The effect of temperature on a ceria-alumina-baria-cordierite monolith combination under oxidising and reducing conditions

摘要

XRD and surface area determinations have been carried out on samples of a commercial ceria-baria-alumina on cordierite monolith heated in air or H_2/He at temperatures from 500 to 1200deg C.Below 700deg C the fall in surface area is slightly greater in the reducing atmosphere but stability is better above 1000deg C.Sintering in air proceeds with continuous growth of CeO_2 particles and the development of #alpha#-Al_2O_3 above 1000deg C.Traces of a previously unreported phase are evident after 24h above 1100deg C.CeO_2 is barely detectable after treatment in H_2/He and less #alpha#-Al_2O_3 is formed.Instead the predominant crystalline phase (apart from cordierite) is CeAlO_3 at temperatures to 1100deg C while the new phase forms at 1200deg C in considerably greater amounts than under oxidising conditions.Sintering for different periods of time at 1200deg C shows that the new phase arises with consumption of #alpha#-Al_2O_3 and CeAlO_3.It is fluorescent in ultraviolet light and can be separatedas a heavy fraction by gravimetric settling.The XRD pattern closely resembles that of a known cerium terbium magnesium hexaaluminate phosphor and electron microprobe analysis suggests an approximate comosition of Ce(Ba_(1-x)Mg_x)Al_(11)O_(19)(x approx 0.87).The presence of magnesium shows that its formation involves mingling of cordierite and washcoat components in a process driven by the conversion of Ce~(4+) to Ce~(3+) in a reducing atmosphere.The new hexaaluminate phase was observable by XRD in four out of 19 three-way converters recovered from used vehicles.