Chemisorption Adsorption, Catalysis

The solid-gas interface and the important topics of physical adsorption, chemisorption, and catalysis are addressed in Chapters XVI-XVIII. These subjects marry fundamental molecular studies with problems of great practical importance. Again the emphasis is on the basic aspects of the problems and those areas where modeling complements experiment. 

What was evident in 1950 was that very few surface-sensitive experimental methods had been brought to bear on the question of chemisorption and catalysis at metal surfaces. However, at this meeting, Mignolet reported data for changes in work function, also referred to as surface potential, during gas adsorption with a distinction made between Van der Waals (physical) adsorption and chemisorption. In the former the work function decreased (a positive surface potential) whereas in the latter it increased (a negative surface potential), thus providing direct evidence for the electric double layer associated with the adsorbate. 

Note that the position of the Fermi level uniquely determines the concentration of the electron and hole gases on the surface of the crystal. This explains the physical significance of the part played by the Fermi level in the phenomena of chemisorption and catalysis, and at the same time establishes a characteristic correlation between the adsorptivity and catal3rtic activity of the surface, on the one hand, and the surface concentration of free electrons and holes on the other. 

The influence of the surface polarity of powders on their adsorption and dispersion properties can be profound, as is discussed in Sec. VIII,A. The values of F are likely to be put to many uses as more of them are measured. The electrostatic surface fields are doubtless involved in the phenomena of chemisorption and catalysis, capable of inducing polarization or electron shift of adsorbing molecules. For silica-alumina catalysts, the production of active M-O-M surface groups must be considered the most important factor responsible for chemisorption and catalj ic activity. 

In the preceding example it was deliberately assumed that the zinc oxide crystal used as starting material was perfect and thus, in particular, of stoichiometric composition. This was done solely in order to show that the crystal became an impurity semiconductor following the adsorption process otherwise the reasoning did not involve in any way the behavior of the bulk of the crystal, and the mechanism of the adsorption process did not depend upon the existence or nonexistence of semiconductivity prior to the uptake of hydrogen. If this were a general situation, further examination of the effect of semiconductivity on chemisorption and catalysis would hardly seem profitable. However cases where semiconductivity may play a direct role can easily be imagined. 

It is apparent that the conclusions just summarized will have a bearing on the concepts currently being used to correlate chemisorption and catalysis. There are two more aspects in which these experiments with the ion gauge and flash filament will influence the interpretation and the theories for previous experiments in adsorption and catalysis. 

The role of electron transfer during chemisorption, and its importance in adsorption, catalysis, and solid state physics, has been demonstrated repeatedly during recent years, examples occurring in every phase of surface study. 

Even when the solid-state physical methods do not indicate that properties of A-in-B are very different from those of A-in-A, it can still be possible that small changes in the electronic structure (a ligand effect on A) can be important enough for chemisorption and catalysis [25]. This should in principle be seen by (i) IR spectra of adsorbed molecules (ii) adsorption calorimetry (iii) changes in the activation energy of a simple catalytic reaction. There is currently experimental information available on all three points. 

The well characterized and stable surface phases observed on the Sn-Pt(l 11) have provided researchers in the chemisorption and catalysis field with a substrate of great interest for studying the properties of bimetallic interfaces. Simple probe gases such as CO have been studied after adsorption on this system [45] as well as a variety of organic molecules such as acetylene [46], cyclohexane and benzene [47, 48], butane and isobutane [49], methanol, ethanol and water [50]. Several surface reactions of the above gases were also studied. 

Langmuir covers the broad area of surface and colloid chemistry. Topics include micelles, emulsions, surfactants, vesicles, wetting and interfacial films, chemisorption and catalysis, electrochemistry, and physical adsorption on liquid and solid surfaces. Original research articles and letters arc accepted. There are occasional invited review tvpe articles. The journal is intended to provide a comprehensive coverage of the field of chemistry at interfaces. 

It has long been suspected that chemisorption and catalysis may actually cause major changes in the shapes of small crystallites from those observed in the absence of adsorption. The reality of such changes has been demonstrated dramatically by van t Blik et al. (134) for the chemisorption of CO on well-reduced crystallites of 6-10 Rh atoms supported on alumina. On adsorption the particles are broken up to form Rh(CO)2 bonded to the oxygens of the support. In the absence of CO, a raft structure for such Rh particles has been observed (135a). However, in the presence of CO this structure is destroyed and the Rh atoms oxidized, on alumina. 

Apart from front-end purification by means of adsorption and catalysis and purification by means of rectification, also the downstream -purification via chemisorption, adsorption and catalysis is applied. This is preferably done to convert technical nitrogen from standard to high purity. 

After reviewing the experimental techniques involved, we survey the principal applications of the infrared method under the headings surface characterization, physical adsorption, and chemisorption and catalysis. 

The English translation of this book by D. Smith and N. G. Adams provides a detailed account of theoretical approaches and experimental techniques of adsorption. The subject matter, essentially comprising physical chemistry, includes defined substances, defined surfaces and their preparation, methods for studying the texture of adsorbents, methods of studying adsorption, the surface structure of solids, theories of adsorption forces, adsorption kinetics and thermodynamics, theories of adsorption equilibria, the mechanisms of physical adsorption and chemisorption, adsorption from flowing gases and liquids, practical applications of adsorption, adsorption from solutions and the relationship between adsorption and catalysis. 

There is a very rich literature and a comprehensive book6 on the role of promoters in heterogeneous catalysis. The vast majority of studies refers to the adsorption of promoters and to the effect of promoters on the chemisorptive state of coadsorbed species on well characterized single crystal surfaces. A... 

It is most convenient to explain catalysis using an example. We have chosen a hydrogenation catalysed by nickel in the metallic state. According to the schematic of Fig. 3.1 the first step in the actual catalysis is adsorption . It is useful to distinguish physisorption and chemisorption . In the former case weak, physical forces and in the latter case relatively strong, chemical forces play a role. When the molecules adsorb at an active site physisorption or chemisorption can occur. In catalysis often physisorption followed by chemisorption is the start of the catalytic cycle. This can be understood from Fig. 3.2, which illustrates the adsorption of hydrogen on a nickel surface. 

In the foregoing it has been discus.sed how a metal can dissociate H2. Fig. 3.6 explains the principle of catalysis with an example of the hydrogenation of ethylene, for which dissociative chemisorption of hydrogen is an elementary step in the catalytic cycle. The adsorption of alkenes, on the other hand, is non-dissociative. 

Quantitative studies of such processes are of great interest for understanding the mechanism of chemisorption and a number of heterogeneous catalytic reactions, because it is superstechiometric (admixture) atoms (ions) of metals become active centers of adsorption of different particles (radicals, molecules) on metal oxides, or centers of catalysis. Such... 

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