Oxidation State of Ce in CeO₂‑Promoted Rh/Al₂O₃ Catalysts during Methane Steam Reforming: H₂O Activation and Alumina Stabilization

The cerium oxidation state in ceria-doped Rh/Al₂O₃ catalysts during the methane steam reforming (MSR) reaction was determined by <i>in situ</i> X-ray absorption spectroscopy and online mass spectrometry at 773 K. The catalysts were characterized by electron microscopy and X-ray diffraction. The oxidation states of rhodium and cerium during MSR are related. A total of 25% of Ce⁴⁺ reduces to Ce³⁺ under a CH₄/H₂O/He flow when the catalyst is active in MSR (773 K). A slight reoxidation occurs when the catalyst is sequentially exposed to steam diluted in helium at high temperatures, showing that only a small fraction of CeO_2–<i>x</i> may be involved in the water activation step by reoxidation. The main role of ceria during reaction, besides the activation of water, is to stabilize the structure of alumina by forming CeAlO₃ and to maintain the dispersion of rhodium