This review highlights various aspects of current palladium membrane research and serves as a comprehensive bibliography covering palladium membrane preparation methods and applications. There are many promising uses for palladium membranes, although widespread use of the available technologies is constrained primarily by the high cost of palladium, lack of durability due to hydrogen embrittlement, and susceptibility to fouling. Various researchers in the field are tackling these problems and fabricating thinner palladium alloy composite membranes that better withstand contaminantion and thermal cycling. What has been accomplished to address these issues and the directions presently being explored are discussed.

Innovations in palladium membrane research

Modelling of packed bed membrane reactors for autothermal production of ultrapure hydrogen

Ultra-thin, high performance composite palladium membrane, developed via a novel electroless plating method, was applied to construct a membrane reactor for methane steam reforming reaction, which was investigated under the following working conditions: temperature 723–823 K, pressure 300–900 kPa, gas hourly space velocity (GHSV) 4000–8000  mL g cat − 1 h − 1 , steam-to-carbon feed ratio (S/C, mol/mol) 2.5–3.5 and sweep ratio (defined as the ratio between flux of sweep gas to that of methane at the inlet of catalyst bed) 0–4.3. In contrast with previous investigations using commercial catalysts activated at lower temperatures, the catalyst applied in this work was a nickel-based one pre-reduced at 1023 K. The results indicated that selective removal of H2 from reaction zone obtained methane conversion much higher than thermodynamic control ones and CO selectivity significantly lower than thermodynamic control values. For instance, 98.8% methane conversion, over 97.0% selectivity to CO2 and over 95.0% H2 recovery rate could be obtained under mild working conditions. The much higher performance of membrane reactor was attributed to the combination of hydrogen ultra-permeable Pd-based membrane, highly active catalyst for methane steam reforming with countercurrent sweep gas flux design. Further work on stability investigation may develop an efficient onsite route of hydrogen production for application to proton exchange membrane fuel cells.

Efficient production of hydrogen from natural gas steam reforming in palladium membrane reactor

Hydrogen separation from binary gas mixtures, Ar−H2 and CO−H2, using a double-tube type of palladium membrane reactor (PMR) was carried out and analyzed by solving mathematical models taking into account mixing diffusion of hydrogen in the radial direction of the catalyst-packed bed. The experiment showed that carbon monoxide prevented hydrogen permeation through the membrane at temperatures less than 280 °C. The decline in hydrogen permeation could be estimated by solving a model describing the concentration polarization and the hindrance by cabon monoxide. Furthermore, the mathematical models were applied to analyze methanol decomposition in the same PMR, showing that a drop in hydrogen permeation due to both the factors had a significant influence on the performance of the PMR.

Decline in Hydrogen Permeation Due to Concentration Polarization and CO Hindrance in a Palladium Membrane Reactor

A carbon membrane reactor

A previous study has shown that a tubular packed-bed type of palladium membrane reactor can considerably promote the catalytic dehydrogenation reaction of cyclohexane by selectively separating hydrogen produced in the course of reaction through palladium membrane. Effects of radial diffusion on the hydrogen separation from a low-flow-rate gas mixture containing hydrogen was examined under atmospheric pressure at 473 K using an annular packed-bed type of palladium membrane reactor. The axial profiles of hydrogen concentration (partial pressure) in the packed bed were experimentally measured and mathematically analyzed using two flow models, ideal and radial diffusion models. The ideal flow model could not explain the experimental results very well. On the other hand, the radial diffusion model could successfully simulate the profiles of hydrogen partial pressure formed along the packed bed of palladium membrane reactor. It became clear that there exists a gradient of hydrogen partial pressure in the radial direction of the packed bed due to its permeation through palladium membrane. In addition, Peclet numbers at low Reynolds numbers were determined and were explainable by the extended Wen and Fan equation.

Wei Chun Xu

Papers

Radial mixing diffusion of hydrogen in a packed-bed type of palladium membrane reactor