Unraveling the Principles of Lattice Disorder Degree of Bi2B2O7 (B = Sn, Ti, Zr) Compounds on Activating Gas Phase O2 for Soot Combustion

With the combination of experimental methods and DFT calculations, the influence of lattice disorder degree on the catalytic property of Bi2B2O7 compounds with different B-site cations (B = Sn, Ti, Zr) has been elucidated in this work. The crystal structures of the samples vary from a well-ordered pyrochlore (Bi2Sn2O7) to a less-ordered pyrochlore (Bi2Ti2O7), and eventually to a disordered defect fluorite (Bi2Zr2O7) phase, hence the lattice disorder degree increases. DFT calculation and oxygen ion conductivity results have testified that the surface oxygen vacancy formation energies (EO-f) of the catalysts decrease with the increase of the disorder degree, accompanying the improvement of the lattice ion mobility and the formation of more surface oxygen vacancies. Isotopic O-18(2) tracing experiments have demonstrated that the adsorbing and activating of gas phase O-2 molecules mainly follow an R-2 mechanism requiring two adjacent surface vacancies. With the synergistic actions of surface oxygen vacancies and lattice O2- anions, both active O-2(-) and O-2(2-) sites contributing vitally to soot combustion can be formed. Therefore, the abundance of surface oxygen vacancies is of great importance for this process, which is intimately related to the lattice disorder degree of the compounds. From this point of view, it is concluded that the lattice disorder degree is the inherent factor to determine the redox property and reaction performance of Bi2B2O7 compounds for soot combustion.