CH/CH2 Group Clusters Doping Methane Hydrate Cages

Methane hydrate is a crystalline compound with methane molecules as guest species trapped in host water cages. In this study, we detected methane hydrate with water cages doped by (C_aromatic-H)₅ clusters, (C_aromatic-H)₆ clusters, and (3C_aliphatic-H₂ + 2H₂O) clusters using current spectroscopic techniques and differential scanning calorimetry (DSC). Methane molecules are trapped in the doped cages with type sI forming in nanoscale silica gel pores. The relative quantity ratio of host carbon to guest carbon in the doped hydrate sample reaches approximately 3.58. Methane hydrate doped by CH/CH₂ group clusters greatly improves the ability of the hydrate unit cell to store methane and increases the stability of methane hydrate. Fast proton diffusion in the doped methane hydrate was confirmed. The results of this study will provide efficient and energy saving technical support for disruptive changes in hydrate storage and transportation of methane gas technology with a doped and dense solid phase.

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 Pt^δ− 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 Pt^δ− 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 Pt^δ− species; positively charged H₃O⁺ migrates to negatively charged Pt^δ− and accumulates around Pt^δ− nanoparticles due to the electrostatic attraction, thus creating a local acidic environment in the alkaline medium.

While catalysts have intrinsic activities toward reactions, such performances often require further optimization. Here, authors engineer an acid-like environment in alkaline media by fine-tuning the reaction environment of platinum nanoparticles on oxide nanosheets for H₂ evolution electrocatalysis.

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.

Hydration structure of flat and stepped MgO surfaces

We investigate the solvation structure of flat and stepped MgO(001) in neutral liquid water using ab initio molecular dynamics based on a hybrid density functional with dispersion corrections. Our simulations show that the MgO surface is covered by a densely packed layer of mixed intact and dissociated adsorbed water molecules in a planar arrangement with strong intermolecular H-bonds. The water dissociation fractions in this layer are >20% and >30% on the flat and stepped surfaces, respectively. Slightly above the first water layer, we observe metastable OH groups perpendicular to the interface, similar to those reported in low temperature studies of water monolayers on MgO. These species receive hydrogen bonds from four nearby water molecules in the first layer and have their hydrophobic H end directed toward bulk water, while their associated protons are bound to surface oxygens. The formation of these OH species is attributed to the strong basicity of the MgO surface and can be relevant for understanding various phenomena from morphology evolution and growth of (nano)crystalline MgO particles to heterogeneous catalysis.

Reply to the 'Comment on "Proton transport in barium stannate: classical, semi-classical and quantum regime"'

We respond to the erroneous criticisms about our modeling of proton transport in barium stannate [G. Geneste et al., Phys. Chem. Chem. Phys., 2015, 17, 19104]. In this previous work, we described, on the basis of density-functional calculations, proton transport in the classical and semi-classical regimes, and provided arguments in favor of an adiabatic picture for proton transfer at low temperature. We re-explain here our article (with more detail and precision), the content of which has been distorted in the Comment, and reiterate our arguments in this reply. We refute all criticisms. They are completely wrong in the context of our article. Even though a few of them are based on considerations probably true in some metals, they make no sense here since they do not correspond to the content of our work. It has not been understood in the Comment that two competitive configurations, associated with radically different transfer mechanisms, have been studied in our work. It has also not been understood in the Comment that the adiabatic regime described for transfer occurs in the protonic ground state, in a very-low barrier configuration with the protonic ground state energy larger than the barrier. Serious confusion has been made in the Comment with the case of H in metals like Nb or Ta, leading to the introduction of the notion of (protonic) "excited-state proton transfer", relevant for H in some metals, but (i) that does not correspond to the (ground state) adiabatic transfers here described, and (ii) that does not correspond to what is commonly described as the "adiabatic limit for proton transfer" in the scientific literature. We emphasize, accordingly, the large differences between proton transfer in the present oxide and hydrogen jumps in metals like Nb or Ta, and the similarities between proton transfer in the present oxide and in acid-base solutions. We finally describe a scenario for proton transfer in the present oxide regardless of the temperature regime.

Hydroxylation Induced Alignment of Metal Oxide Nanocubes

Water vapor is ubiquitous under ambient conditions and may alter the shape of nanoparticles. How to utilize water adsorption for nanomaterial functionality and structure formation, however, is a yet unexplored field. Herein, we report the use of water vapor to induce the self-organization of MgO nanocubes into regularly staggered one-dimensional structures. This transformation evolves via an initial alignment of the MgO cubes, the formation of intermediate elongated Mg(OH)₂ structures, and their reconversion into MgO cubes arranged in staggered structures. Ab initio DFT modelling identifies surface-energy changes associated with the cube surface hydration and hydroxylation to promote the uncommon staggered stacked assembly of the cubes. This first observation of metal oxide nanoparticle self-organization occurring outside a bulk solution may pave novel routes for inducing texture in ceramics and represents a great test-bed for new surface-science concepts.

Water dissociation on MnO(1 × 1)/Ag(100)

In this work we utilize experimental and simulation techniques to examine the molecular level interaction of water with a MnO(1 × 1) thin film deposited onto Ag(100). The formation of MnO(1 × 1)/Ag(100) was characterized by low energy electron diffraction and scanning tunneling microscopy. Density functional theory (DFT) shows MnO(1 × 1) is thermodynamically more stable than MnO(2 × 1) by ∼0.4 eV per MnO. Upon exposure to 2.5 Torr water vapor at room temperature, X-ray photoemission spectroscopy results show extensive surface hydroxylation attributed to reactivity at MnO(1 × 1) terrace sites. DFT calculations of a water monomer on MnO(1 × 1)/Ag(100) show the dissociated form is energetically more favorable than molecular adsorption, with a hydroxylation activation barrier 0.4 eV per H₂O. These results are discussed and contrasted with previous studies of MgO/Ag(100) which show a stark difference in behavior for water dissociation.

Water at Interfaces

The interfaces of neat water and aqueous solutions play a prominent role in many technological processes and in the environment. Examples of aqueous interfaces are ultrathin water films that cover most hydrophilic surfaces under ambient relative humidities, the liquid/solid interface which drives many electrochemical reactions, and the liquid/vapor interface, which governs the uptake and release of trace gases by the oceans and cloud droplets. In this article we review some of the recent experimental and theoretical advances in our knowledge of the properties of aqueous interfaces and discuss open questions and gaps in our understanding.

Promoting the "water-wire" mechanism of double proton transfer in [2,2'-bipyridyl]-3,3'-diol by porous gold nanoparticles

The effect of nanopores in porous gold nanoparticles (Au NPs) on excited-state double proton transfer (DPT) in [2,2'-bipyridyl]-3,3'-diol (BP(OH)2) in an aqueous environment is the main focus of the present work. DPT in BP(OH)2 is known to take place through two mechanisms. In a bulk environment, an open solvated molecule facilitates the process and emits at 460 nm whereas, in a confined situation, formation of a "water wire" between the prototropic centers leads to the transfer of protons. It has been shown spectroscopically in the present study that in the nanovessels provided by nanoporous Au NPs, the unconventional mechanism of DPT through the formation of a "water wire" is promoted due to the presence of a limited number of water molecules around the probe. Experiments in the presence of solid pure Au, Ag and Au/Ag NPs were performed to support our proposition. Time-resolved fluorescence spectral changes confirm our findings.

Solvent-Induced Proton Hopping at a Water-Oxide Interface

Despite widespread interest, a detailed understanding of the dynamics of proton transfer at interfaces is lacking. Here, we use ab initio molecular dynamics to unravel the connection between interfacial water structure and proton transfer for the widely studied and experimentally well-characterized water-ZnO(101̅0) interface. We find that upon going from a single layer of adsorbed water to a liquid multilayer, changes in the structure are accompanied by a dramatic increase in the proton-transfer rate at the surface. We show how hydrogen bonding and rather specific hydrogen-bond fluctuations at the interface are responsible for the change in the structure and proton-transfer dynamics. The implications of this for the chemical reactivity and for the modeling of complex wet oxide interfaces in general are also discussed.