This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes.
Article Cite This: ACS Omega 2018, 3, 10843−10850
http://pubs.acs.org/journal/acsodf
Ultrafine Pd Nanoparticles Anchored on Nitrogen-Doping Carbon for Boosting Catalytic Transfer Hydrogenation of Nitroarenes Longkang Zhang,† Xiaotong Liu,† Xin Zhou, Shutao Gao, Ningzhao Shang,* Cheng Feng, and Chun Wang* College of Science, Hebei Agricultural University, Baoding 071001, China
ACS Omega 2018.3:10843-10850. Downloaded from pubs.acs.org by 95.85.80.15 on 09/08/18. For personal use only.
S Supporting Information *
ABSTRACT: The catalytic performance of metal particles is closely related to the particle size. In this article, ultrafine palladium nanoparticles anchored on nitrogendoping carbon support (Pd/N-XC72R) were fabricated, wherein the N-XC72R was prepared through low-temperature annealing of Vulcan XC72R carbon with urea at 300 °C. Nitrogen dopant on the surface of carbon support can remarkably strengthen the affinity of the metal nanoparticles onto the support. Compared with the Vulcan XC-72R-supported Pd catalyst, the prepared Pd/N-XC72R delivered superior catalytic activity for the transfer hydrogenation of nitroarenes with formic acid as the hydrogen donor at ambient temperature. Our strategy may provide an effective and feasible approach to fabricate N-functionalized carbon materials and construct high-performance ultrasmall metal nanoparticle heterogeneous catalysts.
1. INTRODUCTION Supported metal catalysts have attracted considerable attention in the field of heterogeneous catalysis.1 Recent research studies have demonstrated that the catalytic performance of the metal nanoparticles go hand in hand with the size of the metal particles.2 Usually, ultrafine metal nanoparticles deliver better catalytic properties than bulk metals. Because of the high ratio of surface to volume and large fraction of surface atoms, ultrafine metal nanoparticles provide large amount of active center available for catalysis and provide the surface atoms with high activity, thus significantly improving the atom efficiency and reducing the dosage of precious-metal catalysts.3−6 However, the high surface energy of metal nanoparticles may cause serious aggregation of metal nanoparticles because of Oswald ripening, which can result in the loss of their catalytic performance and reusability.7 Therefore, it is still a challenging work to prepare the size-controlled metal nanoparticles. To date, some synthetic routes have been developed for the preparation of ultrafine metal nanoparticle catalysts, such as utilizing the confinement effect of the supports, strengthening the interactions between the metal nanoparticles and support, and using excess capping agents. For example, a simple nonnoble metal sacrificial approach was developed to prepare highly dispersed AgPd anchored on reduced graphene oxide, which exhibited superior catalytic behavior for the dehydrogenation of formic acid (FA)8 and transfer hydrogenation of nitroarenes.9 Hokenek et al. prepared monodispersed Pd particles using a one-pot polyol method, which were supported onto silica and employed for the catalytic decomposition of methanol.10 Xu and co-workers using a double-solvent way successfully anchored both mono- and bimetallic nanoparticles in the pores of metal−organic frameworks.11 Catalysts © 2018 American Chemical Society
obtained by this method delivered high-catalytic performance as well as excellent durability for CO oxidation reaction. However, this method is only suitable for porous supports with hydrophilic pores, which restrict its applications widely.11 Kempe et al. reported a chemical vapor deposition technique, in which Ni/Pd precursor was introduced into the pores of the carrier by sublimation. The prepared catalyst has a high catalytic activity for the hydrogenation of dialkyl ketones.12 Despite recent progress, a facile and general method for the preparation of ultrafine metal nanoparticle catalysts is still highly desirable and a challenging task. Nitrogen (N)-doped carbon has emerged as a promising catalyst support and found extensive applications in heterogeneous catalysis.13 The incorporation of nitrogen into the carbon architecture can improve the wetting properties of the supports, which is desirable for the metal deposition in solution, for example, by impregnation.14,15 Moreover, nitrogen dopant can act as coordination sites for anchoring of metal nanoparticles, which can stabilize the small-size metal nanoparticles.16 Furthermore, the electronic structure of the nitrogen doping carbon can be altered owing to the doped nitrogen and thus further enhance their catalytic performance. Generally, nitrided carbons can be prepared through arcdischarge/vaporization process, chemical vapor deposition, and plasma treatment under NH3 atmosphere or in the presence of other nitrogen sources.17 Recently, ultrasmall noble metal nanoclusters were prepared with nitrogenfunctionalized porous carbon as a support, which was obtained Received: May 26, 2018 Accepted: August 28, 2018 Published: September 7, 2018 10843
DOI: 10.1021/acsomega.8b01141 ACS Omega 2018, 3, 10843−10850
ACS Omega
Article
2. RESULTS AND DISCUSSION Nitrogen-doped carbon was prepared by one-step lowtemperature pyrolysis of Vulcan XC72R carbon with urea at 300 °C. Nitrogen-containing species can be incorporated into the carbon architecture via the reaction with HCNO and NH3 produced from thermal decomposition of urea,18 which can act as basic coordination sites to strengthen the affinity of metal nanoparticles onto the carbon support and stabilize the highly dispersed metal nanoparticles. 2.1. Characterization of Catalysts. Transmission electron microscopy (TEM) and scanning TEM (STEM) were used to investigate the structure and morphology of the prepared catalysts. Figure 1a,c reveals that well-dispersed Pd nanoparticles are homogeneously distributed throughout the nitride carbon support. In addition, the lattice spacing is measured to be 0.22 nm (Figure 1b), which is attributed to the (111) plane of face-centered cubic (fcc) Pd (0.224 nm).26 The mean size of Pd nanoparticles in the Pd/N-XC72R-6-300 sample was about 2.5 nm (Figure 1d). In contrast, Pd particles with a mean size of 5.3 nm are observed in the Pd/XC72R catalyst. Moreover, the particle size of Pd in Pd/N-XC72R-2300 and Pd/N-XC72R-6-200 was significantly smaller than that in Pd/XC72R (Figure S1, Supporting Information). Furthermore, the STEM energy-dispersive X-ray (EDX) mapping indicates the presence of nitrogen and welldistributed Pd in the carrier (Figure 1e−h). The results revealed that the nitrogen doping of the carbon support played a key role for the highly dispersed Pd nanoparticles; the nitrogen-containing species can stabilize the small-size metal nanoparticles. To understand more about the structure and surface properties of Pd/N-XC72R-6-300 catalyst, we studied the Xray photoelectron spectroscopy (XPS), and the spectra are described in Figure 2. The XPS survey spectrum of Pd/NXC72R-6-300 catalyst indicated that the catalysts consist of mainly N, C, O, and Pd. Binding energy (BE) calibration of the XPS spectra was carried out with the help of the graphitic sp2 signal located at 285 eV. A stronger peak was located at 289 eV for Pd/N-XC72R-6-300, corresponding to a higher proportion of sp3 carbon with C−N and C−O bonds (Figure 2b).27 The N-XC72R-6-300 has a large nitrogen percentage of 12% and shows a three-state doublet corresponding to XPS spectra. Three nitrogen species located at 398.7, 400.0, and 401.3 eV are detected in the Pd/N-XC72R-6-300 (Figure 2c). The maximum BE of the n atom is 400.0 eV, which are assigned to amine and amide groups, for example, C−NH and C−NH2. The peaks at 401.3 and 398.7 eV can be attributed to the graphitic N and pyridine N atoms, respectively.18,28 Respectively, the Pd 3d5/2 and 3d3/2 peaks with a BE of 336.6 and 341.9 eV are attributed to Pd0 (metallic palladium), whereas the BE of 338.3 and 343.9 eV are related to Pd2+ (palladium oxide). The XPS data indicated that 70% Pd on the surface of the catalyst existed as Pd0, which can be attributed to the fact that the incorporation of amine groups in the carrier can prevent the oxidation of Pd0 in air atmosphere.29−31 In addition, the higher BE of Pd0 in Pd/N-XC72R-6-300 sample at 336.6 and 341.9 eV than that in Pd/XC72R at 336.2 and 341.5 eV (Figure S2) indicates the negative-charged Pd, which can be ascribed to the strong electronic coupling between Pd and N-XC72R-6-300 support.32 Compared with the X-ray diffraction (XRD) images of XC72R, N-XC72R-2-300, N-XC72R-4-300, and N-XC72R-6-
by two-step low-temperature annealing with urea. The prepared catalysts showed superior catalytic behavior and recyclability for the methanol oxidation,18 dehydrogenation of FA,19 and selective oxidation of styrene.20 However, these strategies often suffer from various problems, such as tedious, rigorous, and multistep preparative processes, requiring high temperature or long pyrolysis time. Of special note, aromatic amine derivatives are important chemical intermediate for the preparation of azo dyes, pharmaceuticals, pigments, and agrochemicals.21 The conventional route for the production of aniline is the reduction of aromatic nitro compounds.22 Tradition catalysts, such as Raney Ni, mostly use hydrogen as a hydrogen source. However, the application of this technology has been limited because of the demand for specialized equipment and safety issues related to H2 storage, transportation, and handling. Using Fe and HCl as a reductant may generate large amounts of waste acids and toxic residues, which lead to serious environmental problems.23 Pd metals deliver high catalytic activity for hydrogenation of nitroarenes.24 Catalytic transfer hydrogenation (CTH) of hydrogen donor nitro compounds with hydrogen donor is an environmentally friendly and sustainable way, which avoids the use of dangerous H2. Among the hydrogen sources available for CTH, formic acid (FA), a major byproduct of biomass processing, has been considered as a safe, abundant, sustainable, and promising hydrogen source because of its nontoxicity, excellent stability, and high energy density and can be regenerated by the hydrogenation of CO2.15 During the past few years, CTH with FA as a hydrogen donor has attracted considerable interest.25 Here, a simple and effective method was developed for preparing nitrogen-functionalized carbon (N-XC72R) through one-step low-temperature pyrolysis of commercial available Vulcan XC72R with urea at 300 °C. The catalyst preparation process was illustrated in Scheme 1. Vulcan XC72R possesses Scheme 1. Preparation Progress of Pd/N-XC72R
large specific surface area and high porosity, which make it an ideal candidate for nitrogen doping. Well-dispersed and ultrafine Pd nanoparticles were immobilized onto the NXC72R support via a wet chemical reduction method, which displayed superior catalytic activity and stability for the CTH of nitrocompounds at room temperature with FA as the hydrogen donor. 10844
DOI: 10.1021/acsomega.8b01141 ACS Omega 2018, 3, 10843−10850
ACS Omega
Article
Figure 1. TEM (a,b) and STEM images (c) of Pd/N-XC72R-6-300 catalyst and the corresponding size distribution of Pd nanoparticles (d), EDX mappings of Pd/N-XC72R-6-300: Pd, N, O, and C (e−h).
Figure 2. XPS data of Pd/N-XC72R-6-300 catalyst. Survey spectrum (a), high-resolution spectrum of C 1s (b), N 1s (c), and Pd 3d (d).
Figure 3. (a) XRD images of different supports, (b) FT-IR spectra of XC72R (top) and N-XC72R-6-300 (bottom), (c,d) XRD images and the N2 adsorption−desorption isotherms of XC72R, Pd/XC72R, NXC72R-6-300, and Pd/N-XC72R-6-300.
400, a new sharp peak located at 2θ = 27.8° appeared for NXC72R-6-200, N-XC72R-6-300, and N-XC72R-8-300 samples (Figures 3a and S3), which is corresponded to the (002) interlayer stacking of nitrided graphite (g-C3N4). For NXC72R-6-400 sample, the peak at 2θ = 27.8° disappeared because the graphite nitride can be decomposed with the calcination temperature increasing to 400 °C.19,33 Fourier transform infrared spectra revealed that there are two peaks at 3360 and 1509 cm−1 (Figure 3b), which can be attributed to the stretching and bending of H−N, respectively. The strong peaks at 1731 cm−1 are assigned to the stretching of CO, which is originated from thermolysis products of urea, that is, HNCO.19,34 It is notable that no obvious diffraction peak for Pd can be detected at 2θ ≈ 40° for Pd/N-XC72R-6-300 sample, indicating the well-dispersity of Pd nanoparticles in the catalyst. In contrast, the XRD pattern of Pd/XC72R sample revealed distinct diffraction peaks at 2θ = 40.1° because of the (111) reflection of Pd, which confirms the formation of much larger Pd nanoparticles (Figure 3c). These observations are consistent with the TEM results (Figures 1 and S1). The
surface areas and pore structures of the materials were measured by N2 sorption measurements. Surface areas of Pd/N-XC72R-6-300, N-XC72R-6-300, Pd/XC72R, and XC72R were 106, 61, 207, and 253 m2 g−1, respectively (Figure 3d). Compared with the XC72R, the appreciable decrease of the Brunauer−Emmett−Teller surface area of NXC72R-6-300 indicates a large number of thermal decomposition products of urea, such as ammeline, ammelide, and cyanuric acid, successfully dispersed inside the pores of XC72R. During the preparation of Pd/N-XC72R-6-300 catalyst, the N-XC72R-6-300 carrier was treated with sodium hydroxide solution, some of the decomposition products of urea in the N-XC72R-6-300 will be dissolved, which results that the specific surface area of Pd/N-XC72R-6-300 is higher than that of N-XC72R-6-300. 2.2. The CTH of nitroarenes. To investigate the performance of the catalysts, the hydrogenation reaction of nitrobenzene was carried out at 25 °C with ethanol−water 10845
DOI: 10.1021/acsomega.8b01141 ACS Omega 2018, 3, 10843−10850
ACS Omega
Article
Table 1. Screening of Catalystsa entry
catalysts
solvents
time (min)
1 2 3 4 5 6 7 8 9 10 11 12 13 14
XR72R N-XR72R Pd/XR72R Pd/N-XR72R-6-200 2.6 nm Pd/XR72R Pd/N-XR72R-6-300 Pd/N-XR72R-6-400 Pd/N-XR72R-2-300 Pd/N-XR72R-4-300 Pd/N-XR72R-8-300 Pd/N-XR72R-6-300 Pd/N-XR72R-6-300 Pd/N-XR72R-6-300 Pd/N-XR72R-6-300
EtOH/water (4:1) EtOH/water (4:1) EtOH/water (4:1) EtOH/water (4:1) EtOH/water (4:1) EtOH/water (4:1) EtOH/water (4:1) EtOH/water (4:1) EtOH/water (4:1) EtOH/water (4:1) EtOH MeOH MeOH/water (4:1) water
90 90 180 60 60 10 60 30 25 12 45 45 20 30
conversion (%)
selectivity (%)
TOF [h−1]
77 99 99 99 99 99 99 99 96 94 99 99
0 0 10.8 190 190 1200 200 400 500 1000 256 250.7 600 400
21 (29)b 95 96 99 99 99 99 99 99 99 99 99
Reaction conditions: AF (4 mmol), nitrobenzene (1 mmol), catalyst (0.5 mol %), FA (4 mmol), 25 °C. b16 h.
a
literature for the CTH of nitrobenzene (Table S2). With further increase in the amount of FA or AF, no obvious improvement can be observed (Table S1, entries 4−5). When using FA/HCOONa or FA/HCOOK as the hydrogen source, longer reaction time was required (Table S1, entries 6−7). In addition, the effect of the amount of catalyst was also discussed. When the amount of catalyst was increased from 0.5 to 1 mol %, 99% conversion of nitrobenzene was obtained after 4 min (Table S1, entry 8). Conversion (59%) of nitrobenzene was obtained after 90 min with decreasing the amount of catalyst to 0.25 mol % (Table S1, entry 9). The effect of reaction solvent was investigated also (Table 1, entries 11−15). When methanol and ethanol were used as the solvent, the desired product was obtained after 45 min reaction (Table 1, entries 11−12). We were glad to find that the reaction occurred efficiently in 10 min with 99% conversion and selectivity using ethanol−water (4:1, v/v) as the solvent, which was superior than that with water and methanol−water (4:1, v/v) as the solvent (Table 1, entries 13−14). The stability and recyclability of the catalyst are important for their practical application. The catalytic performance of Pd/N-XC72R-6-300 was well-retained after five consecutive runs, demonstrating the reusability of the catalyst. In contrast, the catalytic performance of the recycled Pd/XC72R decreased significantly (Figure S4). In addition, the TEM image of the used catalyst revealed that the size and morphology of palladium nanoparticles did not change significantly after five runs (Figure S5), verifying that the N-functionalized carbon prepared by the one-step low-temperature pyrolysis can tremendously enhance the stability of metal nanoparticles. In addition, the XRD test results indicated that the XRD pattern of the fresh and the reused one has no significant difference up to five runs (Figure S6). Furthermore, a filtration test was also performed after 5 min of reaction, the Pd/N-XC72R-6-300 catalyst was separated, and the solution continuously reacted at room temperature for 16 min. The conversion did not increase further even after 30 min of reaction under identical conditions (Figure S7), indicating that the metal was firmly attached to the carrier and the Pd/N-XC72R-6-300 possessed a truly heterogeneous nature. The above results suggested that the Pd/N-XC72R-6-300 was stable enough under the reaction conditions. Under optimal reaction conditions, a series of nitrosubstrates was applied to expand the reaction over Pd/N-
(4:1: v/v) as the solvent and FA and ammonium formate (AF) as the hydrogen source. In the blank test, no conversion can be observed with N-XR72R and XR72R as the catalyst (Table 1, entries 1−2), suggesting that Pd nanoparticles play a decisive role in the hydrogenation reaction. For the Pd/XR72R catalyst, the nitrobenzene conversion was only 21% after 3 h reaction under the same reaction conditions (Table 1, entry 3). To our surprise, Pd/N-XC72R-6-300 with 2.5 nm size Pd revealed excellent catalytic performance for the transfer hydrogenation of nitrobenzene, 99% of conversion and selectivity were obtained after 10 min at room temperature (Table 1, entry 4). To clarify the effect of N, Pd/XR72R sample with 2.6 nm size was prepared with sodium citrate as a stabilizing agent according to a reported procedure.35 The catalytic activity of Pd/XR72R with 2.6 nm size was much higher than that of Pd/ XR72R with 5.3 nm size (Table 1, entries 3, 5); however, it is still lower than that of Pd/N-XC72R-6-300 with 2.5 nm size. The results showed that as the size of the metal nanoparticles decreases, the catalytic activity increases significantly because smaller particles have more active sites available for the reactants. Meanwhile, the interaction of metal nanoparticle and the support also play an important role for the catalytic performance. The calcination temperature and dosage of urea may also affect the type and amount of nitrogen-containing species in the support. When the calcination temperature was 200 and 400 °C, the catalytic activity of the corresponding catalysts, Pd/N-XC72R-6-200 and Pd/N-XC72R-6-400, decreased significantly, the reaction time required about 60 min to complete the reaction (Table 1, entries 4, 7). When the mass ratio of urea and XC-72 carbon was 2:1, 4:1, 6:1, and 8:1, the reaction can be completed after 30, 25, 10, and 12 min (Table 1, entries 6, 8−10), respectively, which revealed that the mass ratio of 6:1 was enough to achieve the high activity of the catalyst. The influence of hydrogen source and catalyst amount on the CTH catalyzed by Pd/N-XC72R-6-300 at 25 °C was also investigated (Table S1, Supporting Information). Employed FA or AF were used as the hydrogen source; the conversion of nitrobenzene was 79 and 73% after 90 min, respectively (Table S1, entries 1−2). When FA/AF at 1:1 (molar ratio) was used (Table S1, entry 3), 99% aniline was yielded with excellent selectivity at 25 °C after 10 min reaction. The TOF value was 1200 mol mol−1 Pd h−1 (Table 1, entry 6), surpassing most of the heterogeneous catalysts that have been reported in the 10846
DOI: 10.1021/acsomega.8b01141 ACS Omega 2018, 3, 10843−10850
ACS Omega
Article
Table 2. Series of Substrates Reducing to the Corresponding Aminea
Substrate 1.0 mmol, cat (0.5 mol %), EtOH/H2O (v/v = 4:1) (5 mL), FA (4 mmol), HCOONH4 (4 mmol) 25 °C, n-decane as an internal standard for GC analysis. b50 °C. a
13, 14), which indicated that obviously steric hindrance in the reactant can be tolerated by the catalytic system. 2.3. Kinetic Study. Kinetic of the CTH of nitrobenzene was studied over Pd/N-XC72R-6-300 catalyst. The kinetic data were collected under the 0, 15, and 25 °C. The apparent rate constant (k) can be defined as the following eq 1, where k is the rate constant, Ct is the nitrobenzene concentration in time, C0 is the initial nitrobenzene concentration, and t is the reaction time.
XC72R-6-300 catalyst. Different nitro-compounds are converted to the anilines with high yields under mild conditions. In addition, various electron donor substituents attached to the aromatic ring, such as p-amino, o-methyl, p-methyl, p-hydroxyl, and o-hydroxyl, were able to be reduced to corresponding amines with high yield (99%) after 15 min (Table 2, entries 2− 6). The rate of reaction was affected slightly by the steric hindrance (Table 2, entries 3,6). What is more, p-nitrobenzene and p-nitroanisole can be completely reduced to the relevant anilines by prolonging the reaction time to 60 and 30 min with high yield (99%) (Table 2, entries 7, 8). However, the electron-withdrawing substituents of ketone made the nitroarene reduction a little bit slow (Table 2 entries 9, 10). For the hydrogenation of 1-nitronaphthalene and o-nitroacetophenone, the yields of the product were only 63 and 66%, respectively, even after 180 min because of the space steric hindrance (Table 2, entries 11−12). When the temperature rose to 50 °C, the hydrogenation reaction of o-nitroacetophenone and 1nitronaphthalene can be completed with 99% conversation and selectivity within 30 and 120 min, respectively (Table 2, entry
ln
Ct = −kt C0
k = A e−Ea / RT
i E y1 ln k = jjj− a zzz + ln A k R {T
(1) (2)
(3)
The k increases with the increase of temperature, as shown in Figure S7. According to the Arrhenius equation (eqs 2 and 3), the apparent activation energies of the CTH of nitrobenzene on Pd/N-XC72R-6-300 are calculated to be 52.6 kJ· 10847
DOI: 10.1021/acsomega.8b01141 ACS Omega 2018, 3, 10843−10850
ACS Omega
Article
mol−1, the relative lower activation energy makes the reaction be conducted at lower temperatures.36 2.4. Plausible Mechanism for the Hydrogenation of Nitrobenzene. According to the reported documents, the hydrogenation of nitrobenzene is generally conducted via two routes.37,38 One way is to directly reduce the nitro group through hydroxylamine and nitrosoarene as the intermediates. The other one is performed via azobenzene and 1,2-diphenylhydrazine as the intermediates. To gain insight into the possible mechanism of the CTH of nitrobenzene catalyzed by Pd/N-XC72R, the CTH of azobenzene was conducted under the identical conditions. The results indicated that the conversion of azobenzene was much slower than that of nitrobenzene, which excluded the possibility route via azobenzene as the intermediate for the Pd/ N-XC72R system. Therefore, it is exactly said that the transfer hydrogenation of nitrobenzene was performed through the direct reduction route.38 No nitrosoarene and hydroxylamine intermediates can be detected during the reaction process (based on GC analysis), which can be ascribed to that the reactions were conducted rapidly, and so the intermediates cannot be detected. Figure 4 shows the possible reaction
4. EXPERIMENTAL SECTION 4.1. Synthesis of Pd/N-XC72R. Typically, 1.0 g of XC72R and 6 g of urea were physically grinded and then transferred to a ceramic crucible and heated in a muffle furnace at 300 °C for 6 h. After cooling to ambient temperature, the mixture was washed with water and ethanol for three times. The obtained solid was named as N-XC72R-6-300. A series of nitrided carbon, N-XC72R-X-T (X represents the mass ratio of urea and XC-72R; T stands for the pyrolysis temperature), was obtained through changing the calcined temperature and the mass ratio of urea and XC-72R. The Pd nanocatalyst supported on N-XC72R was synthesized by a facile wet chemical impregnation and reduction method. Typically, 100 mg of N-XC72R-6-300 was ultrasonically dispersed in 5.0 mL of water and then mixed with 4.38 mL of H2PdCl4 aqueous solution (2 mg mL−1, 0.049 mmol Pd). The resulted aqueous suspension was stirred for 1 h. Then, 30.0 mg of NaBH4 dissolved in 1.0 mL of 2.0 mol L−1 NaOH solution was added into the above suspension. The mixture was stirred for 1.5 h at room temperature. The obtained catalyst was named as Pd/N-XC72R-6-300. A series of catalysts, Pd/N-XC72R-X-T and Pd/XC72R, were prepared by the same method except that the corresponding support was used. The actual palladium concentration was measured by inductively coupled plasma atomic emission spectrometry. 4.2. Catalytic Reduction of Nitro-Compounds. A mixture of AF (4 mmol), nitro compound (1 mmol), catalyst (0.5 mol % Pd), FA (4 mmol), and solvent (5 mL, EtOH/H2O = 4:1) was mixed. The mixture was stirred at 25 °C for 10− 180 min (Table 2). The reaction process was monitored by TLC. After completion of the reaction, the products were extracted by dichloromethane (3 × 5 mL). The structure of the products was confirmed by GC-MS.
■
Figure 4. Mechanism of hydrogenation of nitroaromatics over Pd/NXC72R.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b01141.
mechanism; first, the catalytic sites that are Pd nanoparticles, activated HCOOH to yield Pd−H species, then the hydrogenation of nitrobenzene was performed by depleting six hydrogen atoms bonded on the surface of the Pd/N-XC72R catalyst. The nitrogen-containing species in the N-XC72R may act as base sites, which may facilitate the adsorption of the nitrobenzene and the corresponding oxygen-containing intermediate molecules onto the surface of N-XC72R via the electrostatic interaction between the electron-deficient nitrogen in nitrobenzene and the electron-rich base sites in NXC72R.
3. CONCLUSIONS In summary, a facile and efficient process was established to prepare nitrogen doping carbon by a one-step low-temperature pyrolysis of commercially available carbon with urea. The nitrogen-containing species incorporated into the carbon architecture enhanced the affinity of metal nanoparticles onto the carbon support, thus preventing aggregation and overgrowth of nanocrystals. Highly dispersed Pd supported on NXC72R-6-300 showed excellent catalytic behavior for the CTH of nitroarenes. Our strategy may provide a promising alternative approach to fabricate nitrogen-doped carbon and ultrasmall metal nanoparticles catalyst, which may find extensive applications in heterogeneous catalysis.
■
Chemical raw materials and products; characterization instrument; Tof calculation; TEM images of Pd/XC72R catalyst; Pd XPS spectra of Pd/N-XC72R-6-300 and Pd/ XC72R; XRD images of urea, XC72R, and N-XC72R-6300; durability test; TEM pattern of the reused Pd/NXC72R-6-300; XRD pattern of fresh and recycled Pd/NXC72R-6-300; filtration test for the reduction of nitrobenzene over Pd/N-XC72R-6-300; kinetic curves of the CTH of nitrobenzene at different reaction temperatures; Pd/N-XC72R-6-300 catalyst catalyzes the reduction of nitrobenzene to aniline; and various reported catalysts tested for reduction of nitroarenes into anilines (PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (N.S.). *E-mail:
[email protected] (C.W.). ORCID
Ningzhao Shang: 0000-0001-9070-0024 Chun Wang: 0000-0002-9366-3668 10848
DOI: 10.1021/acsomega.8b01141 ACS Omega 2018, 3, 10843−10850
ACS Omega
Article
Author Contributions
hydroxide nanoparticles on co,n-codoped carbon nanoframes as efficient bifunctional catalysts for rechargeable zinc-air batteries. Adv. Energy Mater. 2017, 7, 1700467. (15) Li, Z.; Xu, Q. Metal-nanoparticle-catalyzed hydrogen generation from formic acid. Acc. Chem. Res. 2017, 50, 1449−1458. (16) Choi, B.; Yoon, H.; Park, I.-S.; Jang, J.; Sung, Y.-E. Highly dispersed pt nanoparticles on nitrogen-doped magnetic carbon nanoparticles and their enhanced activity for methanol oxidation. Carbon 2007, 45, 2496−2501. (17) Li, M.; Xu, F.; Li, H.; Wang, Y. Nitrogen-doped porous carbon materials: Promising catalysts or catalyst supports for heterogeneous hydrogenation and oxidation. Catal. Sci. Technol. 2016, 6, 3670−3693. (18) Liu, B.; Yao, H.; Song, W.; Jin, L.; Mosa, I. M.; Rusling, J. F.; Suib, S. L.; He, J. Ligand-Free Noble Metal Nanocluster Catalysts on Carbon Supports via ″Soft″ Nitriding. J. Am. Chem. Soc. 2016, 138, 4718−4721. (19) Li, Z.; Yang, X.; Tsumori, N.; Liu, Z.; Himeda, Y.; Autrey, T.; Xu, Q. Tandem nitrogen functionalization of porous carbon: Toward immobilizing highly active palladium nanoclusters for dehydrogenation of formic acid. ACS Catal. 2017, 7, 2720−2724. (20) Liu, B.; Wang, P.; Lopes, A.; Jin, L.; Zhong, W.; Pei, Y.; Suib, S. L.; He, J. Au-Carbon Electronic Interaction Mediated Selective Oxidation of Styrene. ACS Catal. 2017, 7, 3483−3488. (21) Grirrane, A.; Corma, A.; Garcia, H. Gold-catalyzed synthesis of aromatic azo compounds from anilines and nitroaromatics. Science 2008, 322, 1661−1664. (22) Shalom, M.; Molinari, V.; Esposito, D.; Clavel, G.; Ressnig, D.; Giordano, C.; Antonietti, M. Sponge-like nickel and nickel nitride structures for catalytic applications. Adv. Mater. 2014, 26, 1272−1276. (23) Blaser, H.-U.; Steiner, H.; Studer, M. Selective catalytic hydrogenation of functionalized nitroarenes: An update. ChemCatChem 2009, 1, 210−221. (24) Zhang, S.; Chang, C.-R.; Huang, Z.-Q.; Li, J.; Wu, Z.; Ma, Y.; Zhang, Z.; Wang, Y.; Qu, Y. High Catalytic Activity and Chemoselectivity of Sub-nanometric Pd Clusters on Porous Nanorods of CeO2 for Hydrogenation of Nitroarenes. J. Am. Chem. Soc. 2016, 138, 2629−2637. (25) Wu, F.-P.; Peng, J.-B.; Qi, X.; Wu, X. F. Palladium-catalyzed carbonylative sonogashira coupling of aryl diazonium salts with formic acid as the co source: The effect of 1,3-butadiene. Catal. Sci. Technol. 2017, 7, 4924−4928. (26) Xiong, Y.; McLellan, J. M.; Yin, Y.; Xia, Y. Synthesis of palladium icosahedra with twinned structure by blocking oxidative etching with citric acid or citrate ions. Angew. Chem. 2007, 119, 804− 808. (27) Li, X.; Wang, H.; Robinson, J. T.; Sanchez, H.; Diankov, G.; Dai, H. Simultaneous nitrogen doping and reduction of graphene oxide. J. Am. Chem. Soc. 2009, 131, 15939−15944. (28) Chizari, K.; Vena, A.; Laurentius, L.; Sundararaj, U. The effect of temperature on the morphology and chemical surface properties of nitrogen-doped carbon nanotubes. Carbon 2014, 68, 369−379. (29) Radkevich, V. Z.; Senko, T. L.; Wilson, K.; Grishenko, L. M.; Zaderko, A. N.; Diyuk, V. Y. The influence of surface functionalization of activated carbon on palladium dispersion and catalytic activity in hydrogen oxidation. Appl. Catal., A 2008, 335, 241−251. (30) Huo, J.; Johnson, R. L.; Duan, P.; Pham, H. N.; MendivelsoPerez, D.; Smith, E. A.; Datye, A. K.; Schmidt-Rohr, K.; Shanks, B. H. Stability of pd nanoparticles on carbon-coated supports under hydrothermal conditions. Catal. Sci. Technol. 2018, 8, 1151−1160. (31) Fan, H.; Huang, X.; Shang, L.; Cao, Y.; Zhao, Y.; Wu, L.-Z.; Tung, C.-H.; Yin, Y.; Zhang, T. Controllable synthesis of ultrathin transition-metal hydroxide nanosheets and their extended composite nanostructures for enhanced catalytic activity in the heck reaction. Angew. Chem. 2016, 55, 2167−2170. (32) Zhang, C.; Leng, Y.; Jiang, P.; Li, J.; Du, S. Immobilizing palladium nanoparticles on nitrogen-doped carbon for promotion of formic acid dehydrogenation and alkene hydrogenation. ChemistrySelect 2017, 2, 5469−5474.
†
These authors contributed equally to this work.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS Financial supports from the National Natural Science Foundation of China (21603054), the Natural Science Foundation of Hebei Province (B2015204003, B2016204131, and B2018204145), the Young Top-notch Talents Foundation of Hebei Provincial Universities (BJ2016027), and the Natural Science Foundation of Hebei Agricultural University (ZD201613, LG201709, and LG201711) are gratefully acknowledged.
■
REFERENCES
(1) Hutchings, G. J.; Kiely, C. J. Strategies for the synthesis of supported gold palladium nanoparticles with controlled morphology and composition. Acc. Chem. Res. 2013, 46, 1759−1772. (2) Campbell, C. T. PHYSICS: The Active Site in Nanoparticle Gold Catalysis. Science 2004, 306, 234−235. (3) Shang, L.; Bian, T.; Zhang, B.; Zhang, D.; Wu, L.-Z.; Tung, C.H.; Yin, Y.; Zhang, T. Graphene-supported ultrafine metal nanoparticles encapsulated by mesoporous silica: Robust catalysts for oxidation and reduction reactions. Angew. Chem., Int. Ed. 2014, 53, 250−254. (4) Zhu, Q.-L.; Xu, Q. Immobilization of ultrafine metal nanoparticles to high-surface-area materials and their catalytic applications. Chem 2016, 1, 220−245. (5) Dhakshinamoorthy, A.; Garcia, H. Catalysis by metal nanoparticles embedded on metal-organic frameworks. Chem. Soc. Rev. 2012, 41, 5262−5284. (6) Wu, B.; Kuang, Y.; Chen, X.; Chen, J. Noble metal nanoparticles/carbon nanotubes nanohybrids: Synthesis and applications. Nano Today 2011, 6, 75−90. (7) Oliveira, R. L.; Oliveira, C. S.; Landers, R.; Correia, C. R. D. Pd nanoparticles immobilized on graphene oxide/silica nanocomposite: Efficient and recyclable catalysts for cross-coupling reactions. ChemistrySelect 2018, 3, 535−543. (8) Chen, Y.; Zhu, Q.-L.; Tsumori, N.; Xu, Q. Immobilizing highly catalytically active noble metal nanoparticles on reduced graphene oxide: A non-noble metal sacrificial approach. J. Am. Chem. Soc. 2015, 137, 106−109. (9) Liu, X.; Wang, C.; Cheng, S.; Shang, N.; Gao, S.; Feng, C.; Wang, C.; Qiao, Y.; Wang, Z. Agpd nanoparticles supported on reduced graphene oxide: A high catalytic activity catalyst for the transfer hydrogenation of nitroarenes. Catal. Commun. 2018, 108, 103−107. (10) Hokenek, S.; Kuhn, J. N. Methanol decomposition over palladium particles supported on silica: Role of particle size and cofeeding carbon dioxide on the catalytic properties. ACS Catal. 2012, 2, 1013−1019. (11) Aijaz, A.; Karkamkar, A.; Choi, Y. J.; Tsumori, N.; Rönnebro, E.; Autrey, T.; Shioyama, H.; Xu, Q. Immobilizing highly catalytically active pt nanoparticles inside the pores of metal-organic framework: A double solvents approach. J. Am. Chem. Soc. 2012, 134, 13926− 13929. (12) Hermannsdörfer, J.; Friedrich, M.; Miyajima, N.; Albuquerque, R. Q.; Kümmel, S.; Kempe, R. Ni/pd@mil-101: Synergistic catalysis with cavity-conform ni/pd nanoparticles. Angew. Chem., Int. Ed. 2012, 51, 11473−11477. (13) He, L.; Weniger, F.; Neumann, H.; Beller, M. Synthesis, characterization, and application of metal nanoparticles supported on nitrogen-doped carbon: Catalysis beyond electrochemistry. Angew. Chem., Int. Ed. 2016, 55, 12582−12594. (14) Wang, Q.; Shang, L.; Shi, R.; Zhang, X.; Zhao, Y.; Waterhouse, G. I. N.; Wu, L.-Z.; Tung, C.-H.; Zhang, T. Nife layered double 10849
DOI: 10.1021/acsomega.8b01141 ACS Omega 2018, 3, 10843−10850
ACS Omega
Article
(33) Zhao, Q.; Hou, M.; Jiang, S.; Ai, J.; Zheng, L.; Shao, Z. Enhanced sulfur dioxide electrooxidation performance on a modified xc-72 carbon catalyst. J. Solid State Electrochem. 2017, 21, 3113−3120. (34) Schaber, P. M.; Colson, J.; Higgins, S.; Thielen, D.; Anspach, B.; Brauer, J. Thermal decomposition (pyrolysis) of urea in an open reaction vessel. Thermochim. Acta 2004, 424, 131−142. (35) Li, J.; Chen, W.; Zhao, H.; Zheng, X.; Wu, L.; Pan, H.; Zhu, J.; Chen, Y.; Lu, J. Size-dependent catalytic activity over carbonsupported palladium nanoparticles in dehydrogenation of formic acid. J. Catal. 2017, 352, 371−381. (36) Fountoulaki, S.; Daikopoulou, V.; Gkizis, P. L.; Tamiolakis, I.; Armatas, G. S.; Lykakis, I. N. Mechanistic studies of the reduction of nitroarenes by nabh4 or hydrosilanes catalyzed by supported gold nanoparticles. ACS Catal. 2014, 4, 3504−3511. (37) Tuteja, J.; Nishimura, S.; Ebitani, K. Base-free chemoselective transfer hydrogenation of nitroarenes to anilines with formic acid as hydrogen source by a reusable heterogeneous pd/zrp catalyst. RSC Adv. 2014, 4, 38241−38249. (38) Li, X.-H.; Cai, Y.-Y.; Gong, L.-H.; Fu, W.; Wang, K.-X.; Bao, H.L.; Wei, X.; Chen, J.-S. Photochemically engineering the metalsemiconductor interface for room-temperature transfer hydrogenation of nitroarenes with formic acid. Chemistry 2014, 20, 16732−16737.
10850
DOI: 10.1021/acsomega.8b01141 ACS Omega 2018, 3, 10843−10850
