Effect of Crystallite Size

The dependence of the selectivity on the conversion at low conversions has been investigated by varying the amount of catalysts used and the flow rate of the feed. For conversions below 10%, the selectivity is slightly higher at lower conversion, although the difference is only a few percent. The areal rate (rate per unit surface area of iron oxide) of butene oxidation is slightly higher 

Finally, the smallest crystallite size sample was subjected to severe thermal treatment. After calcining to 450, 500, 600, 780, and 920°C, the catalyst was 

It has been reported that by heating in the presence of water vapor, silicon can be incorporated into the lattice of y-Fe203 (55,64). It is unlikely that this effect occurs under the butene oxidation conditions since a physical mixture of large size iron oxide and silica gel retains its selectivity for butadiene after a prolonged reaction time without any indication of changes in the catalyst. If silicon substitution takes place, the water from the oxidation reaction could catalyze the substitution. In fact, deliberate pretreatment of such a physical mixture with water vapor before reaction does not change the activity or selectivity (54). 

In summary, the data shown in Fig. 8 indeed illustrate the dependence of selectivity on the crystallite size of a-Fe203. There is no evidence for the presence of y-Fe203, iron hydroxide, or iron silicate that could affect the data. 

The reasons for the crystallite size effect are not known. There are several possibilities. If the selective oxidation reaction is crystal face specific, then the size effect is due to the fact that different proportions of various crystal faces are present on crystallites of different sizes. It is known that small crystallites supported on silica are more difficult to reduce than large crystallites. This different reducibility can contribute to the size effect. It is possible that the charge transfer ability of small crystallites is different. Since charge transfer is involved in the activation of gaseous oxygen which is active in degradation of surface intermediates, this could also be a contribution. It is clear that further work is needed to distinguish the possibilities. 

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The effect of crystallite size and shape of K-L zeolite on the dispersion of Pt was examined by a variety of techniques by Resasco and coworkers [143], They obtained multiple overlapping CO bands on these samples and were able to assign them to Pt clusters located inside the zeoHte pores (<2050cm ), near the pore mouth (2050-2075 cm" ) and outside the pores (>2075 cm" ). They were able to correlate high -octane aromatization activity with the K-L zeolite samples with short channels where most of the Pt is inside the pores. 

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. 

Figure 8. Effect of crystallite size on o-xylene isomerization reaction paths...

While the effect of crystallite size has been investigated for reactions on iron oxide, the dependence of the activity and selectivity of other oxidation reactions on the nature of the exposed surface planes has been investigated for reactions on Mo03 and V2Os catalysts. A list of these reactions, the catalysts used, and the major conclusions are listed in Table IX. It appears that all the reactions listed are structure sensitive, that is, different crystal faces catalyze different reactions, or special active sites are required. 

During the course of studying the effect of crystallite sizes, attempts were made to produce very small unsupported iron oxide powders by lowering the calcination temperature of the iron hydroxyl gel that was precipitated from iron nitrate with ammonium hydroxide. However, catalysts calcined below 300°C still contain hydroxide, and they show high selectivity in butadiene production. For this reason, two catalysts, calcined at 250°C and 300°C, respectively, were studied in more detail. 

Unfortunately another factor complicates the measurement of true crystallite size. The K parameter is also a function of both crystallite size and of lattice distortion. We recently studied the effect of crystallite size and distortion on the K parameter using optical transform methods with simulated lattice images drawn by a computational method (26). The Scherrer K parameters given in Table X may be used to obtain a true number average crystallite size by any of the methods quoted, but will only be valid for crystallites with a number average size in the range 8 to 15 layer planes and a lattice distortion of 4-6%. The mixed function method (3) appears to give the best estimate of true crystallite size (K = 1.0) and the Hosemann method (4) the best estimate of lattice distortion. 

A brief review of the diffraction phenomenon and the effect of crystallite size is presented. Applications of XRD to catalyst characterization are illustrated, including correlation of XRD powder patterns to molecular structural features, determination of Pt crystallite size and others. Factors that affect the appearance of XRD powder patterns, such as framework structure perturbations, extra-framework material, crystal morphology, impurities, sample preparation, instrument configurations, and x-ray sources, are discussed. 

Figure 10.5 Effect of crystallite size reduction by ball-milling on the hydrogen desorption rates i vacuum at 300°C. Reproduced with permission from [2], published by Elsevier, 1999.

E. Van Looij and J.W. Geus, The Reactivity of Carbon-Containing Species on Nickel, Effects of Crystallite Size and Related Phenomena, Catalysis Letters, Vol.45, 1997, pp.209-213. 

LAN 93] LANGFORD J.I., BOULTIF A., AUFFREDIC J.P., LOUER D., The use of pattern decomposition to study the combined x-ray diffraction effects of crystallite size and stacking faults in ex-oxalate zinc oxide , J. Appl. Cryst, vol. 26, p. 22-33, 1993. 

The correlation is not so clear when the comparison is made with catalysts that have been pre-oxidized before the reaction. However, most studies seem to agree that the TOF increases with particle size. For example, Hicks et a/., working at 335 °C on pre-reduced Pd/Al203 and PdA"-Zr02 samples, observed a steady state TOF of about 1.3 s for large particles and 0.02 s for small particles. Similarly, Muller et a/.examined the effect of crystallite size on Pd/Zr02 catalysts. The crystallite size, as measured by XRD, was varied by using different... 

The effect of crystallite size on the optical yield was first observed by Vedenyapin et al. in the hydrogenation of ethyl acetoacetate into ethyl 3-hydrojg butyrate during progressive loading of Ru in Ru-silica catalysts that had been modified with (2R,3R)-tartaric acid. The reaction proved to be structure sensitive. The most effective catalyst proved to be the one with a 4.5 nm crystallite size, while catalysts with crystallite sizes 1.6 and 8.0 nm revealed lower asymmetric abilities. The sizes of the Ru crystallites were increased by increasing reduction times of the catalysts during their preparations. 

MAA)will diminish optical activity of the resulting 3-hydro g butyrates. So, in order to make an accurate evaluation of the effect of crystallite size, the extent of asymmetric reaction needs to be controlled (for details see later in Part 5.5 ). 

Klofla, T., Buttner, W.J., and Armstrong, N.R. (1986) Effect of crystallite size and hydrogen and oxygen uptake in the photoelectrochemistries of thin films of chlorogallium phthalocyanine./. Electrochem. Soc.,... 

Numerous authors have emphasized the effects of crystallite size on diffraction data and these effects include primary extinction for well-crystallized phases such as quartz and calcite, particle statistics, microabsorption, and preferred orientation. 

IN SITU FTIR, EXAFS, AND ACTIVITY STUDIES OF THE EFFECT OF CRYSTALLITE SIZE ON SILICA-SUPPORTED PT OXIDATION CATALYSTS... 

Gracia, F. J., Bollmann, L., Wolf, E. E., Miller, J. T. Kropf, A. J. In situ FTIR, EXAFS, and activity studies of the effect of crystallite size on silica-supported Pt oxidation catalysts. Journal of Catalysis 220, 382-391 (2003). 

Haga, F., Nakajima, T., Yamashita, K., Mishima, S. (1998). Effect of crystallite size on the catalysis of alumina-supported cobalt catalyst for steam reforming of ethanol. Reaction Kinetics and Catalysis Letters, 63, 253 —259. 

Islam MN, Pradhan A, Kumar S (2005) Effects of crystallite size distribution on the Raman-scattering profiles of silicon nanostructures. J Appl Phys 98 024309 Islam MN, Ram SK, Kumar S (2007) Band edge discontinuities and carrier transport in c-Si/porous silicon heterojunctions. J Phys D Appl Phys 40 5840 Islam MN, Ram SK, Kumar S (2009) Mott and Efros-Shklovskii hopping conductions in porous silicon nanostructures. Physica E 41 1025... 

Dahl IM, Wendelbo R, Andersen A, Akporiaye D, Mostad H, Fuglerud T The effect of crystallite size on the activity and selectivity of the reaction of ethanol and 2-propanol over SAPO-34, Microporous Mesoporous Mafer 29 159—171, 1999a. 

The effect of crystallite size has been investigated by Kockrick et who compared soot combustion over cerium dioxide nanoparticles prepared by an inverse micro-emulsion synthesis and by a simple precipitation route. The results show that, besides there being a great difference in crystal size, the catalytic activity for fresh... 

An example of structure sensitivity in deactivation is found in the work of Ostermaier et al. (3A) for 2-15 nm Pt/Al203 and Pt black in ammonia oxidation at 368-473 K. The effects of crystallite size and temperature in deactivation were investigated it was found that the extent of deactivation increased with decreasing temperature and there was a difference in the Arrhenius behavior between sintered and unsintered materials. Deactivation was more severe with smaller crystallites, but the surface could be completely reactivated by H2 at 673 K. It was suggested that PtO was the deactivated surface, and an excellent correlation of activity was provided by ... 

Some kinetic work has been carried out on the decomposition of (pseudo)-boehmite. Callister et al. [19] reported on the effect of the water pressure. Tsuchlda et al. [20] reported on the effect of crystallite size and confirmed the effect of water pressure as determined by Callister et al. [19]. A consistent kinetic model for the decomposition of pseudoboehmite, which is applicable over the entire temperature range of interest in calcination (20 - 800 °C) and which takes into account the effects of particle size and water pressure is not available in the literature. Therefore the model of Leyko et al. [21] has been fitted to the data of Fig. 6 as a first approximation ... 

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