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

Abstract

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.