Theoretical Investigation On The Structures And Catalytic Activity Of Au And Pd Clusters

Compared to bulk materials,metal clusters with decreasing sizes exhibit a wide range of novel properties,such as high ratio of surface-to-volume,obvious quantum size effects,sharply increased surface free energy and unsaturated coordination of surface atoms,which have aroused increasingly more interest of metal clusters.New developments in experimental techniques make it possible to precisely control the sizes and components of metal clusters.Meanwhile,theoretical calculations are extremely valuable in providing microscopic information of structural,electronic and catalytic properties of metal clusters.Because of the variety in structures of metal clusters and the complexity of the catalysis under realistic reaction conditions,the structures,stability and important factors that govern the catalytic reactions of metal clusters remain unclear.In this dissertation,we investigated the stability of Aum clusters and explored the building units and construction rules of Aum clusters.We also discussed the stability and CO oxidation activity of Pds clusters with and without TiO2(110)substrate and figured out significant factors which influenced the catalytic reactivity.Our results will shed light on the understanding of the relationship between structures and properties of other clusters,and also provide guidance for the rational design of new heterogeneous catalysts.The main contents and results are listed as below:1.The stability and electronic structures of Aum clusters How clusters grow and aggregate into the larger ones remains as an unsolved problem.When the Aum cluster is smaller than Au11,i.e.m≤11,the cluster adopts planar structures.The aromaticity of these planar clusters has not been investigaed as far as we know.We combined the genetic algorithm with the density functional theory(DFT)method to investigate structures of Aum clusters,and analyze the aromaticity of clusters along with their energetic,geometric,vibrational,nuclear magnetic,and electronic density properties.Compared to the Au2 dimer,the planar Aum clusters possess more favorable normalized atomization energy.For the first time,we proposed that each planar Aum cluster could be constructed from several three-membered ring(3MR)Au3(2e)building blocks.With the increase of the number of shared edges between two neighboring Au3 units,the nucleus-independent chemical shift(NICS)values of Au3 units decrease,so does the local aromaticity,which reminded us of the Clar’s rule for the polycyclic π-aromatic polybenzoides.Hence,we named this phenomenon-the dilution of the aromaticity of planar clusters upon ring fusing,as like-Clar’s rule.From the analysis of the vibrational and electronic density properties and charge distribution,we found that the stabilization of planar clusters required a flow of negative charge from internal 3MR with electron-deficient bonding to peripheral 3MR units with stronger Au-Au bonds.The structure with the larger number of rings with dominant 3 c-2e character and a smaller degree of 3 c-3e character dominates the global minimum of the Aum potential energy surface,which is consistent with the above mentioned like-Clar’s rule.This indicates that 3MR Au3(2e)can act as the structural unit of planar Aum clusters.However,with the introduction of ligands,the aromatic and stability study of Au clusters becomes much more complicated.The electronic structures and aromaticity indexes of a series of Aum(SH)n(m,n = 5-12)were comprehensively investigated through energetic,vibrational,magnetic,and electronic density properties,which are highly sensitive to the size and topological structure of the cluster.Generally,computational results of energy gap,normalized atomization energy(NAE),and electron localization function(ELF)-σ values exhibit the odd-even effect,in which clusters with the even number of free valence electrons(being reflected by the value of m-n)possess relatively higher stability than the odd one.However,it is difficult to describe the stability of the cluster with the sophisticated three-dimensional structure through one single aromaticity index such as nucleus-independent chemical shift(NICS)value.Principal component analysis and clustering analysis of the calculation results of Aum(SR)n clusters suggest that the m-n value and the Au4 unit are important for predicting the stability of the Au clusters.2.Planar versus non-planar Pd8 clusters:Stability and CO oxidation activity of Pds clusters with and without TiO2(110)substrateThe different arrangement of valence electrons for Au and Pd atoms leads to distinctly different electronic structures and properties of Au and Pd clusters.The Pdm clusters exhibit non-planar structures when m ≥ 3.Based on theoretical calculations and clustering analysis,we noticed that Pdm behaved quite differently in structures and properties between m<8 and m ≥ 8.Hence,we focused on the Pds cluster in this work.The catalytic activity of metal clusters supported on metal oxide surfaces is sensitive to sizes and shapes of nanoparticles,interfacial interaction with substrates.The structures and CO oxidation activity of unsupported and supported Pd8 clusters on TiO2(110)(denoted as Pd8/TiO2(110))were studied with density functional theory,respectively.The free-standing Pds cluster prefers a non-planar three dimensional(3D)structure.However,when the Pds cluster is dispersed on the TiO2(110)surface,the most stable planar two dimensional(2D)Pd8/TiO2(110)system is 2.30 eV lower in energy than the 3D one.The lattice matching makes the 3D Pdsstay closer to the TiO2(110)surface than the planar cluster.In contrast,2D Pd8 interacts with the upmost electronnegative O atoms of TiO2(110)surface,resulting in more evident interfacial charge than the non-planar 3D one.The increasing number of the adsorbed CO is hard to transform 3D Pd8/TiO2(110)into 2D Pd8/TiO2(110),different from what was reported for Pd7/TiO2(110)system.The CO oxidation paths of both 3D Pd8/TiO2(110)and 2D Pd8/TiO2(110)are favorable thermodynamically and kinetically.Taking advantage of the charge transfer,the reaction with planar Pd8is more favorable than the 3D cluster on TiO2 surface.The 2D Pd8 reconstructed into another truncated triangle 2D appearance during the reaction.In addition,the reactivity of single-atom catalyst of Pdi/TiO2(110)is demonstrated to have a different reaction pathway and higher activation barrier than that of the Pd8/TiO2(110)system.Therefore,the reaction species not only changed the shape but also the CO oxidation activity of Pd8 clusters,and the charge transfer played an important role in the CO oxidation on the Pd8/TiO2(110)systems.We hope our study will provide useful information for rational design of heterogeneous catalysts.