Side-on Configuration Research Articles

Metal-Coordinating Single-Boron Sites Confined in Antiperovskite Borides for N2-to-NH3 Catalytic Conversion

The concept of single-atom catalysts (SACs) takes our understanding of active sites of heterogeneous catalysts to greater heights. While the knowledge about the SACs containing isolated metal centers has rapidly enriched over the last decade, a few examples of the SACs that feature single-nonmetal centers are studied, and their application potentials are also poorly explored. Herein, we report a family of 358 model surfaces featuring metal-coordinated single-boron sites for investigating the effects of boron-centered local environment on N2-to-NH3 catalytic conversion. We demonstrate these model surfaces with (001) surfaces of antiperovskite borides (M3M′B, M, and M′ represent different metal atoms), where the surface-isolated boron atoms are spacially separated by M atoms, but they do not directly bond to M′ atoms. The surface-isolated boron atoms are found to be necessary for the adsorption and activation of dinitrogen, and the types of M atoms determine dinitrogen adsorption configurations. When M atoms are the late transition metals (e.g., Ni), dinitrogen prefers to adsorb on the top-site of the surface B atom with an end-on configuration; when M atoms are alkaline-earth metals, early transition metals and lanthanide elements (e.g., Sr, Zr, and La) and dinitrogen bonds to M–B–M three-center site with a side-on configuration. Additionally, during the catalytic conversion of dinitrogen to ammonia, the M atoms cooperate with the isolated boron atom to costabilize NxHy intermediates (x = 1, 2; y = 0–4), and the M′ atoms finely regulate the electronic structure of B–M ensembles and thereby tune the adsorption property and catalytic activity. Furthermore, the adsorption free energy of *N is found to be applied as a descriptor of the limiting potential of dinitrogen reduction reaction, and its value is correlated with orbital hybridization between M and B atoms apart from the inherent electronic properties of M and B atoms. Finally, we predict Rh3GeB, Rh3SbB, and Ni3LiB to be promising ammonia synthesis catalysts with high activity that exceed that of the stepped Ru(0001) surface, a prominent benchmarking surface for ammonia synthesis.

Mo2C-MoO2 Heterostructure Quantum Dots for Enhanced Electrocatalytic Nitrogen Reduction to Ammonia.

The electrocatalytic nitrogen reduction reaction (NRR) has been regarded as a promising strategy for producing ammonia (NH3) at ambient conditions. However, the development of the NRR is severely hindered by the difficult adsorption and activation of N2 on the catalyst surface and the competitive hydrogen evolution reaction (HER) due to the lack of efficient NRR electrocatalysts. Herein, Mo2C-MoO2 heterostructure quantum dots embedded in reduced graphene oxide (RGO) are proposed as efficient catalysts for the electrocatalytic NRR. The ultrasmall size of the quantum dot is beneficial for exposing the active sites for the NRR, and the synergetic effect of Mo2C and MoO2 can promote N2 adsorption and activation and suppress the competitive HER simultaneously. As a result, a well-balanced NRR performance is achieved with a high NH3 yield rate of 13.94 ± 0.39 μg h-1 mg-1 at -0.15 V vs RHE and a high Faradaic efficiency of 12.72 ± 0.58% at -0.1 V vs RHE. Density functional theory (DFT) calculations reveal that the Mo2C (001) surface has a strong N2 adsorption energy of -1.47 eV with the side-on configuration, and the N≡N bond length is elongated to 1.254 Å, indicating the enhanced N2 adsorption and activation on the Mo2C surface. On the other hand, the low ΔGH* for HER over the MoO2 (-111) surface demonstrates the impeded HER process for MoO2. This work may provide effective catalyst-design strategies for enhancing the electrocatalytic NRR performance of Mo-based materials.

Heterogeneous Two-Atom Single-Cluster Catalysts for the Nitrogen Electroreduction Reaction

The electrocatalytic dinitrogen reduction reaction (NRR) is promising to realize the decentralized production of ammonia by using renewable energies, which contrasts with the energy-intensive Haber–Bosch process. The key to achieve it is to find stable, efficient and selective catalysts. Recently, the heterogeneous single-cluster catalysts (SCCs) have emerged as a promising class of catalysts for electrochemical reactions due to their atomically precise active site, abundant active atoms and atomic level controllability. Herein, the NRR catalyzed by the two-atom SCCs consisting of homonuclear 3d transition metal (TM) dimers over the N-doped graphene, denoted as M2-N6G, is systematically investigated by using density functional theory (DFT). Our results indicate that the ability of metal dimer to capture N2 is related to the reducibility of the catalyst and the orbital interaction between the N-2p states and the TM-3d states. Subsequently, comparing with those metals which overbind N2 through side-on configurations, the M2-N6G SCCs with end-on adsorption of N2 work better. Furthermore, we obtain a linear relationship between the adsorption free energies of *N2H (ΔadsG*NH2) and that of *NH2 (ΔadsG*NH2). Based on this scaling relationship, we propose a compromised strategy for screening efficient two-atom SCCs for NRR. Finally, by comparing the stability, activity and selectivity of various M2-N6G SCCs, the Cr2-N6G and Mn2-N6G are predicted to be most active for NRR with low limiting potential and high suppression to hydrogen evolution reaction (HER). The present work not only provides experimentally synthesizable electrocatalyst candidates for NRR, but also gives insight into the development of the two-atom SCCs.

Palladium-based single atom catalysts for high-performance electrochemical production of hydrogen peroxide

Electrochemical two-electron oxygen reduction reaction (2e-ORR) offers a green route for production of hydrogen peroxide (H2O2). However, it remains a key challenge to design and develop the catalysts possessing both high activity and selectivity. Herein, we design ten Palladium-based single atom catalysts (Pd-based SACs) and report the underlying mechanisms of the activity and selectivity for 2e-ORR to produce H2O2. The intermediate of *OOH is used as a descriptor to describe the catalytic activity of 2e−/4e− ORR through a volcano diagram due to a linear scaling relationship between *OOH and *O (or *OH). We find that a defect graphene binding with one Pd single atom (C4Pd), which is thermodynamically favorable, exhibits an ultralow overpotential (0.044 V) for production of H2O2. In particular, this catalyst displays excellent activity and selectivity for H2O2 in a faintly acid or neutral electrolyte instead of alkaline. Furthermore, the Pd atomic clusters binding on defect graphene (C4Pdx, x = 2, 3, 4, 5) exhibit poor performance for production of H2O2 than that of C4Pd. The computational results reveal that the side-on adsorption configurations of O2 molecule and *H constitute the limiting factors for the selectivity of H2O2. These findings offer new insights for interpreting the mechanisms of 2e-ORR and pave the way for future designs and developments of excellent catalysts for the direct synthesis of H2O2.

Rich B active centers in Penta-B2C as high-performance photocatalyst for nitrogen reduction

The photocatalytic nitrogen reduction reaction (NRR) has mild reaction conditions and only requires sunlight energy as a driving force to replace the traditional ammonia synthesis method. We herein investigate the catalytic activity and selectivity on Penta-B2C for NRR by using density functional theory calculations. Penta-B2C is a semiconductor with an indirect bandgap (2.328 eV) and is kinetically stable based on molecular dynamic simulations. The optical absorption spectrum of Penta-B2C is achieved in the ultraviolet and visible range. Effective light absorption is more conducive to generate photo-excited electrons and improving photocatalytic performances. Rich B atoms as activation sites in Penta-B2C facilitate capturing N2. The activated N2 molecule prefers the side-on adsorption configuration on Penta-B2C, which facilitates the subsequent reduction reaction. Among considered NRR mechanisms on Penta-B2C, the best pathway prefers the enzymatic mechanism, only required a low onset potential of 0.23 V. The hydrogen evolution reaction is inhibited when the hydrogen adsorption concentration is increased or N2 molecules first occupy the adsorption sites. Our results indicate Penta-B2C is a highly reactive and selective photocatalyst for NRR. Our work provides theoretical insights into the experiments and has guiding significance to synthesize efficient NRR photocatalysts.

Ultrasmall size FeNi Prussian blue analogue on rGO with accurate heteronuclear adsorption sites toward efficient electrochemical nitrogen fixation

The electrochemical NRR to produce ammonia under ambient conditions represents an attractive prospect in nitrogen fixation which can greatly ease the energy crisis and environmental pollution. However, suffering from awful N2 activation results in low ammonia yield rate and Faradaic efficiency in electrochemical NRR. In this work, we devote to construct FeNi heteronuclear sites in FeNi-PBA with accurate coordination structure to facilitate N2 activation employing a side-on configuration. Control experiments have verified N2 activation on FeNi heteronuclear sites was enhanced compared with that on FeFe sites. Meanwhile, the particle size of PBA was decreased to boost active sites. FeNi-PBA nanocrystals with ultrasmall size (10 ± 2.8 nm) through microwave-assisted synthesis achieve an ammonia yield rate of 17.7 mgNH3 gcat.−1 h−1 and Faradaic efficiency of 27.5% at a bias of −0.2 V (V vs. RHE) in 0.05 M H2SO4 solution. Faradaic efficiency achieved in this work well exceeds than that of most Fe based transition metal electrocatalysts.

Theoretical insights into bimetallic atoms supported on PC6 as highly efficient electrocatalysts for N2 electroreduction to NH3

Electrochemical N2 reduction under mild conditions has been regarded as a promising alternative to the conventional Haber-Bosch process. In the electrochemical reaction, a highly efficient catalyst plays an important role. Herein, we studied a series of bimetallic atoms catalysts, namely, two transition metal atoms doped on the PC6 monolayer (TM2@PC6, TM = Cr, Mn, Fe, Co. Ni, Mo, Ru, Rh, W, and Ir), to examine the electrocatalytic performance for nitrogen reduction reaction by employing the density functional theory computations. Co2@PC6 is screened out to be the best candidate for the N2 reduction, which has high Faradaic efficiency and lowest limiting potential of −0.28 V, followed by Mo2@PC6, and Fe2@PC6 with limiting potentials of −0.46, and −0.50 V, respectively. These predicted candidates have comparable or even better NRR catalytic activity than the other common two-dimensional materials. The N2H* formation is the potential-determining step for these catalysts. N2 with side-on adsorption configuration (occurred only for Co2@PC6) has the best NRR performance. This work not only provides the potential NRR catalyst but also gives insight into the mechanism to advance the development of efficient NRR catalysts.

Density functional theory study of transition metal single-atoms anchored on graphyne as efficient electrocatalysts for the nitrogen reduction reaction.

Ammonia (NH3) is the main raw material for the organic chemical industry and a critical feedstock for the fertilizer industry with great significance for the global economy. The NH3 demand has gradually increased with modern society development. Moreover, the electrocatalytic nitrogen reduction reaction (NRR) is a promising NH3 synthesis technology. However, the design of efficient electrocatalysts for the NRR is still challenging. In this study, we systematically analyzed transition metal (TM) single-atoms (Ti, V, Cr, Mn, Zr, Nb, and Mo) anchored on graphyne (GY) as NRR catalysts using density functional theory calculations. The calculation results for the first and last hydrogenation steps (*NNH formation and *NH3 desorption, respectively) revealed that Mn@GY (with an end-on configuration) and V@GY (with a side-on configuration) were the most suitable catalytic substrates for the NRR. The free-energy profiles of the TM@GY catalysts indicated that Mn@GY was the best NRR electrocatalyst owing to its distal pathway with a minimum free-energy barrier of 0.36 eV. In addition, the electronic properties, namely the Bader charge, charge density difference, partial density of states, and crystal orbital Hamilton population, of the TM@GY catalysts were analyzed in detail, and the results further confirmed that Mn@GY was an efficient electrocatalyst. The insights obtained from this comprehensive study can provide useful guidelines for designing new and efficient electrocatalysts.

Computational screening of MBene monolayers with high electrocatalytic activity for the nitrogen reduction reaction.

As an emerging family of two-dimensional (2D) materials, transition metal borides (MBenes) have attracted increasing interest due to their potential applications in electrochemistry, especially electrocatalysis. In this work, we addressed six MB (M = Sc, Ti, V, Cr, Mo and W) monolayers as catalysts to explore their electrocatalytic activity for the nitrogen reduction reaction (NRR) using first-principles calculations. Our results demonstrated that N2 molecules could be strongly adsorbed on these MB monolayers to provoke the NRR process. Furthermore, we examined five possible catalytic reaction pathways of the NRR, i.e., the alternating, distal, and three mixed pathways, on the MB monolayers with N2 adsorption (both side-on and end-on) configurations, and screened out three highly efficient NRR catalysts: VB, CrB, and MoB monolayers with the onset potential of -0.396, -0.277, and -0.403 V, respectively. By comparison of the limiting potentials, the most effective reaction pathways of the NRR were ascertained to be the alternating pathway on the VB monolayer with the end-on configuration and the mixed I pathway on the CrB monolayer with the end-on configuration and on the MoB monolayer with the side-on configuration. Our work sheds light on the electrocatalytic mechanisms of the NRR on 2D MBenes, and provides a theoretical foundation for developing highly efficient MBene electrocatalysts for the NRR.

Unraveling Catalytic Mechanisms for CO Oxidation on Boron-Doped Fullerene: A Computational Study.

By means of spin-polarized density functional theory (DFT) computations, we unravel the reaction mechanisms of catalytic CO oxidation on B-doped fullerene. It is shown that O2 species favors to be chemically adsorbed via side-on configuration at the hex-C–B site with an adsorption energy of −1.07 eV. Two traditional pathways, Eley–Rideal (ER) and Langmuir–Hinshelwood (LH) mechanisms, are considered for the CO oxidation starting from O2 adsorption. CO species is able to bind at the B-top site of the B-doped fullerene with an adsorption energy of −0.78 eV. Therefore, CO oxidation that occurs starting from CO adsorption is also taken into account. Second reaction of CO oxidation occurs by the reaction of CO + O → CO2 with a very high energy barrier of 1.56 eV. A trimolecular Eley–Rideal (TER) pathway is proposed to avoid leaving the O atom on the B-doped fullerene after the first CO oxidation. These predictions manifest that boron-doped fullerene is a potential metal-free catalyst for CO oxidation.

Enhanced photocatalytic degradation of perfluorooctanoic acid using carbon-modified bismuth phosphate composite: Effectiveness, material synergy and roles of carbon

Per- and polyfluoroalkyl substances (PFAS) such as perfluorooctanoic acid (PFOA) are recalcitrant to conventional wastewater treatment processes. In search for a more effective technology, we prepared a carbon sphere (CS) modified bismuth phosphate composite photocatalyst (BiOHP/CS) through a hydrothermal process. BiOHP/CS was able to adsorb > 99% of PFOA in 2 h (dosage = 1.0 g/L, initial PFOA = 200 µg/L, pH = 7.0), and subsequently, in situ decompose adsorbed PFOA nearly completely in 4 h of UV irradiation. The PFOA degradation rate was ~18 times greater than that by neat BiOHP. Advanced spectroscopic analyses and density functional theory calculations revealed that the carbon modification not only enhances the PFOA adsorption capacity, but also facilitates a side-on adsorption configuration of PFOA on the photoactive sites. While only the head carboxylate group was involved in PFOA adsorption on neat BiOHP, both the CF2/CF3 entities and the head carboxylate group participated in adsorption of PFOA on BiOHP/CS, which facilitates the subsequent photocatalytic cleavage of the CF bonds. Photoluminescence emission analysis and density of states calculations confirmed the CS-facilitated side-on adsorption mode and CS-enhanced separation of electrons and holes. In addition, the CS modification facilitates activation of PFOA by changing the electron distribution of the C-F bond, and enhances the stability and reusability of BiOHP by preventing photo-corrosion of the composite material. Overall, the findings provide important insights into the roles of carbonaceous modification and adsorption configurations in enhanced photocatalytic destruction of PFOA, which may guide future design and fabrication of environment-friendly adsorptive photocatalysts.

Enhancing hydrogen storage by metal substitution in MIL-88A metal-organic framework

MIL-88A metal-organic framework with the unsaturated Fe metal coordination sites has demonstrated to be a promising material for gas storage and capture. However, the hydrogen storage capacity of MIL-88A has to be improved to meet the practical level at the ambient conditions. In this research, we elucidated the effects of transition metal substitution on the hydrogen storage capability of MIL-88A. The trivalent transition metals including Sc, Ti, V, Cr, Mn, and Ni have been selected to substitute for Fe in MIL-88A. Using the van der Waals dispersion-corrected density functional theory calculations, we explored the most favorable adsorption configurations of the hydrogen molecule in M-MIL-88A (M = Sc, Ti, V, Cr, Mn, Ni). We found that the V-MIL-88A has the strongest binding energy of 17 kJ mol−1 with the hydrogen molecule in the side-on configuration on the metal site. Besides, the grand canonical Monte Carlo simulations showed that the metal substitution greatly influences not only the favorable adsorption configuration and energy but also the hydrogen uptake due to the modification of the H2@MIL-88A interaction. Sc-MIL-88A was found to offer the highest gravimetric H2 uptake compared to the other M-MIL-88A. The value is 5.13 wt% at (cryogenic temperature 77 K, 50 bar) and 0.72 wt% at (room temperature 298 K, 100 bar) for the absolute; 4.63 wt% at (77 K, 10 bar) and 0.29 wt% at (298 K, 100 bar) for the excess capacity. Furthermore, Sc-MIL-88A also exhibited the highest volumetric uptake up to 52 g L−1 at 77 K and 7.1 g L−1 at 298 K for the absolute; 46 g L−1 at 77 K and 2.8 g L−1 at 298 K for the excess loading.

Edge-sited Fe-N4 atomic species improve oxygen reduction activity via boosting O2 dissociation

The development of low-cost, efficient, and stable electrocatalysts toward the oxygen reduction reaction (ORR) is urgently demanded for scalable applications in fuel cells or zinc-air batteries (ZABs), but still remains a challenge. Herein, carbon materials with edge-sited Fe-N4 atomic species (E-FeNC) were synthesized from pyrolysis of abundant Fe-containing biomass using silica spheres as hard template. The E-FeNC delivers remarkable ORR performance with a half-wave potential of 0.875 V (vs. reversible hydrogen electrode (RHE)), much better than Pt/C (0.859 V), attributed to atomically dispersed Fe-N4 moieties nearby graphitic edges. The density functional calculations reveal that O2 molecule adsorbs on Fe-N4 sites with an energetically favorable side-on configuration with elongated OO bond rather than end-on form, boosting the subsequent dissociation pathway with a direct 4e reaction route. Using E-FeNC as cathode catalyst, the primary ZAB exhibits high specific capacity of 710 mA h g−1 and power density of 151.6 mW cm−2. The rechargeable ZAB by coupling E-FeNC and NiFe layered double hydroxide (LDH) demonstrates long-term capacity retention over 200 h, superior to that using noble Pt/C and RuO2. This unique carbon material with atomically dispersed metal sites opens up an avenue for the design and engineering of electrocatalysts for energy conversion systems.

Exploring reaction mechanisms for CO oxidation on boron-doped carbon nanotubes: A computational approach

Spin-polarized first-principles computations are carried out to illustrate the catalytic mechanisms of CO oxidation on the (5, 5) boron-doped carbon nanotube (BCNT). BCNTs with two kind of boron content of 1 and 2 atom % are studied. The calculations show that O2 species prefers to partially reduce and chemically bind at CB site via the side-on configuration with the adsorption energies of −0.85 eV for the BCNT with boron content of 1 atom % and −1.27 ~ −1.29 eV for the BCNT with boron content of 2 atom %. The partially reduced O2 species can further react with CO via the Eley-Rideal (ER) mechanism by overcoming a small energy barrier of 0.34 eV for the BCNT with boron content of 1 atom % and 0.42 eV for the BCNT with boron content of 2 atom %. Second reaction of CO oxidation occurs by the reaction of CO + O → CO2 with a slight energy barriers of 0.12 and 0.14 eV for the BCNT with boron content of 1 and 2 atom %, respectively. Our results manifest that the boron-doped CNT can be an advanced metal-free catalyst for CO oxidation.

Giant light deflection via electro-mechanical modulation of liquid crystals

Liquid crystals (LCs) are matter with fluidity and anisotropy and have been used in various electro-optic devices for their capability to modulate the refractive index by voltage. Here, we show that LCs are capable of electro-mechanically modulating light to cause giant light deflection at low voltages (exceeding 64° at 1.0 V). We use a composite material where polymerized cholesteric LC particles that show optical Bragg reflection float in a nematic LC medium. The polymer-particles are elastically coupled with the host director through their surface molecular anchoring and rotate from a face-on to side-on configuration at the Frederik transition. Rigid-body rotation of the reflection plane causes light deflection, which is well reproducible and can be modelled theoretically. Our findings demonstrate the capability of LCs as a micro-electrical-mechanical system platform, which are potentially useful for large-area light-controlling applications.

Alkyne-Functionalized Ruthenium Nanoparticles: Impact of Metal–Ligand Interfacial Bonding Interactions on the Selective Hydrogenation of Styrene

In the present study, ruthenium nanoparticles functionalized with terminal and internal alkynes were prepared, and it was found that internal alkynes formed a η2 side-on configuration on the surfac...

Design and synthesis of novel porphyrinated polyimide nanofibers as dissolved oxygen sensor: The role of electro-activity and orientation

Two novel porphyrinated polyimides, 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA)/4,4′-Oxydianiline (ODA)/iron 5,10-bis(4-aminophenyl)-15,20-diphenylporphyrin (FeDATPP) and 4,4′-hexafluoroisopropylidene diphthalic anhydride (6FDA)/ODA/FeDATPP, abbreviated as BOFe and 6OFe, were synthesized and electrospun into nanofibers for dissolved oxygen (DO) sensing. The effects of electro-activity (i.e., electron-withdrawing ability of BPDA, 6FDA and FeDATPP when these units were binding with DO) and the roles of the nanofiber orientation in DO sensing were investigated. Electrochemical characterization indicates that the current values in Cyclic Voltammograms (CV) exhibits linear correlation with the DO concentration, with the correlation coefficient R2 varying from 0.9461 to 0.9958, suggesting the excellent DO sensing performance of the synthesized porphyrinated polyimides. Meanwhile, it is found that the oxygen reduction behaviors in DO sensing were vastly altered due to the different electro-activity and orientation structures of the porphyrinated polyimides nanofibers. An unusual side-on porphyrin-O2 configuration and charge transfer mechanism during DO sensing were clarified upon molecular simulation. The present work provides two novel porphyrinated polyimides for DO sensor applications.

Hydrogen adsorption, dissociation and diffusion on two-dimensional Ti2C monolayer

The adsorption, dissociation and diffusion of hydrogen on two-dimensional (2D) Ti2C monolayer with and without carbon vacancies have been investigated by first-principles calculations based on density functional theory. Weak molecular physisorption was observed for the adsorption of H2 on 2D Ti2C monolayer with end-on configuration. However, for the side-on configuration, the H2 molecule decomposes spontaneously and then the two H atoms prefer to bond to the surface Ti atoms at the monolayer. Based on the chemical bonding analysis, the mechanism of the dissociation of H2 has been elucidated. Moreover, the hydrogen diffusion on the Ti2C surface with and without carbon vacancies was investigated. A better hydrogen diffusion property was found for the Ti2C monolayer with carbon vacancies. The energetics for H penetration from surface to the solute site in the bulk was also studied.

Ultrafast kinetics of linkage isomerism in Na2[Fe(CN)5NO] aqueous solution revealed by time-resolved photoelectron spectroscopy.

The kinetics of ultrafast photoinduced structural changes in linkage isomers is investigated using Na2[Fe(CN)5NO] as a model complex. The buildup of the metastable side-on configuration of the NO ligand, as well as the electronic energy levels of ground, excited, and metastable states, has been revealed by means of time-resolved extreme UV (XUV) photoelectron spectroscopy in aqueous solution, aided by theoretical calculations. Evidence of a short-lived intermediate state in the isomerization process and its nature are discussed, finding that the complete isomerization process occurs in less than 240 fs after photoexcitation.

Hydrogen Adsorption on Fe-based Metal Organic Frameworks: DFT Study

We study the interaction of H2 molecule with Fe-based MOF cluster which has square paddle-wheel cage structure using the density functional theory-based (DFT-based) calculations. We find that H2 molecule can be adsorbed on the Fe site of MOF-Fe cluster with a side-on configuration and MOF-H2 distance of about 2.071 Å. This adsorption is facilitated by the interaction of 1σ orbital of the molecule and dz2 -like orbital of MOF-Fe cluster. We also find that the subsitution of MOF's metal center from Cu to Fe could significantly strengthen the hydrogen interaction with the MOF. This is because one of important antibonding state between 1σ orbital of H2 with dz2 -like orbital of MOF is only partially occupied in MOF-Fe system, while it is fully occupied on MOF-Cu.