Intrinsic precursor

Frequently, adsorption proceeds via a mobile precursor, in which the adsorbate diffuses over the surface in a physisorbed state before finding a free site. In such cases the rate of adsorption and the sticking coefficient are constant until a relatively high coverage is reached, after which the sticking probability declines rapidly. If the precursor resides only on empty surface sites it is called an intrinsic precursor, while if it exits on already occupied sites it is called extrinsic. Here we simply note such effects, without further discussion. 

Figure 14. Schematic representation of direct and precursor-mediated processes on a surface [129, 130]. Processes occurring along the surface normal are plotted along the abszissa. The processes are correlated with the potential energy diagram of Fig. 7(b) (ex = extrinsic precursor, in —intrinsic precursor, nc = number of impinging particles from the gas phase, a and a" are fractions of trapped molecules, p = probabilities. p"m = migration probability along the surface).

A following stage of chemisorption process on the solid surface is a chemical reaction of the reactant immediately from the gas phase (Eley-Rideal mechanism) or between the intrinsic precursor and active sites (Langmuir-Hinshelwood mechajiism). Possible mechanisms of these reactions and formal kinetic equations have been discussed previously. 

Even when the result of a gas—solid collision is the formation of a stable chemisorbed species, weakly bound precursor states can play a major role in the kinetic process. Evidence for such precursor states has recently been reviewed by Cassuto and King, [21] who draw a distinction between intrinsic precursor states, which exist at empty surface sites, and extrinsic precursor states, which exist over sites filled with chemisorbed species. The ability of colliding species to be trapped in these states and to be efficiently transported across the surface is an important mechanistic feature in adsorption. A confusion in nomenclature can arise when a metastable, or virgin , chemisorbed state can be formed on the surface as an intermediate between physisorbed and stable chemisorbed states for example, at low temperatures, a virgin, non-dissociatively chemisorbed state of CO is formed on tungsten which can be converted to a dissociatively bound state on heating [102]. In the few cases that have been investigated,... 

Experimental evidence for the existence of intrinsic precursor states is rather more difficult to come by. The common observation that the initial sticking probability, s0, often decreases with increaing substrate temperature is consistent with the existence of such a state, as discussed here. Indirect evidence is also provided by molecular beam studies, for example, Hayward and Walters [401] (for H2 on W 001 ) and Engel [402] (for 02 on Pd 111 ) have observed scattered particle intensity distributions which, even at a fractional coverage in the chemisorbed layer close to zero, exhibit a strong directional lobe in the specular direction superimposed on a cosine law distribution. The specular lobe clearly contains molecules scattered at the first collision, while the cosine law component is most readily attributed to the particles which are trapped in the precursor state and then scattered back into the gas phase. Of... 

Here, ka, kd and km are the rate coefficients for adsorption, desorption, and migration from the intrinsic precursor state, k m and are the rate coefficients for migration and desorption from the extrinsic precursor state, kv is the rate coefficient for transfer from the chemisorbed state to the intrinsic precursor state, and a and a are the trapping probabilities for molecules incident at intrinsic and extrinsic precursor sites, respectively. Direct transfer from gas phase to chemisorbed state or vice versa is included through the probability, sc, for adsorption and the rate coefficient, ftc, for desorption [427]. In order to generalise the rate expressions, we now introduce a group of terms F(0) which are only functions of the surface coverage 0. For a particular case, such as non-dissociative adsorption, these terms may be evaluated and inserted into the appropriate rate expression. 

Fm is the probability that an intrinsic precursor, in hopping, moves to a site configuration where an extrinsic precursor state can exist (=0 for non-dissociative adsorption). 

F is the probability that a collision takes place at an intrinsic precursor site [= (1 — 0) for non-dissociative adsorption]. 

Thus, apart from the term a the rate expressions can be reduced to a form which only contains rate coefficients referred to the intrinsic precursor state. 

Although not explicitly stated, two different models have been used in the literature to describe the nature of the intrinsic precursor state not surprisingly, these model assumptions lead to different rate expressions. Kisliuk [426] and Gorte and Schmidt [297] assume that the intrinsic precursor occupies a single empty chemisorption site, while King and Wells [46] assume that is occupies an n.n. pair of empty chemisorption sites. 

We note that Fa is the probability than an intrinsic precursor, on a single empty site, is in a n.n. configuration to a second empty site.) Thus, from eqns. (49), (50) and (52), we have... 

In this model, the rates of adsorption, migration and desorption from the intrinsic precursor are fea[A2], km [A ], and fc A ], respectively, and it follows that the normalised rate coefficients defined earlier are identical with the probabilities fc, fm and fd defined by King and Wells [46] and equation (65) is readily transformed into their rate expression for dissociative adsorption derived by the statistical method. 

Thus, apart from the term a, this is now in a form which contains rate coefficient referred to the intrinsic precursor. As discussed earlier in Sect. 3.2.3, a = a and ac = 0 (i.e. ignoring direct transfer) so that... 

The Ps represent normalised rate coefficients. If the intrinsic precursor occupies a single site, for dissociative adsorption and no lateral interactions, the function F simply become Fd = 92, Fm =6, and Fa = (1 — 6)2 as shown previously (Sect. 3.2.3). Thus... 

Activated adsorption is primarily found with dissociative adsorption as can be rationalized on the basis of Fig. 1.4. Adsorption in the molecular state, A2,ad (he., trapping), is sometimes denoted as "intrinsic" precursor from where the activation barrier for dissociation has to be surmounted. Usually at least two neighboring free adsorption sites are required for this process, so for random distribution of the adsorbates in the Langmuir picture the sticking coefficient is expected to vary with coverage as s =Sq(1 —5). Again, as with nondissociative adsorption, such a situation is found only in exceptional cases since usually various complications (such as the influence of defects or the need for more than two adjacent vacant sites, etc.) come into play. 

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