The Metallic Phase

Surface segregation of Pd in Pd—Rh catalysts suppresses NOx reduction [61]. De Sarkar and Khanra studied the segregation difference between Pd—Rh and Pt—Rh nanoparticles, and the influence of sulfur in fuel on CO oxidation and NO. They used Monte-Carlo (MC) simulation to predict the surface composition of PtsoRhso and PdsoRhso particles (2406 atoms for 4nm particles). TTiey used a micro-kinetic model to compare the activities of both soHds for reactions of CO -i- O2, CO -I- NO and CO -1- NO -1- O2, and found that Pt and Pd segregate to the particles surface, especially in the Pd catalyst, which is clearly better for CO oxidation, while Pt—Rh is a better catalyst for NO reduction. For both reactions, sulfur poisons the Pd—Rh catalyst more than the Pt—Rh catalyst [62]. 

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The chemical potential pi, has been generalized to the electrochemical potential Hj since we will be dealing with phases whose charge may be varied. The problem that now arises is that one desires to deal with individual ionic species and that these are not independently variable. In the present treatment, the difficulty is handled by regarding the electrons of the metallic phase as the dependent component whose amount varies with the addition or removal of charged components in such a way that electroneutrality is preserved. One then writes, for the ith charged species. 

In obtaining Eq. V-60, it must be remembered that dfi = dfig - J dr< since electrons are confined to the metal phase. Canceling and combining terms. 

Sla.g ReHning. Unwanted constituents can be removed by transfer into a slag phase. Slag refining is also used for operations in which the Hquid metal is maintained in contact with a slag or a molten salt. This second immiscible Hquid is usually more oxidizing than the metallic phase and selective oxidation of the impurities renders them soluble in the slag or molten salt. Impurities that are less easily oxidized remain in the Hquid metal. 

Superimposed on this simple equiUbrium are complex reactions involving the oxides and hydrides of the respective metals. At about 400°C, the metal phase resulting from the reaction of sodium and potassium hydroxide contains an unidentified reaction product that precipitates at about 300°C (15). 

When only one phase is forming eddy cunents, as when a gas is blown across the surface of a liquid, material is uansported from the bulk of the metal phase to the interface and dris may reside there for a short period of time before being submerged again in die bulk. During this residence time t, a quantity of matter, will be U ansported across die interface according to the equation... 

The production of metals which form very stable oxides by tire aluminothermic process, such as manganese, clrromium and vanadium is carried out with reactants at room temperature which react to provide enough heat to raise the temperature of the products to high temperatures at which the whole system is liquid. The metal phase which is produced can therefore separate from the liquid slag which is formed. The production of clrromium serves as a useful... 

In addition to all the metallic phase diagrams, a series of volumes devoted to ceramic systems have been published since 1964 by the American Ceramic Society and is still continuing. The original title was Phase Diagrams for Ceramists, now it is named Phase Equilibria Diagrams. Some 25,000 diagrams, binary and ternary mostly, have been published to date. There is no compilation for polymeric systems, since little attention has been devoted to phase diagrams in this field up to now. 

The orientational structure of water near a metal surface has obvious consequences for the electrostatic potential across an interface, since any orientational anisotropy creates an electric field that interacts with the metal electrons. Hydrogen bonds are formed mainly within the adsorbate layer but also between the adsorbate and the second layer. Fig. 3 already shows quite clearly that the requirements of hydrogen bond maximization and minimization of interfacial dipoles lead to preferentially planar orientations. On the metal surface, this behavior is modified because of the anisotropy of the water/metal interactions which favors adsorption with the oxygen end towards the metal phase. 

The aforementioned inconsistencies between the paralinear model and actual observations point to the possibility that there is a different mechanism altogether. The common feature of these metals, and their distinction from cerium, is their facility for dissolving oxygen. The relationship between this process and an oxidation rate which changes from parabolic to a linear value was first established by Wallwork and Jenkins from work on the oxidation of titanium. These authors were able to determine the oxygen distribution in the metal phase by microhardness traverses across metallographic sections comparison of the results with the oxidation kinetics showed that the rate became linear when the metal surface reached oxygen... 

Electrodes such as Cu VCu which are reversible with respect to the ions of the metal phase, are referred to as electrodes of the first kind, whereas electrodes such as Ag/AgCl, Cl" that are based on a sparingly soluble salt in equilibrium with its saturated solution are referred to as electrodes of the second kind. All reference electrodes must have reproducible potentials that are defined by the activity of the species involved in the equilibrium and the potential must remain constant during, and subsequent to, the passage of small quantities of charge during the measurement of another potential. 

The metals are obtained from the metallic phase of the sulphide matte or the anode slime from electrolytic refining of nickel. In the traditional process for the platinum metals, their separation was facilitated by their solubility in aqua regia and convertibility into PdCl - or PtCl - salts. Nowadays, substantial amounts are obtained using solvent extraction. 

For a multi-stage counter-current extraction operated in true equilibrium without side reactions, the partitioning of a solute into the metal phase (fm) and the salt phase (fs) is defined as follows ... 

PUCI3, and MgCl2 to form a 50/50 mole % NaCl-CaCl salt phase and a molten Am-Pu-Mg-Ca alloy which is immiscible in the above salt(lO). After cooling, the metal phase is cleaved away from the salt phase and the salt phase is analyzed. Little, if any, Am or Pu remains in the salt phase and the salt residues can be discarded to waste. Metal recovery begins by evaporating magnesium and calcium from the residual metal button at about 800°C in vacuum. The americium can then be distilled away from the plutonium in a vacuum still operated at 1200°C, using yttria ceramic vessels to contain the molten metal fraction. The bottoms fraction contains the plutonium which is recycled back into the main plutonium stream. 

After a study of the three alternatives we concluded that pyroredox offered the most promise for anode residue recovery. Pyroredox is a molten-salt process in which plutonium metal is oxidized chemically into the salt phase and then reduced chemically into the metal phase. Most of the impurities are not oxidized and remain in the metal residue. Thus, for a Pu-Ga anode residual, the reactions would be ... 

While from a structural point of view metal/solution and metal/vac-uum interfaces are qualitatively comparable even if quantitatively dissimilar, in the presence of ionic adsorbates the comparability is more difficult and is possible only if specific conditions are met.33 This is sketched in Fig. 7. A UHV metal surface with ions adsorbed on it is electrically neutral because of a counter-charge on the metal phase. These conditions cannot be compared with the condition of a = 0 in an electrochemical cell, but with the conditions in which the adsorbed charge is balanced by an equal and opposite charge on the metal surface, i.e., the condition of zero diffuse-layer charge. This is a further complication in comparing electrochemical and UHV conditions and has been pointed out in the case of Br adsorption on Ag single-crystal faces.88... 

The full MSA expression for the capacitance is complex. However, at low ce it is composed of concentration-independent and concentration-dependent terms. 1 The concentration-independent term is not associated with any specific region of the interface, but quantitative agreement between experimental and theoretical values of capacitance at low ceJ is achieved only if the contribution of the metal phase is included. 

The capacity of the metal phase (CM) and the potential drop in the thin metal surface layer have been discussed by Amokrane and Badiali,122,348 as well as by Damaskin et a/.349 353 The value of was found to increase in the order Ga < In(Ga) < Tl(Ga) Hg if it was assumed that the capacity of a solvent monolayer C = const. The negative value of the surface charge density <7, at which the Cs,ff curve has a maximum, decreases in the order Ga > In(Ga) > Hg, i.e., as the hydrophilicity of the electrode decreases. 

The temperature dependence of the inner-layer properties has been studied by Vaartnou etal.m m over a wide interval, -0.15°C < T< 50°C. The inner-layer integral capacitance Kh a curves have been simulated using the Parsons308 and Damaskin672,673 models. The experimental Kj, T dependence has a minimum at T = 20°C. The influence of the potential drop in the metal phase has been taken into account. 

The C, values for Sb faces are noticeably lower than those for Bi. Just as for Bi, the closest-packed faces show the lowest values of C, [except Bi(lll) and Sb(lll)].28,152,153 This result is in good agreement with the theory428,429 based on the jellium model for the metal and the simple hard sphere model for the electrolyte solution. The adsorption of organic compounds at Sb and Bi single-crystal face electrodes28,152,726 shows that the surface activity of Bi(lll) and Sb(lll) is lower than for the other planes. Thus the anomalous position of Sb(lll) as well as Bi(lll) is probably caused by a more pronounced influence of the capacitance of the metal phase compared with other Sb and Bi faces28... 

In the spectrum from classical intermetaUics to valence compounds to insulators, a smooth transition in their chemical bonding (metallic to ionic) is observed. At the border between Zind phases and metaUic phases, the typical properties of Zind phases diminish and metallic conductivity appears. However, it is inaccurate to impose and define a sharp boundary between classical Zind phases and the metallic phases (e.g.. Laves and Hume-Rothery phases), and it is in the overlapping regimes where much chemistry stiU remains to be discovered and understood. 

Fig. 4 Oxygen Is XPS spectra including curve-fitted components for (a) Catalyst I, (b) Catalyst I after reduction In Fig. 2, a marble-like pattern was observed, which is attributable to solid solution phase of CoO and MgO, because XRD measurement on Catalyst II showed the existence of CoO-MgO solid solution phase [7, 8]. On the other hand, for Catalyst I, no solid solution phase of CoO-MgO was observed. In addition, XRD pattern of Catalyst I indicated the existence of CoO or C03O4. These results suggest that in the case of Catalyst I, Co is loaded on the surface of MgO as CoO or C03O4 phase. Magnified TEM image of Catalyst I after reduction is shown in Fig. 3. In this figure, crystalline lattice image was observed. It is likely that the observed lattice corresponds to the metal phase of Co, because XRD measurement on Catalyst I after reduction showed the existence of Co metal phase [7, 8].

For a supported metal catalyst, the BET method yields the total surface area of support and metal. If we perform our measurements in the chemisorption domain, for example with H2 or CO at room temperature, adsorption is limited to the metallic phase, providing a way to determine the dispersion of the supported phase. 

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