Electrochemical Reduction of Molybdenum(VI) Compounds in Molten Lithium Chloride-Potassium Chloride Eutectic

Molybdenum(VI) oxide reacts with molten LiC1-KCI eutectie at 450 ~ to form MOO2C12, which probably is present as an anion MoO2Cl4 =, and pyromolybdate, Mo207 =. Both of these species are electrochemically reduced to MOO2, which can be reoxidized to MoO2CI2 by current reversal. A second reduction step, observed whether MoO3 or Mo2072is added to the melt, can be attributed to the reduction of MoO4 =, formed as a secondary reaction product in the first step. The reduction of molybdate proceeds in two steps, the first at --0.85V and the second at --1.75V vs. the IM P t ( I I ) / P t reference electrode. The first step shows an abnormally short transition time, at tr ibutable to a slow equilibrium. The second step corresponds to a diffusion-controlled reduction with n = 0.5, yielding a product of the empirical formula LisMo2Os. The l i terature contains very litt le fundamental information on the behavior of molybdenum compounds in molten systems. Stavenhagen and Engels (1) described a sodium molybdenum bronze in the form of a dark bluish-grayish powder formed by electrolytic reduction of fused sodium molybdate. Cannery (2) reported the preparation of lithium, sodium, and potassium molybdenum bronzes using the same method. However, the next year Burgers and van Liempt (3) showed that the bronzes mentioned by Cannery were * E lec t rochemica l Society Act ive Member . K e y words : m o l y b d e n u m , m o l y b d e n u m ( V I ) oxide, m o l y b d e n u m oxych lo r ide , l i t h i u m pyromolybdate, l ithium molybdate, molten salts, electrochemistry. mixtures consisting of molybdenum(IV) oxide and molybdenum blue. According to Magneli, only molybdenum oxides were obtained by Hagg in similar experiments (4, 5). Wold, Kunmann, Arnot, and Ferret t i (6) prepared pure MoO2 crystals by electrolytic reduction of MoO3-Na2MoO4 mixtures at 675~ In addition, both sodium molybdenum bronze and potassium molybdenum bronze crystals were grown from molybdenum(IV) oxide-alkal i molybdate melts under carefully controlled conditions. Senderoff and Brenner (7, 8) have investigated the reduction of molybdate dissolved in molten alkali halides. They found that the bulk of the reaction product was dispersed throughout Downloaded 16 Jun 2011 to 129.252.106.20. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp Vol. 120, No. 10 ELECTRO C H EMI C A L REDUCTION OF Mo(VI ) 1347 the melt as a black, water insoluble powder containing about 77% molydenum, corresponding to MoO2 (75% Mo). The present investigation was under taken in order to study the reduction of molybdenum(VI) compounds and to characterize any reduction products observed. It was also considered possible that some aspects of molybdenum chemistry and interactions between molybdenum(VI) compounds and chloride melt could be deduced.