Electrochemical activation of C–H by electron-deficient W2C nanocrystals for simultaneous alkoxylation and hydrogen evolution

The activation of C–H bonds is a central challenge in organic chemistry and usually a key step for the retro-synthesis of functional natural products due to the high chemical stability of C–H bonds. Electrochemical methods are a powerful alternative for C–H activation, but this approach usually requires high overpotential and homogeneous mediators. Here, we design electron-deficient W2C nanocrystal-based electrodes to boost the heterogeneous activation of C–H bonds under mild conditions via an additive-free, purely heterogeneous electrocatalytic strategy. The electron density of W2C nanocrystals is tuned by constructing Schottky heterojunctions with nitrogen-doped carbon support to facilitate the preadsorption and activation of benzylic C–H bonds of ethylbenzene on the W2C surface, enabling a high turnover frequency (18.8 h−1) at a comparably low work potential (2 V versus SCE). The pronounced electron deficiency of the W2C nanocatalysts substantially facilitates the direct deprotonation process to ensure electrode durability without self-oxidation. The efficient oxidation process also boosts the balancing hydrogen production from as-formed protons on the cathode by a factor of 10 compared to an inert reference electrode. The whole process meets the requirements of atomic economy and electric energy utilization in terms of sustainable chemical synthesis.


Introduction
Direct activation of C−H bonds via selective oxidation of hydrocarbons is of great interest for organic hydrocarbons 1,2 . As a typical and important transformation path of C−H bonds, selective dehydrogenation of C−H bonds has been widely used for the production of high value-added compounds such as alcohols, and ketones and ethers [3][4][5] . However, the chemical stability of C−H bonds without activating neighborhood effects makes C−H activation quite challenging, and either extreme and rather toxic oxidants as chromium or selenium compounds or noble-metal-catalysts (based on rhodium or palladium) at high temperatures have to be applied to obtain acceptable conversions [6][7][8] . Moreover, the asformed side product water from the cleavage of C−H bonds via oxydehydrogenation is free of value. As a result, novel and sustainable strategies are highly desirable to further decrease the economic and environmental footprints of C−H activation processes.
Electrochemical transformation is recognized as an environmentally friendly method for the production of various functional molecules driven by electricity under mild conditions [9][10][11] . Pioneering works of electrochemical synthesis using homogeneous catalysts have demonstrated the advantages of this technique for C−H activation 11 , which includes selective oxidation 12 , amination 13 , epoxidation 14 and dehydrogenative coupling reactions 15 . Most of these reactions have high atom economy and excellent compatibility with ow reactors for continuous synthesis 16,17 . The current strategies to boost the transformation of speci c substrates mainly rely on the involvement of functional additives 11 (e.g., organic ligands, bases and mediators), a high work potential and/or sacri cial transition metal electrodes 18 , which all will severely limit real-industry applications. Principally, the preparation of cost-effective and active electrode materials is at least as important as the development of a new methodology for selective C−H bonds activation [19][20][21][22][23][24][25][26] , and only non-targeted, commercial rst generation electrodes (such as carbon rod, platinum and reticulated vitreous carbon) are applied as the current collectors in electrochemical organic synthesis at the moment. The signi cant progress reported using well-designed reaction-speci ed electrodes in improving the catalytic activity for water splitting, nitrogen reduction reactions and even carbon dioxide reduction reactions [27][28][29][30] further manifests the huge gap between the design of novel electrode materials and the requirements of sustainable electrochemical organic synthesis.
Herein, we present the proof-of-concept application of electron-de cient W 2 C nanocrystal-based electrodes for the highly e cient electrochemical activation of C-H bonds, highlighting the key importance of the modi ed physicochemical properties of electrode materials in boosting additive-free C-H activation reactions. A nanoheterojunction composed of W 2 C nanocrystals and nitrogen-doped carbons has been rationally designed to control the number of electrons owing from W 2 C nanocrystals to nitrogen-doped carbons by increasing the doping concentration in the carbon supports to enhance the interfacial Schottky effect. The as-formed electron-de cient W 2 C nanocrystal-based electrode acts as a functional anode to simultaneously facilitate the alkoxylation of ethylbenzene with methanol on the anode and the balancing hydrogen evolution reaction on the cathode. Both the experimental and theoretical results indicate the key role of the electron de ciency of the W 2 C nanocrystals in capturing ethylbenzene on the anode to substantially increase the reaction rates of alkoxylation and hydrogen evolution reaction processes simultaneously and ensure the long-term stability of the anode without scarifying the current collector.
The W 2 C/NC catalysts were prepared via a modi ed nanocon nement method ( Supplementary Fig. 1) from a mixture of dicyandiamide and ammonium tungstate, followed by N 2 -protected thermal pyrolysis at high temperatures. The nitrogen contents (x at.%) of the W 2 C/N x C samples could be tuned from 3.0 via 2.3 to 1.4 at.% (Supplementary Fig. 2  Transmission electron microscopy (TEM) observations (Fig. 1a-c and Supplementary Fig. 5-7) further reveal the presence of few-layer-graphene-supported W 2 C nanocrystals with a mean size of 2.5 nm ( Fig.   1a and Supplementary Fig. 8) and a typical lattice fringe of 0.24 nm (Fig. 1b), which corresponds to the (002) plane of α-W 2 C 31,32 . The formation of W 2 C is doubly con rmed by its X-ray diffraction (XRD) pattern ( Supplementary Fig. 9), matching well with that of typical α-W 2 C (JCPDS# 35-776) 31 . Detailed elemental mapping images (Fig. 1c) exhibit nanometer-sized W-rich areas with a homogeneous distribution of N atoms along with the whole carbon support, indicating an integrated structure of W 2 C nanocrystals on the nitrogen-doped carbons.
The highly coupled structure of W 2 C/NC dyads makes it possible to form a rectifying interface for modulation of the electron density of W 2 C nanocrystals. The density functional theory (DFT) calculation results ( Supplementary Fig. 10,11) predict electron transfer from W 2 C to nitrogen-doped carbons, resulting in more pronounced electron-de cient regions in W 2 C nanocrystals suggested by the charge density difference (CDD) stereograms (Fig. 1d) of the same W 2 C model supported on pristine carbons (W 2 C/C). The mean number of electrons transferred from the W 2 C nanocrystal to the nitrogen-doped carbon support (Fig. 1e) increases from 0.338 to 0.397 as more nitrogen atoms (from 1.4 to 3.0 at.%) are doped into the carbon support models ( Supplementary Fig. 12), which were constructed based on the Xray photoelectron spectroscopy (XPS) analysis results 33 . As depicted in Fig. 1f, the nanoheterojunction of W 2 C and NC has a rectifying contact, with electrons owing from the W 2 C side with a lower Fermi level (E F ) to the NC side, generating electron-de cient W 2 C due to the interfacial Schottky barrier 34,35 . Indeed, the electron donation from the W 2 C nanocrystals to the nitrogen-rich carbon supports is experimentally con rmed by the gradual shift in W 4f XPS peaks to higher energy ( Inspired by the success in modifying the electron density of W 2 C nanocrystals, we further evaluated the possible catalytic activity of W 2 C/N x C catalysts for electrochemical alkoxylation of ethylbenzene with methanol under mild conditions as a model reaction. Considering that the reported methods for alkoxylation of C-H bonds usually require highly active additives/oxidants and/or a high reaction temperature, we initially tested the possibility of additive-free alkoxylation of ethylbenzene with methanol using only a simple electrolyte containing lithium perchlorate and W 2 C/N x C-based electrodes under ambient conditions ( Fig. 2 and Supplementary Fig. 15). No product was detected without applying a working potential for various electrodes in our electrochemical system (Supplementary Fig. 16), illustrating that the methoxylation reaction cannot proceed spontaneously. Surprisingly, a complete conversion of ethylbenzene can be achieved on the W 2 C/N 3.0 C electrode with high selectivity to the target product (1-methoxyethyl)benzene (Fig. 2a,b and Supplementary Fig. 17) and a total carbon balance of approximately 95%, con rming the possibility of highly e cient alkoxylation of C-H bonds on a welldesigned heterogeneous electrode without scarifying additives. The fact that control electrodes with the same amount of bare NC sample, W 2 C catalyst or a mechanical mixture of the two components ( Fig. 2e) give much lower conversions of ethylbenzene than the W 2 C/N 3.0 C electrode under xed conditions further indicates a synergistic effect between W 2 C and N 3.0 C components in facilitating the transformation of ethylbenzene.
Unlike the oxidative alkoxylation reaction of C-H bonds by using various oxidants for dehydrogenation to generate water 37 , our heterogeneous electrochemical system could achieve the full use of as-formed protons from the activation of C-H bonds and methanol for subsequent hydrogen evolution reactions, generating hydrogen gas bubbles on the cathode ( Supplementary Fig. 18). Moreover, the calculated Faradaic e ciencies ( Fig. 2d and Supplementary Fig. 19) are similar for the conversion of ethylbenzene to (1-methoxyethyl)benzene on the W 2 C/N 3.0 C anode (F E : 42-46%) and hydrogen production on the Ti cathode (F E : 42-55%), implying a cascade transformation of protons generated from the anode into hydrogen gas on the cathode. Even with an excess amount of methanol in the reactor, only a trace amount of formaldehyde (0.006 mmol) formed during the conversion of 0.5 mmol of ethylbenzene ( Supplementary Fig. 20), well explaining the comparable Faradaic e ciencies for the reactions on anode and cathode without the obvious contribution of methanol dehydrogenation to the total F E for hydrogen evolution reactions. Remarkably, the electron-de cient W 2 C in the W 2 C/N 3.0 C-based electrode substantially promotes the hydrogen evolution rate on the Ti cathode to 880 μmol (Fig. 2c), which is above 10 times that on the same Ti cathode (85 μmol) when using bare carbon cloth as the anode. The constant current density of the W 2 C/N 3.0 C anode under xed conditions with different cathodes (Fig. 2f), including Pt mesh, Ti mesh and carbon rod, further demonstrates that the activation and deprotonation of ethylbenzene on the W 2 C/N 3.0 C electrode is the rate dominating step for the whole reaction. Indeed, the alkoxylation reaction could be selectively quenched by butylated hydroxytoluene (BHT) ( Supplementary   Fig. 21), indicating a radical-based pathway on the W 2 C/N 3.0 C anode, as indicated in Fig. 2a 18 .
The role of the electron-de cient W 2 C nanocrystals and the interfacial effect of the heterojunction catalysts on the electrochemical alkoxylation of C-H bonds were simulated via theoretical calculations and then validated by experimental evidence (Fig. 3). The optimized geometry (Fig. 3a,c) of ethylbenzene presents preferred adsorption of benzylic C-H bonds on the W 2 C surface dependent of the electronde ciency of W 2 C, indicating the feature role of W 2 C as an active component. This role was further validated by more negative onset potentials (<1.4 V versus SCE) for the electrochemical alkoxylation reaction on W 2 C/N x C anodes than that (>1.6 V versus SCE) of the bare carbon cloth electrode ( Supplementary Fig. 22). However, the polarization of adsorbed C-H bonds is enhanced by the electronde cient surface of the W 2 C-0.08emodel, as re ected by the more pronounced electron density difference (Hirshfeld charge) of the preadsorbed C-H bonds ( Fig. 3c and Supplementary Fig. 23) and a much lower calculated adsorption energy for ethylbenzene (Fig. 3e). Such strong adsorption of ethylbenzene molecules over the electron-de cient W 2 C surface was then experimentally validated by the temperature-programmed desorption (TPD) analysis results (Fig. 3f), exhibiting gradually elevated adsorption capacities over those of more electron-de cient W 2 C/N x C samples with similar surface areas.
It should be noted that the bare carbon support (NC sample in Fig. 3f) provides a low adsorption capacity, only 21% of the best-in-class W 2 C/N 3.0 C sample ( Supplementary Fig. 24). More importantly, the electron de ciency-induced adsorption behavior of ethylbenzene on the nal W 2 C/N x C-based anodes under a xed bias in the electrochemical reactor was well expressed with the same trend in adsorption capacities (Fig.  3g) as that revealed by TPD results, making successive C-H dissociation process more favorable.
Indeed, the stronger interaction between preadsorbed ethylbenzene molecules and electron-de cient W 2 C signi cantly reduces the Gibbs free energy of each step of the whole alkoxylation reaction pathway (Fig.   3e). The dissociation of C-H bonds of ethylbenzene on the electron-de cient W 2 C catalyst (W 2 C-0.08emodel) is the rate-limiting step with a free energy change of only 0.34 eV, and the subsequent coupling of *C 8 H 9 and *CH 3  This electron-de ciency-dependent promotion effect on the activity of W 2 C was then unambiguously con rmed by the gradually increased catalytic activities (Fig. 3h) and F E values (Fig. 3i) for producing (1methoxyethyl)benzene on more electron-de cient W 2 C/N x C-based anodes under xed work potential.
The W 2 C/N 3.0 C anode also shows excellent electrochemical stability for long-term use. The composition ( Supplementary Fig. 25) and morphology ( Supplementary Fig. 26) of the used W 2 C/N 3.0 C materials were maintained well. Most importantly, the W 2 C/N 3.0 C anode can be recycled at least four times without an obvious decrease in F E (41-46%) ( Fig. 4b and Supplementary Fig. 27). It should be noted that inert metal anodes for alkoxylation of ethylbenzene, including stable metals (exempli ed by Ti mesh) and active metals (exempli ed by Ni plate), decompose rapidly within 5 h ( Fig. 4a and Supplementary Fig. 28), illustrating the key importance of the high activity of the W 2 C/N 3.0 C anode to keep itself from corroding.
As a durable anode, the electron-de cient W 2 C electrode exhibits satisfying activity for electrochemical alkoxylation of various aromatic C-H bonds using a series of aliphatic alcohols (Supplementary Table 2) with good to high conversions and high selectivity in 18 h, suggesting an excellent tolerance of our electrode material to various functional groups. As the best-in-class anode in this work, the W 2 C/N 3.0 C electrode provides a high turnover frequency (TOF) value of 18.8 h -1 , which is comparable to or even higher than the reported values, mostly of homogeneous catalysts, for similar alkoxylation reactions ( Fig.   4c and Supplementary In summary, we have demonstrated the key role of electron-de cient W 2 C nanocrystals as electrode materials in boosting the activity and durability for electrochemical activation of C-H bonds via a heterogeneous pathway. We successfully tuned the electron density of W 2 C nanocrystals by constructing Schottky heterojunctions with nitrogen-doped carbons to achieve preferred adsorption of benzylic C-H bonds of ethylbenzene on the W 2 C surface and facilitate subsequent C-H activation, which is the ratelimiting step. Unlike conventional oxidative alkoxylation to generate water, the as-formed protons on the W 2 C anode could be simultaneously converted to hydrogen gas in our additive-free electrochemical reactor under mild conditions. This two-birds-with-one-stone strategy illustrates the signi cant potential of powerful designer electrode materials to substantially increase catalytic e ciency, atomic economy and electricity utilization for organic electrosynthesis and hydrogen energy production in one electrocatalytic system. In addition to the hydrogen evolution reaction, the reduction process might be compatible with other important reactions (e.g. carbon dioxide reduction reaction or N 2 /NO x reduction reactions) to create novel or more complex cascade reaction pathways for the production of high valueadded compounds from abundant hydrocarbons and even waste gases. This work may also boost the development of zero-additive and zero-emission electrosynthesis systems through the design of novel electrode materials.

Methods
Synthesis of W 2 C/N x C: All chemicals were analytical grade and used as received without further puri cation. A homogenous mixed solution including (NH 4 ) 6 H 2 W 12 O 40 •nH 2 O (0.75 g), DCDA (15 g) and deionized water (150 mL) was heating at 80 °C, and the mixed solution was evaporated at a constant temperature with stirring. The obtained mixture after evaporating water was transferred into a cylindrical crucible with lid, and heated at 1000, 1100 and 1200 °C for 2 h with a heat rate of 2.5 °C min -1 under high purity nitrogen atmosphere. The nal products after cooling down to room temperature were named W 2 C/N x C (x represents the nitrogen contents) and used for further experiments and characterizations.
The control group of nitrogen-doped carbon (NC) was prepared with the same procedure as W 2 C/N x C without metal precursor. And W 2 C and NC mixture was obtained by mixing W 2 C powder and NC mechanically.
Materials Characterization: Powder X-ray diffraction patterns (XRD) was performed using a Bruker D8 Advance X-ray diffractometer equipped with a Cu Kα radiation source (λ = 1.5418 Å) and operated at a scan rate of 6° min −1 . Scanning electron microscopy (SEM) were operated on FEI Nova NanoSEM 450 eld emission scanning electron microscope. Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray (EDX) analysis were measured on a JEM-2100F microscope with an acceleration voltage of 200 kV. Temperature programmed desorption (TPD) was carried out on a Micromeritics Autochem II chemisorption analyzer with ethylbenzene probe molecules at 110 °C. Nitrogen adsorption-desorption isotherms were acquired on Quantachrome NOVA-2200e at 77 K. Prior to the measurement, the samples were degassed at 200 °C for 12 h with a gas ow of nitrogen. X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) experiments were recorded at a Kratos Axis Ultra DLD spectrometer and ESCALAB 250 photoelectron spectrometer (Thermo Fisher Scienti c), respectively. The inductively coupled plasma atomic emission spectroscopy (ICP-AES) measurement was conducted on an iCAP6300 spectrometer for tungsten element analysis.
Electrochemical measurements: All of the electrochemical experiments were performed in a standard three-electrode system on an electrochemical station (CHI 660E, Shanghai CH Instruments Company).
Working electrodes were composed of catalysts supported by carbon cloth. The catalyst ink was prepared by sonicating and dispersing 5 mg of catalyst into a solution containing 700 μL of ethanol, 350 μL of deionized water and 160 μL of 5% Na on solution. The working electrodes were prepared by evenly dipping 150 μL of ink onto carbon cloth (1 × 1 cm) and dried at 120 °C for 1 h in the oven. Titanium (Ti) mesh with a size of 1 × 1 cm and saturated calomel electrode (SCE) were employed as counter and reference electrode, respectively. The control electrodes with different sample loading were prepared by dipping 50, 100 and 200 μL of ink on carbon cloth, respectively. The electrocatalytic reactions were conducted in 15 mL of methanol with 61 μL of ethylbenzene (0.5 mmol) and 0.106 g of lithium perchlorate (1 mmol) at room temperature in a home-made electrolyzer, in which methanol was not only used as solvent, but also simultaneously used as reactant. The electrocatalytic stability tests of catalyst for the reaction between ethylbenzene and methanol were evaluated using the same reaction potential for four consecutive cycles, and electrodes would have to dry at 120 °C before the next reaction.
Electrode adsorption experiments were conducted in electrolyzer containing 1.5 mmol of ethylbenzene, 15 mL of methanol and 1 mmol of lithium perchlorate with W 2 C/N x C anodes, Ti mesh cathode and saturated calomel electrode. To accurately measure the adsorption volume of ethylbenzene before transforming (1-methoxyethyl)benzene, we performed at 2 V versus SCE for 10 min. After the completion of adsorption, the residual volume of ethylbenzene in solution was obtained by extraction and analyzed in gas chromatography-mass spectrometry (GC-MS, Shimadzu QP2010SE) with dodecane as internal standard. And the adsorption volume for ethylbenzene on W 2 C/N x C (C ads ) was calculated using the equation of n ads = n ini -n res (n ini and n res represent the initial and residual contents in solution, respectively) 40 .
Products analysis: After the reaction, 500 μL of solution was rstly taken from electrolyzer and transferred into an extracted solution containing 500 μL of dichloromethane, 500 μL of deionized water and 0.2 μL of dodecane, and then dried by magnesium sulfate anhydrous. Finally, the extracted solution was analyzed in GCMS to determine the components of products and calculate the conversion and selectivity.
The Faradaic e ciency (F E ) was calculated as follow: where N i is the number of moles for the speci c product (mole); n is the number of electrons exchanged for product formation, which is 2 e in this reaction; F is the Faradaic constant of 96487 C mol -1 ; Q is the passed charge.
Turnover frequency (TOF) was de ned by the following equation: where n is the number of moles for product; N is the number of moles of active metal sites determined from ICP-AES.
Theoretical calculation: The Spin polarization density functional theory (DFT) calculations were performed by the DMol3 program on Materials Studio. The generalized gradient approximation method with Perdew-Burke-Ernzerhof functional (GGA-PBE) was used for describing the exchange-correlation interaction among electrons 41 . The double numerical plus polarization (DNP) basis set was employed, while an accurate DFT semi-core pseudopots (DSPP) was adapted to describe the metal atoms 42,43 . The 6 × 6 × 1 k-points was used for sampling the Brillouin zone. Hexagonal W 2 C (0001) facets were modeled in terms of the slabs of 5 × 5 supercells with 14.76 × 14.76 Å and 120 o and W 2 C cluster model was placed above a 6 × 6 supercell of graphene lattice 44 . The vacuum slab was set as 20 Å to calculated all periodical models. The contents of graphitic N and pyridinic N in the graphene lattice were estimated according to the XPS elements analysis results.
The Gibbs free energy change (ΔG) for each step of ethylbenzene activation was calculated as follows: where ΔE, ΔZPE and ΔS are the changes in the reaction energy, zero-point energy and entropy, respectively.

Data availability
The data that support the findings of this study are available from the corresponding authors upon request.     Fig. 15).

Figure 3
Effect of the electron de ciency of W2C nanocrystals on the conversion of ethylbenzene. a and c, Charge density difference stereograms of benzylic C-H bonds of ethylbenzene on the surface of W2C model (a) and electron-de cient W2C model (W2C-0.08e-) (b). The slice is perpendicular to the plane of benzylic C-H bonds, and electron-rich (red) and electron-de cient (blue) areas are presented. Colour code: C, grey; N, blue; H, white; W, purple. c and d, Calculated absorption con gurations of each step of the ethylbenzene activation process on W2C (c) and W2C-0.08e-(d). e, Gibbs free energy diagrams of each step of the ethylbenzene activation process on W2C (black and top) and W2C-0.08e-(red and bottom).   Stability and e ciency of C-H alkoxylation. a, Current density change of W2C/N3.0C, Ti mesh and Ni plate electrodes during the reaction. b, Reusability of W2C/N3.0C under standard conditions within 3 h. c, TOF values (for details, please see Supplementary Table 3) for methoxylation of benzylic C-H bonds on W2C/N3.0C-based heterogeneous system (solid sphere) and by state-of-the-art homogeneous catalyst (Ir* (III) complex, black circle) and photocatalyst (CuCl, blue circle).

Figure 4
Stability and e ciency of C-H alkoxylation. a, Current density change of W2C/N3.0C, Ti mesh and Ni plate electrodes during the reaction. b, Reusability of W2C/N3.0C under standard conditions within 3 h. c, TOF values (for details, please see Supplementary Table 3) for methoxylation of benzylic C-H bonds on W2C/N3.0C-based heterogeneous system (solid sphere) and by state-of-the-art homogeneous catalyst (Ir* (III) complex, black circle) and photocatalyst (CuCl, blue circle).

Supplementary Files
This is a list of supplementary les associated with this preprint. Click to download.