Steering CO2 electroreduction pathway toward ethanol via surface-bounded hydroxyl species-induced noncovalent interaction.

Selective electroreduction of carbon dioxide (CO2RR) into ethanol at an industrially relevant current density is highly desired. However, it is challenging because the competing ethylene production pathway is generally more thermodynamically favored. Herein, we achieve a selective and productive ethanol production over a porous CuO catalyst that presents a high ethanol Faradaic efficiency (FE) of 44.1 ± 1.0% and an ethanol-to-ethylene ratio of 1.2 at a large ethanol partial current density of 501.0 ± 15.0 mA cm-2, in addition to an extraordinary FE of 90.6 ± 3.4% for multicarbon products. Intriguingly, we found a volcano-shaped relationship between ethanol selectivity and nanocavity size of porous CuO catalyst in the range of 0 to 20 nm. Mechanistic studies indicate that the increased coverage of surface-bounded hydroxyl species (*OH) associated with the nanocavity size-dependent confinement effect contributes to the remarkable ethanol selectivity, which preferentially favors the *CHCOH hydrogenation to *CHCHOH (ethanol pathway) via yielding the noncovalent interaction. Our findings provide insights in favoring the ethanol formation pathway, which paves the path toward rational design of ethanol-oriented catalysts.

Intermetallic Compounds: Liquid-Phase Synthesis and Electrocatalytic Applications.

Characterized by long-range atomic ordering, well-defined stoichiometry, and controlled crystal structure, intermetallics have attracted increasing attention in the area of chemical synthesis and catalytic applications. Liquid-phase synthesis of intermetallics has arisen as the promising methodology due to its precise control over size, shape, and resistance toward sintering compared with the traditional metallurgy. This short review tends to provide perspectives on the liquid-phase synthesis of intermetallics in terms of both thermodynamics and methodology, as well as its applications in various catalytic reactions. Specifically, basic thermodynamics and kinetics in the synthesis of intermetallics will be first discussed, followed by discussing the main factors that will affect the formation of intermetallics during synthesis. The application of intermetallics in electrocatalysis will be demonstrated case by case at last. We conclude the review with perspectives on the future developments with respect to both synthesis and catalytic applications.

In Situ Radical Polymerization and Grafting Reaction Simultaneously Initiated by Fluorinated Graphene.

Fluorinated graphene (FG) showed interesting electrochemical, electronic, and mechanical properties, as well as chemical reactivity for multifarious functionalization of graphene material. This work reported a free radical polymerization and grafting from polymerization of a styrene monomer directly initiated by FG, which simultaneously provided free polymers and functionalized graphene with polymer chains grafted. The FG exhibited an almost comparative initiation efficiency to equivalent commercial initiator azodiisobutyronitrile under similar conditions, resulting in a high yield of free polystyrene (40.9%) with a high molecular weight ( Mn = 114.7 kg/mol). It was demonstrated that FG-triggered polymerization presented some special characteristics, such as a long lifetime of chain radical centers even when the reaction was stopped and insensitivity to oxygen molecules. The mechanistic study indicated that the polymerization was initiated by single-electron transfer reaction between FG and a monomer leading to formation of primary radicals; in addition, FG also played an important role in chain transfer and termination reactions during the polymerization process.

Low temperature preparation of highly fluorinated multiwalled carbon nanotubes activated by Fe3O4 to enhance microwave absorbing property.

Conventional approaches to preparing highly fluorinated multiwalled carbon nanotubes (MWCNTs) always require a high temperature. This paper presents a catalytic approach to realizing the effective fluorination of MWNCTs at room temperature (RT). Fe3O4/MWCNTs composites with Fe3O4 loaded on MWCNTs were first prepared using the solvothermal method, followed by fluorination treatment at RT. The attachment of Fe3O4 changes the charge distribution and dramatically improves the fluorination activity of MWCNTs. Consequently, the fluorine content of fluorinated Fe3O4/MWCNTs (F-Fe3O4/MWCNTs) can reach up to 17.13 at% (almost six times that of the unloaded sample) only after fluorination at room temperature, which leads to an obvious decrease in permittivity. Besides, the partial fluorination of Fe3O4 brings about abnormally enhanced permeability due to strengthened exchange resonance. Benefiting from the lower permittivity and higher permeability, F-Fe3O4/CNTs composite exhibits increased impedance matching and thus an enhanced microwave absorption property with a minimal reflection loss of -45 dB at 2.61 mm when the filler content is 13 wt%. The efficient absorption bandwidth (<-10 dB) reaches 4.1 GHz when the thickness is 2.5 mm. This work illustrates a novel catalytic approach to preparing highly fluorinated MWCNTs as promising microwave absorbers, and the design concept can also be extended to the fluorination of other carbon materials.

Crystallization of silica promoted by residual hydrogen bonding interactions at high temperature.

A novel approach to prepare crystalline silica through calcination of the composite of silica and highly fluorinated graphene at a relatively low temperature is demonstrated. Silica and its composites with graphene and its derivatives (graphene, graphene oxide and graphene with various degrees of fluorination) were synthesized and then calcined at 900 °C in an air atmosphere. The results of X-ray-diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy reveal that cristobalite was produced through calcining composites of silica and highly fluorinated graphene under ambient air at a relatively low temperature (900 °C), while for the composites of silica and graphene and its derivatives, the calcined products are all amorphous. Thermal gravimetric analysis results indicate that the maximum decomposition temperature of functional groups in highly fluorinated graphene at air temperature is 457 °C, which is higher than that in medium fluorinated graphene, lower fluorinated graphene and graphene oxide (411.3 °C, 313.4 °C and 238.9 °C). A high degradation temperature of highly fluorinated graphene contributes to strong residual hydrogen bonding interactions at high temperature. FTIR results further illustrate that many residual hydrogen bonding interactions in composites of silica and highly fluorinated graphene at higher temperature result in enough linear structures. As a consequence, stronger residual hydrogen bonding interactions at high temperature in composites of silica and highly fluorinated graphene restrain the self-condensation of Si-OH groups and promote the formation of crystalline structures.

Characterization of the thermal/thermal oxidative stability of fluorinated graphene with various structures.

Considering practical applications, the thermal/thermal oxidative stability of fluorinated graphene should be given sufficient attention. Herein, X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FT-IR) were used to investigate in detail the differences in the thermal stabilities of two types of fluorinated samples, fluorinated graphene (FG) and fluorinated porous graphene (FPG) with various fluorine contents, respectively, as well as the reasons for these differences. It was demonstrated that the thermal stability of FG and FPG was improved upon increasing the fluorine content, which was mainly caused by the enhancement of bond energy of the covalent C-F bonds. Moreover, compared to that of the raw graphene samples, the thermal oxidative stability of FG was reduced due to the defects brought by fluorination, while the thermal oxidative stability of FPG was improved, originating from the inflaming retarding effect of the fluorine element. Interestingly, the thermal oxidative stability of the fluorinated samples was even better than their thermal stability. Using a comparison of the two types of fluorinated samples and support from the computational simulations of the model molecules, it was suggested that a greater amount of CFn (n = 2, 3) groups or defects in the FG samples resulted in its relatively worse thermal stabilities. Furthermore, electron paramagnetic resonance (EPR) spectroscopy was introduced to analyze the thermal stabilities of the fluorinated graphene samples as a novel method. The changes in the spin centers in samples after thermal treatment were studied, which indicated that the lower amount of the more stable spin centers of FPG was another reason leading to its more outstanding thermal stabilities in comparison to FG samples.

Radical mechanism of a nucleophilic reaction depending on a two-dimensional structure.

The mechanism of nucleophilic substitution deserves more investigation to include more reaction systems such as two-dimensional (2D) materials. In this study, we used fluorinated graphene (FG) as a representative 2D material to reveal the in-depth mechanism of its defluorination and nucleophilic substitution reaction under attack of common nucleophiles to explore the chemistry of 2D materials and enrich the research on the nucleophilic substitution reaction. DFT calculations and electron paramagnetic resonance spectroscopy (EPR) demonstrated that defluorination of FG occurred via a radical mechanism after a single electron transfer (SET) reaction between the nucleophile and C-F bond, and a spin center was generated on the nanosheet and fluorine anion. Moreover, neither the SN1 nor SN2 mechanism was suggested to be appropriate for the substitution reaction of FG with a 2D structure due to the corresponding kinetics or thermodynamics disadvantage; hence, its nucleophilic substitution was proved to occur via a radical mechanism initiated by the defluorination step. The proposed substitution mechanism of FG demonstrates that nucleophilic substitution via a radical mechanism can also be applied to the attacking process of common nucleophiles without any particular conditions. Furthermore, it has been discovered that triethylamine without active hydrogen can be covalently attached to graphene nanosheets via a nucleophilic substitution reaction with FG; this further indicates a radical process for the nucleophilic substitution of FG rather than an SN1 or SN2 mechanism. The detailed process of the nucleophilic substitution reaction of FG was revealed to occur via a radical mechanism depending on the 2D structure of FG, which could also represent the typical characteristic of 2D chemistry.

Investigation of the dispersion behavior of fluorinated MWCNTs in various solvents.

The investigation of the dispersion behavior of fluorinated MWCNTs (F-MWCNTs) is very important to understand their structure and take full advantage of their good properties. In this present paper, the dispersion behavior of F-MWCNTs with a low content and a high content of fluorine (denoted as lF-MWCNTs and hF-MWCNTs) was explored in 18 kinds of common solvents. The surface of hF-MWCNTs is considered to be a heterostructure consisting of fluorinated regions and aromatic regions, while lF-MWCNTs are inclined to be a homogeneous structure on the basis of their dispersion behavior. According to dispersion theory based on surface energy and Hansen solubility parameters (HSPs), it was indicated that the corresponding preferable solvents are different for different regions. As a result, good solvents of hF-MWCNTs are distributed in a quite wide scope while lF-MWCNTs can be dispersed only in a significantly narrow range of solvents. The HSPs of lF-MWCNTs and hF-MWCNTs are determined to be δD = 17.6 MPa1/2, δP = 11.8 MPa1/2, δH = 8.8 MPa1/2 and δD = 16.9 MPa1/2, δP = 9.3 MPa1/2, δH = 13.5 MPa1/2, respectively. As a result, mixed solvents of acetone and water were carefully tuned to be compatible with hF-MWCNTs. The dispersion behaviors of lF-MWCNTs and hF-MWCNTs in epoxy were also predicted according to HSPs. It was found that hF-MWCNTs maintain a stable dispersion in epoxy due to their heterogeneous structure at elevated temperatures.

Effects of the oxygenic groups on the mechanism of fluorination of graphene oxide and its structure.

A facile way to prepare fluorinated graphene (FG) with a high fluorine content and controllable structure is important to achieve its full potential application. In this work, it was found that the fluorine to carbon (F/C) ratio of fluorinated graphene oxide (FGO) was nearly twice as much as that of fluorinated chemically reduced graphene oxide (FCrGO) after fluorination at the same temperature. Concerning the detailed effects of oxygenic groups on the fluorination and structure of fluorinated graphene (FG), graphene oxides with different oxygen contents were fluorinated under the same conditions. It was shown that oxygenic groups promote the fluorination reaction by activating the surrounding aromatic regions and taking part in the substitution reaction with fluorine radicals, among which, hydroxyls and carbonyls tend to be replaced by fluorine atoms. Moreover, the fluorination mainly occurs at the edges and defects of graphene sheets with a low oxygen content, while the highly oxidized graphene sheets are fluorinated both at the edges and basal planes simultaneously. This indicates that the quantity and location of the C-F bonds in FGO can be controlled by adjusting the species and content of oxygenic groups in the precursor graphene oxide.

Defluorination and covalent grafting of fluorinated graphene with TEMPO in a radical mechanism.

Fluorinated graphene (FG) can be regarded as the representative two-dimensional (2D) material to study the characteristics of "2D chemistry", whereas its derivative reaction mechanism is still required to be revealed for the destination of deciduous fluorine atoms after defluorination of FG. Herein, we proposed a particular derivative reaction of FG by employing 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) as the attacking reagent, and the products were characterized via Electron Paramagnetic Resonance Spectroscopy (EPR), Mass Spectrometry (MS) and X-ray photoelectron spectroscopy (XPS). It was demonstrated that the defluorination caused by TEMPO occurred in a radical mechanism, thus leading to formations of new spin centers on graphene nanosheets as well as C[double bond, length as m-dash]C bonds. More importantly, the deciduous fluorine atoms after defluorination, which existed in TEMPO fluoride molecules, have been detected for the first time. Meanwhile, some TEMPO molecules were covalently grafted on the nanosheet, which resulted from the coupled reaction between TEMPO radical and the spin center on the FG nanosheet. These findings deepen the research of derivative reactions of FG, meanwhile providing a particular view to investigate the chemistry characteristics of 2D materials from a radical mechanism.