Preparation and effect of Mo-V-Cr-Bi-Si oxide catalysts on controlled oxidation of methane to methanol and formaldehyde

Mo-Cr-V-Bi-Si multi-component oxide catalysts were synthesized by three different coprecipitation methods and used in the controlled oxidation of methane to methanol and formaldehyde. It was shown that Mo content in Mo-V-Cr-Bi-Si oxides and the performance of these catalysts were strongly influenced by different coprecipitation methods. The highest methanol and formaldehyde selectivity of 80.2% could be achieved at a methane conversion of 10 % for the catalyst prepared by a particular method. The results of XRD indicated that the crystalline phase structures of catalysts were sensitive to Mo, V and Bi loadings. Bi(III) could combine with V(V) and Mo(VI) to form BiVO₄ and γ-Bi₂MoO₆, whereas Cr seemed to form a single Cr₂O₂ crystalline phase in the presence of Bi. The effects of Mo and Cr loading on controlled methane oxidation were also investigated. Mo(VI) oxide appears to favor the formation of partial oxidation products and Cr(III) oxide seems to enhance the conversion of methane.     

Glass Formation

The ability to form a glass, i.e., the ability to form bonds which lead to a vitreous network, appears periodically in the classification chart of the elements. Thus simple glasses containing only one kind of atom are formed by the elements of Group VI of the periodic table (O, S, Se, and Te). Only these elements are known to form monatomic (primary) glasses and they retain the ability to form a vitreous network when mixed or chemically bound to each other. The elements of Group VI also form binary glasses, i.e., glasses containing two kinds of atoms, with the elements of Groups III, IV, and V. Experimental evidence of the glassforming capacities of these elements and compounds is given; about twenty new glasses suggested by the foregoing were prepared in the author's laboratory. Binary glasses are also known to exist composed of an element of Group VII (F, Cl, I, or Br) and an element of one of Groups II, III, and IV or a transition element. Experimental evidence for these glasses is presented.     

Rare earth oxides as carbon oxidation catalysts

The behavior of a number of rare earth oxides as catalysts for the oxidation of graphite in air has been investigated by the methods of thermal analysis. Of the oxides studied, only CeO₂ showed significant activity in accelerating the gasification of graphite by oxygen between 500 and 1000°C. Cerium salts, which decompose to a finely dispersed oxide phase at low temperatures, e.g. Ce (III) nitrate and ammonium Ce (IV) nitrate, were found to be very active catalysts. The catalytic effect may be due to a redox process involving the cyclic conversion of the oxide from the Ce (IV) to the Ce (III) oxidation state, or the oxide particles may provide sites for the dissociative chemisorption of oxygen.     

XPS and EIS study of the passive film formed on orthopaedic Ti-6Al-7Nb alloy in Hank's physiological solution

The composition and structure of the passive film formed on Ti-6Al-7Nb alloy by electrochemical oxidation in Hank's physiological solution were studied using X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS). The oxide layer was predominantly TiO₂, but contained small amounts of suboxides TiO and Ti₂O₃ at potentials more negative than 0.75 V. At more positive potentials, TiO₂ was the only form. The formation of suboxides in the lower potential range is less pronounced than in Ti-6Al-4V alloy. The passive range in Hank's physiological solution is broad and extends up to 6.0 V. Aluminium oxide Al₂O₃, and niobium oxides, Nb₂O₅, and NbO and/or NbO₂, are incorporated in the passive layer. Angular resolved XPS analysis confirmed that they are located mainly at the outer oxide/solution interface of the TiO₂ matrix. The thickness of the oxide layer was dependent on the oxidation potential and, after oxidation at 5.75 V, it reached 9.4 nm. EIS measurements correlate well with the XPS data. The incorporation of the oxides of alloying elements into the TiO₂ layer is reflected in the increase in the outer layer resistance at high anodic potentials and longer immersion times. The consequences of this process are beneficial for the overall stability and high corrosion resistance of the Ti-6Al-7Nb alloy under physiological conditions.     

用于环戊烯氧化合成戊二醛反应的钨基催化剂的表征

Tungsten-containing hexagonal mesoporous silica （W-HMS） supported tungsten oxide catalysts （WOx/W-HMS） was prepared for the selective oxidation of cyclopentene with aqueous hydrogen peroxide to glutaraldehyde. X-ray diffraction （XRD） results indicated that the crystal form of the active phase （tungsten oxide） of the WOx/W-HMS catalysts was dependent on the W loading and calcination temperature. X-ray photoelectron spec- troscopy （XPS） analysis revealed that the dispersed tungsten oxides on the surface of W-HMS support consisted of a mixture of W（V） and W（VI）. It was found that a high content of amorphous W species in （5＋） oxidation state resuited in the high catalytic activity. When the W loading was up to 12% （by mass） or the catalyst precursor was treated at temperature of 623 K, the catalytic activity decreased due to the presence of WO3 crystallites and the oxidation of W（V） to W（VI） on the catalyst surface. Furthermore, NH3-temperature-programmed-desorption （NH3-TPD） analysis showed that the effects of W loading and calcination temperature on the acidity of the catalysts were related to the catalytic activity. A high selectivity of 80.2% for glutaraldehyde with a complete conversion of cyclopentene was obtained over 8%WOx/W-HMS catalyst calcined at 573 K after 14 h of reaction.     

Electrochemical behaviour and glass formation in molten bisulphates

Molten bisulphates form an interesting class of low-temperature fused salts which quench to form glasses; some technical and theoretical applications of these systems are discussed. The NaHSO₄KHSO-₄ eutectic (46.5% KHSO₄, m.p. 125°C,T _g 25°C) is easy to study since at lower temperatures decomposition of the melt is retarded and supercooling readily occurs. The solvent properties of this eutectic towards both inorganic and organic solutes have been investigated. The oxidation states available to transition and post-transition metals were studied spectrophotometrically and voltametrically; the results show a very restricted pattern of redox chemistry. The (abbreviated) electrochemical series is V(V)/V(IV)>Mn(III)/Mn(II)>H(I)/H₂(O)>Ag(I)/Ag(O)>Mo(VI)/Mo(V)> Hg(I)/Hg(O)>Cu(II)/Cu(O)>Zn(II)/Zn(O). The Hammett acidity function,H ₀, was measured as in the range –1 to –2. Various factors influencing redox reactions in the melt are discussed.     

Vanadia-molybdena catalysts for the oxidation of 2-picoline

A study has been made of the influence of catalyst composition on the gas phase oxidation of 2-picoline over mixed vanadium and molybdenum oxides supported on kieselguhr. It is shown that the formation of partial oxidation products is associated with the existence of a mixed oxide phase. The selectivity for the formation of pyridine-2-aldehyde reaches a maximum when one in six vanadium(V) ions in the V₂O₅ lattice is replaced by a molybdenum(VI) ion. It is suggested that the origin of the selectivity is the formation of isolated (V⁴⁺-O) species.     

Catalytic combustion of soot on metal oxides and their supported metal chlorides

Gravimetric temperature programmed oxidation was used to study the combustion of a soot mixed with various metal oxides and their supported metal chloride catalysts. It is found that the catalytic effect of metal oxide on soot combustion varies depending on property of oxides. CuO and Cr₂O₃ are better catalysts. Addition of some chloride salts (FeCl₃, NaCl and KCl) increases the catalytic activity and KCl exhibits the highest promoting effect by reducing the T_max for about 200 °C. Metal chlorides can also show a synergistic effect on soot combustion. FeCl₃–KCl/CuO can reduce the T_max of carbon oxidation from 780 to 500 °C. Investigation also demonstrates that FeCl₃–KCl/CuO is effective for NO reduction at low temperatures.     

Recent Progress in Ion-Exchange Studies on Insoluble Salts of Polybasic Metals

Abstract

Zirconium phosphate is perhaps the first insoluble salt of a polybasic metal to be used as an ion exchanger. It is the most studied and probably the most useful of all the materials of this type. These materials are prepared by combining oxides of group IV with the more acidic oxides of groups V and VI of the periodic table, as shown in Table 1.     

Formation and catalytic activity of partially oxidized Pd nanoparticles

Combining multi molecular beam (MB) experiments and in-situ time-resolved infrared reflection absorption spectroscopy (TR-IRAS), we have studied the formation and catalytic activity of Pd oxide species on a well-defined Fe₃O₄ supported Pd model catalyst. It was found that for oxidation temperatures up to 450 K oxygen predominantly chemisorbs on metallic Pd whereas at 500 K and above (~10⁻⁶ mbar effective oxygen pressure) large amounts of Pd oxide are formed. These Pd oxide species preferentially form a thin layer at the particle/support interface. Their formation and reduction is fully reversible. As a consequence, the Pd interface oxide layer acts as an oxygen reservoir providing oxygen for catalytic surface reactions. In addition to the Pd interface oxide, the formation of surface oxides was also observed for temperatures above 500 K. The extent of surface oxide formation critically depends on the oxidation temperature resulting in partially oxidized Pd particles between 500 and 600 K. It is shown that the catalytic activity of the model catalyst for CO oxidation decreases significantly with increasing surface oxide coverage independent of the composition of the reactants. We address this deactivation of the catalyst to the weak CO adsorption on Pd surface oxides, leading to a very low reaction probability.     

Densification and high-temperature oxidation behavior of SiC sintered with multicomponent rare-earth additives

This study examined the effects of rare-earth (RE) elements such as Sc, Y, Ce, and Yb on the densification and oxidation of SiC. After adding binary or ternary RE nitrates in liquid form to β-SiC, hot pressing was performed at 1750 °C for 2 h under 20 MPa. RE nitrate was transformed into RE oxide and formed a liquid phase during sintering by a reaction with SiO₂ present on the SiC surface, where the total amount of RE oxide was fixed at 5 wt%. RE-based silicate melts acted as sintering additives without decomposing SiC at high sintering temperatures. SiC containing Sc–Y as an additive showed a much higher density (≥ 99%) than SiC containing the conventional Al–Y additive (∼95%). The multicomponent RE additive with a melting point (T_m) < 1550 °C had a relatively lower density than that with a higher T_m, owing to the evaporation of the additive at 1750 °C. The density of SiC also depended on the additive composition. The oxidation test, conducted at 1300 °C for up to 168 h in air, exhibited a parabolic weight gain. The SiC sample sintered with the Sc–Yb additive achieved the highest resistance of 3.23 × 10⁻⁵ mg/cm⁴·s.     

Partial oxidation of methane over fine particles of various amorphous oxides including lanthanum

Partial oxidation of methane was carried out over fine particles of amorphous lanthanum oxides including 3B element and amorphous yttrium aluminum oxide. At low temperatures and low methane/oxygen ratios products were CO, CO₂, C2 hydrocarbon, hydrogen and water, and no formaldehyde was detected. The discussion on the contribution of catalyst components to the reaction suggested that 3B elements substantially influenced methyl radical formation and consumption while rare earth elements sufficiently affected formaldehyde decomposition.     

Self‐Generating High‐Temperature Oxidation‐Resistant Glass‐Ceramic Coatings for C–C Composites Using UHTCs

Carbon–carbon (C–C) composites are ideal for use as aerospace vehicle structural materials; however, they lack high‐temperature oxidation resistance requiring environmental barrier coatings for application. Ultra high‐temperature ceramics (UHTCs) form oxides that inhibit oxygen diffusion at high temperature are candidate thermal protection system materials at temperatures >1600°C. Oxidation protection for C–C composites can be achieved by duplicating the self‐generating oxide chemistry of bulk UHTCs formed by a “composite effect” upon oxidation of ZrB₂–SiC composite fillers. Dynamic Nonequilibrium Thermogravimetric Analysis (DNE‐TGA) is used to evaluate oxidation in situ mass changes, isothermally at 1600°C. Pure SiC‐based fillers are ineffective at protecting C–C from oxidation, whereas ZrB₂–SiC filled C–C composites retain up to 90% initial mass. B₂O₃ in SiO₂ scale reduces initial viscosity of self‐generating coating, allowing oxide layer to spread across C–C surface, forming a protective oxide layer. Formation of a ZrO₂–SiO₂ glass‐ceramic coating on C–C composite is believed to be responsible for enhanced oxidation protection. The glass‐ceramic coating compares to bulk monolithic ZrB₂–SiC ceramic oxide scale formed during DNE‐TGA where a comparable glass‐ceramic chemistry and surface layer forms, limiting oxygen diffusion.     

THE SEPARATION OF NEPTUNIUM AND PLUTONIUM FROM NITRIC ACID USING n-OCTYL(PHENYL)-N,N DIISOBUTYLCARBAMOYLMETHYLPHOSPHINE OXIDE EXTRACTION AND SELECTIVE STRIPPING†

The extraction of neptunium and plutonium in several oxidation states was studied as a function of nitric acid concentration for 0.1M n-octyl(phenyl)-N, N-diisobutylcarbamoyl -methylphosphine oxide in 1.4M tributylphosphate with dodecane diluent. Np(V) is only weakly extractable over the range of acid concentrations studied while Np(IV) and Np(VI) are highly extractable. Pu(IV) and Pu(VI) are also highly extractable while Pu(III) was extracted but with lower efficiency. An Fe(II) reductant was used to reduce neptunium to Np(IV) and plutonium to Pu(III) for the initial extraction. Pu(III) was then stripped with dilute HNO₃ in the presence of a holding reductant leaving the Np(IV) in the organic phase. Neptunium may then be recovered to an aqueous phase with one of a number of complexing agents.     

Influence of the composition of high-lead phosphate glasses on the location of the UV transmission edge and technological quality

This paper reports on the results of an investigation into the influence of the purity of the initial materials used for preparing vitreous lead metaphosphate, the acidity of the phosphate matrix, and the contents of additives of Group I–III and V elements and the second glass-former on the location of the UV transmission edge of simple binary and ternary lead phosphate glasses. It is shown that, even for a binary glass of the composition (mol %) 50PbO · 50P₂O₅, the location of the UV transmission edge can be shifted by ～50 nm on the wavelength scale depending on the purity of the initial reactants. The shift of the UV transmission edge toward the UV spectral range for ternary glasses containing no variable-valence elements other than lead is considerably larger than that for the high-lead phosphate glass of the previously proposed composition involving antimony, niobium, and cerium oxides. It is established that the addition of niobium oxide Nb₂O₅ to lead phosphate glasses brings about a red shift of the UV transmission edge and a change in the crystallization ability of the glasses. Niobium oxide at a content up to 1.5 mol % increases the crystallization ability of the glass, whereas a change in the niobium oxide content from 1.5 to 3.1 mol % results in a decrease in the crystallization ability. It is demonstrated that the crystallization ability of high-lead phosphate glasses increases at a boron oxide content higher than 5 mol %.     

Molecular structure and reactivity of the Group V metal oxides

The physical, electronic and reactivity properties of bulk and supported Group V metal oxides (V, Nb, Ta and Db) were compared at the molecular level. Dubnium is a very short-lived element, 60 s, whose properties have not been extensively studied, but can be predicted from knowledge of the other members of the Group V metal oxides. Bulk V₂O₅ possesses platelet morphology with the active surface sites only located at the edges: primarily surface redox sites and some surface acidic sites. Bulk Nb₂O₅ and Ta₂O₅, as well as to be expected for bulk Db₂O₅, possess isotropic morphologies and the active surface sites relatively homogeneously dispersed over their surfaces: only surface acidic sites. However, the bifunctional bulk V₂O₅ was found to exhibit a much higher specific acidic catalytic activity than the acidic bulk Nb₂O₅ and Ta₂O₅, the latter being almost identical in their specific acidic catalytic activity. The bulk properties of the Group V metal oxides were essentially transferred to the analogous supported Group V metal oxides, where the active Group V metal oxides were present as a two-dimensional monolayer on various oxide supports (e.g., Al₂O₃, TiO₂, ZrO₂ as well as Nb₂O₅ and Ta₂O₅). For supported vanadia catalysts, the active surface sites were essentially redox sites, with the exception of supported V₂O₅/Al₂O₃ that also contained strong acidic sites. For supported niobia and tantala catalysts, as well as to be expected for supported dubnia catalysts, the active surface sites were exclusively acidic sites. However, the TOF_redox for the supported vanadia catalysts and the TOF_acidic for the supported niobia and tantala catalysts varied over several orders of magnitude as a function of the specific oxide support with the electronegativity of the oxide support cation. However, the TOF_redox varied inversely to that of the TOF_acidic variation because of the opposite requirements of these active surface sites. Surface redox sites are enhanced by reduction and surface acidic sites are enhanced by stabilization (lack of reduction). The current fundamental understanding of the Group V metal oxides allows for the molecular engineering of their metal oxide applied catalytic materials.     

EXTRACTION OF NEPTUNIUM AND PLUTONIUM NITRATES WITH n-OCTYL(PHENYL)-N,N-DIISOBUTYL-CARBAMOYLMETHYLPHOSPHINE OXIDE∗

Distribution equilibria of Np and Pu in various valence states and stripping of Np(V) were studied in a system involving 0.20 M n-octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide + 1.2 M tributyl phosphate in dodecane as a solvent. Np(V) is weakly extractable at <2 M nitric acid but more effectively extractable at >3 M nitric acid, due to the disproportionation of Np(V) to Np(IV) and Np(VI). Addition of nitrous acid to solutions containing Np(V) increases the over-all extractabllity of Np, due to its partial oxidation to Np(VI). This is also the case In the presence of oxalic acid, both at room temperature and at 40^°C. Both Np(IV) and Np(VI) are highly extractable, but the reduction of Np(V,VI) to Np(IV) is slow even with a reductant as strong as sodium formaldehyde sulfoxylate. Pu(IV) is highly extractable and its reduction to Pu(III) with sulfoxylate Is incomplete in the two-phase system if the aqueous phase contains >0.5 M nitric acid. Extracted Np(VI) can be stripped by reduction to Np(V). The stripping rate is, however, slow with nitrous acid as the reductant at low nitric acid concentration. The reduction of Np(VI) by hydrogen peroxide is fast, but is followed by further reduction to Np(IV). Sulfurous acid reduces Np(VI) rapidly and, if no iron is present, only to Np(V), but Fe(II) induces further reduction to Np(IV).     

Reduction of smoke formation from and flammability of thermoplastic polymers by metal oxides

The properties of various metal oxides have recently been investigated in terms of flame retardance and smoke suppression. None of the metal oxides is an effective flame retardant on its own, but their use in conjunction with halogen compounds often results in systems which are effective as flame retardants, typically antimony (III) oxide, anhydrous aluminium (III) oxide and hydrous tin (IV) oxide phases. The two latter systems show joint flame-retardant and smoke-suppressant effectiveness. Most of the metal oxides examined have some effectiveness as smoke suppressants and, in this connection, magnesium (II) oxide hydrate and silicon (IV) oxide are outstanding, while useful influences can be obtained from anhydrous magnesium (II) oxide, molybdenum (VI) oxide and titanium (IV) oxide. A large proportion of the flame-retardant effectiveness is often derived from the volatilization of a halogenated species or of a metal halide. Some condensed-phase action is always essential at the corret temperature, as shown by the fact that synergism exists between halogen compounds and metal oxides which are catalytic but do not form volatile halides, such as iron (III) oxide, while antagonism is found with some metal oxides with easily volatile halides, such as zinc (II) oxide.     

高熵氧化物的制备及应用研究进展

高熵氧化物作为近几年发展起来的新型氧化物体系，打破了传统掺杂氧化物的设计理念，由五种及以上氧化物以等摩尔或近等摩尔构成，因其具有简单的结构和优异的性能等受到国内外研究人员的广泛关注。高熵氧化物由于多主元且主元之间混乱排列，易形成岩盐型、氟化钙型、尖晶石型或钙钛矿等固溶体结构，从而表现出优异的性能，尤其在能源存储材料和磁性材料方面有十分广阔的应用前景，但目前对高熵氧化物应用研究较少。本工作介绍了国内外高熵氧化物的制备方法，主要包括固相法、热解法、共沉淀法、水热合成法和液相燃烧合成法等，比较了各方法的优缺点和发展前景；归纳了高熵氧化物作为锂离子电极材料、巨介电材料、磁性材料和催化材料等方面的应用；指出了高熵氧化物目前研究存在的问题，讨论了解决措施，展望了高熵氧化物未来的发展趋势。     

Properties of dexsil 300 rubbers

The physical and elastomeric properties of several DEXSIL 300 (10-SiB-3) samples were investigated. Modulus—temperature studies were used to determine the glass transition temperature T_g, the melting temperature T_m, and the oxidative crosslinking temperature T_ox. Stress relaxation in air at elevated temperatures was used to compare the oxidative stability of the various formulations. It was found that the T_g of DEXSIL 300 is some 30°C lower than that of DEXSIL 200 (10-SiB-2) polymers, extending the elastomeric properties of DEXSIL 300 to lower temperatures. At high temperatures, both silica filler and ferric oxide are found to increase T_ox to an ultimate value of 320°C. The effects of cure were also investigated, and γ-radiation-cured samples exhibit a slight degree of crystallinity with a melting temperature T_m = +40°C. No crystallinity was detected in similar peroxide-cured samples. Stress relaxation results are presented in support of the modulus—temperature studies. Formulations with a low T_ox show oxidative effects earlier than those with a higher oxidation temperature. Silica-and ferric oxide-filled samples exhibit improved oxidative stability, as do samples filled with diphenylsilanediol.