Monte Carlo simulations of segregation in Pt-Re catalyst nanoparticles (original) (raw)
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Quantitative prediction of surface segregation in bimetallic Pt–M alloy nanoparticles (M=Ni,Re,Mo)
Progress in Surface Science, 2005
This review addresses the issue of surface segregation in bimetallic alloy nanoparticles, which are relevant to heterogeneous catalysis, in particular for electrocatalysts of fuel cells. We describe and discuss a theoretical approach to predicting surface segregation in such nanoparticles by using the Modified Embedded Atom Method and Monte Carlo simulations. In this manner it is possible to systematically explore the behavior of such nanoparticles as a function of component metals, composition, and particle size, among other variables. We chose to compare Pt 75 Ni 25 , Pt 75 Re 25 , and Pt 80 Mo 20 alloys as example systems for this discussion, due to the importance of Pt in catalytic processes and its high-cost. It is assumed that the equilibrium nanoparticles of these alloys have a cubo-octahedral shape, the face-centered cubic lattice, and sizes ranging from 2.5 nm to 5.0 nm. By investigating the segregation of Pt atoms to the surfaces of the nanoparticles, we draw the following conclusions from our simulations at T= 600 K. (1) Pt 75 Ni 25 nanoparticles form a surface-sandwich structure in which the Pt atoms are strongly enriched in the outermost and third layers while the Ni atoms are enriched in the second layer. In particular, a nearly pure Pt outermost surface layer can be achieved in those nanoparticles. (2) Equilibrium Pt 75 Re 25 nanoparticles adopt a core-shell structure: a nearly pure Pt shell surrounding a more uniform Pt-Re core. (3) In Pt 80 Mo 20 nanoparticles, the facets are fully occupied by Pt atoms, the Mo atoms only appear at the edges and vertices, and the Pt and Mo atoms arrange themselves in an alternating sequence along the edges and vertices. Our simulations quantitatively agree with previous experimental and theoretical results for the extended surfaces of Pt-Ni, Pt-Re, and Pt-Mo alloys. We further discuss the reasons for the different types of surface segregation found in the different alloys, and some of their implications.
Monte Carlo simulations of the structure of Pt-based bimetallic nanoparticles
Acta Materialia, 2012
Pt-based bimetallic nanoparticles have attracted significant attention as a promising replacement for expensive Pt nanoparticles. In the systematic design of bimetallic nanoparticles, it is important to understand their preferred atomic structures. However, compared with unary systems, alloy nanoparticles present more structural complexity with various compositional configurations, such as mixed-alloy, core-shell, and multishell structures. In this paper, we developed a unified empirical potential model for various Pt-based binary alloys, such as Pd-Pt, Cu-Pt, Au-Pt, and Ag-Pt. Within this framework, we performed a series of Monte Carlo (MC) simulations that quantify the energetically favorable atomic arrangements of Pt-based alloy nanoparticles: an intermetallic compound structure for the Pd-Pt alloy, an onion-like multi-shell structure for the Cu-Pt alloy, and core-shell structures (Au@Pt and Ag@Pt) for the Au-Pt and Ag-Pt alloys. The equilibrium nanoparticle structures for the four alloy types were compared with each other, and the structural features can be interpreted by the interplay of their material properties, such as the surface energy and heat of formation. PACS numbers: 61.46.+w, 36.40.Ei, 64.70.Nd I.
2012
In our facile synthesis method, poly(vinylpyrrolidone)-protected Pt and Pt−Pd bimetallic nanoparticles with controllable polyhedral core−shell morphologies are precisely synthesized by the reduction of Pt and Pd precursors at a certain temperature in ethylene glycol with silver nitrate as structure-controlling agent. The Pt nanoparticles exhibited well-shaped polyhedral morphology with highly fine and specific nanostructures in the size range of 20 nm. Important evidences of core−shell configurations of the Pt−Pd core−shell nanoparticles were clearly characterized by high-resolution transmission electron microscopy (HRTEM) measurements. The results of HRTEM images showed that the core−shell Pt−Pd nanoparticles in the size range of 25 nm with polyhedral morphology were synthesized with the thin Pd shells of ∼3 nm in thickness as the atomic Pd layers grown on the Pt cores. Very interesting characteristics of surface structure of Pt nanostructures and Pt−Pd core−shell nanostructures with surface defects were observed. The high-resolution TEM images of Pt−Pd bimetallic nanoparticles showed that the Frank−van der Merwe and Stranski−Krastanov growth modes coexist in the nucleation and growth of the Pd shells on the as-prepared Pt cores. It is predicted that the FM growth becomes the main favorable growth compared with the SK growth in the formation of the thin Pd shells of Pt−Pd core−shell nanoparticles. The experimental evidence of the deformations of lattice fringes and lattice-fringe patterns was found in Pt and Pt−Pd core−shell nanoparticles. The interesting renucleation and recrystallization at the attachments between the nanoparticles are revealed to form a good lattice match. In addition, our novel ideas of the largest surface-area superlattices and promising utilization of them are proposed for next generations of various fuel cells with low cost. Finally, the products of Pt−Pd core−shell nanoparticles can be potentially utilized as highly efficient catalysts in the realization of polymer electrolyte membrane fuel cell and direct methanol fuel cell using the very low Pt loading with better cost-effective design.
Expansion of interatomic distances in platinum catalyst nanoparticles
Acta Materialia, 2010
We study the atomistic structure of Pt catalyst nanoparticles using HRTEM (high-resolution transmission electron microscopy). The particles exhibit a faceted, cubo-octahedral shape, extended planar defects, and mono-atomic surface steps. HRTEM imaging with negative spherical aberration yielded atomic-resolution images with a minimum of artifacts. Combining digital image processing, quantitative image analysis, and HRTEM image simulations to determine local variations of the spacing between neighboring Pt atom columns, we have found an expansion of the lattice parameter in the particle core and even larger, locally varying expansion of Pt-Pt next-neighbor distances at the particle surface. The latter likely originates from an amorphous oxide on the nanoparticle surface and/or dissolution of oxygen on subsurface sites. These structural features may significantly impact the catalytic activity of Pt nanoparticles.
M onometallic and bimetallic nanoparticles (NPs) are key components in many catalytic, optical, and magnetic devices. The ability to control the composition, shape, 1Ϫ3 and architecture 4Ϫ6 of multicomponent NP systems is of increasing importance in tailoring the resulting properties. Synthetic approaches to NP synthesis have been evolving over the last 100 years, but most structural/compositional information is gleaned from TEM and XRD analysis. 7Ϫ10 These methods are quite satisfactory for large NPs (Ͼ10 nm), but obtaining similar information for smaller, catalytically relevant particles of less than 5 nm is more challenging. 11Ϫ15 The vast majority of bimetallic NP catalysts are prepared by impregnation/deposition methods that often give well-dispersed, highly active systems, but the details of structure and local composition are difficult to ascertain and often remain ill-defined. For example, the structure and oxidation states of Pt and Ru in active PtRu fuel cell electrocatalysts remain a contentious topic. We have recently shown that bimetallic particles of the same size, shape, and composition show significant differences in activity when configured into different architectures, such as alloy, coreϪshell, or monometallic mixtures (see drawing). 5 In particular, the coreϪshell structure has emerged as an attractive catalytic component due to the ability to tune the activity of the shell metal through interactions with the core. 4,5,19Ϫ23 However, rational design and control of the particle's activity requires precise synthetic methodology and full knowledge of structure/composition. The structure, composition, and architecture of bimetallic nanoparticles are defined by many parameters. For example, the nanoparticle shape (e.g., cuboctahedral versus truncated octahedral) is a distinct structural characteristic, in addition to the crystal structure (i.e., fcc vs hcp). 24Ϫ27 In addition, bimetallic nanoparticles with the same composition, shape, and crystal structure can have different architectures ABSTRACT A comprehensive structural/architectural evaluation of the PtRu (1:1) alloy and Ru@Pt core؊shell nanoparticles (NPs) provides spatially resolved structural information on sub-5 nm NPs. A combination of extended X-ray absorption fine structure (EXAFS), X-ray absorption near edge structure (XANES), pair distribution function (PDF) analyses, Debye function simulations of X-ray diffraction (XRD), and field emission transmission electron microscopy/energy dispersive spectroscopy (FE-TEM/EDS) analyses provides complementary information used to construct a detailed picture of the core/shell and alloy nanostructures. The 4.4 nm PtRu (1:1) alloys are crystalline homogeneous random alloys with little twinning in a typical face-centered cubic (fcc) cell. The Pt atoms are predominantly metallic, whereas the Ru atoms are partially oxidized and are presumably located on the NP surface. The 4.0 nm Ru@Pt NPs have highly distorted hcp Ru cores that are primarily in the metallic state but show little order beyond 8 Å. In contrast, the 1؊2 monolayer thick Pt shells are relatively crystalline but are slightly distorted (compressed) relative to bulk fcc Pt. The homo-and heterometallic coordination numbers and bond lengths are equal to those predicted by the model cluster structure, showing that the Ru and Pt metals remain phase-separated in the core and shell components and that the interface between the core and shell is quite normal.
Infrared spectroscopy on size-controlled synthesized Pt-based nano-catalysts
Surface Science, 2006
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New Journal of Chemistry, 2012
In this paper, the polyhedral Pt nanoparticles were prepared by a modified polyol method using AgNO 3 as a nanostructure-shaping agent. TEM and HRTEM images of Pt nanoparticles show the particle size in the 10 nm range for the well-controlled case. In contrast, Pt nanoparticles have the particle size in the 50 nm range for the uncontrolled case. To understand the important issues of morphology, size, surface and structure, the as-prepared Pt nanoparticles were investigated through UV-vis spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), and high-resolution (HR)TEM measurements. In the two cases, the as-prepared Pt nanoparticles with and without the control procedures exhibit surface attachment, aggregation or agglomeration and assembly. The mechanisms can lead to the formation of the ultra-porous mesostructure of the as-prepared Pt nanoparticles by using various sophisticated control methods. Therefore, the experimental findings and observations showed the formations of the porous Pt nanostructures as the new Pt textures from self-aggregation or self-agglomeration and self-assembly of extreme importance in designing great superlattices under experimentally chemical and physical methods. This also proved the important role of PVP polymer in the protection of the as-prepared nanoparticles. In particular, a new phenomenon was found in the randomly natural collapse and self-breaking in the Pt nanostructures originating from the naked Pt nanoparticles without protective polymer agents. As a result, a porous meso-nanostructure was formed by the structural changes of Pt nanoparticles without stabilization of the PVP polymer. Finally, the discoveries of surface structure changes of polyhedral Pt shapes and morphologies in future are very important in further catalysis investigation.
Displacement Pt on Ru nanoparticle
The displacement reaction of Pt on Ru to form a Ru core and Pt shell (Ru@Pt) bimetallic structure is investigated by immersing the carbon-supported Ru nanoparticles in hexachloroplatinic acids with pH of 1, 2.2, and 8, followed by a hydrogen reduction treatment. Results from inductively coupled plasma mass spectrometry suggest that the dissolution of Ru is mostly caused by the reduction of Pt cations. Images from transmission electron microscopy demonstrate a uniform distribution of Ru@Pt in size of 3-5 nm. Spectra from X-ray absorption near edge structure and extended X-ray absorption fine structure confirm that the pH value of hexachloroplatinic acid determines the type of ligands complexing the Pt cations that affects their activity and consequently the severity of displacement reaction and alloying degree of Ru@Pt nanoparticles. As a result, the samples from pH 1 bath reveal a desirable core-shell structure that displays a reduced onset potential in CO stripping and stable catalytic performance for H 2 oxidation while the samples from pH 8 bath indicate the formation of Pt clusters on the Ru surface that leads to poor CO stripping and H 2 oxidation characteristics.
Surface Structures of Cubo-Octahedral Pt−Mo Catalyst Nanoparticles from Monte Carlo Simulations
The Journal of Physical Chemistry B, 2005
The surface structures of cubo-octahedral Pt-Mo nanoparticles have been investigated using the Monte Carlo method and modified embedded atom method potentials that we developed for Pt-Mo alloys. The cubo-octahedral Pt-Mo nanoparticles are constructed with disordered fcc configurations, with sizes from 2.5 to 5.0 nm, and with Pt concentrations from 60 to 90 at. %. The equilibrium Pt-Mo nanoparticle configurations were generated through Monte Carlo simulations allowing both atomic displacements and element exchanges at 600 K. We predict that the Pt atoms weakly segregate to the surfaces of such nanoparticles. The Pt concentrations in the surface are calculated to be 5 to 14 at. % higher than the Pt concentrations of the nanoparticles. Moreover, the Pt atoms preferentially segregate to the facet sites of the surface, while the Pt and Mo atoms tend to alternate along the edges and vertices of these nanoparticles. We found that decreasing the size or increasing the Pt concentration leads to higher Pt concentrations but fewer Pt-Mo pairs in the Pt-Mo nanoparticle surfaces.