Interplay between the metal-support interaction and stability in Pt/Co3O4(111) model catalysts

Abstract

The interplay between the metal-support interaction and stability with respect to sintering has been investigated for Pt nanoparticles supported on well-ordered Co₃O₄(111)/Ir(100) films in UHV and under oxidizing conditions by means of synchrotron radiation photoelectron spectroscopy (SRPES) and near ambient pressure X-ray photoelectron spectroscopy (NAP XPS). The electronic metal-support interaction between Pt and Co₃O₄(111) associated with charge transfer results in partial reduction of Co₃O₄(111) yielding partially oxidized Pt^δ+ species at the interface. The stability of the supported Pt particles is coupled with the oxidation state of Pt^δ+ species, which can be reduced or oxidized depending on the Pt coverage and reactive environment. Annealing of Pt/Co₃O₄(111)/Ir(100) in UHV triggers the reduction of Pt^δ+ species. At higher temperature, reverse spillover of oxygen to the Pt nanoparticles is accompanied by reduction of Co₃O₄(111). Under these conditions, the oxidation state of Pt^δ+ species depends strongly on Pt coverage. Thus, at low Pt coverage (0.3 ML Pt), Pt^δ+ is converted to Pt⁴⁺, at intermediate coverage (1.3 ML Pt), Pt^δ+ remains stable, and at high Pt coverage (1.93 ML), Pt^δ+ is reduced to Pt⁰. Sintering of Pt particles is associated with the reduction of the Pt^δ+ species. This process is prevented under oxidizing conditions due to the formation of an interfacial oxide PtO_x. The formation of an interfacial PtO_x is observed at 300 K under exposure to 1 × 10⁻⁶ mbar O₂ at Pt coverages below 1.3 ML. Using NAP XPS, we observe the formation of an interfacial PtO_x at high Pt coverage (2.0 ML) in an oxygen atmosphere (1 mbar) at 300 K while the formation of surface PtO_x is kinetically hindered and occurs above 550 K only.