Activation barrier chemisorbed

If we move the chemisorbed molecule closer to the surface, it will feel a strong repulsion and the energy rises. However, if the molecule can respond by changing its electron structure in the interaction with the surface, it may dissociate into two chemisorbed atoms. Again the potential is much more complicated than drawn in Fig. 6.34, since it depends very much on the orientation of the molecule with respect to the atoms in the surface. For a diatomic molecule, we expect the molecule in the transition state for dissociation to bind parallel to the surface. The barriers between the physisorption, associative and dissociative chemisorption are activation barriers for the reaction from gas phase molecule to dissociated atoms and all subsequent reactions. It is important to be able to determine and predict the behavior of these barriers since they have a key impact on if and how and at what rate the reaction proceeds. 

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... 

Table VIII lists the total bond energies of reactants and products as well as conceivable intermediates in the gas-phase (D) and chemisorbed (D + Q) states on Pt(lll), Pd(lll), Ni(lll). Tables IX and X summarize the activation barriers of the conceivable elementary steps leading to CH4 and CH3OH, respectively.

Tables XI and XII list total bond energies in the gas phase (D) and chemisorbed (D + Q) states for all C2HX species (x = 0-6). The calculated activation barriers AE for C—C and C—H bond cleavage and recombination for chemisorbed C2H species are summarized in Table XIII. All the discussion below will refer to chemisorbed species if not stated otherwise.

Activation Barriers for Forward and Reversed Reactions of Chemisorbed C,H"... 

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.

Studies of the C—H activation of alkanes on platinum surfaces are limited by the very weakly bound chemisorbed state. The low surface temperatures required to accommodate the molecule are insufficient to activate the C—H bonds. It is interesting that C—H activation is apparently occurring on platinum clusters under ambient low-pressure conditions. The rate of reaction appears to be 1-10% of gas kinetic. This suggests that the activation barrier for alkanes on platinum clusters is low, and does not involve significant steric hindrance. One intriguing possibility is that the cluster inserts into the RC—H bond, forming species such as RC—M —H. 

As illustrated in fig. 2, diffusion occurs in the chemisorbed state and its rate is a strong function of temperature in the usual way, being an activated process. Generally the activation barrier to diffusion is something less than half the desorption activation energy, but can be very low indeed. 

Along the same lines, it is worthwhile mentioning that even the two-dimensional contour plot obscures some important features of the true multidimensional PES. In order to illustrate this fact, consider the following argument. Some of the features in the PES can be probed by experiments the weakly physisorbed, molecularly and atomically chemisorbed species by surface spectroscopies and the activation barriers by molecular beam scattering experiments. Thus, much of the PES can be determined and the reader may wonder why one does not simply join these individual regions together and find the PES ... 

The transition from physisorbed H2 to chemisorbed 2H involves dissociation. Dissociative chemisorption turns out to be one of the most important surface-related steps in the formation of hydrides of intermetaUic compounds [42]. Dissociation and chemisorption are called activated if the H2 molecule approaching the surface has to overcome an activahon barrier (Figure 4.11). Activation barriers of up to 1 eV were found on s-electron metals (Mg [37]) d-electrons tend to reduce the barrier [80]. 

A special situation is found with chemisorbed hydrogen atoms where quantum effects come into play. The activation barrier for surface diffusion as sketched in Fig. 1.9b can become surmounted not only by thermal activation, but also (with low probability) by tunneling that manifests itself by the indepen-... 

Even nondissociative (molecular) adsorption may be accompanied by an activation barrier if, for example, the reaction proceeds from a physisorbed state into a chemisorbed state (trapping-mediated adsorption). In the system 02/Pt(l 11), for example, the O2 molecule may be chemisorbed either in a superoxo-like or a peroxo-like state [18]. It was found that with low kinetic energies of the incident molecules, both types of surface species are formed, while at higher kinetic energies the more strongly held peroxo-like species is favored, thus reflecting correlations between incident translational energy and preferred trajectories for adsorption [19]. 

The role of alkali promoters has been subject to many studies, and it has been commonly accepted that the effect of alkali is to lower the activation barrier for the dissociation of chemisorbed N2. Recently, the role of potassium has been accounted for by density functional theory calculations indicating that on iron catalysts destabilisation of NHx species, creating more free sites for N2 adsorption, may be the main reason for the rate enhancement observed, whereas on Ru and Co-based catalysts promotion becomes effective through weakening of the N=N bond of the adsorbed species (38). 

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