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Deactivation and regeneration of ZnO and TiO2 nanoparticles in the gas phase photocatalytic oxidation of n-C7H16 or SO2
Applied Catalysis A: General 275 (2004) 49–54 www.elsevier.com/locate/apcata
Deactivation and regeneration of ZnO and TiO2 nanoparticles in the gas phase photocatalytic oxidation of n-C7H16 or SO2 Jing Liqianga,b,1, Xin Baifua, Yuan Fulonga, Wang Baiqib, Shi Keyinga, Cai Weiminb, Fu Hongganga,* a
School of Chemistry and Material Sciences,The Key Laboratory of Physical Chemistry, Heilongjiang University, Harbin 150080, China b Department of Environmental Sciences and Engineering, Harbin Institute of Technology, Harbin 150001, China Received 27 December 2003; received in revised form 7 July 2004; accepted 21 July 2004 Available online 2 September 2004
Abstract In this paper, we examined the lifetimes of made-in-home ZnO and TiO2 nanoparticles in the gas phase photocatalytic oxidation of nC7H16 or SO2, and especially investigated the deactivation mechanism by utilizing surface photovoltage spectrum (SPS) and X-Ray photoelectron spectroscopy (XPS) testing techniques and by considering semiconductor chemical properties. The results showed that ZnO could almost be deactivated in the gas phase photocatalytic oxidation of n-C7H16, while TiO2 could keep most of its activity. In the gas phase photocatalytic oxidation of n-C7H16 or SO2, ZnO and TiO2 both could almost be deactivated. The deactivation mainly resulted from semiconductor surface conduction type change from N-type before the photocatalytic reaction to P-type after the deactivation because of the adsorption of the oxidation products such as H2O, CO2 and SO3 on the semiconductor photocatalyst surface. In addition, the activity of the deactivated photocatalyst could be regenerated to a nearly full extent by washing and drying. # 2004 Elsevier B.V. All rights reserved. Keywords: ZnO; TiO2; Deactivation and regeneration; Photocatalytic oxidation; n-C7H16; SO2
1. Introduction In recent years, semiconductor photocatalysis is becoming more and more attractive and important since it has a great potential to contribute to environmental problems extensively [1–5]. One of the most important aspects of environmental photocatalysis is the selection of materials such as ZnO and TiO2, which are close to being two kinds of ideal photocatalysts in several respects. For example, they are relatively inexpensive, and they provide photo-generated holes with high oxidizing power due to their wide band gap energy [6,7]. Kormann compared the photocatalytic activity of ZnO, Fe2O3 and TiO2, indicating that ZnO and TiO2 exhibited higher activity than Fe2O3 during the process of photocatalytic oxidation of gas phase chloridized hydrocarbon [8]. Heptane (n-C7H16) and sulfur dioxide (SO2) are * Corresponding author. Tel.: +86 451 8660; fax: +86 451 86673647. 1 E-mail address: [email protected] ( j. Liqiang). 0926-860X/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.07.019
two most important organic and inorganic contaminants in the atmosphere, respectively. Moreover, the homogeneous photoreactions between the two compounds can result in a photochemical smog, a badly polluting phenomenon [9]. Therefore, to eliminate these pollutants in the atmosphere is very meaningful to us. Although, ZnO and TiO2 has been proved to be very active in the photocatalytic oxidation of different pollutants, generally higher in the gas phase than in the liquid phase [4,8], the lifetimes, deactivation and regeneration of semiconductor photocatalysts is also very crucial in environmental photocatalysis. Despite the great body of work about the photocatalysis on ZnO or TiO2 until now, few papers are devoted solely to the deactivation and regeneration of ZnO and TiO2 photocatalysts in the gas phase photocatalytic reactions, especially in the photocatalytic oxidation of hydrocarbon or SO2. In this paper, we examined the lifetimes of ZnO and TiO2 nanoparticles in the gas phase phototcatalytic oxidation of n-C7H16 or SO2, and investigated the de-
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J. Liqiang et al. / Applied Catalysis A: General 275 (2004) 49–54
activation and regeneration. This paper should be of great significance in environmental photocatalysis.
2. Experimental 2.1. Preparation of ZnO and TiO2 nanoparticles ZnO nanoparticles were prepared by thermal decomposition of the precursor, zinc carbonate hydroxide, at 320 8C for 1 h [10,11]. The ZnO sample had a wurtzite structure attached to hexagonal system with the average size of 13 nm; TiO2 nanoparticles were fabricated by the sol-gel procedure. Finally, TiO2 nanoparticles were obtained by thermal treatment of TiO2 gel precursor at 600 8C for 2 h. The TiO2 sample contained about 80% anatase and 20% rutile with the average size of 25 nm [12,13].
reactor to a little higher than one atmospheric pressure. Thus, the reactants were n-C7H16 (or SO2) (4%, balance nitrogen) and oxygen (21%, balance nitrogen). When adsorption reached equilibrium, as measured by gas chromatography (GC), the reaction was started when we turned on the lamp. Subsequently, the concentration of n-C7H16 (or SO2) in the reactor, obtained by a gastight syringe from the sample port, was measured every 30 min by a Varian 3700 GCI equipped with a flame ionization defector and a SE-54 quartz capillary column at 60 8C (or a GC II equipped with a thermal conductivity detector and a GDX-102 stainless steel column at 100 8C). The reaction was ended by our turning off the lamp [17–19]. Three hours was considered as a reactive period during the lifetime measurement process. The next reactive period would begin after the earlier reactive period was finished. The above process was repeated until the concentration of n-C7H16 (or SO2) did not change. The total of reactive times was referred to as lifetime.
2.2. Characterization of deactivated ZnO and TiO2 nanoparticles 3. Results and discussion Surface photovoltage spectroscopy (SPS) of the samples was carried out with a locally built apparatus that has been described elsewhere [13–16]. SPS is a very effective way to study the charge separation and transfer behavior at an interface. The generation of photovoltage arises from the creation of electron-hole pairs, followed by the separation under a built-in electric field (space-charge layer). The difference between the surface potential barrier in the light and that in the dark is the SPS signal. Electric-field-induced surface photovoltage spectroscopy (EFISPS) is a technique that combines the field-effect principle with SPS. The external electric field is applied to two sides of the samples and is regarded as positive when its direction is identical to that of the incident light. The surface compositions and chemical states of the samples were examined with a VG ESCALAB MKII X-ray photoelectron spectrometer (XPS) using a monochromatic aluminum X-ray source. The pressure was maintained at 6.3 10 5 Pa. The binding energies were calibrated with respect to the signal for adventitious carbon (binding energy = 284.6 eV).
3.1. Photocatalyst lifetime Fig. 1 showed the photocatalytic oxidation curves of nC7H16 (A) and SO2 (B) on the ZnO or TiO2 nanoparticle photocatalysts. It could be seen that the photocatalytic
2.3. Lifetime measurement of ZnO and TiO2 nanoparticle photocatalysts The light source used in this experiment was a 400W high pressure mercury lamp (lmax = 365 nm). The UV light was transmitted to the specimen through a quartz tube placed between the UV lamp and the photoreactor to absorb the heat and transmit UV light. The reactor used was a 300 ml cylindrical quartz tube. Photocatalyst (0.1 g) was spread uniformly over the internal surface of the reactor as a thin layer. After this, the reactor was vacuum-packed and 1.2 ml of n-C7H16 (or SO2) and 63 ml of oxygen were injected into the reactor through a sample port. Then the ultrapure nitrogen (99.9999%) was mixed with the reactant in the
Fig. 1. The photocatalytic oxidation curves of n-C7H16 (A) and SO2 (B) on ZnO or TiO2 nanoparticle photocatalysts.
J. Liqiang et al. / Applied Catalysis A: General 275 (2004) 49–54
activity of TiO2 was higher than that of ZnO for the oxidizing gas phase of either n-C7H16 or SO2, which was in good agreement with the literature results [4,7]. Photocatalytic reactions mainly take place on the surface of the photocatalyst. Therefore, the adsorption of pollutants in advance is one of the most important steps in photocatalytic reactions [2–5]. If the products of the photocatalytic reactions firmly adsorb on the surface of the photocatalyst, the surface charge carrier transportation and pollutant adsorption can be influenced greatly so that the photocatalysts can degrade the activity or even be deactivated. In the experiment of gas phase photocatalytic oxidation of n-C7H16, ZnO nanoparticles nearly lost all activity after the photocatalytic reaction was continuously carried out for 6 reactive periods (18 h), while the activity of TiO2 nanoparticles did not change much after the photocatalytic reaction was continuously carried out for 10 reactive periods (30 h). In the experiment of gas phase photocatalytic oxidation of SO2, ZnO and TiO2 nanoparticles both lost nearly all activity after the photocatalytic reaction was continuously carried out for five reactive periods (15 h).
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3.2.1. In the photocatalytic oxidation of n-C7H16 The lifetime measurements indicated that ZnO nanoparticles could be deactivated in the photocatalytic oxidation of n-C7H16, while TiO2 nanoparticles could retain most of their activity. Fig. 2 shows the transfer of photo-induced charge carriers on the semiconductor surface or interface. In the case of an N-type semiconductor, the built-in electric field (SCR) with the direction from the interior to the outer drives photogenerated holes toward the surface or interfacial region and electrons toward the interior of the bulk. But the reverse process can take place at a P-type semiconductor, namely, the built-in electric field with the direction from the outer to the interior drives photo-generated electrons toward the surface or interfacial region and holes toward the interior of
the bulk [16,20]. Thus, we can utilize the oxidizing ability of photo-generated holes on an N-type semiconductor or the reducing ability of photo-generated electrons on a P-type semiconductor. Fig. 3 shows the SPS responses of ZnO nanoparticles before the photocatalytic reaction and after the deactivation. The peak response was at about 360 nm, with its onset at 380 nm. This could be assigned to the electron transition from O2p to Zn3d, corresponding to transition from the valence band to the conduction band according to the energy band structure of ZnO [10,21]. In addition, the direction of SPS response of ZnO nanoparticles after the deactivation was the reverse of that before the photocatalytic reaction, indicating that semiconductor surface conduction type of ZnO nanoparticles had changed from N-type before the phtotocatalytic reaction to P-type after the deactivation [22]. This must result in the deactivation of ZnO nanoparticle in that the photo-induced holes on the surface of N-type semiconductor were mainly utilized to oxidize the pollutants in the photocatalytic reactions, while the photo-induced electrons on the surface of P-type semiconductor were mainly utilized to reduce O2 or other substances. If a positive external electric field is applied to the two sides of a P-type semiconductor, the surface barrier can increase because the external electric field and the built-in electric field (SCR) act in the same direction, which can result in the enhancement of the SPS signal. On the contrary, if a positive external electric field is applied to the two sides of an N-type semiconductor, the surface barrier can decrease because the external electric field and the built-in electric field (SCR) act in the opposite directions, which can result in the decline of the SPS signal [20,21]. Fig. 4 shows the EFISPS responses of ZnO nanoparticles before the photocatalytic reaction (A) and after the deactivation (B). Remarkable changes of SPS response of ZnO nanoparticles could be found if an external electric field is applied. In Fig. 4(A), the higher the positive external electric field, the weaker the SPS response, and the SPS response was even reversed. However, the higher the negative external electric field, the stronger the SPS response. These are common
Fig. 2. The transfer of photo-induced charge carrier on the semiconductor surface or interface (Vs8: the surface barrier before illumination; Vs*: the surface barrier after illumination).
Fig. 3. The SPS responses of ZnO nanoparticles before the photocatalytic reaction and after the deactivation.
3.2. Deactivation mechanism
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Fig. 4. The EFISPS responses of ZnO nanoparticles before the photocatalytic reaction (A) and after the deactivation (B).
characteristics of an N-type semiconductor. In Fig. 4(B), however, the higher the positive external electric field, the stronger the SPS response, while the higher the negative external electric field, the weaker the SPS response, and the SPS response was even reversed. These are common characteristics of a P-type semiconductor. Therefore, the results of EFISPS were in good agreement with that of SPS. Moreover, the SPS response range was broadened under the external electric field, due to the electron transitions related to the surface states [19–21]. Fig. 5 shows XPS spectra of O1s on the surface of ZnO nanoparticles before the photocatalytic reaction and after the deactivation. The XPS peaks were both asymmetric (the right side was wider than the left), indicating that at least two kinds of oxygen species were present at the near-surface region. The peak at about 530 eV was due to crystal lattice oxygen, while the peak at about 532 eV was attributed to adsorbed oxygen [10]. It could be seen that O1s XPS peak after the deactivation shifted to higher binding energy as compared with that before the photocatalytic reaction, indicating that the relative content of adsorbed oxygen after the deactivation was larger than that before the photocatalytic onreaction. The adsorbed oxygen after the deactivation mainly resulted from the photocatalytic products such as H2O and CO2 and O2 molecule. ZnO easily adsorbed the products like H2O and CO2 to possibly produce zinc carbonate or zinc carbonate hydroxide in the light of its chemical properties [23]. Thus, a firm adsorption layer could
gradually be formed on ZnO nanoparticle surfaces as the photocatalytic reaction was continuously carried out. Thus, the surface charge carrier transportation and pollutant adsorption in advance must greatly be influenced due to the firm adsorption layer. Moreover, the surfaces of ZnO nanoparticles could carry some positive charges after adsorbing a certain amount of H2O, since the isoelectric point of ZnO was about pH = 9 [24]. Thus, the carried positive charges could greatly hold back the transportation of photo-induced holes to the surface. Therefore, the firm adsorption layer and the carried positive charge would deactivate ZnO nanoparticles and made a surface conduction change. In addition, in the photocatalytic oxidation of gas phase n-C7H16, we reported the effects of partial pressure of added steam on the activity of ZnO nanoparticle photocatalyst [16]. The results showed that too much steam retarded the photocatalytic reaction. This could be explained by the firm adsorption layer and carried positive charges mentioned above. The appropriate amount of steam could promote the photocatalytic reaction, which could be attributed to adsorbed H2O as the capturer of the photo-induced holes to produce the strong oxidant, hydroxyl radical group,OH. Compared with ZnO, TiO2 did not easily adsorb the products like H2O and CO2 to produce titanium carbonate or titanium carbonate hydroxide [23]. Thus, the firm adsorption layer could not gradually be formed on TiO2 surfaces as the photocatalytic reaction was continuously carried out. Moreover, TiO2 nanoparticle surfaces could carry some negative charges after adsorbing a certain amount of H2O, since the isoelectric point of TiO2 was about pH = 6 [25]. Thus, the carried negative charges could promote the transportation of photo-induced holes to the surface. Therefore, the above factors made for the persistence of photocatalytic activity of TiO2 nanoparticles. 3.2.2. In the photocatalytic oxidation of SO2 The lifetime measurements indicated that ZnO and TiO2 nanoparticles both could be deactivated in the photocatalytic oxidation of SO2. Fig. 6 shows the SPS responses of TiO2
Fig. 5. XPS spectra of O1s on the surface of ZnO nanoparticles before the photocatalytic reaction and after the deactivation.
J. Liqiang et al. / Applied Catalysis A: General 275 (2004) 49–54
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nearly be regenerated to a full degree by ultrasonically washing with deionized water and drying at 70 8C for 24 h after centrifugation. Moreover, this also demonstrated that the deactivation of the investigated photocatalysts should be mainly attributed to the adsorption of the oxidation products such as H2O, CO2 and SO3.
4. Conclusions
Fig. 6. The SPS responses of TiO2 nanoparticles before the photocatalytic reaction and after the deactivation.
nanoparticles before the photocatalytic reaction and after the ‘deactivation. The peak response was at about 360 nm, with its onset at 380 nm, which arose from the electron transition from O2p to Ti3d, corresponding to the valence band to conduction band transition according to the energy band structure of TiO2 [13,26]. In addition, the direction of SPS response of TiO2 nanoparticles after the deactivation was opposite to that before the photocatalytic reaction, indicating that semiconductor surface conduction type of TiO2 nanoparticles had changed from N-type before the photocatalytic reaction to P-type after the deactivation. This was responsible for the deactivation of TiO2 nanoparticles [22]. Fig. 7 shows the XPS spectrum of S2p on the surface of TiO2 nanoparticles after the deactivation, indicating that S element mainly existed as the form of +6 chemical state according to the principle of the XPS method. SO3 was the only oxidization product of SO2. Thus, we conclude that the change of surface conduction type, and even deactivation, mainly resulted from the adsorption of SO3, which is in agreement with the literature [22]. 3.3. Activity regeneration In the experiment of gas phase photocatalytic oxidation of n-C7H16 (or SO2), the photocatalytic activity of deactivated ZnO or TiO2 nanoparticle photocatalysts could
In this paper, we reported that ZnO could be deactivated in the gas phase photocatalytic oxidation of n-C7H16, while TiO2 could nearly retain its activity. In the gas phase photocatalytic oxidation of SO2, ZnO and TiO2 could both be deactivated. The deactivation mainly resulted from a semiconductor surface conduction change from N-type before the photocatalytic reaction to P-type after the deactivation due to semiconductor photocatalyst surface adsorbates studied by means of SPS and XPS testing techniques as well as semiconductor chemical properties. Moreover, the activity of deactivated photocatalysts could be regenerated to a nearly full extent by washing and drying. This paper suggested that the photocatalytic performance of TiO2 is superior to that of ZnO in the light of the activity and lifetime, and that the techniques of photocatalytic oxidation on semiconductors have promising prospects in the degradation of atmosphere pollutants from the points of activity and regeneration.
Acknowledgements This work was supported by the National Nature Science Foundation of China (No. 20171016, 20301006), the Nature Science Foundation of Heilongjiang Province of China (No. E00-16, B0305), the Science Foundation for Excellent Youth of Heilongjiang Province of China (2002), the supporting plan of Education Bureau of Heilongjiang Province (1054G035), and the Science Foundation for Excellent Youth of Heilongjiang University of China (2003).
References
Fig. 7. XPS spectrum of S2p on the surface of TiO2 nanoparticles after the deactivation.
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Deactivation and regeneration of ZnO and TiO2 nanoparticles in the gas phase photocatalytic oxidation of n-C7H16 or SO2 Jing Liqianga,b,1, Xin Baifua, Yuan Fulonga, Wang Baiqib, Shi Keyinga, Cai Weiminb, Fu Hongganga,* a
School of Chemistry and Material Sciences,The Key Laboratory of Physical Chemistry, Heilongjiang University, Harbin 150080, China b Department of Environmental Sciences and Engineering, Harbin Institute of Technology, Harbin 150001, China Received 27 December 2003; received in revised form 7 July 2004; accepted 21 July 2004 Available online 2 September 2004
Abstract In this paper, we examined the lifetimes of made-in-home ZnO and TiO2 nanoparticles in the gas phase photocatalytic oxidation of nC7H16 or SO2, and especially investigated the deactivation mechanism by utilizing surface photovoltage spectrum (SPS) and X-Ray photoelectron spectroscopy (XPS) testing techniques and by considering semiconductor chemical properties. The results showed that ZnO could almost be deactivated in the gas phase photocatalytic oxidation of n-C7H16, while TiO2 could keep most of its activity. In the gas phase photocatalytic oxidation of n-C7H16 or SO2, ZnO and TiO2 both could almost be deactivated. The deactivation mainly resulted from semiconductor surface conduction type change from N-type before the photocatalytic reaction to P-type after the deactivation because of the adsorption of the oxidation products such as H2O, CO2 and SO3 on the semiconductor photocatalyst surface. In addition, the activity of the deactivated photocatalyst could be regenerated to a nearly full extent by washing and drying. # 2004 Elsevier B.V. All rights reserved. Keywords: ZnO; TiO2; Deactivation and regeneration; Photocatalytic oxidation; n-C7H16; SO2
1. Introduction In recent years, semiconductor photocatalysis is becoming more and more attractive and important since it has a great potential to contribute to environmental problems extensively [1–5]. One of the most important aspects of environmental photocatalysis is the selection of materials such as ZnO and TiO2, which are close to being two kinds of ideal photocatalysts in several respects. For example, they are relatively inexpensive, and they provide photo-generated holes with high oxidizing power due to their wide band gap energy [6,7]. Kormann compared the photocatalytic activity of ZnO, Fe2O3 and TiO2, indicating that ZnO and TiO2 exhibited higher activity than Fe2O3 during the process of photocatalytic oxidation of gas phase chloridized hydrocarbon [8]. Heptane (n-C7H16) and sulfur dioxide (SO2) are * Corresponding author. Tel.: +86 451 8660; fax: +86 451 86673647. 1 E-mail address: [email protected] ( j. Liqiang). 0926-860X/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.07.019
two most important organic and inorganic contaminants in the atmosphere, respectively. Moreover, the homogeneous photoreactions between the two compounds can result in a photochemical smog, a badly polluting phenomenon [9]. Therefore, to eliminate these pollutants in the atmosphere is very meaningful to us. Although, ZnO and TiO2 has been proved to be very active in the photocatalytic oxidation of different pollutants, generally higher in the gas phase than in the liquid phase [4,8], the lifetimes, deactivation and regeneration of semiconductor photocatalysts is also very crucial in environmental photocatalysis. Despite the great body of work about the photocatalysis on ZnO or TiO2 until now, few papers are devoted solely to the deactivation and regeneration of ZnO and TiO2 photocatalysts in the gas phase photocatalytic reactions, especially in the photocatalytic oxidation of hydrocarbon or SO2. In this paper, we examined the lifetimes of ZnO and TiO2 nanoparticles in the gas phase phototcatalytic oxidation of n-C7H16 or SO2, and investigated the de-
50
J. Liqiang et al. / Applied Catalysis A: General 275 (2004) 49–54
activation and regeneration. This paper should be of great significance in environmental photocatalysis.
2. Experimental 2.1. Preparation of ZnO and TiO2 nanoparticles ZnO nanoparticles were prepared by thermal decomposition of the precursor, zinc carbonate hydroxide, at 320 8C for 1 h [10,11]. The ZnO sample had a wurtzite structure attached to hexagonal system with the average size of 13 nm; TiO2 nanoparticles were fabricated by the sol-gel procedure. Finally, TiO2 nanoparticles were obtained by thermal treatment of TiO2 gel precursor at 600 8C for 2 h. The TiO2 sample contained about 80% anatase and 20% rutile with the average size of 25 nm [12,13].
reactor to a little higher than one atmospheric pressure. Thus, the reactants were n-C7H16 (or SO2) (4%, balance nitrogen) and oxygen (21%, balance nitrogen). When adsorption reached equilibrium, as measured by gas chromatography (GC), the reaction was started when we turned on the lamp. Subsequently, the concentration of n-C7H16 (or SO2) in the reactor, obtained by a gastight syringe from the sample port, was measured every 30 min by a Varian 3700 GCI equipped with a flame ionization defector and a SE-54 quartz capillary column at 60 8C (or a GC II equipped with a thermal conductivity detector and a GDX-102 stainless steel column at 100 8C). The reaction was ended by our turning off the lamp [17–19]. Three hours was considered as a reactive period during the lifetime measurement process. The next reactive period would begin after the earlier reactive period was finished. The above process was repeated until the concentration of n-C7H16 (or SO2) did not change. The total of reactive times was referred to as lifetime.
2.2. Characterization of deactivated ZnO and TiO2 nanoparticles 3. Results and discussion Surface photovoltage spectroscopy (SPS) of the samples was carried out with a locally built apparatus that has been described elsewhere [13–16]. SPS is a very effective way to study the charge separation and transfer behavior at an interface. The generation of photovoltage arises from the creation of electron-hole pairs, followed by the separation under a built-in electric field (space-charge layer). The difference between the surface potential barrier in the light and that in the dark is the SPS signal. Electric-field-induced surface photovoltage spectroscopy (EFISPS) is a technique that combines the field-effect principle with SPS. The external electric field is applied to two sides of the samples and is regarded as positive when its direction is identical to that of the incident light. The surface compositions and chemical states of the samples were examined with a VG ESCALAB MKII X-ray photoelectron spectrometer (XPS) using a monochromatic aluminum X-ray source. The pressure was maintained at 6.3 10 5 Pa. The binding energies were calibrated with respect to the signal for adventitious carbon (binding energy = 284.6 eV).
3.1. Photocatalyst lifetime Fig. 1 showed the photocatalytic oxidation curves of nC7H16 (A) and SO2 (B) on the ZnO or TiO2 nanoparticle photocatalysts. It could be seen that the photocatalytic
2.3. Lifetime measurement of ZnO and TiO2 nanoparticle photocatalysts The light source used in this experiment was a 400W high pressure mercury lamp (lmax = 365 nm). The UV light was transmitted to the specimen through a quartz tube placed between the UV lamp and the photoreactor to absorb the heat and transmit UV light. The reactor used was a 300 ml cylindrical quartz tube. Photocatalyst (0.1 g) was spread uniformly over the internal surface of the reactor as a thin layer. After this, the reactor was vacuum-packed and 1.2 ml of n-C7H16 (or SO2) and 63 ml of oxygen were injected into the reactor through a sample port. Then the ultrapure nitrogen (99.9999%) was mixed with the reactant in the
Fig. 1. The photocatalytic oxidation curves of n-C7H16 (A) and SO2 (B) on ZnO or TiO2 nanoparticle photocatalysts.
J. Liqiang et al. / Applied Catalysis A: General 275 (2004) 49–54
activity of TiO2 was higher than that of ZnO for the oxidizing gas phase of either n-C7H16 or SO2, which was in good agreement with the literature results [4,7]. Photocatalytic reactions mainly take place on the surface of the photocatalyst. Therefore, the adsorption of pollutants in advance is one of the most important steps in photocatalytic reactions [2–5]. If the products of the photocatalytic reactions firmly adsorb on the surface of the photocatalyst, the surface charge carrier transportation and pollutant adsorption can be influenced greatly so that the photocatalysts can degrade the activity or even be deactivated. In the experiment of gas phase photocatalytic oxidation of n-C7H16, ZnO nanoparticles nearly lost all activity after the photocatalytic reaction was continuously carried out for 6 reactive periods (18 h), while the activity of TiO2 nanoparticles did not change much after the photocatalytic reaction was continuously carried out for 10 reactive periods (30 h). In the experiment of gas phase photocatalytic oxidation of SO2, ZnO and TiO2 nanoparticles both lost nearly all activity after the photocatalytic reaction was continuously carried out for five reactive periods (15 h).
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3.2.1. In the photocatalytic oxidation of n-C7H16 The lifetime measurements indicated that ZnO nanoparticles could be deactivated in the photocatalytic oxidation of n-C7H16, while TiO2 nanoparticles could retain most of their activity. Fig. 2 shows the transfer of photo-induced charge carriers on the semiconductor surface or interface. In the case of an N-type semiconductor, the built-in electric field (SCR) with the direction from the interior to the outer drives photogenerated holes toward the surface or interfacial region and electrons toward the interior of the bulk. But the reverse process can take place at a P-type semiconductor, namely, the built-in electric field with the direction from the outer to the interior drives photo-generated electrons toward the surface or interfacial region and holes toward the interior of
the bulk [16,20]. Thus, we can utilize the oxidizing ability of photo-generated holes on an N-type semiconductor or the reducing ability of photo-generated electrons on a P-type semiconductor. Fig. 3 shows the SPS responses of ZnO nanoparticles before the photocatalytic reaction and after the deactivation. The peak response was at about 360 nm, with its onset at 380 nm. This could be assigned to the electron transition from O2p to Zn3d, corresponding to transition from the valence band to the conduction band according to the energy band structure of ZnO [10,21]. In addition, the direction of SPS response of ZnO nanoparticles after the deactivation was the reverse of that before the photocatalytic reaction, indicating that semiconductor surface conduction type of ZnO nanoparticles had changed from N-type before the phtotocatalytic reaction to P-type after the deactivation [22]. This must result in the deactivation of ZnO nanoparticle in that the photo-induced holes on the surface of N-type semiconductor were mainly utilized to oxidize the pollutants in the photocatalytic reactions, while the photo-induced electrons on the surface of P-type semiconductor were mainly utilized to reduce O2 or other substances. If a positive external electric field is applied to the two sides of a P-type semiconductor, the surface barrier can increase because the external electric field and the built-in electric field (SCR) act in the same direction, which can result in the enhancement of the SPS signal. On the contrary, if a positive external electric field is applied to the two sides of an N-type semiconductor, the surface barrier can decrease because the external electric field and the built-in electric field (SCR) act in the opposite directions, which can result in the decline of the SPS signal [20,21]. Fig. 4 shows the EFISPS responses of ZnO nanoparticles before the photocatalytic reaction (A) and after the deactivation (B). Remarkable changes of SPS response of ZnO nanoparticles could be found if an external electric field is applied. In Fig. 4(A), the higher the positive external electric field, the weaker the SPS response, and the SPS response was even reversed. However, the higher the negative external electric field, the stronger the SPS response. These are common
Fig. 2. The transfer of photo-induced charge carrier on the semiconductor surface or interface (Vs8: the surface barrier before illumination; Vs*: the surface barrier after illumination).
Fig. 3. The SPS responses of ZnO nanoparticles before the photocatalytic reaction and after the deactivation.
3.2. Deactivation mechanism
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Fig. 4. The EFISPS responses of ZnO nanoparticles before the photocatalytic reaction (A) and after the deactivation (B).
characteristics of an N-type semiconductor. In Fig. 4(B), however, the higher the positive external electric field, the stronger the SPS response, while the higher the negative external electric field, the weaker the SPS response, and the SPS response was even reversed. These are common characteristics of a P-type semiconductor. Therefore, the results of EFISPS were in good agreement with that of SPS. Moreover, the SPS response range was broadened under the external electric field, due to the electron transitions related to the surface states [19–21]. Fig. 5 shows XPS spectra of O1s on the surface of ZnO nanoparticles before the photocatalytic reaction and after the deactivation. The XPS peaks were both asymmetric (the right side was wider than the left), indicating that at least two kinds of oxygen species were present at the near-surface region. The peak at about 530 eV was due to crystal lattice oxygen, while the peak at about 532 eV was attributed to adsorbed oxygen [10]. It could be seen that O1s XPS peak after the deactivation shifted to higher binding energy as compared with that before the photocatalytic reaction, indicating that the relative content of adsorbed oxygen after the deactivation was larger than that before the photocatalytic onreaction. The adsorbed oxygen after the deactivation mainly resulted from the photocatalytic products such as H2O and CO2 and O2 molecule. ZnO easily adsorbed the products like H2O and CO2 to possibly produce zinc carbonate or zinc carbonate hydroxide in the light of its chemical properties [23]. Thus, a firm adsorption layer could
gradually be formed on ZnO nanoparticle surfaces as the photocatalytic reaction was continuously carried out. Thus, the surface charge carrier transportation and pollutant adsorption in advance must greatly be influenced due to the firm adsorption layer. Moreover, the surfaces of ZnO nanoparticles could carry some positive charges after adsorbing a certain amount of H2O, since the isoelectric point of ZnO was about pH = 9 [24]. Thus, the carried positive charges could greatly hold back the transportation of photo-induced holes to the surface. Therefore, the firm adsorption layer and the carried positive charge would deactivate ZnO nanoparticles and made a surface conduction change. In addition, in the photocatalytic oxidation of gas phase n-C7H16, we reported the effects of partial pressure of added steam on the activity of ZnO nanoparticle photocatalyst [16]. The results showed that too much steam retarded the photocatalytic reaction. This could be explained by the firm adsorption layer and carried positive charges mentioned above. The appropriate amount of steam could promote the photocatalytic reaction, which could be attributed to adsorbed H2O as the capturer of the photo-induced holes to produce the strong oxidant, hydroxyl radical group,OH. Compared with ZnO, TiO2 did not easily adsorb the products like H2O and CO2 to produce titanium carbonate or titanium carbonate hydroxide [23]. Thus, the firm adsorption layer could not gradually be formed on TiO2 surfaces as the photocatalytic reaction was continuously carried out. Moreover, TiO2 nanoparticle surfaces could carry some negative charges after adsorbing a certain amount of H2O, since the isoelectric point of TiO2 was about pH = 6 [25]. Thus, the carried negative charges could promote the transportation of photo-induced holes to the surface. Therefore, the above factors made for the persistence of photocatalytic activity of TiO2 nanoparticles. 3.2.2. In the photocatalytic oxidation of SO2 The lifetime measurements indicated that ZnO and TiO2 nanoparticles both could be deactivated in the photocatalytic oxidation of SO2. Fig. 6 shows the SPS responses of TiO2
Fig. 5. XPS spectra of O1s on the surface of ZnO nanoparticles before the photocatalytic reaction and after the deactivation.
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nearly be regenerated to a full degree by ultrasonically washing with deionized water and drying at 70 8C for 24 h after centrifugation. Moreover, this also demonstrated that the deactivation of the investigated photocatalysts should be mainly attributed to the adsorption of the oxidation products such as H2O, CO2 and SO3.
4. Conclusions
Fig. 6. The SPS responses of TiO2 nanoparticles before the photocatalytic reaction and after the deactivation.
nanoparticles before the photocatalytic reaction and after the ‘deactivation. The peak response was at about 360 nm, with its onset at 380 nm, which arose from the electron transition from O2p to Ti3d, corresponding to the valence band to conduction band transition according to the energy band structure of TiO2 [13,26]. In addition, the direction of SPS response of TiO2 nanoparticles after the deactivation was opposite to that before the photocatalytic reaction, indicating that semiconductor surface conduction type of TiO2 nanoparticles had changed from N-type before the photocatalytic reaction to P-type after the deactivation. This was responsible for the deactivation of TiO2 nanoparticles [22]. Fig. 7 shows the XPS spectrum of S2p on the surface of TiO2 nanoparticles after the deactivation, indicating that S element mainly existed as the form of +6 chemical state according to the principle of the XPS method. SO3 was the only oxidization product of SO2. Thus, we conclude that the change of surface conduction type, and even deactivation, mainly resulted from the adsorption of SO3, which is in agreement with the literature [22]. 3.3. Activity regeneration In the experiment of gas phase photocatalytic oxidation of n-C7H16 (or SO2), the photocatalytic activity of deactivated ZnO or TiO2 nanoparticle photocatalysts could
In this paper, we reported that ZnO could be deactivated in the gas phase photocatalytic oxidation of n-C7H16, while TiO2 could nearly retain its activity. In the gas phase photocatalytic oxidation of SO2, ZnO and TiO2 could both be deactivated. The deactivation mainly resulted from a semiconductor surface conduction change from N-type before the photocatalytic reaction to P-type after the deactivation due to semiconductor photocatalyst surface adsorbates studied by means of SPS and XPS testing techniques as well as semiconductor chemical properties. Moreover, the activity of deactivated photocatalysts could be regenerated to a nearly full extent by washing and drying. This paper suggested that the photocatalytic performance of TiO2 is superior to that of ZnO in the light of the activity and lifetime, and that the techniques of photocatalytic oxidation on semiconductors have promising prospects in the degradation of atmosphere pollutants from the points of activity and regeneration.
Acknowledgements This work was supported by the National Nature Science Foundation of China (No. 20171016, 20301006), the Nature Science Foundation of Heilongjiang Province of China (No. E00-16, B0305), the Science Foundation for Excellent Youth of Heilongjiang Province of China (2002), the supporting plan of Education Bureau of Heilongjiang Province (1054G035), and the Science Foundation for Excellent Youth of Heilongjiang University of China (2003).
References
Fig. 7. XPS spectrum of S2p on the surface of TiO2 nanoparticles after the deactivation.
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