Surface reconstruction coverage dependence

Finally, although both temperature-programmed desorption and reaction are indispensable techniques in catalysis and surface chemistry, they do have limitations. First, TPD experiments are not performed at equilibrium, since the temperature increases constantly. Secondly, the kinetic parameters change during TPD, due to changes in both temperature and coverage. Thirdly, temperature-dependent surface processes such as diffusion or surface reconstruction may accompany desorption and exert an influence. Hence, the technique should be used judiciously and the derived kinetic data should be treated with care ... 

The steady state reaction of NO with H2 and NH3 on Pt(l 0 0) has been studied using REMPI detection of the N2 product state distributions [134]. These reactions show very complex kinetics, with strongly coverage dependent reaction rates which lead to kinetic oscillations [135] and explosive desorption [136]. This surface also shows a phase transition between the clean surface hex reconstructed phase and... 

Another model that involves subsurface oxygen was developed by Vish-nevskii for Pt(lOO) (60,70,71). The mathematical formulation, though developed independently, is very similar to that of Bassett and Imbihl, and is also a hybrid between Eigenberger-type models and coverage-dependent activation energy models. This model, however, is in conflict with a surface reconstruction model developed by Ertl et al., which will be discussed later. 

Although the Langmuir theory of adsorption is used frequently for technical process development it is a crude approximation, as surface reconstruction frequently occurs. Adsorbed molecules change the structure of the surface layer and the catalytic properties of surface sites are not equal in the ability to bind chemisorbed molecules. The rate is dependent on spatial arrangement and the heat of adsorption depends on coverage (Figures 2.27, 2.28). 

Thus, the mechanism of catalytic processes near and far from the equilibrium of the reaction can differ. In general, linear models are valid only within a narrow range of (boundary) conditions near equilibrium. The rate constants, as functions of the concentration of the reactants and temperature, found near the equilibrium may be unsuitable for the description of the reaction far from equilibrium. The coverage of adsorbed species substantially affects the properties of a catalytic surface. The multiplicity of steady states, their stability, the ordering of adsorbed species, and catalyst surface reconstruction under the influence of adsorbed species also depend on the surface coverage. Non-linear phenomena at the atomic-molecular level strongly affect the rate and selectivity of a heterogeneous catalytic reaction. For the two-step sequence (eq.7.87) when step 1 is considered to be reversible and step 2 is in quasi-equilibria, it can be demonstrated for ideal surfaces that... 

FIG. 10 Phase diagram of the ZGB model with adsorbate induced global reconstructions of the surface. The plots show the dependence of the rate of AB production (7 ab) and the surface coverages with A and B species and 9q, respectively) with the adsorption probabihty of A species given by Fa-... 

However, before we accept this conclusion as the definitive one, a word of caution is necessary. Due to the CO-CO interactions the heats of adsorption depend on the coverage 0. Shek et al (17) compared Pt and alloys at a constant dosage and this could mean that the coverage of Pt was (at the same dosage) lower on Pt than on alloys, and consequently - the heat of adsorption higher. Shek et al report the above mentioned data for the (111) faces of Pt and alloys. They studied also the (110) faces, but there the effect of alloying is masked by the reconstruction of the surface upon CO-adsorption (18). 

Kawamura etal. [421] have studied the effect of electrolytic conditions on the coverage of an Au(lll) electrode with underpotentially deposited layer of Cd. The Cd atomic layer was located on the Au(lll) surface similar to the reconstructed structure. Also, a layer of sulfate anions was positioned on the Cd atomic layer. Cd coverage was found to be dependent on the concentration of H 2 S O4. 

---