Density Functional Theory Study Of W_mx_n(x=cu,mo) Bimetallic Alloy Clusters And Its Adsorption Of Micromolecule

Geometric and electronic properties of W_mCu_n（n+m≤7）,W_mMo_n（n+m≤7）,W_mCu_n H₂（n+m≤7）,W_mMo_n NO（n+m≤6）and W_mX_nCO（m+n≤6,X=Cu,Mo）clusters have been systematically calculated by density functional theory at the generalized gradient approximation level in Dmol³ software package supported by Material Studio.Results are as follows:（1）Geometric and electronic structures of W_mCu_n（m+n≤7）clusters have been systematically calculated by density functional theory at the generalized gradient approximation level for ground-state structures.WCu cluster is linear.W₂Cu,WCu₂,W₂Cu₂,WCu₃,WCu₄ and WCu₆ clusters are planar.The remaining clusters all possess three dimensional configurations.The order of bond length is W-W<Cu-Cu<W-Cu.The calculated second-order differences in energies and HOMO-LOMO energy gaps show pronounced odd-even oscillating behaviors.It is found that Cu 4p and W 6p orbitals appear electronic charge and both Cu 4s and W 6s orbitals lose electronic charge to the W 5d orbital,which leads to the extension of W-Cu bond length.The magnetism of cluster mainly comes from W atoms.（2）Geometric and electronic structures of W_mMo_n（m+n≤7）clusters have been systematically calculated by density functional theory at the generalized gradient approximation level for ground-state structures.Geometry optimization shows that clusters are almost bipyramid structures with m+n>4.E_b of clusters is mainly dominated by W atoms.And the substitution of atoms between W and Mo in Mo_n or W_n（n≤7）clusters enhances the stability of the original clusters.The calculated IE shows that W_1,3,5Mo,W_1,3,5Mo₂,W_1,2,3Mo₃ and WMo_4,5 are relatively more stable in the chemical reaction.In addition,the magnetism of clusters mainly comes from valance d orbitals.（3）The ground state structures of W_mCu_nH₂（m+n≤7）clusters were systematically investigated by using density functional theory（DFT）method at the generalized gradient approximation（GGA）level.Stability and electronic properties were also calculated based on the most stable structures.For all W-Cu clusters,H atoms prefer to attach to W atoms in this system during adsorption because nearly half-full d orbital in W atom enhances the s-d hybridization between W-H atoms.Compared with pure copper clusters and gold-copper clusters,tungsten-copper bimetallic clusters are better for hydrogen storage.Hydrogen adsorption and dissociation process take place more efficiently at the W₂Cu₃H₂ cluster.And due to high thermodynamic stability and adsorption energy of W₅CuH₂ cluster,W₅Cu is more suitable for hydrogen storage.Analysis of electronic properties shows that s orbits of H and W atoms interact with each other to form the bond and more electrons transfer from H atom to W atom with the growth of the cluster which benefits the hydrogen storage.（4）The ground state structures of nitric oxide adsorption on WmMon（m+n≤6）clusters were systematically investigated based on bare W-Mo clusters by using density functional theory（DFT）method at the generalized gradient approximation（GGA）level.Stability and electronic properties were also calculated and analyzed.The adsorption mode of NO molecule is nitrogen end bridging a tungsten atom for W_1,2Mo_1,2,3 and W₃Mo₂ clusters.While NO prefers to locate on the hollow site for WMo₅,W₂Mo₄ and W₃Mo₃ clusters.And dissociation of NO molecule happens on W₃Mo.N-O bond lengths expand with the increasing number of tungsten atoms originating from metal→π^*back-donation.The binding energy is mainly controlled by element W,and it increases with the growth of the clusters except for that of W₄MoNO cluster.W_1,4MoNO,W₂Mo₂NO,W_1,3Mo₃NO and WMo₄NO are more stable and have lower chemical reactivity.The adsorption energy trend fits the variation of the N-O bond length and adsorption energy goes up with the increasing size of the cluster except for W₃MoNO and W₃Mo₃NO clusters.The increase of electron density in NO coming from W-Mo system to theπ^*anti-bonding orbital of NO molecule which leads to the elongation of N-O bond and strengthens the bond between W-Mo and NO molecule.（5）The ground state structures,stability and electronic properties of carbon monoxide adsorption on W_mCu_n and W_mMo_n（m+n≤6）clusters were investigated on the basis of bare tungsten-copper and tungsten-molybdenum clusters with density functional theory method at the generalized gradient approximation level.Slight structural distortions are observed in the extent of the adsorption of CO molecule with the exception of W₂Cu,W₃Cu₂,W₂Cu₄,WCu₅,W₅Mo and W₃Mo₃ clusters.W₂Cu,WCu₂,W₂Cu₂,WCu₃ and WCu₄ whose structures are planar maintain their original configurations,which is associated with the scalar relativistic effect of outer shell electrons of Cu atoms.CO molecule favors to be adsorbed with its carbon end because electron transfer tends to occur for carbon atom than oxygen atom.And carbon atom prefers to bound to tungsten atoms for W-Cu system,while it prefers to bound to molybdenum atoms for W-Mo system.Analyses of electronic property show that metal atoms bonded with carbon atom all possess positive charge for both WCu₄CO and WMo₅CO systems.And charge transfer occurs from WCu₄ cluster to CO molecule for WCu₄CO system and from CO to WMo₅ cluster for WMo₅CO system.