Single-component Oxides Research Articles

Electrocatalytic performance investigation of NiCo2O4 nanostructures prepared by hydrothermal method and thermal post-annealing treatment

Nickel-cobalt oxide (NiCo2O4), also known as nickel–cobalt ore, is a spinel-structured mixed oxide. It is more environmentally friendly, abundant in nature, has better electrical conductivity, and has higher electrochemical activity than two single-component oxides, nickel oxide and cobalt oxide. Here, NiCo2O4 is synthesized using a simple hydrothermal method and then annealed in the air for 3 h at 300, 350, 400 and 450 °C. The resulting materials were characterized using X-ray diffraction (XRD), Fourier-Transform-Infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), ultraviolet–visible (UV–vis) spectroscopy and electrochemical techniques. A single-phase face-centered cubic spinel structure with a nanorod-like morphology is obtained at 350 °C, and samples above this temperature show mixed phases consisting of nanowires and nanorods.. The sample annealed at 350 °C demonstrated excellent electrochemical performance for hydrogen evolution reaction (HER), with an overpotential of 435 mV at a current density of 10 mA/cm2, a pseudo-capacitance of 0.287 mF/cm2, a Tafel slope of 71 mV/dec, an exchange current density of 3.5 × 10-3 mA/cm2, and a minimum charge transfer resistance of 98 O. The study thus demonstrated the efficacy of the hydrothermal method and thermal post-annealing treatment in producing NiCo2O4 nanostructures with excellent electrocatalytic performance for energy conversion and storage applications.

Coupling effect of reaction conditions on the catalytic activity of Cu-Mn composite oxide catalysts for toluene.

Volatile organic compounds (VOCs) are one of the major components of air pollution. Catalytic combustion is a promising technology for the treatment of VOCs and at its center is the preparation of efficient and cheap catalysts. In this study, by loading copper (Cu) and manganese (Mn) on Santa Barbara Amorphous-15 (SBA-15) molecular sieve, the Cux-Mny/SBA-15 (x = 1, 2; y = 1, 2) composite metal oxide catalyst was prepared using the equal volume impregnation method. Their performance in the toluene catalytic combustion reaction was investigated by adjusting the molar ratio (x : y), and the loading of Cu and Mn. The results of the Brunner-Emmett-Teller (BET), X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) analyses show that the CuMnO spinel phase can be detected in the Cu-Mn composite metal oxide catalyst doped with a low concentration of Cu. The overall rod-like structure of the fibrous network structure provides a large specific surface area, and the particle crystallinity is low and the dispersion is good. Due to the synergistic effect of Cu and Mn, the greater the amount of Mn3+ and adsorbed oxygen species (Oads) that are available, and the higher the turnover frequency (TOF) value, the better and more superior catalytic performance and excellent stability is obtained, when compared with the single-component oxides used in toluene catalytic combustion. After a continuous catalytic reaction for 12 h, the toluene conversion rate remained above 95%. The coupling effect of the catalytic reaction temperature and concentration of oxygen on the catalytic combustion of toluene was also studied. At a low reaction temperature (<250 °C), the increase of the concentration of oxygen played a superior role in promoting the conversion of toluene. The kinetic analysis of the toluene catalytic combustion process showed that the catalytic combustion of toluene by Cu-Mn/SBA-15 followed both the Mars-Van Krevelen (MVK) and Langmuir-Hinshelwood (L-H) reaction mechanisms. With the increase of the Oads amount caused by the decrease of the Cu ratio, the proportion of the L-H reaction mechanism increases.

Chromium-Rich Core-Shell Cr x Ir1-X O2 Nanotubes for Highly Efficient Oxygen Evolution Reaction in Alkaline Solution

Considering the growing attention to oxygen evolution reaction (OER) as a key role in clean energy through electrocatalytic water splitting, advanced materials for water oxidation are central to the area of renewable energy. In general, the OER process, involved four electron and four proton coupled electrochemical reaction, demands higher kinetics to overcome an overpotential than HER (hydrogen evolution reaction). For this aspect, significant research efforts have been devoted to developing efficient OER electrocatalyst with enhanced electrocatalytic kinetics and long-term durability.Here we present the synthetic processes and physical/electrochemical characterization of Cr x Ir1-x O2 nanotubes (NTs). Those NTs are synthesized through electrospinning, a simple way to facilitate 1D nanostructure, and post-thermal annealing process with different annealing speed (°C /min). As-prepared Cr x Ir1-x O2 NTs at the optimized conditions have shown outstanding OER electrocatalytic activity in alkaline media from the viewpoints of overpotential and Tafel slope as well as durability. The morphology and compositions of Cr x Ir1-x O2 were characterized by a field emission scanning electron microscope (FESEM) equipped with an energy dispersive X-ray spectrometer (EDS), high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS). The crystalline structures of the prepared Cr x Ir1-x O2 are investigated through high-resolution X-ray diffraction (XRD). Electrochemical activities for oxygen evolution reaction (OER) are studied with rotating disk electrode (RDE) voltammetry in N2-saturated 1.0 M KOH (aq); and compared with those of iridium single component oxides (IrOx and IrO2) and commercial iridium (cIr, counterparts in OER electrocatalysts).This work was financially supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT or by the Ministry of Education (NRF-2019R1F1A1059969, NRF-2018R1A6A1A03025340, NRF-2020R1A2B5B01001984, and NRF-2019R1F1A1062799).

Superior phase stability of high entropy oxide ceramic in a wide temperature range

A high entropy fluorite oxide (Hf0.2Zr0.2Ce0.2Y0.2Yb0.2)O2-δ (HEFO) was investigated to reveal its ultra-wide-temperature phase stability in air. The HEFO exhibited a single-phase fluorite structure, and its lattice constant, a, was evaluated to be 0.527 ± 0.02 nm from room temperature (RT) to > 2573 K. No precipitation of single-component oxides, such as HfO2, ZrO2, Y2O3 and CeO2, was experimentally observed. Since both Gibbs free energy (G) change of possible precipitation reactions and the second derivative of GHEFO with respect to all single-component oxides were positive, the HEFO exhibits no precipitation and decomposition at temperatures above 1700 K. The sluggish dynamics, which was verified by a long-term annealing experiment at 1473 K for 175 h showing a few Yb2O3 precipitation, contributed to the single-phase solution nature of HEFO below 1700 K. Such a superior phase stability in ultrawide temperature range help accelerate the engineering application of HEFO to be thermal structural components.

Identifying the Activation Mechanism and Boosting Electrocatalytic Activity of Layered Perovskite Ruthenate

SrRuO3 as a rare conductive perovskite ruthenate has attracted increasing attention for application in energy conversion. Here, the electrocatalytic activity for the hydrogen evolution reaction (HER) of thermally synthesized layered SrRuO3 is investigated and shows a considerable activation during cathodic polarization in alkaline solution. The analysis demonstrates the electrode activation is caused by the increased hydrophilicity of SrRuO3 surface, revealing the influence of the surface properties on HER performance. For further improving the catalytic activity of perovskite ruthenate, the RuO2 /SrRuO3 (RSRO) heterostructure is fabricated in situ by reducing the thermal decomposition temperature of 1000 °C for SrRuO3 to 600 °C. The appropriate lattice parameter of SrRuO3 ensures a good lattice match, which results in a strong interaction between SrRuO3 and RuO2 . Hence, the RSRO substantially outperforms the corresponding single-component oxides. In addition, the increased active sites induced by the rapid improvement of hydrophilicity of RSRO surface further highlight its structural advantage for catalytic hydrogen generation. The experimental and theoretical computation results consistently validate the positive synergistic effect among SrRuO3 and RuO2 in tuning the atomic and electronic configuration.

A New Type of Supercapacitor Oxide ZnMn2O4 Showing Combined Pseudocapacitance and Battery Behaviors

Supercapacitors have attracted great attention due to their rapid charge-discharge rates, long cycle life and high power density. Binary metal oxides, specially spinel structures as supercapacitor have been investigated to show better electrochemical performances than single component oxides owing to their available multiple oxidation states. In this paper, a simple and scalable process has been developed for synthesizing ultrafine ZnMn2O4/carbon nanostructures through a solution combustion method. Effect of polyethylene glycol (PEG) weight percentage with low and high molecular weights, as carbon source and pore size controller, on the structure and physical properties of the ZnMn2O4/carbon were investigated in details and well characterized using XRD, FESEM, TEM, HRTEM and N2 adsorption/desorption. Electrochemical characterization of the oxide electrode indicated unique combination of pseudocapacitance and battery behaviors, exhibiting large capacitances (> 150 F/g) and an intriguing dependence on electrolytes. The charge-transfer mechanisms have been characterized by in-situ XRD and X-ray absorption analyses.

The peroxidase and oxidase-like activity of NiCo2O4 mesoporous spheres: Mechanistic understanding and colorimetric biosensing

The synthesized NiCo2O4 mesoporous spheres (MS) displayed intrinsic peroxidase and oxidase-like activity were firstly reported. The catalytic mechanism of the oxidase-like activity of NiCo2O4 MS was analyzed in detail using the electron spin resonance (ESR) method. It is found that NiCo2O4 MS could directly oxidize 3,3′,5,5′-tetramethylbenzidine (TMB) but did not produce 1O2 and ·OH. And the mechanism of the peroxidase-like activity of NiCo2O4 MS was also verified that the oxidation of TMB stemmed from not only ·OH but also 1O2. Based on the NiCo2O4 MS showed excellent peroxidase-like activity over a broad temperature range, especially at normal body temperature, a detection tool was designed for glucose determination in diabetics' serum samples. And this detection method based on NiCo2O4 MS gave a lower limit of detection than the method using Co3O4 NPs and NiO NPs, as the single-component oxides of NiCo2O4. Our study may open up the possibility to make a great influence on the next generation of enzyme mimetics system.

Preparation of nanoparticles of oxides by the citrate–nitrate process

Abstract This paper reports preparation of nanoparticles of oxides by the citrate–nitrate process and the effect of metal ions on the thermal decomposition characteristics of the corresponding citrate–nitrate gel precursors. In order to understand the effect of metal ions on the thermal decomposition characteristics of the precursors, we have prepared a series of single component oxides such as MO, where M = Zn, MO2, where M = Sn, Ce, Zr, and M2O3 where M = Al, Fe, Bi. In all the cases the citrate to nitrate ratio was fixed at 0.3. In order to ascertain the decomposition characteristics of the gel samples, TG/DTA studies were performed on the dried gel samples. After complete physico-chemical characterization of the precursors and the calcined products, it could be concluded that the nature of decomposition of the precursors depends largely on the nature of the metal ions. Finally, the advantages of the citrate–nitrate process such as its high degree of reproducibility, its potential for large-scale produ...

Sensing characterization of Sn/In/Ti nanocomplex oxides for CO, CH4 and NO2

The nanocomplex oxides of Sn-In and Sn-In-Ti were prepared by controlled co-precipitation method as sensing materials of semiconductor gas sensors for detection of CO, CH4 and NO2. Through manipulating the Sn/In cation ratio, metal salt total concentration, precipitation pH value and aging time, the nanocrystalline powders were successfully derived with chemical homogeneity and superior thermal stability, compared with the single component oxides. The particle size and morphology, surface area, and thermal and phase stabilities were characterized using TEM, TG-DTA, BET and XRD. The sensing tests showed that the Sn-In composites exhibit high sensitivity and selectivity for CO and NO2. The introduction of TiO2 enhanced CH4 sensitivity and selectivity, particularly, additives of Pd and Al2O3 as a dopant and surface modification greatly enhanced the sensing properties. The sensitivity depended on the composition of composites, calcination temperature and operating temperature. The optimal values were (25%In2O3-75%SnO2)-20%TiO2 for temary composite, 600 and 300°C, respectively. Temperature-programmed desorption (TPD) studies were employed to explain the gas adsorption behavior displayed by the surface of nanocomposites and X-ray photoelectron spectroscopic (XPS) analysis was used to confirm the electronic interactions existing between oxide components. The sensing mechanism of the nanocomposites was attributed to chemical and electronic synergistic effects.

Preparation, characterization and gas-sensing properties of SnO2–In2O3 nanocomposite oxides

This paper aims at preparing gas-sensing nanocomposites of SnO2–In2O3 using a chemically controlled co-precipitation method and testing their sensing properties for detection of toxic gases. Through manipulating the Sn/In cation ratio, metal salt total concentration, precipitation pH value and aging conditions, the nanocrystalline composite powder was successfully derived with chemical homogeneity and superior thermal stability compared to the single-component oxides. The experimental results showed that these nanocomposites exhibited high sensitivity and selectivity for the detection of CO and NOx, and the sensitivity depended on the composition of the composites, calcination temperature and operating temperature. Their sensing performance was further enhanced through the introduction of a small amount of metals or other oxides as dopants and surface coatings. The gas-sensing mechanism of the nanocomposites was also discussed by X-ray photoelectron spectroscopic (XPS) analysis and temperature-programmed desorption (TPD) studies at gas-sensing operating temperatures.

Refractory Selection for High‐Temperature Black Liquor Gasification

A methodology has been established for the selection of materials for refractory linings in black liquor gasifiers that are to be employed by the pulp and paper industry. As a first step, a thermodynamic software package was used to determine that black liquor smelt was composed of a liquid solution of primarily Na2CO3 and Na2S under the operating conditions of the gasifier (950°C, 1 atm). Next, the software was used to predict the interaction of the black liquor smelt with various ceramics such as aluminosilicates, single component oxides, and binary oxides that are candidates for the application. Finally, experiments were performed to verify or disprove the predictions. Using sessile drop testing, contact angles were determined for molten Na2CO3 on candidate refractory compounds. All of the candidate ceramics were wet by molten Na2CO3. Among the candidates, MgAl2O4 was found to have the highest contact angle (∼13°). Post‐mortem analysis was performed on sessile drop specimens using X‐ray diffraction analysis and scanning electron microscopy with energy dispersive spectroscopy to determine if molten Na2CO3 had penetrated into or reacted with the ceramics. From the candidates, MgO and CeO2 were found to have the best resistance to attack, while MgAl2O4 was also found to be a promising candidate.

Effect of Interionic Interaction on the Electronic Polarizability, Optical Basicity and Binding Energy of Simple Oxides.

Interaction parameter A proposed in Yamashita-Kurosawa's theory has been calculated for numerous single component oxides. It has been assumed that in the case of simple oxides it represents, through the electronic polarizabilities of the ionic pair the interaction between oxide ion and the corresponding cation due to overlapping of their outermost electronic orbitals to form a chemical bond. It has been proposed that the parameter A is closely related to the polarizability of the oxide ion as well as the optical basicity of the oxides. The obtained almost linear distribution of the basicity in respect to Yamashita-Kurosawa's parameter A could be used as an optical basicity scale for simple oxides. Systematic decrease of the O1s and outermost cation binding energies with decreasing interaction parameter has been established. It has been associated with decreased charge overlapping between electronic shells of the oxide ion and cation, which relates to large electronic polarizabilities of both cation and oxide ion and increased optical basicity, that is increased ionicity of the chemical bond.

Mocvd of Polycrystalline and Epitaxial Complex Oxides by Liquid Delivery

ABSTRACTA unique approach to MOCVD of complex oxides enables deposition of a number of materials of technological importance through the use of liquid delivery of metalorganic precursors. Methodologies for control of composition and exploration of. process space are compared for two film systems, one in a relatively mature state of development ((Ba,Sr)Ti03), the other in an early state of development (Ni-ferrite). In both cases, composition was controlled by mixing metalorganic precursors dissolved in solvents using a liquid delivery system. Films with excellent crystalline quality were deposited in both cases. Polycrystalline BST films displayed properties suitable for DRAM applications: charge storage densities &gt; 80 fF/μm2 and leakage current density &lt; 10−8 A/cm2 for films as thin as 15 nm. Growth mechanisms and rates were determined for the single component oxides of the ferrite films. Epitaxial NiFe204 films were deposited on MgO single crystal substrates at 650°C; x-ray rocking curves yielded FWHM values of 0.046°, commensurate with the substrate.

Electronic oxide polarizability and optical basicity of simple oxides. I

The average electronic oxide polarizability α02− of numerous single component oxides has been calculated on the basis of two different properties: linear refractive index n0 and energy gap Eg, which have demonstrated remarkable correlation. The optical basicity Λ of the oxides has been estimated on the basis of average electronic oxide polarizability calculated from the refractive index Λ(n0) and the energy gap Λ(Eg). A good agreement has been observed between the optical basicity data obtained using independent initial quantities. The simple oxides have been separated into three groups according to the values of their oxide polarizability. The α02− values (above 3 Å) obtained for PbO, Sb2O3, and Bi2O3 have been attributed to the high cation polarizability and the presence of a lone pair in the valence shell of the cation.

Titania-zirconia mixed oxide aerogels as supports for hydrotreating catalysts

Supercritical fluid (SFC) extraction was used to make aerogels of TiO2, ZrO2 and two TiO2/ZrO2 mixed oxides, having surface areas from two to five times greater than their conventionally prepared equivalents; additionally the mixed oxides have higher surface acidities than the two single component oxides. Heat treatments, either during catalyst preparation or reactor testing, always resulted in small to significant decreases in surface areas in the aerogel-containing samples. These samples were used as supports for Mo-Ni catalysts for the hydroprocessing of gas oil in a pilot-plant scale reactor. The high ZrO2 containing materials were found to be unstable under reaction conditions and were nearly inactive; in contrast, the high TiO2 containing catalysts, while somewhat unstable, are more active on a surface area basis than Al2O3 or conventional TiO2 equivalent supported Mo-Ni catalysts. This improvement is attributed to properties inherent in the SCF prepared supports; these results also indicate that support acidity contributes to hydrotreating activity.

Oxidation of ketones over metal oxide catalysts: I. Catalytic synthesis of biacetyl from methyl ethyl ketone

The gas-phase oxidation of methyl ethyl ketone was studied on metal oxide catalysts in the presence of water vapor. Two types of competitive partial oxidations, i.e., biacetyl formation and oxidative scission reaction leading to acetaldehyde and acetic acid, took place on every oxide studied at 400–500 K. An approximate linear relationship was observed between the selectivity of each reaction and the acid-base property of the oxides; the former reaction was accelerated on the basic oxides such as Co 3O 4, while the latter reaction became predominant on the acidic oxides. As Co 3O 4 was the most effective biacetyl former of single-component oxides, modification of Co 3O 4 was examined to develop more effective catalysts for biacetyl synthesis. Scission reaction took the place of biacetyl formation over a catalyst where Co 2+ ions were located in Y zeolite by an ion-exchanged method. Scission reaction was suppressed when Co oxide was supported on basic oxides such as MgO or CaO; however, the selectivity to biacetyl was slightly decreased due to the occurrence of a new reaction, acetone formation. The addition of Na 2O or Li 2O to Co 3O 4 was found to improve the selectivity to biacetyl without loss of catalytic activity. The maximum efficiency (13%) in biacetyl formation was attained at a Li content of ca. 7 at.%.