- Email: [email protected]
Effect of Preparation Conditions on Visible Photocatalytic Activity of Titania Synthesized by Solution Combustion Method*
Chin. J . C h m . Eng., 15(2) 178-183 (2007)
Effect of Preparation Conditions on Visible Photocatalytic Activity of Titania Synthesized by Solution Combustion Method* CHENGYouping(%k%), SUN Hongqi(%k$!), JW Wanqin(&Ti%)**and XU Nanping(& k 7 )
Membrane Science and Technology Research Center, Nanjing University of Technology, Nanjing 2 10009, China
Abstract Titania catalysts were synthesized by a solution combustion method (SCM). Photodegradation of 4-chlorophenol (4-CP) using the synthesized catalysts was studied under both visible light (L2420nm) and sunlight irradiation. The effect of preparation conditions on photocatalytic activities of the synthesized catalysts was investigated. The optimal photocatalytic activity of the catalyst (denoted as A1 1 was obtained under the following synthesis conditions: ignition temperature of 350"C, fuel ratio ( 4 ) of 1 and calcination time of lh. The degradation and mineralization ratio of 4-CP were 78.2%and 53.7% respectively under visible light irradiation for 3h using catalyst A l . And the catalyst A1 also showed high photocatalytic activity under sunlight irradiation. Keywords TiOz, solution combustion method, photodegradation, visible light
1 INTRODUCTION Semiconductor-based photocatalysis is an advanced technology for removal of organic and inorganic pollutants in air or solution[l]. Many studies have been focused on Ti02 because of its high efficiency, long-time photostability and low cost[2]. Ti02 is the most widely used photocatalyst in practical applications such as self-cleaning window panes and paints[3]. However, it is activated only in the UV region (<387nm)because of its large band gap of about 3.2eV. The cost and inaccessibility of UV photons make it desirable to develop photocatalysts, which are active under visible-light excitation using the solar spectrum. Many attempts have been made to shift the Ti02 absorption from the ultraviolet to the visible region by transition-metal doping, but no appreciable change in band-gap energy of Ti02 has been observed[4]. At the same time, many efforts have been devoted to researching nonmetal ion-doped TiOz, which showed an exciting shift to the visible region[5,6]. Besides, several methods have been employkd for the synthesis of titania photocatalyst such as solution combustion[7], sol-gel method[8], hydrothermal progress[9J and spray pyrolysis[lO]. Usually the sol-gel technique requires further annealing treatment of the amorphous precipitates, to induce crystallization, and the hydrothermal and spray pyrolysis commonly take place under high temperature. However the solution combustion method (SCM) is a simple process to prepare titania catalysts, whch demands shorter time and lower temperature. Despite the contributions from Madras' group[7,11] for their successfully applied catalysts prepared by SCM, little attention has been paid to the effect of the preparation conditions on the photocatalytic activity. Ignition temperature may influence the phase transformation of catalysts, and the combustion temperature changes with the amount of fuel may probably have an effect on the crystallinity of catalysts. The effect of calcination time on the photocatalytic activity is a significant factor especially when Received 2006-03-31, accepted 2006-09-15.
titania is prepared by the liquid phase reaction route. The crystallinity, surface area and surface property of titania, which possibly play an important role in photocatalytic activity, are changed with varying calcination time. Therefore, deep studies of the effect of preparation conditions on photocatalytic activity are necessary. The objective of this study is to investigate the effect of preparation conditions on visible photocatalytic activity for photodegradation of 4-chlorophenol (4-CP) using the catalysts synthesized by SCM. The 4-CP was selected because it is listed among top priority pollutants, and commonly known as a toxic and nonbiodegradable organic compound. It is also widely used for the production of dyes, drugs, and fungicide[ 121. In this paper, the preparation conditions including ignition temperature, calcination time and the amount of fuel have been studied in detail.
2 EXPERIMENTAL 2.1 Catalysts preparation Tetrabutyl titanate of lOml as a precursor was slowly dropped into deionized water of lOOrnl in an ice-water bath with vigorous stirring to cany out hydrolysis. White precipitate of TiO(OH)2 was formed. After continuously stirring for 20min, the precipitate reacted with nitric acid to synthesize into transparent titanyl nitrate solution. Assuming that the titanyl nitrate and glycine undergo complete reaction, the formation of Ti02 can be described in the follow equation. 9TiO(NO,),,,, + lOC,H,O,N(,,3 9Ti0,(,, + (1) 14%,, +2OCO,(,) +25H20(,, 2.5024g of glycine was dissolved in the titanyl nitrate solution. Finally, this new solution was calcinated for lh in a muffle furnace, which was preheated to a fixed temperature of 350°C. A yellow powder was obtained. The ignition temperature, calcination time, and the fuel ratio ( qj ) were changed to obtain various Ti02
* Supported by the Key Laboratory of Material-Oriented Chemical Engineering of Jiangsu Province and Ministry of Education. ** To whom correspondence should be addressed. E-mail: [email protected]
Effect of Preparation Conditions on Visible PhotocatalyticActivity of Titania Synthesized by Solution CombustionMethod 179 powders. According to the stoichiometric ratio of titanyl nitrate and glycine of Eq.(l), it was defined that 4 =9n/10rn, where m and n are the molar mass of titanyl nitrate and glycine, respectively.
I
T
20
25
0
Y
2.2 Characterization The crystal structures of the prepared catalysts were observed with the help of an X-ray diffraction (XRD)instrument (Bruker D8, Germany) using a Cu target K, ray (2=0.15405nm). The accelerating voltage and the applied current were 40kV and 30mA, respectively. The diffraction patterns were collected at room temperature in the range of 20"-80". The average crystallite sizes were calculated with the Scherrer equation with the full width at half maxina (FWHM) data. The Brunauer-Emmett-Teller(BET) surface areas were obtained from the nitrogen adsorption apparatus (CHEMEET-300, America). UV-Vis reflectance spectra of samples were measured with the help of a UV-V is spectrophotometer (UV-2401, Shimadzu, Japan) using BaS04 as background. The carbon states in the combustion-synthesized Ti02 catalyst were measured by X-ray photoelectron spectroscopy (ESCALB MK-11, VG, Britain) using Mg K, radiation as X-ray source. FT-JX studies were carried out with the help of infrared spectroscopy (NEXUS 670, Nicolet, America). 2.3 Photocatalytic degradation experiments Photocatalytic activities of the as-synthesized Ti02 catalysts were evaluated by measuring the degradation efficiencies of 4-CP (O.l5mmol~L-') under both visible light and sunlight irradiation sources. The experiments under visible light irradiation were carried out with lOOmg of catalyst suspended in lOOml 4-CP solution in a 250ml reactor. A recycle water jacket was used to keep the reactor temperature constant at (30*1)"C. A 250-W Xe lamp (L25, Amax= 470nm) was used as visible light source. The light irradiated outside (at a distance of 25cm from the solution surface) through a 420nm cutoff filter (L40, Libang) upon the reactor. The light intensity near the solution surface was about 30mW.~m-~. The experiment for evaluation of sunlight photocatalytic activity of the prepared catalysts was as follows: a catalyst of 200mg was suspended in 200ml4-CP (0.15mmol.L-') solution in a beaker and exposed to sunlight after stirred for l0min. The absorbance spectrum of 4-CP was recorded on a UV-Vis spectrophotometer instrument (LAMBDA 35, Perkin Elmer, USA). The concentrations of 4-CP were determined by a HPLC (Agilent 1100, USA), equipped with a ZORBAX Eclipse XDB-C18 reversed phase column. The mobile phase was a mixture of water and methano! (50 : 50 by volume) with a flow rate of 1.0ml.min- . The total organic carbon (TOC) was measured using a TOC instrument (TOC-VCPH, Shimadzu, Japan), to evaluate the photomineralization degree of 4-CP. 3 RESULTS AND DISCUSSION 3.1 Effect of ignition temperature Figure 1 shows the effects of ignition tempera-
.0
-.-E .+
30
35
40 45 28, ("1
50
55
60
Figure 1 XRD patterns of catalysts prepared at different ignition temperatures T, "C : 1-300; 2-350; 3--400,4-500; 5-00; 6-700 ' Ianatase: rutile
tures on the phase structures of the synthesized Ti02 samples at 300, 350,400, 500 and 600"C, respectively. The temperature of 300°C was chosen as the lowest ignition temperature, because the decomposition temperature of glycine is 262°C. A trace anatase phase was observed for the sample ignited at 300°C. This was because the combustion of the fuel can reach a high temperature of 70Ck8OO"C for a short time. The high temperature induces the material to form the anatase phase. No significant phase changes of the catalysts were observed when the ignition temperature changed from 350°C to 500°C. But when the ignition temperature increased to 600"C, some of the anatase phase changed into the rutile phase. According to previous reports[13,141, the activity of the anatase phase of Ti02 for photodegradation of various pollutants is in general much higher than that of rutile. Therefore the temperature of 350°C is suitable for the ignition temperature. From the Scherrer equation, the average particle size of the catalyst was estimated to be 46nm on the anatase (101) diffraction peaks when combustion was carried out at 350°C.
3.2 Effect of calcination time Figure 2 illustrates the XRD patterns of catalysts prepared at various calcination times. Apparently, the samples have higher crystallinity with increasing the
.2.
3
* .0
.*
-2 20
30
40
50 60 28, ("1
70
80
Figure 2 XRD patterns of catalysts prepared with various calcination times t , min: 1-10; 2-30; 3-60; 4-90; 5-120; 6-180 Chin. J. Ch. E. 15(2) 178 (2007)
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calcination time. The color of the catalysts gradually changed from brown to light yellow with an increasing in the calcination time. The change of color is because of that the organic residual is removed from the catalysts by prolonging the calcinations time. But long calcination time also resulted in larger average particle size and smaller surface area. The results are shown in Table 1. The TiOz catalyst combusted at 350,"C f"'1h with @ of 1 shows a surface area of 215.0m .g- . The phase content, crystallite size and BET surface area of pure TiOz (Degussa P25) is also given in the table for comparison. Photocatalytic activities of the as-synthesized Ti02 catalysts prepared with different calcination times were evaluated by measuring the degradation efficiencies of 4-CP (0.1Smmol.L-') under visible light irradiation. No observable degradation of 4-CP was observed either without catalyst or without irradiation. The adsorption capacity of the as-synthesized catalysts was evaluated in aqueous solution. The result indicated that the adsorption was less than 2% when the synthesized Ti02 was used in the dark over a whole day. The adsorption was not appreciable within 3-4h. Hence, the initial concentration was taken to be Co in all cases. Figure 3 shows the photodegradation and mineralization rates of 4-CP under visible light ( A 3420nm) irradiation with various catalysts. It is obvious that the photocatlytic activity for degradation of 4-CP rapidly decreased with increase in calcination time. The decreased activity is not just because of surface area decrease[l5]. It is commonly thought that the photogenerated valence band hole is quickly converted to hydroxyl radical upon oxidation of surface water, and the hydroxyl radical is the major reactant responsible for oxidation of organic substrates[16,171. The amount of surface hydroxyl groups in catalysts, which play an important role in photocatalytic activity, may decrease with an increase in calcination time. Reasons for a decreased in photocatalytic activity with an increase in calcination time need further investigation. According to the results above, the crystallinity, surface area, and the color of the catalysts changed with varying calcination time. Prolonging the calcination time promoted the gGowth of crystalline and reduced the surface area. It could be suggested that some changes such as the amount of fuel, should be made to enable the photocatalytic activity and surface
0.8
.-
0.6 0.4 0.2
0
0.4 1
0
120 180 240 irradiation time, min (a) Remaining concentration rate 60
,
120 180 irradiation time, min (b) Remaining TOC rate
60
240
Figure 3 Photodegradation of 4-CP with various catalysts under visible light (A3420nm) irradiation Al;
A2; A A3
area to be compensated.
3.3 Effect of fuel ratio The effect of fuel ratio on the phase structures of the synthesized TiOz catalysts was also investigated. When other preparation conditions were kept identical, it was found that the peak intensities of anatase were nearly the same when the value varied from one to three. However, their surface areas become large with an increasing of @ value, as shown in Table 1. This phenomenon was on account of the generation of a significant amount of gas when more fuel was used. The color of the catalysts gradually changed from light yellow to brown with increasing the 4 value. Figure 4 shows the effect of #J on the photocatalytic activities of the catalysts in photodegradation of 4-CP
Table 1 Effect of calcination time and fuel ratio on the phase content, crystallite size and BET surface area Catalyst A1
Calcination time, h 1
4
Phase"
CrystaIIite size, nm'
1.o
A
Surface area, mz.g-* 215.0
A2
2
1.o
A
4.66 5.42
A3
3
A
5.58
119.0
B3
3 3
A A A A(75%), R(25%)
5.37 4.57 4.13 17.8
137.4
c3 D3
1.o 1.5 2.0 3.0
3
P25
0 A: anatase phase; R: m i l e phase. 0 Calculated by applying the Schemer equation. April, 2007
179.5
148.5
207.2 49.2
Effect of Preparation Conditions on Visible PhotocatalyticActivity of Titania Synthesized by Solution Combustion Method 181
0.2 I 0
lytic activity. Some reports[ 18,191 have already illustrated that superoxide radical anion .O, results in lowering the rates of recombination. According to Lettmann's research[l8], an electron can be transferred directly from the excited photosensitiser to triplet oxygen to generate the superoxide radical anion -O,, and the carbonaceous species can act as a photosensitiser. According to this, the higher photocatalytic activity for catalyst D3, prepared with higher fuel ratio, may be attributed to the formation of a more coke-like residua during the combustion process.
I
I
120 180 240 irradiation time, min (a) Remaining concentration rate
60
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7
0.4
0
I
I
60
120 180 irradiation time, min (b) Remaining TOC rate
I
240
Figure 4 Photodegradation of 4-CP using various catalysts prepared with different 4 values under visible light (2 3420nm) irradiation A3; oB3; A c 3 ; r D 3
under visible-light (;1>42Onm) irradiation. As shown in Fig.4, both the photodegradation and mineralization rates increased gradually with the increasing of Q value. In the present of the catalyst D3 prepared with 9 of 3, it showed higher degradation and mineralization rates within 3h of irradiation. There were no significant change in the photodegradation and mineralization rates after that. For other catalysts synthesized at a lower Q value, the photodegradation rate of 4-CP changed almost linearly with irradiation time. The changes of surface areas alone are not sufficient to explain the phenomenon. It is commonly thought that the heterogeneous photocatalysis is a surface phenomenon. The photogenerated electrodhole pairs must be separated effectively to get high photocata-
3.4 Combined effect of calcination time and fuel ratio The effect of the calcination time and the fuel ratio on photocatalytic activity for degradation of 4-CP have been discussed separately above. Higher fuel ratio results in a larger amount of carbonaceous species, but requires longer calcination time to remove the residual soluble organic. However, prolonging calcination time results in the decrease of surface hydroxyl group and surface area. An optimal preparation condition may exist in a region. To find the optimal coordination between the two preparation factors, a series of catalysts were prepared with various calcination times and fuel ratios. Photodegradation of 4-CP under visible-light (A3420nm) and irradiation for 3h were carried out to investigate photocatalytic activities of the synthesized catalysts. The results are listed in Table 2. The highest photocatalytic activity for photodegradation of 4-CP was obtained for the catalyst prepared with 4 = 1 and calcination time of 1h.
3.5 Solar photocatalyticactivity Photodegradation of organic contaminates using sunlight irradiation can be very much more economical than using artificial light irradiation. Investigations have been made to study the photocatalytic activity of the synthesized catalysts under sunlight. Figure 5 shows the sunlight irradiation time-dependence of the absorption spectra of 0.15mmol-L-' 4-CP solution using catalyst Al. The monotonic decreases in the absorbance at 225nm, clearly revealing the decomposition of 4-CP. The increase in absorbance in the region of 240-270nm is attributed to the formation of reaction intermediates,
Table 2 Photodegradationand mineralizationrate of 4-CP under visible light irradiation with series catalysts synthesized with different preparation conditions'
4 = 1.5
$ = 1.0
Calcination time,
h
n
M
n
1
78.2 45.5
53.7
X
4 = 2.0 M
n
M
n
M
X
X
X
X
X
48.1
X
X
X
X
66.1
35.3
74.9
47.9
63.6 20.9 49.8 29.8 15.7 3 30.0 0 X :useless catalyst due to the residual soluble organics in the catalysts.
2
4 = 3.0
Chin. J. Ch. E. 15(2) 178 (2007)
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1
1
wa\eleng\h. nm
Figure 5 Sunlight irradiation time-dependence of the absorption spectra of O.lSmmol-L-' 4-CP solution using catalyst A1 in suspension l-rnin; 2-30min; 3-60min; 4-120min; 5-180min: 6-300min
such as hydroquinone and hydroxyhydroquinone. With an increase in the irradiation time, the generated intermediates were degraded gradually. Fig.6 shows both the photodegradation and mineralization of 4-CP under sunlight irradiation. For the catalyst A l , the degradation and mineralization rates are 93% and 75% in 4h, respectively. 1 .0
absorption spectra of combustion-synthesized TiOz and pure Ti02 (Degussa P25). The pure Ti02 almost has no absorption above 400nm. However, the combustion-synthesized Ti02 results in obvious absorption up to 800nm. Noticeable shifts of the absorbance edge to the visible light region of the combustion-synthesized Ti02 may attribute to carbon modification. This absorption feature suggests that the combustion-synthesized Ti02 can be activated by visible light. To investigate the carbon states in the combustion-synthesized Ti02 catalyst, C 1s core levels were measured by X-ray photoelectron spectroscopy (XPS), as shown in Fig.8. There are two peaks at 285.5 and 289.5eV. The peak at 285.5eV occurs as a result of elemental carbon[6], the latter peak at 289.5eV can be assigned to the formation of carbonate species[20,21]. Khan er a1.[22] prepared carbon-modified rutile titania by controlled flame pyrolysis of Ti metal, and found that the carbon substituted for some of the lattice oxygen atoms. However, Sakthivel et a1.[6] synthesized carbon-modified titanium dioxide by hydrolysis of titanium tetrachloride with tetrabutylammonium hydroxide followed by calcinations at 400°C, and observed new peaks at 287.5eV and 288.5eV. However, the carbon-modified Ti02 prepared by SCM has only one peak, indicating the presence of only one lund of carbonate species. These reports indicated that the carbon oxidation state in carbon-modified titania generally depend on preparation method.
5 1400 I
0
60
120 180 irradiation time. min
240
Figure 6 Remaining concentration and TOC rate of photocatalyticdegradation of 4-CP under sunlight irradiation using catalyst A1 C/C"; A TOCROCo
3.6 Physical properties of combustion-synthesized TiOz Figure 7 gives the UV-Vis diffusive reflectance ~.
Degussa P25
2 si 0 "
1
300
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400
500 600 wavelength, nin
700
800
Figure 7 UV-Vis diffusive reflectance absorption spectra of the combustion-synthesizedTi02( A l ) and pure TiOz (Degussa P25) April, 2007
$
1200
800 600 400
I
292
I
290
1
I
,
288 286 284 binding energy, eV
2 12
Figure 8 C 1s core level spectra of catalyst A1
Figure 9 shows the FT-IR spectra of combustion synthesized TiO2. The IR spectrum of pure Ti02 (Degussa P25) is also given for a comparison. A broad band at 3389cm-: is attributed to bound water. The peak at 1624cm- is because of vibration frequency corresponding to the bending of the water molecule. The low-intensity peak at1403cm-' which does not exists in pure TiOzcan be assigned to carbonate ion. This result agrees well with the XPS. Therefore, our results indicated that the formation of carbonate species lead to the narrowing of the band gap in the combustion-synthesized Ti02. Further investigation is need to determine if the carbon substitute of the lattice oxygen in the titania or only occupy the surface of the titania.
Effect of Preparation Conditionson visible PhotocatalyticActivity of Titania Synthesized by Solution Combustion Method 183 Asahi, R., Morikawa, T., Ohwaki, T., Aoki, K., Taga, Y . , “Visible-light photocatalysis in nitrogen-doped titanium oxides”, Science, 293,269-271(2001). 6 Sakthivel, S., Kisch, H., “Daylight photocatalysis by carbon-modified titanium dioxide”, Angew. Chem. Int. Ed., 42,4908-491 l(2003). 7 Sivalingam, G., Nagaveni, K., Hegde, M.S., Madras, G., “Photocatalytic degradation of various dyes by combustion synthesized nano anatase TiOi’, Appl. Catal. B: Environ., 4523-38(2003). 8 Liu, H., Yang, W., Ma, Y., Cao, Y . , Yao, J., Zhang, J., Hu, T., “Synthesis and characterization of titania prepared by using a photoassisted sol-gel method”, Langmuir, 19, 3001-3005(2003). 9 Liu, H., Gao, L., “(Sulfur, Nitrogen)-codoped rutile-titanium dioxide as a visible-light-activated photocatalyst”, J. Am. Ceram. SOC.,87, 1582-1584(2004). 10 Li, D., Haneda, H., Hishita, S., Ohashi, N., “Visible-light-driven N-F-codoped TiOz photocatalysts ( I ) Synthesis by spray pyrolysis and surface characterization”, Chem. Muter., 17,2588-2595(2005). 11 Priya, M.H., Madras, G., “Photocatalytic degradation of nitrobenzenes with combustion synthesized nano-TiOi’, J. Photochem. Photobiol. A: Chem., 178, 1-7(2006). 12 Zhao, D., Xu, X., Lei, L., Wang, D., “Degradation of 4-chlorophenol solution by synergetic effect of dual-frequency ultrasound with fenton reagent”, Chin. J . Chem. Eng., 13,204-210(2005). 13 Brown, J.D., Williamson, L.D., Nozik, A.J., “Moessbauer study of the kinetics of iron(3+) photoreduction on titanium dioxide semiconductor powders”, J . Phys. Chem., 89,3076-3080(1985). 14 Fox, M.A., Dulay, M.T., “Heterogeneous photocatalysis”, Chem. Rev., 93,341-357(1993). 15 Rachel, A., Subrahmanyam, M., Boule, P., “Comparison of several titanium dioxides for the photocatalytic degradation of benzenesulfonic acids”, Appl. Catal. B: Environ., 37,293-300(2002). 16 Du, Y.K., Rabani, J., “The measure of Ti02 photocatalytic efficiency and the comparison of different photocatalytic titania”, J . Phys. Chem. B , 107, 1197011978(2003). 17 Jung, K.Y., Park, S.B., Anpo, M., “Photoluminescence and photoactivity of titania particles prepared by the sol-gel technique: Effect of calcination temperature”, J. Photochem. Photobiol. A: Chem., 170,247-252(2005). 18 Lemnann, C., Hildenbrand, K., Kisch, H., Macyk, W., Maier, W.F., “Visible light photodegradation of 4-chlorophenol with a coke-containing titanium dioxide photocatalyst”, Appl. Catal. B: Environ., 32,215-227(2001). 19 Moonsiri, M., Rangsunvigit, P., Chavadej, S., Gula$ E., “Effects of Pt and Ag on the photocatalytic degradation of 4-chlorophenol and its by-products”, Chem. Eng. J., 97,241-248(2004). 20 Papirer, E., Lacroix, R., Donnet, J.B., Nanst, G., Fioux, P., “XPS study of the halogenation of carbon black (11) Chlorination”, Carbon, 33, 63-72( 1995). 21 Gopinath, C.S., Hegde, S.G., Ramaswamv, A.V., Mahapatra, S., “Photoemission studies of polymorphic CaC03 materials”,Muter. Res. Bull., 37, 1323-1332(2002). 22 Khan, S.A.M, Ingler, W.B., “Efficient photochemical water splitting by a chemically modified n-TiOy’, Science, 297,2243-2245(2002). 5
I
I
I
I
I
I
4000 3500 3000 2500 2000 1500 1000 500 wavenumber, cm-’
Figure 9 FT-IR spectrum of the combustion-synthesized Ti02 (Al) and pure TiOz (Degussa P25) combustion synthesized TiO,; - - - - Degussa P25 ~
4 CONCLUSIONS The titanium dioxide catalyst synthesized by SCM showed absorption to the visible region for the modification of the carbonate species. Photocatalytic activity of Ti02 was found to be dependent on the preparation conditions. Calcination time greatly affected the photocatalytic activity of the catalysts for degradation of 4-CP. It was also found that the fuel ratio affects the BET surface area and the amount of carbonaceous species. The ignition temperature did not show significant effect on the crystallinity of catalysts. An optimal preparation condition [ignition temperature of 350°C,fuel ratio ( 4 ) of 1 and calcination time of lh] was obtained. NOMENCLATURE M mineralization rate [M=(1-TOCi/rOCo) q
X 100%J
degradation rate [q=(l- Ci/Co)X loo%]
REFERENCES Zhao, W., Ma, W., Chen, C., Zhao, J., Shuai, Z., “Efficient degradation of toxic organic pollutants with Ni,O2/TiO7.,B, under visible irradiation”, J. Am. Chem. S&.,d126,;?8&4783(2004). Shi, Z . , Fan, Y., Xu, N., Jun, S., “Kinetics of photocatalytic degradation of methylene blue over Ti02 particles in aqueous suspensions”, Chin. J. Chem. Eng., 8, 1519(2000). Tryk, D.A., Fujishima, A., Honda, K., “Recent topics in photoelectrochemist: achievements and future prospects”, Electrochim. Acta, 45,2363-2376(2000). Di Paola, A., Marci, G., Palmisano, L., Schiavello, M., Uosaki, K., Ikeda, S., Ohtani, B., “Preparation of polycrystalline Ti02 photocatalysts impregnated with various transition metal ions: characterization and photocatalytic activity for the degradation of 4-nitrophenol”, J. Phys. Chem. B, 106,637-645(2002).
Chin. J. Ch. E. 15(2) 178 (2007)
Effect of Preparation Conditions on Visible Photocatalytic Activity of Titania Synthesized by Solution Combustion Method* CHENGYouping(%k%), SUN Hongqi(%k$!), JW Wanqin(&Ti%)**and XU Nanping(& k 7 )
Membrane Science and Technology Research Center, Nanjing University of Technology, Nanjing 2 10009, China
Abstract Titania catalysts were synthesized by a solution combustion method (SCM). Photodegradation of 4-chlorophenol (4-CP) using the synthesized catalysts was studied under both visible light (L2420nm) and sunlight irradiation. The effect of preparation conditions on photocatalytic activities of the synthesized catalysts was investigated. The optimal photocatalytic activity of the catalyst (denoted as A1 1 was obtained under the following synthesis conditions: ignition temperature of 350"C, fuel ratio ( 4 ) of 1 and calcination time of lh. The degradation and mineralization ratio of 4-CP were 78.2%and 53.7% respectively under visible light irradiation for 3h using catalyst A l . And the catalyst A1 also showed high photocatalytic activity under sunlight irradiation. Keywords TiOz, solution combustion method, photodegradation, visible light
1 INTRODUCTION Semiconductor-based photocatalysis is an advanced technology for removal of organic and inorganic pollutants in air or solution[l]. Many studies have been focused on Ti02 because of its high efficiency, long-time photostability and low cost[2]. Ti02 is the most widely used photocatalyst in practical applications such as self-cleaning window panes and paints[3]. However, it is activated only in the UV region (<387nm)because of its large band gap of about 3.2eV. The cost and inaccessibility of UV photons make it desirable to develop photocatalysts, which are active under visible-light excitation using the solar spectrum. Many attempts have been made to shift the Ti02 absorption from the ultraviolet to the visible region by transition-metal doping, but no appreciable change in band-gap energy of Ti02 has been observed[4]. At the same time, many efforts have been devoted to researching nonmetal ion-doped TiOz, which showed an exciting shift to the visible region[5,6]. Besides, several methods have been employkd for the synthesis of titania photocatalyst such as solution combustion[7], sol-gel method[8], hydrothermal progress[9J and spray pyrolysis[lO]. Usually the sol-gel technique requires further annealing treatment of the amorphous precipitates, to induce crystallization, and the hydrothermal and spray pyrolysis commonly take place under high temperature. However the solution combustion method (SCM) is a simple process to prepare titania catalysts, whch demands shorter time and lower temperature. Despite the contributions from Madras' group[7,11] for their successfully applied catalysts prepared by SCM, little attention has been paid to the effect of the preparation conditions on the photocatalytic activity. Ignition temperature may influence the phase transformation of catalysts, and the combustion temperature changes with the amount of fuel may probably have an effect on the crystallinity of catalysts. The effect of calcination time on the photocatalytic activity is a significant factor especially when Received 2006-03-31, accepted 2006-09-15.
titania is prepared by the liquid phase reaction route. The crystallinity, surface area and surface property of titania, which possibly play an important role in photocatalytic activity, are changed with varying calcination time. Therefore, deep studies of the effect of preparation conditions on photocatalytic activity are necessary. The objective of this study is to investigate the effect of preparation conditions on visible photocatalytic activity for photodegradation of 4-chlorophenol (4-CP) using the catalysts synthesized by SCM. The 4-CP was selected because it is listed among top priority pollutants, and commonly known as a toxic and nonbiodegradable organic compound. It is also widely used for the production of dyes, drugs, and fungicide[ 121. In this paper, the preparation conditions including ignition temperature, calcination time and the amount of fuel have been studied in detail.
2 EXPERIMENTAL 2.1 Catalysts preparation Tetrabutyl titanate of lOml as a precursor was slowly dropped into deionized water of lOOrnl in an ice-water bath with vigorous stirring to cany out hydrolysis. White precipitate of TiO(OH)2 was formed. After continuously stirring for 20min, the precipitate reacted with nitric acid to synthesize into transparent titanyl nitrate solution. Assuming that the titanyl nitrate and glycine undergo complete reaction, the formation of Ti02 can be described in the follow equation. 9TiO(NO,),,,, + lOC,H,O,N(,,3 9Ti0,(,, + (1) 14%,, +2OCO,(,) +25H20(,, 2.5024g of glycine was dissolved in the titanyl nitrate solution. Finally, this new solution was calcinated for lh in a muffle furnace, which was preheated to a fixed temperature of 350°C. A yellow powder was obtained. The ignition temperature, calcination time, and the fuel ratio ( qj ) were changed to obtain various Ti02
* Supported by the Key Laboratory of Material-Oriented Chemical Engineering of Jiangsu Province and Ministry of Education. ** To whom correspondence should be addressed. E-mail: [email protected]
Effect of Preparation Conditions on Visible PhotocatalyticActivity of Titania Synthesized by Solution CombustionMethod 179 powders. According to the stoichiometric ratio of titanyl nitrate and glycine of Eq.(l), it was defined that 4 =9n/10rn, where m and n are the molar mass of titanyl nitrate and glycine, respectively.
I
T
20
25
0
Y
2.2 Characterization The crystal structures of the prepared catalysts were observed with the help of an X-ray diffraction (XRD)instrument (Bruker D8, Germany) using a Cu target K, ray (2=0.15405nm). The accelerating voltage and the applied current were 40kV and 30mA, respectively. The diffraction patterns were collected at room temperature in the range of 20"-80". The average crystallite sizes were calculated with the Scherrer equation with the full width at half maxina (FWHM) data. The Brunauer-Emmett-Teller(BET) surface areas were obtained from the nitrogen adsorption apparatus (CHEMEET-300, America). UV-Vis reflectance spectra of samples were measured with the help of a UV-V is spectrophotometer (UV-2401, Shimadzu, Japan) using BaS04 as background. The carbon states in the combustion-synthesized Ti02 catalyst were measured by X-ray photoelectron spectroscopy (ESCALB MK-11, VG, Britain) using Mg K, radiation as X-ray source. FT-JX studies were carried out with the help of infrared spectroscopy (NEXUS 670, Nicolet, America). 2.3 Photocatalytic degradation experiments Photocatalytic activities of the as-synthesized Ti02 catalysts were evaluated by measuring the degradation efficiencies of 4-CP (O.l5mmol~L-') under both visible light and sunlight irradiation sources. The experiments under visible light irradiation were carried out with lOOmg of catalyst suspended in lOOml 4-CP solution in a 250ml reactor. A recycle water jacket was used to keep the reactor temperature constant at (30*1)"C. A 250-W Xe lamp (L25, Amax= 470nm) was used as visible light source. The light irradiated outside (at a distance of 25cm from the solution surface) through a 420nm cutoff filter (L40, Libang) upon the reactor. The light intensity near the solution surface was about 30mW.~m-~. The experiment for evaluation of sunlight photocatalytic activity of the prepared catalysts was as follows: a catalyst of 200mg was suspended in 200ml4-CP (0.15mmol.L-') solution in a beaker and exposed to sunlight after stirred for l0min. The absorbance spectrum of 4-CP was recorded on a UV-Vis spectrophotometer instrument (LAMBDA 35, Perkin Elmer, USA). The concentrations of 4-CP were determined by a HPLC (Agilent 1100, USA), equipped with a ZORBAX Eclipse XDB-C18 reversed phase column. The mobile phase was a mixture of water and methano! (50 : 50 by volume) with a flow rate of 1.0ml.min- . The total organic carbon (TOC) was measured using a TOC instrument (TOC-VCPH, Shimadzu, Japan), to evaluate the photomineralization degree of 4-CP. 3 RESULTS AND DISCUSSION 3.1 Effect of ignition temperature Figure 1 shows the effects of ignition tempera-
.0
-.-E .+
30
35
40 45 28, ("1
50
55
60
Figure 1 XRD patterns of catalysts prepared at different ignition temperatures T, "C : 1-300; 2-350; 3--400,4-500; 5-00; 6-700 ' Ianatase: rutile
tures on the phase structures of the synthesized Ti02 samples at 300, 350,400, 500 and 600"C, respectively. The temperature of 300°C was chosen as the lowest ignition temperature, because the decomposition temperature of glycine is 262°C. A trace anatase phase was observed for the sample ignited at 300°C. This was because the combustion of the fuel can reach a high temperature of 70Ck8OO"C for a short time. The high temperature induces the material to form the anatase phase. No significant phase changes of the catalysts were observed when the ignition temperature changed from 350°C to 500°C. But when the ignition temperature increased to 600"C, some of the anatase phase changed into the rutile phase. According to previous reports[13,141, the activity of the anatase phase of Ti02 for photodegradation of various pollutants is in general much higher than that of rutile. Therefore the temperature of 350°C is suitable for the ignition temperature. From the Scherrer equation, the average particle size of the catalyst was estimated to be 46nm on the anatase (101) diffraction peaks when combustion was carried out at 350°C.
3.2 Effect of calcination time Figure 2 illustrates the XRD patterns of catalysts prepared at various calcination times. Apparently, the samples have higher crystallinity with increasing the
.2.
3
* .0
.*
-2 20
30
40
50 60 28, ("1
70
80
Figure 2 XRD patterns of catalysts prepared with various calcination times t , min: 1-10; 2-30; 3-60; 4-90; 5-120; 6-180 Chin. J. Ch. E. 15(2) 178 (2007)
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calcination time. The color of the catalysts gradually changed from brown to light yellow with an increasing in the calcination time. The change of color is because of that the organic residual is removed from the catalysts by prolonging the calcinations time. But long calcination time also resulted in larger average particle size and smaller surface area. The results are shown in Table 1. The TiOz catalyst combusted at 350,"C f"'1h with @ of 1 shows a surface area of 215.0m .g- . The phase content, crystallite size and BET surface area of pure TiOz (Degussa P25) is also given in the table for comparison. Photocatalytic activities of the as-synthesized Ti02 catalysts prepared with different calcination times were evaluated by measuring the degradation efficiencies of 4-CP (0.1Smmol.L-') under visible light irradiation. No observable degradation of 4-CP was observed either without catalyst or without irradiation. The adsorption capacity of the as-synthesized catalysts was evaluated in aqueous solution. The result indicated that the adsorption was less than 2% when the synthesized Ti02 was used in the dark over a whole day. The adsorption was not appreciable within 3-4h. Hence, the initial concentration was taken to be Co in all cases. Figure 3 shows the photodegradation and mineralization rates of 4-CP under visible light ( A 3420nm) irradiation with various catalysts. It is obvious that the photocatlytic activity for degradation of 4-CP rapidly decreased with increase in calcination time. The decreased activity is not just because of surface area decrease[l5]. It is commonly thought that the photogenerated valence band hole is quickly converted to hydroxyl radical upon oxidation of surface water, and the hydroxyl radical is the major reactant responsible for oxidation of organic substrates[16,171. The amount of surface hydroxyl groups in catalysts, which play an important role in photocatalytic activity, may decrease with an increase in calcination time. Reasons for a decreased in photocatalytic activity with an increase in calcination time need further investigation. According to the results above, the crystallinity, surface area, and the color of the catalysts changed with varying calcination time. Prolonging the calcination time promoted the gGowth of crystalline and reduced the surface area. It could be suggested that some changes such as the amount of fuel, should be made to enable the photocatalytic activity and surface
0.8
.-
0.6 0.4 0.2
0
0.4 1
0
120 180 240 irradiation time, min (a) Remaining concentration rate 60
,
120 180 irradiation time, min (b) Remaining TOC rate
60
240
Figure 3 Photodegradation of 4-CP with various catalysts under visible light (A3420nm) irradiation Al;
A2; A A3
area to be compensated.
3.3 Effect of fuel ratio The effect of fuel ratio on the phase structures of the synthesized TiOz catalysts was also investigated. When other preparation conditions were kept identical, it was found that the peak intensities of anatase were nearly the same when the value varied from one to three. However, their surface areas become large with an increasing of @ value, as shown in Table 1. This phenomenon was on account of the generation of a significant amount of gas when more fuel was used. The color of the catalysts gradually changed from light yellow to brown with increasing the 4 value. Figure 4 shows the effect of #J on the photocatalytic activities of the catalysts in photodegradation of 4-CP
Table 1 Effect of calcination time and fuel ratio on the phase content, crystallite size and BET surface area Catalyst A1
Calcination time, h 1
4
Phase"
CrystaIIite size, nm'
1.o
A
Surface area, mz.g-* 215.0
A2
2
1.o
A
4.66 5.42
A3
3
A
5.58
119.0
B3
3 3
A A A A(75%), R(25%)
5.37 4.57 4.13 17.8
137.4
c3 D3
1.o 1.5 2.0 3.0
3
P25
0 A: anatase phase; R: m i l e phase. 0 Calculated by applying the Schemer equation. April, 2007
179.5
148.5
207.2 49.2
Effect of Preparation Conditions on Visible PhotocatalyticActivity of Titania Synthesized by Solution Combustion Method 181
0.2 I 0
lytic activity. Some reports[ 18,191 have already illustrated that superoxide radical anion .O, results in lowering the rates of recombination. According to Lettmann's research[l8], an electron can be transferred directly from the excited photosensitiser to triplet oxygen to generate the superoxide radical anion -O,, and the carbonaceous species can act as a photosensitiser. According to this, the higher photocatalytic activity for catalyst D3, prepared with higher fuel ratio, may be attributed to the formation of a more coke-like residua during the combustion process.
I
I
120 180 240 irradiation time, min (a) Remaining concentration rate
60
I
I
7
0.4
0
I
I
60
120 180 irradiation time, min (b) Remaining TOC rate
I
240
Figure 4 Photodegradation of 4-CP using various catalysts prepared with different 4 values under visible light (2 3420nm) irradiation A3; oB3; A c 3 ; r D 3
under visible-light (;1>42Onm) irradiation. As shown in Fig.4, both the photodegradation and mineralization rates increased gradually with the increasing of Q value. In the present of the catalyst D3 prepared with 9 of 3, it showed higher degradation and mineralization rates within 3h of irradiation. There were no significant change in the photodegradation and mineralization rates after that. For other catalysts synthesized at a lower Q value, the photodegradation rate of 4-CP changed almost linearly with irradiation time. The changes of surface areas alone are not sufficient to explain the phenomenon. It is commonly thought that the heterogeneous photocatalysis is a surface phenomenon. The photogenerated electrodhole pairs must be separated effectively to get high photocata-
3.4 Combined effect of calcination time and fuel ratio The effect of the calcination time and the fuel ratio on photocatalytic activity for degradation of 4-CP have been discussed separately above. Higher fuel ratio results in a larger amount of carbonaceous species, but requires longer calcination time to remove the residual soluble organic. However, prolonging calcination time results in the decrease of surface hydroxyl group and surface area. An optimal preparation condition may exist in a region. To find the optimal coordination between the two preparation factors, a series of catalysts were prepared with various calcination times and fuel ratios. Photodegradation of 4-CP under visible-light (A3420nm) and irradiation for 3h were carried out to investigate photocatalytic activities of the synthesized catalysts. The results are listed in Table 2. The highest photocatalytic activity for photodegradation of 4-CP was obtained for the catalyst prepared with 4 = 1 and calcination time of 1h.
3.5 Solar photocatalyticactivity Photodegradation of organic contaminates using sunlight irradiation can be very much more economical than using artificial light irradiation. Investigations have been made to study the photocatalytic activity of the synthesized catalysts under sunlight. Figure 5 shows the sunlight irradiation time-dependence of the absorption spectra of 0.15mmol-L-' 4-CP solution using catalyst Al. The monotonic decreases in the absorbance at 225nm, clearly revealing the decomposition of 4-CP. The increase in absorbance in the region of 240-270nm is attributed to the formation of reaction intermediates,
Table 2 Photodegradationand mineralizationrate of 4-CP under visible light irradiation with series catalysts synthesized with different preparation conditions'
4 = 1.5
$ = 1.0
Calcination time,
h
n
M
n
1
78.2 45.5
53.7
X
4 = 2.0 M
n
M
n
M
X
X
X
X
X
48.1
X
X
X
X
66.1
35.3
74.9
47.9
63.6 20.9 49.8 29.8 15.7 3 30.0 0 X :useless catalyst due to the residual soluble organics in the catalysts.
2
4 = 3.0
Chin. J. Ch. E. 15(2) 178 (2007)
Chin. J. Ch. E. (Vol. 15, No.2)
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1
1
wa\eleng\h. nm
Figure 5 Sunlight irradiation time-dependence of the absorption spectra of O.lSmmol-L-' 4-CP solution using catalyst A1 in suspension l-rnin; 2-30min; 3-60min; 4-120min; 5-180min: 6-300min
such as hydroquinone and hydroxyhydroquinone. With an increase in the irradiation time, the generated intermediates were degraded gradually. Fig.6 shows both the photodegradation and mineralization of 4-CP under sunlight irradiation. For the catalyst A l , the degradation and mineralization rates are 93% and 75% in 4h, respectively. 1 .0
absorption spectra of combustion-synthesized TiOz and pure Ti02 (Degussa P25). The pure Ti02 almost has no absorption above 400nm. However, the combustion-synthesized Ti02 results in obvious absorption up to 800nm. Noticeable shifts of the absorbance edge to the visible light region of the combustion-synthesized Ti02 may attribute to carbon modification. This absorption feature suggests that the combustion-synthesized Ti02 can be activated by visible light. To investigate the carbon states in the combustion-synthesized Ti02 catalyst, C 1s core levels were measured by X-ray photoelectron spectroscopy (XPS), as shown in Fig.8. There are two peaks at 285.5 and 289.5eV. The peak at 285.5eV occurs as a result of elemental carbon[6], the latter peak at 289.5eV can be assigned to the formation of carbonate species[20,21]. Khan er a1.[22] prepared carbon-modified rutile titania by controlled flame pyrolysis of Ti metal, and found that the carbon substituted for some of the lattice oxygen atoms. However, Sakthivel et a1.[6] synthesized carbon-modified titanium dioxide by hydrolysis of titanium tetrachloride with tetrabutylammonium hydroxide followed by calcinations at 400°C, and observed new peaks at 287.5eV and 288.5eV. However, the carbon-modified Ti02 prepared by SCM has only one peak, indicating the presence of only one lund of carbonate species. These reports indicated that the carbon oxidation state in carbon-modified titania generally depend on preparation method.
5 1400 I
0
60
120 180 irradiation time. min
240
Figure 6 Remaining concentration and TOC rate of photocatalyticdegradation of 4-CP under sunlight irradiation using catalyst A1 C/C"; A TOCROCo
3.6 Physical properties of combustion-synthesized TiOz Figure 7 gives the UV-Vis diffusive reflectance ~.
Degussa P25
2 si 0 "
1
300
I
400
500 600 wavelength, nin
700
800
Figure 7 UV-Vis diffusive reflectance absorption spectra of the combustion-synthesizedTi02( A l ) and pure TiOz (Degussa P25) April, 2007
$
1200
800 600 400
I
292
I
290
1
I
,
288 286 284 binding energy, eV
2 12
Figure 8 C 1s core level spectra of catalyst A1
Figure 9 shows the FT-IR spectra of combustion synthesized TiO2. The IR spectrum of pure Ti02 (Degussa P25) is also given for a comparison. A broad band at 3389cm-: is attributed to bound water. The peak at 1624cm- is because of vibration frequency corresponding to the bending of the water molecule. The low-intensity peak at1403cm-' which does not exists in pure TiOzcan be assigned to carbonate ion. This result agrees well with the XPS. Therefore, our results indicated that the formation of carbonate species lead to the narrowing of the band gap in the combustion-synthesized Ti02. Further investigation is need to determine if the carbon substitute of the lattice oxygen in the titania or only occupy the surface of the titania.
Effect of Preparation Conditionson visible PhotocatalyticActivity of Titania Synthesized by Solution Combustion Method 183 Asahi, R., Morikawa, T., Ohwaki, T., Aoki, K., Taga, Y . , “Visible-light photocatalysis in nitrogen-doped titanium oxides”, Science, 293,269-271(2001). 6 Sakthivel, S., Kisch, H., “Daylight photocatalysis by carbon-modified titanium dioxide”, Angew. Chem. Int. Ed., 42,4908-491 l(2003). 7 Sivalingam, G., Nagaveni, K., Hegde, M.S., Madras, G., “Photocatalytic degradation of various dyes by combustion synthesized nano anatase TiOi’, Appl. Catal. B: Environ., 4523-38(2003). 8 Liu, H., Yang, W., Ma, Y., Cao, Y . , Yao, J., Zhang, J., Hu, T., “Synthesis and characterization of titania prepared by using a photoassisted sol-gel method”, Langmuir, 19, 3001-3005(2003). 9 Liu, H., Gao, L., “(Sulfur, Nitrogen)-codoped rutile-titanium dioxide as a visible-light-activated photocatalyst”, J. Am. Ceram. SOC.,87, 1582-1584(2004). 10 Li, D., Haneda, H., Hishita, S., Ohashi, N., “Visible-light-driven N-F-codoped TiOz photocatalysts ( I ) Synthesis by spray pyrolysis and surface characterization”, Chem. Muter., 17,2588-2595(2005). 11 Priya, M.H., Madras, G., “Photocatalytic degradation of nitrobenzenes with combustion synthesized nano-TiOi’, J. Photochem. Photobiol. A: Chem., 178, 1-7(2006). 12 Zhao, D., Xu, X., Lei, L., Wang, D., “Degradation of 4-chlorophenol solution by synergetic effect of dual-frequency ultrasound with fenton reagent”, Chin. J . Chem. Eng., 13,204-210(2005). 13 Brown, J.D., Williamson, L.D., Nozik, A.J., “Moessbauer study of the kinetics of iron(3+) photoreduction on titanium dioxide semiconductor powders”, J . Phys. Chem., 89,3076-3080(1985). 14 Fox, M.A., Dulay, M.T., “Heterogeneous photocatalysis”, Chem. Rev., 93,341-357(1993). 15 Rachel, A., Subrahmanyam, M., Boule, P., “Comparison of several titanium dioxides for the photocatalytic degradation of benzenesulfonic acids”, Appl. Catal. B: Environ., 37,293-300(2002). 16 Du, Y.K., Rabani, J., “The measure of Ti02 photocatalytic efficiency and the comparison of different photocatalytic titania”, J . Phys. Chem. B , 107, 1197011978(2003). 17 Jung, K.Y., Park, S.B., Anpo, M., “Photoluminescence and photoactivity of titania particles prepared by the sol-gel technique: Effect of calcination temperature”, J. Photochem. Photobiol. A: Chem., 170,247-252(2005). 18 Lemnann, C., Hildenbrand, K., Kisch, H., Macyk, W., Maier, W.F., “Visible light photodegradation of 4-chlorophenol with a coke-containing titanium dioxide photocatalyst”, Appl. Catal. B: Environ., 32,215-227(2001). 19 Moonsiri, M., Rangsunvigit, P., Chavadej, S., Gula$ E., “Effects of Pt and Ag on the photocatalytic degradation of 4-chlorophenol and its by-products”, Chem. Eng. J., 97,241-248(2004). 20 Papirer, E., Lacroix, R., Donnet, J.B., Nanst, G., Fioux, P., “XPS study of the halogenation of carbon black (11) Chlorination”, Carbon, 33, 63-72( 1995). 21 Gopinath, C.S., Hegde, S.G., Ramaswamv, A.V., Mahapatra, S., “Photoemission studies of polymorphic CaC03 materials”,Muter. Res. Bull., 37, 1323-1332(2002). 22 Khan, S.A.M, Ingler, W.B., “Efficient photochemical water splitting by a chemically modified n-TiOy’, Science, 297,2243-2245(2002). 5
I
I
I
I
I
I
4000 3500 3000 2500 2000 1500 1000 500 wavenumber, cm-’
Figure 9 FT-IR spectrum of the combustion-synthesized Ti02 (Al) and pure TiOz (Degussa P25) combustion synthesized TiO,; - - - - Degussa P25 ~
4 CONCLUSIONS The titanium dioxide catalyst synthesized by SCM showed absorption to the visible region for the modification of the carbonate species. Photocatalytic activity of Ti02 was found to be dependent on the preparation conditions. Calcination time greatly affected the photocatalytic activity of the catalysts for degradation of 4-CP. It was also found that the fuel ratio affects the BET surface area and the amount of carbonaceous species. The ignition temperature did not show significant effect on the crystallinity of catalysts. An optimal preparation condition [ignition temperature of 350°C,fuel ratio ( 4 ) of 1 and calcination time of lh] was obtained. NOMENCLATURE M mineralization rate [M=(1-TOCi/rOCo) q
X 100%J
degradation rate [q=(l- Ci/Co)X loo%]
REFERENCES Zhao, W., Ma, W., Chen, C., Zhao, J., Shuai, Z., “Efficient degradation of toxic organic pollutants with Ni,O2/TiO7.,B, under visible irradiation”, J. Am. Chem. S&.,d126,;?8&4783(2004). Shi, Z . , Fan, Y., Xu, N., Jun, S., “Kinetics of photocatalytic degradation of methylene blue over Ti02 particles in aqueous suspensions”, Chin. J. Chem. Eng., 8, 1519(2000). Tryk, D.A., Fujishima, A., Honda, K., “Recent topics in photoelectrochemist: achievements and future prospects”, Electrochim. Acta, 45,2363-2376(2000). Di Paola, A., Marci, G., Palmisano, L., Schiavello, M., Uosaki, K., Ikeda, S., Ohtani, B., “Preparation of polycrystalline Ti02 photocatalysts impregnated with various transition metal ions: characterization and photocatalytic activity for the degradation of 4-nitrophenol”, J. Phys. Chem. B, 106,637-645(2002).
Chin. J. Ch. E. 15(2) 178 (2007)