The indirect channel accommodated precursor

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 has been tentatively suggested that the unusually large pre-exponential factor for desorption over dissociation of the molecular chemisorbed precursor on Pt(l 11) may be a result of the role of defects 

Here kCil is the rate of dissociation from the molecular chemisorbed state and ac is the trapping probability into the molecular chemisorbed state. Assuming the rates kca and kd can be expressed in an Arrhenius form (e.g. kd = udexp (—Ed/kT )) and ac is independent of surface temperature, Eq. (4) can be written in terms of the ratio of pre-exponentials, ud/uca and the difference in barrier heights AE = (Ed — ECd) (Eq. (5)). 

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 

Here aP is the trapping probability into the physisorbed state, vd and vpc are the pre-exponentials for desorption and molecular chemisorption respectively, and AE is the difference in activation energy between desorption and chemisorption (Ed — Epc) from the physisorbed precursor. A best fit of Eq. (6) to the experimental data on Pt(5 3 3) gives AE = 120 meV and vd/vpc = 80, with aP = 0.33. 

---