Chemisorbed state

Mention was made in Section XVIII-2E of programmed desorption this technique gives specific information about both the adsorption and the desorption of specific molecular states, at least when applied to single-crystal surfaces. The kinetic theory involved is essentially that used in Section XVI-3A. It will be recalled that the adsorption rate was there taken to be simply the rate at which molecules from the gas phase would strike a site area times the fraction of unoccupied sites. If the adsorption is activated, the fraction of molecules hitting and sticking that can proceed to a chemisorbed state is given by exp(-E /RT). The adsorption rate constant of Eq. XVII-13 becomes... 

The microcalorimetric measurements of Della Gatta and his co-workers in their investigation of the interaction of water vapour with highly dehydroxylated y-alumina confirm that in this system also, the nondissocia-tive chemisorption of water is nonactivated, whilst the dissociative chemisorption is always activated. Thus the pseudo-equilibrium between the two chemisorbed states is displaced towards dissociative chemisorption as the temperature is increased above 150 C. 

In most cases surface reactions proceed according to well-established elementary steps, as schematized in Fig. 1. The first one comprises trapping, sticking, and adsorption. Gaseous reactants atoms and/or molecules are trapped by the potential well of the surface. This rather weak interaction is commonly considered as a physisorbed precursor state. Subsequently, species are promoted to the chemisorbed state, that is, a much stronger... 

As the system passes from the active to the passive state the initial interaction depends on the composition of the aqueous phaseAn initial chemisorbed state on Fe, Cr and Ni has been postulated in which the adsorbed oxygen is abstracted from the water molecules. This has features in common with the metal/gaseous oxygen interaction mentioned previously. With increase in anodic potential a distinct phase oxide or other film substance emerges at thicknesses of 1-4 nm. Increase in the anodic potential may lead to the sequence... 

One of the m jor attractions in the metal-atom synthesis of dimer and cluster species is the ability to isolate highly unsaturated species, M Lm, that may then be considered to be models for chemisorption of the ligand, L, on either a bare, or a supported, metal surface (,100). It is quite informative to compare the spectral properties of these finite cluster-complexes to those of the corresponding, adsorbed surface-layers (100), in an effort to test localized-bonding aspects of chemisorption, and for deciphering UPS data and vibrational-energy-loss data for the chemisorbed state. At times, the similarities are quite striking. 

In Langmuir-Hinshelwood kinetics is it assumed that all species are adsorbed and accommodated (in thermal equilibrium) with the surface before they take part in any reactions. Hence, species react in the chemisorbed state on the surface. This is the prevailing situation in heterogeneous catalysis. 

Equation (12) also contains a pre-exponential factor. In Section 3.8.4 we treated desorption kinetics in terms of transition state theory (Figure 3.14 summarizes the situations we may encounter). If the transition state of a desorbing molecule resembles the chemisorbed state, we expect pre-exponential factors on the order of ek T/h = 10 s . However, if the molecule is adsorbed in an immobilized state but desorbs via a mobile precursor, the pre-exponential factors may be two to three orders of magnitude higher than the standard value of 10 s . ... 

In case of adsorption of donor particles whose chemisorbed state is described by the ionization potential ltd and concentration Ntd < b we obtain... 

CO occupies the atop site. The authors argue that adsorption, even at 4 K, is in the chemisorbed state with the molecular axis oriented perpendicular to the surface. In a physisorbed state, variations in the orientation, including where the C-0 axis is parallel to the surface, would be expected to maximise the van der Waals interaction. The oxidation of CO at Cu(110) is discussed elsewhere (Chapter 5). 

It was an approach that enabled bond energies of chemisorbed states Z)M-h to be estimated [eqn (2)] provided that the heat of chemisorption AH was known, with/) [2the H2 bond energy ... 

When the nanoclusters are pretreated with atomic hydrogen, a much stronger chemisorbed state of adsorbed thiophene is present. Beam -like structures are observed protruding about 0.4 A above the basal plane in the... 

The reaction mechanism of SCR of NOx with decane on acid and iron-exchanged MFI-type zeolite was also investigated by operando FTIR spectroscopy and a special reactor cell enabling the use of water-containing gas mixtures. Brosius et al. found that water has a dramatic influence on the reaction pathway, while the formation of organic nitro and nitrite compounds does not proceed via chemisorbed states of NO, as observed in SCR with dry gases [174],... 

Another aspect of rate measurements that is useful in discriminating between the two types of adsorption involves studies of the rate of desorption. The activation energy for desorption from a physically adsorbed state is seldom more than a few kilocalories per mole, whereas that for desorption from a chemisorbed state is usually in excess of 20 kcal/mole. Consequently, the ease with which desorption occurs on warming from liquid nitrogen temperature to... 

In the two previous sections, evidence has been presented concerning the chemisorbed states formed when benzene interacts with metal surfaces. It is not the intention in this Section to discuss benzene hydrogenation in detail, but rather to enquire whether studies of this hydrogen-addition reaction provide information about the chemisorbed state of benzene. 

Chemistry studies of alkanes on platinum surfaces under UHV conditions are limited by the very weakly bound molecularly chemisorbed state(59). The low surface temperatures required to... 

Reversible electrochemical reactivity is present in the chemisorbed states of some compounds. Characteristics shared by each of these compounds reported to date are as follows ... 

The exchange between the gas-phase and chemisorbed states of small molecules plays a vital role in such technologically important fields as heterogeneous catalysis and corrosion. The dynamics involved in these processes, however, are not currently well understood. Molecular-beam studies combined with classical trajectory calculations have proven to be a successful tool for understanding the underlying features of atomic-scale motion in the gas phase. The extension of these techniques to surfaces has also helped in elucidating the details of gas-surface reactions. 

In the dissociative mechanism, the tt complex adsorbed aromatic reacts with a metal radical (active site) by a substitution process. During this reaction [Eq. (9)] the molecule rotates through 90°, and changes from its horizontally tt complex adsorbed position to a vertically cr-bonded chemisorbed state ... 

It should be noted that particles in the chemisorbed state may differ in nature from the corresponding molecules in the gaseous phase, representing not these molecules themselves, but just parts of them, which lead an independent existence on the surface. In other words, the very act of adsorption may in some cases be accompanied by dissociation of the molecule this may be considered an experimentally established fact. Such adsorption accompanied by dissociation, requires an activation energy, as was shown by Lennard-Jones (16) on the example of the Hj molecule. The mechanism of such dissociation, which is one of the simplest examples of a heterogeneous reaction has, however, until recently not been investigated. 

As we have seen, the participation of the free electrons and holes of the catalyst in chemisorption bonds results in the chemisorbed particle spending a part of its lifetime on the surface in a radical state. Thus, the very fact that a molecule goes over from the gaseous phase to the chemisorbed state leads to an increase in its reactivity. 

The measurement of the equilibrium between the gaseous and the chemisorbed state is frequently difficult because of the very low equilibrium pressures required to saturate the surface. Often, in the case of strong interactions the monolayer is completed at very low pressures, even less than 1 torr, as shown in Fig. 19.1. 

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