Oxygen chemisorption energy

Figure 4.14. Illustration of the interpolation principle. Full DFT calculations for oxygen chemisorption energies are compared to two simple interpolation models for a series of surface alloys. Adapted from Ref. [54].

Figure 3.3.11 Computational volcano curve of the ORR activity of well-defined metal surfaces versus their oxygen chemisorption energy, A/i0. Metals on the left bind oxygen too strongly, resulting in low ORR activity metals on the right bind too weakly, also resulting in low ORR rates. Pt is the most active monometallic ORR catalyst. The top of the volcano curve represents an unknown optimal catalyst and can be achieved by lowering the O chemisorption energy of Pt somewhat (arrow and red dotted lines). Figure adapted from [15].

Using this estimate gives typical mean absolute errors on the data set (dark gray points) in Figure 8.17 of the order 0.1 eV relative to full DFT calculations of the oxygen chemisorption energy. 

Surface Bond Energies Thermochemical data are very scant in the area of oxygen chemisorption (57). These data would be of great value for interpreting spectroscopic and kinetic data and for the analysis of reaction mechanisms. The vast majority of the available data are for low oxidation state systems (55). Although calorimetry offers a means for direct measurements, for analysis of reaction pathways it is necessary to have detailed values for many types of species (M-OH, MO-H, M-OR, M-R, M-O, M-H), and these are usually... 

Figure 2.16. Calculated dissociative nitrogen ( ), carbon monoxide ( ), and oxygen ( ) chemisorption energies over different 3d transition metals plotted as a function of the center of the transition metal rf-bands. A more negative adsorption energy indicates a stronger adsorbate-metal bond. Reproduced from [32].

As a first example of the use of the d band model, consider the trends in dissociative chemisorption energies for atomic oxygen on a series of 4d transition metals (Figure 4.6). Both experiment and DFT calculations show that the bonding becomes... 

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".

Since the d orbitals are not allowed to relax in a one electron ECP it may appear that a third prerequisite is that the frozen d approximation should be valid, i.e. the relaxation of the d orbitals should not influence the bonding appreciably. In reality the effect of d-shell relaxation on various metal cluster properties is appreciable e.g. the d-shell relaxation effect on chemisorption energy of oxygen on a Nis cluster is about 40 kcal/mol[23]. However, a small d orbital relaxation is not a necessary prerequisite for the development of a one electron ECP provided that the relaxation is not dominated by covalency effects. The covalent contribution to the bonding of an oxygen atom to a Cus cluster (where the d-shell relaxation contribution to the binding energy is 17 kcal/mol) is only a few kcal/mol[24]. 

The Cu ions have a weak absorption spectrum that partially overlaps with the emission band of Cu, resulting in resonant energy transfer. In fact the time course of oxygen chemisorption could be followed by monitoring the Cu1 photoluminescence quantum efficiency with the time of exposure of Cu Y to oxygen. 

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