PdC formation in ultra-thin Pd membranes during separation of H2/CO mixtures

The influence of typical syngas CO concentrations (20–33%) on the stability of 3 μm thick Pd membranes was investigated between 593 and 773 K in the limit of maximum H2 extraction. Two kinds of CO inhibition effects were observed: CO adsorption and carbon deposition resulting from catalytic decomposition of CO on the Pd surface. CO adsorption reduced the H2 permeation rate in particular below 623 K due to reversible blocking of Pd surface sites. Carbon deposition became more severe at higher temperatures and feed gas velocities and accumulated with time. In particular, the formation of a Pd1−xCx phase was observed above 723 K with x = 0.1. The propensity to PdC formation increased with temperature and the CO concentration in the retentate. Although the carbon incorporated into the Pd membrane could be effectively removed by H2 and O2 at 723 K, an irreversible increase of the N2 flux after the removal of the carbon was observed in particular after treatment with O2.