96 to 57.4%. The plot of the double reciprocal of initial rate against initial concentration gave a straight line with a slope and an intercept of 2.84 and 0.30, respectively, as shown in Figure 5. This exhibits that the relationship can be expressed as
played an important role for elevating photodecolorization of methyl orange. However, when the pH value exceeded 8.75, the Ag+ ion precipi6ated with OH- to form Ago0 based on eq 21, and the reaction rate of photodecolorization
1/Ri = 2.84(1/[MOb]) + 0.30
decreased. The experimental results were comparable different with those in the presence of 0 2 and the absence of Ag+ ion, which showed a minimum reaction rate at pH 7 (Chen and Chou, 1993b). Both the initial reaction rates were 3.79 pM/min at reaction conditions of pH 8.75 and 392 pM Ag+ ion loading and of pH 7 and 706 pM Ag+ ion loading, respectively. However, the percentages of decolorization of methyl orange of a 30-min run were 81.5 and 67.9 5% for the former and the latter reaction conditions, respectively. This indicates that OH- was more efficient for the reaction of the later stage than Ag+ion. The results might be due to that Ag+ ion deposited on the Ti02 surface and slowed down the transfer of electron to the electrolyte. The surface of illumination was decreased, and led to a slower reaction rate in the later stage. Effect of Calcination Temperature and Structure of TiOz. With increase of the calcination temperature of Ti02 catalyst from 25 to 600 "C, the decolorization rate insignificantly increased from 3.03 to 3.24 pM/min in the presence of 392 pM Ag+ ion. With further increase of the calcination temperature of Ti02 to 1000 "C, the decolorization rate decreased to 3.06 pMImin in the presence of 392 pM Ag+ ion. On the other hand, in the absence of Ag+ ion, with increase of the calcination temperature from 25 to 600 OC, the decolorization rate increased from 0.17 to 0.29 pM/min and then decreased to 0.03 pMlmin when the calcination temperature increased to 1000 "C as shown in Table 3. Increasing the calcination temperature increased the surface area of Ti02 and the extent of rutile form (Chen and Chou, 1993a). However, the increase of surface area made little improvement in the reaction rate since only the exterior surface area could effectively increase the reaction rate. Accordingly, the increase in reaction rate was mainly from the change of the Ti02 structure. The extent of rutile form increased with
(18)
Equation 18 demonstrates the Langmuir-Hinshlewood adsorption model which can also be applied to the photodecolorization of methyl orange in the presence of Ag+ ion. Equation 17 could be rearranged to 1 1 1 1/R = -;i + -k, k,"K EMOI,
(19)
The where k," is equal to k,'[TiOz10.24p0~46[Ag+10~46. parameters k," and K can be solved to be 3.37 and 0.11 by combining eqs 15-19. The parameter k,' was found as 6.32 X 10-3 since the loading of Ag+ ion, the loading of TiOz, and the power intensity are 392 pM, 6.67 g/L, and 800 W, respectively. Accordingly, eq 17could be rewritten as
Effect of Initial pH. Increasing pH from 1.78 to 2.48, the initial decolorization rate and percentage of decolorization of methyl orange in a 30-min run slightly decreased from 0.37 to 0.34 pMImin and 9 to 7.6%, respectively, and reached the maxima of 3.79 pMlmin and 81.6%, respectively, at pH 8.75. However, further increasing the pH to 11.1, the decolorization rate and percentage of decolorization of methyl orange decreased to 3.12 pM/min and 73 76, respectively, as shown in Figure 6. When pH was less than 2.48, the surface of Ti02 was abundant with H+ which interfered with the transfer of holes to OH- and electrons to Ag+ ions. The results indicated that OH- concentration in the bulk solution
2Ag'
+ 20H-
Q
Ag,O
+ H,O
(21)
Ind. Eng. Chem. Res., Vol. 33, No. 6, 1994 1441 Table 3. Effect of Calcination Temperature of Ti02 on the Photodecolorization of Methyl Orange. init rate, rM/min calcination temp, OC
with Ag+ ion*
without Ag+ ion
25 400 600 1000
3.03 3.06 3.24 3.06
0.17 0.27 0.03
a Concentration of methyl orange = 89 f 1 pM; reaction temperature = 27 f 1 OC; volume of solution = 150 mL; stirring rate = 500 rpm; pH = 7.0 f 0.2; Nz flow rate = 200 mL/min. b Concentration of Ag+ ion = 392 @Me
Table 4. Comparisons of Experimental and Calculated APQY Results. [Mob], rM 87.2 89.5 87.4 87.0 86.1 92.0 91.9 86.8 88.5 87.6 60.0 47.5 45.1 31.0 21.3
[Ag+b], rM 392 392 392 392 392 392 706 1100 1550 392 392 392 392 392 392
[TiOz], glL 1.67 3.33 6.67 9.99 13.32 6.67 6.67 6.67 6.67 6.67 6.67 6.67 6.67 6.67 6.67
APQY, % 2.61 3.28 3.86 4.10 4.29 3.82 4.84 5.69 7.10 3.82 3.67 3.59 3.59 3.32 2.93
Ri$
rMlmin deviation,
expt 2.06 2.57 3.03 3.25 3.37 3.00 3.79 4.56 5.57 3.00 2.88 2.81 2.81 2.60 2.30
calc 2.17 2.57 3.03 3.34 3.57 3.04 3.99 4.87 5.60 3.03 2.90 2.80 2.78 2.57 2.31
%
5.3 0.0 0.0 2.8 5.9 1.3 5.3 6.8 0.5 1.0 0.7 0.4 1.1 1.2 0.4
Temperature = 27 f 1 "C; stirring rate = 500 rpm; illumination power = 800 W. a
calcination temperature, and the reaction rate upgraded with calcination temperature until 600 "C. However, when the calcination temperature increased to 1000 "C, Ti02 almost converted to rutile form as confirmed by X-ray diffraction and the reaction rate decreased with calcination temperature. These results indicated that the existence of a small amount of rutile Ti02 could efficiently increase the reaction rate. However, if the extent of rutile form was above the critical value, the reaction rate would decrease. The calcination temperature of Ti02 had an insignificant effect on the photodecolorization of methyl orange in the presence of Ag+ ion compared to that in the absence of Ag+ ion. This implies that the photoelectron generated in rutile form of Ti02 was more difficult to remove than that in the anataseform. However, the rutile form showed comparable activity to the anatase form in the presence of Ag+ ion. Apparent Primary Quantum Yield andComparison of Experimental and Calculated Results. Since the true quantum yield was difficult to obtain in this system, the apparent primary quantum yield is used and defined as the ratio of number of methyl orange molecules decolorized to the number of photon emitted by the ultraviolet source. The experimental results are summarized in Table 4. The maximum and minimum apparent primary quantum yields are 7.1 and 2.692, respectively. These values are larger than the corresponded values reported in our previous study (Chen and Chou, 1993b) without addition of Ag+ ion in neutral solutions. The results of comparisons of the initial decolorization rate calculated from eq 9 with the experimental data are also shown in Table 4.
Conclusions The experimental results indicated that Ag+ ion was more favorable than Cup+, Co2+, Fe3+, and Ce4+ions for
promoting photodecolorization of methyl orange. The counterion of Nos- was better than Sod2- for this decolorization. The results revealed that Ag+ ion could effectively trap the photogenerated electrons to avoid the recombination of electrons and holes. The initial decolorization rate increased with pH and reached a maximum at pH 8.75. The precipitation of AgpO was found when pH exceeded 8.75. On the basis of the experimental results, OH- ion played a more important role than Ag+ ion in the later stage of the run. The calcination temperature and structure of Ti02 showed little effect on the decolorization of methyl orange in the presence of Ag+ ion. The theoretical analysis correlated the experimental results well. The kinetic equations were obtained. It was demonstrated that the reaction orders are 0.24 and 0.46 with respect to the loadings of Ti02 and Ag+ ion, respectively. The effect of initial concentration of methyl orange can be described by the Langmuir-Hinshelwood adsorption model. Oxygen and nitrogen showed little influence on the reaction in the presence of Ag+ ion. Methyl orange was decolorized insignificantly by the electron transition of ligand to metal without Ti02 in the presence of illumination.
Acknowledgment The support of the National Science Council of ROC (NSC82-0416-E-006-299) and the support of National Cheng Kung University are gratefully acknowledged.
Nomenclature [Mob] = concentration of methyl orange in the bulk phase, PM [MO,] = concentration of methyl orange on the surface of catalyst, pmol/g of Ti02 tAg+b] = concentration of Ag+ ion in the bulk phase, pM [Ag+,] = concentration of Ag+ ion on the surface of catalyst, pmol/g of Ti02 [Ti021 = loading of TiOz, g/L A, = irradiated surface, m2 APQY = apparent primary quantum yield, % Ii = intermediate i K = equilibrium constant, k&M/kllS, pM-l KM = adsorption equilibrium constant of methyl orange, (umol/(g of Ti02 uM)) KA'=adsorption equilibrium constant of Ag+, (pmol/(g of Ti02 PM)) KOH= constant, defined in eq 22, W-0.5 m-1 min-l (pmol/g of TiOdO.S k, = rate constant, KAo.~KOH/KM,~ - 0 . 5m-1 min-1 pM0.5 k,' = rate constant, K,Ap0.5/[Ti0210.5,W-O.5min-l pMO.5 (g/L)-0.5pM0.5 kl = rate constant of eq 1, (pmollg of Ti0z)(W.m2.min)-l ki = i = 2-10, rate constants of eqs i, (min)-'(pmol/g of TiOz)-l kll = rate constant of eq 11, (min.m2)-1 k ~= i rate constant, (min)-l (pmol/g of TiOz)-l P = illumination power, W Ri = initial decolorization rate, pM/min S = inert surface, m2 Subscripts b = bulk phase cb = conductive band s = surface of Ti02 vb = valence band
Appendix If both the internal and external mass transfer resistance are negligible (Chen and Chou, 1993a), the material
1442 Ind. Eng. Chem. Res., Vol. 33, No. 6, 1994
balances of HO',, hfvb,and e-cbat pseudo steady state give eqs Al-A3, respectively. d[HO' ] = k,[OH-,I [h+,bI + k6[H,Os1 [h+,bl dt k,[HO',l [MO,,)] - k8[HO',]S - zk,i[Ii(,)I [HO',I = 0 (AI)
[MOA and [Ag+,l can be assumed to equal to KMCMO~I and K~[Ag+bl,where K M and KA are the adsorptive equilibrium constants for methyl orange and Ag+ ion, respectively. Substitute eq A10 into eq A4 to obtain
where R is the decolorization rate and the terms kTKhl/ k& and KA~.~KOH/KM are equal to K and k,, respectively. The intermediate products were negligible at the initial stage of a run; then eq A12 can be rearranged to
--d[M0s3 - k,[MO,I[HO',] dt
where P and A, are the intensity of power and irradiated area, respectively. The term Ck1i[IJ [HO'J means the reaction of the adsorbed intermediates with HO' free radicals. Equation A3 is rearranged to
where Ri is the initial decolorization rate. The irradiated area A, is proportional to the loading of Ti02 if the UV light is enough for the loading of Ti02 which is homogeneously distributed in the reactor. Then eq A13 becomes
Substituting eq A5 into eq A2 gives where k,' = k,Afi5/ [ T i 0 ~ 1 ~ . ~ .
Literature Cited k,[OH-,I [h+,bI - k6[H@,l [h+,bl = 0 (A6) Equation A6 is simplified and rearranged to eq A I by assuming k2 >> k3, since the recombination rate of electron and hole is very fast compared with the trapping reaction of electron by Ag+ ion. The regeneration of Ag+ by eq 4 is insignificant at the initial stage of a run. klk,ApPIAg+,l - [h+,b12(k&2[OH-sl+ @,[H@,])
=0 (A7)
Then eq A7 becomes [h+,I = (k,k3A,P[Ag+,l/(k5kz[OH-sl + k6k,[H,0,1))0'5 (-48) Equation A1 can be rearranged to
Substituting eq A8 into eq A9, [HO',] can be expressed by
Since the kinetic reaction is the rate-determining step,
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Received for review February 7, 1994 Accepted March 20, 1994' Abstract published in Advance ACS Abstracts, May 1,1994.
