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挥发性有机化合物(volatile organic compounds,VOC)是室内最常见的环境污染物之一,而甲醛(HCHO)是其中一种典型污染物[1]。当人体处在甲醛质量浓度大于0.1 mg·m-3环境中时,即可能引发呼吸系统受损,而长期暴露会对人体其他器官造成损害[2]。甲醛已被列为第一类致癌物[3-4]。目前,防治室内甲醛污染的方法有物理法、生物法和化学法,其中物理法包括自然通风和吸附法等,生物法包括植物生态净化和微生物降解等,化学法包括等离子体法、催化氧化技术等[5-7]。物理法成本低、效率高,但不能彻底解决甲醛污染问题,亦可能造成二次污染[8];生物法绿色环保,处理成本低,无二次污染,但存在受环境温/湿度、菌种选择等条件限制,效率低等问题[9];等离子体技术虽受环境影响小,但需施加外部条件,耗能高,安全性差[10-11];催化氧化技术已被证明是一种行之有效的甲醛去除方法,其具体包括热催化、光催化、常温催化、臭氧净化等[12-14]。其中,常温催化氧化技术具有操作简单、耗能低、无二次污染、净化效率高等优点而备受广泛关注,其在实现去除室内低浓度气态甲醛方面有着重要应用价值。
常温催化氧化催化剂可分为两大类:一是贵金属催化剂;二是过渡金属氧化物催化剂。贵金属如Pt、Pd、Au和Ag等,通常以TiO2、Al2O3、SiO2、Co3O4、CeO2等高比表面积为载体制备而成,其活性高、降解性能优越[15-17],但是贵金属资源稀缺、价格昂贵,限制了其广泛范围。过渡金属氧化物因资源丰富、氧化活性高、稳定性良好等优势而具备替代贵金属的潜力,且过渡金属锰氧化物(MnOx)在降解甲醛性能方面表现优异,氧化能力强、CO2选择性高,且晶型、价态、形貌可调,应用前景广泛[18-23]。LU等[24]通过水热法制备了石墨烯-MnO2复合纳米材料,相较于单金属催化剂MnO2,复合催化剂完全催化甲醛所需温度降低了75 ℃,催化性能提高,并且石墨烯增强了Mn3+和Mn4+间的电荷转移,其催化剂表面产生了丰富的氢氧自由基,提升了甲醛催化氧化中间产物-甲酸盐的降解活性。ZHU等[25]通过氧化还原法及铈改性制备了掺杂型MnO2,研究发现随着Ce掺杂量的增加,MnO2结晶度降低,催化剂比表面积增大。当Ce-MnO2中铈锰摩尔比达1∶10时,催化剂常温催化性能稳定,且在100 ℃下可将低浓度甲醛完全氧化为CO2和H2O,这是由于Ce掺杂的MnO2具有更多的表面吸附氧,减小了CeO2纳米团簇与MnO2间的晶界间距,提升了表面氧和晶格氧的氧化还原能力和迁移速率。LI等[26]利用Pechini法制备了MnxCe1-xO2 (x=0.3~0.9) 催化剂,其中Mn0.5Ce0.5O2催化剂活性最佳,且随着质量分数为5% CuOx的掺入,催化剂可进一步降低甲醛100%转化时所需的反应温度。氧化铜的引入增强了氧化物间的协同作用,提高了材料分散性和氧化能力,促进催化材料中氧的释放,进而优化了催化氧化甲醛性能。
如今催化氧化技术已取得长足进展,但探求高效稳定、抗水蒸气能力强、低成本、常温条件去除低浓度甲醛催化剂仍面临挑战。类水滑石化合物(LDHs)是近年来发展迅速的阴离子型粘土材料[27],其通式表示为[M2+1-xM3+x(OH)2]x+[An-x/n]·mH2O,结构如图1所示。其中,M2+是二价金属阳离子,M3+是三价金属阳离子,An-是层间阴离子,通常为无机阴离子Cl-、CO32-等。因其具有耐酸碱性、热稳定性、吸附性、层间阴离子可交换性及高比表面积等特点,在吸附、催化、阻燃、医药等领域已被广泛应用[28-29]。为此本研究提出采用共沉淀法制备了MnOx/类水滑石催化剂,以类水滑石为载体、片状氧化锰为活性组分,探究载体各组分、煅烧温度、氧化锰负载量等对催化剂常温催化氧化性能的影响,运用XRD、SEM、BET、EPR、XPS等技术表征分析探究催化剂微观结构,考察并分析其常温催化氧化甲醛活性和稳定性,以克服锰基氧化物常温条件下因大量吸附水蒸气而稳定性降低的缺点,研制一种高效稳定的过渡金属氧化物催化剂,为常温催化氧化催化剂的研制提供参考。
高表面氧MnOx/类水滑石催化剂常温催化低浓度甲醛的性能
Performance of high surface oxygen MnOx/ hydrotalcite-like catalyst for the catalysis of low concentration formaldehyde at ambient temperature
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摘要: 常温催化氧化技术是去除室内低浓度甲醛(HCHO)的一种有效方法,其中过渡金属氧化物锰基催化剂在催化降解方面性能优异,相较于贵金属催化剂,具有成本低、活性高、稳定性良好、应用前景广等优点。采用共沉淀法制备了类水滑石载体和锰基/类水滑石催化剂,将类水滑石与锰基氧化物相结合用于常温催化氧化,克服了锰基氧化物常温条件下因大量吸附水蒸气而导致稳定性降低的缺点,同时研究考察了载体组成、煅烧温度、氧化锰负载量等对催化剂常温催化氧化性能的影响,并通过XRD、SEM、BET、EPR、XPS等技术对催化剂进行了微观表征。结果表明,当类水滑石中n(Mg)∶n(Al)=4∶1,氧化锰负载量 (质量分数) 为40%,煅烧温度为200 ℃时,所述MnOx/Mg4Al1催化剂性能最佳,其48 h甲醛去除率高达98.5%,甲醛质量浓度由1.000 mg·m-3降至0.015 mg·m−3。类水滑石载体表面存有大量羟基基团,有利于吸附低浓度甲醛及中间产物进而利于催化剂催化氧化性能提升。通过XPS分析可知,相较于MnOx催化剂,MnOx/Mg4Al1催化剂中Oβ/(Oα+Oβ)相对含量比例由35.7%提升至88.0%,表面氧含量(Oβ)大幅提升,其在甲醛常温催化氧化过程中发挥着极其重要作用。本研究结果可为常温催化氧化甲醛提供参考。Abstract: Catalytic oxidation at ambient temperature is an effective method for the removal of low-concentrated formaldehyde (HCHO). The transition metal oxide manganese based catalyst has excellent catalytic degradation performance. Compared with the precious metal catalyst, it has the advantages of low cost, high activity, good stability and wide application prospect. In this study, the hydrotalcite-like and manganese oxides/ hydrotalcite-like catalysts were synthesized by the co-precipitation method. The combination of hydrotalcite-like and manganese oxides for catalytic oxidation of HCHO at ambient temperature overcomed the disadvantage of stability reduction caused by large amount of water vapor adsorption of mangan-based oxides at room temperature. Meanwhile, the effects of carrier composition, calcination temperature and manganese oxides supporting, etc. on the catalytic oxidation at ambient temperature were investigated and characterized by XRD, SEM, BET, EPR and XPS. The results demonstrated that the highest performance of the catalyst was MnOx/Mg4Al1 with n(Mg)∶n(Al)=4∶1, 40 % manganese oxides supporting and calcining at 200 °C. The removal rate of HCHO was up to 98.5% within 48 h, and the concentration was decreased from 1.000 mg·m-3 to 0.015 mg·m-3. The presence of a large number of hydroxyl groups on the surface of the hydrotalcite-like was beneficial to the adsorption of low concentration of HCHO and intermediate by-products, which also improved the oxidation activity of the catalysts. According to XPS analysis, compared to the MnOx catalyst, the Oβ/(Oα+Oβ) ratio of the MnOx/Mg4Al1 was increased from 35.7% to 88.0% and the surface oxygen content (Oβ) also enhanced significantly. The results suggested that the high surface oxygen content played an important role in the catalytic oxidation of HCHO at ambient temperature. This study can provide a reference for the reduction for the catalytic oxidation of HCHO at ambient temperature.
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表 1 催化剂的比表面积、孔体积和平均孔径
Table 1. Specific surface area, pore volume and average pore diameter of catalysts
样品 比表面积(BET)/(m2·g−1) 孔体积/(cm3·g−1) 平均孔径/nm MnOx-200 22.6 0.222 28.4 Mg4Al1-200 58.2 0.109 5.6 MnOx/Mg4Al1-100 62.0 0.231 13.0 MnOx/Mg4Al1-200 78.8 0.437 14.2 MnOx/Mg4Al1-300 81.4 0.497 19.1 MnOx/Mg4Al1-500 66.3 0.566 16.4 MnOx/Mg1Al1-300 79.0 0.187 6.5 MnOx/Mg1Al4-300 156.7 0.438 8.7 -
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