(600f) Sol-Gel Derived Fe2O3-Based, Al2O3 Supported Oxygen Carriers for Hydrogen Production Via a Chemical Looping Cycle

Sol-gel derived Fe₂O₃-based, Al₂O₃ supported oxygen carriers for hydrogen production via a chemical looping cycle

Nur Sena Yüzbasi, Qasim Imtiaz, Christoph R. Müller*

Laboratory of Energy Science and Engineering, ETH Zurich, Leonhardstrasse 27, 8092

Zurich, Switzerland

*Corresponding author: muelchri@ethz.ch

Keywords: Hydrogen production, sol-gel, iron oxide, iron precursors.

Current technologies for hydrogen production such as the steam methane reforming (SMR) process have high production capacities of  ̴15 kte H₂/year or greater. However, due to issues associated with the storage and distribution of H₂, processes that allow the distributed production of H₂ on a small scale may be advantageous.¹ For example, the distributed production of high purity hydrogen from biomass (with simultaneous CO₂ capture) can be achieved by modifying the conventional thermo-chemical looping combustion process. Here, biomass is first gasified with steam to produce a synthesis gas i.e. a mixture of predominantly CO, H₂, CO₂ and H₂O.  The synthesis gas subsequently reduces iron oxide to a lower oxidation state, thereby producing CO₂ and H₂O which can be separated via condensation. Subsequent re-oxidation of the reduced iron oxide with steam yields high purity H₂. One of the key challenges of the above described looping process is the synthesis of iron oxide-based oxygen carriers that possess (i) a high reactivity, (ii) stable redox characteristics over many cycles and (iii) tolerance to typical synthesis gas impurities e.g. sulphurous components or tarry substances. Recent studies have demonstrated that pure iron oxide, i.e. unsupported iron oxide, deactivates after only a few cycles when reduced completely to Fe. ^{1, 2}

A possibility to stabilize the redox characteristics of iron oxide based oxygen carriers is to stabilize Fe₂O₃ with a support material e.g. Al₂O₃,³ ZrO₂,⁴ MgAl₂O₄.⁵ With regards to synthesis techniques, sol-gel methods have received some attention recently since they ensure a homogeneous mixing of the different components and allow the control of the pore structure and surface area of the material. However, there is only limited work on Fe-based oxygen carriers synthesized via sol-gel techniques.^{3, 7} In this study, we prepared different Fe₂O₃-based, Al₂O₃ supported oxygen carriers (containing 60 wt. % Fe₂O₃) to assess critically the influence of the iron precursor, viz. iron nitrate (Fe(NO₃)₃·9H₂O), iron chloride (FeCl₃) or iron acetylacetonate (Fe(C₅H₇O₂)₃), on the yield of H₂. In addition, the effect of the pH on the structure of the synthesized materials was studied. For comparison, pure iron oxide, i.e. unsupported iron oxide manufactured via mechanical mixing technique, was used. The oxygen carriers were characterized using (i) a thermogravimetric analyser (TGA), (ii) a packed bed reactor, (iii) X-ray diffraction (XRD), (iv) N₂ adsorption, (v) high resolution scanning electron microscopy (HR-SEM) and (vi) Fourier transform infrared spectroscopy (FTIR). It was found that the oxygen carriers synthesized using iron chloride or iron acetylacetonate as the iron precursor possessed the highest surface areas and pore volumes. In addition, SEM images showed that the use of iron nitrate or iron acetylacetonate as the iron precursor resulted in oxygen carriers that possessed a homogenous and nano-structured morphology. The cyclic H₂ production of the oxygen carriers was tested over 15 cycles in a fixed bed reactor. Pure Fe₂O₃ possessed a high initial H₂ yield of i.e.~ 13 mmol/g oxygen carrier.  However, after the first cycle the quantity of H₂ produced decreased substantially. On the other hand, Al₂O₃-supported oxygen carriers showed stable redox characteristics over 15 cycles. Oxygen carriers synthesized using iron nitrate as the iron precursor showed the highest H₂ yield, i.e. ~ 6 mmols/g oxygen carrier which is nonetheless less than the theoretically expected H₂ yield. The lower H₂ yield is linked to formation of hercynite, FeO·Al₂O₃ (as confirmed by the XRD) since its re-oxidation with steam is thermodynamically limited. ⁷

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