cerium

Ce. Atomic number 58. Electron configuration [Xe] ^{4f15d16s2 .} A member of the lanthanide group ³ of the periodic table. A member of the cerium group of rare earth elements. Atomic weight 140.116(1). Four stable isotopes with mass numbers 136 (0.185(2)%), 138 (0.251(2)%), 140 (88.450(51)%), and 142 (11.114(51)%) are known, as well as radioactive isotopes with mass numbers 119 to 157. It was discovered in 1803 by JJ Berzelius, MH Klaproth, and W. Hisinger. The element was named after the asteroid Ceres, discovered in 1801. Ceres is the Roman goddess of the agricultural harvest.
Ce is the most abundant rare earth element, and about half of the rare earth elements in monazite [CAS 1306-41-8] are Ce. It is found in minerals such as cerium sulphate and gadolinite, but monazite and bastnasite [CAS 68909-13-7] are the most important minerals in terms of resources. Its abundance in the earth's crust is 33 ppm. Of Japan's total imports of cerium oxide and cerium compounds of 13,000 t (2005), China accounted for 10,000 t, followed by France with 1,500 t. China is by far the largest producer, with the Baiyun Obo mine in Inner Mongolia, which has a mixed deposit of bastnasite and monazite, accounting for about half of the world's production, mainly of light rare earth elements such as cerium, but in terms of confirmed reserves, China accounts for about 30%, Russia 22%, and the United States 15%. The ore is treated with hydrochloric or sulfuric acid (bastnasite) or with sodium hydroxide (monazite), then roasted to produce Ce ^IV , which is then separated by precipitation or solvent extraction. The grey metal is obtained by electrolysis of molten salts such as chlorides. Melting point 799 °C, boiling point 3426 °C. There are four modifications: α, β, γ, and δ (β → γ, 168 °C), with α, γ, and δ having cubic close-packed structures and β having hexagonal close-packed structure. Density α 8.24; β 6.749; γ 6.773; δ 6.70 g cm ^-3 . Standard electrode potential Ce ³⁺ /Ce -2.34 V, Ce ⁴⁺ /Ce ³⁺ 1.71 V. First ionization energy 5.539 eV. Oxidation states 3 and 4. Ce is the only rare earth element with the stable oxidation state of 4. The electronic configuration of Ce ⁴⁺ is [Xe]. It is easily oxidized in air. It reacts with hot water to produce hydrogen. Most Ce ^III salts are colorless, but Ce ^IV salts are yellow to red. Standard electrode potential: Ce ^III /Ce -2.34 V, Ce ^IV /Ce ^III 1.71 V. Ce ^IV is a strong oxidizing agent and is used in redox titrations. Ce ^III compounds are similar to lanthanum compounds, and fluorides, oxalates, carbonates, and phosphates are poorly soluble in water. Ce ⁴⁺ has almost the same ionic radius (Ce ⁴⁺ 0.109 nm, Th ⁴⁺ 0.108 nm) as Th ⁴⁺ (Rn), which has a similar electronic configuration and closed shell, and their properties are similar. For example, both phosphates and iodides are insoluble in 4 mol dm ^-3 HNO ₃ . Of the cerium oxides, CeO ₂ [CAS 1306-38-3] is more stable than Ce ₂ O ₃ [CAS 1345-13-7].
The largest use of cerium in Japan is as an abrasive, where it is used in the form of cerium oxide (IV) CeO ₂ (ceria) for polishing liquid crystal glass and semiconductor wafers. CeO ₂ is also used as a support and promoter together with alumina in three-way catalysts for purifying automobile exhaust gases, as a UV absorbing additive in UV filter glass (automotive glass), and as phosphors for fluorescent lamps and CRTs. Cerium is also added to high-temperature superalloys to prevent oxidation, to aluminum alloys, and to catalysts for petroleum refining. Although not highly toxic, "cerium and its compounds" are designated as harmful air pollutants under the Air Pollution Control Act. [CAS 7440-45-1]

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

Ce．原子番号58の元素．電子配置[Xe]4f ¹5d¹6s²の周期表3族ランタノイド元素．希土類元素セリウム族の一つ．原子量140.116(1)．質量数136(0.185(2)％)，138(0.251(2)％)，140(88.450(51)％)，142(11.114(51)％)の4種の安定同位体と，質量数119～157の放射性同位体が知られている．1803年，J.J. Berzelius(ベルセリウス)，M.H. Klaproth，W. Hisingerにより発見された．元素名は1801年に発見された小惑星セレス(Ceres)にちなんで命名された．セレスは農業収穫を司るローマ神話の女神である．
希土類元素中もっとも多く存在する元素で，モナズ石[CAS 1306-41-8]の希土類元素の約半分はCeである．セル石，ガドリン石などに含まれて産出するが，モナズ石とバストネス石[CAS 68909-13-7]が資源的に重要な鉱物である．地殻中の存在度33 ppm．わが国の酸化セリウム，セリウム化合物全輸入量13000 t(2005年度)のうち中国10000 t，ついでフランス1500 t．生産量は圧倒的に中国で，バストネス石，モナズ石混合鉱床をもつ内蒙古の白雲鄂博(Baiyun Obo)鉱山だけで，セリウムなど軽希土類を中心に世界の生産量の約半分を占めているが，確認埋蔵量の順位では中国約30％，ロシア諸国22％，アメリカ15％ などである．鉱石を塩酸または硫酸で処理(バストネス石)するか，水酸化ナトリウムで処理(モナズ石)した後，ばい焼して Ce^Ⅳにして沈殿法または溶媒抽出法で分離する．塩化物などの溶融塩電解により灰色の金属が得られる．融点799 ℃，沸点3426 ℃．α，β，γ，δ(β→γ，168 ℃)の4変態があり，α，γ，δは立方最密構造，βは六方最密構造．密度α 8.24；β 6.749；γ 6.773；δ 6.70 g cm^－3．標準電極電位 Ce^3＋/Ce －2.34 V，Ce^4＋/Ce^3＋1.71 V．第一イオン化エネルギー5.539 eV．酸化数3，4．希土類元素でCeのみが安定な酸化数4の状態をもつ．Ce^4＋ の電子配置は[Xe]である．空気中で容易に酸化される．熱水と反応して水素を発生する．Ce^Ⅲ塩は無色のものが多いが，Ce^Ⅳ塩は黄色～赤色．標準電極電位 Ce^Ⅲ/Ce －2.34 V，Ce^Ⅳ/Ce^Ⅲ1.71 V．Ce^Ⅳは強い酸化剤で酸化還元滴定に用いられる．Ce^Ⅲの化合物はランタンの化合物に似ていて，フッ化物，シュウ酸塩，炭酸塩，リン酸塩などが水に難溶．Ce^4＋ は，電子配置が同様に閉殻の Th^4＋(Rn)とイオン半径がほぼ等しく(Ce^4＋0.109 nm，Th^4＋0.108 nm)，性質も似ている．たとえば，リン酸塩，ヨウ化物は，ともに4 mol dm^－3HNO₃に不溶．セリウムの酸化物はCeO₂[CAS 1306-38-3]のほうがCe₂O₃[CAS 1345-13-7]より安定である．
わが国におけるセリウムの最大の用途は研磨材用で，酸化セリウム(Ⅳ)CeO₂(セリア)の形で液晶ガラス研磨や半導体ウェファー研磨に用いられる．このほか，CeO₂は自動車排ガス浄化用の3元触媒にアルミナとともに担体・助触媒や，UVフィルターガラス(自動車用ガラス)のUV吸収用添加剤，蛍光灯・CRT用蛍光体にも用いられる．そのほか，セリウムは高温用超合金酸化防止用，アルミニウム合金用，石油精製用触媒に添加される．毒性は高くないが，「セリウム及びその化合物」は大気汚染防止法の有害大気汚染物質に指定されている．[CAS 7440-45-1]

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