Decoding the Reactivity Enhancement of Ln2Ce2O7 Compounds (Ln = Yb, Y, Tb, and Gd) for Soot Combustion: The Remarkable Contribution of Variable Valence A-Sites

The impact of variable valence A-sites on the redox property and reactivity of Ln₂Ce₂O₇ compounds in soot particulate combustion has been investigated. It was observed that Yb₂Ce₂O₇, Y₂Ce₂O₇, and Gd₂Ce₂O₇ formed a rare earth C-type phase, while Tb₂Ce₂O₇ formed a solid solution phase. Both Tb₂Ce₂O₇ and Yb₂Ce₂O₇ possess dual valence state A-sites, resulting in significantly more surface vacancies. Additionally, the advantageous solid solution phase structure of Tb₂Ce₂O₇ leads to even more surface vacancies than Yb₂Ce₂O₇, which is crucial to generate active oxygen sites. Moreover, the introduction of NO into the reaction feed enhances combustion activity by producing active surface monodentate nitrates. A catalyst with higher numbers of surface vacancies exhibits improved NO oxidation ability and better NO₂ utilization efficiency. Consequently, the Tb₂Ce₂O₇ compound demonstrates not only the best soot combustion activity, but also an optimal NO_x-assistance effect. Therefore, it is concluded that variable valence A-site is the intrinsic factor to improve the reactivity of Ln₂Ce₂O₇ catalysts.