On CH 4 decomposition during separation from H 2 mixtures with thin Pd membranes

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 2H 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 0.9C 0.1 and/or Pd 0.85C 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.