Infrared Characterization of Hydroxyl Groups on MgO: A Periodic and Cluster Density Functional Theory Study

Characterisation of Basic Sites on Ga2O3, MgO, and ZnO with Preadsorbed Ethanol and Ammonia-IR Study

The effect of adsorption of ethanol and ammonia on the basicity of Ga2O3, MgO, and ZnO was examined via IR studies of CO2 adsorption. Ethanol reacts with OH groups on Ga2O3, and MgO, forming ethoxyl groups. The substitution of surface hydroxyls by ethoxyls increases the basicity of the neighbouring oxygen. The ethoxyl groups that also form on ZnO do not contain surface OH groups, but the mechanism of their formation is different. On ZnO, ethoxy groups are formed by the reaction of ethanol with surface oxygens. The presence of ethoxyls on ZnO decreases the basicity because some surface oxygens are already engaged in the bonding of ethoxyl groups. The effect of ammonia adsorption on basicity is different for each oxide. For Ga2O3, ammonia adsorption increases the basicity of neighbouring oxygen sites. Ammonia is not adsorbed on MgO; therefore, it does not change the basicity of this oxide. Ammonia adsorbed on ZnO forms coordination bonds with Zn sites; it does not change the number of basic sites but changes how carbonate species are bonded to surface sites.

Water Film-Driven Brucite Nanosheet Growth and Stacking

Thin water films that form by the adhesion and condensation of air moisture on minerals can initiate phase transformation reactions with broad implications in nature and technology. We here show important effects of water film coverages on reaction rates and products during the transformation of periclase (MgO) nanocubes to brucite [Mg(OH)2] nanosheets. Using vibrational spectroscopy, we found that the first minutes to hours of Mg(OH)2 growth followed first-order kinetics, with rates scaling with water loadings. Growth was tightly linked to periclase surface hydration and to the formation of a brucite precursor solid, akin to poorly stacked/dislocated nanosheets. These nanosheets were the predominant forms of Mg(OH)2 growth in the 2D-like hydration environments of sub-monolayer water films, which formed below ∼50% relative humidity (RH). From molecular simulations, we infer that reactions may have been facilitated near surface defects where sub-monolayer films preferentially accumulated. In contrast, the 3D-like hydration environment of multilayered water films promoted brucite nanoparticle formation by enhancing Mg(OH)2 nanosheet growth and stacking rates and yields. From the structural similarity of periclase and brucite to other metal (hydr)oxide minerals, this concept of contrasting nanosheet growth should even be applicable for explaining water film-driven mineralogical transformations on other related nanominerals.

High Efficiency of Formaldehyde Removal and Anti-bacterial Capability Realized by a Multi-Scale Micro-Nano Channel Structure in Hybrid Hydrogel Coating Cross-Linked on Microfiber-Based Polyurethane

Inspired by the transpiration in the tree stem having a vertical and porous channel structure, high efficiency of formaldehyde removal is realized by the multi-scale micro-nano channel structure in a hybrid P(AAm/DA)-Ag/MgO hydrogel coating cross-linked on microfiber-based polyurethane. The present multi-scale channel structure is formed by a joint effect of directional freezing and redox polymerization as well as nanoparticles-induced porosity. Due to the large number of vertically aligned channels of micrometer size and an embedded porous structure of nanometer size, the specific surface area is significantly increased. Therefore, formaldehyde from solution can be rapidly adsorbed by the amine group in the hydrogels and efficiently degraded by the Ag/MgO nanoparticles. By only immersing in formaldehyde solution (0.2 mg mL^-1) for 12 h, 83.8% formaldehyde is removed by the hybrid hydrogels with a multi-scale channel structure, which is 60.8% faster than that observed in hydrogels without any channel structure. After cross-linking the hybrid hydrogels with a multi-scale channel structure to microfiber-based polyurethane and exposing to the formaldehyde vapor atmosphere, 79.2% formaldehyde is removed in 12 h, which is again 11.2% higher than that observed in hydrogels without any channel structure. Unlike the traditional approaches to remove formaldehyde by the light catalyst, no external conditions are required in our present hybrid hydrogel coating, which is very suitable for indoor use. In addition, due to the formation of free radicals by the Ag/MgO nanoparticles, the cross-linked hybrid hydrogel coating on polyurethane synthetic leather also shows good anti-bacterial capability. 99.99% of Staphylococcus aureus can be killed on the surface. Based on the good ability to remove formaldehyde and to kill bacteria, the obtained microfiber-based polyurethane cross-linked with a hybrid hydrogel coating containing a multi-scale channel structure can be used in a broad field of applications, such as furniture and car interior parts, to simultaneously solve the indoor air pollution and hygiene problems.

MgO nanocube hydroxylation by nanometric water films

Hydrophilic nanosized minerals exposed to air moisture host thin water films that are key drivers of reactions of interest in nature and technology. Water films can trigger irreversible mineralogical transformations, and control chemical fluxes across networks of aggregated nanomaterials. Using X-ray diffraction, vibrational spectroscopy, electron microscopy, and (micro)gravimetry, we tracked water film-driven transformations of periclase (MgO) nanocubes to brucite (Mg(OH)₂) nanosheets. We show that three monolayer-thick water films first triggered the nucleation-limited growth of brucite, and that water film loadings continuously increased as newly-formed brucite nanosheets captured air moisture. Small (8 nm-wide) nanocubes were completely converted to brucite under this regime while growth on larger (32 nm-wide) nanocubes transitioned to a diffusion-limited regime when (∼0.9 nm-thick) brucite nanocoatings began hampering the flux of reactive species. We also show that intra- and inter-particle microporosity hosted a hydration network that sustained GPa-level crystallization pressures, compressing interlayer brucite spacing during growth. This was prevalent in aggregated 8 nm wide nanocubes, which formed a maze-like network of slit-shaped pores. By resolving the impact of nanocube size and microporosity on reaction yields and crystallization pressures, this work provides new insight into the study of mineralogical transformations induced by nanometric water films. Our findings can be applied to structurally related minerals important to nature and technology, as well as to advance ideas on crystal growth under nanoconfinement.

Surface Hydroxyl Chemistry of Titania- and Alumina-Based Supports: Quantitative Titration and Temperature Dependence of Surface Brønsted Acid-Base Parameters

Surface hydroxyl groups on metal oxides play significant roles in catalyst synthesis and catalytic reactions. Despite the importance of surface hydroxyls in broader material applications, quantitative measurements of surface acid-base properties are not regularly reported. Here, we describe direct methods to quantify fundamental properties of surface hydroxyls on several titania- and alumina-based supports. Comparing commercially available anatase, rutile, P25, and P90 titania, thermogravimetric analysis (TGA) indicated that the total surface hydroxyl density varied by a factor of 2, and each surface hydroxyl is associated with approximately one weakly adsorbed water molecule. Proton-exchange site densities, determined at 25 °C with slurry acid-base titrations, led to several conclusions: (i) the intrinsic acidity/basicity of surface hydroxyls were similar regardless of the titania source; (ii) differences in the surface isoelectric point (IEP) were primarily attributable to differences in the surface concentration of acid and base sites; (iii) rutile has a higher surface concentration of basic hydroxyls, leading to a higher IEP; and (iv) P25 and P90 titania have slightly higher surface concentrationsof acidic hydroxyls relative to anatase or rutile. Temperature effects on surface acid-base properties are rarely reported yet are significant: from 5 to 65 °C, IEP values change by roughly one pH unit. The IEP changes were associated with large changes to the intrinsic acid-base equilibrium constants over this temperature range, rather than changes in the composition or concentration of the surface sites.

Unravelling the Mechanism of Intermediate-Temperature CO2 Interaction with Molten-NaNO3 -Salt-Promoted MgO

The optimization of MgO-based adsorbents as advanced CO₂ -capture materials is predominantly focused on their molten-salt modification, for which theoretical and experimental contributions provide great insights for their high CO₂ -capture performance. The underlying mechanism of the promotion effect of the molten salt on CO₂ capture, however, is a topic of controversy. Herein, advanced experimental characterization techniques, including in situ environmental transmission electron microscopy (eTEM) and CO₂ chemisorption by diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS), transient ¹⁸ O-isotopic exchange, and density functional theory (DFT), are employed to elucidate the mechanism of the CO₂ interaction with molten-salt-modified MgO in the 250-400 °C range. Herein, eTEM studies using low (2-3 mbar) and high (700 mbar) CO₂ pressures illustrate the dynamic evolution of the molten NaNO₃ salt promoted and unpromoted MgO carbonation with high magnification (<50 nm). The formation of ¹⁸ O-NaNO₃ (use of ¹⁸ O₂ ) and C¹⁶ O¹⁸ O following CO₂ interaction, verifies the proposed reaction path: conversion of NO₃ ^- (NO₃ ^-  → NO₂ ⁺  + O^2- ), adsorption of NO₂ ⁺ on MgO with significant weakening of CO₂ adsorption strength, and formation of [Mg²⁺ … O^2- ] ion pairs preventing the development of an impermeable MgCO₃ shell, which largely increases the rate of bulk MgO carbonation.

Fourier-transform infrared and X-ray diffraction analyses of the hydration reaction of pure magnesium oxide and chemically modified magnesium oxide

The magnesium hydroxide/magnesium oxide (Mg(OH)₂/MgO) system is a promising chemical heat storage system that utilizes unused heat at the temperature range of 200–500 °C. We have previously reported that the addition of lithium chloride (LiCl) and/or lithium hydroxide (LiOH) promotes the dehydration of Mg(OH)₂. The results revealed that LiOH primarily catalyzed the dehydration of the surface of Mg(OH)₂, while LiCl promoted the dehydration of bulk Mg(OH)₂. However, the roles of Li compounds in the hydration of MgO have not been discussed in detail. X-ray diffraction (XRD) and Fourier-transform infrared (FT-IR) techniques were used to analyze the effects of adding the Li compounds. The results revealed that the addition of LiOH promoted the diffusion of water into the MgO bulk phase and the addition of LiCl promoted the hydration of the MgO bulk phase. It was also observed that the concentration (number) of OH⁻ affected hydration. The mechanism of hydration of pure and LiCl- (or LiOH)-added MgO has also been discussed.

The effect of LiCl and LiOH on the hydration of MgO was investigated by XRD and FT-IR measurements, which can help to identify dopants that can effectively catalyze the Mg(OH)₂ dehydration and MgO hydration processes.

Peering into buried interfaces with X-rays and electrons to unveil MgCO3 formation during CO2 capture in molten salt-promoted MgO

The addition of molten alkali metal salts drastically accelerates the kinetics of CO₂ capture by MgO through the formation of MgCO₃ However, the growth mechanism, the nature of MgCO₃ formation, and the exact role of the molten alkali metal salts on the CO₂ capture process remain elusive, holding back the development of more-effective MgO-based CO₂ sorbents. Here, we unveil the growth mechanism of MgCO₃ under practically relevant conditions using a well-defined, yet representative, model system that is a MgO(100) single crystal coated with NaNO₃ The model system is interrogated by in situ X-ray reflectometry coupled with grazing incidence X-ray diffraction, scanning electron microscopy, and high-resolution transmission electron microscopy. When bare MgO(100) is exposed to a flow of CO₂, a noncrystalline surface carbonate layer of ca. 7-Å thickness forms. In contrast, when MgO(100) is coated with NaNO₃, MgCO₃ crystals nucleate and grow. These crystals have a preferential orientation with respect to the MgO(100) substrate, and form at the interface between MgO(100) and the molten NaNO₃ MgCO₃ grows epitaxially with respect to MgO(100), and the lattice mismatch between MgCO₃ and MgO is relaxed through lattice misfit dislocations. Pyramid-shaped pits on the surface of MgO, in proximity to and below the MgCO₃ crystals, point to the etching of surface MgO, providing dissolved [Mg²⁺…O^2-] ionic pairs for MgCO₃ growth. Our studies highlight the importance of combining X-rays and electron microscopy techniques to provide atomic to micrometer scale insight into the changes occurring at complex interfaces under reactive conditions.

Binding and stability of MgO monomers on anatase TiO2(101)

In catalysis, MgO is often used to modify the acid-base properties of support oxides and to stabilize supported metal atoms and particles on oxides. In this study, we show how the sublimation of MgO powder can be used to deposit MgO monomers, hither on anatase TiO₂(101). A combination of x-ray electron spectroscopy, high-resolution scanning tunneling microscopy, and density functional theory is employed to gain insight into the MgO monomer binding, electronic and vibrational properties, and thermal stability. In the most stable configuration, the Mg and O of the MgO monomer bind to two surface oxygens and one undercoordinated surface titanium, respectively. The additional binding weakens the Mg-O monomer bond and makes Mg more ionic. The monomers are thermally stable up to 600 K, where the onset of diffusion into the TiO₂ bulk is observed. The monomeric MgO species on TiO₂(101) represent an ideal atomically precise system with modified acid-base properties and will be employed in our future catalytic studies.

Two-Step Triethylamine-Based Synthesis of MgO Nanoparticles and Their Antibacterial Effect against Pathogenic Bacteria

Magnesium oxide nanoparticles (MgO NPs) were obtained by the calcination of precursor microparticles (PM) synthesized by a novel triethylamine-based precipitation method. Scanning electron microscopy (SEM) revealed a mean size of 120 nm for the MgO NPs. The results of the characterizations for MgO NPs support the suggestion that our material has the capacity to attack, and have an antibacterial effect against, Gram-negative and Gram-positive bacteria strains. The ability of the MgO NPs to produce reactive oxygen species (ROS), such as superoxide anion radicals (O2•-) or hydrogen peroxide (H2O2), was demonstrated by the corresponding quantitative assays. The MgO antibacterial activity was evaluated against Gram-positive Staphylococcus aureus and Gram-negative Escherichia coli bacteria, with minimum inhibitory concentrations (MICs) of 250 and 500 ppm on the microdilution assays, respectively. Structural changes in the bacteria, such as membrane collapse; surface changes, such as vesicular formation; and changes in the longitudinal and horizontal sizes, as well as the circumference, were observed using atomic force microscopy (AFM). The lipidic peroxidation of the bacterial membranes was quantified, and finally, a bactericidal mechanism for the MgO NPs was also proposed.

Investigations on the Temperature-Dependent Interaction of Water Vapor with Tin Dioxide and Its Implications on Gas Sensing

This work presents an operando infrared spectroscopic study of the temperature-dependent water adsorption on pristine SnO₂ surfaces and discusses the possible implications on the oxygen ionosorption and gas-sensing mechanism. The impact of water on the sensor resistance, CO-sensing performance, and CO conversion was studied, and the obtained phenomenological results provide the basis for discussing the operando spectroscopic investigation findings. The provided information allows identification of three different water adsorption regimes ranging from physisorption and dominant associative adsorption to mainly dissociative water adsorption. In these regions, water has different impacts on the surface composition, sensor resistance, and gas-sensing performance.

Origin of enhanced boric acid adsorption in light-burned magnesium oxide

Light-burned magnesium oxide (MgO) possesses a high surface area and has attracted interest as a promising candidate for boron adsorption materials; however, the detailed molecular structures decisive for enhancing the adsorption performance have not yet been elucidated. Here, the origin of enhanced boric acid adsorption for the light-burned MgO is studied by multiple probes, including positronium (Ps) annihilation spectroscopy, Fourier transform infrared spectroscopy, and sorption experiments coupled with molecular simulations. The state-of-the-art technique of open space analysis using Ps revealed the detailed structure of the interfaces between MgO nanograins: ∼10 Å and ∼30 Å open spaces, participating in the chemisorption of B(OH)₄ ^- and BO₃ ^3- simultaneously with the physisorption of neutral B(OH)₃ molecules. Furthermore, in addition to the fraction of open spaces, a proton quasi-layer formed on the interior surfaces of the above-mentioned angstrom-scale open spaces was identified to be attributable for enhancing both the chemisorption and physisorption.

Influence of MgO-Lignin Dual Component Additives on Selected Properties of Low Density Polyethylene

The presented study describes the application of lignin-based dual component fillers into low-density polyethylene (LDPE) and an examination of their selected properties. The main experimental procedure was focused on the preparation of thin sheet films using polyethylene and its composites with 5% by wt. of fillers: MgO, MgO-lignin dual phase systems with varying amounts of lignin and pristine lignin. Analysis of morphology revealed that elongated voids appeared in the structure for hybrid filler with a higher content of lignin (min. 50% by wt. of lignin versus MgO) and also for pristine lignin. Moreover, the prepared sheets were subjected to the thermoforming process by using the positive forming method (male mold). The thermoforming ability of all composites was evaluated by means of a comparison of wall thickness distribution on thermoformed shapes. The most noticeable percentage of wall thinning occurred for films which consisted of LDPE/MgO-lignin (5:1 wt./wt.) composite. In contrast, the best material arrangement and the highest mean percentage wall thickness were observed in the case of the shape formed with LDPE/MgO-lignin (1:5 wt./wt.). In addition, as part of research studies, the measurements of the contact angle have been conducted. The analysed LDPE films, in particular LDPE/MgO-L, have been recognized as materials with high wettability.

Catalytic activity, water formation, and sintering: Methane activation over Co- and Fe-doped MgO nanocrystals

Microstructure, structure, and compositional homogeneity of metal oxide nanoparticles can change dramatically during catalysis. Considering the different stabilities of cobalt and iron ions in the MgO host lattice [M. Niedermaier et al., J. Phys. Chem. C 123, 25991 (2019)], we employed MgO nanocube powders with or without transition metal admixtures for the oxidative coupling of methane (OCM) reaction to analyze characteristic differences in catalytic activity and sintering behavior. Undoped MgO nanocrystals exhibit the highest C₂ selectivity and retain the nanocrystallinity of the starting material after 24 h time on stream. For the Co-Mg-O nanoparticle powder, which exhibits the highest activity and CO_x selectivity and where OCM-induced coarsening is strongest, we found that the Co²⁺ ions remain homogeneously distributed over the MgO lattice. Trivalent Fe ions migrate to the surface of Fe-Mg-O nanoparticles where they form a magnesioferrite phase (MgFe₂O₄) with a characteristic impact on catalytic performance: Fe-Mg-O is initially less selective than MgO despite its lower activity. An increase in C₂ selectivity and a decrease in the CO₂/CO ratio with time on stream are attributed to the increasing fraction of coarsened particles that become depleted in redox active Fe. Surface water is a by-product of the OCM reaction, favors mass transport across the particle surfaces, and serves as a sintering aid during catalysis. The characteristic changes in size and morphology of MgO, Co-doped, and Fe-doped MgO particles can be consistently explained by activity and C₂ selectivity trends. The original morphology of the nanocubes as a starting material for the OCM reaction does not impact the catalytic activity.

Comparison of the Effect of Coaddition of Li Compounds and Addition of a Single Li Compound on Reactivity and Structure of Magnesium Hydroxide

Mg(OH)₂ is a chemical heat storage material that is studied for the utilization of 300-350 °C waste heat. In this study, LiCl and LiOH were coadded to Mg(OH)₂, and the reactivity and structural evolution were investigated. In the hydration of samples at 200 °C subsequent to dehydration at 270 °C, Mg(OH)₂ with coadded LiCl and LiOH showed excellent hydration reactivity, with a heat output density of 1053 kJ kg^-1. The coaddition of LiCl and LiOH enhanced both the dehydration and the hydration reactivity of Mg(OH)₂. X-ray diffraction analysis indicated that the addition of LiOH to Mg(OH)₂ promoted the decomposition of Mg(OH)₂ and the diffusion of water on the surface of Mg(OH)₂, whereas the addition of LiCl to Mg(OH)₂ promoted these processes in the bulk phase of Mg(OH)₂.

O2 reduction on a Au film electrode in an ionic liquid in the absence and presence of Mg2+ ions: Product formation and adlayer dynamics

Aiming at a detailed understanding of the interaction between an ionic liquid, O₂, and electrodes in Mg-air batteries, we performed a combined differential electrochemical mass spectrometry and in situ infrared spectroscopy model study on the interaction between the ionic liquid (IL) 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl) imide (BMP-TFSI) and a gold film electrode in the presence and absence of O₂ and Mg²⁺ ions in the potential range relevant for the oxygen reduction reaction (ORR) and evolution reaction. Detailed information on the dynamic exchange of adsorbed ions, on the stability/decomposition of the ionic liquid, and on the activity/selectivity/reversibility of the ORR is derived from measurements performed under potentiodynamic and potentiostatic conditions. In neat BMP-TFSI, we find the dynamics of the potential induced exchange of adsorbed ions to depend significantly on the exchange direction. In the presence of O₂, the anions formed in the ORR distinctly affect the adsorption characteristics of the IL ions and the exchange dynamics. Furthermore, the ORR changes from reduction to superoxide anions at moderate potentials to reduction to peroxide anion at more negative potentials. In the additional presence of Mg²⁺ ions, dominant magnesium peroxide and oxide formation result in an irreversible ORR, in contrast to the requirements of an efficient re-chargeable Mg-air battery. In addition, these ions result in the increasing formation of a blocking adlayer, reducing the coverage of adsorbed IL species.

Fewer Defects in the Surface Slows the Hydrolysis Rate, Decreases the ROS Generation Potential, and Improves the Non-ROS Antimicrobial Activity of MgO

Magnesium oxide (MgO) is recognised as exhibiting a contact-based antibacterial activity. However, a comprehensive study of the impact of atomic-scale surface features on MgO's antibacterial activity is lacking. In this study, the nature and abundance of the native surface defects on different MgO powders are thoroughly investigated. Their impacts on the hydrolysis kinetics, antibacterial activity against Escherichia coli (ATCC 47076), Staphylococcus epidermidis and Pseudomonas aeruginosa and the reactive oxygen species (ROS) generation potential are determined and explained. It is shown that a reduction in the abundance of low-coordinated oxygen atoms on the surface of the MgO improves its resistance to both hydrolysis and antibacterial activity. The ROS generation potential, determined in-situ using a fluorescence microplate assay and electron paramagnetic resonance spectroscopy, is not an inherent property of the studied MgO, rather it is a side product of hydrolysis (only for the most highly defected MgO particles) and/or a consequence of the MgO/bacteria interaction. The evaluation of the mutual correlations of the hydrolysis, the antibacterial activity and the ROS generation, with their origin in the surface defects' peculiarities, led to the conclusion that the acid/base reaction between the MgO surface and the bacterial wall contributes considerably to the MgO's antibacterial activity.

Effective remediation of low-concentration cadmium in groundwater using nano-scale magnesia

Cadmium (Cd), one of the hazardous elements in groundwater, is a severe threat to human health and ecological systems even at low concentrations. This study explores the effectiveness of commercial and self-synthesized nano-scale magnesia (NMgO) for remediating low-concentration Cd in groundwater as well as their associated removal mechanisms. The sorption kinetic data for both NMgOs were well fitted to the pseudo-second-order model and the calculated q _e values matched the experimental q _e values for both commercial and self-synthesized NMgOs. The sorption equilibrium data for both NMgOs were well fitted to the Langmuir isotherm model, with the maximum Cd sorption capacity (q _e) of 19.25 and 16.54 mg/g at an initial concentration range of 5-200 μg/L and a temperature of 25 °C, for commercial and self-synthesized NMgOs, respectively. The combined sorption kinetics and equilibrium data suggest that the sorption onto both NMgOs follows a monolayer chemisorption. The scanning electron microscope-energy dispersive X-ray (SEM-EDX), Fourier transform infrared spectroscopy (FTIR), and X-ray diffractometer (XRD) analyses show that the chemisorption of Cd onto commercial NMgO is due to the formation of Cd(OH)₂, which precipitates on the sorbent surface. For the self-synthesized NMgO, it was demonstrated that the hydroxyl group plays a role in the chemisorption process and the amount of Cd sorbed on the sorbent was quantified. The results of batch experiments showed that both NMgOs removed Cd effectively, obtaining a removal efficiency of more than 99%, under different experimental conditions of pH, sorbent dosage, co-existing ions, and simulated groundwater. Results from both the sorption isotherm and desorption experiments indicated strong bonding between Cd and both NMgOs, suggesting that NMgOs are safe, effective, and practical sorbents to remediate Cd in groundwater.

Predicting size-dependent emergence of crystallinity in nanomaterials: titania nanoclusters versus nanocrystals

Bottom-up and top-down derived nanoparticle structures refined by accurate ab initio calculations are used to investigate the size dependent emergence of crystallinity in titania from the monomer upwards. Global optimisation and data mining are used to provide a series of (TiO₂)_N global minima candidates in the range N = 1-38, where our approach provides many new low energy structures for N > 10. A range of nanocrystal cuts from the anatase crystal structure are also considered up to a size of over 250 atoms. All nanocrystals considered are predicted to be metastable with respect to non-crystalline nanoclusters, which has implications with respect to the limitations of the cluster approach to modelling large titania nanosystems. Extrapolating both data sets using a generalised expansion of a top-down derived energy expression for nanoparticles, we obtain an estimate of the non-crystalline to crystalline crossover size for titania. Our results compare well with the available experimental results and imply that anatase-like crystallinity emerges in titania nanoparticles of approximately 2-3 nm diameter.

Surface chemistry of MgO/SiO2 catalyst during the ethanol catalytic conversion to 1,3-butadiene: in-situ DRIFTS and DFT study

1,3-Butadiene is an important commodity chemical and new, selective routes of catalytic synthesis using green feedstocks, such as ethanol, is of interest.

Participation of interfacial hydroxyl groups in the water-gas shift reaction over Au/MgO catalysts

OH at 3750 cm⁻¹ reacts with CO only in the presence of Au, indicating the importance of the Au/MgO interface.

Homogeneity of Surface Sites in Supported Single-Site Metal Catalysts: Assessment with Band Widths of Metal Carbonyl Infrared Spectra

Determining and controlling the uniformity of isolated metal sites on surfaces of supports are central goals in investigations of single-site catalysts because well-defined species provide opportunities for fundamental understanding of the surface sites. CO is a useful probe of surface metal sites, often reacting with them to form metal carbonyls, the infrared spectra of which provide insights into the nature of the sites and the metal-support interface. Metals bonded to various support surface sites give broad bands in the spectra, and when narrow bands are observed, they indicate a high degree of uniformity of the metal sites. Much recent work on single-site catalysts has been done with supports that are inherently nonuniform, giving supported metal species that are therefore nonuniform. Herein we summarize values of ν_CO data characterizing supported iridium gem-dicarbonyls, showing that the most nearly uniform of them are those supported on zeolites and the least uniform are those supported on metal oxides. Guided by ν_CO data of supported iridium gem-dicarbonyls, we have determined new, general synthesis methods to maximize the degree of uniformity of iridium species on zeolites and on MgO. We report results for a zeolite HY-supported iridium gem-dicarbonyl with full width at half-maximum values of only 4.6 and 5.2 cm^-1 characterizing the symmetric and asymmetric CO stretches and implying that this is the most nearly uniform supported single-site metal catalyst.

Unique reaction mechanism of preferential oxidation of CO over intermetallic Pt3Co catalysts: surface-OH-mediated formation of a bicarbonate intermediate

A unique and novel reaction mechanism for the preferential oxidation of CO involving surface-OH-derived bicarbonate as an intermediate is reported.

Enhancing the Heat Transfer Efficiency in Graphene-Epoxy Nanocomposites Using a Magnesium Oxide-Graphene Hybrid Structure

Composite materials, such as organic matrices doped with inorganic fillers, can generate new properties that exhibit multiple functionalities. In this paper, an epoxy-based nanocomposite that has a high thermal conductivity and a low electrical conductivity, which are required for the use of a material as electronic packaging and insulation, was prepared. The performance of the epoxy was improved by incorporating a magnesium oxide-coated graphene (MgO@GR) nanomaterial into the epoxy matrix. We found that the addition of a MgO coating not only improved the dispersion of the graphene in the matrix and the interfacial bonding between the graphene and epoxy but also enhanced the thermal conductivity of the epoxy while preserving the electrical insulation. By adding 7 wt % MgO@GR, the thermal conductivity of the epoxy nanocomposites was enhanced by 76% compared with that of the neat epoxy, and the electrical resistivity was maintained at 8.66 × 10(14) Ω m.

Ternary Ag/MgO-SiO2 catalysts for the conversion of ethanol into butadiene

Ternary Ag/Magnesia-silica catalysts were tested in the direct synthesis of 1,3-butadiene from ethanol. The influence of the silver content and the type of silica source on catalytic performance has been studied. Prepared catalysts were characterized by (29) Si NMR, N2 sorption, small-angle X-ray scattering measurements, XRD, environmental scanning electron microscopy with energy dispersive X-ray analysis (ESEM/EDX), FTIR spectroscopy of adsorbed pyridine and CO2 , temperature-programmed desorption of CO2 and UV/Vis diffuse reflectance spectroscopy. Based on these characterization results, the catalytic performance of the catalysts in the 1,3-butadiene formation process was interpreted and a tentative model explaining the role of the different catalytically active sites was elaborated. The balance of the active sites is crucial to obtain an active and selective catalyst to form 1,3-butadiene from ethanol. The optimal silver loading is 1-2 wt% on a MgO-silica support with a molar Mg/Si ratio of 2. The silver species and basic sites (Mg−O pairs and basic OH groups) are of prime importance in the 1,3-butadiene production, catalyzing mainly the ethanol dehydrogenation and the aldol condensation, respectively.

Magnesium-based systems for carbon dioxide capture, storage and recycling: from leaves to synthetic nanostructured materials

Magnesium is used as leitmotif in this review in order to explore the systems involved in natural and artificial CO₂ cycles.

On the relationship between the basicity of a surface and its ability to catalyze transesterification in liquid and gas phases: the case of MgO

The influence of the basic properties of MgO is not the same for liquid and for gas phase transesterification.

Vibrational Analysis of Brucite Surfaces and the Development of an Improved Force Field for Molecular Simulation of Interfaces

We introduce a nonbonded three-body harmonic potential energy term for Mg-O-H interactions for improved edge surface stability in molecular simulations. The new potential term is compatible with the Clayff force field and is applied here to brucite, a layered magnesium hydroxide mineral. Comparisons of normal mode frequencies from classical and density functional theory calculations are used to verify a suitable spring constant (k parameter) for the Mg-O-H bending motion. Vibrational analysis of hydroxyl librations at two brucite surfaces indicates that surface Mg-O-H modes are shifted to frequencies lower than the corresponding bulk modes. A comparison of DFT and classical normal modes validates this new potential term. The methodology for parameter development can be applied to other clay mineral components (e.g., Al, Si) to improve the modeling of edge surface stability, resulting in expanded applicability to clay mineral applications.

Electronic and chemical properties of a surface-terminated screw dislocation in MgO

Dislocations represent an important and ubiquitous class of topological defect found at the surfaces of metal oxide materials. They are thought to influence processes as diverse as crystal growth, corrosion, charge trapping, luminescence, molecular adsorption, and catalytic activity; however, their electronic and chemical properties remain poorly understood. Here, through a detailed first-principles investigation into the properties of a surface-terminated screw dislocation in MgO we provide atomistic insight into these issues. We show that surface dislocations can exhibit intriguing electron trapping properties which are important for understanding the chemical and electronic characteristics of oxide surfaces. The results presented in this article taken together with recent experimental reports show that surface dislocations can be equally as important as more commonly considered surface defects, such as steps, kinks, and vacancies, but are now just beginning to be understood.