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TiO2 with the oxidation of gaseous benzene
Journal of Hazardous Materials 168 (2009) 1632–1635
Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Short communication
Characterization and photocatalytic property of Pd/TiO2 with the oxidation of gaseous benzene Jun Bo Zhong a,∗ , Yan Lu a , Wei Dong Jiang a , Qing Ming Meng a , Xi Yang He a , Jian Zhang Li a , Yao Qiang Chen b a b
College of Chemistry and Pharmaceutical Engineering, Sichuan University of Science and Engineering, Zi gong, 643000, PR China College of Chemistry, Sichuan University, Cheng du, 610064, PR China
a r t i c l e
i n f o
Article history: Received 24 May 2008 Received in revised form 2 January 2009 Accepted 27 February 2009 Available online 11 March 2009 Keywords: Titanium dioxide Pd Benzene Photocatalytic oxidation VOCs
a b s t r a c t Increasing environmental pollution caused by the volatile organic compounds due to their toxicity is a matter of great concern. So it is crucial to develop processes which can destroy these compounds effectively. It has been found that the photocatalytic activity of TiO2 towards the decomposition of gaseous benzene can be greatly enhanced by loading Pd on the surface of TiO2 . The results show that the optimum palladium loading is 0.25 wt.%. The prepared photocatalysts were characterized by XRD, UV–vis diffuse reflectance and XPS. XRD results indicate that no peaks of Pd are detected in the 2 region from 10◦ to 90◦ . Pd/TiO2 absorbs much more light than TiO2 in the visible light region. The XPS spectrum shows that there are Ti, O, C and Pd elements on the surface of the Pd/TiO2 , the binding energy values of Ti2p of 0.25%Pd/TiO2 transfer to a lower value. In addition, the photocatalytic performance of 0.25%Pd/TiO2 was investigated, the result illustrates that 0.25%Pd/TiO2 demonstrates 2.32 times the photocatalytic activity of pure TiO2 . The reason of promotion of photocatalytic performance was discussed. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The volatile organic compounds (VOCs) are widely used in industrial process and domestic activities. These extensive uses lead to water and air pollution, particularly in indoor air pollution. Many VOCs are known to be toxic and considered to be carcinogenic. The most significant problem related to the emission of VOCs is centered on the potential production of photochemical oxidants; for example, ozone and peroxyacetyl nitrate [1]. Emissions of VOCs also contribute to localized pollution problems of toxicity and odor. Many VOCs are implicated in the depletion of the stratospheric ozone layer and may contribute to global warming [2]. As a result of all these problems, VOCs have drawn considerable attention in recent years. Among the technologies developed for the treatment of VOCs, the photocatalytic oxidation process is considered to be a promising technology. However, the decomposition of volatile organic compounds using photocatalytic technology has been difficult because of the low conversion and the common deactivation of photocatalyst [3–7]. Therefore it is crucial to prolong the lifetime of the photocatalyst and
∗ Corresponding author. Tel.: +86 813 5505601; fax: +86 813 5505601. E-mail address: [email protected] (J.B. Zhong). 0304-3894/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2009.02.158
enhance its photocatalytic activity. Various techniques have been developed for development and modification of the TiO2 -based photocatalyst [8], including preparation it as nanosized particles [9], and the incorporation of transition [10] or noble metals [11,12]. It has been reported that the supported metal/TiO2 catalysts improve considerably their performance for the photocatalytic oxidation/reduction of different organic chemicals [13–15]. Electronic transport through the Schottky barrier formed in the metal–semiconductor interface could account for this beneficial effect, since the electrons transferred to the metal particles can favor reduction reactions on their surface and prevent hole–electron recombination [16]. Pd/TiO2 is a potentially interesting catalyst for the elimination of air pollutants, as palladium can promote effectively the photocatalytic activity of titanium dioxide [15]. Benzene is a major indoor and industrial air pollutant, and it is recommended as one of eight representative indoor VOCs. Thus, in the present paper, benzene was chosen as the model VOCs to investigate the capability with Pd/TiO2 . The objective of this work is to prolong the lifetime of photocatalyst and enhance its photocatalytic activity by loading Pd onto TiO2 . This paper presents the experimental results and discusses the influence of loading Pd onto TiO2 on photocatalytic activity of TiO2 .
J.B. Zhong et al. / Journal of Hazardous Materials 168 (2009) 1632–1635
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2. Experimental 2.1. Preparation of Pd/TiO2 The pure anatase TiO2 was donated by Pan Zhi Hua Steel Groups with mean particle size around 30 nm and specific surface area of 122 m2 /g. Pd(NO3 )2 was obtained from Guang Ming Chemical Industries Ltd (Cheng du). Deionized water was used throughout the experiments. The photocatalysts of Pd/TiO2 with different Pd loading were prepared by pore impregnating method using Pd(NO3 )2 solution, then baked at 723 K for 2 h. The pure anatase TiO2 was also dealt with the same procedure as mentioned above without the presence of Pd(NO3 )2 .
Fig. 1. Benzene conversion over Pd promoted TiO2 catalysts and lifetime of catalysts with varying palladium loading: (line) () conversion of benzene; () lifetime of photocatalyst.
2.2. Characterization of Pd/TiO2 X-ray diffraction (XRD) patterns were recorded on a DX-1000 Xray diffractometer using Cu K␣ ( = 0.15406 nm) radiation equipped with a graphite monochromator. The X-ray tube was operated at 45 kV and 25 mA. Samples were scanned from 2 equal to 10◦ up to 90◦ and the X-ray diffraction line positions were determined with a step size of 0.03◦ and a slit of 1. X-ray photoelectron spectroscopy (XPS) measurement was carried out on a spectrometer (XSAM-800, KRATOS Co.) equipped with two ultrahigh-vacuum chambers, the pressure in the chambers during experiment was about 10−7 Pa. A Mg K␣ X-ray source was used. The analysis was operated at 20 eV pass energy for high resolution spectra and 50 eV for survey. The X-ray photoelectron spectrum was referenced to the C1s peak (Eb = 284.80 eV) resulting from adventitious hydrocarbon (i.e. from the XPS instrument itself) present on the sample surface. The UV–vis diffuse reflectance spectrum was performed in a spectrometer (TU-1907) using barium sulphate as the reference. 2.3. Evaluation of the photocatalysis Evaluation of the photocatalysis was carried out in a closed stainless steel reactor, following the literatures [17]. The volume of reactor is 105 L. An electric fan and three 10 W black light lamps with the maximum wavelength of 365 nm were installed on a bracket. 1.5 g of photocatalyst powder was well dispersed to form a thin layer over two aluminum foils (2 × 25 cm (L) × 20 cm (W)). The initial concentration of benzene was kept at 400 mg/m3 for all the experiments. The conversion was calculated by (C0 − C)/C0 , where C is the concentration of the reactant after irradiation, C0 is the concentration of the reactant after adsorption equilibrium and before the irradiation in the presence of catalyst.
The conversion of benzene increases and then drops with Pd loading. The highest efficiency 65.3% is obtained when Pd loading is 0.25%, then the photocatalytic degradation efficiency drops sharply at 1.0%. The decline in photocatalytic activity above the optimum Pd loading on one hand may be largely due to shading of the photosensitive surface of TiO2 , on the other hand, when Pd loading is below the optimum Pd loading, electronic transport through the Schottky barrier formed in the metal–semiconductor interface could account for this beneficial effect, since the electrons transferred to the metal particles can favor reduction reactions on their surface and prevent hole–electron recombination, while when loading is above the optimum Pd loading, the Pd would be the hole–electron recombination center [16]. 3.3. Characterization of the catalyst The X-ray diffraction patterns of the as-received photocatalyst powder samples are shown in Fig. 2. Only X-ray diffraction peak of TiO2 is observed in these photocatalyst samples (TiO2 and 1.0%Pd/TiO2 were used as the reference). The peaks of Pd are not detected by XRD in the 2 region from 10◦ to 90◦ , we believe that two factors can cause this result, one is that the deposited Pd is highly dispersed in the support matrix, the other is the percentage of Pd is too low. Since the pure and Pd/TiO2 samples were used in the photocatalytic reaction, their UV–vis diffuse reflective properties may have had a strong effect on the photocatalytic activity. As shown in Fig. 3, from 200 nm to 800 nm, spectra have an obvious difference. The presence of Pd clearly changes the spectra of TiO2 in the visible light region; Pd/TiO2 absorbs much more visible light than TiO2 . The results indicate that Pd/TiO2 can be excited by visible light. The absorbance in visible light region increases with
3. Results and discussion 3.1. Blank experiments The control experiments were carried out in two conditions: one with illumination but no catalyst, the other with catalyst (pure TiO2 and Pd/TiO2 ) but no illumination. The results show negligible change of initial concentration of benzene in both cases during period of 8 h, which could be ignored. The blank tests prove the stability of benzene rings without illumination or photocatalyst. 3.2. Photocatalytic activity of different TiO2 loading In heterogeneous photocatalysis, the activity of catalyst depends not only on property of loading species but also on the amount of loaded compound. Fig. 1 shows that loading of Pd on the TiO2 surface improves the photocatalytic activity under UV illumination.
Fig. 2. XRD patterns of (1) TiO2 ; (2) 0.25%Pd/TiO2 ; (3) 1.0%Pd/TiO2 photocatalysts.
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Fig. 3. UV–vis diffuse reflectance spectra of (1) TiO2 ; (2) 0.25%Pd/TiO2 ; (3) 1.0%Pd/TiO2 photocatalysts.
the amount of Pd and the photocatalyst exhibits the photocatalytic activity under visible light irradiation. The XPS spectrum shows that there are Ti, O, C and Pd elements on all the surface of the Pd/TiO2 samples. The Ti element results from the TiO2 . The O element is assigned to the precursor TiO2 . The Pd element is assigned to Pd(NO3 )2 . The C element is assigned to the C pollution in the instrument. The high resolution XPS of Ti2p spectra corresponding to the surface are shown in Fig. 4. All Ti2p spectra of photocatalysts are characterized by a main doublet composed of two symmetric peaks. For TiO2 , the Eb (Ti2p3/2 ) is 459.2 eV and Eb (Ti2p1/2 ) is 464.3 eV, respectively, the binding energy difference, Eb = Eb (Ti2p1/2 ) − Eb (Ti2p3/2 ) is 5.7 eV, as previously reported in the literature [18], the main doublet is assigned to Ti(IV) (titanium in the IV oxidation state). However, for 0.25%Pd/TiO2 and 1.0%Pd/TiO2 , the binding energy values of Ti2p of two catalysts all shift to a lower value, which indicates that there is stronger interaction between Pd and TiO2 . Compared to 1.0%Pd/TiO2 , binding energy value of Ti2p3/2 of 0.25%Pd/TiO2 shifts to lower value to 0.7 eV, the result shows that the interaction between Pd and TiO2 in 0.25%Pd/TiO2 is stronger than that of 1.0%Pd/TiO2 , which may promote the catalytic activity of TiO2 . Further information of the surface can be also obtained from the Pd (3d) XPS spectrum. The Pd (3d) peaks of Pd/TiO2 are presented in Fig. 5. The peaks at 342.0 eV and 336.7 eV are assigned to Pd2+ [19,20]. However, for 1.0%Pd/TiO2 , the binding energy of Pd (3d)
Fig. 4. XPS (Ti2p) spectra of (1) TiO2 ; (2) 0.25%Pd/TiO2 ; (3) 1.0%Pd/TiO2 photocatalysts.
Fig. 5. XPS (Pd3d) spectra of (1) 0.25%Pd/TiO2 ; (2) 1.0%Pd/TiO2 photocatalysts.
shifts to a higher value, which indicates that the oxidation state of Pd in 1.0%Pd/TiO2 is higher than that of Pd in 0.25%Pd/TiO2 , the result shows that the interaction between Pd and TiO2 in 0.25%Pd/TiO2 is stronger than that of 1.0%Pd/TiO2 , this result is consentient with the results of Ti2p of two photocatalysts. 3.4. Comparison photocatalytic activity of 0.25%Pd/TiO2 and TiO2 For practical applications of photocatalytic oxidation, it is important to have catalysts with very high activity. This can be achieved by optimizing the preparation procedure including optimization of the percentage of palladium. The photocatalytic activity of the fabricated photocatalysts was investigated by the oxidation of benzene under UV light irradiation. The photocatalytic activity of 0.25%Pd/TiO2 and TiO2 was compared and presented in Fig. 6. As it is shown in Fig. 6, in the process of decomposing benzene, the pure TiO2 deactivates after 3.0 h, and the maximum conversion of 28.2% is reached after 3.0 h, while the 0.25%Pd/TiO2 catalyst exhibits its superiority stability. In the whole process of reaction, the conversion keeps increasing until 6.0 h when the photocatalytic activity begins to decline and the maximum conversion of 65.3% is reached. Moreover, the conversion of benzene is 2.32 times of the pure TiO2 . For 1.0%Pd/TiO2 , the maximum conversion of benzene is 35.7%, the deactivation time is 3.5 h. It is clear that 0.25%Pd/TiO2 can prolong the life of photocatalyst and enhance the photocatalytic activity of catalyst. The results of photocatalytic activity are consistent with the results of XPS. 0.25%Pd/TiO2 catalyst exhibits its
Fig. 6. Benzene conversion on Pd/TiO2 catalysts as a function of reaction time: TiO2 (); 0.25%Pd/TiO2 (); 1.0%Pd/TiO2 (); the relative humidity was 50%.
J.B. Zhong et al. / Journal of Hazardous Materials 168 (2009) 1632–1635
superiority photocatalytic performance, we believe that it is mainly due to the binding energy value of Ti2p3/2 of 0.25%Pd/TiO2 shifts to the lower value and lower oxidation state of Pd in 0.25%Pd/TiO2 . The deactivation of TiO2 -based photocatalyst in gas-phase photocatalysis is mainly assigned to the accumulation of some intermediates on the surface of photocatalyst, these intermediates have strong adsorption ability on the surface of TiO2 , which blocks the active sites of TiO2 . The result shows that palladium could contribute to prolong the life of photocatalyst and enhance the photocatalytic activity of catalyst, but the photocatalyst still deactivates after 6 h, in this respect it is worth noting that the generation of some intermediates is apparently related to the deactivation of the catalyst, however, the present data do not allow to establish if palladium promotes the mineralization of some intermediates or if alternatively it hinders their formation to some extent. 4. Conclusions The present study indicates that the Pd/TiO2 photocatalysts can be applied to the removal of benzene vapor in enclosed atmospheres. The addition of palladium stabilizes the catalysts against the deactivation. The paper reveals that the optimal loading amount of Pd on TiO2 in our experimental condition for the degradation of gaseous benzene is 0.25 wt.%. XRD results show that no peaks of Pd are detected in the 2 region from 10◦ to 90◦ . Pd/TiO2 absorbs much more light than TiO2 in the visible light region. The XPS spectrum shows that there are Ti, O, C and Pd elements on the surface of the Pd/TiO2 , the binding energy values of Ti2p of 0.25%Pd/TiO2 are 462.2 eV and 458.5 eV, these values shift to lower value to 0.7 eV. In the process of decomposing of benzene, the pure TiO2 deactivates after 3.0 h, and the maximum conversion of 28.2% is reached after 3.0 h, the conversion of benzene on 0.25%Pd/TiO2 keeps increasing until 6.0 h when the photocatalytic activity begins to decline and the maximum conversion of 65.3% is reached. 0.25%Pd/TiO2 demonstrates 2.32 times the photocatalytic activity of pure TiO2 , the lifetime of 0.25%Pd/TiO2 is 2.0 times of that of TiO2 . The promotion of the catalytic performance is mainly due to the binding energy value of Ti2p3/2 of 0.25%Pd/TiO2 shifts to the lower value and lower oxidation state of Pd in 0.25%Pd/TiO2 . Acknowledgements This project was supported financially by the Specialized Research Fund for the Doctoral Program of Sichuan University of Science and Engineering (no. 07ZR13) and Ministry of Education of Sichuan province (no. 07zz020)
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References [1] S.B. Kim, S.C. Hong, Kinetic study for photocatalytic degradation of volatile organic compounds in air using thin film TiO2 photocatalyst, Appl. Catal. B: Environ. 35 (2002) 305–315. [2] R.M. Alberic, W.F. Jardim, Photocatalytic destruction of VOCs in the gas-phase using titanium dioxide, Appl. Catal B: Environ. 14 (1997) 55–68. [3] R.M. Alberici, M.C. Canela, M.N. Eberlin, W.F. Jardim, Catalyst deactivation in the gas phase destruction of nitrogen-containing organic compounds using TiO2 /UV–vis, Appl. Catal. B: Environ. 30 (2001) 389–397. [4] Q.C. Zhang, F.B. Zhang, G.L. Zhang, Reaction mechanism of gas-phase photocatalytic oxidation of benzene on TiO2 , Chin. J. Catal. 25 (2004) 39–43. [5] J.Y. Wang, J.B. Zhong, M.C. Gong, M. Wang, Y.Q. Chen, Gas-phase photocatalytic oxidation of benzene over titanium dioxide loaded on strontium cerate, Chin. J. Catal. 26 (2005) 171–172. [6] G. Marc‘ı, M. Addamo, V. Augugliaro, S. Coluccia, E. Garcˇııa-López, V. Loddo, G. Martra, L. Palmisano, M. Schiavello, Photocatalytic oxidation of toluene on irradiated TiO2 : comparison of degradation performance in humidified air, in water and in water containing a zwitterionic surfactant, J. Photochem. Photobiol. A: Chem. 160 (2003) 105–114. [7] Q.C. Zhang, F.Y. Zhang, G.L. Zhang, X.P. Zhang, Gas-phase photo catalytic reaction properties of benzene on TiO2 , Chin. Environ. Sci. 23 (2003) 661–664. [8] T.K. Kim, M.N. Lee, S.H. Lee, Y.C. Park, C.K. Jung, J.H. Boo, Development of surface coating technology of TiO2 powder and improvement of photocatalytic activity by surface modification, Thin Solid Films 475 (2005) 171–177. [9] A.J. Maira, K.L. Yeung, J. Soria, J.M. Coronado, C. Belver, C.Y. Lee, V. Augugliario, Gas-phase photo-oxidation of toluene using nanometer-size TiO2 catalysts, Appl. Catal. B. 29 (2001) 327–336. [10] M.I. Litter, Heterogeneous photocatalysis transition metal ions in photocatalytic systems, Appl. Catal. B: Environ. 23 (1999) 89–114. [11] J.C. Kennedy, A.K. Datye, Photothermal heterogeneous oxidation of ethanol over Pt/TiO2 , J. Catal. 179 (1998) 375–389. [12] A.V. Vorontsov, I.V. Stoyanova, D.V. Kozlov, V.I. Simagina, E.N. Savinov, Study of TiO2 deactivation during gaseous acetone photocatalytic oxidation, J. Catal. 186 (1999) 318–324. [13] H. Tada, T. Ishida, A. Takao, I. Seishiro, Drastic enhancement of TiO2 photocatalyzed reduction of nitrobenzene by loading Ag clusters, Langmuir 20 (2004) 7898–7900. [14] B. Sun, P.G. Smirniotis, P. Boolchand, Visible light photocatalysis with platinized rutile TiO2 for aqueous organic oxidation, Langmuir 21 (2005) 11397– 11403. [15] M. Hosseini, S. Siffert, H.L. Tidahy, R. Cousin, J.F. Lamonier, A. Aboukais, A. Vantomme, M. Roussel, B.L. Su, Promotional effect of gold added to palladium supported on a new mesoporous TiO2 for total oxidation of volatile organic compounds, Catal. Today 122 (2007) 391–396. [16] H. Einaga, S. Futamura, T. Ibusuki, Complete oxidation of benzene in gas phase by platinized titania photocatalysts, Environ. Sci. Technol. 35 (2001) 1880– 1884. [17] J.B. Zhong, T. Lin, M.C. Gong, J.L. Wang, Z.M. Liu, M. Zhao, Y.Q. Chen, Effect of metal doping into Ce0.5 Zr0.5 O2 on photocatalytic activity of TiO2 /Ce0.45 Zr0.45 M0.1 OX (M = Y, La, Mn), J. Hazard. Mater. 143 (2007) 516–521. [18] J.G. Yu, X.J. Zhao, Q.N. Zhao, J.C. Du, XPS of study of TiO2 photocatalytic thin film prepared by the sol–gel method, Chin. J. Mater. Res. 14 (2000) 203–209. [19] G. Veser, A. Wright, R. Caretta, On the oxidation-reduction kinetic of palladium, Catal. Lett. 58 (1999) 199–206. [20] A.M. Venezia, L.F. Liotta, G. Deganello, Z. Schay, L. Guczi, Characterization of pumice-supported Ag–Pd and Cu–Pd bimetallic catalysts by X-ray photoelectron spectroscopy and X-ray diffraction, J. Catal. 182 (1999) 449–455.
Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Short communication
Characterization and photocatalytic property of Pd/TiO2 with the oxidation of gaseous benzene Jun Bo Zhong a,∗ , Yan Lu a , Wei Dong Jiang a , Qing Ming Meng a , Xi Yang He a , Jian Zhang Li a , Yao Qiang Chen b a b
College of Chemistry and Pharmaceutical Engineering, Sichuan University of Science and Engineering, Zi gong, 643000, PR China College of Chemistry, Sichuan University, Cheng du, 610064, PR China
a r t i c l e
i n f o
Article history: Received 24 May 2008 Received in revised form 2 January 2009 Accepted 27 February 2009 Available online 11 March 2009 Keywords: Titanium dioxide Pd Benzene Photocatalytic oxidation VOCs
a b s t r a c t Increasing environmental pollution caused by the volatile organic compounds due to their toxicity is a matter of great concern. So it is crucial to develop processes which can destroy these compounds effectively. It has been found that the photocatalytic activity of TiO2 towards the decomposition of gaseous benzene can be greatly enhanced by loading Pd on the surface of TiO2 . The results show that the optimum palladium loading is 0.25 wt.%. The prepared photocatalysts were characterized by XRD, UV–vis diffuse reflectance and XPS. XRD results indicate that no peaks of Pd are detected in the 2 region from 10◦ to 90◦ . Pd/TiO2 absorbs much more light than TiO2 in the visible light region. The XPS spectrum shows that there are Ti, O, C and Pd elements on the surface of the Pd/TiO2 , the binding energy values of Ti2p of 0.25%Pd/TiO2 transfer to a lower value. In addition, the photocatalytic performance of 0.25%Pd/TiO2 was investigated, the result illustrates that 0.25%Pd/TiO2 demonstrates 2.32 times the photocatalytic activity of pure TiO2 . The reason of promotion of photocatalytic performance was discussed. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The volatile organic compounds (VOCs) are widely used in industrial process and domestic activities. These extensive uses lead to water and air pollution, particularly in indoor air pollution. Many VOCs are known to be toxic and considered to be carcinogenic. The most significant problem related to the emission of VOCs is centered on the potential production of photochemical oxidants; for example, ozone and peroxyacetyl nitrate [1]. Emissions of VOCs also contribute to localized pollution problems of toxicity and odor. Many VOCs are implicated in the depletion of the stratospheric ozone layer and may contribute to global warming [2]. As a result of all these problems, VOCs have drawn considerable attention in recent years. Among the technologies developed for the treatment of VOCs, the photocatalytic oxidation process is considered to be a promising technology. However, the decomposition of volatile organic compounds using photocatalytic technology has been difficult because of the low conversion and the common deactivation of photocatalyst [3–7]. Therefore it is crucial to prolong the lifetime of the photocatalyst and
∗ Corresponding author. Tel.: +86 813 5505601; fax: +86 813 5505601. E-mail address: [email protected] (J.B. Zhong). 0304-3894/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2009.02.158
enhance its photocatalytic activity. Various techniques have been developed for development and modification of the TiO2 -based photocatalyst [8], including preparation it as nanosized particles [9], and the incorporation of transition [10] or noble metals [11,12]. It has been reported that the supported metal/TiO2 catalysts improve considerably their performance for the photocatalytic oxidation/reduction of different organic chemicals [13–15]. Electronic transport through the Schottky barrier formed in the metal–semiconductor interface could account for this beneficial effect, since the electrons transferred to the metal particles can favor reduction reactions on their surface and prevent hole–electron recombination [16]. Pd/TiO2 is a potentially interesting catalyst for the elimination of air pollutants, as palladium can promote effectively the photocatalytic activity of titanium dioxide [15]. Benzene is a major indoor and industrial air pollutant, and it is recommended as one of eight representative indoor VOCs. Thus, in the present paper, benzene was chosen as the model VOCs to investigate the capability with Pd/TiO2 . The objective of this work is to prolong the lifetime of photocatalyst and enhance its photocatalytic activity by loading Pd onto TiO2 . This paper presents the experimental results and discusses the influence of loading Pd onto TiO2 on photocatalytic activity of TiO2 .
J.B. Zhong et al. / Journal of Hazardous Materials 168 (2009) 1632–1635
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2. Experimental 2.1. Preparation of Pd/TiO2 The pure anatase TiO2 was donated by Pan Zhi Hua Steel Groups with mean particle size around 30 nm and specific surface area of 122 m2 /g. Pd(NO3 )2 was obtained from Guang Ming Chemical Industries Ltd (Cheng du). Deionized water was used throughout the experiments. The photocatalysts of Pd/TiO2 with different Pd loading were prepared by pore impregnating method using Pd(NO3 )2 solution, then baked at 723 K for 2 h. The pure anatase TiO2 was also dealt with the same procedure as mentioned above without the presence of Pd(NO3 )2 .
Fig. 1. Benzene conversion over Pd promoted TiO2 catalysts and lifetime of catalysts with varying palladium loading: (line) () conversion of benzene; () lifetime of photocatalyst.
2.2. Characterization of Pd/TiO2 X-ray diffraction (XRD) patterns were recorded on a DX-1000 Xray diffractometer using Cu K␣ ( = 0.15406 nm) radiation equipped with a graphite monochromator. The X-ray tube was operated at 45 kV and 25 mA. Samples were scanned from 2 equal to 10◦ up to 90◦ and the X-ray diffraction line positions were determined with a step size of 0.03◦ and a slit of 1. X-ray photoelectron spectroscopy (XPS) measurement was carried out on a spectrometer (XSAM-800, KRATOS Co.) equipped with two ultrahigh-vacuum chambers, the pressure in the chambers during experiment was about 10−7 Pa. A Mg K␣ X-ray source was used. The analysis was operated at 20 eV pass energy for high resolution spectra and 50 eV for survey. The X-ray photoelectron spectrum was referenced to the C1s peak (Eb = 284.80 eV) resulting from adventitious hydrocarbon (i.e. from the XPS instrument itself) present on the sample surface. The UV–vis diffuse reflectance spectrum was performed in a spectrometer (TU-1907) using barium sulphate as the reference. 2.3. Evaluation of the photocatalysis Evaluation of the photocatalysis was carried out in a closed stainless steel reactor, following the literatures [17]. The volume of reactor is 105 L. An electric fan and three 10 W black light lamps with the maximum wavelength of 365 nm were installed on a bracket. 1.5 g of photocatalyst powder was well dispersed to form a thin layer over two aluminum foils (2 × 25 cm (L) × 20 cm (W)). The initial concentration of benzene was kept at 400 mg/m3 for all the experiments. The conversion was calculated by (C0 − C)/C0 , where C is the concentration of the reactant after irradiation, C0 is the concentration of the reactant after adsorption equilibrium and before the irradiation in the presence of catalyst.
The conversion of benzene increases and then drops with Pd loading. The highest efficiency 65.3% is obtained when Pd loading is 0.25%, then the photocatalytic degradation efficiency drops sharply at 1.0%. The decline in photocatalytic activity above the optimum Pd loading on one hand may be largely due to shading of the photosensitive surface of TiO2 , on the other hand, when Pd loading is below the optimum Pd loading, electronic transport through the Schottky barrier formed in the metal–semiconductor interface could account for this beneficial effect, since the electrons transferred to the metal particles can favor reduction reactions on their surface and prevent hole–electron recombination, while when loading is above the optimum Pd loading, the Pd would be the hole–electron recombination center [16]. 3.3. Characterization of the catalyst The X-ray diffraction patterns of the as-received photocatalyst powder samples are shown in Fig. 2. Only X-ray diffraction peak of TiO2 is observed in these photocatalyst samples (TiO2 and 1.0%Pd/TiO2 were used as the reference). The peaks of Pd are not detected by XRD in the 2 region from 10◦ to 90◦ , we believe that two factors can cause this result, one is that the deposited Pd is highly dispersed in the support matrix, the other is the percentage of Pd is too low. Since the pure and Pd/TiO2 samples were used in the photocatalytic reaction, their UV–vis diffuse reflective properties may have had a strong effect on the photocatalytic activity. As shown in Fig. 3, from 200 nm to 800 nm, spectra have an obvious difference. The presence of Pd clearly changes the spectra of TiO2 in the visible light region; Pd/TiO2 absorbs much more visible light than TiO2 . The results indicate that Pd/TiO2 can be excited by visible light. The absorbance in visible light region increases with
3. Results and discussion 3.1. Blank experiments The control experiments were carried out in two conditions: one with illumination but no catalyst, the other with catalyst (pure TiO2 and Pd/TiO2 ) but no illumination. The results show negligible change of initial concentration of benzene in both cases during period of 8 h, which could be ignored. The blank tests prove the stability of benzene rings without illumination or photocatalyst. 3.2. Photocatalytic activity of different TiO2 loading In heterogeneous photocatalysis, the activity of catalyst depends not only on property of loading species but also on the amount of loaded compound. Fig. 1 shows that loading of Pd on the TiO2 surface improves the photocatalytic activity under UV illumination.
Fig. 2. XRD patterns of (1) TiO2 ; (2) 0.25%Pd/TiO2 ; (3) 1.0%Pd/TiO2 photocatalysts.
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Fig. 3. UV–vis diffuse reflectance spectra of (1) TiO2 ; (2) 0.25%Pd/TiO2 ; (3) 1.0%Pd/TiO2 photocatalysts.
the amount of Pd and the photocatalyst exhibits the photocatalytic activity under visible light irradiation. The XPS spectrum shows that there are Ti, O, C and Pd elements on all the surface of the Pd/TiO2 samples. The Ti element results from the TiO2 . The O element is assigned to the precursor TiO2 . The Pd element is assigned to Pd(NO3 )2 . The C element is assigned to the C pollution in the instrument. The high resolution XPS of Ti2p spectra corresponding to the surface are shown in Fig. 4. All Ti2p spectra of photocatalysts are characterized by a main doublet composed of two symmetric peaks. For TiO2 , the Eb (Ti2p3/2 ) is 459.2 eV and Eb (Ti2p1/2 ) is 464.3 eV, respectively, the binding energy difference, Eb = Eb (Ti2p1/2 ) − Eb (Ti2p3/2 ) is 5.7 eV, as previously reported in the literature [18], the main doublet is assigned to Ti(IV) (titanium in the IV oxidation state). However, for 0.25%Pd/TiO2 and 1.0%Pd/TiO2 , the binding energy values of Ti2p of two catalysts all shift to a lower value, which indicates that there is stronger interaction between Pd and TiO2 . Compared to 1.0%Pd/TiO2 , binding energy value of Ti2p3/2 of 0.25%Pd/TiO2 shifts to lower value to 0.7 eV, the result shows that the interaction between Pd and TiO2 in 0.25%Pd/TiO2 is stronger than that of 1.0%Pd/TiO2 , which may promote the catalytic activity of TiO2 . Further information of the surface can be also obtained from the Pd (3d) XPS spectrum. The Pd (3d) peaks of Pd/TiO2 are presented in Fig. 5. The peaks at 342.0 eV and 336.7 eV are assigned to Pd2+ [19,20]. However, for 1.0%Pd/TiO2 , the binding energy of Pd (3d)
Fig. 4. XPS (Ti2p) spectra of (1) TiO2 ; (2) 0.25%Pd/TiO2 ; (3) 1.0%Pd/TiO2 photocatalysts.
Fig. 5. XPS (Pd3d) spectra of (1) 0.25%Pd/TiO2 ; (2) 1.0%Pd/TiO2 photocatalysts.
shifts to a higher value, which indicates that the oxidation state of Pd in 1.0%Pd/TiO2 is higher than that of Pd in 0.25%Pd/TiO2 , the result shows that the interaction between Pd and TiO2 in 0.25%Pd/TiO2 is stronger than that of 1.0%Pd/TiO2 , this result is consentient with the results of Ti2p of two photocatalysts. 3.4. Comparison photocatalytic activity of 0.25%Pd/TiO2 and TiO2 For practical applications of photocatalytic oxidation, it is important to have catalysts with very high activity. This can be achieved by optimizing the preparation procedure including optimization of the percentage of palladium. The photocatalytic activity of the fabricated photocatalysts was investigated by the oxidation of benzene under UV light irradiation. The photocatalytic activity of 0.25%Pd/TiO2 and TiO2 was compared and presented in Fig. 6. As it is shown in Fig. 6, in the process of decomposing benzene, the pure TiO2 deactivates after 3.0 h, and the maximum conversion of 28.2% is reached after 3.0 h, while the 0.25%Pd/TiO2 catalyst exhibits its superiority stability. In the whole process of reaction, the conversion keeps increasing until 6.0 h when the photocatalytic activity begins to decline and the maximum conversion of 65.3% is reached. Moreover, the conversion of benzene is 2.32 times of the pure TiO2 . For 1.0%Pd/TiO2 , the maximum conversion of benzene is 35.7%, the deactivation time is 3.5 h. It is clear that 0.25%Pd/TiO2 can prolong the life of photocatalyst and enhance the photocatalytic activity of catalyst. The results of photocatalytic activity are consistent with the results of XPS. 0.25%Pd/TiO2 catalyst exhibits its
Fig. 6. Benzene conversion on Pd/TiO2 catalysts as a function of reaction time: TiO2 (); 0.25%Pd/TiO2 (); 1.0%Pd/TiO2 (); the relative humidity was 50%.
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superiority photocatalytic performance, we believe that it is mainly due to the binding energy value of Ti2p3/2 of 0.25%Pd/TiO2 shifts to the lower value and lower oxidation state of Pd in 0.25%Pd/TiO2 . The deactivation of TiO2 -based photocatalyst in gas-phase photocatalysis is mainly assigned to the accumulation of some intermediates on the surface of photocatalyst, these intermediates have strong adsorption ability on the surface of TiO2 , which blocks the active sites of TiO2 . The result shows that palladium could contribute to prolong the life of photocatalyst and enhance the photocatalytic activity of catalyst, but the photocatalyst still deactivates after 6 h, in this respect it is worth noting that the generation of some intermediates is apparently related to the deactivation of the catalyst, however, the present data do not allow to establish if palladium promotes the mineralization of some intermediates or if alternatively it hinders their formation to some extent. 4. Conclusions The present study indicates that the Pd/TiO2 photocatalysts can be applied to the removal of benzene vapor in enclosed atmospheres. The addition of palladium stabilizes the catalysts against the deactivation. The paper reveals that the optimal loading amount of Pd on TiO2 in our experimental condition for the degradation of gaseous benzene is 0.25 wt.%. XRD results show that no peaks of Pd are detected in the 2 region from 10◦ to 90◦ . Pd/TiO2 absorbs much more light than TiO2 in the visible light region. The XPS spectrum shows that there are Ti, O, C and Pd elements on the surface of the Pd/TiO2 , the binding energy values of Ti2p of 0.25%Pd/TiO2 are 462.2 eV and 458.5 eV, these values shift to lower value to 0.7 eV. In the process of decomposing of benzene, the pure TiO2 deactivates after 3.0 h, and the maximum conversion of 28.2% is reached after 3.0 h, the conversion of benzene on 0.25%Pd/TiO2 keeps increasing until 6.0 h when the photocatalytic activity begins to decline and the maximum conversion of 65.3% is reached. 0.25%Pd/TiO2 demonstrates 2.32 times the photocatalytic activity of pure TiO2 , the lifetime of 0.25%Pd/TiO2 is 2.0 times of that of TiO2 . The promotion of the catalytic performance is mainly due to the binding energy value of Ti2p3/2 of 0.25%Pd/TiO2 shifts to the lower value and lower oxidation state of Pd in 0.25%Pd/TiO2 . Acknowledgements This project was supported financially by the Specialized Research Fund for the Doctoral Program of Sichuan University of Science and Engineering (no. 07ZR13) and Ministry of Education of Sichuan province (no. 07zz020)
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