Iron oxide acidic sites

Soil types tested for Pb species All Pb released to soils— PbS04, PbCOa, PbS, PbCIBr, etc.—is connected to adsorbed complexed forms binding sites are clays, organic acids, hydrous iron oxide Various binding systems Chaney et al. (1988)... 

The aluminum is incorporated in a tetrahedral way into the mesoporous structure, given place to Bronsted acidic sites which are corroborated by FTIR using pyridine as probe molecule. The presence of aluminum reduces the quantity of amorphous carbon produced in the synthesis of carbon nanotubes which does not happen for mesoporous silica impregnated only with iron. It was observed a decrease in thermal stability of MWCNTs due to the presence of more metal particles which help to their earlier oxidation process. 

An interesting variation on sulfated metal oxide type catalysts was presented by Sun et al. (198), who impregnated a dealuminated zeolite BEA with titanium and iron salts and subsequently sulfated the material. The samples exhibited a better time-on-stream behavior in the isobutane/1-butene alkylation (the reaction temperature was not given) than H-BEA and a mixture of sulfated zirconia and H-BEA. The product distribution was also better for the sulfated metal oxide-impregnated BEA samples. These results were explained by the higher concentration of strong Brpnsted acid sites of the composite materials than in H-BEA. 

One of the earliest reports of LO inhibition concerned the effects of ortho-dihydroxybenzene (catechol) derivatives on soybean 15-LO [58]. Lipophilic catechols, notably nordihydroguaiaretic acid (NDGA) (19), were more potent (10 /zM) than pyrocatechol itself. The inactivation was, under some conditions, irreversible, and was accompanied by oxidation of the phenolic compound. The orfAo-dihydroxyphenyl moiety was required for the best potency, and potency also correlated with overall lipophilicity of the inhibitor [61]. NDGA and other phenolic compounds have been shown by electron paramagnetic resonance spectroscopy to reduce the active-site iron from Fe(III) to Fe(II) [62] one-electron oxidation of the phenols occurs to yield detectable free radicals [63]. Electron-poor, less easily oxidized catechols form stable complexes with the active-site iron atom [64]. 

In general, the adsorption behaviour of gases on iron oxides has been investigated either because the gases are atmospheric pollutants (NO, SO2, CO2), or because they act as probe molecules and provide information about surface sites. The adsorption sites on iron oxides are usually the Lewis acid sites. 

Pernicone et al. [253,254] bring forward some evidence that surface acidity also plays a role with iron molybdate catalysts. Hammett indicators adsorbed over the molybdate assume the acid colour. Pyridine poisons the oxidation of methanol to formaldehyde. A correlation is reported between acidity and activity [253]. The authors agree with Ai that the acid sites are connected with Mo6+ ions. 

Hydrocarbon production and selectivities at comparable CO conversion are given in Table 19.2. The ultrafine iron oxide catalyst had a very poor C2-C4 olefin selectivity while the olefin selectivity of the precipitated catalyst was slightly higher than the iron carbide catalyst. This is surprising because Rice et al. report higher olefin selectivity for a similar iron carbide catalyst than a conventional Fe/Co catalyst.6 Soled et al. have subsequently reported that the conventional catalyst contains acidic sites which... 

Iron oxides have been known to adsorb either acids or bases (25). The ratio of acidic to basic sites can be estimated by a method developed by Manson and Straume (31), based on the adsorption of... 

The acidic sites on iron oxides are believed to be FeOH sites (32), much like the well-known SiOH sites on silica. Heats of adsorption on iron oxide of bases of known Cg and Eg, having appreciably different ratios of Cg to Eg ("hardness" or "softness"), allow estimation of the and for the acidic sites of iron oxide. Our initial studies were done by measuring adsorption isotherms at two or more temperatures (Figure 7) and from the temperature coefficient of the equilibrium constant K the enthalpy of adsorption was calculated. In Figure 7 the adsorption data is plotted as a Langmuir isotherm ... 

J. Bolger proposed that the isoelectric pH in water could be used to rank the acidic or basic surface properties of oxides (37) and there is much merit in the suggestion. Certainly the basicity of aluminum oxide is predicted by its isoelectric pH of 9.0 and the acidity of silica is predicted by its isoelectric pH of 3.0. However when a surface has both basic and acidic sites like many iron oxides, the isoelectric pH s near 7 do not really tell the whole story. 

ETFE resins have a good thermal stability however, for high-temperature applications thermal stabilizers are often added67 A wide variety of compounds, mostly metal salts, such as copper oxides and halides, aluminum oxide, and calcium salts, will act as sacrificial sites for oxidation. Addition of certain salts can alter the decomposition from oligomer formation to dehydrofluorination. Iron and other transition metal salts accelerate the dehydrofluorination. Hydrofluoric acid itself destabilizes ETFE at elevated temperatures and the degradation becomes self-accelerating. For that reason, extrusion temperatures higher than 380°C (716°F) should be avoided.68... 

Using IR spectroscopy, Tanabe et al. proposed a structure for the acid sites on sulfur-promoted iron oxide to be chelating bidentate complexes (158-160). 

Freeze-quenching technique in combination with ESR and Mossbauer spectroscopy was used for monitoring intermediates in the reaction of substrate free 57Fe-P450C8Itl with peroxy acetic acid (Schunemann et al., 2000). In such a condition, the oxidant oxidized the enzyme active site iron (III) to iron (VI) and Tyr 96 into tyrosine radical, 90% and 10% from the starting material, respectively. Thus the tyrosine residue may be involved in the catalytic process. 

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