Adsorption molybdates

Molybdate also functions as an effective inhibitor by slowing down pitting corrosion through a mechanism of adsorption onto the pit wall. 

Finally, trimetallic compounds have been developed to enhance the electroactivity of Pt-based catalysts, for either methanol or ethanol electro-oxidation. A long time ago, it was reported that adsorption of molybdates (Na2Mo04) at a Pt black electrode... 

Figures 11(a) and 11(b) [112] show the variation of Ni-Ge-P deposition rate and Ge content as a function of aspartic acid and Ge(IV) concentration, respectively. A relatively low P content, ca. 1-2 at%, was observed in the case of films exhibiting a high concentration of Ge (> 18 at%). Like other members of its class, which includes molybdate and tungstate, Ge(IY) behaves a soft base according to the hard and soft acids and bases theory (HSAB) originated by Pearson [113, 114], capable of strong adsorption, or displaying inhibitor-like behavior, on soft acid metal surfaces. In weakly acidic solution, uncomplexed Ge(IV) most probably exists as the hydrated oxide, or Ge(OH)4, which, due to acid-base reactions, may be more accurately represented as [Gc(OH)4 nO ] ".

Roy WR, Hassett JJ, Griffin RA (1986) Competitive coefficient for the adsorption of arsenate, molybdate, and phosphate mixtures by soils. Soil Sci Soc Am J 50 1176-1182... 

Wu CH, Shang LL, Cheng FL, Chao YK (2001) Modeling competitive adsorption of molybdate, sulfate and selenate on y-Al203 by the triple-layer model. J Colloid Interf Sci 233 259-264... 

Yoshimura et al. [193] carried out microdeterminations of phosphate by gel-phase colorimetry with molybdenum blue. In this method phosphate reacted with molybdate in acidic conditions to produce 12-phosphomolybdate. The blue species of phosphomolybdate were reduced by ascorbic acid in the presence of antimonyl ions and adsorbed on to Sephadex G-25 gel beads. Attenuation at 836 and 416 nm (adsorption maximum and minimum wavelengths) was measured, and the difference was used to determine trace levels of phosphate. The effect of nitrate, sulfate, silicic acid, arsenate, aluminium, titanium, iron, manganese, copper, and humic acid on the determination were examined. 

Manning, B.A. Goldberg, S. (1996) Modeling competitive adsorption of arsenate with phosphate and molybdate on oxide minerals. Soil Sci. Soc. Am. J. 60 121-131 Manning, B.A. Fendorf S.E. Goldberg, S. (1998) Surface structures and stability of ar-senic(lll) on goethite spectroscopic evidence for inner-sphere complexes. Environ. Sci. Techn. 34 2383-2388... 

Reyes, E.D. Jurinak, J.J. (1967) A mechanism of molybdate adsorption on a-Fe203. Soil Sci. 

Zhang, P.C. Sparks, D.L. (1989) Kinetics and mechanism of molybdate adsorption/desorp-tion at the goethite/water interface using pressure-jump relaxation. Soil Sci. Soc. Am. 

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. 

Hexacyanoferrates were immobilized on Au covered with SAM of 3,3 -thiodipropionic acid [86]. It has been found from voltammetric studies that the surface coverage of hexacyanoferrate is close to one monolayer and such an electrode exhibits very good surface redox behavior. Cheng et al. [87] have described the formation of an extremely thin multilayer film of polybasic lanthanide heteropolytungstate-molybdate complex and cationic polymer of quaternary poly(4-vinylpyridine), partially complexed with osmium bis(2,2 -bipyridine) on a gold electrode precoated with a cysteamine SAM. Consequently, adsorption of inorganic species might also be related to the properties of SAMs. This problem will be discussed in detail in a separate section later. 

Iron molybdates, well known as selective methanol oxidation catalysts, are also active for the propene oxidation, but not particularly selective with respect to acrolein. Acetone is the chief product at low temperature (200°C), whereas carbon oxides, besides some acrolein, predominate at higher temperatures [182,257], Firsova et al. [112,113] report that adsorption of propene on iron molybdate (Fe/Me = 1/2) at 80—120°C causes cation reduction (Fe3+ -> Fe2+) as revealed by 7-resonance spectroscopy. Treatment with oxygen at 400°C could not effect reoxidation (in contrast to similarly reduced tin molybdate). The authors assume that this phenomenon is related to the low selectivity of iron molybdate. 

Mann and Ko [202] likewise examined the selective oxidation of isobutene on bismuth molybdate. With an integral flow reactor, the highest selectivity was obtained at over 30% conversions for an oxygen/olefin ratio of 2/1 and a W/F = 2.5 g h mol-1 (390°C). The data were correlated with a rather complicated Langmuir—Hinshelwood expression inconsistent with a redox mechanism. This was based on a rate-controlling step between adsorbed isobutene and adsorbed oxygen, and included an inhibiting effect of methacrolein by competitive adsorption with isobutene, viz. 

Investigating a bismuth molybdate catalyst with Bi Mo = 1 oxidizing pentenes to isoprene, Watanabe and Echigoya [344] found that isomers of pentenes were less reactive than those of the corresponding butenes in flow experiments, but the reverse was true in pulse experiments. Heat of adsorption measurements make it clear that the active sites are not uni-... 

There is considerable evidence that surface acidity influences the catalytic activity of iron molybdate [254]. It was found by studying the adsorption of ammonia using infrared spectroscopy that, under reaction conditions, the acidity is due to Lewis sites. The conclusion is that surface acidity is a necessary, but not a sufficient, property. 

Molybdate s low toxicity to fish and other aquatic life has helped to gain recognition in recent years as a corrosion inhibitor. Molybdate forms its protective film by adsorption on metal surfaces. When chloride and sulfate anions are present in the cooling water environment, they compete for adsorption, and high concentrations of molybdate are needed for effective passivation of the metal surfaces. In order to be able to reduce the molybdate concentration for cost-effective levels, synergistic blends are made up that include other inhibitors such as phosphorates and zinc. 

When fine powders of vitreous silica, quartz, tridymite, cristobalite, coesite, and stishovite of known particle-size distribution and specific surface area are investigated for their solubility in aqueous suspensions, final concentrations at and below the level of the saturated concentration of molybdate-active silicic acid are established. Experimental evidence indicates that all final concentrations are influenced by surface adsorption of silicic acid. Thus, the true solubility, in the sense of a saturated concentration of silicic acid in dynamic equilibrium with the suspended silica modification, is obscured. Regarding this solubility, the experimental final concentration represents a more or less supersaturated state. Through adsorption, the normally slow dissolution rates of silica decrease further with increasing silicic acid concentrations. Great differences exist between the dissolution rates of the individual samples. 

Further evidence supporting the bismuth center as a site of propylene activation comes from the analysis of the rates of formation and product distribution of propylene oxidation over bismuth oxide, bismuth molybdate, and molybdenum oxide. Bismuth molybdate is highly active and selective for the conversion of propylene to acrolein. However, the interaction of propylene with its component oxides yields very different results. Haber and Grzybowska (//. ), Swift et al. 114), and Solymosi and Bozso 115) showed that in the absence of oxygen, propylene is converted to 1,5-hexadiene over bismuth oxide with good selectivity and at a high rate, whereas molybdenum oxide is known to be a fairly selective but a nonactive catalyst for acrolein formation. The formation of 1,5-hexadiene over bismuth oxide can be explained if the adsorption of propylene on a bismuth site yields a ir-allylic species. Two of these allylic intermediates can then combine to give 1,5-hexadiene. 

A somewhat different picture emerges from the adsorption studies of Matsuura and Schuit 117). They have attempted to elucidate the active adsorption sites on -y-bismuth molybdate by measuring the adsorption equilibria of butene, propylene, oxygen, water, butadiene, acrolein, and... 

Another communication has presented a comparison of the adsorption properties of the a, /3, and y phases of bismuth molybdate (120). The / phase exhibited two types of adsorption a slow, activated type for butadiene and a fast, weak type for butadiene and butene. The two types of adsorption were similar, but not completely identical, to the A and B sites on the y phase of bismuth molybdate. The slow and strong adsorption on the /3 phase was stronger than on the y phase and the weak adsorptions were single site instead of dual site. 

Weak- and single-type adsorptions of butene and butadiene were reported for the a phase of bismuth molybdate. The total amount of adsorption was low and the strong adsorption of butadiene was not observed. A previous comparison of the adsorption properties of the a phase and the y phase also revealed that for the a phase, the number of B sites undergoes a drastic decrease without changing their specific properties (106). On the other hand, the concentration of A sites on the a phase was observed to remain essentially the same but their properties were changed substantially from the y phase. In particular, the A sites on the a phase lost their property to adsorb butadiene. 

---