Effects of Alloy Formation

NO adsorption. NO dissociates on the surface, resulting in oxygen-induced Rh surface segregation. The presence of sulfur also causes Rh surface segregation 123). 

In view of the relative thermal stabilities of the oxides (Rh203 PdO Pt02), it is not surprising that under oxidizing conditions and at temperatures higher than 870 K, metallic Pt particles exist in combination with Rh oxide 132,133) and that PdO crystallites separate from Pt crystallites in Pd-Pt alloys in the temperature range 670-870 K 134). A more complicated behavior was found for Pd-Rh 135). 

Adsorption and reactivity investigations of Pt-Rh alloy surfaces were 

At temperatures of about 210 K, complete dissociation of NO occurs on Rh(lOO) and on Pt-Rh(lOO) alloy surfaces at low NO coverages, and partial 

In contrast to the behavior on Rh(lOO) and Pt-Rh(lOO) surfaces, the fraction of NO decomposing on Pt(lOO) does not change dramatically with coverage. The behavior of the Pt-Rh(lOO) surface resembles that of the pure Rh(lOO) surface at low NO coverage and that of pure Pt(lOO) at high coverage. This pattern illustrates that Rh atoms on the surface are very effective in NO dissociation and that NO dissociation occurs mainly on Rh sites. These results also indicate that mixed Pt-Rh sites are not very reactive in NO bond breaking 141). 

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A more delicate item may be the effect of alloy formation between noble metals and the metal components of the supports. In the case of Pt/Sn02, low-temperature (at 120 °C) reduction is required, which leads to both the formation of Pt-Sn alloys and the formation of surface hydroxyls at the perimeter [68]. On the other hand, in the case of Au/Ti02, vacuum evacuation or reduction dramatically suppresses the initial catalytic activity, which can be recovered gradually during CO oxidation in excess O2. The removal of oxygen species at the perimeter interface is deleterious to supported Au catalysts. 

Casanovas, A., Llorca, I., Homs, N., Fierro, J.L.G., and Ramirez de la Piscina, P Ethanol reforming processes over ZnO-supported palladium catalysts Effect of alloy formation. Journal of Molecular Catalysis A Chemical, 2006, 250 (1-2), 44. 

Certain depolarization of M reduction is expected, which is consistent with the energetic effects of alloy formation. A shift in the equilibrium potential value can be obtained from the thermodynamic relationship... 

The information on the effect of alloy formation on the catalytic activity is generally conflicting. The effect of Sc during superheating of the catalyst is interpreted as an activity increase due to reduction of SC2O3 to Sc followed by an activity decrease due to Sc + Fe alloy formation [271]. Small amounts of Co increases the rate of NH3 synthesis and N2-chemisorption [123]. Small amounts of Ni increases only the rate of NH3 synthesis, not the rate of N2 chemisorption [123]. 

Many liquid alloys, in particular, the alkali-group IV alloys, exhibit (Zintl) anion clustering and show strong effects of compound formation. A typical example of such Zintl systems are sodium-tin alloys. In the solid NaSn crystal the Zintl anions Sn appear [1]. An interesting question is the stability of these anions in the liquid. Furthermore, the electrical conductivity of these alloys shows a strong dependence on composition [2] For the limiting (sodium-rich or tin-rich) cases a metallic (small) conductivity appears, but for the nearly equimolar compositions a semi-metallic behavior - with a considerably smaller conductivity - is observed. 

Ru provides sites for water activation as well as having an electronic effect on the Pt atoms, such that CO is less strongly adsorbed. In situ XAS measurements have been used to determine the structure of PtRu catalysts, to assess the magnitude of any electronic effect that alloy formation may have on the Pt component of the catalyst, and to provide evidence in support of the bifunctional mechanism. 

If the above conclusion is correct, i.e., if the 3Cay complexes are, indeed, preferentially formed on the summit of atoms, we can deduce from the selectivity effects of alloying and of C and S deposition that the formation of the 2Ca/j complexes requires one or, even more likely, two valley positions. We shall turn to this point later. 

Platinum-based bi-metallics (Pt M, M = Ti, Cr, V, Mn, Fe, Co, Ni, Cu, etc.) have been shown to exhibit enhanced activity toward the OER. Several rationales have been proposed including (1) enhanced chemisorption of intermediates (2) a lattice change of Pt that results in the shortening of Pt-Pt interatomic distances by alloying (3) the formation of skin Pt which has increased d-electron vacancy of the thin Pt surface layer caused by the underlying alloy and the anchor effect of alloy metals on a carbon carrier.93,94... 

Iwasa, N., Masuda, S., Ogawa, N., and Takezawa, N. Steam reforming of methanol over Pd/ZnO Effect of the formation of PdZn alloys upon the reaction. Applied Catalysis. A, General, 1995, 125 (1), 145. 

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