Efficient Preparation and Catalytic Activity of MgO(111) Nanosheets

Facile Synthesis of Ni-MgO/CNT Nanocomposite for Hydrogen Evolution Reaction

In this study, the pristine MgO, MgO/CNT and Ni-MgO/CNT nanocomposites were processed using the impregnation and chemical vapor deposition methods and analyzed for hydrogen evolution reaction (HER) using the electrochemical water splitting process. Furthermore, the effect of nickel on the deposited carbon was systematically elaborated in this study. The highly conductive carbon nanotubes (CNTs) deposited on the metal surface of the Ni-MgO nanocomposite heterostructure provides a robust stability and superior electrocatalytic activity. The optimized Ni-MgO/CNT nanocomposite exhibited hierarchical, helical-shaped carbon nanotubes adorned on the surface of the Ni-MgO flakes, forming a hybrid metal-carbon network structure. The catalytic HER was carried out in a 1M alkaline KOH electrolyte, and the optimized Ni-MgO/CNT nanocomposite achieved a low (117 mV) overpotential value (ɳ) at 10 mA cm-2 and needed a low (116 mV/dec) Tafel value, denotes the Volmer-Heyrovsky pathway. Also, the high electrochemical active surface area (ECSA) value of the Ni-MgO/CNT nanocomposite attained 515 cm2, which is favorable for the generation of abundant electroactive species, and the prepared electrocatalyst durability was also performed using a chronoamperometry test for the prolonged duration of 20 h at 10 mA cm-2 and exhibited good stability, with a 72% retention. Hence, the obtained results demonstrate that the optimized Ni-MgO/CNT nanocomposite is a highly active and cost-effective electrocatalyst for hydrogen energy production.

Mechanistic insights into the selective adsorption of phosphorus from wastewater by MgO(100)-functionalized cellulose sponge

Metal oxides have remained state-of-the-art adsorbents for recovering phosphorus from aqueous solutions, but their practical application is still limited by their unsatisfactory adsorption capacities and selectivities in wastewater. Here, using MgO as a model metal oxide, the strategy of employing porous cellulose sponge to support metal oxides featuring exposed specific crystal facets was proposed to develop promising phosphate adsorbents. The phosphate adsorption isotherms and kinetics were measured and the phosphate adsorption mechanism was explored. The results show that cellulose sponge-supported MgO(100) (C-MgO(100)) has a saturation capacity of 28.3 mg P/g, over ten times higher than MgO(100) particles. Importantly, the phosphate adsorption properties of C-MgO(100) are almost not affected in wastewater, demonstrating its exceptional selectivity for phosphate adsorption. In contrast, the saturation capacity of MgO(111)-functionalized cellulose sponge is obviously declined in wastewater. Experimental together with theoretical analyses indicate that phosphate is chemically adsorbed on C-MgO(100) with obvious electrons transfer from the p-orbital of phosphate, and the adsorption energy of C-MgO(100) towards phosphate is maintained in the presence of coexisting anions. Ultimately, regeneration experiments reveal that a regenerant formulation composed of KOH (wt.1 %) and tap water is suitable for the regeneration of C-MgO(100) with >82.6 % phosphate desorption efficiencies after 5 cycles, further confirming its potential in practical application for the treatment of real water.

Dispersed surface Ru ensembles on MgO(111) for catalytic ammonia decomposition

Ammonia is regarded as an energy vector for hydrogen storage, transport and utilization, which links to usage of renewable energies. However, efficient catalysts for ammonia decomposition and their underlying mechanism yet remain obscure. Here we report that atomically-dispersed Ru atoms on MgO support on its polar (111) facets {denoted as MgO(111)} show the highest rate of ammonia decomposition, as far as we are aware, than all catalysts reported in literature due to the strong metal-support interaction and efficient surface coupling reaction. We have carefully investigated the loading effect of Ru from atomic form to cluster/nanoparticle on MgO(111). Progressive increase of surface Ru concentration, correlated with increase in specific activity per metal site, clearly indicates synergistic metal sites in close proximity, akin to those bimetallic N2 complexes in solution are required for the stepwise dehydrogenation of ammonia to N2/H2, as also supported by DFT modelling. Whereas, beyond surface doping, the specific activity drops substantially upon the formation of Ru cluster/nanoparticle, which challenges the classical view of allegorically higher activity of coordinated Ru atoms in cluster form (B5 sites) than isolated sites.

A theory for the stabilization of polar crystal surfaces by a liquid environment

Polar crystal surfaces play an important role in the functionality of many materials and have been studied extensively over many decades. In this article, a theoretical framework is presented that extends existing theories by placing the surrounding solution environment on an equal footing with the crystal itself; this is advantageous, e.g., when considering processes such as crystal growth from solution. By considering the polar crystal as a stack of parallel plate capacitors immersed in a solution environment, the equilibrium adsorbed surface charge density is derived by minimizing the free energy of the system. In analogy to the well-known diverging surface energy of a polar crystal surface at zero temperature, for a crystal in solution it is shown that the "polar catastrophe" manifests as a diverging free energy cost to perturb the system from equilibrium. Going further than existing theories, the present formulation predicts that fluctuations in the adsorbed surface charge density become increasingly suppressed with increasing crystal thickness. We also show how, in the slab geometry often employed in both theoretical and computational studies of interfaces, an electric displacement field emerges as an electrostatic boundary condition, the origins of which are rooted in the slab geometry itself, rather than the use of periodic boundary conditions. This aspect of the work provides a firmer theoretical basis for the recent observation that standard "slab corrections" fail to correctly describe, even qualitatively, polar crystal surfaces in solution.

Electrochemical Growth of Mg(OH) x Layered Films Stacked Parallel to the Substrates and Their Thermal Conversion to (111)-Oriented Nanoporous MgO Films

Stacking layered metal hydroxide films parallel to a substrate is challenging. Here, we demonstrate a simple and rapid electrodeposition method for stacking magnesium hydroxide layered films. Room-temperature cathodic electrolysis (40 mA cm^-2) in a Mg(NO₃)₂ aqueous solution induces the deposition of ⟨001⟩-oriented Mg(OH) _x layered films stacked parallel to the substrate at the deposition rate of ∼2 μm min^-1. The obtained Mg(OH) _x layered films undergo an overall oriented transformation by heat treatment to form ⟨111⟩-oriented nanoporous MgO films.

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 O^2- 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.

Slow and Sustained Release of Carbonate Ions from Amino Acids for Controlled Hydrothermal Growth of Alkaline-Earth Carbonate Single Crystals

Alkaline-earth metal carbonate materials have attracted wide interest because of their high value in many applications. Various sources of carbonate ions (CO₃ ^2-), such as CO₂ gas, alkaline-metal carbonate salts, and urea, have been reported for the synthesis of metal carbonate crystals, yet a slow and sustained CO₃ ^2- release approach for controlled crystal growth is much desired. In this paper, we demonstrate a new chemical approach toward slow and sustained CO₃ ^2- release for hydrothermal growth of large alkaline-earth metal carbonate single crystals. Such an approach is enabled by the multiple hydrolysis of a small basic amino acid (arginine, Arg). Namely, the amino groups of Arg hydrolyze to form OH^- ions, making the solution basic, and the hydrolysis of the guanidyl group of Arg is hydrothermally triggered to produce urea and ammonia, followed by the hydrolysis of urea to produce CO₂ and ammonia and then the release of CO₃ ^2- because of the reaction between CO₂ and the OH^- ions hydrolyzed from ammonia. Such a CO₃ ^2- release behavior enables the slow and controlled growth of various carbonate single crystals over a wide range of pH values. The growth of uniform rhombohedron MgCO₃ single crystals with variable morphologies and crystal sizes is studied in detail. The influences of reaction temperature, solution pH, precursor type, and concentration on the morphology and size of the resulting MgCO₃ crystals are elucidated. The crystal evolution mechanism is also proposed and discussed with various supportive data.

Photocatalytic water splitting by N-TiO2 on MgO (111) with exceptional quantum efficiencies at elevated temperatures

Photocatalytic water splitting is attracting enormous interest for the storage of solar energy but no practical method has yet been identified. In the past decades, various systems have been developed but most of them suffer from low activities, a narrow range of absorption and poor quantum efficiencies (Q.E.) due to fast recombination of charge carriers. Here we report a dramatic suppression of electron-hole pair recombination on the surface of N-doped TiO₂ based nanocatalysts under enhanced concentrations of H⁺ and OH^-, and local electric field polarization of a MgO (111) support during photolysis of water at elevated temperatures. Thus, a broad optical absorption is seen, producing O₂ and H₂ in a 1:2 molar ratio with a H₂ evolution rate of over 11,000 μmol g^-1 h^-1 without any sacrificial reagents at 270 °C. An exceptional range of Q.E. from 81.8% at 437 nm to 3.2% at 1000 nm is also reported.

Carbon Capture by Metal Oxides: Unleashing the Potential of the (111) Facet

Solid metal oxides for carbon capture exhibit reduced adsorption capacity following high-temperature exposure, due to surface area reduction by sintering. Furthermore, only low-coordinate corner/edge sites on the thermodynamically stable (100) facet display favorable binding toward CO₂, providing inherently low capacity. The (111) facet, however, exhibits a high concentration of low-coordinate sites. In this work, MgO(111) nanosheets displayed high capacity for CO₂, as well as a ∼65% increase in capacity despite a ∼30% reduction in surface area following sintering (0.77 mmol g^-1 @ 227 m² g^-1 vs 1.28 mmol g^-1 @ 154 m² g^-1). These results, unique to MgO(111), suggest intrinsic differences in the effects of sintering on basic site retention. Spectroscopic and computational investigations provided a new structure-activity insight: the importance of high-temperature activation to unleash the capacity of the polar (111) facet of MgO. In summary, we present the first example of a faceted sorbent for carbon capture and challenge the assumption that sintering is necessarily a negative process; here we leverage high-temperature conditions for facet-dependent surface activation.

Characterization of the surface properties of MgO using paper spray mass spectrometry

RATIONALE

Significant advances have been made in the preparation of different morphologies of magnesium oxide (MgO), but the relationship between MgO morphology and its interactions with therapeutic drugs is rarely studied. Herein, we investigated the interactions between different morphologies of MgO and therapeutic drugs using paper spray mass spectrometry.

METHODS

Different morphologies of MgO including trapezoidal, needle-like, flower-like and nest-like structures were prepared through a facile precipitation method. The as-obtained MgO particles were then coated onto the surface of filter paper via vacuum filtration strategy. The coated papers with different morphologies of MgO were used as the substrates for paper spray mass spectrometry to explore the interactions between different MgO and therapeutic drugs.

RESULTS

Through investigating the interactions between different morphologies of MgO coated papers and therapeutic drugs, it demonstrated that, in contrast to the trapezoidal, needle-like and nest-like MgO coated papers, different drugs in dried blood spots (DBS) were more favourably eluted off from the paper coated with flower-like MgO due to its weaker surface basicity. Also, the signal intensities of different drugs during paper spray were highly dependent on their elution behaviours.

CONCLUSIONS

Paper spray mass spectrometry (MS) provides an avenue to elaborate the surface properties of MgO with different structures. The surface basicity of MgO played a crucial role in determining the elution behaviours of therapeutic drugs in DBS, and a more favourable elution behaviour tended to result in a higher MS signal. Copyright © 2016 John Wiley & Sons, Ltd.

Surfactant controlled magnesium oxide synthesis for base catalysis

Magnesium oxide catalysts were used to investigate the influence of novel preparative techniques for surface site control on activity.

Faceted metal and metal oxide nanoparticles: design, fabrication and catalysis

The review addresses new advances in metal, bimetallic, metal oxide, and composite particles in their nanoregime for facet-selective catalytic applications. The synthesis and growth mechanisms of the particles have been summarized in brief in this review with a view to develop critical examination of the faceted morphology of the particles for catalysis. The size, shape and composition of the particles have been found to be largely irrelevant in comparison to the nature of facets in catalysis. Thus selective high- and low-index facets have been found to selectively promote adsorption, which eventually leads to an effective catalytic reaction. As a consequence, a high density of atoms rest at the corners, steps, stages, kinks etc on the catalyst surface in order to host the adsorbate efficiently and catalyze the reaction. Again, surface atomic arrangement and bond length have been found to play a dominant role in adsorption, leading to effective catalysis.

Calcination temperature-dependent surface structure and physicochemical properties of magnesium oxide

The electrochemical performance of MgO particles is highly dependent on their crystal structures resulting from calcination at different temperatures.

Shape evolution of parallelogrammic magnesium oxalate controlled by phosphate species

The shape evolution of magnesium oxalate dihydrate to parallelogrammic micro-particles is closely related to the amount of Na₅P₃O₁₀ participating in the self-assembly of the layer-like parallelogram.

Substrate engineering in stabilizing epitaxial MgO(1 1 1) polar ultrathin films: first-principles calculations

Growing MgO(1 1 1) polar ultrathin films (PUFs) on heterogeneous substrates is technologically challenging. By using first-principles calculations, we show that while the O-termination of the supported PUF can be perfectly passivated by H adatoms, the compensation of the Mg-termination at the interface is the key to the stabilization. Proper charge transfer across the interface is highly required, which is dominated by the work function difference between the two surfaces that form the interface. Taking Ag(1 1 1) as an example, we propose that a thin Pt buffer layer can increase the work function of the substrate,and improve the PUF quality significantly.

Catalysing sustainable fuel and chemical synthesis

Concerns over the economics of proven fossil fuel reserves, in concert with government and public acceptance of the anthropogenic origin of rising CO2 emissions and associated climate change from such combustible carbon, are driving academic and commercial research into new sustainable routes to fuel and chemicals. The quest for such sustainable resources to meet the demands of a rapidly rising global population represents one of this century's grand challenges. Here, we discuss catalytic solutions to the clean synthesis of biodiesel, the most readily implemented and low cost, alternative source of transportation fuels, and oxygenated organic molecules for the manufacture of fine and speciality chemicals to meet future societal demands.

Heterogeneous catalysis for sustainable biodiesel productionviaesterification and transesterification

Low temperature catalytic conversion of triglycerides and fatty acids sourced from renewable feedstocks represents a key enabling technology for the sustainable production of biodiesel through energy efficient, intensified processes.

Extension of metal oxide laser ionization mass spectrometry to analytes with varied chemical functionalities

RATIONALE

Applications of metal oxide laser ionization mass spectrometry (MOLI MS) have been limited to compounds that provide protons for ionization upon adsorption to a metal oxide surface. The addition of a small molecule, which can act as a proton source and extend MOLI MS applications to a wider variety of analytes, was investigated.

METHODS

Mass spectrometric measurements were made with a PerSeptive Biosystems Voyager DE-STR MALDI mass spectrometer, using NiO-based MOLI to generate ions. NiO was pretreated in this study with a number of small molecules to act as proton sources during ionization. Peptides, monosaccharides, and a crude oil sample were analyzed with pretreated and untreated NiO.

RESULTS

Candidate small molecule proton donors were evaluated based on their effect on signal-to-noise ratio and peak intensity for tristearin. The best performing proton source was used for subsequent analyses. The analysis of monosaccharides, peptides and compounds in heavy crude oil was enhanced by the use of proton source treated NiO for MOLI MS. Proton/cation exchange following ionization was observed.

CONCLUSIONS

Addition of a proton source to NiO enhanced the MOLI MS response for peptides, carbohydrates, and compounds in a crude oil sample. Efficient ionization of these samples effectively extended applications of MOLI MS beyond lipid analytes containing esters.