Tuning SMSI Kinetics on Pt-loaded TiO$_2$(110) by Choosing the Pressure: A Combined UHV / Near-Ambient Pressure XPS Study

TL;DR

This study investigates how varying oxygen pressure influences the metal-support interaction and encapsulation behavior of Pt/TiO₂ catalysts using near-ambient pressure XPS, bridging the gap between surface science and practical catalysis.

Contribution

It demonstrates how O₂ pressure controls the SMSI state and encapsulation of Pt on TiO₂, providing insights into catalyst behavior at realistic pressures.

Findings

Encapsulation grows rapidly at low O₂ pressure in the presence of metallic Pt.

Higher O₂ pressures lead to less effective encapsulation and oxidized Pt.

Oxidation and reduction of Pt are reversible, but encapsulation remains once formed.

Abstract

Pt catalyst particles on reducible oxide supports often change their activity significantly at elevated temperatures due to the strong metal-support interaction (SMSI), which induces the formation of an encapsulation layer around the noble metal particles. However, the impact of oxidizing and reducing treatments at elevated pressures on this encapsulation layer remains controversial, partly due to the 'pressure gap' between surface science studies and applied catalysis. In the present work, we employ synchrotron-based near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) to study the effect of O$_2$ and H$_2$ on the SMSI-state of well-defined Pt/TiO$_2$(110) catalysts at pressures of up to 0.1 Torr. By tuning the O$_2$ pressure, we can either selectively oxidize the TiO$_2$ support or both the support and the Pt particles. Catalyzed by metallic Pt, the encapsulating oxide overlayer grows rapidly in 1x10$^{-5}$ Torr O$_2$, but orders of magnitudes less effective at higher O$_2$ pressures, where Pt is in an oxidic state. While the oxidation/reduction of Pt particles is reversible, they remain embedded in the support once encapsulation has occurred.