High-temperature water-gas shift over Fe/Ce/Co spinel catalysts: Study of the promotional effect of Ce and Co

Abstract

A set of Fe/Ce/Co ferrite spinels was synthesized and characterized to investigate the effect of Ce and Co atomic contents in the catalyst on the high-temperature water-gas shift (HT-WGS) performance. It was found that both the Ce and Co atomic compositions in the Fe/Ce/Co catalyst have a significant influence on the surface and structural properties and thereby HT-WGS activity. The Fe/Ce/Co (10:1:1.5) exhibited the best activity owing to preferable properties among the catalysts with other Ce and Co atomic contents. Besides, it showed an excellent time-on-stream (TOS) stability in HT-WGS for 100h of reaction even at lower steam to CO ratio of 1.5. XRD results suggested that Ce and Co co-doping could make the lattice strain in the magnetite lattice because of difference in the ionic sizes of Fe and dopants. Raman spectroscopy demonstrated that the lattice disorder was introduced with the addition of Ce and Co. The magnitude of the lattice strain/disorder depends on the promoters’ (Ce and Co) atomic composition in the catalyst. Our H2-TPR findings indicate that the degree of reduction for hematite-to-magnetite conversion increased until the Ce and Co atomic concentration reached to 1.0 and 1.5, respectively. The XPS analysis proves that both Ce and Co as dopants have the ability to facilitate the surface Fe3+↔Fe2+ redox cycle over the catalysts because of their redox nature (Ce4+/Ce3+, Co2+/Co3+). We observed the changes in the quantity of surface Fe3+/Fe2+ ratio as a function of the Ce and Co atomic contents. The results also revealed that the surface area, lattice strain/disorder, reducibility, and surface Fe3+/Fe2+ redox pair were the decisive factors for the HT-WGS reaction. A combination of techniques demonstrated that there is a strong synergistic interaction between Fe and dopants (Ce and Co) in the Fe/Ce/Co (10:1:1.5) which could enhance its surface area, lattice strain/disorder, surface Fe3+/Fe2+ redox capability, and thus leading to the elevated HT-WGS activity among other catalysts. XPS investigations on the spent Fe/Ce/Co (10:1:1.5) after 100h of reaction show that the catalyst remains in magnetite phase like the reduced one and is not reduced or oxidized during reaction. Our TEM images confirm that the Fe/Ce/Co (10:1:1.5) catalyst particles are not agglomerating/sintering during the TOS study which is an important characteristic for its long term stability.