Nanoscale Interactions in a Multifunctional Ni–Fe–Ca–Mg–Al–O Chemical Looping Material

Abstract

As an alternative to using distinct materials with a single function, a multifunctional material, Ni/CaO/Fe₂O₃–MgAl₂O₄, was synthesized to integrate three functions for combined chemical looping processes at the nanoscale, i.e., CO₂ sorption (Ca), redox activity for CO₂ conversion (Fe), and redox activity for heat generation (Ni). Sorption and redox testing of the mixed oxide show that the material holds all 3 functionalities: a CO₂ capture capacity of 1–0.3 mmol_CO2/g, which is retained after sequential redox cycles at 950 °C by both CO₂ and O₂, redox activity for CO₂ reduction into CO, and in situ heat generation upon O₂ oxidation. To assess the structural changes upon treatment, in situ XRD, in situ X-ray absorption spectroscopy (XAS), and in situ X-ray Raman scattering (XRS) were used. The dominant crystalline phases were Mg_0.7Fe_0.23Al_1.97O₄ and Ca₂Fe_2–xAl_xO₅, next to a considerable amorphous fraction of 40%. Under H₂-TPR, Ni and Fe oxides were reduced to form NiFe alloy, and CaCO₃ became CaO. CO₂ reoxidation returned CaO into CaCO₃, while decarbonization realized the opposite. To include the amorphous material in the analysis, in situ XAS was applied for Fe and Ni, while in situ XRS looked into the light elements Ca and O. XAS found Ni fully reduced after H₂ reduction, whereas slight oxidation was observed after CO₂ oxidation. In contrast, Fe was always in a mixed oxidation state, either dominantly metallic after reduction or close to oxidized after reoxidation. The Ca L_2,3-edge XRS spectra showed only minor variation during sequential treatments, reproducible by simulations considering an average contraction of the Ca octahedra. The bulk-averaged O K-edge spectra varied more strongly with each treatment. This was reproduced by linear combination fitting with simulations for the dominant phases, each having a semiempirical screening parameter to reflect the degree of electron transfer between oxygen and its 3d transition metal neighbors. This insight into the interplay between structure and function offers clear design guidelines for next-generation multifunctional looping materials, enabling more efficient integration of the CO₂ capture and conversion processes.