Crystalline MgO Research Articles

Stability of Cs/Ru/MgO Catalyst for Ammonia Synthesis as a Hydrogen and Energy Carrier

The Cs/Ru/MgO catalyst was synthesized by sequential impregnation of Ru and Cs on MgO support using Ru(NO3)3 and CsNO3 precursors. Catalytic ammonia synthesis was carried out in a fixed-bed flow reactor using H2 and N2 as reactants. The stability of the catalyst was measured at 350 °C, 2.5 MPa gauge pressure, and SV as 1200 h−1 using the H2/N2 ratio 3 as a reactant feedstock. The Cs/Ru/MgO catalyst retained its ammonia synthesis activity while conducting experiments at mild reaction conditions of 325 °C and 350 °C. An increase in experimental temperature to 375–425 °C decreased the ammonia synthesis activity retaining only to 42% of the initial activity after 680 h of time on stream. The deformation of the catalyst’s structure, which was caused by Cs leaching and redistribution of the Ru and increased crystallinity of MgO at high-temperature conditions, was considered the plausible reason for the drastic decrease in ammonia synthesis activity.

Atomically Dispersed Platinum in Surface and Subsurface Sites on MgO Have Contrasting Catalytic Properties for CO Oxidation.

Atomically dispersed metals on metal oxide supports are a rapidly growing class of catalysts. Developing an understanding of where and how the metals are bonded to the supports is challenging because support surfaces are heterogeneous, and most reports lack a detailed consideration of these points. Herein, we report two atomically dispersed CO oxidation catalysts having markedly different metal-support interactions: platinum in the first layer of crystalline MgO powder and platinum in the second layer of this support. Structural models have been determined on the basis of data and computations, including those determined by extended X-ray absorption fine structure and X-ray absorption near edge structure spectroscopies, infrared spectroscopy of adsorbed CO, and scanning transmission electron microscopy. The data demonstrate the transformation of surface to subsurface platinum as the temperature of sample calcination increased. Catalyst performance data demonstrate the lower activity but greater stability of the subsurface platinum than of the surface platinum.

Structural, electrical and optical properties of MgO-reduced graphene oxide nanocomposite for optoelectronic applications

MgO-reduced graphene oxide nanocomposites (NCs) were synthesized by a simple two-step chemical method. The microstructure, surface morphology, and composition of the prepared samples have been studied. X-ray diffractometer (XRD) analysis confirmed the crystalline cubic MgO nanoparticle and rGO sheets. Scanning electron microscope (SEM) showed the spherical MgO nanoparticles well dispersed over the graphene sheets. UV–visible spectroscopy analysis demonstrated that a red shift in the wavelength dependent absorbance curve. The band gap of the samples was found to be decreased with the increase of rGO content. The dielectric studies have been examined in the frequency range 500 Hz−5 MHz and found significant improvement in the dielectric constant, dielectric loss, and electric properties due to rGO addition.This is mainly attributed to the strong interfacial polarization (Maxwell–Wagner polarization) between MgO and rGO sheets. Further, the modulation of charge carrier density with rGO additions help to enhance the electrical conductivity of NCs and thus, encouraging to have wider application in electronic and energy technologies.

Tuning Optical Bandgap of Crystalline Mgo by Mev Co Ion Implanted Embedded Defects

Tuning Optical Bandgap of Crystalline Mgo by Mev Co Ion Implanted Embedded Defects

Tuning Optical Bandgap of Crystalline Mgo by Mev Co Ion Implanted Embedded Defects

Tuning Optical Bandgap of Crystalline Mgo by Mev Co Ion Implanted Embedded Defects

Recovering local structure information from high-pressure total scattering experiments.

High pressure is a powerful thermodynamic tool for exploring the structure and the phase behaviour of the crystalline state, and is now widely used in conventional crystallographic measurements. High-pressure local structure measurements using neutron diffraction have, thus far, been limited by the presence of a strongly scattering, perdeuterated, pressure-transmitting medium (PTM), the signal from which contaminates the resulting pair distribution functions (PDFs). Here, a method is reported for subtracting the pairwise correlations of the commonly used 4:1 methanol:ethanol PTM from neutron PDFs obtained under hydro-static compression. The method applies a molecular-dynamics-informed empirical correction and a non-negative matrix factorization algorithm to recover the PDF of the pure sample. Proof of principle is demonstrated, producing corrected high-pressure PDFs of simple crystalline materials, Ni and MgO, and benchmarking these against simulated data from the average structure. Finally, the first local structure determination of α-quartz under hydro-static pressure is presented, extracting compression behaviour of the real-space structure.

A Theory-Guided X-ray Absorption Spectroscopy Approach for Identifying Active Sites in Atomically Dispersed Transition-Metal Catalysts.

Atomically dispersed supported metal catalysts offer new properties and the benefits of maximized metal accessibility and utilization. The characterization of these materials, however, remains challenging. Using atomically dispersed platinum supported on crystalline MgO (chosen for its well-defined bonding sites) as a prototypical example, we demonstrate how systematic density functional theory calculations for assessing all the potentially stable platinum sites, combined with automated analysis of extended X-ray absorption fine structure (EXAFS) spectra, leads to unbiased identification of isolated, surface-enveloped platinum cations as the catalytic species for CO oxidation. The catalyst has been characterized by atomic-resolution imaging and EXAFS and high-energy resolution fluorescence detection X-ray absorption near edge spectroscopy. The proposed platinum sites are in agreement with experiment. This theory-guided workflow leads to rigorously determined structural models and provides a more detailed picture of the structure of the catalytically active site than what is currently possible with conventional EXAFS analyses. As this approach is efficient and agnostic to the metal, support, and catalytic reaction, we posit that it will be of broad interest to the materials characterization and catalysis communities.

Enhanced mineral carbonation at room temperature through MgO nanocubes synthesized by self-combustion

Carbon dioxide is one of well-known greenhouse gases, which cause the global warming and climate change. Mineral carbonation with calcium or magnesium oxide (CaO or MgO) based adsorbents has been investigated to mitigate the impact of climate change on the human health and environment by reducing CO2 concentration in the flue gas and ambient air through adsorption. However, previous studies were generally conducted at higher pressure or higher temperature for applying the adsorbents to the post-capture process. In this study, we synthesized nanometer-sized crystalline MgO particles with a cubic shape (nanocubes) by an aerosol method (Self-combustion), and investigated the adsorption characteristics of the MgO nanocubes at room temperature (25 °C) by comparing with commercial MgO and CaO powders. The CO2 adsorption experiments for the three adsorbents were conducted using a batch type chamber after mixing the adsorbents with deionized water for making slurries. Based on the results of X-ray diffraction (XRD), scanning electron microscopy (SEM), and thermal gravimetric analysis (TGA), the MgO nanocubes showed higher captured CO2 per unit mass of the adsorbent than the commercial MgO and CaO adsorbents at the room temperature. Based on the comparison of the ambient hydration and carbonation processes of the two MgO adsorbents, commercial MgO showed similar carbonation kinetics with the commercial CaO. However, the MgO nanocubes showed enhanced carbonation kinetics than the other two commercial metal oxides at room temperature.

Interface-driven magnetic anisotropy of epitaxial [formula omitted] thin films

The effect of film-substrate interface on the magnetic anisotropy (MA) of epitaxial Fe4N thin films grown on single crystalline MgO (100) and MgO (111) substrates has been studied in this work. Both samples were grown simultaneously using a reactive dc magnetron sputtering process at a substrate temperature of 250°C. Single phase Fe4N samples were found to grow epitaxially on (100) and (111) oriented MgO substrates as confirmed from out-of-plane x-ray diffraction and in-plane ϕ-scan measurements. The MA of (100) and (111) oriented Fe4N samples was measured using longitudinal magneto-optical Kerr effect (MOKE). In spite of being grown simultaneously, the MA of both these samples was found to be different. Secondary ion mass spectroscopy and x-ray reflectivity measurements revealed the formation of a dense layer at the film-substrate interface. The MA of this dense layer along with the MA of Fe4N has a dramatic effect on magnetic reversal depending on the orientation of the substrate. Further, from the MOKE measurements, the difference in the magnetization switching processes was found to depend on the microstructure and orientation of Fe4N thin films.

Hydration structure of flat and stepped MgO surfaces.

We investigate the solvation structure of flat and stepped MgO(001) in neutral liquid water using ab initio molecular dynamics based on a hybrid density functional with dispersion corrections. Our simulations show that the MgO surface is covered by a densely packed layer of mixed intact and dissociated adsorbed water molecules in a planar arrangement with strong intermolecular H-bonds. The water dissociation fractions in this layer are >20% and >30% on the flat and stepped surfaces, respectively. Slightly above the first water layer, we observe metastable OH groups perpendicular to the interface, similar to those reported in low temperature studies of water monolayers on MgO. These species receive hydrogen bonds from four nearby water molecules in the first layer and have their hydrophobic H end directed toward bulk water, while their associated protons are bound to surface oxygens. The formation of these OH species is attributed to the strong basicity of the MgO surface and can be relevant for understanding various phenomena from morphology evolution and growth of (nano)crystalline MgO particles to heterogeneous catalysis.

Corrosion resistance and degradation behavior of anodized Mg-Gd alloys: A comparative study

Abstract A conventional anodic oxidation process was conducted on two Mg-Gd alloys to study the corrosion resistances and degradation behaviors of their anodized films, and a commercial AZ31 alloy was used for comparison. It was found that both the structures and degradation properties of the anodized films of Mg-Gd alloys were quite different from AZ31 alloy. For Mg-Gd alloys, the anodized films contained more crystalline MgO, Mg2SiO4 and Mg3(PO4)2 with lower porosity and small pore size compared with anodized AZ31 alloy. Although the anodized AZ31 alloy showed slightly higher initial corrosion resistance, its long-term degradation performance was uncompetitive compared with two anodized Mg-Gd alloys. And the scanning electrochemical microscopy (SECM) tests also pointed to more uniform degradation behaviors of anodized Mg-Gd alloys. The better long-term degradation performances of anodized Mg-Gd alloys, especially anodized Mg-1Zn-1Gd alloy, were believed to benefit from their different structures, compositions and corrosion products of anodized films compared with anodized AZ31 alloy.

Intensity and wavelength dependence of anisotropic nonlinear absorption inside MgO

We report an experimental measurement of nonlinear absorption by using the near-infrared femtosecond laser and a crystalline MgO sample. We show that the nonlinear absorption inside MgO presents a four-fold modulation as a function of the crystal alignment to the laser polarization. In different intensities and wavelengths, we observe that the modulation amplitude is changed. We introduce a model based on the calculation of the carrier density to explain the phenomenon. It shows that the varied modulation is due to the variation of effective electron mass, which is related to the crystal structure. Our work makes a step forward in the application of nonlinear absorption in exploring solid structure.

Ultrathin Single Crystalline MgO(111) Nanosheets*.

Synthesizing high-quality two-dimensional nanomaterials of nonlayered metal oxide is a challenge, especially when long-range single-crystallinity and clean high-energy surfaces are required. Reported here is the synthesis of single-crystalline MgO(111) nanosheets by a two-step process involving the formation of ultrathin Mg(OH)2 nanosheets as a precursor, and their selective topotactic conversion upon heating under dynamic vacuum. The defect-rich surface displays terminal -OH groups, three-coordinated O2- sites and low-coordinated Mg2+ sites, as well as single electrons trapped at oxygen vacancies, which render the MgO nanosheets highly reactive, as evidenced by the activation of CO molecules at low temperatures and pressures with formation of strongly adsorbed red-shifted CO and coupling of CO molecules into C2 species.

Dominant effect of anharmonicity on the equation of state and thermal conductivity of MgO under extreme conditions

Understanding heat transfer in the earth mantle region is of great scientific interest but highly challenging due to dissimilar conditions from our daily lives. Thermal conductivity of highly pressured and thermally activated materials can be completely different from that at ambient condition. In this study we calculate the equation of state and thermal conductivity of cubic crystalline MgO, a major component of the mantle, in such an environment. It is shown that the material properties are not accurately captured unless rigorous treatment of anharmonicity of phonon vibrational motion is explicitly applied. To predict renormalized phonon dispersion relations, interatomic force constants of high-order expansion are required, especially at elevated temperature. The anharmonicity plays an increasingly substantial role for regulating volume expansion as temperature rises, because the Gibbs free energy of MgO considerably deviates from what was predicted by the quasiharmonic approximation. We find that the lattice thermal conductivity of MgO and the heat transfer of the earth's mantle are reliably estimated only when both the temperature-dependent renormalization of phonon frequencies and four-phonon scattering effects are taken into consideration. We map the characteristics at Geotherm into contour diagrams in which equilibrium volumes and the thermal conductivities are described as functions of temperature and pressure. The thermal conductivity values of MgO in this study are smaller than those in previous theoretical studies due to the additional incorporation of four-phonon scatterings in the calculations. Specifically, deviation between the values in previous studies and in this study becomes larger as thermodynamic variables reach those near the core-mantle boundary. For instance, thermal conductivities of 52.15 and 23.67 $\phantom{\rule{0.16em}{0ex}}{\mathrm{Wm}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ are estimated at ambient and extreme conditions ($T=4000\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and $P=135\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$), respectively.

Effect of chloride ingress on self-healing recovery of smart cementitious composite incorporating crystalline admixture and MgO expansive agent

In this study, the effect of chloride environment containing various concentrations of chloride ion (Cl−) on self-healing performance of pre-cracked cementitious composite containing crystalline admixture (CA) and MgO expansive agent (MEA) was investigated under wet-dry cycles in chloride solutions. The results revealed that the Cl− changed the mineralogy of self-healing products, and consequently, affected the crack closure ratio and mechanical strength recovery. When self-healing occurred in distilled water, a large amount of ettringite (AFt) were detected, whereas in Cl− solution, monosulfate (AFm) was consumed by Cl− to form Friedel's salt (Fs), and then the Fs was decomposed to Al(OH)3(AH3) due to carbonation. During the multiphase conversion process, hydroxide (OH−) was released into crack solution, therefore the dissolved carbon dioxide (CO2) concentration was increased. The carbonation of the crystals formed in cracks was accelerated with the volume expansion, which achieved rapid crack sealing but contributed little to the mechanical performance recovery.

Influencing Martensitic Transition in Epitaxial Ni-Mn-Ga-Co Films with Large Angle Grain Boundaries

Magnetocaloric materials based on field-induced first order transformations such as Ni-Mn-Ga-Co are promising for more environmentally friendly cooling. Due to the underlying martensitic transformation, a large hysteresis can occur, which in turn reduces the efficiency of a cooling cycle. Here, we analyse the influence of the film microstructure on the thermal hysteresis and focus especially on large angle grain boundaries. We control the microstructure and grain boundary density by depositing films with local epitaxy on different substrates: Single crystalline MgO(0 0 1), MgO(1 1 0) and AlO(0 0 0 1). By combining local electron backscatter diffraction (EBSD) and global texture measurements with thermomagnetic measurements, we correlate a smaller hysteresis with the presence of grain boundaries. In films with grain boundaries, the hysteresis is decreased by about 30% compared to single crystalline films. Nevertheless, a large grain boundary density leads to a broadened transition. To explain this behaviour, we discuss the influence of grain boundaries on the martensitic transformation. While grain boundaries act as nucleation sites, they also lead to different strains in the material, which gives rise to various transition temperatures inside one film. We can show that a thoughtful design of the grain boundary microstructure is an important step to optimize the hysteresis.

Date pits extracts assisted synthesis of magnesium oxides nanoparticles and its application towards the photocatalytic degradation of methylene blue

In the present work, a novel bio-synthesis of highly crystalline cubic MgO nanoparticles from aqueous extracts obtained from date pits is reported. The synthesised cubic MgO nanoparticles were annealed at 500 and 700 °C for 2 h in air. Characterization of the samples was carried out using various characterization techniques. To show the photocatalytic effectiveness of date pits mediated MgO nanoparticles, degradation ability with Methylene blue (MB) under visible irradiation was evaluated.HRTEM analysis of the samples reveals a spherical-like structure of the nanoparticles while selected area electron diffraction and XRD patterns showed that the annealed nanoparticles are crystalline. The photocatalysis efficiency showed 64 and 52% degradation ability for MB of MgO nanoparticles annealed at 500 and 700 °C, respectively.

Evolution of PEO coatings on AM50 magnesium alloy using phosphate-based electrolyte with and without glycerol and its electrochemical characterization

Abstract PEO coatings were synthesized from phosphate-based (bP-PEO) and glycerol added phosphate-based (gP-PEO) electrolytes with different processing times. For both bP-PEO and gP-PEO coatings treated with different processing time its morphology, elemental and phase composition, and electrochemical behaviour has been comparatively investigated. For this, scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDXS), X-ray diffraction, electrochemical impedance spectroscopy (EIS), and potentiodynamic polarization techniques were employed. The gP-PEO coatings were observed to have higher pore density with reduced pores size, surface porosity, and average coating thickness compared to bP-PEO for all the PEO processing time. XRD studies revealed that glycerol addition resulted in the fostering of crystalline MgO (periclase) phase and promoting Mg3(PO4)2 (farringtonite) phase amorphization. In general, electrochemical behaviour showed improved corrosion behaviour for gP-PEO compared to bP-PEO.

The Electronic and Magnetic Properties of Ultrathin γ-Fe2O3 Films

.In this work, iron oxide γ-Fe2O3 films with thickness of 10 nm were grown on a single crystalline MgO (001) substrate by molecular beam epitaxy using oxidation of an iron thin film at substrate temperature 250 °C. The crystal structure, electronic, and magnetic properties of the ultrathin epitaxial γ-Fe2O3 films were investigated. X-ray photoemission spectroscopy, Low energy electron diffraction, and X-ray diffraction confirmed that films were single crystalline quality with the same orientation as the MgO substrates and had only a single phase of γ-Fe2O3. Furthermore, the magnetization measurements at 300 K showed that the ultrathin films were ferromagnetic with a magnetization value of 270 emu cm−3. These results show that it is possible to synthesize high-quality ultrathin γ-Fe2O3 films with good properties, which are promising as a spin filtering tunnel junction and for application in other spintronic devices, using the iron oxidation method.

Correlation between crystallization and oxidation process of ScN films exposed to air

Oxidation and hydrolysis are two spontaneous reactions occurring when scandium nitride (ScN) is exposed to air (oxygen and humidity). In order to evaluate the detrimental effect of the oxidation on functional properties of the material, ScN films were prepared under ultrahigh vacuum conditions at 700 °C onto two different substrates; crystalline MgO(0 0 1) and amorphous fused silica. X-ray Photoelectron Spectroscopy analysis was performed to study the oxidation reactions. The study also employed vacuum transferred pristine samples. Crystalline quality and oxidation resistance were found to be strongly correlated. Degradation of electrical and optical properties was observed for polycristalline films prepared on fused silica. On the other hand, highly ordered crystalline films grown on MgO(0 0 1) demonstrated surface passivation preventing further progress of the oxidation. Post-deposition exposure to air was found to be a principal source of the observed oxidation.