Palladium, group VIII of the fifth cyclePlatinum series element, element symbol Pd.Pure palladium is silver whitetransition metal, soft, goodDuctilityandPlasticityIt can be forged, calendered and drawn.Lump metal palladium can absorb a large amount ofhydrogen, making the volume significantly expand, become brittle and even break into pieces.
Palladium was produced by British chemists in 1803WollastonfromPlatinum oreIt is one of the indispensable key materials in high-tech fields such as aerospace, aviation and automobile manufacturing.
In 1803, British chemistWollastonA new element has been discovered from platinum ores.He dissolved the natural platinum ore in aqua regia, removed the acid, and drippedMercuric cyanide（Hg(CN)two）Solution to obtain yellow precipitate.takesulfur、boraxHeating together with the precipitate can obtain bright metal particles.He called it palladium, and the element symbol was Pd.The word comes from the asteroid discovered at that timeZhishen StarPallas，It originates from Balas Pallas, the goddess of wisdom in Greek mythology.
An important step for Wollaston to discover palladium is to selectMercuric cyanide。Although mercury cyanide solution contains almost no cyanide ion (CN-）However, when palladium ion (Pd2+）When you meet it, it immediately generates yellowishPalladium cyanide (II)（Pd(CN)two）Precipitation, and other platinum elements will not form this cyanide precipitation.
Distribution
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It can be separated from the natural alloy of platinum[1]。Palladium reserves on the earth are scarce, so it is difficult to mine and smelt. It belongs to the rare and precious metal series gold, silverplatinum, Palladiumruthenium、iridiumCategory of.The content of palladium in the crust is 0.0006ppm, and it is often dispersed in various minerals of alluvial deposits and placer deposits (such as original platinum ore, nickel copper sulfidePentlanditeEtc.).Independent minerals include hexagonal palladium ore, palladium platinum ore diversion, one lead four palladium ore, antimony palladium ore, bismuth lead palladium ore, tin palladium ore, etc. Natural palladium is also formed in free state.
The melting point of palladium is the lowest among platinum group metals.
Physical and chemical properties
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physical property
Palladium metal
Palladium is a silvery white transition metal, which is soft, has good ductility and plasticity, and can be forged, calendered and drawn.Lump metal palladium can absorb a large amount ofhydrogen, making the volume significantly expand, become brittle and even break into pieces.Ultrafine palladium powder is gray black powder.Palladium powder with micron size, scaly, amorphous and spherical.The loose packing density of flake palladium powder is 0.67 ～ 0.95g/cmthree。Shake density 1.6~2.00g/cmthree。Average particle size0.35～0.48μm，Specific surface area3.3～5.2mtwo/g。The electrical performance is stable, and the hydrogen adsorption capacity is large.[8]
1 volume at room temperatureSponge palladiumIt can absorb 900 volumes of hydrogen, and 1 volume of colloidal palladium can absorb 1200 volumes of hydrogen.When heated to 40~50 ℃, most of the absorbed hydrogen is released, which is widely used as a gas reaction, especially a hydrogenation or dehydrogenation catalyst, and can also be used to make resistance wires, clocks and watches alloys, etc.
The fifth cycle Group VIII platinum elements
Element type: metal element
Atomic number: 46
Proton number: 46
Atomic weight: 106.42
Melting point: 1554 ℃
Boiling point: 2970 ℃
Density: 12.02g/cmthree（20℃）
Mohs hardness: 4.75
Sound propagation speed: 3070m/s
Element content: 0.003ppm in the sun, 0.000000019ppm in the Pacific Ocean surface, 0.0006ppm in the crust
chemical property
Reaction of Palladium with Air
Palladium oxide (Ⅱ) (PdO) can be formed by heating palladium in oxygen.[7]
It is heated to 800 ℃ in air to form a dull oxide film, which will decompose when the temperature is higher than this temperature.[8]
Reaction of palladium with halogen
Palladium andFluorine gas（Ftwo）React under controlled conditions and generatePalladium trifluoride。This compound is not Pd (Ⅲ) fluoride, but a mixed salt of Pd (Ⅱ) and Pd (Ⅳ) [Pd] [PdFsix]。[7]Pd (Ⅱ) in the compound is of high spin configuration, so "Pd (Ⅲ) fluoride" isParamagnetismchemical compound.Pd[PdFsix]And SeFfourCommon reflux and distillation can obtainPalladium difluoridePdFtwo。[9]
Palladium in red hot state reacts with chlorine to formPalladium chloridePdCltwoPdCl at 600 ℃twoStart to sublimate, and decompose into palladium and chlorine at the same time;At 738 ℃, itsDecomposition pressureIt reaches 101kPa.[9]
Reaction with acid
Resistance at room temperaturehydrofluoric acid、phosphoric acid, hydrochloric acid and sulfuric acid.Soluble in aqua regia, hot nitric acid, sulfuric acid, slightly soluble in hydrochloric acid.Insoluble in cold and hot water.[8]
Other properties
Palladium powder can burn in case of high temperature and open fire.Andformic acidorSodium borohydrideThe reaction gives off hydrogen.AndIsopropanolSevere reaction occurs.[8]
Preparation method
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1. Industrial production can be made from ore by dry method;Orcopper、nickelThe by-products generated in the production process of producing copper and nickel from sulphide ore are used as raw materials and are produced by hydrometallurgy.The residual components after nickel and copper extraction are used as raw materials for wet process, and thenAqua regiaExtract, filter and add to the filtrateammoniaandhydrochloric acidReact and generateAmmonium chlorpalladiumSedimentation.After refining and filtering, ammonium chlorpalladium is reduced with hydrogen to obtain 99.95% finished palladium.
2. At PdCltwoOr NatwoPdClfourNH added infourCl generates possible trace PtAmmonium chloroplatinate(NHfour)twoPtClsixSedimentation.Add excessive ammonia into the filtrate and boil it, and filter it again if necessary.Then acidified with hydrochloric acid to produce pure yellowPalladium dichlorodiamine[PdCltwo(NHthree)two]Sedimentation.If the precipitate is dark yellow, it contains a small amount ofRhodium chloride pentaammonia[RhCl(NHthree)five]Cltwo。The precipitate can be boiled with ammonia, the filtrate can be taken out and acidified with hydrochloric acid again to obtain bright yellow very pure [PdCltwo(NHthree)two]Crystal.Place the crystal at HtwoBurn and restore in the air flow to obtain light graySponge palladium。[8]
application area
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Palladium is an indispensable key material in high-tech fields such as aerospace, aviation, navigation, weapons and nuclear energy, as well as in the automobile manufacturing industry. It is also an investment product that cannot be ignored in the international precious metal investment market.
Palladium is mainly used as catalyst in chemistry;Palladium andruthenium、iridium、silver, gold, copper and other alloys can improve the resistivity, hardness and strength of palladium, and can be used to manufacture precision resistors, jewelry and ornaments.The most common and marketable alloy of palladium jewelry is palladium.
It is mainly used for making catalysts, dental materials, watches and surgical instruments.
Related palladium products
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Palladium
As a member of the platinum group, the element symbol Pd is similar to platinum in appearance, with silver white metallic luster and bright color.The specific gravity is 12, lighter than platinum, and the ductility is strong.The melting point is 1554 ℃, the hardness is 4-4.5, slightly harder than platinum.It is chemically stable, insoluble in organic acid, cold sulfuric acid or hydrochloric acid, but soluble in nitric acid and aqua regia.It is not easy to oxidize and lose luster under normal conditions.Palladium is one of the platinum series metals.Platinum series metals include ruthenium, rhodium, palladium, platinum, etc.Many of them are more expensive than gold. They are typical noble families in metal[2]。
Palladium has excellent physical and chemical properties, high temperature resistance, corrosion resistance, wear resistance and strong extensibility. It can replace platinum and gold in purity, rarity and durability.Palladium is the rarest in the worldnoble metalOne is that the content in the crust is about 1/100 million, much less than gold.In the world, only a few countries such as Russia and South Africa produce gold, and the total annual output is less than 5 ‰ of gold, which is also rare than platinum.Palladium is extremely tough. Jewelry made of palladium not only has the charming luster of platinum like nature, but also can withstand the tempering of years, as new as ever.Palladium is almost free of impurities, extremely pure and shining with white light.The purity of palladium is also very suitable for the skin and will not cause skin allergy.
Palladium membrane
It is usually rolled from palladium alloy and can be made into diaphragm (called palladium membrane) and membrane tube (called palladium tube)[3]。The film thickness is usually 50-100 microns.It is mainly used for hydrogen purification. Its principle is dissolution diffusion mode. The driving force of diffusion is the hydrogen partial pressure difference on both sides of the membrane.At 300-500 ℃, when the raw hydrogen is pressurized into one side of the membrane, the hydrogen molecule first chemisorbs on the surface of the membrane and dissociates into hydrogen atoms, the latter dissolves in the palladium alloy to form hydride, and the small hydrogen atoms are located in the gap of the palladium alloy lattice and can move freely.Driven by the concentration gradient, hydrogen atoms diffuse to the other side of the membrane and precipitate, and then re combine into hydrogen molecules and then desorb.Except for hydrogen and its isotopes, no other gas can pass through the palladium membrane, so ultra-high purity hydrogen can be obtained by using the palladium membrane.Hydrogen permeation rate versus temperature, membrane thickness and hydrogen partial pressure difference on both sides of the membrane △PofIncrease temperature, increase △PAnd reducing the film thickness will increase the hydrogen permeability.However, if the temperature is too high, it will increase energy consumption and reduce the physical strength of the membrane.Therefore, the temperature is usually controlled at about 400 ℃.Some impurities can lead to membrane poisoning, reduce the hydrogen permeability of the membrane, and even damage the membrane.Substances that can cause palladium poisoning include:mercury、Arsenide、halide, oil vapor, includingsulfurAnd ammonia containing substances and dust.Palladium alloy can be made into tubular (palladium tube) or diaphragm (palladium membrane).
Palladium alloy tube
be commonly calledPalladium tube, used for hydrogen purification.Pure palladium has poor mechanical properties and is prone to hydrogen embrittlement, so the material of palladium tube is generally an alloy formed by palladium and 1B and VIII elements.Among the most commonly used palladium tube materials, silver accounts for about 25%, and the content of other ingredients (such as gold) is less than 5%[4]。
Palladium alloy
Alloys composed of palladium and other elements, mainly including[5]：
（1）Palladium alloyPd Au alloy, an alloy containing more than 20% gold Au, is insoluble in nitric acid. Because of its high melting point and high corrosion resistance, this kind of alloy is used to manufacture chemical utensils;The alloy containing 20% Pd-30% Au is used to make artificial fiber drawing dies.
（2）Palladium silver alloyPd Ag alloy: the corrosion resistance of the alloy containing less than 50% silver Ag is close to that of palladium. Adding gold and platinum can improve the performance of the alloy. Pd Ag alloys containing 50% Ag and 10% Pt or Au can be used to manufacture corrosion resistant parts and watch cases of optical instruments.
（3）Palladium iridium alloyPd Ir alloy,iridiumIr can significantly improve the corrosion resistance of Pd, and Pd Ir alloy is used as electrical contact point.
Colloidal palladium
The earliest colloidal palladium was invented by Shipley[6], byPalladium chlorideandStannous chlorideReaction preparation.The diameter of colloidal palladium particles is between 1-100nm. The smaller the size of palladium particles, the higher the catalytic activity and the better the stability.The biggest feature of colloidal palladium activation solution is that sensitization and activation are completed simultaneously in the impregnation process of one solution.When the copper clad foil after palladium drilling and cleaning is immersed in it, colloidal metal palladium particles are adsorbed on the hole wall insulation material and the copper foil surface to form a catalytic layer.On the surface of activated copper foil, there is no palladium ion that can be replaced, so there is no loose copper replacement layer.
Pd/Sn colloidal catalyst is the most widely used colloidal palladium in industrytwoAnd SnCltwoIt is prepared by reaction in acid solution. The colloidal core is palladium, the outer layer is hydrated divalent and tetravalent tin ions, and excess Sn2+It plays a key role in the stability of the catalytic liquid.In modern colloidal catalysts, there are also a lot of acids or salts.
