Electronic interaction, between support surface

Another example of electronic interaction between the graphite and ionic solutes is the retention of metal ions on PGC supports. Such electronic interaction, either electron donation or acceptance, is presumably between the available orbitals of metal ions and the electronic cloud of the flat graphite surface. The addition of a small concentration of oxalic acid into aqueous mobile phase tends to modify the graphite surface and to increase, through complexation, the range of metal ions retained. 

The carbon nanostructured support provides both a high activity and a high selectivity when compared to what is usually observed on traditional supports such as alumina or activated charcoal. Such catalytic behavior is attributed to the presence of a peculiar electronic interaction between the carbon nanofilaments and the metal which constitutes the active phase. This leads to a metallic site with unexpected catalytic performances [6,7]. In addition, due to their small dimensions, typically of about hundred of nanometers or less, the carbon nanofilaments display an extremely high external surface area which makes them a catalyst support of choice for liquid phase reactions. Due to the low difiusion coefficients of gaseous reactants in liquids, mass transfer phenomena become predominant in the liquid phase. The l%h external surfece area considerably decreases the... 

Taking into account all these factors, it has been found that the most appropriate supports for PFMFCs catalysts are carbon blacks of ca. 250 m g BFT surface area, and the most widely used is Vulcan XC-72R commercialized by Cabot. ° ° Due to the importance of the surface chemistry of these supports, and its influence in the supported active metal phase, ° ° different chemical modifications of the support have been also investigated. The chemical nature of the carbon surface produces different electronic interactions between the noble metals and the carbon support, and affects the metal particle morphology, and influences the catalytic activity. Additionally, during the last few years, new alternative materials to carbon blacks have also been used, especially on the basis of their porous structure (nanotubes, mosoporous carbons) or their microstructure (nano- and microfibers, and microspheres). 

The electrocatalytic ORR activity of platinum monolayers supported onAu(lll),Rh lll), Pd lll), Ru(OOOl), andlr lll) surfaces in a 0.1 M HCIO4 were also studied, as shown in Figure 7.4(A). There was a volcano-type dependence of monolayer catalytic activity on the substrate, and the most active was PtML/Pd lH), as shown in Figure 7.4(B). The weighted center of the d-band (cd) was believed to play a decisive role in determining a surface reactivity. The positions of the cd of the Pt monolayers depend both on the strain (geometric effects) and on the electronic interaction between... 

The easily reduced noble metals usually are supported on metal oxides which cannot be reduced. The noble metal oxides are first reduced to form metal particles and the surfaces of these small grains have different crystal faces and different properties. The size of metal particles are also influenced by the chemical—physical property of supports, enrichment and agglomeration of metals, the reduction temperature and the formation process of metals grain. There is electronic interaction between the support, promoters and metal particles, and so can affect the activity. 

The infrared spectra demonstrate that there is a strong interaction between CR and the PS segment of SBS and between SBS and BR. Scanning electron micrographs of the fracture surfaces further support the conclusion. The fracture surface of 70 30 BR/CR shows layer-shaped morphology and 30 70 blends show hillock-shaped protmsions. Addition of 5% SBS reduces the particle size, makes the particles more spherical, and enhances uniform distribution. This provides further evidence of the compatibilizing effect of SBS. 

In many cases there is an interaction between the carrier and the active component of the catalyst so that the character of the active surface will change. For example, the electronic character of the supported catalyst may be influenced by the transfer of electrons across the catalyst-carrier interface. In some cases the carrier itself has a catalytic activity for the primary reaction, an intermediate reaction, or a subsequent reaction, and a dual-function catalyst is thereby obtained. Materials of this type are widely employed in reforming processes. There are other cases where the interaction of the catalyst and support are much more subtle and difficult to label. For example, the crystal size and structure of supported metal catalysts as well as the manner in which the metal is dispersed can be influenced by the nature of the support material. 

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