Molybdenum compared

The close similarity between adsorption of CO on tungsten and molybdenum is shown by the work of Jackson and Hooker (82) in a LEED study on the Mo(llO) face when they observed a complex ordered structure formed by heating the crystal to 1300°K in the presence of CO. This surface structure shows a marked resemblance to that obtained by May and Germer on W(llO) (71) after annealing the crystal to between 600-900°K. On molybdenum the removal of the additional structure was not observed until about 1800°K, whereas on W(llO) CO was observed to desorb at 1100°K. This result is puzzling in the light of the flash filament studies and calorimetric heats of adsorption on polycrystalline samples which indicate a weaker adsorption process for CO on poly crystalline molybdenum compared with tungsten. 

Increased frequency of the L(, , ) phonon for molybdenum compared to niobium is caused by the development of directional bonding from the additional occupied d states. This gives rise to bonding forces, which help to restore the equilibrium position of molybdenum atoms. 

Molybdenum Oxide. Molybdenum compounds incorporated into flexible PVC not only increase flame resistance, but also decrease smoke evolution. In Table 10 the effect of molybdenum oxide on the oxygen index of a flexible PVC containing 50 parts of a plasticizer is compared with antimony oxide. Antimony oxide is the superior synergist for flame retardancy but has Httle or no effect on smoke evolution. However, combinations of molybdenum oxide and antimony oxide may be used to reduce the total inorganic flame-retardant additive package, and obtain improved flame resistance and reduced smoke. 

Fused Salt Electrolysis. Only light RE metals (La to Nd) can be produced by molten salt electrolysis because these have a relatively low melting point compared to those of medium and heavy RE metals. Deposition of an alloy with another metal, Zn for example, is an alternative. The feed is a mixture of anhydrous RE chlorides and fluorides. The materials from which the electrolysis cell is constmcted are of great importance because of the high reactivity of the rare-earth metals. Molybdenum, tungsten, tantalum, or alternatively iron with ceramic or graphite linings are used as cmcible materials. Carbon is frequently used as an anode material. 

The oxidative dehydration of isobutyric acid [79-31-2] to methacrylic acid is most often carried out over iron—phosphoms or molybdenum—phosphoms based catalysts similar to those used in the oxidation of methacrolein to methacrylic acid. Conversions in excess of 95% and selectivity to methacrylic acid of 75—85% have been attained, resulting in single-pass yields of nearly 80%. The use of cesium-, copper-, and vanadium-doped catalysts are reported to be beneficial (96), as is the use of cesium in conjunction with quinoline (97). Generally the iron—phosphoms catalysts require temperatures in the vicinity of 400°C, in contrast to the molybdenum-based catalysts that exhibit comparable reactivity at 300°C (98). 

Titanium disulfide has been proposed as a soHd lubricant. The coefficient of friction between steel surfaces is 0.3, compared to only 0.2 for molybdenum disulfide. However, because it does not adhere strongly to metal surfaces, TiS2 is generally less effective than molybdenum sulfide. 

Chromium, 122 ppm of the earth s crustal rocks, is comparable in abundance with vanadium (136 ppm) and chlorine (126 ppm), but molybdenum and tungsten (both 1.2 ppm) are much rarer (cf. Ho 1.4 ppm, Tb 1.2 ppm), and the concentration in their ores is low. The only ore of chromium of any commercial importance is chromite, FeCr204, which is produced principally in southern Africa (where 96% of the known reserves are located), the former Soviet Union and the Philippines. Other less plentiful sources are crocoite, PbCr04, and chrome ochre, Cr203, while the gemstones emerald and ruby owe their colours to traces of chromium (pp. 107, 242). 

One of the most serious corrosion problems associated with type 316 stainless steel is its susceptibility to crevice corrosion. The incidence and extent of this type of corrosion in surgical implants was stressed by Scales eta/. who reported the presence of crevice corrosion in 24% of type 316L bone plates and screws examined after removal from patients. This record however compared favourably with the presence of crevice corrosion in 51 % of 18-8 stainless plates, demonstrating the superiority of the molybdenum-containing grade. 

At ambient temperatures the strength of molybdenum is comparable to that of normalised low-alloy steel and moderately higher than that of the austentic stainless steels. However, whereas the low-alloy steels are limited to use at service temperatures of about 550°C and stainless steels to about... 

Other alloys of molybdenum which have been investigated for their corrosion resistance contain 10-50% Ta and were found to have excellent resistance to hydrochloric acid. Ti-Mo alloys were found to resist chemicals that attack titanium and Ti-Pd alloys, notably strong reducing acids such as hot concentrated hydrochloric, sulphuric, phosphoric, oxalic, formic and trichloroacetic. For example, a Ti-30Mo alloy has the following corrosion rates in boiling 20% hydrochloric acid, 0-127-0-254 mm/y in 10% oxalic acid at 100°C, 0-038 mm/y, which compares favourably with the respective rates of 19-5 and 122 mm/y for the Ti-0-2Pd alloy. 

The above procedure may be adapted to the determination of molybdenum in steel. Dissolve a 1.00 g sample of the steel (accurately weighed) in 5 mL of 1 1 hydrochloric acid and 15 mL of 70 per cent perchloric acid. Heat the solution until dense fumes are evolved and then for 6-7 minutes longer. Cool, add 20 mL of water, and warm to dissolve all salts. Dilute the resulting cooled solution to volume in a 1 L flask. Pipette 10.0 mL of the diluted solution into a 50 mL separatory funnel, add 3 mL of the tin(II) chloride solution, and continue as detailed above. Measure the absorbance of the extract at 465 rnn with a spectrophotometer, and compare this value with that obtained with known amounts of molybdenum. Use the calibration curve prepared with equal amounts of iron and varying quantities of molybdenum. If preferred, a mixture of 3-methylbutanol and carbon tetrachloride, which is heavier than water, can be used as extractant. 

Sulphuric acid is not recommended, because sulphate ions have a certain tendency to form complexes with iron(III) ions. Silver, copper, nickel, cobalt, titanium, uranium, molybdenum, mercury (>lgL-1), zinc, cadmium, and bismuth interfere. Mercury(I) and tin(II) salts, if present, should be converted into the mercury(II) and tin(IV) salts, otherwise the colour is destroyed. Phosphates, arsenates, fluorides, oxalates, and tartrates interfere, since they form fairly stable complexes with iron(III) ions the influence of phosphates and arsenates is reduced by the presence of a comparatively high concentration of acid. 

Discussion. In acid solution, toluene-3,4-dithiol (dithiol) forms a red compound when warmed with tin(II) salts (compare molybdenum, Section 17.30). Tin(IV) also reacts, but more slowly than tin(II) thioglycollic acid may be employed to reduce tin(IV) to tin(II). The reagent is not stable, being easily reduced, and hence should be prepared as required. A dispersant is generally added to the solution under test. 

Most preparations of the intermediate racemic 3-allyl(dicarhonyl)cyclopentadieny molybdenum complexes7 2 start from sodium or potassium tricarbonyl(cyclopentadienyl)molybdate8 9 10 (4) for a simple preparation see ref 11, p 493. The allylation of 4 hy homoallylic halides, such as 4-hromo-l-butene, is accompanied by rearrangement and decarhonylationI2. (Z)-if-2-butenyl(dicarbonyl)cyclopentadienylmolybdenum (5), like other comparable complexes, exists as a mixture of endo/exo- conformers, which interconvert rapidly at room temperature12. 

Compared to the tungsten-catalyzed reaction, however, the regiochemistry of the molybdenum-catalyzed alkylation is highly dependent on the structure of the nucleophile89. 

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