(526d) Investigation Into The Role Of Various Dopant Ions On The Ht-Wgs Activity Of Modified Ferrites
A series of modified ferrites [MOx/Fe2O3] were prepared by doping iron oxide with various transition/non-transition and inner transition metal ions [M = Cr, Mn, Fe, Co, Ni, Cu, Zn & Ce] during synthesis. All the modified ferrites thus obtained exhibit remarkably high surface areas, greater than that of pure iron oxide (Fe2O3) sample. The efficacy of the dopant ions in modifying the resultant specific surface area, could be directly related to variations in the rate of crystal growth. The nature and concentration of the foreign cations present in the system govern this variation. Interestingly a narrow pore size distribution was observed in all the modified ferrite based catalyst systems, in the range of 49 ? 250 Å. XRD analysis revealed the existence of singe phase ? hematite (Fe2O3) in all the fresh catalyst samples. The X-ray diffraction experiments performed on spent catalysts, confirmed the existence of Fe3O4 phase. It is well established fact that rapid electron hoping between Fe2+ and Fe3+ in the Fe3O4 lattice system is needed for carrying out the water gas shift reaction. It was observed that, small variation in the substitution degree of Fe(II) or Fe(III) leads to major changes of the intrinsic physico-chemical properties and also in the sample morphology. Mössbauer spectroscopy indicates the preference of dopant ion ?M' for octahedral positions. The substitution level depends on the nature of the substituting dopant ion ?M'. The WGS reaction was performed in the temperature range of 350 to 550 °C using designated amounts of steam and CO [S/CO = 1; 3.5; 7.0]. A space velocity of 60,000 h-1 was maintained in all the experiments. The catalysts in general exhibit less activity at 350 °C, a fact which can be related to their difficulty in producing the active phase in situ, in accordance with TPR results. The WGS activity increased with the temperature, due to kinetic factors. Also the WGS activity was found to increase as the steam-to-CO ratio was increased. On the whole characterization data indicated that the differences in the catalytic performance could be due to changes in catalyst reducibility and magnetite crystallite growth. Among all the catalysts tested Fe/Cr, Fe/Ce, Fe/Cu, and Fe/Ni exhibit remarkable activity, therefore they seem to be promising candidates for membrane reactor operations.