Effects of the Ce and Cr Contents in Fe-Ce-Cr Ferrite Spinets on the High-Temperature Water-Gas Shift Reaction

A series of Fe-Ce-Cr ternary oxide catalysts with different Fe/Ce/Cr atomic ratios have been synthesized using a coprecipitation method. The effects of the contents of both Ce and Cr in the Fe-Ce-Cr catalyst on the high-temperature water-gas shift (HT-WGS) reaction were investigated. Among the Fe-Ce-Cr catalyst with different atomic concentrations of Ce and Cr, the catalyst with a 10:1:1 atomic ratio showed the best performance in terms of HT-WGS activity. We found a strong synergistic effect between the dopants (Ce and Cr) and Fe that contributes the highest lattice strain and lowest crystallite size in Fe-Ce-Cr with an atomic ratio of 10:1:1 compared to other compositions. The Fe/Ce/Cr atomic ratio of 10:1:1 generated the highest lattice disorder in the Fe-Ce-Cr catalyst, as determined by Raman analysis. Our H-2 TPR results confirmed that the reduction temperature of hematite (Fe2O3) to magnetite (Fe3O4) increased upon the addition of Cr. Conversely, the incorporation of Ce significantly decreased the reduction temperature for hematite-to-magnetite conversion until the Fe/Ce atomic ratio reached 10:1. The XPS results demonstrated that only Ce acts as a chemical advocate for iron oxide in the HT-WGS by facilitating the surface Fe2+/Fe3+ redox cycle through its Ce3+/Ce4+ redox couple. However, Cr does not chemically support the iron oxide catalyst because of its single 3+ oxidation state. The Fe2+/Fe3+ redox cycle was greatly facilitated over the surface of the catalyst at an Fe/Ce/Cr atomic ratio of 10:1:1 compared to the other investigated samples with different Fe/Ce and Fe/Cr compositions. The better HT-WGS performance of the Fe-Ce-Cr (10:1:1) catalyst compared to all of the other catalysts can be mainly ascribed to its high lattice strain or disorder and excellent surface redox properties, linked to the Fe2+/Fe3+ and Ce3+/Ce4+ redox pairs.