Because of their various potential applications, non-porous, selective oxygen permeable ceramic membranes with mixed ionic and electronic conductivity (MIECs) have received significant interest over the past decade [1-8]. In particular, the incorporation of these materials into catalytic reactors for the oxidation of hydrocarbons is being investigated as a way to eliminate costly air separation steps and to achieve staged addition of oxygen [1-7]. Previous studies have typically focused on the use of ceramic MIEC membranes for the partial oxidation of methane and have shown that the presence of a catalyst near the permeate surface of a membrane can increase oxygen flux up to ten times by reducing the permeate side oxygen partial pressure [1,2]. However, currently attainable oxygen fluxes remain insufficient for widespread industrial-scale partial oxidation applications.
This work describes the novel use of a mixed conducting oxygen permeable SrFeCo0.5Ox (SFC) membrane for the CO2 reforming of CH4 and demonstrates that oxygen evolved from a membrane in direct contact with a reforming catalyst can significantly enhance catalyst performance. Additionally, semiconductor processing techniques have been used to deposit catalyst particles and thin catalyst films directly on the high pO2 surfaces (source side) of SFC membranes, and the effect of deposited catalyst on both membrane oxygen flux and catalyst performance is reported.
Flux and reaction experiments were conducted in custom-built stainless steel and quartz CSTR-type reactors with disk-shaped SFC membranes. For the CO2 reforming experiments, a traditional powder catalyst (0.43 wt% Pt/ZrO2) was dispersed in a thin layer across the membrane's permeate side to ensure good contact with the membrane surface. The baseline reaction set includes tests with a small amount of co-fed oxygen to examine the effect of the mode of oxygen introduction. These reactions were performed over a stainless steel “blank” membrane coated with the same inert BN3 paint used to coat the interior of the stainless steel reactor. No catalyst reduction was performed prior to any of the reactions.
For deposited catalyst particle studies, the SFC membranes were polished to a near optical finish before being patterned with Pt using a bi-layer lift-off photolithography procedure common to the semiconductor processing industry. Unpatterned membranes were also polished to maintain consistent surface area within a study.
The powder Pt/ZrO2 catalyst was chosen for the CO2 reforming studies because it exhibits relatively rapid deactivation under the reaction conditions studied (800°C, 10 mL/min each of CH4 and CO2 with 5mL/min of Ar). As expected, the catalyst lost most of its activity within 2 hours of operation on the “blank” membrane, which is consistent with this catalyst's performance in a packed-bed reactor configuration. The activity trends with 1 mol% oxygen in the feed stream are nearly identical, indicating a similar rate and extent of catalyst deactivation with and without co-fed oxygen. As confirmed by the parallel packed-bed reactor study, a slightly higher steady state methane conversion occurs as a result of the added oxygen. However, replacing the stainless steel blank with an SFC membrane produces much slower and less extensive deactivation as well as higher initial activity than with co-fed oxygen. This observation supports the conclusion that oxygen alone does not retard the deactivation of this Pt/ZrO2 catalyst and suggests that the addition of oxygen via the membrane is more beneficial than oxygen added to the reactor feed stream.
Catalyst deactivation can result either from loss of catalyst surface area via platinum particle sintering or from carbon deposition and/or adsorption of other species. After six hours of reaction without oxygen over the blank membrane, 1 mol% oxygen was added temporarily to the reactor feed. During this period, methane conversion was comparable to the conversion with continuously co-fed oxygen in the feed (1 mol%) at the corresponding reaction time, which confirms the assumption of similar catalyst deactivation behavior with and without co-fed oxygen. After seven hours of reaction over the blank with continuous co-fed oxygen, the reactant feed was stopped and the catalyst was exposed to 1% oxygen in argon for 2 hours before restarting the reactant feed. The catalyst showed no significant increase in activity upon resuming the CO2 reforming reaction, which implies irreversible catalyst deactivation that could be caused by platinum sintering. This hypothesis was supported by similar results from packed-bed reactions, which included post-run temperature-programmed oxidations (TPOs) that indicated negligible carbon deposition. Platinum particle sintering in the used powder catalyst is currently being evaluated by transmission electron microscopy.
Flux studies were performed using air as the oxygen source and argon as the sweep gas. As described previously [9], oxygen flux was determined at temperatures between 500 and 800 °C for both an unpatterned polished membrane and an identical membrane patterned with 5 μm diameter platinum circles spaced by 3 μm (120 nm deposition thickness), and the platinum features were observed to increase membrane oxygen flux by approximately 100% at all temperatures that exhibited flux. Additionally, the platinum pattern reduced the temperature at which oxygen permeation could first be detected from 600 °C to 550 °C. Subsequent work has been conducted on the deposition of very thin films (~0.7 nm) of platinum and palladium on the oxygen source side of SFC membranes including surface roughness analysis by atomic force microscopy to obtain estimates of the true membrane surface area. Oxygen flux and catalyst activity profile results for these thin film-coated membranes will be presented.
<>References
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