Long-term WGS stability of Fe/Ce and Fe/Ce/Cr catalysts at high and low steam to CO ratios—XPS and Mössbauer spectroscopic study
Long term time on stream stability of Fe/Ce and Fe/Ce/Cr catalysts has been investigated for high temperature water gas shift (WGS) reaction. Excellent WGS stability was observed for the Fe/Ce catalyst at steam to CO ratio of 3.5 and temperature of 500 °C for 30 days. The Fe/Ce catalyst also exhibited remarkable stability in presence of sulfur since no deactivation was observed in the presence of 400 ppm of sulfur. However, at steam to CO ratio 1.5, Fe/Ce deactivated continuously with time due to continuous formation of carbon and methane. X-ray diffraction measurements reveal that all the catalysts are stable in terms of composition during the WGS reaction irrespective of the steam to CO ratio. However, sintering of the magnetite phase is rapid in the experiment at steam to CO ratio of 1.5. XPS and Mössbauer spectroscopic measurements show that surface and local structural rearrangement of iron ions are taking place in the experiment at steam to CO ratio 1.5. XPS measurements also indicate the carbonate formation during the activation of Fe/Ce catalyst. The present study shows that Ce is a good stabilizer for iron oxide for high temperature WGS reaction only for higher steam to CO ratio applications. At low steam to CO ratio, Ce does not stabilize the Fe3+/Fe2+ redox couple during the WGS reaction. Addition of Cr to the Fe/Ce catalyst improves the WGS long term stability of Fe/Ce catalyst at steam to CO ratio 1.5. Remarkably, no deactivation was observed for the Fe/Ce/Cr catalyst at steam to CO ratio 1.5 and in presence of 400 ppm of sulfur. Structural and surface characterization measurements suggest that both Ce and Cr prevent the magnetite phase from sintering of and local structural rearrangement of iron ions during the WGS reaction at steam to CO ratio 1.5.
Graphical abstractFigure optionsDownload full-size imageDownload high-quality image (124 K)Download as PowerPoint slideHighlights► Long term stability of Fe/Ce/Cr investigated for high temperature WGS reaction. ► At S/CO = 1.5 Ce do not stabilize Fe3+/Fe2+ redox couple during the WGS reaction. ► Addition of Cr to the Fe/Ce catalyst improves the WGS activity at S/CO = 1.5. ► XPS indicate that Cr and Ce stabilizes the magnetite during WGS reaction. ► XRD and Mössbauer results suggest that Cr enters at M-modified octahedral sites.
Journal: Applied Catalysis A: General - Volumes 415–416, 16 February 2012, Pages 101–110