Dissociative chemisorption precursor mechanism

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. 

General rate expressions of the form given in equations and have been experimentally verified for many types of LH reactions. Similar but more complicated rate expressions are easily derived assuming different (non-Langmuir) isotherms, higher-order reaction steps, or dissociative chemisorption of the reactants. In the ER mechanism, surface reaction takes place between a chemisorbed species and a nonchemisorbed species, e.g., Aads + Bg products. The nonchemisorbed species may be physisorbed or weakly held in a molecular precursor state. In this case, the rate expression for the surface reaction becomes... 

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 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... 

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