Crystal structures molybdates

S2Og2- has been used to oxidize Ni2+(aq) in the presence of molybdate ion. The NiIV product obtained was crystallized as (NH4)6[NiMo9032]-6II20 and its crystal structure (determined independently by two groups) revealed a severely distorted octahedral 06 environment for the Ni atom, with an average Ni—O bond length of 1.876(5)A or 1.872(2)A.177,178... 

Molybdates and tungstates have likewise often been investigated, some of which have come out of our research group (84). These compounds can be subdivided according to crystal structure and these structures considered individually. 

Normal isopoly- and peroxymolydates of ammonium and several metals are known. The normal or orthomolybdates may be considered as salts of molybdic acid having formulas H2Mo04 xH20 or M20 Mo03 xH20. They are either of monoclinic or scheelite type crystal structure and obtained as hydrated salts. 

Crystal structures of the bismuth molybdate and of the mixed iron and cobalt solid solution molybdate samples were controlled by X-ray diffraction (10). The chemical compositions of the samples were determined by atomic absorption and their surface areas measured by nitrogen adsorption using the BET method. 

TABLE 111 Crystal Structure of Divalent-Metal Molybdate (81) ... 

Point defects in the form of cation vacancies () were introduced by Aykan et al. (93-95) into molybdates, tungstates, and vanadates with scheelite-type crystal structures. The authors studied the catalytic properties of more than 30 scheelite-structure phases represented by the formula A1 x< xM04 (M = molybdenum, tungsten, and/or vanadium and A may include Li, Na, K, Ag, Ca, Sr, Ba, Cd, Pb, Bi, and/or arare earth element in quantities appropriate to achieve charge balance for the normal oxidation states). It was found that the defects can be introduced... 

In addition to the 12-molybdocerate complexes described in Section 2.1, and the Peacock-Weakley analogs listed in Table 4, there have been other attempts to synthesize rare-earth complexes of molybdates. In view of the inadequacies of 95Mo-NMR caused by low sensitivity and quadrupolar relaxation, and the ambiguities of other spectroscopic methods, it has so far been impossible to confirm whether those compounds that reveal discrete polyoxoanions in crystal structure analysis yield solutions that retain the same structures. 

Lead chromate occurs in nature as crocoite, an orange-red mineral. Synthetically prepared lead chromate and its solid solutions with lead sulfate and lead molybdate represent a hue range from primrose yellow to red. The various hues of chrome yellow, chrome orange, and molybdate orange depend not only on composition but also on crystal structure and particle size. 

Precipitation conditions determine crystal structure. Lemon chrome yellows are precipitated hot with excess lead to form the monclinic crystal. Primrose yellow is preciptated at a lower temperature to form the orthorhombic crystal. Chrome oranges are precipitated under alkaline conditions. The higher the alkalinity, the larger the particle. The precipitation of molybdate orange is complicated by the polymorphic nature of two of the components of the solid solution precipitate lead chromate and lead sulfate. As with primrose yellow, the crystal is stabilized after precipitation to prevent conversion. 

Several different processes have been used, the simplest being by the reaction of hydrogen sulphide with molybdenum pentachloride, or the reaction of sulphur vapour with molybdic oxide or molybdenum metal. The last of these processes has been called the SHS process (Self-Propagating High-Temperature Synthesis) and Russian workers have reported that the product is less contaminated with impurities and has almost identical lubricating properties to natural molybdenum disulphide. The crystal structure is considered in more detail later, but it seems probable that the initial product of syntheses has a disordered... 

The crystal structure determination of tetraphenylphosphonium tetra-cyano[JV-o-tolylhydroxylaminato(2-)-0,JV]molybdate(VI) indicates the same mode of bonding of the -N-O substituent as that for dioxygen in the case of the above-mentioned dioxygen complexes (219). 

Aluminium molybdate, Al2(Mo04)3, is an important compound in catalyst technology. Although the crystal structure contains four equally populated inequivalent Al sites, an... 

P.Y. Zavalij and M.S. Whittingham, The crystal structure of layered methylammonium molybdate (CH3NH3)2Mo7022 from X-ray powder data, Acta Cryst. C53, 1374 (1997). 

Lindqvist Y, Schneider G, Vihko P (1994) Crystal structures of rat acid phosphatase com-plexed with the transition-state analogs vanadate and molybdate. Implications for the reaction mechanism. Eur J Biochem 221 139-142... 

The selective oxidation of methanol to give formaldehyde is in practice performed in two different processes, one using metallic silver, the other using iron molybdate as catalyst. Vanadium oxide has been shown to be a good selective catalyst in a variety of oxidation processes (refs. 1-2) and we have previously shown that it is also selective for methanol oxidation (refs. 3-5) when the V Og is applied as a very thin layer (monolayer) on different supports the support can have a significant influence on the activity and selectivity of these monolayer catalysts, as was shown by Roozeboom (ref. 6). In a previous paper (ref. 5), it was shown that both the type of support (A Og or TiC ) and the crystal structure of the TiO have an influence on the selectivity of the catalyst for the production of formaldehyde in general, production of the formaldehyde increases with a decrease in the reducibility of the vanadia. 

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