Nanoparticle exsolution - the growth of metallic nanoparticles directly from an oxide support in which host cations have been substituted to some degree by cations of the active component(s) – has demonstrated great promise as a method of active catalyst dispersal in the preparation of heterogeneous catalyst materials, producing fine, well-distributed nanoparticle catalysts that are anchored in their oxide host, endowing them with excellent stability against deactivation. The properties of these nano-catalysts, such as their morphology, size and composition, demonstrate a strong dependence on the conditions imposed to drive exsolution and the composition of the oxide host, with both providing the opportunity to precisely tailor the properties of these exsolved heterogeneous nanoparticle catalysts to a given application. In the case of systems substituted with multiple active components, careful control of their relative proportions and the conditions imposed to drive exsolution could yield alloy nano-catalysts with enhanced activity, selectivity and stability relative to their pure metallic counterparts; however, we must develop our understanding of the processing-composition-property relationship for this to be achieved in practise.

In this work, we take a systematic approach to studying the exsolution behaviour of ruthenium, iron and their bimetallic alloy from defect fluorite-type yttrium zirconate – a host structure type that has not been extensively employed in exsolution studies, but which presents a particularly interesting alternative host structure for exsolution, as defect fluorites exhibit a high intrinsic concentration of oxygen vacancies, which are well established to play an important role in exsolution. We combine both in situ and ex situ X-ray photoelectron spectroscopy to probe how the electronic structure evolves as the reducing conditions are varied during exsolution, gaining valuable insight into the sequence of chemical state changes that take place in the initial stages of exsolution, and how the distribution of these states – and consequently, the extent of exsolution - depends on the conditions imposed during reduction. High-resolution TEM and STEM-EDX measurements reveal significant differences in the size and dispersion of exsolved ruthenium and iron, and provide unique insight into the influence of alloy composition on particle size and dispersion in the ruthenium-iron binary alloy system, with exsolved RuFe alloy nanoparticles demonstrating a significantly decreased particle size and increased dispersion relative to that of exsolved iron. Through developing our understanding of the independent exsolution behaviour of ruthenium and iron, and how this compares with the exsolution behaviour of their alloy, these findings move us a step closer towards the rational design of exsolved bimetallic ruthenium-iron nanoparticle catalysts that perform better than the sum of their parts.