| With the promulgation of the 14th Five-Year Plan,our country further emphasizes the importance of ecological protection,and green production by the end of 2035 will be come true,so that the significant improvement in ecological environment can be achieved and the harmonious co-existence of man and nature can be promoted.With the successful achievements of the 13th Five-Year Plan,our atmosphere environment has been improved to a certain extent,and volatile organic compounds(VOCs)have been basically controlled after several years of treatment efforts,but atmosphere pollutions in some areas are still present,for example,high-concentration PM2.5,O3 pollution,and hazy days exhibit an increasing trend.Atmosphere pollution,especially VOCs emissions,is one of the environmental problems in China.In February 2021,the Ministry of Ecology and Environment released the"14th Five-Year Plan"atmosphere pollution prevention and control strategies,VOCs will replace SO2 for the first time to be one of the"14th Five-Year Plan"air-quality improvement indicators,which demonstrates that in the period of the"14th Five-Year"or even longer time,VOCs removal is focus of the most important issues that must control,and the industries including chemical,petroleum,electron manufacturing,printing,pharmaceutical,and coating are the main enterprises that are strictly controlled.VOCs are a class of organic compounds with very complex compositions,which are also responsible for the formation of high concentrations of the precursors of PM2.5 and O3.Among them,chlorinated volatile organic compounds(CVOCs)are formed when hydrogen atoms in hydrocarbons are replaced by chlorine atoms.CVOCs are widely used as solvents in chemical processes due to their good stability and solubility,and are contained in many industrial emissions due to their volatility.CVOCs not only have a negative impact on the environment,but also pose a significant risk to human health.Catalytic oxidation is thought to be an effective method for the oxidative elimination of CVOCs due to its high oxidation efficiency,low energy consumption,and no secondary pollution,but the key to catalytic oxidation lies in the design and preparation of high-performance catalysts.Considering the characteristics of high toxicity of CVOCs,inertness,and poisoning to the catalyst by Cl-containing species,the development of novel and efficient catalysts for the elimination of CVOCs is therefore of great importance.Transition metal oxides are inexpensive and simple to prepare,noble metal-loaded catalysts have the advantages of resistance to poisoning and good low-temperature activity.The porous materials have high surface areas and well-developed pore channels for easy mass transfer.By combining the advantages of porous materials and noble metal nanoparticles(NPs),a series of noble metal alloy NPs-loaded catalysts were prepared and their catalytic performance for the oxidation of typical CVOCs was investigated.The activity and selectivity of the noble metal-loaded catalysts are related to their noble metal particle size,particle morphology,support nature,preparation method,interaction between noble metal NPs and support,and the acidity/basicity.we investigated the catalyst poisoning,thermal stability,product selectivity,adsorption,intermediate types and reaction mechanism as well as effects of H2O,SO2 CO2 or HCl addition on catalytic activity of the as-prepared catalysts for the oxidation of typical CVOCs.It is therefore of interest to examine these factors in influencing the catalyst performance.In this thesis,three-dimensionally ordered macroporous(3DOM)or mesoporous oxide supports and their supported noble metal alloy or transition metal oxide catalysts were prepared by the polymethyl methacrylate(PMMA)templating,solvothermal,impregnation,polyvinyl alcohol(PVA)-protected Na BH4 reduction,and/or oil phase reduction methods.Catalytic activities and selectivities of the catalysts for typical CVOCs(e.g.,TCE and 1,2-DCE)oxidation were evaluated and their structure-performance relationships were revealed,catalytic reaction mechanisms were clarified and kinetic equations for the reactions were established.The results of the study can be used as a reference for the design and preparation of new high-performance catalysts in industrial applications.The main conclusions are as follows:(1)Transition metal oxides(MOx;M=Cr,Mn,Fe,Ni,Cu)-doped titania solid solution catalysts(10 wt%MOx傍i O2,denoted as 10MOx傍i O2)were prepared by the coprecipitation method.The techniques of XRD,TPR,TPD,XPS,TPSR,and in situ DRIFTS were used to characterize physicochemical properties of the materials,and their catalytic activities were evaluated for the oxidation of 1,2-DCE.The introduction of MOx enhanced adsorption and activation of oxygen molecules,mobility of surface lattice oxygen,and low-temperature reducibility.The 10MOx傍i O2 catalysts showed good performance,with 10Cr Ox傍i O2 exhibiting the highest catalytic activity(reaction rate=2.35ﾗ10-7 mol/(gcat s)and apparent activation energy(Ea)=35k J/mol at space velocity=40,000 m L/(g h))and good resistance to chlorine poisoning,The mechanism of 1,2-DCE oxidation over 10Cr Ox傍i O2 was also discussed based on the results of TPSR and in situ DRIFTS characterization.It is concluded that strong acidity and redox ability,high adsorbed oxygen species concentration,and strong interaction between Ti O2 and Cr Ox were accountable for the good performance of10Cr Ox傍i O2.(2)The 3DOM CeO2-supported Au-Pd alloys(x Au Pdy/3DOM CeO2,x is the total loading(wt%)of Au and Pd,and y is the Pd/Au molar ratio)were synthesized using the PMMA-templating and PVA-protected reduction methods.The samples were evaluated for the oxidation of TCE.It is found that the x Au Pdy/3DOM CeO2samples displayed a good-quality 3DOM architecture,and the noble metal NPs were highly dispersed on the surface of 3DOM CeO2.Among all of the samples,2.85Au Pd1.87/3DOM CeO2 exhibited the highest catalytic activity,with the temperature at a TCE conversion of 90%(T90%)being 415 oC at a space velocity of20,000 m L/(g h).Furthermore,the 2.85Au Pd1.87/3DOM CeO2 sample possessed the lowest apparent activation energy(33 k J/mol).In addition,the synergistic effect of Au and Pd in the 2.85Au Pd1.87/3DOM CeO2 catalyst also contributed to the superior thermal stability,water resistance and resistance to Cl.It was also found that the alloying of Au and Pd improved the adsorption and activation of oxygen molecules and enhanced the interaction between the Au Pd alloy NPs and the carrier,resulting in a significant increase in catalytic activity and resistance to chlorine poisoning.We conclude that the excellent catalytic performance for TCE combustion of2.85Au Pd1.87/3DOM CeO2 was associated with good low-temperature reducibility,high adsorbed oxygen species concentration,highly dispersed Au Pd NPs,and strong interaction between Au Pd NPs and 3DOM CeO2 as well as the high-quality 3DOM structure and high surface acidity.(3)Ruthenium-based materials are promising catalysts for the removal of CVOCs due to their good ability in cleaving the C-Cl bonds and decreasing the Cl deposition.However,the roles of noble bimetals alloyed with Ru have not been clarified clearly.In this work,the bimetallic RuyM(M=Au,Pd or Pt)alloys supported on 3DOM CeO2(xRuyM/3DOM CeO2;RuyM loading(x)=0.90-0.93 wt%,and Ru/M molar ratio(y)=2.77-2.87)were fabricated by the PMMA-templating and PVA-protected reduction methods,and their physicochemical properties were well measured using a number of techniques.Oxidation of trichloroethylene(TCE)was used to evaluate catalytic performance of the as-obtained samples with a good-quality3DOM structure and a surface area of 39-44 m~2/g.The Ru,Pd,and RuyM NPs with an average size of 2.9-3.5 nm were uniformly dispersed on the skeleton surface of3DOM CeO2.The 0.93Ru2.87Pd/3DOM CeO2 sample showed the highest catalytic activity and the lowest apparent activation energy(34 k J/mol),with the temperatures(T50%and T90%)at TCE conversions of 50 and 90%being 237 and 298 oC at a space velocity of 20,000 m L/(g h),respectively.Simultaneously,0.93Ru2.87Pd/3DOM CeO2exhibited the highest TCE oxidation rate(30.3?mol/(gNoble metal s))and the highest TOFNoble metal(3.1ﾗ10-3 s-1)at 250 oC.The 0.93Ru2.87Pd/3DOM CeO2 sample also possessed a better hydrothermal stability than the 0.85Ru/3DOM CeO2 sample after hydrothermal ageing treatment at 750 oC.Effects of H2O,CO2,and HCl on catalytic activity of 0.93Ru2.87Pd/3DOM CeO2 were also examined.The presence of Ru in the samples was favorable for generation of HCl and Cl2 and reduction of the by-products in TCE oxidation,and tetrachloroethylene was the main by-product.The possible catalytic mechanism over 0.93Ru2.87Pd/3DOM CeO2 was also proposed.The outstanding catalytic efficiency of 0.93Ru2.87Pd/3DOM CeO2 could be assigned to the high adsorbed oxygen species,good low-temperature reducibility,strong TCE adsorption and activation ability,and formation of the intimate nanointerface between Ru Pd NPs and 3DOM CeO2.(4)The nature of porous support(mesoporous Al2O3(meso-Al2O3),mesoporous Mg O(meso-Mg O),and microporous HZSM-5)on catalytic performance of the bimetallic RuCo NPs was investigated for the oxidation of 1,2-dichloroethane(1,2-DCE).Redox and acid properties and reaction intermediates of the samples were measured by means of various techniques.The order of activity of the catalysts were:RuCo/HZSM-5(T50%=238 oC and T90%=281 oC)>Ru/HZSM-5(T50%=260 oC and T90%=308 oC)>Co/HZSM-5(T50%=285 oC and T90%=329 oC)>RuCo/meso-Al2O3(T50%=363 oC and T90%=391 oC)>RuCo/meso-Mg O(T50%=403 oC and T90%=445oC),with the more acidic RuCo/HZSM-5 showing the highest catalytic activity,with a reaction rate of 18.7?mol/(gNoble metal s)at 270 oC,TOFNoble metal=3.6?10-3 s-1,and Ea=36 k J/mol.The excellent oxidation performance is related to the abundant acidic sites,pore structure and redox centrers on the surface of HZSM-5.The large specific surface area and pore structure of the catalyst facilitate the adsorption of 1,2-DCE molecules,and then the 1,2-DCE can remove HCl at the acidic sites to obtain C2H3Cl.The loading of RuCo NPs as a strong oxidation centre facilitates the deep oxidation of the intermediate products,thus effectively inhibiting the formation of the toxic by-product C2H3Cl and reducing secondary contamination.The XANES and EXAFS results confirm that the Ru O2 species were dominantly present in the bimetallic sample,and doping of the Co species could increase the length of the Ru-O bond.Over the RuCo/HZSM-5 sample,the partial deactivation induced by water vapor or HCl addition was reversible,while that induced by SO2 introduction was irreversible.Based on the characterization results,we believe that the oxidation of 1,2-DCE over RuCo/meso-Al2O3,RuCo/meso-Mg O or RuCo/HZSM-5 might take place concurrently via the Langmuir-Hinshelwood and Mars-van Krevelen mechanisms.The synergistic promotion of the redox,the surface acid and the strong interaction of the highly dispersed RuCo NPs with HZSM-5 improve the oxidation performance of the catalyst. |
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