Photocatalytic Degradation of Dyes and Organics with Nanosized

J. Phys. Chem. C 2007, 111, 1665-1674

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Photocatalytic Degradation of Dyes and Organics with Nanosized GdCoO3 Partha Mahata,† T. Aarthi,‡ Giridhar Madras,*,‡ and Srinivasan Natarajan*,† Framework Solids Laboratory, Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India, and Department of Chemical Engineering, Indian Institute of Science, Bangalore 560 012, India ReceiVed: September 26, 2006; In Final Form: NoVember 16, 2006

GdCoO3 nanoparticles with sizes of 3, 12, and 200 nm were prepared from single source mixed-metal organic framework (MOF) precursor, [Gd(H2O)3Co{C5N1H3(COO)2}3]. The photocatalytic degradation studies, carried out for the first time in the presence of nanosized GdCoO3 oxides, on dyes, phenols, and substituted phenols indicate favorable activity. The intermediates formed during the photocatalytic reactions were analyzed, and possible reaction mechanisms have been proposed. The results of the present study indicate that the degradation appear to be faster in the presence of 3 nm GdCoO3 compared to P-25 Degussa TiO2 catalyst. The reduced formation of unwanted intermediates in the presence of GdCoO3 indicates that this material would prove to be a good photocatalyst in environmental photocatalysis.

Introduction Nanoscale structures have attracted extensive attention as a result of their novel size-dependent properties.1-4 The challenge of developing nanoscale devices for optical, electronic, thermal, mechanical, and catalytic applications depends largely on the ability to synthesize and characterize the nanostructures. Most of the recent research efforts were concentrated on binary metal oxide nanoparticles,5-8 and very little is known about the ternary system.9,10 It is likely that the control of the stoichoimetry and size of the mixed metal oxide could pose significant problems for the successful synthesis of such ternary nanostructures. Mixed metal oxides of the perovskite family, ABO3 where A ) rare-earth and B ) transition element, are important in advanced technologies such as fuel cells,11 catalysis,12 electrode materials,13 and sensors.14 It has been observed that the perovskite oxides also function as good photocatalysts.15 In the past three decades, semiconductor-based photocatalysts have attracted much attention for harnessing the solar energy.16-18 Intense research activity in this area has resulted in many novel mixed metal oxide photocatalyts such as SrTiO3, SrNb2O7, ZnNb2O6, BiVO4, etc.19-23 These oxides have been found to be useful in splitting water into H2 and O2 by utilizing the solar energy. There is a need for developing new materials with improved photophysical/chemical properties, and in this context the newly developed nanoparticles of mixed oxides is expected to be of much interest.24-26 Phenols and substituted phenols pose a serious environmental concern because of their toxicity, as chlorophenols,27 methylphenols,28 and nitrophenols29 are produced as intermediates in a variety of industrial processes. Many technologies such as Fenton oxidation, electrolysis, sorption, etc., have been developed for the treatment of these effluents. Photocatalysis is being considered as a serious alternative for treating many of the industrial effluents as it appears to be advantageous for the following reasons: complete mineralization, low cost, no sludge * Corresponding authors. E-mail: [email protected], [email protected]. † Solid State and Structural Chemistry Unit. ‡ Department of Chemical Engineering.

disposal, mild temperature and pressure conditions.30,31 There are numerous studies in the literature about the photodegradation of phenols,32-36 substituted phenols,32,37,38 their mixtures,39 and multisubstituted phenols.40-42 The development of new photocatalysts that can provide faster degradation of the pollutants and produce lesser concentration of the intermediates and those that can be easily separated from the system is an emerging task in achieving efficient photocatalytic systems. It is likely that the use of nanoparticles of mixed metal oxides as a photocatalyst would be of much interest as such studies are just beginning to emerge.15,43 In this study, we have prepared nanoparticles of GdCoO3 from a mixed-metal organic framework (MOF), [Gd(H2O)3Co{C5N1H3(COO)2}3], precursor. This approach, to our knowledge, has been used for the first time for preparing nanosized GdCoO3. In this paper, we report the synthesis, characterization, and photocatalytic studies of nanosized GdCoO3. The photodegradation studies of dyes, phenols, and substituted phenols, investigated for the first time in the presence of GdCoO3, indicate that GdCoO3 has activity comparable to that of commercial Degussa P-25 TiO2 catalyst. Experimental Section Materials. The organic dyes and other compounds for the photocatalytic experiments, Rhodamine B (RB), Rhodamine Blue (RBL), Orange G (OG), and Remazol Brilliant Blue R (RBBR) (S. D. Fine-Chem. Ltd, India), phenol, 4-methylphenol, 4-nitrophenol, and 4-chlorophenol (Merck, India), were used as received. The water used was double distilled filtered through a Millipore membrane. Synthesis and Initial Characterization. Mixed-metal pyridine-2,3-dicarboxylate precursor, [Gd(H2O)3Co{C5N1H3(COO)2}3])] (MOF), was prepared employing the hydrothermal method.44 For the large-scale synthesis of the MOF precursor, 3.43 g of Gd(NO3)3, 2.52 g of Co(OAc)2·4H2O, and 3.338 g of pyridine-2,3-dicarboxylic acid were dissolved in 70 mL of water, and the mixture was homogenized at room temperature for 30 min. The final reaction mixture was sealed in a 140 mL PTFElined stainless autoclave and heated at 150 °C for 72 h. The

10.1021/jp066302q CCC: $37.00 © 2007 American Chemical Society Published on Web 01/10/2007

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Figure 1. (a) TGA studies of the MOF precursor in air, (b) powder XRD patterns of the MOF precursor heated ex-situ at various temperatures in air, and (c) powder XRD pattern of GdCoO3 formed at 700 °C. (The XRD lines are indexed based on the pervoskite structure.)

resulting product contained dark pink rodlike crystals of the precursor (yield ∼ 85%). Nanosized GdCoO3 was synthesized by calcining the MOF in air for 10 h at three different temperatures: 400, 700, and 800 °C. The heating of the MOF at the above temperatures directly produced 3, 12, and 200 nm sized GdCoO3 mixed-metal oxide, respectively. The calcined samples were characterized using a powder X-ray diffractometer (Philips, X’part-Pro) with a Cu KR radiation. The shape and size of the particles were analyzed using transmission electron

microscopy (JEOL, JEM3010). BET analysis (Quantachrome Nova 2200) by N2 sorption was carried out to determine the surface area of the materials. The surface area was found to be 28, 9, and 5 m2/g respectively for 3, 12, and 200 nm sized GdCoO3. Photocatalytic Experiments. Photochemical Reactor. The photochemical reactor employed in this study consisted of a jacketed quartz tube of 3.4 cm i.d., 4 cm o.d., and 21 cm length and an outer Pyrex glass reactor of 5.7 cm i.d. and 16 cm length.

Photocatalytic Degradation of Dyes and Organics

J. Phys. Chem. C, Vol. 111, No. 4, 2007 1667

The UV light was generated by using a 125 W high-pressure mercury vapor lamp (Philips, India). The ballast and capacitor were connected in series with the lamp to avoid fluctuations in the input power supply. Water was circulated through the annulus of the quartz tube to avoid heating of the solution due to the dissipative loss of the UV energy. The solution was taken in the outer reactor and continuously stirred using a magnetic stirrer to ensure that the suspension of the catalyst was uniform during the course of the reaction. The lamp radiated predominantly at 365 nm, corresponding to an energy of 3.4 eV. Further details of the experimental setup can be found elsewhere.45,46 Degradation Experiments. The phenolic compounds and dyes (model pollutants) were dissolved in double distilled Millipore filtered water. The degradation reactions were performed using 1.0 kg/m3 of the catalyst with a variety of concentrations of the pollutant. Samples were collected from the catalytic reactor at regular intervals for the analysis. Control experiments were conducted without the catalyst under UV radiation and in the presence of the catalyst without the UV radiation. The degradation studies were carried out at natural pH conditions and different initial concentrations for all the phenols and the dyes. Sample Analysis. The molar absorptivity of the dyes employed was measurably high even in dilute solutions; hence, photometric studies were carried out. The UV-vis spectrophotometer (Shimazdu, UV 2100) with quartz cuvettes was used in the range of 190-700 nm. The λmax for RB, RBL, OG, and RBBR was 553, 665, 475, and 590 nm, respectively. The UV spectrum before and after degradation showed a decrease in the peak intensity corresponding to the decrease in the concentration of the dye but did not show any new peaks. This indicates that no detectable intermediates were formed during the decomposition studies of the organic dyes. The Beer-Lambert law was used to calibrate and quantify the concentration of the dyes, before and after the photocatalytic experiments. The products of the photodegradation, in the case of phenols and substituted phenols, were filtered through Millipore membrane filters and centrifuged to remove the catalyst particles prior to the analysis. To quantify the photodegradation, the samples were analyzed by HPLC. The HPLC consisted of an isocratic pump (Waters 501), a Rheodyne injector, a C-18 column, a UV detector (Waters 2487), and a data acquisition system. The eluent system consisted of 90 vol % water and 10 vol % methanol pumped at 0.5 mL/min for analyzing phenol and 4-chlorophenol and 60 vol % water/40 vol % methanol for analyzing methylphenol and nitrophenol. The substituted phenols were found to elute only after 30-45 min, after injection in the HPLC, which ensures that the peaks of the intermediates can be separated easily. Samples were injected in a Rheodyne valve with a sample loop of 50 µL, and the UV absorbance was at 270, 280, 280, and 320 nm for phenol, 4-chlorophenol, 4-methylphenol, and 4-nitrophenol, respectively. The chromatographic areas were converted to concentration values by using the calibration curves based on the pure compounds. Results and Discussion Thermal Decomposition Studies. Thermogravimetric studies (TGA, Metler-Toledo) in flowing air (flow rate ) 50 mL/min) on the MOF compound indicated a weight loss in the temperature range 100-200 °C, which may be due to the loss of the bound water molecules. The second weight loss in the temperature range 330-360 °C corresponds to the loss of the carboxylate. No further weight loss was observed up to 850 °C. The final product after the TGA studies was found to be

Figure 2. TEM images of the sample formed at three different temperatures: (a) 400, inset shows SAED pattern, (b) 700, and (c) 800 °C.

crystalline by powder XRD and corresponds to GdCoO3 (JCPDS: 00-025-1057) (Figure 1a). Since we did not observe


Mahata et al.

Figure 3. Concentration profiles of the various dyes (a) OG, (b) RBBR, (c) RB, and (d) RBL, degraded using 3 and 200 nm GdCoO3 and Degussa P-25. The closed, open, and crossed points correspond to 3 and 200 nm GdCoO3 and Degussa P-25, respectively.

any change in weight in our TGA after 360 °C, it is likely that the GdCoO3 has been formed at that temperature. The MOF precursor was heated, ex-situ, in air at various temperatures, and the decomposition process was monitored by powder XRD (Figure 1b). It may be noted that the sample heated at 400 °C was predominantly amorphous. The sample, however, exhibited sufficient crystallinity when heated at 700 °C, which was found to be GdCoO3 (Figure 1c). TEM Investigation. From the ex-situ XRD studies, it is clear that the decomposition of the MOF precursor follows an X-ray amorphous phase route to form the perovskite oxide. The temperature range for the X-ray amorphous phase is 350-650 °C. TEM of the samples prepared at 400, 700, and 800 °C were studied to investigate the shape and size of the particles. For all the samples, the EDAX analysis clearly indicated a ratio of 1:1 for La:Co, which is also the ratio present in the MOF as well as in GdCoO3. The results are presented in Figure 2a-c. The selected area electron diffraction (SAED) pattern indicates that the 3 nm particle (decomposed at 400 °C) is crystalline (inset of Figure 2a). It is clear that the heating temperature has a profound effect on the particle size of the GdCoO3 phase. We have obtained particle sizes of 3 ( 2, 12 ( 3, and 200 ( 20 nm for the sample heated at 400, 700, and 800 °C, respectively. This indicates that the growth of the particles is rather sluggish in the temperature range 400-700 °C. It may be noted that the XRD pattern for the 700 °C heated sample showed the GdCoO3 perovskite phase. The particle size of the sample heated at 800 °C appears to show a considerable increase in the size. This behavior indicates that the sample reaches a critical size and then aggregates to form larger particles. Similar behavior has also been observed previously in the in-situ studies on the growth of PbTiO3 phase.47 This is typical of nucleation and growth mechanism proposed for the formation of phases.

The main result of the present study is that we have been able to form nanosized GdCoO3, where the average particle sizes are 3, 12, and 200 nm, respectively, by decomposing the precursor at different temperatures. Photocatalytic Investigations. The photocatalytic control experiments indicated that there was no appreciable degradation of the dyes or phenol substances over GdCoO3 either in the absence of UV irradiation or in the absence of the catalyst. All the compounds were stirred in dark for 30 min prior to UV irradiation to ensure that the equilibrium adsorption/desorption of the substrate on the catalyst was attained as well as eliminating the effect of adsorption during photocatalytic reactions. During this step, a significant decrease in the concentration of the model pollutants was observed, when 3 nm GdCoO3 was used. For 12 and 200 nm GdCoO3 and Degussa P-25 catalysts the adsorption effect was found to be less than 2% for an initial concentration of the model pollutant of 100 mg L-1. This indicates that the 3 nm catalyst adsorbs the dye more effectively than the 12 and 200 nm GdCoO3 or Degussa P-25. The corresponding concentration of the dye/ phenol substances (model pollutants) after adsorption was taken as the initial concentration for all the subsequent photocatlytic reactions. The variation of the concentration of the dyes, viz. OG, RBBR, RB, and RBL, at different initial concentrations, in the presence of 3 and 200 nm GdCoO3 and Degussa P-25 photocatalysts is presented in Figure 3a-d. The degradation profile of the dyes in the presence of 12 nm GdCoO3 was found to be in between that of 3 and 200 nm GdCoO3 and was also similar to that of Degussa P-25. Further evaluation of the behavior of the photocatalysts was conducted only with 3 and 200 nm sized GdCoO3 and compared with Degussa P-25. The initial rates of the reactions were determined by extrapolating the tangent



Figure 4. Variation of the inverse of the initial rate with the inverse of the initial concentration for the various dyes (a) RBL, (b) RB, (c) RBBR, and (d) OG using the 3 and 200 nm GdCoO3. The closed and open symbols correspond to 3 and 200 nm GdCoO3, respectively.

TABLE 1: Observed Rate Constants for the Photocatalytic Degradation of Dyes RB, RBL, RBBR, and OG Using 3 and 200 nm GdCoO3 3 nm GdCoO3 dye RB RBL RBBR OG

k0

(min-1) 0.065 0.078 0.019 0.053

K1 (L

TABLE 2: Comparison of the Photocatalytic Degradation Rate of the Dyes RB, RBL, RBBR, and OG Using 3 and 200 nm GdCoO3 and Degussa P-25

200 nm GdCoO3 mg-1)

0.009 0.009 0.002 0.048

k0

(min-1) 0.008 0.013 0.004 0.020

K1 (L

mg-1)

0.000 0.002 0.001 0.015

(based on the linear fit of the first three or four points) of the concentration profile back to the initial conditions. The slopes were found to be nearly constant, indicating the accuracy of the initial rates reported in this study. The heterogeneous photocatalytic degradation appears to follow Langmuir-Hinshelwood (LH) kinetics r0 ) k0C0/(1 + K1C0), where r0 indicates the initial photocatalytic degradation rate and C0 is the initial concentration of the model pollutant. For the degradation of the dyes catalyzed by 3 and 200 nm GdCoO3, the inverse of initial rates, r0, varies linearly with the inverse of the initial concentration [C0], as shown in Figure 4a-d. The values of k0 and K1 for GdCoO3 were obtained from the linear regression and are presented in Table 1. In the case of 200 nm GdCoO3, the value of equilibrium adsorption coefficient, K1, was found to be almost negligible. The equilibrium adsorption coefficient for degradation of dyes in the presence of 3 nm GdCoO3 was considerable and was at least twice that observed for 200 nm.

dye

C0 (mg L-1)

r0, 3 nm (mg L-1 min-1)

r0, 200 nm (mg L-1 min-1)

r0, DP-25 (mg L-1 min-1)

RB RBL RBBR OG

9.3 13.6 109 46.8

0.56 0.94 1.74 0.77

0.08 0.17 0.37 0.54

0.66 0.70 0.41 0.69

The 3 nm GdCoO3 was found to be consistently more active in degrading the four dyes compared to the 200 nm GdCoO3. Correspondingly, the values of k0 and K1 for the 3 nm GdCoO3 were higher than those obtained for the 200 nm GdCoO3, for all the dyes investigated in this study. The photocatalytic decomposition rate (k0) for the dyes, RB, RBL, RBBR, and OG, was found to 7.8, 6.1, 5.3, and 2.6 times higher for the 3 nm GdCoO3 compared to the 200 nm sample. The photodegradation rates obtained for the 3 and 200 nm GdCoO3 were compared with that obtained for the commercial catalyst, i.e., Degussa P-25 in Table 2. The photocatalytic degradation rate (k0) obtained for 3 nm GdCoO3 was higher than that obtained for Degussa P-25 by 1.3, 4.2, and 1.1 times respectively for the decomposition of RBL, RBBR, and OG. For RB, the Degussa P-25 was found to be 1.2 times higher than the 3 nm GdCoO3 catalyst. It was observed recently that the photocatalytic degradation of methylene blue in aqueous system decreases with increasing


Mahata et al.

Figure 5. Photocatalytic degradation profiles of phenol, 4-chlorophenol, 4-methylphenol, and 4-nitrophenol in the presence of 3 and 200 nm GdCoO3 and Degussa P-25. The closed, open, and crossed points correspond to 3 and 200 nm GdCoO3 and Degussa P-25, respectively.

TABLE 3: Rate Constants for the Photocatalytic Degradation of Phenol, 4-Chlorophenol, 4-Methylphenol, and 4-Nitrophenol in the Presence of 3 and 200 nm GdCoO3 3 nm GdCoO3

Figure 6. Variation of the inverse of the initial rate with the inverse of the initial concentration for phenol, 4-chlorophenol, 4-methylphenol, and 4-nitrophenol in the presence of 3 nm GdCoO3.

size of titania (12 nm to 49 µm, BET surface area varying from 127 to 12 m2/g).48 In the present study, we also find that the 3 nm GdCoO3 (surface area of 28 m2/g) shows much higher photocatalytic activity compared to the 200 nm GdCoO3 (5 m2/ g), consistent with the earlier observation.

200 nm GdCoO3

pollutant

k0 (min-1)

K1(L mg-1)

k0 (min-1)

phenol 4-chlorophenol 4-methylphenol 4-nitrophenol

0.021 0.061 0.069 0.056

0.015 0.003 0.032 0.057

0.007 0.010 0.016 0.009

The photocatalytic studies on the degradation of the phenols and related compounds were carried out for 3 and 200 nm GdCoO3 and Degussa P-25 catalysts. The results of the studies are shown in Figure 5. The decomposition of the pollutants, again, follow the Langmuir-Hinshelwood (LH) kinetics. The variation of the inverse of the initial rate with respect to the inverse of the initial concentration for the 3 nm GdCoO3catalyzed reactions is shown in Figure 6. It was observed that the 200 nm GdCoO3 sample show negligible surface adsorption for phenols and substituted phenols. It was observed that r0 was linear with respect to C0, indicating that K1 is negligible. Hence, the photodegradation experiments were done at only one initial concentration for 4-nitrophenol and 4-chlorophenol. The order of the degradation rate coefficient, k0, for the various model pollutants is as follows: 4-methylphenol > 4-chlorophenol > 4-nitrophenol > phenol, in the case of both 3 and 200 nm



TABLE 4: Comparison of the Photocatalytic Degradation Rates of Phenol, 4-Chlorophenol, 4-Methylphenol, and 4-Nitrophenol in the Presence of 3 and 200 nm GdCoO3 and Degussa P25

pollutant

C0 (mg L-1)

r0, 3 nm (mg L-1 min-1)

r0, 200 nm (mg L-1 min-1)

r0, DP-25 (mg L-1 min-1)

phenol 4-chlorophenol 4-methylphenol 4-nitrophenol

49 130 49 25

0.585 5.777 1.329 0.553

0.343 1.255 0.784 0.225

0.459 3.759 0.619 0.480

TABLE 5: Formation and Consumption Rate Constants for the Various Intermediates Formed with the Phenol and Substituted Phenols in the Presence of 3 and 200 nm GdCoO3 and Degussa P-25 catalyst 3 nm GdCoO3

pollutant phenol 4-chlorophenol

4-methylphenol 200 nm GdCoO3 phenol 4-chlorophenol

4-methylphenol Degussa P-25

phenol 4-chlorophenol 4-methylphenol

kc kf intermediates (min-1) (min-1) kc/kf hydroquinone catechol hydroquinone catechol chlorocatechol phenol hydroquinone catechol hydroquinone catechol hydroquinone catechol chlorocatechol hydroquinone catechol hydroquinone catechol hydroquinone catechol hydroquinone catechol phenol

0.034 0.200 0.036 0.200 0.075 0.984 0.127 0.393 0.023 0.074 0.035 0.050 0.017 0.092 0.160 0.036 0.036 0.070 0.051 0.046 0.045 0.045

0.005 0.007 0.002 0.018 0.004 0.011 0.011 0.009 0.004 0.007 0.0238 0.007 0.1161 0.007 0.004 0.013 0.009 0.010 0.018 0.013 0.022 0.030

7.4 29.8 17.8 11.1 20.8 93.6 12.5 43.8 6.2 10.6 0.68 6.83 7.5 12.9 37.0 2.7 3.9 7.0 2.8 3.5 2.0 1.5

GdCoO3. The results are also presented in Table 3. The overall photocatalytic degradation rate for all the three catalytic systems, however, follow the order 4-chlorophenol > 4-methylphenol > 4-nitrophenol or phenol, and the observed rates are presented in Table 4. This observation appears to be consistent with the trend reported previously.37 The higher degradation rate in the case of the substituted phenols may be attributed to the preferential attack at the substituent, which can easily be hydroxylated compared to a simple aromatic compound without such a substituent group. Comparing GdCoO3 with Degussa P-25, for the photocatalytic decomposition of phenol and substituted phenols, indicates that the rate obtained for the 3 nm GdCoO3 was the highest, and the trend is as follows: 3 nm GdCoO3 > Degussa P-25 > 200 nm GdCoO3. The kinetics of the reaction, the rates of formation, and consumption of the primary intermediates were determined using the equation developed previously for the concentration of the intermediate.49 In the case of the degradation of phenol, methylphenol (cresol), and chlorophenol, two intermediates, namely catechol and hydroquinone, were formed in the presence of all three catalysts. In the presence of Degussa P-25, however, phenol was found to be the additional intermediate during the degradation of methylphenol. Further, in the presence of 3 nm GdCoO3, phenol and chlorocatechol were observed as additional intermediates during the degradation of chlorophenol. In the case of nitrophenol, no intermediates were detected for all the three catalysts during the photocatalytic reactions. Though the

Figure 7. Concentration profile of the intermediates (a) hydroquinone and (b) catechol formed during degradation of phenol by 3 and 200 nm GdCoO3 and Degussa P-25, respectively. The closed, open, and crossed points correspond to 3 and 200 nm GdCoO3 and Degussa P-25, respectively. The numbers on the graph indicates the initial concentration of phenol. Lines indicate the model fit.

intermediates observed for the three catalysts were similar, the rate at which they were formed and disappeared was different. This confirms that the mechanisms of degradation of the various substituted phenols are comparable. It is to be noted that the mechanism for the degradation of various substituted phenols is well established and reported in many studies.28,37,50-52 In photocatalysis, a frequently discussed issue is the oxidative pathway, i.e., direct hole attack or/and OH radical oxidation. In the case of GdCoO3, OH radicals are present in the phenol solution, indicating that GdCoO3-mediated degradation of phenol and substituted phenols occurs by the direct hole attack as well as OH radical oxidation. The degradation mechanism seems to be similar to that observed in the presence of titania. The perovskite compounds can accommodate bigger lattice distortions leading to a decrease of the recombination rate of the electron-hole pairs, thereby increasing the photocatalytic activity. The correlation of photocatalytic activity to perovskite structure, however, is not yet fully understood. In the perovskite oxides, the A-site (Gd) cations influence the catalytic activity because of the partly occupied 4f levels of Gd, and the B-site (Co) cations are catalytically active because the conduction band and valence band primarily consist of Bd orbitals and Op orbitals, respectively.15 During the present studies, we observed catechol and hydroquinone as the primary intermediates, which can


Mahata et al.

Figure 8. Concentration profile of the intermediates (a) hydroquinone, (b) catechol, (c) phenol, and (d) chlorocatechol, formed during degradation of 4-chlorophenol by 3 and 200 nm GdCoO3 and Degussa P-25. The closed, open, and crossed points correspond to 3 and 200 nm GdCoO3 and Degussa P-25. The numbers on the graph indicates the initial concentration of 4-chlorophenol. Lines indicate the model fit.

undergo secondary hydroxylation and then ring-opening reaction to give rise to aliphatic intermediates, which can further degrade giving rise to CO2 and water vapor. The concentration of all the intermediates increased initially and then decreased. The maximum in the concentration of the intermediates is observed due to the competition between the primary hydroxylation and the secondary hydroxylation step. The degradation pathway for the pollutants thus can be described as follows

Cint,max, and the corresponding time, tmax, can be obtained from the above equation and are given by

() () ( )

Cint,max kf ) CA0 kc

ln

tmax )

kc

where A denotes the initial pollutant, B, C, and D denote the primary hydroxylated intermediates, E and F denote the secondary hydroxylated intermediates, and ROF denotes the ringopened fragments. Assuming all the reactions to be first order, for a series reaction, the concentration of the intermediate is given by49Cint/ CA0 ) kf/kc - kf(exp(-kft) - exp(-kct)), where Cint is the concentration of the intermediate at any time t, CA0 is the initial concentration of the pollutant A, and kf and kc denote the formation and the consumption rate constant of the intermediate, respectively. The maximum concentration of the intermediate,

1/(1-kf/kc)

kf kc

kf -1 kc

(1)

(2)

The concentration of the intermediates obtained experimentally is fitted with eqs 1 and 2 to determine kf and kc. The values of kf and kc for the primary hydroxylated intermediates are listed in Table 5. The concentration profiles of the intermediates for the phenols and substituted phenols in the presence of different photocatalysts along with corresponding model fit are shown in Figures 7-9. As can be noted, the proposed model fits well with the experimental data. In all the cases, the value of kc is higher than kf, indicating that the secondary hydroxylation and subsequent reaction steps are faster than the primary hydroxylation. This could be a reason that no detectable amount of secondary hydroxylated species was observed. However, in all the cases, a peak corresponding to the organic acids, primarily


J. Phys. Chem. C, Vol. 111, No. 4, 2007 1673 slower compared to that of Degussa P-25, the intermediates observed are lower in concentration. Since these intermediates are also pollutants and are formed in lesser quantities, one can conclude that GdCoO3 can be a good photocatalyst in environmental by relevant application. Conclusions Nanosized GdCoO3 pervoskite oxide with different particle sizes was prepared by the thermal decomposition of a single source mixed-metal carboxylate framework (MOF) precursor. Photocatalytic studies on the decomposition of organic dyes, phenol, and substituted phenols as model pollutant indicate that 3 nm GdCoO3 was more efficient than the Degussa P-25 catalyst. The intermediates formed during the decomposition of phenols were similar for both the GdCoO3 and the P-25 catalysts, indicating that the possible mechanisms of degradation are same. The formation of lesser concentration of the intermediates in the presence of GdCoO3 shows that it is beneficial for environmental photocatalytic applications. Acknowledgment. The authors thank the Nano Center at IISc for help with TEM Studies. S.N. thanks the Department of Science and Technology (DST), Government of India, for the award of a research grant. The authors also thank the Nanoscience and Technology Initiative (NSTI, DST), Government of India, for financial support. References and Notes

Figure 9. Concentration profile of the intermediates (a) hydroquinone, (b) catechol, and (c) phenol, formed during degradation of 4-methylphenol by 3 and 200 nm GdCoO3 and Degussa P-25. The closed, open, and crossed points correspond to 3 and 200 GdCoO3 and Degussa P-25, respectively. The numbers on the graph indicates the initial concentration of 4-methylphenol. Lines indicate the model fit.

a mixture of maleic acid and oxalic acid, was also detected. These acids are formed as a result of ring opening during the photodegradation.40 The values of kc/kf for all the intermediates are much higher in 3 nm GdCoO3 catalyzed system than that obtained in the Degussa P-25 catalyzed system which indicates a minimal presence and formation of the intermediates during photodegradation. Even for 200 nm GdCoO3, which degrades phenol

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