Controllable Synthesis of Pure-Phase Rare-Earth Orthoferrites Hollow Spheres with a Porous Shell and Their Catalytic Performance for the CO + NO Reaction

Pure perovskite LnFeO3 (Ln = La, Pr−Tb) hollow spheres with porous shell and solid spheres (Ln = Dy−Yb, Y) have been successfully synthesized via calcination of a Ln-Fe citrate complex precursor, which was prepared via a convenient and effective hydrothermal method. The reaction parameters to obtain the Ln-Fe citrate complex with Ln/Fe = 1:1 can be calculated using a diagram of reaction species distribution in the solution. The calculated value was coincident well with the experimental one. The composition and the thermal decomposition processes of the Ln-Fe citrate complex were investigated carefully, using elemental analysis, inductively coupled plasma analysis (ICP), energy-dispersive spectroscopy (EDS), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD). The formation mechanism of the Ln-Fe citrate complex hollow spheres was proposed as an acidic digestion ripening process. Furthermore, all 13 pure-phase LnFeO3 microspheres had an excellent catalytic performance for the NO + CO reaction. The experimental results showed that the completed conversion of NO and specific selectivity of N2 were achieved, which could be comparable with the noble metal catalysts and better than that of LnFeO3 nanoparticles synthesized via the Pechini process. In addition, the LnFeO3 microspheres exhibited good chemical stability and almost invariable catalytic activity during the lifetime test lasting for 96 h.