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1. Stability of adsorption of Mg and Na on sulfur-functionalized MXenes

   https://doi.org/10.1039/d1cp03433b  

   Chaney, G.; Çakır, D.; Peeters, F. M.; Ataca, C. (November 2021, Physical Chemistry Chemical Physics)

   Two-dimensional materials composed of transition metal carbides and nitrides (MXenes) are poised to revolutionize energy conversion and storage. In this work, we used density functional theory (DFT) to investigate the adsorption of Mg and Na adatoms on five M 2 CS 2 monolayers (where M = Mo, Nb, Ti, V, and Zr) for battery applications. We assessed the stability of the adatom ( i.e. Na and Mg)-monolayer systems by calculating adsorption and formation energies, as well as voltages as a function of surface coverage. For instance, we found that Mo 2 CS 2 cannot support a full layer of Na nor even a single Mg atom. Na and Mg exhibit the strongest binding on Zr 2 CS 2 , followed by Ti 2 CS 2 , Nb 2 CS 2 and V 2 CS 2 . Using the nudged elastic band method (NEB), we computed promising diffusion barriers for both dilute and nearly full ion surface coverage cases. In the dilute ion adsorption case, a single Mg and Na atom on Ti 2 CS 2 experience ∼0.47 eV and ∼0.10 eV diffusion barriers between the lowest energy sites, respectively. For a nearly full surface coverage, a Na ion moving on Ti 2 CS 2 experiences a ∼0.33 eV energy barrier, implying a concentration-dependent diffusion barrier. Our molecular dynamics results indicate that the three (one) layers (layer) of the Mg (Na) ion on both surfaces of Ti 2 CS 2 remain stable at T = 300 K. While, according to voltage calculations, Zr 2 CS 2 can store Na up to three atomic layers, our MD simulations predict that the outermost layers detach from the Zr 2 CS 2 monolayer due to the weak interaction between Na ions and the monolayer. This suggests that MD simulations are essential to confirm the stability of an ion-electrode system – an insight that is mostly absent in previous studies. 

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2. Synthesis and magnetic studies of pentagonal bipyramidal metal complexes of Fe, Co and Ni

   https://doi.org/10.1039/c8dt05074k  

   Deng, Yi-Fei; Yao, Binling; Zhan, Peng-Zhi; Gan, Dexuan; Zhang, Yuan-Zhu; Dunbar, Kim R. (March 2019, Dalton Transactions)

   Three mononuclear metal complexes [M II (L-N 3 O 2 )(MeCN) 2 ][BPh 4 ] 2 (M = Fe, 1 ; Co, 2 ; Ni, 3 ) were isolated and structurally characterized. Magnetic studies revealed uniaxial magnetic anisotropy for 1 ( D = −17.1 cm −1 ) and 3 ( D = −14.3 cm −1 ) and easy-plane magnetic anisotropy for 2 ( D = +36.9 cm −1 ). Slow magnetic relaxation was observed for complexes 1 and 2 under an applied magnetic field, both of which are dominated by a Raman process. 

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3. The roles of chalcogenides in O ₂ protection of H ₂ ase active sites

   https://doi.org/10.1039/D0SC02584D  

   Yang, Xuemei; Darensbourg, Marcetta Y. (September 2020, Chemical Science)

   null (Ed.)

   At some point, all HER (Hydrogen Evolution Reaction) catalysts, important in sustainable H 2 O splitting technology, will encounter O 2 and O 2 -damage. The [NiFeSe]-H 2 ases and some of the [NiFeS]–H 2 ases, biocatalysts for reversible H 2 production from protons and electrons, are exemplars of oxygen tolerant HER catalysts in nature. In the hydrogenase active sites oxygen damage may be extensive (irreversible) as it is for the [FeFe]–H 2 ase or moderate (reversible) for the [NiFe]–H 2 ases. The affinity of oxygen for sulfur, in [NiFeS]–H 2 ase, and selenium, in [NiFeSe]–H 2 ase, yielding oxygenated chalcogens results in maintenance of the core NiFe unit, and myriad observable but inactive states, which can be reductively repaired. In contrast, the [FeFe]–H 2 ase active site has less possibilities for chalcogen-oxygen uptake and a greater chance for O 2 -attack on iron. Exposure to O 2 typically leads to irreversible damage. Despite the evidence of S/Se-oxygenation in the active sites of hydrogenases, there are limited reported synthetic models. This perspective will give an overview of the studies of O 2 reactions with the hydrogenases and biomimetics with focus on our recent studies that compare sulfur and selenium containing synthetic analogues of the [NiFe]–H 2 ase active sites. 

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4. Combined computational and experimental investigation of high temperature thermodynamics and structure of cubic ZrO2 and HfO2

   https://doi.org/10.1038/s41598-018-32848-7  

   Hong, Qi-Jun; Ushakov, Sergey V.; Kapush, Denys; Benmore, Chris J.; Weber, Richard J. K.; van de Walle, Axel; Navrotsky, Alexandra (October 2018, Scientific Reports)

   Abstract Structure and thermodynamics of pure cubic ZrO₂and HfO₂were studied computationally and experimentally from their tetragonal to cubic transition temperatures (2311 and 2530 °C) to their melting points (2710 and 2800 °C). Computations were performed using automatedab initiomolecular dynamics techniques. High temperature synchrotron X-ray diffraction on laser heated aerodynamically levitated samples provided experimental data on volume change during tetragonal-to-cubic phase transformation (0.55 ± 0.09% for ZrO₂and 0.87 ± 0.08% for HfO₂), density and thermal expansion. Fusion enthalpies were measured using drop and catch calorimetry on laser heated levitated samples as 55 ± 7 kJ/mol for ZrO₂and 61 ± 10 kJ/mol for HfO₂, compared with 54 ± 2 and 52 ± 2 kJ/mol from computation. Volumetric thermal expansion for cubic ZrO₂and HfO₂are similar and reach (4 ± 1)·10⁻⁵/K from experiment and (5 ± 1)·10⁻⁵/K from computation. An agreement with experiment renders confidence in values obtained exclusively from computation: namely heat capacity of cubic HfO₂and ZrO₂, volume change on melting, and thermal expansion of the liquid to 3127 °C. Computed oxygen diffusion coefficients indicate that above 2400 °C pure ZrO₂is an excellent oxygen conductor, perhaps even better than YSZ. 

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5. Mechanisms of Complete Dissociation of CO ₂ on Iron Clusters

   https://doi.org/10.1002/cphc.202200277  

   Gutsev, Gennady_L; Tibbetts, Katharine_M; Gutsev, Lavrenty_G; Aldoshin, Sergey_M; Ramachandran, Bala_R (August 2022, ChemPhysChem)

   Abstract Dissociation of CO₂on iron clusters was studied by using semilocal density functional theory and basis sets of triple‐zeta quality. Fe₂, Fe₄, and Fe₁₆clusters were selected as the representative host clusters. When searching for isomers of Fe_nCO₂,n=2, 4 and 16 corresponding to carbon dioxide attachment to the host clusters, its reduction to O and CO, and to the complete dissociation, it was found that the total spin magnetic moments of the lowest energy states of the isomers are often quenched with respect to those of initial reagents Fe_n+CO₂. Dissociation pathways of the Fe₂+CO₂, Fe₄+CO₂, and Fe₁₆+CO₂reactions contain several transition states separated by the local minima states; therefore, a natural question is where do the spin flips occur? Since lifetimes of magnetically excited states were shown to be of the order of 100 fs, the search for the CO₂dissociation pathways was performed under the assumption that magnetic deexcitation may occur at the intermediate local minima. Two dissociation pathways were obtained for each Fe_n+CO₂reaction using the gradient‐based methods. It was found that the Fe₂+CO₂reaction is endothermic with respect to both reduction and complete dissociation of CO₂, whereas the Fe₄+CO₂and Fe₁₆+CO₂reactions are exothermic to both reduction and complete dissociation of carbon dioxide. The CO₂reduction was found to be more favorable than its complete dissociation in the Fe₄case. 

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