Design of non-noble metal A2B2O7 compounds to catalyze soot particulate combustion: Deciphering the inherent factors contributing to the excellent activity with the integration of experiments and DFT calculations

Abstract

A₂B₂O₇ is a class of novel and promising catalytic materials, but there is still a lack of in-depth understanding regarding their catalytic chemistry, which deserves to be further explored. To design efficient and non-noble metal catalysts for soot particulate combustion, four three-dimensionally ordered macro-porous (3DOM) A₂B₂O₇ (A = La, Bi; B = Ce, Zr) complex oxides with a fluorite phase are designed and studied by the integration of experimental tools and DFT calculations. The combination of reducible A- and B- elements promotes greatly the activity due to the presence of dual redox cycles. Furthermore, a catalyst with a weaker average M−O bond strength and larger unit free cell volume (V_f) forms richer surface defects/vacancies. Isotopic ¹⁸O₂ tracing experiments have proven that the chemisorbed oxygen species are generated predominantly via an R² mechanism on a more defective compound, and its lattice O^2– becomes more mobile, which is favorable to the activity. The more defective catalyst has also a better NO_x-assistance effect due to the improved NO₂ utilization efficiency. Bi₂Ce₂O₇ compound possesses dual Bi³⁺/Bi⁰ and Ce³⁺/Ce⁴⁺ redox cycles and rich surface defects, thus showing excellent soot combustion activity competitive to noble metal catalysts.