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

The influence of high CH4 concentrations (8–31%) on the H2 permeation through a 2 μm thick Pd membrane was investigated between 473 and 823 K at 100 kPa pressure difference at the maximum H2 extraction limit, with the retentate H2 concentration always staying above 50%. The permeate flux was slightly higher in CH4/H2 mixtures than in equimolar N2/H2 mixtures. This is attributed to decomposition of CH4 on the Pd surface as indicated by trace amounts of C2H6. At 723 K and below H2 permeation remained stable during 6 h experiments in CH4/H2 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 CH4. Following CH4 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 CH4 exposure above 723 K and graphite platelets after treatment at 823 K. Furthermore, metastable Pd0.9C0.1 and/or Pd0.85C0.15 phases were detected after CH4 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 N2 between 623 and 723 K. Mechanisms for deactivation of the Pd membrane during separation of CH4/H2 mixtures at elevated temperatures are discussed.