Activity alloy phase

Here W(a,(i) is the p a transition probability for which we accept the conventional thermally activated atomic exchange model . Below we briefly review several recent works on the general formulation of this approach and on its applications to studies of alloy phase transformatious. 

Bauer et ah, 1986). It is also a significant corrosion product of Fe alloy phases on Antarctic meteorites where its formation is induced by the chloride ions coming from airborne seaspray and/or volcanic activity (Buchwald and Clarke, 1989). In these meteorites, akaganeite is located adjacent to the corroding surface and beneath a layer of goethite/spinel into which it eventually transforms. 

A stated objective of many of the reported studies of the catalytic properties of alloys has been to elucidate the significance of the band structure of the metallic phase (i.e., the energy levels of the d electrons) in determining the energetics of reaction (i.e., the value of E). While significant correlations of the values of E with band structures have been found in several instances [e.g., (25,255)], the interpretation of results is not always straightforward (237) and it may be necessary to incorporate due allowance for other factors that may exert some control over the mechanisms of reactions. Such factors include the possible presence of more than one alloy phase (207), dissolution of hydrogen in the alloy (207), and the composition and disposition of elements in the active outer surface of the alloy under reaction conditions (28,113,208). 

The development of new and improved electrocatalysts begins with the discovery of materials displaying improved intrinsic electrochemical activity. Intrinsic activity is best observed and compared in a well-controlled catalyst environment where variables that may disguise the intrinsic activity trends are minimized or absent. Particle size, particle size distribution, surface area, catalyst utilization and the distribution of crystallographic phases are parameters that are typically difficult to control. Vapor deposition of unsupported thin film electrocatalysts eliminates many of these variables. This method provides a controlled synthetic route to smooth, single-phase or multi-phase, ordered or disordered metal alloy phases depending on deposition and processing conditions. 

Clusters and alloys are molecular species that may show different catalytic activity, selectivity and stability than bulk metals and alloys. Small metal clusters and alloy clusters have been studied reeendy for potential use as catalysts, ceramic precursors, and as thin films. Several fundamental questions regarding such clusters are apparent. How many atoms are needed before metallic properties are observed How are steric and electronic properties related to the number, type and structure of such clusters Do mixed metal clusters behave like bulk alloy phases ... 

The thermodynamics of corrosive alkali salt-oxide interaction is not well established. In an assessment of research needs for materials in coal conversion, the need for carbonate-silicate melt studies, including activity and phase equilibrium measurements, was stressed (31 ). The lack of thermodynamic data for fused salts, and their reactions with oxides and alloys leading to models of hot corrosion, was also indicated. 

Total metal loading. The active phase was a bcc alloy phase with SO/SO Fe/Rh atomic ratio. 

The electro-catalytic oxidation of hydrogen, and reduction of oxygen, at carbon supported platinum based catalysts remain essential surface processes on which the hydrogen PEM fuel cell relies. The particle size (surface structure) and promoting component (as adsorbate or alloy phases) influence the activity and tolerance of the catalyst. The surface chemical behavior of platinum for hydrogen, oxygen, and CO adsorption is considered, in particular with respect to the influence of metal adsorbate and alloy components on close packed and stepped (defect) platinum surfaces. Dynamical measurements (employing supersonic molecular beams) of the... 

The additive effect of noble metals to NiO-MgO solid solution on the activity and on the resistance to the carbon deposition in oxidative steam reforming of methane under pressurized condition (I.O MPa) was investigated. It was found that the addition of small amount of Rh or Pd was effective to enhance the activity and to suppress the carbon deposition. The Pd and Rh K-edge EXAFS analysis indicated that M-Ni alloy phase (M=Rh or Pd) was formed, and FTIR spectra of CO adsorption suggests that the alloy could be formed on the surfece of bimetallic particles. 

Careful investigation of the catalyst indicates that the active phase is a Ru3Sn7 alloy phase. Ru3Sn7 is a cubic structure of the Im3m spatial group, which contains tin antiprisms as characteristic structure. 

Supported silver catalysts are relatively commonly used in gas phase oxidations of alcohols.74,75 Benzyl alcohol can be selectively oxidised to benzaldehyde using a 0.6% Ag/pumice catalyst76 with 100% selectivity, although its activity is less than a similar Pd material. However, a mixed Pd-Ag/pumice bimetallic increases the activity whilst retaining the 100% selectivity to benzaldehyde. The authors of this study concluded that the role of the Pd was to activate the substrate whereas the highly dispersed silver particles served to activate the oxygen. Hence, the mechanism was one of cooperation between the Ag° and Pd° sites, the alloy phase, detected by EXAFS, was considered not to play an important role. 

The other approach, which so far has had the greater practical impact, is primarily empirical. The CALPHAD (from CALculated PHAse Diagrams) approach was pioneered by L. Kaufman, a more recent presentation is by Saunders and Miodownik The work of the CALPHAD group focuses on calculations based on thermochemical data such as those compiled by the Scientific Group Thermodata Europe (SGTE), the ASM-NBS Alloy Phase Data Activity, PHACOMP (see above), and CALPHAD itself, but also uses the results of ab initio structural energy calculations for elements and compounds as these become available. 

According to the results of in situ Mossbauer spectroscopy the formation and stabilization of the PtSn (1 1) alloy phase has been shown under condition of room temperature CO oxidation (see results presented in Table 12). One of the most active surfaces of the PtSn (1 1) alloy phase, the (110) phase, was chosen to model the interaction of the CO molecule with the metal surface. The computer modeling and the related calculations were made on density functional level. In this model a small cluster of the (110) surface of the PtSn phase as shown in Figure 25a, was selected in order to calculate and investigate the interaction of the CO molecule with the metal surface. 

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