A First-principles Study On The Adsorption Of Small Molecules Onto Gold Cluster And Gold-based Binary Alloy Cluster

Recently, due to the distinctive electronic, optical, magnetic and chemical properties, the low dimension nano-cluster materials have attracted broad conceren of scientists. In cluster, many atoms are located at surface and the unpaired electrons have low probability to be paired, large number of dangling bonds exists. Compared with corresponding bulk materials, these nano-clusters have large surface-volume ratio and surface energies, curious active chemical reactivities, good catalytic properties and reaction selectivities. All these properties have important applications in the physical or chemical processes of phase transition, adsorption, catalysis and diffusion. Now, metallic complex clusters, compound clusters and noble metal clusters play important role in the catalytic science.As an important model used for the research of chemisorption, the interaction between cluster and small molecules is the most important step in catalytic reactions. The studies on the interaction between cluster and molecule and the adsorption of small molecule onto clusters have important meanings for the understanding of catalytic reaction mechanism, designing of new cluster catalytic materials, controlling of the reaction process and sarching for the suitable catalysis.In this paper, under the framework of DFT, we have performed a first principles all-electron scalar relativistic calculation on the geometrical structures, properties and adsorption behavior of gold clusters and gold-based binary alloy clusters toward CO, NO and H₂S molecules at GGA-PW91 level. The main conclusions are summarized as follows:(1) Based on the all-electron scalar relativistic calculations on gold clusters, we find that the gold clusters can keep the planar structures up to the size of n=13 and this size is obviously larger than that of the calculation results without including the relativistic effect. Meanwhile, we can also see that the relativistic effect may promote the charge transfers between atoms among gold cluster and the sd hybridization in gold atom, it also strengthens the Au-Au interaction and lowers the energy level of frontier orbital, enhances the degree of delocation, and thus makes the clusters have lower total energy, larger binding energy, shorter bond lengths and higher stability. With the increasing size of gold clusters, the atomic coordination number increases and the interaction between atoms becomes balance gradually, and then leads to the increase of binding energy and the enhancement of stability. Due to the influence of electron-pairing effects, the odd-even oscillations of HOMO-LUMO gap (HLG) and second-order difference of cluster energies can be observed clearly, and the even-numbered gold cluster with closed-shell electronic structure is relatively more stable than the adjacent odd-numbered gold cluster with open-shell electronic structure.(2) The CO, H₂S and NO molecules prefer to bond with gold cluster at the single fold coordination site by carbon, sulphur and nitric respectively. After adsorption, the gold cluster still keeps the planar structure and is distorted slightly. The CO and H₂S molecules also keep the former structure, but, the NO molecule is bent and the O atom moves closer to the cluster. Meanwhile, the Au-Au bond is strengthened and the C-O, S-H and N-O bonds are weakened to some extent, the reactivities of these molecules are enhanced. The higher adsorption energies and relatively shorter Au-C bond lengths can be observed in the adsorptions of Au₂, Au₃ and Au₄ cluster toward CO molecule. From n=5 to n=13, the adsorption energies and Au-C bond lengths are fluctuated. For the adsorption toward H₂S molecule, the higher adsorption energy, larger charge transfer and shorter Au-S bond length indicate that the H₂S molecule is more favorable to be adsorbed by the even-numbered gold cluster. Compared with adjacent even-numbered Au_nNO cluster with open-shell electronic structure, the odd-numbered Au_nNO cluster with closed-shell electronic structure has higher adsorption energy, shorter Au-N bond length, larger charge transfer and higher vibrational frequency of Au-N mode, the NO molecule is more favorable to be adsorbed by the odd-numbered gold cluster. Due to the electron pairing effect, the even-numbered Au_nCO cluster and Au_nH₂S cluster with closed-shell structure have the lower HOMO energy level, higher LUMO energy level, larger HOMO-LUMO gap and larger vertical ionization potential compared with the adjacent odd-numbered Au_nCO cluster and Au_nH₂S cluster with open-shell structure, the even-numbered Au_nCO cluster and Au_nH₂S cluster are relatively more stable. Similarly, the odd-numbered Au_nNO cluster is relatively more stable than the adjacent even-numbered Au_nNO cluster. After adsorption of these small molecules, the delocation of electron cloud for HOMO in gold cluster is still obvious. The delocation caused by the relativistic effect in gold cluster makes the distribution of electron cloud more uniform and then promotes the charge transfer between gold cluster and CO molecule and the charge transfer between C and O, strengthens the Au-C bond and weakens the C-O bond by the C-O anti-bonding effect. It is favorable to the adsorption of gold cluster toward CO molecule and the reactivity enhancement of CO molecule. (3) In order to check the influence of impurity on the adsorption of small gold cluster toward CO molecule, first we study the the structures and properties of Au_nPt(n=1-12)clusters and Au_nAg(n=1-12)clusters. All the lowest energy geometries of Au_nPt (n=1-12) clusters may be generated by substituting Pt atom for one gold atom of the Au_n+1 cluster at the highest coordinated site. The Au-Pt bonds are stronger and most Au-Au bonds far from Pt atom are weaker than the corresponding Au-Au bonds in pure Au_n+1 cluster. By substituting Pt atom for one gold atom of Au_n+1 cluster at the highest coordinated site, the relatively stable and inactive odd-numbered Au_n+1 cluster becomes the relatively unstable and reactive odd-numbered Au_nPt cluster, and the relatively unstable and reactive even-numbered Au_n+1 cluster becomes the relatively stable and inactive even-numbered Au_nPt cluster chemically and electronically. The odd-even alteration of cluster stability between Au_n+1 cluster and Au_nPt cluster caused by the impurity Pt is very obvious. Different from Au_nPt cluster, the lowest energy geometries of Au_nAg (n=2-12) clusters may be generated by substituting Ag atom for one gold atom of the Au_n+1 cluster at the relatively lower coordinated site. All these substitution sites are at the edge of plane and believed to be the active site for substitution and adsorption. The introduction of impurity Ag weakens the total stability, electronic structure stability and chemical stability, but, it almost has no influence on the odd-even oscillation of relative stability.(4) The lowest energy geometries of Au_nPtCO (n = 1-12) clusters show that the CO molecule would like to bond with Pt atom at the on-top and single fold coordination site. The Pt-C bond length in Au_nPtCO (n = 1-12) clusters is shorter than the Au-C bond length in corresponding Au_n+1CO (n = 1-12) clusters, and the C-O bond length in Au_nPtCO (n = 1-12) clusters is longer than the C-O bond length in corresponding Au_n+1CO (n = 1-12) clusters. The introduction of impurity Pt strengthens the charge transfer between cluster and CO molecule and the charge transfer between C and O, and then promotes adsorption toward CO molecule and the reactivity enhancement of CO molecule. After introduction of impurity Ag, the lowest energy geometries of Au_nAgCO (n = 1-12) cluster still belong to the structure generated by binding C with Au atom at the single-fold bonding site. The Au-C bond length and C-O bond length of the lowest energy geometry of Au_nAgCO (n = 1-12) cluster are longer than those of corresponding Au_n+1CO cluster. Even though the introduction of impurity Ag weakens the adsorption toward CO molecule, it strengthens the charge transfer between C and O, and thus promotes the C-O anti-bonding effect and the reactivity enhancement of CO molecule. Because the Au and Ag have similar valence electronic structure, the introduction of impurity Ag almost has no influence on the odd-even oscillation of cluster stability.(5)By strengthening the charge transfer between cluster and adsorbate, the relativistic effect may promote the adsorption of gold cluster toward small molecules and the activity of adsorbate. After introduction of the Pt impurity, the CO molecule is adosbed on the Pt site. The influence of relativistic effect on the selectivity of adsorption site almost can not be found and the influence on the adsorption strength is also weakened. After introduction of the Ag impurity, the CO is still adsorbed on the Au site. The influence of relativistic effect on the selectivity of adsorption site is obvious and the enhancement of adsorption strength and the activity of adsorbate caused by the relativistic effect can be observed.