现代化工

现代化工 ›› 2025, Vol. 45 ›› Issue (2) : 122-128. DOI: 10.16606/j.cnki.issn0253-4320.2025.02.023

科研与开发

δ-MnO₂在常温湿式空气氧化反应中的催化性能研究

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Catalytic performance of δ-MnO₂ in wet air oxidation reaction at room temperature

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Received		Published
2024-04-02		2025-02-20
Issue Date	Revised Date
2025-02-14	2024-04-02

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摘要

采用水热法合成了δ-MnO₂,考察了其作为常温湿式空气氧化反应的催化剂对模式污染物双酚A(BPA)的催化降解性能。结果表明,在一般环境的催化湿式空气氧化(AP-CWAO)系统中,50 mg/L的BPA在3 h内的去除率为52.0%,而在外电场辅助的催化湿式空气氧化(ECWAO)系统中,BPA在80 min内即可完全去除;在AP-CWAO中δ-MnO₂的表面催化机理主要是通过形成单线态氧(¹O₂)对BPA实施氧化降解,在ECWAO中主要通过活化O₂分子生成活性氧种[O^-/O₂^-]^*;BPA在不同反应系统中的降解途径不同,但降解中间体产物的毒性均明显降低;将δ-MnO₂作为ECWAO的催化剂应用于实际污泥滤液的处置,可使废水的COD去除率达到80.8%且循环稳定性良好。

Abstract

δ-MnO₂ is synthesized via a hydrothermal method,and its catalytic performance as a catalyst is evaluated in wet air oxidation degradation of bisphenol A at room condition.Results show that in the catalytic wet air oxidation system under general environmental conditions (AP-CWAO),the removal rate of BPA with a concentration of 50 mg·L^-1 reaches 52.0% in 3 h.In the catalytic wet air oxidation system assisted by an external electric field (ECWAO),the removal rate of BPA can reach 100% in 80 min.Mechanism analysis show that the catalytic oxidation degradation of BPA over δ-MnO₂ in AP-CWAO performs mainly through the formation of singlet oxygen (¹O₂),while in ECWAO,the catalytic oxidation degradation of BPA goes mainly through the formation of reactive oxygen species [O^-/O₂^-]^* by activating O₂ molecules.Although the degradation pathway for BPA is different in these two systems,the toxicity of all degraded intermediates becomes significantly lower than BPA.As the ECWAO system with δ-MnO₂ as the catalyst is applied to the degradation of actual sludge filtrates,the removal rate of COD reaches 80.8% and the catalyst presents a good catalytic stability.

Graphical abstract

关键词

δ-MnO₂ / 机理 / 降解 / 污泥滤液 / 催化湿式空气氧化

Key words

δ-MnO₂ / mechanism / degradation / sludge filtrate / catalytic wet air oxidation

Author summay

徐甜甜(1998-),女,硕士生,研究方向为催化湿式空气氧化技术开发与应用,2631745465@qq.com。

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postcode=null, companyName=null, departmentName=null, remark=School of Chemistry and Chemical Engineering, Hefei University of Technology, Hefei 230009, China), AuthorCompanyExt(id=1241353143931335661, tenantId=1226480274814496768, journalId=1226484258170171394, articleId=1240719926794612784, companyId=1241353143914558444, language=CN, country=null, province=null, city=null, postcode=null, companyName=null, departmentName=null, remark=合肥工业大学化学与化工学院,安徽合肥 230009)])])] 徐甜甜,武明明,胡召健,翟林峰. δ-MnO₂在常温湿式空气氧化反应中的催化性能研究[J]. 现代化工, 2025, 45(2): 122-128 DOI:10.16606/j.cnki.issn0253-4320.2025.02.023

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湿式空气氧化(WAO)是一种利用空气中的O₂对废水中的有机污染物实施氧化降解的高级氧化技术,该技术因氧化效率高且洁净稳定等特点而获得关注。然而WAO需要高温(150~350℃)、高压(2~10 MPa)条件来引发O₂对有机污染物的氧化,这不仅对反应设备提出了苛刻的要求,而且能耗也非常高^[1]。催化湿式空气氧化法(CWAO)可以显著降低反应的温度和压力,但仍然需要一定的温度(80~180℃)和压力(1~5 MPa)^[2-3]。因此,开发一般环境下的CWAO(AP-CWAO)系统或高活性的催化剂材料,进一步降低CWAO反应系统的能耗,使其在常温、常压条件对有机污染物实施高效降解一直是该领域的研究热点。

电助催化湿式空气氧化(ECWAO)是一种将微弱的外加正电场施加于催化剂表面来促进电子的传递和对O₂的活化,从而实现常温下水体中有机污染物有效降解的新系统^[4]。具有二维层状结构的δ-MnO₂因层间距离大、氧迁移率高且廉价易得等优点被广泛应用于氧气的催化活化反应^[5⇓-7]。笔者采用水热法合成了δ-MnO₂并考察其在AP-CWAO和ECWAO不同系统中的催化性能和对模式污染物BPA降解机理,为CWAO反应提供新的理论依据,并为实现常温条件下有机污染的高效降解提供新技术。

1 材料与方法

1.1 实验原料

双酚A(BPA)、硫酸锰(MnSO₄·H₂O)、苯醌(p-BQ)、5,5-二甲基-1-吡咯啉-N-氧化物(DMPO)、2,2,6,6-四甲基哌啶(TEMP),阿拉丁化学试剂有限公司生产;高锰酸钾(KMnO₄),上海振企化学试剂有限公司生产;叔丁醇(TBA),上海易恩化学技术有限公司生产;糠醇(FFA)、聚乙二醇-4000(PEG-4000)、无水硫酸钠(Na₂SO₄),国药集团化学试剂有限公司生产;压滤污泥滤液来自含水率为98%的市政污泥压滤滤液(COD=69 mg/L);去离子水为实验室自制二次蒸馏水,所有试剂均未经纯化处理。

1.2 δ-MnO₂粉末和电极材料的制备

将0.3 g KMnO₄和0.053 g MnSO₄·H₂O溶于60 mL的去离子水中(摩尔比为6∶1),搅拌均匀后转移至100 mL的高压反应釜中,160℃下反应12 h,冷却后抽滤、洗涤并于60℃烘箱中干燥12 h后得到δ-MnO₂粉末^[8]。

将20 mg δ-MnO₂粉末加入到10 mL质量分数为3%的PEG-4000的溶液中,超声30 min后得分散均匀的悬浮液。再将厚度为3 mm的石墨毡(4×2.5 cm)浸入上述悬浮液中,后取出并于105℃烘箱内烘干。重复上述浸渍-烘干的操作直至锰氧化物全部转移至石墨毡表面。最后,将负载锰氧化物的石墨毡置于马弗炉内350℃热处理2 h得石墨毡负载δ-MnO₂催化剂的电极材料。

1.3 污染物降解实验

AP-CWAO系统:常温、常压、磁力搅拌下,120 mL柱状玻璃容器内将20 mg δ-MnO₂粉末催化剂加入到60 mL 50 mg/L的BPA溶液中,反应过程中在溶液底部以0.1 L/min不断鼓入空气。

ECWAO系统:三电极体系中,120 mL柱状玻璃容器内,以石墨毡负载的δ-MnO₂为工作电极,铂丝为对电极,饱和甘汞电极为参比电极,50 mmol/L的Na₂SO₄溶液为电解质,60 mL 50 mg/L的BPA为模式污染物,以0.1 L/min从反应器底部不断鼓入空气,+1 V恒电压下驱动系统运行。

定时取样1 mL后用0.22 μm聚醚砜膜过滤。利用高效液相色谱仪(Waters,美国)检测反应物浓度,流动相为乙腈和水,V(乙腈)/V(水)=2/3。

1.4 分析方法

利用X射线衍射仪(XRD,Rigaku D/MAX2500VL/PC,日本)和透射电镜(TEM,Jeol JEM-2100F,日本)分析催化剂的晶体结构;利用扫描电子显微镜(SEM,Zeiss GeminiSEM 300,德国)观察材料的形貌;利用气体吸附分析仪(Kubo-1108,中国)分析材料的比表面积和孔隙结构;利用X射线光电子能谱仪(XPS,Thermo ESCALAB250,美国)对锰氧化物的组成和价态进行分析;利用电子自旋共振波谱仪(ESR,Bruker Magnettech ESR 5000,美国)检测活性氧;利用气相色谱-质谱联用仪(GC-MS,Thermo Trace 1300,美国)对降解过程中的中间产物进行分析。

2 结果与讨论

2.1 催化剂的表征

δ-MnO₂的XRD谱图如图1所示。从图1中可以看出,所制备催化剂材料在2θ为11.8、24、36.8、66.3°处出现明显的特征衍射峰,分别对应于δ-MnO₂的(001)、(002)、(110)、(020)晶面(PDF#80-1098)^[9],而无明显的其他晶相特征峰,表明实验条件下所制备的催化剂材料主要是具有单斜晶系结构的δ-MnO₂。

δ-MnO₂的SEM、TEM及其元素映射图如图2所示。从图2(a)和图2(b)中可以看出,δ-MnO₂主要呈现为二维纳米片状结构,这些褶皱状纳米片通过自组装形成了片层花式结构的形貌^[10]。δ-MnO₂这种独特的层状结构有助于增加材料的比表面积且易于反应物的吸附和传质反应^[5]。从图2(c)和图2(d)中可以看出,这2种元素在δ-MnO₂上分布均匀,也反映了催化剂材料具有较纯的单一晶相结构。

通过N₂吸附-脱附的方法对δ-MnO₂催化剂的比表面积和孔径分布进行分析,结果如图3所示。从图3中可以看出,该吸附-脱附等温线属于Ⅳ型且伴有H3型滞后环,表明制备的δ-MnO₂催化剂具有介孔结构^[6]。孔径分布结果表明,δ-MnO₂的孔径主要在2~13 nm范围,这种较小的介孔结构赋予催化剂材料高比表面积(82.44 m²/g),有利于催化活性位点均匀分散,并且可充分发挥催化剂孔道的限域效应和活性位点的活化作用,而催化剂表观具有的片层花式大孔结构则有利于反应物的高效传质或提供充足的储存空间,从而为其优异的催化降解性能提供了支撑^[11-12]。

2.2 BPA在CWAO体系中的降解效果分析

为了消除吸附对BPA降解的干扰,在降解实验前将催化剂置于无氧BPA水溶液中进行静态吸附12 h至饱和,其对BPA的去除率均小于8.2%。δ-MnO₂在不同常温CWAO体系中对BPA的降解效率对比图如图4(a)、表1所示。从图4(a)、表1中可以看出,在AP-CWAO中,δ-MnO₂催化下3 h时对50 mg/L的BPA和TOC的去除率分别为52.0%和45.0%;而在ECWAO系统中,80 min时BPA的去除率为100%,TOC的去除率达到98.6%。

通过对降解数据进行动力学拟合发现,CWAO过程中有机物降解遵循一级动力学。其动力学方程为:

(1)

(d [B P A]) / d t = - k' [B P A]

其中:[BPA]为反应过程中BPA的浓度;t为反应时间;k'为伪一级动力学反应速率常数。

从图4(b)中可以看出,相同温度下,δ-MnO₂在AP-CWAO和ECWAO系统中催化降解BPA的速率常数k'分别为0.006 3 min^-1和0.089 3 min^-1,表明电场的引入大大降低了反应的活化能,从而提高了反应速率。

2.3 降解机理分析

2.3.1 元素组成分析

通过XPS对反应前后δ-MnO₂催化剂的表面进行化学组成分析,结果如图5所示。从图5中可以看出,δ-MnO₂的Mn 2p光谱分别在642.4 eV和654.2 eV处分裂为Mn 2p_3/2和Mn 2p_1/2峰。在Mn 2p_3/2谱中,643.1、642.1 eV和640.8 eV的特征峰分别归属于Mn^Ⅳ、Mn^Ⅲ和Mn^Ⅱ^[13]。由图5(a)中还可以看出,不同体系中反应后的Mn^Ⅲ均明显地增多。有研究指出,低价态锰与MnO₂的氧化反应活性呈正相关^[14]。低价态锰的含量还可以通过计算Mn 3s的平均氧化态(AOS)进行描述^[15]:

(2)

A O S = 8.956 - 1.126 * Δ E

其中:ΔE为Mn 3s光谱2个特征峰之间的结合能差值。

由图5(b)中可以看出,经CWAO反应后的平均氧化态均有所降低,且在ECWAO反应后平均氧化态最低(3.43),这一结果与Mn 2p的结果得到了印证。

2.3.2 活性氧的鉴定

为了探究反应的活性物质,采用ESR技术对反应过程中产生的活性氧进行鉴定,结果如图6所示。从图6(a)和图6(b)中可以看出,AP-CWAO中观察到TEMP-¹O₂加合物的三重态(1∶1∶1),证实了 ¹O₂的生成。此外,

D M P O - O 2 · -

的信号表明

O 2 · -

的存在^[16]。在ECWAO体系中,尽管观察到微弱的TEMP-¹O₂和DMPO-

O 2 · -

信号,但强度显著低于 AP-CWAO体系,表明ECWAO体系中 ¹O₂和

O 2 · -

的含量低于AP-CWAO。值得注意的是,在2个CWAO体系中均没有观察到DMPO被·OH氧化产生的DMPO-·OH(1∶2∶2∶1)典型的四线峰,说明体系中没有·OH的生成。

为进一步验证自由基对降解反应的影响,对反应过程进行淬灭实验。选用TBA、FFA和p-BQ分别作为·OH、¹O₂和

O 2 · -

的淬灭剂^[17-18]。从图6(c)和图6(d)中可以看出,在AP-CWAO体系中观察到FFA和p-BQ对BPA的抑制作用都很明显,且p-BQ的抑制作用略强于FFA,说明 ¹O₂在反应中是由

O 2 · -

转化而来。而TBA的加入对降解过程几乎没有抑制作用,这与ESR没有检测到·OH的结果相一致。而在ECWAO体系中,¹O₂、

O 2 · -

和·OH在反应中几乎不起作用,虽然能检测到微弱的TEMP-¹O₂和

D M P O - O 2 · -

信号,但这并不是反应的主要活性物种,由此初步推断,在ECWAO中有机物的降解是通过其他非自由基的方式产生活性物种进而降解污染物。

2.4 电化学分析

淬灭实验证实了AP-CWAO和ECWAO的反应均为非自由基为主导的反应,即AP-CWAO体系主要是以 ¹O₂为主导的反应,而ECWAO体系是反应过程中形成了其他的活性氧物种,因此需进一步验证。不同气氛下δ-MnO₂的循环伏安曲线如图7(a)所示。从图7(a)中可以看出,O₂氛围、0.5 V左右在δ-MnO₂的表面出现1个氧化峰^[19],而在N₂氛围下却无氧化峰出现,这是由于O₂在δ-MnO₂表面发生活化反应。

通过计时电流分别考察了O₂和BPA对电流强度的影响,如图7(b)所示。从图7(b)中可以看出,ECWAO系统在N₂饱和的Na₂SO₄溶液中稳定运行一段时间后瞬时加入BPA溶液,在计时电流曲线中可以观察到电极的电流强度迅速增大,但随后又回到原来的电流值水平。而当通入O₂,电极表面的电流值不仅迅速升高,并且经过一段时间后仍会保持较大的电流状态。说明电场的作用主要是通过先活化O₂生成一种活性氧种[O^-/O₂^-]^*,而后该活性氧物种对有机物实施降解,而不是电场的直接电化学氧化^[20]。

2.5 降解过程分析

通过GC-MS推测BPA降解的中间产物,在不同条件下对BPA的降解产物进行测定。在不同CWAO条件下的降解均可发现2条路径,结果如图8所示。从图8中可以看出,路径1中,连接苯环的C—C键首先断裂生成P1和P2,P2连接苯环的C—C键再次断裂生成P1,P1发生羟基化反应生成P3,P3进一步氧化生成P4,P4的C=O键被破坏并发生羧基化反应成P5,P5的C=C键断裂生成小分子酸P6;路径2中,BPA发生羟基化反应生成P7,P7上连接苯环的C—C键发生断裂生成P2和P8,P8被氧化生成P9,P9的C=O键被破坏并发生羧基化反应成P10,接着P10被氧化成P11。最后P6和P11被矿化成CO₂和H₂O。

针对BPA产生的中间产物毒性利用Toxicity Estimation Software Tool软件(TEST)进行毒性分析^[21-22],比较了BPA降解过程中产生的副产物(P1~P11)的致死剂量(LD 50)、致突变性(Mutagenicity)和发育毒性(Development toxicane),如图9所示。从图9(a)中可以看出,大鼠口服BPA的LD 50约为2 877.04 mg/kg,处于“毒性”水平。降解中间体产生了比BPA毒性更大的物质(P1、P3、P4、P7、P8、P9),可见P1、P3的口服LD 50达到了“剧毒”水平。然而,随着苯环的开环,与BPA相比,P6和P10的毒性显著降低,LD 50分别为3 467.62 mg/kg和 6 231.28 mg/kg。从图9(b)中可以看出,在BPA催化降解BPA的初始阶段,部分致突变性水平比BPA本身的致突变性水平更高,但随着BPA降解的进行,大部分中间体出现更低甚至负的致突变性水平^[22]。此外,从图9(c)中可以看出,除P5外,其他全部中间体的发育毒性都低于BPA。也就是说,虽然在BPA降解初期的中间毒性超过了BPA,但在进一步的降解中,中间毒性明显降低。结果表明,这种催化反应不仅可以有效地消除BPA,还可以减少已知的由降解中间体的生物毒性对水环境造成的潜在二次污染。

2.6 实际废水降解实验

δ-MnO₂作为CWAO反应的催化剂,在BPA去除过程中展示了高的催化活性,尤其是电场的作用可以进一步提升δ-MnO₂对有机污染物的催化降解活性,为此考察了其在实际有机污染物废水中的应用。以起始COD为69 mg/L的污泥滤液为目标污染物和电解质,探究其在ECWAO体系中COD的去除效果,结果如图10(a)所示。从图10(a)中可以看出,尽管实际废水中有机物的浓度较大,但在ECWAO体系中反应3 h后,废水的COD去除率可达到80.8%。

δ-MnO₂在ECWAO体系中的循环稳定性实验结果如图10(b)所示。从图10(b)中可以看出,催化剂的催化稳定性能良好。在前5次连续循环运行过程中,污泥滤液COD的降解率变化不明显,均维持在80%左右。从第6个循环开始,催化剂的催化活性开始有较为明显的降低,污泥滤液的COD去除率下降至67.82%。这是由于在经过多次循环后电极表面的活性位点被未降解污染物所包裹而导致活性位点密度降低所致。

3 结论

通过水热法合成了具有层状结构的δ-MnO₂,且在BPA的去除中展现出优异的性能,BPA降解产生的中间体的生物毒性有所降低,减少了对水环境的二次污染,具有良好水处理潜力。不同的CWAO条件下的降解机理会有所区别,在AP-CWAO下的反应为 ¹O₂为主导的反应,而ECWAO反应则主要是通过活化O₂产生了[O^-/O₂^-]^*。δ-MnO₂在处理污泥滤液废水时表现出良好的去除能力以及循环稳定性,对实际废水的修复具有重要的科学意义。

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