Oxidation, atomistics

Finally, we want to describe two examples of those isolated polymer chains in a sea of solvent molecules. Polymer chains relax considerably faster in a low-molecular-weight solvent than in melts or glasses. Yet it is still almost impossible to study the conformational relaxation of a polymer chain in solvent using atomistic simulations. However, in many cases it is not the polymer dynamics that is of interest but the structure and dynamics of the solvent around the chain. Often, the first and maybe second solvation shells dominate the solvation. Two recent examples of aqueous and non-aqueous polymer solutions should illustrate this poly(ethylene oxide) (PEO) [31]... 

Figure 5.9 Schematic cyclic voltammogram showing the electro-oxidation of the electrode (dashed box). The curve was generated from measurements by Jerkiewicz et al. [2004] of Pt in 0.5 M H2SO4 with a reversible hydrogen reference electrode (RHE). For each separable potential range, an atomistic model of the electrode structure is shown above.

Before we can apply the extended ab initio atomistic thermodynamics approach to the oxygen-covered surface or the surface/bulk oxide, we have to investigate the structure of the bulk electrode. 

We have also discussed two applications of the extended ab initio atomistic thermodynamics approach. The first example is the potential-induced lifting of Au(lOO) surface reconstmction, where we have focused on the electronic effects arising from the potential-dependent surface excess charge. We have found that these are already sufficient to cause lifting of the Au(lOO) surface reconstruction, but contributions from specific electrolyte ion adsorption might also play a role. With the second example, the electro-oxidation of a platinum electrode, we have discussed a system where specific adsorption on the surface changes the surface structure and composition as the electrode potential is varied. 

Mackrodt, W.C. (1988) Atomistic simulation of oxide surfaces. Phys. Chem. Min. 15 228-237... 

Experiments have shown that Aoxide spinel formation is on the order of 10 4cm at ca. 1000°C [C.A. Duckwitz, H. Schmalzried (1971)]. Using Eqns. (10.45) and (10.46) with the accepted cation diffusivities (on the order of 10 10 cm2/s), one can estimate from j% that each A particle crosses the boundary about ten times per second each way. In other words, quenching cannot preserve the atomistic structure of a moving interface which developed during the motion by kinetic processes. This also means that heat conduction is slower than a structural change on the atomic scale, unless one quenches extremely small systems. 

A majority of literatures on atomistic modeling of PEFC are about Nation polymer electrolyte based systems. The predominant issues are (1) OER at cathode, (2) oxidation of CO and methanol, and (3) transport processes in Nation polymer electrolyte. 

So far, the atomistic modeling on oxidation of CO and methanol has been aimed to elucidate mechanisms for (1) the bifunctional effect, in which the unique catalytic properties of each of the elements in the alloy combine in a synergetic fashion to yield a more active surface and (2) the ligand or electronic effect, in which the interaction between dissimilar atoms yield alters electronic states and hence results in a more active catalytic surface. In parallel to the study on the OER, study of oxidation of CO and methanol has seen a progress from vapor phase models to liquid phase models. However, polymer cluster has not been involved in the ab initio models. 

The reason why atomistic modeling study of PEFCs is considered as being in its initial stage of progress is that the system size and timescale in the simulations are often very limited. With the AIMD approach that is considered to be exact can reveal some very important mechanisms for OER, oxidation of CO and methanol, and proton transfer in Nafion. However, because the simulations are conducted with very small system (<200 atoms) and with a timescale of several picoseconds, it may be an overstatement when one claims that a phenomenon observed experimentally can be explained with mechanisms found in the simulations. 

Abbreviations A(hkl), surface area of the simulation cell in atomistic approaches (m2) am. lattice parameter of an AM oxide (A) , fivefold coordinated, n-valent anion , four-... 

A tolerance factor [9,10] can be used to determine the phase transition in AB03 perovskite oxides, as given by t — (rA + > o )/V2(J b + ro), where rA, rB, and rQ are the ionic radii [11] of the A, B, and O ions, respectively. This indicates that the spatial margin relates to the type of phase transition. However, the atomistic explanation has not been given for the factor in order to distinguish between ferroelectric and antiferrodistortive phase transitions in AB03 perovskite oxides. 

In terms of atomistic description of the phenomenon of catalysis, the border between these two approaches is determined by the rate of attenuation of the electron perturbation in a solid with the distance from an adsorbed molecule as well as by the degree of similarity between electron structures of the whole surface of a solid and of a fragment considered as a model of the adsorption or of an active site. For a long time this problem lacked an unambiguous solution. Therefore both approaches were equally widely used to describe catalytic phenomena, often without proper regard to specific features of the systems considered. Thus, active sites on metal catalyst surfaces were frequently modeled by individual metal atoms. On the contrary, catalysis on insulator oxide surfaces was sometimes discussed in terms of their cooperative electron properties. 

Reuter, K. et al., Atomistic description of oxide formation on metal surfaces The example of ruthenium, Chem. Phys. Lett., 352, 311, 2002. 

Anodic aluminum oxide, 312, 401 Antiferromagnetic-like interactions, 141 Antiferromagnetism, 433 Arc discharge method, 448 Ar ion sputtering, 449 Arrhenius, Carl Axel, 4 Arrhenius-Uke behaviour, 134 Asteriod hypothesis, 46 Atomic force microscope (AFM), 397 Atomistic simulation, 296,297 Au(llO), 137 Au(lll), 135... 

Maciel, G. E., L. Simeral, and J. J. H. Ackerman (1977). Effect of complexatlon of zinc (II) on zinc-67 chemical shifts. J. Phys. Chem. 81, 263-67. Mackrodt, W. C. (1988). Atomistic simulation of oxide surfaces. Phys. Chem. Mineral. 15, 228-37. 

Chemical vapor deposition (CVD) is an atomistic surface modification process where a thin solid coating is deposited on an underlying heated substrate via a chemical reaction from the vapor or gas phase. The occurrence of this chemical reaction is an essential characteristic of the CVD method. The chemical reaction is generally activated thermally by resistance heat, RF, plasma and laser. Furthermore, the effects of the process variables such as temperature, pressure, flow rates, and input concentrations on these reactions must be understood. With proper selection of process parameters, the coating structure/properties such as hardness, toughness, elastic modulus, adhesion, thermal shock resistance and corrosion, wear and oxidation resistance can be controlled or tailored for a variety of applications. The optimum experimental parameters and the level to which... 

The periodic approach is not the only one available for atomistic simulations of these materials and we should first mention that much progress has been made in the application of molecular quantum chemical methods using cluster representations of the local structure of oxide materials [1, 2], More recently, this has given way to mixed quantum mechanics/molecular mechanics (QM/MM) calculations. In QM/MM simulations the important region, the active site for catalysis, is represented at a quantum chemical level while the influence of its environment, the extended solid, is represented using the computationally less-demanding atomistic force field approach. This allows complex structures such as metal particles supported on oxides to be tackled [3]. 

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