Particle size effects supported metal catalysts

This latter was accepted as evidence for possible aggregation, since some literature data indicate that hydrogenolysis, being a structure-sensitive reaction, involves multiple adsorption (/58-161). However, two recent review articles on particle size effects on metal catalysts (162, 163) emphasize the inconsistency of the rather scarce data on nickel and warn of the difficulties connected with this problem. These data, although scarce, support the conclusion that the structure in the precrystallization state is the most favorable for catalytic activity, but detailed understanding of the phenomenon requires further clarification. 

S. H. Overbury, L. Ortiz-Soto, H. G. Zhu, B. Lee, M. D. Amiridis, and S. Dai, Comparison of Au catalysts supported on mesoporous titania and silica Investigation of Au particle size effects and metal-support interactions, Catal. Lett. 95(3-4), 99-106 (2004). 

Thus, complex high-area catalysts are typically not the best for fundamental investigations at the atomic or molecular level. Although many broadly important characteristics of heterogeneous catalysis, such as metal particle size effects, support effects, metal—support interaction, and the influence of the promoters and poisons... 

Both the metal particle size and the crystallite size of supported metal catalysts have decisive effects on enantioselectivity, but other parameters of the reaction often become important as well for example, the nature of the metal, the nature of the support, the method of preparation, the salt used for preparation, the mode of catalyst reduction, and the nature of catalyst pretreatment, such as high temperature heating or sintering. 

Abstract To date, microcalorimetry of CO adsorption onto supported metal catalysts was mainly used to study the effects induced by the nature and the particle size of supported metallic clusters, the conditions of pretreatment and the support materials on the surface properties of the supported metallic particles. The present chapter focuses on the employ of adsorption microcalorimetry for studying the interaction of carbon monoxide with platinum-based catalyst aimed to be used in proton exchange membrane fuel cells (PEMFCs) applications. 

As on previous occasions, the reader is reminded that no very extensive coverage of the literature is possible in a textbook such as this one and that the emphasis is primarily on principles and their illustration. Several monographs are available for more detailed information (see General References). Useful reviews are on future directions and anunonia synthesis [2], surface analysis [3], surface mechanisms [4], dynamics of surface reactions [5], single-crystal versus actual catalysts [6], oscillatory kinetics [7], fractals [8], surface electrochemistry [9], particle size effects [10], and supported metals [11, 12]. 

In many catalytic systems, nanoscopic metallic particles are dispersed on ceramic supports and exhibit different stmctures and properties from bulk due to size effect and metal support interaction etc. For very small metal particles, particle size may influence both geometric and electronic structures. For example, gold particles may undergo a metal-semiconductor transition at the size of about 3.5 nm and become active in CO oxidation [10]. Lattice contractions have been observed in metals such as Pt and Pd, when the particle size is smaller than 2-3 nm [11, 12]. Metal support interaction may have drastic effects on the chemisorptive properties of the metal phase [13-15]. Therefore the stmctural features such as particles size and shape, surface stmcture and configuration of metal-substrate interface are of great importance since these features influence the electronic stmctures and hence the catalytic activities. Particle shapes and size distributions of supported metal catalysts were extensively studied by TEM [16-19]. Surface stmctures such as facets and steps were observed by high-resolution surface profile imaging [20-23]. Metal support interaction and other behaviours under various environments were discussed at atomic scale based on the relevant stmctural information accessible by means of TEM [24-29]. 

Based on TEM studies of supported metal catalysts, several workers have concluded that their catalysts were made of two-dimensional discs or rafts , where virtually all atoms are at the particle surface. However, sample tilting experiments in TEM have shown that great care should be exercised in the interpretation of TEM images of small particles (<2 nm in size), since phase contrast effects may dominate and variations in the particle contrast with specimen orientation can occur as a result of amplitude contrast effects (Treacy and Howie 1980). Sample tilting is therefore necessary to ensure correct interpretations of TEM images of metal-particle catalysts. This will be discussed further in the following sections. 

HREM methods are powerful in the study of nanometre-sized metal particles dispersed on ceramic oxides or any other suitable substrate. In many catalytic processes employing supported metallic catalysts, it has been established that the catalytic properties of some structure-sensitive catalysts are enhanced with a decrease in particle size. For example, the rate of CO decomposition on Pd/mica is shown to increase five-fold when the Pd particle sizes are reduced from 5 to 2 nm. A similar size dependence has been observed for Ni/mica. It is, therefore, necessary to observe the particles at very high resolution, coupled with a small-probe high-precision micro- or nanocomposition analysis and micro- or nanodiffraction where possible. Advanced FE-(S)TEM instruments are particularly effective for composition analysis and diffraction on the nanoscale. ED patterns from particles of diameter of 1 nm or less are now possible. 

In the mentioned studies the selective hydrogenations were made with the aim to obtain kinetical data, and the catalysts characterization was scarce. On the other hand, It is known that metal supported catalysts exhibit important particle size and support effects in the selectivity patterns (ref. 7). 

For supported-metal catalysts, the questions of interaction with and location of the metal on the support are of important concern, since these factors may be instrumental in determining, for example, the metal particle size and size distribution, the particle size stability to thermal and chemical treatments, and the accessibility of the metal to the reactants of the catalytic process. That these questions are amenable to study using the Mbssbauer effect is the topic of this section. 

This paper focuses on the influence of the support on the H/D exchange of CP over supported Pt catalysts. It will be shown that kinetics and selectivities are largely affected by the support material. Particle size effects are separated from support effects. The activity shows a compensation effect, and the apparent activation energy and pre-exponential factor show an isokinetic relationship . This can be explained by different adsorption modes of the CP on the metallic Pt surface. The change in adsorption modes is attributed to a change in the electronic structure of the Pt particles, which in turn is induced by changes in the acid/base properties of the support. 

As is shown in Figure 6 (experiments) and Table 4 (Monte-Carlo analysis), a general trend is that Pt catalysts with supports of higher acidity lead to a higher contribution of the a-T)1 (Dl) and di-o-T)2 (D2) intermediates. As the ASA and LTL supports have similar metal particle sizes, this cannot be explained by particle size effects. Apparently, acidic supports enhance... 

The efficiency and selectivity of a supported metal catalyst is closely related to the dispersion and particle size of the metal component and to the nature of the interaction between the metal and the support. For a particular metal, catalytic activity may be varied by changing the metal dispersion and the support thus, the method of synthesis and any pre-treatment of the catalyst is important in the overall process of catalyst evaluation. Supported metal catalysts have traditionally been prepared by impregnation techniques that involve treatment of a support with an aqueous solution of a metal salt followed by calcination (4). In the Fe/ZSM-5 system, the decomposition of the iron nitrate during calcination produces a-Fe2(>3 of relatively large crystallite size (>100 X). This study was initiated in an attempt to produce highly-dispersed, thermally stable supported metal catalysts that are effective for synthesis gas conversion. The carbonyl Fe3(CO) was used as the source of iron the supports used were the acidic zeolites ZSM-5 and mordenite and the non-acidic, larger pore zeolite, 13X. 

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. 

For the majority of industrial catalysts, the sizes of supported metal particles arc less than the mean free path of the electrons analysed. All the metal in the particles is effectively analysed. For highly dispersed systems, XPS surface analysis and bulk X-ray nuorcsccncc analysis therefore give similar results. Comparing information from these two techniques can be used to show a change in the distribution of metals on the surface due, for example, to sintering or to the inclusion of one of the metals into the carrier structure. 

Model catalysts, consisting of metal particles supported on thin-film or single-crystal metal oxide surfaces, have been utilized successfully for more than a decade in an effort to understand particle size and support effects in catalysis [1 ]. However, a... 

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