Platinum‐Iron(II) Oxide Sites Directly Responsible for Preferential Carbon Monoxide Oxidation at Ambient Temperature: An Operando X‐ray Absorption Spectroscopy Study**

Abstract

Operando X‐ray absorption spectroscopy identified that the concentration of Fe2+ species in the working state‐of‐the‐art Pt−FeOx catalysts quantitatively correlates to their preferential carbon monoxide oxidation steady‐state reaction rate at ambient temperature. Deactivation of such catalysts with time on stream originates from irreversible oxidation of active Fe2+ sites. The active Fe2+ species are presumably Fe+2O−2 clusters in contact with platinum nanoparticles; they coexist with spectator trivalent oxidic iron (Fe3+) and metallic iron (Fe0) partially alloyed with platinum. The concentration of active sites and, therefore, the catalyst activity strongly depends on the pretreatment conditions. Fe2+ is the resting state of the active sites in the preferential carbon monoxide oxidation cycle.

Highly Active MnCoOx Catalyst toward CO Preferential Oxidation

“…Furthermore, in order to explore the change of coordination environment and electronic structure of the active sites, in situ XAFS (Figures d–f and S22–S25) measurements were carried out in various atmospheres (CO, O 2 , or H 2 ) at 80 and 150 °C, respectively, which has been widely used to explore the evolution of catalyst structure under different conditions in previous studies. ,, According to the reported absolute octahedral site preference energies in the spinel Co 2 MnO 4 structure: ,, Co 2+ (31.0 kJ mol –1 ) < Co 3+ (79.5 kJ mol –1 ) < Mn 3+ /Mn 4+ (95.2 kJ mol –1 ), and Mn 3+ /Mn 4+ and Co 3+ tend to occupy the octahedral sites whereas Co 2+ preferentially occupies the tetrahedral sites. For the spinel-like MnCoO x -T samples in this work, the Fourier-transform Co K-edge EXAFS spectra (Figure S26) display that the Co–O bond (either at the tetrahedral or octahedral site) is located at ∼1.5 Å; the characteristic octahedra (Co oct –Co/Mn oct ) and tetrahedra (Co tet –Co/Mn oct ) scattering paths (O-bridged), in which the metallic ion is stabilized by oxygen ligands in geometry configuration, are detected at ∼2.5 and ∼3 Å, respectively. ,, After in situ preoxidation of the MnCoO x -300 catalyst at 300 °C followed by He purging, the normalized Co K-edge XANES spectrum was obtained at 80 and 150 °C (Figures d and S23a, black line).…”