Effect of External Electric Field on Nitrogen Activation on a Trimetal Cluster.

Efficient nitrogen (N2) fixation and activation under mild conditions are crucial for modern society. External electric fields (Felectric) can significantly affect N2 activation. In this work, the effect of Felectric on N2 activation by Nb3 clusters supported in a sumanene bowl was studied by density functional theory calculations. Four typical systems at different stages of N-N activation were studied, including two intermediates and two transition states. The impact of Felectric on various properties related to N2 activation was investigated, including the N-N bond length, overlap population density of states (OPDOS), total energy of the system, adsorption energy of N2, decomposition of energy changes, and electron transfer. The sumanene not only functions as a support and protective substrate, but also serves as a donor or acceptor under different Felectric conditions. Negative Felectric is beneficial to N-N bond activation because it promotes electron transfer to the N-N region and improves the d-π* orbital hybridization between metals and N2 in the activation process. Positive Felectric improves d-π* orbital hybridization only when the N-N is nearly dissociated. The microscopic mechanism of Felectric's effects provides insight into N2 activation and theoretical guidance for the design of catalytic reaction conditions for nitrogen reduction reactions (NRR).

Catalysts with Trimetallic Sites on Graphene-like C2N for Electrocatalytic Nitrogen Reduction Reaction: A Theoretical Investigation.

Electrocatalytic nitrogen reduction reaction (NRR) is a green and highly efficient way to replace the industrial Haber-Bosch process. Herein, clusters consisting of three transition metal atoms loaded on C2N as NRR electrocatalysts are investigated using density functional theory (DFT). Meanwhile, Ca was introduced as a promoter and the role of Ca in NRR was investigated. It was found that Ca anchored to the catalyst can act as an electron donor and effectively promote the activation of N2 on M3. In both M3@C2N and M3Ca@C2N (M = Fe, Co, Ni), the limiting potential (UL) is less negative than that of the Ru(0001) surface and has the ability to suppress the competitive hydrogen evolution reaction (HER). Among them, Fe3@C2N is suggested to be the most promising candidate for NRR with high thermal stability, strong N2 adsorption ability, low limiting potential, and good NRR selectivity. The concepts of trimetallic sites and alkaline earth metal promoters in this work provide theoretical guidance for the rational design of atomically active sites in electrocatalytic NRR.

Exploring the stability and aromaticity of rare earth doped tin cluster MSn16- (M = Sc, Y, La).

Rare earth elements have high chemical reactivity, and doping them into semiconductor clusters can induce novel physicochemical properties. The study of the physicochemical mechanisms of interactions between rare earth and tin atoms will enhance our understanding of rare earth functional materials from a microscopic perspective. Hence, the structure, electronic characteristics, stability, and aromaticity of endohedral cages MSn16- (M = Sc, Y, La) have been investigated using a combination of the hybrid PBE0 functional, stochastic kicking, and artificial bee colony global search technology. By comparing the simulated results with experimental photoelectron spectra, it is determined that the most stable structure of these clusters is the Frank-Kasper polyhedron. The doping of atoms has a minimal influence on density of states of the pure tin system, except for causing a widening of the energy gap. Various methods such as ab initio molecular dynamics simulations, the spherical jellium model, adaptive natural density partitioning, localized orbital locator, and electron density difference are employed to analyze the stability of these clusters. The aromaticity of the clusters is examined using iso-chemical shielding surfaces and the gauge-including magnetically induced currents. This study demonstrates that the stability and aromaticity of a tin cage can be systematically adjusted through doping.

Heteronuclear Trimetallic MFe2 and M2 Fe (M=V, Nb, and Ta) Clusters for Dinitrogen Activation.

Catalysts with heteronuclear metal active sites may have high performance in the nitrogen reduction reaction (NRR), and the in-depth understanding of the reaction mechanisms is crucial for the design of related catalysts. In this work, the dissociative adsorption of N2 on heteronuclear trimetallic MFe2 and M2 Fe (M=V, Nb, and Ta) clusters was studied with density functional theory calculations. For each cluster, two reaction paths were studied with N2 initially on M and Fe atoms, respectively. Mayer bond order analysis provides more information on the activation of N-N bonds. M2 Fe is generally more reactive than MFe2 . The coordination mode of N2 on three metal atoms can be end-on: end-on: side-on (EES) for both MFe2 and M2 Fe. In addition, a unique end-on: side-on: side-on (ESS) coordination mode was found for M2 Fe, which leads to a higher degree of N-N bond activation. Nb2 Fe has the highest reactivity towards N2 when both the transfer of N2 and the dissociation of N-N bonds are taken into account, while Ta-containing clusters have a superior ability to activate the N-N bond. These results indicate that it is possible to improve the performance of iron-based catalysts by doping with vanadium group metals.

Trimetallic clusters in the sumanene bowl for dinitrogen activation.

It is of great importance to find catalysts for the nitrogen reduction reaction (NRR) with high stability and reactivity. A series of M3 clusters (M = Ti, Zr, V, and Nb) supported on sumanene (C21H12) were designed as potential catalysts for the NRR by taking advantage of the high reactivity of trimetallic clusters and the unique geometric and electronic properties of sumanene, a bowl-like organic molecule. Detailed mechanisms of NN bond cleavage on C21H12-M3 were investigated by DFT calculations and compared with those on bare M3 clusters. M3 in the sumanene bowl is very stable with large binding energies, which prohibits the cohesion of M3 into M6. In the bowl, M3 has a (quasi-) equilateral triangle structure with lengthened M-M bonds, which is particularly beneficial to the N2 transfer process on Ti3 and V3 clusters. The N-N bond can be dissociated by both M3 and C21H12-M3 clusters without the overall energy barriers. A blurring effect is found in which some geometric and electronic properties of different metal types become similar when M3 is supported on the substrate. Our work demonstrates that sumanene is a suitable substrate to support M3 in the activation of N2 with enhanced stability and maintained a high level of reactivity compared to bare M3.

Comparison of Nitrogen Activation on Trinuclear Niobium and Tungsten Sulfide Clusters Nb3 Sn and W3 Sn (n=0-3): A DFT Study.

The front cover artwork is provided by Prof. Xun-Lei Ding's group at North China Electric Power University (NCEPU). The image shows the cleavage of the triple bond of a dinitrogen molecule on trinuclear metal clusters with sulfide ligands, which is the critical step in nitrogen reduction reactions (NRR). Read the full text of the Research Article at 10.1002/cphc.202200124.

Comparison of Nitrogen Activation on Trinuclear Niobium and Tungsten Sulfide Clusters Nb3 Sn and W3 Sn (n=0-3): A DFT Study.

The reaction of N2 with trinuclear niobium and tungsten sulfide clusters Nb3 Sn and W3 Sn (n=0-3) was systematically studied by density functional theory calculations with TPSS functional and Def2-TZVP basis sets. Dissociations of N-N bonds on these clusters are all thermodynamically allowed but with different reactivity in kinetics. The reactivity of Nb3 Sn is generally higher than that of W3 Sn . In the favorite reaction pathways, the adsorbed N2 changes the adsorption sites from one metal atom to the bridge site of two metal atoms, then on the hollow site of three metal atoms, and at that place, the N-N bond dissociates. As the number of ligand S atoms increases, the reactivity of Nb3 Sn decreases because of the hindering effect of S atoms, while W3 S and W3 S2 have the highest reactivity among four W3 Sn clusters. The Mayer bond order, bond length, vibrational frequency, and electronic charges of the adsorbed N2 are analyzed along the reaction pathways to show the activation process of the N-N bond in reactions. The charge transfer from the clusters to the N2 antibonding orbitals plays an essential role in N-N bond activation, which is more significant in Nb3 Sn than in W3 Sn , leading to the higher reactivity of Nb3 Sn . The reaction mechanisms found in this work may provide important theoretical guidance for the further rational design of related catalytic systems for nitrogen reduction reactions (NRR).

Facile N≡N Bond Cleavage by Anionic Trimetallic Clusters V3-x Tax C4- (x=0-3): A DFT Study.

Activation of N2 on anionic trimetallic V3-x Tax C4- (x=0-3) clusters was theoretically studied employing density functional theory. For all studied clusters, initial adsorption of N2 (end-on) on one of the metal atoms (denoted as Site 1) is transferred to an of end-on: side-on: side-on coordination on three metal atoms, prior to N2 dissociation. The whole reaction is exothermic and has no global energy barriers, indicating that the dissociation of N2 is facile under mild conditions. The reaction process can be divided into two processes: N2 transfer (TRF) and N-N dissociation (DIS). For V-series clusters, which has a V atom on Site 1, the rate-determining step is DIS, while for Ta-series clusters with a Ta on Site 1, TRF may be the rate-determining step or has energy barriers similar to those of DIS. The overall energy barriers for heteronuclear V2 TaC4- and VTa2 C4- clusters are lower than those for homonuclear V3 C4- and Ta3 C4- , showing that the doping effect is beneficial for the activation and dissociation of N2 . In particular, V-Ta2 C4- has low energy barriers in both TRF and DIS, and it has the highest N2 adsorption energy and a high reaction heat release. Therefore, a trimetallic heteronuclear V-series cluster, V-Ta2 C4- , is suggested to have high reactivity to N2 activation, and may serve as a prototype for designing related catalysts at a molecular level.

Non-Dissociative Activation of Chemisorbed Dinitrogen on One or Two Vanadium Atoms Supported by a Mo6 S8 Cluster.

Adsorption of N2 on Mo6 S8 q _Vx clusters (x=0, 1, 2; q=0, ±1) were systematically studied by density functional theory calculations with dispersion corrections. It was found that the N2 can be chemisorbed and undergo non-dissociative activation on single or double metal atoms. The adsorption and activation are influenced by metal types (V or Mo), N2 coordination modes and charge states of the clusters. Particularly, anionic Mo6 S8 - _V2 clusters have remarkable ability to fix and activate N2 . In Mo6 S8 - _V2 , two V atoms prefer to adsorb on two adjacent S-Mo-S hollow sites, leading to the formation of a supported V…V unit. The N2 is bridged side-on coordinated with these two V atoms with high adsorption energy and significant charge transfer. The bond order, bond length and vibration frequency of the adsorbed N2 are close to those of a N-N single bond.

Non-stoichiometric molybdenum sulfide clusters and their reactions with the hydrogen molecule.

Structures of non-stoichiometric MoxSy clusters (x = 2-4; y = 2-10) were studied by density functional calculations with global optimization. Besides 1T phase like structures, a novel regular grid structure in which Mo atoms are well separated by S atoms was found, which might be used as a building-block to construct a new type of two-dimensional molybdenum sulfide monolayer. The hydrogen molecule prefers to be adsorbed onto Mo atoms rather than S atoms, and Mo atoms with less S coordination have a higher ability to adsorb H2. In addition, the reaction pathways for H2 dissociation were studied on two clusters with the highest H2 adsorption energy (Mo2S4 and Mo3S3). The vacant bridge site of Mo-Mo in S-deficient clusters, which corresponds to the sulfur vacancy in the bulk phase MoS2, is favored by H atom adsorption and plays an important role in the H atom transfer on MoxSy clusters. Our results provide a new aspect to understand the reason why S defect in MoS2 and MoS2 with an Mo-edge could enhance the catalytic performance in the hydrogen evolution reaction.

Methane activation by heteronuclear diatomic AuRh+ cation: comparison with homonuclear Au2+ and Rh2.

The ability to activate methane differs appreciably for different transition metals, and it is attractive to find the most suitable metal for the direct conversion of methane to value-added chemicals. Herein, we performed a comparative study on the reactions of CH4 with Au2+, AuRh+ and Rh2+ cations by mass-spectrometry based experiments and DFT-based theoretical analysis. Different reactivity has been found for these cations: Au2+ has the lowest reactivity, and it can activate methane but only produce H-Au2-CH3+ without H2 release; Rh2+ has the highest reactivity, and it can produce both carbene-type Rh2-CH2+ and carbyne-type H-Rh2-CH+ with H2 release; AuRh+ also has high reactivity to produce only AuRh-CH2+ with H2, avoiding the excessive dehydrogenation of CH4. Our theoretical results demonstrate that Rh is responsible for the high reactivity, while Au leads to selectivity, which may be caused by the unique intrinsic bonding properties of the metals.

Synthesis of 2D MoS2(1-x)Se2x semiconductor alloy by chemical vapor deposition.

Alloying/doping in two-dimensional (2D) materials is emerging as an increasingly important strategy due to the wide-range bandgap tunability and versatility of these materials. Monolayer 2D transition metal dichalcogenide (TMD) alloy has been investigated both theoretically and experimentally in recent years. Here, we synthesized a bilayer MoS2(1-x)Se2x semiconductor alloy via the chemical-vapor deposition technique. The as-grown triangular MoS2(1-x)Se2x flakes with size of roughly 10 µm were observed by optical microscope and scanning electron microscope (SEM). The 1.4-1.9 nm thickness of the samples, as measured by AFM, means that bilayer MoS2(1-x)Se2x alloys were grown. The characteristic Raman modes related to Mo-S and Mo-Se vibrations were observed in the Raman spectrum. Two emission peaks were respectively found, corresponding to the A and B excitons in the photoluminescence (PL) spectrum. XPS measurements confirmed the Se doping of the alloy. The first-principles calculation results show a contraction of the band gap value with the increase of Se doping in the MoS2 lattice. Compared with monolayer MoS2(1-x)Se2x alloy, the band bending effect is more obvious, and the bilayer MoS2(1-x)Se2x alloy still shows the direct band gap luminescence characteristic, which has certain guiding significance for the growth of two-dimensional materials and for device preparation.

Side-on-End-on Coordination of Dinitrogen on a Polynuclear Vanadium Nitride Cluster Anion [V5 N5 ].

The side-on-end-on coordination of N2 can be very important to activate and functionalize this very stable molecule. However, such coordination has rarely been reported. This study reports a gas-phase species (a polynuclear vanadium nitride cluster anion [V5 N5 ]- ) that can capture N2 efficiently (12 %), and the quantum chemistry modelling suggests an unusual side-on-end-on coordination. The cluster anions were generated by laser ablation and the reaction with N2 has been characterized by mass spectrometry, photoelectron imaging spectroscopy, and density functional theory calculations. The back-donation interactions between the localized d-d bonding orbitals on the low-coordinated dual metal (V) sites and the antibonding π* orbitals of N2 are the driving forces to adsorb N2 with a high binding energy (about 2.0 eV).

Activity of Atomically Precise Titania Nanoparticles in CO Oxidation.

Understanding the property evolution of atomically precise nanoparticles atom by atom along the size continuum is critical for selecting potential candidates to assemble nanomaterials with desired functionality, but it is very challenging experimentally especially for systems having mixtures of elements such as metal oxides. In this work, the capability to oxidize carbon monoxide has been measured experimentally for titania nanocluster anions of (TiO2 )n Om - (-3≤m≤3) across a broad size range in the gas phase. Stoichiometric (TiO2 )n O- exhibits superior oxidative activity over other clusters of (TiO2 )n Om - (m≠1) even when the cluster dimensions are scaled up to n=60, indicating that each atom still influences the chemical behavior of titania nanoparticles composed of ≈180 atoms. The fascinating result not only identifies a promising building block of Tin O2n+1 for devising new nanoscale titania materials with desirable oxidative activity, but also provides compelling molecular-level evidence for the Mars-van Krevelen mechanism of CO oxidation over titania supports.

Mechanistic Variants in Methane Activation Mediated by Gold(I) Supported on Silicon Oxide Clusters.

Cationic gold has been frequently identified as a suitable reactive species for activating methane in condensed-phase studies. However, it is far from clear how the coordination site manipulates the activity of such species. Herein, by anchoring AuI on silicon oxide cluster supports of variable sizes, the site-specific methane activation by AuI -Ox has been clarified by mass spectrometry in conjunction with quantum chemistry calculations. An unexpected mechanistic switch in C-H activation was identified for the cluster anions Au(SiO2 )n O- (n=1-3) that selectively activate one of the four C-H bonds of methane with different reaction efficiencies: a low efficiency was observed for the two-fold-coordinated gold ion (AuI, 2f ), which was anchored on an AuSiO3 - or AuSi2 O5 - cluster, through an oxidative addition mechanism (a homolytic process), and high efficiency was observed for the one-fold-coordinated gold ion (AuI, 1f ), which was supported on an AuSi3 O7 - cluster, through Lewis acid/base pairs mechanism (AuI, 1f ⋅⋅⋅O2- , a heterolytic process). Fine regulation of the 5d orbital level of the Au atom by the oxygen ligands accounted for the mechanistic difference between AuI, 2f and AuI, 1f species. The mechanistic understanding of the reactivity of AuI -Ox at a strictly molecular level can be used to clarify the dissimilar activity of gold anchored on different oxide supports.

Small stoichiometric (MoS2)n clusters with the 1T phase.

Stoichiometric (MoS2)n clusters (n = 1-6) were systematically studied by density functional theory calculations with hybrid B3LYP and pure GGA PW91 functionals. The most stable structures of these clusters were obtained by global optimizations with a genetic algorithm. A triangle of Mo3 capped with a S atom was found to be a favourite building block, which can construct the most stable structures (or at least candidates) for n = 3-6 clusters. These types of structures can be viewed as fragments of MoS2 monolayer with the 1T phase, which is an important phase that has unique reactivity and conductivity properties. Electronic structures were analyzed by means of density of states and frontier molecular orbitals (FMOs), which suggest that both Mo and S contribute to the FMOs below the Fermi energy while Mo atoms contribute more toward the unoccupied FMOs.

Support effects on adsorption and catalytic activation of O2 in single atom iron catalysts with graphene-based substrates.

The adsorption and catalytic activation of O2 on single atom iron catalysts with graphene-based substrates were investigated systematically by density functional theory calculation. It is found that the support effects of graphene-based substrates have a significant influence on the stability of the single atom catalysts, the adsorption configuration, the electron transfer mechanism, the adsorption energy and the energy barrier. The differences in the stable adsorption configuration of O2 on single atom iron catalysts with different graphene-based substrates can be well understood by the symmetrical matching principle based on frontier molecular orbital analysis. There are two different mechanisms of electron transfer, in which the Fe atom acts as the electron donor in single vacancy graphene-based substrates while the Fe atom mainly acts as the bridge for electron transfer in double vacancy graphene-based substrates. The Fermi softness and work function are good descriptors of the adsorption energy and they can well reveal the relationship between electronic structure and adsorption energy. This single atom iron catalyst with single vacancy graphene modified by three nitrogen atoms is a promising non-noble metal single atom catalyst in the adsorption and catalytic oxidation of O2. Furthermore, the findings can lay the foundation for the further study of graphene-based support effects and provide a guideline for the development and design of new non-noble-metal single atom catalysts.

H2 Oxidation Mediated by Au1-Doped Vanadium Oxide Cluster Cation AuV2O5+: A Comparative Study with AuCe2O4.

To clarify the relationship between the type of the oxide support and the activity of the gold-doped oxide clusters toward H2 oxidation, a suitable closed-shell system AuV2O5+ is chosen to have a comparative study with AuCe2O4+, the first closed-shell cluster that is reactive toward H2 oxidation. The reaction of AuV2O5+ with H2 was characterized by mass spectrometry and density functional theory calculations. The AuV2O5+ cluster is reactive toward H2 leading to the major product of V2O5H2+ (+ Au), whereas the product of AuV2O4+ (+ H2O) is completely absent in the experiment. This is in sharp contrast with the similar reaction system of AuCe2O4+ with H2, in which the formation of H2O was experimentally evidenced. Theoretical calculations revealed that the distinct reaction behaviors between AuV2O5+ and AuCe2O4+ can be attributed to the gold-metal bond strength, which plays an important role in anchoring the gold atom. The weaker Au-V bond promotes the evaporation of Au, which has a negative effect on the total oxidation of H2 to H2O. This comparative study provides molecular-level mechanisms to understand the important roles of the gold-metal bond in the oxidation of hydrogen molecule over metal oxide supports.

A theoretical investigation on doping superalkali for triggering considerable nonlinear optical properties of Si12 C12 nanostructure.

In this work, we designed a series of superalkali-doped Si12 C12 nanocage M3 O@Si12 C12 (M = Li, Na, K) with donor-acceptor framework. Density functional theory calculations demonstrated that the HOMO-LUMO gap of the complexes conspicuously narrowed with increase of atomic number of the alkali metal, the value decreased from 5.452 eV of pure Si12 C12 nanocage to 3.750, 2.984, and 2.634 eV of Li3 O@Si12 C12 , Na3 O@Si12 C12 , and K3 O@Si12 C12 , respectively. This finding shows that the pristine Si12 C12 cluster could be transformed to n-type semiconductor by introduction of the superalkali M3 O. We also showed that the superalkali doping remarkably enhanced the first hyperpolarizability of Si12 C12 . Among the studied systems, K3 O@Si12 C12 not only has the narrowest gap but also has the strongest nonlinear optical (NLO) properties, its first hyperpolarizability reached as high as 21695 a.u. The striking results presented in this work will be beneficial for potential applications of the Si12 C12 -based nanostructure in the electronic nanodevices and high-performance NLO materials. © 2017 Wiley Periodicals, Inc.

Geometric and electronic properties of gold clusters doped with a single oxygen atom.

The adsorption behaviour of a single O atom on Aunq clusters (n = 1-8, q = 0, ±1) was systematically investigated by DFT calculations. Both hybrid and pure GGA functionals (B3LYP and PBE) were used to provide reliable conclusions. The most stable structures of AunOq clusters were obtained by using global optimizations with a genetic algorithm. Cationic clusters tend to become three-dimensional for large clusters, as for Au8O+. The binding of O in AunOq clusters is quite strong, especially in the anionic clusters. The O atom can be bound to one, two, or three Au atoms, obtaining nearly one electron from gold atoms. Similarities have been found between AunOq and Aun+1q in terms of geometric structures and binding energies. Frontier molecular orbitals and the distribution of unpaired spin density on the O atom were discussed, both of which have a close relationship with the activity of the clusters.