In Situ Preparation Of Nickel Foam Monolithic Catalysts And Its Catalytic Application For VOCs Removal

Volatile organic compounds(VOCs)not only directly affect environment and human health,but also cause photochemical ozone/smog pollution as precursors.Benzene,toluene,ethylbenzene,and xylene(BTEX)are kinds of representative VOCs which show the following characteristics:various pollution sources,strong volatility,wide distribution,high difficulty in degradation,moreover,these pollutants can be harmful to skin and can cause neurological,respiratory,and central nervous system damage with prolonged exposures.For the consideration of economic benefit and practicability,catalytic oxidation,an effective strategy to completely remove VOCs,is undergoing a transformation from noble metal catalyst to transition metal oxide catalyst,and from powder catalyst to monolithic catalyst.For monolithic catalysts,the intrinsic activity of the active component and the anchoring force between the active phase and the carrier are important factors affecting the catalytic activity.Compared with the traditional impregnation and coating process,the in-situ synthesis strategy can not only avoid the uneven dispersion of the acative components,but also effectively improve the peel resistance of the active components.Different from the single radial heat transfer mode of ceramic,the metal carrier exhibits excellent diathermancy and electron trannsport ability,which will reveal great applications prospect in the field of synergetic catalysis with photosource or electricity.In this paper,a series of transition metal-based monolithic catalysts were prepared by using flexible nickel foam as a carrier through an in-situ synthesis strategy,expected to be applied in the field of micro VOCs emission source treatment and indoor environmental purification.The catalytic activity of the catalyst was optimized step by step from the intrinsic activity of the active component and the anchoring force between the active component and the carrier.The specific research contents and results are as follows:(1)Self-sacrificing template was introduced in this work,which resulted in nickel-cobalt layered double hydroxides regularly arranged in vertical orientation,then leading to a hollow nanocage structure with rich specific surface area.After calcination,nickel-cobalt layered double hydroxide is transformed into NiCo₂O₄ spinel phase with a nanoarray structure that has been completely inherited.The effect of nickel ions concentration,hydrothermal duration and calcination temperature on the surface defects of the catalyst was studied,and Ni-Co-2:1-6h-350 sample possesses optimal catalytic activity and stability,achieving 90%toluene conversion at 229? under a space velocity of 60000 mL g-1 h-1.The excellent catalytic performance is attributed to abundant surface trivalent cobalt,active oxygen species,and moderately strong acid sites.Finally,the NiCo₂O₄@NiO/NF monolithic catalyst was prepared by in-situ hydrothermal method under the optimal synthesis conditions.The active component of the monolithic catalyst maintained the micro-morphology of the powder catalyst,but the activity of the monolithic catalyst needed to be improved due to the low loading mass.(2)In this work,a series of NiCo₂O₄ nickel foam based monolithic catalysts were successfully prepared by in-situ electro-deposition strategy.The activity phase was strongly and vertically anchored on nickel foam,leading catalyst to a stable structure,abundant surface areas and excellent electronic transmission capacity.Moreover,the as-prepared catalysts exhibit abundant structural defects which can be attributed to the violent reaction.All the samples assuredly possess excellent benzene oxidation performance,and the sample of CoNi-NF-2:1 exhibits superior activity,oxidizing about 90%benzene at 198? and 227?(after ultrasonic exfoliation experiment)under a gaseous hourly space velocity of 6000 h-1.It can be rationally drawn the conclusion that the superior catalytic performance and stability of the catalysts are primarily attributed to the strong anchoring force,rich oxygen vacancies,and the higher ratio of Co3+/Co2+.(3)MnOx/NF monolithic catalyst was successfully prepared by an in-situ electro-deposition strategy,amorphous MnOx active phase uniformly coated on three-dimensional nickel foam.At the space velocity of 6000 h-1,MnOx-NF-250 and MnOx-NF-350 catalysts reached 90%benzene removal rate at 212 and 217?,respectively.The high activity of these two catalysts was attributed to the strong interphase anchoring force and abundant surface active oxygen species,respectively.After ultrasonic exfoliation experiment,MnOx-NF-250 catalyst oxidizing about 90%benzene at 219?,which was only 7? higher than fresh catalyst,indicating that the strong interphase anchoring force was produced between the MnOx active component and the nickel foam support of MnOx-NF-250catalyst.(4)In this work,the strategy of alternating anode and cathode electro-deposition was creatively proposed,a hierarchical monolithic catalyst that three-dimensional nickel foam with an open-pore structure at the bottom,structural flexible MnOx crystal in the middle layer,and vertically oriented small size NiCo₂O₄ spinel phase at the top was successfully prepared.Consequently,NiCo₂O₄-MnOx-NF-250 monolithic catalyst exhibits excellent performance,oxidizing about 90%benzene at 196? and 204?(after 2 h ultrasonic exfoliation experiment)under a gaseous hourly space velocity of 6000 h-1.Then,by optimizing the electro-deposition parameters and changing the deposition mass of the two active phases,NiCo₂O₄-MnOx/NF monolithic catalyst with rich phase interface was constructed.This catalyst exhibits a rich concentration of surfae active oxygen species,and its catalytic performance is significantly improved,achieving 90%benzene conversion under 200? at a space velocity of 12000 h-1.