Study On Structure Modification And Catalytic Performance Of Formaldehyde Oxidation Over Ag/γ-Al₂O₃ Catalysts

Exposure to indoor air containing formaldehyde（HCHO）to a certain extent may cause serious health hazards.Much effort has been devoted to eliminate HCHO,among which catalytic oxidation is one of the most effective methods for lower energy consumption,simple operation and free secondary pollution.Recently,supported Ag catalyst has been proved effective for HCHO catalytic oxidation by microstructure adjustment of metal particles and supports.However,it is still an inevitable challenge for practical application respect to the desires for lower temperature and economy.Therefore,to enhance the activity at low temperatures,this study had focused on the microstructure and properties of the supports and active components by adjusting crystalline phases,selecting precursor and modifying with metal oxide.The mechanism of catalytic oxidation of formaldehyde was also studied.The main content and results of the paper are as follows:（1）The effect of different crystalline phases of Al₂O₃（γ-Al₂O₃,γ,θ-Al₂O₃）on HCHO oxidation over Ag/γ-Al₂O₃（8 wt.%）catalysts was investigated,and the silver state and particle size strongly depend electron state and the crystalline phase.The various coordinatively unsaturated Al³⁺（AlⅢ,AlⅤ）involves in the formation of Ag-O-Al entities.Ag species is mainly anchored on AlⅤsites forγ-Al₂O₃,whereas mainly on AlⅢsites forγ,θ-Al₂O₃.The weaker interaction of Ag,O and AlⅤonγ-Al₂O₃compared with that of Ag,O and AlⅢonγ,θ-Al₂O₃and the larger surface area ofγ-Al₂O₃result in the formation of more Ag⁰with high dispersion.The synergistic effect of OH group and the high activation ability for oxygen on smaller Ag⁰particles benefit the high HCHO oxidation activity on Ag/γ-Al₂O₃(T₁₀₀=110^oC).（2）Ag/γ-Al₂O₃-Nb₂O₅（4 wt.%）were prepared by the addition of Nb₂O₅,and the structure and activity of the Ag/γ-Al₂O₃-Nb₂O₅ catalysts were investigated.The addition of Nb₂O₅onγ-Al₂O₃,supplies oxygen vacancy and more oxygen species on the surface of the catalysts,and then enhance the activation of oxygen species.Asisited by the enriched surface oxygen,much more DOM formed than on Ag/γ-Al₂O₃,high capability of oxygen activation by Ag⁰made it possible to degradate DOM at low temperature,then HCHO converted completely to CO₂and water at 80^oC.（3）Guided by this viewpoint that hydroxyl groups are considered as anchoring sites for active components on catalysts,4 wt.%Ag/γ-Al₂O₃ catalyst derived from Boehmite（Al OOH）was prepared.The results of ¹H MAS NMR,UV-vis spectra and Transmission electron microscopy（TEM）images suggest that the abundant bridging OH groups prominently contributed to Ag anchoring and the formation of a large amount of small sized metallic Ag（mean size:2.6 nm）with high dispersion and Ag_n^δ+cluster,and thus facilitating the catalytic oxidation of formaldehyde.The Ag/γ-Al₂O₃catalyst showed 100%formaldehyde conversion to CO₂ at 75^oC and high stability.The conversion of HCHO remain 75%at 70^oC and 100%at75^oC for 24 h.（4）Further modifying the catalyst by addition of Nb₂O₅,the catalyst Ag/γ-Al₂O₃-Nb₂O₅were designed and prepared based on the catalyst 4 wt.%Ag/γ-Al₂O₃（from Al OOH）.The Ag⁰remains the main species and the mean size is at 2-3 nm.The increase of oxygen vacancy benefits the adsorption and activation of oxygen species.The sinergistic effect of the high ability of oxygen activation of small size Ag⁰ and adsorbed oxygen are responsible for enhancement of the catalytic activity for HCHO oxidation.The resullt of in situ DRIFTS test suggests that DOM is much more on Ag/γ-Al₂O₃-Nb₂O₅than on Ag/γ-Al₂O₃,and DOM can be degradated at lower temperature,thus the catalyst show higher activity for HCHO oxidation.In addition,the study found that the catalytic oxidation activity of formaldehyde is related to the mass ratio of Ag/γ-Al₂O₃ and Nb₂O₅.When the ratio of Ag/γ-Al₂O₃:Nb₂O₅is 10:3,the catalyst displays the 100%HCHO conversion at 70^oC and 85%at 65^oC.Compared with the conversion of Ag/γ-Al₂O₃ catalyst at 65^oC,this is an increase of 3.7 times.