On CH4 decomposition during separation from H2 mixtures with thin Pd membranes

• DOI: 10.1016/j.memsci.2008.06.056
• Publication Date: 2008-07-10T10:09:22Z
• Authors: Hui Li; Andreas Goldbach; Wenzhao Li; Hengyong Xu
• Keywords: Carbon fibers

Abstract The influence of high CH 4 concentrations (8–31%) on the H 2 permeation through a 2 μm thick Pd membrane was investigated between 473 and 823 K at 100 kPa pressure difference at the maximum H 2 extraction limit, with the retentate H 2 concentration always staying above 50%. The permeate flux was slightly higher in CH 4 /H 2 mixtures than in equimolar N 2 /H 2 mixtures. This is attributed to decomposition of CH 4 on the Pd surface as indicated by trace amounts of C 2 H 6 . At 723 K and below H 2 permeation remained stable during 6 h experiments in CH 4 /H 2 mixtures, whereas it gradually decreased with time above that threshold. TPO, SEM and XRD revealed several types of carbon forming on the surface of membrane pieces during exposure to pure CH 4 . Following CH 4 treatment below 723 K two types of carbon deposits were found, which are designated as carbidic carbon and disordered carbon aggregates. Carbonaceous filaments were observed after CH 4 exposure above 723 K and graphite platelets after treatment at 823 K. Furthermore, metastable Pd 0.9 C 0.1 and/or Pd 0.85 C 0.15 phases were detected after CH 4 treatment at 573, 623, and 773 K. Carbidization of the membrane was most severe at 623 K, but the carbides could be readily decomposed by heat treatment in N 2 between 623 and 723 K. Mechanisms for deactivation of the Pd membrane during separation of CH 4 /H 2 mixtures at elevated temperatures are discussed.