Formation of Supernarrow Borophene Nanoribbons.

Borophenes have sparked considerable interest owing to their fascinating physical characteristics and diverse polymorphism. However, borophene nanoribbons (BNRs) with widths less than 2 nm have not been achieved. Herein, we report the experimental realization of supernarrow BNRs. Combining scanning tunneling microscopy imaging with density functional theory modeling and ab initio molecular dynamics simulations, we demonstrate that, under the applied growth conditions, boron atoms can penetrate the outermost layer of Au(111) and form BNRs composed of a pair of zigzag (2,2) boron rows. The BNRs have a width self-contained to ∼1 nm and dipoles at the edges to keep them separated. They are embedded in the outermost Au layer and shielded on top by the evacuated Au atoms, free of the need for post-passivation. Scanning tunneling spectroscopy reveals distinct edge states, primarily attributed to the localized spin at the BNRs' zigzag edges. This work adds a new member to the boron material family and introduces a new physical feature to borophenes.

Anisotropic Growth of One-Dimensional Carbides in Single-Walled Carbon Nanotubes with Strong Interaction for Catalysis.

Tungsten and molybdenum carbides have shown great potential in catalysis and superconductivity. However, the synthesis of ultrathin W/Mo carbides with a controlled dimension and unique structure is still difficult. Here, inspired by the host-guest assembly strategy with single-walled carbon nanotubes (SWCNTs) as a transparent template, we reported the synthesis of ultrathin (0.8-2.0 nm) W2C and Mo2C nanowires confined in SWCNTs deriving from the encapsulated W/Mo polyoxometalate clusters. The atom-resolved electron microscope combined with spectroscopy and theoretical calculations revealed that the strong interaction between the highly carbophilic W/Mo and SWCNT resulted in the anisotropic growth of carbide nanowires along a specific crystal direction, accompanied by lattice strain and electron donation to the SWCNTs. The SWCNT template endowed carbides with resistance to H2O corrosion. Different from normal modification on the outer surface of SWCNTs, such M2C@SWCNTs (M = W, Mo) provided a delocalized and electron-enriched SWCNT surface to uniformly construct the negatively charged Pd catalyst, which was demonstrated to inhibit the formation of active PdHx hydride and thus achieve highly selective semihydrogenation of a series of alkynes. This work could provide a nondestructive way to design the electron-delocalized SWCNT surface and expand the methodology in synthesizing unusual 1D ultrathin carbophilic-metal nanowires (e.g., TaC, NbC, ß-W) with precise control of the anisotropy in SWCNT arrays.

Aldehyde Hydrogenation by Pt/TiO2 Catalyst in Aqueous Phase: Synergistic Effect of Oxygen Vacancy and Solvent Water.

The aldehyde hydrogenation for stabilizing and upgrading biomass is typically performed in aqueous phase with supported metal catalysts. By combining density functional theory calculations and ab initio molecular dynamics simulations, the model reaction of formaldehyde hydrogenation with a Pt/TiO2 catalyst is investigated with explicit solvent water molecules. In aqueous phase, both the O vacancy (Ov) on support and solvent molecules could donate charges to a Pt cluster, where the Ov could dominantly reduce the Pt cluster from positive to negative. During the formaldehyde hydrogenation, the water molecules could spontaneously protonate the O in the aldehyde group by acid/base exchange, generating the OH* at the metal-support interface by long-range proton transfer. By comparing the stoichiometric and reduced TiO2 support, it is found that the further hydrogenation of OH* is hard on the positively charged Pt cluster over stoichiometric TiO2. However, with the presence of Ov on reduced support, the OH* hydrogenation could become not only exergonic but also kinetically more facile, which prohibits the catalyst from poisoning. This mechanism suggests that both the proton transfer from solvent water molecules and the easier OH* hydrogenation from Ov could synergistically promote aldehyde hydrogenation. That means, even for such simple hydrogenation in water, the catalytic mechanism could explicitly relate to all of the metal cluster, oxide support, and solvent waters. Considering the ubiquitous Ov defects in reducible oxide supports and the common aqueous environment, this synergistic effect may not be exclusive to Pt/TiO2, which can be crucial for supported metal catalysts in biomass conversion.

Pseudo-adsorption and long-range redox coupling during oxygen reduction reaction on single atom electrocatalyst.

Fundamental understanding of the dynamic behaviors at the electrochemical interface is crucial for electrocatalyst design and optimization. Here, we revisit the oxygen reduction reaction mechanism on a series of transition metal (M = Fe, Co, Ni, Cu) single atom sites embedded in N-doped nanocarbon by ab initio molecular dynamics simulations with explicit solvation. We have identified the dissociative pathways and the thereby emerged solvated hydroxide species for all the proton-coupled electron transfer (PCET) steps at the electrochemical interface. Such hydroxide species can be dynamically confined in a "pseudo-adsorption" state at a few water layers away from the active site and respond to the redox event at the catalytic center in a coupled manner within timescale less than 1 ps. In the PCET steps, the proton species (in form of hydronium in neutral/acidic media or water in alkaline medium) can protonate the pseudo-adsorbed hydroxide without needing to travel to the direct catalyst surface. This, therefore, expands the reactive region beyond the direct catalyst surface, boosting the reaction kinetics via alleviating mass transfer limits. Our work implies that in catalysis the reaction species may not necessarily bind to the catalyst surface but be confined in an active region.

Lattice oxygen self-spillover on reducible oxide supported metal cluster: the water-gas shift reaction on Cu/CeO2 catalyst.

In this work we have tackled one of the most challenging problems in nanocatalysis namely understanding the role of reducible oxide supports in metal catalyzed reactions. As a prototypical example, the very well-studied water gas shift reaction catalyzed by CeO2 supported Cu nanoclusters is chosen to probe how the reducible oxide support modifies the catalyst structures, catalytically active sites and even the reaction mechanisms. By employing density functional theory calculations in conjunction with a genetic algorithm and ab initio molecular dynamics simulations, we have identified an unprecedented spillover of the surface lattice oxygen from the ceria support to the Cu cluster, which is rarely considered previously but may widely exist in oxide supported metal catalysts under realistic conditions. The oxygen spillover causes a highly energetic preference of the monolayered configuration of the supported Cu nanocluster, compared to multilayered configurations. Due to the strong metal-oxide interaction, after the O spillover the monolayered cluster is highly oxidized by transferring electrons to the Ce 4f orbitals. The water-gas-shift reaction is further found to more favorably take place on the supported copper monolayer than the copper-ceria periphery, where the on-site oxygen and the adjacent oxidized Cu sites account for the catalytically active sites, synergistically facilitating the water dissociation and the carboxyl formation. The present work provides mechanistic insights into the strong metal-support interaction and its role in catalytic reactions, which may pave a way towards the rational design of metal-oxide catalysts with promising stability, dispersion and catalytic activity.

Using general computational chemistry strategy to unravel the reactivity of emerging pollutants: An example of sulfonamide chlorination.

Increasing number of emerging pollutants in environments requires an effective approach which can facilitate the prediction of reactivity and provide insights into the reaction mechanisms. Computational chemistry is exactly the tool to fulfill this demand with its good performance in theoretical investigation of chemical reactions at molecular level. In this study, chlorination of sulfonamide antibiotics is used as an illustration to present a systematic strategy demonstrating how computational chemistry can be applied to investigate the reaction behavior of emerging pollutants. Sulfonamides is a class of micropollutants that contain the common structure of 4-aminobenzenesulfonmaide while differ in their heterocycles. Based on the calculated conceptual DFT indices, the reactive sites of sulfonamide are successfully predicted, which locate on their common structure of 4-aminobenzenesulfonmaide. Therefore, all sulfonamides follow the similar reaction pathway. Product identification by LTQ-Orbitrap MS further verifies the in silico prediction. Three critical pathways are discovered, i.e., S-N bond cleavage, Cl-substitution onto aniline-N, and the following rearrangement to lose -SO2- group, among which Cl-substitution is the key step due to its lowest free energy barrier. Heterocycles impact the reaction rate by affecting the electronic density of aniline group. In general, the more electron-donating the heterocycle is, the more readily sulfonamides to be chlorinated.

Carbon Monoxide Gas Induced 4H-to-fcc Phase Transformation of Gold As Revealed by In-Situ Transmission Electron Microscopy.

The hexagonal 4H phase gold nanostructures shows great potential for catalysis, optical, and biomedical fields. However, its phase stability remains largely unclear. Here, we report the 4H-to-face-centered cubic (fcc) phase transformation of gold induced by CO gas interactions and an electron beam observed through in-situ transmission electron microscopy (in-situ TEM). The atomic scale transformation mechanism is revealed experimentally and supported by first-principle calculations. Density functional theory calculations show that the 4H-to-fcc phase transformation processes via the transition of layer sliding with expanded layer spacing, which can be facilitated by both the adsorbed CO molecules and the extra electron provided by the electron beam. The transformation first takes place at the edges of the nanorods with the collective assistance of both CO and extra electrons, and then the inner portion of the bulk crystal follows with extra electrons as the lubricant. These results promote the understanding of the toxic effect of CO gas and shining light on the structural conversion and atomic migration of noble metal catalysts when they interact with CO molecules.

Enantioselective photoinduced cyclodimerization of a prochiral anthracene derivative adsorbed on helical metal nanostructures.

The generation of molecular chirality in the absence of any molecular chiral inductor is challenging and of fundamental interest for developing a better understanding of homochirality. Here, we show the manipulation of molecular chirality through control of the handedness of helical metal nanostructures (referred to as nanohelices) that are produced by glancing angle deposition onto a substrate that rotates in either a clockwise or counterclockwise direction. A prochiral molecule, 2-anthracenecarboxylic acid, is stereoselectively adsorbed on the metal nanohelices as enantiomorphous anti-head-to-head dimers. The dimers show either Si-Si or Re-Re facial stacking depending on the handedness of the nanohelices, which results in a specific enantiopreference during their photoinduced cyclodimerization: a left-handed nanohelix leads to the formation of (+)-cyclodimers, whereas a right-handed one gives (-)-cyclodimers. Density functional theory calculations, in good agreement with the experimental results, point to the enantioselectivity mainly arising from the selective spatial matching of either Si-Si or Re-Re facial stacking at the helical surface; it may also be influenced by chiroplasmonic effects.

Gas-assisted transformation of gold from fcc to the metastable 4H phase.

The metastable hexagonal 4H-phase gold has recently attracted extensive interest due to its exceptional performance in catalysis. However, gold usually crystallizes to its lowest free energy structure called face-centered cubic (fcc). The phase transformation from the stable fcc phase to the metastable 4H phase is thus of great significance in crystal phase engineering. Herein, we report this unusual phenomenon on a 4H gold nanorod template with the aid of CO gas and an electron beam. In situ transmission electron microscopy was used to directly visualize the interface propagation kinetics between the 4H-Au-nanorod and fcc-Au nanoparticle. Epitaxial growth was initiated at the contact interface, and then propagated to convert all parts of these fcc nanoparticles to 4H phase. Density functional theory calculations and ab initio molecular dynamics simulations show that the CO molecules can assist the Au diffusion process and promote the flexibility of Au particles during the epitaxial growth. The phase transformation was driven by the reduction of Gibbs free energy by eliminating the interface between fcc and 4H phases.

Structural inhomogeneity as a factor promoting the homogenous catalysis of CO2 hydrogenation by (PMe3)4RuH2.

During homogenous catalysis by organometallic complexes, the dissociation of a ligand to produce an unsaturated site on the metal center is often invoked as the first step of activation, especially when photo-excitation is involved. In this theoretical study, we demonstrated that under mild conditions, a thermodynamically unstable yet dynamically favorable active intermediate could be produced by the inhomogeneity of the solvent distribution around the catalyst rather than by ligand dissociation. This occurred at the end of the first catalytic cycle when the product was eliminated. The empty site was immediately filled by one of the additive molecules aggregated around the reaction center even when the intermediate complex was unstable, producing a transient and more active catalyst. This process accounted for the accelerated reaction rate observed in the landmark CO2 hydrogenation catalyzed by (PMe3)4RuH2 in supercritical CO2 when H2O, MeOH, or HNMe2 was added. This also suggests a new way to exploit the structural inhomogeneity around an organometallic complex for the design of superior catalysts.

A comparative study on the CO2 hydrogenation catalyzed by Ru dihydride complexes: (PMe3)4RuH2 and (Me2PCH2CH2PMe2)2RuH2.

The phosphine complexes of Ru dihydride are model catalysts for CO2 hydrogenation. Despite many theoretical studies, important questions remain unresolved regarding the underlying catalytic mechanisms. We report a comparative study by using density functional theory on two catalysts, (PMe3)4RuH2 and (dmpe)2RuH2, with dmpe = Me2PCH2CH2PMe2, for which very different mechanisms have been suggested in previous studies. By comparing their energy profiles along all possible reaction paths side by side, we are able to clarify the similarity and difference between them, and provide a consistent account for all the experimental observations reported, including the kinetic models, the cis to trans transformation of (dmpe)2RuH2, and the significant enhancement of the catalytic rate in supercritical CO2 for (PMe3)4RuH2 and its lack thereof for (dmpe)2RuH2. The crucial difference between the two mechanisms involves the formation of an intermediate, in which a formate ion binds to Ru as a bidentate ligand. When this step results in the dissociation of a ligand, the reaction rate is enhanced under supercritical conditions, due to the increase in entropy, which should be a valid consideration for other catalytic reactions as well.

Facile Assembly of Chiral Metallosquares by Using Enantiopure Tribenzotriquinacene Corner Motifs.

A pair of enantiomerically pure metallosquares based on linear platinum-diacetylene edges and tribenzotriquinacene corner units was synthesized. Their structures were characterized by (1) H-, (13) C- and (31) P NMR spectroscopy as well as MALDI-TOF mass spectrometry and circular dichroism. Based on DFT calculation, the optimized geometry possesses a distorted square conformation in which the four edges are not sitting on the same plane. The molecular square further self-assembled in the solid state to afford microspheres with diameter of approximately 300 nm, as determined by scanning electron microscopy.

Facile Assembly of Chiral Metallosquares by Using Enantiopure Tribenzotriquinacene Corner Motifs.

Invited for the cover of this issue are Wen-Rong Wu, Guang-Jie Xia, and Hak-Fun Chow of The Chinese University of Hong Kong, Xiao-Ping Cao of Lanzhou University and Dietmar Kuck of Bielefeld University. The image depicts how different routes can lead to the same goal. Read the full text of the article at 10.1002/chem.201501556.