Alloy catalysts surface composition

The ascertaining by classical and modern methods that the surface composition of alloys can strongly deviate from the composition of the bulk. These methods have opened up the possibility of correlating catalytic phenomena with catalyst surface composition, thus removing a major drawback of older studies with alloy catalysts. 

Over the past 35 years, much has been learned about the electrooxidation of methanol on the surface of noble metals and metal alloys, in particular platinum and ruthenium [2, 4, 6, 7]. Significant overpotential losses occur in the reaction due to poisoning of the alloy catalyst surface by carbon monoxide. Yet, Pt-based metal alloys are still the most popular catalyst materials in the development of new fuel cell electrocatalysts, based on the expectation that a more CO-tolerant methanol catalyst will be developed. The vast ternary composition space beyond Pt-Ru catalysts has not been adequately explored. This section demonstrates how the ternary space can be explored using the high-throughput, electrocatalyst workflow described above. 

Cui C, Gan L, Li HH, Yu SH, Heggen M, Strasser P (2012) Octahedral PtNi nanoparticle catalysts exceptional oxygen reduction activity by tuning the alloy particle surface composition. Nano Lett 12 5885-5889... 

With Equation 5.24, we can calculate a useful approximation of the reversible potential on a specific catalyst surface using the adsorption energies of each species involved in the reaction, if we know the reversible potential of a redox reaction in aqueous solution, or we can calculate the shift of the onset reversible potential between two unlike catalyst surface compositions if the absolute reversible potential in aqueous phase for a specific redox reaction is not known. For example, the change in onset reversible potential due to a Pt surface and an alloy will be... 

For the study of the electrocatalytic reduction of oxygen and oxidation of methanol, our approach to the preparation of catalysts by two-phase protocol " provides a better controllability over size, composition or surface properties in comparison with traditional approaches such as coprecipitation, deposition-precipitation, and impregnation. " The electrocatalytic activities were studied in both acidic and alkaline electrolytes. This chapter summarizes some of these recent results, which have provided us with further information for assessing gold-based alloy catalysts for fuel cell reactions. 

It should be noted that, in the interpretation of activity patterns of alloy catalysts, extreme care is needed to ensure that the surface composition is known. It has been shown [321,322] with copper—nickel alloys, which show two phases in the composition range 2—80% copper, that, within this miscibility gap, the surface composition remains constant at 80% Cu—20% Ni, independent of the nominal bulk composition. Furthermore, the surface composition may vary depending upon the catalyst pretreatment [322], No miscibility gap occurs with palladium—gold or palladium-silver alloys [323]. 

Anderson et al. (33) found that the surface composition can be a function of particle size in supported Pt-Cu and Rh-Ag alloys. Bartholomew and Boudart (17) did not find enrichment in highly dispersed Pt-Fe catalysts. 

Other workers (165) used X-ray photoelectron spectroscopy (XPS) to examine the influence of ammonia oxidation on the surface composition of alloy gauzes. After several months on stream, the surface was covered by the same types of highly faceted structures noted by others. As illustrated in Fig. 14, XPS analysis provides evidence that the top microns, and in particular the top 100 A of the surface, were enriched in rhodium. This enrichment was attributed to the preferential volatilization of platinum oxide. The rhodium in the surface layers was present in the oxide form. Other probes confirm the enrichment of the surface in rhodium after ammonia oxidation (166). Rhodium enrichment has been noted by others (164, 167), and it has been postulated that in some cases it leads to catalyst deactivation (168). 

As a whole, considerable experimental data have been accumulated in the literature concerning the influence of the gas phase on the surface state of adsorbents and catalysts. The surface state is used specially to preliminarily produce the required surface composition of a catalyst. At the same time, quantitative characteristics of these changes are mainly available only for binary alloys contacting with molecules of H2, CO, and 02 [41,42]. Investigations conducted in recent years reveal that the influence of adsorbed particles on the state of a solid surface is apparently more significant than is customarily considered at present. The structural transformations in the surface layers (an example is the rearrangement of the surface layer of platinum in the adsorption of CO and 02 molecules [43]) and the processes of formation of new phases in them, which are similar to three-dimensional topochemical processes [44], may be of a major significance. 

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