High-throughput Catalytic Abatement Of Co And O₃ In Air:Thin-felt Al?Cu?-fiber Structured Catalysts And Their Catalytic Performance

In modern times,the extensive using of fossil fuels and the massive exhaust emissions from both the chemical industry and motor vehicles have led to serious air pollution problems?O₃,CO,NO_x,etc.?,which poses a great threat to human health.With the increasing awareness of environmental protection,the voice of controlling the emission of pollution sources and environment governance is becoming more and more intense.Environmental catalysis is a key technology in environmental control.However,structured catalysts prepared by traditional coating process which meet the requirements of high flux and low pressure drop operation in environmental catalysis process have the problems of exfoliation of coatings,binder contamination and high cost of catalyst preparation.In this dissertation,based on the“Top-Down”design philosophy,namely reactor?Top,hydrodynamics and transfer?-catalyst?Down,surface/interface reaction?,and fully utilizing the advantages of metal-fiber structured supports with 3D open networks?optimizing mass transfer/heat transfer,high flux,low pressure drop,large-scale preparation?,we developed microfiber structured catalysts via in-situ hydrothermal and impregnation methods.These catalysts can hold active components solidly on the surface and exhibited good catalytic activity and stability for CO oxidation and O₃ decomposition reactions,which realized the collaborative coupling of shape controllable design of structured catalysts and catalytic functionalization of substrate surface.The detailed contents and main results of this dissertation consisted of four parts as following:?1?Thin-felt microfibrous-structured Au-?-Fe2O3/ns-?-Al2O3/Al-fiber catalyst for high-throughput CO oxidationA thin-sheet aluminum microfiber felt?denoted as Al-fiber,16 mm diameter,2mm thickness,60?m fiber diameter?with large void fraction?15 vol%Al fibers and85 vol%voidage?,porous structure,large specific surface area,good permeability,enhanced mass transfer/heat transfer and flexible geometry was employed as substrate.The thin-sheet aluminum microfiber felt was first hydrothermally treated for in situ growth of AlOOH nanosheets?ns-AlOOH?along with the Al-fiber.As-obtained ns-AlOOH/Al-fiber was calcined in air at 600 ^oC for 2 h to form ns-?-Al₂O₃/Al-fiber.Subsequently,the?-Fe₂O₃/ns-?-Al₂O₃/Al-fiber support was obtained by directly growth of nano-sheet FeOOH on the ns-?-Al₂O₃ firmly attached on the Al-fiber substrate by hydrothermal synthesis followed by 600 ^oC calcination in air for 2 h.Finally,the Au NPs were deposited onto the?-Fe₂O₃ structured on the ns-?-Al₂O₃/Al-fiberusingDP-ureamethodtoproductthe Au-?-Fe₂O₃/ns-?-Al₂O₃/Al-fiber catalyst.The hydrothermal conditions for fabricating the?-Fe₂O₃/ns-?-Al₂O₃/Al-fiber support are very critical to the final catalyst performance,and the optimal variables are screened out as follows:T_HT-Fe=180 ^oC,t_HT-Fe=12 h,n_u/Fe=10.4.Such microfibrous-structured catalyst?0.2 wt%Au loading?performs robustly for at least 232 h with two temperature changing cycles?0 ^oC?25^oC?150 ^oC?.After first testing cycle for 120 h,whereas the low temperature activity is declined to some extent?CO conversion:⁴0%to 23%at 0 ^oC;>99.9%to⁶8%at25 ^oC?,the catalyst still shows good CO conversion maintenance within second testing cycle.Compared with particulate Au-?-Fe₂O₃ catalyst,the structured Au-?-Fe₂O₃/ns-?-Al₂O₃/Al-fiber catalyst showed higher catalytic efficiency?TOF?and utilization ratio of active components,and only very low pressure drop was produced.SEM and EDX characterizations revealed that the uniform surface morphology of Au-?-Fe₂O₃/ns-?-Al₂O₃/Al-fiber catalyst and the highly dispersed active components of Au-?-Fe₂O₃ on the surface of Al-fiber substrate were responsible for the improved activities for low temperature CO oxidation.?2?Copper-fiber-structured M-CuOx?M=Co,Ce and Mn?:Preparation,catalytic performance and kinetic study in low-temperature CO oxidationThe thin-sheet sinter-locked microfibrous structure consisting of 15 vol%Cu fibers?60?m fiber diameter?and 85 vol%voidage,with entirely open 3D porous network,were prepared through wet layup paper making/sintering processes to use as substrates.Firstly,the thin-felt CuO_x/Cu-fiber supports?8 mm diameter,2 mm thickness?are obtained through endogenous growth of strip-like CuO_x onto a three-dimensional?3D?network of 60?m-Cu-fiber thin felt with the aid of acid etching,followed by calcination.Subsequently,5M-CuO_x/Cu-fiber catalysts?M=Co,Ce and Mn?were prepared using impregnation method,followed by calcination.The experiment results showed that 5Mn-CuO_x/Cu-fiber exhibited the best catalytic activities for CO oxidation:T₅₀ value for CO oxidation of the as-obtained5Mn-CuO_x/Cu-fiber catalyst is⁷8 ^oC in a feed gas of 1 vol%CO in air with a GHSV of 6000 mL g^-1_cat.h^-1.The largest surface area,the smallest Cu₂O particle size ad the most regular surface morphology together with the improved redox ability and oxygen mobility are paramount for the remarkable improvement of CO oxidation activity of the 5Mn-CuO_x/Cu-fiber catalyst,as verified by BET,XRD,Raman,SEM,H₂-TPR,O₂-TPD and CO-TPD measurements.Besides,the results of kinetic experiment showed that the 5Mn-CuO_x/Cu-fiber catalyst has the lowest activation energy.And by comparing the reaction orders of CO and O₂ over 5Mn-CuO_x/Cu-fiber catalyst at different temperatures,it can be seen that the CO oxidation reaction on5Mn-CuO_x/Cu-fiberatlowertemperatures?<75^oC?followesthe Langmuir-Hinshelwood mechanism,while at higher temperatures?>150 ^oC?,the reaction followes the Mars-van Krevelen mechanism.?3?High-performance Co-MnOx composite oxide catalyst structured onto Al-fiber felt for high-throughput O3 decompositionThe thin-sheet aluminum microfiber felt?denoted as Al-fiber?mentioned in part?1?was employed as substrate.Firstly,the thin-felt ns-?-Al₂O₃/Al-fiber supports are obtained through endogenous growth of free-standing boehmite nanosheets onto a three-dimensional?3D?network of 60?m-Al-fiber thin felt with the aid of steam-only hydrothermal oxidation reaction between Al metal and H₂O,followed by calcination.Co-Mn oxide composites were then place onto the pore surface of as-obtained ns-?-Al₂O₃/Al-fiber supports by co-impregnation method using Co and Mn nitrates as precursors followed by wetting with Na₂CO₃ solution and subsequent calcination.The initial activity for O₃ decomposition and stability of the Co-MnO_x/ns-?-Al₂O₃/Al-fiber?denoted as Co-MnO_x-Al?catalysts are sensitive to their Co/Mn molar ratio and catalyst calcination temperature.The best catalyst is the Co-MnO_x?0.36?-Al with Co/Mn molar ratio of 0.36 and Co-MnO_x loading of 5 wt%?Co₂O₃+MnO₂?after calcining at 500 ^oC in air,providing unique combination of high activity,good stability,promising moisture resistance,and low pressure drop at high-throughput operation.For example,this catalyst achieves full O₃ conversion and is stable for at least 12 h for a dry feed gas with 1000±30 ppm O₃ in O₂/Ar mixture at a high linear velocity of 10.5 cm s^-1;whilst,even in case for a feed gas?1000±30 ppm O₃?with50%relative humidity,an initial O₃ conversion of 88%is obtainable but smoothly slides to a flat of⁶6%within 90 min.In nature,CoO_x additives introduced into the Co-MnO_x-Al catalyst system at an appropriate Co/Mn molar ratio results in enhanced interactions between CoO_x and MnO_x thereby leading to improved redox properties of MnO_x and facilitating the formation of low-valent manganese(Mn³⁺and Mn²⁺)species on the catalyst surface,as verified by XRD,Raman,H₂-TPR,XPS and EPR measurements.The Co-MnO_x?0.36?-Al achieves the highest the fraction of surface low-valent manganese species,that is,the highest amount of oxygen vacancies that are suggested to be the active sites for decomposition of O₃.?4?Thin-felt Al-fiber-structured Pd-Co-MnOx/Al2O3 catalyst with high moisture resistance for high-throughput O3 decompositionTo further improve the O₃ decomposition activities under humid conditions of Co-MnO_x-Al catalyst?Co/Mn molar ratio of 0.36 and Co-MnO_x loading of 5 wt%?prepared in part?3?,we developed Pd-modified Co-MnO_x-Al catalyst.And Pd was introduced into Co-MnO_x-Al catalyst using impregnation method and the as-prepared Pd-Co-MnO_x-Al catalyst exhibited markedly improved activity,stability,and enhanced moisture resistance.The most promising 0.1Pd-Co-MnO_x-Al?0.1 wt%Pd,0.36 Co/Mn molar ratio,5 wt%Co-MnO_x loading?catalyst remains full O₃conversion for at least 4 h at 25 ^oC for a feed gas containing 1500±45 ppm O₃ even at a high relative humidity?RH?of 50%and 70%,using a high gas hourly space velocity of 48000 mL g_ca^-1 _t.h^-1;the full O₃ conversion quickly slides to a flat of⁹6%during 4h testing at 90%RH whereas it is retrievable immediately after switching the feed gas to a dry one.The remarkable improvement of activity,stability and moisture resistance by Pd-doping of Co-MnO_x-Al is,in nature,due to the highly improved and stabilized low-valent-Mn related oxygen vacancies?i.e.,active sites?and markedly weakened H₂O adsorption on the catalyst surface,which are verified by XRD,H₂-TPR,O₂-TPD,H₂O-TPD and XPS measurements.