MnO_2催化剂用于催化氧化含氯芳香烃的形貌效应与反应机制研究（英文）

含氯挥发性有机物(Chlorinated VOCs)被广泛应用于工业、农业、医药、有机合成等领域,在使用过程中会通过挥发、泄漏、废气排放等途径进入大气环境中,造成臭氧层破坏与光化学烟雾,且很难被生物降解,对人体具有很强的"三致"效应.在众多治理方法中,催化燃烧因高效低能耗的特点而被认为是具有应用前景的含氯VOCs处理方式,然而催化剂中毒以及毒副产物生成极大限制了该技术的工业应用.锰基催化剂由于价格低廉、来源广泛以及价态多变等特点被广泛应用于环境催化领域,包括甲醛、甲苯、CO催化氧化以及选择性催化还原脱硝等.MnO_2的晶体形貌与其催化性能息息相关,二者的构效关系已有广泛研究,但在含氯VOCs催化氧化中,MnO_2的形貌特征与催化活性、反应稳定性、副产物等的关系尚不明晰.因此,本文通过水热法制备了纳米棒状γ-MnO_2,纳米管状α-MnO_2以及具有层状结构的δ-MnO_2,系统研究了这三种形貌结构在氯苯催化氧化中的反应特征,利用XRD,XPS,TPR,TPD,吡啶-IR等手段对催化剂的形貌、表面元素价态、氧化还原性能以及表面酸性等进行了表征,获得了MnO_2在含氯VOCs催化氧化应用中的构效关系.XRD以及SEM分析结果表明,三种形貌的MnO_2样品均由水热法成功制得.H_2-TPR和O_2-TPD测试分析显示,MnO_2催化剂的氧化还原性能按如下顺序递减δ-MnO_2≥γ-MnO_2>α-MnO_2,与这些催化剂活性测试中的氯苯转化率结果一致,但与其CO_2选择性的结果不一致.氧化还原能力最佳的δ-MnO_2上CO_2选择性表现最差,即使提高温度仍无法提升.XPS结果表明,三种催化剂的Mn元素平均价态高低顺序为δ-MnO_2(3.80)>α-MnO_2(3.75)≥γ-MnO_2(3.74).δ-MnO_2催化剂表面因具有最丰富的Mn^(4+),反应过程中易生成强的Mn-Cl键,从而抑制了Cl与解离水反应生成HCl,导致催化剂富集氯失活,CO_2选择性差.对反应尾气及催化剂表面产物分析后发现,三种MnO_2催化剂均生成了具有更高毒性的多氯副产物,其中主要有CHCl_3,CCl_4,C_2HCl_3,C_2Cl_4等,尤其在α-MnO_2催化剂表面发现了二氯苯存在,其可能通过进一步的亲核取代生成氯苯酚,并最终聚合成二噁英类物质.

超临界水热合成过渡金属改性铈基催化剂应用于CO-SCR脱硝研究（英文）

氮氧化物(NO_x)作为煤炭燃烧过程主要污染物之一,可直接或间接引起如光化学烟雾、酸沉降、平流层臭氧损耗和全球气候变化等大气环境污染问题.NO_x的选择性催化还原技术(SCR)被认为是目前处理固定源NO_x的最有效方法之一.由于燃煤工业锅炉烟气中还有1%~3%的CO,远高于NO_x的0.02%~0.04%,因此,以CO为还原剂进行CO-SCR脱硝具有现实意义,它可在反应过程中同时消除CO和NO两种有害气体,但对催化剂的活性及抗毒性提出更高要求.CeO_2作为一种常用的稀土材料,因具有良好的储放氧能力而广泛应用于SCR反应中.过渡金属改性可进一步改善CeO_2的物化性能,从而可能达到CO-SCR的应用要求.本文利用超临界水热技术合成了MOx-CeO_2(M=Co,Fe,Cu)固溶体催化剂,并利用X射线衍射(XRD),氢气程序升温还原(H2-TPR),傅里叶变换原位红外(DRFTIR)等探究了催化剂在CO-SCR反应中的催化活性与作用机制.CO-SCR反应活性测试表明,CuO-CeO_2催化剂活性明显优于FeOx-CeO_2和CoO_x-CeO_2催化剂,在126°C NO去除率即可达到90%;其N_2选择性也可在179°C时达到90%.为了进一步探究MOx-CeO_2(M=Co,Fe,Cu)催化剂的CO-SCR反应途径,本文随后进行了系列原位DRFTIR实验,发现NO在三种催化剂表面均能被高效吸附,其吸附态中间产物主要为双齿硝酸根,桥式硝酸根,桥式硝基和亚硝酰基等.另外,在CuO-CeO_2催化剂表面还存有螯合硝基和单齿硝酸根.CO在催化剂表面主要以CO_x,碳酸根和羧酸根等形式存在.值得注意的是,在CuO-CeO_2表面,CO因吸附于Cu^+而形成Cu^+-CO,在2100 cm.1左右形成明显的特征峰.当催化剂表面吸附CO至饱和后再通入NO发现,CO的吸附特征峰逐渐被NO的特征吸附峰取代;而当NO被吸附至饱和后再通入CO,NO的特征峰则不出现明显变化.这表明NO和CO在催化剂表面存在竞争吸附,NO可能优先于CO吸附在催化剂表面.当NO和CO同时通入红外反应仓时发现,在CoOx-CeO_2和FeOx-CeO_2催化剂表面只观察到NO的吸附峰,而在CuO-CeO_2催化剂表面观察到Cu^+-CO的特征峰,说明在CO-SCR反应过程中,CO可以在Cu+表面被有效吸附,其与吸附于CeO_2表面的NO物种反应生成N_2和CO_2,遵循Langmuir-Hinshelwood反应机理.而在CoOx-CeO_2和FeOx-CeO_2催化剂表面,因NO的竞争吸附。

Atmospheric heterogeneous reaction of chlorobenzene on mineralα-Fe_(2)O_(3)particulates:a chamber experiment study

Despite the large emission of chlorinated volatile organic compounds(CVOCs)into the atmosphere,the ultimate fate of these compounds remains largely unknown.Herein,we explore the photochemical conversion of an important class of CVOCs,namely chlorobenzene(CB),on mineralα-Fe_(2)O_(3)particulates under atmospheric relevant conditions.A series of chamber reactions composed of the CB with/without SO_(2)or NO_(2)are performed,followed by in situ diffuse reflectance infrared Fourier transform spectroscopy measurements and density functional theory calculations.We show that CB can be considerably degraded byα-Fe_(2)O_(3)under light irradiation,whereas the reaction is markedly suppressed by adding SO_(2)or NO_(2)owing to their competitive adsorption and surface acidification.In particular,we discover that CB can be ultimately converted into polychlorinated dibenzo-p-dioxins and dibenzofurans(PCDD/Fs)under dark state or light irradiation,suggesting a possible origin of atmospheric PCDD/Fs from this overlooked photochemical source.

A mechanic insight into low-temperature catalytic combustion toward ethylene oxide over Pt-Ru/CuCeO_(x) bimetallic catalyst

The catalytic oxidation performance toward ethylene oxide(EO)and the consequent mechanism were investigated on the Pt-Ru/CuCeO_(x)bimetallic catalyst,which was prepared by a distinct method combining stepwise adsorption and subsequent impregnation.The catalytic tests show that the introduction of Ru into the Pt catalyst,so as to form Pt-Ru bimetallic active sites,can greatly increase the oxidation activity of the catalyst,as evidenced by the extremely lower full oxidation temperature(120℃)when compared with that of the Pt/CeO_(2) catalyst(160℃).The XPS spectra show that the Ru species(mainly RuO_(x))have strong interaction with the CuCeO_(x) support,which can therefore affect the electron transfer between the Pt species and the support.As a result,the oxygen activation on Pt species is obviously facilitated and catalytic activity is enhanced.Finally,in situ diffuse reflectance infrared Fourier transform spectroscopy(DRIFTs)was used to track the reaction mechanism.It is found that the catalytic oxidation process follows the MvK catalytic mechanism at low temperature and the L-H catalytic mechanism when the temperature moves to higher range.

Catalytic destruction of chlorobenzene over K-OMS-2: Inhibition of high toxic byproducts via phosphate modification

In the process of catalytic destruction of chlorinated volatile organic compounds(CVOCs),the catalyst is prone to chlorine poisoning and produce polychlorinated byproducts with high toxicity and persistence,bringing great risk to atmospheric environment and human health.To solve these problems,this work applied phosphate to modify K-OMS-2 catalysts.The physicochemical properties of catalysts were determined by using X-ray powder diffraction(XRD),scanning electron microscope(SEM),X-ray photoelectron spectroscopy(XPS),hydrogen temperature programmed reduction(H_(2)-TPR),pyridine adsorption Fouriertransform infrared(Py-IR)and water temperature programmed desorption(H_(2)O-TPD),and chlorobenzene was selected as a model pollutant to explore the catalytic performance and byproduct inhibition function of phosphating.Experimental results revealed that 1 wt.%phosphate modification yielded the best catalytic activity for chlorobenzene destruction,with the 90%conversion(T90)at approximately 247℃.The phosphating significantly decreased the types and yields of polychlorinated byproducts in effluent.After phosphating,we observed significant hydroxyl groups on catalyst surface,and the active centerwas transformed into Mn(IV)-O…H,which promoted the formation of HCl,and enhanced the dechlorination process.Furthermore,the enriched Lewis acid sites by phosphating profoundly enhanced the deep oxidation ability of the catalyst,which promoted a rapid oxidation of reaction intermediates,so as to reduce byproducts generation.This study provided an effective strategy for inhibiting the toxic byproducts for the catalytic destruction of chlorinated organics.

Enhanced CO_2 adsorptive performance of PEI/SBA-15 adsorbent using phosphate ester based surfactants as additives

In this study,a series of polyetherimide/SBA-15： 2-D hexagonal P6 mm,Santa Barbara USA（PEI/SBA-15） adsorbents modified by phosphoric ester based surfactants（including tri（2-ethylhexyl）phosphate（TEP）,bis（2-ethylhexyl） phosphate（BEP） and trimethyl phosphonoacetate（TMPA））were prepared for CO2 adsorption.Experimental results indicated that the addition of TEP and BEP had positive effects on CO2 adsorption capacity over PEI/SBA-15.In particular,the CO2 adsorption amount could be improved by around 20% for 45PEI-5TEP/SBA-15 compared to the additive-free adsorbent.This could be attributed to the decrease of CO2 diffusion resistance in the PEI bulk network due to the interactions between TEP and loaded PEI molecules,which was further confirmed by adsorption kinetics results.In addition,it was also found that the cyclic performance of the TEP-modified adsorbent was better than the surfactant-free one.This could be due to two main reasons,based on the results of in situ DRIFT and TG-DSC tests.First and more importantly,adsorbed CO2 species could be desorbed more rapidly over TEP-modified adsorbent during the thermal desorption process.Furthermore,the enhanced thermal stability after TEP addition ensured lower degradation of amine groups during adsorption/desorption cycles.

Unveiling the importance of reactant mass transfer in environmental catalysis:Taking catalytic chlorobenzene oxidation as an example

To date,investigations onto the regulation of reactants mass transfer has been paid much less attention in environmental catalysis.Herein,we demonstrated that by rationally designing the adsorption sites of multi-reactants,the pollutant destruction efficiency,product selectivity,reaction stability and secondary pollution have been all affected in the catalytic chlorobenzene oxidation(CBCO).Experimental results revealed that the co-adsorption of chlo robenzene(CB)and gaseous O_(2)at the oxygen vacancies of CeO_(2)led to remarkably high CO_(2)generation,owning to their short mass transfer distance on the catalyst surface,while their separated adsorptions at Bronsted HZSM-5 and CeO_(2)vacancies resulted in a much lower CO_(2)generation,and produced significant polychlorinated byproducts in the off-gas.Howeve r,this separated adsorption model yielded superior long-term stability for the CeO_(2)/HZSM-5 catalyst,owning to the protection of CeO_(2)oxygen vacancies from Cl poisoning by the preferential adsorption of CB on the Bronsted acidic sites.This work unveils that design of environmental catalysts needs to consider both of the catalyst intrinsic property and reactant mass transfer;investigations of the latter could pave a new way for the development of highly efficient catalysts towards environmental pollution control.

Tailoring the simultaneous abatement of methanol and NO_(x) on Sb-Ce-Zr catalysts via copper modification

Simultaneously removal of NO_(x)and VOCs over NH3-SCR catalysts have attracted lots of attention recently.However,the presence of VOCs would have negative effect on deNOx efficiency especially at low temperature.In this study,copper modification onto Sb_(0.5)CeZr_(2)O_(x)(SCZ)catalyst were performed to enhance the catalytic performance for simultaneous control of NNO_(x)and methanol.It was obtained that copper addition could improve the low-temperature activity of both NOx conversion and methanol oxidation,where the optimal catalyst(Cu_(0.05)SCZ)exhibited a deNOx activity of 96%and a mineralization rate of 97%at 250℃,which are around 10%higher than that of Cu free sample.The characterization results showed that copper addition could obviously enhance the redox capacity of the catalysts.As such,the inhibition effect of methanol incomplete oxidation on NO adsorption and NH3 activation were then lessened and the conversion of surface formamide species were also accelerated,resulting in the rising of NOx conversion at low temperature.However,excessive copper addition would damage the Sb-Ce-Zr oxides solid solution structure owing to Cu-Ce strong interactions,decreasing the surface area and acidity.Meanwhile,due to easier over-oxidation of NH3 with more Cu addition,the temperature window for NOx conversion would become quite narrow.These findings could provide useful guidelines for the synergistic removal of VOCs over SCR catalyst in real application.

Unveiling the secondary pollution in the catalytic elimination of chlorinated organics: The formation of dioxins

Since the discovery of polychlorinated dibenzo-p-dioxins and dibenzofurans(PCDD/Fs)in the process of municipal solid waste incineration(MSWI),a large number of researches have been conducted to reveal their formation mechanisms and emission characteristics.As one of national priority control pollutants,chlorinated organics are inclined to transfer into PCDD/Fs in the heterogeneously catalyzed process,which has been considered to be one of great challenges in environmental catalysis.However,so far direct evidences to support such a conversion process are insufficient,and the reaction mechanisms are lack of exploration.This study investigated the catalytic elimination of chlorobenzene(CBz)over a range of industrially applied active species including Pt,Ru,V,Ce and Mn oxides,and explored their reaction byproducts,chlorine adsorption/desorption behaviors and PCDD/F formations.We found that all of these species could generate the PCDD/Fs,amongst which,Mn species were the most active for PCDD/F formation.Approximately 140 ng I-TEQg-1 PCDD/Fs were detected on the Mn-CNT surface after ageing at250℃for 30 h.Even using the dichloromethane(DCM)as a precursor,significant PCDD/Fs were still detected.The Ru and V species were shown to generate much less polychlorinated byproducts and PCDD/Fs,owning to their sufficiently high abilities in Cl desorption,which were through the semi-Deacon and Br(?)nsted H reactions,respectively.