Design And Application Of Composite Nanocatalytic Materials

With the progress of science and technology,as well as the increasing stringent environmental issues,integration chemical synthesis with multistep reactions under mild conditions aiming at the high efficiency and low emissions is highly desired.Therefore,greater demands are being placed on the catalytic activity and the control of selectivity to target product.For example,catalytic hydrogenation under the acidic conditions,the catalyst should not only have the catalytic hydrogenation activity but also the acid resistance and low loss.Both the ability of CO hydrogenation and the inhibition of CO2 production are required for iron-based Fischer-Tropsch(FT)catalyst.The additional requirements for the performance of the catalysts motivate the study of the composite nanocatalytic materials.Based on the above topics,this paper studied the performances of nickel silicides and silicon carbide encapsulated nickel for one-pot hydrogenation of nitrobenzene to p-aminophenol(PAP)in sulfuric acid,the influences of the electronic state in the semiconductor/metal reverse heterojunction on the FT performances of supported iron-based catalyst,and the structural stability of y-Al2O3 supported catalysts in catalytic oxidation of toluene.The contents are summarized as following:(1)The hydrogenation of nitrobenzene in sulfuric acid to produce PAP,an important intermediate,involves partial hydrogenation of nitrobenzene to phenylhydroxylamine,followed by in-situ acid catalyzed Bamberger rearrangement.The Pt/C or Pt/SiO2 is found to produce much aniline in the above process and thus the PAP selectivity is hardly to reach 85%.To address the above issue,Ni-based catalyst has been attempted to replace noble metals using solid acids,however,the results are not attractive.In this work,in order to improve the acid-resistance of Ni in sulfuric acid,Si was inserted into the Ni crystal structure to form nickel silicides.The as-prepared supported nickel silicides nanoparticles,especially NiSi2 nanoparticles supported on acid-treated active carbon,exhibit excellent acid-resistance and perform high hydrogenation activity and selectivity to PAP.The structural characterization indicates the electron doping from Ni to Si in the nickel silicides,which favors the stability in acid solution and the activity of hydrogen activation is partially reserved.Thus,the catalysts exhibit excellent stability in alloy structure,morphology,and catalytic performance in reuses,suggesting the NiSi2 nanoparticles as the promising substitute for the precious metal for PAP manufactures.(2)In our previous work,carbon nitride encapsulated Ni/Al2O3(CN/Ni/Al2O3)with semiconductor/metal reverse heterojunction was evidenced as a material in electron-rich state and possessed H2-dissociative adsorption activity due to the electron doping from underlying nickel.According to the above material structure,catalyst supported on 6 h acid-treated active carbon(SiC/Ni/AC6)with semiconductor(SiC)/metal(Ni)reverse heterojunction was prepared.The structural characterization suggests the electron interaction within SiC/Ni/AC6 enhances the hydrogenation reaction activity and inert SiC protects the catalyst from acid etching.Hydrogenation of nitrobenzene to PAP was also used as the probe,and the influence of reaction conditions on catalytic performance was studied.The catalyst shows excellent hydrogenation reaction activity,stability,and selectivity to PAP in 1.5 mol/L H2SO4.(3)Iron oxide loaded on the CN/NiAl2O3 was synthesized and investigated by FT synthesis to test the special effect of electron-rich support on the catalytic activity of iron oxide.The Fe/CN/Al2O3 and CN/Ni/Al2O3 samples were accordingly synthesized for comparison.In Fe/CN/Ni/Al2O3,the iron oxide was reduced to magnetite by syngas as evidenced by the in-situ x-ray photoelectron spectroscopy measurements and x-ray diffraction patterns.Compared with Fe/CN/Al2O3,more light hydrocarbons over Fe/CN/Ni/Al2O3 are observed.It should be understood by the enhanced H2 adsorption ability and the interaction between iron oxide and support mainly due to the effect of electron-rich support.In addition,such a novel support facilitates the CO conversion and retards the water-gas shift reaction and CO2 formation.(4)Catalysts loaded on y-Al2O3(a very important support)were reported to rehydrate in the aqueous reactions,especially in the hot water,with changes in the composition,structure and the morphology,and thus reducing the catalytic activity and stability.In this paper,y-Al2O3 was used to prepare the supported xPd/Al2O3 with different metal contents.Then xPd/Al2O3 was treated with silane to form Si-xPd/Al2O3.Characterization indicates that silicon species formed during the silane treatment and the silica produced on the surfaces of Si-xPd/Al2O3 due to oxidation during the reaction effectively prevent the rehydration of the support and avoid the performance degradation caused by metal particles aggregation.Therefore,Si-xPd/Al2O3 exhibits good structural and catalytic stability in the catalytic oxidation of toluene for the preparation of more valuable functionalized derivatives.