Reactivity of Solids最新文献

Sponge iron or direct reduced iron (DRI) is known to be very susceptible to loss of metallization by reoxidation in moist oxygen-containing atmospheres, even resulting in spontaneous ignition under specific situations. Different mechanisms have been put forward so far for such this phenomenon. On the basis of experimental investigations carried out extensively with coal-based DRI produced through a pilot kiln, it has been brought to light that the reoxidation process follows a logarithmic law at low temperatures and a first-order law at high temperatures. It is also shown that both laws may be derived from similar expressions for a porous material such as DRI.

The conversion of MnCr₂O₄ to sulphides was investigated at 1073 K, 1173 K and 1273 K in H₂-H₂O-H₂S mixtures. From thermogravimetric studies on the sulphidation of the bulk spinel, the stability limit of MnCr₂O₄ was determined. When a certain sulphur pressure is exceeded corresponding to the reaction MnCr₂O₄ + 3/2S₂ = MnS + 2CrS + 2O₂, the spinel is converted to a porous layer of mixed sulphides (Mn,Cr)S and (Cr,Mn)S. The partial pressure dependences indicate that the reaction rate is controlled by a surface reaction, i.e. the sulphur transfer from H₂S at the interface sulphide-spinel which is easily accessible through the large pores in the sulphide layer. Similar kinetics and morphology of the reaction products were observed when MnCr₂O₄ layers obtained by preoxidation of Fe-Cr-Mn alloys were sulphidized. The results provide information on the stability of oxide layers formed on Mn-containing high temperature alloys in oxidizing-sulphidizing atmospheres.

The 2-furaldehyde thiosemicarbazone complexes of Co^II, Ni^II and Pd^II have been subjected to thermal decomposition studies in air using TG, DTG and DTA techniques. The kinetic parameters for both stages of decomposition of these complexes were computed by a weighted least-squares method using the general approach as well as the Coats-Redfern, Freeman-Carroll and Horowitz-Metzger equations. The results indicate that the values of the kinetic parameters obtained by these different methods agree well. It was also found that both stages of decomposition of these complexes follow first-order kinetics.

The interaction between gaseous oxygen and the surface of sintered polycrystalline Fe₂O₃ was studied by using “in situ” work function measurements at 1140 K (867°C). The treatment of the specimen (after preliminary sintering at 1673 K in air) involved polishing of the oxide surface with a diamond powder resulting in the formation of a high density of grooves of different orientations along the surface. Sorption of oxygen on the freshly polished surface resulted in the formation of a positive surface charge which has been considered in terms of the formation of subsurface microdipoles, induced either by specific active centres formed during the mechanical treatment, or by donors formed at the same centres. The centres were closely related to the presence of grooves. The charge decreased during the thermal treatment and finally changed in sign. It was also observed that the treatment led to disappearance of grooves except some with specific orientation. The time of the equilibration, whose rate is controlled by the surface diffusion, was about 300 h at 1140 K.

The influence of particle size of α-Fe₂O₃ on the formation of ZnFe₂O₄, which proceeds by an initial rapid surface reaction followed by the diffusion process, was studied at 700–1000°C. Four kinds of α-Fe₂O₃ samples were used: a ball-milled and sieved sample with average particle size of d₀ =1.2 μm (designated as M-1) and another with d₀ = 16.8 (M-2), and an unmilled and sieved sample with d₀ = 19.2 μm (I-1) and another with d₀ = 73.5 μ (I-2). With sample M - 1, the initial surface reaction accounts for at least 35% of the reacted fraction at temperatures higher than 700 ° C and with the other samples, the surface reaction is of importance above 800°C.

The M - 2, I - 1 and I - 2 samples had for the diffusion process almost the same activation energy (about 180 kJ mol⁻¹), indicating the particle size had no influence on the diffusion process, whereas sample M - 1 showed a much lower activation energy, this reduction presumably due to the ball-milling. Scanning electron microscopy of the surfaces of α-Fe₂O₃ particles showed a characteristically textured ferrite surface with the wavy lines arising in the initial period from the rapid specific migration of the ZnO component leading to the surface reaction. This texture changed to an interwoven pattern as the reacted fraction increased.

Cationic and H-forms of clinoptilolite and ferrierite were irradiated with -γ-rays. Presorbed triethylamine (TEA) on the unirradiated and irradiated zeolites was subjected to oxidation in a differential scanning calorimetry (DSC) cell in flowing oxygen, making use of the programmed temperature increase of the DSC system. The sum of ΔH for all DSC effects obtained for TEA oxidation in a thermogram was taken as a measure for the zeolite chemisorptive capacity (acidity). The data given by DSC and the acidities determined thermogravimetrically (TG) for presorbed TEA as well as those determined from the IR spectra of presorbed pyridine (Bronsted pyridine “BPY” and Lewis pyridine “LPY”) show certain discrepancy due to evaluation of some high-temperature DSC effects by extrapolation beyond the DSC maximum temperature. Improvement of the DSC data is discussed. However, similar trends of the DSC ΔH values and the TG and IR values with irradiation dose are observed. The temperature at which the maximum of the principal oxidation DSC effect can be taken as a criterion for correlating the zeolite reactivity to oxidise TEA. In general, γ-irradiation was found to enhance the zeolite reactivity to chemisorb TEA (acidity) and to enhance the rate of oxidizing the presorbed base.

The biologically active square planar Co^II- and Cu^II-metformin complexes show a phase change upon heating in the solid state. This phase change was studied by various analytical and spectral methods and some kinetic parameters of the transformation reaction calculated. The obtained phase change was also discussed in terms of solid state thermal isomerisation.

The hydrothermal treatment in NaOH solution of clays activated by grinding leads to the formation of different zeolites, whose nature depends on the amorphization of the clay obtained by applying a periodic stress by means of an oscillating mill. In this way, hydroxysodalite, mixtures of hydroxysodalite and NaA zeolite, or NaA pure zeolite can be obtained. In the present paper, it is shown that the product varies in composition, and is enriched in NaA zeolite as the crystalline structure of the clay is destroyed by grinding. It is proved that the activation by impact grinding is an alternative method to the classical calcination for the synthesis of the NaA zeolite.

Mechanically alloyed nickel aluminide of composition Ni₃₅Al₆₅ retains its CsCl structure after being treated with 20% KOH solution. The Raney nickel catalyst thus obtained was identified as having a CsCl-type structure. Annealing of this metastable material at 120–150°C led to the formation of a cubic nickel structure. This phase transformation was accompanied by the appearance of strong ferromagnetism.

CuO-Fe₂O₃ sample of equimolar ratio was prepared by coprecipitation. The mixed hydroxides were studied using differential thermal analysis. The thermal products at different temperatures up to 1000°C were characterized by means of X-ray diffraction analysis. The morphological changes were followed through scanning electron microscopy and complemented by X-ray photoelectron spectroscopy. The thermal treatment of the mixed hydroxides at temperatures between 200 and 1000°C produced almost a crystalline CuO-phase. Scanning electron microscopy showed two distinct types of species. The first (major) is microspheres and the second is needle-like type. Electron spectroscopy for chemical analysis revealed that this sample contained sodium ions which covered the surface of the CuO-Fe₂O₃ species in the form of sodium sulphate layer. This layer prevents the solid-solid interaction. Washing the sample thoroughly leads to the removal of sodium sulphate layer and increases the reactivity of solids to form a crystalline copper-ferrite phase.