Toward High Activity and Durability: An Oxygen Rich Boron Nitride

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Toward High Activity and Durability: An Oxygen Rich Boron Nitride Supported Au Nanoparticles for 4-nitrophenol Hydrogenation Bo Yu, Bo Han, Xinrui Jiang, Chenggang Zhou, Kaisheng Xia, Qiang Gao, and Jinping Wu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00600 • Publication Date (Web): 01 Apr 2019 Downloaded from http://pubs.acs.org on April 1, 2019

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The Journal of Physical Chemistry

Toward High Activity and Durability: An Oxygen Rich Boron Nitride Supported Au Nanoparticles for 4-nitrophenol Hydrogenation Bo Yu#, Bo Han*#, Xinrui Jiang#, Chenggang Zhou*, Kaisheng Xia, Qiang Gao, Jinping Wu Engineering Research Center of Nano-Geomaterials of Ministry of Education, Faculty of Materials Science and Chemistry, China University of Geosciences, 388 Lumo Road, Wuhan 430074, P. R. China *Email: [email protected] (B. H.) [email protected] (C. Z.) #

B. Y., B. H., and X. J. contributed equally to this work.

Abstract Catalytic hydrogenation of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) is widely recognized as one of the most effective solution to deal with the water related environmental issues, where catalysts with high activity and long cycle life are always pursued. In this paper, oxygen rich boron nitride (BNO) supported Au nanoparticles (Au/BNO) were proposed to serve as catalysts for 4-NP hydrogenation. Our results show that the Au nanoparticles are well dispersed on the highly porous BNO substrate with a small particle size of 2.2 nm. Under the optimal reaction conditions, the 4-NP molecules could be completely reduced to 4-AP in 2.5 min over Au/BNO with a relatively high conversion rate constant of 2.25 min-1. Moreover, the as-synthesized Au/BNO is also highly stable with no visibly efficiency decay after been reused for several times. The excellent durability of the catalyst could be ascribed to the strong anchoring strength between Au nanoparticles and BNO substrate, leading to negligible metal loss or particle agglomeration upon cycling. The role of O heteroatoms in promoting the activity and durability of Au/BNO catalyst were further confirmed by first-principles calculations. Our simulation results indicate that the high activity of Au/BNO could be attributed to the strong π-π attraction between BNO and 4-NP, which facilitates the fast diffusion of 4-NP molecules to the surface of Au/BNO. The long cycle life of Au/BNO is correlated to the partially destroyed electronic conjugation of the substrate, leading to relatively strong coordination interaction between BNO and Au atom to stabilize the metal nanoparticles. Introduction The 4-nitrophenol (4-NP) is widely recognized as a toxic pollutant that causes severe environmental problems.1-2 Removing 4-NP from water is of essential importance for the health and safety issues of human beings. A lot of efforts, including biological degradation, physical adsorption, photocatalytic degradation, and catalytic hydrogenation, have been made to effectively remove 4-NP from water.3-8 Among these methods, catalytic hydrogenation of 4-NP to 4-aminophenol (4-AP), also known as catalytic reduction, is one of the most promising solution due to its high efficiency.9-10 However, the hydrogenation efficiency is highly dependent on the activity and durability of the involving catalyst. In the past decade, it was found that transition metals nanoparticles, such as Ni, Ag, Cu, Pt, Pd and Au, are highly active in catalytic converting 4-NP to 4-AP. 11-17 In particular, because of the superior catalytic activity, gold nanoparticles (AuNPs) become one of the most attractive research field in recent years.18 Generally, AuNPs need to be carefully protected to avoid aggregation. 1

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Using surface stabilizers has been recognized as a feasible method to prevent AuNPs from agglomeration.19-21 However, separating the homogenously dispersed AuNPs from water for reuse is still a challenge due to the small particle size of AuNPs. Alternatively, loading Au onto supporting materials with large particle size and high surface area should be one of the most promising way to effectively stabilize AuNPs.22-24 Mostafa Farrag22 compared the performance of two typical supporting materials (Al2O3 and TiO2) for gold clusters in 4-NP hydrogenation. Their results showed that Au nanoparticles loaded on Al2O3 exhibited higher catalytic efficiency than that loaded on TiO2. Taketoshi et al synthetized gold clusters on various metal oxides, such as MnO2, Al2O3, and TiO2. It was found that AuNPs loaded on the Al2O3 showed the highest activity in CO catalytic oxidation among these metal oxide candidates.24 These studies clearly show that the substrate materials could significantly affect the catalytic activity of metal nanoparticles. Therefore, it is necessary to choose a suitable supporting substrate for a certain catalytic reaction. Generally, besides the large specific surface area and rich loading sites, the substrate should be stable enough to prevent itself from degradation during the catalytic process. In addition, it is also a beneficial nature if the carrier could selectively adsorb the reactant molecules to promote the catalytic process. To meet these requirements, the hexagonal boron nitride (h-BN) with graphite-like structure was proposed to serve as a new substrate for metal nanoparticles due to its superior chemical and thermal stability.25 Owing to its unique 2D structure, the electrons of h-BN are highly delocalized due to the strong conjugation effect, which would improve its affinity toward the benzene rings of 4-NP molecules via the π-π interactions. In fact, recent studies have shown that the loading AuNPs onto h-BN substrate could considerably promote the catalytic activity of catalyst in 4-NP hydrogenation.26-27 For example, polydopamine was proposed by Roy and co-workers to serve as “glue layer” to stabilize AuNPs on h-BN substrate.26 Their results showed that when the h-BN is grafted by polydopamine, the anchoring strength between metal and substrate could be significantly enhanced, leading to good dispersion stability of AuNPs and excellent catalytic efficiency for 4-NP hydrogenation. Alternatively, the polydopamine coated h-BN could be also used as substrate for metal oxide (such as Fe3O4).28 However, directly loading AuNPs onto h-BN is still a challenge due to the weak anchoring interaction between metal and h-BN.29 Gao and co-workers studied the interactions between Au and h-BN based on first-principles calculations. It was found that the weak anchoring strength between Au and h-BN could be attributed to the strong electrons conjugation on h-BN plane, which suppressed the formation of Au-N covalent bond.29 When defects or heteroatoms were introduced to h-BN substrate, the conjugation of h-BN could be partially destroyed and the anchoring strength toward metal nanoparticles could be significantly enhanced.30 In this paper, oxygen heteroatoms are introduced into h-BN lattice to enhance the interaction between Au and h-BN for directly loading AuNPs on h-BN. Our results show that the oxygen rich h-BN (denoted as BNO) is high defective and porous. Through a typical adsorption-reduction procedure, the AuNPs could be directly loaded onto BNO with excellent stability and dispersity. The as prepared Au/BNO exhibits excellent activity in catalytic hydrogenation of 4-NP, which could be completely converted to 4-AP in 2.5 min. Moreover, the catalyst also shows long cycle life with negligible efficiency decay when being reused for 5 times. Theoretical calculations were also conducted to address the role of O heteroatoms in BNO substrate. Comparing with h-BN, the 2


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anchoring strength of BNO toward both Au and 4-NP could be significantly enhanced upon O doping, leading to superior durability and activity of Au/BNO catalyst. Experimental Section Materials. Boric acid (AR, 99.5%), melamine (CR, 99.0%), HAuCl4·4H2O (AR, 99.0%), hydrochloric acid (AR, 36.0–38.0%), NH3.H2O (AR, 25.0–28.0%), NaBH4(AR, 96.0%) and p-nitrophenol (AR, 99.0%) were purchased from Sinopharm Chemical Reagent Co., Ltd. in China and were used as received. The hexagon boron nitride (h-BN, AR, 99.9%) was purchased from Aladdin Industrial Corporation to serve as control sample. Sample Preparation. The oxygen rich mesoporous boron nitride (BNO) substrate was synthesized from boric acid and melamine. The details could be found in our previous work31. Briefly, the g-C3N4 intermediate was obtained from melamine in the first step. Boric acid was then introduced and mixed with the g-C3N4 intermediate with a molar ratio of 2:1 to serve as precursor. Subsequently, the mixed precursor was heated up to 900 ℃ in a N2 atmosphere, washed with dilute hydrochloric acid and ultrapure water, dried and ground into powder. Eventually, the resulting product was heated to 600 ℃ for 2 h in a close roaster to remove residual carbon, yielding the BNO support. Au/BNO composite was prepared by a typical adsorption-reduction method. Generally, 98 mg of the as-obtained BNO powder was dispersed into 20 mL ultrapure water. Then, HAuCl4 aqueous solution (416 μL, 24.28 mM) was added into the above dispersion. After stirred for 5 min, ammonia (5 wt%) was added into the mixture to adjust pH to 9.3. After stirred for 12 h in the dark, the mixed solution was washed with ultrapure water for several times to get the precipitate, which was then redispersed into 20 mL ultrapure water. The resulting mixture changed gradually from white to purple with the dropwise addition of NaBH4 solution (0.03M, 4 mL), implying the formation of gold nanoparticles. After stirred for 1 h, the product was collected by membrane filtration, washed and dried for 4 h in an oven. For comparison, Au/h-BN was also prepared by the same procedure, where the BNO substrate was substituted by h-BN. Sample Characterization. The structural characteristics of the samples were investigated by X-ray diffraction (XRD-6000, Shimadzu, Japan; Cu Kα, 40 kV, scan rate of 3 °/min), PHI Quantera II X-ray Photoelectron Spectroscopy (Ulvac-PHI, Japan), the nitrogen physisorption isotherms on a 3020 TriStar II (Micromeritics, USA). The morphological features were collected using Tecnai G2 F30 (FEI, USA) and SU8010 SEM (Hitachi, Japan). The load amount of Au nanoparticles was checked by Optima 8300 ICP-OES (PerkinElmer, USA). The contact angle was tested by JC2000C contact-angle system (Powereach, China), and average value were measured from three points on the same surface. Catalytic hydrogenation of 4-nitrophenol. Typically, p-nitrophenol solution (0.5 mL, 50 mM) and 5.0 mg Au/BNO composite were first added into 10 mL ultrapure water with continuous stirring until the catalyst was dispersed. Then, NaBH4 solution (5 mL, 0.3 M, freshly prepared) was added into the above mixture to serve as reducing agent. The reaction process was monitored by the concentration of 4-NP, which was recorded in the Specord 200p UV-vis spectrophotometer 3



(Anlytikiena, Germany) at a wavelength of 400 nm. For recycling experiment, the catalysts were collected by membrane filtration after the reaction, dried at 65°C, and then reused. The recyclability test was carried out for five catalytic cycles. Results and Discussions

Figure 1. XRD patterns of the samples Au/BNO composite and BNO powders. In our experiments, the graphite-like carbon nitride (g-C3N4, obtained from melamine32) was employed as nitrogen source to react with boric acid. The planar structure of g-C3N4 could also serve as a template for BNO growth, where the in-plane holes of g-C3N4 would facilitate the O doping during the reaction. The XRD patterns of the as-obtained BNO powders are shown in Figure 1. The feature peaks of BNO match well with the standard diffraction peaks of layered h-BN (JADE card no. 45-0896) without any impurity peak, suggesting that the as-synthesized BNO is pure with a typical hexagonal phase structure. When Au nanoparticles were loaded on the substrate, a relatively small diffraction peak located at 38.25°, which could be assigned to the (111) facet of Au (JADE card no. 65-8601), was detected in the XRD patterns of Au/BNO. Such a weak diffraction peak could be ascribed to the relatively small loading amount of Au, as confirmed by the ICP-OES characterization that only 1.2 wt% Au was detected on the BNO substrate (Table S1).

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Figure 2. (a) XPS spectra of the element survey scans of BNO and Au/BNO composites, (b) XPS spectra of Au. Figure 2a shows the XPS results of BNO and Au/BNO. Besides the peaks of B 1s and N 1s, a strong O 1s peak was observed for both BNO and Au/BNO, implying that a certain amount of O atoms has been successfully introduced into the substrate. Indeed, the calculated atomic concentration of BNO clearly shows that a relatively high O concentration of 15.7 at% could be achieved in our experiments (Table S2). High-resolution XPS results of BNO (Figure S1) reveal that there are three types of O atoms (C-O, O-H and B-O-B) in the BNO substrate, suggesting that the O atoms are located in both the lattice points and edges of BNO. In contrast, only two types of O atoms corresponding to C-O and O-H were detected for h-BN, reflecting the fact that O atoms are mainly located at the edges of h-BN. Table S2 also indicates that only a relatively small Au amount of 0.05 at% (or 0.76 wt%) was loaded on the BNO substrate, leading to negligible variation of element content upon Au loading. We should note that the low loading amount of Au in Table S2 is largely attributed to the nature of XPS characterization, which focuses on the valance state of the atoms in the surface region at a certain depth of the catalyst. The Au 4f peaks are located at 83.8 and 87.5 eV (Figure 2b), demonstrating that the supported Au nanoparticles are completely reduced to Au0 oxidation state. The SEM and TEM characterizations were conducted to examine the morphologies of BNO and Au/BNO (Figure 3). It was found that the BNO sample was consisted of small particles with average particle size of 50 nm (Figure 3a). No further agglomeration was observed upon Au loading, indicating that the substrate itself is largely stable (Figure 3b). In addition, the small loading amount of Au is also responsible for the negligible structure changes. TEM image of Au/BNO (Figure 3c) shows that the Au nanoparticles are well dispersed on the substrate with an average particle size of 2.2 nm. It is believed that such a small particle size would substantially improve the active sites of Au/BNO, leading to superior catalytic performances. High resolution TEM image of Au/BNO clearly shows that the Au particles are directly loaded on BNO surface. In particular, the lattice fringes of Au exhibit a distance of 0.235 nm, which is the typical lattice spacing of Au (111) facet, reflecting the fact that Au particles are well crystalized and completely reduced to Au0 oxidation state.

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Figure 3. The SEM images of (a) BNO powders and (b) Au/BNO composites. TEM images of (c) fresh Au/BNO catalyst and (d) Au/BNO been used for 5 times.

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Figure 4. The textural structure characterization of BNO and Au/BNO. (a) The N2 adsorption/desorption loop. (b) The pore size distribution. The textural parameters of BNO and Au/BNO were identified by N2 adsorption/desorption measurements. As shown in Figure 4, a typical type-IV isotherm pattern with an H4 type broad hysteresis loop33 was observed for both the BNO and Au/BNO composites, indicating the two samples are highly porous. The BNO exhibits a relatively large surface area of 220 m2 g-1, which is much higher than the typical BN without O doping. When Au nanoparticles were loaded, the surface area was slightly reduced to 193 m2 g-1, indicating that some pores were occupied by Au particles. Indeed, the calculated pore size distribution of BNO and Au/BNO (Halsey method, Figure 4b) clearly showed that the pores of BNO were partially inhabited by Au particles, as evidenced by the reduction in both pore size and pore volume.

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Figure 5. Time dependent absorption spectra for the catalytic hydrogenation of 4-NP by NaBH4 in the presence of 5.0 mg catalysts: (a) Au/BNO composites, (b) Au/h-BN composites, (c) the BNO powers, and (d) the plot of A/A0 versus reaction time. Conditions: [4-NP] = 0.05 M, [NaBH4] =0.3 M, 25 °C. The catalytic performance of Au/BNO was evaluated by the hydrogenation of 4-NP to p-aminophenol (4-AP) with NaBH4 as reducing agent. Figure S2 displays the color change during the hydrogenation process. The color of the 4-NP solution changed from light yellow to bright yellow when NaBH4 was introduced, suggesting that the 4-NP molecules are converted to p-nitrophenolate ions under alkaline condition.34 When the catalyst was dispersed in the solution, the bright yellow color rapidly diminished to light purple (the color of Au/BNO composite) within 2.5 min, suggesting the fast hydrogenation of 4-NP. The time-dependent UV–vis absorption spectra was collected in Figure 5. It was found that the absorbance peak at 400 nm corresponding to 4-NP decreased rapidly over time. Meanwhile, a new peak at about 300 nm attributed to 4-AP gradually increased (Figure 5a), indicating the 4-NP molecules were largely converted to 4-AP. Control experiments with Au/h-BN and BNO were also carried out to identify the catalytic contribution BNO substrate. Comparing with Au/BNO, the catalytic activity of Au/h-BN is much lower (Figure 5b). Only 71% of 4-NP could be hydrogenated over Au/h-BN catalyst after 3 min. Even if the reaction time extended to 10 min, there were still 10% 4-NP in the solution, indicating the poor catalytic efficiency of Au/h-BN. Such a low efficiency could be attributed to the low surface area of h-BN and poor adsorption interaction between 4-NP molecules and h-BN substrate. It is interesting to note that about 8% of 4-NP molecules were removed in 10 minutes when pure BNO powders were added into the mixture solution, indicating the relatively strong adsorption between BNO and 4-NP (Figure 5c). Such a strong interaction will lead to the enrichment of 4NP 8


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molecules on the surface of the catalyst, leading to higher catalytic activity. To address the 4-NP enrichment effect near Au/BNO surface, we conducted a catalytic reaction of 4-NP without stirring (Figure S3). The results show that the hydrogenation efficiency of 4-NP without stirring is equivalent to that with stirring, demonstrating that the 4-NP molecules could be attracted by Au/BNO and be enriched near the surface of catalyst. First-principles simulations (supporting information for computational details) were carried out to further address this issue (Figure S4 and Table S3). Our results showed that the π-π interactions between 4-NP molecules and h-BN substrate were relatively weak with the calculated adsorption energy of 0.733 eV. In contrast, a strong anchoring strength between 4-NP and BNO was observed with the calculated adsorption energy of 2.312 eV. Such a high adsorption strength should be the driving force in promoting the concentration of 4-NP near the substrate, leading to the improved catalytic efficiency of Au/BNO. In fact, the projected secondary differential charge distribution (Figure S5) also clearly shows that, during the adsorption process, the BNO surface would loss its electrons (0.726 e) to the adsorbed 4-NP molecule via the strong π-π interaction between 4-NP and BNO. However, when the 4-NP was reduced to 4-AP, the π-π interactions between 4-AP molecules and BNO substrate was somewhat weakened with the adsorption energy of 1.823 eV. The lowered adsorption energy of 4-AP indicates that the hydrogenation product could be easily removed from the catalyst by new 4-NP molecules via a competitive adsorption process, which facilitates the 4-AP desorption. It is worth noting that the BNO also exhibits much better hydrophilicity than h-BN with a moderate water adsorption strength of 0.867 eV, implying that the Au/BNO could be easily dispersed in aqueous environments. Indeed, the improved wettability of Au/BNO was proved by the water contact angle measurements (Figure S6), where the contact angles were 36° and 53° for Au/BNO and Au/h-BN, respectively. Figure 5d displays the calculated conversion rate of the three samples, where the superior catalytic efficiency of Au/BNO could be clearly observed. Here, the reaction rate constant (kapp) of 4-NP hydrogenation on Au/BNO is determined to be 2.25 min-1, which is much higher than the values of other supported AuNPs (Table 1). The high reaction rate of Au/BNO also gives rise to its high turnover frequency (TOF) of 32.13 mol4-NP molAu-1 min-1, which is also better than the values of other BN based catalysts. Table 1. Comparison of reduction time and rate constant values for the 4-NP hydrogenation to 4-AP using various catalysts. Catalyst

[4-NP]/mM

[NaBH4]/mM

Time/min

k/min-1

TOF

Ref.

Au/PEG-PEI-PCL

0.2

100

8

0.37

NA

19

Au/graphene hydrogel

0.1

100

12

0.19

0.189

35

Au-Ag nanocages/GO

0.1

10

7

0.394

NA

36

Au/PANF-g-HPEI

60

150

13

0.48

NA

37

Au-Ag nanowires

4

200

5

0.204

NA

38

Au/h-BN

0.1

100

17

NA

2.3

39

Ag/h-BN

0.4

400

10

0.163

16.59

40

Cu2O/h-BN

0.2

100

12

NA

0.035

41

Au/BNO

50

250

2.5

2.25

32.13

This work

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Figure 6. (a) The relationship between ln(A/A0) and reaction time, and (b) the calculated kapp under various NaBH4 concentrations. Conditions: Au/BNO=5 mg, [4-NP] =0.05 M, 25 °C. We next focus on reaction conditions that might affect the efficiency of 4-NP hydrogenation. Here, the effect of NaBH4 amount on the hydrogenation rate was studied for identifying its appropriate concentration. As shown in Figure 6a, a good linear relationship between ln(A/A0) and reaction time was observed for all the involved NaBH4 concentrations, indicating the first-order reaction nature of the hydrogenation process.42 Figure 6b displays the calculated reaction rate constant (kapp), it is found that the reaction rate increased with the increase of NaBH4 concentration in the range of 0.03~0.25 M, after which it becomes insensitive to NaBH4 concentration in the range of 0.25–0.3 M. Therefore, we choose a NaBH4 concentration of 0.25 M, which is 50 times higher than the concentration of 4-NP, to ensure that the reaction rate of catalytic process is independent of the NaBH4 concentration. Based on this optimal ratio of NaBH4 to 4-NP, the effect of 4-NP concentration was also evaluated. As shown in Figure 7, when a small concentration of 4-NP is introduced, the amount of NaBH4 is also decreased accordingly, resulting in a low hydrogenation efficiency due to the low contacting probability between catalyst particles and reduction agent molecules. As the concentration of 4-NP increasing, the hydrogenation efficiency increases rapidly and tops at the 4-NP concentration of 50 mM (insert in Figure 7), after which the efficiency is slightly reduced due to the high concentration of 4-NP. It is interesting to note that even if the 4-NP concentration is as high as 150 mM (far higher than the reported concentration ranging from 0.1 to 60 mM), the Au/BNO is still capable of converting all 4-NP to 4-AP within 3 min.

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Figure 7. Effect of 4-NP concentration on catalytic hydrogenation of 4-NP. Conditions: Au/BNO=5 mg, 25 °C.

Figure 8. The relationship between ln(A/A0) and reaction time at various temperatures. Conditions: Au/BNO=5 mg, [4-NP] =0.05 M. To address the kinetics of the reaction, the catalytic process was conducted under various temperatures ranging from 15 to 35 ℃. As shown in Figure 8a, the catalytic process fits well with the first-order reaction. The calculated rate constant gradually increases from 1.29 to 4.17 min-1 with respect to the temperature. The relationship between lnk and 1/T is displayed in Figure 8b, where a small activation energy of 47.50 kJ mol-1 is determined according to the Arrhenius theory. Such a small activation barrier should be the driven force of the superior catalytic efficiency of Au/BNO catalyst. In contrast, the Au/h-BN showed a relatively high activation barrier of 61.18 kJ mol-1 (Figure S7), in line with its poor catalytic efficiency in Figure 5d. Therefore, we could rationally conclude that the Au/BNO should be a promising heterogeneous catalyst for effectively hydrogenation of 4-NP.

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Figure 9. The durability of Au/BNO catalyst. Conditions: Au/BNO=5 mg, [4-NP] =0.05 M, 0.5 mL, [NaBH4] =0.25 M, 5 mL, 25°C. To address the stability of Au/BNO, the catalyst was reused for five successive runs. As shown in Figure 9, the Au/BNO remains highly active during each run. After five runs, the catalyst retains almost 100% of its initial activity, indicating the excellent stability and durability of Au/BNO. The superior durability of Au/BNO could be ascribed to the strong anchoring strength between Au and BNO substrate. In fact, the ICP-OES results clearly showed that only a negligible metal loss (~3.1%) was detected for Au/BNO after 5 runs (Table S1). The morphology variations of Au/BNO during the catalytic process were also determined by TEM measurement (Figure 3e). It was found that the Au nanoparticles are still well dispersed on BNO surface with no visibly agglomeration. When the catalyst has been reused for 5 times, the Au particle size only slightly increased to 3.3 nm. Moreover, HRTEM image (Figure 3f) also shows that the morphology and the crystallinity of the cycled Au particles are virtually identical to those of the fresh catalyst, indicating the excellent stability of Au/BNO.

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Figure 10. The calculated adsorption structures of Au atom on (a) h-BN and (b) BNO substrate. The strong interactions between Au and BNO substrate were further confirmed by first-principles simulations. As shown in Figure 10a, the atop site of N atoms on h-BN substrate is determined to be the energetically most favorable site for Au atom. However, the anchoring strength between Au and h-BN is relatively weak with a long Au-N distance of 2.593 Å. When the Au-N bond is formed, the N atom tend to adopt sp3 hybridization. Unfortunately, such a hybridization variation is prohibited by the strong conjugation effect in h-BN plane, leading to the weak anchoring strength with a small adsorption energy of 0.125 eV. For BNO substrate (Figure 10b), Au atom prefers to interact with B atom in adjacent of the O dopant, forming a strong Au-B bond with bond length of 2.156 Å. Upon the formation of Au-B bond, the involving B atom also needs to change its hybridization to sp3, which is promoted by the partially destroyed conjugation of the substrate upon O doping. As a result, the B atom could slightly rise above the BNO plane to adopt the typical configuration of sp3 hybridization. The calculated Au adsorption energy of 3.470 eV on BNO substrate is much higher than that on h-BN, revealing the superior anchoring stability of Au/BNO catalyst. Conclusions In summary, an oxygen rich boron nitride (BNO) supported Au nanoparticles were synthesized to serve as catalyst (Au/BNO) for 4-NP hydrogenation. The XRD, SEM, TEM, FT-IR, XPS and BET analyses were employed to characterize the structure and morphology of the as-prepared Au/BNO catalyst. Our results show that the Au nanoparticles are well dispersed on the highly porous BNO substrate, which provides sufficient active sites for 4-NP molecules. Indeed, a rapid hydrogenation of 4-NP could be achieved over Au/BNO catalyst in 2.5 min with a relatively high conversion rate constant of 2.25 min-1, which is more efficient than other supported Au nanoparticles. The high activity of Au/BNO is ascribed to the strong π-π interactions between BNO substrate and 4-NP molecules, which could enrich the concentration of 4-NP near the surface of catalyst. In addition, the small activation energy (47.50 kJ mol-1) of 4-NP hydrogenation on Au/BNO is also responsible for the superior catalytic efficiency. Owning to the strong anchoring strength between Au and BNO, the Au/BNO also exhibits excellent durability with negligible efficiency decay when being 13



reused for several times. First-principles simulations clearly reveal that the strong interaction between Au and BNO is highly correlated to the O heteroatoms, which partially destroy the conjugation of the BNO substrate to facilitate its interaction with Au. The excellent activity and durability of Au/BNO make itself a promising heterogeneous catalyst for water treatment. More importantly, the proposed porous BNO could be also used as supporting materials for other metal nanoparticles or metal oxides. Supporting Information The Supporting Information is available free of charge on the ACS Publications website. The loading amount of Au in Au/BNO catalyst; the element concentrations in BNO and Au/BNO; the O1s XPS spectra of BNO and h-BN; the optical photographs of the color change during the reaction process; the reaction activity of Au/BNO with/without stirring; the adsorption structures of 4-NP on h-BN and BNO; the secondary differential charge distribution of BNO upon 4-NP adsorption; the calculated adsorption energies of 4-NP, 4-AP and H2O on h-BN and BNO substrate; the water contact angles of (a) Au/BNO and (b) Au/h-BN; the kinetic results of Au/h-BN; and the computational methods.

Conflicts of Interest There are no conflicts of interest to declare. Acknowledgements The authors gratefully acknowledge financial support from National Natural Science Foundation of China (No. 21773217), National Key R&D Program of China (No. 2018YFF0215404), Wuhan Science & Technology Project (No. 2018010401011276), Natural Science Foundation of Zhejiang Province (No. LQY19E020001), and the Open Fund of the Guangdong Provincial Key Laboratory of Advance Energy Storage Materials (No. AESM201815). Support from the high-performance computing platform of China University of Geosciences is also gratefully acknowledged.

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Toward High Activity and Durability: An Oxygen Rich Boron Nitride

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