Spectroscopic and Microscopic Characterization of Iron- and/or Manganese-Promoted Sulfated Zirconia

Abstract

The structures of iron-promoted sulfated zirconia (FSZ), manganese-promoted sulfated zirconia (MSZ) and iron- and manganese-promoted sulfated zirconia (FMSZ) were investigated by ultra-violet-visible (UV-vis) diffuse reflectance, ESR, laser Raman, and X-ray photoelectron (XP) spectroscopies and by transmission electron microscopy with energy dispersive X-ray microanalysis. The bulk structure of tetragonal zirconia and the surface sulfate structure of SZ remained virtually unchanged when the promoters were added to SZ, as shown by laser Raman spectroscopy. The UV-vis spectra show that iron was present in aggregated structures and not as atomically isolated species. The Raman spectra identify these dispersed structures as Fe2O3, and the electron micrographs confirm the presence of particulate structures on the surfaces of FSZ and FMSZ, but not SZ and MSZ. The XP spectra provide confirming evidence of Fe3+, consistent with the presence of Fe2O3, and the ESR spectra show the presence of Fe3+ions in a broad distribution of coordination environments. The evidence of particulate Fe2O3is in agreement with the results of J. E. Tabora and R. J. Davis (J. Chem. Soc. Faraday Trans.91, 1825 (1995)), whose Fe-edge extended X-ray absorption fine structure spectra showed the lack of Fe–Zr and Fe–Fe contributions, consistent with particulate iron oxide clusters and inconsistent with iron in the bulk zirconia. Manganese in MSZ and FMSZ is identified by the ESR spectra as Mn2+, with the resolved hyperfine structure indicating that part of the Mn2+ions were highly dispersed in still unknown locations in FMSZ. The results are not sufficient to establish the nature of the promoter effects or to resolve the nature of the interactions between Mn and Fe indicated by catalyst performance data (F. C. Langeet al., Catal. Lett.41, 95 (1996)), showing that FMSZ is much more active than either FSZ or MSZ for isomerization ofn-butane.