Precursor states

Note that the van der Waals forces tliat hold a physisorbed molecule to a surface exist for all atoms and molecules interacting with a surface. The physisorption energy is usually insignificant if the particle is attached to the surface by a much stronger chemisorption bond, as discussed below. Often, however, just before a molecule fonus a strong chemical bond to a surface, it exists in a physisorbed precursor state for a short period of time, as discussed below in section AL7.3.3. 

Note that chemisorption often begins with physisorption into a weakly bound precursor state. While in this... 

The chemisorption of a molecule is often a precursor [31] to fiirther reactions such as dissociation (see section A3,9.5.2). that is, the molecule must reside in the precursor state exploring many configurations until finding that leading to a reaction. Where there is more than one distinct chemisorption state, one can act as a precursor to the other [32], The physisorption state can also act as a precursor to chemisorption, as is observed for the 02/Ag(l 10) system [33],... 

The presence of a precursor breaks the dynamical motion into tliree parts [34], First, there is the dynamics of trapping into the precursor state secondly, there is (at least partial) thennalization in the precursor state and, thirdly, the reaction to produce the desired species (possibly a more tightly bound chemisorbed molecule). 

U ] Rettner C T and Stein H 1987 Effect of the translational energy on the chemisorption of N2 on Fe(111) activated dissociation via a precursor state Phys. Rev. Lett. 59 2768... 

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... 

To further demonstrate the power of the kinetic lattice gas approach we review briefly the work on precursor-mediated adsorption and desorption [60,61]. We consider an adsorbate in which, in addition to the most strongly bound chemisorbed (or physisorbed) adsorbed state, the adparticles can also be found in intrinsic or extrinsic precursor states. One introduces three sets of occupation numbers, , = 0 or 1, = 0 or 1, and /, = 0 or 1, depending... 

The constants rc, u ic, etc. are specified in terms of microscopic parameters and the functions fc, f, f c tc. account for the various lateral interactions between the particles in the adsorbed and precursor states. We have factored out an explicit dependence on the coverages so that in the absence of any lateral interactions these functions are all equal to one. 

After some time in the mobile precursor state, the atom finds a free site and forms a true chemical bond with the surface ... 

If the molecule adsorbs via a physisorbed precursor state in which it is free to move across the surface, while rotating and vibrating (with possibly modified frequencies and rotational modes), we obtain ... 

Hence, according to the transition state theory, adsorption becomes more likely if the molecule in the mobile physisorbed precursor state retains its freedom to rotate and vibrate as it did in the gas phase. Of course, this situation corresponds to minimal entropy loss in the adsorption process. In general, the transition from the gas phase into confinement in two dimensions will always be associated with a loss in entropy and the sticking coefficient is normally smaller than unity. 

Clearly, the sticking coefficient for the direct adsorption process is small since a considerable amount of entropy is lost when the molecule is frozen in on an adsorption site. In fact, adsorption of most molecules occurs via a mobile precursor state. Nevertheless, direct adsorption does occur, but it is usually coupled with the activated dissociation of a highly stable molecule. An example is the dissociative adsorption of CH4, with sticking coefScients of the order 10 -10 . In this case the sticking coefficient not only contains the partition functions but also an exponential... 

On the basis of entropy changes, why is the direct adsorption of a molecule on a surface site less probable than indirect adsorption through a precursor state ... 

Figure I Possible precursor states to the selective enantioface adsorption of tiglic acid...

Dissociative chemisorption was considered to be either direct, when the incoming diatomic molecule has sufficient energy to surmount the barrier without being trapped into the molecular state, or indirect, when it passes via the molecular (precursor) state into the dissociated state. If the dissociated state is not immediately equilibrated with the lattice, the fragments will move across... 

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