Ceria-zirconia stabilised tungsten oxides for the production of hydrogen by the methane-water redox cycle

The generation of essentially pure hydrogen through a redox cycle using methane and then water has been investigated for a series of tungsten oxides stabilised by ceria-zirconia. The calcined starting materials were largely monoclinic WO₃ with CeO₂ and ZrO₂ undetectable by XRD. Samples containing 80% and especially 69% WO₃ showed additional XRD lines due to a phase of unknown composition. Temperature programmed reduction to 750 °C in methane converted samples containing WO₃ alone to WC together with small amounts of tungsten metal and a WC_1-x phase. Reoxidation in water at the same temperature then produced CO and H₂ in corresponding amounts. Under the same conditions the 69% WO₃ sample was reduced only as far as WO ₂ and reoxidation yielded H₂ largely free of CO. The reoxidation product was not WO₃ but consisted of various non-stoichiometric oxides with composition WO_2+x (x = 0.72, 0.83, etc). The reduction-reoxidation cycle could be repeated many times without loss of hydrogen production efficiency.