Low Activation Barrier Research Articles

Surface modification of Fe5C2 by binding silica-based ligand: A theoretical explanation of enhanced C2 oxygenate selectivity

Elucidating the role(s) of support-like ligands remains a challenge in catalytic reaction like the Fischer-Tropsch Synthesis (FTS) catalyzed by the iron catalyst supported on silica. We herein theoretically investigated surface modification of Fe5C2 by silica-based ligand and its influence on C2 oxygenate selectivity in FTS, by carrying out DFT calculations on dissociation of CO and formations of CH4 and C2 on ha-SiO2/Fe5C2(510). To mimic the structure of surface modification, the ha-SiO2/Fe5C2(510) model was built up by binding the silica cluster (the ha-SiO2 ligand) to Fe5C2(510). DFT calculations elucidated that the C + CH coupling with the lowest activation barrier among all the possible routes of C2 formation on Fe5C2(510) is suppressed after modification with the ha-SiO2 ligand because the binding ha-SiO2 ligand limits the geometry relaxation caused by the C+CH coupling. However, CO molecule is anchored by the ha-SiO2 ligand via hydrogen bond, suppressing the CO cleavage because the d-valence band center of Fe5C2(510) lowers in energy by surface modification with the ha-SiO2 ligand, but facilitating the C + CO coupling with the lowest activation barrier among all the possible routes of C2 formation. Also, CH4 formation in the ha-SiO2/Fe5C2(510) case is not so easy as that on Fe5C2(510). Therefore, C2 oxygenate is formed more easily in the ha-SiO2/Fe5C2(510) case than in the Fe5C2(510) case. The result agrees with the experimental observation that C2 oxygenate selectivity became high for iron-based FTS catalyst after surface modification of by silica-based ligand.

Intramolecular Proton-Coupled Hydride Transfers with Relatively Low Activation Barriers.

We report that bifunctional molecules containing hydroxyl and carbonyl functional groups can undergo an effective transfer hydrogenation via an intramolecular proton-coupled hydride transfer (PCHT) mechanism. In this reaction mechanism, a hydride transfer between two carbon atoms is coupled with a proton transfer between two oxygen atoms via a cyclic bond rearrangement transition structure. The coupled transfer of the two hydrogens as Hδ+ and Hδ- is supported by atomic polar tensor charges. The activation energy for the PCHT reaction is strongly dependent on the length of the alkyl chain between the hydroxyl and carbonyl functional groups but relatively weakly dependent on the functional groups attached to the hydroxyl and carbonyl carbons. We investigate the PCHT reaction mechanism using the Gaussian-4 thermochemical protocol and obtain high activation energy barriers (ΔH‡298) of 210.5-228.3 kJ mol-1 for chain lengths of one carbon atom and 160.2-163.9 kJ mol-1 for chain lengths of two carbon atoms. However, for longer chain lengths containing 3-4 carbon atoms, we obtain ΔH‡298 values as low as 101.9 kJ mol-1. Importantly, the hydride transfer between two carbon atoms proceeds without the need for a catalyst or hydride transfer activating agent. These results indicate that the intramolecular PCHT reaction provides an effective avenue for uncatalyzed, metal-free hydride transfers at ambient temperatures.

Confined Ru Sites in a 13X Zeolite for Ultrahigh H2 Production from NH3 Decomposition.

Catalytic NH3 synthesis and decomposition offer a new promising way to store and transport renewable energy in the form of NH3 from remote or offshore sites to industrial plants. To use NH3 as a hydrogen carrier, it is important to understand the catalytic functionality of NH3 decomposition reactions at an atomic level. Here, we report for the first time that Ru species confined in a 13X zeolite cavity display the highest specific catalytic activity of over 4000 h-1 for the NH3 decomposition with a lower activation barrier, compared to most reported catalytic materials in the literature. Mechanistic and modeling studies clearly indicate that the N-H bond of NH3 is ruptured heterolytically by the frustrated Lewis pair of Ruδ+-Oδ- in the zeolite identified by synchrotron X-rays and neutron powder diffraction with Rietveld refinement as well as other characterization techniques including solid-state nuclear magnetic resonance spectroscopy, in situ diffuse reflectance infrared transform spectroscopy, and temperature-programmed analysis. This contrasts with the homolytic cleavage of N-H displayed by metal nanoparticles. Our work reveals the unprecedented unique behavior of cooperative frustrated Lewis pairs created by the metal species on the internal zeolite surface, resulting in a dynamic hydrogen shuttling from NH3 to regenerate framework Brønsted acid sites that eventually are converted to molecular hydrogen.

The Molecular Mechanism of H2O2 Decomposition in a Reaction with an Au25(SCH3)12 Cluster

The reactions of neutral and anionic Au25(SCH3)12 clusters with one H2O2 molecule (mechanism I) and with its dimer (H2O2)2 (mechanism II) have been studied within the framework of the density functional theory (DFT). It has been established that all processes proceed with low activation barriers and a large gain in energy during the formation of products, and also that mechanisms I and II are interconnected. Based on the calculated data, the structure of gold clusters with the most probable active centers for further interaction with methane, which contain one or two O atoms, is proposed. In this case, clusters containing the O2 fragment can form not only in the reaction of the initial cluster Au25(SCH3)12 with hydrogen peroxide, but also with molecular oxygen, since the O2 adsorption energy is low and the process is close to equilibrium.

Latent Carbene in Diaminomethylation of Benzenes: Mechanism and Practical Application

Silylformamidine 1 exists in equilibrium with its carbenic form 1' due to an easy migration of the silyl group. The reaction of 1 with variously substituted fluorobenzenes proceeds as an insertion of the nucleophilic carbene 1' into the most acidic C-H bond upon mixing the reagents and does not require any catalyst. According to DFT calculations, the classical interpretation of the insertion reaction proceeding via a three-membered transition state structure requires high activation energy. Instead, low activation barriers are predicted for a transfer of the most acidic proton in the aromatic substrate to the carbene carbon. As the next step, a barrierless rearrangement of the formed ion pair toward the product completes the process. The reactivity of substituted benzenes in the reaction with silylformamidine can be roughly assessed by calculated pKa (DMSO) values for the C-H hydrogens. Benzene derivatives having pKa approx. less than 31 can undergo C-H insertion. The reaction provides aminals as the first products, which can easily be transformed into the corresponding aldehydes via acidic hydrolysis. As silylformamidine 1 is tolerant to many functional groups, the reaction can be applied to numerous benzene derivatives, making it a reliable strategy for application in organic synthesis.

Effect of Halogen at the Divalent Sulfur Atom on the Properties of Complexes with a Chalcogen and Hydrogen Bond

Binary complexes with a chalcogen (A complexes) and hydrogen (B complexes) bond formed by SHX molecules (X = F, Cl, Br, OH) of divalent sulfur and a water molecule have been calculated by the MP2/aug-cc-pVTZ quantum chemical method. An NBO analysis was performed for complexes of both types along with the topological analysis of electron density and decomposition of the binding energy into components. The quantum chemical calculations showed that the binding energies, interorbital interaction energies of monomers, and electron densities at the critical point (3, –1) of intermolecular contact are close in the A and B complexes. The main contribution to stabilization of the complexes is made by the electrostatic interaction; in the B complexes, however, the contribution of the charge transfer component is also significant. The dispersion energy plays a significant role in the binding of monomers in complexes of both types. According to the calculations, the interconversion of A and B complexes occurs with a very low activation barrier.

Theoretical Study of the Hydrolysis of Iron–Sulfur–Nitrosyl Complex [Fe(NO)2(SCH2)2

The mechanisms of hydrolysis of a model iron–sulfur–nitrosyl complex (ISNC) [Fe(NO)2(SCH2)2]+ 1 with thioformaldehyde ligands have been studied using the density functional theory and polarizable continuum model of water. Quantum chemical calculations employed the TPSSH and M06 functionals and def2-TZVP basis set and took into account interactions with water medium. Hydrolysis of 1 was found to be an exothermic process with small activation energy whereas exchange of NO for H2O is thermodynamically unfavorable. The calculations have predicted lower activation barrier for the associative mechanism with concerted replacement of SCH2 by H2O than for dissociative mechanism with homolytic bond cleavage of the Fe–S coordination bond in water. The mechanism of hydrolysis that involves participation of OH– was found to be less probable at pH 7. The calculation results show that ISNC 1 is of {Fe1+(NO•)2}9 type and retains its tetrahedral structure that is typical for crystals of ISNC with thiocarbonyl ligands.

Understanding the Reactivity of Diazaborinines towards the Activation of σ-Bonds.

The poorly understood factors governing the small molecule activation reactions mediated by diazaborinines have been computationally explored in detail using quantum chemical tools. To this end, the activation of E-H σ-bonds (E = H, C, Si, N, P, O, S) has been investigated. These reactions, which proceed in a concerted manner, are exergonic and, in general, associated with relatively low activation barriers. In addition, the barrier becomes lower for the E-H bonds involving the heavier element in the same group (ΔG≠ : C>Si; N>P; O>S). This reactivity trend together with the mode of action of the diazaborinine system are quantitatively analyzed by means of the activation strain model of reactivity in combination with the energy decomposition analysis method.

Anion-induced differential assembly and structural transformation of supramolecular coordination cages

The intimate host-anion interactions will regulate thermodynamics and kinetics in the self-assembly of cationic cages mimicking biological counterparts. Herein, we report construction and transformation of three Pd(II)-based metal-organic cages (MOCs) depending on different anions. Stoichiometric conversions of the lantern-shaped MOC-34 into either octahedral MOC-35 or tricapped trigonal prism MOC-36 are induced by BF4‒ or NO3‒, respectively. MOC-36 is kinetically favored and can undergo quantitative conversion to the thermodynamically preferred MOC-35 upon heating, accelerated by excess BF4‒ to motivate dissociative dynamics of Pd-vertices and lower activation barrier of cage transformation. The guest encapsulation behaviors of MOC-35 and MOC-36 have also been tested. These results manifest a significance of host-anion dynamics beyond complementary anion template, shedding light on the understanding of intricate anion recognition in nature.

Interaction of Atomic Deuterium with Rutile TiO2(011)-(2×1)

Inspired by the significance of hydrogen–solid interaction in hydrogen energy and catalysis, adsorption, diffusion, and desorption behaviors of deuterium atoms in rutile TiO2(011) have been investigated by temperature-programmed desorption (TPD) and ultraviolet photoelectron spectroscopy (UPS). Upon exposure, a small portion of D atoms adsorb at surface oxygen sites, resulting in the band gap states at 1.35 eV below the Fermi level and desorbing as water at ∼400 K. Most of the D atoms will diffuse into the bulk due to the relatively low activation barrier and the huge capacity of the solid material. These bulk D species desorb as D2/HD between 500 and 800 K. While the desorbing D2O from surface hydroxyls saturates at ∼0.10 monolayer (ML), the yielding D2 is about 96 ML (equivalent coverage) at the largest atomic D exposure of 4.54 langmuir and no saturation trend has been observed in the present work. Detailed analysis indicates the bulk D will diffuse back to the surface and recombine as D2 at elevated temperatures. The differences between the behavior of H(D) in rutile TiO2(110) and TiO2(011) have been discussed by considering the presence of additional bridging oxygen atoms between the in-plane and topmost ones on the latter surface. The striking finding that most surface D atoms diffuse into the bulk of rutile TiO2(011) will not only broaden our understanding of the interaction of H/D with the prototypical metal oxide material but also provide clues to investigate the mechanism of H/D involving reactions over TiO2 catalysts, for example, hydrogen evolution and hydrogenation.

Non-oxide supported Pt-metal-group catalysts for efficiently CO and toluene co-oxidation: Difference in water resistance and degradation intermediates

Herein, Pd and Pt-catalysts loaded on non-oxide support (g-C3N4) were prepared and compared for CO and toluene co-oxidation, and Pt/C3N4 presented excellent catalytic performance. A series of characterizations including TEM, H2-TPR, O2-TPR and XPS etc., indicated that Pt/C3N4 exhibited the smaller Pt NPs, better low temperature reduction ability, more active oxygen species and positive charged Pt species, which could account for its better catalytic performance. Additionally, DFT calculations further confirmed that the lower activation barrier of O2 and the higher adsorption for CO and toluene triggered the efficient CO and toluene degradation over Pt/C3N4. Interestingly, the introduction of H2O (10.0 vol%) suppressed the catalytic performance of Pd/C3N4, while expedited CO and toluene oxidation over Pt/C3N4, which was revealed by DFT calculations. Furthermore, the difference in degradation intermediates, corresponding distributions and toxicity generated during toluene oxidation under different conditions over Pd/C3N4 and Pt/C3N4 were exposed. Compared with Pd/C3N4, less variety, toxicity and content of intermediates produced over Pt/C3N4. This work would inspire many rational designs of supported noble metal catalysts for environmentally friendly catalysis.

Diffusion of oxygen vacancies formed at the anatase (101) surface: An activation-relaxation technique study

${\mathrm{TiO}}_{2}$ is a technologically important material. In particular, its anatase polymorph plays a major role in photocatalysis, which can also accommodate charged and neutral vacancies. There is, however, scant theoretical work on the vacancy charge and associated diffusion from surface to subsurface and bulk in the literature. Here, we aim to understand +2 charge and neutral vacancy diffusion on anatase (101) surface using 72- and 216-atom surface slabs employing a semilocal density functional and the Hubbard model. The activation-relaxation technique nouveau coupled with quantum espresso is used to investigate the activated mechanisms responsible for the diffusion of oxygen vacancies. The small-slab model overstabilizes the +2 charged topmost surface vacancy, which is attributed to the strong Coulomb repulsion between vacancy and neighboring ${\mathrm{Ti}}^{+4}$ ions. The larger slab allows atoms to relax parallel to the surface, decreasing the +2 charged topmost surface vacancy stability. The calculated surface-to-subsurface barriers for the +2 charged vacancy and diffusion of the neutral vacancy on a slab of 216 atoms are 0.82 and 0.52 eV, respectively. Furthermore, the bulk vacancy prefers to migrate toward the subsurface with relatively low activation barriers 0.19 and 0.27 eV and the reverse process has to overcome 0.38- and 0.40-eV barriers for the +2 charged and the neutral vacancies. This explains the experimentally observed high concentration of vacancies at the subsurface sites rather than in the bulk, and the dynamic diffusion of vacancies from the bulk to the subsurface and from the subsurface to the bulk is highly likely on the surface of anatase (101). Finally, we provide a plausible explanation for the origin of recently observed subsurface-to-surface diffusion of oxygen vacancy from the calculated results.

Thermochemistry, Tautomerism, and Thermal Stability of 5,7-Dinitrobenzotriazoles

Nitro derivatives of benzotriazoles are safe energetic materials with remarkable thermal stability. In the present study, we report on the kinetics and mechanism of thermal decomposition for 5,7-dinitrobenzotriazole (DBT) and 4-amino-5,7-dinitrobenzotriazole (ADBT). The pressure differential scanning calorimetry was employed to study the decomposition kinetics of DBT experimentally because the measurements under atmospheric pressure are disturbed by competing evaporation. The thermolysis of DBT in the melt is described by a kinetic scheme with two global reactions. The first stage is a strong autocatalytic process that includes the first-order reaction (Ea1I = 173.9 ± 0.9 kJ mol-1, log(A1I/s-1) = 12.82 ± 0.09) and the catalytic reaction of the second order with Ea2I = 136.5 ± 0.8 kJ mol-1, log(A2I/s-1) = 11.04 ± 0.07. The experimental study was complemented by predictive quantum chemical calculations (DLPNO-CCSD(T)). The calculations reveal that the 1H tautomer is the most energetically preferable form for both DBT and ADBT. Theory suggests the same decomposition mechanisms for DBT and ADBT, with the most favorable channels being nitro-nitrite isomerization and C-NO2 bond cleavage. The former channel has lower activation barriers (267 and 276 kJ mol-1 for DBT and ADBT, respectively) and dominates at lower temperatures. At the same time, due to the higher preexponential factor, the radical bond cleavage, with reaction enthalpies of 298 and 320 kJ mol-1, dominates in the experimental temperature range for both DBT and ADBT. In line with the theoretical predictions of C-NO2 bond energies, ADBT is more thermally stable than DBT. We also determined a reliable and mutually consistent set of thermochemical values for DBT and ADBT by combining the theoretically calculated (W1-F12 multilevel procedure) gas-phase enthalpies of formation and experimentally measured sublimation enthalpies.

Uncovering the Function of a Five‐Membered Heterocyclic Solvent‐Based Electrolyte for Graphite Anode at Subzero Temperature

AbstractEthylene carbonate (EC) is taken as the essential electrolyte component in lithium‐ion batteries (LIBs) due to its high permittivity and film‐forming ability. However, its high melting point (36.4 °C) and strong solvation energy severely hinder Li+ transportation and Li+ desolvation process under low temperatures, resulting in capacity loss and even Li plating on graphite anode. Herein, a five‐membered heterocyclic compound isoxazole (IZ), similar to EC molecule, is well‐formulated to substitute EC for low‐temperature operation of graphite anode. It is revealed that IZ with dispersed charge distribution exhibits a weaker solvation ability than EC with highly polar carbonyl group, which induces relatively more anions into the solvation sheath to form contact ion pairs and aggregates. The tamed electrolyte not only exhibits high ionic conductivities over wide‐temperature range but also generates an inorganic‐rich interphase with low activation barrier for smooth Li+ ions threading. This enables graphite anode with an impressive reversible capacity of 263 mAh g‐1 at the low temperature of −30 °C (a room‐temperature retention of as high as 71.5%), nearly twice higher than graphite with EC‐based electrolyte. This study provides an alternative electrolyte recipe to relieve the anxiety of LIBs operated under harsh conditions.

Comparative computational study of CO2 hydrogenation and dissociation on metal-doped Pd clusters

Density functional theory calculations are executed to research CO2 adsorption and initial conversion on Pd12M (M = Fe, Co, Ni, Cu, Zn, Ru, Rh, Pd, Ag, Cd) bimetallic clusters, with a focus on finding optimal ancillary metal to Pd element. The stability analysis of the structures demonstrates that the transition metals with higher surface energy (compared to Pd element) doped Pd13 clusters possess better structural stability. For CO2 adsorption, the structural parameters of CO2* have remarkable linear relation with the amount of charges transfer. For CO2 initial conversion, it can be concluded that the Pd12Cu cluster is the best candidate with a lower activation barrier (0.69 eV) for the initial hydrogenation of CO2. According to the electronic structure analysis, the elevated HOMO caused by the doping of Cu atom is the main reason for the enhanced catalytic activity. In addition, the PdCu bimetallic clusters with larger size are constructed and their activity and selectivity towards CO2 initial conversion are also studied. Compared with Pd32Cu6 and Pd42Cu13 bimetallic clusters, Pd12Cu has more advantages in activity and selectivity. Therefore, Pd12Cu is the best choice, that is, it inhibits the generation of by-product and improves the catalytic activity, which makes it become an excellent bimetallic candidate for CO2 initial hydrogenation.

Modulating the Energetics of C–H Bond Activation in Methane by Utilizing Metalated Porphyrinic Metal–Organic Frameworks

In recent years, much effort has been directed toward utilizing metal-organic frameworks (MOFs) for activating C-H bonds of light alkanes. The energy demanding steps involved in the catalytic pathway are the formation of metal-oxo species and the subsequent cleavage of the C-H bonds of alkanes. With the intention of exploring the tunability of the activation barriers involved in the catalytic pathway of methane hydroxylation, we have employed density functional theory to model metalated porphyrinic MOFs (MOF-525(M)). We find that the heavier congeners down a particular group have high exothermic oxo-formation enthalpies ΔHO and hence are associated with low N2O activation barriers. Independent analyses of activation barriers and structure-activity relationship leads to the conclusion that MOF-525(Ru) and MOF-525(Ir) can act as an effective catalysts for methane hydroxylation. Hence, ΔHO has been found to act as a guide, in the first place, in choosing the optimum catalyst for methane hydroxylation from a large set of available systems.

Strain-Sensitive On-Surface Ladderization by Non-Dehydrogenative Heterocyclization.

On-surface cyclodehydrogenation recently became an important reaction to planarize π-conjugated molecules and oligomers. However, the high-activation barrier to cleave the C-H bond often requires high-temperature annealing, consequently restricting structures of precursor molecules and/or leading to random fusion at their edges. Here, we present a synthesis of pyrrolopyrrole-bridged ladder oligomers from 11,11,12,12-tetrabromo-1,4,5,8-tetraaza-9,10-anthraquinodimethane molecules on Ag(111) with bond-resolved scanning tunnelling microscopy. This non-dehydrogenative cyclization between pyrazine and ethynylene/cumulene groups has a low-activation barrier for forming intermediary dimeric oligomer containing dipyrazinopyrrolopyrrolopyrazine units, thus giving new insight into the strain-sensitive in ladder-oligomer formation.

A Theoretical Study on Rate Constants of Excited State Proton Transfer Reaction in Anthracene-Urea Derivatives

Abstract We elucidate the origin of the two types of observed fluorescence in a complex consisting of nPUA (1-anthracen-n-yl-3-phenylurea; n = 1, 2, 9) and an acetate ion. By calculating the molecular properties related to the proton transfer reaction in the excited state, we clarified a correlation between the acid dissociation constant in the excited state (pKa) in the urea moiety and the rate constant of the excited state proton transfer reaction. The computed pKa suggests that the proton on the anthracenyl group side is transferred in the case of 1PUA and 2PUA, whereas the proton on the phenyl group side is transferred in 9PUA. Low pKa and activation barriers were calculated for 9PUA, which causes the stability of 9PUA due to the absence of the planarity after the proton transfer reaction.

Gas-Phase Characterization of Hypervalent Carbon Compounds Bearing 7-6-7-Ring Skeleton: Penta- versus Tetra-Coordinate Isomers.

In this study, we afford explicit characterizations of the electronic and geometrical structures of recently reported hypervalent penta-coordinate carbon compounds by using gas-phase characterization techniques: photodissociation spectroscopy (PDS) and ion mobility-mass spectrometry (IM-MS). In particular for a compound with moderately electron-donating ligands, bearing p-methylthiophenyl substituents, the coexistence of tetra- and penta-coordinate isomers is confirmed, consistent with solution characterizations. It is in sharp contrast to the exclusive tetra-coordinate form (with normal valence of the central carbon atom) in the single crystal. This suggests that a non-polar environment makes the penta-coordinate structure thermodynamically most stable. This delicate difference between the tetra- and penta-coordinate structures, which depends on the environment, is a close reflection of the lower activation barrier of the SN 2 reaction found in neutral solvent or gas-phase reactions.

Synergetic effects of Fe[sbnd]N[sbnd]C and LiFePO4 in modified separator on improved adsorption and catalytic conversion reaction of soluble LiPSs for Lithium-sulfur batteries

“Shuttle effect” is a serious obstacle in the development of high-performance lithium-sulfur batteries and modified separator is considered an effective strategy to prevent the shuttle effect of soluble polysulfides (LiPSs). Herein, we demonstrate a modified polypropylene (PP) separator composed of nitrogen-doped carbon coated LiFePO4 (NC-LFP), Ketjen black carbon (KB) and polyvinylidene fluoride (PVDF) (NC-LFP interlayer), which can improve adsorption and catalytic conversion toward LiPSs. The analysis shows that NC-LFP interlayer could effectively adsorb LiPSs and therefore suppress its shuttle effects due to the FeNC functional groups in the nitrogen-doped carbon coated on LFP polar particles. The DFT analysis and the electrochemical performance test demonstrate the FeNC functional groups in NC-LFP interlayer provide lower activation barrier for catalytic conversion reaction of LiPSs to Li2S and vice versa. The coin cells with NC-LFP modified separator exhibit an initial discharge specific capacity of 907 mAh/g at 1C. After 500 cycles, the specific capacity is 702 mAh/g with a retention rate of 79.5 %. Moreover, at 8C rate, the discharge-specific capacity of 667.5 mAh/g is achieved. The use of NC-LFP interlayer modified separators realizes lithium-sulfur batteries with long-cycle life and high-rate performance, providing an emerging strategy for high-performance Li-S batteries.