Ce/Cr and Ce/Co modified ferrite catalysts for high temperature water-gas shift reaction at elevated pressures

Abstract

In this work, Ce and Cr/Co co-doped iron oxide (FeCeMOx, where M = Cr or Co) ferrites were investigated for high-temperature water-gas shift (HT-WGS) under industrially relevant pressures (20 bars). The Ce and Cr/Co bi-doping has greatly promoted the catalytic performance in comparison to the single Ce-doping. Especially, the FeCeCoOx ferrite demonstrated the best HT-WGS activity without any deactivation at a steam to CO ratio of 3.5. The ternary FeCeCrOx and FeCeCoOx catalysts also exhibited reasonably stable performance with a slight methanation activity (<2.5%) even under low steam to CO ratio of 1.5. The hematite phase (Fe2-xCex/2Mx/2O3, where M = Cr or Co and × = 0.33) of the fresh (calcined) catalysts became [A(1-δ)Bδ]T[AδB(2-δ)]OO4 type magnetite spinel structure (Fe3-xCex/2Mx/2O4, where M = Cr or Co and × = 0.5) during the activation process, which is regarded as the active phase for the WGS reaction. The co-doping of Cr or Co increased the stability of the active magnetite phase against sintering and suppressed the coke formation during the WGS reaction, which should be responsible for the stable activity. The addition of Cr or Co to iron oxide also delays the formation and over-reduction of active magnetite phase. Mössbauer spectra confirmed that the Cr or Co ions were incorporated into the octahedral sites of [A(1-δ)Bδ]T[AδB(2-δ)]OO4 spinel framework during the activation and modifies the local structure. The superparamagnetic behavior of the spinel ferrites has significantly enhanced upon the co-doping of Ce and Cr/Co into the iron oxide lattice, indicating a high fraction of nanoparticles in the ternary spinel ferrites. The Cr or Co addition resulted in the decreased surface Fe3+/Fe2+ ratio which could be due to the replacement of Fe3+ ions by Cr or Co ions. Most importantly, no evident change was observed in the surface structure of ternary spinel ferrites even after the reaction, which could be responsible the stable performance of the catalysts.