Oxygen adsorbed iron oxide

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 passivation by oxygen of a commercial ammonia synthesis catalyst was studied with adsorption microcalorimetry by Tsarev and co-workers (240). Two types of adsorbed oxygen at 293 K were found to participate in the formation of a passivating layer. One type was characterized by differential heats of adsorption near 420 kJ mol" that were close to the heat of iron oxidation and which were independent of surface coverage for several mono-layers. The other form was obtained after a large dose, sufficient for coverage of the entire metal surface with a molecular monolayer. Subsequent adsorp-... 

We are investigating bifunctional catalysts in which one component of the catalyst adsorbs or oxidizes CO and the other component dissociates water. Our present research is focusing on metal-support combinations to promote this bifunctional mechanism. The metallic component is chosen to adsorb CO at intermediate adsorption strengths (platinum [Pt], Ru, palladium [Pd], PtRu, PtCu, cobalt [Co], ruthenium [Ru], silver [Ag], iron [Fe], copper [Cu], and molybdenum [Mo]). The support is chosen to adsorb and dissociate water, typically a mixed-valence oxide with redox properties or oxygen... 

However, these results must be carefully evaluated since contamination of the systems with molecular oxygen cannot be excluded, especially in the early studies. Thus, the experimental conditions and systems reported in this literature are not directly comparable to our study where predominantly non mixed-valence iron oxides were investigated under strict exclusion of molecular oxygen. Direct electron transfer between adsorbed Fe(II) and structural Fe(III) at the goethite surface seems unlikely as Fe(III) within the crystal structure is stabilized by neighbouring oxygen ligands. 

J. Moreau and J. Benard [C. K Acad. Sci (Paris) 242,1724 (1956)] showed, through observation of the metal surface in H2O-H2 mixtures at elevated temperatures, that oxygen adsorbed on an 18% Cr stainless steel is thermodynamically more stable than the metal oxide. For analogous data on iron, see A. Pignocco and G. Pellissier, J. Electrochem. Soc. 112,1118 (1965) E. Hondros, Acta Metall. 16, 1377 (1968). 

The oxygen signal (figure 3) is composed of three peaks the first at 530.2 0.1 eV can be assigned to the iron oxide the second peak at 531.8 0.1 eV is assigned to the iron oxy-hydroxides and the third, at 533.1 0.1 eV, having the lower intensity in comparison with the others, is attributable to adsorbed water. 

In the non-contact area, a thin film of adsorbed organic thiophosphate and iron oxy-hydtoxides is clearly identified. The iron oxide-related oxygen clearly dominates outside the contact area, whereas the contact region consists mostly of phosphate-type oxygen. These results illustrate the complexity of the surface chemical analysis of a tribostressed sample. 

The gas product derived from reduction should be expelled from the reaction region along the same path. Here, the reducing agent contacts with iron oxide, namely, it is adsorbed on the surface of the solid and starts surface reactions. The reduction reaction includes seizure of oxygen from oxides, the formation and growth of crystal nuclei of the product. The continuous growth of product layer is maintained by solid state reaction and the diffusion in solid state. 

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