Chemisorption surface molecule concept

Chemisorption is a phenomenon of importance in catalysis which may be treated by MO theory. Experimental studies have been carried out for a variety of systems, but theoretical descriptions of the electronic features of chemisorption beyond simple considerations are in a primitive stage. There are several factors responsible for this state of affairs. One is, of course, the complexity of the substrate system to be modeled, which has forced theorists to work with a small-size representation for the surface, as implied by the surface molecule concept of localized interactions. Although some early work has been done by... 

The work function plays an important role in catalysis. It determines how easily an electron may leave the metal to do something useful for the activation of reacting molecules. However, strictly speaking, the work function is a macroscopic property, whereas chemisorption and catalysis are locally determined phenomena. They need to be described in terms of short-range interactions between adsorbed molecules and one or more atoms at the surface. The point we want to make is that, particularly for heterogeneous surfaces, the concept of a macroscopic work function, which is the average over the entire surface, is not very useful. It is more meaningful to define the work function as a local quantity on a scale with atomic dimensions. 

The use of CO as a chemical probe of the nature of the molecular interactions with the surface sites of metallic catalysts [6] was the first clear experimental example of the transposition to surface science and in particular to chemisorption of the concepts of coordination chemistry [1, 2, 5], In fact the Chatt-Duncanson model [7] of coordination of CO, olefins, etc. to transition metals appeared to be valid also for the interactions of such probes on metal surfaces. It could not fit with the physical approach to the surface states based on solid state band gap theory [8], which was popular at the end of 1950, but at least it was a simple model for the evidence of a localized process of chemical adsorption of molecules such as olefins, CO, H, olefins, dienes, aromatics, and so on to single metal atoms on the surfaces of metals or metal oxides [5]. 

The theoretical chemical application of surface chemical bonding theory, highlighted next, is related to formal chemisorption theory as developed in surface physics, but concentrates on quantum chemical concepts as the electron distribution over bonding and antibonding orbital fragments [5, 6]. It will be seen that both approaches complement each other. The notion of a surface molecule relates to the surface physicists concept of surface state. 

In this subsection, we will analyze CO and H chemisorption using the Bethe lat tice approximation (section 2.5.1) for d- and s-metal valence electrons. First we will discuss chemisorption of CO with atop and bridge coordination to the (111) surface of platinum. Initially the interaction with the d- and s-valence electrons will be considered separately. We will focus on an analysis that makes explicit the concepts of weak chemisorption and the quasi-surface molecule limit that we also... 

More recently, Silva et a/.447,448 have found that the temperature coefficients of dEa /dT for a number of stepped Au surfaces do not fit into the above correlation, being much smaller than expected. These authors have used this observation to support their view of the hydrophilicity sequence the low 9 (rs0/97 on stepped surfaces occurs because steps randomize the orientation of water dipoles. Besides being against common concepts of reactivity in surface science and catalysis, this interpretation implies that stepped surfaces are less hydrophilic than flat surfaces. According to the plot in Fig. 25, an opposite explanation can be offered the small BEod0/dT of stepped surfaces is due to the strong chemisorption energy of water molecules on these surfaces. 

The energy of an adsorbed species is the same anywhere on the surface and is independent of the presence or absence of nearby adsorbed molecules. This assumption implies that the forces between adjacent adsorbed molecules are so small as to be negligible and that the probability of adsorption onto an empty site is independent of whether or not an adjacent site is occupied. This assumption usually implies that the surface is completely uniform in an energetic sense. If one prefers to use the concept of a nonuniform surface with a limited number of active centers that are the only points at which chemisorption occurs, this is permissible if it is assumed that all these active centers have the same activity for adsorption and that the rest of the surface has none. 

H2, N2, or CO dissociates on a surface, we need to take two orbitals of the molecule into account, the highest occupied and the lowest unoccupied molecular orbital (the HOMO and LUMO of the so-called frontier orbital concept). Let us take a simple case to start with the molecule A2 with occupied bonding level a and unoccupied anti-bonding level a. We use jellium as the substrate metal and discuss the chemisorption of A2 in the resonant level model. What happens is that the two levels broaden because of the rather weak interaction with the free electron cloud of the metal. 

The zeolites are also known as molecular sieves because of their capacity to discriminate between molecules they find numerous uses in separation and catalytic processes. Although they appear to be solid particles to the naked eye, they are highly porous, with a typical specific surface area of about 1000 m2/g. Catalysis is discussed in Chapter 9, but the scope of that chapter does not permit detailed discussions of the various types of catalysts and the role of physisorption and chemisorption in catalysis this vignette provides a glimpse of the rationale used in the molecular design of new materials of interest in surface chemistry and how the concepts introduced in Chapter 1 and Chapter 9 fit into the larger scheme. 

Chemisorption [9] is an adsorptive interaction between a molecule and a surface in which electron density is shared by the adsorbed molecule and the surface. Electrochemical investigations of molecules that are chemisorbed to electrode surfaces have been conducted for at least three decades. Why is it, then, that the papers that are credited with starting the chemically modified electrode field (in 1973) describe chemisorption of olefinic substances on platinum electrodes [10,11] What is it about these papers that is different from the earlier work The answer to this question lies in the quote by Lane and Hubbard at the start of this chapter. Lane and Hubbard raised the possibility of using carefully designed adsorbate molecules to probe the fundamentals of electron-transfer reactions at electrode surfaces. It is this concept of specifically tailoring an electrode surface to achieve a particularly desired goal that distinguishes this work from the prior literature on chemisorption, and it is this concept that launched the chemically modified electrode field. 

From this example it is clear that the selectivity for (a) dehydrogenation, (b) isomerization, and (c) cracking is likely to be related to the relative concentrations of mono-, di-, and tri-adsorbed complexes, etc. More generally, the selectivity of a catalytic reaction will depend on the relative chance for a molecule adsorbed on -surface atoms either to desorb or become adsorbed on (n + 1) surface atoms. This idea easily permits us to understand that dilution of an element A, capable of forming chemisorption bonds with a given molecule, with an inert element B will lower the ratio of poly- to monoadsorbed molecules and have an effect on catalytic selectivity. We will call this concept the primary ensemble effect. 

At sufficiently high temperatures, due to not too strong cohesion, the surface Pd atoms may acquire convenient positions to form a bond with reacting hydrocarbon molecule (189). This concept, called extractive chemisorption, was introduced by Burwell et al. (190, 191) as a possible cause of absence of steric hindrance in adsorption and reaction of some complex organic molecules. It was proposed that in chemisorption one or two metal atoms were displaced above the initial planar level, leading to increased bonding to the surface for low-dispersion catalysts. An extension of this concept to the problem of structure sensitivity allows one to explain several cases of the relatively mild (or absent) structure sensitivity in many reactions catalyzed by Pd catalysts. 

The aim of specific poisoning is the determination of the chemical nature of catalytically active sites and of their number. The application of the HSAB concept together with eight criteria that a suitable poison should fulfill have been recommended in the present context. On this basis, the chemisorptive behavior of a series of hard poisoning compounds on oxide surfaces has been discussed. Molecules that are usually classified as soft have not been dealt with since hard species should be bound more strongly on oxide surfaces. This selection is due to the very nature of the HSAB concept that allows only qualitative conclusions to be drawn, and it is by no means implied that compounds that have not been considered here may not be used successfully as specific poisons in certain cases. Thus, CO (145, 380-384), NO (242, 381, 385-392, 398), and sulfur-containing molecules (393-398) have been used as probe molecules and as specific poisons in reactions involving only soft reactants and products (32, 364, 368). 

As noted above, it is widely adopted that trigonal aluminum is one of the most important chemisorption and catalytic sites in aluminosilicates. Formation of these centers is usually associated with dehydroxylation. In the preceding section this concept was used to discuss different types of BASs formed as a result of water adsorption on the dehydroxylated surface of a model aluminosilicate fragment. The activity of the trigonal aluminum atoms was particularly manifested in the strong activation of the coordinatively bonded water molecules. In the chemical sense, such a site comprises a typical Lewis acid, which is also confirmed by quantum-chemical calculations. 

The question still in doubt concerns the nature of the activation barrier. In the classical treatment, the only one described in Trapnell s monograph (11), a gas molecule diving to an adsorption site must surmount an activation barrier. As pointed out previously by Taylor in his 1932 paper introducing the concept of activated adsorption, this simple picture immediately raises the question of a very small probability factor for adsorption, of the order of 10"6. Of course this small probability factor may be explained away if it is identified with the small fraction of active sites available at the surface. Another possibility is the relatively large negative value of the activation entropy that can be obtained for certain models of the activated complex. A treatment of chemisorption by absolute rate theory was first given in 1940 (16), but the use of the... 

The double-strand structure of an oligonucleotide is shown schematically in Fig. 6-1. Anticipating discussion in later Sections, the molecule is shown in a upright orientation attached to an atomically planar metallic electrode surface (Au(lll), cf below) by chemisorption via a hexamethylenethiol group. Fig. 6-1 shows the four nucleobases presently in focus. We discuss first concepts and formalism of electron and hole transport of DNA-based molecules in homogeneous solution and at electrochemical interfaces. We then focus on DNA-based molecules in electrochemical nanogaps and STM in electrochemical environments in situ STM). Some case examples illustrate accordance and limitations of current theoretical views of DNA-conductivity. This adds to the comprehensive overview of interfacial electrochemical ET of DNA-based molecules by O Kelly and Hill in Chapter 5. 

A rather extensive study of acetylene and its derivatives adsorbed on alumina and silica has been made by Yates and Lucchesi (74). Their results are explained in terms of acid-base concepts. Strong chemisorption is observed on alumina which has acid sites on its surface, but only physisorption is found on silica. The acetylenic CH vibration of the chemisorbed acetylene produces an absorption band at 3300 cm-1 which lies between the values for the corresponding vibration in gaseous acetylene and in monosubstituted acetylenes in fact, its value is very close to that of liquid methyl acetylene (3305 cm-1). This implies that the chemisorbed species is held normal to the surface. The C=C frequency (2007 cm-1), of the adsorbate also supports this picture since this value lies between those for monodeuteracetylene (1851 cm-1) and methyl acetylene (2142 cm-1) suggesting that one end of the molecule has an effective mass between 2 and 15. 

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