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 LnFeO 3 (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 LnFeO 3 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 N 2 were achieved, which could be comparable with the noble metal catalysts and better than that of LnFeO 3 nanoparticles synthesized via the Pechini process. In addition, the LnFeO 3 microspheres exhibited good chemical stability and almost invariable catalytic activity during the lifetime test lasting for 96 h.