Crystallite size effect

The hydrogenation of para-substituted anilines over rhodium catalysts has been investigated. An antipathetic metal crystallite size effect was observed for the hydrogenation of /Moluidinc suggesting that terrace sites favour the reaction. Limited evidence was found for catalyst deactivation by the product amines. Catalysts with pore diameters less than 13.2 nm showed evidence of diffusion control on the rate of reaction but not the cis trans ratio of the product. 

Crystallite Size Effects upon AP Catalyst Selectivity. Previous studies have shown that with the pellet sizes investigated, gross particle size does not affect activity or selectivity. If there are diffusional limitations, they must be intracrystalline and therefore a function of the crystallite size of the zeolite component. 

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. 

For fuel-cell technology development, it has been important to understand the characteristics and operation of highly dispersed platinum and platinum alloy electrocatalysts. A series of papers on platinum crystallite size determinations in acid environments for oxygen reduction and hydrogen oxidation was published together by Bert, Stonehart, Kinoshita and co-workers.5 The conclusion from these studies was that the specific activity for oxygen reduction on the platinum surface was independent of the size of the platinum crystallite and that there were no crystallite size effects. 

This paper examines the hydrogenation of aniline, /Moluidinc. and 4-fcrt-butylanilinc over a series of 2.5 % Rh/Si02 catalysts, comparing reaction rates and product selectivities. Further studies concentrated on examining support particle size and average metal crystallite size effects on /Moluidinc hydrogenation and the support pore size effects on 4-tert-butylaniline hydrogenation. 

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... 

In this chapter we review studies, primarily from our laboratory, of Pt and Pt-bimetallic nanoparticle electrocatalysts for the oxygen reduction reaction (ORR) and the electrochemical oxidation of H2 (HOR) and H2/CO mixtures in aqueous electrolytes at 274—333 K. We focus on the study of both the structure sensitivity of the reactions as gleaned from studies of the bulk (bi) metallic surfaces and the resultant crystallite size effect expected or observed when the catalyst is of nanoscale dimension. Physical characterization of the nanoparticles by high-resolution transmission electron microscopy (HRTEM) techniques is shown to be an essential tool for these studies. Comparison with well-characterized model surfaces have revealed only a few nanoparticle anomalies, although the number of bimetallics... 

Structure Sensitivity of the ORR and Implications for a Crystallite Size Effect... 

Kinoshita has also shown that ORR data for supported catalysts in hot, concentrated H3PO4 (180 °C, 97-98% acid) reported in three different studies were also fit by this model. Since the physical basis for the crystallite size effect in sulfuric acid is anion adsorption, it would be a considerable reach to suggest that the same physical basis applies to this size effect, i.e., structure-sensitive anion adsorption. There are, nonetheless, indications that this is the case. Anion adsorption in dilute phosphoric [43] has a very similar structure sensitivity as sulfate adsorption, i.e., strongest adsorption on the (111) face, and on poly-Pt anion adsorption and/or neutral molecule adsorption in dilute phosphoric has a strongly inhibiting effect on the kinetics of the ORR [43]. Sattler and Ross [16] report a similar crystallite size dependence of the ORR on supported Pt in dilute phosphoric acid at ambient temperature as that found in hot, concentrated acid with the same catalysts. But it is unclear whether similar adsorption chemistry would exist in the extreme conditions of hot, concentrated phosphoric acid. 

With differences in activity between crystal faces of only a factor of five or less for oxygen reduction in perchloric acid, the particle models of Kinoshita indicate that there would be little or no crystallite size effect for the ORR in a nonadsorbing acid, such as trifluoromethane sulfonic acid or closely related derivatives [44]. In KOH, because the (111) face is the most active, an increase in activity with the smallest particles (e.g., 1 nm)—ideally (uniform size, perfect geometry) would be predicted by the Vt hkl) data—would be by about a factor of five. With real catalysts, where all particles are neither uniformly sized nor perfectly facetted, the effect might be much less, perhaps only a factor of two or so. Unfortunately, we do not know of any studies of crystallite size effects for supported Pt catalysts in either KOH or... 

The third concern of those who are searching for better catalysts might be the metal crystallite size effect. As shown in Figure 5, with a decrease in the diameter of metal particles the fractions of edges, corners, and surface exposed atoms increase, markedly in the range below 2 nm. The electronic state of metal particles may also... 

G. Tamizhmani, J.P. Dodelet, D. Guay, Crystallite size effects of carbon-supported platinum on oxygen reduction in liquid acids. J. Electrochem. Soc. 1996, 143, 18-22. 

Three aspects of the performance of supported catalysts are also discussed in this Chapter. With the development of techniques, as outlined above, for the characterization of supported metal catalysts, it seems timely to survey studies of crystallite size effect/structure sensitivity with special reference to the possible intrusion of adventitious factors (Section 5). Recently there has been considerable interest in the existence of (chemical) metal-support interactions and their significance for chemisorption and catalytic activity/ selectivity (Section 6). Finally, supported bimetallic catalysts are discussed for various reactions not involving hydrocarbons (hydrocarbon reactions over alloys and bimetallic catalysts have already been reviewed in this Series with respect to both basic research and technical applications ). References to earlier reviews (including some on techniques) that complement material in this Chapter are given in the appropriate sections. It might be useful, however, to note here some topics not discussed that also form part of the vast subject of supported metal and bimetallic catalysts and for which recent reviews are available, viz, spillover, catalyst deactivation, the growth and... 

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