Mixed Oxide Research Articles

The high‐temperature creep behavior transition in stoichiometric nanocrystalline (Pux,U1−x)O2 mixed oxides

AbstractThe study of high‐temperature creep properties of (Pu,U)O2 mixed oxides (MOX) ceramics is an increasingly important and attractive issue in the field of nuclear fuels. Using the molecular dynamics method, we study the creep behavior and creep mechanism transition of nanocrystalline (NC) MOX affected by temperature, stress, grain size, and Pu contents. It is found that the creep of NC MOX is dominated by diffusional creep and larger stress can activate the grain boundary sliding. At the Bredig transition temperature of MOX, its anionic sublattice pre‐melts, which increases the creep rate due to intensified ions diffusion. Thus, the increase of temperature and stress leads to a transition in the creep mechanism from Coble creep with grain boundary sliding to Nabarro–Herring creep. Based on the method of analogy, we develop a theoretic model of the cationic vacancy concentration rate for high‐temperature diffusional creep of NC MOX. This model can well predict the cationic vacancy concentration in the secondary creep stage. The findings of this study could not only give us a deep understanding of the creep behaviors of MOX ceramics at higher temperatures, but provide valuable insights in the variations of cationic vacancy concentration during steady‐state creep stage of NC MOX.

Revealing Activity of Symmetrical and Asymmetrical Ag2O‐MOx (M = Cu, Fe, Zn) Catalysts via DFT Calculations for Direct Propylene Epoxidation

AbstractDirect propylene epoxidation (DPE) with molecular oxygen is an effective process of conversion of propylene into propylene oxide (PO), which is an essential intermediate compound for many petrochemical products such as propylene glycol, propylene carbonate, polyurethane foams, etc. The modification of Ag‐based catalysts which prevents overoxidation and improves catalytic activity for PO production is studied by simulating Ag2O and mixed metal oxides of Ag2O‐CuO (Ag2CuO2), Ag2O‐ZnO (Ag2ZnO2) and Ag2O‐Fe2O3 (Ag2Fe2O4) catalysts using density functional theory (DFT). Instantaneous adsorption of propylene was observed over Ag2ZnO2 and Ag2CuO2. DPE with oxygen via the Mars‐Van Krevelen mechanism (MVKM) was investigated considering propylene and oxygen as alternatively adsorbed species and molecular species. DPE with molecular oxygen and adsorbed propylene required 1.7 times more activation energy than DPE with molecular propylene and adsorbed oxygen. DPE with oxygen via the Lagmuir‐Hinshelwood mechanism (LHM) required 3–4 times more activation energy than MVKM for Ag2O and Ag2CuO2, whereas it is comparable for Ag2ZnO2. This showed that both mechanisms are followed for DPE over Ag2ZnO2. Ag2ZnO2 was found to be the optimum catalyst for DPE with instantaneous adsorption of propylene and instantaneous desorption of PO.

Mixed metal oxide and boron-doped diamond anodes for the treatment of 1-butyl-3-methylimidazolium chloride in single- and dual-flow configuration

Mixed metal oxide and boron-doped diamond anodes for the treatment of 1-butyl-3-methylimidazolium chloride in single- and dual-flow configuration

Experimental and Theoretical X-ray Absorption Near Edge Structure Study of UOx Systems at the U L3 Edge.

Uranium forms a large number of oxides and its electronic state in them is of great fundamental interest. We employ X-ray absorption spectroscopy at the U L3 edge to differentiate between mixed oxide phases in uranium compounds. By combining experimental XANES spectra with theoretical modeling using the FEFF code, we analyze five uranium oxides: UO2, U4O9, U3O8, UO3, and U3O7. Our study reveals how variations in spectral features correspond to changes in oxidation states, local structure, and electronic information. These findings demonstrate a study that consolidates and systematically compares the entire series UOx, providing analysis of changes in electronic structure as a function of oxidation state and local structure which is important for nuclear materials research.

Forced Dynamic Operation of Propylene Selective Oxidation to Acrolein on Bismuth-Molybdate Structured Catalysts

Forced dynamic operation (FDO) of selective oxidation of propylene to acrolein is evaluated on a structured alumina foam coated with a mixed metal oxide catalyst (bismuth molybdate-based). Reactor experiments examine feed composition modulation as a strategy to increase the yield of acrolein. At lower temperatures, separating the oxidant (O2) and reductant (C3H6) enhances propylene conversion and acrolein selectivity. Variation in the reaction-regeneration switching amplitude and frequency gives a 40% higher cycle-averaged acrolein yield compared to steady-state operation (SSO) for the same overall feed composition. The results suggest that FDO maintains a higher concentration of selective oxygen species during the propylene feed, while pre-oxidation of the catalyst maximizes the amount of selective oxygen species, leading to increased acrolein yield at the beginning of the reaction cycle. Experiments at lower temperature for both commercial promoted and in-house, unpromoted catalysts reveal both catalysts are most active and selective at their highest oxidation states.

Mo–V–Al–K–O catalyst for low-temperature oxidative dehydrogenation of propane

Compared with propane dehydrogenation (PDH) technology, oxidative dehydrogenation (ODH) of propane is an effective way to break through the thermodynamic constraints, which has changed the endothermic reaction into exothermic. Meanwhile, coking in PDH was significantly reduced by the presence of an oxidant in ODH of propane. Therefore, ODH of propane has attracted wide interest and has been studied in recent years. However, the contradiction between the conversion of propane and the selectivity of propene was the intrinsic problem of ODH of propane that required further study. In this work, a novel aluminum/potassium-doped mixed metal oxide catalyst Mo1V0.5Al0.02K0.03Ox was synthesized using a simple co-impregnation method, whose selectivity for propene (83.2 %) and conversion of propane (12.7 %) at 300 °C were the best among all other alkali metal/alkaline-earth metal-doped catalysts. The further discussion illustrated that doping potassium and alumina could synergistically adjust the density of acid sites on the catalyst, establishing an equilibrium between high propane conversion and the ideal propene selectivity. After modification, the density of Brønsted acid decreased, leading to an enhancement in the selectivity of propene. Simultaneously, the addition of steam in the feed gas promotes propene to oxygenates and improves the conversion of propane under the condition of constant contact time.

Oxidation of Ni(H2O)6@MoO3 to Ni2O3/MoO3 Composites by Aerial Annealing: Electrocatalytic Hydrogen Evolution.

Molybdenum trioxide (α-MoO3) is a promising and inexpensive alternative to platinum group metals (PGMs), for electrocatalytic hydrogen evolution reaction (HER). However, to make it a viable candidate for electrocatalytic systems, we must address the hurdles associated with its inferior electrical conductivity and lack of active sites. Unlike Mo-based compounds such as MoS2 and MoSe2, which possess catalytically active edges, α-MoO3 lacks inherent active sites for HER. Previous studies have employed various strategies to activate MoO3 for HER, yet its activation in near-neutral conditions remain largely unexplored. In this study, a previously known α-MoO3 intercalating {Ni(H2O)6}2+, [MoVI2O6(CH3COO){NiII(H2O)6}0.5]·H2O (Ni(H2O)6@MoO3) is prepared via a simple and scalable room-temperature aqueous synthesis. In the subsequent aerial thermal annealing process at 300, 400 and 500 °C, Ni(H2O)6@MoO3 acts as a self-sacrificial template, yielding mixed metal oxide composites of nickel and molybdenum (named as MoO3-300, MoO3-400 and MoO3-500). The HR-TEM and XPS analyses confirm the formation of the Ni2O3 phase alongside the orthorhombic α-MoO3. The annealing temperature plays a key role in the crystallinity, phase, morphology, and electrocatalytic performance of the resulting composites. The composite formed at 400 °C (MoO3-400) shows the best electrocatalytic performance among them.

Microstructure and magnetic properties of Fe-Al2O3 nanocomposites prepared by a versatile facile combustion-based route

Fe-Al2O3 nanocomposites with Al2O3 nanoparticles uniformly dispersed have been successfully prepared via a versatile facile and scalable combustion-based route with two steps. Firstly, the homogenous mixed oxide precursor composed with α-Fe2O3, γ-Fe2O3 and Al2O3 was prepared by solution combustion synthesis. Subsequently, Fe-Al2O3 nanocomposites with Al2O3 nanoparticles evenly dispersed were prepared by hydrogen reduction of the mixed oxide precursor. The effects of the various reduction temperatures on the phase composition, morphology, grain size and magnetic properties of the Fe-Al2O3 nanocomposites were investigated in details. The results show that the Fe-Al2O3 nanocomposites prepared at as low as 400 °C present a connected network structure with an average grain size of about 17 nm, which have a saturation magnetization of 125.3 emu/g, and a coercivity of 721.2 Oe. With the increase of reduction temperature, the average grain size, and saturation magnetization of the Fe-Al2O3 nanocomposites increase gradually, while the coercivity decrease. When the reduction temperature is increased to 500 °C and 600 °C, the grain size increases to about 27 nm and 37 nm, the saturation magnetization increases to 153.6 emu/g and 177.1 emu/g, and the coercivity decreases to 634.6 Oe and 467.8 Oe, respectively.

Conversion of glycerol into renewable hydrocarbons employing platinum and iron catalysts supported on mixed zirconium and aluminum oxides

Conversion of glycerol into renewable hydrocarbons employing platinum and iron catalysts supported on mixed zirconium and aluminum oxides

>100 m fiber-coupled microchip laser–induced breakdown spectroscopy for remote elemental analysis applications

This study aims to develop ultralong fiber-optic cable (FOC) coupled microchip laser – induced breakdown spectroscopy (mLIBS) to reveal the elemental distribution and local composition of nuclear fuel debris in an accident-damaged reactors at Fukushima Daiichi Nuclear Power Station (FDNPS). Currently at FDNPS, the distance between the area where humans can safely work in a sufficient space and the nuclear fuel debris is expected to be >100 m. Therefore, it becomes crucial to analyze the light transmittance performance of FOC-coupled mLIBS systems over such long distances in a high-radiation environment. Therefore, this study focuses on the influence of gamma radiation dose and FOC length on the transmittance of the visible and near-infrared (NIR) wavelengths through FOCs. Compared with an FOC with low-OH groups, that with high-OH groups exhibits better light transmittance performance in both wavelength regions in a high-radiation environment. Furthermore, the light transmittance performance of the high-OH FOC extended up to 500 m in length is tested in a radiation-free environment. From the measurement results, the maximum FOC length for microchip crystal oscillation is estimated to be >800 m, although attenuation is observed with the increase in FOC length. Finally, we analyze the ultralong FOC–coupled mLIBS system through the detection of gadolinium (Gd) in mixed oxide samples. The strong emission lines of Gd were only available in the visible wavelength region. Therefore, quantitative analysis of Gd is successfully performed in the visible region using the mLIBS system coupled with up to 300-m-long FOCs, with the limits of detection being between 0.1% and 0.2%.

Inquisition on enhanced photocatalytic performance of ternary cerium manganese nickel metal oxide nanocomposites

The hydrothermal approach was used to successfully incorporate ternary Cerium Manganese Nickel mixed metal oxides (CMN-MMOs) with various Ce/Mn/Ni ratios. The improved crystallanity of incorporated CMN-MMOs was discovered using powder X-ray diffraction. From Ultra Violet (UV)–visible diffuse reflection spectroscopy (DRS), CMN-MMOs displays a broad absorption in the visible range. Scanning Electron Microscope (SEM) reveals that CMN-MMOs exhibit in nanoslab. Image of Transmission Electron Microscope (TEM) expose that synthesized samples are in hexagonal structure with average size of 20–30 nm. The photocatalytic activity was appraised for calcined samples of CMN-MMOs (1: 1: 2). It indicates the greatest photocatalysic activity with 89 % degradation of methylene blue (MB) dye. CMN- MMOs (1: 1: 2) also had a strong degradation effect on the non-dye contaminant phenol. Methylene Blue degradation followed a pseudo-first-order kinetic trend. The mechanism of photo degradation of CMN- MMOs was also explored.

Isothermal CO2 separation Enabled by redox-active mixed oxide sorbents

This study reports an isothermal CO2 capture strategy using redox-active perovskite oxides and demonstrates its application in sorption-enhanced hydrogen production. Using a thermodynamic analysis, we investigated the relationship between the equilibrium oxygen partial pressure and the extent of CO2 sorption as well as the theoretical hydrogen purity. To validate the concept, calcium and cobalt co-doped SrFeO3 (SrxCa1-xFeyCo1-yO3-δ) samples were screened experimentally to assess their (isothermal) CO2 sorption capacity and reducibility. The optimal sorbent, Sr0.6Ca0.4Fe0.7Co0.3O3-δ (SCFC-6473), underwent further characterizations and was used for isothermal sorption-enhanced steam reforming (iSESR) of glycerol to produce H2, achieving 93 vol% H2 purity. In addition, we present iSESR of biogas and isothermal sorption-enhanced gasification (iSEG) of pulverized pine by Sr0.875Ba0.125MnO3-δ (SBM 718) to validate the generalizability of the proposed concept for a variety of applications. Nearly 90 % syngas efficiency was demonstrated for iSEG of biomass while producing a hydrogen enriched (73 vol%) syngas product.

Synthesis of sustainable aviation fuels via (co–)oligomerization of light olefins

Within this study, the (co–)oligomerization of methanol-based olefins in the C2-4 range was investigated. The main objective was to increase the yield of oligomers with carbon chain lengths in the range of kerosene (C9-16). Commercially available mesoporous amorphous mixed silicon-aluminum oxides, optionally modified with nickel species, were applied as catalysts. Initially, single olefin feeds were employed, i.e. homo-oligomerization reactions of pure propylene and pure 1-butylene were studied at 120 °C and 32 bar olefin partial pressure, respectively. The co-oligomerization of olefin mixtures (C3+4 and C2+3+4), which can be obtained in Methanol-to-Olefins (MtO) processes, yields product mixtures with reduced selectivities to specific chain lengths. However, selectivities to kerosene-like olefins up to 85 % have been achieved and the main side product is gasoline. Investigations with varying reaction conditions reveal comparable effects as in the case of homo-oligomerization. The use of nickel-free catalysts resulted in the highest selectivities of kerosene-like olefins, but no ethylene was converted. The negative effects of nickel catalysts on fuel quality can be compensated by two consecutive catalyst beds, the first catalyst bed with a nickel-loaded silicon-aluminum oxide for ethylene conversion followed by a catalyst bed of neat silicon-aluminum oxide for the synthesis of highly branched, long chain oligomers. The reaction network for olefin oligomerization reactions is depicted, which can be simplified remarkably in the case of catalysts without nickel. A long-term experiment lasting for more than 200 h was conducted revealing a deactivation of the acid sites of the catalysts, but also the possibility of reactivation. Selectivity to kerosene-like olefins remained above 63 % and fuel characterization showed that the resulting kerosene fraction will be suitable for blending with conventional fuels.

Selective hydroxylation of benzene via enhanced generation and utilization of hydroxyl radicals with CuZnSbO photocatalyst

Photocatalytic selective benzene hydroxylation via activation of C(sp2)–H under visible light remains a challenging reaction. Copper-incorporated mixed metal oxide photocatalysts have shown promise in addressing this difficulty by enabling visible light harvesting, controlled reactive oxygen species (ROS) generation, effective ROS utilization, and reactant adsorption. However, copper incorporated metal oxide catalysts were suffered from poor catalytic activity and selectivity due to leaching of Cu species. To solve this problem, herein, a novel stable mixed metal oxide (CuZnSbO) was prepared for the first time by applying a facile method. Copper introduction brought about the suitable band gap energy for a wide range of visible light harvesting of the CuZnSbO catalyst. The copper species play a key role in activating H2O2 to produce hydroxyl radicals (•OH) for benzene oxidation by controlling charge recombination. The mixed metal oxide of zinc and antimony supports the included copper strongly enabling copper stability. The CuZnSbO not only generates available hydroxyl radicals but also facilitates efficient hydroxyl radical consumption to initiate C(sp2)–H activation, forming benzene radical intermediates en route to phenol. That is ever reported. CuZnSbO delivered 31.51 % benzene conversion and 100 % phenol selectivity. This work demonstrates the promise of engineered mixed metal oxide that boosts Cu species utilization for H2O2 and benzene activation. The photocatalyst showed good activity in selective oxidative hydroxylation reactions through harnessing visible light and controlled ROS generation. Importantly, Cu cations govern the photocatalytic •OH generation mechanisms as shown by XPS and active site deactivation analysis.

Hydrogen Production Through Water Splitting Reactions Using Zn-Al-In Mixed Metal Oxide Nanocomposite Photocatalysts Induced by Visible Light

In this study, the synthesis of hybrid photocatalysts of Zn-Al-In mixed metal oxides were activated by using visible light, derived from Zn-Al-In layered double hydroxide (ZnAlIn-LDH), and these nanocomposites demonstrated high efficiency for photocatalytic H2 production under UV light when using methanol as a sacrificial agent. The most active photocatalytic material produced 372 μmol h−1 g−1 of H2. The characterization of these materials included X-ray diffraction (DRX), infrared spectroscopy (FTIR), X-ray fluorescence spectroscopy (XRF), X-ray spectroscopy (XEDS), scanning electron microscopy analysis (SEM), transmission electron microscopy (TEM), diffuse reflectance spectroscopy, and N2- physisorption. In addition, the materials were characterized by photoelectrochemical techniques to explain the photocatalytic behavior. Subsequently, the photocatalytic performance for the water-splitting reactions under visible irradiation was evaluated. The ZnAlIn-MMOs with an In/(Al + In) molar ratio of 0.45 exhibited the highest photocatalytic activity in tests under visible light, attributed to the efficient separation and transport of photogenerated charge carriers originating from the new nanocomposite. This discovery indicates a method for developing new types of heteronanostructured photocatalysts which are activated by visible light.

On some distinct characteristics of reactive and normal flash sintering: A case study using low-temperature synthesized p-type cuprous delafossite, CuCrO2

We report the rapid synthesis of the high temperature cuprous delafossite (CuCrO2) through a single-step Reactive Flash (RF) method at furnace temperature, Tf=270 ℃ and dc electric field, E≈160 V/cm in ≈ 6.5 min from mixed oxides of Cu and Cr. Besides successful synthesis, we also investigate the larger question of the effect of external dc fields and current on nucleation and growth (N-G) type systems with accompanying compositional changes, typical of chemical reactions. CuCrO2 powders obtained from a hydrothermal route with an abundance of product nuclei were used to investigate the effect on N-G systems with insignificant compositional change (fine-grained/amorphous → coarse-grained). Temperature dependent current measurements towards the end of stage I indicate processes with activation energies of ∼3 eV, suggesting incipient electrochemical and ionic activity. In both scenarios (nucleation, growth, and chemical reactions), the influence of the E-field (Stage I) is far surpassed by the electric current (Stage III) yielding almost single-phase microstructures and p-type conductivity with [h] ≈1014/cm3. Subtle distinctions are identified in the flash characteristics using descriptors including widths of diffraction peaks and average voltage fluctuations.

Evolution of structural, magnetic and transport properties of 3d-5d based double perovskites Nd2–xSrxCoIrO6

The evolution of the structural, magnetic and transport properties of the intermediate compounds Nd2-xSrxCoIrO6withx= 0.2, 0.4, 0.6, 1 and 1.5 have been studied to establish important roles of sizes and oxidation states of cations on various phases. The replacement of Nd3+by Sr2+primarily influences the oxidation states of Co (Co2+→Co3+) and Ir (Ir4+→Ir5+) ions to maintain the charge neutrality in the entire system. The Sr dopants give rise to an increasing Co/Ir antisite disorder (ASD) to accommodate the variation of charge state and ionic radius of Co and Ir. The nature of magnetic interaction induced by Sr changes from being a ferrimagnetic (FIM) to a more dominant antiferromagnetic. The suppression of the second magnetic transition below 30 K in samples forx>0.2 is entirely due to dilution of the Nd-Nd magnetic interaction. The combined effects of ASD and mixed oxidation state of Co and Ir ions generate various types of magnetic exchange pathways and create competitive magnetic interactions to stabilize a particular magnetic ground state. In the middle compound NdSrCoIrO6, a Griffith like phase in the temperature region 65-150 K and exchange bias field of 658 Oe at 2.3 K under a cooling field of 50 kOe has been observed. The compounds show an insulating kind of behaviour, and with hole doping the value of room temperature resistivity drastically decreases. The nature of conduction is found to follow three dimensional Mott's variable range hopping process.

Comparative performance analysis of unmixed and mixed metal oxide sensors for dual-sensing leveraging machine learning.

This paper presents the synthesis of mixed metal oxide (BaTiO3: ZnO) (B: Z) sensors with various molar ratios using a low- temperature hydrothermal method for dual sensing applications (gas and acceleration). The sensor developed with an equal molar ratio of 1B:1Z, showcases superior performance compared to unmixed and alternative mixed metal oxide sensors. This equilibrium in ratios optimally enhances synergistic effects between elements B and Z, resulting in improved sensing properties. Furthermore, it contributes to structural stability, enhancing performance in gas and acceleration sensing. A decreased band gap of 2.82eV and a rapid turn-on voltage of 0.18V were achieved. The acceleration performance of 1B:1Z sensor exhibits a maximum voltage of 2.62 V at a 10 Hz resonant frequency and an output voltage of 2.52 V at 1 g acceleration, achieving an improved sensitivity of 3.889 V/g. In addition, the proposed gas shows a notable sensor response of ~63.45% (CO) and 58.29% (CH4) at 10 ppm with a quick response time of 1.19s (CO) and 8.69s (CH4) and recovery time of 2.09s (CO) and 8.69s (CH4). Challenges in selectivity are addressed using machine learning, employing various classification algorithms. Linear Discriminant Analysis (LDA) achieves superior accuracy in differentiating between CO and CH4, reaching 96.6 % for CO and 74.6 % for CH4 at 10 ppm. Understanding these concentration-dependent trends can guide the optimal use of the sensors in different current applications.&#xD.

CO2 utilization for the synthesis of propylene carbonate from propylene oxide using CeO2–La2O3 mixed oxide under solventless condition

One of the most promising strategies for removing excess carbon dioxide from the atmosphere is CO2 utilization. Here, we describe the effective synthesis of propylene carbonate utilizing CeO2–La2O3 (Ce–La) mixed oxide, combining propylene oxide with carbon dioxide. The addition of La2O3 (La) to CeO2(Ce) resulted in an increase in oxygen vacancy, reducibility, and support–support interaction. Modifying the La:Ce ratio leads to a fine–tuning of the reducibility of the oxide, which can be explained by the formation of oxygen vacancies in the ceria lattice upon La addition. This modification also permits tailoring of the oxide morphology (lattice parameter, pore structure, particle size, and surface area). Powder X–ray diffraction (P–XRD), temperature programmed reduction by hydrogen (H2–TPR), Raman spectroscopy, BET surface area, and X–ray photoelectron spectroscopy (XPS) were used to analyze the physico–chemical properties of the developed materials. Additionally, it demonstrates excellent catalytic activity for the solvent–free cycloaddition of CO2 with propylene oxide, yielding 86%, and it maintains its high stability over five cycles without changing its shape.

N-alkylation of amines with alcohols over hydrothermally prepared Nb-W mixed oxides

Hydrothermally synthesized Nb-W mixed oxides (Nb-W-O) have been employed as a cost-effective and recyclable alternative to noble metals and homogeneous catalysts for the N-alkylation of amines with alcohols, with the model reaction of aniline and benzyl alcohol. The synergistic roles of Nb and W and the influence of calcination conditions were explored. Nb-W-O exhibited a linear packing of corner-sharing octahedra, analogous to the hexagonal tungsten bronze (HTB) structure, but without the long-range order of the HTB in the ab plane, characteristic of pseudo-crystalline materials. Calcination at 773 K in an N2 flow preserved the pseudo-crystalline phase and generated additional Brønsted acid sites, associated with reduced tungsten oxide species stabilized by Nb. The primary reaction pathway involved the activation of the benzyl alcohol by Brønsted acid sites, followed by a nucleophilic attack of the formed carbocation by the aniline. Nb-W-O demonstrated superior activity to other solid acids including metal oxides, silica-alumina, and zeolites in the N-alkylation reaction.