Physisorbed precursor

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. 

If adsorption occurs via a physisorbed precursor, then the sticking probability at low coverages will be enhanced due to the ability of the precursor to diflfiise and find a lattice site [30]. The details depend on parameters such as strength of the lateral interactions between the adsorbates and the relative rates of desorption and reaction of the precursor. In figure Al.7,8 an example of a plot of S versus 0 for precursor mediated adsorption is presented. 

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... 

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. 

Figure 9.6 Simulated CVD cluster morphology for coverages of -0.07 and 0.25 monolayer, showing the effect of differential dissociation probability, (a) and (b), versus the effect of a uniform dissociation probability, (c) and (d) the physisorbed precursor molecules are mobile in each case. Note, (c) and (d) are equivalent to the simulated morphology for growth by PVD. (Reproduced with permission from Reference [73].)...

STM experiments are in general agreement with the picture above. STM directly observes the two different molecular precursor states at low Ts and observes their dissociation by thermal annealing, photochemistry and viatunneling electrons [153,326]. The molecularly adsorbed states and dissociated show a strong Ts dependent clustering at low Ts, and this is evidence for a very mobile physisorbed precursor since the molecularly chemisorbed states are not significantly mobile at these Ts [326,327]. 

The rapid decrease in So(E ) observed below 0.15 eV on Pt(5 3 3) (Fig. 18) has also been observed on the Pt(l 1 1) surface [134] and is consistent with a trapping mechanism where the need to dissipate energy limits the probability of adsorption, and subsequent dissociation, via the physisorbed precursor. In order to assess the contribution of the physisorption mediated channel, the contribution to sticking directly via the chemisorbed channel must be subtracted from the measured So. The proportion of So derived from the direct chemisorption channel on Pt(5 3 3) at Ex = 0.05 eV is significantly higher than on Pt(l 1 1) (ca. 10%) [137]. Once this direct contribution is subtracted, the dependence S0(Ts)can be used to obtain kinetic parameters relating to the partition of the physisorbed precursor. This is achieved... 

Some structures reach a minimal energy with little charge transfer and no dissociation of the molecule, pointing to the formation of a physisorbed precursor on the cluster. 

F. 3. Illustration of the activation energy E of adsorption between a physisorbed precursor and a chemisorbed state in a one-dimensional potential energy diagram. 

Silica hydration as a chemisorption process promoted by a physisorbed precursor... 

Calculations on the reaction between adsorbed ammonia and the O2 molecule in a physisorbed precursor state as sketched in Figure 6.11a indicate that this mechanism is thermodynamically feasible. The 0-0 bond is significantly weaker than the Cu-O bond, and consequently the reaction of Figure 6.11b is exothermic with an overall heat of reaction of 80 kJ/mol. The OOHads peroxide decomposes in OHads and Oads, and the latter reacts rapidly with NH2,ads and NHads- Such reactions are thermodynamically much more favorable than the one between NH3 ads and Oads because of the much weaker N-H bond in these species. 

---