Oxidation State of Ce in CeO<sub>2</sub>‑Promoted
Rh/Al<sub>2</sub>O<sub>3</sub> Catalysts during Methane Steam Reforming:
H<sub>2</sub>O Activation and Alumina Stabilization
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Abstract
The cerium oxidation
state in ceria-doped Rh/Al<sub>2</sub>O<sub>3</sub> 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 Ce<sup>4+</sup> reduces to Ce<sup>3+</sup> under a CH<sub>4</sub>/H<sub>2</sub>O/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<sub>2–<i>x</i></sub> 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 CeAlO<sub>3</sub> and to maintain
the dispersion of rhodium