Sulfur tolerant metal doped Fe/Ce catalysts for high temperature WGS reaction at low steam to CO ratios – XPS and Mössbauer spectroscopic study

High temperature water gas shift reaction (WGS) at low steam to CO ratios has been investigated over Fe2.4Ce0.3M0.3O4 (M = Cr, Co, Zr, Hf, and Mo) type of spinels in the temperature region of 400–550 °C and in the presence of sulfur. Remarkably, both Cr- and Co-doped Fe/Ce catalysts show excellent activity at steam to CO ratio as low as 1.5. Other dopants namely Zr, Hf and Mo have little or no effect on the WGS activity of Fe/Ce. All the catalysts exhibit excellent stability in the presence of sulfur. X-ray diffraction (XRD) measurements reveal the formation of hematitic phase in as-prepared catalysts and magnetite phase in activated and spent catalysts. After the water gas shift reaction, we observe a decrease in the cell parameter of the magnetite lattice for the spent Fe/Ce and Co-, Zr-, Hf-, and Mo-doped Fe/Ce catalysts compared to the activated catalysts. For Cr-doped Fe/Ce catalyst, no change in the lattice parameter was observed after the WGS reaction at a steam/CO ratio = 1.5 and in the presence of sulfur. Temperature-programmed reduction (TPR) measurements suggest that the addition of Zr and Hf to the Fe/Ce lowers the reduction temperature of surface ceria. X-ray photoelectron (XPS) spectra show that both Cr and Co inhibit the carbonate formation in the magnetite during the activation as well as the WGS reaction, whereas Zr, Hf, and Mo can inhibit the formation of carbonate in the magnetite during the activation but not during the WGS reaction. Mössbauer spectral analysis shows that both Cr and Co occupy the octahedral sites of the magnetite during the activation of the catalysts and exhibit higher WGS activity. On the other hand, Zr, Hf, and Mo have little or no effect on the structure and catalytic properties of magnetite either during the activation or during the WGS reaction.