Oxygen permeation studies in surface Pd-activated asymmetric Ce0.9Gd0.1O1.95 membranes for application in CO2 and CH4 environments

Abstract

Oxygen Transport Membranes (OTMs) present a high potential for being considered in the integration of O2 supply systems in oxyfuel installations, as well as for the conduction of chemical reactions when operating Catalytic Membrane Reactors (CMRs). Several solutions are being prospected for overcoming the main drawbacks regarding materials stability and membrane performance. A highly stable material such as Ce0.9Gd0.1O1.95 (CGO) doped with 2% mol. Co was studied as a 40 µm thick CGO supported CGO membrane. This membrane was characterized by studying its performance as oxygen permeation membrane for the production of oxygen under oxyfuel conditions and for the conduction of chemical reactions involving CH4. In order to improve oxygen surface reactions and consequently, the oxygen permeation, the membrane was surface activated with the addition of Pd nanoparticles. A broad characterization consisting of the study of O2 production under different environments simulating real application conditions was conducted by subjecting the membrane to Ar, CO2 and CH4 environments in the temperature range of 750 to 1000 ºC. A peak oxygen flux of 7.8 ml•min -1 •cm -2 was obtained at 1000 ºC when using a sweep consisting of 75% CH4 in Ar. This flux corresponds to a 16-fold improvement in the O2 permeation at 1000 ºC when sweeping with Ar, with an oxygen flux of 0.47 ml•min -1 •cm -2 . An oxygen flux of 1.2 ml•min -1 •cm -2 was obtained at 1000 ºC when feeding with pO2 = 1 atm in feed side. Membrane performance under CO2-containing environments showed a positive effect of CO2 on permeation at 1000-900 ºC, reaching up to 0.59 ml•min -1 •cm -2 O2 at 1000 ºC. A continuous exposure of CO2 during 48 hours at 750 ºC resulted in a slight J(O2) increase, with a reversible reduction in performance when returning to clean conditions, thus demonstrating high stability of CGO membranes.

Ice-Templating for the Elaboration of Oxygen Permeation Asymmetric Tubular Membrane with Radial Oriented Porosity

“…Specifically, Figure 9b presents the variation in J O2 as a function of pO 2 at 1000 • C. At this temperature, J O2 increases linearly with pO 2 and reaches a value of 1.07 mL•min −1 •cm −2 under pure oxygen, which would correspond to a 5 bar air feeding. Again, these values are close to the obtained for the asymmetric CGO-Co membrane [32], and much lower than the 17.5 mL•min −1 •cm −2 obtained with a planar LSCF freeze-cast membrane under similar conditions [6]. Figure 9c depicts the evolution of O 2 permeation flux as a function of the feed (air) flow rate at 1000 • C. For this scenario, the oxygen permeation increases linearly with the increase of the inlet flow for values lower than 150 mL•min −1 where it is of 0.22 mL•min −1 •cm −2 .…”

“…It can be seen that oxygen permeation increases with temperature and reaches 0.31 mL•min −1 •cm −2 at 1000 • C while it is of 0.08 mL•min −1 •cm −2 at 850 • C. The obtained results are far from the expected performance as previously reported in planar all-LSCF membranes deposited on freeze-cast [6,31] and tape-cast [5] porous supports, where oxygen fluxes of 6.8 and 4.3 mL•min −1 •cm −2 were obtained for 30 µm-thick membranes. Instead, the tested tubular membrane presents a similar performance to the data observed in a 40 µm-thick asymmetric CGO-Co membrane [32], as can be seen in Figure 9a. This can be ascribed to a low porosity of the 80 µm-thick CGO-Co layer ( Figure 5), which impedes feed gas diffusion to the LSCF layer, becoming a hindrance for oxygen permeation.…”

Evaluation of Er Doped CeO2-δ as Oxygen Transport Membrane

“…The permeation values are in agreement with similar ceria doped membranes, as can be seen in Table 1. Garcia-Fayos et al [33] measured a CGO thin film membrane, surface activated with Pd, in which no electronic carriers were introduced. By extrapolating the results to a membrane of thickness similar to this study (769 µm) with the Wagner's equation, an O 2 flux of 0.024 mL•min −1 •cm −2 at 1000 • C is estimated.…”

“…These results enlighten the influence of the electronic contribution and nature of the trivalent dopant to the performance of doped-ceria materials as oxygen transport membranes. CGO thin film (Pd activated) 0.024 40 1000 [33] The influence of the O 2 gradient through the membrane was evaluated by pure O 2 as feed for the CeEr20 + Co sample. As it is observed in Figure 5b, O 2 flux increases with the pO 2 in the feed, i.e., pO 2 gradient across the membrane, as it is postulated by the Wagner equation [34,35].…”