Crystal face catalysts

Since in some cases the previously described alloy, dealloyed, and controlled-crystal-face catalysts also develop porous/hollow structures, it is of particular interest to determine to what extent the hollow structure affects the high ORR activities seen in those catalysts. Focus points for future research should include (i) developing scalable synthesis techniques and (ii) determining whether the surface and bulk diffusion rates of Pt in these hollow structures, relative to the fuel cell life, are sufficiently slow for this type of catalyst to be practical. 

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. 

In heterogeneous catalysis, the catalyst often exists in clusters spread over a porous carrier. Experimentally, it is well established that reactivity and selectivity of heterogeneous reactions change enormously with cluster size. Thus, theoretical studies on clusters are particularly important to establish a basis for the determination of their optimal size and geometry. Cluster models are also important for studying the chemistry and reactivity of perfect crystal faces and the associated adsorption and desorption processes in heterogeneous catalysis (Bauschlicher et al, 1987). 

Active crystal face of vanadyl pyrophosphate for selective n-butane oxidation catalyst preparation, 157-158 catalyst weight vs. butane oxidation, 162,163/ catalytic activity, 162,1 (At catalytic reaction procedure, 158 experimental description, 157 flow rate of butane vs. butane oxidation, 162,163/ fractured SiOj-CVO PjO scanning electron micrographs, 160,161/ fractured scanning electron... 

The six sections following the overview chapter deal with aspects of selective oxidation that range from theories and concepts to state-of-the-art engineering applications. Several chapters describe the synthesis, characterization, and performance of potentially attractive new catalytic materials. These catalysts range from single crystals with well-defined crystal faces to highly dispersed or amorphous solids. Most of the actual catalytic reactions studied involve the oxidation of hydrocarbons in the range from to C. ... 

Reactions which may occur on sites consisting of one or two atoms only on the surface of the catalyst are generally known as facile reactions. Reactions involving hydrogenation on metals are an example. Eor such reactions, the state of dispersion or preparation methods do not greatly affect the specific activity of a catalyst. In contrast, reactions in which some crystal faces are much more active than others are called structure sensitive. An example is ammonia synthesis (discovered by Fritz Haber in 1909 (Moeller 1952)) over Fe catalysts where (111) Fe surface is found to be more active than others (Boudart 1981). Structure-sensitive reactions thus require sites with special crystal structure features, which... 

Varying the conditions of deposition of the film in CVD can alter the morphology of the nanocrystals formed Figure 11.1(a) and Figure 11.1(b) show nanosized diamond crystals in diamond films grown with 111 (octahedral) and 100 (cubic) faces. Techniques for producing specific morphologies could be very important in the production of catalysts because different crystal faces can catalyse very specific reactions. 

There are a great many examples of non-activated dissociation at surfaces. H2 dissociation on Pd(100) and the other single crystal faces of Pd are perhaps the best studied examples, both experimentally and theoretically. Part of the interest in these systems is the importance of Pd as a hydrogenation catalyst. The other major technological impetus is that bulk Pd readily absorbs large amounts of H, so is a prototype for metal-hydride hydrogen storage systems. 

The LH recombination of + CO to form C02 on Pt(lll) has been extensively studied over the years, in part because this reaction is the basic oxidation process occurring in the automotive exhaust catalyst. The reaction on other crystal faces of Pt, e.g., Pt(110), exhibit a rich variety of nonlinear dynamics [357]. Even for the simpler reaction on Pt(lll), the oxidation is complex because it shows a strong dependence on the relative morphology of the two adsorbates on the surface, and hence on a wide variety of experimental parameters. Although the simple associative... 

When foreign molecules are adsorbed on a crystal face, those situated in hollow corners of the catalyst surface (c in Fig. 14) are tied most... 

Our article has concentrated on the relationships between vibrational spectra and the structures of hydrocarbon species adsorbed on metals. Some aspects of reactivities have also been covered, such as the thermal evolution of species on single-crystal surfaces under the UHV conditions necessary for VEELS, the most widely used technique. Wider aspects of reactivity include the important subject of catalytic activity. In catalytic studies, vibrational spectroscopy can also play an important role, but in smaller proportion than in the study of chemisorption. For this reason, it would not be appropriate for us to cover a large fraction of such work in this article. Furthermore, an excellent outline of this broader subject has recently been presented by Zaera (362). Instead, we present a summary account of the kinetic aspects of perhaps the most studied system, namely, the interreactions of ethene and related C2 species, and their hydrogenations, on platinum surfaces. We consider such reactions occurring on both single-crystal faces and metal oxide-supported finely divided catalysts. 

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. 

The adsorption and ordering characteristics of the various hydrocarbon molecules on the low Miller index platinum surfaces are discussed in great detail elsewhere. These two surfaces appear to be excellent substrates for ordered chemisorption of hydrocarbons, which permit one to study the surface crystallography of these important organic molecules. The conspicuous absence of C-H and C-C bond breaking during the chemisorption of hydrocarbons below 500 K and at low adsorbate pressures (10 9-10-6 Torr) clearly indicates that these crystal faces are poor catalysts and lack the active sites that can break the important C-C and C-H chemical bonds with near zero activation energy. 

In addition, the same studies that were carried out on the Pt(lll) crystal face result in reaction rates identical to those found on stepped crystal surfaces of platinum. These observations support the contention that well-defined crystal surfaces can be excellent models for polycrystalline supported metal catalysts. It also tends to verify Boudart s hypothesis that cyclopropane hydrogenolysis is an example of a structure-insensitive reaction. The initial specific reaction rates, which were reproducible.within 10%, are within a factor of two identical to published values for this reaction on highly dispersed platinum catalysts. The activation energies that were observed for this reaction, in addition to the turnover number, are similar enough on the various platinum surfaces so that we may call the agreement excellent. 

We have been able to identify two types of structural features of platinum surfaces that influence the catalytic surface reactions (a) atomic steps and kinks, i.e., sites of low metal coordination number, and (b) carbonaceous overlayers, ordered or disordered. The surface reaction may be sensitive to both or just one of these structural features or it may be totally insensitive to the surface structure, The dehydrogenation of cyclohexane to cyclohexene appears to be a structure-insensitive reaction. It takes place even on the Pt(l 11) crystal face, which has a very low density of steps, and proceeds even in the presence of a disordered overlayer. The dehydrogenation of cyclohexene to benzene is very structure sensitive. It requires the presence of atomic steps [i.e., does not occur on the Pt(l 11) crystal face] and an ordered overlayer (it is poisoned by disorder). Others have found the dehydrogenation of cyclohexane to benzene to be structure insensitive (42, 43) on dispersed-metal catalysts. On our catalyst, surfaces that contain steps, this is also true, but on the Pt(lll) catalyst surface, benzene formation is much slower. Dispersed particles of any size will always contain many steplike atoms of low coordination, and therefore the reaction will display structure insensitivity. Based on our findings, we may write a mechanism for these reactions by identifying the sequence of reaction steps ... 

When the catalytic properties of metals are examined, the importance of the non-uniformity of sites depends on the reaction under study. For some reactions, the activity of the metal catalyst depends only on the total number of sites available and these are termed structure-insensitive reactions. For other reactions, classified as structure-sensitive reactions, activity may be much greater on sites associated with a particular crystal face or even with some type of defect structure. The alternative names of facile or demanding have been used to describe structure-insensitive or structure-sensitive reactions, respectively. 

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