For the separation of hydrogen and impurities.
The principle of palladium hydrogen purification is that, when the hydrogen to be purified is passed into the side of the palladium tube at 300-500°C, hydrogen is adsorbed on the palladium tube wall.
The 4d electronic layer of palladium lacks two electrons. It can generate unstable chemical bonds with hydrogen (the reaction of palladium and hydrogen is reversible). Under the action of palladium, hydrogen is ionized into protons with a radius of 1.5×10 15 m. The lattice constant of palladium is 3.88×10-10m (at 20°C), so the palladium can act as a proton to combine with electrons and re-form hydrogen molecules to escape from the other side of the palladium tube. On the surface of the palladium tube, gas that has not been dissociated is impermeable, so that high purity hydrogen can be obtained using a palladium tube. Although palladium has unique permeation properties for hydrogen, pure palladium has poor mechanical properties, is easily oxidized at high temperatures, has a low recrystallization temperature, and easily deforms and embrittles the palladium tube. Therefore, pure palladium cannot be used as a permeation membrane. Adding a proper amount of Group IB and Group VIII elements to palladium to make a palladium alloy improves the mechanical properties of palladium.
In the currently used palladium alloys, about 20-30% of silver is used, and the content of other components (such as gold, etc.) is <5%. The rate at which hydrogen permeates the palladium alloy is related to the temperature, the thickness of the membrane, and the pressure difference (ΔP) between the raw hydrogen and the pure hydrogen on both sides of the penetrating contact. Increasing the temperature, increasing the ΔP, and decreasing the thickness of the membrane will increase the hydrogen transfer rate. However, the increase in temperature will reduce the tensile strength of the permeable membrane. Therefore, the use temperature of the palladium tube is usually controlled at about 450°C.
Some impurities can lead to palladium poisoning, deteriorate the permeability of the gas, and even destroy the membrane. Substances that can cause palladium poisoning include: mercury, arsenides, halides, oil vapors, sulfur and ammonia-containing substances, and dust. The palladium alloy can be made into a tube (called a palladium tube) or a diaphragm (called a palladium film).