Nanoscale materials nanoparticle catalysts

The lure of new physical phenomena and new patterns of chemical reactivity has driven a tremendous surge in the study of nanoscale materials. This activity spans many areas of chemistry. In the specific field of electrochemistry, much of the activity has focused on several areas (a) electrocatalysis with nanoparticles (NPs) of metals supported on various substrates, for example, fuel-cell catalysts comprising Pt or Ag NPs supported on carbon [1,2], (b) the fundamental electrochemical behavior of NPs of noble metals, for example, quantized double-layer charging of thiol-capped Au NPs [3-5], (c) the electrochemical and photoelectrochemical behavior of semiconductor NPs [4, 6-8], and (d) biosensor applications of nanoparticles [9, 10]. These topics have received much attention, and relatively recent reviews of these areas are cited. Considerably less has been reported on the fundamental electrochemical behavior of electroactive NPs that do not fall within these categories. In particular, work is only beginning in the area of the electrochemistry of discrete, electroactive NPs. That is the topic of this review, which discusses the synthesis, interfacial immobilization and electrochemical behavior of electroactive NPs. The review is not intended to be an exhaustive treatment of the area, but rather to give a flavor of the types of systems that have been examined and the types of phenomena that can influence the electrochemical behavior of electroactive NPs. 

Chemical reactions in general can be accelerated to go in a forward direction using catalysts which do not participate directly in the reaction. Ihe type of catalyst used depends on the nature of reactants in the reaction and the different materials used are Pd, Pt, Ag, Ni, TiO, ZnO and Fe-Oxides. The inherent catalytic property of these materials can be further enhanced by increasing their specific surface area available for reactions, i.e., by reducing the particle size to nanodimensions. However, agglomeration of the nanoscale materials in their innate state is a serious limitation which reduces the effective surface area available for reaction. The aggregates are easy to recover and recycle. These limitations can be overcome mainly in two separate ways (i) immobilization of the nanoparticles in a porous support or carrier, and (ii) synthesis of the catalytic material as a nanoporous network-like structiu e using different types of templates. Bacterial cellulose has been used extensively as a support material to host the catalytic nanoparticles, while in some cases it has also been used as a template to synthesize catalyst network structure. Some typical studies wherein BC has been used as a support to hold PdCu, Pd, TiO and CdS nanoparticles are discussed first, followed by template structure based composites. 

We find that a layer model analysis can adequately describe the Pt NMR spectrum of nanoscale electrode materials. The shifts of the surface and sub-surface peaks of Pt NMR spectra correlate well with the electronegativity of various adsorbates, while the Knight shift of the adsorbate varies linearly with the f-LDOS of the clean metal surface. The Pt NMR response of Pt atoms from the innermost layers of the nanoparticles does not show any influence of the adsorbate present on the surface. This provides experimental evidence, which extends the applicability of the Friedel-Heine invariance theorem to the case of metal nanoparticles. Further, a spatially-resolved oscillation in the s-like E( -LDOS was observed via Pt NMR of a carbon-supported Pt catalyst sample. The data indicate that much of the observed broadening of the bulk-like peak in Pt NMR spectra of such systems can be attributed to spatial variations of the A( f). The oscillatory variation in A(A) beyond 0.4 nm indicates that the influence of the metal surface goes at least three layers inside the particles, in contrast to the predictions based on the Tellium model. 

Chemical properties. Increased surface area increases the chemical activity of a material. For example, a metal in bulk form may not be a catalyst the same metal in nanoscale particles may be an excellent catalyst. Important research measures pH, oxidation and reduction characteristics, and surface properties. An important concern is how nanostructures can change the chemical mechanisms of such key processes as hydrolysis and catalytic responses as well as differing hydrophobic, hydrophilic, or amphipathic surface properties. The atomic structures of high-energy surface sites and various types of defect sites on nanocrystals are needed, as well as their effect on reactivity. An initial priority is to gain exploitable knowledge of the physical chemistry of various nanoparticle surfaces. 

This scenario calls for a deep characterization of Pt based materials in terms of how many alloyed phases and segregated phases are present in the mixture as well as their composition. The more accurately the bulk of the Pt based materials is characterized the more precise the characteristics of surface are because the bulk and surface should not be so dissimilar, above the micrometric dimension, in terms of phases and composition. However, the same should not be strictly expected for nanoscale dimensions. Platinum nanoparticle based materials may not form a true alloy but a surface composition much dissimilar from the core given the equalized quantity of both the bulk and surface free energy. To complicate the picture even more, the electrochemical results often depend on the technique used to evaluate the activity of the catalysts. 

The processes occurring at the interface between the catalyst and electrolyte are manifold and strongly influenced by the surrounding environment and the external parameters (temperature, pressure and electrode potential). In addition, these external parameters can affect the morphology and composition of the CL, especially in cases where nanoparticles are used as catalyst materials. A consistent theoretical description of such complex catalytic systems is a real challenge. We have proposed a novel model within a continuum framework to describe in a detailed way the electrochemical interface at the vicinity of the catalyst under non-equilibrium conditions.This nanoscale model, which is a key part of MEMEPhys , comprises a ID-dififiise layer sub-model and a ID-inner layer submodel, as represented in Fig. 11.13. 

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