Nanostructured high-surface-area carbon materials are ubiquitous in many applications, including catalysis, energy storage, and separations. [1][2][3] As supports for catalytic nanoparticles, nanostructured carbons can provide conductive substrates with high surface areas and excellent dispersion characteristics, which are important both for optimizing the synergistic nanoparticle-support interactions and for maximizing the mass activity of expensive precious metal catalysts. [4,5] This is particularly important for platinumbased nanoparticle catalysts, the benchmark electrocatalysts for proton-exchange-membrane fuel cells (PEMFC) that facilitate hydrogen oxidation at the anode and the oxygen reduction reaction (ORR) at the cathode.[6] The ORR is critically important for fuel-cell applications because it is the reaction that prevents maximum efficiency from being realized. [7] General strategies for improving ORR catalysis focus on increasing the accessible surface area of the catalyst and enhancing activity by size reduction, nanostructure control, and alloying. [8][9][10] Herein, we describe a significant improvement in apparent platinum mass activity for the ORR in 0.5 m H 2 SO 4 by anchoring platinum nanoparticles (Pt NPs) onto a new type of nano-engineered carbon support: monodisperse spherical nanoshells of graphitic carbon that are intermediate in size between C 60 and most other hollow graphitic nanomaterials. Nanostructured carbon materials are often prepared by carbon replication of sacrificial colloidal and porous templates, [1,2,11] chemical leaching of metals from metal carbides, [12] pyrolysis of carbon-rich organic materials, [1] and a variety of other methods.[13] For all of these methods, hightemperature (> 600 8C) processing is typically required to produce graphitic carbon. [12] Our approach to graphitic carbon nanoshells, shown schematically in Figure 1, is inspired by carbide-derived carbons (CDCs), which are formed by high-temperature/ high-pressure extraction of non-carbon elements from metal carbides.[12] For example, silicon and titanium can be leached from SiC and TiC, respectively, using chlorine gas. The leaching process introduces large numbers of micropores and mesopores, which result in high surface areas. However, the high temperatures and large particle sizes make it difficult to exploit CDCs as highly-dispersible supports for catalytic nanoparticles. Our nanostructure design strategy (Figure 1) yields highly dispersible uniform nanoshells of graphitic carbon with high surface areas by synergistically merging several important materials capabilities from colloidal nanoscience and solid-state chemistry. Recent reports have described the synthesis of nanocrystalline Ni 3 C [14] and NPs of the hcp allotrope of nickel, [15] both of which have similar XRD patterns. The product of a reaction claimed to yield near-monodisperse particles of hcp Ni, [15] when heated, behaves as would be expected for Ni 3 C, decomposing at about 420 8C.[16] In nanoparticle form, this decomposition results i...

Nanostructured high-surface-area carbon materials are ubiquitous in many applications, including catalysis, energy storage, and separations. [1][2][3] As supports for catalytic nanoparticles, nanostructured carbons can provide conductive substrates with high surface areas and excellent dispersion characteristics, which are important both for optimizing the synergistic nanoparticle-support interactions and for maximizing the mass activity of expensive precious metal catalysts. [4,5] This is particularly important for platinumbased nanoparticle catalysts, the benchmark electrocatalysts for proton-exchange-membrane fuel cells (PEMFC) that facilitate hydrogen oxidation at the anode and the oxygen reduction reaction (ORR) at the cathode. [6] The ORR is critically important for fuel-cell applications because it is the reaction that prevents maximum efficiency from being realized. [7] General strategies for improving ORR catalysis focus on increasing the accessible surface area of the catalyst and enhancing activity by size reduction, nanostructure control, and alloying. [8][9][10] Herein, we describe a significant improvement in apparent platinum mass activity for the ORR in 0.5 m H 2 SO 4 by anchoring platinum nanoparticles (Pt NPs) onto a new type of nano-engineered carbon support: monodisperse spherical nanoshells of graphitic carbon that are intermediate in size between C 60 and most other hollow graphitic nanomaterials. Nanostructured carbon materials are often prepared by carbon replication of sacrificial colloidal and porous templates, [1,2,11] chemical leaching of metals from metal carbides, [12] pyrolysis of carbon-rich organic materials, [1] and a variety of other methods. [13] For all of these methods, hightemperature (> 600 8C) processing is typically required to produce graphitic carbon. [12] Our approach to graphitic carbon nanoshells, shown schematically in Figure 1, is inspired by carbide-derived carbons (CDCs), which are formed by high-temperature/ high-pressure extraction of non-carbon elements from metal carbides. [12] For example, silicon and titanium can be leached from SiC and TiC, respectively, using chlorine gas. The leaching process introduces large numbers of micropores and mesopores, which result in high surface areas. However, the high temperatures and large particle sizes make it difficult to exploit CDCs as highly-dispersible supports for catalytic nanoparticles. Our nanostructure design strategy (Figure 1) yields highly dispersible uniform nanoshells of graphitic carbon with high surface areas by synergistically merging several important materials capabilities from colloidal nanoscience and solid-state chemistry. Recent reports have described the synthesis of nanocrystalline Ni 3 C [14] and NPs of the hcp allotrope of nickel, [15] both of which have similar XRD patterns. The product of a reaction claimed to yield near-monodisperse particles of hcp Ni, [15] when heated, behaves as would be expected for Ni 3 C, decomposing at about 420 8C. [16] In nanoparticle form, this decomposition resul...

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A two-dimensional model has been developed to study the flame structure above composite propellants using a vorticity-velocity formulation of the transport equations. This formulation allows for a more stable, robust, accurate, and faster solution method compared with the compressible Navier-Stokes equations in the low Mach flow regime. The model includes mass and energy coupling between the condensed and gas phases. The condensed-phase model is based on previously reported one-dimensional models and includes distributed decomposition and multistepreaction kinetics. The model uses a detailed gas-phase kinetic mechanism consisting of 37 species and 127 reactions. The kinetic mechanism and species diffusion determine the flame structure of the system; no assumptions are made beforehand, aside from appropriate boundary conditions. Numerical studies have been performed to examine the flame structure above an ammonium-perchlorate/hydroxy-terminated-polybutadiene propellant. The predicted flame structure was found to be qualitatively similar to the Beckstead-Derr-Price model with both premixed and diffusion flames present. Results present significant insight into ammonium perchlorate's ability to control a propellant's burning rate and illustrate the importance of the primary diffusion flame in composite propellant combustion.Nomenclature c p = heat capacity, erg=g=K g = gravitational acceleration vector, cm=s 2 h = specific enthalpy, erg=g i = species index M = Mach number r = radial distance, cm r = radial direction T = temperature, K t = time, s v = velocity vector, cm=ŝ v = diffusion velocity, cm=s W = molecular mass, g=mol _ w = chemical production rate, g=cm 3 =s Y = mass fraction z = axial direction = thermal conductivity, erg=s=cm=K = viscosity, poise = density, g=cm 3 = dissipation function ! = vorticity vector, 1=s