Revisiting Acido-basicity of the MgO Surface by Periodic Density Functional Theory Calculations: Role of Surface Topology and Ion Coordination on Water Dissociation

Bird's-Eye View of the Activity Distribution on a Catalyst Surface via a Machine Learning-Driven Adequate Sampling Algorithm

Rational design of catalysts relies on a deep understanding of the active centers. The structure and activity distribution of active centers on a surface are two of the central issues in catalysis and important targets of theoretical and experimental investigations. Herein, we report a machine learning-driven adequate sampling (MLAS) framework for obtaining a statistical understanding of the chemical environment near catalyst sites. Combined strategies were implemented to achieve highly efficient sampling, including the decomposition of degrees of freedom, stratified sampling, Gaussian process regression, and well-designed constraint optimization. The MLAS framework was applied to the rate-determining step in NH3 synthesis, namely the N2 activation process. We calculated the produced population function, PA, which provides a comprehensive and intuitive understanding of active centers. The MLAS framework can be broadly applied to other more complicated catalyst materials and reaction networks.

Optimising the acid-base ratio of Mg-Al layered double oxides to enhance CO2 capture performance: the critical role of calcination conditions

The effect of calcination conditions (ramp rate, calcination temperature and time) on the formation of Mg2Al layered double oxides (Mg2Al LDOs) as well as their CO2 capture performance, has been systematically investigated. This study explores novel insights into the intricate relationship between these calcination conditions and the resulting surface characteristics, which play a vital role in CO2 capture efficiency. Notably, it is revealed that a rapid ramp rate (100 °C min-1) significantly increases surface area and hydroxyl concentration, leading to a 69% increase in CO2 capture efficiency compared to slower ramp rate. Conversely, short calcination times (1 h) and fast ramp rates (100 °C min-1) are observed to compromise CO2 adsorption due to the presence of dehydrated LDHs. A critical acid : base ratio of 0.37, achieved from a fast ramp rate (100 °C min-1) at 400 °C for 2 h, was found as a key threshold for optimising surface properties, effectively balancing favourable hydroxyl and less favourable strong acid sites, thereby maximizing CO2 capture performance.

Photodriven Methane Conversion on Transition Metal Oxide Catalyst: Recent Progress and Prospects

Methane as the main component in natural gas is a promising chemical raw material for synthesizing value-added chemicals, but its harsh chemical conversion process often causes severe energy and environment concerns. Photocatalysis provides an attractive path to active and convert methane into various products under mild conditions with clean and sustainable solar energy, although many challenges remain at present. In this review, recent advances in photocatalytic methane conversion are systematically summarized. As the basis of methane conversion, the activation of methane is first elucidated from the structural basis and activation path of methane molecules. The study is committed to categorizing and elucidating the research progress and the laws of the intricate methane conversion reactions according to the target products, including photocatalytic methane partial oxidation, reforming, coupling, combustion, and functionalization. Advanced photocatalytic reactor designs are also designed to enrich the options and reliability of photocatalytic methane conversion performance evaluation. The challenges and prospects of photocatalytic methane conversion are also discussed, which in turn offers guidelines for methane-conversion-related photocatalyst exploration, reaction mechanism investigation, and advanced photoreactor design.

Raman spectroscopy investigation of magnesium oxide nanoparticles

We investigate Raman spectra (100 cm-1 to 3900 cm-1) of magnesium oxide nanoparticles with nominal sizes of 10 nm, 20 nm, 40 nm, 50 nm, and 300 nm. The crystal structure of MgO prohibits first-order modes and yet, there are numerous reports of relatively intense peaks throughout the literature. Raman signals at approximately 278 cm-1 and 445 cm-1 that were attributed to MgO nanoparticles by previous authors are shown to belong to layers of Mg(OH)2 formed on the surface of MgO nanoparticles. Through an annealing process at 400 °C in an O2 atmosphere, we observe that modes in the 3700 cm-1 spectral region, which are a signature of OH groups, disappear together with modes at 278 cm-1 and 445 cm-1, thus establishing a necessary criterion to associate all of these peaks to the presence of OH groups on the surface.

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.

A combined periodic DFT and QM/MM approach to understand the radical mechanism of the catalytic production of methanol from glycerol

The production of methanol from glycerol over a basic oxide, such as MgO, using high reaction temperatures (320 °C) is a promising new approach to improving atom efficiency in the production of biofuels. The mechanism of this reaction involves the homolytic cleavage of the C₃ feedstock, or its dehydration product hydroxyacetone, to produce a hydroxymethyl radical species which can then abstract an H atom from other species. Obtaining a detailed reaction mechanism for this type of chemistry is difficult due to the large number of products present when the system is operated at high conversions. In this contribution we show how DFT based modelling studies can provide new insights into likely reaction pathways, in particular the source of H atoms for the final step of converting hydroxymethyl radicals to methanol. We show that water is unlikely to be important in this stage of the process, C-H bonds of C₂ and C₃ species can give an energetically favourable pathway and that the disproportionation of hydroxymethyl radicals to methanol and formaldehyde produces a very favourable route. Experimental analysis of reaction products confirms the presence of formaldehyde. The calculations presented in this work also provide new insight into the role of the catalyst surface in the reaction showing that the base sites of the MgO(100) are able to deprotonate hydroxymethyl radicals but not methanol itself. In carrying out the calculations we also show how periodic DFT and QM/MM approaches can be used together to obtain a rounded picture of molecular adsorption to surfaces and homolytic bond cleavage which are both central to the reactions studied.

Machine Learning in Computational Surface Science and Catalysis: Case Studies on Water and Metal-Oxide Interfaces

The goal of many computational physicists and chemists is the ability to bridge the gap between atomistic length scales of about a few multiples of an Ångström (Å), i. e., 10⁻¹⁰ m, and meso- or macroscopic length scales by virtue of simulations. The same applies to timescales. Machine learning techniques appear to bring this goal into reach. This work applies the recently published on-the-fly machine-learned force field techniques using a variant of the Gaussian approximation potentials combined with Bayesian regression and molecular dynamics as efficiently implemented in the Vienna ab initio simulation package, VASP. The generation of these force fields follows active-learning schemes. We apply these force fields to simple oxides such as MgO and more complex reducible oxides such as iron oxide, examine their generalizability, and further increase complexity by studying water adsorption on these metal oxide surfaces. We successfully examined surface properties of pristine and reconstructed MgO and Fe₃O₄ surfaces. However, the accurate description of water–oxide interfaces by machine-learned force fields, especially for iron oxides, remains a field offering plenty of research opportunities.

First-principles-informed energy span and microkinetic analysis of ethanol catalytic conversion to 1,3-butadiene on MgO

First-principles-informed models elucidate the impact of energetic and kinetic limitations on selectivity and activity of ethanol conversion to 1,3-butadiene.

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.

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.

A DFT study on the aldol condensation reaction on MgO in the process of ethanol to 1,3-butadiene: understanding the structure–activity relationship

The mechanism of aldol condensation on MgO surfaces with different structures was investigated to illustrate the structure–activity relationship.

Correlation between the acid-base properties of the La2O3 catalyst and its methane reactivity

Density functional theory and coupled cluster theory calculations were carried out to study the effects of the acid-base properties of the La2O3 catalyst on its catalytic activity in the oxidative coupling of methane (OCM) reaction. The La(3+)-O(2-) pair site for CH4 activation is considered as a Lewis acid-Brönsted base pair. Using the Lewis acidity and the Brönsted basicity in the fluoride affinity and proton affinity scales as quantitative measures of the acid-base properties, the energy barrier for CH4 activation at the pair site can be linearly correlated with these acid-base properties. The pair site consisting of a strong Lewis acid La(3+) site and a strong Brönsted base O(2-) site is the most reactive for CH4 activation. In addition, the basicity of the La2O3 catalyst was traditionally measured by temperature-programmed desorption of CO2, but the CO2 chemisorption energy is better regarded as a combined measure of the acid-base properties of the pair site. A linear relationship of superior quality was found between the energy barrier for CH4 activation and the CO2 chemisorption energy, and the pair site favorable for CO2 chemisorption is also more reactive for CH4 activation, leading to the conflicting role of the "basicity" of the La2O3 catalyst in the OCM reaction. The necessity for very high reaction temperatures in the OCM reaction is rationalized by the requirement for the recovery of the most reactive acid-base pair site, which unfortunately also reacts most readily with the byproduct CO2 to form the very stable CO3(2-) species.

Insight on Tricalcium Silicate Hydration and Dissolution Mechanism from Molecular Simulations

Hydration of mineral surfaces, a critical process for many technological applications, encompasses multiple coupled chemical reactions and topological changes, challenging both experimental characterization and computational modeling. In this work, we used reactive force field simulations to understand the surface properties, hydration, and dissolution of a model mineral, tricalcium silicate. We show that the computed static quantities, i.e., surface energies and water adsorption energies, do not provide useful insight into predict mineral hydration because they do not account for major structural changes at the interface when dynamic effects are included. Upon hydration, hydrogen atoms from dissociated water molecules penetrate into the crystal, forming a disordered calcium silicate hydrate layer that is similar for most of the surfaces despite wide-ranging static properties. Furthermore, the dynamic picture of hydration reveals the hidden role of surface topology, which can lead to unexpected water tessellation that stabilizes the surface against dissolution.

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.

Density functional theory simulations of complex catalytic materials in reactive environments: beyond the ideal surface at low coverage

Advanced DFT models of complex catalysts, such as amorphous silica–alumina and supported subnanometric platinum particles, bridge the gap between the ideal surface model and the industrial catalyst.

A molecular approach for unraveling surface phase transitions: sulfation of BaO as a model NO(x) trap

SO(3) -induced surface reconstruction: The SO(3) molecule as a multidentate ligand induces remarkable surface reconstruction phenomena on alkaline earth oxide surface. By using ab initio computations, adsorption properties are derived to elucidate the thermodynamics of the SO(3) -BaO system.

Hydration of calcium oxide surface predicted by reactive force field molecular dynamics

In this work, we present the parametrization of Ca-O/H interactions within the reactive force field ReaxFF, and its application to study the hydration of calcium oxide surface. The force field has been fitted using density functional theory calculations on gas phase calcium-water clusters, calcium oxide bulk and surface properties, calcium hydroxide, bcc and fcc Ca, and proton transfer reactions in the presence of calcium. Then, the reactive force field has been used to study the hydration of the calcium oxide {001} surface with different water contents. Calcium oxide is used as a catalyzer in many applications such as CO(2) sequestration and biodiesel production, and the degree of surface hydroxylation is a key factor in its catalytic performance. The results show that the water dissociates very fast on CaO {001} bare surfaces without any defect or vacancy. The surface structure is maintained up to a certain amount of water, after which the surface undergoes a structural rearrangement, becoming a disordered calcium hydroxyl layer. This transformation is the most probable reason for the CaO catalytic activity decrease.

Tuning surface basic properties of nanocrystalline MgO by controlling the preparation conditions

Different nanocrystalline magnesias were synthesized by precipitation and hydrothermal treatments of aqueous salt solutions in an attempt to tune their surface basicity. CO(2) was chosen as an acidic molecule to probe the basic sites by both temperature-programmed desorption and infrared spectroscopy. All samples were shown to be crystalline, and except that obtained by nitrate decomposition, they all possessed high surface areas. The oxides presented different basic site distributions, evidencing the significant role of the preparation conditions on tuning the surface basicity: while medium-strength basic centers are dominant in the samples prepared by precipitation aging or hydrothermal treatment, the one obtained by precipitation features a roughly equal concentration of medium-strength and strong centers. Infrared spectra revealed that hydrogen carbonate and monodentate and bidentate carbonates were formed in distinct proportion on all oxides. However, the bidentate complexes were shown to have different thermal stabilities; the more stable species are thought to be formed on acid-base pair centers associated with an anionic vacancy. Distinct morphological and structural characteristics were also observed by high-resolution transmission electron microscopy. It was consistently found that the high-surface area samples are formed by aggregates of nanoparticles (2-5 nm) randomly oriented and with a high concentration of structural defects. These findings allowed us to conclude that the surface heterogeneity promoted during synthesis increases the concentration of basic sites and plays an important role in tuning the basicity of the solids.