Niobium oxide promoted with alkali metal nitrates for soot particulate combustion: elucidating the vital role of active surface nitrate groups.

With the target of developing efficient base metal oxide catalysts for soot particulate combustion, Nb 2 O 5 catalysts promoted using different alkali metal nitrates have been prepared via an impregnation method. The activity of all the modified catalysts is better than that of the pure Nb 2 O 5 , and follows the sequence of CsNb1-9 > KNb1-9 > NaNb1-9 > LiNb1-9 > Nb 2 O 5 . It has been discovered that the original LiNO 3 and NaNO 3 precursors were decomposed into inert Li 2 O and Na 2 O on LiNb1-9 and NaNb1-9 during the calcination process. However, the KNO 3 and CsNO 3 precursors were intact on KNb1-9 and CsNb1-9 due to the strong stabilization effect of the K + and Cs + cations. As confirmed using different means, surface nitrates are the predominant active centers that contribute to the soot oxidation activity, through the redox cycles between nitrate (NO 3 - ) and nitrite (NO 2 - ) groups. Due to the existence of a large quantity of active surface NO 3 - groups, KNb1-9 and CsNb1-9 thus exhibit a much better reaction performance than LiNb1-9 and NaNb1-9.