Chemical potential of oxygen

The actual surface termination of oxides not only is a result of relaxation or reconstruction but also depends critically on the environment. Preparation of oxide surfaces for UHV studies often requires high-temperature or high-pressure oxygen treatment. Such factors can lead to dynamic adsorption/desorption processes, which determine the surface structure under these conditions. At given experimental parameters, the surface structure with the lowest surface free energy will be the most stable one. A formahsm has been developed that allows the calculation of the surface free energy from ah initio methods as a function of pressure and 

Very similar concepts of surface stability criteria, as those discussed, have been described for surfaces of compound semiconductors. In this respect, the concept of autocompensation, or electron counting, has been introduced. This refers to lowering the surface energy by pairing of dangling bonds leading to completely filled anion states, and completely empty cation states, respectively. The reader is referred to Refs. [22-25]. 

---

Figure 7.14. Schematic representation of the spatial variation of electrode potential, chemical potential of oxygen and electrochemical potential of O2 for the cell 02, M1YSZ1M, 02 (=1 atm).

Schematics of an oxygen membrane reactor for catalytic POx of methane. A blown up section on the left-hand side shows the details of the ceramic membrane wall explaining the mechanism of oxygen permeation across the membrane. /- is the chemical potential of oxygen and ai and

The chemical potential of oxygen can now be derived and the related quantity log pC>2 expressed as a function of <5 ... 

The electrical conductivity of CU2O is expected to vary as experimentally it is found that the conductivity varies as Po , suggesting that equation (8.10) roughly represents the true state of affairs in CU2O. Since the chemical potential of oxygen, P2, is given by RT n, the expression for the so-called rational-rate constant, is written... 

In Figure 1.8(b) the spectroscopic consequences of the heating in various chemical potentials of oxygen can be seen directly. The reduction causes a donor state band to... 

The oxidation of propylene oxide on porous polycrystalline Ag films supported on stabilized zirconia was studied in a CSTR at temperatures between 240 and 400°C and atmospheric total pressure. The technique of solid electrolyte potentiometry (SEP) was used to monitor the chemical potential of oxygen adsorbed on the catalyst surface. The steady state kinetic and potentiometric results are consistent with a Langmuir-Hinshelwood mechanism. However over a wide range of temperature and gaseous composition both the reaction rate and the surface oxygen activity were found to exhibit self-sustained isothermal oscillations. The limit cycles can be understood assuming that adsorbed propylene oxide undergoes both oxidation to CO2 and H2O as well as conversion to an adsorbed polymeric residue. A dynamic model based on the above assumption explains qualitatively the experimental observations. 

FIGURE 6 Schematic representation of an oxide catalyst with its functional compartments in various structural states for high (back) and low (front) chemical potentials of oxygen. The arrows and the question mark indicate the complex distribution of oxygen in its dual role as a reactant at the surface and as a constituent of the catalyst material in the bulk. Its abundance is controlled by the presence of reducing species in the gas phase leading to a dependence of the results of XRD structural analysis on the availability of reducing gas-phase species. For details and references, see the text. 

Since the chemical potential of oxygen (in the tracer case we refer to the chemical potential of the isotope) is spatially (A) constant in equilibrium, it holds that... 

As discussed in table 10.1, the mobile species within a fuel cell are ions, which necessitate the electrolyte being an ionic conductor and electronic insulator. If the oxygen ions are the only charge carriers, the electron motive force (EMF) of the cell is determined from the chemical potential of oxygen (i.e., oxygen activity), which is expressed by the Nernst equation as... 

Riess et al." measured the chemical potential of oxygen in non-stoichiometric CeOi., as a function of x (10 x < 10 ) and temperature (873K-1073K), by... 

Here, A is the area of the surface unit-cell, and Go surf and Gsurf are the Gibbs free energies of the oxidized and clean surface, respectively, juq and jiiM are the chemical potentials of oxygen and metal atoms, and No is the number of oxygen atoms contained in the oxidized surface structure for which AG jjo) is computed. Finally, AIVm is the difference in the number of metal atoms between the reference clean surface and the oxidized surface structural model. 

Fig. 5.2 GO phase diagram with respect to the chemical potentials of oxygen and hydrogen. Insets show the atomic structures of the corresponding GO phases (Adapted from Ref [46]. Copyright (2010) by the American Physical Society)...

Wang et al. [45,46] have constructed a phase diagram of GO at different chemical potentials of oxygen and hydrogen. They found four thermodynamically stable GO structures as shown in Fig. 5.2, which are all fully oxidized and can only exist at extreme 0-rich and H-poor conditions. Genetic algorithm has also been used to study GO structure [47]. In the epoxy-only case, the ground state contains normal epoxy, unzipped epoxy, and epoxy pair. 

If the SE and RE of the zirconia-based sensor are exposed to different oxygen partial pressures, P02 (gas) and 7 02 (reference), this induces different chemical potentials of oxygen ions in zirconia at the interfaces with gas phases. In order that the electrochemical potential remains constant, the electrical potential has to be different. Therefore, the output emf of the electrochemical cell (3.2), represented as a difference between potentials on the RE and SE, obeys the well-known Nemst s law ... 

---