Enriching surface-ordered defects on WO3 for photocatalytic CO2-to-CH4 conversion by water.

Defect engineering has been widely applied in semiconductors to improve photocatalytic properties by altering the surface structures. This study is about the transformation of inactive WO3 nanosheets to a highly effective CO2-to-CH4 conversion photocatalyst by introducing surface-ordered defects in abundance. The nonstoichiometric WO3-x samples were examined by using aberration-corrected electron microscopy. Results unveil abundant surface-ordered terminations derived from the periodic {013} stacking faults with a defect density of 20.2%. The {002} surface-ordered line defects are the active sites for fixation CO2, transforming the inactive WO3 nanosheets into a highly active catalyst (CH4: O2 = 8.2: 16.7 µmol h-1). We believe that the formation of the W-O-C-W-O species is a critical step in the catalytic pathways. This work provides an atomic-level comprehension of the structural defects of catalysts for activating small molecules.

The role of Mo species in Ni-Mo catalysts for dry reforming of methane.

The Ni-Mo catalyst has attracted significant attention due to its excellent coke-resistance in dry reforming of methane (DRM) reaction, but its detailed mechanism is still vague. Herein, Mo-doped Ni (Ni-Mox) and MoOx adsorbed Ni surfaces (MoOx@Ni) are employed to explore the DRM reaction mechanism and the effect of coke-resistance. Due to the electron donor effect of Mo, the antibonding states below the Fermi level between Ni and C increase and the adsorption of C decrease, thereby inhibiting the carbonization of Ni. On account of the strong Mo and O interaction, more O atoms gather around Mo, which inhibits the oxidation of Ni and may promote the formation of MoOx species on the Ni-Mo catalyst. The presence of Mo-O species promotes the carbon oxidation, forming a unique redox cycle (MoOx ↔ MoOx-1) similar to the Mars-van Krevelen (MvK) mechanism, explaining the excellent anti-carbon deposition effect on the Ni-Mo catalyst.

Nitrogen reduction on crystalline carbon nitride supported by homonuclear bimetallic atoms.

Electrocatalytic nitrogen reduction reaction (eNRR) is a new method for sustainable NH3 production, which has attracted much attention in recent years. However, the low Faradaic efficiency due to the competitive hydrogen evolution reaction (HER) and inert N≡N triple bond activation hinders its practical application. To find highly efficient electrocatalysts with excellent activity, stability and selectivity, we have studied a series of transition metal dimers (TM2) loaded on poly triazine imide, (PTI) a crystalline carbon nitride, by density functional theory calculations. The results show that most of the metal dimers have good stability. Finally, among 26 homonuclear diatomic catalysts, Mo2@PTI, Re2@PTI, and Pt2@PTI exhibit strong capability for suppressing HER, with a favorable limiting potential of -0.53, -0.36, and -0.63 V, respectively, and hence, can be used as efficient electrocatalysts for NRR. In this study, a homonuclear diatomic eNRR catalyst was designed and screened to provide not only a theoretical basis for the experiments but also an alternative approach for sustainable synthesis of ammonia.

Atomistic Observation of Temperature-Dependent Defect Evolution within Sub-stoichiometric WO3-x Catalysts.

Tunable crystalline defects endow WO3-x catalysts with extended functionalities for a broad range of photo- and electric-related applications. However, direct visualization of the defect structures and their evolution mechanism is lacking. Herein, aberration-corrected and in situ transmission electron microscopy was complemented by theoretical calculations to investigate the effect of temperature on the defect evolution behavior during hydrogenation treatment. Low processing temperature (100-300 °C) leads to the occurrence of randomly distributed oxygen vacancies within WO3-x nanosheets. At higher temperatures, oxygen vacancies become highly mobile and aggregate into stacking faults. Planar defects are prone to nucleate at the surface and develop in a zigzag form at 400 °C, while treating at 500 °C promotes the growth of {200}-type stacking faults. Our work clearly establishes that the atomic configuration of the defects in WO3-x samples could be manipulated by regulating the hydrogenation temperature. This study not only expands our understanding of the structure-function relationships of sub-stoichiometric tungsten oxides but also unlocks their full potential as advanced catalysts by tuning stoichiometry in a controlled manner.

Fully Condensed Poly (Triazine Imide) Crystals: Extended π-Conjugation and Structural Defects for Overall Water Splitting.

Conventional polymerization for the synthesis of carbon nitride usually generates amorphous heptazine-based melon with an abundance of undesired structural defects, which function as charge carrier recombination centers to decrease the photocatalytic efficiency. Herein, a fully condensed poly (triazine imide) crystal with extended π-conjugation and deficient structure defects was obtained by conducting the polycondensation in a mild molten salt of LiCl/NaCl. The melting point of the binary LiCl/NaCl system is around 550 °C, which substantially restrain the depolymerization of triazine units and extend the π-conjugation. The optimized polymeric carbon nitride crystal exhibits a high apparent quantum efficiency of 12 % (λ=365 nm) for hydrogen production by one-step excitation overall water splitting, owing to the efficient exciton dissociation and the subsequent fast transfer of charge carriers.

Unraveling the mechanisms of S-doped carbon nitride for photocatalytic oxygen reduction to H2O2.

An in-depth understanding of the microscopic reaction mechanism on a nonmetal-doped catalytic system at the atomic level is one of the critical approaches to developing new efficient catalysts. Herein, the effects of S-doping on melon-based carbon nitride (CN) for the photocatalytic selective oxygen reduction reaction (ORR) have been comprehensively investigated by first-principles calculations. The configurations, electronic properties, optical properties, and the reaction performance of the S-doped melon-based CN have been studied and discussed. The results demonstrate that the decoration with S atoms exhibited substantial effects, involving the redistribution of the charge density and tuning of the bandgap, which promote the photocatalytic selective ORR activity. Accordingly, O2 is activated on the S-doped system with about 0.4 e of charge obtained from catalytic surfaces, leading to the thermodynamically feasible H2O2 and H2O formation, which is in good agreement with the experimental results. Our results provide theoretical insights into the design and development of polymeric carbon nitride (PCN) as well as other metal-free photocatalysts for the selective ORR.

Facile Synthesis of Unsolvated Alkali Metal Octahydrotriborate Salts MB3 H8 (M=K, Rb, and Cs), Mechanisms of Formation, and the Crystal Structure of KB3 H8.

A facile synthesis of heavy alkali metal octahydrotriborates (MB3 H8 ; M=K, Rb, and Cs) has been developed. It is simply based on reactions of the pure alkali metals with THF⋅BH3 , does not require the use of electron carriers or the addition of other reaction media such as mercury, silica gel, or inert salts as for previous procedures, and delivers the desired products at room temperature in very high yields. However, no reactions were observed when pure Li or Na was used. The reaction mechanisms for the heavy alkali metals were investigated both experimentally and computationally. The low sublimation energies of K, Rb, and Cs were found to be key for initiation of the reactions. The syntheses can be carried out at room temperature because all of the elementary reaction steps have low energy barriers, whereas reactions of LiBH4 /NaBH4 with THF⋅BH3 have to be carried out under reflux. The high stability and solubility of KB3 H8 were examined, and a crystal structure thereof was obtained for the first time.

Theoretical insight on ethylene polymerization catalyzed by carborane-containing metal complexes: the role of carborane.

In this study, density functional theory calculations have been used to investigate the decisive role that the carborane (Cab) ligand plays in ethylene polymerization catalyzed by Cab-containing metal complexes. In the catalytic process, a large amount of charge transfer (CT) from Cab to the metal centre was found to facilitate ethylene insertion. Besides this, the (Cab)B-HH-C(alkyl) dihydrogen bond interaction was demonstrated to be the main factor in stabilizing the intermediates.

Solvent-promoted catalyst-free regioselective N-incorporation multicomponent domino reaction: rapid assembly of π-functionalized [60]fullerene-fused dihydrocarbolines.

Described here is the first example of solvent-promoted catalyst-free N-incorporation multicomponent domino reaction for the direct construction of novel π-extension [60]fullerene-fused dihydrocarbolines from simple hydrocarbons. This unprecedented transformation is proposed to proceed by a sequential N-incorporation/[4+2] cycloaddition/elimination process, involving multiple types of bond cleavage and formation in a single chemical operation. Mechanistic studies uncover that solvent is crucial for the success of the transformation, and multiple solvent-aided deprotonation and proton transfer are involved as key steps.

The Reactivity of Mercapto Groups against Boron Hydrides in Pincer Ligated Nickel Mercapto Complexes.

Several pincer ligated nickel mercapto complexes, [2,6-(R2 PCH2 )2 C6 H3 ]NiSH (R=tBu, 1 a; iPr, 1 b), [2,6-(R2 PO)2 C6 H3 ]NiSH (R=tBu, 2 a; iPr, 2 b) and [4-MeOCO-2,6-(tBu2 PO)2 C6 H2 ]NiSH (3 a), were synthesized and fully characterized. The reactivity of the mercapto groups against boron hydrides and organic bases was investigated. It was found that the mercapto groups are difficult to be deprotonated by boron hydrides or organic bases. The treatment of complex 2 a or 2 b with an excess amount of catecholborane (HBcat) afforded the corresponding pincer ligated nickel borohydride complexes and the HBcat degradation product. The treatment of complex 1 a, 2 a or 2 b with an excess amount of BH3 ⋅THF produced the corresponding nickel borohydride species and the S-bridged triborane species THF⋅BH2 -µ2 -S(B2 H5 ) (5). No reactions between these complexes and organic bases were observed. DFT calculations were carried out to understand this reactivity and get mechanistic insights into the reactions.

Elucidation of the Formation Mechanisms of the Octahydrotriborate Anion (B3H8-) through the Nucleophilicity of the B-H Bond.

Boron compounds are well-known electrophiles. Much less known are their nucleophilic properties. By recognition of the nucleophilicity of the B-H bond, the formation mechanism of octahydrotriborate (B3H8-) was elucidated on the bases of both experimental and computational investigations. Two possible routes from the reaction of BH4- and THF·BH3 to B3H8- were proposed, both involving the B2H6 and BH4- intermediates. The two pathways consist of a set of complicated intermediates, which can convert to each other reversibly at room temperature and can be represented by a reaction circle. Only under reflux can the B2H6 and BH4- intermediates be converted to B2H5- and BH3(H2) via a high energy barrier, from which H2 elimination occurs to yield the B3H8- final product. The formation of B2H6 from THF·BH3 by nucleophilic substitution of the B-H bond was captured and identified, and the reaction of B2H6 with BH4- to produce B3H8- was confirmed experimentally. On the bases of the formation mechanisms of B3H8-, we have developed a facile synthetic method for MB3H8 (M = Li and Na) in high yields by directly reacting the corresponding MBH4 salts with THF·BH3. In the new synthetic method for MB3H8, no electron carriers are needed, allowing convenient preparation of MB3H8 in large scales and paving the way for their wide applications.