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
TL;DR: In this paper, it was shown that freshly formed metal accelerates the rate of reduction of ferric oxide in the presence of water vapour, and this effect was explained on the basis of the spillover of hydrogen from the metal sites to the oxide phase through "portholes" of water.
Abstract: Freshly 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.
Citations
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TL;DR: In this article, 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).
65 citations
TL;DR: In this paper, the effect of water vapour on the kinetics of the steam-iron process was investigated and two models, the competitive model and the inhibitive model, were proposed to predict the behavior of the solid subjected to successive reductive and oxidative cycles.
35 citations
TL;DR: In this article, 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.
14 citations
TL;DR: In this article, the Verwey transition temperature (Tv) was found to be the most sensitive indicator of low-temperature oxidation of magnetite, and also lattice constants correlate well with the shift of Tv.
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.
10 citations
TL;DR: In this paper, the hydrogen reduction behavior of iron oxide composite pellets containing Ni, Fe, and Mn was compared with iron oxide and Al2O3 containing reference composite pellets to determine the effect of metallic species on the kinetics of IR reduction.
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.
10 citations
References
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TL;DR: In this paper, the reduction of WO 3 by H 2 to a blue form proceeds readily above 400°C and the acceleration by water is ascribed to a marked increase in the rate of diffusion of the reducing species.
168 citations
TL;DR: In this article, the role of water in the Pt-WO3 system was investigated and it was shown that if the WO3 was mixed with platinum black and also if water was preadsorbed on the mixture prior to admitting hydrogen to the system, the reduction did not take place at all at room temperature.
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.
134 citations
TL;DR: In this paper, it was shown 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.
39 citations