Adsorption Associative chemisorption

Looking at the trends in dissociation probability across the transition metal series, dissociation is favored towards the left, and associative chemisorption towards the right. This is nicely illustrated for CO on the 4d transition metals in Fig. 6.36, which shows how, for Pd and Ag, molecular adsorption of CO is more stable than adsorption of the dissociation products. Rhodium is a borderline case and to the left of rhodium dissociation is favored. Note that the heat of adsorption of the C and O atoms changes much more steeply across the periodic table than that for the CO molecule. A similar situation occurs with NO, which, however, is more reactive than CO, and hence barriers for dissociation are considerably lower for NO. 

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. 

Two types of adsorption are usually postulated, the one being the physical adsorption which occurs at a lower temperature and is associated with smaller adsorption heats of the magnitude of the latent heat of evaporation of the adsorbate, and the other the chemical adsorption or chemisorption occurring usually at higher temperatures with larger heats of the magnitude of heats of chemical reactions. 

The model can be extended to predict the direction of adsorbate-induced shifts in cluster IP upon the chemisorption of other reactant molecules as well. To illustrate this, consider the chemisorption of ammonia. In this case the net charge donation is from a filled nonbonding orbital of NH3 to the metal cluster (metal-acceptor interaction), resulting in associative chemisorption of NH3 onto the cluster. In contrast to the situation for H2, adsorption of NH3 can be viewed as a reductive addition process with respect to the metal cluster, thus resulting in an increase in the Fermi level and a decrease in IP. This prediction is in excellent agreement with recent data for NH3 chemisorbed on iron clusters, which indicate that the IPs of the fully ammoniated (saturated) clusters are as much as 2 eV lower than those of the corresponding naked clusters. 

Extensive studies of hydrocarbon chemisorption have been made by Eischens and Pliskin (1). In a series of studies on olefins and paraffins chemisorbed on silica-supported nickel they were able to show that both associative and dissociative adsorption could occur, depending on catalyst pretreatment. Associative chemisorption of olefins is observed when hydrogen is left on the nickel surface dissociative absorption, when the hydrogen has been pumped off at an elevated temperature before chemisorption. The associative mechanism is deduced from the fact that the only absorption bands found when ethylene is added to a hydrogen-covered surface are in the C-H stretching region characteristic of saturated hydrocarbons, and that a C-H deformation band at 1447 cm-1 characteristic of two hydrogens on a carbon is also observed. 

A note on terminology is in order the term adsorption means chemisorption unless prefixed by the word physical the term physisorption is not much used by people working on catalysis. Purists have pointed out that the word desorption is etymologically unsound the prefix meaning away from is dis-. Thus we have associate and dissociate so the opposite of adsorption should be dissorp-tion but it is probably too late to do much about it. The solid is referred to as the adsorbent and the gaseous adsorbing species as the adsorptive this species when adsorbed is referred to as the adsorbate. 

The consensus is that organic compounds inhibit corrosion by adsorbing at the metal/solu-tion interface. Three possible types of adsorption are associated with organic inhibitors n-bond orbital adsorption, electrostatic adsorption, and chemisorptions. A more simplistic view of the mechanism of corrosion inhibitors can be described as controlled precipitation of the inhibitor from its environment (water and hydrocarbons) onto metal surfaces. During the past decade, the primary improvements in inhibitor technology have been the refinement of formulations and the development of improved methods of applying inhibitors (Totlani and Athavale 2000 Farquhar et al. 1994). 

Atoms and molecules frequently adsorb on surfaces, where they may decompose and/or react with other adsorbed species. Modern technology is increasingly dependent on surface chemistry which underlies many industrially important processes as well as destructive processes such as corrosion. It is useful to distinguish two types of adsorption physical adsorption, or physisorption, and chemical adsorption, or chemisorption. Physisorption is similar in nature to condensation and involves little chemical interaction with the surface, being associated with van der Waals forces. Chemisorption involves a tme chemical interaction with the surface, with the formation of a chemical bond. Thus, the enthalpy of physisorption is usually of the order of 20 kJ mol while for chemisorption values are in the region of200 kJ mol . A chemisorbed molecule may either remain intact in molecular chemisorption, or fall apart in dissociative chemisorption. In an important recent development, it is now possible to identify individual molecular bonds of adsorbed molecules using STM... 

In this chapter I present the current state of three aspects in physicochemistry of nanoparticles critical for understanding structure and properties of nanocomposites. This also relates to adsorption and chemisorption of macromolecules on nanoparticle surfaces from solutions the generation of interfaces phenomena of surface conductivity and specific interactions that depend on the chain origin and length, its conformation, the composition of copolymers and so on. In polyelectrolytes similarly charged with nanoparticles, hydrophobic polymers are inclined to associate ionic groups and form domains as microphases of ion regions. 

Since in chemisorption systems it is reasonable to suppose that the strong adsorbent-adsorbate interaction is associated with specific adsorption sites, a situation that may arise is that the adsorbate molecule occupies or blocks the occupancy of a second adjacent site. This means that each molecule effectively requires two adjacent sites. An analysis [106] suggests that in terms of the kinetic derivation of the Langmuir equation, the rate of adsorption should now be... 

Here we shall be concerned with the interaction of inacming diatomic molecules (H-/ 0.) with either types of potential energy wells The molecular InteractJjon (responsible for elastic and direct-inelastic scattering with extremely short residence times of the irpinglng molecules in the potential) and the chemisorptive interaction (leading to dissociative adsorption and associative desorption, reflectively, and associated with H (D) atoms trapped in the chemisorption potential for an appreci le time). 

The temperature dependence of the extent of adsorption was not interpreted, except that the results were considered to be consistent with the magnetic measurements of Selwood (see Section II,C) which indicate that the number of carbon-metal bonds between adsorbed species and the surface increases threefold between 120°and 200°C due to extensive dissociative chemisorption. The authors proposed that two forms of chemisorbed benzene exist at the nickel surface, (i) an associatively adsorbed form which can be displaced by further benzene, and which may be w- or hexa-dissociatively adsorbed form that requires the presence of hydrogen to bring about its removal from the surface. 

To summarize, the use of heavy water as a deuterium source has provided a wealth of experimental information. Evidence for the associative ir-adsorption of benzene [species (I) J is secure (2). Evidence for hydrogen exchange in the benzene ring by an abstraction-addition mechanism is less well established, partly because of uncertainties that surround the mode of chemisorption and reaction of water at metal surfaces. Nevertheless, it would be wrong to deny that Scheme 6 is consistent with a large body of experimental work. 

The conclusions from this work were (i) that the mechanism that operates is of wide applicability, (ii) that exchange proceeds by either the dissociative chemisorption of benzene or by the dissociation of benzene which has previously been associatively chemisorbed, and (iii) that M values of about 2 indicate that further dissociation of surface-area measurements. Surface areas of metal films determined by the chemisorption of hydrogen, oxygen, carbon monoxide, or by physical adsorption of krypton or of xenon concur... 

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