(353c) Synthesis Gas Generation Using Ionic/Electronic Oxygen Permeable Membranes

The incorporation of non-porous, selective, oxygen permeable membranes with both ionic and electronic conductivity into catalytic reactors has received significant interest as a way to eliminate costly air separation steps and to achieve staged addition of oxygen. Many of the studies have focused on the use of the membranes for the partial oxidation of methane, and it has been shown that the presence of a catalyst near the surface of a membrane greatly increases the oxygen flux by reducing the oxygen concentration on the reaction side [1,2]. However, currently attainable oxygen fluxes are insufficient for large-scale industrial applications. This work investigates using semiconductor processing techniques to pattern a partial oxidation catalyst on the surface of a SrFeCo_0.5O_x selective oxygen permeable membrane and the effect of the deposited catalyst on oxygen flux and reaction. The membranes were studied with the depositions serving as a partial oxidation catalyst and also as an oxygen dissociation catalyst on the low and high oxygen partial pressure sides of the membrane, respectively.

Reaction and flux experiments were carried out in a stainless steel CSTR type reactor with a reactor volume of approximately 0.5 cm³. Reaction studies were conducted at 700 and 750 °C with a 1:1 mixture of CO₂ and CH₄ at a total feed flow rate of 10 cm³/min. The catalysts and membranes were reduced in-situ prior to reaction. For comparison, reactions were carried out with 8 mg of a 0.43 wt% Pt / ZrO₂ traditional powder catalyst dispersed over the membrane surface and also performed using a stainless steel ?blank? coated with inert BN₃ paint to isolate the effect of the membrane from the reactor configuration. The effluent of the reactor was analyzed using a gas chromatograph and mass spectrometer simultaneously.

SrFeCo_0.5O_x membranes were prepared by Dr. Balachandran at Argonne National Laboratory using a technique previously reported in the literature [2]. The sintered membranes were then polished to a near optical finish before being patterned with metals and supports such as Pt, Ni, CeO₂, and ZrO₂. The patterning follows a bi-layer lift off lithography procedure commonly accepted in the semiconductor processing field. Two layers of photoresist are coated on the membranes and exposed in the desired pattern using a manual UV mask aligner. The pattern is developed in the appropriate solution and catalyst metal/oxides are deposited with an electron beam evaporator designated specifically for this project. Lift off occurs over a period of 1 hour in a microposit remover solution. Pre and post reaction membrane characterization is done using scanning electron microscopy (SEM) and elemental dispersive x-ray (EDX) analysis.

The addition of a patterned Pt catalyst to the high oxygen partial pressure side of the membrane has shown to increase oxygen flux through the membrane an average of two to three fold over the 500 to 750 °C tested temperature range. Multicomponent metal/mixed oxide and other more effective oxygen dissociation catalysts are currently under investigation for this application.

Methane conversion of the powdered catalyst in the stainless steel reactor without the SrFeCo_0.5O_x membrane present was initially 40% but deactivated to 8% after only 2 hours of reaction. For comparison, the conversion without catalyst or membrane was less than 2% under than same conditions. When the reaction was performed with powdered catalyst and in the presence of the membrane, the initial conversion achieved was only 12% but was very stable during the 5-hour reaction. The low initial catalyst activity is believed to be caused by surface oxidation of the Pt during the temperature ramp prior to reaction. Although the catalyst was initially reduced at 400 °C, an increase in the oxygen flux through the membrane was observed while heating from 500 °C to the reaction temperature under argon, resulting in an oxidizing environment in the reaction chamber. Different pretreatment conditions are currently being explored to eliminate the oxidation of the catalyst prior to reaction.

After testing the membranes with the powdered Pt/ZrO₂ catalyst, metal and metal/mixed oxide patterns were fabricated on the membrane surface. Pt, Ni, Au, CeO₂, and combinations thereof have been successfully deposited with feature sizes ranging from 2 to 150 microns and have been shown to be stable under both oxidizing and reducing conditions at 500°C. Patterned membranes exhibited higher initial activity than the plain membrane and a higher turnover frequency compared to reaction with the powdered catalyst. Deactivation was observed on the patterned membrane with the final conversion simlar to the plain membrane. Post reaction membrane characterization using scanning electron microscopy and energy dispersive x-ray analysis revealed membrane surface morphology changes during reaction and flux studies both with and without catalyst; investigation into this phenomena is underway.

This work has shown that metal/oxide patterns can be uniformly created on membrane surfaces via a reproducible fabrication technique. The ability of the deposited catalyst to increase oxygen flux through the membrane has been demonstrated, and investigation into its ability to enhance reaction is ongoing.

References

1. C. Y. Tsai, A. G. Dixon, W. R. Moser, and Y. H. Ma, AIChE Journal, 43 (11A), 2741 (1997).

2. U. Balachandran, J. T. Dusek, P. S. Maiya, B. Ma, R. L. Mieville, M. S. Kleefisch, and C. A. Udovich, Catal. Today, 36, 265 (1997).