High Temperature Water Gas Shift Reaction over Nanocrystalline Copper Codoped-Modified Ferrites

Iron-containing spinels, also known as ferrospinels, are widely used catalysts for the water gas shift (WGS) reaction. The catalytic activity of spinels containing transition metal ions as dopants or modifiers is influenced by the acid base and redox properties of these ions as well as by their site occupancy amongst the octahedral and tetrahedral sites in the spinel structure. It has been established that the octahedral sites are almost exclusively exposed at the surface of the spinel crystallites and that the catalytic activity is mainly due to octahedral cations. In our earlier study, we modified iron oxide spinel (Fe:M) with selected transition metals (M = Cr, Mn, Co, Ni, Cu, Zn, and Ce) and catalytically examined them for the high temperature WGS reaction. Among the various M-modified catalysts investigated, both Fe:Cr and Fe:Ce spinels in atomic ratio of 10:1 exhibited the highest activity. It is well-known that Cu is a promoter for both the low and the high temperatureWGS reactions, although its promotional function is still debated. One argument is that Cu provides active sites via formation of active Cu alloys. Cu doping into the iron oxide modifies the acid base and redox properties of the spinel lattice. Rhodes and Hutchings reported that Cu codoping into the Fe/Cr oxide decreases the reduction temperature of hematite to magnetite phase. The physical and chemical state of the Cu dopant on the surface plays an important role in the WGS reaction. Other recent studies have also revealed that active Cu metal with partial ionic character present in the interface of nanoparticles and the support influences the activity of the catalyst. 10 Higher loading of CuO on oxide supports can lead to bigger particles and reduced activity. Moreover, pyrophoricity also increases with Cu loading. Usually, impregnation methods are employed formetal loading on the surface of the supports. However, the coprecipitation method stabilizes the Cu in its ionic form. In this approach, a simultaneous precipitation of component hydroxides in the precursor may aid in the surface incorporation of Cu2þ ions. Hence, all of the catalysts were prepared by coprecipitation method. The present study mainly aimed at developing new series high temperature catalysts for the WGS reaction that can operate in extreme conditions, as posed in the membrane reactor regime. For this purpose, a series of Cu codoped-modified ferrites of type Fe:M:Cu withM =Cr, Ce, Ni, Co, Mn, and Zn with the chemical composition 10:1:0.25 atomic ratios were synthesized. High temperature WGS has been carried out over these modified ferrites in the temperature region of 400 550 C and steam to