Specific Particle Size Effects

In summary, apparent metal-support interactions may arise through the operation of a specific particle size effect, or of bifunctionality or spillover, or through the support acting as a source or sink of a catalytic poison. Deliberately added promoters constitute an additional complication. Real interactions not due to these or similar causes may be attributed either to electronic or geometric effects, the latter embracing possible differences in crystal habit, or to the creation of phases which contain the active component in some form but which are hard to reduce. 

It is now appropriate to consider in somewhat greater detail some of those phenomena which can easily be mistaken for true metal-support interactions. 

Takezawa, H. Kobayashi, Y. Kamegai, and M. Shimokawake, Appl. Catal., 1982, 3, 381. 

Fagherazzi, L. Schiffini, 1. W. Bassi, G. Vlaic, S. Galvagno, and G. Parravano, J. Phys. Chem., 1979, 83, 2S21. 

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In general, the filler industry recognises these limitations, and tries to use a few relatively simple parameters that, taken in combination, give an approximate, working definition of morphology appropriate to the application in mind. The parameters that are most likely to be encountered are specific surface area, average particle size, effective top size and oil adsorption. The measurement and application of these are discussed in more detail below. 

Next, let us consider the fact that a given solid of known crystal structure has at least two additional degrees of freedom which may change its behavior. The presence of lattice defects, such as dislocations, and any alteration of particle size or specific surface will change its Gibbs energy. Since our present knowledge of the influence of lattice defects on solubility is rather limited, we shall restrict ourselves to a discussion of the particle size effect only. 

The improvements in the activation polarization defined as either mass-specific activity or site-specific activity (activity/number of specific crystal planes on the surface) were reported, especially for the kinetically difficult ORR. Wealth of prior data on both ORR as well as direct methanol oxidation (both multielectron reduction and oxidation processes) showed clear particle-size effects. Bulk of these... 

The former phenomenon is usual referred to as particle-size effect and is pronounced for structure-sensitive reactions [1,2], i.e., catalytic reactions where the rate and/or selectivity is significantly different from one crystallographic plane to another. Structure-sensitive reactions (e.g., isomerizations) frequently occur on catalytic sites consisting of an ensemble of surface atoms with specific geometry. It is thus reasonable to expect that as the active-phase crystallite size decreases, there will be a different distribution of crystallographic planes on the catalyst surface, with the possible disappearance of ensemble sites, so that both the catalyst activity and... 

In this context, it should be mentioned that once the Pt surface really starts being oxidized, i.e. when the oxygen coverage becomes noticeable, the methanol oxidation rate goes down as well. This is the origin of a particle-size effect for this reaction [151] when the particle diameter falls below 4 nm, the increased strength of adsorption of 0(H), and by the way, also CO(H) [155], causes the specific MeOH electroxidation activity to decrease with decreasing Pt particle size. 

The baffle at the bottom has an apparently neutral effect on agitation it does, however, allow particle separation. It was observed that particles with a mean weight of 0.01 g are incorporated into the flow described previously. At the same time, particles with a mean weight of 0.06 g settled. This section of the bioreactor should be adjusted to allow the separation of a specific particle size to satisfy specific needs, such as large sludge particles that could lose efficiency. 

A controversial issue related to cobalt catalysts in Fisher-Tropsch synthesis is the structure-sensitive character of this reaction. Iglesia and co-workers [126,127] reported a large increase in activity when the cobalt particle size was decreased from 200 nm to 9 nm, whereas the specific activity [turnover frequency (TOF)] was not influenced by the cobalt particle size. However, other authors have reported that the TOF suddenly decreased for catalysts with cobalt particle sizes smaller than 10 nm [122,128]. Bezemer et al. [125] were the first to investigate the influence of cobalt particle size in the range 2.6 to 27 nm on performance in Fischer-Tropsch synthesis on well-defined catalysts supported on carbon nanofibers. It was found that the TOF for CO hydrogenation was independent of cobalt particle size for catalysts with particles larger then 6 nm (at atmospheric pressure) or 8 nm (at 35 bar). But both the TOF and the C5+ selectivity decreased for catalysts with smaller particles. It was proposed that the cobalt particle size effects could be attributed to a strucmre-sensitivity characteristic of the reaction, together with a CO-indnced reconstmction of the cobalt surface. 

The effect of carbon aerogel pore texture and method of preparation of the supported Pt catalysts on their activity has recently been reported [85]. Two different Pt precursors were used H2[PtCl6] and [Pt(NH3)4](OH)2. For a given Pt precursor, the pore texture of carbon aerogels used had no influence in Pt surface area and ORR activity. The best ORR specific activity was obtained with catalysts prepared with H2[PtCl6]. By contrast, the activity of catalysts prepared from [Pt(NH3)4](OH)2 was low, despite the fact that the Pt dispersion value reached was the highest. These authors [85] indicate that this is probably due to the particle size effect on ORR activity, with a smaller Pt particle size showing a lower activity. 

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