Article

Stability of platinum nanoparticles supported on surface-treated carbon black

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Abstract

Platinum nanoparticles supported on carbon black (CB) are used as electrocatalysts in polymer electrolyte fuel cells, and their stability is important for the durability of the catalyst system. Here, we investigated the stability of Pt nanoparticles supported on surface-treated CB with a thermochemically developed surface nanostructure. The protruding structure of surface-treated CB was able to support Pt nanoparticles synthesized via the sodium tetrahydroborate (NaBH4) reduction of hexachloroplatinic(IV) acid hexahydrate (H2PtCl6 center dot 6H(2)O) with polyvinylpyrrolidone (PVP). The dependence of Pt stability on the deposition locations on the CB surface was examined using in situ transmission electron microscope observations during heating up to 800 degrees C. We found that the protruding parts on the CB surface function as local sites for stably supporting Pt nanoparticles. Hence, we suggest that the stability of the Pt nanoparticles can be improved using the CB surface nanostructure.

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Agglomeration is encountered in many natural or industrial processes, like growth of aerosol particles in the atmosphere and during material synthesis or even flocculation of suspensions, granulation, crystallization, and with colloidal particle processing. These particles collide by different mechanisms and stick together forming irregular or fractal-like agglomerates. Typically, the structure of these agglomerates is characterized with the fractal dimension, Df, and pre-exponential factor, kn, of simulated agglomerates of monodisperse primary particles (PP) for ballistic or diffusion-limited particle–cluster and cluster–cluster collision mechanisms. Here, the effect of PP polydispersity on Df and kn is investigated with agglomerates consisting of 16-1024 PP with closely controlled size distribution (geometric standard deviation, σg = 1–3). These simulations are in excellent agreement with the classic structure (Df and kn) of agglomerates consisting of monodisperse PPs made by four different collision mechanisms as well as with agglomerates of bi-, tridisperse, and normally distributed PPs. Broadening the PP size distribution of agglomerates decreases monotonically their Df, and for sufficiently broad PP distributions (σg > 2.5) the Df reaches about 1.5 and kn about 1 regardless of the collision mechanism. Furthermore, with increasing PP polydispersity, the corresponding projected area exponent, Dα, and pre-exponential factor, ka, decrease monotonically from their standard values for agglomerates with monodisperse PPs. So Df as well as Dα and ka can be an indication for PP polydispersity in mass mobility and light scattering measurements if the dominant agglomeration mechanism is known, like diffusion-limited and/or ballistic cluster–cluster coagulation in aerosols. Copyright 2012 American Association for Aerosol Research
Article
Controlling the size, dispersion, and shape of nanoparticles (NPs) in the high-temperature range is a key topic for the development of new technologies with applications in the particular fields of catalysis and energy storage. In this article, we present an approach combining in situ transmission electron microscopy (TEM), electron tomography (ET), and molecular dynamics (MD) calculations for assessing the evolution of Pt NPs deposited onto few-layer graphene supports. Spherical Pt NPs with average sizes of 2 nm located preferentially at the support topographical defects (e.g., steps and edges) diffuse and coalesce along these defects, such that, after annealing to 700 °C, the nanoparticles were located exclusively here. Their dispersion remained significant; only the particle size distribution changed from mono- to bimodal. This statistical variation is discussed herein by reviewing fundamental issues such as the NP–support interaction and NP faceting, diffusion, and subsequent sintering in the high-temperature range. Fundamental MD simulations are reported here as reinforcements of the experimental findings and as a means to provide deeper insight into the phenomenological issues behind the behavior of the system investigated.
Article
Colloidal silver-platinum alloys have been prepared by NaBH4 reduction of silver(I) bis(oxalato)platinate(II) (Ag2[Pt(C2O4)2]) in ethylene glycol and characterized by means of EDX, HRTEM, XRD, and optical absorption spectra. The HRTEM micrographs show that the metal particles thus prepared have homogeneous {111} lattice spacing of 2.31 +/- 0.02 angstrom, which is an intermediate value between those of monometallic Ag and Pt colloids, indicating that they are Ag-Pt alloys. The same trend is found in the XRD profiles. Optical spectra of the Ag-Pt alloy colloid show a plasma resonance band at 354 nm. The peak wavelength shifts in the range of 330-410 nm by varying the Ag/Pt ratio. The alloy formation has been studied by step-by-step reduction of Ag2[Pt(C2O4)2]. It is suggested that the reduction of [Pt(C2O4)2]2- takes place by the catalytic action of the Ag particles to form the Ag-Pt alloy.
Article
High-angle annular dark-field scanning transmission electron microscopy tomography is applied to the study of Pt and PtCr nanoparticles supported on carbon black, which are used as heterogeneous catalysts in the electrodes of proton exchange membrane fuel cells. By using electron tomography, the three-dimensional architecture of the heterogeneous catalyst system can be determined, providing high-spatial-resolution information about the shapes, faceting and crystallographies of 5–20 nm single and multiply twinned catalyst particles, as well as their positions with respect to the carbon support. Approaches that can be used to provide improved information about the distribution and orientation of the particles on their support are proposed and discussed. Our results show that electron tomography provides important information that is complementary to high-resolution lattice imaging. Both techniques are required to understand fully the nature and role of the surfaces of faceted catalyst particles.
Article
A semi-empirical thermodynamic model for size dependency of melting point of nano particles and wires has been proposed by introducing a size dependency of surface energy. The model predicts the size dependency of melting point of nano particles and wires for a wide range of elements: fcc (Au, Pt, Ni), hcp (Mg), and bcc (W), all in good agreement with experimental data and/or molecular dynamics simulations. Since the model is free from adjustable parameters, it is applicable to a wider range of materials.
Article
Carbon nanotube supported platinum (Pt/CNTs) catalysts prepared by different Pt deposition methods and on different CNT supports were studied. Colloidal based methods were demonstrated to be more effective than other wet chemistry deposition methods (e.g., impregnation and precipitation) for the preparation of highly dispersed Pt/CNTs. Pt catalyst supported on CNTs with a dispersion uniformity comparable to that supported on carbon powder was achieved using a zwitterionic surfactant 3-(N,N-dimethyldodecylammonio) propanesulfonate (SB12) as stabilizer in a monitored pH environment. It was experimentally observed that oxygen-containing surface functionalities on CNTs can greatly affect the catalyst particle dispersion by manipulating Pt anchoring and/or nucleating sites. Furthermore, it was revealed that the performance of Pt/CNTs based fuel cell is strongly dependent on the electrode fabrication method.
Article
Fundamental understanding about the thermal stability of nanoparticles and deliberate control of structural and morphological changes under reactive conditions is of general importance for a wide range of reaction processes in heterogeneous and electrochemical catalysis. Herein, we present a parametric study of the thermal stability of carbon-supported Pt nanoparticles at 80 °C and 160 °C, with an initial particle size below 3 nm, using in situ high-temperature X-ray diffraction (HT-XRD). The effects on the thermal stability of carbon-supported Pt nanoparticles are investigated with control parameters such as Brunauer-Emmet-Teller (BET) surface area, metal loading, temperature, and gas environment. We demonstrate that the growth rate exhibits a complex, nonlinear behavior and is largely controlled by the temperature, the initial particle size, and the interparticle distance. In addition, an ex situ transmission electron microscopy study was performed to verify our results obtained from the in situ HT-XRD study.
Article
This study addresses the sintering mechanism of Pt nanoparticles dispersed on a planar, amorphous Al(2)O(3) support as a model system for a catalyst for automotive exhaust abatement. By means of in situ transmission electron microscopy (TEM), the model catalyst was monitored during the exposure to 10 mbar air at 650 degrees C. Time-resolved image series unequivocally reveal that the sintering of Pt nanoparticles was mediated by an Ostwald ripening process. A statistical analysis of an ensemble of Pt nanoparticles shows that the particle size distributions change shape from an initial Gaussian distribution via a log-normal distribution to a Lifshitz-Slyozov-Wagner (LSW) distribution. Furthermore, the time-dependency of the ensemble-averaged particle size and particle density is determined. A mean field kinetic description captures the main trends in the observed behavior. However, at the individual nanoparticle level, deviations from the model are observed suggesting in part that the local environment influences the atom exchange process.
Article
The stability of Ni, Cu, Mo and Au transmission electron microscope (TEM) grids coated with ultra-thin amorphous carbon (alpha-C) or silicon monoxide film is examined by in-situ heating up to a temperature in the range 500-850 degrees C in a transmission electron microscope. It is demonstrated that some grids can generate nano-particles either due to the surface diffusion of metal atoms on amorphous film or due to the metal evaporation/redeposition. The emergence of nano-particles can complicate experimental observations, particularly in in-situ heating studies of dynamic behaviours of nano-materials in TEM. The most widely used Cu grid covered with amorphous carbon is unstable, and numerous Cu nano-particles start to form once the heating temperature reaches 600 degrees C. In the case of Ni grid covered with alpha-C film, a large number of Ni nano-crystals occur immediately when the temperature approaches 600 degrees C, accompanied by the graphitization of amorphous carbon. In contrast, both Mo and Au grids covered with alpha-C film exhibit good stability at elevated temperature, for instance, up to 680 and 850 degrees C for Mo and Au, respectively, and any other metal nano-particles are detected. Cu grid covered Si monoxide thin film is stable up to 550 degrees C, but Si nano-crystals appear under intensive electron beam. The generated nano-particles are well characterized by spectroscopic techniques (EDXS/EELS) and high-resolution TEM. The mechanism of nano-particle formation is addressed based on the interactions between the metal grid and the amorphous carbon film and on the sublimation of metal.