Sticking zero coverage

The probability for sticking is known as the sticking coefficient, S. Usually,. S decreases with coverage. Thus, the sticking coefficient at zero coverage, the so-called initial sticking coefficient,. S q, reflects the interaction of a molecule with the bare surface. 

The sticking coefficient at zero coverage, Sq T), contains the dynamic information about the energy transfer from the adsorbing particle to the sohd which gives rise to its temperature dependence, for instance, an exponential Boltzmann factor for activated adsorption. 

Figure 6.41. Reactivity of a pseudomorfic overlayer of Ni deposited on Ru(OOOl) for the dissociative adsorption of methane. At zero coverage the measurements reveal the sticking of methane on pure Ru. When nickel atoms are deposited on the surface, the dissociation...

The reader is left to make this trivial conversion. Please note that the slope of the uptake curve at zero coverage equals So(T), and that the above derivation implicitly assumes that the adsorbates do not interact, which is seldom the case. Hence, sticking coefRcients in the limit of zero coverage are the most meaningful quantity. 

Fig. 27. Variation of the sticking coefficient at zero coverage, S0, with temperature for oxygen adsorption on Pd(lll) (101).

The rate constants kx and k for CO and 02 adsorption, respectively, are determined by the respective initial sticking coefficients (i.e., at zero coverage). The coverage dependence of CO adsorption follows a precursor kinetics that can be modeled by the exponent r [being between 3 and 4 for Pt(110) (22)]. The kinetics of oxygen adsorption is modeled by the requirement for two neighboring empty adsorption sites, which can be occupied by either O or CO. We will return to this model later at various points. 

Prior to a discussion of adsorption kinetic rate laws and mechanistic theories, we review the very large amount of data that has been gathered in recent years. We consider first the values reported in the literature for the sticking probabilities at zero coverage, s0, for a variety of gas—metal systems secondly, the variations of s0 with gas temperature, Tg, and surface temperature, Ts and thirdly, the dependence of s on surface coverage. 

The basic effects with which we will be concerned are the manner in which the surface electronic structure influences adsorption and how it is modified by the adsorption process, thus defining the adsorption site. In purely kinetic terms, the important parameters are the sticking coefficient at zero coverage, the saturation coverage and the transition from oxygen adsorption to oxide growth. The amount of kinetic information is very limited, but this is more than compensated by the extent of ELS, UPS and XPS studies. 

Figure 6 shows the variation in the relative surface concentration of adsorbed N atoms with N2 exposure at 693 K for the Fe(llO), Fe(lOO), and Fe(lll) surfaces from which data for the initial (i.e., extrapolated to zero coverage) sticking coefficient was found to vary from 7 x 10 to 2 x 10 to 4 x 10 for the respective surfaces (21). The value for Fe(lll) is of the same order of magnitude as that derived by Emmett and Brunauer (22) for the doubly promoted catalyst. Even more remarkably, these numbers are also of the same order as the reaction probabilities derived in Somoijai and co-workers work (26). This agreement shows that kinetic parameters derived from single-crystal studies are transferable across the pressure gap (rate measurements were performed at 20 bar, whereas that on adsorption kinetics at 10 mbar ) and are also consistent with the behavior of a real catalyst. 

Figure 5.27 Alkali-metal-induced change of the zero-coverage H2-sticking coefficient on a reconstructing (Ti= 370 K) and nonreconstructing (T rlOO K) Cu(llO) surface. Note the close resemblance of the depicted curves to the alkali-induced surface-state shift shown in Figure 5.25.

---