Co-adsorption effects

An influence of co-adsorption effect on the reaction rates is illustrated in Figure 3 by typical dependencies of the rate of bimolecular interaction vkox on presence of H2O vapour in the atmosphere. Presence of water vapour in the atmosphere significantly affects the rate of the bimolecular interaction only on the surfaces modified by Pt (3) or composed of mixed oxides of tin and molybdenum (1). On these surfaces, the rate Vkox is comparatively high in dry atmosphere but drastically decreases if relative humidity of the atmosphere increases. It must be noted that the other rate parameters are also dependent on the co-adsorption effect. Consequently, a gaseous mixture will be distinguished from a single gas by a change in the parameter set. 

P-23 - Comparative properties of modified HEMT and HY zeolites from the FTIR study of CO adsorption effect of the dealumination and amorphous debris on the Bronsted acidity... 

The adsorption isotherms relate the local fluid phase composition within the particles to the amount adsorbed on the surface. The amount of any one species that is adsorbed depends on the local temperature and on the partial pressure or concentration of that component, and because of co-adsorption effects, on the partial pressures or concentrations of all the other components. 

Restructuring of a surface may occur as a phase change with a transition temperature as with the Si(OOl) surface [23]. It may occur on chemisorption, as in the case of oxygen atoms on a stepped Cu surface [24]. The reverse effect may occur The surface layer for a Pt(lOO) face is not that of a terminal (100) plane but is reconstructed to hexagonal symmetry. On CO adsorption, the reconstruction is lifted, as shown in Fig. XVI-8. 

For alkali modified noble and sp-metals (e.g. Cu, Al, Ag and Au), where the CO adsorption bond is rather weak, due to negligible backdonation of electronic density from the metal, the presence of an alkali metal has a weaker effect on CO adsorption. A promotional effect in CO adsorption (increase in the initial sticking coefficient and strengthening of the chemisorptive CO bond) has been observed for K- or Cs-modified Cu surfaces as well as for the CO-K(or Na)/Al(100) system.6,43 In the latter system dissociative adsorption of CO is induced in the presence of alkali species.43... 

Besides the effect of the presence of alkali on CO adsorption, there is also a stabilizing effect of adsorbed CO on the adsorption state of alkali. Within the high alkali coverage range the number of CO molecules adsorbed on promoted surface sites becomes practically equal to the number of alkali metal species and their properties are not dependent on the CO coverage. In this region CO adsorption causes also stabilization of the adsorbed alkali, as indicated by the observed high temperature shift of the onset of alkali desorption. 

The effect induced by different electronegative additives is more pronounced in the case where the additive adatoms occupy the most coordinated sites forming ordered structures (e.g Cl addition onNi(lOO)). In this case (Fig. 2.28) one modifier adatom affects 3-4 CO adsorption sites and complete disappearance of the CO p2-peak is observed above modifier coverages of -0.25 or less. The lack of ordering and the tendency of the modifier to form amorphous islands (e.g. P on Ni(100)) diminishes the effect. Thus in the case of P on Ni(100) the disappearance of the CO p2-peak is observed at P coverages exceeding 0.6. 

Figure 6.14. CO chemisorption on a transition metal. Molecular orbitals and density of states before (a,b) and after (c and d) adsorption. Effect of varying 0 and EF on electron backdonation (c) and donation (d). Based on Fig. 4 of ref. 98. See text for discussion. Reprinted with permission from Elsevier Science.

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). 

Dederichs, F., Friedrich, K F. and Daum, W. (2000) Sum-frequency vibrational spectroscopy of CO adsorption on Pt(l 11) and Pt(llO) electrode surfaces in perchloric acid solution effects of thin-layer electrolytes in spectro-electrochemistry. J. Phys. Chem. B, 104, 6626-6632. 

However, the CO tolerance at Pt-Co degraded at 70 °C. As seen in Fig. 10.9, the HOR activity of Pt-Co at a given dco is close to that of pure Pt, although the deceleration effect on the CO adsorption rate was still observed to some extent at 70 °C. Such a deactivated electrode cannot recover the original CO tolerance. This can certainly be ascribed to a severe dealloying of the nonprecious metal component (Co) in hot acid solution. We will discuss this in Section 10.3.2. 

Lemire C, Meyer R, Shaikhutdinov S, Freund HJ. 2004. Do quantum size effects control CO adsorption on gold nanoparticles Angew Chem Int Ed 43 118-121. 

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