Evolution of Size-Selected Pt Cluster Catalysts on Prototypical Oxide Supports

Abstract

The current quest for new pathways into sustainable, efficient and durable energy conversion technologies makes the need for a fundamental understanding of the atomic-scale phenomena underlying catalytic processes ever more pressing. In this context, characterizing catalyst particles in situ provides valuable information about the evolution of their composition, structure, oxidation state and charge state during an ongoing process. To disentangle the influence of individual parameters – temperature, pressure, gas composition, cluster size, as well as support acidity, redox state and defect density –, it is crucial to control them precisely and separately in experiments. At the example of size-selected Pt_n clusters – i.e. sub-nm particles defined to the exact number of atoms – on flat oxide supports, we follow their rich evolution phenomena via (synchrotron-based) X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM) during temperature ramps and in various gas environments. Here, we present our experience with these highly defined, yet complicated-to-create samples in ultra-high vacuum (UHV) and at mbar pressures. We discuss their stability during transport to synchrotrons and under reaction conditions on three prototypical oxide supports and show the various phenomena that can be disentangled. Pt_n clusters on the non-reducible silicon dioxide, SiO₂, remain size-selected and show remarkable stability. Electronic doping strongly influences the size-dependent binding energy shifts and changes the Pt response to oxidative and reaction conditions, which we attribute to different cluster geometries: p-doping leads to wetting, enhanced sinter resistance and a diminished response to oxidative environments, compared to clusters on n-doped samples. We compare the system with our previous findings of a similar change in dimensionality for Pt₂₀ clusters on the reducible ceria CeO₂(111) support, induced by a modulation of its O vacancy density. The Pt_n/CeO₂ system is particularly interesting for strategies to stabilize and redisperse Pt clusters dynamically. Finally, we study the evolution of Pt_n clusters on another reducible oxide support, magnetite Fe₃O₄(001), in 0.1 mbar alternating redox conditions at RT and elevated temperature. In analogy to findings previously reported for Pt/TiO₂(110), the clusters either become encapsulated by a thin oxide film via strong metal-support interaction (SMSI) or deeply buried in the magnetite. Overall, our approach of following the evolution of size-selected clusters on oxide supports leads to fundamental atomic-scale insights on nano-scale catalyst materials, on our path to sustainable, dynamic and self-repairing catalysts.