Structure Design And Mechanism Of Nickel-based Ammonia Decomposition Catalyst

There is widespread consensus that fossil fuel reserves,especially oil reserves,will be exhausted to a large extent in the course of the current century,possibly leading to shortages relatively soon.Hydrogen is considered to be the most ideal fossil fuel alternatives as an efficient,clean,sustainable energy carrier.At the same time,hydrogen fuel cells gained more and more attention as a prospect of hydrogen energy use.However,how to store safely and release efficiently is the main obstacle of hydrogen utilization.In order to solve this problem,in situ hydrogen production technology has become a meaningful alternative way.Ammonia with high hydrogen storage density and easy liquefaction properties was regarded as ideal hydrogen-stored materials.At present,ammonia decomposition under low-temperature still has many problems to be solved.It is of practical significance to investigate the economically effective ammonia decomposition catalyst.Designing and synthesizing ammonia decomposition catalyst with high catalytic activity and high stability is the key to solve these problems.Herein,the density functional theory was used to design the catalyst and the oleylamine system was used for the experimental study.The main conclusions are as follows:(1)Four different size nickel nanoclusters,Ni19,Ni44,Ni85,and Ni146,were used to investigate the nanoscale size effect throughout DFT calculation.The predicted catalytic activities of these nickel nanoclusters decrease in the sequence of Ni44＞Ni146≈Ni85＞Ni19,based on the analysis of adsorption energies of NH3 decomposition intermediates and landscapes of NH3 decomposition pathways.Besides,the reaction heat of Ni146 nanocluster is only 0.65 eV in the process of N2 desorption,showing the nature of easily desorbed out of the surface of the catalyst.Moreover,molecular dynamics calculations show that large-size nanoclusters perform better on thermal stability.Combined with dynamics analysis,we found that Ni146 nanocluster possesses not only high thermodynamic stability but also good catalytic activity.(2)We prepared Ni nanocrystals of different sizes by thermodynamic control.By using oleylamine system,uniform size 1.5～8 nm Ni metal nanocrystals were obtained by controlling the reduction temperature,the amount of metal salts,the concentration of reductant and the order of reduction process.The influence of nickel particle size in the range of 1.5～8.0 nm in the process of ammonia decomposition reaction has been investigated using well-defined catalysts based on an inert MCF-17 support material.The size effect of nickel particles is more obvious as the temperature increases,moreover,Ni nanoparticles possess the excellent catalytic activity with particle size between 3 nm and the catalytic activity is probablely affected by the concentration of B5 active sites.(3)By means of density functional theory(DFT),the ammonia decomposition reactions catalyzed by Ni13,Cu13,and Ni12Cu clusters have been studied and compared.We firstly investigated the structural stability of these clusters,and then systematically investigated their ammonia decomposition activity by analyzing the adsorption property of reaction intermediates and the relative energy diagram.The results show that the adsorption energy of reaction intermediate N on Ni12Cu cluster is-5.93 eV,which is very close to the optimal value(-5.81 eV)of ammonia decomposition volcano curve.The reaction energy diagram shows that the dehydrogenation of NH intermediate is the rate-determining step for these clusters due to the positive reaction heat.Furthermore,the catalytic property of Ni12Cu cluster is interpreted by density of states.It indicates that the adsorption energies of reaction intermediates mainly depend on the d-band center of the clusters.