RESUMO
Thorium oxide has many important applications in industry. In this article, theoretical calculations have been carried out to explore the hydrolysis reactions of the ThOn (n=1-3) clusters. The reaction mechanisms of the O-deficient ThO and the O-rich ThO3 are compared with the stoichiometric ThO2 . The theoretical results show good agreement with the prior experiments. It is shown that the hydrolysis mainly occurred on the singlet potential surface. The overall reactions consist of two hydrolysis steps which are all favourable in energy. The effects of oxygen content on the hydrolysis are elucidated. Interestingly, among them, the peroxo group O2 2- in ThO3 is converted to the HOO- ligand, behaving like the terminal O2- in the hydrolysis which is transformed into the HO- groups. In addition, natural bond orbital (NBO) analyses were employed to further understand the bonding of the pertinent species and to interpret the differences in hydrolysis.
RESUMO
Two new chemically stable metalloporphyrin-bridged metal-catechol frameworks, InTCP-Co and FeTCP-Co, were constructed to achieve artificial photosynthesis without additional sacrificial agents and photosensitizers. The CO2 photoreduction rate over FeTCP-Co considerably exceeds that obtained over InTCP-Co, and the incorporation of uncoordinated hydroxyl groups, associated with catechol, into the network further promotes the photocatalytic activity. The iron-oxo coordination chain assists energy band alignment and provides a redox-active site, and the uncoordinated hydroxyl group contributes to the visible-light absorptance, charge-carrier transfer, and CO2 -scaffold affinity. With a formic acid selectivity of 97.8 %, FeTCP-OH-Co affords CO2 photoconversion with a reaction rate 4.3 and 15.7 times higher than those of FeTCP- Co and InTCP-Co, respectively. These findings are also consistent with the spectroscopic study and DFT calculation.
RESUMO
Ni-rich layered oxides, like LiNi0.8Co0.1Mn0.1O2 (NCM811), have been widely investigated as cathodes due to their high energy density. However, gradual structural transformation during cycling can lead to capacity degradation and potential decay of cathode materials. Herein, we doped high-valence transition metal (TM) ions (V5+, Nb5+, and Zr4+) at the Ni site of NCM811 by first principles simulations and explored the mechanism of doping TMs in NCMs for enhancing the electrochemical performance. Analysis of the calculations shows that doping V, Nb and Zr has an efficient influence on alleviating the Ni oxidation, reducing the loss of oxygen, and facilitating Li+ migration. Moreover, V doping can further suppress the lattice distortion due to the radius of V5+ being close to the radius of Mn4+. In particular, compared with the barrier of the pristine NCM in Li divacancy, the barrier of V-doped NCM reaches the lowest. In conclusion, V is the most favorable dopant for NCM811 to improve the electrochemical properties and achieve both high capacity and cycling stability.
RESUMO
Based on first-principle calculations, we proposed a one two-dimensional (2D) blue AsP (b-AsP) monolayer as an ideal anode material for lithium/sodium-ion (Li/Na-ion) batteries for the first time. The b-AsP monolayer possesses thermal and dynamic stabilities. The system undergoes the transition from semiconductor to metal after Li/Na atoms are embedded, which ensures good electric transportation. Most remarkably, our results indicate that the b-AsP monolayer exhibits high theoretical capacities of 1011.2 mA h g-1 (for Li) and 1769.6 mA h g-1 (for Na), low average open circuit voltages of 0.17 eV for Li4AsP and 0.14 eV for Na7AsP systems and ultrafast diffusivity with the low energy barriers of 0.17/0.15 eV and 0.08/0.07 eV of the P/As sides for Li and Na, respectively. Given these exceptional properties, the synthesis of a buckled b-AsP monolayer is desired to achieve a promising electrode material for Li- and Na-ion batteries.
RESUMO
Three classical Fe-MOFs, viz., MIL-100(Fe), MIL-101(Fe), and MIL-53(Fe), were synthesized to serve as platforms for the investigation of structure-activity relationship and catalytic mechanism in the selective conversion of H2S to sulfur. The physicochemical properties of the Fe-MOFs were characterized by various techniques. It was disclosed that the desulfurization performances of Fe-MOFs with well-defined microstructures are obviously different. Among these, MIL-100(Fe) exhibits the highest catalytic performance (ca. 100% H2S conversion and 100% S selectivity at 100-180 °C) that is superior to that of commercial Fe2O3. Furthermore, the results of systematic characterization and DFT calculation reveal that the difference in catalytic performance is mainly because of discrepancy in the amount of Lewis acid sites. A plausible catalytic mechanism has been proposed for H2S selective conversion over Fe-MOFs. This work provides critical insights that are helpful for rational design of desulfurization catalysts.
RESUMO
Density functional theory (DFT) calculations have been performed to explore the gas-phase hydrolysis reaction of mononuclear thorium halide clusters ThX4 (X = F, Cl). We have found that the hydrolysis of ThCl4 is easier than that of ThF4. Furthermore, their hydrolysis reactions favor pathways of direct dehydration of Th(OH)4 instead of further hydrolysis of ThOX2. There are some differences between the hydrolysis of ThCl4 and that of MCl4 (M = Ti, Zr and Hf). The X-HY (X = F, Cl; Y = F, Cl and OH) hydrogen bonds play an important role in the hydrogen transfer process of the hydrolysis reaction. The differences in the steric effects and bonding may be important factors that are related to the disparities in the hydrolysis of the above-mentioned metal halides.
RESUMO
Density functional theory (DFT) calculations are employed to investigate the reactivity of tungsten oxide clusters towards carbon monoxide. Extensive structural searches show that all the ground-state structures of (WO3)n(+) (n = 1-4) contain an oxygen radical center with a lengthened W-O bond which is highly active in the oxidation of carbon monoxide. Energy profiles are calculated to determine the reaction mechanisms and evaluate the effect of cluster sizes. The monomer WO3(+) has the highest reactivity among the stoichiometric clusters of different sizes (WO3)n(+) (n = 1-4). The reaction mechanisms for CO with mono-nuclear stoichiometric tungsten oxide clusters with different charges (WO3(-/0/+)) are also studied to clarify the influence of charge states. Our calculated results show that the ability to oxidize CO gets weaker from WO3(+) to WO3(-) as the negative charge accumulates progressively.
RESUMO
In excitonic solar cells (XSC), power conversion efficiency (PCE) depends critically on the interface band alignment between donor and acceptor materials. Graphene or silicene is not suitable for donor materials due to their semimetallic features (zero band gaps); it is therefore highly desired to open an energy gap in graphene or silicene to extend their application in optoelectronic devices, especially in photovoltaics. In this paper, based on the global particle-swarm optimization algorithm and the density functional theory methods, we predict a novel SiC2 siligraphene (g-SiC2) with a direct band gap of 1.09 eV showing infinite planar geometry, in which Si and C atoms adopt sp(2) hybridization and C atoms form delocalized 4 C-domains that are periodically separated by Si atoms. Such a g-SiC2 siligraphene (with a global minimum of energy) is 0.41 eV/atom lower and thermally stabler than the isomeric pt-SiC2 silagraphene containing planar 4-fold coordinated silicon (3000 K vs 1000 K). Interestingly, the derivative (n, 0), (n, n) nanotubes (with diameters greater than 8.0 Å) have band gaps about 1.09 eV, which are independent of the chirality and diameter. Besides, a series of g-SiC2/GaN bilayer and g-SiC2 nanotube/ZnO monolayer XSCs have been proposed, which exhibit considerably high PCEs in the range of 12-20%.
RESUMO
Two novel vanadoborate compounds, [Cu(en)2]3[Li(H2O)]4[Li(H2O)3]2[V12B18O50(OH)10(H2O)]2·33.5H2O (1) and (H2en)4[Li(H2O)]4[V12B18O55(OH)5(H2O)]·14H2O (2), were synthesized via hydrothermal synthesis under identical conditions except for temperature. Structural analysis revealed that although both contain [V12B18O60]n- cluster anion, the different countercations potentially lead to variations in the [V12B18O60]n- cluster anion skeletons. In compound 1, the V4+/V5+ ratio was 10:2; while in compound 2 the ratio was 11:1. It is speculated that different countercations may influence the valence states of cluster anions. In this study, quantum chemical calculations revealed that the aromaticity and activity of the two compounds were different, and two-dimensional correlation infrared spectroscopy (2D-COS-IR) under magnetic perturbation confirmed that distinct response peaks of functional group vibrations to the magnetic field due to the different V4+/V5+ ratios and aromaticity of the two compounds. An electrochemical analysis revealed that compound 2 exhibits higher electrocatalytic activity. The results of quantum chemical calculations are aligned not only with the changes in the 2D-COS-IR spectra but also with the conclusions obtained from experiments on electrochemical properties. Overall, this work proposes a novel strategy for interpreting the alteration of vanadoborate anionic skeleton due to the introduction of different countercations by combining 2D-COS-IR with quantum chemical calculations.
RESUMO
Cis- and trans-isomeric heterotrinuclear-metallic complexes and their two-electron-oxidation products, cis-/trans-[Cp(dppe)Fe(µ-CN)Ru(bpy)2(µ-CN)Fe(dppe)Cp][PF6]2 (cis-/trans-1[PF6]2) and cis-/trans-[Cp(dppe)Fe(µ-CN)Ru(bpy)2(µ-CN)Fe(dppe)Cp][PF6]4 (cis-/trans-1[PF6]4), have been synthesized and structurally characterized. To the best of our knowledge, the complexes are the first example of a cis-/trans-isomer with multistates. Although separated by the diamagnetic cyanido-metal bridge, the two distant paramagnetic metal centers in both the oxidized complexes exhibit quite strong magnetic couplings. As a unique example, cis-1[PF6]4 is antiferromagnetic, and trans-1[PF6]4 is ferromagnetic. Density functional theory (DFT) calculations suggest that the spin-delocalization mechanism should be responsible for the magnetic interactions between the two distant paramagnetic Fe(III) centers across the diamagnetic cyanido-metal in both cis- and trans-1(4+). Most importantly, the DFT calculations revealed that the type (antiferromagnetic or ferromagnetic) and strength (J) of the magnetic interactions in such compounds can be controlled by the variation (cis or trans) of the diamagnetic central metal configurations.
RESUMO
Density functional theory (DFT) calculations are employed to investigate the structural and electronic properties of bare tritungsten clusters (W3, W3(-), W3(2-)) and tritungsten oxide clusters W3Ox(-/0) (x = 1, 2). Generalized Koopmans' theorem is applied to predict the vertical detachment energies and simulate the photoelectron spectra (PES) for W3Ox(-) (x = 0-2) clusters. Extensive DFT calculations are performed in search of the lowest energy structures for both the anions and the neutrals. The bare tritungsten clusters are predicted to be triangular structures with D3h ((3)A1'), C2v ((2)A1) and D3h ((1)A1') symmetry for W3, W3(-) and W3(2-), respectively. For W3O(-) and W3O clusters, the oxygen atom occupies the terminal site, while the next added oxygen atom is found to be a bridging one in both W3O2(-) and W3O2 clusters. Molecular orbital analyses are carried out to elucidate the chemical bonding of these clusters and provide insights into the sequential oxidation from W3(-) to W3O2(-). Partial σ- and δ-aromaticity are revealed in the neutral W3 (D3h, (3)A1'), while the anion W3(2-) (D3h, (1)A1') possesses only δ-aromaticity.
RESUMO
Density functional theory (DFT) and coupled cluster theory (CCSD(T)) calculations are carried out to investigate the electronic and structural properties of a series of monomolybdenum sulfide clusters, MoSn(-/0) (n = 1-5). Generalized Koopmans' theorem is applied to predict the vertical detachment energies and simulate the photoelectron spectra (PES). We found that the additional sulfur atoms have a tendency to successively occupy the terminal sites in the sequential sulfidation until the Mo reaches its maximum oxidation sate of +6. After that, the polysulfide ligands (viz., S2 and S3) emerge in the MoS4 and MoS5(-/0) clusters. The MoS4 (C2, (1)A) is predicted to be the ground state and may be used as a neutral model for the sulfur-rich edge sites of the fresh MoS2 catalysts. Molecular orbital analyses are performed to analyze the chemical bonding in the monomolybdenum sulfide clusters and to elucidate their electronic and structural evolution.
RESUMO
A new chiral sulfide family, Ln(4)InSbS(9) (Ln = La, Pr, Nd), with its own structure type in space group P4(1)2(1)2 or its enantiomorph P4(3)2(1)2 has been synthesized by solid-state reaction. Remarkably, the La member shows the strongest Kleinman-forbidden second harmonic generation to date, with an intensity 1.5 times that of commercial AgGaS(2) at a laser wavelength of 2.05 µm, and exhibits type-I phase-matchable behavior. Density functional theory calculations and ab initio molecular dynamics simulations suggest that lattice vibrations may be responsible for the origin and magnitude of the strong SHG effect.
RESUMO
The combination of lone-pair effects on Pb(2+) cations and the smaller electronegativity of I(-) anions into the pentaborate framework generates a phase-matchable material, Pb(2)B(5)O(9)I, with the largest powder SHG response among borates, about 13.5 times that of KDP (KH(2)PO(4)), and transparency over the near-UV to middle-IR region. DFT calculations on electronic structure and cutoff-energy-dependent SHG coefficients confirm these origins.
RESUMO
Density functional theory (DFT) calculations are carried out to investigate the electronic and structural properties of a series of tetratungsten oxide clusters, W(4)O(n)(-/0) (n = 10-13). Generalized Koopmans' theorem is applied to predict the vertical detachment energies and simulate the photoelectron spectra (PES). A large energy gap (approximately 2.9 eV) is observed for the stoichiometric W(4)O(12) cluster, which reaches the bulk value. The calculations suggest that W(4)O(12)(-/0) have the planar eight-membered ring structures, in which each tungsten atom is tetrahedrally coordinated with two bridging O atoms and two terminal O atoms. W(4)O(10)(-/0) and W(4)O(11)(-) can be viewed as removing two and one terminal O atoms from W(4)O(12)(-/0), respectively. The W(4)O(11) neutral is an interesting species, which possesses the pentabridged structure. We show that W(4)O(11)(-) contains a localized W(3+) site, which can readily react with O(2) to form the W(4)O(13)(-) cluster, whereas the corresponding neutral W(4)O(13) can be viewed as replacing a terminal oxygen in W(4)O(12) by a peroxo O(2) unit. Molecular orbital analyses are performed to analyze the chemical bonding in the tetratungsten oxide clusters and to elucidate their electronic and structural evolution.
RESUMO
The electronic and structural properties of a series of triniobium oxide clusters, Nb(3)O(n)(-) and Nb(3)O(n) (n = 3-8), are investigated using photoelectron spectroscopy (PES) and density functional theory (DFT) calculations. PES spectra are obtained for Nb(3)O(n)(-) (n = 3-8) at various photon energies and are used to compare with the DFT calculations. A trend of sequential oxidation is observed as a function of O content until Nb(3)O(8)(-), reaching the highest oxidation state of Nb. DFT calculations are performed to search for the lowest energy structures for both the anionic and neutral clusters. For Nb(3)O(3)(-), the three O atoms are shown to prefer the bridging sites of a triangular Nb(3), leading to two nearly degenerate cyclic structures of C(s) symmetry. The next three O atoms from n = 4-6 each occupy a terminal site directly bonded to Nb, resulting in a symmetric Nb(3)O(6)(-) with C(3v) symmetry and a low-lying isomer of C(s) symmetry. The seventh O atom is bonded to two Nb atoms forming a double bridge, whereas the eighth O atom is bonded to a terminal site so that in Nb(3)O(8)(-) each Nb atom reaches its maximum oxidation state of +5. The structures and electronic states for the triniobium oxide clusters are significantly different from the corresponding tritantalum oxide clusters, in particular, for Nb(3)O(3)(-), Nb(3)O(5)(-), and Nb(3)O(7)(-). Molecular orbital analyses are performed to elucidate the chemical bonding and the electronic and structural evolution in these triniobium oxide clusters.
RESUMO
H2S is abundantly available in nature, and it is a common byproduct in industries. Molybdenum sulfides have been proved to be active in the catalytic decomposition of hydrogen sulfide (H2S) to produce hydrogen. In this study, density functional theory (DFT) calculations are carried out to explore the reaction mechanisms of H2S with MS3 (M = Mo, W) clusters. The reaction mechanism of H2S with MoS3 is roughly the same as that of the reaction with WS3, and the free-energy profile of the reaction with MoS3 is slightly higher than that of the reaction with WS3. The overall driving forces (-ΔG) are positive, and the overall reaction barriers (ΔG b) are rather small, indicating that such H2 productions are product-favored. MS3 (M = Mo, W) clusters have clawlike structures, which have electrophilic metal sites to receive the approaching H2S molecule. After several hydrogen-atom transfer (HAT) processes, the final MS4·H2 (IM-4) complexes are formed, which could desorb H2 at a relatively low temperature. The singlet product MS4 clusters contain the singlet S2 moiety, similar to the adsorbed singlet S2 on the surface of sulfide catalysts. The theoretical results are compared with the experiments of heterogeneous catalytic decomposition of H2S by MoS2 catalysts. Our work may provide some insights into the optimal design of the relevant catalysts.
RESUMO
Because of the high efficiency and mild reaction conditions, electrocatalytic CO2 reduction (ECR) has attracted significant attention in recent years. However, the specific mechanism of the formation of the two-electron production (CO or HCOOH) in this reaction is still unclear. Herein, with density functional theory calculation and experimental manipulation, the specific mechanism of the selective two-electron reduction of CO2 has been systematically investigated, employing the polyphenolate-substituted metalloporphyrinic frameworks, ZrPP-1-M (M = Fe, Co, Ni, Cu, and Zn), as model catalysts. Experimental observations and theoretical calculations discovered that ZrPP-1-Co is a more favorable catalyst for ECR among them. Compared with the formation of HCOOH, electroreduction of CO2 into CO has more beneficial thermodynamic and kinetic routes with ZrPP-1-Co as a catalyst. After introducing the r-GO for improving the conductivity, the Faradaic efficiency for CO formation is 82.4% at -0.6 v (vs RHE).
RESUMO
We present a theoretical study on the potential thermoelectric performance of antimony nanoribbons (SNRs). Based on density functional theory and the semiclassical transport model, the thermoelectric figure of merit ZT was calculated for various Sb nanoribbon sizes and different chiralities. The results indicated that the chemical-bond-driven edge reconstruction of nanoribbons (denoted as SNRs-recon) eliminated all of the dangling bonds and passivated all of the boundary antimony atoms with 3-fold coordination. SNRs-recon are the most energy favorable compared to the ribbons with unsaturated edge atoms. Semimetal to semiconductor transition occurred in SNRs-recon. The band gap was width-dependent in armchair SNRs (denoted as ASNRs-recon), whereas it was width-independent in zigzag SNRs (ZSNRs-recon). After nanolization and reconstruction, the TE properties of SNRs were enhanced due to higher Seebeck coefficient and lower thermal conductivity. The thermoelectric properties of n-doped ASNRs-recon and p-doped ZSNRs-recon showed width-dependent odd-even oscillation and eventually resulted in ZT values of 0.75 and 0.60, respectively. Upon increasing the ribbon width, ZT of n-doped ASNRs-recon decreased and approached a constant value of about 0.85. However, n-doped ZSNRs-recon exhibited poor TE performance compared with the others. Importantly, the ZT value could be optimized to as high as 1.91 at 300 K, which was larger than those of Sb-based bulk materials and 100 times that of thin Sb films. These optimizations make the materials promising room-temperature high-performance thermoelectric materials. Furthermore, the proposed new concept of chemical-bond-driven edge reconstruction may be useful for many other related systems.
RESUMO
A series of zirconium polyphenolate-decorated-(metallo)porphyrin metal-organic frameworks (MOFs), ZrPP-n (n = 1, 2), featuring infinite ZrIV -oxo chains linked via polyphenolate groups on four peripheries of eclipse-arranged porphyrin macrocycles, are successfully constructed through a top-down process from simulation to synthesis. These are the unusual examples of Zr-MOFs (or MOFs in general) based on phenolic porphyrins, instead of commonly known carboxylate-based types. Representative ZrPP-1 not only exhibits strong acid resistance (pH = 1, HCl) but also remains intact even when immersed in saturated NaOH solution (≈20 m), an exceptionally large range of pH resistance among MOFs. The metallation at the porphyrin core gives rise to materials with enhanced sorption and catalytic properties. In particular, ZrPP-1-Co, with precise and uniform distribution of active centers, exhibits not only high CO2 trapping capability (≈90 cm3 g-1 at 1 atm, 273 K, among the highest in Zr-MOFs) but also high photocatalytic activity for reduction of CO2 into CO (≈14 mmol g-1 h-1 ) and high selectivity over CH4 (>96.4%) without any cocatalyst under visible-light irradiation (λ > 420 nm). Given the strong chemical resistance under extreme alkali conditions, these catalysts can be recycled without appreciable loss of activity. The possible mechanism for photocatalytic reduction of CO2 -to-CO over ZrPP-1-Co is also proposed.