Saturated hydrocarbons, or alkanes, are major constituents of natural gas and oil. Directly transforming alkanes into more complex organic compounds is a value-adding process, but the task is very ...difficult to achieve, especially at low temperature. Alkanes can react at high temperature, but these reactions (with oxygen, for example) are difficult to control and usually proceed to carbon dioxide and water, the thermodynamically stable byproducts. Consequently, a great deal of research effort has been focused on generating and studying chemical entities that are able to react with alkanes or efficiently activate C–H bonds at lower temperatures, preferably room temperature. To identify low-temperature methods of C–H bond activation, researchers have investigated free radicals, that is, species with open-shell electronic structures. Oxygen-centered radicals are typical of the open-shell species that naturally occur in atmospheric, chemical, and biological systems. In this Account, we survey atomic clusters that contain oxygen-centered radicals (O–•), with an emphasis on radical generation and reaction with alkanes near room temperature. Atomic clusters are an intermediate state of matter, situated between isolated atoms and condensed-phase materials. Atomic clusters containing the O–• moiety have generated promising results for low-temperature C–H bond activation. After a brief introduction to the experimental methods and the compositions of atomic clusters that contain O–• radicals, we focus on two important factors that can dramatically influence C–H bond activation. The first factor is spin. The O–•-containing clusters have unpaired spin density distributions over the oxygen atoms. We show that the nature of the unpaired spin density distribution, such as localization and delocalization within the clusters, heavily influences the reactivity of O–• radicals in C–H bond activation. The second factor is charge. The O–•-containing clusters can be negatively charged, positively charged, or neutral overall. We discuss how the charge state may influence C–H bond activation. Moreover, for a given charge state, such as the cationic state, it can be demonstrated that local charge distribution around the O–• centers can also significantly change the reactivity in C–H bond activation. Through judicious synthetic choices, spin and charge can be readily controllable physical quantities in atomic clusters. The adjustment of these two properties can impact C–H bond activation, thus constituting an important consideration in the rational design of catalysts for practical alkane transformations.
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The reaction of N2 with trinuclear niobium and tungsten sulfide clusters Nb3Sn and W3Sn (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 Nb3Sn is generally higher than that of W3Sn. 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 Nb3Sn decreases because of the hindering effect of S atoms, while W3S and W3S2 have the highest reactivity among four W3Sn 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 Nb3Sn than in W3Sn, leading to the higher reactivity of Nb3Sn. 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).
Dissociations of N−N bonds on trinuclear niobium and tungsten sulfide clusters Nb3Sn and W3Sn (n=0–3) are all thermodynamically allowed. Nb3Sn clusters have higher reactivity than W3Sn clusters. As the number of S atoms increases, the reactivity of Nb3Sn clusters decrease, while that for W3Sn the reactivity first rises and then falls.
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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 V5N5−) 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).
N2 fixation: The polynuclear vanadium nitride cluster anion V5N5− can capture N2 efficiently (12 %) with the unusual side‐on‐end‐on coordination. 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).
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
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.
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.
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 M2Fe (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. M2Fe 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 M2Fe. In addition, a unique end‐on: side‐on: side‐on (ESS) coordination mode was found for M2Fe, which leads to a higher degree of N−N bond activation. Nb2Fe 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.
The end‐on: end‐on: side‐on (EES) coordination mode of N2 on three metal atoms was found for both MFe2 and M2Fe (M=V, Nb, and Ta), while the unique end‐on: side‐on: side‐on (ESS) coordination mode was found only for M2Fe, which leads to a higher degree of N−N bond activation.
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Adsorption of N2 on Mo6S8q_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 Mo6S8−_V2 clusters have remarkable ability to fix and activate N2. In Mo6S8−_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.
Activation of dinitrogen: Two V atoms supported on a Mo6S8 cluster can activate N2 effectively. Different coordination modes of N2 may cause great differences in the activation of N−N bonds. The net charge in the system also has a great influence and anionic clusters have higher activity. The N−N bonds in the adsorbed systems can be approximately regarded as single, double, or triple bonds.
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Activation of N2 on anionic trimetallic V3−xTaxC4− (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 V2TaC4− and VTa2C4− clusters are lower than those for homonuclear V3C4− and Ta3C4−, showing that the doping effect is beneficial for the activation and dissociation of N2. In particular, V−Ta2C4− 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−Ta2C4−, is suggested to have high reactivity to N2 activation, and may serve as a prototype for designing related catalysts at a molecular level.
Activation of N2 is critical in nitrogen reduction reactions (NRR). The activation of N2 on trimetallic anionic clusters V3−xTaxC4− (x=0–3) was studied by density functional theory calculations. VTa2C4−, a heteronuclear cluster, can activate N2 through N2 transfer and N−N dissociation processes with low energy barriers.
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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)nOm− (−3≤m≤3) across a broad size range in the gas phase. Stoichiometric (TiO2)nO− exhibits superior oxidative activity over other clusters of (TiO2)nOm− (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 TinO2n+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.
Bigger is not always better: The oxidative activity of (TiO2)nO− clusters is superior to that of the cluster series (TiO2)nOm− (m≠1) across a broad size range. Each atom contributes to the chemical behavior of titania nanoparticles for clusters containing up to 60 Ti atoms, and the Mars–van Krevelen mechanism of CO oxidation over titania is supported by molecular‐level evidence.
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