The Structures, Catalytic Properties And Stabilities Of Pt-Ru Nanoparticles:a First Principles Investigation

Bimetallic nanoparticles usually have unique physicochemical properties that are different from the bulk and their corresponding monometallic nanoparticles. The structure informations of bimetallic nanoparticles are very useful, since all the electronic and catalytic properties are determined by the structure. However, up to now, the structures of small nanoparticles still cannot be exactly detected only by the method of experiment, while accurate theoretical calculations can provide reliable informations for the structures of small naoparticles. We have studied the structural properties of icosahedral PtnRu55-n bimetallic nanopartices using first principles calculations. Icosahedral Ru55 nanoparticle is selected to catalyze the dissociation of H2O monomer and H2O dimer. The Ru55 nanoparticles have shown a high catalytic activity for the dissociation of H2O monomer and H2O dimer. However, nanoparticles are only kinetic stable but not thermodynamic stable in solution. In thermodynamics, their agglomeration and formation of bulk metal, which makes the loss of their unique properties of nanoparticles, are favored. Stabilizer must be added to prevent the agglomeration of nanoparticles. We use ionic liquids as the stabilizer of nanoparticles and try to clarify the mechanism about the stabilization of metal nanoperticles in ionic liquids. The main findings are listed as follows:(1) In icosahedral PtnRu55_n bimetallic nanoparticles, the Ru atoms prefer to occupy the core, while the Pt atoms tend to distribute on the surface. The storage of Ru is much richer that of Pt in nature, and the price of Ru is much cheaper than that of Pt. Therefore, catalysts with Ru in the core and Pt on the surface can reduce the usage of Pt and cut the cost of production. In the surface of PtnRu55-n nanoparticle, which has the Ru13 core, Pt and Ru atoms compete to occupy the vertex site. When the number of surface Pt (Ru) atoms is fewer than 6, Pt (Ru) atoms prefer to occupy the vertex sites. When the number of Pt (Ru) atoms on surface is in the range of 6 to 36, the structures with six Pt atoms and six Ru atoms on the vertex sites are more stable. In this case, one half of the vertex sites are occupied by Pt, while the other half of the vertex sites are occupied by Ru. For PtnRu55-n nanoparticles that have the core of Ru13, the nanoparticle which has more Pt atoms is more stable, and when all the surface sites are occupied by Pt atoms, the obtained core shell nanoparticle Ru13@Pt42 is the most stable one.(2) After DFT-D2 correction, the energy barrier for the dissociation of H2O monomer on t1 site of Ru55 is 0.43 eV and the energy barrier on t2 site is 0.34 eV, indicating the t2 site is more active than the t1 site. The energy barrier for H2O dimer dissociation on the t2 site of Ru55 is only 0.11 eV, indicating Ru55 is a good catalyst for H2O dissociation. Moreover, in the dissociation of H2O dimer, the H atom can automatically move from the H2O locating on t1 site to the more active t2 site.(3) The interaction strengths between [BMIM][PF6] ionic liquid and PtnRu55-n (n=0,13,42,55) nanoparticles are very different from each other. The maximum interaction energy is -19 times of the minimum interaction energy. In the weakest interaction system of Ru13Pt42-[BMIM][PF6], only the anion of ionic liquid can form bond with the nanoparticle. However, in the other three stronger interaction systems, both the anion and cation of ionic liquids can form bonds with metallic nanoparticle. Especially in Pt13Ru42-[BMIM][PF6], which is the strongest interaction system, the interaction between [BMIM][PF6] and Pt13Ru42 nanoparticle is so strong that it results in the original core-shell structure of Pt13Ru42 distorted. Some of the core Pt atoms move to the surfaces. With the increase of the interaction energies between [BMIM][PF6] ionic liquid and PtnRu55-n (n=0,13,42,55) nanoparticles, the stability enhancement degree of nanoparticles increase after interaction. In Pt13Ru42-[BMIM][PF6], the energy of the distorted Pt13Ru42 nanoparticles is 8.50 eV lower than that of the original core shell one.(4) When Ru7Pt6 cluster interacts with different ionic liquids of [BMIM][Br], [BMIM][BF4], [BMIM][PF6], [BMIM][CF3SO3], and [BMIM][NTf2], the ionic liquids with median size anions (PF6-, CF3SO3-) can play a more effective role than those with the small anions (Br-, BF4-) and the large anion (NTf2-) in improving the stability of Ru7Pt6 cluster. For the Ru4Pt9, Ru7Pt6, and Ru9Pt4 clusters interacting with the same ionic liquid of [BMIM][CF3SO3], the stronger the interaction is, the more enhancement of the clusters stability are after interaction. In the case of Ru7Pt6-n{[BMIM][CF3SO3]} (n=1,2,3,4), the total interaction energy is nearly proportional to the number of ionic liquid molecules. In Ru7Pt6-[BMIM][CF3SO3] system, which has one ionic liquid molecule on surface of the cluster, the stabilization of metal cluster in ionic liquid mainly attributes to the interaction between ionic liquid and cluster. With the increase of ionic liquid molecules, in Ru7Pt6-4{[BMIM][CF3SO3]}, the metallic cluster is fully wrapped by ionic liquids, and the protective layer formed by ionic liquids plays the primary role in stabilizing the cluster.