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 WC1−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 WO2 and reoxidation yielded H2 largely free of CO. The reoxidation product was not WO3 but consisted of various non-stoichiometric oxides with composition WO2+x (x = 0.72, 0.83, etc). The reduction–reoxidation cycle could be repeated many times without loss of hydrogen production efficiency.