palladium

Pd. Atomic number 46. Platinum group element in periodic table, with electron configuration [Kr]4d ^10. Atomic weight 106.42(1). Six stable isotopes are known, with mass numbers 102 (1.02(1)%), 104 (11.14(8)%), 105 (22.33(8)%), 106 (27.33(3)%), 108 (26.46(9)%), and 110 (11.72(9)%), as well as radioactive isotopes with mass numbers 91-124. Pd was discovered in 1802 by the British WH Wollaston in a solution obtained after precipitating platinum as (NH ₄ ) ₂ Pt ^IV Cl ₆ from an aqueous solution, and was named after the asteroid Pallas, discovered in the same year. Pallas was the Greek goddess of wisdom, Pallas Athene. In his book "Semitsu Kaisyo" published in 1837, Udagawa Yoan refers to it as Parradium.
Its abundance in the earth's crust is 0.001 ppm, but it is more abundant than platinum among the platinum group elements. It occurs in platinum deposits as minerals such as palladium ore (Pd ₃ Sb), paolovo ore (Pd ₂ Sn), zvyagintsevite (Pd ₃ Pb), blagite (Pt, Pd, Ni)S, and platinum ore (Pt, Pd)S. Of the 71,000 t of extractable platinum group elements, South Africa has 63,000 t, followed by Russia with 6,200 t, and the United States and Canada combined with 1,200 t. South Africa accounts for 25-45% of the palladium in the platinum group metals (depending on the ore deposit), while Russia's Ural deposits account for 71%, making South Africa the country with the largest palladium resources. Of the total supply of 230 t in 2007, Russia and South Africa each accounted for approximately 40%. Refined platinum ore, or anode mud from nickel or copper electrolytic refining is dissolved in aqua regia or Cl ₂ -HCl solution, etc., and the Pd salt is separated and refined by precipitation, recrystallization, solvent extraction, and distillation, after which the metal is obtained by thermal decomposition. A malleable silvery-white metal with excellent ductility. Cubic close-packed structure. Density 12.02 g cm ^-3 . Lightest of the platinum group metals. Lowest melting point of the platinum group metals at 1552 °C, boiling point 3140 °C. Hardness 4.8. First ionization energy 8.34 eV. Standard electrode potential Pd ²⁺ /Pd 0.915 V. Easily soluble in aqua regia, nitric acid, and hot concentrated sulfuric acid. Absorbs nearly 900 times its volume of hydrogen as a hydride. This can be used to purify hydrogen. Usual oxidation number is 2 to 4. It mainly forms divalent compounds, such as PdO (black), PdS (black-brown), PdX ₂ (X = F (brown), Cl (dark red), Br (reddish-brown), I (black)), Pd(NO ₃ ) ₂ (yellow-brown), and PdSO ₄ (reddish-brown). The chlorides, nitrates, and sulfates are soluble in water, while the oxides, sulfides, bromides, iodides, and cyanides are insoluble in water. Pd ^III compounds and Pd ^IV compounds are unstable. Like platinum, it easily forms complexes, and complexes with oxidation numbers 0 and 1 also exist. Many Pd II complexes with planar tetracoordinate structures are known, such as ^Cl- , ^Br- , ^I- , ^CN- , ^SCN- and [Pd ^II _X4 ] ^2- , _NH3 and [Pd ^II _X4 ] ²⁺ , and ethylenediamineen and [ ^{Pd II (} en) ₂ ] ²⁺ .
Palladium is never used as pure palladium, but is used in various alloys and catalyst materials (see [Alternative terms] Palladium catalysts). Its largest use is in automobile exhaust gas purification and three-way catalysts, where it is used together with platinum and rhodium. As exhaust gas regulations become stricter, the amount of platinum group metals used per vehicle has increased, and as of 2007, it is about 5 g per small car. Because it is cheaper than Pt, efforts are being made to replace it with Pd wherever possible. Palladium is alloyed with gold and silver for dental purposes. Due to its corrosion resistance, alloys with gold are called white gold and are used in ornamental items such as electrical contacts. In 2005, Japan's demand was 20 t for three-way catalysts, 16 t for dental use, 8 t for electrical parts, and 5 t for ornamental items. Of the total amount of palladium imported in Japan in 2005 of about 70 t, about 20 t each was imported from Russia, South Africa, and the United States. [CAS 7440-05-3]

Source: Morikita Publishing "Chemical Dictionary (2nd Edition)" Information about the Chemical Dictionary 2nd Edition

Pd．原子番号46の元素．電子配置[Kr]4d¹⁰の周期表10族貴金属または白金族元素．原子量106.42(1)．質量数102(1.02(1)％)，104(11.14(8)％)，105(22.33(8)％)，106(27.33(3)％)，108(26.46(9)％)，110(11.72(9)％)の6種の安定同位体と，質量数91～124の放射性同位体が知られている．1802年イギリスのW.H. Wollastonが王水溶液から白金を(NH₄)₂Pt^Ⅳ Cl₆として沈殿分離したあとの溶液中に発見し，同じ年に発見された小惑星パラスにちなんで命名した．パラスはギリシア神話知恵の女神Pallas Athene．宇田川榕菴は天保8年(1837年)に出版した「舎密開宗」で，巴爾刺冑母(パルラヂウム)としている．
地殻中の存在度0.001 ppm であるが，白金族元素のなかでは白金より多い．鉱物としては，安パラジウム鉱Pd₃Sb，パオロボ鉱Pd₂Sn，ズビャジンツェフ鉱Pd₃Pb，ブラグ鉱(Pt，Pd，Ni)S，硫白金鉱(Pt，Pd)Sなど，白金鉱床中に産出する．白金族元素の可掘埋蔵量71000 t 中，63000 t が南アフリカ，ついでロシア6200 t，アメリカ・カナダあわせて1200 t の順である．白金族中のPdの割合は南アフリカが25～45％(鉱脈による)で，ロシアのウラル鉱床が71％ であるから南アフリカが最大のPd資源国である．2007年の全供給量230 t 中，ロシア，南アフリカが約40％ ずつを占めている．精製白金鉱，またはニッケル，銅電解精製過程の陽極泥を王水，Cl₂-HCl溶液などに溶解後，沈殿，再結晶，溶媒抽出，蒸留によりPd塩を分離精製後，熱分解で金属を得る．展延性に富む可鍛性の銀白色の金属．立方最密構造．密度12.02 g cm^－3．白金族中もっとも軽い．融点は白金族中もっとも低く1552 ℃，沸点3140 ℃．硬さ4.8．第一イオン化エネルギー8.34 eV．標準電極電位 Pd^2＋/Pd 0.915 V．王水に易溶，硝酸，熱濃硫酸に可溶．体積で900倍近くの水素を水素化物として吸収する．これを利用して水素を精製することができる．通常の酸化数2～4．主として二価で化合物をつくり，PdO(黒)，PdS(黒褐)，PdX₂(X ＝ F(褐)，Cl(暗赤)，Br(赤褐)，I(黒))，Pd(NO₃)₂(黄褐)，PdSO₄(赤褐)などがある．塩化物，硝酸塩，硫酸塩は水に可溶，酸化物，硫化物，臭化物，ヨウ化物，シアン化物は水に不溶．Pd^Ⅲ化合物，Pd^Ⅳ化合物は不安定である．白金と同じく錯体を形成しやすく，酸化数が0と1の錯体も存在する．Cl^－，Br^－，I^－，CN^－，SCN^－と [Pd^ⅡX₄]^2－，NH₃と [Pd^ⅡX₄]^2＋，エチレンジアミンenと [Pd^Ⅱ(en)₂]^2＋ など，平面四配位構造の Pd^Ⅱ錯体が多く知られている．
純パラジウムとして使用されることはなく，種々の合金，触媒材料に利用される([別用語参照]パラジウム触媒)．最大の用途は自動車排気ガス浄化，三元触媒用で，白金，ロジウムとともに使用される．排気ガス規制強化とともに1台当たりの白金族使用量が増加し，2007年現在，小型車1台当たり約5 g とされる．Ptより安価なため，可能なかぎりPdへの置き換えが進められている．歯科用に金銀との合金が使用される．耐腐食性から，電気接点など，金との合金はホワイトゴールドとよばれ装飾品に使われる．わが国の需要は，2005年では三元触媒用20 t，歯科用16 t，電気部品用8 t，装飾品用5 t など．わが国は2005年には全量約70 t のうちそれぞれ約20 t をロシア，南アフリカ，アメリカから輸入している．[CAS 7440-05-3]

出典　森北出版「化学辞典（第2版）」化学辞典 第2版について　情報