Catalyst-support interactions mechanism

An important question frequently raised in electrochemical promotion studies is the following How thick can a porous metal-electrode deposited on a solid electrolyte be in order to maintain the electrochemical promotion (NEMCA) effect The same type of analysis is applicable regarding the size of nanoparticle catalysts supported on commercial supports such as Zr02, Ti02, YSZ, Ce02 and doped Zr02 or Ti02. What is the maximum allowable size of supported metal catalyst nanoparticles in order for the above NEMCA-type metal-support interaction mechanism to be fully operative ... 

The model of electrochemical promotion regards the phenomenon as catalysis in presence of an electrically controlled double layer formed by spillover-backspillover mechanism at the gas-exposed catalyst surface. This shows strong analogy with catalyst-support interactions... 

The good qualitative agreement between eUwR variation and O0 variation shown in Figure 11.11 for the various supports used, underlines again the common promotional mechanism of electrochemically promoted and metal-support interaction promoted metal catalysts. 

In situ ETEM permits direct probing of particle sintering mechanisms and the effect of gas environments on supported metal-particle catalysts under reaction conditions. Here we present some examples of metals supported on non-wetting or irreducible ceramic supports, such as alumina and silica. The experiments are important in understanding metal-support interactions on irreducibe ceramics. 

The synthesis of paratolunitrile (PTN) and terephtalonitrile (TPN) by reaction of paraxylene with nitrogen monoxide was studied over a series of aerogel chromium oxide alumina catalysts. The stabilization of the active phase was interpreted on the basis of Cr O support interactions. Kinetic studies show that the reaction follows a "redox" mechanism for the formation of PTN and a Langmuir Hinshelwood mechanism for the production of TPN. 

Well-defined peptides of known sequence have been used to shed light on the mechanism of catalysis in the epoxidation of enones with hydrogen peroxide [91, 93-95]. The peptide sequences of the catalysts have been systematically varied and correlated with catalytic activity and selectivity. From the many variations investigated it was concluded (i) that the N-terminal region of the peptides harbors the catalytically active site, and that (ii) a helical conformation is required for the peptide catalysts to be active. The latter conclusion is supported both by the dependence of catalytic activity on chain-length and by IR investigations [91, 94]. NMR data that might aid further elucidation of catalyst structure, interaction with the substrate enones, etc., are, unfortunately, not yet available. 

The concept of mechanical fixation of metal on carbon makes catalytic applications at high temperatures possible. These applications require medium-sized active particles because particles below 2nm in size are not sufficiently stabilised by mechanical fixation and do not survive the high temperature treatment required by the selective etching. Typical reactions which have been studied in detail are ammonia synthesis [195, 201-203] and CO hydrogenation [204-207]. The idea that the inert carbon support could remove all problems associated with the reactivity of products with acid sites on oxides was tested, with the hope that a thermally wellconducting catalyst lacking strong-metal support interactions, as on oxide supports, would result. 

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