Theoretical Investigation On Structures And Properties Of Confined Transition Metals And Complexes

When microscopic particles,such as electrons,atoms,molecules,etc.,are confined in a definite space,they will exhibit properties different from their free-moving counterparts.This phenomenon is commonly called confinement effect.Electrons that participate in bonding will be confined by the nucleus and electrostatic interactions.A directly related example is metal atoms anchored on support,where metal electrons are confined at the metal-support interface by the interfacial bonds.This interfacial confinement will alter the energy levels and spatial distribution of metal states and affect the adsorption and reaction of adsorbates.On the other hand,in host-guest composites,guest molecules are staying in the cavities and their motion is limited by the host material.This kind of spatial confinement will induce the redistribution of charges within the guest molecule to exhibit properties different from either the host or the freestanding guest molecules.In this thesis,on one hand,we used formaldehyde oxidation and selective hydrogenation of 1,3-butadiene as probes to investigate the impact of interfacial confinement on the electronic structure and catalytic performance of transition metal atoms confined on graphene.On the other hand,we also constructed host-guest systems composed by hexagonal boron nitride(h-BN)and Pt(II)complexes to highlight significant role of host materials on the photophysical properties of these spatially confined transition metal complexes.The main contents of this thesis are as the following:We firstly investigated the electronic structure of Fe atoms anchored on defective graphene and their potential catalytic performance in formaldehyde oxidation by first principles based calculations.We found that the vacancy defects bind strongly with Fe atoms.The formed interactions not only stabilize these mono-dispersed Fe atoms from sintering into less reactive large particles but also effectively modulate the energy levels of Fe atomic states for effective activation of the adsorbed reaction species.The catalytic formaldehyde oxidation over Fe atoms anchored on defective graphene involves the formation and the dissociation of 5-membered peroxide ring intermediate,the formation of ?1-OCHO,the isomerization of ?1-OCHO and hydride transfer for formation of H2 O and CO2.The calculated energy barriers for formation and dissociation of 5-membered peroxide ring intermediate are 0.43 and 0.40 eV,respectively,while those for isomerization and hydride transfer are 0.47 and 0.13 eV,respectively.These findings suggest the potential high performance of these isolated Fe atoms for formaldehyde oxidation.Further investigations show the important role of Fe in stabilizing the high spin reactive species and facilitating the spin switching for the progression of the reaction.Then,we also investigated the electronic structure of Pd atoms confined on defective graphene and the role of these Pd atoms in selective hydrogenation of 1,3-butadiene.The results showed that strong interactions are formed between Pd atoms and C atoms associated with dangling bonds around the defects.Associated with these interactions,electrons of Pd atoms are partially confined at the interface resulting in redistribution of energy levels and spatial distribution of Pd atomic states and promoted selectivity to 1-butene.According to the calculated reaction barriers,the hydrogenation of 1,3-butadiene over these confined Pd atoms would take place through the 1,2-addition mechanism making 1-butene as the major product.The calculated barriers for the hydride transfer are 0.08 and 0.02 eV,respectively,showing not only the superiority of these confined Pd atoms for selective hydrogenation of 1,3-butadiene but also the role of interfacial confined Pd atoms in differentiating the reaction barriers along competitive reaction paths.Finally,we investigated change of structures and photophysical properties of a series of Pt(II)complexes when adsorbed on,confined into h-BN or solved in solutions based on extensive first-principles-based calculations at B3LYP/LANL2DZ/6-31g(d,p)level.The h-BN host has only negligible impact on the structure of these complexes,due to the nearly van der Waals interaction among them.However,charge redistribution is induced within the host-guest composites and inhibits the metal to ligand,intra-ligand and inter-ligand charge transfer,as evidenced by the blue-shifted the maximum adsorption wave-lengths as compared with those of complexes in solution,and this shift is nearly proportional to the interaction with the host.Further investigations highlight the significant role of the host materials on inducing the charge redistribution and polarization of the guest molecules to exhibit photophysical properties different from their solved counterparts.