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Vanadium(V) oxide

Vanadium oxide

Vanadium(V) oxide (vanadia) is the chemical compound with the formula V2O5. Commonly known as vanadium pentoxide, this orange solid is the most important[citation needed] compound of vanadium. Upon heating it reversibly loses oxygen. Related to this ability, V2O5 catalyses several useful aerobic oxidation reactions, the largest scale of which underpins the production of sulfuric acid from sulfur dioxide. It is a poisonous orange solid which, because of its high oxidation state, is both an amphoteric oxide and an oxidising agent. Unlike most metal oxides, it dissolves slightly in water due to hydrolysis.

Vanadium(V) indicates that vanadium is in the +5 oxidation state. The oxygen atoms in the compound are in the -2 oxidation state.

Natural form of this compound, mineral shcherbinaite, is found extremely rarely, almost always among some fumaroles.
Chemical properties

Acid-base reactions
V2O5 is an amphoteric oxide. Thus it reacts with strong non-reducing acids to form solutions containing the pale yellow salts containing dioxovanadium(V) centers:

V2O5 + 2 HNO3 → 2 "VO2(NO3)" + H2O
It also reacts with strong alkali to form polyoxovanadates, which have a complex structure that depends on pH.[1] If excess aqueous sodium hydroxide is used, the product is a colourless salt, sodium orthovanadate, Na3VO4. If acid is slowly added to a solution of Na3VO4, the colour gradually deepens through orange to red before brown hydrated V2O5 precipitates around pH 2. These solutions contain mainly the ions HVO42− and V2O74− between pH 9 and 13, but below pH 9 more exotic species such as V4O124− and HV10O285− predominate.

Thionyl chloride converts it to VOCl3:

V2O5(s) + 3 SOCl2(l) → 2 VOCl3(l) + 3 SO2(g)

Redox reactions
V2O5 is easily reduced in acidic media to the stable vanadium(IV) species, the blue vanadyl ion (VO(H2O)52+). This conversion illustrates the redox properties of V2O5. For example, hydrochloric acid and hydrobromic acid are oxidised to the corresponding halogen, e.g.,

V2O5(s) + 6 HCl + 7 H2O → 2 [VO(H2O)5]2+ + 4 Cl− + Cl2
Solid V2O5 is reduced by oxalic acid, CO, and SO2 to give vanadium(IV) oxide, VO2 as a deep-blue solid. Further reduction using hydrogen or excess CO can lead to complex mixtures of oxides such as V4O7 and V5O9 before black V2O3 is reached. Vanadates or vanadyl(V) compounds in acid solution are reduced by zinc amalgam through the interestingly colorful pathway -

colorless "VO3−" → yellow "VO2+" → blue "VO2+" → green "V3+" → purple "V2+" The ions are of course hydrated to varying degrees.
Preparation
Technical grade V2O5 is produced as a black powder used for the production of vanadium metal and ferrovanadium.[1] A vanadium ore or vanadium-rich residue is treated with sodium carbonate to produce sodium metavanadate, NaVO3. This material is then acidified to pH 2-3 using H2SO4 to yield a precipitate of "red cake" (see above). The red cake is then melted at 690 °C to produce the crude V2O5.

Vanadium(V) oxide is also the main product when vanadium metal is heated with excess oxygen, but this product is contaminated with other lower oxides. A more satisfactory laboratory preparation involves the decomposition of ammonium metavanadate at around 200 °C:

2 NH4VO3 → V2O5(s) + 2 NH3 + H2O

Uses

Sulfuric acid production
The most important[citation needed] use of vanadium(V) oxide is in the manufacture of sulfuric acid, an important industrial chemical with an annual production of 165 million metric tons in 2001, with an approximate value of US$8 billion. Vanadium(V) serves the crucial purpose of catalysing the mildly exothermic oxidation of sulfur dioxide to sulfur trioxide by air in the contact process:

2 SO2 + O2 ⇌ 2 SO3
The discovery of this simple reaction, for which V2O5 is the most effective catalyst, allowed sulfuric acid to become the cheap commodity chemical it is today. The reaction is performed between 400 and 620 °C; below 400 °C the V2O5 is inactive as a catalyst, and above 620 °C; it begins to break down. Since it is known that V2O5 can be reduced to VO2 by SO2, one likely catalytic cycle is as follows:

SO2 + V2O5(s) → SO3(g) + 2 VO2(s) followed by
2 VO2(s) +1/2 O2(g) → V2O5
Paradoxically, it is also used as catalyst in the selective catalytic reduction of NOx emissions in some power plants. Due to its effectiveness in converting sulfur dioxide into sulfur trioxide, and thereby sulfuric acid, special care must be taken with the operating temperatures and placement of a power plant's SCR unit when firing sulfur-containing fuels.

Other oxidations
Maleic anhydride is another important industrial material, used for the manufacture of polyester resins and alkyd resins.[2] Vanadium(V) oxide can catalyse its production from a variety of organic starting materials such as n-butane, furfural and benzene, the last of which is the usual commercial method. In a related process, phthalic anhydride, used for making plasticisers for PVC manufacture, may be produced by V2O5 catalysed oxidation of ortho-xylene or naphthalene at 350-400°C.

Other applications
In terms of quantity, the major use for vanadium(V) oxide is in the production of ferrovanadium (see above). The oxide is heated with scrap iron and aluminium, producing the iron-vanadium alloy along with alumina as a by-product. In 2005 a shortage of V2O5 caused a price rise to around $40/kg, which in turn caused a rise in the price of ferrovanadium.

Due to its high thermal coefficient of resistance, vanadium(V) oxide finds use as a detector material in bolometers and microbolometer arrays for thermal imaging.

It also find an application as ethanol sensor in ppm levels (up to 0.1 ppm).

Possible new uses include the preparation of bismuth vanadate ceramics for use in solid oxide fuel cells.[3] Another new application is in vanadium redox batteries, a type of flow battery used for energy storage, including large power facilities such as wind farms.

五氧化二钒

   理化常数
　　国标编号
　　61028
　　CAS号
　　1314-62-1
　　中文名称
　　五氧化二钒
　　英文名称
　　Vanadium pentoxide
　　别 名
　　钒(酸)酐
　　摩尔质量
　　181.88 g /mol
　　外观
　　橙黄色固体
　　分子式
　　V2O5 外观与性状 橙黄色或红棕色结晶粉末
　　分子量
　　181.88
　　熔 点
　　690℃ （963 K）
　　溶解性
　　微溶于水，不溶于乙醇，溶于浓酸、碱
　　密 度
　　相对密度(水=1)3.35 稳定性 稳定
　　在水中的溶解度
　　0.8 g/100 mL （20 °C）
　　危险标记
　　13(剧毒品)
    化学性质
　　酸碱反应：五氧化二钒酸性显著，碱性表现不强，既可溶于酸生成相应的黄色偏钒酸根，在PH2左右可沉淀为偏钒酸，也可溶于碱生成偏钒酸盐溶液。
　　氧化还原反应：五氧化二钒中的正五价钒可被还原，在酸性介质得到VIV，蓝色的VO(H2O)52+钒氧基离子。以盐酸和氢溴酸作还原剂，氧化产物是相应的卤素单质。
[编辑本段]用途
　　钒是一种稀有黑色金属，五氧化二钒广泛用于冶金、化工等行业，主要用于冶炼钒铁用作合金添加剂，占五氧化二钒总消耗量的80%以上，其次是用作有机化工的催化剂，即触媒，约占总量的10%，另处用作无机化学品、化学试剂、搪瓷和磁性材料等约占总量的10%
    生产工艺
　　生产原料
　　工业上提取五氧化二钒的原料主要是含钒石煤矿和钒钛磁铁矿，含钒石煤矿五氧化二钒品位一般在0.5-1.5%,钒钛磁铁矿五氧化二钒品位一般在1.0-5.0%.
　　矿分解
　　钒矿的分解方法有二类，一是烧结法，即将粉碎的矿粉与纯碱或钠盐混合并经高温烧结，使其中的低价态的钒氧化并转化为五价的钒酸钠，然后再用水浸取，使钒溶解进入溶液；二是酸溶法，即将粉碎的矿粉用稀酸（主要是稀硫酸）加热浸取，使钒直接分解进入溶液。
　　浸取
　　将烧结料用水浸取，使钒以偏钒酸钠形式进入溶液，过滤洗涤，滤液为粗偏钒酸钠溶液，再经化学沉淀、溶剂萃取等工艺进一步提纯分离杂质，使钒富集到一定浓度，一般在15克/升以上。
　　沉钒
　　将纯偏钒溶液调节PH值至6.5-7.5后，加入四倍理论量的碳酸氢铵或氯化铵溶液作沉淀剂，控制温度在30度以下，析出偏钒酸铵沉淀，过滤，用5%的碳酸氢铵溶液洗涤2-3次，再用30%乙醇洗1-2次，烘干，得纯偏钒酸铵产品。
　　
影响沉淀的因素

　　工业生产中浸出液的ph值大约在8-9 之间，加入沉淀剂氯化铵与浓硫酸调节PH值到2-2.5之间，钒可以偏钒酸铵形式沉定，但沉淀受以下条件的影响。
　　
　　钒溶液的浓度
　　原液中的烦的浓度越高，钒沉淀时候水解时候的晶核的生长速度也就越快，沉淀速度晶核生长速度快，所以晶体结构不完整，沉淀的颗粒不紧密，吸附的杂质较多水分多，品味地，有时候控制不好很容易出现粘料的情况。总之钒的浓度越高，沉淀速度越快，沉淀物的品位就越低，一般情况下原液的浓度控制在15-20g/L。
　　酸度的影响
　　试验证明 ph值=3.6时候钒的沉淀开始进行，当随着酸度的增加，钒的反应速度就越快。但是随着酸度的增大当增大到一定值的时候，就伴随着副反应的的发生，也就是多钒酸氨的反溶解
　　铵盐量的影响
　　铵盐的加入方式，一般在碱性溶液下铵盐易于分解，因此工业中卡将溶液ph值调节到4-5 之间，再加入铵盐，然后在搅拌的条件下，在调节ph至道1.9-2.1左右，再加热沉淀
　　沉淀时间的影响
　　当加氨系数固定在0.65时候，ph值等于2.2时，时间达到25分钟以上时候，沉淀率可达到98%以上，实际生产中控制在30-40分钟以上是可以保证沉淀率的。
　　温度的影响
　　在其余条件完全相同的情况下，温度的升高可加快钒从溶液中析出的速度，温度对反析出的速度，物理形态、多钒酸氨的品位也有很大的影响。低温下进行沉淀时候，沉淀物的表观颗粒细，颜色红、品位高。高温下得到的沉淀物颗粒大，颜色深，品位相应的低。
　　沉淀温度大于80摄氏度，沉淀率即可达到99%，实际生产中用蒸汽煮沸沉淀，是完全可以满足沉钒的要求的。
　　煅烧
　　将烘干后的偏钒酸铵于回转炉中缓慢升温至300度，保持1小时，再升至500-550度（不能超过650度，否则五氧化二钒熔融），保持2小时，最后降至450度保持1小时冷却出炉，即得组成准确、恒定的五氧化二钒产品。煅烧反应为：2 NH4VO3 → V2O5+ 2 NH3 + H2O
   毒性及影响
　　健康影响
　　侵入途径：吸入、食入、经皮吸收。
　　健康危害：对呼吸系统和皮肤有损害作用。急性中毒：可引起鼻、咽、肺部刺激症状，多数工人有咽痒、干咳、胸闷、全身不适、倦怠等表现，部分患者可引起肾炎、肺炎。 慢性中毒：长期接触可引起慢性支气管炎、肾损害、视力障碍等。但微量的钒（50微克/升以下）有降血糖作用，可作治疗治糖尿病的微量元素药物。
　　毒理学资料及环境行为
　　毒性：属高毒类。
　　急性毒性：LD5010mg/kg(大鼠经口)
　　危险特性：未有特殊的燃烧爆炸特性。
　　燃烧(分解)产物：可能产生有害的毒性烟雾。
[编辑本段]环境监测方法
　　N-肉桂酰-邻-甲苯羟胺比色法《空气中有害物质的测定方法》(第二版)杭士平主编
　　苯甲酰苯基羟胺比色法《空气中有害物质的测定方法》(第二版)杭士平主编
　　火焰原子吸收法《空气中有害物质的测定方法》(第二版)杭士平主编
    环境标准
　　中国(TJ36-79)车间空气中有害物质的最高容许浓度 0.1mg/m3[烟]；0.5mg/m3[粉尘]
　　前苏联(1977)大气质量标准 0.02mg/m3
    应急处置方法
　　一、泄漏应急处理
　　隔离泄漏污染区，周围设警告标志，建议应急处理人员戴正压自给式呼吸器，穿化学防护服。不要直接接触泄漏物。避免扬尘，用清洁的铲子收集于干燥净洁有盖的容器中，转移到安全场所。也可以用水泥、沥青或适当的热塑性材料固化处理再废弃。如大量泄漏：收集回收或无害处理后废弃。
　　二、防护措施
　　呼吸系统防护：空气中浓度超标时，应该佩带防毒口罩。必要时佩带自给式呼吸器。
　　眼睛防护：戴化学安全防护眼镜。
　　身体防护：穿相应的防护服。
　　手防护：戴防护手套。
　　其它：工作现场禁止吸烟、进食和饮水。工作后，淋浴更衣。单独存放被毒物污染的衣服，洗后再用。进行就业前和定期的体检。
　　三、急救措施
　　皮肤接触：脱去污染的衣着，立即用流动清水彻底冲洗。
　　眼睛接触：立即提起眼睑，用流动清水冲洗。
　　吸入：迅速脱离现场至空气新鲜处。注意保暖，必要时进行人工呼吸。就医。
　　食入：误服者给饮大量温水，催吐，就医。
　　灭火方法：不燃。火场周围可用的灭火介质。