1. Laboratory of Environmental Science and Technology, Xinjiang Technical Institute of Physics & Chemistry, Chinese Academy of Sciences, Urumqi 830011, Xinjiang, P. R. China;
2. University of Chinese Academy of Sciences, Beijing 100049, P. R. China
*Corresponding author: WANG Chuanyi, E-mail: cywang@ms.xjb.ac.cn
Abstract: Nanosized ZnO particles have been synthesized by sol-gel method and studied for photocatalytic activities toward gaseous CH2O degradation. It was found that CH2O adsorbed on ZnO nanoparticles can be oxidized to CO2 under UV light irradiation with the coexistence of either O2 or H2O. However, different from aqueous condition where H2O molecules are first oxidized to be free radicals then react with CH2O, direct photoactivation of O2 becomes the dominant step in promoting photocatalytic degradation efficiency of gaseous CH2O. The results reveal a new reaction mechanism that can be important for further modification of photocatalyst for gaseous CH2O degradation.
Key words: ZnOphotocatalytic degradation of CH2Ophotoactivation of O2
甲醛气体在ZnO表面光降解的机理研究
李玲1,2, 杜云舒1,2, 王璇1,2, 韩建1,2, 王传义1
1. 中国科学院 新疆理化技术研究所 环境科学与技术研究室, 新疆 乌鲁木齐 830011;
2. 中国科学院大学, 北京 100049
2017-04-08 收稿, 2017-05-03 录用
*通讯作者: 王传义, E-mail: cywang@ms.xjb.ac.cn
摘要: 本文采用溶胶-凝胶法制备纳米氧化锌粒子，并对其降解气相甲醛的光催化活性进行研究。研究发现，在紫外光照射下，当水或氧气存在时，氧化锌可以将甲醛氧化生成二氧化碳；但是在两种不同气氛条件下的反应过程是不同的。气相水参与反应的过程中，首先是水分子被光活化生成羟基自由基，然后再与甲醛进行反应；而在氧气参与反应的过程中，是光活化的氧直接与甲醛反应，这一步骤在氧气参与的甲醛光催化降解反应中占据主导地位。这一反应机理的发现，对研究甲醛的光催化降解可能具有重要意义。
关键词: ZnO光催化甲醛光活化氧
1 IntroductionIn the recent decades, environmental problem has become one of the focusing issues in people's daily life. As a typical indoor pollutant, CH2O has attracted increasing attentions than others due to its special harmfulness to human health and slow release rate from wood and wood composites[1-3]. Effective removal of gaseous CH2O is of significance for either environment protection or scientific interest. Much efforts have been paid to remove gaseous CH2O including catalytic oxidation at room temperature[4-7], adsorption techniques[8, 9] and photocatalytic oxidation[10, 11], etc. Among those methods, photocatalysis exhibits unique prospects due to its superiority in avoidance of secondary pollution, technical convenience and abundance of solar resources[12-16]. As one of important photocatalysts, ZnO has a similar band gap to TiO2, with high exciton binding energy and electron mobility[17-20]. Besides, ZnO has been widely used as catalyst in heterogeneous catalytic reactions such as methanol synthesis, and CH2O was proposed to be one of the important intermediates[21-23]. It means, on one hand, the ZnO shows good ability to adsorb CH2O (neither too strong nor too weak), on the other hand, CH2O on ZnO surface can be easily catalyzed. Large number of ZnO-based photocatalysts have been synthesized and intensively studied for CH2O degradation[10, 13, 24]. However, a systematic study on the the influence of environment on photocatalytic reactivity were rarely reported.
In this work, we synthesized ZnO nanoparticles by sol-gel method with the structure and morphology being characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). The photocatalytic activities toward gaseous CH2O degradation under UV irradiation have been studied with the coexistence of various amount of H2O and O2. The results revealed that photoactivation of O2 may be involved and dominates in the photo-oxidation of CH2O over ZnO nanoparticles while H2O has barely influence on the photocatalytic efficiency, which possibly due to the freezing of the mobility of ·OH free radicals.
2 ExperimentThe ZnO nanoparticles used in this work were synthesized by sol-gel method. Zn(Ac)2·2H2O (10.98 g, 50 mmol) was dissolved in 500 mL C2H5OH (99.7%) followed with stirring at 333 K for 30 min. H2C2O4·2H2O (12.55 g, 100 mmol) was dissolved in 200 mL C2H5OH, and then added to the warm solution of Zn(Ac)2·2H2O. The mixture solution was stirred for 2 h at 333 K, and dried at 353 K overnight to get dry xerogel. ZnO nanoparticles were then obtained after the dry xerogel was calcined at 673 K in atmosphere for 2 h. X-ray diffraction (XRD) characterization was carried out by a powder X-ray diffractometer (Bruker D8-ADVANCE, Germany) with a CuKa anode over scattering angle from 5° to 80°. The morphologies of ZnO nanoparticles were investigated by a field emission scanning electron microscope (SEM) (ZEISS SUPRA 55VP). The photocatalytic reactions were carried out with a homemade gas sorption analyzer(GSA) connected with a quartzphotoreactor (diameter: 60 mm, inner volume: 30 mL). Reaction gases and products were monitored by a quadrapole mass spectrometer. The GSA can work under either vacuum (better than 10-3 mbar) or near ambient pressure, which allows delicate comparison experiments and systematic studies. The pressure in the GSA system are measured by a Parani gauge (5.0×10-5 mbar to 1000 mbar) and capacitance gauge (0.1 mbar to 1000 mbar). To perform a photocatalytic characterization, about 50 mg ZnO nanoparticles mixing with water are painted homogeneously on the bottom of photoreactor and dried on a hot plate at 353 K before connecting to the GSA system. The ZnO sample was usually heated to 393 Kunder vacuum for 2 h followed with direct irradiation from a 300 W Xeon lamp for 1 h. The purpose of first step is to remove residual gas and organic contamination, and the second to remove the possible photosensitive contaminations mixed in the ZnO nanoparticles. After the pretreatment, reaction gases are introduced to the photoreactor (e.g. different ratio of H2O/CH2O or O2/CH2O mixtures). The photoreactor is then checked by mass spectrometer every 15 min. Usually, the reactor is checked for four times before UV light irradiation to either stabilize the system or check whether a black reaction occurs. The light irradiated on sample was filtered by a 350 nm low-pass optical filter and the light intensity was calibrated to be about 15 mW/cm2.
3 Results and DiscussionsFigure 1 presents the X-ray diffraction (XRD) patterns of ZnO nanoparticles as prepared. The diffraction patterns are corresponding to a wurtzite ZnO structure (JCPDS No. 36-1451). No other miscellaneous peak is observed indicating a high purity of wurtzite phase. Figure 2 exhibits the SEM image of the ZnO sample. The ZnO nanoparticles are in shape of sphere with a very narrow size distribution. An average diameter is estimated to be 30 nm.
图 1
Fig. 1
 | Fig.1 XRD patterns of ZnO nanoparticles |
图 2
Fig. 2
 | Fig.2 SEM images of ZnO nanoparticles with different magnification ratios |
Figure 3 presents the temperature programmed desorption (TPD) result of CH2O on ZnO nanoparticles. The molecularly desorption of CH2O has two maximum desorption rate centered at 340 K and 440 K. At higher temperature the adsorbates undergo decomposition process. At 562 K, decomposition products CO, CO2 and H2 were investigated and at 592 K the decomposition product are CO and H2. The TPD results indicate that the ZnO nanoparticles has good ability to adsorb CH2O, which is consistent with reported models.
图 3
Fig. 3
 | Fig.3 TPD result of CH2O adsorbed on ZnO nanoparticles at room temperature The heating rate was 20 K/min. Each desorption curve was indicated by either name or mass to charge ratio |
To compare with the photodegradation process occurs in solution, where ·OH free radicals were proposed to be a key intermediate to oxidize CH2O[25, 26], we have checked the influence of H2O pressure on the photodegradation over ZnO nanoparticles. In this work, as the product of CH2O photooxidation on ZnO, CO2 has been chosen to indicate the photocatalytic activities since the CH2O can be slightly polymerized. The photoreactor was put on a 323 K hot plate to avoid the influence of environment temperature. As shown in Figure 4, with the increasing partial pressure of H2O in H2O/CH2O mixture, formation of CO2 has a slight increase in the first 30 min, while the evolution rate of CO2 approaches to nearly a constant after 30 min. Indeed, because the H2O molecules may be adsorbed on the wall of photoreactor, UV light irradiation leads to desorption of H2O that can consequently exchange with background CO2 adsorption. The results reveal that H2O partial pressure has barely influence on the photocatalytic degradation rate of gaseous CH2O. This is reasonable since H2O molecular adsorbed on ZnO nanoparticle, even an ·OH free radical is produced under UV light irradiation, has lower mobility compared to aqueous condition. In addition, the same evolution rate of CO2 under different H2O partial pressure indicates that the gas phase H2O is not involved in the reaction as an adsorption state.
图 4
Fig. 4
 | Fig.4 The evolution of CO2 amount produced during photocatalytic oxidation of gaseous CH2O with coexistence of different amount of H2O The pressure of CH2O was 2 mbar with the equivalent gas (helium) of 100 mbar, the partial pressure of H2O was A: 5 mbar, B: 13 mbar, C: 20 mbar. All experiments were carried out at 323 K |
When the reaction gas was switched to O2/CH2O mixture, the situation became completely different. As shown in Figure 5, the amount of CO2 product under UV light irradiation was plotted versus time for different partial pressure of O2 in mixtures. The result of H2O/CH2O was also shown for comparison purpose. It can be clearly seen that adoption of O2 can effectively improve the photodegradation efficiency of gaseous CH2O and the degradation rate increases monotonously with the elevating partial pressure of O2. This indicates that the O2 was directly involved in the photodegradation of CH2O over ZnO nanoparticles. Note that black reaction has been checked before switching on the UV light and no increasing of CO2 product was observed. It can be concluded that this is surely not a thermal oxidation of CH2O either adsorbed on ZnO surface or in gas phase at room temperature. In our experimental condition, the activation of O2 is the rate-determining step in the charge-induced surface reactions since it dominates the production of CH2O oxidation. A further improvement on photodegradation of gaseous CH2O over ZnO-based photocatalysts would be either elevating O2 partial pressure or improving the adsorption ability of O2 with modifying the photocatalyst, such as mixing with metal nanoparticles.
图 5
Fig. 5
 | Fig.5 The evolution of CO2 amount produced during photocatalytic oxidation of gaseous CH2O with coexistence of different amount of O2 The pressure of CH2O was 2 mbar with the equivalent gas (helium) of 100 mbar. The partial pressure of O2 in was B: 10 mbar, C: 30 mbar, D: 60 mbar. Mixture with 20 mbar H2O was shown as curve A for comparison purpose All experiments were carried out at 323 K |
In conclusion, we have successfully synthesized ZnO nanoparticles by sol-gel method and studied their photocatalytic activities towards gaseous CH2O degradation. It was found that O2 can effectively improve the degradation rate, while during the freezing of the mobility of ·OH free radicals, the mixing of gaseous H2O has barely promotion effect. Indeed, the experimental condition perfectly matches the situation that trace amount of CH2O mixed in air can be degraded directly to other harmless product rather than adsorbed physically. The microscopic step of O2 photoactivation involved in photodegradation of gesous CH2O over ZnO-based photocatalysts will be studied in the forthcoming work.
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