Cr- and Ce-Doped Ferrite Catalysts for the High Temperature Water−Gas Shift Reaction: TPR and Mossbauer Spectroscopic Study

Cr and Ce-doped Fe2O3 catalysts are investigated for high temperature water gas shift (WGS) reaction. The chosen doping atomic ratios of Fe/metal for this investigation ranged from 10:0.2 to 10:2.5. Precipitation of Fe(III) nitrates with Cr- or Ce-metal nitrate(s) at different concentrations resulted in formation of high surface area nanosized catalysts. The WGS reaction was carried out in a fixed bed reactor at 400 and 500 °C and at atmospheric total pressure. Of all catalysts tested, Fe/Cr (10:0.5) and Fe/Ce (10:2) exhibited the highest activity within the specified activation conditions. X-ray diffraction (XRD) measurements revealed the formation of hematite (Fe2O3) phase in fresh catalysts, and magnetite (Fe3O4) phase in the activated and used catalysts after the WGS reaction. Lattice parameters reveal that the dopant ions enter the hematite structure under the present synthesis conditions. A lattice expansion occurs in the case of the Fe/Ce catalysts while a lattice contraction for the Fe/Cr catalysts. Temperature programmed reduction measurements suggest that the addition of Cr to hematite increases the transformation temperature, T max of Fe2O3 → Fe3O4, whereas the addition of Ce decreases T max of Fe2O3 → Fe3O4 transformation. Mössbauer spectroscopy studies show changes in internal magnetic field at iron octahedral sites with characteristic isomer shift “δ” resulting due to lattice contraction or expansion of the hematitic phase. These effects strongly depend on the nature and the amount of the incorporated metal cation. Mössbauer spectroscopy measurements also reveal that a rearrangement of Fe2+ and Fe3+ ions occurs between the octahedral (Oh) and tetrahedral (Td) sites during the WGS reaction of pure Fe2O3. No such rearrangement was noted in the doped catalysts except for the Fe/Cr (10:0.2) case.