Crystallites, surface structures

A wide variety of solid materials are used in catalytic processes. Generally, the (surface) structure of metal and supported metal catalysts is relatively simple. For that reason, we will first focus on metal catalysts. Supported metal catalysts are produced in many forms. Often, their preparation involves impregnation or ion exchange, followed by calcination and reduction. Depending on the conditions quite different catalyst systems are produced. When crystalline sizes are not very small, typically > 5 nm, the metal crystals behave like bulk crystals with similar crystal faces. However, in catalysis smaller particles are often used. They are referred to as crystallites , aggregates , or clusters . When the dimensions are not known we will refer to them as particles . In principle, the structure of oxidic catalysts is more complex than that of metal catalysts. The surface often contains different types of active sites a combination of acid and basic sites on one catalyst is quite common. 

Saltier ML, Ross PN. 1986. The surface structure of Pt crystallites supported on carbon black. 

Composition range 30-80% Rh. In this composition range phase separation occurs, and the structure of such Pd-Rh alloy films has been reviewed (Section II). Phase I varied in composition and phase II contained 88 5% Rh. It was proposed that these results could be explained by the preferential nucleation of rhodium so that the crystallites consisted of a phase II kernel surrounded by an outer shell (phase I), the Rh content of which increased with an overall increase in the Rh content of the alloy film. Note the essential difference to the Cu-Ni films (38, 33) discussed in Section IV.A where complete separation into two phases of fixed equilibrium composition is envisaged, and over a wide composition range the crystallite surfaces have the same composition. 

EXAFS has provided detailed information about the local environment of the active Co and Ni sites and the Mo atoms to which they are attached in terms of the types of atoms within two atomic shells away from the atom being characterized. Cobalt and nickel were shown to be definitely bonded to the surfaces of small MoS2 crystallites. Representative structures for the environments of Mo and Co are illustrated in the following diagram. In such structures, Mo has a coordination number (CN) of 6, with six nearneighbor sulfur atoms, three nearby Mo atoms, and one nearby Co or Ni atom. Co-S configurations were either CN = 5 (square pyramidal) or CN = 6 (octahedral), with either one nearby Mo atom (low HDS activity) or two nearby Mo atoms (high HDS activity) (62). 

Surface properties of mesoporous materials are sometimes important and TEM surface profile imaging is often used to investigate the surface structures of these materials. The advantages of this method are that it can be used to study the surfaces of small crystallites of almost any morphology, that the specimen preparation is as simple as that for the studies of bulk structures without requiring any special treatment and that, unlike scanning tunnelling... 

Starch changes during cooking of pasta are reported to vary from a hydration-driven gelatinization process in the outer layer to a heat-induced crystallite melting in the center.525 It is speculated that both the state of the starch and the surface structure contribute to the development of the elastic texture and stickiness of pasta. Interactions between starch and the surrounding protein matrix are evident in the outer and intermediate layer. In the center of cooked pasta, wheat starch granules retain their shape due to limited water diffusion, and the protein network remains dense. 

The catalyst particle is usually a complex entity composed of a porous solid, serving as the support for one or more catalytically active phase(s). These may comprise clusters, thin surface mono- or multilayers, or small crystallites. The shape, size and orientation of clusters or crystallites, the extension and arrangement of different crystal faces together with macrodcfects such as steps, kinks, etc., are parameters describing the surface topography. The type of atoms and their mutual positions at the surface of the active phase or of the support, and the type, concentration and mutual positions of point defects (foreign atoms in lattice positions, interstitials, vacancies, dislocations, etc.) define the surface structure. 

Lombardo and Bell (1991) reviewed stochastic models of the description of rate processes on the catalyst surface, such as adsorption, diffusion, desorption, and surface reaction, which make it possible to account for surface structure of crystallites, spatial inhomogeneities, and local fluctuations of concentrations. Comparison of dynamic MC and mean-field (effective) description of the problem of diffusion and reaction in zeolites has been made by Coppens et al. (1999). Gracia and Wolf (2004) present results of recent MC simulations of CO oxidation on Pt-supported catalysts. 

Once a carbon support has been prepared, it is desirable to post-treat it to modify the surface structure in order to confer certain properties. Since the electrocatalyst is to be platinum on the carbon, even dispersion of the platinum crystallites over the carbon surface and minimal loss of surface area during fuel cell operation are important concerns. 

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