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TiO2 photocatalysts promoted by alkali metals
Applied Catalysis B: Environmental 55 (2005) 221–226 www.elsevier.com/locate/apcatb
TiO2 photocatalysts promoted by alkali metals Beata Zielin´skaa, Antoni Waldemar Morawskib,* b
a Natural Science Faculty, University of Szczecin, Felczaka 3A, 71-412 Szczecin, Poland Institute of Chemical and Environment Engineering, Technical University of Szczecin, Pułaskiego 10, 70-322 Szczecin, Poland
Received 24 May 2004; received in revised form 24 August 2004; accepted 31 August 2004 Available online 12 October 2004
Abstract The new photocatalysts have been obtained by calcination titanium dioxide of anatase type (Tytanpol A11, ‘‘Police’’ Chemical Factory, Poland) with hydroxides (Li, Na, K, Ba) and carbonate (Sr). The obtained materials have been characterised by several analytical methods, like XRD, FT-IR/DRS, DR–UV–vis. The photocatalytic oxidation of phenol as model contamination has been investigated over obtained alkali metals–TiO2. From all investigated photocatalysts only materials based on titanates in perovskite forms (BaTiO3, SrTiO3) have higher activity than pure titanium dioxide A11. # 2004 Elsevier B.V. All rights reserved. Keywords: Photocatalytic oxidation; Modified titanium dioxide; Phenol
1. Introduction Photocatalytic materials use the photon energy of the light to catalyze a chemical reaction. Applications of photocatalysts include the decomposition of water into hydrogen and oxygen and the complete oxidation of organic contaminants in aqueous environments. In the past few years, many catalysts like TiO2, ZnO, WO3, SnO2, ZrO2, CeO2, CdS and ZnS have been tried for photocatalytic oxidation of water environmental contaminants [1]. Among these catalysts TiO2 has been proved to be an excellent catalyst in the photodegradation of organic pollutants, because is an effective, photostable, reusable, inexpensive, non-toxic and easily available. Also many metals modification of TiO2 were described in [2]. The aim of activation was concerned on the preparation of stable and active photocatalysts without corrosion and on the reduction of recombination between electron/hole pairs. Above modification conducts to shifting of absorption edge to the higher wavelength were visible light could be utilized.
* Corresponding author. Tel.: +48 91 449 44 74; fax: +48 91 449 46 86. E-mail address: [email protected] (A.W. Morawski). 0926-3373/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2004.08.015
However, the number of publication on modification of anatase by alkali metals is rather limited [2]. The aim of the present study is to investigate the photocatalytic decomposition of phenol in water over modified titanium dioxide of anatase type (Tytanpol A11) produced on Polish market. We reported that titanates at perovskite forms are more active than pure titanium dioxide. The perovskite structure is named after the mineral CaTiO3. This structure is composed of organic corner-sharing TiO6 octahedra, with Ca ions in the large cavities at the corner of the unit cell.
2. Experimental 2.1. Materials The precursor used for the preparation of modified photocatalysts was titanium dioxide supplied by ‘‘Police’’ Chemical Factory (TiO2 – Tytanpol A11) and contain 99.5% of anatase. Detailed characteristics of this material were given elsewhere [3]. TiO2 – Tytanpol A11 showed good activity for photocatalytic decomposition of dyes, phenols
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and oil [3–7]. Through text TiO2 – Tytanpol A11 is shortly marked as A11. Alkali metals in hydroxide forms (Li, Na, K, Ba) and carbonate form (Sr) were used for TiO2 modification.
appropriate time intervals, in order to determine the phenol concentration in the solution. The change of concentration of phenol was measured using UV–vis spectrophotometer (Jasco, Japan) with calibrated curve at wavelength, at 270 nm.
2.2. Preparation of modified photocatalyst samples The first step of A11 photocatalyst modification was the impregnation with aqueous solution of alkali metals. The weight ratio of A11/Me (Me – metals) was 4:1. After impregnation catalysts were dried at temperature of 110 8C and calcinated at temperature of 550 8C (Li, Na, K, Ba hydroxides) and 1200 8C (strontium carbonate). The calcination temperature was chosen on the basis of previous investigation [5]. Calcination proceeded for 6 h, including heating to the demanded temperature of calcination. After calcination the samples were powdered and washed with distilled water until the neutral pH of filtrate was obtained. Such prepared photocatalysts were marked as Me/A11 for all used metals, where Me (Li, Na, K, Ba, Sr). 2.3. Experimental procedures and techniques Prepared photocatalysts were characterized by: (1) XRD technique using HZG-diffractometer, (2) DR–UV–vis spectroscopy using SPECORD M40 spectrometer (Carl– Zeiss, Germany) equipped with an integrating sphere accessory for diffuse reflectance (BaSO4 was used as a reference), (3) IR method using Jasco FT-IR spectrometer with Praying Mantis attachment for diffuse reflection measurements FT-IR/DRS and (4) nitrogen gas adsorption method using Micrometrics ASAP 2010 apparatus. The band gap energies of A11 and modified photocatalysts were determined using DR–UV–vis method and calculated according to EG ¼
hc l
(1)
where EG is the band gap energies (eV), h the Planck’s constant, c the light velocity (m/s) and l is the wavelength (nm). The photocatalytic activities of the photocatalysts for phenol degradation were carried out in a batch reactor. The scheme of apparatus is presented elsewhere [5]. All the experiments of photocatalytic decomposition of phenol were conducted with the initial phenol concentration of 50 mg/ dm3 and 0.2 g of catalyst. Photoreactions were carried out at atmospheric pressure using air as an oxidant in bath photoreactor. Reaction was performed with using 500 cm3 of phenol solution and the proper amount of powdered photocatalyst. Reaction mixture was stirred with the magnetic stirrer before and during UV–vis illumination. The suspension was mixed for 15 min in the dark (for adsorption of phenol onto photocatalyst surface) and then the reaction mixture was exposed to the UV–vis light. The samples of reaction mixture were taken from reactor, at
3. Results and discussion 3.1. Characteristics of photocatalysts The commercial titanium dioxide (TiO2 – Tytanpol A11) contains only one, clear anatase phase (JPCDS card no. 211272) and it is being very well crystallised with crystallite size of 40 nm. The XRD pattern of this material is presented elsewhere [3]. Materials obtained after reaction of anatase with hydroxides and carbonate contained a mixture of titanates and titanium dioxide. Photocatalysts modified by Li, Na, K, Ba showed anatase phase of titanium dioxide with different contamination and following titanates phases: Li2TiO3, Na8Ti5O14, K2Ti6O13 and BaTiO3, respectively. Because of the anatase phase domination, the XRD patterns of Li/A11, Na/A11 K/A11 are not presented here. Photocatalyst prepared by calcination of A11 impregnated by strontium carbonate showed a mixture of rutile phase of TiO2 and titanate SrTiO3 (Fig. 1). Transformation of anatase phase to rutile was caused by high temperature of calcination (1200 8C). The samples of Ba/A11 and Sr/A11 have titanates of perovskite form (JPDS card no. 5-626 and JPCDS card no. 35-734). Fig. 2 shows the UV–vis absorption spectra of A11 original and modified by alkali metals. The absorption edges of Li/A11 and Na/A11 seem to shift toward shorter wavelength. The absorption edge of K/A11 and Sr/A11 are shifted toward longer wavelength. In case of Sr/A11 photocatalyst, the shifting of the absorption edge mainly indicated from the presence of the rutile phase in the sample. It is indicated from the fact, that the rutile has smaller band gap energy than anatase phase. For photocatalysts Ba/A11, the absorption spectra overlapped with spectra of pure titanium dioxide and the absorption edge of Ba/A11 has different intensity than the spectrum of A11. The first derivatives of UV–vis absorption spectra are shown in Fig. 2b. The absorption edges of these samples were determined from the apparent peak positions. Li/A11 and Sr/A11 photocatalysts showed two peaks on the plot of the first derivative of DR–UV–vis spectrum. It means that they consist of two clear phases of TiO2. The band gap energies (EG) of photocatalysts calculated from Eq. (1) are collected in Table 1 in relation to other properties, including phase composition. In Fig. 3, FT-IR/DRS spectra of A11 original and modified by alkali metals are presented. The spectrum of A11 show absorption bands at 3695 cm1, a broad band in the region of 3600–2600 cm1, band at 1637 cm1 and a broad band in the range of 1500–500 cm1. It was reported
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Fig. 1. Diffractograms of photocatalysts: (a) Ba/A11, (b) Sr/A11 (* – anatase, * – rutile, & – BaTiO3, ^ – SrTiO3).
Table 1 Properties of the original titanium dioxide Tytanpol A11 and modified by alkali metals Photocatalyst
Phase composition
BET surface area (m2/g)
Mean pore diameter (nm)
Band gap energy (eV)
A11 Li/A11 Na/A11 K/A11 Sr/A11 Ba/A11
Anatase Li2TiO3, anatase Na8Ti5O14, anatase K2Ti6O13, anatase SrTiO3, rutile BaTiO3, anatase
11.4 12.58 16.57 8.19 2.04 10.07
7.7 10.48 8.12 8.92 7.33 7.87
3.312 4.136, 3.402 3.343 3.244 3.333, 3.035 3.331
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Fig. 2. The DR–UV–vis absorption spectra (a) and the first derivatives of UV–vis absorption (b) of photocatalysts: (1) A11, (2) Li/A11, (3) Na/A11, (4) K/A11, (5) Ba/A11, and (6) Sr/A11. Fig. 3. FT-IR/DRS spectra of A11 original and modified by alkali metals: (1) A11, (2) Li/A11, (3) Na/A11, (4) K/A11, (5) Ba/A11, and (6) Sr/A11.
that adsorbed water shows bands at around 3400 and 1630 cm1, while Ti–OH band is around 3563, 3172 and 1600 cm1 [8]. Therefore, it is maintained that observed peaks on FT-IR spectra correspond to the hydroxyl groups and water adsorbed on the catalyst surface. All modified photocatalysts show also characteristic vibrations at about 3600–2600, 1500–500 and 1630 cm1. The same bands were found for original A11. In case of Li/ A11, Ba/A11, Sr/A11 photocatalysts the band at 1630 cm1 disappeared. Additionally, for all modified photocatalysts, a new absorption bands arisen. From Fig. 3, it is seen that in case of photocatalysts Ba/A11 and Sr/A11 intensity of a broad band in the region of 3600–2600 cm1 decrease. It is assumed that those bands form new sites on the photocatalyst surface, caused by Li, Ba and Sr presence. Some properties of A11 original and modified by alkali metals are presented in Table 1. According to the manufactures data, A11 has a specific surface area of 11.4 m2/g. BET surface areas of A11 modified by Li and Na were higher than original A11 but photocatalysts modified by K, Sr, Ba had BET surface area lower than precursor. Sr/ A11 photocatalyst had the lowest surface area from all investigated materials. It was caused by the high temperature of calcination [8]. The photocatalysts of lower surface area were of better ability to decantation. The resistance to water of Li, Na and K modified were of lower stability in water.
3.2. Photocatalytic reactions Firstly, the blank test (without photocatalyst) was carried out under UV–vis irradiation. A small amount of phenol decomposition was observed due to photolysis. This value was taken to the account during activity calculation.
Fig. 4. Dependence of ln(C0/C) = kt vs. time for phenol photodecomposition on TiO2 – Tytanpol A11 original and modified by alkali metals.
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Table 2 Rate constants of phenol photodecomposition and linear regression coefficients from a plot of ln(C0/C) = kt Photocatalyst
R2
k (min1)
A11 Li/A11 Na/A11 K/A11 Sr/A11 Ba/A11
0.9884 0.9859 0.9841 0.9813 0.9926 0.9901
0.0119 0.0085 0.0102 0.0095 0.0128 0.0147
Next experiments were conducted with using A11 original and modified by alkali metals photocatalysts. The kinetics of phenol photodecomposition on the catalysts surface can be described by the first order reaction (Eq. (2)) [9–11]: C0 ln ¼ kt C
(2)
where k is the rate constant (min1), C0 and C are the initial and relative phenol concentration at time t, respectively. In Fig. 4, the linear relation of ln(C0/C) versus irradiation time for phenol is presented. Rate constants (k) were determined for all tested photocatalysts from the slope of the line, as presented in Fig. 4. In Table 2, rate constants for phenol decomposition on all investigated photocatalysts are listed together with the coefficient of the linear regression. The rate constants for all studied samples are presented in Fig. 5. From these figures, it is seen that from all investigated photocatalysts only materials based on titanium dioxide and barium hydroxide and strontium carbonate was of higher activity than pure precursor. It appeared that the most activity at photocatalytic decomposition of phenol has titanium dioxide modified with barium hydroxide (Ba/A11). Ba/A11 photocatalysts has similar properties as a pure precursor. BET surface area of Ba/A11 was about 0.6 m2/g lower and band gap energy was about 0.019 eV higher than A11. Sr/A11 photocatalyst characterised higher EG in comparison to A11 of about 0.019 eVand lower BET surface area of about 9 m2/g. Also it is worth to point out that
Fig. 6. The relationship constant rate of phenol decomposition on Me/TiO2 vs. density of used alkali metals.
materials with Ba and Sr were of higher stability and better decantability. The higher activities of Ba/A11 and Sr/A11 photocatalysts in comparison to the original A11 are probably caused by the presence of titanates at perovskite forms as it was show the diffractograms of Ba/A11 and Sr/A11 photocatalysts (Fig. 1). For samples Ba/A11 and Sr/A11, a new absorption bands appeared on FT-IR/DRS spectra (Fig. 3, spectra 5, 6). In case of sample Ba/A11, we obtained new sites at 2451, 1749 and 1469 cm1, and for Sr/A11 at 2341 and 1769 cm1. These new bands and smaller intensity of broad bands in the region of 3600–2600 cm1 and bands at 1630 cm1 can be explained by creation of new active sites in the photocatalysts, which results in higher activity of Ba and Sr modified TiO2. In the case of material based on A11 and strontium carbonate (Sr/A11), its activity results from SrTiO3 phase only, because rutile form of titanium dioxide has no activity in the photocatalytic process. Rutile obtained by calcination of anatase at temperature of 1200 8C showed insignificant activity for phenol decomposition (below 0.5%). Therefore, this activity is negligible. The constant rate recalculated on mass of pure SrTiO3 phase would be of much higher than for both anatase and SrTiO3 phases. In Fig. 6 presented is relation of constant ratio with density of doped metals. We can see from this figure that activities of modified photocatalysts grow with increase of density alkali metals. The metals with higher density (or atomic mass) seem to be better modificator of TiO2. Finally, the anatase modified by divalent alkali metals forms photocatalysts of higher activity than the bare anatase.
4. Conclusion
Fig. 5. Rate constants of phenol decomposition on TiO2 – Tytanpol A11 original and modified by alkali metals.
Several materials based on the commercial titanium dioxide of anatase type (Tytanpol A11) have been tested for the photocatalytic oxidation of phenol. Higher activity of A11 modified by barium hydroxide and strontium carbonate in comparison to A11 original was found. This higher
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activity resulted from the presence of the titanates at perovskite forms (BaTiO3 and SrTiO3). The relationship between constant rate on modified photocatalysts and alkali metals density was found.
References [1] A.I. Kokorin, D.W. Bahnemann, Chemical Physics of Nanostructured Semiconductors, Brill Academic Publishers, Leden, The Netherlands, 2003. [2] M. Koneko, I. Okura (Eds.), Photocatalysis, Science and Technology, Kodansha–Springer, 2002. [3] B. Zielinska, J. Grzechulska, B. Grzmil, A.W. Morawski, Photocatalytic degradation of Reactive Black 5. A comparison between TiO2 – Tytanpol A11 and TiO2 – Degussa P25 photocatalysts, Appl. Catal. B: Environ. 35 (2001) L1–L7. [4] M. Hamerski, J. Grzechulska, A.W. Morawski, Photocatalytic purification of soil contaminated with oil using modified TiO2 powders, Solar Energy 66 (6) (1999) 395–399.
[5] J. Grzechulska, M. Hamerski, A.W. Morawski, Photocatalytic decomposition of oil in water, Water Res. 34 (5) (2000) 1644–1683. [6] B. Zielinska, J. Grzechulska, R.J. Kalen´ czuk, A.W. Morawski, The pH influence on photocatalytic decomposition of organic dyes over A11 and P25 titanium dioxide, Appl. Catal. B: Environ. 45 (2003) 293– 300. [7] B. Zielin´ ska, J. Grzechulska, A.W. Morawski, Photocatalytic decomposition of textile dyes on TiO2 – Tytanpol A11 and TiO2 – Degussa P25, J. Photochem. Photobiol. A: Chem. 157 (2003) 65–70. [8] Z. Ding, G.Q. Lu, P.H. Greenfield, Role of the crystallite phase of TiO2 in heterogeneous for phenol oxidation in water, J. Phys. Chem. B 104 (2000) 4815–4820. [9] J.M. Herrmann, H. Tahiri, Y. Ait-Ichou, G. Lassaletta, et al. Characterization and photocatalytic activity in aqueous medium of TiO2 and Ag–TiO2 coatings on quartz, Appl. Catal. B: Environ. 13 (1997) 219–228. [10] K.-H. Wang, Y.-H. Hsieh, M.-Y. Chou, C.-Y. Chang, Photocatalytic degradation of 2-chloro and 2-nitrophenol by titanium dioxide suspension in aqueous solution, Appl. Catal. B: Environ. 21 (1999) 1–8. [11] A. Sobczyn´ ski, J. Gimenez, S. Cervere-March, A fluidized photoreactor design for water detoxification by photocatalysis, Polish J. Appl. Chem. XL (z.1–2) (1996) 103–114.
TiO2 photocatalysts promoted by alkali metals Beata Zielin´skaa, Antoni Waldemar Morawskib,* b
a Natural Science Faculty, University of Szczecin, Felczaka 3A, 71-412 Szczecin, Poland Institute of Chemical and Environment Engineering, Technical University of Szczecin, Pułaskiego 10, 70-322 Szczecin, Poland
Received 24 May 2004; received in revised form 24 August 2004; accepted 31 August 2004 Available online 12 October 2004
Abstract The new photocatalysts have been obtained by calcination titanium dioxide of anatase type (Tytanpol A11, ‘‘Police’’ Chemical Factory, Poland) with hydroxides (Li, Na, K, Ba) and carbonate (Sr). The obtained materials have been characterised by several analytical methods, like XRD, FT-IR/DRS, DR–UV–vis. The photocatalytic oxidation of phenol as model contamination has been investigated over obtained alkali metals–TiO2. From all investigated photocatalysts only materials based on titanates in perovskite forms (BaTiO3, SrTiO3) have higher activity than pure titanium dioxide A11. # 2004 Elsevier B.V. All rights reserved. Keywords: Photocatalytic oxidation; Modified titanium dioxide; Phenol
1. Introduction Photocatalytic materials use the photon energy of the light to catalyze a chemical reaction. Applications of photocatalysts include the decomposition of water into hydrogen and oxygen and the complete oxidation of organic contaminants in aqueous environments. In the past few years, many catalysts like TiO2, ZnO, WO3, SnO2, ZrO2, CeO2, CdS and ZnS have been tried for photocatalytic oxidation of water environmental contaminants [1]. Among these catalysts TiO2 has been proved to be an excellent catalyst in the photodegradation of organic pollutants, because is an effective, photostable, reusable, inexpensive, non-toxic and easily available. Also many metals modification of TiO2 were described in [2]. The aim of activation was concerned on the preparation of stable and active photocatalysts without corrosion and on the reduction of recombination between electron/hole pairs. Above modification conducts to shifting of absorption edge to the higher wavelength were visible light could be utilized.
* Corresponding author. Tel.: +48 91 449 44 74; fax: +48 91 449 46 86. E-mail address: [email protected] (A.W. Morawski). 0926-3373/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2004.08.015
However, the number of publication on modification of anatase by alkali metals is rather limited [2]. The aim of the present study is to investigate the photocatalytic decomposition of phenol in water over modified titanium dioxide of anatase type (Tytanpol A11) produced on Polish market. We reported that titanates at perovskite forms are more active than pure titanium dioxide. The perovskite structure is named after the mineral CaTiO3. This structure is composed of organic corner-sharing TiO6 octahedra, with Ca ions in the large cavities at the corner of the unit cell.
2. Experimental 2.1. Materials The precursor used for the preparation of modified photocatalysts was titanium dioxide supplied by ‘‘Police’’ Chemical Factory (TiO2 – Tytanpol A11) and contain 99.5% of anatase. Detailed characteristics of this material were given elsewhere [3]. TiO2 – Tytanpol A11 showed good activity for photocatalytic decomposition of dyes, phenols
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and oil [3–7]. Through text TiO2 – Tytanpol A11 is shortly marked as A11. Alkali metals in hydroxide forms (Li, Na, K, Ba) and carbonate form (Sr) were used for TiO2 modification.
appropriate time intervals, in order to determine the phenol concentration in the solution. The change of concentration of phenol was measured using UV–vis spectrophotometer (Jasco, Japan) with calibrated curve at wavelength, at 270 nm.
2.2. Preparation of modified photocatalyst samples The first step of A11 photocatalyst modification was the impregnation with aqueous solution of alkali metals. The weight ratio of A11/Me (Me – metals) was 4:1. After impregnation catalysts were dried at temperature of 110 8C and calcinated at temperature of 550 8C (Li, Na, K, Ba hydroxides) and 1200 8C (strontium carbonate). The calcination temperature was chosen on the basis of previous investigation [5]. Calcination proceeded for 6 h, including heating to the demanded temperature of calcination. After calcination the samples were powdered and washed with distilled water until the neutral pH of filtrate was obtained. Such prepared photocatalysts were marked as Me/A11 for all used metals, where Me (Li, Na, K, Ba, Sr). 2.3. Experimental procedures and techniques Prepared photocatalysts were characterized by: (1) XRD technique using HZG-diffractometer, (2) DR–UV–vis spectroscopy using SPECORD M40 spectrometer (Carl– Zeiss, Germany) equipped with an integrating sphere accessory for diffuse reflectance (BaSO4 was used as a reference), (3) IR method using Jasco FT-IR spectrometer with Praying Mantis attachment for diffuse reflection measurements FT-IR/DRS and (4) nitrogen gas adsorption method using Micrometrics ASAP 2010 apparatus. The band gap energies of A11 and modified photocatalysts were determined using DR–UV–vis method and calculated according to EG ¼
hc l
(1)
where EG is the band gap energies (eV), h the Planck’s constant, c the light velocity (m/s) and l is the wavelength (nm). The photocatalytic activities of the photocatalysts for phenol degradation were carried out in a batch reactor. The scheme of apparatus is presented elsewhere [5]. All the experiments of photocatalytic decomposition of phenol were conducted with the initial phenol concentration of 50 mg/ dm3 and 0.2 g of catalyst. Photoreactions were carried out at atmospheric pressure using air as an oxidant in bath photoreactor. Reaction was performed with using 500 cm3 of phenol solution and the proper amount of powdered photocatalyst. Reaction mixture was stirred with the magnetic stirrer before and during UV–vis illumination. The suspension was mixed for 15 min in the dark (for adsorption of phenol onto photocatalyst surface) and then the reaction mixture was exposed to the UV–vis light. The samples of reaction mixture were taken from reactor, at
3. Results and discussion 3.1. Characteristics of photocatalysts The commercial titanium dioxide (TiO2 – Tytanpol A11) contains only one, clear anatase phase (JPCDS card no. 211272) and it is being very well crystallised with crystallite size of 40 nm. The XRD pattern of this material is presented elsewhere [3]. Materials obtained after reaction of anatase with hydroxides and carbonate contained a mixture of titanates and titanium dioxide. Photocatalysts modified by Li, Na, K, Ba showed anatase phase of titanium dioxide with different contamination and following titanates phases: Li2TiO3, Na8Ti5O14, K2Ti6O13 and BaTiO3, respectively. Because of the anatase phase domination, the XRD patterns of Li/A11, Na/A11 K/A11 are not presented here. Photocatalyst prepared by calcination of A11 impregnated by strontium carbonate showed a mixture of rutile phase of TiO2 and titanate SrTiO3 (Fig. 1). Transformation of anatase phase to rutile was caused by high temperature of calcination (1200 8C). The samples of Ba/A11 and Sr/A11 have titanates of perovskite form (JPDS card no. 5-626 and JPCDS card no. 35-734). Fig. 2 shows the UV–vis absorption spectra of A11 original and modified by alkali metals. The absorption edges of Li/A11 and Na/A11 seem to shift toward shorter wavelength. The absorption edge of K/A11 and Sr/A11 are shifted toward longer wavelength. In case of Sr/A11 photocatalyst, the shifting of the absorption edge mainly indicated from the presence of the rutile phase in the sample. It is indicated from the fact, that the rutile has smaller band gap energy than anatase phase. For photocatalysts Ba/A11, the absorption spectra overlapped with spectra of pure titanium dioxide and the absorption edge of Ba/A11 has different intensity than the spectrum of A11. The first derivatives of UV–vis absorption spectra are shown in Fig. 2b. The absorption edges of these samples were determined from the apparent peak positions. Li/A11 and Sr/A11 photocatalysts showed two peaks on the plot of the first derivative of DR–UV–vis spectrum. It means that they consist of two clear phases of TiO2. The band gap energies (EG) of photocatalysts calculated from Eq. (1) are collected in Table 1 in relation to other properties, including phase composition. In Fig. 3, FT-IR/DRS spectra of A11 original and modified by alkali metals are presented. The spectrum of A11 show absorption bands at 3695 cm1, a broad band in the region of 3600–2600 cm1, band at 1637 cm1 and a broad band in the range of 1500–500 cm1. It was reported
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Fig. 1. Diffractograms of photocatalysts: (a) Ba/A11, (b) Sr/A11 (* – anatase, * – rutile, & – BaTiO3, ^ – SrTiO3).
Table 1 Properties of the original titanium dioxide Tytanpol A11 and modified by alkali metals Photocatalyst
Phase composition
BET surface area (m2/g)
Mean pore diameter (nm)
Band gap energy (eV)
A11 Li/A11 Na/A11 K/A11 Sr/A11 Ba/A11
Anatase Li2TiO3, anatase Na8Ti5O14, anatase K2Ti6O13, anatase SrTiO3, rutile BaTiO3, anatase
11.4 12.58 16.57 8.19 2.04 10.07
7.7 10.48 8.12 8.92 7.33 7.87
3.312 4.136, 3.402 3.343 3.244 3.333, 3.035 3.331
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Fig. 2. The DR–UV–vis absorption spectra (a) and the first derivatives of UV–vis absorption (b) of photocatalysts: (1) A11, (2) Li/A11, (3) Na/A11, (4) K/A11, (5) Ba/A11, and (6) Sr/A11. Fig. 3. FT-IR/DRS spectra of A11 original and modified by alkali metals: (1) A11, (2) Li/A11, (3) Na/A11, (4) K/A11, (5) Ba/A11, and (6) Sr/A11.
that adsorbed water shows bands at around 3400 and 1630 cm1, while Ti–OH band is around 3563, 3172 and 1600 cm1 [8]. Therefore, it is maintained that observed peaks on FT-IR spectra correspond to the hydroxyl groups and water adsorbed on the catalyst surface. All modified photocatalysts show also characteristic vibrations at about 3600–2600, 1500–500 and 1630 cm1. The same bands were found for original A11. In case of Li/ A11, Ba/A11, Sr/A11 photocatalysts the band at 1630 cm1 disappeared. Additionally, for all modified photocatalysts, a new absorption bands arisen. From Fig. 3, it is seen that in case of photocatalysts Ba/A11 and Sr/A11 intensity of a broad band in the region of 3600–2600 cm1 decrease. It is assumed that those bands form new sites on the photocatalyst surface, caused by Li, Ba and Sr presence. Some properties of A11 original and modified by alkali metals are presented in Table 1. According to the manufactures data, A11 has a specific surface area of 11.4 m2/g. BET surface areas of A11 modified by Li and Na were higher than original A11 but photocatalysts modified by K, Sr, Ba had BET surface area lower than precursor. Sr/ A11 photocatalyst had the lowest surface area from all investigated materials. It was caused by the high temperature of calcination [8]. The photocatalysts of lower surface area were of better ability to decantation. The resistance to water of Li, Na and K modified were of lower stability in water.
3.2. Photocatalytic reactions Firstly, the blank test (without photocatalyst) was carried out under UV–vis irradiation. A small amount of phenol decomposition was observed due to photolysis. This value was taken to the account during activity calculation.
Fig. 4. Dependence of ln(C0/C) = kt vs. time for phenol photodecomposition on TiO2 – Tytanpol A11 original and modified by alkali metals.
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Table 2 Rate constants of phenol photodecomposition and linear regression coefficients from a plot of ln(C0/C) = kt Photocatalyst
R2
k (min1)
A11 Li/A11 Na/A11 K/A11 Sr/A11 Ba/A11
0.9884 0.9859 0.9841 0.9813 0.9926 0.9901
0.0119 0.0085 0.0102 0.0095 0.0128 0.0147
Next experiments were conducted with using A11 original and modified by alkali metals photocatalysts. The kinetics of phenol photodecomposition on the catalysts surface can be described by the first order reaction (Eq. (2)) [9–11]: C0 ln ¼ kt C
(2)
where k is the rate constant (min1), C0 and C are the initial and relative phenol concentration at time t, respectively. In Fig. 4, the linear relation of ln(C0/C) versus irradiation time for phenol is presented. Rate constants (k) were determined for all tested photocatalysts from the slope of the line, as presented in Fig. 4. In Table 2, rate constants for phenol decomposition on all investigated photocatalysts are listed together with the coefficient of the linear regression. The rate constants for all studied samples are presented in Fig. 5. From these figures, it is seen that from all investigated photocatalysts only materials based on titanium dioxide and barium hydroxide and strontium carbonate was of higher activity than pure precursor. It appeared that the most activity at photocatalytic decomposition of phenol has titanium dioxide modified with barium hydroxide (Ba/A11). Ba/A11 photocatalysts has similar properties as a pure precursor. BET surface area of Ba/A11 was about 0.6 m2/g lower and band gap energy was about 0.019 eV higher than A11. Sr/A11 photocatalyst characterised higher EG in comparison to A11 of about 0.019 eVand lower BET surface area of about 9 m2/g. Also it is worth to point out that
Fig. 6. The relationship constant rate of phenol decomposition on Me/TiO2 vs. density of used alkali metals.
materials with Ba and Sr were of higher stability and better decantability. The higher activities of Ba/A11 and Sr/A11 photocatalysts in comparison to the original A11 are probably caused by the presence of titanates at perovskite forms as it was show the diffractograms of Ba/A11 and Sr/A11 photocatalysts (Fig. 1). For samples Ba/A11 and Sr/A11, a new absorption bands appeared on FT-IR/DRS spectra (Fig. 3, spectra 5, 6). In case of sample Ba/A11, we obtained new sites at 2451, 1749 and 1469 cm1, and for Sr/A11 at 2341 and 1769 cm1. These new bands and smaller intensity of broad bands in the region of 3600–2600 cm1 and bands at 1630 cm1 can be explained by creation of new active sites in the photocatalysts, which results in higher activity of Ba and Sr modified TiO2. In the case of material based on A11 and strontium carbonate (Sr/A11), its activity results from SrTiO3 phase only, because rutile form of titanium dioxide has no activity in the photocatalytic process. Rutile obtained by calcination of anatase at temperature of 1200 8C showed insignificant activity for phenol decomposition (below 0.5%). Therefore, this activity is negligible. The constant rate recalculated on mass of pure SrTiO3 phase would be of much higher than for both anatase and SrTiO3 phases. In Fig. 6 presented is relation of constant ratio with density of doped metals. We can see from this figure that activities of modified photocatalysts grow with increase of density alkali metals. The metals with higher density (or atomic mass) seem to be better modificator of TiO2. Finally, the anatase modified by divalent alkali metals forms photocatalysts of higher activity than the bare anatase.
4. Conclusion
Fig. 5. Rate constants of phenol decomposition on TiO2 – Tytanpol A11 original and modified by alkali metals.
Several materials based on the commercial titanium dioxide of anatase type (Tytanpol A11) have been tested for the photocatalytic oxidation of phenol. Higher activity of A11 modified by barium hydroxide and strontium carbonate in comparison to A11 original was found. This higher
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activity resulted from the presence of the titanates at perovskite forms (BaTiO3 and SrTiO3). The relationship between constant rate on modified photocatalysts and alkali metals density was found.
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