Corrosion catalysis

The sour natural gas sulfur recovery industry covers virtually the entire gamut of chemistry. From the sour gas reservoir to the Claus plant end product problems are encountered in thermodynamics, kinetics, corrosion, catalysis, redox, rheology and the environment - plus all the rest In reviewing recent developments in such a wide ranging field it is only possible to select examples. It is hoped, however, that these highlights will serve to illustrate the dynamism of the industry in recent years and the progress it has made in developing a new source of one of the world s most basic and essential elements in an environmentally acceptable manner. 

Additives have been routinely used in corrosion catalysis and electrodeposition (3,A),flelds In which metals Interface with electrolytic solutions. Studies In these areas are part of the field of modification of metal surfaces In order to change the rates of processes occurring at the surface. In recent years there has been a good deal of work on what Is known as chemical modifications of electrodes (. While these semipermanent modifications have Involved seme sophisticated Investigations, the additive field Is largely studied by a trial and error process. The work In our laboratories has been aimed at obtaining an understanding of the role of additives In these... 

While a knowledge of surface mobility is of great interest in physical adsorption, it becomes essential in chemisorption phenomena. For instance in calorimetric work a curve of differential heats of adsorption versus surface coverage will be horizontal if adsorption is localized but shows the customary slope from high to low values of the heat of adsorption if the adsorbed layer is mobile Furthermore if a chemisorbed intermediate takes part in a surface reaction (crystal growth, corrosion, catalysis), it is essential to know whether, after adsorption anywhere on the surface, it can migrate to a locus of reaction (dislocation, etch pit, active center). Yet here again, while Innumerable adsorption data have been scrutinized for their heat values, very few calculations have been made of the entropies of chemisorbed layers. A few can be found in the review of Kemball (4) and in the book of Trapnell (11). 

The oxides of iron are of broad interest because of their importance in such diverse fields as corrosion, catalysis, geochemistry and magnetism. Despite their different structures and metal oxidation states, the oxides of iron — FeO, Fe304, and a- and y-Fe203 - have in common a close-packed plane of oxygen anions in the (111) orientation in which the nearest-neighbor distances are rather similar (within 5%). The various phases differ in the distribution of Fe within the cation planes that lay between the oxygen planes. FeO is rocksalt, whereas... 

Historically, electron spectroscopy has matured In two separate but related areas. One has been the use of electron spectroscopy as applied to analytical problems, especially those that relate to surfaces, such as failure analysis, corrosion, catalysis, or tribology. In such studies, the technique Is often used In conjunction with other techniques such as low energy electron diffraction (LEED), secondary Ion mass spectrometry (SIMS), or Ion scattering spectroscopy (ISS). Another related area Is the use of electron spectroscopy to examine the electronic structure of materials or chemical species. 

Because of the widespread interest in growth of material systems by deposition, the subject of surface diffusion is one of enormous current interest. The example of surface diffusion being taken up here is of interest to our overall mission for several different reasons. First, as noted above, surfaces are one of the most important sites of communication between a given material and the rest of the world. Whether we interest ourselves in oxidation and corrosion, catalysis, the crystal surface is the seat of tremendous activity, most of which is mediated by diffusion. A second reason that we have deemed it important to consider the role of surface diffusion is that our analysis will reveal the dangers that attend the use of transition state theory. In particular, we will appeal to the existence of exchange mechanisms for diffusion that reveal that the diffusion pathways adopted on some crystal surfaces are quite different than those that might be suggested by intuition. 

The proper ways to model a metallic surface in the presence of water are described in Chapter 3 by Drs. John C. Shelley and Daniel R. Berard. Considering all the situations in which metal comes in contact with water, it is clear that the understanding of interfacial regions between water and metals has implications for electrochemistry, corrosion, catalysis, and other phenomena. Effective methods for performing molecular dynamics and Monte Carlo simulations on interfaces are explained. Heat baths and other pertinent techniques for calculation and analysis are described. 

Metal oxides are used as pigments, electrolytes, and coatings. Oxides are also intimately involved in the processes of corrosion, catalysis, energy production, and pollution control. There is therefore considerable incentive to understand oxide properties so that rational materials selection and process optimization may be undertaken. Attainment of these twin objectives increasingly entails realistic force field based atomic modeling. Applications of such methods to oxide systems are briefly reviewed. 

In the quarter of a century or so that has elapsed since the surface analysis methods XPS, AES and SIMS became commercially available they have moved from their original home in the laboratory of the surface chemist and surface physicist to become methods of applied surface analysis. The wide range of disciplines within which these methods have been successfully applied can be readily appreciated by the diverse contents of this book. Some areas, such as corrosion, catalysis and polymers, have featured in the literature since the early 1970s while other disciplines have come to surface analysis rather more recently. 

Studies of surfaces and surface properties can be traced to the early 1800s [1]. Processes that involved surfaces and surface chemistry, such as heterogeneous catalysis and Daguerre photography, were first discovered at that time. Since then, there has been a continual interest in catalysis, corrosion and other chemical reactions that involve surfaces. The modem era of surface science began in the late 1950s, when instmmentation that could be used to investigate surface processes on the molecular level started to become available. 

A3.10 Reactions on surfaces corrosion, growth, etching and catalysis... 

This chapter will explore surface reactions at the atomic level. A brief discussion of corrosion reactions is followed by a more detailed look at growth and etchmg reactions. Finally, catalytic reactions will be considered, with a strong emphasis on the surface science approach to catalysis. 

The most direct effect of defects on tire properties of a material usually derive from altered ionic conductivity and diffusion properties. So-called superionic conductors materials which have an ionic conductivity comparable to that of molten salts. This h conductivity is due to the presence of defects, which can be introduced thermally or the presence of impurities. Diffusion affects important processes such as corrosion z catalysis. The specific heat capacity is also affected near the melting temperature the h capacity of a defective material is higher than for the equivalent ideal crystal. This refle the fact that the creation of defects is enthalpically unfavourable but is more than comp sated for by the increase in entropy, so leading to an overall decrease in the free energy... 

Acetylene is condensed with carbonyl compounds to give a wide variety of products, some of which are the substrates for the preparation of families of derivatives. The most commercially significant reaction is the condensation of acetylene with formaldehyde. The reaction does not proceed well with base catalysis which works well with other carbonyl compounds and it was discovered by Reppe (33) that acetylene under pressure (304 kPa (3 atm), or above) reacts smoothly with formaldehyde at 100°C in the presence of a copper acetyUde complex catalyst. The reaction can be controlled to give either propargyl alcohol or butynediol (see Acetylene-DERIVED chemicals). 2-Butyne-l,4-diol, its hydroxyethyl ethers, and propargyl alcohol are used as corrosion inhibitors. 2,3-Dibromo-2-butene-l,4-diol is used as a flame retardant in polyurethane and other polymer systems (see Bromine compounds Elame retardants). 

Chemical appHcations of Mn ssbauer spectroscopy are broad (291—293) determination of electron configurations and assignment of oxidation states in stmctural chemistry polymer properties studies of surface chemistry, corrosion, and catalysis and metal-atom bonding in biochemical systems. There are also important appHcations to materials science and metallurgy (294,295) (see Surface and interface analysis). 

The polymer-supported catalysts are thus important conceptually in linking catalysis in solutions and catalysis on supports. The acid—base chemistry is fundamentally the same whether the catalytic groups are present in a solution or anchored to the support. The polymer-supported catalysts have replaced acid solutions in numerous processes because they minimise the corrosion, separation, and disposal problems posed by mineral acids. 

Polymer-supported catalysts incorporating organometaUic complexes also behave in much the same way as their soluble analogues (28). Extensive research has been done in attempts to develop supported rhodium complex catalysts for olefin hydroformylation and methanol carbonylation, but the effort has not been commercially successful. The difficulty is that the polymer-supported catalysts are not sufftciendy stable the valuable metal is continuously leached into the product stream (28). Consequendy, the soHd catalysts fail to eliminate the problems of corrosion and catalyst recovery and recycle that are characteristic of solution catalysis. 

XPS has been used in almost every area in which the properties of surfaces are important. The most prominent areas can be deduced from conferences on surface analysis, especially from ECASIA, which is held every two years. These areas are adhesion, biomaterials, catalysis, ceramics and glasses, corrosion, environmental problems, magnetic materials, metals, micro- and optoelectronics, nanomaterials, polymers and composite materials, superconductors, thin films and coatings, and tribology and wear. The contributions to these conferences are also representative of actual surface-analytical problems and studies [2.33 a,b]. A few examples from the areas mentioned above are given below more comprehensive discussions of the applications of XPS are given elsewhere [1.1,1.3-1.9, 2.34—2.39]. 

Apart from the application of XPS in catalysis, the study of corrosion mechanisms and corrosion products is a major area of application. Special attention must be devoted to artifacts arising from X-ray irradiation. For example, reduction of metal oxides (e. g. CuO -> CU2O) can occur, loosely bound water or hydrates can be desorbed in the spectrometer vacuum, and hydroxides can decompose. Thorough investigations are supported by other surface-analytical and/or microscopic techniques, e.g. AFM, which is becoming increasingly important. 

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