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1.
J Am Chem Soc ; 138(36): 11560-7, 2016 09 14.
Artigo em Inglês | MEDLINE | ID: mdl-27414757

RESUMO

Wet conditions in heterogeneous catalysis can substantially improve the rate of surface reactions by assisting the diffusion of reaction intermediates between surface reaction sites. The atomistic mechanisms underpinning this accelerated mass transfer are, however, concealed by the complexity of the dynamic water/solid interface. Here we employ ab initio molecular dynamics simulations to disclose the fast diffusion of protons and hydroxide species along the interface between water and ceria, a catalytically important, highly reducible oxide. Up to 20% of the interfacial water molecules are shown to dissociate at room temperature via proton transfer to surface O atoms, leading to partial surface hydroxylation and to a local increase of hydroxide species in the surface solvation layer. A water-mediated Grotthus-like mechanism is shown to activate the fast and long-range proton diffusion at the water/oxide interface. We demonstrate the catalytic importance of this dynamic process for water dissociation at ceria-supported Pt nanoparticles, where the solvent accelerates the spillover of ad-species between oxide and metal sites.

2.
ACS Nano ; 10(3): 3334-41, 2016 Mar 22.
Artigo em Inglês | MEDLINE | ID: mdl-26891421

RESUMO

MXene, a family of layered compounds consisting of nanosheets, is emerging as an electrode material for various electrochemical energy storage devices including supercapacitors, lithium-ion batteries, and sodium-ion batteries. However, the mechanism of its electrochemical reaction is not yet fully understood. Herein, using solid-state (23)Na magic angle spinning NMR and density functional theory calculation, we reveal that MXene Ti3C2Tx in a nonaqueous Na(+) electrolyte exhibits reversible Na(+) intercalation/deintercalation into the interlayer space. Detailed analyses demonstrate that Ti3C2Tx undergoes expansion of the interlayer distance during the first sodiation, whereby desolvated Na(+) is intercalated/deintercalated reversibly. The interlayer distance is maintained during the whole sodiation/desodiation process due to the pillaring effect of trapped Na(+) and the swelling effect of penetrated solvent molecules between the Ti3C2Tx sheets. Since Na(+) intercalation/deintercalation during the electrochemical reaction is not accompanied by any substantial structural change, Ti3C2Tx shows good capacity retention over 100 cycles as well as excellent rate capability.

3.
Phys Rev Lett ; 109(26): 266102, 2012 Dec 28.
Artigo em Inglês | MEDLINE | ID: mdl-23368586

RESUMO

We adopt fringe counting from classical moiré interferometry on moiré patterns observed in scanning tunneling microscopy of strained thin films on single crystalline substrates. We analyze inhomogeneous strain distribution in islands of CeO2(111) on Cu(111) and identify a generic source of strain in heteroepitaxy--a thickness-dependent lattice constant of the growing film. This observation is mediated by the ability of ceria to glide on the Cu substrate. The moiré technique we are describing has a potential of nanometer-scale resolution of inhomogeneous two dimensional strain in incommensurate layered systems, notably in supported graphene.

4.
J Chem Phys ; 133(23): 234705, 2010 Dec 21.
Artigo em Inglês | MEDLINE | ID: mdl-21186882

RESUMO

The thermodynamic, structural and electronic properties of Cu-CeO(2) (ceria) surfaces and interfaces are investigated by means of density functional theory (DFT+U) calculations. We focus on model systems consisting of Cu atoms (i) supported by stoichiometric and reduced CeO(2) (111) surfaces, (ii) dispersed as substitutional solid solution at the same surface, as well as on (iii) the extended Cu(111)/CeO(2)(111) interface. Extensive charge reorganization at the metal-oxide contact is predicted for ceria-supported Cu adatoms and nanoparticles, leading to Cu oxidation, ceria reduction, and interfacial Ce(3+) ions. The calculated thermodynamics predict that Cu adatoms on stoichiometric surfaces are more stable than on O vacancies of reduced surfaces at all temperatures and pressures relevant for catalytic applications, even in extremely reducing chemical environments. This suggests that supported Cu nanoparticles do not nucleate at surface O vacancies of the oxide, at variance with many other metal/ceria systems. In oxidizing conditions, the solid solutions are shown to be more stable than the supported systems. Substitutional Cu ions form characteristic CuO(4) units. These promote an easy and reversible O release without the reduction of Ce ions. The study of the extended CeO(2)(111)/Cu(111) interface predicts the full reduction of the interfacial ceria trilayer. Cu nanoparticles supported by ceria are proposed to lie above a subsurface layer of Ce(3+) ions that extends up to the perimeter of the metal-oxide interface.

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