Development of Nb-based alloy membranes with improved resistance to corrosion by H2S

Abstract

At present, the interest in non-Pd hydrogen permeation membranes mainly focuses on group 5 B metals (Nb, V, Ta) and their alloys. To solve the problem of easy oxidation and weak dissociation of hydrogen molecules, a "Pd catalytic layer" needs to be deposited on the membrane surface. Due to its limited stability towards sulfur, issues of irreversible poisoning by sulfur compounds must be addressed. To this end, Pd-, Pd65Ag35-, Pd90Cu10-, Pd80Au20-, and Pd55Ag35Au10-coated Nb30Hf35Co35 membranes were firstly synthesised by magnetron sputtering, and their hydrogen permeation behaviour in pure H2 and/or in different H2/H2S mixtures for up to 1000 ppm at 673 K were evaluated. At these conditions, the permeabilities of the Pd-coated membranes decrease rapidly with increasing the H2S concentration and cannot recover after removal of H2S from the feed mixtures. In comparison, the Pd–Ag-containing membrane greatly increases the pure H2 permeability, whereas the membrane doped with Au can effectively improve the response to H2S. Specifically, the Pd65Ag35-coated membrane shows the highest pure H2 permeability, but it loses maximum permeability value on exposure to H2/H2S feed mixtures. In contrast, Pd80Au20-containing membranes exhibit good tolerance of H2S poisoning and displayed a high fractional recovery (∼83%) of initial H2 permeability. The degradation of permeability for Pd- or Pd65Ag35-coated membranes after the H2S exposure can be ascribed to the deterioration of the structure caused by the possible formation of bulk Pd4S, which has been corroborated with EDS analysis, XRD, and XPS after testing. Finally, to improve both permeability and chemical tolerance, a Pd55Ag35Au10-coated membrane was successfully fabricated, and it possesses excellent comprehensive performance with respect to the permeability and H2S response. At 120 ppm H2S in the feed, the H2 flux equals ca. 43% of its original value at 673 K and can be stable for at least 166 h. Bulk sulfides did not form on the membrane surface after the testing. This paper not only developed a new type of group 5 B alloy membrane with improved resistance to corrosion by H2S, but also further confirmed the feasibility of improving H2S corrosion resistance by adjusting the catalytic layer type or composition of the outermost coating.