Synthesis Of Nickel、Iron And Their Alloys Catalysts And Their Application In Ammonia Decomposition

With economic development and increasing population,the current reserves of fossil fuels and other non-renewable energies are unable to meet increasing demand.In addition,the exhaust gas generated from fossil fuels is likely to pollute the environment and leads to a greenhouse effect.Therefore,it is imminent to develop alternative clean energy.Hydrogen is an ideal alternative energy source.It is burned to water without any pollution and any other side reaction.However,hydrogen is difficult to store and transport,not to mention a high-risk explosion range of 4-75%when mixed with air.Ammonia has attractive properties as a carbon-free hydrogen-containing molecule.At certain mass and volume,liquefied ammonia contains higher hydrogen content than liquefied hydrogen,and the explosion range of ammonia in air is only 16-25%,far narrower than the hydrogen explosion range.Therefore,ammonia is considered to be safer than hydrogen in use.The decomposition rate of ammonia gas is usually very low.The conversion of ammonia in the empty pipe at 600℃ is less than 1%.In order to effectively convert ammonia,we aim to design non-nobel metal catalyst for ammonia decomposition with good activity and stability.Nickel and iron were selected as the active components of catalyst,but the naked nickel and iron nanoparticles tend to agglomerate at high temperatures,leading to decrease in specific surface area and active sites,and consquently catalyst deactivation.In order to maintain catalyst stability at high reaction temperature,building up the core-shell structure is an effective way.Through encapsulating a layer of stable silica outside the active component,we can isolate the active nanoparticles and maintain a high degree of metal dispersion at high temperatures.In this thesis,binary Ni-Fe core-shell catalysts were developed and further applied in ammonia decomposition for pure hydrogen production.The following conclusions can be obtained:1.The nickel-based core-shell nanostructured catalysts were prepared by microemulsion method,and the conditions were optimized to achieve the uniform core-shell structure.The preparation conditions have been optimized as follows:the concentration of nickel nitrate is 2 mol·L-1,the volume of cyclohexane solvent is 30 mL,and the surfactant dosage is 2 mL.Under these conditions,the resulting core-shell structure is significant and uniform,with satisfactory dispersion.2.By fixing the overall metal content being 10%,the Ni/Fe ratio is systematically tuned in a series of Ni-Fe@SiO2 catalysts prepared under the optimized conditions.It was found that except for 10Fe@SiO2,all of the other catalysts exhibited typical core-shell structure with good particle dispersion.Among these catalysts,9Ni1Fe@SiO2 displays the highest activity,while 7Ni3Fe@SiO2 also shows good activity.3.By adjusting the total metal content of n(7Ni3Fe)@SiO2(n=1,1.5,2,and 2.5),the maximum metal content at which the core-shell structure can be retained is determined.It was found that 14Ni6Fe@SiO2 can yield a uniform core-shell structure and high catalytic activity.4.The effects of dopants on the 20Ni@SiO2 catalyst have been studied.Upon doping alkali metals K,Cs or lanthanide metals La and Ce of 2wt%equialvant Ni-Fe content,the activity of K-20Ni@SiO2 is slightly improved while the other element-doped catalysts show declined activity,indicating insignificant promotion effect by the employed additives on the current system.5.Based on the characterizations of TEM,XRD,BET,H2-TPR,H2-TPD,it is thought that the activity of core-shell catalysts is determined by the overall metal content,the Ni/Fe ratio,and the specific preparation conditions.Especially,the microcapsule-like structure generated upon in situ reduction is benifical to enrich the reactant molecules for surface adsorption/reaction on the active cores,thereby increasing ammonia conversion.