Synthesis, Characterization And Catalytic Performance Of Highly Dispersed Supported Nickel Catalyst From Layered Double Hydroxides

In the field of heterogeneous reactions, carbon materials have been usedas catalyst supports to disperse and stabilize metal nanoparticles because oftheir outstanding properties, which can be listed as (1) excellent electrical andthermal conductivity,(2) high chemical stability,(3) low thermal expansioncoefficient,(4) light weight,(5) possible to have various pores,(6) non toxic,etc. Among various carbon-based support materials, CNTs are very attractivesince1991. To date, a large number of metals and metal oxides, such as Pt, Au,Ru, Pd, and RuO2have been deposited onto CNTs. However, the adhesion ofguest materials on CNTs matrix is often not strong enough to survive themechanical shaking involved in reaction by using the conventionalimpregnation of CNTs into the precursor solution. The applied main strategyfor efficient immobilization of active metal components is to utilize surfacemodification of CNTs with a variety of chemical functional groups or organicbinders, which facilitates enhancing the interactions between supports anddesired metal precursors. Layered double hydroxides (LDHs) belong to a family of highly orderedtwo-dimensional layered materials, where different M²⁺and M³⁺metal cationsuniformly distribute and orderly prearrange in the brucite-like sheets, andvarious charge-compensating anions (A^n-) are present in the interlayer space.According to the fact that LDHs can accommodate a large number of tunableM²⁺and M³⁺ions within the layers or in the interlayer space in the form ofmetal complexes, well-dispersed supported metal catalysts can be obtained byreducing calcined LDHs with desired active metal species. This kind ofstructural transformation endows LDH materials with extraordinary capabilityas catalyst precursors in various metal-catalyzed reactions. In comparison withother metal-supported catalysts, LDH-derived supported metal catalysts havetwo advantages:(i) Active components with adjustable content can beuniformly integrated into the LDH structure;(ii) metal nanoparticles withtunable particle size can be formed in a controllable manner.(1) Highly dispersed supported Ni nanoparticles were successfully preparedby reduction of nitric acid modified CNTs-supported NiAl-LDH. Ninanoparticles were uniformly dispersed on the external surface of nitric acidmodified CNTs with a narrow size distribution centered at around5.1nm. Arelatively strong metal-support interaction existed between the Ninanoparticles and the graphite edges sites of CNTs. Ni/L-HCNTs exhibitedexcellent catalytic performance in liquid phase hydrogenation with97.1%conversion of o-CNB and98.3%selectivity to o-CAN, which performed much better than CNTs supported Ni catalysts prepared by incipient wetnessimpregnation. The unusual behavior of as-prepared supported Ni catalyst isattributed to the higher dispersion of active component and strong interactionbetween nanoparticles and support.(2) A dispersion-enhanced nickel catalyst, Ni-L/P-CNTs, was prepared fromhybrid nanocomposite of NiAl-LDH and PAA-functionalized CNTs. InNi-L/P-CNTs, Ni nanoparticles with a narrow size distribution centered ataround6.0nm were highly dispersed on the external surface of support.Furthermore, supported Ni catalysts did not present both internal and externalmass transport limitations in the hydrogenation of o-CNB. Ni-L/P-CNTsexhibited superior catalytic performance in liquid phase hydrogenation,compared with that prepared by the conventional impregnation method. Theunusual behavior of Ni-L/P-CNTs catalyst is attributed to high dispersion ofNi0species as well as the strong metal–support interaction. And theappropriate reaction temperature and the Ni loading could improve theconversion of o-CNB by accelerating the rate of hydrogenation. Moreimportantly, owing to the intrinsic properties such as high surface area,chemical/thermal stability and low cost, such type of support Ni-based catalystmay work as an alternative to noble metal, which can be used widely in thefield of heterogeneous hydrogenation catalysis.(3) We established a new synthesis of supported Ni/C catalysts with enhanceddispersion derived from NiAl-LDH/C nanocomposite precursors, which were assembled through a separate nucleation and aging process accompanied bythe carbonization of glucose. XRD, HT-XRD and TG-DTA-MS results clearlyrevealed in situ reduction process of Ni²⁺to Ni⁰by carbon component incomposite as reducing agent. TEM, BET and XPS measurements indicatedthat the crystallite size of Ni nanoparticles for as-synthesized Ni/C catalystswas smaller than that for the catalyst prepared by traditional incipient wetnessimpregnation, and nanocomposite-derived Ni/C catalysts possessed highernickel dispersion. Studies of the liquid phase HDC of CB showed that2.5Ni/Ccatalyst exhibited excellent catalytic performance with CB conversion up to99.3%. This is attributed to the presence of more accessible Ni active sitesoriginating from appropriate metal dispersion and loading amount, as well asstrong metal-support interaction.(4) Carbon nanofiber supported nickel carbide (NiC/CNFs) with enhancementof dispersion was prepared by CVD over Ni/C catalyst under acetylene.2NiC/CNFs exhibited excellent catalytic performance in liquid phasehydrogenation with96.2%conversion of o-CNB and96.0%selectivity too-CAN. The unusual behavior of2NiC/CNFs catalyst is attributed to highdispersion of Ni⁰species as well as the existence of nickel carbide.