Adsorption precursor state

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... 

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...

It has been proposed that the precursor state [81, 82] for the adsorption-desorption reaction consists of weakly physisorbed CO. This can be CO sitting on an occupied site (COad-CO) or on an sterically unfavorable Pt site. According to Ertl [81], the desorption process occurs through a trapping mechanism on such sites if the surface is saturated by chemisorbed CO the desorption channel involves either a COad-CO potential well or a Pt-CO attractive well which is sterically weakened by the presence of pre-absorbed CO . 

The first step always occurs since, attractive forces between the undissociated molecule and the surface usually exist. This step may involve adsorption into a so-called precursor state where the molecule is mobile and diffuses across the... 

In order to understand why the activation energies differ between the two pathways, Mui et al. examined the transition state geometries [279]. They found that as electron density is donated from the amine lone pair to the down silicon atom upon adsorption into the precursor state, the up Si atom in the dimer becomes electron rich. At this stage, the dative bonded precursor can be described as a quaternary ammonium ion. The N—H dissociation pathway can thus be interpreted as the transfer of a proton from the ammonium ion to the electron-rich up Si atom through a Lewis acid-base reaction. In the transition state for this proton transfer, the N—H and Si—H... 

Fio. 16. Schematic representation of adsorption and desorption including a precursor state, ft is the probability for adsorption in the precursor statand i. 1a are the probabilities for diffusion and desorption from the precursor and chemisorbed states, respectively. f, and fa are the probabilities for the transition from the precursor state to the chemisorbed state and for the inverse process, respectively 101). 

If r0 and m are known quantities, the activation energy for desorption may be simply determined from the temperature, Tp, at which the maximum rate of desorption occurs (117). For associatively adsorbed CO the reaction order for desorption may be safely assumed to be one and frequently vo = 1013 sec-1 is assumed to be a reasonable value. If the resulting data for d are compared with values for the isosteric heats of adsorption a (these should be equal since the kinetics of adsorption is nonactivated), very often deviations by several kcal/mol occur (91) that indicate the weakness of this assumption. More sophisticated techniques for analyzing thermal desorption spectra (118-121) allow the independent determination of both parameters, v and d. The results demonstrate that vQ may deviate considerably from 1013 sec-1. For example, for the system CO/Ru(001) Menzel et al. (122) came to the conclusion that v0 may reach values up to 1018 sec-1, whereas a rather small number of 1011 sec-1 was derived by Weinberg et at. (76) for CO desorption from an oxidized Ir(l 10) surface. An additional complication arises from the fact that analysis of thermal desorption spectra on the basis of (4) may yield misleading results if desorption takes place via transition to a precursor state (102). which may be the case for adsorbed CO. 

Mitterdorfer A., Gauckler L.J., 1999. Reaction kinetics of the Pt, 02(g) c-Zr02 system Precursor-mediated adsorption. Solid State Ionics 120(1/4), 211-225. 

Equations similar to eqns. (5), (6) and (8) were obtained by Zhdanov [104] to describe the monomolecular adsorption and associative desorption and Eley-Rideal s bimolecular reaction. He examined the dependence of the rate constants of these processes on the surface coverages and discussed various approximations applied previously to describe the effect of lateral interaction of adsorbed molecules on the desorption rate constant. He also considered the effect of the lateral interaction on the pre-exponential factor of the rate constants for various processes, and in terms of the "precursor state model, the effect of ordering the adsorbed molecules on the sticking coefficient and the rate constant of monomolecular desorption. 

Adsorption-desorption of CO. CO adsorption is monomolecular. On all the Pt metals except Ir it proceeds through the pre-adsorbed ("precursor ) state [17, 93], The activation energy is practically zero and the initial sticking coefficient is high (0.5-1.0). Oxygen does not inhibit CO adsorption [55, 94], The sticking coefficient is weakly dependent on the surface concentration of CO. During the adsorption on Ru and Ir, surface carbides can form. 

In the case of hydration, an asymmetric stepwise low energy pathway to dissociative adsorption is generally accepted. The mechanism initially involves molecular adsorption as a precursor state. A secondary interaction through hydrogen bonding between neighbor dimer OH groups, that seems to affect further surface reactions of adsorbed OH and H, has been illustrated by theoretical and experimental studies. 

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