Insight Into The Structural And Electronic Properties Of Pdni Bimetallic Clusters And The Adsorption And Decomposition Of H₂S On Their Surface:A Theoretical Study

Using the first-principle methods, we investigate the structural and electronic properties of PdNi bimetallic clusters, and the adsorption and the first dissociation step of H2S on their surface. The main results are listed as follows:1. The structural and electronic properties of the icosahedral Pd55-nNin(n = 0-55) clusters have been investigated within the density functional theory. According to the calculated excess energy, we found that the core-shell Ni13@Pd42 cluster is the most stable structure, and Pd44Nin cluster possesses the highest relative stability among the random Pd55-nNin clusters. The calculated first-neighbor distances of Pd-Pd and Ni-Ni in Pd28Ni27 and Pdi3Ni42 clusters are consistent with the experimental results. The electronic property analysis reveals that the interaction between Pd-4d and Ni-3d orbitals makes the main contribution and a strong d-d self-interaction among the Ni-3d. With an increasing number of Ni atoms, the d-band center of the surface Pd atoms shifts far away from the Fermi level, while the root mean squared d-band width broadens. This feature is also observed in the partial density of states of the surface Pd atoms. Through analyzing the Lowdin charge population, there is significant charge transfer from Ni atoms at the top sites of surface to Ni atoms in the core, and partial charge is transferred from Pd to Ni in the bimetallic cluster.2. Density functional theory calculation was employed to investigate the structural and electronic properties of the adsorption H2S on the (111) facet of Pd55-nNin (n=0,7,13,25,43,55) clusters. According to calculate the adsorption energy, we obtained three kinds of the most stable structure, which are identified with S atom binding at the vertex site of Pd (or Ni), at the edge site of Pd (or Ni) and at the vertex and edge sites of Ni atoms, respectively. After adsorption, the angel H-S-H internal angles and H-S bond lengths are 88.54-93.29° and 1.36-1.38 A, which are consistent with the experimental and theoretical results. We found that the adsorption energies (3.80 and 4.23 eV) of the most stable sites of Pd55-nNin (n=25,43) clusters are higher than that (3.61 eV) of the pure Pd55. This confirms that the interactions between Pd-based bimetallic clusters and H2S are strong. Thus, M-d (M:metal) and S-p orbitals make the main contribution in the partial density of state. For the change of d-band center before and after adsorption, the surface activity of Pd55.nNin (n=0,25,43) clusters are improved. Finally, according to calculate the difference charge density and bader charge, there is charge transfer from S to Ni, and some charge is transferred from S to H and from adsorbed Ni to the other atoms of cluster.3. Density functional theory was employed to investigate the first dissociation step of H2S on the (111) facet of Pd55-nNin (n=0,25,43) clusters. According to calculate the adsorption energy, we found that H2S prefers to adsorb on the top site. On the different (111) facet, the order of the adsorption energy of the most stable adsorption site is PdNi(111)> NiPd(111)> Pd(111). And the Ni atom can improve the interaction between H2S and cluster. HS and S both prefer to adsorb on the fcc site, and H atom occupies preferentially the hcp site. For the co-adsorption of HS/H, HS and H adsorb respectively on the bridge and hcp sites of Pd(111), while they occupy respectively on the bridge and fcc sites of PdNi(111). According to analyze the first dissociation step of H2S, the dissociation barrier and reaction energy are 0.12 and-1.19 eV on Pd(111), and those of PdNi(111) are 0.33 and-1.19 eV. Obviously, Ni atoms replacing the Pd atom can inhibit the dissociation of H2S on the PdNi(111), and effectively reduce the poisoning phenomenon of surfur. Finally, we analyze the density of state of HS/H compared with H2S. It is found that the orbital of S-p overlaps with the whole Pd-d after dissociation on Pd(111), which suggests that the interaction between S and Pd enhances. In addition, there is significant interaction between the partial of Pd-d and H-s. On PdNi(111), H-s and Ni-d orbitals have remarkable overlap at-7.0 and-6.2 eV, implying the formation of H-Ni bond.