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

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


Imtiaz, Q. - Presenter, ETH Zürich
Yüzbasi, N. S., ETH Zurich
Broda, M., ETH Zurich
Kierzkowska, A., ETH Zurich
Müller, C. R., ETH Zurich
Abdala, P., ESRF
van Beek, W., ESRF

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.


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[3] R.
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D. Bohn, C. R. M?ller, J. P. Cleeton, A. N. Hayhurst, J. F. Davidson, S. A.
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[5] S.
Wang, G. Wang, F. Jiang, M. Luo  and H. Li, Energy Environ. Sci. 2010,
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