Metal molybdates, structural properties

Modification of the metal itself, by alloying for corrosion resistance, or substitution of a more corrosion-resistant metal, is often worth the increased capital cost. Titanium has excellent corrosion resistance, even when not alloyed, because of its tough natural oxide film, but it is presently rather expensive for routine use (e.g., in chemical process equipment), unless the increased capital cost is a secondary consideration. Iron is almost twice as dense as titanium, which may influence the choice of metal on structural grounds, but it can be alloyed with 11% or more chromium for corrosion resistance (stainless steels, Section 16.8) or, for resistance to acid attack, with an element such as silicon or molybdenum that will give a film of an acidic oxide (SiC>2 and M0O3, the anhydrides of silicic and molybdic acids) on the metal surface. Silicon, however, tends to make steel brittle. Nevertheless, the proprietary alloys Duriron (14.5% Si, 0.95% C) and Durichlor (14.5% Si, 3% Mo) are very serviceable for chemical engineering operations involving acids. Molybdenum also confers special acid and chloride resistant properties on type 316 stainless steel. Metals that rely on oxide films for corrosion resistance should, of course, be used only in Eh conditions under which passivity can be maintained. 

Fven though the literature on this topic has been mainly focussed on the structural and chemical-physical properties of Bi molybdates, and on the reachvity of its various polymorphs (the a, P and y structures), the industrial catalyst consists of several divalent and trivalent metal molybdates. Indeed, Bi is present in minor amounts in catalyst formulahons. The two classes of molybdate contribute differently to catalyhc performance (1) trivalent Bi/Fe/Cr molybdates, having the Schee-lite-type shucture, contain the catalytically active elements while (2) divalent Ni/Go/Fe/Mg molybdates, having the Wolframite-type structure, mainly enhance the catalyst re-oxidahon rate. 

One possible conclusion is that under reducing conditions, metal cation movement occurs. Another possible conclusion is that despite the similar surface layer composition of bismuth and molybdenum for the three phases of bismuth molybdate, the three bismuth molybdate phases possess different catalytic activities, catalytic selectivities, adsorption properties, surface oxomolybdenum species, and reducibilities because the surface properties of the active bismuth molybdates are dependent upon the foundation upon which they exist, i.e., upon the bulk structure and its chemical and electronic properties. 

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... 

Catalytic results for a structure-sensitive reaction show that this control of Pd dispersion may be used to tune the activity of Pd-supported systems. However, they also show that the presence of surface molybdates has other effects on Pd particles than simple size control. They also point at the need of a still finer characterisation of M0/7AI2O3 precursors, especially concerning the presence of Mo-O-Al bonds. Altogether, this work presents encouraging prospects for fine-tuning the properties of supported metal catalysts by prior surface engineering of the support. 

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