| Pt-based bimetallic nanoparticles (NPs) have been widely used in many chemical reactions because of their excellent activity. For example, Pt-based bimetallic NPs show higher catalytic activity than other metals in fuel cell. But the catalytic activity, stability, selectivity and CO tolerance of Pt-based bimetallic NPs should be improved in actual applications. The Pt-Fe bimetallic NPs were studied because of high stability and activity for CO oxidation. In many theoretical and experimental studies, the coverage or pressure of CO is very low to avoid or decrease the CO poison, and the possible reaction between adsorbed CO is not considered at high CO coverage. The catalytic activity of Pt-Fe bimetallic NPs for CO oxidation relates to the Pt/Fe ratio, but the mechanism is still not known. On the other hand, bimetallic NPs may segregate under actual atmosphere at high temperature, but few studies considered the segregation when studing CO oxidation on bimetallic NPs.Many studies have investigated the relationship between segregation and thermodynamics, but the kinetic factors are still not clear. Based on the density functional theory, the main work and results can be summarized as:1. By studying the dimerization and trimerization of CO on icosahedral Pt55, it is found that the products of CO polymerization depend on the different active sites of the metal surface and CO coverage. C2O2 can be adsorbed on either two neighboring Pt atoms or one Pt atom, and the former case is preferred. The preference can be ascribed to the stronger interaction between the 8s orbital of C2O2 and 5d orbitals of Pt in the former case. Two neighboring adsorbed CO molecules (CO*) can capture one free CO to form a ring-opening CO trimer on the Pt surface. High CO coverage can facilitate the dimerization and trimerization of CO and change the preferred adsorption site of C2O2, and highly-coordinated Pt atoms present the superior chemical activity for CO polymerization at high CO coverage. The CO dimerization by two CO* needs to overcome a high energy barrier of 1.92 eV, but one CO*can capture one free CO molecule to form the C2O2 with overcoming a much lower energy barrier of 0.87 eV. The energy barrier of CO trimerization is 1.14 eV.2. Then the larger CO polymers were studied to demonstrate if they can be formed on Pt55. Firstly, some randomly generated oxocarbons, ranging from C2 species to C9 species, are obtained on the Pt cluster at high CO coverage.The high stability of those adsorbed oxocarbons is demonstrated by ab initio molecular dynamics simulations, and the high CO coverage can hinder the decomposition of oxocarbons especially for C2O2. By comparing the favorite structures of C4O4, C5O5 and C6O6 before and after adsorbed on the Pt cluster,it is found that all the oxocarbons preferred to be linear rather than cyclic or lactone-like when adsorbed on the Pt cluster because of the stronger interaction between linear oxocarbons and Pt surface. Several different mechanisms of oxocarbon growth are also investigated, and we find that small oxocarbons prefer to couple with co-adsorbed CO to form larger oxocarbons with overcoming an energy barrier of ?1 eV. The high stability and low energy barrier of oxocarbon growth indicate the utilization of Pt cluster can significantly overcome the drawbacks of CO polymerization in diamond anvil cell: extreme condition and very low yield.3. The adsorption behaviors of PtnFe(55-n) (n=0, 11, 13, 18, 28, 37, 42, 44 and 55) NPs on single vacancy graphene (SVG) were studied. It is found that the Pt-Fe bimetallic NPs prefer to be adsorbed on the SVG through the Fe atoms when Pt and Fe atoms are both on the surfaces, because the interactions between Fe atoms and SVG are stronger than those between Pt atoms and SVG. Based on the density of states, molecular orbital and charge difference density analyses, the Fe atoms in the NPs can strongly interact with the sp2 dangling bonds and the ? orbitals near the vacancy in the SVG at the same time. The interactions between the Pt atoms and the ? orbitals are very weak.Meanwhile, due to the interactions between the Pt and Fe, the d-band centers of the Pt and Fe atoms in the NPs shift to lower energy, and the adsorption energies of PtnFe(55-n) (n=18, 37, 42 and 44) NPs on the SVG are decreased compared with that of Pt55.4. The CO oxidation mechanisms on PtnFe(55-n) (n=0, 11, 13, 18, 28, 37,42, 44 and 55) NPs were investigated. By comparing the adsorption properties of CO and O2 on the different NPs, it is found that their adsorption energies are decreased on the bimetallic NPs because of the interaction between Fe and Pt. The adsorption selectivity of O2 on Fe is higher than that on Pt, although the CO adsorption energies on Pt and Fe are both larger than those of O2. By comparing the segregation energies of Pt-Fe bimetallic NPs with or without CO and 02 adsorption, it is found that the CO and 02 adsorption can decrease the segregation energy, and all the fully segregated Pt-Fe NPs are more stable when the O2 coverage increased to 1. Meanwhile, only fully segregated PtnFe55-n (n=37, 42, 44 and 55) are more stable when the CO coverage increased to 1. The energy barriers of CO oxidation on PtnFe55-n(n=0, 11, 37,42 and 55) were calculated, and then the reaction rates were obtained combining with the micro-kinetic model. The CO coverage has significant effect on the CO oxidation rate. When the CO coverage is increased, the degree of segregation of Pt-Fe NPs will be increased, and the CO and O2 adsorption energies are decreased, which can change the energy barriers. By comparing the CO oxidation rates at high CO coverage, the CO oxidation rate increases with the increasing the Fe/Pt ratio and reaches the maximum on Pt13@Fe12Pt30. Then the oxidation rate decreases when the Fe/Pt ratio is larger than that in Pt13@Fe12Pt30. The most important reason is that the CO oxidation rate on the Pt-Fe NPs is dominated by the adsorption selectivity of O2 rather than the energy barriers, and there is a linear relationship between the logarithm of CO oxidation rate and the logarithm of the ratio of adsorption equilibrium constants of O2 and CO.5. At last, the segregation mechanism of Fe13@Pt42 under O2 were investigated. By comparing the structures of Fe13@Pt42 with or without 20 or 40 O atoms adsorbed, it is found that the geometric structures of Fe13@Pt42O20 and Fe13@Pt42O32 are dramatically distorted, and the volumes are increased.The structural change can enhance the adsorption strength of O atoms. After long time ab inito molecular dynamics simulations of Fe13@Pt42O20 and Fe13@Pt42O32, all the Fe atoms remain in the core, but two Pt atoms migrate to the sub-surface and interact with the central Fe atom, namely the alloying process occurs. By using CI-NEB method, the energy barrier of one Fe atom segregating to the surface of simulated Fe13@Pt42O20 is found to be about 1.47 eV, and the segregated Fe atom can remain at surface when it interacts with O atom, which is demonstrated by ab inito molecular dynamics simulations.Compared with the segregation process and energy barriers of Fe13@Pt42, the energy barriers of Fe13@Pt42O20 are much lower, which can be ascribed to the changes of stress caused by 0 adsorption. |
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