Oxygen adsorption, from

Hydrogen adsorption from solution Oxygen adsorption from solution Underpotential deposition of metals Adsorption of probe molecules from solution ... 

It has been possible to demonstrate that for the lower rank coals that there are very few (more than 10 /g but less than 10 /g) reactive sites for strong oxygen adsorption. From the data collected we wish to propose the following mechanism for low temperature oxidation. 

The oxide film is often the first line of defense a material has against the corrosive influences of its environment. A passive oxide film forms upon oxygen adsorption from the environment. These films can be beneficial, serving to slow or block metal dissolution and further corrosion. This effect is called oxide passivation. On the other hand, metals may continue to oxidize/corrode, even after an oxide film has formed or this film may break down leading to active corrosion. Because of these drastically different outcomes, an understanding of how these films are formed under different environmental conditions is needed to understand how to mitigate corrosion. The focus of this chapter is the use of quantum mechanics simulations to understand the thermodynamics of passive film formation. In particular, we look at calculations of first-principles phase diagrams as a function of environmental conditions and how these studies fit with experimental data. We focus on Pd, Mg, and Pt metal systems as they are well studied, represent different types of oxide film formation, and fall within the authors areas of expertise. 

Oxygen Adsorption from NO2. In this second example, we imagine a surface dosed with oxygen through the equilibrated reaction of N02(g) and NO(g), eqn (2.37). The equilibrium assumption allows us to write the oxygen chemical potential, po, as ... 

Two other methods worth discussing are wet air oxidation and regeneration by steam. Wet oxidation may be defined as a process in which a substance in aqueous solution or suspension is oxidized by oxygen transferred from a gas phase in intimate contact with the liquid phase. The substance may be organic or inorganic in nature. In this broad definition, both the well known oxidation of ferrous salts to ferric salts by exposure of a solution to air at room temperature and the adsorption of oxygen by alkaline pyrogallol in the classical Orsat gas analysis would be considered wet oxidations. 

The simplest example of oxygen spillover is found in the adsorption of oxygen on carbon. The spillover oxygen migrates from the basal carbon (donor) to carbon atoms exposed at steps between layers of the graphite surface, where it reacts with the edge carbons (acceptor).71 In this case the donor and acceptor phase consist of the same material with different surface properties. 

Figure 4.43. Thermal desorption spectra after gaseous oxygen adsorption on a Pt film deposited on YSZ at 673 K and an 02 pressure of 4x 10"6 Torr for 1800 s (7.2 kL) followed by electrochemical O2 supply (I=+15 pA) for various time periods.29-30 Reprinted from ref. 30 with permission from Academic Press.

Figure 5.39. Characterization of the spillover species by photoelectron spectra of the Ols region taken from a 0.02 pm2 spot on the Pt surface (a) The residual O Is spectrum after the cleaning cycles (b) The Ols spectrum measured in 02 atmosphere (pO2=lxI0 6 mbar) (c) The Ols spectrum obtained during electrochemical pumping in vacuum with UWr = 1.1 V. R1 and R2 are the components which are formed by adsorption from the gas phase and by electrochemical pumping. The fitting components of the residual oxygen are shown with dashed lines. Photon energy = 643.2 eV, T 350-400°C.67 Reprinted with permission from Elsevier Science.

SIMS Cluster Ion Characterization During Oxygen Adsorption and Oxidation. For heavy oxidation, that is essentially bulk oxide films, the oxidation state of the metal can be determined from the positive and negative SIMS intensity distributions (1 ). Though similar attempts have been made to characterize the nature of the surface during the early stages of oxygen interactions (14,15), we now know from the extensive information available from other techniques that such interpretations are incorrect. We use the by now well-characterized W(100)/O and Ni(100)/0 systems as examples. 

Sulfiir-anchored SAMs and thin films, mostly from organosulfiir precursors, have been discussed at length by a number of authors [10, 181]. SAMs of organosulfiir compounds (thiols, disulfides, sulfides) form on gold substrates by spontaneous adsorption from either the liquid or the vapor phase. A number of experimental factors can affect the formation and structure of SAMs such as choice of solvent, temperature, concentration, immersion time, purity of adsorbate, oxygen concentration in solution, cleanliness, and structure of the adsorbate. Interestingly, the... 

Oxygen adsorption that occurs at platinum at potentials more positive than 0.9 to 1.0 V is irreversible, in contrast to hydrogen adsorption. Oxygen can be removed from the surface by cathodic current, but the curves obtained in the anodic and cathodic scan do not coincide cathodic oxygen desorption occurs within a narrower region of potentials, and these potentials are more negative than the region where the... 

The enhanced adsorption of anions and other substances that occurs at increasingly positive potentials causes a gradual displacement of water (or other solvent) molecules from the electrolyte layer next to the electrode. This leads to a markedly slower increase in the rate of oxygen evolution from water molecules and facilitates a further change of potential in the positive direction. As a result, conditions arise that are favorable for reactions involving the adsorbed species themselves (Fig. 15.9). In particular, adsorbed anions are discharged forming adsorbed radicals ... 

The specific adsorption of bisulfate anions is observed in H2SO4 in both EXAFS and XANES data and, in agreement with voltammetry, is seen to impede oxygen adsorption. Significant specific anion adsorption was found in 6 M TFMSA, but not in 1 M TFMSA [Teliska et al., 2007]. As mentioned above, this specific anion adsorption suppresses OH adsorption (particularly the formation of subsurface O), causes the Pt nanoparticle to become more round, and weakens the Pt-Pt bonding at the smface. The specific anion adsorption becomes site-specific only after lateral interactions from other chemisorbed species such as OH force the anions to adsorb into specific sites. 

Considering the above discussion, the currents in Fig. 15.5 are normalized to the surface areas estimated from CO stripping. Similar to massive electrodes, CVs for Pt nanoparticles show three characteristic potential regions (all potentials vs. RHE) (i) an Hupd region 0.05 Y < E < 0.40 V, followed by (ii) the so-called doublelayer region 0.40V < E< 0.60V and (iii) for E > 0.7 V, the oxygen adsorption/ desorption region. 

It should be noted that dissociation of surface complexes of oxygen in polar solvents on semireduced ZnO films is presumably justified from the thermodynamic point of view as oxygen adsorption heat on ZnO and electron work function are [58] 1 and approximately 5 eV respectively while the energies of affinity of oxygen molecules to electron, to solvation of superoxide ion and surface unit charge zinc dope ions are 0.87, 3.5, and higher than 3 eV, respectively [43]. 

Kds are the constants of rates of chemical reactions of oxygen adsorption and desorbtion from ZnO film and Aq are electron work function from ZnO before oxygen gets adsorbed and its variation caused by dipole moment of adsorbed complexes being formed U is the adsorption activation energy of non-electrostatic nature [ M] is the concentration of solvent molecules. Apparently we can write down the following expression for the stationary system ... 

The method of semiconductor sensors allows one to determine the flux of atoms, to which the sensor was exposed, from electric conductivity measurements (provided coefficients of ionization and reflection of oxygen atoms from zinc oxide films are known). In other words, the sensor technique can be used in this case as an absolute method [21]. Indeed, variation of electric conductivity of a semiconductor film Acrpi due to adsorption is known to be caused by variation of carrier concentration An in the film, rather than by variation of their mobility / [21] ... 

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