Study On Rare Earth-containing Porous Materials Supported Noble Metal Nanocrystalline Catalysts For Catalytic Combustion Of VOCs

Volatile organic compounds（VOCs）are important components of air pollutants and they are emitted into our environment from various urban and industrial areas.Most VOCs are toxic,and they can cause great damage to human life and cause irreversible defects during long-lasting exposure.One of the most efficient ways to eliminate VOCs is catalytic combustion,which has the advantages of large treating capacity,high efficiency,low light-off temperature and easy product control.Although the cost of supported noble metal catalysts is high,they have been widely applicated because of their low loading,preferable catalytic activity and stability.Supporting materials also play an important role in catalysts.Porous materials have the characteristics of high specific surface area（SBET）and easy mass transfer of reactant molecules.Rare earth elements（REEs）play the role of a promoter that can improve thermal stability,selectivity,anti-poisoning ability and catalytic activity.Therefore,the rare earth-containing porous material supported noble metal nanocrystalline catalysts are expected to show excellent catalytic performance for VOCs oxidation.In this paper,γ-Al₂O₃,MCM-22 and CeO₂ with large SBET and pore volume（VP）were prepared as supports by sol-gel method,static hydrothermal crystallization method and direct calcination method,respectively.A high-temperature solution-phase reduction method was used to synthesize a series of noble metal nanocrystalline catalysts.The effects of different preparation methods,different Pd/Pt molar ratios,addition of REEs and different supports on the catalytic combustion performance of low-concentration VOCs were investigated.A series of characterization techniques were adopted to study the crystal structure,particle size,morphology,valence and VOCs adsorption and desorption properties of the catalysts.The main findings are as follows:1.Mesoporousγ-Al₂O₃ prepared by a sol-gel method was used as support and a high-temperature solution-phase reduction method was used to synthesize a series of Pd-Pt nanocrystalline catalysts.Comparing the Pd（Pt）/γ-Al₂O₃ synthesized by traditional impregnation method,the results show that the catalyst synthesized by high-temperature solution-phase reduction method has better catalytic activity.γ-Al₂O₃exhibits a rod structure.Pd Ox（x=0,1）and Pt Ox（x=0,2）nanoparticles has a small diameter（about 2 nm）with good disersion on the surface of mesoporousγ-Al₂O₃.The addition of Ce O₂ can enhance the catalysts’abilities to store and release oxygen through the interaction and conversion of Ce⁴⁺/Ce³⁺,the dispersibility of noble metal particles,the adsorption capacity and adsorption strength.Therefore,Pd-Pt（1:1）/10%Ce/γ-Al₂O₃has the highest catalytic activity,and it can completely oxidize benzene at 200℃.2.Micro-mesoporous MCM-22 prepared by a static hydrothermal crystallization method was used as support and a high-temperature solution-phase reduction method was used to synthesize a series of REEs modified MCM-22-supported Pd Ox（x=0,1）nanocrystalline catalysts.The catalytic combustion properties of low-concentration toluene was studied.The results show that catalysts with an appropriate yttrium content greatly improves the catalytic activity of Pd/MCM-22.Pd/7.5%Y/MCM-22 exhibits the highest catalytic performance and it can completely oxidize toluene at 220℃.Meanwhile,the catalyst maintains 95%conversion after continuous reaction at 210℃for 100 h,indicating that the catalyst has good durability.Pd Ox nanoparticles with a size of 3.6-6.8 nm are uniformly dispersed on the surface of MCM-22.Efficient electron transfer from the Pd²⁺/Pd⁰ redox cycle facilitates the catalytic oxidation process,and the interaction between Pd Ox and Y₂O₃ promotes the high dispersion of the particles.3.CeO₂ prepared by commercial purchase,direct calcination method and Ce-MOF thermal decomposition method were used as supports,which were denoted as CeO₂-1,CeO₂-2,and CeO₂-3,respectively.A high-temperature solution-phase reduction method was used to synthesize a series of Pd Ox（x=0,1）nanocrystalline catalysts.The catalytic combustion performance of low-concentration benzene on three kinds of different Pd/CeO₂ was investigated.The results show that the interaction between Pd Ox and CeO₂ is crucial to the redox performance and catalytic activity.Meanwhile,CeO₂nanoparticles with the smallest size is formed in CeO₂-2,which mainly expose{100}crystal faces.Pd/Ce O₂-2 has the highest Ce³⁺concentrations,appropriate Pd²⁺/(Pd²⁺+Pd⁰)and the best low-temperature reducibility.Therefore,it has the highest catalytic activity and it can completely oxidize benzene at 260℃.