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In this paper, introductory to the Symposium, an attempt is made to summarize and rationalize the available information on the reforming reactions of hydrocarbons on metals and alloys. It is shown that three important aspects of hydrocarbon adsorption, viz. chemisorption bond strength, reactivity towards C-H bonds and the ability to form multiple bonds with the group VIII metals, are not dramatically changed by alloying. Therefore, various changes due to alloying observed in the selectivity of alloys are ascribed to the availability of ensembles of active sites on the alloy surface (ensemble size effects). It is shown that also on pure metals various intermediates are identified which differ in the number of C- atoms and metal-atoms involved in the formation of activated complexes. By alloying a consistent shift is observed from mechanisms and intermediates requiring big ensembles to mechanisms which can also operate on small ensembles. As a rule mechanisms involving big ensembles are faster. The identity of alloy components in their chemisorption and catalytic behaviour is preserved to a high extent also in alloys and is a more important feature in catalysis than the effects caused by mutual influencing of alloy components.

In this review, the most relevant aspects of skeletal isomerization of n-butene to isobutene are discussed: the nature of the active sites, the prevailing mechanism of the skeletal isomerization, and the relation of this to that of n-butane. It is concluded that the prevailing mechanism of skeletal isomerization of n-butene is monomolecular (in contrast to butane isomerization) and requires Br⊘nsted acid (OH) active sites. The selectivity and catalytic stability can be influenced by the shape selectivity of zeolites and zeotypes. These effects are explained on the basis of the knowledge on the prevailing mechanism.

Adsorption of acetic acids has been studied by FT-IR on a series of adsorbents, known as good catalysts for the selective hydrogenation of carboxylic acids to corresponding aldehydes: SnO2, TiO2, Pt SnO 2 , Pt TiO 2 . Very similar adsorbed species are observed by FT-IR with all mentioned catalysts. The catalysts differ mainly by their behaviour upon increasing temperature program. The strongly bound unidentate or the bidentate, bound to the oxide, are the possible intermediates of the catalytic reaction to acetaldehyde.

The present study gives the results of our attempts to gain more knowledge on the interaction between an aromatic carboxylic acid and an oxide catalyst. There is quite a difference in the coordination of benzoic (and salicylic) acid with an acid-base catalyst (γ-Al2O3) or a redox one (α-Mn3O4). When a substituent is incorporated into the ring, the difference becomes even larger. These differences can perhaps explain the difficulties in the catalytic reduction from (substituted) benzoic acids to the corresponding aldehydes.

The structure of metallic copper oxide La2Sr6Cu8O18-δ (δ≈0.4) has been determined by electron diffraction and neutron powder diffraction. The correct space group was found to be P 4/mbm with lattice parameters a=10.8502 (3) A≈2√2ap and c=3.7658 (1) A≈ap, where ap is the lattice parameter of the cubic perovskite. The oxygen vacancies are ordered both in the basal plane, along [001], and in the z = 1 2 plane, along [110]. The idealised framework of [Cu8O18] consists of square planar CuO4 groups and CuO5 square pyramids by sharing their corner oxygen, leading to the formation of hexagonal tunnels along the c-axis. La3+ and Sr2+ ions are located in these tunnels in a disordered manner.

Reactions of n-hexane on three bimetallic (alloy) systems: Pt-Re, Pt-Ir, Pt-Co have been studied with sulphur-free and sulphided catalysts. A shift is observed in the selectivity pattern from “metallic: (on sulphur free catalysts) to “acid catalysed” reactions (on sulphur modified catalysts), which is explained by the shift in the formation of certain intermediates of the reaction.

When a mixture of carbon monoxide and hydrogen is contacted with some metals (Co, Ru, Ni, Fe), a great number of reactions occur. Some of them are schematically described by the following stiochio-metric equations Reaction (1) with m=1 was discovered by Sabatier [1] and is usually called methanation. Reactions (1) to (3) with m < 1 are the Fischer-Tropsch [2] synthesis of hydrocarbons. There are several very good review papers [3–8] on this subject, and it is not intended to add another paper to this collection. Rather, results obtained during the last few years at Leiden will be reviewed. The mechanism suggested is still somewhat speculative; however, some experimental evidence exist for all aspects of the mechanism.

The practical importance of alloy surfaces in catalysis, corrosion andother aspects of materials performance is widely recognized. What is needed now is sufficient knowledge of the relationship between externally controllable factors — alloy composition, temperature, environment — and surface properties — composition, structure, chemical activity — to control materials performance in these applications. Our purpose here is to review progress in determining and predicting the relationship between one surface property, composition, and certain externally controllable variables: overall composition, temperature, environment and physical form. We find that theoretical treatments of metal alloy surface composition now include essentially all significant physical effects and can predict values for most parameters of interest. Though improvements are still possible, the accuracy of predictions is more often limited by uncertainties or absence of the basic data for the calculations (e.g., thermochemical values) than by the models themselves. Alloy surface composition can now be measured well. The first monolayercomposition of large alloy slabs can be determined quantitatively over a wide temperature range in ultra-high vacuum. Difficulties with specimens of practical interest still challenge experimentalists. Among these are supported catalysts, surfaces under chemisorbed layers and composition of layers below the first. Significant progress is being made and we expect the next few years will see success.

There are probably very few other systems in industrial chemistry which equal the CO/H2 reaction system in the variety of possible products. When using pure Ni or Co catalysts one can produce almost pure CH, or a mixture of saturated hydrocarbons; a promoted (K2O) iron catalyst produces, in addition to these products, also alcohols, aldehydes, and other oxygen-containing products (oxygenates); and almost pure CH3OH can be obtained when applying Cu/ZnO/Al2O3 or Pd-MgO promoted SiO2 catalysts. Some catalysts (Rh/V2O3) can lead the reaction of syngas toward C2-oxygenates, and others to a mixture of C2+-alcohols and aldehydes (Co, K, Cr, Fe multicomponent catalysts). All this is schematically shown in Fig. 1. It is a remarkable, but not unusual, situation that while the economic interest follows the order (Fig. 1) III > II > I, the fundamental knowledge on the reactions mentioned follows just the reverse order: I > II > III. Although the leading ideas for preparing selective catalysts are onIy slowly...

Historically, the interest in catalysis by alloys shows periods of growth and periods of stagnation. The first period of growing interest was in the thirties when alloys were used to disclose the role of the order-disorder phenomena (see, e.g., Refs. 1–3). In the fifties alloys were used intensively to establish the role of the electronic structure of solids in catalysis (see, e.g., Refs 3–5). It was a period of very fast development in catalysis, the penetration of theoretical and experimental physics into catalysis initiated by the authors of The Electronic Theory of Catalysis (Dowden [5], Rienacker [2], Schwab [6], Eley [TI, Wolkenstein [a], Hauffe [9], and others), it brought much news and the experimental material on catalysis increased substantially. However, as far as the research on alloys itself is concerned, its success was debatable. The President of the IVth Congress on Catalysis in 1968, when closing the congress said about this period: “These systems (i.e. alloys) have been a danger...

One of the basic parameters characterizing the bond between a chemisorbed atom (or molecule) and the surface is its dissociation energy. It is difficult, however, to determine the bond energy and therefore it becomes necessary to turn to some other quantity, which is sufficiently close to the bond energy and which can be determined without much difficulty. Such a quantity is the heat of adsorption. Heat of adsorption is defined as the difference between the total energy of the adsorption system prior to and after the adsorption process, i.e., it equals the sum of all energy changes connected with adsorption. Thus the experimentally determined value for the heat of adsorption is obviously closely related to the bond energy of the chemisorbed particle, though this relation is not quite simple [1,2].

Hydrogenation of benzene and cyclohexene has been studied on Pt and four Pt-Au alloys. The overall behaviour −- activity, selfpoisoning and regeneration was similar to that of Ni−Cu alloys. The surface of the two-phase Pt−Au alloys had, according to the kinetic data a composition of the β-alloy (Au-rich phase). Hydrogenation of benzene was 103 to 104 times slower than the hydrogenation of cyclohexene. The most dilute alloys are active in cyclohexene hydrogenation but they are completely inactive in the low temperature hydrogenation of benzene. The main effect of alloying is the decrease in the pre-exponential term of the rate. The effect is explained in geometric terms: activation of a benzene molecule for the low temperature hydrogenation requires larger ensembles than those for cyclohexene activation and the larger ensembles are missing on the surface of the most dilute alloys.