Fe K‐Edge X‐ray Absorption Fine Structure Determination of γ‐Al₂O₃‐Supported Iron‐Oxide Species

Abstract

The structure of FeOx species supported on γ-Al2 O3 was investigated by using Fe K-edge X-ray absorption fine structure (XAFS) and X-ray diffraction (XRD) measurements. The samples were prepared through the impregnation of iron nitrate on Al2 O3 and co-gelation of aluminum and iron sulfates. The dependence of the XRD patterns on Fe loading revealed the formation of α-Fe2 O3 particles at an Fe loading of above 10 wt %, whereas the formation of iron-oxide crystals was not observed at Fe loadings of less than 9.0 wt %. The Fe K-edge XAFS was characterized by a clear pre-edge peak, which indicated that the FeO coordination structure deviates from central symmetry and that the degree of FeOFe bond formation is significantly lower than that in bulk samples at low Fe loading (<9.0 wt %). Fe K-edge extended XAFS oscillations of the samples with low Fe loadings were explained by assuming an isolated iron-oxide monomer on the γ-Al2 O3 surface.

Sulfur Tolerant Subnanometer Fe/Alumina Catalysts for Propane Dehydrogenation

“…6 For 15 Fe/Al 2 O 3 , a satellite peak at 650 °C was observed which may be due to the formation of a minority 3D-Fe 2 O 3 species due to the higher coverage. 30,50,51 This is consistent with XAS results presented in section 3.3, which indicates some agglomeration at higher Fe loading. From Figure 5b, it is observed that the peak temperature (Tp) of the supported Fe oxide species appears to be relatively independent of Fe loading over Al 2 O 3 .…”

“…Similarly, other studies also suggested that iron can be dispersed between 4 and 5.5 Fe atoms nm −2 over γ-Al 2 O 3 support. 30,51 Accordingly, the XRD and PDF results in this work indicate the possibility of forming a highly dispersed Fe-phase. Notably, temperatures higher than 1000 °C are usually required to form aluminate spinel (FeAlO 3 ) or crystalline hercynite (FeAl 2 O 4 ) from Fe 2 O 3 and Al 2 O 3 .…”

“…This trend has previously been documented in dispersed Fe on Al 2 O 3 and was attributed to the formation of extended iron oxide species, and the current results are consistent with this interpretation. 30 On the basis of the surface area of Al 2 O 3 used in this study, monolayer coverage of iron is expected to be reached at a Fe loading of 6.4 wt %; however, the intensity increase between 7115 and 7118 eV appears to increase proportionally with loading rather than abruptly when Fe loading surpasses monolayer coverage. The gradual trend can be rationalized through the formation of a minority species of crystalline Fe 2 O 3 or 2D/oligomeric iron oxide species, both of which would allow for Fe−Fe orbital hybridization responsible for the intensity increase, consistent with STEM results.…”

“…A previous report on Fe/Al 2 O 3 EXAFS similarly modeled one short and one long Fe−O bond distance and obtained similar Fe−O coordination numbers reported here. 30 For the short Fe−O bond, the coordination number decreased from 3.9 to 3.3 as Fe loading was increased from 1.5 to 6.4%. The long Fe−O bond also showed small variation between the catalysts (0.9−1.3) though the change was not systematic.…”

Effect of FeOx Modification of Al2O3 on Its Supported Au Catalyst for Hydrogenation of 5-Hydroxymethylfurfural

“…FeO x in the above case, forms various structures depending on its loading amount. 28 The structural variation of modifying species is expected to change catalytic activity of its supported metal catalysts. Actually, Tomita et al demonstrated that the variation of FeO x on Al 2 O 3 with the Fe loading significantly affects the catalytic activity of its supported Pt nanoparticles for CO oxidation reaction.…”