Adsorption energy of oxygen

Figure 4.42. The turnover frequencies for the low-temperature WGS reaction as a function of adsorption energies of oxygen and carbon monoxide. The positions of the step sites on noble and late transition metals are shown. As observed experimentally only copper appears to be a suitable pure metal catalyst for the process. Adapted from [139].

Hyman and Medlin also examined intermediates adsorption on model Pt(l 11)-alloy surfaces [65]. Their study illustrated that the primary mechanism of OH destabilization on a Pt/PtsM surface is due to compressive strain, which also destabilizes adsorption of all of the intermediates. This is consistent with the study on Pd alloy [58]. Whereas shifts in binding energy due to strain correlate well for all of the intermediates examined, shifts in O adsorption energy resulting from ligand contributions were found not to correlate with the other intermediates. The authors also pointed out that the adsorption energy of oxygenate intermediates does not depend solely on the f-band center of the surface, but is also dependent on the electron density near the Fermi level. 

In principle, reaction and activation free energies required in Equation 3.59 could be obtained from ab initio studies of reaction pathways and mechanisms, such as those performed in Rossmeisl et al. (2005), Jacob (2006), and Roudgar et al. (2010). Values of these energy parameters for the appropriate sequence of steps, evaluated at equilibrium, will determine the effective activation potential and the exchange current density of the ORR. These relations are complicated by adsorbate interaction effects and site-specific dispersion of adsorption energies of oxygen intermediates. Currently, these effects are not accounted for in ab initio studies. 

Fig. 6.12 (a) Schematic potential energy curve of ORR at an electrode potential of 0 V vs. RHE black line), 1.23 V vs. RHE (equilibrium electrode potential, red line), and at 0.8 V vs. RHE LGJe, blue line). Energy difference of every step at 0 V vs. RHE is marked with AGj. (b) Dependency of AGi and AG4 on the dissociated adsorption energy of oxygen. The fourth step limits the activity of ORR on the Pt(l 11) facet. The dashed line indicates the equihbtium electrode potential of ORR. Modified with permission from ref [53]... 

The simplest model would be to use data of pure metal surfaces alone and not taking into account strain and ligand effects induced by the host material. Let AB JB be the model surface alloy system we want to describe. Here, B is the host material and A is the solute and x is the fractional amount of A in the surface layer. The adsorption energy of oxygen on the surface of our model system can now be approximated as... 

FIG. 4 Normalized oxygen density profile perpendicular to the surface from simulations of pure water with adsorption energies of 12, 24, 36, and 48 kJ/mol (from bottom to top). The lower curves are shifted downwards by 0.5, 1.0, and 1.5 units. The inset shows the height of the first (diamonds) and second peak (crosses) as a function of adsorption energy. Water interacts with the surface through a Morse potential. (From Ref. 98.)... 

Fig. 5(a) contains the oxygen and hydrogen density profiles it demonstrates clearly the major differences between the water structure next to a metal surface and near a free or nonpolar surface (compare to Fig. 3). Due to the significant adsorption energy of water on transition metal surfaces (typically of the order of 20-50kJmoP see, e.g., [136]), strong density oscillations are observed next to the metal. Between three and four water layers have also been identified in most simulations near uncharged metal surfaces, depending on the model and on statistical accuracy. Beyond about...

Table 1 All adlayer structures for which the adsorption energy of the oxygen atoms have been determined. Also given are the coverage 6 and the adsorption energy per oxygen atom Eads (in kJjmol). Data is reproduced from references. ...

We will now compare the N2 system to the much more studied isoelectronic CO molecule adsorbed on Ni(100). Like N2, CO adsorbs in a c(2 x 2) overlayer structure on Ni(100), occupying on-top sites with the carbon end down with a C—Ni distance of 1.73 A, see Chapter 1 for details. However, the adsorption energy of 1.2 eV [63] is much higher in comparison to that of N2. It is therefore very interesting to see how the difference in electronegativity of the carbon and oxygen atoms influences the surface-chemical bond in comparison to the isoelectronic N2. 

Fig. 1. Energy scheme of chemisorption and physical adsorption of oxygen vs. distance from the surface according to Lennard-Jones. E tt is the electron affinity of atomic oxygen, Eo the dissociation energy of oxygen molecules, Ecu the chemisorption energy, and Exot the activation energy. Position A is that of physically adsorbed O2, and position B is that of chemisorbed O".

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