The Stability And Gas-phase Reactivity Of Co/Ni Clusters

Cluster science is an interdiscipline covering the research fields in atomic and molecular physics,condensed matter physics,quantum chemistry,surface chemistry,and nanomaterials etc.In-depth study of the structural stability and reactivity of size-dependent clusters is helpful to reveal the laws of material structure evolution and microscopic mechanisms of nanocatalysis,it is also a prerequisite and cornerstone for the development of new materials and catalysts with functional cluster genes.Transition metal clusters having special d or f electronic structures often exhibit some interesting and diverse physical and chemical properties,they have become one of the important research objects in research field of the cluster nowdays.However,the metal-metal bond energies of these metals are much smaller compared to their metal-oxygen bond energies,so they are easy to form oxides or load other non-metallic impurities,leading to a big challenge in preparations of their pure metal clusters in the gas phase.In this thesis,however,the normal distribution mass spectra of anions,cations,and neutral species of the 3d light transition metal cobalt/nickel clusters with 1-50 atoms have been measured by using a home-built spectrometer,the stability and gas-phase reactions of these size-dependent cobalt/nickel clusters have been systematically investigated.The cobalt/nickel are abundant in the earth,cheap,and readily available.There is a wide range of applications of them in industrial catalysis,optoelectronic devices,energy conversion,and molecular electronics.Therefore,the study of reactions between the cobalt/nickel metal clusters and small molecules is beneficial to deepen understanding of the microscopic mechanisms about them on surface adsorption,chemical reaction processes,and the formation of specific stable products,the results of which will provide useful information to the design and preparation of novel functional cluster genetic materials and devices.The following are the main research contents,the significant findings,and conclusions of the study in each section of this thesis.(1)The reaction of Con±/0 clusters with nitrogen:superatom Co6+ and superatomic complex Co5N6+In the study of gas-phase clusters,researchers have discovered many new species of clusters with highly symmetric structures and special stability,and have proposed"Special and General Superatoms"(the special sense refers to bare metal clusters following the jellium model and the general sense includs heteroatom/ligand-protected metal clusters).Based on the observation to reactions of Con±/0 clusters with nitrogen,it is found that the Con±clusters are more reactive with N2 than anionic and neutral cobalt clusters,and can adsorb one or more nitrogen molecules.Interestingly,the Co6+clusters do not form any Co6Nm+ products in a large amount of nitrogen environment,showing exceptional stability;in contrast,the Co5+ clusters are more reactive and the Co5N6+ complexes dominate the mass distrubution.Combined with DFT calculations,it is revealed that the highly D3d symmetric Co6+ clusters possess properties of open-shell superatomic stability of 1S21P3?1D0,aromaticity of the delocalized electrons,a large HOMO-LUMO energy gap,and a weaker Co-N interaction.In addition,it is elaborated that the stable superatomic complexes Co5N6+ have superatomic orbital characteristics similar to the Co5+ clusters and an increased HOMO-LUMO energy gap.This work provides a typical example in study on the special and general superatoms,and provides reliable experimental information for the design of stable and efficient cobalt-based materials.(2)Hydrogen evolution reaction of pure metal cluster Con?3+ with ammoniaCatalytic decomposition of ammonia to produce hydrogen has the advantage of being pollution-free,it is an important research for the development of clean,storable,and sustainable energy sources at present.Based on the observation to reactions of Con±/0 clusters with ammonia,it is found that the anionic Con-cluster is inert,the neutral Con clusters can adsorb multiple NH3 molecules,but the cation Con?3+ has the best performance in catalytic activation of N-H bonds.Results in the DFT calculations in energetics,charge analysis,partial density of states,potential energy scan,and natural bond orbitals confirm that the charge-dependent reactivity of cobalt clusters is in an order of Con+>Con>Con-.In addition,calculations of dynamic paths explain that Co+ and Co2+ are unable to provide active sites to the transfer of the second hydrogen atom in the reaction with ammonia,while the Con?3+ cluster can provide synergistic active sites for ammonia dehydrogenation,thus revealing the size-dependent reactivity of cobalt clusters.This study offers useful theoretical guidance in studying the mechanism of hydrogen evolution reaction from metal surfaces adsorbed with ammonia and in designing ammonia fuel cells.(3)The reaction of neutral Con clusters with oxygen:the finding of metalloxocubes Co13O8It is an interesting but challenging work to explore stable clusters to understand the structural evolution from atoms to macroscopic matter and construct new materials.Utilizing the DUV laser ionization mass spectrometry technique in study the reactions of neutral Con clusters with oxygen,a new species of especially stable Co13O8 has been discovered.DFT calculations confirm that Co13O8 clusters have a cubic core-shell structure as well as a large magnetic moment(30?B),and explain the rational process of the reaction of the icosahedral metal Co13 cluster with O2 in forming Co13O8 clusters.Using molecular dynamics simulations,it is revealed that the oxygen-saturated Co13O8 is stable at high temperatures and under the perturbation of more oxygen molecules.Combined with the electronic structure and orbital analysis,the special stability of Co13O8 is revealed to be closely related to its unique cubic structure,large HOMO-LUMO gap,and cubic aromaticity.We named these new oxygen passivated metal clusters as "metalloxocubes",which is suitable to be the candidates for the construction of new genetic materials.(4)The reaction of Nin± clusters with oxygen:metalloxocubes Ni13O8±Based on the experimental study on the reaction of Nin± clusters and oxygen,their reaction patterns and the stability mechanism of nickel metal oxides are explored.In the experiments,the Ni13O8± clusters are found to dominate the mass distribution of the reaction between Nin± clusters and oxygen.The DFT calculations and analysis show that the Ni13O8± cluster has a similar cubic structure and aromaticity as Co13O8.Comparing with the value of 30?B in the Co13O8 clusters,the Ni1308-and Ni13O8+ clusters have relatively low spin magnetic moments of 16?B and 12?B,corresponding to 15 and 11 single electrons,respectively.The spin population analysis reveals that Ni13O8-clusters are ferromagnetic,while the Ni13O8+ clusters are ferrimagnetic.In addition,it is shown that the stability of such magnetic clusters is related to their bonding nature of superatomic compounds.These stable ferromagnetic/ferrimagnetic "metalloxocubes"with tunable spin electron numbers may provide ideal candidates for the studies of spintronics,molecular electronics,and catalysis of genetic materials.