Size effects in chemistry of heterogeneous systems

Уваров, Н. Ф.; Болдырев, В. В.

doi:10.1070/rc2001v070n04abeh000638

Cited by 130 publications

(48 citation statements)

References 112 publications

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“…It is well-established that a change in size of a metal nanoparticle results into: (i) a variation of the mean coordination number, 1 (ii) a change of the equilibrium shape, 2 (iii) a shift of the d-band center relative to the Fermi level, 2−5 and (iv) an expansion or a contraction of the lattice parameter. 6,7 Strikingly, despite specific cases such as the oxidation of carbon monoxide (CO) on Au nanoparticles, 8,9 the intrinsic catalytic activity is usually depreciated when decreasing the metal nanoparticle size. 4,5 This holds particularly true in proton-exchange membrane fuel cells (PEMFCs), where Ptbased nanoparticles supported onto a high-surface area carbon electrocatalyze the hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR).…”

Section: ■ Introductionmentioning

confidence: 99%

Tuning the Performance and the Stability of Porous Hollow PtNi/C Nanostructures for the Oxygen Reduction Reaction

et al. 2015

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Due to their increased surface area to volume ratio and molecular accessibility, microporous and mesoporous materials are a promising strategy to electrocatalyze the cathodic oxygen reduction reaction (ORR), the key reaction in proton-exchange membrane fuel cells (PEMFC). Here, we synthesized and provided atomically resolved pictures of porous hollow PtNi/C nanocatalysts, investigated the elemental distribution of Ni and Pt atoms, measured the Pt lattice contraction, and correlated these observations to their ORR activity. The best porous hollow PtNi/C nanocatalyst achieved 6 and 9-fold enhancement in mass and specific activity for the ORR, respectively over standard solid Pt/C nanocrystallites of the same size. The catalytic enhancement was 4 and 3-fold in mass and specific activity, respectively, over solid PtNi/C nanocrystallites with similar chemical composition, Pt lattice contraction, and crystallite size. Furthermore, 100% of the initial mass activity at E = 0.90 V vs RHE (0.56 A mg −1 Pt) of the best electrocatalyst was retained after an accelerated stress test composed of 30 000 potential cycles between 0.60 and 1.00 V vs RHE (0.1 M HClO 4 T = 298 K), therefore meeting the American Department of Energy targets for 2017−2020 both in terms of mass activity and durability (0.44 A mg −1 Pt, mass activity losses < 40%). The better catalytic activity for the ORR of hollow PtNi/C nanocatalysts is ascribed to (i) their opened porosity, (ii) their preferential crystallographic orientation ("ensemble effect"), and (iii) the weakened oxygen binding energy induced by the contracted Pt lattice parameter ("strain effect").

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Section: ■ Introductionmentioning

confidence: 99%

Tuning the Performance and the Stability of Porous Hollow PtNi/C Nanostructures for the Oxygen Reduction Reaction

et al. 2015

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“…It depends greatly on the production method: either by nanocluster aggregation, in particular, under conditions close to thermodynamic equilibrium [226,227], or by dispersion from the consolidated substance during non-equilibrium thermodynamic processes while applying outside powerful energy fluxes. The latter method increases greatly the energy saturation of nanoparticles, which may lead to their specific behavior and cause the threshold phenomena (see, for example: [228][229][230][231][232][233][234]). In all cases, a significant role in forming the nano-structure stability is played self-organization phenomena [235] including those happening at the formation level of the above-mentioned non-autonomous phases with particular physical chemistry.…”

Section: Dissipative Phenomena Chemical Thermal Current and Magnetmentioning

confidence: 99%

High-temperature superconductivity: From macro- to nanoscale structures

Коваленко¹

2016

Nanosystems: Phys. Chem. Math.

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The analysis of achievements, problems and prospects of high-temperature superconductivity (HTSC) in the macro-and nanostructured materials has been given. The main experimental results and theoretical models describing the physical mechanisms of the superconductivity appearance at phenomenological and microscopic levels, including change in the energy spectrum of atoms in these materials with the advent of the 'superconducting' gap at temperatures below the critical transition, as well as the above-critical temperature 'pseudo-gap' have been analyzed.Although the origin of the pseudo-gap is not completely understood, it can be considered as an independent phase transition in the substance prior to the transition to the zero resistance state and insusceptibility (or insensitivity) to external magnetic field in high-temperature superconductors. Features of multi-gap and gapless superconducting materials as well as their ability to further increase the temperature of supercritical transition are discussed. Large resources to create the necessary electron and phonon spectra in the process of high-temperature superconductivity formation are associated with the use of nanoscale structures and nanoparticles of conductors and dielectrics. Electrical conductive contacts between nanoparticles and tunnel chains of nanoclusters, where delocalized electron spectra form similar energy shells to the atomic or nuclear shells, play a significant part here. It is required to further investigate the occurrence of high-temperature superconductivity at the level of interphase layers (non-autonomous phases) in nanostructures containing a large fraction of the substance in this condition. It is essential to develop adequate methods for synthesis of nanoparticles of variable size, structure and morphology, as well as techniques for their consolidation that would ensure the preservation of superconductivity of individual nanoparticles, their chemical, thermal, magnetic and current stability in the dissipative processes with functional and fluctuation effects.

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“…In addition, neglecting the difference in the Grüneisen parameter ␥ under isochoric conditions can also potentially result in a large deviation because the lattice vibrations in nanoparticles associated with the Grüneisen parameter ␥ are influenced to a certain degree by the interfaces of nanoparticles ͑size effect͒. 24 The comparison of the predicted Hugoniots for porous 2024 Al, Cu, and Fe with the available experimental data clearly show that the deviation of predicted results with experimental data are generally increasing with increasing porosity, which is linked to the effects of the specific internal energy and other thermodynamic properties. In more recent works, 25,26 the Wu-Jing model 12 has been extended by the authors to incorporate thermal pressure of electrons and correlate the calculated Hugoniot of highly porous metals including tungsten, copper, iron, lead, and aluminum, with experimental data available in the Russian literature.…”

Section: Model Calculationsmentioning

confidence: 99%

“…The principal cause for the ineffectiveness of the Wu-Jing method is most possibly the neglect of the characteristic properties of Fe nanoparticles, in particular, the contribution of the very large specific surface energy associated with the internal energy of the system. 24 The applicability of the Wu-Jing method for predicting the Hugoniot nanopowder compact can possibly be improved by incorporating the difference in internal energy between the powder compact and the solid, and the size effect of nanoparticles on lattice vibrations as they relate to the Grüneisen parameter. The experimental data obtained in this work are particularly valuable for probing the shock densification of Fe nanopowder, for validating the models for nanopowder Hugoniot prediction, and for modeling the complete EOS of iron over a relatively wide range of initial density and temperature.…”

Section: Fig 9 Example Of a Typical Voltagementioning

confidence: 99%

Dynamic densification behavior of nanoiron powders under shock compression

Dai

Eakins

Thadhani

2008

Journal of Applied Physics

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The dynamic densification behavior of nanoiron powder ͑ϳ25 nm particle size͒ prepressed to ϳ35% and ϳ45% of solid density was determined based on measurements of shock input stress and wave velocity by using piezoelectric stress gauges. The experimentally determined shock densification response is observed to be sensitive to the initial density ͑or porosity͒ of prepressed nanoiron powder compacts. Hugoniot measurements show an obvious densification-distension transition at ϳ2 GPa for the ϳ35% dense and ϳ6 GPa for the ϳ45% dense powder compacts. The densification and shock compression responses of the nanoiron powders are also calculated by using isobaric and isochoric models. Correlations of the model calculations with the measured data indicate that the shock Hugoniot of nanoiron powders cannot be correctly described by the currently available analytical models that are otherwise capable of predicting the Hugoniot of highly porous materials ͑prepressed compacts͒ of micron-sized powders.

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Size effects in chemistry of heterogeneous systems

Cited by 130 publications

References 112 publications

Tuning the Performance and the Stability of Porous Hollow PtNi/C Nanostructures for the Oxygen Reduction Reaction

Tuning the Performance and the Stability of Porous Hollow PtNi/C Nanostructures for the Oxygen Reduction Reaction

High-temperature superconductivity: From macro- to nanoscale structures

Dynamic densification behavior of nanoiron powders under shock compression

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