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

Steam reforming of methane at 650 degrees C under water-deficient conditions was investigated over Ir/Ce0.9Gd0.1O2-x nanocatalysts (noted Ir/CGO hereafter) with very low Ir loading (0.1 wt%). Our previous works showed 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 degrees 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 O2 (10 ppm) maximizes the initial activity. In contrast, overoxidation appears detrimental. Systematic investigations by aberrationcorrected transmission electron microscopies, X-ray photoelectron spectroscopy and synchrotron X-ray absorption spectroscopy at the Ir L3 edge, including operando XANES, allow us to explain the enhancement by identifying the key restructuring phenomena and the active species. Whereas single Ir cations and metallic Ir nanoparticles are spectator species, oxidic Ir clusters exhibit high activity. Subtle changes in O2 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.