Unexpected Behavior of Copper in Modified Ferrites during High Temperature WGS Reaction-Aspects of Fe~(3+) - Fe~(2+) Redox Chemistry from Mossbauer and XPS Studies

摘要

We report dynamic alternation of the redox chemistry of the Fe~(3+)/ Fe~(2+) couple in magnetite during high temperature water-gas shift reaction in Cu codoped M-modified ferrite catalysts. Various hematitic solid solutions of the type Fe2O3-M_xO_y-CuO_x with M = Cr, Ce, Ni, Co, Mn, and Zn are synthesized using the industrially economical and environmentally friendly coprecipitation method. Interestingly, Cu shows an unusual effect in the M-modified ferrites during the high temperature WGS reaction. Remarkably, our shift activity measurements reveal that Cu act as a promoter for all M-modified ferrites with M = Cr, Ni, Co, Mn, and Zn except M = Ce. For the latter case, Cu acts as an inhibitor for the hightemperature WGS reaction. Temperature programmed reduction measurements (TPR) show that Cu selectively promotes the reduction of hematite (Fe2O3) to magnetite (Fe3O4) in all modified ferrite catalysts. However, Cu does not promote the reduction of magnetite to wustite or reduction of other metal oxide present in the ferrite expect for M = Ce. Mossbauer effect studies show distortions in the Fe local environments when Cu is codoped in magnetite. These distortions are reflected in the internal magnetic field at octahedral (O_h) sites with characteristic isomer shift "5". The Mossbauer spectra and XPS measurements show that Cu plays a different role on Fe~(3+)/Fe~(2+) redox chemistry in the bulk and surface. During the activation, some of the Cu enters at the O_h sites of the magnetite and replaces Fe~(2+) ions and the remaining Cu forms metallic Cu species except for the Fe/Ce. This dual promotional role is responsible for the observed high temperature WGS activity in Cu codoped M-modified ferrites. For M = Ce, characterization studies show that both Ce and Cu enter the iron oxide lattice substitutionally upon activation and form the wustite (FeO) phase along with the magnetite phase. The formation of wustite (FeO) is responsible for the decreased WGS activity upon Cu codoping of Fe/Ce catalyst.