Controllable synthesis, property investigation of hexagonal boron nitride micromesh and its functionalization by Ag nanoparticles

Abstract

Hexagonal boron nitride (h-BN) micromeshes (defined as “BNMM”) of high crystallinity (with diameters of up to 100 μm and pore size of 2.5 μm on average) have been synthesized by using Li₂B₄O₇, Mg and NaN₃ in stainless steel autoclaves at 500 °C for 12 h. Through tuning the experimental parameters, BN materials with various morphologies (such as nanospheres or thin films) could also be selectively prepared. The thermal gravimetric analysis (TGA) results of the as-grown BNMM reveal their high thermal stability not only in ambient atmosphere but also in nitrogen atmosphere below 1050 °C. The BNMM were also analyzed by thermomechanical analysis (TMA) in nitrogen atmosphere. The as-obtained BNMM were applied for oxidation of benzyl alcohol and also functionalized by monodispersed Ag particles with potential applications as catalysts for carbon monoxide (CO) oxidation. The results indicate that the BNMM have 37.85% catalytic activity and nearly 100% selectivity in translating benzyl alcohol to benzaldehyde, and the Ag/BNMM composites have catalytic activity for 70.50% of carbon monoxide oxidation.

---

1. Introduction

Hexagonal boron nitride (h-BN) is a structure analogue of graphite, which has many characteristic properties, such as low density, low conductivity, high chemical stability, high melting point and high thermal conductivity.¹ Owing to these unique advantages, boron nitride can be used as electrical insulators, refractory, catalyst supports, composites matrixes and so forth.^2–4

Compared with 2D-structured h-BN nanoplates,⁵ h-BN mesh has extraordinary structure and properties.^6,7 Boron nitride nanomesh with a pore size of several nanometers has been firstly synthesized by a self-assembly process on a Rh (111) single crystalline surface.⁸ Since then, mesh-like boron nitride materials have prompted extensive research interest owing to their special structural characteristics and potential technological applications. Recently, BN micromeshes with diameters of up to 100 μm and pore size of 2.8 μm on average have been fabricated via an in situ produced MgO template process at 450 °C for 12 h.⁹ The meshy BN product with a high surface area of 220 m² g⁻¹ has been prepared by using NaBH₄ and CS(NH₂)₂ at 550 °C for 10 h.¹⁰

During the past decades, the applications and functionalizations of boron nitride materials have attracted increasing attention, particularly in the areas of molecule absorption and catalysis. There have been increasing studies on developing the chemical functionalization of BNNTs due to different interactions, such as covalent bonding, π stacking interactions, cation-π and other interactions.^11,12 It has also been reported that the molecules could be isolated and trapped in the holes of the h-BN nanomesh on Rh (111), such as Cu-phthalocyanine molecules and Xe molecules.¹³ Moreover, BN nanomeshes could also be utilized as an oxygen- and carbon-free template for the production of nanocatalysts, nanomagnets against severe reaction conditions.^7,14,15 Furthermore, Berner et al. exemplified that BN nanomeshes could be used as a template for the growth of stable and well-ordered molecular arrays from a corrugated monolayer.¹⁶ Besides, the functionalizations and composites of BN mesh-like materials with metals have also been studied. Goriachko et al. reported that Au nanoparticles could be dispersed uniformly due to the regular BN nanomeshes.¹⁷

The extensive interest in the catalytic oxidation of carbon monoxide and volatile organic compounds has stemmed from their relevances in many fields, especially the application in air-purification and pollution control industries.^18,19 To date, some transition metals, bi-metals or composites of oxides with loading metals have been used for catalytic oxidation of benzyl alcohol and carbon monoxide. These materials show high activity and selectivity towards benzyl alcohol oxidation and CO oxidation.^20–25 However, when confronted with rigorous circumstances such as acid conditions, humid atmosphere or high temperature, degradation or less efficiency (or invalidation) usually inevitably occurred on these catalysts. Meanwhile, compared with other noble metals, such as Pt, Au, Pd, Ag is more economic. Therefore, hexagonal BN materials with low density, high chemical stability, high melting point and high thermal conductivity could be used as a superior catalyst support under harsh conditions.^1,26–29 To the best of our knowledge, there are few reports about the catalytic performances of hexagonal BN micromeshes and Ag-loaded BN micromeshes. Therefore, it is significant to investigate their performance in catalytic oxidation processes of benzyl alcohol and carbon monoxide, respectively.

In this research, a convenient synthesis of crystalline BN micromeshes was reported. Typically, BN micromeshes with an average pore size of 2.5 μm were produced by Li₂B₄O₇, NaN₃ and Mg at 500 °C for 12 h. When LiBO₂ and LiBH₄ were used instead of Li₂B₄O₇, BN materials with different morphologies could also be produced. The as-obtained boron nitride micromeshes perform high thermal stability both in ambient and nitrogen atmosphere by TGA. These hexagonal boron nitride micromeshes were firstly used for the catalytic oxidation of benzyl alcohol, which show 37.85% oxidation activity at the beginning, and maintain nearly 100% selectivity in the whole oxidation process at 240 °C. Through a wet chemical method, Ag nanoparticles with sizes of 50–200 nm were well dispersed on the surfaces of BNMM, and the as-produced Ag/BNMM composites display a conversion rate of 70.5% at 450 °C in carbon monoxide oxidation.

2. Experimental

2.1. Preparation of BNMM

All chemical reagents were analytical-grade purity and used without further purification. The NaN₃ powders were purchased from Tianjing Kaitong Chemical Co., Ltd. Li₂B₄O₇ and Mg powder were purchased from Shanghai Jingchun Industry Co., Ltd. In a typical synthesis process, Li₂B₄O₇ (1.69 g), NaN₃ (2.60 g) and Mg powder (1.20 g) were mixed and put into a 20 ml stainless steel autoclave. The autoclave was tightly sealed and heated from room temperature to 500 °C with an increasing rate of 10 °C min⁻¹, and then maintained at the target temperature for 12 h. After that, the autoclave was cooled to room temperature naturally. In order to remove the byproduct, the raw product was collected and washed with anhydrous ethanol, distilled water and dilute hydrochloric acid several times. Finally, the end-product (defined as “S₁”) was dried in vacuum at 80 °C for 8 h.

2.2. Preparation of Ag/BNMM

A wet chemical method³⁰ was used for the preparation of Ag/BNMM composites. In order to acquire the desired metal loading, AgNO₃ (AR) and h-BN micromeshes were added into 50 mL ethylene glycol (Sinopharm Chemical Reagent Co., Ltd. AR). Then 10 mL ammonia water was added dropwise, and this solution was stirred slowly at room temperature for 6 h. After that, the solution was added into a 50 mL Teflon-lined stainless steel autoclave. The autoclave was put into the electronic furnace and was heated from room temperature to 200 °C and maintained at 200 °C for 12 h. Finally, the brown product was obtained via washing and drying for usage as catalyst.

2.3. Sample characterization

The h-BN micromeshes and Ag/BNMM composites were determined by Bruker D8 advanced X-ray diffractometer with Cu–K_α irradiation. Fourier transformation infrared spectroscopy (FTIR) was recorded with a VERTEX-70 Fourier transform infrared spectrometer by using a pressed KBr disk. The morphology and structure of the samples were evaluated by transmission electron microscopy (TEM, JEM-1011), field emission scanning electron microscopy (FESEM, JEOL JSM-7600F) and high-resolution TEM (HRTEM, JEM-2100, 200 kV). Thermal gravimetric analysis (TGA) was taken on a SDT Q600 V20.9 Build 20 thermal analyzer apparatus under ambient air and nitrogen atmosphere. Thermomechanical analysis (TMA) was conducted by a TMA Q400 V22.5 Build 31 thermal analyzer apparatus under N₂ atmosphere.

2.4. Catalytic performances characterization of BNMM and Ag/BNMM

Catalytic activity was measured by using a continuous flow fixed-bed microreactor at atmospheric pressure. In a typical experiment of oxidation of alcohols, benzyl alcohol (Acros, 99.0%) was confirmed by gas chromatography (GC) analysis, mixed with high-purity oxygen and nitrogen gases were used as receiver. In all experiments, BNMM catalysts were manually added into and mixed with 25–50 mesh quartz sand (2 g) to ensure sufficient contact between the reactants and the catalysts. The gas-phase selective oxidation of benzyl alcohol was carried out at atmospheric pressure in a KLCP4010 fixed-bed vertical quartz tubular reactor (id = 4 mm), and heated by an electronical temperature programmed tube furnace. The gas streams (44 mL min⁻¹) were supplied by mass flow controllers and benzyl alcohol (0.44 mmol min⁻¹) was fed continuously through a syringe pump. Liquid vaporization occurred in the preheater prior to the catalytic reaction bed. The condensable reaction products and the unreacted benzyl alcohol were cooled and collected using a cold trap (0 °C) and then analyzed by GC with a GC9790II equipped with a flame ionization detector, using a SGE-30QC2/AC5 capillary column and N₂ as a carrier gas. 1-Butanol was employed as internal standard. GC-MS (Thermo Trace GC Ultra DSQ) was also employed to determine the reaction products. The first two sets of data were recorded at 0.5 h and 1 h, separately, and the next data were continuously recorded every 1 h. As the experiment of CO oxidation, the system was firstly purged with high purity N₂ gas and then a gas mixture of 1% CO-10% O₂-89% N₂ with a flow rate of 66.7 mL min⁻¹ was introduced into the reactor which contained 0.3 g samples. The concentrations of CO₂ and CO were analyzed with an online infrared gas analyzer (Gasboard-3121, China Wuhan Cubic Co.). The results were further confirmed with a Shimadzu Gas Chromatograph (GC-14C).

3. Results and discussion

Fig. 1a shows a typical XRD pattern of the final product (S₁) prepared by using Li₂B₄O₇, NaN₃ and Mg at 500 °C for 12 h. All diffraction peaks can be indexed to be hexagonal BN. The calculated lattice constant (a = 2.507 Å and c = 6.655 Å) is consistent with the reported value (JCPDS card no. 34-0421). No noticeable peaks of other impurities such as Li₂B₄O₇, B₂O₃ and B are detected in the pattern. Fig. 1b displays a typical XRD pattern of S₁ before the hydrochloric acid treatment. In Fig. 1b, the distinct diffraction peaks of MgO (JCPDS card no. 65-0476) can be found except h-BN. Fig. 1c presents a typical FT-IR spectrum of S₁, in which the peaks located at 815.95 and 1375.19 cm⁻¹ can be attributed to the TO mode of the sp²-bonded BN.^31,32 The former should be indexed as B–N–B bonding vibrations, while the latter can be identified as the B–N stretching vibrations. The weak peak centers at around 2500 cm⁻¹ may originate from the adsorption of carbon dioxide on the surfaces of S₁.

Fig. 1 Typical XRD patterns of BN after (a, the standard XRD patterns of h-BN is drawn in red) and before (b) the acid treatment of S₁ [● and ○ indicate the diffraction peaks from h-BN (JCPDS card no. 34-0421) and MgO (JCPDS card no. 65-0476), respectively]. (c) Typical FT-IR spectrum of S₁.

The morphology of the sample was characterized by TEM and SEM. Fig. 2 display TEM and SEM images of the final h-BN micromeshes (S₁). From Fig. 2a and b, we can see that the product is mainly composed of BN micromeshes, and the diameters of the micromeshes are in the range of tens of micrometers to 100 micrometers. Fig. 2c presents the SEM image of an individual mesh-like sheet. Fig. 2d shows the SEM image of a single pore with a pore size of about 2.3 μm.

Fig. 2 Typical TEM image (a) and magnified SEM image (c) of the final product h-BN micromeshes (S₁). (d) SEM image of an enlarged view of a pore. (b) SEM image of large scale h-BN micromeshes.

The thermal stability of the as-grown boron nitride micromeshes was studied using TGA and TMA (Fig. 3). Fig. 3A and B show typical TGA curves of S₁ in ambient and nitrogen atmosphere, respectively. In Fig. 3A and B, the slight weight loss of the product that occurred below 400 °C might be attributed to the loss of small molecules that adsorbed on the surfaces, which was consistent with the previous report.³³ From Fig. 3A, it is found that the weight of the product has not changed significantly from 400 to 1050 °C. The results were comparable to that of the previous TGA report in ambient atmosphere,³⁴ revealing the same high thermal stability of the present sample as some nano-structured BN materials. When the temperature was raised further from 1050 to 1200 °C, the related part in curve (a) shows a fast increasing rate, the possible reason is that S₁ suffered continuous and fast oxidation on its surfaces in ambient atmosphere. In Fig. 3B, curve (c) has minor changes as compared to curve (a) from 400 to 1200 °C, which implies that S₁ has higher thermal stability in nitrogen atmosphere than in ambient atmosphere especially when temperature was above 1000 °C. Thermomechanical analysis (TMA) was used in a nitrogen atmosphere to investigate the plastic capacity of BNMM using a compressive load of 0.1 N from the temperature range of 23–1000 °C. From the TMA curve (Fig. 3C), it is observed that the BNMM has slight shrinks in the thickness and a low dimensional change (3.5%) below 750 °C, which can be potentially applied in the preparation of non-oxide ceramic workpieces.^35–37 Therefore, the h-BN micromeshes have excellent thermal stability and anti-oxidation properties, which may be used as promising materials in high temperature environment.³⁸

Fig. 3 TGA curves of S₁: (A) in ambient atmosphere and (B) in nitrogen atmosphere. (C) TMA curve for BNMM in a nitrogen atmosphere with a heating rate of 5 °C min⁻¹.

In order to investigate the effects of the reaction conditions on the formation of BN micromeshes, a series of comparative experiments were carried out (see Table 1). It is found that Mg powder and temperature play important roles in this experiment. In the absence of Mg powder or when another metals (such as Fe, Zn, Ni) used instead of Mg, no BN meshes could be obtained (see Table 1). When less amounts of Mg powder were used, the phase of the product was a mixture of h-BN and r-BN (Fig. 4a). Otherwise, BN films (Fig. 4b), plates (Fig. 4c) and sphere-like particles (Fig. 4d) co-existed when the used amount of Mg powder was 0.5 g. Along with increasing the amounts of Mg powder used, the morphologies and quantities of BNMM were improved. However, BN micromeshes could not be found when the amount of Mg powder exceeded 3.0 g. When the temperature was set below 400 °C, BN micromeshes could not be observed. The higher the reaction temperature, the more uniform morphology of the final product of BN micromeshes could be obtained. However, as the temperature was above 600 °C, there was no significant improvement.

Table 1 Influence of metals, temperature and time on the final products

Sample No.	Metal (g)	Temperature (°C)	Reaction time (h)	Final products
S4	—	500	12	h-BN and r-BN with film-like morphology
S5	Mg(0.5)	500	12	h-BN and r-BN with sphere-like and film-like morphology
S6	Mg(1.0)	500	12	h-BN micromesh
S7	Mg(1.2)	500	12	h-BN micromesh
S8	Mg(3.0)	500	12	h-BN no micromesh
S9	Mg(1.2)	400	12	Little amount of h-BN micromesh
S10	Mg(1.2)	<400	12	No product
S11	Mg(1.2)	500	2	h-BN with film-like morphology
S12	Fe(2.8)	500	12	No product
S13	Zn(3.25)	500	12	No product
S14	Ni(2.9)	500	12	h-BN and r-BN with film-like morphology

---

Fig. 4 XRD patterns and TEM images of the products obtained using different amounts of Mg powder but the other reactants are the same as those in S₁. (a) XRD patterns of the samples [□ and ■ indicate the diffraction peaks from h-BN (JCPDS card no. 34-0421) and r-BN (JCPDS card no. 45-1171), respectively]. (b) Typical TEM image of the BN films. (c) Typical TEM image of the BN plates. (d) Typical TEM image of the sphere-like BN particles. (e) TEM image of S₁ before the acid treatment.

When Li₂B₄O₇ was replaced by other boron sources that contained element Li (such as LiBO₂ and LiBH₄, with equal numbers of boron moles), while other experimental parameters remain unchanged, h-BN micromeshes could not be found (see Table 2). This might be attributed to the different physical properties of boron sources containing Li, such as different melting points and structures. Moreover, no BN product was found when using LiBH₄ as the boron source.

Table 2 Effect of the boron sources on the final morphology of h-BN

Sample no.	Reactants	Reactant amounts (mmol)	Temperature (°C)	Morphology
S1	Li2B4O7 + NaN3 + Mg	10 + 50 + 40	500	h-BN micromesh
S2	LiBo2 + NaN3 + Mg	40 + 50 + 40	500	Film-like
S3	LiBH4 + NaN3 + Mg	40 + 50 + 40	500	no BN

---

According to the XRD pattern of the product without treatment with hydrochloric acid, MgO (JCPDS card no. 65-0476) can be found besides BN (See Fig. 1b). As we know, Li, Li₂O, Na and Na₂O are sensitive to oxygen and water in the air during the XRD characterization, it is difficult to detect whether the raw products contain them or not. From the TEM image of the raw product (Fig. 4e), it can be clearly observed that some microparticles (marked with an arrow) are embedded in the micromeshes. Therefore, it is inferred that the synthetic mechanism is an in situ MgO template process.⁹ In addition, when the autoclaves were opened after the reaction, some gases were released. Thus, the possible reaction equation involved in this experiment can be tentatively written as follows:

Li₂B₄O₇ + 4NaN₃ + 6Mg = 4BN + 6MgO + Li₂O + 4Na + 4N₂

The performances of catalytic oxidation of benzyl alcohol to benzaldehyde by using pure BNMM are shown in Fig. 5. In order to investigate the gas-phase catalytic oxidation and the activity and selectivity of BN micromeshes, temperature was maintained at 240 °C (the boiling point of benzyl alcohol is 206 °C). From Fig. 5, it is clear that the pure BNMM catalysts exhibited an efficient catalytic activity (Fig. 5A), the highest selectivity (Fig. 5B) and better stability (with stable conversion for over 5 h) when the test was carried out at 240 °C with 21% O₂ and 79% N₂. As we can see, the highest conversion rate occurred during the first 0.5 h, and was decreased with the reaction time, but selectivity was nearly not changed throughout the whole process. To address the effect of various amounts of catalyst, three different samples (marked BNMM₁₀, BNMM₂₀ and BNMM₄₀) were studied. As shown in Fig. 5, pure BNMM₂₀ (20 mg) was the best catalyst amounts, it displayed a conversion rate of 37.85% and nearly 100% selectivity during the first 0.5 h, which was competitive material to porous carbon materials,³⁹ and more efficient than pure hexagonal mesoporous silica.⁴⁰ Then, the conversion rate was decreased with the reaction time, and the average conversion rate was 28.12% at 5 h, while selectivity was almost unchanged. The behavior trend of sample BNMM₁₀ was similar to that of BNMM₂₀. Besides that, BNNM₄₀ had the lowest average conversion rate (12.75%), but could also maintain a relatively stable conversion rate.

Fig. 5 Conversion (A) and selectivity (B) of benzyl alcohol as a function of reaction time at 240 °C. ((a) BNMM₁₀ with 10 mg, (b) BNMM₂₀ with 20 mg, (c) BNMM₄₀ with 40 mg).

The reproducibility of BNMM catalyst was also investigated. BNMM with 40 mg was chosen as study target and the related data were shown in Fig. 6A. In the first run, the conversion rate was about 33.34% during the first 0.5 h, and then decreased to 19.76% and finally maintained a relatively stable conversion rate (curve a in Fig. 6A). The second run (curve b in Fig. 6A) of the sample was taken after 7 days later, and the conversion rate still remained almost unchanged, about 12.75% on average. It is found that both in the first run and the second run, the selectivity of the composites is almost 100% (curve c and d in Fig. 6A).

Fig. 6 (A) Reproducibility (conversion curves a and b, selectivity curves c and d) of the oxidation of benzyl alcohol (BNMM with 40 mg). (B) Behavior of the oxidation of benzyl alcohol in the absence and presence of gaseous oxygen in 5 h at 240 °C (BNMM with 20 mg). [Conversion (e) and selectivity (g) of benzyl alcohol without O₂, conversion (f) and selectivity (h) of benzyl alcohol with O₂].

To understand the role of oxygen in the reaction, a contrast experiment was carried out. Fig. 6B shows the conversion rate and selectivity of BNMM (20 mg) in absence and presence of gaseous oxygen. It can be seen that benzaldehyde could be formed in the absence of gaseous oxygen, although the conversion rate was very low (6.65% at the first 0.5 h) and decreased continuously in 5 h. A high yield of benzaldehyde could not be detected because of the insufficient oxygen species. Thus, it is believed that the oxygen or related molecules adsorbed on the surfaces of BN may be largely responsible for the oxidation of benzyl alcohol.

It is well known that benzyl alcohol is hardly oxidated at 240 °C without an added catalyst. Zhi et al. reported that many molecules could be attached to BN materials due to the defect sites (such as N-sites and B-sites).¹² Shrinwantu et al. stated that the inherent electron deficiency of boron in BN materials might let BN materials act as Lewis acids.⁴¹ Therefore, it is thought that the BNMM play a significant role in the present reaction. In addition, BN materials with micro- and nano-sizes could act as efficient and economic (recycled) catalysts, indicating their potential applications in the catalytic reactions of alcohols.

Typical XRD pattern and TEM images of Ag/BNMM are shown in Fig. 7. In Fig. 7a, the diffraction peaks can be indexed to be h-BN (JCPDS card no. 34-0421) and cubic Ag (JCPDS card no. 65-2871), with d-spacing of 0.2359, 0.2043, 0.1445 and 0.1232 nm, corresponding to the diffraction crystal-planes of (111), (200), (220) and (311) from cubic Ag. From Fig. 7b, we can see that a large portion of Ag particles with sizes of 50–200 nm were decorated on the h-BN micromeshes. In Fig. 7d the clear rings with an average spacing of 0.2365 nm correspond to the (111) lattice spacings of a cubic Ag crystal. The removal of CO by catalytic oxidation has a great practical significance in many industries, such as air purification and hydrogen fuel cells.

Fig. 7 (a) Typical XRD pattern of Ag/BNMM [● and ○ indicate the diffraction peaks from h-BN (JCPDS card no. 34-0421) and Ag (JCPDS card no. 65-2841), respectively]. (b) Typical TEM image of the 10% Ag/BNMM composites, inset is its SAED pattern. (c) Typical TEM image of 1% Ag/BNMM composites. (d) HRTEM image taken from part of a Ag nanoparticle.

The catalytic activity curve over Ag/BNNM catalyst was depicted in Fig. 8. It is found that the Ag/BNNM catalyst was inactive under 225 °C, but when the temperature was above 250 °C, the CO conversion rate was increased rapidly with increasing temperature. In Fig. 8A, the 15% Ag/BNMM composites reached 70.50% conversion rate, which is much higher than that of 1% Ag/BNMM sample (the maximum conversion rate is 15.79% at 375 °C). Fig. 7b and c show typical TEM images of 10% Ag/BNMM composites and 1% Ag/BNMM composites. Compared to Fig. 7c, silver particles of 10% Ag/BNMM were dispersed more uniformly and the particles size was smaller. In Fig. 7c, it is found that the silver grains were aggregated to some extent. Therefore, we tentatively assumed that the amounts of loading and sizes of silver grains might be one of the main influencing factors. The maximum CO oxidation conversion rate is 70.50% when temperature is about 450 °C. As the reaction temperature continues to rise, the catalyst shows an apparent decrease in activity. We also investigated the reproducibility of Ag/BNMM catalyst. 15% Ag/BNMM composites were chosen as the investigated target. As shown in Fig. 8B, 15% Ag/BNMM displays 70.50% conversion rate in the first run (curve a), while the same sample still has a conversion rate of about 54.8% and 43.2% after 7 and 15 days, respectively (see curve b and curve c in Fig. 8B). As can be seen, the Ag/BNMM catalysts present effective catalytic activity in translating CO to CO₂ within 15 days, with only about 10–15% decrease in catalytic activity. It is thought that the decreasing activity of Ag nanopatricles and the adsorption of other molecules might be the main influencing factors. The Ag/BNMM catalyst was demonstrated to have relative high stability in Fig. 8B, where curves d, e and f all show a stable conversion rate within 10 h. As stability is another important factor for the evaluation of the catalyst, it is believed that the as-obtained BNNM can be a good candidate for catalyst support (such as Ag, Au and other metal nanoparticles) at a relatively low cost under high-temperature conditions.⁴²

Fig. 8 (A) CO oxidation curve of (a) 1% Ag/BNMM composites, (b) 5% Ag/BNMM composites, (c) 10% Ag/BNMM composites and (d) 15% Ag/BNMM composites. (B) The reproducibility (curve a, b and c) and stability (inset illustration, curve d, e and f) of CO oxidation by 15% Ag/BNMM composites after 1 day, 7 days and 15 days, respectively.

4. Conclusions

In summary, crystalline h-BN micromeshes with sizes of up to 100 μm and an average pore size of ∼2.5 μm were synthesized using Li₂B₄O₇, NaN₃ and Mg powder as reactants at 500 °C for 12 h. The use of Mg powder and the temperature played important roles for their formation. The TGA and TMA curves indicated that these h-BN micromeshes have high thermal and thermomechanical stability, which can be potentially used as ceramic materials additives. These hexagonal BN micromeshes were first applied to the oxidation of benzyl alcohol and they performed efficiently and economically. A novel micromesh-structured silver catalyst Ag/BNMM was prepared through the wet chemical method, which was effective for a low-cost application in the oxidation of carbon monoxide at relatively high temperature.

Acknowledgements

This work was supported by the National Nature Science Fond of China and the Academy of Sciences large apparatus United Fond (Nos. 20971079 and 11179043), and the 973 Project of China (No. 2011CB935901). The authors thank Prof. Xiaohong Xu for help in testing the catalytic oxidation of benzyl alcohol and CO.

Notes and references

1. R. T. Paine and C. K. Narula, Chem. Rev., 1990, 90, 73–91.
2. H. Zhang, Y. J. Chen, J. H. Ma, H. X. Tong, J. Yang, D. W. Ni, H. M. Hu and F. Q. Zheng, J. Alloys Compd., 2011, 509, 6616–6620.
3. F. Iskandar, S. G. Kim, A. B. D. Nandiyanto, Y. Kaihatsu, T. Ogi and K. Okuyama, J. Alloys Compd., 2009, 471, 166–171.
4. I. V. Povstugar, A. N. Streletskii, D. G. Permenov, I. V. Kolbanev and S. N. Mudretsova, J. Alloys Compd., 2009, 483, 298–301.
5. M. H. Li, L. Q. Xu, C. H. Sun, Z. C. Ju and Y. T. Qian, J. Mater. Chem., 2009, 19, 8086–8091.
6. O. Bunk, M. Corso, D. Martoccia, R. Herger, P. R. Willmott, B. D. Patterson, J. Osterwalder, J. F. van der Veen and T. Greber, Surf. Sci., 2007, 601, L7–L10.
7. R. Widmer, S. Berner, O. Groning, T. Brugger, J. Osterwalder and T. Greber, Electrochem. Commun., 2007, 9, 2484–2488.
8. M. Corso, W. Auwarter, M. Muntwiler, A. Tamai, T. Greber and J. Osterwalder, Science, 2004, 303, 217–220.
9. L. C. Wang, L. Q. Xu, C. H. Sun and Y. T. Qian, J. Mater. Chem., 2009, 19, 1989–1994.
10. X. L. Meng, N. Lun, Y. Q. Qi, J. Q. Bi, Y. X. Qi, H. L. Zhu, F. D. Han, Y. J. Bai, L. W. Yin and R. H. Fan, Eur. J. Inorg. Chem., 2010, 20, 3174–3178.
11. C. Y. Zhi, Y. Bando, C. C. Tang, S. Honda, K. Sato, H. Kuwahara and D. Golberg, Angew. Chem., Int. Ed., 2005, 44, 7929–7932.
12. C. Y. Zhi, Y. Bando, C. C. Tang, Q. Huang and D. Golberg, J. Mater. Chem., 2008, 18, 3900–3908.
13. H. Dil, J. L. Checa, R. Laskowski, P. Blaha, S. Berner, J. Osterwalder and T. Greber, Science, 2008, 319, 1824–1826.
14. A. Goriachko, A. A. Zakharov and H. Over, J. Phys. Chem. C, 2008, 112, 10423–10427.
15. M. Corso, T. Greber and J. Osterwalder, Surf. Sci., 2005, 577, L78–L84.
16. S. Berner, M. Corso, R. Widmer, O. Groening, R. Laskowski, P. Blaha, K. Schwarz, A. Goriachko, H. Over, S. Gsell, M. Schreck, H. Sachdev, T. Greber and J. Osterwalder, Angew. Chem., Int. Ed., 2007, 46, 5115–5119.
17. A. Goriachko, Y. B. He and H. Over, J. Phys. Chem. C, 2008, 112, 8147–8152.
18. R. J. Farrauto and R. M. Heck, Catal. Today, 2000, 55, 179–187.
19. F. I. Khan and A. K. Ghoshal, J. Loss Prev. Process Ind., 2000, 13, 527–545.
20. D. H. Kim, K. Shin and H. M. Lee, J. Phys. Chem. C, 2011, 115, 24771–24777.
21. Y. S. Sawayama, H. Shibahara, Y. Ichihashi, S. Nishiyama and S. Tsuruya, Ind. Eng. Chem. Res., 2006, 45, 8837–8845.
22. J. P. Mao, M. M. Deng, L. Chen, Y. Liu and Y. Lu, AIChE J., 2010, 56, 1545–1556.
23. J. B. Park, J. Graciani, J. Evans, D. Stacchiola, S. D. Senanayake, L. Barrio, P. Liu, J. F. Sanz, J. Hrbek and J. A. Rodriguez, J. Am. Chem. Soc., 2010, 132, 356–363.
24. D. Q. Han, T. T. Xu, J. X. Su, X. H. Xu and Y. Ding, ChemCatChem, 2010, 2, 383–386.
25. A. M. Alexander and J. S. J. Hargreaves, Chem. Soc. Rev., 2010, 39, 4388–4401.
26. G. Postole, A. Gervasini, M. Caldararu, B. Bonnetot and A. Auroux, Appl. Catal., A, 2007, 325, 227–236.
27. G. Postole, M. Caldararu, N. I. Ionescu, B. Bonnetot, A. Auroux and C. Guimon, Thermochim. Acta, 2005, 434, 150–157.
28. J. A. Perdigon-Melon, A. Auroux, J. M. Guil and B. Bonnetot, Stud. Surf. Sci. Catal., 2002, 143, 227–237.
29. G. Postole, A. Gervasini, C. Guimon, A. Auroux and B. Bonnetot, J. Phys. Chem. B, 2006, 110, 12572–12580.
30. S. K. Mohapatra, M. Misra, V. K. Mahajan and K. S. Raja, J. Phys. Chem. C, 2007, 111, 8677–8685.
31. V. Cholet, L. Vandenbulcke, J. P. Rouan, P. Baillif and R. Erre, J. Mater. Sci., 1994, 29, 1417–1435.
32. S. Sadananda, K. Stefan, I. Lubica, M. Jana, B. Imre and S. Janos, J. Eur. Ceram. Soc., 1998, 18, 1037–1043.
33. J. H. Ma, J. Li, G. X. Li, Y. G. Tian, J. Zhang, J. F. Wu, J. Y. Zheng, H. M. Zhuang and T. H. Pan, Mater. Res. Bull., 2007, 42, 982–988.
34. L. Q. Xu, J. H. Zhan, J. Q. Hu, Y. Bando, X. L. Yuan, T. Sekiguchi, M. Mitome and D. Golberg, Adv. Mater., 2007, 19, 2141–2144.
35. S. Duperrier, R. Chiriac, C. Sigala, C. Gervais, S. Bernard, D. Cornu and P. Miele, J. Eur. Ceram. Soc., 2009, 29, 851–855.
36. J. Li, S. Bernard, V. Salles, C. Gervais and P. Miele, Chem. Mater., 2010, 22, 2010–2019.
37. D. Ye, D. C. Jia, Z. H. Yang, Z. L. Sun and P. F. Zhang, J. Alloys Compd., 2010, 506, 88–92.
38. D. Golberg, Y. Bando, C. C. Tang and C. Y. Zhi, Adv. Mater., 2007, 19, 2413–2432.
39. G. Liu, Y. Liu, X. Y. Zhang, X. L. Yuan, M. Zhang, W. X. Zhang and M. J. Jia, J. Colloid Interface Sci., 2010, 342, 467–473.
40. L. H. Jia, S. Zhang, F. N. Gu, Y. Ping, X. F. Guo, Z. Y. Zhong and F. B. Su, Microporous Mesoporous Mater., 2012, 149, 158–165.
41. P. Shrinwantu, S. R. C. Vivekchand, A. Govindaraj and C. N. R. Rao, J. Mater. Chem., 2007, 17, 450–452.
42. L. Q. Xu, S. L. Li, Y. X. Zhang and Y. J. Zhai, Nanoscale, 2012, 4, 4900–4915.

---