Preparation Of Nickel And Nickel-Based Binary Alloys Supported On Multi-Walled Carbon Nanotubes And Their Applications As Catalysts

Due to the over-exploitation and utilization of nature by human,resources, energy shortages and environmental pollution problems arebecoming more and more serious eventually. Therefore, in the21thcentury, therational use of existing resources and energy, as well as protection of theenvironment has been paid more and more attention by human beings,meanwhile, the development and utilization of new energy has become apressing problem. The concept of "efficiency, energy saving, environmentalprotection" has been paid increasingly attention. Hydrogenation is a keyreaction in the industrial production, the way of improving the efficiency ofindustrial hydrogenation is of significance importance for the efficient andenergy-saving. Meanwhile, the development of more friendly for environmentand more efficient power supply is of great strategy, and fuel cell is widelyregarded as the new technologies to achieve this goal, and the developmentand progress of the above two fields dependent on the development ofcatalysts. At present, the catalysts for industrial hydrogenation and fuel cellelectrode are mainly noble metals. However, the noble metals such asplatinum and palladium have the disadvantages of lacking of resources,expensive, difficult in controlling the cost and long-term utilization. Therefore,the application of non-noble metal catalysts, especially nickel series in thesetwo fields has been paid more and more attention gradually. However, thenickel catalysts have the advantages of lacking of catalytic activity, stabilityand so on, leading to the difficulties of the applications in actual utilization.Based on the background above, this thesis has mainly focused on thestudies of preparation of nickel and nickel-based binary alloy supported onmulti-walled carbon nanotubes and their applications as hydrogenationcatalysts and electrocatalyts. We applied two synthesis methods, electrolessdeposition and optimized liquid phase reduction for the preparation of threenickel-series catalysts, which are amorphous MWCNT/Ni-B, MWCNT/Ni-Pcatalysts and crystalline state MWCNT/Ni catalysts, and systematical studiedthe effect of preparing methods on the behavior of catalysts. The detailphysical properties of the catalysts are characterized by XRD, SEM, HRTEM,XPS, ICP and TG-DTA and obtained the most optimized preparationparameters. Then, we studied the applications of the two amorphous Ni-basedbinary alloys for catalytic hydrogenation of styrene and benzene and their kinetic processes. Finally, we exploited the applications of the three Ni-seriescatalysts in the field of alkaline ethanol fuel cell by researching their catalyticperformances, mechanisms and kinetic processes for ethanol electrocatalyticoxidation. The main creative achievements are exhibited as follows:(1) A novel electroless bath containing NiCl2·6H₂O as predominant salt,ethylenediamine as complexing agent, NaOH as pH regulator and NaBH4asreduction agent has been successfully developed to deposit the Ni-Bnanoparticles on the sensitized-activated MWCNT by controlling the reactiontime. The obtained Ni-B binary alloy nanoparticles are fine spheres comprisedof amorphous structure with the morphologically unique fine-structure likeflowers, and homogenously dispersed with a narrow particle size distributioncentered at around60nm diameter. With the increase of electroless platingreaction time, the Ni loading as well as B content in the Ni-B particlesgradually increase, leading to the crystallization temperature eventuallyincreased to326°C, finally. XPS results indicated that there are electronstransferred from boron to nickel making the surface electro-enrich.(2) A novel electroless bath containing NiCl2·6H₂O as predominant salt,C₆H₅Na₃O₇as complexing agent,(NH4)₂SO₄as special additive, NaOH as pHregulator and NaH₂PO₂·H₂O as reduction agent has been successfullydeveloped to deposit the Ni-P binary alloy nanoparticles on thesensitized-activated MWCNT by optimizations of the reaction time,electroless bath concentration, NaOH added amount and the reactiontemperature. The obtained Ni-P nanoparticles are fine spheres of solid state,and homogenously dispersed with a narrow particle size distribution centeredat around100nm diameter. The tests results showed that the Ni-Pnanoparticles of the optimal catalysts are typical amorphous structure and thecrystallization temperature is about360°C. After the high temperatureannealing, the Ni-P nanoparticles agglomerate and become larger comprisedof Ni and Ni3P mixed phases.(3)The studies of the catalytic hydrogenation performances and reactionkinetics of the two amorphous Ni-based binary alloys MWCNT/Ni-B and theMWCNT/Ni-P catalysts showed that the two amorphous catalysts exhibitedexcellent catalytic activities toward the hydrogenation reaction of styrene,which can be100%selectively hydrogenised transferred into ethylbenzeneunder the condition which can be easily achieved. The kinetic parameters ofthe two amorphous catalysts showed first-order dependence on styreneconcentration for both catalysts. MWCNT/Ni-P catalysts have a smallerreaction order dependence on hydrogen pressure, and styrene can be100%conversed under lower temperature range and showing relatively higher rateconstant k of35122.2·e(-4914.7/T), smaller activation energy E of40.86kJ/mol,indicating better catalytic activity of MWCNT/Ni-P catalysts. Further study ofthe MWCNT/Ni-P catalysts for catalytic hydrogenation of benzene showed itsexcellent catalytic activity for benzene, which can be selectively conversed to cyclohexane. The conversion of benzene increases linearly with the increasesof time, indicating a zero-order dependence on benzene concentration;meanwhile, there is a maximum value of about60%at the reactiontemperature of180°C, which might be caused by: firstly, the coverage ofaromatics at the surface of catalyst decreases as the temperature rises,resulting in maximum value at a certain temperature point; secondly, themaximum temperature point may be caused by the competitive adsorption ofbenzene and its reduced molecules during the reaction.(4) Developed an optimized liquid phase chemical reduction method bythe introduction of acidificated-sensitized-activated multi-walled carbonnanotubes. After controlled the amount of adding surfactant CTAB, pH valueof the system at about10and used hydrazine as the reducing agent reacted at80°C for1hour, we finally got small polycrystalline nickel nanoparticlesuniformly distributed on multi-walled carbon nanotube surface with particlesize of about20nm. Additive agent CTAB can complex with Ni ion atappropriate pH range. If the aqueous solution had low CTAB concentration(<120mM), a bilayer could be formed around Pd nanoparticles built by CTABmolecules, which controlled the concentration of both hydrazine andNi/hydrazine close to the Pd surface. In this case, the reduction rate of nickelwas decresed, making the particle size smaller; If the concentration of CTABincreased to high level (>120mM), the solution inclined to oil state, micellesinstead of bilayers would form on the surface of Pd nanoparticles, which couldinhibit the diffusion of Ni/hydrazine complexes and hydrazine molecules inthe solution, thus prevented the Ni reduction process.(5) The Electrocatalytic behaviors and kinetic studies of three Ni-seriescatalysts showed that all of the three Ni-series catalysts in alkaline mediumexhibited excellent electrocatalytic activities towards ethanol oxidation andthe electrocatalytic process were all controlled by the diffusion of ethanol inthe solution. MWCNT/Ni catalyst had the highest peak current for ethanoloxidation, which possibly caused by the larger reactive surface area fromsmaller particle size. The comparison of kinetic processes on three catalystsshowed that the MWCNT/Ni-B and MWCNT/Ni catalysts had similar valueof ethanol electrooxidation rate, which were both much higher than that ofMWCNT/Ni-P catalysts. The phenomenon could be attributed to the smallparticle size and good dispersion of Ni nanoparticles in MWCNT/Ni catalystsand the electron transfer between B and Ni in MWCNT/Ni-B catalysts.Therefore, if we could successfully synthesize amorphous Ni-B nanoparticlecatalysts with the size less than20nm supported on multi-walled carbonnanotubes, it could be expected to obtain extremely high oxidation current andoxidation rate for ethanol electrooxidation at the same time.