M. Gracia

Cation distribution and magnetic structure of the ferrimagnetic spinel NiCo2O4

The compound NiCo2O4, with spinel-related structure, has been prepared by thermal decomposition of metal nitrates and its bulk structural properties examined by means of magnetic measurements, neutron diffraction, X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). The results suggest a delocalised electron distribution on the octahedral sites with average oxidation states of +3.5 and +2.5 for nickel and cobalt, respectively, and lead to a cation distribution for NiCo2O4 of {Ni3+0.1Co2+0.9}tet[Ni3.5+0.9Co2.5+1.1]octO4. This electronic configuration is consistent with magnetisation measurements if applied magnetic fields cause a charge redistribution on the octahedral sites to favour Co3+ and Ni3+. The surface of NiCo2O4 was examined by X-ray photoelectron spectroscopy (XPS) and found to have a different composition containing Co2+, Co3+, Ni2+, Ni3+ and, probably, Ni4+.

Characterisation by X-ray photoelectron spectroscopy of thin MnxCo3−xO4(1≥x≥0) spinel films prepared by low-temperature spray pyrolysis

Abstract Thin films of MnxCo3−xO4 with x=0; 0.25; 0.5; 0.75; 1, prepared by low temperature (150°C) spray pyrolysis onto a glass substrate with a sublayer of doped SnO2 were examined by X-ray photoelectron spectroscopy. The binding energies of the photoelectron peaks observed for the transition metal ions and the oxygenated ions were recorded. The results indicate that in these materials cobalt is present in both divalent and trivalent states, whereas manganese is found to be Mn3+.

Bricks in historical buildings of Toledo City: characterisation and restoration

Abstract Two different types of ancient bricks (12th to 14th centuries) collected from historical buildings of Toledo (Spain) were characterised by optical microscopy, scanning electron microscopy/energy-dispersive X-ray spectrometers (SEM/EDS), electron probe microanalysis (EM), X-ray diffraction (XRD), differential thermal analysis (DTA) and 57Fe-Mossbauer spectroscopy. Physical properties such as water absorption and suction, porosity, density and compression strength were also determined. Several minerals found in the brick matrix, such as garnet, let us infer raw material sources; calcite, dolomite, illite and neoformed gehlenite and diopside phases, on temperature reached in firing; secondary calcite, on first cooling scenarios; and manganese micronodules, on late pollution environments. XRD and DTA of original and refired samples supply information about firing temperatures. Additional data on firing conditions and type of the original clay are provided by the Mossbauer study. Physical properties of both types of bricks were compared and correlated with raw materials and fabric and firing technology employed. The physicochemical characterisation of these bricks provides valuable data for restoration purposes to formulate new specific bricks using neighbouring raw materials.

Ni0.3Co2.7O4 spinel particles/polypyrrole composite electrode: Study by X-ray photoelectron spectroscopy

In this work we have studied the multilayered polypyrrole/oxide composite electrode on glassy carbon having the structure GC/PPy/PPy(Ni0.3Co2.7O4)/PPy, in which the spinel oxide Ni0.3Co2.7O4 is known to be an electrocatalyst of the oxygen reduction reaction in alkaline medium. The successive GC/PPy, GC/PPy/PPy(Ni0.3Co2.7O4) and GC/PPy/PPy(Ni0.3Co2.7O4)/PPy parts of the electrode were examined by X-ray photoelectron spectroscopy. The results indicate that the electronic structure of the PPy is independent of the thickness of the PPy used to prepare the electrodes. The XPS data also show that cobalt is present in both divalent and trivalent states. The PPy doping degree by Cl− ions (in terms of the Cl−/N) and the oxide/PPy mass ratio observed by XPS were 19 and 4.5%, respectively.

Influence of weathering on the sulphur removal from coal by pyrolysis

Abstract In this paper evidence of the negative effect of weathering on the sulphur removal by pyrolysis is reported. The study of the evolution of sulphur forms during the pyrolysis of a weathered coal has been conducted by chemical analysis, transmission Mossbauer spectroscopy and scanning electron microscopy with energy dispersive X-ray analysis. This study shows that in the pyrolysis of weathered coals some special chemical changes occur when compared with the pyrolysis of unweathered coals. In particular, no accumulation of organic sulphur in chars has been observed, however, inorganic iron sulphates formed by the oxidation of pyrites are decomposed above 500°C and yield complex sulphides which accumulate in chars, decreasing the efficiency of the desulphurization. These sulphides, because of their unreactivity, cannot be determined completely by chemical analysis, which implies an overcalculation of organic sulphur when calculated by difference.

Mössbauer study of the corrosion behaviour of pure iron and weathering steel under a wet-dry cycle

Mossbauer spectroscopy (MBS) and X-ray diffraction (XRD) have been used to establish the composition of the rust layer formed on weathering steel and pure iron under several wet-dry cycles in a SO2-polluted atmosphere. FeSO3−3H2O, FeSO4−4H2O, and poorly crystalline ferrihydrite were identified as the only corrosion products. The Mossbauer spectrum of FeSO3−3H2O is reported.

Surface spectroscopic study of the corrosion of ultrathin 57Fe-evaporated and Langmuir–Blodgett films in humid SO2 environments

Integral conversion electron Mossbauer spectroscopy (ICEMS), x-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and scanning electron microscopy (SEM) have been used to carry out a comparative study of the corrosion resistance against humid SO2-aggressive environments of ultrathin 57Fe films. These films, having a thickness ≤ 2.5 nm, have been prepared by evaporation of 57Fe under vacuum and by 57Fe coating by a Langmuir–Blodgett (LB) technique on SiO2/Si wafers. The results have shown that the corrosion resistance of the evaporated films is similar to that of massive Fe. However, the LB films show a remarkably higher corrosion resistance. Thus, although the Fe films prepared by evaporation have been completely corroded after 6 h of exposure to the deleterious environment, the LB films remain unaltered even after 18 h of exposure to the same aggressive conditions. The higher corrosion resistance of the LB films appears to be related to the existence of a thin surface layer containing Si, Fe2+ and Fe3+ formed on top of the Fe metal film during the thermal treatments subsequent to the LB deposition. Copyright

Corrosion of weathering steel and iron under wet-dry cycling conditions: Influence of the rise of temperature during the dry period

The effect of a dry-hot period on the SO2 corrosion of weatherig steel and pure iron under wet-dry cycling was investigated. Corrosion products were identified by Mössbauer spectroscopy and X-ray powder diffraction. The formation of an intermediate corrosion layer of spm α-FeOOH only on weathering steel was the most significant result.

Corrosion studies of iron and its alloys by means of57Fe Mössbauer spectroscopy

Some of the advantages and limitations of Mössbauer spectroscopy when used in corrosion research are shown by using three examples taken from the work of the authors on (i) the passive layer of iron, (ii) the corrosion of weathering steels by SO2-polluted atmospheres and (iii) the performance of rust converters.

The corrosion of weathering steel by SO2 polluted atmospheres at its very early stages

CEMS was used in conjunction with AES to study the protective film formed on a weathering steel by exposure to a highly SO2-polluted atmosphere. Ferrous species (sulphite) and ferric oxyhydroxides (ferrihydrite and α-FeOOH) were identified as corrosion products. From the correlation of CEMS and AES results the evolution with time of the different compounds is obtained, and a possible reaction sequence is outlined.