Platinum nanocrystals supported by silica, alumina and ceria: metal–support interaction due to high-temperature reduction in hydrogen (original) (raw)
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Pt/ceria thin film model catalysts after high-temperature reduction: a (HR)TEM study
Vacuum, 2003
Well-facetted Pt particles, epitaxially grown on (001) NaCl single crystal surfaces, were supported by thin films of crystalline ceria. Structural and morphological changes due to calcination and reduction have been studied after treating in oxygen (673K, 1h) and hydrogen exposure at increasing temperatures from 673 to 1073K. These changes were followed by (HR)TEM, electron diffraction and EELS, and possible structures were selected with help of density functional calculations. Reduction at 723K causes the transformation of the initially half octahedral Pt particles into cube or platelet shapes with a double lattice periodicity in both HRTEM image and electron diffraction pattern. This can be attributed to the hydrogen-influenced topotactic formation of a Pt3Ce alloy phase of high thermodynamic stability as confirmed by DFT calculations. Further heating of the ceria-based catalyst in hydrogen up to 1073K results in clustering of metal particles on large flat areas of the sintered support with lattice periodicities between 0.8 and 1.2 nm. EELS and X-ray diffraction both confirm partial reduction of ceria and indicate the formation of chemically stable Ce suboxides.
Nanotechnology, 2010
In this paper, Pt nanoparticles with good shapes of nanocubes and nano-octahedra and wellcontrolled sizes in the range 5-7 and 8-12nm, respectively, have been successfully synthesized. The modified polyol method by adding silver nitrate and varying the molar ratio of the solutions of silver nitrate and H2PtCl6 has been used to produce Pt nanoparticles of the size and shape to be controlled. The size and morphology of Pt nanoparticles have been studied by transmission electron microscopy (TEM) and high resolution TEM (HRTEM). The results have shown that their very sharp and good shapes exist in the main forms of cubic, cuboctahedral, octahedral and tetrahedral shapes directly related to the crystal nucleation along various directions of the {100} cubic, {111} octahedral and {111} tetrahedral facets during synthesis. In particular, various irregular and new shapes of Pt nanoparticles have been found. Here, it is concluded that the role of silver ions has to be considered as an important factor for promoting and controlling the development of Pt nanoparticles of {100} cubic, {111} octahedral and {111} tetrahedral facets, and also directly orienting the growth and formation of Pt nanoparticles. © IOP Publishing Ltd.
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.
RSC Advances, 2014
Carbon supported Pt nanoparticles with diameters ranging from 2 to 28 nm have been studied using X-ray diffraction. The unit cell parameter of synthesized Pt/C nanoparticles is always lower than that of bulk Pt. By decreasing the average particle size D to approximately 2 nm, the unit cell parameter a nonlinearly decreases by about 0.03Å which corresponds to a variation of 0.7% in comparison to bulk Pt, and the size effect is predominant for sizes ranging from 2 to 10 nm. The dependence a(1/D) is approximated well using a straight line with a slope of À0.0555 AE 0.0067 nm À1 and an intercept of À3.9230 AE 0.0017 A. For interpreting the obtained experimental dependence of the unit cell parameter of Pt/C nanoparticles, four different theoretical approaches such as the thermal vacancy mechanism, continuous-medium model, Laplace pressure, and bond order-length-strength correlation mechanism, were used. Comparison of the calculated dependencies, based on the above models, with the experimental ones, shows that the continuous-medium model agrees best with the experimentally found unit cell parameter dependence of our carbon supported Pt nanoparticles.
Decomposition of the Precursor Pt(NH3)42, Genesis and Structure of the Metal-Support Interface of Alumina Supported Platinum Particles: A Structural Study Using TPR, MS, and XAFS Spectroscopy
The Journal of Physical Chemistry, 1995
During the preparation of alumina supported platinum catalysts, the precursor [Pt(NH3)4](OH)2 decomposes to a neutral Pt(NH3)zO species during the drying process at 120 "C. Treatment in flowing hydrogen at 180 "C leads to partial reduction of the platinum ammine complex and formation of platinum metal particles. A large increase in metal particle size is observed after a treatment under flowing H2 at 200 "C. The final reduction at 350 "C causes the total disappearance of the platinum precursor with a further increase in platinum particle size. The direct reduction at 350 "C yields the biggest metal particles (35 A) while calcination before reduction produces a much higher dispersion (metal particle diameter 10 A). The beneficial effect of calcination, already observed by many authxs when using [Pt(NH3)4](OH)2 as a precursor for the preparation of highly dispersed WyA1203, can now be explained because this treatment avoids the formation of the mobile neutral Pt(NH3)zO complex. The metal particles produced by treatment in flowing hydrogen at 180 "C present a metal-oxygen contribution at 2.7 A formed at the metal-support interface. This long distance is assumed to be caused by the presence of hydrogen in the metal-support interface based upon our results in combination with other TPD and EXAFS studies. A second metal-oxygen contribution with similar coordination number is detected at 3.86 A. This is a consequence of the presence of the first shell metal-oxygen at 2.7 8, and implies a [ 11 11 epitaxy in the metal-support interface.
Journal of Electronic Materials
Bulk platinum has been knownt ob ec hemically inert, but shows remarkably high activity as ac atalyst when finely dispersed as nano particles (10 nm) in ceramic substrates. In order to understand this mechanism, we have studied the effect of ar educing environmento nt he structure, morphology, and distribution of these Pt nano particles as well as of oxide support in gadolinium-doped ceria catalysts by transmission electron microscopy (TEM) and x-ray photoelectron spectroscopy( XPS) methods. In fresh catalysts prepared by calcinations, the Pt nano particles weref ound to be crystalline with many of them twinned and distributed randomly in the microstructure of crystalline ceria support. The size of the Pt nano particles ranged between 20 and 50 nm. Aspecial featurefound in these catalysts is that no constituting Pt particlei sp artially or fully embedded in the ceria grain.U pon reduction with hydrogen gas at high temperatures, both Pt and ceria crystals showed evidence of crystalline defects. The Pt nano particles in reduced catalysts appeared similar in sizea nd shape to those observed in the fresh catalysts,a nd they resided on the surfaceo ft he ceria crystal. Ther esults ared iscussed in relation to their expectedcatalytic activity in autothermal reforming of iso-octane.
Crystal size and shape analysis of Pt nanoparticles in two and three dimensions
2006
Abstract. The majority of industrial catalysts are high-surface-area solids, onto which an active component is dispersed in the form of nanoparticles that have sizes of between 1 and 20 nm. In an industrial environment, the crystal size distributions of such particles are conventionally measured by using either bright-field transmission electron microscope (TEM) images or X-ray diffraction.
Effect of reducing atmosphere on the structure of ceria-supported nano-pt catalysts
Journal of Electronic Materials, 2006
Bulk platinum has been knownt ob ec hemically inert, but shows remarkably high activity as ac atalyst when finely dispersed as nano particles (10 nm) in ceramic substrates. In order to understand this mechanism, we have studied the effect of ar educing environmento nt he structure, morphology, and distribution of these Pt nano particles as well as of oxide support in gadolinium-doped ceria catalysts by transmission electron microscopy (TEM) and x-ray photoelectron spectroscopy( XPS) methods. In fresh catalysts prepared by calcinations, the Pt nano particles weref ound to be crystalline with many of them twinned and distributed randomly in the microstructure of crystalline ceria support. The size of the Pt nano particles ranged between 20 and 50 nm. Aspecial featurefound in these catalysts is that no constituting Pt particlei sp artially or fully embedded in the ceria grain.U pon reduction with hydrogen gas at high temperatures, both Pt and ceria crystals showed evidence of crystalline defects. The Pt nano particles in reduced catalysts appeared similar in sizea nd shape to those observed in the fresh catalysts,a nd they resided on the surfaceo ft he ceria crystal. Ther esults ared iscussed in relation to their expectedcatalytic activity in autothermal reforming of iso-octane.
Electron tomography and 3D molecular simulations of platinum nanocrystals
Nanoscale
This work reports on the morphology of individual platinum nanocrystals with sizes of about 5 nm. By using the electron tomography technique that gives 3D spatial selectivity, access to quantitative information in the real space was obtained. The morphology of individual nanoparticles was characterized using HAADF-STEM tomography and it was shown to be close to a truncated octahedron. Using molecular dynamics simulations, this geometrical shape was found to be the one minimizing the nanocrystal energy. Starting from the tomographic reconstruction, 3D crystallographic representations of the studied Pt nanocrystals were obtained at the nanometer scale, allowing the quantification of the relative amount of the crystallographic facets present on the particle surface.