Time and frequency domain analysis of hydrogen permeation across PdCu metallic membranes for hydrogen purification

Hydrogen permeation is a process used in the industry for purification purposes. Palladium alloys (PdAg and PdCu) are commonly used as membrane material. In this communication, we report on the kinetics of hydrogen permeation across Pd0.47Cu0.53 metallic membranes which can be used in catalytic crackers of biofuels. The permeation mechanism is a multi-step process including surface chemisorption of molecular hydrogen (upstream side of the membrane), hydrogen diffusion across bulk regions, hydrogen recombination (downstream side of the membrane) and evolution. The role of different operating parameters (temperature, surface state, sample microstructure) is analyzed and discussed using both time and frequency domain experiments. Experimental pneumato-chemical impedance diagrams show that there is no significant rate-limitation at surfaces, except at low temperatures close to room temperature. Diffusion-controlled transport of hydrogen across the membrane is rate-determining. However, the value of the hydrogen diffusion coefficient does not rise exponentially with operating temperature in the 40–400 °C temperature range under investigation, as expected for a thermally activated diffusion process. At temperatures as low as 300 °C, new rate-limitations appear. They can be attributed to recrystallization and/or phase transformation processes induced by temperature and the presence of hydrogen.