<div class="htmlview paragraph">Selective NOx recirculation (SNR), involving adsorption, selective external recirculation and decomposition of the NOx by the combustion process, is itself a promising technique to abate NOx emissions. Three types of materials containing Ba: barium aluminate, barium tin perovskite and barium Y-zeolites have been developed to adsorb NOx under lean-burn or Diesel conditions, with or without the presence of S0₂. All these materials adsorb NO₂ selectively (lean-burn conditions), and store it as nitrate/nitrite species. The desorption takes place by decomposition of these species at higher temperatures. Nitrate formation implies also sulfate formation in the presence of SO2 and SO₃, while the NO₂/SO₂ competition governs the poisoning of such catalysts.</div>