Oxygen chemisorption on metals

Compared to the corresponding carbides the heats of oxygen chemisorption on metals are higher. For example, on metallic tungsten the heat of adsorption is 812 kJ/mole 02, while on metallic chromium it is 730 kJ/ mole 02n. These values are significantly higher that those of the carbides of the same metals (Table 16.2). Thus, carbon atoms, when implanted in the metal lattice, reduce the adsorption affinity of the metal atoms towards oxygen. 

Besenbacher F, Ndrskov JK (1993) Oxygen-chemisorption on metal-surfaces - general trends for Cu, Ni and Ag. Prog Surf Sci 44 5... 

F. Besenbacher, J.K. Norskov, Oxygen-chemisorption on metal-surfaces—general trends for Cu, N1 and Ag. Prog. Surf Sci. 44(1), 5-66 (1993)... 

It is important to notice that the work function, , of a given solid surface changes significantly with chemisorption. Thus oxygen chemisorption on transition metal surfaces causes up to 1 eV increase in while alkali chemisorption on transition metal surfaces causes up to 3 eV decrease in . In general electronegative, i.e. electron acceptor adsorbates cause an increase in 0 while electropositive, i.e. electron donor adsorbates cause a decrease in 0. Note that in the former case the dipole vector P formed by the adsorbate and the surface points to the vacuum while in the latter case P points to the surface (Fig. 4.20). 

Souda R, Aizawa T, Otani S, Ishizawa Y (1991) Oxygen-chemisorption on transition-metal carbide (100) surfaces studied by x-ray photoelectron-spectroscopy and low-energy He" scattering. Surf Sci 256 19... 

In spite of such catalytic effects of the metal cluster, the rate of this reaction remains very low. Nevertheless, this reaction is an example of a very interesting type of homogeneous catalysis. Here the activation of carbon monoxide appears to be achieved by interaction of both the carbon and oxygen atoms with the metal cluster atoms in a similar way to the CO-chemisorption on metals in heterogeneous Fischer-Tropsch processes. 

Eleetroehemieal experiments and surface analyses show that the adsorbed oxygen speeies in solution on most transition metals at 25°C are likely to be hydroxyls. Thermodynamic data obtained from electrochemical experiments are presently available only for on copper [49]. Therefore is considered here on Fe, Ni, and Cr and on Cu. The standard Gibbs energies of formation (chemical potentials) for sulW and oxygen adsorbed on metal surfaces can be calculated [44—46] from literature thermodynamie data for reversible chemisorption at the metal-gas interface (see Chqj. 2). 

Most importantly. Coulomb repulsion between the electron pairs plays a role of dominance, not only in H2O but also in the dynamics of oxygen chemisorption to metal surfaces. STM/VLEED study of oxygen chemisorption on Cu(001) surface [2] (Chap. 7) revealed that the O-Cu bond and the 0 Cu bond relax oppositely in lengths. The O -Cu" contracts to 0.163 nm, while the 0 Cu expands to 0.195 nm in the Cu 0-Cu configuration with creation of Cu dipoles and missing Cu vacancies. 

Perhaps the most fascinating detail is the surface reconstruction that occurs with CO adsorption (see Refs. 311 and 312 for more general discussions of chemisorption-induced reconstructions of metal surfaces). As shown in Fig. XVI-8, for example, the Pt(lOO) bare surface reconstructs itself to a hexagonal pattern, but on CO adsorption this reconstruction is lifted [306] CO adsorption on Pd( 110) reconstructs the surface to a missing-row pattern [309]. These reconstructions are reversible and as a result, oscillatory behavior can be observed. Returning to the Pt(lOO) case, as CO is adsorbed patches of the simple 1 x 1 structure (the structure of an undistorted (100) face) form. Oxygen adsorbs on any bare 1 x 1 spots, reacts with adjacent CO to remove it as CO2, and at a certain point, the surface reverts to toe hexagonal stmcture. The presumed sequence of events is shown in Fig. XVIII-28. 

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