Effect of Copper as a Dopant on the Water Gas Shift Activity of Fe/Ce and Fe/Cr Modified Ferrites
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Effect of Copper as a Dopant on the Water Gas Shift Activity of Fe/Ce and Fe/Cr Modified Ferrites Gunugunuri K. Reddy • Panagiotis G. Smirniotis
Received: 13 May 2010 / Accepted: 12 October 2010 / Published online: 2 November 2010 Ó Springer Science+Business Media, LLC 2010
Abstract The role of copper as a dopant on the WGS activity of Fe/Cr and Fe/Ce catalysts has been investigated. Our catalysts were synthesized via simultaneous precipitation of substituent metal cation/s (Cu2?/Mn?) with Fe3? which leads to the formation of hematitic type of spinels. After activation, the catalysts are transformed into mixed or inverse spinels. Interestingly, impersonal shift activity findings reveal that copper behaves very differently when added to the Fe/Cr in comparison to the Fe/Ce catalyst formulations, namely Cu acts as a promoter for Fe/Cr, while it does not promote the activity of the Fe/Ce catalyst. Our XRD and Mo¨ssbauer studies reveal the formation of wustite phase (FeO) in activated Fe/Ce/Cu samples as the reason for their decreased activity. Moreover, TPR measurements indicate that copper promotes the Fe3O4 ? FeO transformation to a much lower temperature which starts at 200 °C for the Fe/Ce/Cu sample compared to the Fe/Ce sample (starts at 450 °C). In contrast, no such behavior was observed for the Fe/Cr/Cu catalyst, since the reduction of Fe3O4 ? FeO starts at 500 °C. Mo¨ssbauer studies show distortions in the cubic lattice of magnetite due to the incorporation of copper and ceria in the lattice. These distortions are reflected in the internal magnetic field of the iron octahedral sites with characteristic isomer shift ‘d’.
G. K. Reddy P. G. Smirniotis (&) Department of Chemical and Materials Engineering, University of Cincinnati, Cincinnati, OH, USA e-mail: [email protected]
Keywords HT-WGS Effect of copper TPR XRD Modified ferrites
1 Introduction In our previous study [1], we introduced a variety of metal ions (M = Cr, Mn, Co, Ni, Cu, Zn, and Ce) into iron oxide (spinel lattice) and screened them for effectiveness for the high-temperature water–gas shift reaction. The idea was to stimulate the ferrite formation via doping with certain foreign cations and to promote the Fe3? $ Fe2? redox couple. Among the various catalysts tested, Fe/Cr and Fe/Ce were found to be the most active [1]. Cu is a widely used promoter for high temperature WGS catalysts although its promotional function is still debated. One argument is that Cu provides active sites via formation of active Cu alloys [2, 3]. We know that incorporation of copper into the iron oxide modifies the acid–base and redox properties of spinel lattice. Rhodes et al. [4] reported that incorporation of Cu in Fe/Cr oxide decreases the reduction temperature required for the transformation of hematite to magnetite. The physical and chemical state of the copper species on the surface plays an important role in the WGS reaction. Herein, we describe new high temperature catalysts for the WGS reaction that can operate in extreme conditions as posed
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