Magnesia Surface Research Articles

Simulation of fatty acid adsorption at the magnesia surface

In this study, wet surface modification converted the hydrophilic magnesia (MgO) surface to a hydrophobic surface, which is crucial for magnesia additives in polymer fire retardants, inks, cosmetics and paint and paper materials and in the flotation of magnesia. For this reason, the adsorption of fatty acids at the magnesia surface in the wet surface modification process was examined by molecular dynamics simulations. The sessile drop contact angles at the magnesia surface with monolayer adsorption of lauric acid (C12H24O2), oleic acid (C18H34O2) and stearic acid (C18H36O2) and the interfacial water structures at the magnesia surface with sub-monolayer, monolayer and bilayer fatty acid adsorption were analyzed and compared with the experimental results. The sessile drop contact angle at the magnesia surface increased as a function of the carbon (C) chain length with fatty acid adsorption and agreed with the experimental results. All the fatty acids were adsorbed at the magnesia surface, and the interaction energy results agreed with the adsorption phenomenon. A hydrophobic surface was established in monolayer stearic acid adsorption and was transformed into a hydrophilic surface in stearic acid bilayer adsorption.

Novel Alkaline Method for the Preparation of Low-Chromium Magnesia

The typical method for the preparation of chromium-magnesia is by energy-consuming grinding, but the pollution of Cr6+ ions is a problem. However, the chromium-magnesia refractory is still irreplaceable due to its outstanding performance. In this regard, low-chromium magnesia was prepared by the alkaline chromium precipitation method from cheap light-burned magnesia. The impurities in the light-burned magnesia was removed in order to avoid the formation of Cr6+. The chromium was precipitated directly on the light-burned magnesia surface without the grinding process. The low-chromium magnesia was obtained after the calcination. The reaction temperature, solid content, reaction time and stirring speed were examined and low-chromium magnesia with Cr3+ 0.4%, MgO 98.5% was obtained. The effect of temperature on the surface tomography of low-chromium magnesia was also discussed. The present novel alkaline method should be a promising way for the preparation of low-chromium magnesia.

Kinetic control of MgO hydration in refractory castables by using carboxylic acids

Based on a kinetic model that describes the hydration reaction via nucleation and growth mechanisms, formic, acetic or propionic acids were analyzed as hydration site activators on magnesia particles with different reactivity and granulometry used as a main binder source in high-alumina refractory castables. These additives induced the hydroxide generation in MgO suspensions and the hydration degree was proportional to the size of the carboxylate chain. Moreover, a more controlled growth of the brucite crystals could be achieved when formic acid was added to the refractory castables. By adjusting the amount of formic acid in the castable mixtures, large crack-free samples could be prepared, which pointed out the role of this additive to prevent the drawbacks associated with MgO hydration. Therefore, the suitable kinetic control of the nucleation and growth of brucite crystals on the magnesia surface allowed for the development of high-alumina magnesia-bonded castables.

Atomic scale behavior, growth morphology and magnetic properties of CoO on MgO(100) surface: a density functional study

In this study, we investigated the adsorption behaviors of the Co atom and CoO molecules on the MgO(100) surface, including the diffusion processes based on density functional theory calculations. The structure and growth morphology of the deposited cobalt oxide at various coverage levels as well as the magnetic properties are also discussed. The CoO molecule is found to bind more strongly to the surface compared to the isolated neutral Co atom, which is physically adsorbed on top of the surface oxygen site. Different to the isolated Co atom, the diffusion path of CoO molecule predominantly involves 90° rotation movements about its ends, while the surface hollow site acts as the saddle point for the hopping of the Co atom between two adjacent oxygen sites. A two-dimensional growth mode is observed to be the dominant layer growth mechanism for CoO on the magnesia surface, where the antiferromagnetic state is energetically more favorable compared to the ferromagnetic state. Based on the charge transfer and the density of state distribution, the interaction with the magnesia substrate does not affect the magnetic properties of the CoO monolayer.

Effect of SO<sub>2</sub> on Mn-Mg-O<sub>x</sub> Catalyst Treated by Non-Thermal Plasma for NO Catalytic Oxidation

The effect of MgO addition on SO2tolerance of MnOxfor the NO catalytic oxidation was investigated in this work. MgO addition significantly promotes the SO2resistance of MnOxat low temperature range of 50–250°C. The slight decrease in catalytic activity over Mn-Mg-Oxexposure to SO2is attributed to the partially deactivation of the catalytic active site poisoned by SO2. Through XRD and FT-IR characterization, MgO may effectively inhibit the competitive adsorption between NOxand SO2in manganese site, and enhance SO2adsorption on the magnesia surface.

Impact of isoelectric points of nanopowders in electrolytes on electrochemical characteristics of dye sensitized solar cells

Nanoparticle loaded quasi solid electrolytes are important from the view point of developing electrolytes for dye sensitized solar cells (DSSCs) having long term stability. The present work shows the influence of isoelectric point of nanopowders in electrolyte on the photoelectrochemical characteristics of DSSCs. Electrolytes with nanopowders of silica, alumina and magnesia which have widely differing isoelectric points are used in the study. Adsorption of ions from the electrolyte on the nanopowder surface, characterized by zeta potential measurement, show that cations get adsorbed on silica, alumina surface while anions get adsorbed on magnesia surface. The electrochemical characteristics of nanoparticulate loaded electrolytes are examined through cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). DSSCs fabricated using liquid, silica or alumina loaded electrolytes exhibit almost similar performance. But interestingly, the magnesia loaded electrolyte-based cell show lower short circuit current density (JSC) and much higher open circuit voltage (VOC), which is attributed to adsorption of anions. Such anionic adsorption prevents the dark reaction in magnesia loaded electrolyte-based cell and thus, enhances the VOC by almost 100 mV as compared to liquid electrolyte based cell. Also, higher electron life time at the titania/electrolyte interface is observed in magnesia loaded electrolyte-based cell as compared to others.

Chelants to inhibit magnesia (MgO) hydration

Magnesium oxide (MgO) presents excellent refractoriness and high resistance to basic slag. However, in the presence of water, MgO undergoes an expansive hydration reaction generating Mg(OH) 2, which can lead to swelling and cracking. In this work, additives called chelants were added to dead burnt magnesia suspensions in order to check their effectiveness as inhibitors of the magnesium oxide hydration. Zeta potential, ionic conductivity, pH and temperature measurements were used to provide information related to the magnesia surface and the chelant adsorption. Assessment of the hydration degree and volumetric expansion provided indications of the amount of Mg(OH) 2 formed, as well as its likelihood to damage the ceramic bodies. The results showed that citric acid can inhibit hydration to some extent, whereas ethylenediamine tetraacetic acid (EDTA) was more effective in preventing volume expansion. An addition of 0.3 wt% of these chelants was sufficient to prevent hydration and avoid expansion.

Surface pump-probe femtosecond-laser mass spectrometry: Time-, mass-, and velocity-resolved detection of surface reaction dynamics

A detailed account of the experimental methodology of surface pump-probe femtosecond-laser mass spectrometry is presented. This recently introduced technique enables the direct time-resolved investigation of surface reaction dynamics by monitoring the mass and the relative velocity of intermediates and products of a photoinduced surface reaction via multiphoton ionization. As a model system, the photodissociation dynamics of methyl iodide adsorbed at submonolayer coverage on magnesia ultrathin films is investigated. The magnesia surface preparation and characterization as well as the pulsed deposition of methyl iodide are described. The femtosecond-laser excitation (pump) and, in particular, the resonant multiphoton ionization surface detection (probe) schemas are discussed in detail. Results of pump-probe time-resolved methyl and iodine atom detection experiments are presented and the potential of this method for velocity-resolved photofragment analysis is evaluated.

ChemInform Abstract: Ultrathin Magnesia Films as Support for Molecules and Metal Clusters: Tuning Reactivity by Thickness and Composition

AbstractReview: 107 refs.

Femtosecond‐laser photoemission and photodesorption from magnesia supported gold clusters

AbstractNanosize gold clusters were grown by thermal evaporation on ultrathin magnesia films on Mo(100) and investigated by femtosecond (fs)‐laser photoemission spectroscopy. The surface work function in this system was found to increase with increasing nominal gold coverage. Methyl bromide (CH3Br) molecules adsorb molecularly on the magnesia surface as well as on the gold clusters. At sub‐monolayer coverage the CH3Br molecules were, however, considerably stronger bound to the gold particles than to the magnesia surface as revealed by temperature programmed desorption spectroscopy. To investigate the photoreaction dynamics of the adsorbate molecules fs‐laser pump–probe mass spectrometry was applied. In this new approach the methyl fragment appearance was monitored after photoexcitation of the adsorbed CH3Br with fs time resolution. The transient methyl data revealed that the CH3Br dissociation and subsequent methyl desorption proceeded in less than one picosecond. Most interestingly, a part of the transient signal could be unambiguously assigned to the dynamics of methyl desorption from the gold clusters. The appearance of this signal at about 270 fs after photoexcitation illustrates the massive distortion of the CH3Br electronic level structure due to the strong interaction with the gold clusters compared to the bare magnesia surface. magnified image

Ultrathin magnesia films as support for molecules and metal clusters: Tuning reactivity by thickness and composition

AbstractUltrathin metal oxide films have attracted considerable interest in recent years as versatile substrate for the design of nanocatalytic model systems. In particular, it has been proposed theoretically and confirmed experimentally that the electronic structure of adsorbates can be influenced by the layer thickness and the stoichiometry, i.e., the type and number of defects, of the oxide film. This has important consequences on the chemical reactivity of the oxide surface itself and of oxide supported metal clusters. It also opens new possibilities to influence and to control chemical reactions occurring at the surface of these systems. The present feature focuses on very recent experiments that illustrate the effects of a proper adjustment of layer thickness and composition of ultrathin MgO(100) films on chemical transformations. On the magnesia surface itself, the photodissociation dynamics of methyl iodide molecules is investigated via femtosecond‐laser pump–probe mass spectrometry. Furthermore, the catalytic oxidation of carbon monoxide at mass‐selected Au20 clusters deposited on magnesia is explored through temperature programmed reaction measurements. In the latter case, detailed first principles calculations are able to correlate the experimentally observed reactivity with structural dimensionality changes that are induced by the changing thickness and composition of the magnesia support. magnified image

Acid-basic, adsorption, and photosorption properties of magnesia samples

The acid-basic, adsorption, and photosorption properties of differently prepared magnesia samples were studied. It was found that the less basic magnesia surface photosorbs better oxygen and adsorbs better the water vapor.

Promotion of MgO addition on SO2 tolerance of Ag/Al2O3 for selective catalytic reduction of NO with methane at low temperature

Abstract The effect of MgO addition on SO 2 tolerance of Ag/Al 2 O 3 for the CH 4 -SCR was investigated. MgO addition significantly decreases the active temperature window of NO x conversion, and promotes the SO 2 resistance of Ag/Al 2 O 3 at low temperature range of 573–673 K. Silver oxide particles are well dispersed and large silver crystals are present in Ag/MgAl 2 O 4 catalyst. The slight decrease in catalytic activity over Ag/MgAl 2 O 4 exposure to SO 2 is attributed to the partially deactivation of alumina sites poisoned by SO 2 . MgO may effectively inhibit the competitive adsorption between NO x and SO 2 in alumina sites, and enhance SO 2 adsorption on the magnesia surface.

Differences in the metallic film growth mode between perfect and defective MgO surfaces

We have studied theoretically the differences in metallic film growth modes between perfect and defective oxide substrates by combining ab initio hybrid B3LYP periodic calculations on the corresponding 2D slab models of Me/MgO(0 0 1) interfaces (Me = Ag, Cu) with thermodynamic theory of solid solutions. We observed a strong change of the bonding between the metal adatoms and substrate in the vicinity of the surface F s centers (neutral O vacancies), which affects the thermodynamic conditions and the morphology of the growing metallic layer. For a perfect magnesia surface, we confirm the experimentally observed sub-monolayer growth of metallic islands (Ag possesses a higher trend toward such adatom aggregation than Cu). The surface F s centers weaken the trend toward metal atom aggregation and above some critical concentration lead to formation of disordered 2D metallic films; i.e. the island formation mode is replaced by the layer-by-layer growth mode.

Metal film growth on regular and defective MgO(0 0 1) surface: A comparative ab initio simulation and thermodynamic study

In order to understand the difference in metallic film growth modes on perfect and defective oxide substrates, we have combined ab initio B3LYP periodic calculations on the slab models of the corresponding Me/MgO(0 0 1) interfaces (Me = Ag, Cu) with thermodynamic theory of solid solutions. For a defectless magnesia surface, we confirm the experimentally observed submonolayer growth of 3D metallic islands (Ag possesses a higher trend than Cu). Formation of F s centers (neutral O vacancies) on the substrate markedly enhances metal atom adsorption as compared to physisorption over regular sites on a defect-free substrate. For the first time, we predict that the presence of these surface defects (beginning with concentrations of 5% for Cu and 22% for Ag) can stimulate the growth of uniform 2D metallic sublayers.

In SituFormation of Metal Nanoparticles on Ceramic Surface

A new preparation method for nanocomposites is proposed in which nanometer-scale nickel particles are precipitated homogeneously on the magnesia surface. Our approach is based on a conventional method that uses a reaction of difference in oxide stability in reductive atmosphere. We found that aluminum doping of the material precursor enabled preferential precipitation on the oxide substrate. We also have applied the preparation method for the nickel-copper/magnesia system. An addition of copper was effective for reducing the particle diameter. We found that morphology created by the method could be controlled by varying precursor composition.

Influence of magnesia surface on the setting time of magnesia–phosphate cement

The following paper presents the influence of magnesia reactivity in magnesia–phosphate cement. When water is added to cement, monoammonium dihydrogen phosphate (NH4H2PO4 or MAP) goes in solution till saturation while magnesia (MgO) is wetted and starts to dissociate. This dissociation only depends on MgO surface, except when magnesium carbonate is present. In this case, it accelerates the dissolution process, independently of the surface state. For magnesia, the higher the amount of surface defect sites, the higher the MgO wetting. Wetting and nucleation are promoted by a large interface between MgO and MAP, because adsorption probabilities are more important. Therefore, grinding a powder allows a better reactivity. On the contrary, calcination, in a first step, enhances the surface state and then reduces MgO reactivity due to the melting of grains, which reduces the total developed surface.

Theory of the growth mode for a thin metallic film on an insulating substrate

We have developed a novel theory predicting the growth mode of a thin metallic film on an insulating substrate. This combines ab initio electronic structure calculations for several ordered metal/insulator interfaces (varying both coverage and substrate lattice constant), with a thermodynamic approach based on microscopic calculations. We illustrate this approach for Ag film deposited on MgO(001) substrate. Ab initio calculations predict high mobility of adsorbed silver atoms on the perfect magnesia surface even at low temperatures. Our theoretical analysis clearly demonstrates that the growth of metallic islands is predominant at the initial stage of silver deposition, which agrees with the experimental data.

Appearance of Flashover Treeing on Polycrystalline Magnesia Surface

Appearance of Flashover Treeing on Polycrystalline Magnesia Surface

Mechanisms of catalytic activation of methane and ethylene on magnesia surface elucidated by isotopic kinetics

Activation of C nH 4 (n = 1,2) and H 2 on MgO, elucidated by new isotopic method, proceeds as a results of reversible adsorption with extraction of one atom H.