Ind. Eng. Chem. Res. 1998, 37, 4207-4214
4207
KINETICS, CATALYSIS, AND REACTION ENGINEERING Kinetics and Mechanism of Photoelectrochemical Oxidation of Nitrite Ion by Using the Rutile Form of a TiO2/Ti Photoelectrode with High Electric Field Enhancement Chih-Cheng Sun and Tse-Chuan Chou* Department of Chemical Engineering, National Cheng Kung University, Tainan, 70101, Taiwan, Republic of China
The photoelectrochemical oxidation of nitrite ion on a rutile form of a TiO2 particulate film immobilized on a titanium plate (TiO2/Ti) with applying bias potential was studied. The experimental results indicate that the oxidation of nitrite ion is mainly affected by the applied potential and irradiation of power of light, and the oxidation is also independent of both nitrite ion concentration and pH. Theoretical analysis results correlate well with experimental ones, indicating that the reaction is of zero order with respect to the concentrations of nitrite ion and the hydroxyl ion. Meanwhile, the reaction is of 0.30 and 0.80 order with respect to the applying bias potential and the output power of light, respectively. Introduction Nitrite ion is a toxic material in wastewater or fish breeding water. Nitrite ion concentration is a serious problem, particularly in aquatic breeding, even at an extremely low concentration.1,2 Despite the availability of several methods to remove nitrite ion in water,2,3 they have several limitations. Of particular concern is the uneconomic and inefficient treatment of water containing an extremely low concentration of toxic nitrite ion. By using semiconductor photocatalysts, degradation of toxic chemicals by solar energy is a highly promising and economic alternative.4-8 Previous investigations have examined the feasibility of oxidation of nitrite ion by supplying oxygen in an aqueous solution using different semiconductor powders as photocatalysts.9-11 However, the photocatalytic reaction enhanced by the high electric field is not described. Among the semiconductor photocatalyts, TiO2 is the most attractive catalyst for purifying and treating of water and air.12,13 A great deal of reports have been devoted to develop the supported TiO2 catalyst.14-17 The TiO2 film supported on a conductive material exhibits interesting electrochemical and photoelectrochemical properties.17-20 On the TiO2 film electrode, the anatase form is a popular morphology of TiO2.5,21-23 This popularity is attributed to that the larger band gap for anatase and its more negative flatband potential lead to higher photoactivity in suspension heterogeneous photocatalytic reaction.23,24 But both the adhesion of these anatase form of TiO2 films and the applying high bias potential on these film electrodes have seldom been mentioned. In general, increasing the calcination tem* To whom correspondence should be addressed. Tel.: 8866-2757575, ext. 62639. Fax: 886-6-2344496. E-mail: tcchou@ mail.ncku.edu.tw.
perature increases the adhesion of the TiO2 film on the Ti plate; the rutile form of TiO2 film electrode calcinated at high temperature has seldom been discussed.24 The adhesion of rutile form of TiO2 film may be better than that of anatase form, and the photoelectrochemical reaction behavior enhanced by the high electric field using rutile form of the TiO2 film electrode is interesting. The photoelectrochemical decomposition of pollutants using the anatase form of the TiO2 film electrode was paid more attention by several investigators.17-20,25 Although many pollutants can be effectively photodecomposed using the anatase form of TiO2 film, the kinetics and mechanism of photooxidation of the pollutant, especially in the condition of applying high bias potential on the anatase form of the TiO2 electrode, are somewhat unclear. Several kinetic models have been proposed for the oxidation of pollutants using a suspension heterogeneous photocatalyst.26-29 However, relatively few investigations have developed kinetic models of the photoelectrochemical reaction. Hori20 reported the photooxidation of nitrite ion with the low photocurrent density and low applied bias potential using an anatase form of the TiO2 film electrode, and a reaction mechanism was proposed. However, no rate equation, power of light, or applying bias potential was provided in this proposed mechanism. The photoelectrochemical oxidation of nitrite ion by using the rutile form of TiO2 film electrode was not mentioned. Moreover, the reaction mechanism and kinetics of photoelectrochemical oxidation of nitrite ion using rutile form of TiO2 film electrode are unclear. In this work, the photoelectrochemical oxidation of nitrite ion using the rutile form of TiO2/Ti photoelectrode and without oxygen supply will be systematically studied. A reaction mechanism of photoelectrochemical oxidation of nitrite ion at high photocurrent density is
10.1021/ie980222u CCC: $15.00 © 1998 American Chemical Society Published on Web 09/24/1998
4208 Ind. Eng. Chem. Res., Vol. 37, No. 11, 1998
Figure 1. Photoelectrochemical reaction setup: 1, power supply of light; 2, housing of light bomb; 3, divided cell; 4, photoanode (working); 5, reference electrode; 6, sampling valve; 7, Pt electrode (counter); 8, stirrer; 9, potentiostat/galvnnostat.
also proposed. The kinetic model is theoretically and experimentally studied as well. Experimental Section Manufacturing Process of a TiO2/Ti Photoelectrode. The substrate Ti plate with a 30 cm2 surface area was etched by a 18 M hydrogen chloride at 90 °C for 2 h and then washed with distilled water in an ultrasonic bath. The treated Ti plate was dried in oven at 100 °C for 30 min and then immersed in a suspension solution containing 2.00 g of TiO2 (Degussa P-25) in 20 mL of 10% TiCl4 and 90% ethanol. The coated TiO2 film was dried with a constant moisture and then annealed at 650 °C for 2 h. Finally, the morphology of the TiO2coated plate was analyzed by X-ray diffraction. X-ray Diffraction (XRD). An X-ray diffractometer (Rigaku D/max III V XRD) was employed to analyze the structure of photocatalyst on the TiO2/Ti photoelectrode. The radiation source was Cu KR. The applied current and voltage were 30 mA and 40 kV, respectively. The sample was scanned at speed of 0.4°/min from 20 to 80°. Measurement of Photocurrent. The desired NaCl concentration and volume of electrolyte were prepared and added into the cell, respectively. The prepared TiO2/Ti plate was used as working electrode, and the platinum electrode with 4.50 cm2 surface area was used as counter electrode. An experimental apparatus was set up as shown in Figure 1. The photocurrent was recorded by an EG&G 273A potentiostat/galvanostat with 270 Electrochemical Analysis System. All potentials were specified to the reference electrode, Ag/AgCl/ saturated KCl aqueous solution, prepared in our laboratory. Finally, the irradiation light was supplied by a super-high-pressure Hg(Xe) lamp (Oriel) which was operated at different powers by adjusting the output of the power supply. Photoelectrochemical Oxidation of Nitrite Ion. The desired concentration of nitrite ion was added into the reactor containing 200 mL of NaCl electrolyte which is deoxygenated by nitrogen gas. The reactor is a divided cell, and the working and counter electrodes are separated by a porous glass. The photocatalytic oxidation of nitrite ion was proceeded by simultaneously applying a bias potential and irradiation. Samples were periodically taken with a pipet from the reactor and then were analyzed. According to the APAH testing,30 the nitrite ion concentration was determined by a Jasco UV-vis spectrophotometer (Jasco) at a wavelength of
Figure 2. XRD spectra of TiO2/Ti photoelectrode.
543 nm, and a color reagent was added to the sample by the NEDA (N-(1-naphthyl)ethylenediamine dihydrochloride) colorimetric method. Finally, the pH values were determined by a pH meter. Results and Discussion Structure of a TiO2/Ti Photocatalyst. Figure 2 shows the XRD spectra of the Ti plate, TiO2 (Degussa p25), and TiO2 film with different calcination temperatures. The principal peaks at 2θ ) 25.5 and 48.0 in the spectrum of TiO2 are easily identified as the crystal of anatase form, whereas the crystal peaks at 2θ ) 27.6 and 54.5 are also easily identified as the crystal of rutile from. When the calcination temperature is higher than 650 °C, the peak of anatase form disappears and the peak of the rutile form remains. This finding reveals that all the crystal of TiO2 on the TiO2/Ti photoelectrode transfers from the anatase form to the rutile form during the calcination at 650 °C. No anatase form was found in this case. Photocurrent Density on a TiO2/Ti Electrode. Figure 3 summarizes the results of photocurrent density of the TiO2/Ti photoelectrode in a 0.51 M NaCl solution by simultaneously applying a bias potential and irradiation. Increasing the applying bias potential from 0.00 to 4.00 V (vs Ag/AgCl) increases the photocurrent density from 0.34 to 2.65 mA/cm2. In addition, the photocurrent density approaches 80% of the ultimate value at 2.00 V (vs Ag/AgCl) when applying bias potential. No photocurrent was found without irradiation in the applying bias potential in a range from 0.00 to 4.00 V as shown in Figure 3. Therefore, the photocurrent measurement is not interfered by a dark current even applying a high bias potential. In addition, the photogenerated electrons are withdrawn from anode to cathode where cathodic reduction occurs. In this case, the hydrogen gas is generated at the cathode, while the equivalent holes on the TiO2 surface are enhanced, and holes induce the oxidation of nitrite ion. The recombination of electrons and holes decreases obviously when the bias potential is applied. Effect of Applying Bias Potential on Reaction Rate. Increasing the applying bias potential from -0.22 to 3.00 V increases the initial oxidation rate of nitrite ion from 1.67 to 8.04 µM/min. However, further
Ind. Eng. Chem. Res., Vol. 37, No. 11, 1998 4209
Figure 3. Relationship between photocurrent density and applying bias potential at pH ) 5.50: photoanode, 7.06 cm2 TiO2/Ti; cathode, 4.50 cm2 Pt; potential vs Ag/AgCl; temperature, 30 °C; agitation rate, 250 rpm; electrolyte, 0.51 M NaCl; power of light, 400 W.
Figure 4. Relationship between the initial rate of nitrite ion oxidation and applying bias potential at pH ) 5.50: photoanode, 7.06 cm2 TiO2/Ti; cathode, 4.50 cm2 Pt; potential vs Ag/AgCl; temperature, 30 °C; agitation rate, 250 rpm; electrolyte, 0.51 M NaCl; power of light, 400 W.
increasing the applying bias potential from 3.00 to 4.00 V does not increase the reaction rate, which reaches and remains a constant as shown in Figure 4. When the applying is less than 3.00 V, a logarithmic plot of initial reaction rate against the applying bias potential yields a straight line with a slope of 0.30 as shown in Figure 5. This finding suggests that the initial reaction rate is proportional to the 0.30 order of the applying bias potential. Effect of Nitrite Ion Concentration on Reaction Rate. For the increase of the nitrite ion concentration from 5.00 to 30.00 ppm, the nitrite ion decomposition percentage during irradiation is shown in Figure 6. The slope of curve in Figure 6 multiplied by nitrite ion concentration is the oxidation reaction rate of nitrite ion, -d[NO2-]/dt. The logarithmic plot of initial reaction rate against the concentration of nitrite ion yields a
Figure 5. Logarithmic plot of initial rate of nitrite ion oxidation against applying bias potential: concentration of nitrite ion, 20 ppm; temperature, 30 °C; Electrolyte, 0.51 M NaCl; power of light, 400 W.
Figure 6. Decomposition percentage of nitrite ion in photoelectrochemical reaction during the irradiation time at pH ) 5.50: photoanode, 7.06 cm2 TiO2/Ti; cathode, 4.50 cm2 Pt; potential, 2.00 V vs Ag/AgCl; temperature, 30 °C; agitation rate, 250 rpm; electrolyte, 0.51 M NaCl; power of light, 400 W.
straight line with a slope of zero as shown in Figure 7. Accordingly, the reaction is of zero order with respect to the nitrite ion concentration. In this case, absorbed nitrite ion on the surface may be excess for the oxidation by the holes. Although the applying bias potential may promote the mass transfer as well as the adsorption of nitrite ion on the working electrode, the photoelectrochemical oxidation of nitrite ion on the surface may be a rate-determining step. Therefore, when the photoelectrode is simultaneously applied by a positive bias potential and irradiation, the reaction type and photocatalytic efficiency of photoelectrochemical reaction differ with respect to the suspension TiO2 in which no applying potential is used. The photooxidation rate of nitrite ion is of first order with respect to the nitrite ion concentration for a suspension photocatalytic oxidation of nitrite ion.10 On the other hand, the reaction
4210 Ind. Eng. Chem. Res., Vol. 37, No. 11, 1998
Figure 7. Logarithmic plot of initial rate of nitrite ion oxidation against concentration of nitrite ion: pH, 5.50; temperature, 30 °C; electrolyte, 0.51 M NaCl; power of light, 400 W.
Figure 8. Logarithmic plot of initial rate of nitrite ion oxidation against concentration of hydroxide ion: concentration of nitrite ion, 20 ppm; temperature, 30 °C; applying bias potential, 2.00 V vs Ag/AgCl; electrolyte, 0.51 M NaCl; power of light, 400 W.
rate of this system is independent of the nitrite ion concentration. Effect of pH on Reaction Rate. Increasing the pH value from 5.50 to 11.00 decreases the initial reaction rate from 0.33 to 0.08 ppm/min. A logarithmic plot of initial reaction rate against the concentration of OHresults in a straight line with a zero slope in the pH range from 5.50 to 10.30 as shown in Figure 8, and the slope changes when the pH is higher than 10.30. Accordingly, the reaction is of zero order with respect to the concentration of OH- in the pH range from 5.50 to 10.30. This phenomena is similar to that of the NO2absorbed on the reaction surface. In this case, the absorbed hydroxide ion on the reaction surface may be in excess too. Therefore, the reaction rate is independent of the bulk concentration of OH- in the pH range from 5.50 to 10.30. On the other hand, the reaction order changes when the pH is higher than 10.3 as shown
Figure 9. Relationship between the irradiation intensity in the reactor and the output power of light.
Figure 10. Logarithmic plot of initial rate of nitrite ion oxidation against the output power of light: concentration of nitrite ion, 20 ppm; temperature, 30 °C; applying bias potential, 2.00 V vs Ag/ AgCl; electrolyte, 0.51 M NaCl.
with the dashed line in Figure 8. This finding suggests that both the extensive adsorption of OH- and the depletion of OH free radical on the surface dramatically increase in this pH range. Effect of the Power of Light on Reaction Rate. The super-high-pressure Hg(Xe) lamp is initiated by a power supply which can be adjusted to a desired magnitude of its output power of light. The intensity of incident photons is proportional to the output power of light as shown in Figure 9. Increasing the output power of light from 360 to 640 W increases the initial reaction rate from 0.30 to 0.48 ppm/min. A logarithmic plot of initial reaction rate against the output power of light yields a straight line with a slope of 0.80 as shown in Figure 10. This finding implies that the initial reaction rate is proportional to the 0.80 order of the output power of light. This magnitude of reaction order exceeds the 0.25 for the decolorization of methyl orange28 and 0.50 and 0.60 for the degradation of phenol
Ind. Eng. Chem. Res., Vol. 37, No. 11, 1998 4211
reported.31,32 This discrepancy of reaction order can possibly be accounted for by the applying bias potential decreasing the recombination of the generated electrons and holes. Mechanism and Kinetics of Photooxidation of the Nitrite Ion. Applying an anodic bias potential to a photoelectrode provides a potential gradient within the semiconductor film to efficiently drive away the photogenerated holes and electrons at opposite directions.17-19,25,33 The photocatalytic reaction on the TiO2/ Ti photoanode and the Pt cathode by the applying bias potential can be expressed as eqs 1-3. With irradiation,
k10
HO•s + HO•s 98 H2O2s k11
(10)
H2O2s + HO•s 98 HO2•s + H2Os
(11)
H2O2s f 1/2O2 + H2O
(12)
is an important side reaction, which is the reverse reaction of eq 1.34
((TiO2)-e-cb)anode + ((TiO2)-h+vb)anode f heat
(13)
anode: k1
TiO2/Ti 9 8 ((TiO2)-e-cb)anode + ((TiO2)-h+vb)anode (1) hν elc
((TiO2)-e-cb)anode 98 ((Pt)-e-)cathode
(2)
k3
cathode: 2((Pt)-e-)cathode + 2H2O 98 H2 + 2OH-
(3)
electrons and holes are generated on TiO2 as shown in eq 1. The electrons on the TiO2/Ti photoanode are withdrawn by the applying bias potential and transfer through the substrate Ti metal and external circuit to the cathode as shown in eq 2, where the electrons react with water to form H2 and OH- as shown in eq 3. Equation 4 is a combination of eqs 1-3, indicating that the applying bias potential makes more available holes for the photooxidation of nitrite ion. elc
8 1/2H2 + OH- + ((TiO2)-h+)anode TiO2/Ti + H2O 9 hν (4) The mechanism of photoelectrochemical oxidation of nitrite ion by the holes has been proposed in the following. The holes react with the OH- anion or H2O molecular adsorbed on the TiO2 surface, i.e., OH-s and H2Os, to generate the OH free radical.27 k5
OH-s + h+vb 98 OH•s k6
H2Os + h+vb 98 OH•s + H+
(5) (6)
And then the OH free radical is used to oxidize the nitrite ion adsorbed on the TiO2 surface, i.e. NO2-s10,11 k7
OHs + NO2-s 98 NO2•s + OH-s k8
NO2•s + OH•s 98 NO3- + H+
(7) (8)
and the OH free radical on the TiO2 surface may be depleted and lose its activity.28,29 k9
OH•s + (TiO2) 98 inactive species
(9)
In addition, the OH free radical on the TiO2 surface may combine to generate hydrogen peroxide and then generate HO2• radical or oxygen expressed as eqs 10-12. However, the concentration of HO2• radical is lower than the major oxidant, i.e., OH•, in the photocatalytic reaction28,29 In addition to the reaction of reduction by an electron and oxidation by a hole, recombining electrons and holes
Comparing eqs 2 and 13 reveals that the electron is withdrawn to the cathode, i.e. eq 2, or recombines with the hole, i.e. eq 13, in which the available holes on the anode are much greater than those without applying bias potential. In general, the higher the applied bias potential the faster the reaction of eq 2 is. Accordingly, the applying bias potential on the TiO2/Ti electrode increases the available holes for oxidation of nitrite ion and decreases the recombination rate of eq 13. In the photocatalytic oxidation reaction using TiO2 suspension as catalyst, the oxidation of nitrite ion by photogenerated OH free radical was reported as shown in eqs 7 and 8.10,11,20 Accordingly, the reaction rate of photoelectrochemical oxidation of nitrite ion is expressed as eq 14, where R denotes the apparent reaction rate.
R ) k7[NO2-s][OH•s]
(14)
At steady state, the mass balance of OH•, h+, and NO2can be respectively expressed as
d[OH•s] ) k5[OH-s][h+vb] + k6[H2Os][h+vb] dt k7[OH•s][NO2-s] - k8[NO2•s][OH•s] - k9S[OH•s] k10[OH•s] - k11[H2O2s][OH•s] ) 0 (15) d[h+vb] ) k1ApPR - k13Eβ[h+vb][e-cb] dt k5[OH-s][h+vb] - k6[H2Os][h+vb] ) 0 (16) d[NO2•s] ) k7[NO2-s][OH•s] - k8[NO2•][OH•s] ) 0 dt (17) where P and Ap represent the power of light and the TiO2 surface of irradiation, respectively. In addition, the E and S are the applied bias potential and the site of OH free radical depletion on the photoelectrode, respectively. Moreover, R and β represent the order of the power of light and applying bias potential, respectively, in the kinetic reaction equations. On the basis of eq 15, the following equation is derived:
[OH•s] )
(k5[OH-s] + k6[H2O])[h+] k9S + k10 + k11[H2O2s] + 2k7[NO2-s]
(18)
In general, the recombination of photogenerated elec-
4212 Ind. Eng. Chem. Res., Vol. 37, No. 11, 1998
the active sites on the surface of titanium dioxide. KNO2and KOH- are the adsorption equilibrium constants of nitrite and hydroxyl ion, respectively. Next, eqs 22 and 23 are substituted into eq 21. The rate equation is obtained as eq 24.
Table 1. Comparison of the Experimental and Theoretical Analysis Results reaction order
b
material or variation
theoretical
NO2OH- a OH- b P E
0 0 0 R/2 -β/2
exp
[
0 0
a Concentration of OH- when the pH value was less than 10.3. Concentration of OH- when the pH value was higher than 10.3.
}(
k6[H2Os] Rs
trons and holes is rapid, i.e.,
[h+vb][e-cb]
β
k13E
.
k5[OH-s][h+vb]
+
{ (
R ) (KPRE-β)1/2 k7 k5Rs
0.8 0.3
k6[H2Os][h+vb]
[h+vb] )
( ) k13Eβ
2k7 Rs
(19)
Combining eqs 18 and 19 and assuming that the concentration of H2O2 is very low on the TiO2 surface35 yields
[OH•s] )
(k5[OH-s]
R
-β 1/2
+ k6[H2Os])(KP E )
k9S + k10 + 2k7[NO2-s]
(20)
R)
(KPRE-β)1/2 k7(k5[OH-s] + k6[H2Os])[NO2-s] k9S + k10 + 2k7[NO2-s]
) Rs
[OH-s] ) Rs
KNO2-[NO2-b] 1 + KNO2-[NO2-b] KOH- [OH-b] 1 + KOH- [OH-b]
KNO2- [NO2-b] 1 + KNO2- [NO2-b]
)]
KNO2- [NO2-b] . 1
(25)
KOH- [OH-b] . 1
(26)
(KPRE-β)1/2 k7{k5Rs + k6[H2Os]}(Rs) k9S + k10 + 2k7(Rs)
(21)
R ) K′(PRE-β)1/2
(23)
The rate equation of eq 28 reveals that the reaction is of zero order with respect to the concentrations of both NO2- and OH- in the pH range from 5.50 to 10.30. Experimental results indicate that the reaction order
process study Ia
a
Hori, Yoshio, 1990.20
b
(27)
(22)
Table 2. Comparison of the Results in This Study with Those in the Literature
TiO2 form O2 supply applied bias potential photocurrent density reacn order of [NO2-] reacn order of pH reacn order of applied bias potential reacn order of power of light reacn rate eq reacn rate (r) photocatal efficiency (η)
(24)
Comparison of Experimental and Theoretical Analysis Results. Table 1 shows the comparison of the theoretical analysis with the experimental results. According to this table, the power supply, applying bias potential, and reaction orders of species NO2- and OHof the experimental results correlate with the theoretical analysis. The adsorption of both NO2- and OH- is increased by applying a bias potential on the photoelectrode. Then, eq 27 can be simplified and subsequently becomes eq 28, where K′ ) 0.05 (mol/min)-1 W-0.80 V-0.30
If the Langmuir-Hinshelwood adsorption model is applied to nitrite and the hydroxyl ion in this system, then the relationship of NO2-s and OH-s with the bulk concentration of NO2- and OH-, i.e. NO2-b and OH-b, can be expressed as eqs 22 and 23 where Rs represents
[NO2-s]
/ k9S + k10 +
Substituting eqs 25 and 26 into eq 24 subsequently leads to eq 27.
Equation 20 is substituted into eq 14, and the rate equation is obtained as
R)
+
From the experimental data, the reaction rate is zero order with respect to NO2- and OH-; it can be assumed that both NO2- and OH- anions are strongly adsorbed on the photoelectrode and then
1/2
) (KPRE-β)1/2
)] [
1 + KNO2- [NO2-b]
(
)
1 + KOH- [OH-b]
KNO2- [NO2-b]
and eq 16 is simplified and rearranged to eq 19.
k1ApPR
KOH- [OH-b]
anatase yes low (0.00-0.50 V vs SCE) low (∼0.05 mA/cm2)
(1.00-2.00) × 10-5 (A/cm2)c 1/(Ar + 1) A ) 1.88 × 105 (A/cm2)-1
IIb rutile no high (2.00-4.00 V vs Ag/AgCl) high (∼2.00 mA/cm2) zero order zero order 0.30 order 0.8 order R ) 0.05P0.80E0.30 (µmol/(L min)) (5.00-8.00) × 10-4 (A/cm2) 1/(BP0.80E0.30 + C) B ) 1.98 × 10-8 (A/cm2)-1, C ) 3.02
This study. c Experimental data from the literature (function of applied bias potential).
(28)
Ind. Eng. Chem. Res., Vol. 37, No. 11, 1998 4213
of P and E are 0.80 and 0.30, respectively. Therefore, the R and β values are 1.60 and -0.60, respectively. Comparison of the Results in This Study with Those in the Literature. A comparison of the results in this study with those in the literature20 is shown in Table 2. By using the rutile form of the TiO2 film electrode, this study presents a detail discussion of factors such as effect of pH, bias potential and power of light and expresses a reaction rate equation. On the other hand, no reaction order data fitted to the concentration of nitrite ion, pH value, applied bias potential, and power of light or reaction rate equation could be found in the literature, as shown in Table 2. In addition, the photocurrent density of the photoelectrochemical oxidation of nitrite ion in this study is 40 times higher than that in the literature.20 The reaction rate obtained in this study is 5.00 × 10-4-8.00 × 10-4 A/cm2, while the reaction rate was 1.00 × 10-5-2.00 × 10-5 A/cm2 reported in the literature.20 Furthermore, the type of expression of photocatalytic efficiency of this study presented the value at different operation conditions. Conclusions The photoelectrochemical oxidation of nitrite ion on the rutile form of the TiO2/Ti photoelectrode was obtained. The experimental results indicate that the photoelectrochemical oxidation of nitrite ion is efficiently obtained without an oxygen supply and is significantly enhanced by the applying bias potential. The reaction mechanism including the reactions of both the photoelectrochemical anode and the counter cathode is proposed. The reaction rate is independent of the nitrite ion concentration and is zero order with respect to the pH value in the range from 5.50 to 10.30. Both experimental and theoretical results indicate that the reaction rate increases with the applying bias potential “E” and intensity of power “P”, and the reaction rate equation is
R ) K′P0.80E0.30 Acknowledgment The support of National Science Council of the Republic of China (Grant NSC 85-2214-E006-018) and National Chung Kung University is acknowledged. Nomenclature R ) the apparent reaction rate, µmol/L‚min Ri ) initial reaction rate, µmol/L‚min [NO2-s] ) concentration of nitrite ion on the surface of catalyst, µmol/g of TiO2 [NO2-b] ) concentration of nitrite ion in the bulk phase, mM [OH-s] ) concentration of hydroxyl ion on the surface of catalyst, µmol/g of TiO2 [OH-b] ) concentration of hydroxyl ion in the bulk phase, mM P ) power of light, W Ap ) TiO2 surface of irradiation, m2 E ) applied bias potential, V S ) site of OH free radical depletion on the photoelectrode R ) order of power of light in the kinetic reaction equation β ) order of applying bias potential in the kinetic reaction equation
KNO2- ) adsorption equilibrium constant of nitrite ion, (mM)-1 KOH- ) adsorption equilibrium constant of hydroxyl ion, (mM)-1 Rs ) the active site on the surface of titanium dioxide K′ ) reaction rate equation constant, (µmol/L‚min) W-0.80 V-0.30 k1 ) reaction rate constant of eq 1, (µmol/g of TiO2) (W0.80‚m2‚min)-1 ki, i ) 5-8, 10, 11, ) reaction rate constants of eqs i, (min)-1 (µmol/g of TiO2)-1 k9 ) reaction rate constants of eqs 9, (min m2)-1 k13 ) reaction rate constant of eq 13, (min)-1 (µmol/g of TiO2)-1 V0.3 Subscripts b ) bulk solution cb ) conduction band s ) surface of TiO2 vb ) valence band
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Received for review April 10, 1998 Revised manuscript received July 20, 1998 Accepted July 24, 1998 IE980222U
