Early Stage Of Reduction Research Articles

Studies on influence of hydrogen and carbon monoxide concentration on reduction progression behavior of molybdenum oxide catalyst

Temperature programmed reduction (TPR) analysis was applied to investigate the chemical reduction progression behavior of molybdenum oxide (MoO3) catalyst. The composition and morphology of the reduced phases were characterized by X-ray diffraction spectroscopy (XRD), X-ray photoelectron spectroscopy (XPS), and field emission scanning electron microscopy (FE-SEM). The reduction progression of MoO3 catalyst was attained with different reductant types and concentration (10% H2/N2, 10% and 20% CO/N2 (%, v/v)). Two different modes of reduction process were applied. The first approach of reduction involved non-isothermal mode reduction up to 700 °C, while the second approach of reduction involved the isothermal mode reduction for 60 min at 700 °C. Hydrogen temperature programmed reduction (H2-TPR) results showed the reduction progression of three-stage reduction of MoO3 (Mo6+ → Mo5+ → Mo4+ → Mo0) with Mo5+ and Mo4+. XRD analysis confirmed the formation of Mo4O11 phase as an intermediate phase followed by MoO2 phase. After 60 min of isothermal reduction, peaks of metallic molybdenum (Mo) appeared. Whereas, FESEM analysis showed porous crater-like structure on the surface cracks of MoO2 layer which led to the growth of Mo phase. Meanwhile, the reduction of MoO3 catalyst in 10% carbon monoxide (CO) showed the formation of unstable intermediate phase of Mo9O26 at the early stage of reduction. Furthermore, by increasing 20% CO led to the carburization of MoO2 phase, resulted in the formation of Mo2C rather than the formation of metallic Mo, as confirmed by XPS analysis. Therefore, the presented study shows that hydrogen gave better reducibility due to smaller molecular size, which contributed to high diffusion rate and achieved deeper penetration into the MoO3 catalyst compared to carbon monoxide reductant. Hence, the reduction of MoO3 in carbon monoxide atmosphere promoted the formation of Mo2C which was in agreement with the thermodynamic assessment.

Promoted methane conversion to syngas over Fe-based garnets via chemical looping

Fe-based oxygen carries (OCs) have attracted wide attention due to the low cost and environmental compatibility for chemical looping methane partial oxidation. However, they usually suffer from low CH4 reactivity mainly due to insufficient synergy between catalytic CH4 activation and lattice oxygen mobility. In this work, Fe ions with different amount were incorporated into a novel garnet structure (Y3FexAl5-xO12), which exhibited drastically increased CH4 conversion with reduction time. This resulted from in-situ formed Fe0 sites at the early stage of reduction, which led to a more efficient reaction route involving methane catalytic decomposition on Fe0 and the resulted carbon oxidation easier by lattice oxygen, compared with methane directly oxidized by OCs. Moreover, the amount of Fe ions in garnet influenced surface exposed Fe0 sites and oxygen mobility, inducing Y3Fe2Al3O12 performed the best with almost 94 % methane conversion thanks to the combined advantages of larger surface exposed Fe0 and higher lattice oxygen mobility.

Heat treatment in the Still Bay - A case study on Hollow Rock Shelter, South Africa

The Still Bay, with its carefully crafted bifacial points, is one of the most enigmatic technocomplexes in the later Middle Stone Age of the southern African subcontinent. Heat treatment of silcrete has been documented in the Still Bay but it has recently been suggested that its application was restricted to the later stages of the production of points. This would confer a special role to heat treatment in the Still Bay if compared to the following Howiesons Poort technocomplex. In this paper, we analyse the silcrete assemblage from Hollow Rock Shelter for heating proxies to provide a first picture of the prevalence of heat treatment in the Still Bay and to investigate whether points were treated differently in terms of heat treatment than other end-products. Our results show no evidence of later-stage heat treatment but, on the contrary, comprehensive data to support heat treatment in an early stage of reduction. Relatively less silcrete was heated in the Still Bay than in later Howiesons Poort, revealing technological differences between both phases. We found a significant number of silcrete pieces that exploded during heat treatment and were still knapped afterwards, indicating a heating process that involved fast heating rates. We also found that points were not treated differently than the other end-products. These findings have implications for our understanding of the fabrication of bifacial points and the Still Bay chaîne opératoire in general.

Carbothermic Reduction of Misamis Oriental Chromite Ores

This study was conducted to determine the non-isothermal and isothermal reduction of chromium ores in the solid-state by reductants like solid carbon under argon and hydrogen-argon atmosphere. Two different chromite ores from local sources , sandy chromite ore from Opol, Misamis Oriental (SCO) and lumpy chromite ore from Manticao, Misamis Oriental (LCM) were used in the study. Isothermal and non-isothermal experiments were conducted in a fixed bed reactor heated in a vertical tube furnace in the temperature range 800 to 1000°C. Raw chromite and reduced samples were subjected to phase analysis and morphology characterization using X-ray flourescence (XRF), X-ray diffraction (XRD), and energy dispersive x-ray spectroscopy (EDX). It was found that reduction does not go to completion at this temperature range. The early stage of reduction of iron was controlled by nucleation and the later by nucleation or chemical reaction or both. The activation energy at the early stage of reduction is estimated to be 44.76 kJ/mol and the later stage of reduction is 144 kJ/mol for SCO and 76.5 kJ/mol for LCM. The reduction of chromium was controlled by chemical reaction.

Redox Treatment of Orthorhombic Mo29V11O112 and Relationships between Crystal Structure, Microporosity and Catalytic Performance for Selective Oxidation of Ethane

Redox treatments of an orthorhombic Mo29V11O112 catalyst (MoVO) were conducted and its crystal structure, microporosity, and catalytic activity were investigated. TPR and TG revealed that MoVO evolved two kinds of lattice oxygen (α-oxygen and β-oxygen) from the structure by reduction treatment. In the early stage of reduction, α-oxygen was evolved from the structure, causing expansion of the micropore channel. With further reduction, the atoms in the pentagonal [Mo6O21]6– unit moved toward the micropore channel, resulting in a decrease in micropore size. Expansion of the micropore drastically increased catalytic activity for selective oxidation of ethane, but the activity was decreased by a reduction in the micropore channel size. Strong relationships were found between crystal structure, microporosity, and catalytic activity for selective oxidation of ethane.

Kinetic analysis of iron oxide reduction in top gas recycling oxygen blast furnace

Reduction experiments of sinter in a top gas recycling oxygen blast furnace (TGR-OBF) were studied at temperatures ranging from 1173 to 1373 K. The changes in iron oxide reduction were evaluated, and based on the ‘unreacted core model’, the kinetic behaviour of this reduction was also analysed. The results show that with increasing temperature, the reduction rate of sinter increased, the reduction time decreased and the final degree of reduction increased. At 1173, 1273 and 1373 K, the reduction process is controlled by interfacial chemical reactions, and the chemical reaction rate constants are 0·363, 0·591 and 0·787 cm s−1 respectively at an early stage of reduction. Reactions are controlled by interface chemical reactions and internal diffusion at the final stage of reduction, and the chemical reaction rate constants are 0·541, 0·682 and 0·866 cm s−1 respectively, and the diffusion co-efficients of the reduction reaction are 0·463, 1·970 and 2·412 cm2 s−1 respectively. In the TGR-OBF, the range of temperatures is from 1173 to 1373 K. The apparent activation energy is 52·07 kJ mol−1, and the frequency factor is 77·26 cm s−1 at an early stage of reduction. At the final stage, the apparent activation energy is 31·41 kJ mol−1, and the frequency factor is 13·46 cm s−1.

合成クロマイト(Fe<SUB>0.5</SUB>Mg<SUB>0.5</SUB>)(Cr<SUB>0.8</SUB>Al<SUB>0.2</SUB>)<SUB>2</SUB>O<SUB>4</SUB>の溶融還元速度

The kinetics of smelting reduction of (Fe0.5Mg0.5)(Cr0.8Al0.2)2O4 in molten silicate flux with graphite has been investigated at temperatures from 1773 to 1973 K. The reduction rate was described with a first-order-type rate equation: log (1−R)=−k·t. In the early stage of reduction, in which FeCr2O4 component was preferentially reduced, the rate constant k was virtually independent of the flux composition and the wetting of the flux to the graphite. The activation energies were 200∼250 kJ/mol. The reduction rate was controlled by the dissolution of the chromite into the flux.In the late stage of reduction, Mg(Cr0.6Al0.4)2O4 component was reduced. The wider contact of the flux with the graphite increased k. The dependences of k on the flux composition and temperature were complicated. The reduction in the flux with a low melting point at a higher temperature was controlled by the chemical reaction at the flux-graphite interface. The reduction in the flux with a high melting point at a lower temperature was mixed-controlled by both the dissolution and the interfacial reaction.

FeCr<SUB>2</SUB>O<SUB>4</SUB>およびMgCr<SUB>2</SUB>O<SUB>4</SUB>の炭素還元におよぼす酸化物と還元生成物層の性状の影響

The reduction rates of powdered FeCr2O4 and MgCr2O4 in a graphite crucible as reductant have been measured by means of thermo-balance under an argon atmosphere at temperatures from 1773 to 2073 K. Depending on particle size of oxide and temperature, there is the transition from one reaction mechanism to another as the reduction progresses.The rates of reduction increase with increasing particle size, particularly in the early stage of reduction. The overall rate of reduction is controlled by gas dfffusion through the interparticle pores within the powder.At lower temperatures, the reduction of fine particles is drastically retarded by the formation of densely sintered layers of oxide and the reduction product.At higher temperatures, the reduction of FeCr2O4 is markedly accelerated as a consequence of the direct reduction by carbon dissolved in molten Fe-Cr-C alloy. The reduction of MgCr2O4 is sluggish at the later stage because of slower diffusion in the sintered layers of Cr-C alloy and MgO with high melting points.

固相ならびに溶融状態での固体炭素によるFe<SUB>2</SUB>O<SUB>3</SUB>-Cr<SUB>2</SUB>O<SUB>3</SUB>混合物の還元

In relation to the smelting reduction as a new process of producing Fe-Cr alloys, the mechanism of reduction of Cr2O3-Fe2O3 mixtures with solid carbon has been investigated by means of thermogravimetry and metallography. The rate of reduction was measured at high temperatures from 1673 to 1923 K under an argon atmosphere.In the early stage of reduction, Fe2O3 was preferentially reduced to FeO, which immediately reacted with Cr2O3 to yield chromite. Subsequently, nearly simultaneous reduction of FeO and Cr2O3 in the chromite occurred.The reduction of Cr2O3-Fe2O3 mixture in a graphite crucible as a reductant was retarded at low temperatures, since the solid reduction product was formed at the oxide-graphite interface and the rate of reduction was controlled by the diffusion of carbon through the solid product. At high temperatures, the samples was reduced rapidly by forming a molten Fe-Cr-C alloy. The reduction was accelerated by decreasing the Cr/Fe ratio and by the addition of graphite powder. When Cr2O3-Fe2O3 mixtures with a low Cr/Fe ratio were reduced by powdered graphite, the molten Fe-Cr-C alloy was finely dispersed apart from the graphite particle. It is considered that the reduction rate would be under a mixed control of the Boudouard reaction and the counter-current of CO and CO2 between the oxide and the graphite particles. In the sample with a high Cr/Fe ratio, the graphite particle was surrounded by the Fe-Cr-C alloy having a high melting point.

Behavior of constituent minerals in the carbon monoxide reduction of sinter at 900.DEG.C.

The reduction behavior of constituent minerals of iron ore sinter has been investigated microscopically by using two kinds of commercial sinter having different basicity and FeO content. The results are summarized as follows:(1) There is a difference in reducibility between hematite and magnetite, and hematite is reduced faster than magnetite.(i) Both hematite and magnetite are reduced rapidly to wustite, but there is a difference in the rate of reduction from wustite to iron.(ii) The reduction of wustite reduced from hematite does not proceed topochemically but homogeneously. Although the reduction of the wustite is comparatively fast in the almost whole period of reduction, part of the wustite becomes surrounded by dense iron and left unreduced in the final stage of reduction.(iii) In the reduction of wustite reduced from magnetite, almost all grains of wustite are surrounded by dense iron from the early stage of reduction, which causes the retardation of reduction.(2) Calcium ferrite is reduced much faster than the wustites mentioned above, which is shown by the fact that many grains of wustite are left unreduced even after calcium ferrite is reduced completely.(i) The reduction of calcium ferrite proceeds topochemically.(ii) Iron produced by the reduction of calcium ferrite is very porous and not sintered, which does not cause the retardation of reduction.(3) In many cases, reduction begins with the surface of mineral grains facing a macro-pore, which serves as a passage of reducing gas to each mineral grain to be reduced.

ESR evidence for structural rearrangements occurring upon MoO 3 reduction

Reduction of polycrystalline MoO 3 in the course of heat treatment in vacuo has been studied by means of ESR. The formation of two different Mo(V) centers has been identified: center A with g x = 1.946, g y = 1.960, g z = 1.868; and center B characterized by g x = 1.943, g y = 1.954, g z = 1.878. Center B exhibited a well-resolved hyperfine structure resulting from naturally abundant odd isotopes of Mo: A x = 3.2mT, A y = 3.0mT, A z = 7.0mT. Species A formed at an early stage of reduction has been identified as Mo(V) in rhombically distorted square pyramidal surrounding, whereas center B appearing in strongly reduced samples shows a distorted octahedral coordination. The results have been interpreted in terms of the crystallographic shear mechanism.

Surface states of MoO3 on ZrO2 and catalytic properties for the reaction of NO with H2

The adsorption and reaction of nitric oxide on reduced molybdena–zirconia were studied by means of e.s.r., i.r. and u.v. spectroscopies. On the surface, at least two types of nitrosyl complex exist. At an early stage of reduction, a diamagnetic dinitrosyl complex with i.r. bands at ca. 1790 and 1690 cm–1 formed on NO adsorption and was ascribed to Mo4+(NO)2. After reduction at a higher temperature, a paramagnetic dinitrosyl species appeared upon NO adsorption and was tentatively assigned to Mo5+(NO)2. Nitric oxide can adsorb on Mo5+ in tetrahedral coordination, but not on Mo5+ in octahedral coordination. This can be explained by the coordination being limited. For the reaction of NO with H2, the species of Mo5+(NO)2 was concluded to be a main active complex in the catalytic cycle.

酸化鉄ペレット単一球の脈動還元反応の速度論的研究

The fact that the rate of reduction of a hematite pellet under pulsating flow is higher than that under steady flow can de ascribed to the decrease of mass-transfer resistance in the gas film. The rate of reduction under pulsating flow is calculated by the use of an unreacted-core shrinking model and KITAURA'S expression for a mass-transfer coefficient, and the following conclusions are obtained.(1) The rate of reduction under pulsating flow increases as frequency and amplitude of pulsation increase.(2) Pulsating flow accelerates the reduction effectively especially in the early stage of reduction and in a high temperature range. This trend coincides qualitatively with previously reported results.(3) Optimum values of the pellet diameter and time-averaged flow rate are proposed.(4) When the pellet diameter and flow rate are not signiffcantly large, the effect of pulsating flow increases gradually as N2 gas percentage in binary bulk gas, H2-N2, increases.

ヘマタイト鉱石の低温度域における水素還元

Brazilian and Indian ores, high quality hematites, were ground spherically and then reduced with hydrogen at 500° and 550°C. The weight changes were measured by a thermobalance. Obtained data were analyzed by a mixed-controlling mechanism of the intraparticle gaseous diffusion and the chemical reaction at the interface.In Brazilian ore, the crack developes into the network along grain-boundaries in the early stage of reduction and its width is larger than 1 μ. Therefore the intraparticle diffusivity is large and the reducibility is similar to that of the porous pellet. In Indian ore, the size of pore resulted by the reduction is several hundreds Å in radius, indicating that the intraparticle diffusion is akin to the Knudsen diffusion. The diffusing resistance is relatively large and the reducibility is inferior to that of Brazilian ore.

ペレットの還元時のSwellingについて

Two different views were brought by previous works for the swelling of pellets during reduction.“The swelling refers intimately to the transformation of mineral phases during reduction” is the one, and“ The growth of whisker-like fibrous materials cause the swelling like cotton candy” is the another.The present authors carried out series of experiment on the swelling by various kinds of raw material, after the detailed review on the previous works.Following explanations are possible. 1. A generation of some kind of force is supposed to be possible in the relatively early stage of reduction, which will cut off the bonds between the constituent grains, and break the grains themselves down into minute fragments. 2. The growth of fibrous material is observed in later stage of reduction after the breaking down came to end, and is found to form cotton candy like body. The breaking down and growth of fibrous materials are absolutely independent in the stage of reduction. 3. Swellings can be prevented by taking the diameters of pellets small, even for the raw material with remarkable swelling nature, and for the pellets fired under such conditions that will cause swelling or preserve the nature of swelling, also.

Gas chromatographic determination of reversible adsorption of hydrogen: II. The properties of reversibly adsorbed hydrogen on a nickel catalyst

The nature of the reversible adsorption of hydrogen on nickel, the determination of which was reported for the first time in a preceding paper, has been investigated in terms of heat of adsorption and by the effect of catalyst reduction and the effect of water vapor as a carrier gas impurity. The heat of adsorption was found to be about 2 kcal/mole at low temperature (−195–−100 °C) and about 13 kcal/mole at around 0 °C (−30–60 °C), which indicates two different types of adsorption. The adsorption sites for them appeared to be developed in the early stage of reduction. Adsorbed water is likely to activate another type of hydrogen adsorption.