Catalytic behavior molybdates

Catalytic Behavior and Phase Composition of Bismuth-Iron Molybdates... 

The studies performed over promoted manganese molybdate catalysts have shown significant changes in catalytic behavior due to presence of the promoter. The preliminary results suggest that the pronounced differences observed in selectivity and activity may be related to the effect of the promoter cations on the reactivity of the lattice oxygen and the availability of adsorbed oxygen. 

Oxidation with N2O. In the evaluation of the catalytic data, we first compare the catalytic behavior of the metal molybdates with that of Mo03/Si02. Data obtained for this catalyst eailier (7) under the present conditions aie also included in Figure 2 and the appropriate tables. The conversion of ethane on Mo03/Si02 at 823 K was... 

Oxidation with O2. In this case there was a dramatic difference in the catalytic behavior of supported M0O3 and metal molybdate catalysts. Whereas in harmony with previous studies (16), only the complete oxidation of ethane occuiTed on Mo03/Si02 in the temperature range 500-550 K, on metal molybdates, ethylene was... 

Reported in Fig. 2b is an example of the catalytic behavior of a Pd-doped V-phospho-molybdic add catalyst Differently from the above samples, this catalyst shows a high in-... 

Physically, the relationship between catalytic activity and Z f can be understood from a study of single phase bismuth cerium molybdate solid solutions. The results show that maximum activity is achieved when there exists a maximum number and optimal distribution of all the key catalytic components bismuth, molybdenum and cerium in the solid. Therefore, it reasonably follows that the low catalytic activity observed for the two phase compositions where Af Af(min) results from the presence of interfacial regions in the catalysts where the compositional uniformity deviates significantly from the equilibrium distribution of bismuth and cerium cations present in the solid solutions. These compositions may contain areas in the interfacial region which are more bismuth-rich or cerium-rich than the saturated solid solutions. Conversely, at Af(min), the catalyst is similar to an ideal mixture of the two optimal solid solutions. The compositional homogeneity of the interfacial region approaches that of the saturated solid solutions. Therefore, the catalytic behavior of compositions at Af(min) is similar to that of the saturated solid solutions. 

Formaldehyde is nowadays one of the major produced chemicals due to its uses in many fields of chemical industry [1]. The commercial production started in 1890 using metallic copper catalysts. In 1910 copper catalysts were replaced by silver catalysts with higher yields [2]. Although the first report of the excellent catalytic behavior of iron molybdates in selective oxidation of methanol to formaldehyde is of 1931, the related industrial process based on them only went into operation in 1940-50 [1]. A recent report [3] shows that iron molybdates and silver catalysts are nowadays equally used as industrial catalysts for formaldehyde production. 

Several studies of the catalytic behavior of iron molybdates as a function of their physical and chemical properties show that surface acidity, related with Mo ions, is a necessary condition for the catalyst effectiveness for formaldehyde formation [8,9]. The oxidizing function is also necessary, though the activity is not necessarily determined by this function. In a recently study Sun-Kuo et al [6] demonstrated that catalytic behavior of iron... 

Up to now several methods have been used to prepare iron molybdates, the most part of them based on coprecipitation techniques. Previous studies [11] have evidenced that the catalytic behavior of Mo-Fe oxides depends on many variables of the coprecipitation procedure starting compounds, concentration of parent solutions, pH and temperature of coprecipitation step, order of addition of parent solutions, ripening etc. In a typical preparation procedure iron molybdate is coprecipitated from solutions of ferric chloride or ferric nitrate and ammonium molybdate [8]. The control of all the above mentioned procedure variables, strongly difficult the preparation of these catalysts and deviations from the preparation recipe can have very adverse effects on the performances of the catalyst from the standpoint of activity, selectivity and stability. 

Often the catalyst is bonded or compounded with silica, and the presence or absence of silica appears to have little effect on the reactions. Likewise, results are reported both with and without phosphorus which do not differ greatly. Studies of the effect of the Bi/Mo ratio have been made in efforts to find the reasons for the markedly different catalytic behavior of bismuth molybdate, so-called, relative to bismuth and molybdenum oxides. The phase diagram for this system was published by Belyaev and Smolyaninov 144), and was confirmed and refined by... 

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