Chromium promotion of iron oxide based water-gas shift (WGS) catalysts prepared via co-precipitation/calcination was investigated. Mössbauer spectroscopy and XRD evidence that chromium is incorporated in the calcined hematite (α-Fe2O3) precursor irrespective of the doping level (0−12 wt.%). CO-TPR shows chromium delays the reduction of hematite and the active magnetite (Fe3O4) phase. WGS activity was evaluated under realistic conditions for 4 days. Enhanced CO conversion was observed with increased chromium doping. Mössbauer spectra indicate that chromium incorporates into octahedral sites of magnetite and prevents reduction of Fe3+ to Fe2+ during formation of the active phase, leading to an increased Fe3+/Fe2+ ratio in octahedral sites. The higher Fe3+/Fe2+ ratio did not affect the high CO conversion associated with the structural stabilization mechanism of Cr-doping. Interpretation of the Mössbauer spectra was supported by computational modelling of various chromium and vacancy-doped magnetite structures. The bulk structure of an in situ prepared chromium-doped high-temperature WGS catalyst is best described as a partially oxidized chromium-doped magnetite phase. No surface effects of Cr-doping were found.
Bibliographical noteFunding Information:
This publication is part of the project Application of advanced combined in-situ Mössbauer/IR/GC characterisation under industrially relevant conditions to underpin and accelerate development of improved Fe-based catalysts with project number 731.015.419 of the research programme LIFT, which is (partly) financed by the Dutch Research Council (NWO) . The authors would like to thank Arno van Hoof for the TEM measurements and Freddy Oropeza for his assistance and advice on the XPS measurements. Deborah Dodds for her advice and training in catalyst preparation and Michel Steenvoorden for his help with the Mössbauer spectroscopy setup.
© 2021 The Author(s)
- Iron oxide
- Mössbauer spectroscopy
- Water-gas shift reaction