Neutron powder diffraction study of sulfated zirconia catalysts

摘要

In situ neutron powder diffraction method was used to investigate the crystal structures, phase abundance and thermal stability of sulfated zirconia catalysts prepared by impregnation of hydrous zirconium oxide gels with 0.5 M H2SO4 solutions. The sample studied was precipitated at pH = 10 and dried for 5 h at 493 K, and then calcined at 853 K for 3 h. Diffraction data were collected in the temperature range 295-1273 K in an Ar atmosphere and analyzed using the Rietveld method. Only the metastable tetragonal phase was observed below 673 K. Above this temperature, the sample consisted of a mixture of tetragonal (T) and monoclinic (M) phases in the proportions (T:M wt%) 85:15 (1073 K) and 61:39 (1273 K). Surface modification by sulfation was found to retard the onset of the tetragonal-to-monoclinic transformation relative to pure zirconia. The decrease in peak-broadening at the higher temperatures reflects both an increase in crystallite size and a decrease in microstrain.