Air Pollutant Purification Over Monolithic Catalysts With Metal Substrate By Electric-assisted Technique

Volatile organic compounds（VOCs）and ozone（O₃）,as the main air pollutants,seriously threaten human health,and the secondary organic aerosols generated by the interaction of the two are more harmful to the human body.Therefore,promoting the rapid development of catalytic oxidation technology in VOCs and O₃ purification is in line with our country’s major national strategic needs to win the"Blue Sky Defense War".Compared with particulate catalysts,monolithic catalysts are more conducive to meet the actual needs of air purification due to the advantages of small wind resistance and low pressure drop.Among them,noble metal-based and manganese oxide-based monolithic catalysts are the most used for VOCs and O₃ purification,respectively.However,the existing monolithic catalysts still have deficiencies in low-temperature purification activity and water resistance,which limit their practical application in air pollution control.Therefore,it is imperative to develop high-performance catalysts and innovative new models of catalytic purification.In view of this,in this thesis,with the help of a new electric-assisted technology,on the one hand,on the FeCrAl corrugated sheet-supported palladium-based monolith catalyst,the electric-assisted heating mode was used to catalyze the oxidation and purification of VOCs,focusing on improving the catalyst’s rapid light-off and low-temperature purification activities.On the other hand,on the Al honeycomb-supported manganese dioxide monolith catalyst,the electric-assisted purification mode was used to catalyze the oxidation and degradation of O₃at room temperature,focusing on improving the water resistance and stability of the catalyst.Details as follows:On the FeCrAl corrugated sheet metal substrate,the precious metal palladium was slowly grown on the surface of the substrate by electroless plating to form a Pd/Fe Cr Al monolithic catalyst;then the catalyst was directly connected to the DC power supply to realize the electric-assisted heating mode,which was used for VOCs such as toluene pollutant purification research.First,two catalysts,Pd/Fe Cr Al-acid and Pd/Fe Cr Al-base,were obtained in acid and base systems,respectively.The characterization found that spherical Pd O particles with a particle size of about 100-200 nm in both catalysts can be firmly on the surface of the metal substrate,load and distribute evenly.When using the normal heating mode at the same time,the two catalysts can make the toluene conversion rate reach 95%at the reaction temperature of 190-220°C.Then,when the new electric-assisted mode was adopted,the reaction temperature of the Pd/Fe Cr Al-acid and Pd/Fe Cr Al-base catalysts when the toluene conversion rate reached 95%was reduced to 160°C and 130°C,respectively,showing superior low-temperature purification activity.At the same time,in the electric-assisted mode,the catalyst can be raised from room temperature to reaction temperature or lowered from reaction temperature to room temperature in only 90 seconds,showing excellent rapid heating and cooling characteristics.Further series characterization of the catalysts before and after the reaction under different heating modes found that,compared with the normal heating mode,the Pd O active component in the catalyst can better maintain the spherical particle size,surface micromorphology and surface oxidizes when the electric-assisted mode was used.At the same time,the oxidation ability of surface oxidized palladium species and active adsorbed oxygen was better and the content was higher,which make the low-temperature purification activity of the catalyst better in the electric-assisted mode.Finally,on the monolithic catalyst prepared by using Fe Cr Al resistance wire as the metal substrate,the toluene purification temperature of the catalyst can also be effectively reduced by using the electric-assisted mode,which proved the electric-assisted mode had good universality for improving the low-temperature purification activity of metal substrate monolithic catalysts.On the Al honeycomb metal substrate,a simple hydrothermal process controlled the slow growth of manganese oxide（MnO₂）on the surface of the substrate to form a Mn O₂/Al monolithic catalyst;then the catalyst was directly connected to the DC power supply to realize the electric-assisted mode,which was used for ozone room temperature purification studies.First,the optimal Mn O₂/Al catalyst was obtained by adjusting the preparation conditions such as acetic acid concentration,hydrothermal time and pickling pretreatment,characterization found that theα-Mn O₂ nanorods in the catalyst can be interdigitated and distributed on the surface of the metal aluminum substrate to form a uniform network structure;And the catalyst can achieve about 100%ozone conversion under dry atmosphere.Then,the water resistance test found that in the normal purification mode,when the relative humidity（RH）increased to90%,the ozone conversion rate of the catalyst would drop sharply to 20%,however,when switching to the electric-assisted mode,the ozone conversion rate of the catalyst will quickly return to 90%,and the ozone conversion rate can always be maintained above 82%in the 720-minute stability test;Moreover,by further increasing the electric-assisted input current,the ozone conversion rate of the catalyst in the stability test can also be increased to more than90%.In order to explore the reasons why the electric-assisted mode improved the water resistance and stability of the catalyst,a series of characterization methods such as XRD,Raman,SEM,TEM,H₂-TPR,O₂-TPD and XPS were used for analysis.First of all,it was confirmed that compared with the normal purification mode,in the electric-assisted mode,although the catalyst had the advantages of a small increase in its own temperature（<5°C）,more uniform dispersion of Mn O₂ particles and smaller particle size,it will help to improve the overall performance of the catalyst,but it should not be the decisive factor;Then it was confirmed that the deactivation mechanism of Mn O₂/Al catalyst was also due to the competitive adsorption of oxygen vacancies on the surface of manganese oxide by water molecules in the normal purification mode.At the same time,it was confirmed that compared with the normal purification mode,the electric-assisted mode can keep the surface oxygen vacancy density of the catalyst basically unchanged before and after the reaction.It was speculated that the external electric field formed between the metal substrate and the manganese oxide component layer by the electric-assisted mode can effectively prevent the competitive adsorption of polar water vapor molecules on the catalyst surface,thereby effectively improving the water resistance of the catalyst;At the same time,the electron transport loop formed from the DC power supply via the Al metal substrate to the manganese oxide component in the electric-assisted mode can also effectively promote the desorption of intermediate oxygen species on the surface of the catalyst and the recovery of surface oxygen vacancies,effectively improving the stability of the catalyst.