Morphology-dependent adsorption energetics of Ru nanoparticles on hcp-boron nitride (001) surface - a first-principles study.

Active B5-sites on Ru catalysts can be exploited for various catalytic applications; in particular, the epitaxial formation of Ru nanoparticles with hexagonal planar morphologies on hexagonal boron nitride sheets increases the number of active B5-sites along the nanoparticle edges. The energetics of adsorption of Ru nanoparticles on hexagonal boron nitride were investigated using density functional theory calculations. Then, to understand the fundamental reason for this morphology control, adsorption studies and charge density analysis were performed on fcc and hcp Ru nanoparticles heteroepitaxially formed on a hexagonal boron nitride support. Among the explored morphologies, hcp Ru(0001) nanoparticles exhibited the highest adsorption strength of -31.656 eV. To verify the hexagonal planar morphologies of the hcp-Ru nanoparticles, three different hcp-Ru(0001) nanoparticles-Ru60, Ru53, and Ru 41-were adsorbed onto the BN substrate. In agreement with the experimental studies, the hcp-Ru60 nanoparticles exhibited the highest adsorption energy owing to their long-range and perfect hexagonal match with the interacting hcp-BN(001) substrate.

Heteroepitaxial Growth of B5 -Site-Rich Ru Nanoparticles Guided by Hexagonal Boron Nitride for Low-Temperature Ammonia Dehydrogenation.

Ruthenium is one of the most active catalysts for ammonia dehydrogenation and is essential for the use of ammonia as a hydrogen storage material. The B5 -type site on the surface of ruthenium is expected to exhibit the highest catalytic activity for ammonia dehydrogenation, but the number of these sites is typically low. Here, a B5 -site-rich ruthenium catalyst is synthesized by exploiting the crystal symmetry of a hexagonal boron nitride support. In the prepared ruthenium catalyst, ruthenium nanoparticles are formed epitaxially on hexagonal boron nitride sheets with hexagonal planar morphologies, in which the B5 sites predominate along the nanoparticle edges. By activating the catalyst under the reaction condition, the population of B5 sites further increases as the facets of the ruthenium nanoparticles develop. The electron density of the Ru nanoparticles also increases during catalyst activation. The synthesized catalyst shows superior catalytic activity for ammonia dehydrogenation compared to previously reported catalysts. This work demonstrates that morphology control of a catalyst via support-driven heteroepitaxy can be exploited for synthesizing highly active heterogeneous catalysts with tailored atomic structures.

When Bigger Is Not Greener: Ensuring the Sustainability of Power-to-Gas Hydrogen on a National Scale.

As the prices of photovoltaics and wind turbines continue to decrease, more renewable electricity-generating capacity is installed globally. While this is considered an integral part of a sustainable energy future by many nations, it also poses a significant strain on current electricity grids due to the inherent output variability of renewable electricity. This work addresses the challenge of renewable electricity surplus (RES) utilization with target-scaling of centralized power-to-gas (PtG) hydrogen production. Using the Republic of Korea as a case study, due to its ambitious plan of 2030 green hydrogen production capacity of 0.97 million tons year-1, we combine predictions of future, season-averaged RES with a detailed conceptual process simulation for green H2 production via polymer electrolyte membrane (PEM) electrolysis combined with a desalination plant in six distinct scale cases (0.5-8.5 GW). It is demonstrated that at scales of 0.5 to 1.75 GW the RES is optimally utilized, and PtG hydrogen can therefore outperform conventional hydrogen production both environmentally (650-2210 Mton CO2 not emitted per year) and economically (16-30% levelized cost reduction). Beyond these scales, the PtG benefits sharply drop, and thus it is answered how much of the planned green hydrogen target can realistically be "green" if produced domestically on an industrial scale.

Metastable hexagonal close-packed palladium hydride in liquid cell TEM.

Metastable phases-kinetically favoured structures-are ubiquitous in nature1,2. Rather than forming thermodynamically stable ground-state structures, crystals grown from high-energy precursors often initially adopt metastable structures depending on the initial conditions, such as temperature, pressure or crystal size1,3,4. As the crystals grow further, they typically undergo a series of transformations from metastable phases to lower-energy and ultimately energetically stable phases1,3,4. Metastable phases sometimes exhibit superior physicochemical properties and, hence, the discovery and synthesis of new metastable phases are promising avenues for innovations in materials science1,5. However, the search for metastable materials has mainly been heuristic, performed on the basis of experiences, intuition or even speculative predictions, namely 'rules of thumb'. This limitation necessitates the advent of a new paradigm to discover new metastable phases based on rational design. Such a design rule is embodied in the discovery of a metastable hexagonal close-packed (hcp) palladium hydride (PdHx) synthesized in a liquid cell transmission electron microscope. The metastable hcp structure is stabilized through a unique interplay between the precursor concentrations in the solution: a sufficient supply of hydrogen (H) favours the hcp structure on the subnanometre scale, and an insufficient supply of Pd inhibits further growth and subsequent transition towards the thermodynamically stable face-centred cubic structure. These findings provide thermodynamic insights into metastability engineering strategies that can be deployed to discover new metastable phases.

Formic acid dehydrogenation over PdNi alloys supported on N-doped carbon: synergistic effect of Pd-Ni alloying on hydrogen release.

Bimetallic Pd1Nix alloys supported on nitrogen-doped carbon (Pd1Nix/N-C, x = 0.37, 1.3 and 3.6) exhibit higher activities than Pd/N-C towards dehydrogenation of formic acid (HCO2H, FA). Density functional theory (DFT) calculations provided electronic and atomic structures, energetics and reaction pathways on Pd(111) and Pd1Nix(111) surfaces of different Pd/Ni compositions. A density of states (DOS) analysis disclosed the electronic interactions between Pd and Ni revealing novel active sites for FA dehydrogenation. Theoretical analysis of FA dehydrogenation on Pd1Nix(111) (x = 0.33, 1 and 3) shows that the Pd1Ni1(111) surface provides optimum H2-release efficiency via a favorable 'HCOO pathway', in which a hydrogen atom and one of the two oxygen atoms of FA interact directly with surface Ni atoms producing adsorbed CO2 and H2. The enhanced efficiency is also attributed to the blocking of an unfavorable 'COOH pathway' through which a C-O bond is broken and side products of CO and H2O are generated.

An Amine-Borane System Featuring Room-Temperature Dehydrogenation and Regeneration.

Amine-borane complexes have been extensively studied as hydrogen storage materials. Herein, we report a new amine-borane system featuring a reversible dehydrogenation and regeneration at room temperature. In addition to high purity H2 , the reaction between ethylenediamine bisborane (EDAB) and ethylenediamine (ED) leads to unique boron-carbon-nitrogen 5-membered rings in the dehydrogenation product where one boron is tricoordinated by three nitrogen atoms. Owing to the unique cyclic structure, the dehydrogenation product can be efficiently converted back to EDAB by NaBH4 and H2 O at room temperature. This finding could lead to the discovery of new amine boranes with potential usage as hydrogen storage materials.

Three-Dimensional TEM Study of Dendrimer-Encapsulated Pt Nanoparticles for Visualizing Structural Characteristics of the Whole Organic-Inorganic Hybrid Nanostructure.

Here, we report three-dimensional (3-D) visualization of dendrimer-encapsulated Pt nanoparticles (Pt DENs) by using 3-D electron tomography to reveal intricate structural characteristics of their whole organic-inorganic hybrid nanostructure. We reconstructed the 3-D spatial volume of Pt DENs by back-projecting a tilt series of two-dimensional (2-D) projections of Pt nanoparticles encapsulated inside dendrimers negatively stained with uranyl acetate. The direct 3-D visualization of Pt DENs elucidated their encapsulation characteristics with the spatial imaging of Pt nanoparticles embraced inside dendrimers in three dimensions. The encapsulation characteristics of Pt DENs were further verified with selective electrochemical poisoning experiments. In addition, quantitative 3-D structural characterization of Pt DENs provided more accurate and precise size distributions of nanoparticles than those obtained from conventional 2-D transmission electron microscopy analysis relying only on a 3-D structure projected on a 2-D plane.

Top-Down Syntheses of Nickel-Based Structured Catalysts for Hydrogen Production from Ammonia.

We report the fabrication and catalytic performance evaluation of highly active and stable nickel (Ni)-based structured catalysts for ammonia dehydrogenation with nearly complete conversion using nonprecious metal catalysts. Low-temperature chemical alloying (LTCA) followed by selective aluminum (Al) dealloying was utilized to synthesize foam-type structured catalysts ready for implementation in commercial-scale catalytic reactors. The crystalline phases of Ni-Al alloy (NiAl3, Ni2Al3, or both) in the near-surface layer were controlled by tuning the alloying time. The best-performing catalyst was obtained from a Ni foam substrate with a NiAl3/Ni2Al3 overlayer synthesized by LTCA at 400 °C for 20 h. The developed Ni catalyst exhibited an activity enhancement of 10-fold over the nontreated Ni foam and showed outstanding activities of 15 800 molH2molNi-1h-1 (TOF: 4.39 s-1) and 19 978 molH2molNi-1h-1 (TOF: 5.55 s-1) at 550 and 600 °C, respectively. This performance is unprecedented compared with previously reported Ni-based ammonia cracking catalysts with higher-end performance (TOFs of 0.08-1.45 s-1 at 550 °C). Moreover, this catalyst showed excellent stability for 100 h at 600 °C while discharging an extremely low NH3 concentration of 1034 ppm. The NH3 concentration in the exhaust gas was further reduced to 690 and 271 ppm at 700 and 800 °C, respectively, while no deactivation was observed at these elevated temperatures. Through material characterizations, we clarified that controlling the degree of Al alloying in the outermost layer of Ni is a crucial factor in determining the activity and stability because residual Al possibly modifies the electronic structure of Ni for enhanced activity as well as transforming to acidic alumina for increased intrinsic activity and stability.

Pd2+ -Initiated Formic Acid Decomposition: Plausible Pathways for C-H Activation of Formate.

Formic acid (HCOOH, FA) has long been considered as a promising hydrogen-storage material due to its efficient hydrogen release under mild conditions. In this work, FA decomposes to generate CO2 and H2 selectively in the presence of aqueous Pd2+ complex solutions at 333 K. Pd(NO3 )2 was the most effective in generating H2 among various Pd2+ complexes explored. Pd2+ complexes were in situ reduced to Pd0 species by the mixture of FA and sodium formate (SF) during the course of the reaction. Since C-H activation reaction of Pd2+ -bound formate is occurred for both Pd2+ reduction and H2 /CO2 gas generation, FA decomposition pathways using several Pd2+ species were explored using density functional theory (DFT) calculations. Rotation of formate bound to Pd2+ , ß-hydride elimination, and subsequent CO2 and H2 elimination by formic acid were examined, providing different energies for rate determining step depending on the ligand electronics and geometries coordinated to the Pd2+ complexes. Finally, Pd2+ reduction toward Pd0 pathways were explored computationally either by generated H2 or reductive elimination of CO2 and H2 gas.

Repetitively Coupled Chemical Reduction and Galvanic Exchange as a Synthesis Strategy for Expanding Applicable Number of Pt Atoms in Dendrimer-Encapsulated Pt Nanoparticles.

In this study, we report the controllable synthesis of dendrimer-encapsulated Pt nanoparticles (Pt DENs) utilizing repetitively coupled chemical reduction and galvanic exchange reactions. The synthesis strategy allows the expansion of the applicable number of Pt atoms encapsulated inside dendrimers to more than 1000 without being limited by the fixed number of complexation sites for Pt2+ precursor ions in the dendrimers. The synthesis of Pt DENs is achieved in a short period of time (i.e., ∼10 min) simply by the coaddition of appropriate amounts of Cu2+ and Pt2+ precursors into aqueous dendrimer solution and subsequent addition of reducing agents such as BH4-, resulting in fast and selective complexation of Cu2+ with the dendrimers and subsequent chemical reduction of the complexed Cu2+ while uncomplexed Pt2+ precursors remain oxidized. Interestingly, the chemical reduction of Cu2+, leading to the formation of Cu nanoparticles encapsulated inside the dendrimers, is coupled with the galvanic exchange of the Cu nanoparticles with the nearby Pt2+. This coupling repetitively proceeds until all of the added Pt2+ ions form into Pt nanoparticles encapsulated inside the dendrimers. In contrast to the conventional method utilizing direct chemical reduction, this repetitively coupled chemical reduction and galvanic exchange enables a substantial increase in the applicable number of Pt atoms up to 1320 in Pt DENs while maintaining the unique features of DENs.

Amine/Hydrido Bifunctional Nanoporous Silica with Small Metal Nanoparticles Made Onsite: Efficient Dehydrogenation Catalyst.

Multifunctional catalysts are of great interest in catalysis because their multiple types of catalytic or functional groups can cooperatively promote catalytic transformations better than their constituents do individually. Herein we report a new synthetic route involving the surface functionalization of nanoporous silica with a rationally designed and synthesized dihydrosilane (3-aminopropylmethylsilane) that leads to the introduction of catalytically active grafted organoamine as well as single metal atoms and ultrasmall Pd or Ag-doped Pd nanoparticles via on-site reduction of metal ions. The resulting nanomaterials serve as highly effective bifunctional dehydrogenative catalysts for generation of H2 from formic acid.

A highly active and durable Co-N-C electrocatalyst synthesized using exfoliated graphitic carbon nitride nanosheets.

Exfoliated graphitic carbon nitride nanosheets (g-C3N4-NS) were applied for the first time for the preparation of an electrocatalyst for the oxygen reduction reaction (ORR). A less dense structure with increased surface area was observed for g-C3N4-NS compared to bulk g-C3N4 from detailed analyses including TEM, STEM, AFM with depth profiling, XRD, and UV-Vis spectroscopy. The pyrolysis of the prepared g-C3N4-NS with Co and carbon under an inert environment provided an enhanced accessibility to the N functionalities required for efficient interaction of Co and C with N for the formation of Co-N-C networks and produced a hollow and interconnected Co-N-C-NS structure responsible for high durability. The Co-N-C-NS electrocatalyst exhibited superior catalytic activity and durability and further displayed fast and selective four electron transfer kinetics for the ORR, as evidenced by various electrochemical experiments. The hollow, interconnected structure of Co-N-C-NS with increased pyridinic and graphitic N species has been proposed to play a key role in facilitating the desired ORR reaction.

Mechanisms of enhanced sulfur tolerance on samarium (Sm)-doped cerium oxide (CeO2) from first principles.

The role of samarium (Sm) 4f states and Sm-perturbed O 2p states in determining the sulfur tolerance of Sm-doped CeO2 was elucidated by using the density functional theory (DFT) + U calculation. We find that the sulfur tolerance of Sm-doped CeO2 is closely related to the modification of O 2p states by the strong interaction between Sm 4f and O 2p states. In particular, the availability of unoccupied O 2p states near the Fermi level is responsible for enhancing the sulfur tolerance of Sm-doped CeO2 compared to the pure CeO2 by increasing the activity of the surface lattice oxygen toward sulfur adsorption, by weakening the interaction between Sm-O, and by increasing the migration tendency of the subsurface oxygen ion toward the surface.

Convenient metal embedment into mesoporous silica channels for high catalytic performance in AB dehydrogenation.

The infiltration of palladium nanoparticles (PdNPs) into the channels of SBA-15 was conveniently achieved via an incipient wetness procedure employing a tetraglyme solution. Electron tomography demonstrated that PdNPs were outgrown preferentially from the channels. The resultant Pd/SBA-15 showed high performance in the dehydrogenation kinetics of ammonia borane.

Metal-free, polyether-mediated H2-release from ammonia borane: roles of hydrogen bonding interactions in promoting dehydrogenation.

Polyetheral additives were found to be efficient promoters to enhance the rate of H2-release from ammonia borane (AB) at various temperatures. In particular, tetraethylene glycol dimethyl ether (T4EGDE, 29 wt% relative to AB + T4EGDE) exhibited significantly improved activities for AB dehydrogenation, with the material-based hydrogen storage capacity of 10.3 wt% at 125 °C within 40 min. In situ FT-IR spectroscopy indicated the formation of B-(cyclodiborazanyl)amino-borohydride (BCDB), borazine, and µ-aminodiborane as gaseous byproducts. In addition, (11)B nuclear magnetic resonance (NMR) spectroscopy further revealed that diammoniate of diborane (DADB) was initially formed to give polyaminoborane as liquid and/or solid spent-fuel, consistent with previous reports. Density Functional Theory (DFT) calculations suggested that hydrogen bonding interactions between AB and a polyetheral promoter initially played an important role in increasing the reactivity of B-H bonds of AB by transferring electron density from oxygen atoms of the promoter into B-H bonds of AB. These partially activated, hydridic B-H bonds were proposed to help promote the formation of diammoniate of diborane (DADB), which is considered as a reactive intermediate, eventually enhancing the rate of H2-release from AB. In addition, our in situ solid state (11)B magic angle spinning (MAS) NMR measurements further confirmed that the rate of DADB formation from AB with a small quantity of T4EGDE was found to be much faster than that of pristine AB even at 50 °C. This metal-free method for H2-release from AB with an added, small quantity of polyethers would be helpful to develop feasible hydrogen storage systems for long-term fuel cell applications.

Base-promoted ammonia borane hydrogen-release.

The strong non-nucleophilic base bis(dimethylamino)naphthalene (Proton Sponge, PS) has been found to promote the rate and extent of H(2)-release from ammonia borane (AB) either in the solid state or in ionic-liquid and tetraglyme solutions. For example, AB reactions in 1-butyl-3-methylimidazolium chloride (bmimCl) containing 5.3 mol % PS released 2 equiv of H(2) in 171 min at 85 degrees C and only 9 min at 110 degrees C, whereas comparable reactions without PS required 316 min at 85 degrees C and 20 min at 110 degrees C. Ionic-liquid solvents proved more favorable than tetraglyme since they reduced the formation of undesirable products such as borazine. Solid-state and solution (11)B NMR studies of PS-promoted reactions in progress support a reaction pathway involving initial AB deprotonation to form the H(3)BNH(2)(-) anion. This anion can then initiate AB dehydropolymerization to form branched-chain polyaminoborane polymers. Subsequent chain-branching and dehydrogenation reactions lead ultimately to a cross-linked polyborazylene-type product. AB dehydrogenation by lithium and potassium triethylborohydride was found to produce the stabilized Et(3)BNH(2)BH(3)(-) anion, with the crystallographically determined structure of the [Et(3)BNH(2)BH(3)](-)K(+).18-crown-6 complex showing that, following AB nitrogen-deprotonation by the triethylborohydride, the Lewis-acidic triethylborane group coordinated at the nitrogen. Model studies of the reactions of [Et(3)BNH(2)BH(3)](-)Li(+) with AB show evidence of chain-growth, providing additional support for a PS-promoted AB anionic dehydropolymerization H(2)-release process.

Ammonia triborane: a new synthesis, structural determinations, and hydrolytic hydrogen-release properties.

Iodine oxidation of B(3)H(8)(-) in glyme solution to produce (glyme)B(3)H(7), followed by displacement of the coordinated glyme by reaction with anhydrous ammonia provides a safe and convenient preparation of ammonia triborane, NH(3)B(3)H(7) (1). X-ray crystallographic determinations and DFT computational studies of both NH(3)B(3)H(7) and the NH(3)B(3)H(7) x 18-crown-6 adduct demonstrate that while computations predict a symmetric single bridging-hydrogen conformation, NH(3)B(3)H(7) has a highly asymmetric structure in the solid-state that results from intermolecular N-H(+)...H(-)-B dihydrogen bonding interactions. Studies of its hydrolytic reactions have shown that upon the addition of acid or an appropriate transition metal catalyst, aqueous solutions of 1 rapidly release hydrogen, with 6.1 materials wt % H(2)-release being achieved from a 22.7 wt % aqueous solution of 1 at room temperature in the presence of 5 wt % Rh/Al(2)O(3) (1.1 mol% Rh). The rate of H(2)-release was controlled by both the catalyst loadings and temperature.

Computational studies of the reactions of B10H13- with alkynes and olefins: pathways for dehydrogenative alkyne-insertion and olefin-hydroboration reactions.

Quantum mechanical computational studies of possible mechanistic pathways for B10H13(-) dehydrogenative alkyne-insertion and olefin-hydroboration reactions demonstrate that, depending on the reactant and reaction conditions, B10H13(-) can function as either an electrophile or nucleophile. For reactions with nucleophilic alkynes, such as propyne, the calculations indicate that at the temperatures (approximately 110-120 degrees C) required for these reactions, the ground-state B10H13(-) (1) structure can rearrange to an electrophilic-type cage structure 3 having a LUMO orbital strongly localized on the B6 cage-boron. Alkyne binding at this site followed by subsequent steps involving the formation of additional boron-carbon bonds, hydrogen elimination, protonation, and further hydrogen elimination then lead in a straightforward manner to the experimentally observed ortho-carborane products resulting from alkyne insertion into the decaborane framework. A similar mechanistic sequence was identified for the reaction of propyne with 6-R-B10H12(-) leading to the formation of 1-Me-3-R-1,2-C2B10H11 carboranes. On the other hand, both B10H13(-) and 4,6-C2B7H12(-) have previously been shown to react at much lower temperatures with strongly polarized alkynes, and the DFT and IRC calculations support an alternative mechanism involving initial nucleophilic attack by these polyborane anions at the positive terminal acetylenic carbon to produce terminally substituted olefinic anions. In the case of the B10H13(-) reaction, subsequent cyclization steps were identified that provide a pathway to the experimentally observed arachno-8-(NC)-7,8-C2B10H14(-) carborane. The computational study of B10H13(-) propylene hydroboration also supports a mechanistic pathway involving a cage rearrangement to the electrophilic 3 structure. Olefin-binding at the LUMO orbital localized on the B6 cage-boron, followed by addition of the B6-H group across the olefinic double bond and protonation, then leads to the experimentally observed 6-R-B10H13 products.

Ammonia triborane: a promising new candidate for amineborane-based chemical hydrogen storage.

A convenient and safe method for the synthesis of ammonia triborane is reported along with studies of its hydrolytic reactions that demonstrate ammonia triborane is both soluble and stable in water but that upon the addition of acid or an appropriate transition metal catalyst it rapidly releases hydrogen. These studies indicate that ammonia triborane is a promising material for chemical hydrogen storage applications.