Anchoring Copper Single Atoms on Porous Boron Nitride Nano ber to Boost Selective Reduction of Nitroaromatics

Single-atom catalysts have received widespread attention for their fascinating performance in terms of metal atom efficiency as well as their special catalysis mechanisms compared to conventional catalysts. Here, we prepared a high-performance catalyst of single-Cu-atom-decorated boron nitride nanofibers (BNNF-Cu) via a facile calcination method. The as-prepared catalyst shows high catalytic activity and good stability for converting different nitro compounds into their corresponding amines both with and without photoexcitation. By combined studies of synchrotron radiation analysis, high-resolution high-angle annular dark-field transmission electron microscopy studies, and DFT calculations, dispersion and coordination of Cu atoms as well as their catalytic mechanisms are explored. The BNNF-Cu catalyst is found to have a record high turnover frequency compared to previously reported non-precious-metal-based catalysts. While the performance of the BNNF-Cu catalyst is only of the middle range level among the state-of-the-art precious-metal-based catalysts, due to the much lower cost of the BNNF-Cu catalyst, its cost efficiency is the highest among these catalysts. This work provides a choice of support material that can promote the development of single-atom catalysts.


Introduction
Single atoms catalysts (SACs), as one of the most innovative and active research frontiers in the eld of catalysis, have recently attracted a lot of attention due to their compelling merits such as ultra-high activity and selectivity, nearly 100% atom-utilization e ciency, easy identi cation of the reaction mechanisms and so on [1][2][3] . The SACs have good potential for environmental, energy-conversion and industrial utilizations that are traditional dominated by conventional metal catalysts. Different from conventional catalysts, in SACs, the interaction between the metal and the reactive species changes due to the decreased electron density caused by positively charged metal atoms. Moreover, the uniform dispersion of single metal atoms in SACs can also lead to the change of reactive intermediates adsorption mode and prevent side reactions [4][5][6] .
Porous materials such as zeolites, MOFs, and COFs are considered as perfect SACs supports due to their unique pore structures, large surface areas, high defect densities which are bene cial to uniform dispersion and stabilization of isolated metal atoms. Porous boron nitride (p-BN) which possesses the advantages of porous materials and good oxidation resistance as well as excellent physical and chemical inertness 27,28 has attracted wide attention and have been utilized in various elds, such as pollution treatment 27,29 , gases capturing and storage 30,31 , catalysis etc 32,33 . Although some theoretical works have predicted that the BN can stabilize isolated metal atoms and exhibit certain catalytic activity [34][35][36][37] , there is, so far, no report on BN-based SACs.
In this work, for the rst time we successfully prepared a porous BN nano bers/single Cu atom (BNNF-Cu) catalyst via a facile in-situ pyrolysis strategy. Dispersion of Cu single atoms and coordination between BN and Cu atoms were systematically analyzed and con rmed with synchrotron radiation analysis and high-resolution high-angle annular dark-eld transmission electron microscopy. The existence of isolated Cu atoms on BNNF leads to signi cantly enhanced catalytic performance for selectively converting nitro compounds into amines. Notably, the turnover e ciency of BNNF-Cu is considerably higher than and comparable to those of previously reported nonprecious-and preciousmetal-based catalysts respectively. Furthermore, density functional theory (DFT) calculation was also carried out to explore the reaction pathway and explain the mechanism of the catalytic processes. It is believed that the successful preparation of BNNF-Cu provides a facile strategy to simultaneously achieve good dispersion of single metal atoms and high catalytic activity as well as good stability, promoting the development of SACs.

Results
Synthesis and characterization of materials. BNNF-Cu with different copper contents were prepared via a facile method using low-cost raw materials. CuCl 2 , melamine and boric acid were rst reacted to form a gel-like BNNF-Cu precursor. BNNF-Cu was obtained by annealing the precursor at 900 o C as stated in detail in the Methods section. Catalytic activity of the BNNF-Cu was optimized by regulating the amount of copper salt added. As shown in Supplementary Fig. 1-2, the BNNF-Cu samples prepared with different copper concentrations have similar crystal structures and functional groups. Supplementary Fig. 3 shows that BNNF-Cu-2 has the highest catalytic activity. Thus, subsequent studies would be concentrated on BNNF-Cu-2 and it will be simply referred as BNNF-Cu hereafter. Figure 1a shows a schematic diagram of BNNF-Cu's preparation process.
Microstructure and morphology of BNNF-Cu were characterized with eld-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). As shown in Fig. 1b-d, the asprepared sample showed a ber-like structure with a rectangular cross-section (see Supplementary Fig. 4) of about 200 nm width and relatively low crystallinity (Fig. 1c, inset). There is no obvious difference between BNNF-Cu and BNNF synthesized without Cu ( Supplementary Fig. 5), indicating that the addition of Cu 2+ ions does not affect the samples' morphology. Fourier transform infra-red (FT-IR) spectra of BNNF and BNNF-Cu were shown in Supplementary Fig. 6. A spectrum from commercially purchased boron nitride nanosheets (BNNS) was also shown for comparison. All the three samples clearly show two characteristic B-N vibrations at ∼1380 cm − 1 (B − N transverse stretching in plane) and ∼800 cm − 1 (B − N−B bending out of plane). There are also additional peaks in BNNF and BNNF-Cu located between 3200 ∼ 3600 cm − 1 relating to functional groups -OH and ν (N − H) and a weak vibration at 1637 cm − 1 probably due to δ (N − H) 28 . The introduction of these functional groups probably leads to enhancement of adsorption capacity. The counter-phase B − N E 2g Raman vibration mode of boron nitride at ∼1370 cm − 1 can be observed in Raman spectra of all three samples ( Supplementary Fig. 7) 38 . The main difference is that the Raman peak is much shaper in BNNS which is highly crystalline hexagonal boron nitride ( Fig. 1g and Supplementary Fig. 8). The noisy spectra, board and shifted Raman peaks of BNNF and BNNF-Cu are due to their low crystallinity. Supplementary Fig. 9 shows X-ray photoelectron spectroscopy (XPS) survey spectra from BNNF and BNNF-Cu. Other than B and N, signal from O and C are attributed to surface contamination. It is interesting to note that almost no Cu signal was detected even in the BNNF-Cu sample, suggesting that the amount of Cu in it is close to or lower than detection limit of our XPS system.
The above analysis shows that adding Cu 2+ in the reactant will not cause signi cant impact on the morphology, crystal structure nor functional groups of the formed BNNF or BNNF-Cu. We can also conclude that the amount of Cu in the BNNF-Cu should be low (< 1%).
We then employed more sophisticated analytical techniques including synchrotron radiation analysis, high-resolution high-angle annular dark-eld (HAADF) transmission electron microscopy as well as electron paramagnetic resonance (EPR) spectroscopy to determine whether there is really copper in the  30 . Interestingly, comparing the peaks of these three layered samples, the (002) peak of BNNF-Cu is slightly shift to lower angel, meaning that the interlayer distance of BNNF-Cu is expanded probably attributed to the insertion of Cu atoms between BN layers 40 . Furthermore, the decrease of the primary diffraction peak intensity for BNNF-Cu, in contrast to BNNF, suggests that the insertion of Cu atoms does distort the crystal structure of BNNF to some extent 41 . In addition, the speci c surface area of BNNF-Cu was determined to be 153 m 2 g − 1 which is higher than that of BNNS and BNNF (Supplementary Fig. 11 and Supplementary Table 1). The improvement of speci c surface area can be attributed to the existence of Cu atoms providing more adsorption sites and increasing the probability of pore formation.
To further investigate the dispersion of Cu atoms and coordination between Cu atoms and BNNF, synchrotron radiation X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption ne structure (EXAFS) measurement were conducted. Three standard samples of copper foil, CuO and Cu 2 O were used as references. The obtained results reveal that the Cu species are dispersed as isolated single Cu atoms and stabilized by nitrogen atoms of BNNF. As shown in Fig. 2a, the absorption edge for BNNF-Cu is sandwiched between those of CuO and Cu 2 O, clearly demonstrating the electron structure with positive charges in Cu δ+ (1 < δ < 2) 40 . In addition, an electron paramagnetic resonance (EPR) spectrum of BNNF-Cu shows the characteristic Cu 2+ signal (g ∥ =2.33, g ⊥ =2.05) 42 (Fig. 2d) and a XPS high resolution Cu 2p spectrum of BNNF-Cu ( Supplementary Fig. 12) veri ed the existence of Cu 2+ in BNNF-Cu 40 . Figure 2b shows Fourier transform of the Cu K-edge EXAFS of BNNF-Cu and reference samples Cu foil, Cu 2 O and CuO. There is a main peak at ~ 1.6 Å of BNNF-Cu, which is closing to the main peak positions of CuO and Cu 2 O, can be attributed to the Cu-N rst shell. Additionally, no obvious peak at 2.2 Å (metallic Cu-Cu coordination) can be observed. These results reveal that the atomic dispersion of Cu atoms in BNNF-Cu. Moreover, the wavelet transforms (WT) of Cu K-edge EXAFS also present the well dispersion of Cu atoms in BNNF-Cu (Fig. 2c). The WT for the EXAFS signals of BNNF-Cu, Cu foil, CuO 2 , and CuO are achieved by using the complex wavelet developed by H. Hunke 43 . The WT contour plots of BNNF-Cu shows one maximum intensity at point (4 Å −1 , 1.6 Å), which can be ascribed to the Cu-N coordination, and almost no Cu-Cu signals are observed, as compared with that of Cu foil, CuO, and CuO 2 .
Structural parameters of BNNF-Cu were further extracted by tting the Cu K-edge EXAFS spectrum using the Artemis software. Figure 2e and Supplementary Fig. 13 show optimal tting curves in R space and K space, respectively. The optimal tting results are attributed to the rst shell of Cu-N (bond distance: ~ 2.0 Å) and the second shell of Cu-B (bond distance: ~ 2.64 Å), as shown in Supplementary Tables 2 and 3. Moreover, there is a signi cant difference in the coordination number (CN) between the rst shell (Cu-N: 6.26 ± 0.97) and the of second shell (Cu-B: 2.56 ± 0.74), indicating that the coordination structure around the Cu single atoms is very disordered, which is consistent with XRD results. According to these results, we then build a model for density functional theory (DFT) calculation with an isolated Cu atomic surrounded with six Cu-N as shown in Supplementary Fig. 14.
Photocatalytic activity of BNNF-Cu. Aromatic amines are important intermediates with wild usages for synthesis of many chemicals for commercial and biomedical applications. Selective catalytic reduction of nitro compounds into amines with hydrogen or hydrogen donors has been considered as an important chemical reaction in synthetic organic chemistry 44 . Herein, we selected the reduction of p-nitrophenol (PNP) to p-aminophenol (PAP) with borohydride to evaluate the catalytic performance of BNNF-Cu. It is known that after adding NaBH 4 to a PNP solution, 4-nitrophenolate anions with strong visible absorbance at 400 nm will be formed ( Supplementary Fig. 15), and the concentration of 4-nitrophenolate is proportional to the intensity of the absorption peak. Under irradiation, the characteristic absorption peak of PNP gradually disappears, accompanied by the appearance PAP absorption peak centered at about 300 nm during the catalytic reduction of PNP over BNNF-Cu (Fig. 3a). Moreover, the existence of two isosbestic points at 280 and 314 nm indicates complete conversion of PNP to PAP without any byproducts 45 . As shown in Fig. 3b, no matter under irradiation or in dark, the C t /C 0 (where C t is the concentration of PNP at time t) value did not change in BNNS and BNNF, indicating that these two materials cannot catalyze the reduction of PNP. By contrast, the addition of BNNF-Cu caused a rapid decrease of PNP concentration whether under photoexcitation (BNNF-Cu-D-L) or in dark (BNNF-Cu-D). These results suggested that the present of copper is essential for the catalytic action of BNNF-Cu. To further con rm this point, we decorated a BNNF sample with copper nanoparticles (BNNF-Cu-NP) with sonicating a BNNF aqueous dispersion with pre-prepared copper nanoparticle of 50 to 80 nm diameter ( Supplementary Fig. 16). Figure 3b shows that the surface decoration of copper nanoparticles onto BNNF indeed endow it with catalytic activity both under photoexcitation and in dark. On the other hand, it can also be noted that catalytic activity of the BNNF-Cu sample is much higher than that of BNNF-Cu-NP suggesting that the isolated copper atoms or few-atom clusters on BNN-Cu do have better catalytic performance comparing to copper nanoparticles. Since the initial amount of the NaBH 4 added is far more than needed, a pseudo rst-order kinetic equation has been utilized to evaluate reaction rate, written as ln (C t /C 0 ) = -kt, (where k is the apparent rate constant). The calculated rate constant (k) of BNNF-Cu-L (0.65 min − 1 ) is more than twice that of BNNF-Cu-D (0.31 min − 1 ) (Supplementary Fig. 17a, c), indicating a good photocatalytic activity of the BNNF-Cu. To clarify the in uences of visible light excitation on the catalytic performance, a 400 nm lter has been equipped on Xenon lamp to block ultraviolet (UV) light with wavelength below 400 nm. As shown in Fig. 3c, without UV light, the BNNF-Cu performed a moderate catalytic activity (k ≈ 0.43, Supplementary Fig. 17b), indicating that the BNNF-Cu has a certain sensitivity to visible light.
To check the stability of the catalyst, the PNP reduction experiment was repeated by adding the same amount of PNP into the used solution after complete disappearance of original PNP's absorption peak. After 5 cycles, photocatalytic activity of BNNF-Cu shows little observable degrade (Fig. 3d). Compared the XRD pattern and FTIR spectrum of BNNF-Cu after reaction with that of the as-prepared sample, the almost same results ( Supplementary Fig. 18) indicates that the crystallinity and composition of the catalyst did not change during reaction. Meanwhile, there is no observable aggregation of the isolated Cu atoms in BNNF-Cu after use ( Supplementary Fig. 19). Thus, the BNNF-Cu shows a high operation stability under photoexcitation. Moreover, the photocatalytic universality of BNNF-Cu also estimated by reducing other nitroaromatic chemicals. As shown in Fig. 3e, f, the BNNF-Cu can quickly convert o-nitrophenol, 3nitrophenol, 3-methyl-4-nitrophenol, 2-chloro-4-nitrophenol, and 2-methyl-4-nitrophenol to their corresponding reductive amino-aromatic products under irradiation. To show the excellent performance of BNNF-Cu, a comparison of the catalytic activities with that of the recently reported catalysts was made (Fig. 3g). Because the mass of metal in catalysts and the amount of PNP in solution varied largely in different reported works, the normalized turnover frequency (TOF) was employed to evaluate the catalytic activity. As shown in Fig. 3, BNNF-Cu shows a record high TOF comparing to previously reported nonprecious-metal-based catalysts. (left panel of Fig. 3g) and even higher than many precious-metal-based catalysts (right panel of Fig. 3g). Although the TOF of BNNF-Cu is still lower than a few precious-metalbased catalysts, considering the price of copper which is much lower than precious metal (Au, Ag, Pt and Pd) (Cost per TOF are shown in Supplementary Table 4), the BNNF-Cu does possess higher coste ciency. All the above-mentioned excellent performance indicated that the BNNF-Cu catalyst has potential application prospects in the eld of nitro reduction synthesis of amines.
Mechanism of enhanced catalytic performance. To further explain the positive effect of Cu single atoms in BNNF-Cu on catalytic activity, we systematically analyzed the band structure and electrical conductivity of BNNF-Cu. Compared with BNNF, the BNNF-Cu sample have a narrower bandgap, higher valence band position and broader light absorption range ( Supplementary Fig. 20-22), revealing that the introduction of a small amount of Cu single atoms can change the band positions and improve light absorption capacity. Figure 4a shows the time-resolved photoluminescence (TRPL) spectroscopy of BNNF and BNNF-Cu. The PL decay lifetime (τ) of BNNF-Cu (2.40 ns) is slightly shorter than that of BNNF (2.73 ns), indicating an enhancement of charge transfer and separation in the BNNF-Cu system, which are probably ascribed to narrower bandgap and new transfer channels caused by anchoring of isolated Cu atoms 10,40,46 . Higher photocurrent density (Fig. 4b) and lower impedance of BNNF-Cu (Fig. 4c) also support the conclusion that the e ciencies of charge separation and transfer are improved by the induced isolated Cu atoms. As a result, anchoring small amount of isolated Cu atoms on BNNF can simultaneously adjust band structure and enhance the separation and transfer of electron-hole pairs. DFT calculations were applied to gain a deeper understanding of the excellent catalytic performance of BNNF-Cu in the reduction of PNP to PAP. According to some previous works and our calculations, the con gurations of adsorbate on BNNF-Cu were set to be parallel to the catalyst surface 47,48 . There is no energy barrier for the hydrogenation of PNP for BNNF-Cu in the reaction pathway, in contrast, there is a large energy barrier for PNP (*NO 2 ) →*NOOH for BNNF ( Supplementary Fig. 23), indicating that anchoring isolated Cu atom on BNNF makes it suitable for reduction reactions.

Discussion
Via analysis of both the theoretical and the experimental results, the important role of the isolated Cu atoms in BNNF-Cu for the catalytic reduction reaction of PNP can be described as follow: The isolated Cu atoms on BNNF-Cu can provide a high density of active sites for catalytic reduction of PNP to PAP. Moreover, the introduction of isolated Cu atoms can also improve the adsorption capacity of BNNF-Cu, making it easier for PNP molecules to be absorbed onto the catalyst surface. Under irradiation, BNNF-Cu shows an enhanced catalytic activity which is probably ascribed to the assistant of the generated photoelectrons of catalyst. In detail, when irradiating the reaction system, the catalyst will be activated, electrons are excited from the valence band (VB) to conduction band (CB) followed an easy electron transfer from CB to Cu single atoms (the Fermi level of Cu is commonly lower than the CB of BNNF), more electrons will take part in the hydrogeneration steps, thus speeding up the reaction.
In summary, we rstly reported a single atom catalyst, Cu single atoms supported by porous BN nano bers, which is prepared by a facile in situ pyrolysis strategy. The dispersion of Cu single atoms and coordination between BN and Cu atoms were rstly con rmed with STEM, XAFS and other characteristic methods. Due to the uniform dispersion of Cu atoms, the as-prepared sample processes higher speci c surface area, stronger light absorption capacity, faster charge transfer and easier charge separation, leading to an excellent performance in reduction of PNP with NaBH 4 . The good universality and stability of BNNF-Cu catalyst provide potential application in the eld of catalytic reduction. Theoretical simulation systematically illustrated the pathway of the reduction of PNP, and further proved the bene ts of introducing a single atom. This work not only synthesizes an e cient single atom catalyst for the photocatalytic reduction, but also provide the possibility for the application of boron nitride in eld of single atom photocatalyst.

Methods
Materials. Melamine, boric acid, copper chloride dihydrate and sodium borohydride were purchased from Sigma Aldrich. P-nitrophenol, o-nitrophenol, 3-nitrophenol, 2-methyl-4-nitrophenol, 2-chloro-4-nitrophenol, 2-chloro-4-nitrophenol and boron nitride nanosheets were purchased from Aladdin chemical reagent Corp (Shanghai, China). All the chemicals were used as received. Characterizations. X-ray powder diffraction (XRD) patterns were recorded on a Bruker D2 phaser with Cu Kα radiation at 30 kV. Fourier-transform infrared (FT-IR) spectra were measured with a Nicolet iS50 system using a KBr disk method. Scanning electron microscopy (SEM) were carried out with a QUATTRO S eld-emission scanning electron microscope. Raman spectra were recorded on Renishaw inVia Raman spectrometer excited with a 633 nm laser. UV-vis adsorption spectra for solutions and solids were recorded with a Shimadzu 1700 and a Shimadzu solidSpec-3700/3700DUV systems, respectively. X-ray photoelectron spectroscopy (XPS) measurements were performed on Escalab 250Xi spectrometer with an Al Ka source. Photoluminance spectra and time-resolved uorescence decay spectra were recorded with Spectro uorometer (Edinburgh FL980). Electron paramagnetic resonance (EPR) spectra were tested with a Bruker A300-10/12 EPR spectrometer (center eld: 3510.00 G; microwave frequency: 9.85 GHz; and microwave power: 1.87 mW) at room temperature. Nitrogen adsorption-desorption measurements were performed on an automatic gas adsorption analyzer AutoSorb iQ. The Brunauer-Emmett-Teller (BET) method and the Barrett-Joyner-Halenda (BJH) method were applied for determination of speci c surface area and pore size distribution, respectively. Atomic resolution scanning transmission electron microscope (STEM) observation was carried out on a double aberration-corrected FEI Titan G2 60-300 S/TEM. High angle annular dark eld (HAADF) and energy dispersive X-ray spectroscopy (EDS) mapping were performed at an accelerating voltage of 300 kV. Photoelectrochemical measurements were carried out on a CHI 660B electrochemical system (Chenhua Instruments) using a conventional three-electrode cell with Pt as the counter electrode, and Ag/AgCl/KCl electrode as reference electrode. Transient photocurrent responses were measured under intermittent full spectrum light irradiation or intermittent visible light irradiation. Electrochemical impedance spectroscopy experiments were performed in dark.
XAFS analysis. X-ray absorption ne structure (XAFS) spectra at Cu K (E0 = 8979 eV) edge was measured at the BL14W1 beam line of the Shanghai Synchrotron Radiation Facility (SSRF). We calibrated the energy according to the absorption edge of a pure Cu foil. Data extraction and analysis were carried out with respectively Athena and Artemis codes 49 . Based on normalized X-ray absorption near edge spectroscopy (XANES) pro les, with the help of bulk references (Cu 2 O for Cu + and CuO for Cu 2+ ), oxidation state of copper can be determined by the linear combination t referencing to bulk references (Cu 2 O for Cu + and CuO for Cu 2+ ). For the extended X-ray absorption ne structure (EXAFS) part, the Fourier transformed (FT) data in R space were analyzed by applying BN-like model with replaced Cu center for Cu-N/Cu-B. The parameters describing the electronic properties (e.g., correction to the photoelectron energy origin, E0) and local structure environment including coordination number (CN), bond distance (R) and Debye-Waller factor around the absorbing atoms were allowed to vary during the tting process.

Calculation details
The Vienna ab initio simulation package (VASP) 50 was implemented to evaluate the reduction pathway of PNP to PAP. To model the porous BN with single Cu atom, two boron atoms and two nitride atoms were removed from a periodic supercell (5 × 5) of BN nanosheet to provide anchoring sites for a single Cu atom. A vacuum of 20 Å along the z-direction was considered to avoid interaction between the periodically repeated layers. The calculations were performed at the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional within a generalized gradient approximations (GGA) 51,52 . The van der Waals interactions were described using the empirical correction method (DFT-D3). The cut-off energy was set to be 500 eV. The force and energy convergence criterion were set to 0.01 eV/Å and 10 − 5 eV, respectively. A Monkhorst-Pack k-point mesh 3 × 3 × 1 was used for structural optimization 53 . The selection of these computational parameters was based on convergence tests with respect to the total energy of the systems.
Photocatalytic test. Photocatalytic reduction of PNP over as-prepared samples were carried out under irradiation (full spectrum light or visible light) at ambient temperature. In detail, adding 10 mg catalysts in 80 mL PNP solution (10 ppm) followed 1 hour stirring in dark. Then, 11 mg NaBH 4 was added into the solution. Keep stirring and irradiating the reaction solution during the reaction process. At a certain interval, about 2 mL of the reaction solution was taken out and ltered and characterized using a UV-vis absorption spectrophotometer. The photocatalytic reduction of other nitroaromatic chemicals were tested by the same method.

Declarations
Data availability The data that support the plots within the paper and other ndings of this study are available from the corresponding author upon reasonable request.