Al 2 O 3 catalysts has been investigated by IR spectroscopy and temperature-programmed desorption. Upon NO interaction, small amounts of nitrites, nitrates, and hyponitrite species were formed on the Ba-containing samples. The NO x storage capacity of the catalysts was highly enhanced upon adsorption of NO/O 2 mixtures and further upon NO 2 admission. Upon adsorption of NO/O 2 on Pt/Al 2 O 3 sample nitrites, nitrates and NO 2 δ+ species were mainly formed, showing a moderate thermal stability. Barium markedly increased the amount and stability of the stored NO x species, which were bidentate and monodentate nitrites and, in minor amounts, nitrates. Nitrites were removed below 750 K and/or transformed into ionic Ba nitrates, stable up to 800-900 K. Upon NO 2 adsorption, huge amounts of nitrates, but no nitrites, were formed on all the samples. Also in this case, Ba increased the amount and stability of the stored NO x species. The nature and the amounts of the stored NO x species formed upon adsorption of NO, NO/O 2 , or NO 2 were similar on Ba/Al 2 O 3 and Pt-Ba/Al 2 O 3 catalysts, whereas Pt slightly decreased their thermal stability. Bulky Ba nitrate was formed during the thermal desorption of NO 2 (and to a less extent of NO/O 2 ), inducing an extensive decomposition of the Ba carbonate or oxycarbonate phase which was detected on the calcined samples.

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