Hydrogen Extraction with Palladium Based Membranes

Abstract: Palladium membranes are commercially used to purify hydrogen gas and in dehydrogenation reactions. The combination of the catalytic ability of the membrane surface and the selectivity of hydrogen permeation offers a tool to extract pure hydrogen and to shift a dehydrogenation reaction towards the product side. In this thesis, hydrogen extraction over palladium and palladium-silver based membranes both from different gas mixtures and from dehydrogenated organic molecules is investigated. The aim has been to find the optimal conditions for hydrogen extraction in different environments.The hydrogen permeation rate has been shown to depend on both silver concentration on the surface and in the bulk of a palladium based membrane. The diffusion through the membrane is the rate limiting step in the permeation process of most studied membranes. For a palladium membrane with 20 Å silver deposited on the upstream surface, the surface reactions, however, become rate limiting.Co-adsorbed oxygen will inhibit hydrogen permeation by blocking hydrogen adsorption sites and by consuming already adsorbed hydrogen in the water forming reaction on Pd membrane surfaces. On Pd70Ag30 membranes, however, oxygen has no effect on the hydrogen permeation rate, mainly due to an effective hydrogen dissolution into silver and a strongly reduced water formation rate. CO blocks hydrogen adsorption sites on both Pd and PdAg membranes effectively below 150°C, but above 300°C, CO has almost no effect on hydrogen permeation.Hydrogen can also be extracted through the dehydrogenation of organic molecules. A steady and continuous dehydrogenation of methanol and ethanol, and a subsequent hydrogen permeation, can be maintained in the presence of oxygen through both Pd and PdAg membranes. Without oxygen, a blocking contaminating layer is formed from the decomposition products, which prevents alcohol adsorption and thus also the hydrogen permeation. The hydrogen yield is larger over PdAg membranes than over Pd membranes mainly due to a smaller hydrogen consumption in the water forming reaction, but also due to a larger conversion of the alcohol on PdAg.The long time objective of this research has been to develop a method to extract hydrogen from anaerobic bacteria degradation of organic waste material in a co-operation project with microbiologists at the Department of Water and Environmental Studies at Linköping University. The selectivity towards hydrogen permeation in palladium membranes offers a tool to obtain clean hydrogen, which can be used as an energy carrier. By draining the bacteria culture of hydrogen, and thereby reducing the partial pressure of hydrogen, the fermentation process is directed towards a higher production of hydrogen.