Studies on a Urea-Complexed Iron(III) Dichromate, a Precursor of Chromium-Rich Nanospinel Catalysts Prepared for the Reductive Transformation of Carbon Dioxide

Abstract

Energy-saving and cost-efficient reaction routes to prepare highly active catalysts for CO₂ hydrogenation or solid oxide fuel cells (SOFCs) are enormously important. In this paper, we report a detailed study of a dichromate salt of [Fe(urea)₆]³⁺, a member of the [M(urea)₆]³⁺ complex family (M = Fe, Al, Mn, Cr, V, or Ti) with oxidizing anions, which is a promising precursor of a Cr-rich mixed chromium iron oxide catalyst prepared at a low temperature in the solid phase. The single-crystal X-ray structure, various (infrared, ultraviolet–visible, and Raman) spectroscopic studies, and thermal analysis (differential scanning calorimetry and thermogravimetric analysis/mass spectrometry) of [hexakis(urea-O)iron(III)] dichromate {[Fe(urea-O)₆]₂(Cr₂O₇)₃} and its decomposition products confirmed the presence of a quasi-intramolecular redox reaction between the urea ligands and dichromate anions. The redox reactions result in various mixed Cr–Fe oxides with amorphous structure, whereas above 550 °C, the crystal structure and composition of the final products depend on the atmosphere during the thermal decomposition. The iron–chromium mixed oxides are potential catalysts in CO₂ hydrogenation that afford CO, CH₄, C₂H₆, and C₃H₈. Furthermore, our Mössbauer spectroscopy studies show a possible electron hopping between the Fe^II and Fe^III ions at the tetrahedral sites of the spinel structure, which suggests that the formed chromite is also a potential SOFC material. Our study also demonstrates that hexaureairon(III) dichromate is a selective oxidation agent of sulfur-containing organic compounds.