Influence of the selective layer morphology on the permeation properties for Pd-PSS composite membranes prepared by electroless pore-plating: Experimental and modeling study

This work analyzes the relationship between hydrogen permeation behavior and palladium layer morphology, particularly taking into account the palladium introduction into the pores for composite membranes prepared by Electroless Pore-Plating (ELP-PP) over porous stainless steel supports. Fully dense membranes have been achieved with hydrogen permeances in the range 7-18·10-5mol/m2sPa0.5 by varying the hydrazine concentration used for the palladium plating. Permeation measurements for pure gases and mixtures have been carried out at different conditions: pressure driving force, temperature and feed composition. Both morphology and permeation properties are clearly influenced by the concentration of hydrazine, used as reducing agent during the ELP-PP process. In this manner, higher hydrazine concentrations provoke the formation of thicker external palladium films and, consequently, the reduction of both external porosity and roughness. On the other side, lower hydrazine concentrations reduce the required amount of palladium to obtain a fully dense membrane. In the last case, however, part of palladium is introduced into the pores of the support in a huger extension. This fact, in turns, generates an additional hydrogen permeation resistance, which diminishes the permeation flux and introduces a threshold in the minimum trans-membrane pressure value required for a hydrogen permeate flux. A new mathematical model, based on the classical Sieverts' law, is proposed to predict this particular behavior by including a new fitting parameter defined as the ratio of the gas-palladium surface in both retentate and permeate sides of the membrane. This new parameter has been denoted as η, achieving good accuracy between both predicted and experimental data when considering the gap in the trans-membrane pressure. Therefore, the presented model can be used in multiple cases to design and optimize membranes modules, whereas the retentate and permeate gas-palladium surface ratio will be different, as for instance in case of palladium is introduced partially into the pores of supports.