Solid-State Kinetic Investigations of Nonisothermal Reduction of Iron Species Supported on SBA-15

Abstract

Iron oxide catalysts supported on nanostructured silica SBA-15 were synthesized with various iron loadings using two different precursors. Structural characterization of the as-prepared Fe O /SBA-15 samples was performed by nitrogen physisorption, X-ray diffraction, DR-UV-Vis spectroscopy, and Mössbauer spectroscopy. An increasing size of the resulting iron species correlated with an increasing iron loading. Significantly smaller iron species were obtained from (Fe(III), NH 4 )-citrate precursors compared to Fe(III)-nitrate precursors. Moreover, smaller iron species resulted in a smoother surface of the support material. Temperatureprogrammed reduction (TPR) of the Fe O /SBA-15 samples with H 2 revealed better reducibility of the samples originating from Fe(III)-nitrate precursors. Varying the iron loading led to a change in reduction mechanism. TPR traces were analyzed by modelindependent Kissinger method, Ozawa, Flynn, and Wall (OFW) method, and model-dependent Coats-Redfern method. JMAK kinetic analysis afforded a one-dimensional reduction process for the Fe O /SBA-15 samples. The Kissinger method yielded the lowest apparent activation energy for the lowest loaded citrate sample ( ≈ 39 kJ/mol). Conversely, the lowest loaded nitrate sample possessed the highest apparent activation energy ( ≈ 88 kJ/mol). For samples obtained from Fe(III)-nitrate precursors, decreased with increasing iron loading. Apparent activation energies from model-independent analysis methods agreed well with those from model-dependent methods. Nucleation as rate-determining step in the reduction of the iron oxide species was consistent with the Mampel solid-state reaction model.

Influence of Adding Molybdenum on Structure and Performance of FexOy/SBA‐15 Catalysts in Selective Oxidation of Propene

“…These smaller iron oxidic species with strong interactions to the surface of SBA‐15 possessed a lower reducibility. Decreased reducibility with decreased iron oxidic species size was already observed for Fe x O y /SBA‐15 samples obtained from nitrate precursor . Moreover, adding molybdenum and thereby formed Fe−O−Mo structure units may further inhibit reducibility of the Fe x O y system because of a charge transfer between iron and molybdenum species.…”

“…Significant differences in TPR traces were discernible. 10.7 wt% Fe_SBA‐15 showed a two‐step reduction mechanism where the first and second reduction step were assigned to reduction of Fe III oxidic species to Fe II oxidic species and further reduction to Fe 0 , respectively . Adding molybdenum at atomic ratios between 0.07Mo/1.0Fe and 0.21Mo/1.0Fe induced a rising shoulder at the first TPR maxima at higher temperatures while the TPR maxima were shifted to higher temperatures.…”

“…Herein, the fractal dimension D f was determined as a measure of the roughness of the surface . For Mo x O y _Fe x O y /SBA‐15 samples, 1.3 wt% Mo_SBA‐15, 10.7 wt% Fe_SBA‐15, and SBA‐15, the fractal dimension ranged between 2 and 3, indicating a rough surface . In order to elucidate the effect of supported iron and molybdenum species on surface roughness of the support material, ΔD f values were calculated as difference between D f values of SBA‐15 and those of corresponding Mo x O y _Fe x O y /SBA‐15 samples.…”

“…[22,24] For Mo x O y _Fe x O y /SBA-15 samples, 1.3 wt% Mo_SBA-15, 10.7 wt% Fe_SBA-15, and SBA-15, the fractal dimension ranged between 2 and 3, indicating a rough surface. [25] In order to elucidate the effect of supported iron and molybdenum species on surface roughness of the support material, ΔD f values were calculated as difference between The decreased contribution of micropores to the entire surface area of SBA-15 for this sample was in accordance with the smoother surface. Depositing iron and molybdenum oxides in the pores of SBA-15 at higher Mo/Fe atomic ratio resulted in an enhanced micropore filling, accompanied by an enhanced smoothing of the support surface.…”

“…Decreased reducibility with decreased iron oxidic species size was already observed for Fe x O y /SBA-15 samples obtained from nitrate precursor. [25] Moreover, adding molybdenum and thereby formed FeÀ OÀ Mo structure units may further inhibit reducibility of the Fe x O y system because of a charge transfer between iron and molybdenum species. A postulated electron transfer from iron to oxygen, and further to molybdenum [33] yielded a strengthened FeÀ O bond, and thus, hindered the reducibility.…”