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In the present paper, the mechanisms of N 2 O reduction by H 2 were systemically examined over various polyoxometalate-supported single-atom catalysts (SACs) M 1 /PTA (M = Fe, Co, Mn, Ru, Rh, Os, Ir, and Pt; PTA = [PW 12 O 40 ] 3- ) by means of density functional theory calculations. Among these M 1 /PTA SACs, Os 1 /PTA SAC possesses high activity for N 2 O reduction by H 2 with a relatively low rate-determining barrier. The favorable catalytic pathway involves the first and second N 2 O decomposition over the Os 1 /PTA SAC and hydrogenation of the key species after the second N 2 O decomposition. Molecular geometry and electronic structure analyses along the favorable reaction pathway indicate that a strong charge-transfer cooperative effect of metal and support effectively improves the catalytic activity of Os 1 /PTA SAC. The isolated Os atom not only plays the role of adsorption and activation of the N 2 O molecule but also works as an electron transfer medium in the whole reaction process. Meanwhile, the PTA support with very high redox stability has also been proven to be capable of transporting the electron to promote the whole reaction. We expect that our computation results can provide ideas for designing new SACs for N 2 O reduction by using H 2 selective catalytic reduction technology.

Papermaking black liquor (BL) contains a large amount of organic matter, alkali, and salt, which has high value in industrial and agricultural production. In the present paper, hydrothermal carbonization (HTC) technology has been employed to deal with BL and corn stalk (CS) under various conditions. A variety of analytical technologies, including Fourier Transform Infrared Spectrometer, X-Ray Diffraction, element analyzer, comprehensive thermal analyzer, scanning electron microscope, chromatography/mass spectrometry, TOC analyzer, and density functional theory (DFT), were used to characterize the physicochemical properties of hydrochar and aqueous phase product. The effects of experimental parameters (temperature, solid-liquid ratio, and time) on the HTC of BL and CS were systematically investigated. The higher energy yield and densification of hydrochar prepared by our HTC technology were obtained at 260 ℃ in 30 min with a solid-liquid ratio of 1:3. Mechanism study indicates that NaOH, which comes from the original BL, could effectively facilitate the cleavage of β –O–4 bond in lignin and β –1–4 glycosidic bond in cellulose and hemicellulose molecules. Hemicellulose and cellulose are more susceptible to degradation relevant to the lignin in the presence of NaOH. Elementary analysis shows that the dosage of BL is important for improving energy densification of hydrochar. The combustion characteristics show that hydrochar presents a thermochemical behavior, which is close to bituminous coal. Owing to hydroxyl ion in NaOH ion pair was consumed in the series of dehydrogenation reactions, pH of HTC aqueous phase product significantly decreases from 11 to 13 in original BL to 4.8–5. Such results indicate that a completely change for the components of BL could be obtained by our HTC technology, which provide a novel idea for the treatment of Papermaking BL without concentration. • HTC of corn stalk and papermaking black liquor to produce hydrochar. • BL is important for improving energy properties and clean performance of hydrochar. • NaOH could cleave β–O–4 bond and β–1–4 glycosidic bond through DFT calculations. • HTC has potential for large-scale implementation in papermaking industry. [ABSTRACT FROM AUTHOR]

In this work, we rationally designed a series of crystalline and stable dioxin‐linked metallophthalocyanine covalent organic frameworks (COFs; MPc‐TFPN COF, M=Ni, Co, Zn) under the guidance of reticular chemistry. As a novel single‐site catalysts (SSCs), NiPc/CoPc‐TFPN COF exhibited outstanding activity and selectivity for electrocatalytic CO2 reduction (ECR; Faradaic efficiency of CO (FECO)=99.8(±1.24) %/ 96.1(±1.25) % for NiPc/CoPc‐TFPN COF). More importantly, when coupled with light, the FECO and current density (jCO) were further improved across the applied potential range (−0.6 to −1.2 V vs. RHE) compared to the dark environment for NiPc‐TFPN COF (jCO increased from 14.1 to 17.5 A g−1 at −0.9 V; FECO reached up to ca. 100 % at −0.8 to −0.9 V). Furthermore, an in‐depth mechanism study was established by density functional theory (DFT) simulation and experimental characterization. For the first time, this work explored the application of COFs as photo‐coupled electrocatalysts to improve ECR efficiency, which showed the potential of using light‐sensitive COFs in the field of electrocatalysis. [ABSTRACT FROM AUTHOR]

Hydrogen peroxide (H2O2), as clean oxidant, has been disregarded due to its low efficiency and selectivity for the oxidation of olefins. In the present paper, the redox‐important ferrate anion (FeO42−) has been anchored to a silanol‐decorated polyoxometalates (POM) to form a single site‐supported Fe‐POM catalyst. Possible reaction mechanisms for the epoxidation of propylene with H2O2 catalyzed by the Fe‐POM catalyst have been investigated based on density functional theory with the M06L functional. The study of molecular geometry, electronic structure, and bonding feature shows that the Fe‐POM complex can be viewed as a high‐valent Fe‐oxo (FeO) species. The propylene molecule was activated by the Fe‐POM catalyst via an effective electron transfer from propylene to the Fe‐POM catalyst to form a cation propylene radical. Due to the high reactivity of radical species, the calculated activation energy barrier is only 4.50 kcal mol−1 for epoxidation of propylene to epoxypropane catalyzed by the Fe‐POM catalyst. Subsequently, the calculated free energy profiles show that H2O2 was decomposed into a H2O molecule and a surface O species over the Fe‐POM catalyst, and the remaining O atom attaches to the exposed Fe center, resulting in the replenishing of the Fe‐POM catalyst via a two‐state reaction pathway. The calculated activation energy barrier for this process is 23.42 kcal mol−1, and thus, decomposition of H2O2 is the rate‐determining step for the whole reaction. The Fe center serves as an electron acceptor, accepting electrons from the binding propylene molecule to form radical species in the first half of the reaction and, in the role of electron donor in the remaining reaction steps to eliminate the radical feature, reduces the reactivity and stops the reaction at the stage of the desired epoxypropane product. [ABSTRACT FROM AUTHOR]

The discrete anionic structure of polyoxometalates (POMs) at the interface is more like a separate small “island”, which effectively prevents the diffusion of single atoms and prohibits the agglomeration and generation of metal particles; thus, POMs can enhance the sintering-resistant behavior and increase metal loading on the surface of single-atom catalysts (SACs). To explore the catalytic performance of POM-supported SACs for CO oxidation, we employed density functional theory (DFT) calculations to gain an understanding of some important aspects, including the CO adsorption, the formation of oxygen vacancies, and the activity of the surface oxygen species, of the catalytic system. Compared to previous theoretical studies, in which the catalytic behavior of POMs has been investigated based on the anionic unit with the highest negative charge, herein, we have constructed a model of the POM-supported SACs, which are neutral species. Our DFT calculations indicated that in the series of the SACs studied herein, (1) upon anchoring of a single metal atom on the POM surface, four key surface oxygen atoms were lifted from the POM surface to form a new interface, and thus, the surface oxygen species were activated; (2) CO adsorbed more strongly on the Ir, Os, Rh, Pt, and Ru sites than on the Fe, Mn, and Co sites; (3) it was easy to form an oxygen vacancy on the POM surface in the case of the Pt system when compared with the other systems; (4) the difference in the surface oxygen species for CO oxidation was remarkable, and the Oc atom at the catalyst interface had higher reactivity for CO oxidation as compared to the Ob atom in the Pt system studied herein; and (5) the single Pt atom served as an electron reservoir in the CO oxidation along the reaction pathway. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • A complete catalytic cycle including adsorption of reactants, activation of the hydroxylammonium chloride, formation of hyponitrous acid intermediates ((HNO) 2 ), removal of water molecules to generate N 2 , has been proposed based on our mechanism study. • The key intermediate (HNO) 2 toward N 2 is identified, and the rate–limiting step in the activation of NH 2 OH⋅HCl is a proton transfer process. • Electronic structure analysis indicates that the coordinated NO has the high electrophilic activity because the Ru → NO+ back donating bonding interaction. Abstract Reaction mechanism corresponding to reduction of NO to N 2 by hydroxylammonium chloride (NH 2 OH⋅HCl) catalyzed by a ruthenium–substituted Keggin-type polyoxometalates (POMs) has been studied by using density functional theory (DFT) method with the M06L functional. A complete catalytic cycle including adsorption of reactant, activation of the hydroxylammonium chloride, formation of hyponitrous acid intermediates ((HNO) 2 ), removal of water molecules to generate N 2 , has been proposed based on our mechanism study. The key intermediate (HNO) 2 toward N 2 is identified. The Ru center in this POM complex is proposed to be the active site, responsible not only for the formation of (HNO) 2 by the reaction of NO and NH 2 OH⋅HCl but also for the subsequent two dehydration steps to form N 2 , which is well in agreement with available experimental literature. The calculated whole energy profiles indicate that the formation of dinitrogen is controlled by thermodynamic factor. Detailed electronic structure analysis indicates that the coordinated NO is a good electrophile and responsible for the catalytic activity because the Ru → NO+ back donating bonding interaction. These results provide a valuable insight to development of the redox catalyst of NO into inorganic POM field. [ABSTRACT FROM AUTHOR]

The metalation chemistry of expanded porphyrins exhibits wide-ranging applications in numerous fields. In the work, we have investigated the electron spin density, charge transfer, frontier molecular orbitals and UV–Vis absorption spectra of four expanded porphyrin mono-Cu(II) complexes (Cu(II)@P, Cu(II)@HP, Cu(II)@OP and Cu(II)@ d -OP) using density functional theory (DFT) method and time-dependent DFT. The results show that Cu(II)@HP has different electronic properties and absorption spectra compared with other complexes. What is more interesting is that the single electrons of Cu(II)@HP are not only distributed on the Cu atom and the four coordination N atoms, but also on the uncooridnation conjugated pyrroles. However, the single electrons of other complexes are mainly distributed on the Cu atom and the four coordination N atoms. This phenomenon can be successfully explained by the specific mechanism of Cu(II)@HP unlike the Zn(II)@HP. Meanwhile, the calculated values of extended charge decomposition analysis indicate that beta electrons of Cu(II)@HP have more transfer, and its frontier molecular orbitals demonstrate greater involvement of Cu(II) ion compared with other complexes. The particular properties give rise to the different absorption spectra of Cu(II)@HP. The maximum wavelength of Cu(II)@HP displays an unusual red-shift (over 100 nm) compared with Cu(II)@ d -OP. The absorption spectra of Cu(II)@HP can be assigned to π → π ∗ transition and metal-to-ligand charge transfer characteristic. Emission wavelengths corresponding maximum absorption wavelengths of Cu(II)@HP is 610 nm that show Stokes shift with 27 nm. The aim of this work is to provide valuable information to contribute to synthesis of new metal complexes of expanded porphyrins and optical materials. [ABSTRACT FROM AUTHOR]

Crown ether can bind the alkali metal ions, and the binding is related to numerous factors. In present work, we investigated the effect of the species and number of heteroatom on the interaction energy and charge transfer between crown ether and alkali metal ions in the three complexes (12C4O-Li, 15C5O-Na and 18C6O-K) using density functional method. Our results show that the N- or S-substitution is more favorable to the enhancement of charge transfer between the alkali metal ions and crown ethers than P-substitution; furthermore, the interaction energy of N- or S-substitution is also higher than that of P-substitution. The 12C4O has strongest affinity for Li in the three complexes from the perspective of the variation of the species and the number of heteroatom. Thus, take 12C4O for example, it is concluded that the number of N-substitution has slight influence on the interaction energy; however, as the number of P- or S-substitutions increases, the interaction energy values decrease sharply. [ABSTRACT FROM AUTHOR]

Although epoxidation of alkenes by N 2 O catalyzed by Mn–substituted polyoxometalates (POMs) has been studied both experimental and theoretical methods, a complete catalytic cycle has not been established currently. In the present paper, density functional theory (DFT) calculations were employed to explore possible reaction mechanism about this catalytic cycle. Our DFT studies reveal that the reaction pathway starts from a low–valent Keggin-type POM aquametal derivative [PW 11 O 39 Mn III H 2 O] 4− . In the presence of N 2 O pressure, the formation of the active catalytic species [PW 11 O 39 Mn V O] 4− involves a ligand–substituted reaction about replacement of the aqua ligand with N 2 O to generation of POM/N 2 O adduct [PW 11 O 39 Mn III ON 2 ] 4− and dissociation of N 2 from this adduct. The calculated free energy indicates that the ligand–substituted reaction is endergonic both in gas phase or various solvents. The partial optimization method reveals that the dissociation of N 2 from [PW 11 O 39 Mn III O N 2 ] 4− involves crossing of the quintet state with a low-lying triplet state. Due to the high reactivity, the high–valent Mn V –oxo species, [PW 11 O 39 Mn V O] 4− , may react with the excess N 2 O and alkenes. Thus, two alternative reaction pathways corresponding to activation of N 2 O and epoxidation of alkenes have been considered in this work. The calculated free energy profile indicates that epoxidation of alkenes pathway is the favorable routes. Finally, a complete catalytic cycle for this reaction has been proposed. The rate–determining step in this catalytic cycle is the dissociation of N 2 from the low–valent POM/N 2 O adduct according to our DFT–M06L calculations. [ABSTRACT FROM AUTHOR]

Kerogen is irregularly connected by a variety of chemical bonds including C–C bond, C=C bond, C–O bond, C–N bond, C–H bond and C–S bond. It is difficult to identify selective bond breaking events using the existing experimental conditions. In this study, we predict dissociation tendencies of chemical bonds in various kerogen molecules by using 554 small molecular model compounds based on the density functional theoretical (DFT) calculations. Results from our DFT study indicate that the calculated various subtypes of bond dissociation enthalpies (BDEs) changing tendency is that C–S bond < C–N bond < C–O bond < C–C bond < C–H bond < C=C bond. However, for each bond type, the BDE value has a large range and overlaps each other, and the BDE mostly relies on the local environment (the functional groups and position) of chemical bonds and radical stability. For the C–C bond, a linear relationship (R= 0.85) between C–C bond distances and BDE has been achieved. Thus, the bond distance can serve as a good indicator of bond strength for C–C bond linkages. All results support that the BDE is a good choice to evaluate the dissociation tendencies of kerogen and this work provides an effective path to reveal the nature of chemical bonding for large-sized kerogen molecules. [ABSTRACT FROM PUBLISHER]

A molecular switch having large contrast on the second-order nonlinear optical (NLO) properties has been studied based on tetrathiafulvalene (TTF)-π-polynitrofluorene (PTF) molecule. It can afford to switch second-order NLO response across 10 stable states by an effective combination of its photoisomerisation and redox properties. According to density functional theory (DFT) finite field calculations (BHandHLYP/CAM-B3LYP/LC-BLYP/6-31g(d)), the redox-active TTF, TCNQ (7,7′,8,8′-tetracyano-p-quinodimethane) units, and a photoisomerised chromophore (thiophene derivative of perfluorocyclopentene) act as the tuning centre to switch second-order NLO responses. Meanwhile, time-dependent DFT method has been employed to calculate excited-state nature for getting more insights of the second-order NLO responses. [ABSTRACT FROM PUBLISHER]

Highlights: [•] The nonlinear optical properties (NLO) of a series of zinc porphyrin have been analyzed. [•] The NLO properties of these zinc porphyrins are sensitive to the nature of electron donor and porphyrin. [•] These zinc porphyrins also have very good two-dimensional second-order NLO properties. [•] The nature of excited state of zinc porphyrin has been analyzed. [ABSTRACT FROM AUTHOR]

The electronic structure and bonding feature of a series of mono-ruthenium-substituted Keggin-type polyoxometalates (POMs) have been investigated by using density functional theory (DFT) calculation, natural bond orbital (NBO) analysis and energy-decomposition analysis (EDA). A comparison of the electronic properties of two known dimethyl sulfoxide (DMSO)-supported mono-ruthenium-substituted Keggin-type POMs shows that both complexes have the analogous frontier-molecular orbital feature. One of them possesses a relatively small HOMO–LUMO gap because of the high HOMO energy level. This difference comes from a high composition of POM ligand with antibonding feature in HOMO. In addition, three typical Keggin-type POM complexes [{PW11O39}RuII/III(L)]n−(L = H2O, C5H5N, NH3) have also been explored according to our computational studies. The NBO analysis shows that the RuII/III–L bond comes from donor–acceptor interactions between the end ligand and the ruthenium (II/III) centre. The EDA shows that the POM complex [{PW11O39}RuII(C5H5N)]5−has much stronger RuII–L bond than ammonia-supported Keggin-type POM [{PW11O39}RuII(NH3)]5−. And the enhancement of the RuII–L bonding interaction in [{PW11O39}RuII(C5H5N)]5−is mainly due to the large orbital interaction energy ΔEorb. The ammonia-supported Keggin-type POM [{PW11O39}RuII(NH3)]5−and the aqua-ruthenium derivative [{PW11O39}RuII(H2O)]5−have an analogous magnitude of the total bond energy. This result supports a non-aqueous environment for synthesis of an ammonia-supported Keggin-type POM [{PW11O39}RuII(NH3)]5−. [ABSTRACT FROM AUTHOR]

The bonding features and electronic structures of a series of transition metal carbon dioxide complexes have been studied by density functional theory (DFT) calculations combined with natural bond orbital (NBO) analysis and energy-decomposition analysis (EDA). NBO analysis shows that the interaction between the metal center and the carbon atom of the carbon dioxide ligand (M–C) is stronger than the other interaction between the metal center and the carbon dioxide ligand. Natural hybrid orbital (NHO) analysis gives the detailed bonding features of the M–C bond for each complex. The NBO charge distribution on the carbon dioxide unit in all studied complexes is negative, which indicates charge transfer from the metal center to the carbon dioxide ligand for all studied complexes. The hyperconjugation effect of the metal center and the two C–O bonds of the carbon dioxide ligand has been estimated using the NBO second-order perturbation stabilization energy. It was found that the NBO second-order stabilization energy of C–O → nM* is sensitive to the coordinated sphere and the metal center. Frontier molecular orbital (FMO) analysis shows that complexes1and4may be good nucleophilic reagents for activation of the carbon dioxide molecule. However, the EDAs show that the M–CO2bond interaction energy of complex4is about two times as large as that of complex1. The high M–CO2bond interaction energy of complex4may limit its practical application. [ABSTRACT FROM PUBLISHER]

In this paper, electronic structures and third-order nonlinear optical (NLO) properties of a series of Pt-Pt bond-containing metal complexes have been calculated by using the density functional theory (DFT) combining with the sum-over-states (SOS) method. In order to check the reliability of this method, the electron correlations and basis sets have been compared. Our calculated results show that introduction of electron donor, thiophene ring and lengthing of organic conjugated ligand can enhance third-order NLO responses. The electronic structure analysis shows that the [Pt-Pt(H2P(CH2)PH2)2] fragment displays strong electron-withdrawing character in these systems. Meanwhile, the third-order NLO response of Pt-Pt bond also has been estimated by using TDDFT-SOS method. An enhancement of third-order NLO response has been observed because of the introduction of the Pt-Pt bond. This is mainly due to the intense and low-lying metal-to-ligand charge transfer (MLCT) and intraligand (IL) CT transitions. [ABSTRACT FROM AUTHOR]

The second-order nonlinear optical (NLO) properties of a series of expanded porphyrins, donor–acceptor-substituted hexaphyrins(1.1.1.1.1.1) and their mono- and bis-Au(III) complexes have been calculated based on density functional theory. The result shows that large-sized hexaphyrin molecules have a large static first hyperpolarizability when compared with the relatively small porphyrin molecule. However, the small porphyrin molecule is intrinsically more efficient for enhancement of the intrinsic NLO response according to intrinsic hyperpolarizability analysis. On the other hand, the switchable NLO properties of aromatic-to-antiaromatic conversion caused by two-electron oxidation/reduction processes have also been probed. The result shows that aromatic→antiaromatic reduction on the hexaphyrin core decreases the static first hyperpolarizability and the intrinsic hyperpolarizability. The value of the aromatic complex is ∼25 times larger than that of the antiaromatic complex. This indicates that the hexaphyrin ring plays an important role in the determination of the second-order NLO response, and the aromatic-to-antiaromatic changes on the hexaphyrin ring cause switchable second-order NLO responses. [ABSTRACT FROM PUBLISHER]

Density functional theory (DFT) was employed to calculate the bonding feature, electron spectrum, and redox properties of a dumbbell-shaped polyoxometalate (POM) cluster, [ [image omitted](PW11O39)2]8-. Our DFT calculations confirm that the quadruple bond interaction between two ReIII centers consists of σ, 2π, and δ orbital overlaps, and the strength of the bonding interactions is in the order σ-bond > π-bond>δ-bond. Time-dependent (TD) DFT calculations suggest that this cluster possesses one strong and four weak absorption bands between 0.7 and 2.5 eV. Excited state calculations show that the Re-Re bond length increases when one electron of the Re-Re bonding δ orbital is shifted to the Re-Re δ* antibonding orbital. Molecular orbital predictions and spin unrestricted calculations indicate that the two Re atoms form the reduced center. The Re-Re bond length increases in one- and two-electron reduction processes because the Re-Re δ* antibonding orbital becomes occupied. However, these changes in the Re-Re bond length in excitation and reduction processes are not substantial because of the weak δ bond. The substitution effect of the central tetrahedron heteroatom of the POM ligand affects the Re-Re bond length, with the Re-Re distance decreasing with the substitution of X from Al to S, Al (2.345 Å) > Si (2.310 Å) > P (2.282 Å) > S (2.257 Å). [ABSTRACT FROM AUTHOR]

Abstract: Time-dependent density functional theory (TDDFT) was employed to study second-order nonlinear optical (NLO) properties of transition metal fullerene complexes with a face to face model. The results show that the ruthenium species possesses the larger second-order NLO response than that of the iron species. Chemical modifications of the fullerene by carbon–carbon bond scission can tune charge transfer character in these complexes, and increase the static first hyperpolarizability. The electronic structure analysis and TDDFT calculations show that the open-cage complex in the ruthenium species possesses low-lying and strong charge transfer transitions, and thus give the largest second-order NLO response. [ABSTRACT FROM AUTHOR]

• TTF-TC-SP hybrid possesses large NLO responses. • The strongly favorable charge transfer enhances NLO response. • TTF-TC-SP hybrid switches the first hyperpolarizability across four states. A series of novel molecular hybrids, in which tetrathiafulvalene (TTF) unit and spiropyran (SP) core are covalently linked to various typical π-conjugated bridges, has been designed based on auxiliary donors and acceptors model and twisted intramolecular charge transfer (TICT) strategy. And the second-order nonlinear optical (NLO) responses of these molecular hybrids have been investigated by using density functional theory (DFT) calculations. The results show that grafting of the TTF unit and SP core to the twisted chromophore (TC) can effectively enhances second-order NLO responses. The promoting effect of enhancing NLO response can be attributed to a strongly favorable charge transfer in TTF-TC-SP hybrid. More importantly, we have found that this TTF-TC-SP hybrid with donor-π-conjugated bridge-acceptor structure can efficiently switch the first hyperpolarizability across four states via redox and photochemical stimuli. Meanwhile, time-dependent (TD) DFT calculations were carried out to probe the origin of the second-order NLO response of TTF-TC-SP molecular hybrid. All these findings provide a clear and well understanding of the relationship between the tunable first hyperpolarizability and electronic structure. We hope these findings would be very useful to guide the search for potential polymorphic molecular switch. [ABSTRACT FROM AUTHOR]

A whole catalytic cycle for CO oxidation via Rh–assisted Mans–van Krevelen mechanism has been studied by using density functional theory calculations. [Display omitted] • A catalytic cycle for CO oxidation by O 2 catalyzed by the Rh 1 /PTA has been proposed. • Strong adsorption of CO molecules significantly inhibits the adsorption of O 2. • The surface oxygen species of PTA support was activated by the anchoring effect. • The difference of O b and O c atoms of PTA surface is remarkable for CO oxidation. • Rh atom serves as an electron reservoir along the favorable reaction pathway. Recently, there are contradictory picture of the surface processes for polyoxometalates (POMs) supported single–atom catalysts (SACs) in CO oxidation, both Langmuir–Hinshelwood (LH) and Mars–van Krevelen (MvK) mechanisms have been proposed based on experimental and theoretical studies. We here present results of density functional theoretical (DFT) calculations on the neutral phosphotungstic acid (PTA) supported Rh SAC (Rh 1 /PTA), aiming at the identification of an Rh–assisted MvK mechanism. The main finding of our DFT calculations are: (1) two CO molecules adsorb simultaneously at the Rh site with considerable adsorption energy, which inhibits the adsorptions of O 2 ; (2) anchoring of single Rh atom on the PTA surface weakens W–O b /O c bonds and leads to the charge rearrangement of the surface oxygen species; (3) the anchored Rh atom serves as an electron reservoir along the favorable pathway, as supported by the Rh atom acts as electron acceptor in formation of oxygen vacancy and electron donor in the replenishing step; (4) the difference between O b and O c atoms of PTA surface is remarkable, the calculated barrier for vacancy formation by reaction with adsorbed CO and O b atom is lower than that of O c atom (25 kcal mol–1 vs 40 kcal mol–1, however, previous experimental and theoretical studies neglect this remarkable difference of the O b and O c atoms in CO oxidation, which were treated indiscriminately.); (5) although the O 2 molecule was activated by the charge transfer from the anchoring Rh atom to π* antibonding orbital of O 2 , a strong bonding interaction of Rh–O 2 leads to a high barrier of 41 kcal mol–1. Therefore, the dissociation of the adsorbed O 2 molecule is the rate–determining step for the whole catalytic cycle. Consequences of these findings for the Rh–assisted MvK mechanism on the Rh 1 /PTA SAC, can be viewed as a model system for POM–supported SACs. [ABSTRACT FROM AUTHOR]

• The mechanism of CO oxidation by O 2 was studied over two POM-supported SACs (Rh 1 /PTA and Rh 1 /PMA). • The oxidation process involves consecutive oxidation of three CO molecules via a Rh-assisted MvK mechanism. • The second CO molecule was oxidized by the corner O atom of the POM support rather than adsorbed O 2 molecule. • Oxidation state of the Rh center changes among III, II, and I, which promotes electron transfer and the whole reaction. The low and insufficient anchored site on support structure is the bottleneck problem for development of single-atom catalysts (SACs). Owing to the discrete anionic structure, the surface structure of unimolecular polyoxometalates (POMs) in solid is more like the fragments of metal oxide and possesses the unique separate structure, which can be viewed as the separate small "island" and effectively prevent agglomeration of single metal atoms. In the present paper, the mechanism of CO oxidation by O 2 was systematically studied over two POM-supported SACs, Rh 1 /PTA and Rh 1 /PMA (PTA = [PW 12 O 40 ]3− and PMA = [PMo 12 O 40 ]3−) by means of density functional theory (DFT) calculations. The results indicated that this process involves consecutive oxidation of three CO molecules over Rh 1 /PTA and Rh 1 /PMA SACs via a Rh-assisted Mar-van Krevelen (MvK) mechanism. The first CO molecule was oxidized by a bridged O atom of POM support to form CO 2 molecule and an oxygen vacancy. The surface oxygen vacancy was refilled by O 2 molecule, and the second CO molecule was oxidized by a corner O atom of the POM support to form CO 2 molecule and a new oxygen vacancy. Finally, the forming surface oxygen vacancy was replenished by the O 2 molecule to form the key Mo-O-O-Mo unit, which can easily oxidize the third CO to form the CO 2 molecule. Compared with the reported MvK mechanism, the significant difference arises from oxidation of the second CO molecule, in which the second CO molecule was oxidized by the corner O atom of the POM support rather than adsorbed O 2 molecule. This is mainly due to a low reactivity for the adsorbed O 2 molecule, which comes from a bonding interaction between the adsorbed O 2 molecule and the single Rh atom. The atomic charge analysis show that the single Rh atom stabilized on the POM supports promotes the whole catalytic cycle via the change of oxidation state among III, II, and I. All these results show that the presence of single Rh atom on the unimolecular PTA and PMA supports is a key factor for determination of new reaction pathway and high reactivity. We hope our computation results can provide new physical insights to design SACs for CO oxidation by using unimolecular POM cluster. The reaction mechanism of CO oxidation by O 2 was systematically studied over two polyoxometalate-supported single-atom catalysts by means of density functional theory calculations. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract: The second-order nonlinear optical (NLO) properties of a series of the polyoxometalate (POM)-spiropyran derivatives have been studied by using quantum chemical calculations. The photochemical reaction of spiropyran compounds will produce two different species, spiropyran and merocyanine. The merocyanine species has a large second-order NLO responses because of its good π-conjugated character. And thus a large contrast of the first hyperpolarizability has been obtained. According to quantum chemical calculation, this photo-triggered switchable effect derived from the spiropyran-merocyanine conversion has been enhanced because of introduction of the POM unit. Meanwhile, the substituted effect of the POM species also has been considered in this work. It can be found that introduction of the electron donor/acceptor group into the POM derivative does not significantly affect the magnitude of the first hyperpolarizability. [Copyright &y& Elsevier]

Cu catalyst modified by B single-atom obtained unprecedented catalytic property compared to the corresponding pure metal catalyst, which is due to the special active site for water dissociation that the embedded B single-atom creates two heterogeneous Cu atoms with different electronic structures. [Display omitted] • B-doped Cu catalyst exhibits excellent performance compared to pure Cu catalyst in WGSR. • Doping strategy creates heterogeneous Cu atoms with different electronic structures by doping B single-atom in pure Cu catalyst to promote WGSR. • Due to its charming electronic property, B atom can act as a charge transfer medium, freely switching between anionic and cationic states. It is crucial to explore the best active center for a catalyst in heterogeneous catalysis. Here we probe the performance of B single-atom doped Cu catalyst and the corresponding pure metal catalyst in water-gas shift reaction (WGSR) through density functional theory (DFT). The results show that Cu catalyst modified by B single-atom obtained unprecedented catalytic property compared to the corresponding pure metal catalyst, which is due to the special active site for water dissociation that the embedded B single-atom creates two heterogeneous Cu atoms with different electronic structures. Moreover, due to its charming electronic property, B atom can act as a charge transfer medium, freely switching between anionic and cationic states. More importantly, the synergy of heterogeneous Cu atoms reduces the activation energy barrier of the rate-limiting step (water dissociation) by 0.11 eV on B-doped Cu surface in WGSR. Electronic structure analysis emerges a downshifted d -band center of surface Cu atoms coordinated with B atom and an upshifted d -band center of those by Cu-Cu coordination originated from strain and ligand effects. The findings aim to provide deep insights for people to design new catalysts using earth-abundant elements in WGSR. [ABSTRACT FROM AUTHOR]