Characterization and evaluation of Fe2O3/Al2O3 oxygen carrier prepared by sol–gel combustion synthesis

The sol–gel combustion synthesis (SGCS) for oxygen carrier (OC) to be used in chemical looping combustion (CLC) was first designed and experimented in this work, which is a new method of OC synthesis by combining sol–gel technique and solution combustion synthesis. Cheap hydrated metal nitrates and urea were adopted as precursors to prepare Fe2O3/Al2O3 OC at the molar ratio to unity (Fe1Al1), which was characterized through various means, including Fourier transforms infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), differential thermal analysis (DTA), X-ray diffractor (XRD), and N2 isothermal adsorption/desorption method. FTIR analysis on the chemical structure of the dried gel of Fe1Al1 indicated that urea was partly hydrolyzed and the hydrated basic carbonate was formed by the combination of groups such as (Fe(1−yAly)1−xO1−3x, CO32− and –OH–. By analyzing the staged products during SGCS, calcination was found as a necessary step to produce Fe2O3/Al2O3 OC with separate phases of α-Fe2O3 and α-Al2O3. Through TGA–DTA, the decomposition of the dried gel was found to undergo five stages. The analysis of the evolved gases from the gel decomposition using FTIR partially confirmed the staged decomposition and assisted a better understanding of the mechanism of SGCS. XRD identification further substantiated the necessity of calcination to synthesize Fe2O3/Al2O3 OC with separate phases of α-Fe2O3 and α-Al2O3, though it was not necessary for the synthesis of single phase α-Fe2O3 and α-Al2O3. Structural characterization performed on N2 adsorption analyzer displayed that the pore shape of Fe1Al1 particles was heterogeneous. Finally, H2 temperature-programmed reduction (TPR) of Fe1Al1 products in TGA indicated that the reduction reaction of Fe1Al1 OC after calcination was a single step reaction from α-Fe2O3 to Fe, and calcination benefited to improve the transfer rate of the lattice oxygen from the OC to fuel H2. Furthermore, four times of reduction and oxidization (redox) reaction by alternating with H2 and air demonstrated the synthesized OC had good reactivity and sintering-resistance, much suitable to be used in the realistic CLC. Overall, the SGCS method was found superior to other existent methods to prepare Fe2O3/Al2O3 OC for CLC application.