Metal Surface Area, Crystallite Size, and Dispersion

One important aspect of the utilization of the latter method is the appearance of an artifact due to the tensile strength of the adsorbate which causes a false maximum to frequently occur between r values of 1.7—2.0 nm when N2 is used [10]. Although this artifact has been misinterpreted in numerous papers and reported as an actual sharp distribution of small pores, one must not be misled by it. 

3 Metal Surface Area, Crystallite Size, and Dispersion 

Many catalytic studies, perhaps even a majority, have involved metallic systems, either unsupported or supported on a high surface area substrate which is frequently inert in the reaction of interest. Thus the reaction rate is dependent on the specific surface area (m g ) of the metal, not only because the total number of active sites can vary, but also because the average metal crystallite size is dependent on this value and some reactions, now termed structure-sensitive [14], have areal rates (and TOFs) that are dependent on crystallite size [14,15]. Consequently, it is of utmost importance to measure the metal surface areas in these catalysts and calculate metal dispersions and crystallite sizes based on this information. The three most general approaches to accomplish this involve TEM (SEM), XRD, and chemisorption methods. 

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The STEM Is Ideally suited for the characterization of these materials, because one Is normally measuring high atomic number elements In low atomic number metal oxide matrices, thus facilitating favorable contrast effects for observation of dispersed metal crystallites due to diffraction and elastic scattering of electrons as a function of Z number. The ability to observe and measure areas 2 nm In size In real time makes analysis of many metal particles relatively rapid and convenient. As with all techniques, limitations are encountered. Information such as metal surface areas, oxidation states of elements, chemical reactivity, etc., are often desired. Consequently, additional Input from other characterization techniques should be sought to complement the STEM data. 

Table 1 summarizes the information required for a detailed characterization of a supported metal catalyst for supported bimetallics there are additional questions, e.g., the distribution of atoms in bimetallic clusters and the surface composition of larger alloy crystallites. For the support and the prepared catalyst, the total surface area, pore size distribution, and surface acidity are routinely measured, if required, while other characteristics, e.g., thermal and chemical stability, will have been assessed when selecting the support. The surface structure of alumina, silica, charcoal, and other adsorbents used as catalyst supports has been reviewed. Undoubtedly, the most commonly measured property is the metal dispersion, often expressed in terms of the specific metal area and determined by selective chemisorption or titration but, as discussed (Section 2), there is the recurring problem of deciding the correct adsorption stoicheiometry. 

With a supported catalyst system, the information derived by the above methods relates to the composition as a whole. A number of methods have been applied to measure the sizes of metal crystallites on the support surface, and hence estimate the dispersion or surface area of the metal itself. These include electron microscopy. X-ray diffraction and Mdssbauer spectroscopy. More direct measurements of metal surface area have been obtained from adsorption isotherms for hydrogen, carbon monoxide and other adsorbates, by assuming that all the exposed metal surface adsorbs such gases. GSC methods can again be applied. 

In a proper catalytic study, as much as possible should be learned about the physical properties of the catalyst employed. These properties include the total surface area of the catalyst, the average pore size and/or the pore size distribution, the metal surface area and average metal crystallite size, especially in supported metal catalysts, as well as the metal weight loading of the latter. From this information and the packing densities of different crystallographic planes, one can calculate the dispersion of the metal. Dm, defined as the fraction of the total amount of metal atoms, Nm(, that exist as surface atoms, Nm, thus. 

Supported metal catalysts are used in a large number of commercially important processes for chemical and pharmaceutical production, pollution control and abatement, and energy production. In order to maximize catalytic activity it is necessary in most cases to synthesize small metal crystallites, typically less than about 1 to 10 nm, anchored to a thermally stable, high-surface-area support such as alumina, silica, or carbon. The efficiency of metal utilization is commonly defined as dispersion, which is the fraction of metal atoms at the surface of a metal particle (and thus available to interact with adsorbing reaction intermediates), divided by the total number of metal atoms. Metal dispersion and crystallite size are inversely proportional nanoparticles about 1 nm in diameter or smaller have dispersions of 100%, that is, every metal atom on the support is available for catalytic reaction, whereas particles of diameter 10 nm have dispersions of about 10%, with 90% of the metal unavailable for the reaction. 

Physical characteristics of a support, namely porosity and specific surface area, have long been understood to play a key role in stabilizing active components of the catalysts in dispersed state. Explicitly or implicitly, they reflect topological properties of the carbon surface, namely the nature and quantity of (1) traps (potential wells for atoms and metal particles), which behave as sites for nucleation and growth of metal crystallites and (2) hindrances (potential barriers) for migration of these atoms and particles [4,5]. An increase in the specific surface area and the micropore volume results, as a rule, in a decrease in the size of supported metal particles. Formal kinetic equations of sintering of supported catalysts always take into consideration these characteristics of a support [6]. 

Metal Dispersion by Chemisorption and Titration Selective Chemisorption. - This is the most frequently used technique for determining the metal area in a supported catalyst and depends on finding conditions under which the gas will chemisorb to monolayer coverage on the metal but to a negligible extent on the support. Various experimental methods, conditions, and adsorbates have been tried and studies made of catalyst pre-treatment and adsorption stoicheiometry, viz, the (surface metal atom)/(gas adsorbate) ratio, written here as Pts/H, Bh jQO,etc., and reviews to about 1975 are available. A summary is given in Table IV of ref. 2 of methods used to confirm the various adsorption stoicheiometries proposed, sometimes from infrared studies. These include chemisorption on metal powders of known BET area or, more satisfactorily, one of the instrumental methods reviewed in Section 3 for the determination of crystallite size distributions. For many purposes, a relative measurement of metal dispersion is sufficient, conveniently expressed as the ratio (number of atoms or molecules adsorbed)/(totfl/ number of metal atoms in the catalyst), e.g., H/Ptt. 

Metal catalysts are usually applied as small crystallites dispersed on a high-surface-area porous support such as 7-AI2O3, SiOj, or carbon ". The crystallites in such a catalyst are nonuniform in size and structure crystallite dimensions may range from less than 1 nm to tens of nanometers. Crystallites smaller than about 1 nm are often referred... 

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