Catalytic surface activation of ceramic oxygen transport membranes based on BSFC and LSFC

摘要

Oxygen transport membranes based on mixed oxides allow the selective O_2 separation at high temperature. The development of highly permeable materials and membranes would make possible their integration in oxyfuel power plants, which can readily apply CCS technologies. For mixed ionic-electronic conducting (MIEC) membrane, the bulk oxygen ionic diffusion and surface exchange kinetics steps are particularly important for the oxygen transporting process. Oxygen permeation shows Arrhenius behaviour and, the apparent activation energies for oxygen transport often change as a function of the temperature, so the rate limiting step changes according to the temperature range investigated:rn(a) At higher temperatures, the bulk oxygen-ion diffusion is the limiting step for oxygen permeation across the disk membrane, and hence the oxygen permeation flux (J_(O2)) should be given by Wagner equationrn(b) At temperatures below the characteristic thickness, the permeation of oxygen is limited by surface steps, i.e., the rate of oxygen exchange between the gas and the membrane surface. Bouwmeester et al. proposed an empirical power equation.rnOxygen permeation studies and catalytic membrane tests were carried out in the same experimental set-up. The disk-shaped planar membrane (diameter 15 mm, thickness ～ 0.5 - 0.7 mm) was sealed using gold gaskets. Synthetic air was fed to the oxygen-rich membrane compartment, while argon was used as a sweep gas on the permeate (reaction) side. Gas chromatography and mass spectrometry allowed complete analysis of gases at both sides of the membrane.rnPreparation of membranes samples was performed using Ba_(0.5)Sr_(0.5)Fe_(0.8)Co_(0.2)O_(δ_3) (BSFC) and La_(0.58)Sr_(0.4)Fe_(0.8)Co_(0.2)O_(δ-3) (LSFC) powders, respectively. The catalyst formulation of the active layer was based on MIEC (perovskites). Different catalyst formulations were considered, varying the element in the A-site of perovskite structure (ABO_(3-δ)). The perovskite-based compounds were synthesized by EDTA-citrate complexation route. The modification of the membrane surface was done on the permeate side of the membranes by screen-printing and subsequent sintering.