Redox-mediated conversion of atomically dispersed platinum to sub-nanometer particles

Abstract

The stability and the conversion of atomically dispersed Pt²⁺ species to sub-nanometer Pt particles have been investigated as a function of the Sn concentration in Pt–CeO₂ films by means of synchrotron radiation photoelectron spectroscopy, resonant photoemission spectroscopy, and angle-resolved X-ray photoelectron spectroscopy in combination with density functional calculations. The deposition of Sn onto the Pt–CeO₂ films triggers the reduction of Ce⁴⁺ cations to Ce³⁺ yielding Sn²⁺ cations. Consecutively, the redox coupling between the Ce³⁺ and Pt²⁺ species triggers the reduction of Pt²⁺ species yielding sub-nanometer Pt particles. The onset of reduction of Pt²⁺ species is directly related to the concentration of Ce³⁺ centers which, in turn, is controlled by the concentration of Sn²⁺ cations in the Pt–CeO₂ film. On average, the formation of 6Ce³⁺ centers corresponding to the adsorption of 3Sn atoms gives rise to the reduction of one Pt²⁺ species. The analysis of the depth distribution of Sn atoms in the Pt–CeO₂ films revealed preferential adsorption of Sn²⁺ at the surface followed by diffusion of Sn²⁺ ions into the bulk at higher Sn coverages. Density functional modeling suggested that the adsorption of three Sn atoms in the vicinity of the Pt²⁺ species results in a rearrangement of the local coordination accompanied by substantial destabilization of the Pt²⁺ species followed by its conversion to Pt⁰ atoms. The formation of sub-nanometer Pt particles is coupled with re-oxidation of two Ce³⁺ centers per one Pt²⁺ species reduced. Annealing of the Pt–CeO₂ films in the presence of metallic Sn also leads to the reduction of the Pt²⁺ species due to thermally triggered oxidation of metallic Sn residues followed by diffusion of Sn²⁺ into the bulk. Annealing of the Pt–CeO₂ films to temperatures above 600 K results in a loss of Sn yielding sub-nanometer Pt particles supported on nearly stoichiometric and Sn-free CeO₂ films.