Comparative study on the activities of different MgO surfaces in CO2 activation and hydrogenation

Electrostatically tunable interaction of CO2 with MgO surfaces and chemical switching: first-principles theory

Electric field-assisted CO2 capture using solid adsorbents based on basic oxides can immensely reduce the required energy consumption compared to the conventional processes of temperature or pressure swing adsorption. In this work, we present first-principles density functional theoretical calculations to investigate the effects of an applied external electric field (AEEF) within the range from -1 to 1 V Å-1 on the CO2 adsorption behavior on various high and low-index facets of MgO. When CO2 is strongly adsorbed on MgO surfaces to form carbonate species, the coupling of electric fields with the resulting intrinsic dipole moment induces a 'switch' from a strongly chemisorbed state to a weakly chemisorbed or physisorbed state at a critical value of AEEF. We demonstrate that such 'switching' enables access to different metastable states with variations in the AEEF. On polar MgO(111) surfaces, we find a distinct feature of the adsorptive dissociation of CO2 towards the formation of CO in contrast to that on the non-polar MgO(100) and MgO(110) surfaces. In some cases, we observe broken inversion symmetry because of the AEEF that results in induced polarity at the interaction site of CO2 on MgO surfaces. Our results provide fundamental insights into the possibility of using AEEFs in novel solid adsorbent systems for CO2 capture and reduction.

Exploring Mechanisms of Hydration and Carbonation of MgO and Mg(OH)2 in Reactive Magnesium Oxide-Based Cements

Reactive magnesium oxide (MgO)-based cement (RMC) can play a key role in carbon capture processes. However, knowledge on the driving forces that control the degree of carbonation and hydration and rate of reactions in this system remains limited. In this work, density functional theory-based simulations are used to investigate the physical nature of the reactions taking place during the fabrication of RMCs under ambient conditions. Parametric indicators such as adsorption energies, charge transfer, electron localization function, adsorption/dissociation energy barriers, and the mechanisms of interaction of H₂O and CO₂ molecules with MgO and brucite (Mg(OH)₂) clusters are considered. The following hydration and carbonation interactions relevant to RMCs are evaluated: (i) carbonation of MgO, (ii) hydration of MgO, carbonation of hydrated MgO, (iii) carbonation of Mg(OH)₂, (iv) hydration of Mg(OH)₂, and (v) hydration of carbonated Mg(OH)₂. A comparison of the energy barriers and reaction pathways of these mechanisms shows that the carbonation of MgO is hindered by the presence of H₂O molecules, while the carbonation of Mg(OH)₂ is hindered by the formation of initial carbonate and hydrate layers as well as presence of excessed H₂O molecules. To compare these finding to bulk mineral surfaces, the interactions of the CO₂ and H₂O molecules with the MgO(001) and Mg(OH)₂ (001) surfaces are studied. Therefore, this work presents deep insights into the physical nature of the reactions and the mechanisms involved in hydrated magnesium carbonates production that can be beneficial for its development.

Activation of CO2 and CH4 on MgO surfaces: mechanistic insights from first-principles theory

One of the most challenging topics in heterogeneous catalysis is conversion of CH₄ to higher hydrocarbons. Direct conversion of CH₄ to ethylene can be achieved via the oxidative coupling of methane (OCM) reaction. Despite studies which have shown MgO to activate CH₄ and initiate the OCM reaction, its large-scale applications face a significant impediment due to formation of a byproduct, CO₂, and poisoning of the catalyst due to carbonate formation. In the present work, we address two aspects of the OCM reaction on MgO surfaces: carbonate formation on the surface of the catalyst, and (dissociative) adsorption of CH₄. We use first-principles density functional theoretical calculations to determine the energetics and underlying mechanisms of interaction of CO₂ and CH₄ with various surfaces of MgO: (100), (110), and (111) (both Mg- and O-terminations), and the seldom studied, hydroxylated (111) MgO surface with O-termination. We find that the strength of the interaction of CO₂ with MgO surfaces depends on several factors: their surface energies, coordination number of surface O atoms, and ability to donate electrons. However, the O-terminated (111) surface of MgO bucks all aforementioned factors, with only oxygen richness affecting its reactivity towards CO₂. The interaction of CH₄ with MgO surfaces depends primarily on the coordination number of the surface O atoms and the orientation of the CH₄ molecule with respect to the surface. Finally, we provide insights into (a) formation of surface carbonates, which is relevant to CO₂ capture and conversion, and (b) C-H bond activation on MgO surfaces, which is crucial for direct conversion of CH₄ to value-added chemicals.

Hydride species on oxide catalysts

Hydride species on oxide catalysts are widely involved in oxide-catalyzed reactions, and relevant fundamental understanding is important to establish reaction mechanisms and structure-performance relations of oxide catalysts. In this topical review, recent progresses on the formation and reactivity of hydride species on the surface or in the bulk of oxides are briefly summarized. Firstly, characterization techniques for hydride species are introduced. Secondly, formation of hydride species on the surface or in the bulk of various oxides and their reactivity in oxide-catalyzed hydrogenation and dehydrogenation reactions are reviewed. Finally, short summary and outlook are given.