Alloy particles

As mentioned above, nonreducible metal ions can affect the reduction of other ions in the same zeolite by blocking sites or by anchoring the reduced particles to the wall of their cage. On the other hand, the presence of readily reducible ions can enhance the reduction of less easily reducible transition metal ions. 

A case in point is the reduction of Ni ions in NaX that is enhanced by the presence of Pd (199). The activation energy for the reduction of in the presence of Pd is 42 kJ/mol, which is considerably lower than the value of 117 kJ/mol for the reduction of Ni alone in the same zeolite (137,200). 

Number of Pd Ions Per Unit Cell Number of CO Ions Per Unit Cell Calcination Temperature (°C) Co Reduced (%) 

This conclusion is further confirmed by data obtained with Co(NH3)6. In this case, amminated Co ions remain in supercages at calcination temperatures below 300°C (77). Therefore, a pronounced reduction enhancement of Co ions by Pd is found after calcination at low temperatures, e.g., 250°C. Evidence for direct interaction of Pd ions with amminated Co ions is obtained by UV-Vis diffuse reflectance spectroscopy and temperature-programmed oxidation. The oxidation of the ammine ligands of the Co ion is catalyzed by Pd, so that ammine oxidation is complete below 350°C for [Pd(NH3)4 + Co(NH3)6 ]/NaY, whereas 450°C is required in the absence of Pd. Ammine destruction of the [Co(NH3)6] ion is a stepwise process, the last step being the conversion of the tetrahedral complex [CoCOzlsNHs], where stands for framework oxygen at the SII site, to a naked Co ion inside a hexagonal prism. The locations and coordinations of Co and Pd ions in NaY are summarized in Table III. 

In PtSn/ZSM-5 the reduction of Sn is enhanced by Pt due to an interaction of their precursors (207). The TPR maximum of Sn is shifted from about 350°C for Sn/ZSM-5 to 180°C for PtSn/ZSM-5. The interaction of Sn with Pt leads to increased selectivity in propane aromatization. 

---

In the dispersed-phase amalgam an additional phase of an eutectic of the Ag—Cu system and a reaction ring (2one) of Cu Sn [12019-69-1] around the residual silver alloy particles has been detected. If 10% gold is added to conventional alloys at the expense of the silver content, non-y2 amalgams are... 

Fig. 5. A growth model of a nanocapsule partially filled with a crystallite of rare-earth carbide (RCj for R = Y, La,. . . , Lu R,C4 for R = Sc) (a) R-C alloy particles, which may be in a liquid or quasi-liquid phase, are formed on the surface of a cathode (b) solidification (graphitizalion) begins from the surface of a particle, and R-enriched liquid is left inside (c) graphite cage outside equilibrates with RCj (or R3C4 for R = Sc) inside.

Fig. 13. Flypothetical growth process of SW tubes from a metal/carbon alloy particle (a) segregation of carbon toward the surface, (b) nucleation of SW tubes on the particle surface and, (c) growth of the SW tubes.

A third way to increase both the active surface area and the number of oxygenated species at the electrode surface is to prepare alloy particles or deposits and then to dissolve the non-noble metal component. This technique, which is similar to that used to prepare Raney-type catalysts, yields very high surface area electrodes and hence some improvements in the electrocatalytic activities compared with those of pure platinum. However, it is always difficult to be sure whether the mechanism of enhancment of the activities is due to this effect or the possible presence of remaining traces of the dissolved metal. Results with PtyCr and PtSFe were encouraging, although the effect of iron is still under discussion. From studies in a recent work on the behavior of R-Fe particles for methanol electrooxidation, it was concluded that the electrocatalytic effect is due to the Fe alloyed to platinum. ... 

In the presence of tin, the number of active platinum sites seems to be superior compared to catalyst B and thus an increase in the space velocity by a factor of ten does not seem to saturate all the sites. These results show the importance of the role played by tin since platinum loading was the same in both cases. It is reasonable to think that in the case of catalyst B, due to the method of deposition, some aggregates of platinum are formed on the surface of the catalyst. In the presence of tin, a part of the aggregates could disappear and some Pt/Sn alloy particles, better dispersed at the surface, could be formed. 

The particle size depends on the chain length of the aluminium-alkyl group and the concentration of the tri-alkylaluminium compound applied. If mixtures of, e.g., Fe- and Co-carbonyl compounds are used, Fe/Co alloy particles are formed. When the magnetic particles... 

Remarkably the position of the final plasmon peak of the alloy particles is dependent on the molar ratio of gold to silver nanoparticles. When the ratio is shifted favoring either metal, an alloy of any desired composition can be formed. This alloying phenomenon indicates that it is possible for true tuneability of the properties of a set of nanoparticles. 

Figure 16. EDX spectrum of a single Cu/Au alloy particle. (Reprinted with permission from Ref [59], 2006, American Chemical Society.)...

Chemical reduction of metal salts in solution is the most widely used method of preparation of metal nanoparticles, especially in laboratories. In general, the reducing reagents are added into the solution of the precursor ions, but in some cases, a solvent works as a reductant. Various reducing reagents have been proposed to prepare metal nanoparticles. Ethanol or small alcohols can reduce precious metal ions such as Au, Pt", Pd, Ag, and so on [3j. Polymer-stabilized precious metal nanoparticles and their alloy particles can be used as good catalysts for various reactions. Polyols, such as ethylene glycol, were... 

Moving on from preparation of homogenous Pt alloy particles to tailoring of core-and-sheU alloy particles, targeting (i) further lowering of the mass of precious metal per unit power output and (ii) further boost of catalytic activity per square centimeter of catalyst area [Zhang et al., 2004]. 

Irregular shaped alloy particles containing AI2O3... 

Recently, a novel process for the preparation of chromia promoted skeletal copper catalysts was reported by Ma and Wainwright (8), in which Al was selectively leached from CuA12 alloy particles using 6.1 M NaOH solutions containing different concentrations of sodium chromate. The catalysts had very high surface areas and were very stable in highly concentrated NaOH solutions at temperatures up to 400 K (8, 9). They thus have potential for use in the liquid phase dehydrogenation of aminoalcohols to aminocarboxylic acid salts. 

Papavassiliou GC (1976) Surface plasmons in small Au-Ag alloy particles. J Phys F Met Phys 6 L103-L105... 

Ruthenium and copper are not miscible hence, homogeneous alloy particles will not be formed in supported Ru-Cu catalysts. As copper has a smaller surface free energy than ruthenium, we expect that if the two metals are present in one particle, copper will be at the surface and ruthenium in the interior (see also Appendix 1). This is indeed what chemisorption experiments and catalytic tests suggest [40], EXAFS, being a probe for local structure, is of particular interest here because it investigates the environment of both Ru and Cu in the catalysts. 

In 1997 and 1998, the authors306 307 also examined acid leached Raney copper catalysts, whereby the alloy was leached with either nitric or perchloric acid of 5, 14, or 27.5 wt% strength. The acid solution was added dropwise over 15 min to an equal volume of deionized water containing the alloy particles. After leaching at 50 °C, the particles were removed and washed to a pH of 7. Air drying at 120 °C was then carried out for one hour. The dissolution rates of catalyst components were observed to be functions of the extraction time (Table 56). 

Figure 1.7. Shapes of solidified droplets (particles) generated in powder production and spray forming processes, (a) Spherical shape gas-atomized gold alloy particles (b) near-spherical and dendritic shapes water-atomized bronze particles (c) irregular and porous (spongiform) shapes water-atomized zinc particles (d) irregular aggregates water-atomized copper particles (Cour. tesy of Atomizing Systems Ltd., UK.)...

Unreacted alloy particle Partially leached alloy Fully leached alloy... 

Figure 5.1 Schematic representation of the progress of leaching of an alloy particle. A partially leached alloy shows a sharp reaction front and the extent of leaching is given by the depth of the residual catalytic...

As an aside, we should mention that the same principles apply to the formation of bimetallic clusters on a support. In the case of Pt-Re on AI2O3 it has been shown that hydroxylation of the surface favors the ability of Re ions to migrate toward the Pt nuclei and thus the formation of alloy particles, whereas fixing the Re ions onto a dehydroxylated alumina surface creates mainly separated Re particles. As catalytic activity and selectivity of the bimetallic particles differ vastly from those of a physical mixture of monometallic particles, the catalytic performance of the reduced catalyst depends significantly on the protocol used during its formation. The bimetallic Pt-Re catalysts have been identified by comparison with preparations in which gaseous Re carbonyl was decomposed on conventionally prepared Pt/Al203 catalysts. ... 

---