(696d) Hydrogen Permeation in Palladium-Based Membranes in the Presence of Carbon Monoxide

Carbon monoxide is present in steam reforming processes, one of the most promising fields for the application of Pd-based membranes, and it is well known that hydrogen flux through palladium-based membranes is drastically decreased by CO, due to its interaction with the metal surface. In this work a theoretical model to describe hydrogen permeation in palladium and silver-palladium membranes in presence of carbon monoxide is proposed and validated. Unlike the couple of empirical equations proposed to that aim, in the model developed the effects of CO are included in the framework suggested by Ward and Dao whose approach has been modified by considering the competitive adsorption of the different non-inert molecules on the metal interface. The competitive adsorption of CO and H2 has been examined accounting for the spectrum of information available for CO adsorption on palladium, as well as for hydrogen in palladium and palladium-silver alloys. The model has been validated using several literature data spanning in a rather broad range of carbon monoxide partial pressures, temperatures and membrane thicknesses. The data available in the open literature allow the independent estimation of most of the parameters involved in the model so that only one of them needed to be adjusted to obtain a satisfactory representation of the system behaviour, namely the heat of adsorption of CO which is linearly related to the carbon monoxide surface coverage. With the same values of the parameters the model is able to represent well all the different behaviours observed, i.e. the diffusion limited regime obtained with high temperatures and/or large thicknesses, the adsorption limited regime observed with decreasing temperatures in the presence of CO, as well as the desorption limited regime when the effects of CO vanish. The model thus a valuable tool for the prediction of expected membrane behaviour and for membrane modules design. In addition, due to its strong physical bases it can be naturally extended to include the possible presence of other non-inert molecules, also known to affect the H2 permeate fluxes, such as SH2 or water vapor.