Precursor-mediated dissociation

After a molecule traps and thermally equilibrates in a molecularly adsorbed state, the net dissociation probability S is given by competition between the thermal 

---

Luntz A C and Harris J 1992 The role of tunneling in precursor mediated dissociation Alkanes on metal surfaces J. 

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]

For molecular dissociation, the qualitative behavior of S(Et, 0 , Ts) depends greatly on the mechanism of dissociation, i.e., whether it is direct or precursor-mediated. When the dissociation is direct, absolute values of S are generally only weakly dependent on Ts. On the other hand, 5 is a very strong function of Ts for precursor-mediated dissociation (eq. (2.7)), decreasing nearly exponentially with Ts when EcEd. Generally, the entrance to the precursor state is non-activated and S decreases with Ej due to a... 

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.

Figure 3.30. Natural logarithm of the CH4 dissociation probability S0 (= S) on Pt(lll) vs. 1/TS for different incident energies at normal incidence ( = En). If the Ts dependence was due to precursor-mediated dissociation, it would be Arrhenius (a straight line with slope independent of e ) and hence is due to lattice coupling.

It is easy to conclude that activated CH4 dissociation on Ni(100) (and other transition metals) is dominated by direct rather than precursor-mediated processes under molecular beam conditions because of the strong dependence of S on Et and Tv. However, it is not as easy to decide in thermal bulb experiments and there has been considerable controversy over which dominates the thermal CH4 dissociation [59,285,286]. This is especially true since both lattice coupling in direct dissociation and a precursor-mediated dissociation with Ec>Ed (see Section 2.3.2) can cause k(Tg = 300K, Ts) to increase with Ts. One way to distinguish between these two possibilities is to compare isothermal rates k(T = Ts) with non-isothermal... 

The scenario described here, i.e., activated dissociation at terrace sites and precursor-mediated dissociation at step or defect sites, is likely to be a very general one since barriers to dissociation are generally much lower at step sites [353]. There are already many known examples C2H6 dissociation on Ir(lll) [354], CH3OH dissociation on Pt(lll) [355] and neopentane dissociation on Pt(lll) [356], etc. 

Without a doubt, a complete picture of the dynamics of dissociative chemisorption and the relevant parameters which govern these mechanisms would be incredibly useful in studying and improving industrially relevant catalysis and surface reaction processes. For example, the dissociation of methane on a supported metal catalyst surface is the rate limiting step in the steam reforming of natural gas, an initial step in the production of many different industrial chemicals [1]. Precursor-mediated dissociation has been shown to play a dominant role in epitaxial silicon growth from disilane, a process employed to produce transistors and various microelectronic devices [2]. An examination of the Boltzmann distribution of kinetic energies for a gas at typical industrial catalytic reactor conditions (T 1000 K)... 

The primary intent of this chapter is to visit and discuss the present body of research regarding the dynamics and mechanisms involved in precursor-mediated dissociative chemisorption, with a particular emphasis on the dynamics of precursors in so-called activated dissociative chemisorption systems. A variety of supporting data from the literature has been adapted and will be presented to aid in the discussion of these topics. 

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. 

Weaver JF, Hinojosa JA Jr, Hakanoglu C, Antony A, Hawkins JM, Asthagiri A (2011) Precursor-mediated dissociation of n-butane on a PdO(lOl) thin film. Catal Today 160 213-227... 

The molecule interacts only weakly with the surface when trapped in a physis-orption well. At low energies the molecule may be trapped in this well before it dissociates. Such a temporary state is called a precursor state, and the precursor mediated dissociation is expected to exhibit a decrease in dissociative sticking with increasing surface temperature. Higher surface temperature will tend to lower... 

---