A Comprehensive Study on the Phase Structure, Surface Properties, and Active Oxygen Species of A2B2O7 Composite Oxides

A2B2O7 complex oxides have a great potential to be used in high-temperature catalytic processes. Herein, a series of A2B2O7 (A = La, Nd, Sm, Gd, Er, Yb; B = Ti, Sn, Zr, Ce) compounds with all four kinds of typical sub-crystalline phases were synthesized to study their bulk and surface properties. FTIR spectroscopy was adopted as a novel method in this study to identify distinctively these phases. Whereas, it cannot be used to distinguish the subtle structure difference between disordered and ordered pyrochlores, nor that between the disordered defect fluorite and the rare earth. To discriminate these exquisite phase differences, XPS spectra must be supplementarily used. Specifically, it was discovered that the coordination numbers of the A- and B-site cations are the key factor affecting their binding energies. Furthermore, the electronegativity of the A- and B-site elements significantly influences the binding energy of surface lattice oxygen, reflecting their electrophilic and nucleophilic properties, which can thus be used to effectively identify the sub-crystalline phase. The oxygen vacancy concentration of different sub-crystalline phases is the primary factor controlling the amount of surface chemisorbed oxygen species on A2B2O7 compounds, with superoxide anions (O2−) identified as the major species.