Synthesis of Alloy versus Two-phase Catalysts

A challenge for the preparation of FC catalysts is the formation of catalysts that consist of surfaces made, at the atomic level, of homogeneously distributed catalyst components. Pt forms bulk alloys of a wide range of concentrations with a number 

The two-step synthesis (subsequent reduction of the precursor salts) is more likely to result in phase-separated catalysts, possibly core-shell catalysts. Typically a colloid of a particular metal is first formed then subsequently reacted with the salt of the second metal. Numerous examples of such reactions exist in the literature [95, 96]. The deposition of the seeond metal onto the preformed colloid can be driven by a redox reaction if the seeond metal is more noble than the metal of flic preformed colloid, i.e., depending on the difference in the free energy (AG), which is determined by the difference in the equilibrium potentials, Ei and E2 of the redox reactions of the two metal/salts, as follows [97]  

In Equation 9.13, Zi, and Z2 are the number of electrons involved for redox Reaction 9.1 and 9.2, respectively, and F is Faraday s constant. If AG is negative then the oxidized form, Ox], will be reduced by the reductant, Red2. As an example, Pt can be readily deposited onto Cu surfaces and onto Au [97]. Deposition of Pt onto pre-reduced Ru surfaces has also been carried out using flic same principles. Similarly, Ru has been deposited onto Cu surfaces, but to a smaller extent than Pt and Au. This is in fact seen experimentally, by preferential Ru deposition onto the rims of copper particles [97]. The equilibrium potentials, Ei and E2, are given by the metal and metal-salt involved, as defined by Nemst s law. It follows that the anion of the metal salt influences the E value and hence, AG. Through the selection of proper complexing ions it is possible to reverse the host and depositing metals. This has been shown for Pd deposition onto Rh surfaces 

It should be noted that deposition driven by differences in redox potentials results in the partial dissolution of the metal from the pre-formed colloid/particle. The re-dissolved metal salt can be rapidly reduced and re-deposited onto the existing colloid if the reaction is carried out in a reducing environment (as is the case in, e.g., colloidal 2-step synthesis). In such a case, flic shell is likely to consist of a mixture of the two metals rather than a pure metal phase. 

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