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Cu-Zn hydroxycarbonates have been studied as precursors of Cu-ZnO catalysts, with particular reference to the effect on catalyst activity of ageing the precursor prior to decomposition and reduction. The precursor obtained by precipitation from mixed nitrate solution ( C u / Z n molar ratio 2 : 1 ) at 333 K and pH 7.0 consisted of zincian malachite ( C u / Z n = 8.5: 1.5) and aurichalcite. The precursor was aged in the mother liquor at 333 K for various times. Characterisation by XRD, i.r., DTA, electron microscopy, EDAX and XPS showed that ageing led to loss of the aurichalcite and production of a more finely divided copper-enriched ( C u / Z n = 2 : 1) malachite phase. The unaged precursor yielded a catalyst of low activity for both methanol synthesis (studied at 50 bar and at 1 bar) and the reverse water-gas shift reaction. The aged precursor gave catalysts of much higher activity for both reactions. Increased ageing did not change the selectivity ratio for methanol synthesis cs. reverse shift in the CO,+ H, reaction at normal pressure.

The mixed ionic-electronic conducting perovskites (ABO 3 ) have attracted considerable attention for application as oxygen-permeable membranes in pressure-driven air separators, due to their superior oxide ion and electronic conductivities. Some of the highest oxygen permeabilities have been obtained for the strontium-doped lanthanum cobaltate series La 1Ϫx Sr x CoO 3Ϫ␦ . However compositions with a high Sr content exhibit poor thermomechanical behavior and are not stable under low partial pressures of oxygen, and are therefore unsuitable for technical applications. Doping of the B site of the perovskite with iron (La 1Ϫx Sr x Co 1Ϫy Fe y O 3Ϫ␦ ) has been found to produce a much more stable ceramic with only slightly reduced ionic conductivity relative to that of the lanthanum strontium cobaltate, 1 and it is these LSCF materials which appear to be the most promising candidates as air-separation membranes.Obviously one of the most important factors in the utilization of these materials is the long-term stability of the ceramic under the operating conditions of the system. In practice, such an oxygen-separation unit may be operating at 750ЊC or higher, under an aggressive atmosphere of H 2 O/CO 2 /O 2 at the feed side of the membrane, and be subject to a considerable pressure gradient from the high to the low oxygen-partial pressure side. 2 Studies on the related compound Sr(Fe,Co)O x 3 suggest the operating environment can result in both chemical degradation of the material and in mechanical fracture of the membrane. Kim et al. 4 recently reported a reduction in oxygen permeation flux with time for La 0.2 Sr 0.8 Cu 0.4 Co 0.6 O 3Ϫ␦ , due to both chemical degradation at the reduced permeate side of the membrane and a vacancy ordering effect at low oxygen partial pressures, resulting in a transformation from the disordered perovskite to a structure related to Brownmillerite.The present study has been conducted with a view to investigating first the degrading effects of H 2 O/CO 2 /O 2 atmospheres on the surface and bulk compositions of the ceramic, and second, to ascertain the effect of the presence of CO 2 and H 2 O on the oxygen surface-exchange process. ExperimentalDegradation of LSCF materials: preparation of samples.-Samples of LSCF were prepared from commercially obtained LSCF powder (Praxair Speciality Chemicals, Seattle, WA, USA). Dense pellets were obtained by first ballmilling the powder overnight in ethanol, followed by pressing into 13 mm pellets at 1 ton using a uniaxial press, and cold isostatic pressing at 300 MPa. The green pellets were sintered at 1200ЊC for 8 h. Only samples with densities in excess of 95% of the theoretical value were suitable for secondary ion mass spectroscopy (SIMS) analysis. Surface roughness was removed by grinding the surfaces using silicon carbide paper, followed by polishing to a 1/4 m finish using successive grades of diamond polishing compound. The polished samples were then preannealed in oxygen atmospheres at the appropriate temperature and oxygen partial pressure co...

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