Crystallite size kinetics

Fig. 15. Kinetics of the ethylene hydrogenation on Ni and 0-Ni-hydride film catalysts m denotes mass of films, which as known is connected with the thickness and crystallite sizes of the films involved. Blank points—rate of reaction proceeding on Ni film catalysts black points—rate of reaction proceeding on nickel previously exposed to the atomic hydrogen action, i.e. transformed to some extent into /3-Ni-hydride.

The oxidation of cobalt metal to inactive cobalt oxide by product water has long been postulated to be a major cause of deactivation of supported cobalt FTS catalysts.6 10 Recent work has shown that the oxidation of cobalt metal to the inactive cobalt oxide phase can be prevented by the correct tailoring of the ratio Ph2cJPh2 and the cobalt crystallite size.11 Using a combination of model systems, industrial catalyst, and thermodynamic calculations, it was concluded that Co crystallites > 6 nm will not undergo any oxidation during realistic FTS, i.e., Pi[,()/I)i,2 = 1-1.5.11-14 Deactivation may also result from the formation of inactive cobalt support compounds (e.g., aluminate). Cobalt aluminate formation, which likely proceeds via the reaction of CoO with the support, is thermodynamically favorable but kinetically restricted under typical FTS conditions.6... 

The catalysts were synthesized as films, with ceria prepared by spray pyrolysis of 0.1 M solutions of Ce(N03)3 onto nonporous alumina wafers held at 250 °C. The ceria was then calcined at 300 °C, resulting in a crystallite size of 10 nm. Pt, Pd, or Rh was vapor deposited onto the oxide film. For kinetics testing, the temperature was 300 °C. To determine the reaction order of H20, Pco was maintained constant at 0.026 atm. For the reaction order on CO, Ph2o was maintained constant at 0.02 atm. The kinetic parameters are tabulated in Table 69. 

Gillot, B. Rousset, A. Dupre, G. (1978) Influence of crystallite size on the oxidation kinetics of magnetite. J. Solid State Chem. 25 263-271... 

As noted by Carberry in 1987, only phenomenological values can be measured in the laboratory since it is not possible to a priori distinguish between A (the catalytic area) and A (exposed measurable area), per volume of catalyst agent. This yields a structure-sensitive reaction that is dependent on crystallite size. While it is clear that a mechanism cannot be determined from purely kinetic measurements, a proposed mechanism is only accepted after it can predict the observed kinetic measurements. The dominant issue of the observed measurements is whether A or A is being measured. This correct measurement will yield the proper intrinsic kinetics, but will not reveal much insight into the mechanism. Thus, it is imperative to identify and obtain as much information as possible on the nature of intermediate chemical species. 

Effect of Zeolite Crystallite Size on the Selectivity Kinetics of the Heterogeneous Catalyzed Isomerization of Xylenes... 

Quantitative Interpretation of Intracrystalline Diffusional Effects. Since a qualitative effect of crystallite size upon selectivity was observed, the next step was to extract some quantitative values for the intracrystalline diffusional parameters. To do this, we must either know the intrinsic or diffusion-free kinetics or be able to make a simplifying assumption so that the diffusional parameters can be extracted from the available data. 

It is a logical extension that the reduction of the diffusional resistance, such as by decreasing the crystallite size, should result in apparent kinetics that approach that of a simple series reaction scheme. If we compare the apparent kinetics for large and small crystallite catalysts, we find the o-xylene p-xylene path at 400° F is essentially eliminated with small crystallites. 

The first spectrum could be recorded 25 s after admission of alcohol to the catalyst. For all the zeolite samples of various crystallite sizes (Table I) at 296 K, the adsorption was complete within 25 s for sec- and isobutyl alcohols. The dehydration process of these alcohols in the zeolitic pores was, however, slower. For a given alcohol (/ -, sec-, or iso-) the kinetics of water elimination were identical for catalysts of different crystallite sizes. This firmly establishes the absence of any diffusion limitation for dehydration for these three alcohols. 

The picture is different for the bulkier tert-butyl alcohol the kinetics of its adsorption at room temperature are markedly retarded with increasing crystallite sizes of HZSM-5 and they exhibit a t12 dependence (t is the time after adsorption) (see Fig. 3c of ref. 8d). This is symptomatic of a diffusion-influenced process. 

For all four alcohols in the zeolitic catalysts with small enough crystallite sizes—when diffusion limitations also disappear—dehydration kinetics are well approximated by the exponental function, a fact that is explicable in terms of the unimolecular decay of molecules of butyl alcohol adsorbed on identical active sites. With isobutyl alcohol, for example, the rate coefficient k may be written... 

Temperature influences all diffusion processes (cation diffusion in the electrolyte, surface diffusion of the adatoms). If small crystallite sizes are desired the deposition should be performed at ambient temperatures or below in order to slow down the kinetics of recrystallisation of the nuclei. 

Depending on the kinetics of the different elementary processes involved in the formation of the precipitate, a temperature increase might lead to an increase in crystallite size, as was observed for the crystallization of pseudoboehmite [24] or iron molybdates [25]. However, in other cases no influence of the precipitation temperature on the crystallite size of the final catalyst was reported [26], or a decrease was reported, as for the ZnO system [27],... 

C which may be due to crystallization.15 The sample, in fact, gives an X-ray pattern (though ill-defined) when heated at 250°C. The crystallite size is very low ( 100 A) and increases markedly when heated to higher temperatures (fig. 2b) just as for pure anatase (fig. 1 b). The kinetics of the transformation of this anatase sample were studied at different temperatures. 

An issue of debate is the relative roles of internal and external sites in the catalytic process. The effects of shape selectivity, clearly present in product distribution, seem to indicate a predominance of intra-porous hydroxylation. However, the different catechol/hydroquinone ratio in methanol (0.5) and acetone (1.3), could indicate a significant contribution of sites located on the outer surface of the crystals, particularly for crystallite sizes <0.3 xm. Tuel and others, studying the time course of the reaction and the solubility of tarry deposits, went further and concluded that catechol and hydroquinone were produced on different sites, external and internal respectively [49]. The effect of acetone and methanol simply reflected their ability to maintain external sites clean from tar deposits, which are soluble in the former and insoluble in the latter. On the other hand, Wilkenhoner and others concluded, with the support of kinetic constants estimated independently for internal and external sites, that catechol was also produced in the pores over the entire reaction profile, albeit at a lower rate [47]. The contribution of the outer surface for crystal sizes close to 0.1 (xm ranged from 46% in methanol to 69% in acetone. 

Carbon number distributions are similar on all Co catalysts. As on Ru catalysts, termination probabilities decrease with increasing chain size, leading to non-Flory product distributions. The modest effects of support and dispersion on product molecular weight and C5+ selectivity (Table III) reflect differences in readsorption site density and in support pore structure (4,5,14,40,41), which control the contributions of olefin readsorption to chain growth. Carbon number distributions obey Flory kinetics for C30+ hydrocarbons the chain growth probability reaches a constant value (a ) as olefins disappear from the product stream. This constant value reflects the intrinsic probability of chain termination to paraffins by hydrogen addition it is independent of support and metal dispersion in the crystallite size range studied. 

Markovic NM, Gasteiger HA, Ross PN (1997) Kinetics of oxygen reduction on Pt(hkl) electrodes implications for the crystallite size effect with supported Pt electrocatalysts. J Electrochem Soc 144 1591... 

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