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

Abstract EN

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 FexOy/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), NH4)-citrate precursors compared to Fe(III)-nitrate precursors.

Moreover, smaller iron species resulted in a smoother surface of the support material.

Temperature-programmed reduction (TPR) of the FexOy/SBA-15 samples with H2 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 model-independent 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 FexOy/SBA-15 samples.

The Kissinger method yielded the lowest apparent activation energy for the lowest loaded citrate sample (Ea ≈ 39 kJ/mol).

Conversely, the lowest loaded nitrate sample possessed the highest apparent activation energy (Ea ≈ 88 kJ/mol).

For samples obtained from Fe(III)-nitrate precursors, Ea 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.