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Vanadium(V) oxide (V2O5), commonly known as vanadium pentoxide, is the most important compound of vanadium. Upon heating it can reversibly lose oxygen to the air. Related to this is its ability to catalyse the delivery of oxygen from the air to compounds such as sulfur dioxide, benzene and naphthalene, which is the basis for its industrial use as a catalyst in the manufacture of sulfuric acid, maleic anhydride and phthalic anhydride, respectively. 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 is slightly soluble in water.
Chemical properties In this compound, like all others having the vanadium(V) designation, the vanadium is in the +5 oxidation state. All of the oxygen atoms in the compound are in the -2 oxidation state. Vanadium(V) oxide dissolves slightly in water to give an acidic solution, though it is an amphoteric oxide. Thus it reacts with strong non-reducing acids to form solutions containing the pale yellow dioxovanadium(V) ion: V2O5 + 2 HNO3 → 2 VO2(NO3) + H2O Thionyl chloride converts it to VOCl3, since VCl5 is unknown: V2O5(s) + 3 SOCl2(l) → 2 VOCl3(l) + 3 SO2(g) It also reacts with strong alkali to form vanadate salts, which have a complex structure that depends on pH1. If excess aqueous sodium hydroxide is used, the product is a colourless salt, sodium metavanadate, 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. (V2O5) is easily reduced in acidic media to the stable vanadium(IV) species, the blue vanadyl ion (VO2+). This makes V2O5 an oxidising agent. For example, hydrochloric acid and hydrobromic acid are oxidised to the corresponding halogen, e.g., V2O5(s) + 6 HCl(aq) + 7 H2O(l) → Solid V2O5 is reduced by oxalic acid, CO or 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 - yellow VO3- and VO2+ → blue VO2+ → green V3+ → purple V2+ 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 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(s) → V2O5(s) + 2 NH3(g) + H2O(g) Uses The most important use of vanadium(V) oxide is in the manufacture of sulfuric acid, the world's most important industrial chemical with an annual production of 145 million tonnes in 19861. 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(g) + O2(g) → 2 SO3(g) This 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 must be performed between 400°C and 620°C; below 400° 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 at 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. Maleic anhydride is another important industrial material, used for the manufacture of polyester resins and alkyd resins.6 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. 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. Possible new uses include the preparation of bismuth vanadate ceramics for use in solid oxide fuel cells.7 Biological activity Despite being highly toxic in humans, vanadium has an interesting biochemistry, for example in enzymes such as vanabins. Vanadate (VO43−), formed when V2O5 is dissolved in water at alkaline pH, appears to inhibit enzymes that process phosphate (PO43−). However the exact mode of action remains elusive.1 Precautions Vanadium(V) oxide is highly toxic, and should only be handled by trained personnel. Work in a fume hood, and wear gloves and goggles. Keep away from reducing agents. | |||||||||||||
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