Tuning the nature of iridium species supported on gadolinium-doped ceria for accelerating methane steam reforming

• The structure of 0.1 wt% Ir/Ce 0.9 Gd 0.1 O 2-x methane-steam-reforming catalyst was investigated by operando XAS, XPS, and HRTEM. • Pretreating the catalyst at 900 °C in helium flow with only 10 ppm O 2 concentration maximizes the initial activity. • Both the pretreatment and the reaction conditions control the nuclearity and the oxidation state of Ir species. • Partially oxidized Ir clusters are the most active species. Single-cation dispersion or Ir oxide reduction are detrimental to performance. Steam reforming of methane at 650 °C under water-deficient conditions was investigated over Ir/Ce 0.9 Gd 0.1 O 2- x nanocatalysts (noted Ir/CGO hereafter) with very low Ir loading (0.1 wt%). Our previous works have shown the promising performances of Ir/CGO for gradual internal reforming in solid-oxide fuel cells (SOFCs). However, it was observed that the high-temperature (900 °C) conditions of the catalyst pretreatment, as required by SOFC devices, lead to low initial catalytic activity when using a purely neutral gas feed, although a slow activation occurs on stream. Herein, we demonstrate that a pretreatment involving trace amount of O 2 (10 ppm) maximizes the initial activity. In contrast, overoxidation appears detrimental. Systematic investigations by aberration-corrected transmission electron microscopies, X-ray photoelectron spectroscopy and synchrotron X-ray absorption spectroscopy at the Ir L 3 edge, including operando XANES, allow us to explain the enhancement by identifying the key restructuring phenomena and active species. Whereas single Ir cations and metallic Ir nanoparticles are spectator species, oxidic Ir clusters exhibit high activity. Subtle changes in O 2 concentration during the pretreatment dictate the initial nature of the Ir species as well as their evolution on stream through redox and clustering/redispersion phenomena. These results then suggest optimal conditions for the reforming process.