The Influence of Cu and Al Additives on Reduction of Iron(III) Oxide: In Situ XRD and XANES Study

Abstract

The reduction of Fe-based nanocomposite catalysts doped with Al and Cu has been studied using in situ X-ray diffraction (XRD), in situ X-ray absorption near-edge structure (XANES), and temperature-programmed reduction (TPR) techniques. The catalysts have been synthesized by melting of iron, aluminum, and copper salts. According to XRD, the catalysts consist mainly of Fe2O3 and Al2O3 phases. Alumina is in an amorphous state, whereas iron oxide forms nanoparticles with the protohematite structure. The Al3+ cations are partially dissolved in the Fe2O3 lattice. Due to strong alumina–iron oxide interaction, the specific surface area of the catalysts increases significantly. TPR and XANES data indicate that copper forms highly dispersed surface CuO nanoparticles and partially dissolves in iron oxide. It has been shown that the reduction of iron­(III) oxide by CO proceeds via two routes: a direct two-stage reduction of iron­(III) oxide to metal (Fe2O3 → Fe3O4 → Fe) or an indirect three-stage reduction with the formation of FeO intermediate phases (Fe2O3 → Fe3O4 → FeO → Fe). The introduction of Al into Fe2O3 leads to a decrease in the rate for all reduction steps. In addition, the introduction of Al stabilizes small Fe3O4 particles and prevents further sintering of the iron oxide. The mechanism of stabilization is associated with the formation of Fe3–x Al x O4 solid solution. The addition of copper to the Fe–Al catalyst leads to the formation of highly dispersed CuO particles on the catalyst surface and a mixed oxide with a spinel-type crystalline structure similar to that of CuFe2O4. The low-temperature reduction of Cu2+ to Cu0 accelerates the Fe2O3 → Fe3O4 and FeO → Fe transformations but does not affect the Fe3O4 → FeO/Fe stages. These changes in the reduction properties significantly affect the catalytic performance of the Fe-based nanocomposite catalysts in the low-temperature oxidation of CO.

Kinetic and mechanistic study of CO oxidation over nanocomposite Cu−Fe−Al oxide catalysts

“…The fraction of Fe 3+ cations decreases to approximately 27 % at 500 °C (Table ). The obtained results are in full agreement with our recent in situ study of the Cu 5 Fe 78 Al 17 catalyst by XANES and XRD . It was established that during heating in a CO flow, the iron‐containing phases were reduced to Fe 3 O 4 ‐spinel modified by Al 3+ cations, which contained Fe 3+ and Fe 2+ cations.…”

“…The lattice parameter of the spinel is 8.333 Å. This value is between the lattice parameter of CuFe 2 O 4 (8.400 Å, ICSD 39132) and AlFe 2 O 4 (8.204 Å, ICSD 86571), which probably indicates the formation of a mixed oxide (Cu,Al,Fe) 3 O 4 –,. The phase quantification by the Rietveld refinement gives approximately 80 % of Fe 2 O 3 and 20 % of Cu x Al y Fe 3‐x‐y O 4 spinel for the Cu 10 Fe 74 Al 16 catalyst.…”

“…Our previous investigations of these systems by various methods showed that, at low concentrations, copper mainly forms CuO and CuFeO x clusters –,. These clusters are not detected by XRD.…”

Insights into Redox Dynamics of Vanadium Species Impregnated in Layered Siliceous Zeolitic Structures during Methanol Oxidation Reactions

“…[28] Recently, Ek et al [29] explored the facetdependent reactivity of VO x species on TiO 2 previously observed in the selective reduction of NO to NH 3 [30] by visualizing redox dynamic processes over (001) and (101) facets of the oxide applying combined atomic-resolution TEM and electron energy loss spectroscopy (EELS) under alternated oxidizing and reducing environments. By applying in situ spectroscopic [31][32][33] and atomic-resolution imaging [34][35][36] techniques to monitor redox properties under a reaction environment, new ways are opened to move towards a more complete understanding of the reactivity in oxide-catalyzed processes.…”

Highly Active CuFeAl-containing Catalysts for Selective Hydrogenation of Furfural to Furfuryl Alcohol

“…Earlier, we have already tested oxide catalyst systems based on copper and iron for the oxidation of CO [27], in which they showed high activity. Therefore, based on the literature data and previously obtained results, it seemed advisable to investigate copper-iron catalysts in the reduced form in the hydrogenation of furfural, since spinel structures can increase the overall activity of the copper catalyst [28].…”