Selective Oxidation Behavior Research Articles

Facile method for synthesis of nanosized β–MoO3 and their catalytic behavior for selective oxidation of methanol to formaldehyde

β-MoO3 was successfully synthesized from all commercial materials using a fast, effective and simple method and characterized by differential scanning calorimetry, x-ray powder diffraction, field emission scanning electron microscopy, infrared and Raman spectroscopy. The prepared sample was highly active and selective to formaldehyde formation from methanol over a wide range of reaction temperatures. β-MoO3 catalyst also exhibited stable methanol conversion and formaldehyde selectivity at around 84% and over 95% respectively for over 15 operating hours at 320 °C. However, it may be deactivated at elevated reaction temperature due to transformation of metastable to stable phase. It was revealed that the prepared catalyst maintains its high selectivity to formaldehyde during deactivation. This can be considered as an advantage of the prepared MoO3 catalyst in comparison with the industrial one.

Synthesis of α-MoO3 Thin Sheets and Their Catalytic Behavior for Selective Oxidation of Methanol to Formaldehyde

α-MoO3 thin sheets were successfully synthesized from all commercial materials using a fast, effective and simple method and characterized by X-ray diffractometer, scanning electron microscopy, IR and Raman spectroscopy. The prepared catalyst was highly active and selective to formaldehyde formation from methanol over a wide range of reaction temperatures comparing to the commercial one. We found that the high selectivity of formaldehyde over the prepared α-MoO3 was originated from its special morphology, which had abundant exposed edges comparing to the commercial α-MoO3. Additionally, the prepared α-MoO3 showed significant stability due to its stable nature.

The effect of AuAg–MCF and AuAg–NbMCF catalysts pretreatment on the gold–silver alloy formation and the catalytic behavior in selective methanol oxidation with oxygen

MCF (mesostructured cellular foams) and NbMCF were used as supports for gold and silver. Bimetallic (Au–Ag) catalysts were prepared by metals loading (Au/Ag atomic ratio=0.5 and 2) on 3-aminopropyl-trimethoxysilane grafted supports and reduction by NaBH4 before calcination at 773K. The prepared samples were then activated in inert gas at 623K or reduced with hydrogen at 723K. The catalysts treated in these ways were fully characterized by XRD, TEM, UV–vis and XPS techniques. Au–Ag alloy was identified on the surface of AuAg–MCF and AuAg–NbMCF catalysts. The composition of Au–Ag alloy was found to be sensitive to the catalyst pretreatment conditions. The reduction of Au–Ag containing materials with NaBH4 led to the formation of alloy with low silver content. The Au–Ag alloy was enriched in silver partially by the activation in argon flow and to a substantial degree by the reduction with hydrogen. The conditions of catalysts treatment also determined the alloy particle size. Catalytic activity of Au–Ag catalysts was tested in the oxidation of methanol in the gas phase. The results showed that the presence of gold–silver alloy on MCF surface was responsible for high activity of the catalysts as a result of synergetic interactions between Au and Ag species. Niobium in MCF support influenced both the activity of the catalysts and the selectivity to formaldehyde.

Nitrogen-induced selective high-temperature internal oxidation behavior in duplex stainless steels 19Cr–10Mn–0.3Ni–xN

Isothermal high-temperature oxidation of duplex stainless steels (DSSs) 19Cr–10Mn–0.3Ni–xN (x=0.211–0.315wt.%) at 1100°C in air has been investigated mainly by scanning electronic microscopy and Raman spectroscopy. High-temperature oxidation resistance of DSSs became worse with the increase of nitrogen content. Self-repairing behavior of local breakaway regions locating in ferrite diffusion layer occurred in DSSs with lower nitrogen content. High-temperature internal oxidation phenomenon preferred to occur in DSS with higher nitrogen content and penetrate into local breakaway regions in ferrite diffusion layer upon the austenite substrate regions as the increased outward diffusion of nitrogen ions.

The selective oxidation behaviour of WC–Co cemented carbides during the early oxidation stage

The oxidation experimental of WC–Co cemented carbides was carried out at 500°C in air. Selective oxidation phenomenon of Co phase in WC–Co cemented carbides was observed and proved by EPMA and AFM. At first, the O content in Co phase increases quickly, and then tends to be stable after oxidation of 20min. In sharp contrast, no change of O content in WC phase was observed. Moreover, it was found that the oxide scales thickness of Co phase increased parabolically with oxidation time. And thermodynamic and kinetic analysis of oxidation was done.

Ab initio atomistic thermodynamics study on the oxidation mechanism of binary and ternary alloy surfaces.

Utilizing a combination of ab initio density-functional theory and thermodynamics formalism, we have established the microscopic mechanisms for oxidation of the binary and ternary alloy surfaces and provided a clear explanation for the experimental results of the oxidation. We construct three-dimensional surface phase diagrams (SPDs) for oxygen adsorption on three different Nb-X(110) (X = Ti, Al or Si) binary alloy surfaces. On the basis of the obtained SPDs, we conclude a general microscopic mechanism for the thermodynamic oxidation, that is, under O-rich conditions, a uniform single-phase SPD (type I) and a nonuniform double-phase SPD (type II) correspond to the sustained complete selective oxidation and the non-sustained partial selective oxidation by adding the X element, respectively. Furthermore, by revealing the framework of thermodynamics for the oxidation mechanism of ternary alloys through the comparison of the surface energies of two separated binary alloys, we provide an understanding for the selective oxidation behavior of the Nb ternary alloy surfaces. Using these general microscopic mechanisms, one could predict the oxidation behavior of any binary and multi-component alloy surfaces based on thermodynamics considerations.

Influence of Fe Oxidation on Selective Oxidation Behavior of Si and Mn Added in High Strength Sheet Steel

In the process of hot dip galvanizing high tensile strength sheet steels containing Si and Mn, selective surface oxidation of Si and Mn causes coating defects. One promising method for overcoming this problem is an oxidation-reduction process. When the steel surface is exposed to an oxidizing atmosphere, it will react primarily by forming an Fe oxide, which can be reduced by hydrogen in a reduction process that follows. It has been explained that good wettability can be obtained due to the formation of pure iron. However, the mechanism of suppression of selective surface oxidation has not been clearly understood in detail yet. In order to reveal this mechanism, the present study focused on both Mn and Fe oxidation behavior during the oxidation-reduction process for a cold-rolled sheet steel containing 0.25mass%Si-1.8mass%Mn. Surface and cross-sectional analyses were performed by using secondary electron microscopy and transmission electron microscopy. Selective surface oxidation behavior was investigated by glow discharge optical emission spectroscopy. The main results obtained are as follows. First, selective surface oxidation of Mn was suppressed even if soaking was continued after completion of the reduction of the Fe oxide. Second, as the reduction process proceeded, Mn was trapped as an internal oxide under the Fe oxide layer. Moreover, depletion of solute Mn was observed in the matrix. From these results, depletion of solute Mn is supposed to suppress the outer diffusion of Mn during soaking. Therefore, selective surface oxidation of Mn is suppressed even after Fe oxide reduction is completed.

Selective oxidation behavior of an ignition-proof Mg-Y-Ca-Ce alloy

A Mg-Y-Ca-Ce magnesium alloy was optimized for high ignition-proof property, which did not burn in air at 1233 K up to 30 min. Oxidation behavior of the alloy was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and thermodynamics calculation at 673, 773 and 873 K. XRD and SEM analysis indicated that dense and compact oxide films composed of MgO, Y2O3 and CaO formed. The oxidation behavior was characterized by the selective oxidation. Based on Pilling-Bedworth ratio (PBR) and energy dispersive spectrometer (EDS) analysis, Y2O3 contributed more to form the compact surface oxide film, which led to the excellent ignition-proof performance. The thermodynamics analysis and EDS results implied that the Y-rich areas were preferred paths for the selective oxidation.

Molybdenum oxide supported on nanostructured MgO: Influence of the alkaline support properties on MoOx structure and catalytic behavior in selective oxidation

Local structure and catalytic activity of molybdenum oxide supported on alkaline, nanostructured support materials were investigated. The structure of the surface oxide catalysts was significantly affected by the properties of the support. The support material was prepared via two different routes. A direct MgO synthesis approach was compared to SBA-15 coated with magnesium oxide. The structure of the support materials was investigated by XRD and N2 physisorption. Both materials exhibited a high specific surface area with a narrow pore size distribution. The MgO-coated SBA-15 (MgO/SBA-15) exhibited a similar alkaline character as pure nanostructured MgO (nano-MgO) but was more stable under catalytic conditions. The highly dispersed molybdenum species on the support was characterized by XRD, XAS and DR-UV–Vis spectroscopy. The catalytic activity for selective oxidation of propene was investigated by gas chromatography. A considerably different species was present on the alkaline support, compared to more acidic support materials like SBA-15. The structure of the supported molybdenum oxide consisted of predominantly isolated [MoO4] tetrahedra and was largely independent from Mo loadings. The supported catalysts with isolated [MoO4] tetrahedra exhibited only poor catalytic activity compared to the higher polymerized Mo species supported on silica SBA-15.

Fe酸化がSi,Mn選択外部酸化挙動に及ぼす影響

In the process of hot dip galvanizing high tensile strength sheet steels containing Si and Mn, selective surface oxidation of Si and Mn causes coating defects. One promising method for overcoming this problem is an oxidation-reduction process. When the steel surface is exposed to an oxidizing atmosphere, it will react primarily by forming an Fe oxide, which can be reduced by hydrogen in a reduction process that follows. It has been explained that good wettability can be obtained due to the formation of pure iron. However, the mechanism of suppression of selective surface oxidation has not been clearly understood in detail yet. In order to reveal this mechanism, the present study focused on both Mn and Fe oxidation behavior during the oxidation-reduction process for a cold-rolled sheet steel containing 0.25mass%Si-1.8mass%Mn. Surface and cross-sectional analyses were performed by using secondary electron microscopy and transmission electron microscopy. Selective surface oxidation behavior was investigated by glow discharge optical emission spectroscopy. The main results obtained are as follows. First, selective surface oxidation of Mn was suppressed even if soaking was continued after completion of the reduction of the Fe oxide. Second, as the reduction process proceeded, Mn was trapped as an internal oxide under the Fe oxide layer. Moreover, depletion of solute Mn was observed in the matrix. From these results, depletion of solute Mn is supposed to suppress the outer diffusion of Mn during soaking. Therefore, selective surface oxidation of Mn is suppressed even after Fe oxide reduction is completed.

Effects of furnace atmosphere on selective oxidation behaviour of Si-added transformation-induced plasticity steels

The selective oxidation behaviours of Si-added transformation-induced plasticity (TRIP) steels were investigated by controlling the furnace atmosphere, dew point, and annealing temperature. Two steels were used for the study: a sample 2 of 1.6 wt% Mn and 1.5 wt% Si and a sample 1 of 2.4 wt% Mn and 1.0 wt% Si. As the annealing temperature increased, the surface enrichment of Si and Mn increased for a sample 1. However, the surface enrichment of Mn decreased above 1073 K for sample 2. This was correlated with the deterioration of galvanized coating quality. The over-aging at 773 K showed a different trend of Mn enrichment as a function of Si and Mn contents. For sample 1, Mn was enriched continuously at relatively low temperatures (773 K) during the over-aging process, as well as at high temperatures (1073 K) during soaking. This could mean that Mn enrichment increased due to the formation of selective oxides Mn2SiO4 at low temperature. However, Mn enrichment was not dominant for sample 2 during the overaging process.

Study of Selective Oxidation Behavior of a 1.2Si-1.5Mn TRIP Steel during Intercritical Annealing

The present study aims at clarifying the oxides that form on the surface of a 1.2Si-1.5Mn (in wt%) transformation-induced plasticity (TRIP) steel during intercritical annealing. X-ray photoelectron spectroscopy and transmission electron microscopy were employed as complementary tools for analyzing the oxides. The results indicate that both the dew point of the annealing atmosphere and the soaking time play important roles in the oxides forming on the surface. Regarding the steel annealed at 800°C in a low dew point (dp) atmosphere of −70°C, the surface oxides were initially amorphous and were composed of a mixture of SiO2 and xMnO·SiO2 (x<0.7). After soaking for 60 s, a small amount of Mn2SiO4 precipitated in xMnO·SiO2. The steel annealed at 800°C at dp = 0°C showed significant internal oxidation. Initially, a mixture of SiO2, xMnO·SiO2 and Mn2SiO4 formed on the surface. After soaking for 60 s, SiO2 and Mn2SiO4 transformed to xMnO·SiO2 and MnSiO3, respectively. However, some Mn2SiO4 still remained at the grain boundaries.

Effect of dew point on the surface selective oxidation and subsurface microstructure of TRIP-aided steel

In this work we examined the effect of dew point on the surface selective oxidation behavior and the subsurface microstructure of TRIP-aided steel mainly by XPS and FE-SEM. It is shown that the variation of the dew point changed the morphology and chemical composition of the surface oxides. Furthermore, as the dew point increased, ferrite was found in the subsurface region of TRIP-aided steel. Considering the severe degree of internal oxidation, decarburization and consumption of minor alloy elements are proposed to be responsible for this ferrite band.

Effect of Alloy Composition and Exposure Conditions on the Selective Oxidation Behavior of Ferritic Fe–Cr and Fe–Cr–X Alloys

Selective oxidation behavior of ferritic martensitic Fe–Cr base alloys, exposed in various atmospheres containing combinations of O2, CO2, and H2O, were studied at various temperatures relevant to oxy-fuel combustion. This paper begins with a discussion of the required Cr content to form a continuous external chromia scale on a simple binary Fe–Cr alloy exposed in oxygen or air based on experiments and calculations using the classic Wagner model. Then, the effects of the exposure environment and Cr content on the selective oxidation of Fe–Cr alloys are evaluated. Finally, the effects produced by alloying additions of Si, commonly present in various groups of commercially available ferritic steels, are described. The discussion compares the oxide scale formation on simple binary and ternary Fe–Cr base model alloys with that on several commercially available ferritic steels.

Selective Oxidation and Reactive Wetting of 1.0 Pct Si-0.5 Pct Al and 1.5 Pct Si TRIP-Assisted Steels

The effect of oxygen partial pressure on the selective oxidation and reactive wetting behavior of 1.0 pct Si-0.5 pct Al and 1.5 pct Si TRIP-assisted steels was studied. The annealing atmosphere affected the surface chemistry and the oxide morphology, which in turn affected reactive wetting. Similar wetting results were observed for the two TRIP steel compositions with good wetting being observed at the two lower oxygen partial pressure atmospheres (220 K (–53 °C) or 223 K (–50 °C) dew point (dp) and 243 K (–30 °C) dp) with poor reactive wetting at the higher oxygen partial pressure atmosphere (278 K (+5 °C) dp). The differences in wetting were attributed to the oxide morphology. The predominant oxide morphology at the two low oxygen partial pressure atmospheres was larger, widely spaced oxide nodules. At the 278 K (+5 °C) dp, the predominant oxide morphology was smaller, more closely spaced oxide nodules. TEM analysis revealed that at the 243 K (–30 °C) dp, Fe2Al5 was able to form between oxide nodules promoting good reactive wetting. At the 278 K (+5 °C) dp, the more closely spaced nodules may have impeded the formation of Fe2Al5, thereby producing numerous bare spot defects in the zinc coating.

Thermodynamic Analysis of Selective Oxidation Behavior of Si and Mn-added Steel during Recrystallization Annealing

In order to establish the necessary conditions for producing high strength hot-dip Zn galvanized steel sheets for automotive use on a Continuous Galvanizing Line (CGL), a thermodynamic calculation of the selective oxidation behavior of Si, Mn-added high strength steel sheets was introduced, assuming a model in which an equilibrium is reached locally at the outermost surface layer of the steel sheet. The applicability of that model is confirmed by comparison with experimental results. Both the calculated chemical potential diagram for an Fe–Si–Mn–O system and an isothermal pseudo ternary phase diagram for an FeO–SiO2–MnO system, explain the reaction path of the selective oxidation behavior in 1 mass% Si and 0.01–3.01 mass% Mn-added steel. As this simulation model demonstrates good agreement with experimental results, this thermodynamic calculation is extremely appropriate for prediction of the surface oxidation behavior of Si, Mn-added high strength steel sheets. The transition from selective surface oxidation to internal oxidation can be explained by considering the oxygen flux in the oxidation film and the effect of the selective surface oxides on the inward diffusion behavior of oxygen. By thermodynamic calculation of the suppression condition of SiO2 film formation, which deteriorates the molten Zn wettability of the surface of annealed sheets, a process control model for industrially stable production of high strength hot-dip Zn galvanized steel sheets with arbitrary chemical compositions is proposed. Based on this research, a comprehensive surface control technology for high strength steel sheets is established.

Application of AFM in a study of the selective oxidation behavior of materials during the early oxidation stage

The roughening kinetics of the oxide scales formed on a novel electrodeposited nanocomposite Ni–12.6 wt.% Cr having a nanocrystalline Ni matrix with a dispersion of Cr nanoparticles and on a conventional coarse-grained, chromia-forming Ni–20Cr alloy have been studied with atomic force microscopy (AFM). The results show that it is possible to use AFM to check the validity of the model proposed which links the selective oxidation behavior of the two materials with different roughening process.

Thermodynamic Analysis of Selective Oxidation Behavior of Si and Mn-added Steel during Recrystallization Annealing

In order to establish the necessary conditions for producing high strength hot-dip Zn galvanized steel sheets for automotive use on a Continuous Galvanizing Line (CGL), a thermodynamic calculation of the selective oxidation behavior of Si, Mn-added high strength steel sheets was introduced, assuming a model in which an equilibrium is reached locally at the outermost surface layer of the steel sheet. The applicability of that model is confirmed by comparison with experimental results. Both the calculated chemical potential diagram for an Fe–Si–Mn–O system and an isothermal pseudo ternary phase diagram for an FeO–SiO2–MnO system, explain the reaction path of the selective oxidation behavior in 1 mass% Si and 0.01–3.01 mass% Mn-added steel. As this simulation model demonstrates good agreement with experimental results, this thermodynamic calculation is extremely appropriate for prediction of the surface oxidation behavior of Si, Mn-added high strength steel sheets. The transition from selective surface oxidation to internal oxidation can be explained by considering the oxygen flux in the oxidation film and the effect of the selective surface oxides on the inward diffusion behavior of oxygen. By thermodynamic calculation of the suppression condition of SiO2 film formation, which deteriorates the molten Zn wettability of the surface of annealed sheets, a process control model for industrially stable production of high strength hot-dip Zn galvanized steel sheets with arbitrary chemical compositions is proposed. Based on this research, a comprehensive surface control technology for high strength steel sheets is established.

Internal Oxidation during Intercritical Annealing of CMnSi TRIP Steel

The equilibrium internal oxidation of CMnSi TRIP steel at intercritical annealing temperatures in a +3°C dew point N2+10%H2 atmosphere was investigated by means of high resolution transmission electron microscopy of cross-sectional samples. The experimental conditions are considered to lead to the selective internal oxidation of Mn and Si. The intercritical annealing however resulted in the formation of three types of isolated particles on the surface: 200–340 nm size single crystal MnO oxide particles, crystalline 30–60 nm size xMnO·SiO2 (1≤x≤4) and amorphous α-xMnO·SiO2 (0<x<0.9) oxide particles. A thin 25–35 nm film of crystalline xMnO·SiO2 (1≤x≤2) was present between these particles. Thin discontinuous transition oxide layers with a thickness of 25–55 nm were formed between the MnO particles and the Fe matrix. The composition of this xMnO·SiO2 layer varied from 1≤x≤2 for crystalline xMnO·SiO2 to 0<x<0.9 for amorphous α-xMnO·SiO2. In the subsurface region, two distinct types of internal oxidation zones were observed. In the first internal oxidation zone, within the 1 μm range below the surface, fine ferrite grains with a grain size of ~550 nm were observed. Crystalline xMnO·SiO2 (1≤x≤2) oxide particles with a diameter of 80–250 nm were present in this internal oxidation zone. In the second internal oxidation zone, from 1 to 4 μm below the surface, the ferrite grain size was about 4 μm, and similar to the grain size in the bulk. Inter-granular amorphous α-SiO2 particles, 180–300 nm in diameter, and an extensive, apparently continuous, grain boundary network of amorphous α-SiO2 were observed in this second internal oxidation zone. The observed oxide distribution is related to the difference in selective oxidation behavior of the main alloying elements Si and Mn and very likely due to the difference in their diffusivities, oxide stability and oxide solubility during intercritical annealing. The decarburization of the matrix during annealing in a high dew point atmosphere is also likely to play a role. The observed selective oxidation is relevant for the continuous galvanizing of high strength steels. It is expected that the presence of a thinner oxide film is less likely to prevent the formation of the inhibition layer and the wetting of the steel surface by the liquid Zn during hot dip galvanizing. Operation of the annealing furnace at a higher dew point during the processing of Si-bearing steels such as conventional CMnSi TRIP steel may therefore lead to a decrease of galvanizing surface defects.

Effects of Pre-Oxidation on Selective Oxidation Behaviour of Fe-25Cr-7Ni-N Alloy

Effects of Pre-Oxidation on Selective Oxidation Behaviour of Fe-25Cr-7Ni-N Alloy