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, Nb2O5 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 Nb2O5, and follows the sequence of CsNb1-9 > KNb1-9 > NaNb1-9 > LiNb1-9 > Nb2O5. It has been discovered that the original LiNO3 and NaNO3 precursors were decomposed into inert Li2O and Na2O on LiNb1-9 and NaNb1-9 during the calcination process. However, the KNO3 and CsNO3 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 (NO3-) and nitrite (NO2-) groups. Due to the existence of a large quantity of active surface NO3- groups, KNb1-9 and CsNb1-9 thus exhibit a much better reaction performance than LiNb1-9 and NaNb1-9.