Fe surface

In moist enviromnents, water is present either at the metal interface in the fonn of a thin film (perhaps due to condensation) or as a bulk phase. Figure A3.10.1 schematically illustrates another example of anodic dissolution where a droplet of slightly acidic water (for instance, due to H2SO4) is in contact with an Fe surface in air [4]. Because Fe is a conductor, electrons are available to reduce O2 at the edges of the droplets. 

Figure 8.20 Nitrogen Is and oxygen Is X-ray photoelectron spectra of nitric oxide (NO) adsorbed on an iron surface. 1, Fe surface at 85 K 2, exposed at 85 K to NO at 2.65 x 10 Pa for 80 s 3, as for 2 but exposed for 200 s 4, as for 2 but exposed for 480 s 5, after warming to 280 K. (Reproduced, with permission, from Kishi, K. and Roberts, M. W., Proc. R. Soc. Land., A352, 289, 1976)...

Figures The NIE), dN E)/dE, dEN E/dE. and EN E) forms of secondary electron energy spectra from a slightly contaminated Fe surface.

Karlson et found, from on-site experience of cement-producing plant, that corrosion of Fe surfaces may occur in gases containing O2, SO2 and alkali chlorides such as NaCl and KCl between 300°C and 500°C. They reported that the corrosion rates may be extraordinarily high (5-10 mm/ month) implying liquid-phase corrosion. Laboratory simulation of the plant conditions demonstrated the need for both SO2 and the alkali chloride in the environment. The principle corrosion reaction was found to be ... 

A classical example of promotion is the use of alkalis (K) on Fe for the ammonia synthesis reaction. Coadsorbed potassium (in the form of K20) significantly enhances the dissociative adsorption of N2 on the Fe surface, which is the crucial and rate limiting step for the ammonia synthesis5 (Fig. 2.1). 

FIG. 7 Total energy per cross-sectional area as a function of interfacial separation between Fe and A1 surfaces for the clean interface and for monolayer interfacial impurity concentrations of B, C, N, O, and S. Graph (a) is for the case where the impurity monolayer is applied to the free A1 surface prior to adhesion, while graph (h) has the impurity monolayer applied to the free Fe surface prior to adhesion. The curves fitted to the computed points are from the universal binding energy relation. (From Ref. 28. Copyright 1999 hy the American Physical Society.)... 

The Fe-B nanocomposite was synthesized by the so-called pillaring technique using layered bentonite clay as the starting material. The detailed procedures were described in our previous study [4]. X-ray diffraction (XRD) analysis revealed that the Fe-B nanocomposite mainly consists of Fc203 (hematite) and Si02 (quartz). The bulk Fe concentration of the Fe-B nanocomposite measured by a JOEL X-ray Reflective Fluorescence spectrometer (Model JSX 3201Z) is 31.8%. The Fe surface atomic concentration of Fe-B nanocomposite determined by an X-ray photoelectron spectrometer (Model PHI5600) is 12.25 (at%). The BET specific surface area is 280 m /g. The particle size determined by a transmission electron microscope (JOEL 2010) is from 20 to 200 nm. 

Fe/MgO catalysts with 5 to 30 mol % Fe have been prepared by impregnation and coprecipitation. Their reducibility has been measured and a comparison made of their Fe° surface areas. Catalysts prepared via coprecipitation yielded larger iron areas than those via impregnation. The activity and selectivity of the reduced catalysts for the hydrogenation of propanenitrile at 20-30 bar and 473 K and of ethanenitrile at 1 bar and 508 K have been determined. The most active catalysts are those prepared by coprecipitation and they show high selectivity for primary amines. The activity for ethanenitrile hydrogenation correlates with the iron surface area. 

As catalyst, a Pd/Fe system is used, having finely dispersed Pd clusters (< 1 pm) on the Fe surface [20] (see also original citations in [20]). A considerable portion of the surface remains uncovered, exposing Fe for reaction. 

Recently a novel experimental approach using Schottky diodes with ultra-thin metal films (see Fig. 11) makes direct measurement of reaction-induced hot electrons and holes possible. See for example Refs. 64 and 65. The chemical reaction creates hot charge carriers which travel ballistically from the metal film towards the Schottky interface and are detected as a chemicurrent in the diode. By now, such currents have been observed during adsorption of atomic hydrogen and deuterium on Ag, Cu and Fe surfaces as well as chemisorption of atomic and molecular oxygen, of NO and N02 molecules and of certain hydrocarbons on Ag. Similar results have been found with metal-insulator-metal (MIM) devices, which also show chemi-currents for many exothermic surface reactions.64-68... 

The results of this work are given in Table II. It can be seen that the CO/Fe surface ratio varies from sample to sample. This probably results from the presence of two surface species, in different relative amounts, on each sample. This is explained in the discussion section. 

Catalysts consisting of more than one component are often superior to monometallic samples. Model studies with potassium on Fe surfaces revealed, for example, the role of the electronic promoter in ammonia synthesis. A particularly remarkable case was recently reported for a surface alloy formed by Au on a Ni(lll) surface where the combination of STM... 

Iron oxides in the finely divided form have the power to promote (catalyse) a range of redox and photochemical reactions (Tab. 11.7). The preliminary step is the adsorption of the reacting species on the iron oxide. This may be followed either by direct reaction with the Fe surface atoms or surface functional groups or the surface may promote reaction between the adsorbed species and a solution species such as dissolved oxygen. 

The Ea for the dissolution of hematite by mercapto carboxylic acids in acid media in the presence of UV radiation was lower (64 5 kj mol ) than that for dissolution in the absence of radiation (94 8 kJ mol ) (Waite et al. 1986). The reaction in both cases was considered to involve formation of an intermediate organic-Fe surface complex which decomposed as a result of intramolecular electron transfer to release Fe". UV irradiation enhanced the decomposition of the surface complex either through excitation of the ligand field states associated with the free electrons on the S atoms, or through high energy charge transfer states. 

Reactions which may occur on sites consisting of one or two atoms only on the surface of the catalyst are generally known as facile reactions. Reactions involving hydrogenation on metals are an example. Eor such reactions, the state of dispersion or preparation methods do not greatly affect the specific activity of a catalyst. In contrast, reactions in which some crystal faces are much more active than others are called structure sensitive. An example is ammonia synthesis (discovered by Fritz Haber in 1909 (Moeller 1952)) over Fe catalysts where (111) Fe surface is found to be more active than others (Boudart 1981). Structure-sensitive reactions thus require sites with special crystal structure features, which... 

The promotion effects of Mn on unsupported Fe-based F-T catalysts were also studied by Jensen and Massoth. " These authors concluded that the incorporation of Mn chemically and electronically promotes the active Fe surface. More particularly, it appears to alter the CO hydrogenation reaction path by suppressing the direct formation of paraffins from the reactive intermediate, leading to the increased production of higher olefins. Finally, Das et al. also observed that the addition of moderated amounts of Mn promoter to unsupported Fe F-T catalysts promotes the catalytic activity as well as the selectivity towards lower alkenes. ... 

Finally, all reactions may occur, and no simplification of Eq. (7.114) is possible. Both the slope of the plot and the intercept will be potential dependent and the latter will be greater than 1/A. These possibilities are all portrayed in Fig. 7.53. From such results the conclusions drawn for oxygen reduction on a pure Fe surface were that on the bare iron, the rate-determining step involves the formation of Oj, while on the passive layer it is oxygen chemisorption under Temkin conditions. 

The mechanism may change from acids to alkalis in some cases [365], This may be related to the higher sensivity of the Fe surface to oxidation in alkaline solutions [365, 367], Actually, the corrosion of Fe proceeds also under moderate cathodic load [368]. Impedance measurements have suggested that the classical mechanisms of hydrogen evolution is probably inadequate to describe the situation on Fe [377], A surface diffusion step with spillover of hydrogen to sites with lower M-H energy has been suggested. Adsorption of CN- interferes with such a diffusion. 

The HSAB principle states that hard acids (Fe surface) prefer to coordinate with hard bases (oxygen, phosphates). Hard interactions are normally ionic. Iron oxides can be readily formed and the anti wear mechanism starts to interact with the polymeric zinc metaphosphate, Zn(P03)2. 

Figure 3 Scanning electron micrograph of an Fe° grain taken from an FePRB at the Y-12 site at Oak Ridge, TN. The bright spot is mostly U, showing that these deposits are localized on the Fe° surface. These deposits were associated with varying amounts of Fe, S, Si, and Ca. Additional details on the analyses of these samples are in Ref. 76.

Competition for Reactive Sites. Recently, it has become widely recognized that kobs can vary with the concentration of the contaminant. In most cases, this effect has been attributed to saturation of reactive sites on the Fe° surface. One kinetic model that has been used to describe these data is of the form ... 

The overall reaction occurring at an Fe° surface involves a series of steps including (1) mass transport to the reactive site, (2) chemical reaction at the surface (e.g., sorption, electron transfer, etc.), (3) desorption, and (4) mass transport to the bulk solution (recall Fig. 7). Any one of these steps can limit the rate of contaminant removal by Fe°, so the observed... 

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