Studies On Zeolite Supported Noble Metal Catalysts For Catalytic Oxidation Of Formaldehyde At Room Temperature

As we all know,the content of indoor formaldehyde(HCHO)has a significant impact on human health.It will bring discomfort on human body,and even cause cancer,even if exposed to low ppm levels of HCHO.Therefore,how to effectively remove HCHO is an urgent problem at present.To date,several strategies have been adopted for indoor HCHO removal.Among these ways,room-temperature catalytic oxidation is supposed to be the most promising method.And the key of this technology is developing efficient catalyst.At present,there are two kinds of catalysts for catalytic oxidation of HCHO.Transition-metal-oxide catalysts with low price always show poor low-temperature activity and weak oxygen activation ability.However,the supported noble metal catalyst show excellent low-temperature activity,but the most drawback is the high price.In general,catalytic activities of noble-metal supported catalysts strongly depend on the characteristics of supports,dispersion of noble metal,and the valence of the active site.It is common that zeolite supports with large specific surface area and developed porous structures are beneficial for metal dispersion and mass transfer.Hence,in order to eliminate HCHO at room temperature and simultaneously reduce the loading of noble metals,a series of studies on zeolite based catalysts were carried out for HCHO oxidation.The influence of hydrophobicity of support for HCHO oxidation was investigated,and the size effect of active site of Pd in Pd@TS-1 prepared by encapsulation strategy was elucidated.Furthermoer,the specific role of K doping in promoting the performance of Pd@Silicalite-1 catalyst was investigated,and also the effects of different reduction methods on the structure of Pd/TS-1 catalyst for oxidation of HCHO were revealed.The main outcomes obtained in this thesis are as follows:(1)Three types of MFI zeolites supported Pt catalysts(Pt/ZSM-5,Pt/Silicalite-1 and Pt/TS-1)were prepared by surfactant protection and chemical reduction method.The size of Pt nanoparticles(only 2.9 nm)on the sample 0.3Pt/TS-1 was smaller than those on the sample 0.3Pt/ZSM-5 and 0.3Pt/silicalite-1.At the same time,the interaction between Ti and Pt was verified,and there was a high concentration of adsorbed oxygen on Pt/TS-1.Therefore,the 0.3Pt/TS-1 sample with low Pt loading exhibited excellent catalytic performance for catalytic oxidation of HCHO.The HCHO conversion reached 100%at 20℃ at a SV of 60,000 mL/(g×h).The mechanism study showed that the main active species were dimethylene oxide and formate.The excellent catalytic activity of Pt/TS-1 was attributed to the excellent hydrophobicity of TS-1 support,high Pt dispersion,high concentration of surface oxygen and the strong interaction between TS-1 and Pt.(2)Pd@TS-1(x=30,50,and 150)were prepared by in situ encapsulation and ligand stabilized strategy.The distrubution of Pd nanoparticles(2.2-3.5 nm)were uniform in the TS-1,but obviously aggregated Pd NPs(5.1 nm)were observed on the surface of TS-1 for TS-1 supported Pd catalyst.At the same time,the encapsulating strategy can significantly improve Pd dispersion in TS-1 matix(51%).In addition,densities of states and charge density difference based on DFT revealed the electron effect and electron transfer between Pd and Ti.And the framework Ti in TS-1 was favorable for the dispersion of Pd nanoparticles.Hence,the Pd@TS-1(50)sample showed better catalytic activity for HCHO oxidation than those of Pd@TS-1(30),Pd@TS-1(150),and Im-Pd/TS-1(50).Compared with the supported catalyst,Pd@TS-1(50)also displayed good catalytic stability due to the dreaming core-shell structure with hydrophobic silicon-rich extrasurface.(3)The 0.15Pd@M-Silicalite-1 samples with different alkali metal(M=Li,Na,K,Cs)were prepared by the encapsulation strategy.The results showed that the addition of alkali metal had a dramatic promotion effect on the pore structure and surface defect.At the same time,the addition of alkali metals,especially K,significantly increased the dispersion of Pd nanoparticles.Combined with FT-IR characterization,it was found that the addition of alkali metals could replace H in Si-OH of Silicalite-1,thus forming of highly dispersed active center of Si-O-K-Pd.Hence,the activity of Pd@K-Silicalite-lwas better than Pd@Silicalite-1,Pd@Li-Silicalite-1,Pd@Na-silicalite-1 and Pd@Cs-Silicalite-1.And the catalytic oxidation of HCHO at room temperature was achieved on the 0.15Pd@K-Silicalite-1 sample.The excellent catalytic activity was attributed to high dispersion of Pd nanoparticles,good low-temperature reducibility,good HCHO adsorption capacity and excellent ability of CO2 desorption.(4)Pd/TS-1 samples were pretreated with different reducing gases(H2,CO)and liquid phase reducing agents(ascorbic acid,ethylene glycol,sodium borohydride).The results showed that the reduction pretreatment can significantly reduce the size of Pd nanoparticles and improve the Pd dispersion.Besides,the the size of Pd nanoparticles was depend on the way of reduction treatment.In general,the Pd nanoparticles obtained by liquid phase reduction were more relatively uniform than that by gas phase reduction at high temperature.And the small size of Pd nanoparticles with 5.08 nm was obtained by using the NaBH4.Therefore,compared with the Pd/TS-1(air)sample calcined in air,the activity of the reduced sample for catalytic oxidation of HCHO was improved,and the Pd/TS-1(NaBH4)sample could abate HCHO at room temperature.The decrease of the size of Pd nanoparticles was mainly attributed to the enhanced interaction between TS-1 and Pd.Meanwhile,the reduced samples had good low-temperature reducibility and could reduce the aggregation of Pd nanoparticles.The mechanism showed that the metallic Pd in Pd/TS-1 sample was the active center.Compared with the high valence of Pd2+,the metallic Pd was conducive to activate oxygen,transform of intermediate products and inhibite the formation of carbonate.