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Journal ArticleDOI

Hydrogen spillover effects in the reduction of iron oxide

01 Mar 1975-Reaction Kinetics and Catalysis Letters (Kluwer Academic Publishers)-Vol. 2, Iss: 1, pp 51-56

AbstractFreshly formed metal accelerates the rate of reduction of ferric oxide in the presence of water vapour. This effect is explained on the basis of the spillover of hydrogen from the metal sites to the oxide phase through “portholes” of water.

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Citations
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Journal ArticleDOI
Abstract: The reduction kinetics of both non-activated and mechanically activated hematite concentrate in a vibratory mill for different grinding periods have been studied using themogravimetry (TG). Changes in the structure of hematite were studied using X-ray diffraction analysis. The isoconversional method of Kissinger–Akahira–Sunose (KAS) was used to determine the activation energy of the different reactions. The Vyazovkin model-free kinetic method was also used for prediction of kinetic behavior of the samples for a given temperature. Fe2O3 was found to reduce to Fe in a two-step via Fe3O4. Intensive grinding resulted in improved resolution of overlapping reduction events. It was also established that the mechanical activation had a positive effect on the first step of reduction. With increasing the grinding time, the activation energy at lower extent of conversion (α ≤ 0.11) decreased from 166 to 106 kJ mol−1 range in the initial sample to about 102–70 kJ mol−1 in the sample ground for 9 h. The complexity of the reduction of hematite to magnetite and magnetite to iron was illustrated by the dependence of E on the extent of conversion, α(0.02 ≤ α ≤ 0.95). The values of E decreased sharply with α for 0.02 ≤ α ≤ 0.11 range in the initial sample and mechanically activated samples, followed by a slight decrease in the values of E during further reduction by α ≤ 0.85 in the ground samples up to 3 h. A slight increasing dependence of E on α for mechanically activated sample within 9 h in the second step of reduction was observed due to the finely agglomerated particles during intensive milling and subsequently the formation of a dense layer during the reduction processes. In addition, the dependence of ln Aα on α was detected and it was found that the ln Aα shows the same dependence on α as the apparent activation energies.

52 citations

Journal ArticleDOI
Abstract: The presence of low quantities of water vapour can seriously affect the kinetics of reduction of iron oxides when they are used as catalyst or to store and/or purify hydrogen from streams in the steam-iron process. Only 5% (v) of steam should be enough to inhibit the complete reduction of the solids. Since steam is a product of the reduction reaction, small amounts of water present in the reactive atmosphere can slow down the reduction itself. To account for the effect of the steam pressure during the reduction stage of the steam-iron process, two approaches have been considered and the resulting models, i.e. ‘competitive model’ and ‘inhibitive model’ have been tested against experimental measurements. Both models are based on the known Johnson-Mehl-Avrami-Kolmogorov (JMAK) theory. The ‘competitive model’, accounts for the discretization of groups of moles of iron oxide/iron reducing and oxidizing with their own reaction rates. By using the kinetic parameters obtained from independent reduction and oxidation processes, this model is not capable of predicting properly the behaviour of the solid subjected to successive reductive and oxidative cycles. On the contrary, the ‘inhibitive model’, which takes into account the hydrogen and water vapour partial pressures in a Langmuir–Hinshelwood type kinetic constant dependency, seems to be very appropriate to predict correctly the effect of the presence of water in the reducing atmosphere.

29 citations

Journal ArticleDOI
Abstract: The rate of reduction of magnetite by gaseous hydrogen is slightly affected by water vapour (1%). However, this effect of water vapour is significant in iron catalysts for ammonia synthesis of the KM I type. Promoted magnetite is the main component of the iron catalyst and it is concluded therefore that the influence of water is applicable only when promoters are present. The validity of the core-and-shell reduction model, assuming a Langmuir-Hinshelwood kinetic equation which describes the reaction at the oxide/iron interface, is discussed on the basis of the kinetic data for unpromoted and promoted iron catalysts. It is found that the model is generally valid, except for the case of advanced reduction of promoted catalyst in a moist atmosphere.

14 citations

Journal ArticleDOI
Abstract: The hydrogen reduction behavior of iron oxide composite pellets containing Ni, Fe, and Mn from 973 K to 1173 K was compared with iron oxide and Al2O3 containing reference composite pellets to determine the effect of metallic species on the kinetics of iron oxide reduction. The Mn and Ni containing pellets showed slightly faster initial reduction rates compared to the Fe and Al2O3 containing pellets. The effect of the metal phases was found to be more significant at lower temperatures when chemical reaction at the interface is a slower and more controlling factor. From the SEM of partially reduced pellets, a wide intermediate region between an O rich unreacted core and an Fe rich outer shell was observed. Although an initially short topochemical receding interface controlled region exists, the mixed control between the topochemical receding interface and pore diffusion was prevalent. For Fe2O3/Mn composite pellets, the thermodynamic stability of the MnO is higher and Mn can act as a reductant for iron oxide. Thus, the overall metallization of the Fe2O3/Mn composite pellets decreased compared to the other Fe2O3/metal composite pellets. From the temperature dependence of the iron-oxide/metal composite pellets, the apparent activation energy was calculated to be approximately between 15 to 20 kJ/mol, which is typical of a mixed control reduction mechanism of gas diffusion and interface reaction.

9 citations

Journal ArticleDOI
Abstract: Low-temperature oxidation (LTO) of magnetite is an alteration process which occurs under normal atmospheric conditions, causing maghemitization. The use of magnetic properties as palaeoclimate proxies requires improved understanding of how humidity and temperature affect such processes. We exposed natural magnetite, with grain size ranging from <1 to ∼30 μm, to different humidity conditions at room temperature and 70 °C for 1 yr. Changes in room temperature setups were very minor, but in all 70 °C setups alteration was detected by magnetic and mineralogical properties. Lowering of the Verwey transition temperature (Tv) turned out to be the most sensitive indicator of LTO, and also lattice constants correlate well with the shift of Tv. Thermomagnetic curves and XRD-results indicate that LTO affects the entire volume of the particles rather than only surface layers. The sample exposed to high relative humidity (rH) >90 per cent at 70 °C showed the strongest degree of LTO with an increase of the oxidation degree by ∼3 per cent according to Tv, and it was the only setup where partial alteration to hematite was indicated by Mössbauer analysis. The sample with extremely dry conditions (rH of ∼5 per cent) at 70 °C, and the sample that was exposed to cycles of high and low humidity in 2-weeks alternation at 70 °C, both revealed a smaller degree of LTO. The smallest change of the high temperature setups was observed for the sample with intermediate rH of ∼13 per cent. The results suggest a non-linear sensitivity of magnetite alteration to humidity conditions, high humidity strongly favours alteration, but alteration is strongly reduced when extreme humidity alternates with dry conditions, suggesting an importance of seasonality in natural weathering.

6 citations


References
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380 citations

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198 citations

Journal ArticleDOI
Abstract: The reduction of WO 3 by H 2 to a blue form proceeds readily above 400°C. If the WO 3 powder is mixed with platinum black, reduction will start below 100°C. But if this mixture is made to adsorb water, reduction takes place rapidly at room temperature. The catalysis by platinum is apparently due to the dissociation of molecular hydrogen on the metal, followed by diffusion of adsorbed hydrogen atoms across the metal-oxide interface. The acceleration by water is ascribed to a marked increase in the rate of diffusion of the reducing species.

156 citations

Journal ArticleDOI
Abstract: It was found that yellow WOs could be reduced at room temperature by molecular hydrogen to form blue solids, the hydrogen analogs of tungsten bronzes with a molar composition of H0.35WO3. But this was possible only if the WO3 was mixed with platinum black and also if water was preadsorbed on the mixture prior to admitting hydrogen to the system. If either one of these conditions was not satisfied, reduction did not take place at all at room temperature. Additional observations of spillover in this and similar systems are reported in this paper. They throw more light on the role of water in the Pt— WO3 system, show the generality of the phenomenon and stress its importance in adsorption, solid state reactions and surface catalysis.

125 citations

Journal ArticleDOI
Abstract: The ESR investigation has provided evidence that the resonance at g = 2.10, developed upon heating of noble-metal-supported catalysts in hydrogen, results from the reduction of iron oxide to metallic iron also present on the supported catalyst. The mechanism of the oxide reduction evidently occurs by a sequence of events including hydrogen chemisorption on a metal such as palladium, hydrogen atom transfer to the support and then to iron oxide sites, and subsequent coalescing of iron to form ferromagnetic domains. The relative effectiveness of iron oxide reduction resulting from the presence of various supported metals and supports appears to depend on the relative heats of hydrogen atom sorption compared with the activation energy required for the atom transfer from the metal to the support.

38 citations