MgO Surface Research Articles

Mechanistic insights of CO methanation over MgO using DFT

Catalytic CO reduction is of tremendous importance, not only for CO emission reduction but also for generating cleaner fuels. The CO methanation reaction transforms CO into methane, a significant component of natural gas that can also be used as fuel. Methane emits less carbon dioxide upon combustion, making it cleaner than other hydrocarbon fuels. Metal oxide based catalysts are shown to have a high activity level for the reaction. However, the reaction pathways vary based on the formation of the favourable intermediate on the catalyst surface. In this work, we investigated three distinct pathways of CO methanation on the MgO (110) surface: (i) Hydrogenation of both C and O terminal of CO to form HCOH intermediate, (ii) Hydrogenation of O terminal of CO and (iii) CO bond dissociation. The ΔE (in eV) of intermediate and transition species are investigated using DFT. Further, a reaction coordinate diagram was developed with three pathways to find the best possible reaction mechanism for CO methanation on MgO (110) surface.

Revealing the Synergetic Effects between Reactants in Oxidative Coupling of Methane on Stepped MgO(100) Catalyst

The oxidative coupling of methane (OCM) on MgO is often computationally explored via Mars-Krevelen (MvK) mechanism. However, the difficult desorption of CH3 radical at stepped MgO surface shadow the feasibility of mechanism. In this work, density functional theory calculations are performed to unravel the syngenetic effects between reactants which lead to a new Langmuir-Hinshelwood (L-H)-like mechanism. It was found that co-adsorption of reactants pave ways for CH3 radical formation with negligible desorption energy. The role of oxygen molecule is not only to oxidize reduced surface but also decrease the reactivity of Mg-O site which facile CH3 desorption. Electronic structure analysis indicated the distinct feature along pathway between MvK and L-H. The current work clearly indicated the importance of effective interactions between reactants and provided new insights on the reaction mechanism of OCM.

Caught while Dissolving: Revealing the Interfacial Solvation of the Mg2+ Ions on the MgO Surface.

Interfaces between water and materials are ubiquitous and are crucial in materials sciences and in biology, where investigating the interaction of water with the surface under ambient conditions is key to shedding light on the main processes occurring at the interface. Magnesium oxide is a popular model system to study the metal oxide–water interface, where, for sufficient water loadings, theoretical models have suggested that reconstructed surfaces involving hydrated Mg2+ metal ions may be energetically favored. In this work, by combining experimental and theoretical surface-selective ambient pressure X-ray absorption spectroscopy with multivariate curve resolution and molecular dynamics, we evidence in real time the occurrence of Mg2+ solvation at the interphase between MgO and solvating media such as water and methanol (MeOH). Further, we show that the Mg2+ surface ions undergo a reversible solvation process, we prove the dissolution/redeposition of the Mg2+ ions belonging to the MgO surface, and we demonstrate the formation of octahedral [Mg(H2O)6]2+ and [Mg(MeOH)6]2+ intermediate solvated species. The unique surface, electronic, and structural sensitivity of the developed technique may be beneficial to access often elusive properties of low-Z metal ion intermediates involved in interfacial processes of chemical and biological interest.

Theoretical study on the cluster–surface interaction: The case of subnanometer Pt–Re clusters supported on MgO(100)

Bimetallic clusters in gas phase display novel physicochemical properties that are different from those present in their pure metal analogs. When bimetallic clusters are deposited on a substrate, such properties may change depending on the type and strength of the metal–support interaction. Here, we report a theoretical study, based on density functional theory (DFT), for sub-nanometer clusters of Pt–Re on the pristine MgO(100) surface, using the Basin-Hopping DFT algorithm to find the global minima. Then, their structural, energetic, electronic, and vibrational properties are calculated, and a comparison between gas- and supported-phase behavior is performed. It is obtained that the MgO(100) surface has a more relevant effect on the structural and vibrational properties of the Pt–Re bimetallic clusters and pure Pt clusters. For example, the metal–metal bond lengths of the supported clusters increase with respect to those observed in gas phase, also giving rise to red shifts in the corresponding vibrational frequencies. On the other hand, the structural and vibrational properties of pure gas- and supported-phase Re clusters are quite similar. These results are consistent with the adsorption energy calculations indicating a strong interaction between Pt–Re clusters and pure Pt clusters with the MgO(100) surface. Moreover, the metal–support interaction leads to a charge transfer from the metal oxide to the metal clusters and to the hybridization of the d- and s-states of the metal atoms with the p-states of the oxygen atoms present in the substrate. This work contributes to the understanding of the role of the support in bimetallic Pt–Re clusters on metal oxides, which would be of potential interest in the design of novel nanocatalysts.

Periodic Density Matrix Embedding for CO Adsorption on the MgO(001) Surface.

The adsorption of simple gas molecules to metal oxide surfaces is a primary step in many heterogeneous catalysis applications. Quantum chemical modeling of these reactions is a challenge in terms of both cost and accuracy, and quantum-embedding methods are promising, especially for localized chemical phenomena. In this work, we employ density matrix embedding theory (DMET) for periodic systems to calculate the adsorption energy of CO to the MgO(001) surface. Using coupled-cluster theory with single and double excitations and second-order Møller–Plesset perturbation theory as quantum chemical solvers, we perform calculations with embedding clusters up to 266 electrons in 306 orbitals, with the largest embedding models agreeing to within 1.2 kcal/mol of the non-embedding references. Moreover, we present a memory-efficient procedure of storing and manipulating electron repulsion integrals in the embedding space within the framework of periodic DMET.

A method to stabilize the unreconstructed MgO (111) polar surface

The choice and quality of single-crystal substrates for epitaxial thin film growth drastically influence the properties of the synthesized films. Among them, high-quality substrates with a polar surface are particularly difficult to prepare due to their inherent structural instability against electrostatic forces of the polar discontinuity. As a commonly used insulating substrate, the (111) cut of MgO shows alternating planes of Mg2+ and O2− that form a hexagonal unit cell at the surface, which is useful for thin films with similar crystal symmetries. We present a method to prepare such a surface by first introducing (3×3)R30° reconstructed MgO (111) surface via face-to-face annealing in an O2 environment. We then dissolve the top layers in acid to achieve an unreconstructed and also passivated surface. The structure, stoichiometry, and stability of the resulting surfaces are characterized. This work provides an easy and reliable way to prepare a MgO (111) polar surface ready for subsequent epitaxial growth.

Mechanism for the promotional formation of NH4+ by SO2 on different mineral dust surfaces

Ammonium is an important atmospheric particulate component that dictates many environmental processes. The promotion of the heterogeneous conversion of NH3 to NH4+ by SO2 on different mineral dust surfaces displays remarkable discrepancies, especially on MgO and α-Fe2O3 surfaces, however, the underlying mechanisms are not well known. Here, using periodic density functional theory (DFT) calculation and Born-Oppenheimer molecular dynamics (BOMD) simulation, we explored the heterogeneous adsorption of NH3 on MgO (110) and α-Fe2O3 (001) surfaces in the presence and absence of SO2. The results show that on MgO (110) surface, hydrogen-bonding interactions of NH3 on both adsorbed hydroxyl or bisulfite/bisulfate sites are observed no matter whether SO2 is present or not. While, on the α-Fe2O3 (001) surface, significant conversion of NH3 to NH4+ occurs with the coexistence of SO2, which is due to the hydrogen transfer reaction from surface HSO4 to N in NH3. The fundamental reason may be that the stronger electron affinity of Fe3+ than Mg2+ results in adsorbed bisulfate and/or bisulfite with greater acidity on α-Fe2O3 surface than MgO surface. Our results give a molecular-level explanation for the heterogeneous conversion of NH3 to NH4+ on different mineral dust surfaces under complex air pollution conditions. Considering the fact that ammonium is abundant in secondary particulates, this work would help in understanding the rapid conversion of ammonia to ammonium and in developing classification governance policies for the key precursor pollutants in China.

Spin canting of Ni/CoO/Fe films grown on curved MgO(0 0 1) substrate

Using element-resolved x-ray magnetic circular dichroism (XMCD) and x-ray magnetic linear dichroism (XMLD) measurements, we determined the spin orientations of Ni, CoO and Fe films in Ni/CoO/Fe films grown on a curved MgO(001) substrate. We find that the vicinal surface of MgO(001) substrate results in a spin canting towards out-of-plane direction in the Ni and CoO films as a result of the interfacial coupling. The Ni spin canting angle increases monotonically with the vicinal angle in the studied range of 0–17° and the CoO spin canting angle increases more rapidly towards saturation only at a few degrees of the vicinal angle. The uniaxial magnetic anisotropy induced in the Ni layer by the Ni/CoO interfacial coupling is quantitatively determined and is shown to increase monotonically with the vicinal angle. Our result provides a new pathway for tailoring the spin orientation by modifying the substrate surface symmetry in combining with the ferromagnetic/antiferromagnetic interfacial interaction in thin-film based spintronic devices.

PH Dependence of MgO, TiO2, and γ-Al2O3 Surface Chemistry from First Principles

We investigate the pH-dependent surface chemistry of three relevant and widely used oxides, MgO, TiO2, and γ-Al2O3, from ab initio molecular dynamics simulations at the level of density functional theory (DFT). We studied the MgO (001), anatase TiO2 (101), and γ-Al2O3 (001) low-index surfaces and considered solvation effects by explicitly simulating the solid/water interfaces. The analysis of the MgO/H2O, TiO2/H2O, γ-Al2O3/H2O interfaces allowed us to access atomistic details of the structure and the effects induced by water on these surfaces. We also investigated the pH dependence by means of the Grand Canonical formulation of species in solution. This allowed us to evaluate acid–base equilibrium constants and the point-of-zero charge (pHPZC) values. The calculated pHPZC values compare well with available experimental measurements. Based on the above result, we also predicted the fraction of charged species (H+ and OH–) and neutral molecules (H2O) present on the surface as a function of pH, a fundamental aspect if one wants to identify suitable catalysts for water splitting reactions.

Granulation of alkaline metal nitrate promoted MgO adsorbents and the low-concentration CO2 capture performance in the fixed bed adsorber

Magnesium-based adsorbents with the modification of alkaline metal salts are considered the competitive candidates for CO2 capture at intermedium temperature. In this article, the NaNO3 promoted MgO adsorbents were prepared and granulated, and the characterization methods were used to detect the microstructure of the granular adsorbents, including NaNO3 distribution, BET specific surface area, micropore size and microcrystal aggregation. The thermogravimetric analyzer (TGA) tests were carried out to investigate the optimal CO2 adsorption and desorption temperatures and the cyclic stability of the NaNO3 promoted MgO granules. Moreover, the pretreatment of carbonization and calcination was adopted to improve the cyclic stability of the granular adsorbents. Furthermore, the fixed bed adsorber with 40 g NaNO3-MgO granules was used to capture CO2 from 50% CO2 mixture gas with N2 and from 20% CO2 mixture gas with N2, respectively, and the CO2 breakthrough behaviors in the fixed bed were investigated. It was found that NaNO3-MgO adsorbents were suitable for the middle and high-concentration CO2 capture (such as 50% CO2 feed gas) in fixed bed adsorber with the higher CO2 recovery efficiency, but the CO2 recovery efficiency was lower for the low-concentration CO2 capture (such as 20% CO2 feed gas) at ordinary pressure. Because about 10 kPa CO2 partial pressure in the gas phase was needed to overcome CO2 mass transfer resistance in the molten salt layer on MgO surface during adsorption, the appropriate pressurization would improve the low-concentration CO2 capture efficiency in the fixed bed adsorber.

Effect of solute atom adsorption on heterogeneous nucleation by in-situ MgO particles: Experimental and theoretical studies

The introduction of oxide inclusions during the smelting process has a clear promising heterogeneous nucleation potency on Mg-based alloys, but the mechanism has not been explored clearly yet. In the present work, the grain refinement mechanism of MgO in pure Mg, Mg-3Al and AZ31 (Mg-2.9Al-0.9Zn-0.3Mn) alloys is investigated by combing first-principles calculations and experiments. The theoretical results show that solute atoms adsorption will affect the nucleation and the subsequent growth process, which is an important factor affecting the refinement efficiency. A contradiction between the experimental results and the grain growth restriction factor (GRF) theory is observed, that is the refinement ratio of AZ31 is worse than Mg-3Al alloy. This is explained by an adsorption model which reveals that Al promotes the adsorption of Mg on MgO surface so as to stimulate more particles available as nucleating sites. Meanwhile, Fe and Mn also have favorable effects on the adsorption of Mg, Zn may play the opposite role compared to Al. The theoretical analyzes provide a strong support to the experiments that Al benefits the initial nucleation of α-Mg on MgO to promote the grain refining effect of Mg-3Al prior to AZ31 alloy.

Molecular Dynamics Simulations of Nitrate/MgO Interfaces and Understanding Metastability of Thermochemical Materials.

We explore the role of molten nitrate interfaces on MgO surface treatment for improving the reversibility of thermochemical energy storage via sorption and desorption of water or CO2. Our molecular dynamics simulations focus on melts of LiNO3, NaNO3, KNO3, and the triple eutectic mixture Li0.38Na0.18K0.44NO3 on the surface of MgO to provide atomic scale details of adsorbed layers and to rationalize interface energies. On this basis, a thermodynamic model is elaborated to characterize the effect of nitrate melts on the dehydration of Mg(OH)2 and to quantitatively explain the difference in dehydration temperatures of intact and LiNO3-doped Mg(OH)2.

Nano-faceted stabilization of polar-oxide thin films: The case of MgO(111) and NiO(111) surfaces

Molecular beam epitaxy growth of polar MgO(111) and NiO(111) films demonstrates that surface stabilization of the films is achieved via the formation of neutral nano-faceted surfaces. First-principles modeling of the growth of polar MgO(111) films reveals that the growth does not proceed layer-by-layer. Instead, the Mg or O layers grow up to a critical sub-monolayer coverage, beyond which the growth of the next layer becomes energetically favorable. This non-layer-by-layer growth is accompanied by complex relaxations of atoms both at the surface and in the sub-surface, and leads to the experimentally observed surface nano-faceting of MgO and NiO (111) films through formation of neutral nano-pyramids terminated by {100} facets. These facets are limited in size by an asymptotical surface energy relation to their height; with the reconstruction being much more stable than previously reported surface terminations across a wide range of growth conditions. The termination offers access to lower coordinated atoms at the intersection of the neutral {100} planes whilst also increasing the surface area of the film. The unique electronic structures of these surfaces can be utilized in catalysis, as well for forming unique heterostructures for electronic and spintronic applications.

Synthesis of a Solid Base Catalyst Formed from Zinc(II) Acetylacetonate and Consisting of MgO Modified with ZnO

Solid base catalysts consisting of MgO modified with 10 mol% ZnO (ZnO/MgO) were prepared by depositing zinc(II) acetylacetonate (Zn(acac)2) in polar and nonpolar organic solvents on the surface of MgO and Mg(OH)2. The process was followed by thermal decomposition in air at 773 K over a period of 3 h. The activity of MgO modified with Zn(acac)2 for base-catalyzed retroaldol reaction (the decomposition of diacetone alcohol (4-hydroxy-4-methyl-2-pentanone) to acetone) at 299 K was higher than the activity of unmodified MgO prepared by thermal decomposition of Mg(OH)2. The activity of the catalysts prepared by depositing Zn(acac)2 on Mg(OH)2 was lower than that of unmodified MgO. Analysis of X-ray diffraction patterns revealed the presence of highly intense peaks, which were assigned to the ZnO units in the samples with high catalytic activity. ZnO was deposited in the form of large particles on the surface of MgO in the catalysts with high activity.

Self-metalation of a free-base porphyrin on a metal oxide surface mediated by extended defects: Insight from ab initio molecular dynamics simulations

An important pathway for functionalization of porphyrin-based organic-inorganic structures is the metalation of porphyrins. Recently, the porphyrin metalation was demonstrated on different metal oxide surfaces, however, the underlying mechanisms regarding the role of the surface morphology, the substituted metal, and ligands are still under investigation. Here we address the adsorption and self-metalation of H2TPP on a MgO(001) surface with low-coordinated sites. We employ ab initio molecular dynamics simulations around room temperature to provide insight into dynamic steric effects. We observe that H2TPP is mobile on the pristine surface as the steric hindrance by phenyl rings prevents the physisorption of the macrocycle at a specific site. In contrast, step edges or kink sites provide anchor points exposing low-coordinated, reactive oxygen-sites to hydrogens of the macrocycle. We report a spontaneous proton transfer at these sites forming an intermediate complex before the metalation occurs. The energetics of the self-metalation reaction is modeled.

Engineering a local acid-like environment in alkaline medium for efficient hydrogen evolution reaction

Tuning the local reaction environment is an important and challenging issue for determining electrochemical performances. Herein, we propose a strategy of intentionally engineering the local reaction environment to yield highly active catalysts. Taking Ptδ− nanoparticles supported on oxygen vacancy enriched MgO nanosheets as a prototypical example, we have successfully created a local acid-like environment in the alkaline medium and achieve excellent hydrogen evolution reaction performances. The local acid-like environment is evidenced by operando Raman, synchrotron radiation infrared and X-ray absorption spectroscopy that observes a key H3O+ intermediate emergence on the surface of MgO and accumulation around Ptδ− sites during electrocatalysis. Further analysis confirms that the critical factors of the forming the local acid-like environment include: the oxygen vacancy enriched MgO facilitates H2O dissociation to generate H3O+ species; the F centers of MgO transfers its unpaired electrons to Pt, leading to the formation of electron-enriched Ptδ− species; positively charged H3O+ migrates to negatively charged Ptδ− and accumulates around Ptδ− nanoparticles due to the electrostatic attraction, thus creating a local acidic environment in the alkaline medium.

Defect-induced decomposition of energetic nitro compounds at MgO Surface

We studied the role of F0 centers at a MgO surface in decomposition of energetic nitro compounds which is critically important for the control of chemical explosion reactions in these materials as well as for their recycling. We found that the first step of the molecular decomposition is initiated by an electron charge transferring from one of the F0 centers at the surface to the NO2 group of the energetic molecule which results in the cleavage of one of the NO bonds. The O atom released from the nitro molecule goes to the oxygen vacancy thus liquidating the vacancy defect. The process of oxygen trapping in the F0 center is highly exothermic, with a significant energy release. The released energy is sufficient to break other molecular bonds thus facilitating a further decomposition of the energetic molecules and propagating the decomposition reaction. This decomposition initiation mechanism is significantly different from decomposition in gas phase or solid state for all three considered nitro energetic molecules. In particular, the activation barriers for NO cleavage are much lower than activation barriers of the first step of decomposition reactions in gas phase or solid state (molecular crystals) for all three nitro materials.

Do stoichiometric or nonstoichiometric models of a polar surface affect their structural, energetic and electronic structure properties? A DFT case study of Ru/MgO(111)

The structural, energetic and electronic structure properties of stoichiometric and nonstoichiometric slab models of bare MgO(111) and Ru/MgO(111) with different coverages of 1 monolayer (ML), 1/4 ML and 1/16 ML have been investigated using spin-polarized density functional theory. Calculated results show that the structural, energetic properties and charge transfer of both bare MgO(111) and Ru/MgO(111) are independent of the stoichiometric and nonstoichiometric models. In contrast, their density of state (DOS) profiles demonstrate metal and half-metal characters for the stoichiometric and nonstoichiometric bare MgO(111) surfaces, respectively. The Ru–O orbital coupling characters of these two types of Ru/MgO(111) models are also different. This work indicates that for a polar surface model, the calculated features and trends of the structural and energetic properties, charge distributions and magnetic structures might not be affected by their stoichiometric and nonstoichiometric models; however, the detailed features of their DOS features would strongly depend on the models constructed.

Diradical Characters of s-Indaceno[1,2,3-cd;5,6,7-c’d’]Diphenalene with and without Interaction with MgO(001)

Diradical character is important for investigating and predicting the functions of singlet biradical molecules. Singlet biradical molecules possess many unique properties, and their immobilisation by surfaces has been extensively studied for applications in devices, such as quantum computers, single-molecule magnets, single-atom catalysts, organic batteries, and organic superconductivity devices. Recently, an estimation scheme of diradical character from the calculated results of density functional theory with plane-wave basis (DFT/plane-wave), which is a general method for calculating the electronic structures of surfaces, was developed by our research group. This scheme enables us to investigate the diradical character of the open-shell structure stabilised by the surfaces. In this study, DFT/plane-wave calculations for s-indaceno[1,2,3-cd;5,6,7-c’d’]diphenalene molecules, whose diradical character was experimentally observed, were performed. Using the developed scheme, it was verified whether the diradical character was modulated by the interaction with the MgO(001) surface. The calculated results showed that the diradical character increased as the distance from the surface decreased, although the enhancement effect of the surface was inhibited by the steric hindrance of the substituents.

Sequential photocatalytic degradation of organophosphorus pesticides and recovery of orthophosphate by biochar/α-Fe2O3/MgO composite: A new enhanced strategy for reducing the impacts of organophosphorus from wastewater

Photocatalysis is regarded as one of the most promising techniques for degrading organophosphorus pesticides (OPs) from wastewater given its low-cost and environmental benignity. Yet, photocatalytic degradation of OPs is an incomplete process, since PO43− is produced after photocatalysis. Here, biochar-supported α-Fe2O3/MgO composite (BC-α-Fe2O3/MgO) is developed for sequential photocatalytic degradation of OPs and recovery of the produced PO43−. The degradation efficiency of N-phosphonomethyl iminodiacetic acid (NPA) by BC-α-Fe2O3/MgO based photocatalysis reaches 90.1% within 80 min, and the recovery ratio of the produced PO43− by BC-α-Fe2O3/MgO is as high as 82.3%. Besides, the as-prepared BC-α-Fe2O3/MgO used in this integrated photocatalysis and adsorption process maintain a high stability towards NPA degradation and PO43− recovery during five cycling experiments. The mechanistic study reveals that holes, •OH, and •O2– radicals generated in the photocatalytic process are responsible for NPA degradation. UV light can activate persistent free radicals on BC-α-Fe2O3/MgO surface, which provide O2 withelectrons for the generation of •O2– and subsequent yield of •OH. Moreover, the presence of α-Fe2O3 and MgO on the surface of biochar leads to the excellent P adsorption performance of BC-α-Fe2O3/MgO. This study opens new possibilities for the recovery of P resource from organophosphorus polluted wastewater.