Reactions surface-mediated

One of the most fundamental examples of a surface-mediated reaction is the heterogeneous catalytic hydrogenation of olefins and acetylenes on charcoal-supported transition metals like Pd, Pt, Ni, 

It is assumed that the effectiveness of surface-mediated reactions under sol-vent-free condition is due to the following factors  

The pores present in the supports behave as enzyme pockets where both substrate and reactant are constrained, consequently lowering the entropy of activation. 

The surface provides a microenvironment of difl crcni polarity with acidic and basic sites, causing appropriate activation of substrates and stabilization of the intermediates. 

The products are, in general, obtained with higher purity compared to their homogeneous counterparts. 

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Fig. XVIII-22. Schematic illustration of the steps that may be involved in a surface-mediated reaction initial adsorption, subsequent thermalization, diffusion and surface reaction, and desorption. (From Ref. 199 copyright 1984 by the AAAS.)...

It is not surprising then that the first reported surface mediated reaction made use of vicinal dihalides. In addition to poly-p-nitrostyrene immobilized metallo-porphyrins and viologens were found to be catalytically active. Recently,... 

Fourier Transform IR Studies of Surface Adsorbates and Surface-Mediated Reactions... 

Fe(II) goethite and Fe(II) with no mineral phase added. The degradation kinetics of CgCl NO in reactions and the change in sorbed Fe(II), [Fe(II)] as a function of pH are shown in Fig. 16.8. The primary degradation route for C Cl NO occurs through a surface-mediated reaction with Fe(II) the final product is C Cl NH, with an intermediate product believed to be phenylhydroxylamine (Cp NHOH). 

Klupinski et al. (2004) conclude that the reduction of nitroaromatic compounds is a surface-mediated process and suggest that, with lack of an iron mineral, reductive transformation induced only by Fe(II) does not occur. However, when C Cl NO degradation was investigated in reaction media containing Fe(II) with no mineral phase added, a slow reductive transformation of the contaminant was observed. Because the loss of C Cl NO in this case was not described by a first-order kinetic model, as in the case of high concentration of Fe(II), but better by a zero-order kinetic description, Klupinski et al. (2004) suggest that degradation in these systems in fact is a surface-mediated reaction. They note that, in the reaction system, trace amounts of oxidize Fe(II), which form in situ suspended iron oxide... 

In heterogeneous catalysis by metal, the activity and product-selectivity depend on the nature of metal particles (e.g., their size and morphology). Besides monometallic catalysts, the nanoscale preparation of bimetallic materials with controlled composition is attractive and crucial in industrial applications, since such materials show advanced performance in catalytic processes. Many reports suggest that the variation in the catalyst preparation method can yield highly dispersed metal/ alloy clusters and particles by the surface-mediated reactions [7-11]. The problem associated with conventional catalyst preparation is of reproducibility in the preparative process and activity of the catalyst materials. Moreover, the catalytic performances also depend on the chemical and spatial nature of the support due to the metal-support interaction and geometrical constraint at the interface of support and metal particles [7-9]. 

This chapter deals with the selective preparation, TEM/EXAFS/XPS characterization and catalysis of mono- and bimetallic nanowires and nanoparticles highly ordered in silica FSM-16, organosilica HMM-1 and mesoporous silica thin films. The mechanism of nanowire formation is discussed with the specific surface-mediated reactions of metal precursors in the restraint of nanoscale void space of mesoporous silica templates. The unique catalytic performances of nanowires and particles occluded in mesoporous cavities are also reviewed in terms of their shape and size dependency in catalysis as well as their unique electronic and magnetic properties for the device application. 

It has been previously reported [21, 22] that metal colloids are formed by radiochemical reactions in water/alcohol solutions, in which the reduction of metal salts takes place by solvated electrons and free radicals produced under UV or y-ray irradiation. Ichikawa et al. have applied this photoreduction method to the surface-mediated reaction of metallic ions and succeeded in synthesizing metal/aUoy nanowires in the constrained cavities of mesoporous supports such as FSM-16 and MCM-41 [18-20, 23-25]. The adsorbed water and alcohol work not only as solvents in the nanoscale silica void space but also as a source of reducing species for metallic ions to metals under UV-vis and y-ray [11, 18, 19] irradiation. The results indicate the dense formation of Pt nanowires inside the charmels of mesoporous supports, such as FSM-16, which act as the templates. In fact, no any Pt wire is observed on the external surface of FSM-16 or amorphous silica surface. Short wires, 10 nm long, are also observed as a minor species in the samples in the initial stage of UV and y-ray irradiation. 

Et-HMM (HMM-1) and Ph-HMM (or HMM-p), respectively, have a highly ordered 2D-hexagonal structure that exhibits more hydrophobic properties than the sihca FSM-16. They may work as attractive templates for metal nanowires and nanopar-tides by the hydrophobic surface-mediated reaction. 

Figure 15.21 shows a schematic representation of the SCCO2 treatment effect for promoting the internal diffusion of metal ions to prepare Rh and RhPt alloy nanoparticles in mesoporous FS-16 and HMM-1. The supercritical phase displays both liquid and gas properties at the same time. SCFs can also dissolve various metal precursors, which promotes their mobiUty and surface-mediated reaction to form nanoparticles by the hydrogen reduction in the mesoporous cavities of... 

Figure 15.23 Pictorial representation of the step-decoration method for preparing Mo nanowires by surface-mediated reaction on a stepped graphite.

Keywords amine, surface-mediated reaction, silica gel, hydroxylamine... 

Keywords diethyl hydrogen phosphite, aromatic aldehyde, magnesia, surface-mediated reaction, diethyl 1 -hydroxyarylmethylphosphonate... 

Unlike laboratory chemistry, geological chemistry occurs in thermodynamically open systems. That is to say, there is flagrant exchange of materials and energy between the system and the environment Not only is the system as a whole not isolated from the environment, but the materials of interest are rarely isolated from each other. To further the complexity, interaction of mineral catalysts with those forms of energy most likely to affect surface-mediated reactions has barely been characterized in model systems. 

One particularly appealing route for effecting controlled redox reactions involves an array of surface-mediated reactions initiated by ultraviolet irradiation of suspended semiconductor particles [3-13]. Such reactions involve band-gap excitation of the semiconductor, interfacial electron transfer, and secondary dark chemical reactions of singly oxidized and reduced adsorbates. Because the semiconductor surface is restored to its original structure and oxidation level after these transformations, these photoreactions are often called photocatalytic, leaving the light-responsive photocatalyst ready to act as initiator for another cycle. The use of such photocatalysts also obviates the need to acquire expensive electrochemical equipment. 

Anastasio C. and Mozurkewich M. (2002) Laboratory studies of bromide oxidation in the presence of ozone evidence for a glass-surface mediated reaction. J. Atmos. Chem. 41, 135-162. 

The reaction rates observed in surface-mediated reactions are proportional to the surface concentration of the reactant on the surface. Typically this surface concentration can be expressed as a function of its solute concentration by the Langmuir equation that is,... 

If Cl is substituted for I, no reaction occurs in ether or THF. The authors postulate that a surface-mediated reaction takes place, and because no positive Gilman test occurs, no intermediate Grignard reagent is formed. 

The combination of all these factors probably contributes to the selectivity and elSciency often achieved in surface-mediated reactions. Various heterogeneous surfaces such as clay, silica gel and alumina are widely used. 

The surface defines the interface between a mineral and its surroundings. In a dynamic context, reactions that occur between apatites and the environments in which they exist take place at or through their surfaces. This includes, but is not restricted to, crystal growth, dissolution, and surface-mediated reactions such as sorption, surface complexation, and catalysis (Hochella and White 1990). Because this interface is partly defined by the nature of the environment around the crystal, the properties, structure and chemistry of the crystal surface are always different than those of the bulk, and can be quite varied depending on the environment. For example, the crystal surface of apatite may have very different characteristics in contact with an aqueous solution as opposed to a polymerized silicate melt. 

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