Molecules chemisorbed states

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... 

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... 

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 . ... 

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 example of ISTS of a single CeHe molecule chemisorbed on a Ag(llO) surface is illustrated in Fig. 4.6(a). The isolated CeHe molecules exhibit inelastic peaks at 4 and 19 mV, while fully CeHe covered Ag(llO) (Fig. 4.6(b)) exhibits peaks at 7 and 44 mV, where CeHe molecules are in a very weakly adsorbed state. These differences in the spectra between isolated molecules and MLs, where lateral molecule-molecule interactions are present in addition to the... 

In field ionization, hydrogen molecules near the tip region are attracted to the tip surface. They either hop around the tip surface or are field adsorbed on it. As the hopping motion and the field adsorption are dynamical phenomena, some of the ionic species detected may also come from field adsorbed states, not necessarily just from the gas phase. On the other hand, in pulsed-laser stimulated field desorption, where gas pressure is very low, of only 1 X 10-8 Torr, gas molecules are thermally desorbed by laser pulses from their field adsorbed and chemisorbed states. When they pass across the field ionization zone some of them are field ionized. The critical ion energy deficit in pulsed-laser stimulated field desorption of a gas is therefore found to be identical to that found in field ionization. In both pulsed-laser stimulated field desorption and field ionization of hydrogen, the majority of ions detected are H3 and H+. 

Another PES topology for molecular dissociation occurs when an intermediate molecularly chemisorbed state lies parallel to the surface between the physisorption well and the dissociated species as shown in Figure 3.2(b). This molecular state is usually described in terms of a diabatic correlation to a state formed by some charge transfer from the surface to the molecule [16]. In this case, there can be two activation barriers, V] for entry into the molecular chemisorption state of depth Wx and barrier V2 for dissociation of the molecularly chemisorbed state. This PES topology is relevant to the dissociation of some it bonded molecules such as 02 on metals, although this is often an oversimplification since distinct molecularly adsorbed states may exist at different sites on the surface [17]. In some cases, V < 0 so that no separate physisorbed state exists [18]. If multiple molecular chemisorption... 

In summary, the point dipole model with images can be made to account for the experimentally determined effects of intermole-cular dipole coupling. The magnitudes of the effects cannot be predicted from the properties of the free CO molecule but they can be used to estimate the changed values in the chemisorbed state. 

The preference of Eq. (21a) for C02 dissociation may be well anticipated. It has been shown [see, for example, the high-resolution electron energy loss spectroscopy (HREELS) studies of C02 on Re(001) (71a), ultraviolet/ X-ray photoelectron spectroscopy (UPS/XPS) studies of C02 on Fe(lll) and Fe(l 10) (37), and computer simulations for C02 on Pt(l 11) (71b)] that the molecule is practically undistorted (symmetric and linear) in the ground chemisorbed state but strongly distorted (nonsymmetric and bent) as an intermediate preceding the dissociation C02 s — COs + Os. So, there is a good reason to believe that in the transition state the coordinated C—O bond is strongly expanded [by 0.12 A (71b)] and becomes very weak. But the weaker the C—O bond (xc0 — 0), the more accurate is Eq. (21a). 

Figure 1 One-dimension potential energy diagram for precursor-mediated chemisorption of molecule R-H. The dotted line barrier above the vacuum zero is for an activated system, whereas the solid line barrier below the vacuum zero is for a facile system. Ed is the activation energy of the physisorbed state and Er is the activation barrier to the chemisorbed state.

Figure 8 shows the results now the NO scattered pulse shows clearly a demodulation that proves that the residence time of the NO molecules is larger than the rise time of the pulse. In addition N2 is present but no other products (like N20, N02, 02) were detected. The measurable stay time of NO comes from the chemisorption of NO on the Pd clusters. The angular distribution (Fig. 8b, solid circles) shows a clear increase of the cosine component due to chemisorption of NO on the Pd particles. From the pulse shape (Fig. 8a) we see that when the NO beam is turned on, the NO signal increases abruptly then more slowly. The first part called fast component corresponds to NO scattered or desorbed from the clean MgO, while the slow component is associated to NO desorbing (from a chemisorbed state) from the Pd clusters. Then, it is possible to measure the intensity of the two components as a function... 

The rotational energy distributions by the REMPI spectra of desorbed NO from the hep hollow species on Pt(l 1 1), which is induced by 2.3-6.4 e V laser irradiation, are represented by a Boltzmann distribution such as in Fig. 16a. It is impossible to understand that molecules desorbed by a non-thermal process reach thermal equilibrium in the rotational energy distribution during the very short residence time in the excited state for chemisorbed species on metal and semiconductor surfaces (lifetime = 10 16-10-14 s [62, 72, 73]). Besides, no rotational freedom exists in the chemisorbed state and the desorption is... 

Transition of oxygen molecules on Au(110) from the physisorbed to chemisorbed state has been achieved by UV radiation at 28 K.1 This probably excites them from the triplet to the singlet 1Ag state in which all electrons are paired although the bond length is almost unchanged, the dissociation energy (396 kJ mol-1) is much decreased, and it appears that in this state it is more easily chemisorbed, probably without dissociation. Bombardment of this surface by 0+ ions led on TPD to recognition of a number of bound states.1... 

Much of our effort involves studies of the chemical behavior of dusters not only as a function of size, but also as a function of metal type, charge state (neutral, cationic or anionic), and reagent molecule. There are two different operating conditions for which we probe the chemisorption of molecules onto clusters as a function of duster size. The first is such that the rate of reaction is kinetically controlled. Here we obtain information about the rate at which the first reagent molecule chemisorbs onto the otherwise bare cluster. In the second case, chemisorption studies are carried out under near steady-state conditions. In this instance we attempt to determine how many molecules a particular size cluster can bind, i.e. the degree of saturation. 

It was found that chemisorption equilibrium is rapidly attained in most reacting systems through rapid desorption and readsorption. With a few exceptions, chemisorbed molecules can be regarded as immobile since statistical-mechanical calculations of the chemisorption equilibrium agree well with the experiment if two-dimensional translations and rotations of the chemisorbed molecules are assumed to be nonexistent. The chemisorbed state of di- or triatomic molecules can be molecular or atomic, depending on the nature of the adsorbent. For example, the carbon dioxide molecule is chemisorbed with complete dissociation into its three atoms on metallic surfaces, while on oxidic catalysts it is chemisorbed with only partial dissociation. 

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