Mixed oxide and its use as NOx adsorber

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A mixed oxide composition comprises manganese, a rare earth, and aluminium and/or magnesium. The atomic ratio of manganese to aluminium and magnesium ranges from about 100 : 10 to about 100 : 70 and that of manganese to rare earth ranges from about 100 : 8 to about 100 : 80. The rare earth may be Ce, Y or, particularly, La. The composition may have a crystallinity of less than about 80% determined by XRD. A method of making the mixed oxide may comprise mixing manganese source, rare earth source, and aluminium and/or magnesium source to form an aqueous solution in the required ratios, followed by mixing with an alkali solution to form a slurry and isolating the solid. An NOx adsorber may comprise the mixed oxide. The nitrogen oxides adsorber may be used in an exhaust system for internal combustion engines.