Precursor-mediated adsorption

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. 

Note that if sticking is controled by site-exclusion only, i.e., if S 6,T) = 5 o(P)(l — 0), this rate is that of a first-order reaction at low coverage. This simple picture breaks down when either the sticking coefficient depends dilferently on the coverage, as it does for instance for precursor-mediated adsorption, or when lateral interactions become important. It then does not make much physical sense to talk about the order of the desorption process. 

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... 

In this review article we have tried to show that an analytical approach to the thermodynamics and the kinetics of adsorbates is not restricted to simple systems but can deal with rather complicated situations in a systematic approach, such as multi-site and multi-component systems with or without precursor-mediated adsorption and surface reconstruction, including multi-layers/subsurface species. This approach automatically ensures that such fundamental principles as detailed balance are implemented properly. 

For adsorbates out of local equilibrium, an analytic approach to the kinetic lattice gas model is a powerful theoretical tool by which, in addition to numerical results, explicit formulas can be obtained to elucidate the underlying physics. This allows one to extract simplified pictures of and approximations to complicated processes, as shown above with precursor-mediated adsorption as an example. This task of theory is increasingly overlooked with the trend to using cheaper computer power for numerical simulations. Unfortunately, many of the simulations of adsorbate kinetics are based on unnecessarily oversimplified assumptions (for example, constant sticking coefficients, constant prefactors etc.) which rarely are spelled out because the physics has been introduced in terms of a set of computational instructions rather than formulating the theory rigorously, e.g., based on a master equation. 

Eichler A, Mittendorfer F, Hafner J. 2000. Precursor-mediated adsorption of oxygen on the (111) surfaces of platinum-group metals. Phys Rev B 62 4744-4755. 

Figure 3.1. Schematic of bond making/breaking process considered in this chapter (a) atomic adsorption/desorption/scattering, (b) molecular adsorption/desorption/scattering, (c) direct dissocia-tion/associative desorption, (d) precursor-mediated dissociation/associative desorption, (e) Langmuir-Hinschelwood chemistry, (f) Eley-Rideal chemistry, (g) photochemistry/femtochemistry, and (h) single molecule chemistry. Solid figures generally represent typical intial states of chemistry and dashed figures the final states of the chemistry.

Figure 3.3. Schematic of direct and precursor-mediated dissociation processes on a typical adiabatic PES (given by the solid line). Solid arrow labeled S represents direct dissociation and that labeled a represents trapping into a molecular adsorption well. Dashed arrows represent competing thermal (Arrhenius) rates for desorption (kd) and dissociation (kc) from the molecular well.

Translation to lattice energy transfer is the dominant aspect of atomic and molecular adsorption, scattering and desorption from surfaces. Dissipation of incident translational energy (principally into the lattice) allows adsorption, i.e., bond formation with the surface, and thermal excitation from the lattice to the translational coordiantes causes desorption and diffusion i.e., bond breaking with the surface. This is also the key ingredient in trapping, the first step in precursor-mediated dissociation of molecules at surfaces. For direct molecular dissociation processes, the implications of Z,X,Y [Pg.158]

Figure 3.11. Typical experimental behaviors for dissociative adsorption probabilities S with respect to incident energy Et in (a) and with respect to Ts in (b) for limiting dissociation behaviors. The solid lines are for direct (weakly activated) dissociative adsorption and the dashed lines are for a precursor-mediated dissociation.

The interaction of N2 with transition metals is quite complex. The dissociation is generally very exothermic, with many molecular adsorption wells, both oriented normal and parallel to the surface and at different sites on the surface existing prior to dissociation. Most of these, however, are only metastable. Both vertically adsorbed (y+) and parallel adsorption states (y) have been observed in vibrational spectroscopy for N2 adsorbed on W(100), and the parallel states are the ones known to ultimately dissociate [335]. The dissociation of N2 on W(100) has been well studied by molecular beam techniques [336-339] and these studies exemplify the complexity of the interaction. S(Et. 0n Ts) for this system [339] in Figure 3.36 (a) is interpreted as evidence for two distinct dissociation mechanisms a precursor-mediated one at low E and Ts and a direct activated process at higher These results are similar to those of Figure 3.35 for 02/ Pt(lll), except that there is no Ts... 

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. 

For a variety of the adsorption systems investigated in the literature, it has been demonstrated that chemisorption proceeds, primarily, by two mechanisms a direct dissociative mechanism and a precursor-mediated mechanism. 

The kinetic and dynamical aspects of the dissociative adsorption of 02 on the Pt(l 1 1), and surfaces vicinal to Pt(l 11), has been investigated in some detail. It provides a good example of precursor mediated dissociation, but is complicated by the fact that both physisorbed and chemisorbed molecular precursor states are involved, and access to the chemisorbed precursor is activated. It is also a good example of the role of step and defect sites in the overall conversion of the precursor states. The adsorption system has the advantage that the characterisation of a number of molecular and atomic states has also been the subject of considerable attention. 

It appears that H atoms, adsorbed at bridging oxygen atoms or at oxygen vacancies [68], are almost always present on TiOaCl 10)(lxl) surfaces. They can stem from dissociative adsorption of water on oxygen vacancies [64] (which is a precursor-mediated process and can hardly be avoided even in the best vacuum conditions). Possibly, H impurities may also come from the bulk of the crystal [68]. 

Low activation energy values are indicative of corrugated surfaces. From the difference in the found energy barriers, it is also concluded that CO adsorption is the rate-determining step for CO dissociative adsorption, followed by the dissociation step. These findings suggest a precursor-mediated mechanism for CO dissociative adsorption. 

It must be noted that this is a schematic diagram where the abscissa is not a linear distance scale instead it represents the trajectory pathway of an incoming molecule to a surface. Dissociative adsorption can occur from a weakly held molecular state if the net barrier to adsorption is low (precursor mediated) but is of low probability if it is high. Then it is only the hot molecules of the Maxwell Boltzmann distribution of velocities (fig. 9) which can dissociate and they do this by direct passage over the energy barrier (direct activated). The rate of dissociation from a precursor state can be written as follows for the simple case in fig. 9,... 

Often, at least for systems where the activation barrier is not too large, both of these channels can co-exist, though each tend to dominate in different temperature regimes. In general the precursor mediated channel will dominate at low substrate and gas temperatures, while the direct channel will be dominant at high gas temperatures, examples of this being O2 adsorption on Cu(llO) (Pudney and Bowker, 1990 Hodgson et ah, 1993) and N2 on Fe(lll) (Ertl, 1991 Rettner and Stein, 1987), discussed below (sect. 3.3). 

It can be the case that both adsorption channels are important for a particular system. Examples of this are given here for O2 adsorption on Ag and Cu and for N2 dissociation on Fe. In these cases we can generalise and say that the precursor mediated route tends to dominate at low substrate and gas temperatures, while direct activated adsorption dominates at high gas temperatures. Furthermore, in all these cases, molecular chemisorbed states of adsorption can exist which complicate the pathway of adsorption. A one dimensional potential energy profile is shown in fig. 8 for the case of O2 adsorption on Ag taken from the work of Dean and Bowker (1988/89, 1989) and of Campbell (1985), although this is likely to be a general representation for this type of adsorption system with other adsorbate/metal combinations. 

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