Ceramic Membrane Reactor Materials

J. D. Jorgensen, J. P. Hodges; in collaboration with B. Ma and U. Balachandran, Energy Technologies Division, and J. W. Richardson, Jr. and B. Mitchell, Intense Pulsed Neutron Source

The present work is focused on understanding and optimizing a ceramic material that exhibits both electronic and oxygen ionic conduction for use as an oxygen separator in a reactor that is used to convert natural gas to synthesis gas. The target material, called SFC-2, was developed a few years ago at Argonne and found to perform well in the application, but its crystal structure and the basis for its performance (from a fundamental point of view) were not known. Over the last three years we have characterized this material with the aim of optimizing its properties. We first learned that SFC-2 was a mixed phase material consisting of a layered compound (not previously known) with fixed composition and a perovskite compound with variable composition. Because the key to controlling the transport properties is the perovskite compound (by varying its composition) we have focused on this material during the last year.

Structures of the Series of Compounds SrFeO3-1/n (n=2, 4, 8, ð). The perovskite compound in SFC-2 can assume four different crystal structures depending on the oxygen content. These structures differ by the pattern for ordering of oxygen vacancies and the associated displacements and charge states of the surrounding metal ions. Although this series of compounds has been studied for many years, we found that the published structures for the n=4 and n=8 compositions were wrong. The differences are subtle, but are readily seen in high-resolution neutron diffraction. More importantly, when the correct structures are known, one can reconcile Mossbauer measurements on the same materials and can assign charge states to the metal ions. Additionally, the correct structures exhibit well-defined channels which are likely involved in the fast-ion conduction of oxygen.

In situ Neutron Diffraction Studies of Membrane Materials. A key part of our program has been to study the membrane reactor materials, using neutron diffraction, in an environment (temperature and oxygen partial pressure) that simulates a working conversion reactor. We have developed a furnace for doing this at the Intense Pulsed Neutron Source and have performed measurements on the SFC-2 mixed-phase compound and on single-phase perovskite compounds. These experiments show how reversible conversion between the layered phase and perovskite phase can occur under reactor operating conditions and contribute to the mechanical/chemical stability of the material.

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