Pd-Ag thin film membrane for H2 separation. Part 2. Carbon and oxygen diffusion in the presence of CO/CO2 in the feed and effect on the H2 permeability

The effect of CO and CO2 on the performance and stability of Pd-Ag thin film membranes prepd. by electroless plating deposition (EPD) was investigated, observing the presence of dissocn. to carbon and oxygen which slowly diffuse in the membrane influencing also H2 permeability. The effect of the two carbon oxides was investigated both sep. and combined in the 400-450 °C temp. range over long-term cumulative expts. (up to over 350 h) on a membrane that already worked for over 350 h in H2 or H2-N2 mixts. An increase of the H2 permeation flux was obsd. feeding only CO2 in the range 10-20%. This effect was interpreted as deriving from the facilitated H2 flux caused from oxygen diffusion (deriving from CO2 dissocn.) in the membrane. CO induces instead a partial inhibition on the H2 flux deriving from the neg. effect of CO competitive chemisorption as well as C diffusion in the membrane, which overcome the pos. effect assocd. to oxygen diffusion in the membrane. Carbon and oxygen diffuse through the membrane with a rate two order of magnitude lower than hydrogen, and recombinate at the permeate side forming CO, CO2 and CH4 which amt. increases with time-onstream. The effect is reversible and not assocd. with the creation of cracks or defects in the membrane, as supported by leak tests. [on SciFinder(R)]