(743f) Synthesis and Redox Pathways of MgAl2O4-Stabilized, Fe-Cu-Based Oxygen Carriers for Chemical Looping Water Splitting

Hydrogen may become an important energy carrier in the future. The
current technologies for the large scale production of H₂, i.e.
steam reforming of natural gas and steam gasification of coal, are very energy
intensive owing to the need for various separation steps to obtain pure H₂.¹
However, for H₂ to become a major energy carrier it has to be
produced in an efficient and sustainable way. Chemical looping water-splitting
(CLWS) is an emerging technology that can inherently produce a pure stream of H₂,
thus eliminating the need for energy intensive purification steps.²
In CLWS, lattice oxygen of an oxygen carrier, typically a transition metal
oxide, is used to combust a hydro-carbonaceous fuel. In the subsequent step,
the reduced oxygen carrier is re-oxidized with steam to produce a pure stream
of H₂. Iron oxide is an attractive material for the CLWS process
owing to its wide availability, low cost, minimal environmental impact and high
equilibrium partial pressure of H₂.³ Nevertheless, its reactivity
and cyclic redox stability are two major concerns with regards to the CLWS
process. Pure Fe₂O₃ that is completely reduced to Fe
deactivates after a few cycles, probably due to sintering.⁴ To
circumvent this rapid deactivation it has been proposed to stabilize iron oxide
with a high Tammann temperature support, such as Al₂O₃,
ZrO₂, TiO₂, SiO₂ or MgAl₂O₄.
However, it was found that supported Fe₂O₃ has often a
high tendency for carbon deposition. An approach to improve the reactivity of iron
oxide with CH₄ and higher hydrocarbons is the addition of more
reactive transition metal oxides.⁵

Here, we
synthesized MgAl₂O₄-stabilized, Fe-Cu-based oxygen
carriers containing 70 wt. % Fe₂O₃ for CLWS using CH₄
as a fuel. The reactivity, H₂ yield and
carbon deposition characteristics of the oxygen carriers were compared to pure Fe₂O₃
and CuFe₂O₄ and MgAl₂O₄-stabilized
Fe₂O₃. The oxygen carriers were characterized using
powder X-ray diffraction (XRD), CH₄ and H₂ temperature
programmed reduction, N₂ adsorption measurements, scanning electron
microscopy, transmission electron microscopy, Raman spectroscopy and X-ray
absorption spectroscopy (XAS). The redox and carbon deposition characteristics
of the synthesized oxygen carriers were determined at 900 °C using both a
thermo-gravimetric analyzer and a packed bed reactor.

Pure CuFe₂O₄
was found to have a low surface area, compact morphology, slow kinetics and
high tolerance towards carbon formation. On the other hand MgAl₂O₄-supported
Fe₂O₃ possessed a 'grainy' surface texture, high surface
area, high reactivity towards CH₄ and a high activity for carbon
deposition. The surface morphology of MgAl₂O₄-supported,
Fe-Cu-based oxygen carriers were found to be an intermediate of MgAl₂O₄-supported
Fe₂O₃ and CuFe₂O₄. Thermo-gravimetric
and pack bed measurements revealed that the oxidation kinetics and H₂
yield of MgAl₂O₄-stabilized, Fe-Cu-based oxygen carriers were,
respectively, substantially faster and higher than for CuFe₂O₄.
For MgAl₂O₄-stabilized, Fe-Cu-based oxygen carriers in-situ
XRD and XAS measurements revealed that Fe₂O₃ was
stabilized by both a spinel phase (MgAl₂O₄) and a delafossite
phase (CuFeO₂). Additionally, MgAl₂O₄-stabilized,
Fe-Cu-based oxygen carriers effectively inhibited the decomposition of CH₄
and the formation of iron carbide. Scanning electron micrographs and XAS
measurements indicated that the reduced quantity of carbon deposition on the
bimetallic oxygen carriers is due to an alteration of the surface morphology,
impeding the Fe-based activation of CH₄.

References

[1] A.
T-Raissi and D. L. Block, Power and Energy Magazine, IEEE 2004, 2, 40.

[2] A.
Thursfield, A. Murugan, R. Franca and I. S. Metcalfe, Energy Environ. Sci. 2012,
5, 7421.

[3] R.
D. Solunke and G. T. Veser, Ind. Eng. Chem. Res. 2010, 49, 11037.

[4] C.
D. Bohn, C. R. M?ller, J. P. Cleeton, A. N. Hayhurst, J. F. Davidson, S. A.
Scott and J. S. Dennis, Ind. Eng. Chem. Res. 2008, 47, 7623.

[5] S.
Wang, G. Wang, F. Jiang, M. Luo  and H. Li, Energy Environ. Sci. 2010,
3, 1353.