Promotion of mixed protonic–electronic transport in La5.4WO11.1−δ membranes under H2S atmospheres

Abstract

Catalytic membrane reactors (CMR) based on H₂-separation membranes can improve the performance of thermodynamically-limited reactions such as high-pressure steam methane reforming, ammonia cracking, non-oxidative aromatics production, and water gas shift reaction (WGS). In these industrial processes, the membrane surfaces are typically exposed to steam, CO₂, CO, H₂S, and hydrocarbons in combination with high temperatures. Therefore, the membrane materials require long-term thermo-chemical stability under the mentioned conditions. Stability in H₂S is of outstanding importance since its presence, even at ppm level, gives rise to substantial surface poisoning and decomposition of most materials. Here we characterize the influence of H₂S on the crystalline structure, lattice composition, and hydrogen-transport properties of La_5.4WO_11.1−δ, one of the reference protonic membrane materials. The incorporation of sulfide ions in the crystal lattice is ascertained from XRD, XPS, FESEM, WDS, EDS, and FIB-SIMS analyses. UV-vis spectroscopy and EIS measurements illustrate the effect of the incorporated sulfur in the transport properties, i.e., vigorously promoting the electronic conductivity mediated by the concurrent partial reduction of tungsten cations (W⁶⁺). The rise in electronic conductivity allowed an H₂ flux of 0.042 mL cm⁻² min⁻¹ to be reached at 700 °C for a ∼700 μm-thick membrane, in contrast with negligible H₂ permeation in H₂S-free conditions.