Tunable internal structure carbon sphere synthesis driven by water-solubility and its application in gas separation.

A method for synthesizing carbon spheres with a tunable particle size and internal structure from polyfurfuryl alcohol (PFA) was developed. By tuning the concentration of a structure directing agent (polypropylene glycol, PPG), we found a mechanism to tune the inner architecture of carbon spheres driven by water-solubility. A mixture of PFA and PPG transferred from the "water-in-oil" phase to an "oil-in-water" phase with an increasing content of PPG because of a difference in water-solubility between furfuryl alcohol (FA), PFA, and PPG. As a result, the internal morphology of the carbon sphere evolved from a "cheese-like" to a "pomegranate-like" structure, which was accompanied by an increasing specific surface area and pore volume. Furthermore, the separation of C2H2 and C2H3Cl was tested on the 25%-FACS (furfuryl alcohol-based carbon sphere) sample under different activation treatments with CO2 or CO2-NH3, with the coexisting "cheese-like" and "pomegranate-like" inner structures, owing to its moderate pore volume and mechanical strength. The maximum adsorption capacity of C2H3Cl reached 0.77 mmol g-1, while C2H2 was adsorbed in significantly lower quantities. It is believed that the high polarizability and high dipole moment of the C2H3Cl molecule primarily contribute to the excellent performance of C2H2 and C2H3Cl separation, and the introduction of polar N-containing groups on the carbon skeleton further promotes C2H3Cl adsorption.

Xylene Synthesis Through Tandem CO2 Hydrogenation and Toluene Methylation Over a Composite ZnZrO Zeolite Catalyst.

Selective synthesis of specific value-added aromatics from CO2 hydrogenation is of paramount interest for mitigating energy and climate problems caused by CO2 emission. Herein, we report a highly active composite catalyst of ZnZrO and HZSM-5 (ZZO/Z5-SG) for xylene synthesis from CO2 hydrogenation via a coupling reaction in the presence of toluene, achieving a xylene selectivity of 86.5 % with CO2 conversion of 10.5 %. A remarkably high space time yield of xylene could reach 215 mg gcat -1 h-1 , surpassing most reported catalysts for CO2 hydrogenation. The enhanced performance of ZZO/Z5-SG could be due to high dispersion and abundant oxygen vacancies of the ZZO component for CO2 adsorption, more feasible hydrogen activation and transfer due to the close interaction between the two components, and enhanced stability of the formate intermediate. The consumption of methoxy and methanol from the deep hydrogenation of formate by introduced toluene also propels an oriented conversion of CO2 .

Tailored Cl- Ligation on Supported Pt Catalysts for Selective Primary C-H Bond Oxidation.

It is challenging to achieve high selectivity over Pt-metal-oxide catalysts widely used in many selective oxidation reactions because Pt is prone to over-oxidize substrates. Herein, our sound strategy for enhancing the selectivity is to saturate the under-coordinated single Pt atoms with Cl- ligands. In this system, the weak electronic metal-support interactions between Pt atoms and reduced TiO2 cause electron extraction from Pt to Cl- ligands, resulting in strong Pt-Cl bonds. Therefore, the two-coordinate single Pt atoms adopt a four-coordinate configuration and thus inactivated, thereby inhibiting the over-oxidation of toluene over Pt sites. The selectivity for the primary C-H bond oxidation products of toluene was increased from 50.1 to 100%. Meanwhile, the abundant active Ti3+ sites were stabilized in reduced TiO2 by Pt atoms, leading to a rising yield of the primary C-H oxidation products of 249.8 mmol gcat-1. The reported strategy holds great promise for selective oxidation with enhanced selectivity.

Pt Nanoparticles Confined in a 3D Porous FeNC Matrix as Efficient Catalysts for Rechargeable Li-CO2/O2 Batteries.

The cathodic product Li2CO3, due to its high decomposition potential, has hindered the practical application of rechargeable Li-CO2/O2 batteries. To overcome this bottleneck, a Pt/FeNC cathodic catalyst is fabricated by dispersing Pt nanoparticles (NPs) with a uniform size of 2.4 nm and 8.3 wt % loading amount into a porous microcube FeNC support for high-performance rechargeable Li-CO2/O2 batteries. The FeNC matrix is composed of numerous two-dimensional (2D) carbon nanosheets, which is derived from an Fe-doping zinc metal-organic framework (Zn-MOF). Importantly, using Pt/FeNC as the cathodic catalyst, the Li-CO2/O2 (VCO2/VO2 = 4:1) battery displays the lowest overpotential of 0.54 V and a long-term stability of 142 cycles, which is superior to batteries with FeNC (1.67 V, 47 cycles) and NC (1.87 V, 23 cycles) catalysts. The FeNC matrix and Pt NPs can exert a synergetic effect to decrease the decomposition potential of Li2CO3 and thus enhance the battery performance. In situ Fourier transform infrared (FTIR) spectroscopy further confirms that Li2CO3 can be completely decomposed under a low potential of 3.3 V using the Pt/FeNC catalyst. Impressively, Li2CO3 exhibits a film structure on the surface of the Pt/FeNC catalysts by scanning electron microscopy (SEM), and its size can be limited by the confined space between the carbon sheets in Pt/FeNC, which enlarges the better contacting interface. In addition, density functional theory (DFT) calculations reveal that the Pt and FeNC catalysts show a higher adsorption energy for Li2CO3 and Li2CO4 intermediates compared to the NC catalyst, and the possible discharge pathways are deeply investigated. The synergetic effect between the FeNC support and Pt active sites makes the Li-CO2/O2 battery achieve optimal performance.

Theoretical Understandings of Graphene-based Metal Single-Atom Catalysts: Stability and Catalytic Performance.

Research on heterogeneous single-atom catalysts (SACs) has become an emerging frontier in catalysis science because of their advantages in high utilization of noble metals, precisely identified active sites, high selectivity, and tunable activity. Graphene, as a one-atom-thick two-dimensional carbon material with unique structural and electronic properties, has been reported to be a superb support for SACs. Herein, we provide an overview of recent progress in investigations of graphene-based SACs. Among the large number of publications, we will selectively focus on the stability of metal single-atoms (SAs) anchored on different sites of graphene support and the catalytic performances of graphene-based SACs for different chemical reactions, including thermocatalysis and electrocatalysis. We will summarize the fundamental understandings on the electronic structures and their intrinsic connection with catalytic properties of graphene-based SACs, and also provide a brief perspective on the future design of efficient SACs with graphene and graphene-like materials.

Surface-modified Pt1Ni1-Ni(OH)2 nanoparticles with abundant Pt-Ni(OH)2 interfaces enhance electrocatalytic properties.

Pt-Based catalysts for the methanol oxidation reaction (MOR) are highly susceptible to poisoning due to the surface adsorption of reaction intermediates such as COads. Depositing Pt nanoparticles (NPs) on Ni(OH)2 to fabricate Pt-Ni(OH)2 interfaces is considered as a promising method to improve the stability of Pt-based catalysts because Ni(OH)2 could facilitate water dissociation in alkaline electrolytes to form OH adspecies and assist in the oxidative removal of COads on adjacent Pt sites. However, this supported structure rather limited Pt-Ni(OH)2 interfaces because only a small fraction of the Pt NP surface could come into contact with Ni(OH)2. Herein, this work has addressed a simple and efficient strategy to engineer novel-structure catalysts by tuning the properties of the interface of Pt-based NPs with high-index facets (HIFs). Pt1Ni1-Ni(OH)2 nanoparticles (NPs) were synthesized through Ni(OH)2 partially covering the HIFs of monodisperse Pt1Ni1 concave nanocubes (CNCs) in situ. Pt-Ni(OH)2 interfaces were characterized and over 40% of the Pt surface active sites fall within the periphery of Ni(OH)2. Thanks to the synergy of HIFs and abundant Pt-Ni(OH)2 interfaces, Pt1Ni1-Ni(OH)2 NPs exhibited remarkable catalytic performance towards the MOR in alkaline solution.

Promoting effect of nickel hydroxide on the electrocatalytic performance of Pt in alkaline solution.

Despite intense research in the past decades, the lack of high-performance catalysts for fuel cell reactions remains a challenge in realizing fuel cell applications. Herein, we report a novel hybrid nanomaterial of platinum-nickel hydroxide-nanotubes (Pt/Ni(OH)2/CNTs) for improving electrocatalytic performance in alkaline environments. Ni(OH)2 was directly grown on functionalized nanotubes and then, Pt nanoparticles were in situ immobilized by the microwave synthesis method. Due to electronic and synergistic effects, 10 : 2-Pt/Ni(OH)2/N-CNT catalyst exhibited 2.77 times specific activity and 6.27 times mass activity toward methanol oxidation reaction (MOR), which were higher than those of commercial Pt/C in alkaline solution. The CO-stripping experiments and hydrogen evolution reaction (HER) further demonstrated that Ni(OH)2 could promote oxidation removal of carbonaceous poison for MOR via accelerating water dissociation: (i) Ni(OH)2 acted on an H2O molecule, leading to the formation of OHad; (ii) OHad oxidized the intermediate COad to CO2. Furthermore, the 10 : 2-Pt/Ni(OH)2/N-CNT catalyst also exhibited 2.07 times specific activity and 1.67 times mass activity toward oxygen reduction reaction (ORR), which were higher than those of commercial Pt/C in alkaline solution and Pt/N-CNT catalysts. Thus, the preparation of this hybrid nanomaterial provides a new direction for catalyst performance optimization towards next-generation fuel cells in alkaline environments.

Metallic Cobalt Encapsulated in Bamboo-Like and Nitrogen-Rich Carbonitride Nanotubes for Hydrogen Evolution Reaction.

Despite being technically possible, the hydrogen production by means of electrocatalytic water splitting is still practically unreachable mainly because of the lack of inexpensive and high active catalysts. Herein, a novel and facile approach by melamine polymerization, exfoliation and Co(2+)-assisted thermal annealing is developed to fabricate Co nanoparticles embedded in bamboo-like and nitrogen-rich carbonitride nanotubes (Co@NCN). The electronic interaction between the embedded Co nanoparticles and N-rich carbonitride nanotubes could strongly promote the HER performance. The optimized Co@NCN-800 exhibits outstanding HER activity with an onset potential of -89 mV (vs RHE), a large exchange current density of 62.2 µA cm(-2), and small Tafel slope of 82 mV dec(-1), as well as excellent stability (5000 cycles) in acid media, demonstrating the potential for the replacement of Pt-based catalysts. Control experiments reveal that the superior performance should be ascribed to the synergistic effects between embedded Co nanoparticles and N-rich carbonitride nanotubes, which originate from the high pyridinic N content, fast charge transfer rate from Co particles to electrodes via electronic coupling, and porous and bamboo-like carbonitride nanotubes for more active sites in HER.

Theoretical study of the cooperative effects between the triel bond and the pnicogen bond in BF3···NCXH2···Y (X = P, As, Sb; Y = H2O, NH3) complexes.

The interplay between the triel bond and the pnicogen bond in BF3···NCXH2···Y (X = P, As, Sb; Y = H2O, NH3) complexes was studied theoretically. Both bonds exhibited cooperative effects, with shorter binding distances, larger interaction energies, and greater electron densities found for the ternary complexes than for the corresponding binary ones. The cooperative effects between the triel bond and the pnicogen bond were probed by analyzing molecular electrostatic potentials, charge transfer, and orbital interactions. The results showed that the enhancement of the triel bond can mainly be attributed to the electrostatic interaction, while the strengthening of the pnicogen bond can be ascribed chiefly to the electrostatic and orbital interactions. In addition, the origins of both the triel bond and the pnicogen bond were deduced via energy decomposition.

Novel pnicogen bonding interactions with silylene as an electron donor: covalency, unusual substituent effects and new mechanisms.

An analogue of carbene, singlet silylene (H3P=N)2Si, was paired with the mono-substituted phosphines XH2Y (X = P, As, and Sb; Y = F, Cl, Br, and I) to form unconventional pnicogen-bonded complexes. All structures have Cs symmetry except the Sb complex, showing a deviation from this symmetry due to the coexistence of H···H interactions. The P and As complexes have different geometries from conventional pnicogen-bonded ones because the Y-X···Si line shows a large deviation from the molecular plane composed of two N atoms and one Si atom of (H3P=N)2Si. This deviation can be attributed to a new formation mechanism of the pnicogen bond due to the combined result of the LPSi→ BD*X-Y and LPX→ LP*Si orbital interactions. Generally, the pnicogen bond becomes stronger in the order of F < Cl < Br < I and weaker in the order of P > As > Sb, exhibiting an unexpected substitution effect and dependence on the nature of the pnicogen atom. These orders are inconsistent with the MEP on the X atom but can be better explained by the above orbital interactions. The Si···X interaction displays a character of covalent or partially covalent interaction, evidenced by the high interaction energy of -59.9 to -105.4 kJ mol(-1) as well as the negative energy density and the great charge transfer.

Tetrel-hydride interaction between XH3F (X = C, Si, Ge, Sn) and HM (M = Li, Na, BeH, MgH).

A tetrel-hydride interaction was predicted and characterized in the complexes of XH3F···HM (X = C, Si, Ge, Sn; M = Li, Na, BeH, MgH) at the MP2/aug-cc-pVTZ level, where XH3F and HM are treated as the Lewis acid and base, respectively. This new interaction was analyzed in terms of geometrical parameters, interaction energies, and spectroscopic characteristics of the complexes. The strength of the interaction is essentially related to the nature of X and M groups, with both the larger atomic number of X and the increased reactivity of M giving rise to a stronger tetrel-hydride interaction. The tetrel-hydride interaction exhibits similar substituent effects to that of dihydrogen bonds, where the electron-donating CH3 and Li groups in the metal hydride strengthen the binding interactions. NBO analyses demonstrate that both BD(H-M) → BD*(X-F) and BD(H-M) → BD*(X-H) orbital interactions play the stabilizing role in the formation of the complex XH3F···HM (X = C, Si, Ge, and Sn; M = Li, Na, BeH, and MgH). The major contribution to the total interaction energy is electrostatic energy for all of the complexes, even though the dispersion/polarization parts are nonnegligible for the weak/strong tetrel-hydride interaction, respectively.

Complexes between hypohalous acids and phosphine derivatives. Pnicogen bond versus halogen bond versus hydrogen bond.

The complexes of HOBr:PH2Y (Y=H, F, Cl, Br, CH3, NH2, OH, and NO2), HOCl:PH2F, and HOI:PH2F have been investigated with ab initio calculations at the MP2/aug-cc-pVTZ level. Four types of structures (1, 2, 3a, and 3b) were observed for these complexes. 1 is stabilized by an O⋯P pnicogen bond, 2 by a P⋯X halogen bond, 3a by a H⋯P hydrogen bond and a P⋯X pnicogen bond, and 3b by H⋯P and H⋯Br hydrogen bonds. Their relative stability is related to the halogen X of HOX and the substituent Y of PH2Y. These structures can compete with interaction energy of -10.22∼-29.40 kJ/mol. The HO stretch vibration shows a small red shift in 1, a small irregular shift in 2, but a prominent red shift in 3a and 3b. The XO stretch vibration exhibits a smaller red shift in 1, a larger red shift in 2, but an insignificant blue shift in 3a and 3b. The PY stretch vibration displays a red shift in 1 but a blue shift in 2, 3a, and 3b. The formation mechanism, stability, and properties of these structures have been analyzed with molecular electrostatic potentials, orbital interactions, and non-covalent interaction index.

Novel CX⋯π halogen bonds in complexes of acetylene and its derivatives of Na and MPH3 (M=Cu, Ag, Au) with XCCF (X=Cl, Br, I).

Ab initio calculations have been carried out for a variety of model systems with a T-shaped CX⋯π motif. The CX⋯π interaction of acetylene with the halogen donor molecule XCCF (X=Cl, Br, I) is invariably found to be weak with the interaction energy less than 11kJ/mol in magnitude. Substitution of the two protons in acetylene with more electron-donating sodium atoms increases the π electron density in the CC bond and leads to a substantial enhancement in its interaction with the halogen donor. The calculated interaction energies increase to as much as 73kJ/mol in the case of C2Na2-ICCF. The interaction of XCCF with a model coinage metal ethynide, H3PMCCMPH3 (M=Cu, Ag, Au), is intermediate between these two extremes, and the interaction energy is related to the nature of coinage metals. The CX⋯π halogen bonds have been analyzed with natural bond orbital, atoms in molecules, and energy decomposition.

Influence of the nature of hydrogen halides and metal cations on the interaction types between borazine and hydrogen halides.

The interactions between the H atom of borazine and hydrogen halide (HX, X = F, Cl, Br, and I) have been studied systematically. Four structures (a, b, c, and d) have been observed. The cyclic structure a is combined through a NH···X hydrogen bond and a BH···HX dihydrogen bond, a NH···X hydrogen bond and a BH···X halogen-hydride interaction are responsible for the cyclic structure b, structures c and d are maintained by a dihydrogen bond and a halogen-hydride interaction, respectively. Structures a and b are stable in energy, while structures c and d are unstable in energy. Structures a and b can transform each other through structure c or d. The interaction mode and strength are related to the nature of HX. The cation-π interaction of borazine with Li(+) and Mg(2+) causes a change in the interaction mode in structures a and b, and has an enhancing effect on the interaction strength in a and b.

Cooperative and diminutive effects of pnicogen bonds and cation-π interactions.

The interplay between pnicogen bonds and cation-π interactions has been investigated at the MP2/aug-cc-pVDZ level. Interesting cooperative and diminutive effects are observed when pnicogen bonds and cation-π interactions coexist in the same complex. These effects have been analyzed in terms of the structural, energetic, and charge-transfer properties of the complexes. The variations in electron density at critical points of the intermolecular bond have been used to analyze bond strengthening or weakening. The nature of the interactions and the mechanisms of cooperative and diminutive effects have been studied by means of symmetry-adapted perturbation theory and molecular electrostatic potentials.

Is π halogen bonding or lone pair···π interaction formed between borazine and some halogenated compounds?

Borazine, "inorganic benzene", exhibits some different properties from benzene although both of them are isostructural and isoelectronic. It was known that benzene is favorable to form halogen bonds with halogenated molecules. However, borazine more easily forms lone pair-π interactions with halogenated molecules, but for stronger halogen donors it can also form halogen bonds. The halogen bonds formed by borazine are stronger than the corresponding lone pair-π interactions. It was found that the pair-π interactions can be changed into halogen bonds with the increase of interaction strength. The dispersion energy plays a main role in stabilizing the weakly bonded complexes, while the electrostatic energy is dominant in the strongly bonded complexes. This is different from the nature of the respective benzene complexes.

Non-additivity between substitution and cooperative effects in enhancing hydrogen bonds.

Ternary systems XO2F:NCH:NCY and XO2F:CNH:CNY (X = P and As; Y = H and Li) as well as the corresponding binary ones were studied at the MP2/aug-cc-pVTZ level. Interestingly, the π-hole pnicogen bond in the PO2F complex is stronger than that in the AsO2F counterpart. The substituent Li in the Lewis base strengthens the hydrogen bond and pnicogen bond, but the more prominent enhancing effect is found for the pnicogen bond. The substitution effect is governed mainly through electrostatic interaction for the hydrogen bond but a combination of electrostatic and polarization interactions for the pnicogen bond. In the ternary systems, the π-hole pnicogen bond exhibits a positive cooperative effect with the hydrogen bond. Energy decomposition analysis indicates that the cooperativity is mainly attributed to the polarization energy. There is positive non-additivity between the substitution and cooperative effects, which is an effective measure for strengthening the hydrogen bond. The largest interaction energies occur in AsO2F:CNH:CNLi, amounting to -130.24 kJ/mol for the pnicogen bond and -119.90 kJ/mol for the hydrogen bond, and the former is a covalent interaction and the latter is an ion-pair hydrogen bond.

Competition between hydrogen bonds and halogen bonds in complexes of formamidine and hypohalous acids.

Quantum chemical calculations have been per-formed for the complexes of formamidine (FA) and hypohalous acid (HOX, X = F, Cl, Br, I) to study their structures, properties, and competition of hydrogen bonds with halogen bonds. Two types of complexes are formed mainly through a hydrogen bond and a halogen bond, respectively, and the cyclic structure is more stable. For the F, Cl, and Br complexes, the hydrogen-bonded one is more stable than the halogen-bonded one, while the halogen-bonded structure is favorable for the I complexes. The associated H-O and X-O bonds are elongated and exhibit a red shift, whereas the distant ones are contracted and display a blue shift. The strength of hydrogen and halogen bonds is affected by F and Li substitutents and it was found that the latter tends to smooth differences in the strength of both types of interactions. The structures, properties, and interaction nature in these complexes have been understood with natural bond orbital (NBO) and atoms in molecules (AIM) theories.