Size, pressure, and substrate effects on the oxidation of supported Pt particles

Abstract

Transition metal catalysts supported on reducible oxides are known to be complex and highly variable systems at elevated pressures and temperatures, where they commonly reconfigure into a working state different from the one found at room temperature and ultrahigh vacuum. Pt particles on rutile TiO2 in particular are well-known to change their activity significantly under reducing conditions due to the strong metal-support interaction (SMSI), which induces the formation of an encapsulation layer [1]. The effect of oxidizing treatments remains controversial, partly due to the ‘pressure gap’ between surface science studies and applied catalysis, but also due to the often ill-defined defect density in the support. Importantly for oxidation catalysis, platinum itself can be oxidized, though ordered surface oxides are only stable at elevated pressure [2]. Recent publications have also shown that a ‘non-classical’ encapsulation can occur in oxidizing conditions, where nanoparticles are overgrown by a thicker, TiO2-stoichiometric layer [3], though the driving force for this process remains an open question. The analysis is complicated by the interplay between particle-support interactions and redox processes of the oxide and the individual catalyst particles. In the present work, we employ near-ambient pressure scanning tunneling microscopy (NAP- STM) and X-ray photoelectron spectroscopy (NAP-XPS) to study the evolution of Pt particles supported on rutile TiO2(110) in an oxygen pressure ranging from UHV up to 1 mbar. We find that the oxidation state and stability of the platinum particles depends strongly on their size, the support stoichiometry, and the oxygen pressure. On a reduced substrate, Pt clusters are rapidly buried by new TiO2 layers, which are formed due to substrate reoxidation [4]. In contrast, Pt nanoparticles are somewhat resistant to this burial at high O2 pressure, likely because the Pt itself becomes oxidized. When the oxygen pressure is low and Pt remains metallic, even large nanoparticles rapidly become buried by stoichiometric TiO2 layers, though the ‘classical’ TiOx (x<2) SMSI layer is not modified up to that point. No encapsulation is observed for Pt on near-stoichiometric TiO2 substrates, which allows even small Pt clusters to be oxidized at near-ambient oxygen pressure. Finally, we compare our results to Pt-loaded P25 powder catalysts exposed to the same conditions, and discuss advantages and limitations of single crystalline model systems. [1] S. J. Tauster, Acc. Chem. Res. 20, 389 (1987). [2] M. A. Van Spronsen, J. W. M. Frenken, and I. M. N. Groot, Nat. Commun. 8 (2017). [3] A. Beck, X. Huang, L. Artiglia, M. Zabilskiy, X. Wang, P. Rzepka, D. Palagin, M.-G. Willinger, and J. A. van Bokhoven, Nat. Commun. 11, 3220 (2020). [4] F. Kraushofer, M. Krinninger, S. Kaiser, J. Reich, A. Jarosz, M. Füchsl, G. Anand, F. Esch, and B. A. J. Lechner, Nanoscale 16, 17825 (2024).