Selective Oxidation of Carbon Using Iron-Modified Cerium Oxide

Abstract

This research investigated a solid−solid reaction between carbon and CeO2 and the selective oxidation of carbon to CO by air using a cerium iron catalyst. The carbon-containing materials were obtained by reaction of fresh CeO2 or CeO2−Fe2O3 compounds with methane. X-ray powder diffraction (XRD), Raman spectra, scanning electron microscopy (SEM), and temperature-programmed reduction (TPR) procedures were used to study the structure, morphology, and redox performance of fresh and reduced CeO2 or CeO2−Fe2O3 samples and their carbon deposits. XRD analysis showed no significant differences between reduced CeO2 (left overnight) and fresh samples with respect to their lattice parameters. However, some iron and cementite (Fe3C) did appear on the reduced CeO2−Fe2O3 sample. Raman results showed that the carbon formed on the surface of reduced CeO2 was amorphous, whereas a higher degree of graphitization was obtained using the reduced CeO2−Fe2O3 sample. SEM measurements showed filament conformation of the carbon of the reduced CeO2−Fe2O3 and a floc structure on the reduced CeO2 sample. The H2-TPR experiment showed that the redox performance of CeO2 was strongly enhanced by doping with Fe2O3. The temperature-programmed and isothermal reactions indicated that CeO2 could provide oxygen as the sole oxidant in the oxidization of carbon deposits to CO with high selectivity in the absence of gaseous oxygen at an appropriate temperature, such as 800 °C. Moreover, carbon conversion was significantly enhanced in the presence of Fe species; the in situ reoxidation of reduced CeO2−Fe2O3 with air produced mostly CO at 800 °C with a rapid reaction rate. Less CO was obtained using reduced Fe2O3−ZrO2, indicating that the reduced Ce species in association with the reduced Fe species contributed to CO formation during carbon oxidation by air. It also showed that carbon oxidation by air was strongly influenced by the reaction temperature; the selective oxidation of carbon to CO cannot occur once the temperature is too low (e.g., 400 °C). A mechanism describing oxygen adsorption and migration with a cerium iron catalyst and CO formation was proposed based on the selective oxidation of carbon by CeO2 and on the excellent oxygen adsorption capacity of cerium suboxides (anionic vacancies) and reduced Fe sites (e.g., Fe and Fe3C). This mechanism reveals that the carbothermic reduction of CeO2 (i.e., reduction of CeO2 by carbon) along with the active oxygen formed on the reduced Fe sites may play an important role in the selective oxidation of carbon by air at high temperature.