Substoichiometric ultrathin zirconia films cause strong metal-support interaction

TL;DR

This study demonstrates that ultrathin substoichiometric zirconia films can induce strong metal-support interactions on various metal supports, challenging the assumption that zirconia's resistance to reduction prevents SMSI effects.

Contribution

It reveals that zirconia, typically considered non-reducible, can form ultrathin suboxide films that cause SMSI, expanding understanding of oxide-support interactions in catalysis.

Findings

Ultrathin zirconia films form on Rh(111), Pt(111), and Ru(0001) after annealing in UHV.

Zirconia remains in Zr4+ state with electrons transferred to metal supports.

The ultrathin zirconia films are reversible via oxygen annealing.

Abstract

The strong metal-support interaction (SMSI) leads to substantial changes of the properties of an oxide-supported catalyst after annealing under reducing conditions. The common explanation is the formation of heavily reduced, ultrathin oxide films covering metal particles. This is typically encountered for reducible oxides such as TiO2 or Fe3O4. Zirconia (ZrO2), a typical catalyst support, is difficult to reduce and therefore no obvious candidate for the SMSI effect. In this work, we use inverse model systems with Rh(111), Pt(111), and Ru(0001) as supports. Contrary to expectations, we show that SMSI is encountered for zirconia. Upon annealing in ultra-high vacuum, oxygen-deficient ultrathin zirconia films ($\approx$ZrO$_{1.5}$) form on all three substrates. However, Zr remains in its preferred charge state of 4+, as electrons are transferred to the underlying metal. At high temperatures, the stability of the ultrathin zirconia films depends on whether alloying of Zr and the substrate metal occurs. The SMSI effect is reversible; the ultrathin suboxide films can be removed by annealing in oxygen.