Relationship between temperature-programmed reduction profile and activity of modified ferrite-based catalysts for WGS reaction

Abstract

A series of modified ferrites were prepared by doping iron oxide with various transition/non-transition/inner-transition metal ions [M = Cr, Mn, Co, Ni, Cu, Zn and Ce] in situ during synthesis. All the modified ferrites thus obtained exhibit remarkably high surface areas, greater than that of pure iron oxide (Fe 2O 3) 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 all the modified ferrites, exhibit a narrow pore size distribution in the range of 4.9–25 nm. XRD analysis revealed the existence of hematite (Fe 2O 3) phase in all the as-prepared samples. The X-ray diffraction experiments performed on activated catalysts, confirmed the existence of magnetite (Fe 3O 4) phase with a nominal composition of Fe 2.73M 0.27O 4. These inverse or mixed spinels with general formula A (1− δ) B δ [A δ B (2− δ) ]O 4, possess highly facile Fe 3+ ⇔ Fe 2+ redox couple, the degree of facileness depends on the extent of synergistic interaction between iron and the other substitutent metal ion. The rapid electron hopping between Fe 3+ ⇔ Fe 2+ in the Fe 3O 4 lattice system is essential to catalyze WGS reaction. From TPR it was observed that, incorporation of metal cations into the hematite (α-Fe 2O 3) crystal structure alters the reducibility of the hematite particles, which in turn depends on the nature of the incorporated metal cation. A plausible explanation for the WGS activity over various modified ferrites has been attempted with the help of TPR analysis.