Activity alloy components

In pack cementation, the part to be coated is placed in a retort and surrounded with a powdered pack consisting of the coating component and an activator the latter reacts with the coating component to form the carrier vapor, usually a haHde or an inert diluent, to prevent the pack from sintering together and to permit vapor transport of the alloying component through the pack. 

Antimony (Sb) and tin (Sn) are usually not added to the active material, but both are alloying components of the grid. They are gradually released from the grid by corrosion, and permeate the active material by dissolution and diffusion. 

This allows a direct influence of the alloying component on the electronic properties of these unique Pt near-surface formations from subsurface layers, which is the crucial difference in these materials. In addition, the electronic and geometric structures of skin and skeleton were found to be different for example, the skin surface is smoother and the band center position with respect to the metallic Fermi level is downshifted for skin surfaces (Fig. 8.12) [Stamenkovic et al., 2006a] owing to the higher content of non-Pt atoms in the second layer. On both types of surface, the relationship between the specific activity for the oxygen reduction reaction (ORR) and the tf-band center position exhibits a volcano-shape, with the maximum... 

Binder H, Kohling A, Sandstede G. 1972. Effect of alloying components on the catalytic activity of platinum in the case of carbonaceous fuels. In Sandstede G, ed. From Electrocatalysis to Fuel Cells. Seattle University of Washington Press, p. 43. 

Although potential measurements are used primarily to determine activities of electrolytes, such measurements can also be used to obtain information on activities of nonelectrolytes. In particular, the activities of components of alloys, which are solid solutions, can be calculated from the potentials of cells such as the following for lead amalgam ... 

The general field of measuring thermodynamic quantities using gas-phase methods is based simply on Eq. (3.55) where the activity of component i (Oj) is equal to the vapour pressure of i in the alloy divided by the equilibrium vapour pressure of pure i. 

In conclusion, the computational study of ternary Pt-Ru-X alloys suggests that future strategies toward more active electrocatalysts for the oxidation of methanol should be based on a modification of the CO adsorption energy of Pt (ligand effect), rather than on the enhancement of the oxophilic properties of alloy components (enhanced bifunctional effect). 

The reasons for the superior catalytic properties of these bimetallic catalysts are not adequately understood even after 30 years of active research in this area. Many of the explanations for the superior properties of the bimetallic catalysts are based on a structural point of view. Many argue that the bimetallic components form an alloy which has better catalytic properties than Pt alone. For example, alloy formation could influence the d-band electron concentration, thereby controlling selectivity and activity (3). On the other hand, the superior activity and selectivity may be the result of high dispersion of the active Pt component, and the stabilization of the dispersed phase by the second component (4). Thus, much effort has been expended to define the extent to which metallic alloys are formed (for example, 5-18). These studies have utilized a variety of experimental techniques. 

Skeletal catalysts consist of a metal skeleton remaining after the less noble components of an alloy has been removed by leaching with base or acid. The skeleton metals belong almost exclusively to Groups IB and VIIB of the periodic table (Fe, Co, Ni, Cu, and Ag), whereas Al, Zn, Si, and Mg are the most commonly used alloy components. The alloys are prepared by fusion of the components in the proper proportion, Raney pioneered the development of skeleton catalysts. A widely used Ni and Co catalyst, which is highly active... 

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 activity, stability, and tolerance of supported platinum-based anode and cathode electrocatalysts in PEM fuel cells clearly depend on a large number of parameters including particle-size distribution, morphology, composition, operating potential, and temperature. Combining what is known of the surface chemical reactivity of reactants, products, and intermediates at well-characterized surfaces with studies correlating electrochemical behavior of simple and modified platinum and platinum alloy surfaces can lead to a better understanding of the electrocatalysis. Steps, defects, and alloyed components clearly influence reactivity at both gas-solid and gas-liquid interfaces and will understandably influence the electrocatalytic activity. 

Oforka and Argent [237] determined the thermodynamic activities of the alloy components and the integral Gibbs energies of mixing for an isothermal section of the Ni-Cr-Al system at 1423 K. Some values were also reported for the three boundary systems Cr-Ni, Cr-Al, and Ni-Al. 

Investigations covering both the liquid and the solid phase are reported by Neubert [229] for the Li-Pb system, by Ono et al. [136] for the Cr-Cu system, by Rammensee and Fraser [241] for the Fe-Co and Fe-Ni systrans, by Fraser and Rammensee [134] for the Fe-Co-Ni system, and by Hilpert et al. [252] for the Ni-Al system. Thermodynamic activities or activity coefficients of the alloy components were determined by all these investigations. Moreover, enthalpies... 

The term bimetallic was introduced by Sinfelt to account for the fact that a catalyst may contain a multitude of phases containing the active metallic components.22 Of these many phases, a characteristic one is the binary alloy. The term alloy can describe a broad range of situations from well-defined phases or solid solutions to surface alloys in cases where bulk alloys are not thermodynamically favoured but a clearly defined surface local arrangement is obtained. Note that the novel core-shell bimetallic structures are included in this catch-all term. A historical overview of the properties of alloys in connection with catalysis has been published by Ponec.23 At present, a... 

While attempting to use platinum in fuel cells, it has been demonstrated that its surface exhibits important electrocatalytic activities toward the oxidation of organic compounds. However, this effect can sometimes be enhanced by the use of bimetallic surfaces [1-10]. The physical mixture and the electronic interaction of the alloy components lead to a modification in the interaction between the adsorbate and the substrate in an electrocatalytic reaction. As a consequence of the structural changes at the single crystal surfaces during the electrochemical activation (examined with in situ STM) [11], it has been demonstrated that most of the catalysts are constituted by randomly oriented islands [12-14]. 

This treatment increases the surface area. Furthermore, addition of other alloying metals and the use of both phosphorus and silicon as metalloids lead to electrodes with high activity for chlorine and low activity for oxygen evolution. Pd l TisRusPioSis amorphous alloy, surface activated at 573 K is the best of all samples and exhibits a current density of 1300 A/m2 at 1.15 V, which is much higher than the 750 A/m2 of Pt-Ir-Ti, currently the best electrode (102). A surface-activated nine-component alloy exhibits an even higher value (1450 A/m2) (104). 

With alloy evaporation, the vapour composition and thus also the film composition depend on the ratio of the vapour pressure of the alloy components and on the activity coefficients. In the case of varying vapour pressures of the alloy components, depletion of the easily volatile components in the evaporation source results... 

This general method for preparation of catalytically active structures is also applicable to metals other than Ni. It is also useful with alloys [31, 34]. In addition to Al, the alloy component which is leached out may be Si [12], Zn [24] and occasionally Mg [28] (dilute acetic acid is used as the leaching fluid). In determining the optimum composition of the Raney alloy for a specific purpose, one must also take into account the effect of the second metal. 

The selective oxidation of an alloy component, e.g., A1 or Si, requires the alumina or silica to be more stable than the oxides of the other components in the alloy. Figure 2.5 indicates this condition would be met for compounds such as nickel aluminides and molybdenum silicides. However, in the case of Nb- or Ti-base compounds the oxides of the base metal are nearly as stable as those of A1 or Si. This can result in conditions for which selective oxidation is impossible. This situation exists for titanium aluminides containing less than 50 at% A1 as illustrated in Figure 5.27. In this case a two-phase scale of intermixed AI2O3 and I1O2 is generally observed. It should be emphasized that the determination of which oxide is more stable must take into account the prevailing metal activities. 

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