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1.
Chemistry ; 26(41): 9012-9023, 2020 Jul 22.
Artigo em Inglês | MEDLINE | ID: mdl-32428349

RESUMO

Understanding the formation of nanoparticles (NPs) is key to develop materials by sustainable routes. The Co4CatTM process is a new synthesis of precious metal NPs in alkaline mono-alcohols well-suited to develop active nanocatalysts. The synthesis is 'facile', surfactant-free and performed under mild conditions like low temperature. The reducing properties of the solvent are here shown to strongly influence the formation of Pt NPs. Based on the in situ formation of CO adsorbed on the NP surface by solvent oxidation, a model is proposed that accounts for the different growth and stabilization mechanisms as well as re-dispersion properties of the surfactant-free NPs in different solvents. Using in situ and ex situ characterizations, it is established that in methanol, a slow nucleation with a limited NP growth is achieved. In ethanol, a fast nucleation followed by continuous and pronounced particle sintering occurs.

2.
Microsc Microanal ; : 1-15, 2020 Jan 20.
Artigo em Inglês | MEDLINE | ID: mdl-31957636

RESUMO

One of the biggest challenges for in situ heating transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) is the ability to measure the local temperature of the specimen accurately. Despite technological improvements in the construction of TEM/STEM heating holders, the problem of being able to measure the real sample temperature is still the subject of considerable discussion. In this study, we review the present literature on methodologies for temperature calibration. We analyze calibration methods that require the use of a thermometric material in addition to the specimen under study, as well as methods that can be performed directly on the specimen of interest without the need for a previous calibration. Finally, an overview of the most important characteristics of all the treated techniques, including temperature ranges and uncertainties, is provided in order to provide an accessory database to consult before an in situ TEM/STEM temperature calibration experiment.

3.
ChemSusChem ; 12(6): 1229-1239, 2019 Mar 21.
Artigo em Inglês | MEDLINE | ID: mdl-30673164

RESUMO

The recently reported Co4Cat process is a synthesis method bearing ecological and economic benefits to prepare precious-metal nanoparticles (NPs) with optimized catalytic properties. In the Co4Cat process, a metal precursor (e.g., H2 PtCl6 ) is dissolved in an alkaline solution of a low-boiling-point solvent (methanol) and reduced to NPs at low temperature (<80 °C) without the use of surfactants. Here, the Co4Cat process to prepare Pt NPs is described in detail. The advantages of this new synthesis method for research and development but also industrial production are highlighted in a comparison with the popular "polyol" synthesis. The reduction of H2 PtCl6 from PtIV to PtII and further to Pt0 is followed by UV/Vis and XANES/EXAFS measurements. It is demonstrated how the synthesis can be accelerated, how size control is achieved, and how the colloidal dispersions can be stabilized without the use of surfactants. Despite being surfactant-free, the Pt NPs exhibit surprisingly long-term (up to 16 months) stability in water over a wide pH range (4-12) and in aqueous buffer solutions. The Co4Cat process is thus relevant to produce NPs for heterogeneous catalysis, electro-catalysis, or bio/medical applications.

4.
Angew Chem Int Ed Engl ; 57(38): 12338-12341, 2018 Sep 17.
Artigo em Inglês | MEDLINE | ID: mdl-30051948

RESUMO

Compared to conventional preparation methods for supported heterogeneous catalysts, the use of colloidal nanoparticles (NPs) allows for a precise control over size, size distribution, and distribution/location of the NPs on the support. However, common colloidal syntheses have restrictions that limit their applicability for industrial catalyst preparation. We present a simple, surfactant-free, and scalable preparation method for colloidal NPs to overcome these restrictions. We demonstrate how precious-metal NPs are prepared in alkaline methanol, how the particle size can be tuned, and how supported catalysts are obtained. The potential of these colloids in the preparation of improved catalysts is demonstrated by two examples from heterogeneous catalysis and electrocatalysis.

5.
ACS Appl Mater Interfaces ; 8(7): 4548-60, 2016 Feb.
Artigo em Inglês | MEDLINE | ID: mdl-26829707

RESUMO

A microchanneled asymmetric dual phase composite membrane of 70 vol % Gd(0.1)Ce(0.9)O(1.95-δ)-30 vol % La(0.6)Sr(0.4)FeO(3-δ) (CGO-LSF) was fabricated by a "one step" phase-inversion tape casting. The sample consists of a thin dense membrane (100 µm) and a porous substrate including "finger-like" microchannels. The oxygen permeation flux through the membrane with and without catalytic surface layers was investigated under a variety of oxygen partial pressure gradients. At 900 °C, the oxygen permeation flux of the bare membrane was 1.6 (STP) ml cm(-2) min(-1) for the air/He-case and 10.10 (STP) ml cm(-2) min(-1) for the air/CO-case. Oxygen flux measurements as well as electrical conductivity relaxation show that the oxygen flux through the bare membrane without catalyst is limited by the oxygen surface exchange. The surface exchange can be enhanced by introduction of catalyst on the membrane surface. An increase of the oxygen flux of ∼1.49 (STP) mL cm(-2) min(-1) at 900 °C was observed when catalyst is added for the air/He-case. Mass transfer polarization through the finger-like support was confirmed to be negligible, which benefits the overall performance. A stable flux of 7.00 (STP) ml cm(-2) min(-1) was observed between air/CO/CO2 over 200 h at 850 °C. Partial surface decomposition was observed on the permeate side exposed to CO, in line with predictions from thermodynamic calculations. In a mixture of CO, CO2, H2, and H2O at similar oxygen activity the material will according to the calculation not decompose. The microchanneled asymmetric CGO-LSF membranes show high oxygen permeability and chemical stability under a range of technologically relevant oxygen potential gradients.

6.
Nanotechnology ; 23(40): 405705, 2012 Oct 12.
Artigo em Inglês | MEDLINE | ID: mdl-22995859

RESUMO

Carbon nanotube terminated atomic force microscopy (AFM) probes have been used for the imaging of 5 nm wide surface supported Pt nanoclusters by non-contact (dynamic mode) AFM in an ultra-high vacuum. The results are compared to AFM measurements done with conventional Si-tips, as well as with transmission electron microscopy images, which give accurate measures for cluster widths. Despite their ideal aspect ratio, tip-broadening is concluded to be a severe problem even when imaging with carbon nanotube tips, which overestimates the cluster width by several times the nominal width of the nanotube tip. This broadening is attributed to a bending of the carbon nanotubes, and not to pure geometrical factors, which coincidentally results in a significant improvement for relative height measurements of tightly spaced high aspect ratio structures, as compared to what can be achieved with geometrically limited conventional probes. Superior durability also stands out as a defining feature of carbon nanotube terminated probes, allowing them to give results with a greatly enhanced reproducibility.

7.
J Am Chem Soc ; 132(23): 7968-75, 2010 Jun 16.
Artigo em Inglês | MEDLINE | ID: mdl-20481529

RESUMO

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.

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