Highly efficient hydrogen release from formic acid

Applied Surface Science 426 (2017) 605–611

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Highly efficient hydrogen release from formic acid using a graphitic carbon nitride-supported AgPd nanoparticle catalyst Fang Yao a , Xiao Li a , Chao Wan a , Lixin Xu a , Yue An b , Mingfu Ye a , Zhao Lei a,∗ a b

School of Chemistry and Chemical Engineering, Anhui University of Technology, 59 Hudong Road, Ma’anshan 243002, China College of Chemical and Biological Engineering, Zhejiang University, 38 Zheda Road, Hangzhou 310027, China

a r t i c l e

i n f o

Article history: Received 16 April 2017 Received in revised form 12 July 2017 Accepted 20 July 2017 Available online 26 July 2017 Keywords: Hydrogen storage Formic acid Dehydrogenation Palladium Silver g-C3 N4

a b s t r a c t Bimetallic AgPd nanoparticles with various molar ratios immobilized on graphitic carbon nitride (gC3 N4 ) were successfully synthesized via a facile co-reduction approach. The powder XRD, XPS, TEM, EDX, ICP-AES and BET were employed to characterize the structure, size, composition and loading metal electronic states of the AgPd/g-C3 N4 catalysts. The catalytic property of as-prepared catalysts for the dehydrogenation of formic acid (FA) with sodium formate (SF) as the additive was investigated. The performance of these catalysts, as indicated by the turnover frequency (TOF), depended on the composition of the prepared catalysts. Among all the AgPd/g-C3 N4 catalysts tested, Ag9 Pd91 /g-C3 N4 was found to be an exceedingly high activity for decomposing FA into H2 with TOF up to 480 h−1 at 323 K. The prepared catalyst is thus a potential candidate for triggering the widespread use of FA for H2 storage. © 2017 Elsevier B.V. All rights reserved.

1. Introduction In view of the gradual depletion of fossil fuels and increasing environmental problems, alternative green and sustainable energy resources have been attracting considerable attention [1,2]. Hydrogen is a potential clean energy source with promising applications in a number of fields, as it can link all forms of energy use. The potential application of H2 is as the fuel in proton exchange membrane fuel cells (PEMFCs), in which the by-products are only water and small amounts of heat [3–5]. However, the low density of H2 under ambient conditions makes its efficient generation and safe storage difficult, which in turn is a bottleneck in the widespread commercialization of hydrogen as a fuel [6–8]. Therefore, it is a challenging task to seek new hydrogen storage technologies. Conventional hydrogen storage methods, including the use of metal hydrides, compressed hydrogen, sorbent materials, liquid hydrogen, and liquid organic hydrogen carriers, suffer from problems of poor safety, high cost, and weight [1,2,9–11]. Chemical hydrogen storage technology, where hydrogen is produced by the catalytic decomposition of chemicals such as ammonia borane derivatives, hydrazine hydrate, sodium borohydride, and formic acid [12–17], has attracted considerable attention. Formic acid

∗ Corresponding author. E-mail address: [email protected] (Z. Lei). http://dx.doi.org/10.1016/j.apsusc.2017.07.193 0169-4332/© 2017 Elsevier B.V. All rights reserved.

(HCOOH, FA) is considered one of the most competitive hydrogen storage materials to be utilized in the current infrastructure of liquid-based fuels. Furthermore, FA as the main product of biomass oxidation, has been regarded as a convenient and safe hydrogen storage material because of its easy rechargeability, high stability, nontoxicity, and high hydrogen capacity (4.4 wt%) [18–21]. The decomposition of FA follows two principal pathways: one process generates carbon dioxide (CO2 ) and hydrogen (H2 ) via dehydrogenation, and the other produces carbon monoxide (CO) and water (H2 O) via dehydration, as shown below [22–25]. HCOOH(l) → CO2 (g) + H2 (g)G298K = −35.0 kJ mol−1

(1)

−1

(2)

HCOOH(l) → H2 O(l) + CO(g)G298K = −14.9 kJ mol

For the effective application of FA in hydrogen storage, the undesirable dehydration route (Eq. (2)) must be circumvented [26–28]. Therefore, it is desirable to design and prepare high-activity catalysts for the efficient dehydrogenation of FA. Currently, both homogeneous and heterogeneous catalysts are being investigated for the dehydrogenation of FA [29–35]. However, the poor recoverability and reusability of homogeneous catalysts largely impedes their widespread use. Heterogeneous catalysts based on noble metal nanoparticles have attracted much attention in this regard [32–35]. In particular, Pd-based multimetallic NPs has been extensively studied because of their high activity and stability for practical hydrogen production from FA decomposition. Different MOFs and carbon-based catalysts such as AgPd

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without further purification. De-ionized water was used as the solvent. 2.2. Preparation of g-C3 N4 Pure g-C3 N4 was prepared via directly calcining melamine [51]. In a typical process, 5.0 g of melamine powder was placed in the bottom of the alumina crucible. This crucible with a cover was heated to 823 K in the muffle furnace with a heating ramp of 5 K min−1 , at the same time, kept at 823 K for 4 h under nitrogen flow (30 mL min−1 ). The yellow g-C3 N4 sample was ground and obtained after natural cooling. 2.3. Synthesis of AgPd/g-C3 N4 catalysts

Fig 1. Hydrogen generation from FA/SF with different of Ag/Pd content immobilized on g-C3 N4 vs. time (a) Ag4 Pd96 /g-C3 N4 , (b) Ag9 Pd91 /g-C3 N4 , (c) Ag16 Pd84 /g-C3 N4 , (d) Ag21 Pd79 /g-C3 N4 , (e)Pd/g-C3 N4 , (f) Ag/g-C3 N4 at 323 K (nFA = 3 mmol, nSF = 1 mmol).

[36–38], NiAgPd [39], Au@Pd [40], AuPd [41,42], monodisperse AgPd [43], CoAuPd [44], AgAuPd [45], and Ag@Pd [46] have been comprehensively explored for the dehydrogenation of FA. However, there are only limited reports on the use of NPs deposited on graphitic carbon nitride for the catalytic dehydrogenation of FA [47]. Graphitic carbon nitride (g-C3 N4 ), possessing a stacked twodimensional structure, has been considered as N-substituted graphite and has emerged as a promising source for the mass production of free-standing nitrogen-rich carbon materials [48,49]. Owing to its unique combination of multiple physicochemical features, such as thermal and chemical stability, ultrahigh nitrogen composition, and outstanding charge carrier mobility, g-C3 N4 has been proposed as the most promising candidate to complement traditional carbon materials. The use of g-C3 N4 materials in heterogeneous catalysis, photocatalysis, and fuel cells has been reported [50–52]. Recently, we reported that AgPd/mCND/SBA-15 showed much higher catalytic performance in contrast to AgPd/SBA-15 for dehydrogenation of formic acid at 323 K [53]. It can be found that the introduction of a small amount of carbon nitride can significantly improve the catalytic activity of the catalyst. In view of this, AgPd- supported on carbon nitride may exhibit better catalytic activity. Therefore, we report for the first time, a series of AgPd NPs deposited on g-C3 N4 in different molar ratios and investigate the synergistic effect between the composition of the metal and g-C3 N4 for the dehydrogenation reaction of FA. Compared with the catalysts reported in literature, the Ag9 Pd91 /g-C3 N4 catalyst developed by us demonstrates superior performance, with a TOF as high as 480 h−1 at 323 K, and the selectivity for hydrogen released from FA/SF solution up to 100%.

A series of bimetallic AgPd supported on g-C3 N4 catalysts were synthesized as follows: 0.2 g of g-C3 N4 was added to a flask containing 20 mL mixture solution of 0.04 mmol AgNO3 and 0.36 mmol K2 PdCl4 under magnetic stirring for 24 h to impregnate the support with the two metals. Then, fresh NaBH4 aqueous solution (2 mmol) was dropped into the above mixture at 273 K under vigorous stirring. Ag9 Pd91 /g-C3 N4 catalysts could be acquired by centrifugation, washing by ethanol as well as drying in the vacuum at 373 K for 12 h. For comparison, the preparation of Ag4 Pd96 /g-C3 N4 , Ag16 Pd84 /g-C3 N4 , Ag21 Pd79 /g-C3 N4 , Ag/g-C3 N4, and Pd/g-C3 N4 catalysts were following the above analogous process. 2.4. H2 release from FA aqueous solution 2.4.1. Catalytic dehydrogenation of FA The as-synthesized AgPd/g-C3 N4 toward catalytic dehydrogenation of FA was evaluated in a typical water-filled graduated burette system. Typically, the experiment was conducted in a twonecked round-bottomed flask containing 0.1 g catalyst. One neck of the flask was connected to a gas burette to measure the volume of the evolution of gas. The reaction initiated when 2 mL admixture of FA (1.5 M) and SF (0.5 M) was injected into the mixture using a syringe. Dehydrogenation of FA/SF over Ag9 Pd91 /g-C3 N4 catalyst was carried out at different temperatures to acquire the activation energy (Ea ) of the dehydrogenation reaction. For comparison, other monometallic and bimetallic catalysts with different Ag/Pd molar ratios were used for the dehydrogenation. 2.4.2. NaOH trap test The gas released during the reaction was treated with the NaOH trap, and its volume was monitored using the gas burette. 2.5. Stability test The stability tests were conducted repeatedly by adding fresh FA (3 mmol) to the reaction flask after the completion of the previous reaction. 2.6. Characterization

2. Experimental section 2.1. Materials Melamine (C3 H6 N6 , 99%, Aladdin Industrial Inc.), formic acid (HCOOH, 98%, Aladdin Industrial Inc.), silver nitrate (AgNO3 , AR, Nanjing Chemical Reagent Company Ltd.), palladium chloride (PdCl2 , 99%, Nanjing Chemical Reagent Company Ltd.), potassium chloride (KCl, 99%, Nanjing Chemical Reagent Company Ltd.), sodium borohydride (NaBH4 , 98%, Aladdin Industrial Inc.), and sodium formate (HCOONa, 99.5%, Aladdin Industrial Inc.) were used

The metal loadings for the catalysts were determined by an inductively coupled plasma-atomic emission spectrometer (ICP-AES, Thermo iCAP6300). The surface area of the catalysts were measured by Brunauer–Emmett–Teller (BET) method using Micromeritics ASAP2020. Powder X-ray diffraction (XRD) was investigated by a Bruker D8-Advance X-ray diffractometer using a Cu K␣ radiation source (␭ = 0.154178 nm) with a velocity of 4◦ min−1 to identify the crystallinity of the catalysts. X-ray photoelectron spectroscopy (XPS) was performed on an Escalab 250Xi spectrometer using an Al K␣ source. The microstructure and


607

Fig. 2. (a) Time course plots for gas produced by the dehydrogenation of FA/SF by Ag9 Pd91 /g-C3 N4 at different temperatures; (b) Arrhenius plot of ln k vs. 1/T over Ag9 Pd91 /gC3 N4 according to the data of (a) (nFA = 3 mmol, nSF = 1 mmol).

Fig. 3. Powder X-ray diffraction patterns for (a) g-C3 N4 , (b) Ag/g-C3 N4 , (c) Pd/gC3 N4 , (d) Ag4 Pd96 /g-C3 N4 , (e) Ag9 Pd91 /g-C3 N4 , (f) Ag16 Pd84 /g-C3 N4 , (g) Ag21 Pd79 /gC3 N4 .

composition of the samples were observed using FEI Tecnai F20 transmission electron microscope (TEM) equipped with an energy dispersive X-ray detector (EDX) at a working voltage of 200 kV. Detailed analyses for the gas were carried out on GC-9860 II with a thermal conductivity detector (TCD) for CO2 and H2 , a flame ionization detector (FID)-Methanator for CO as well. 3. Results and discussion The g-C3 N4 support was synthesized using a previously reported method [51]. A series of AgPd/g-C3 N4 catalysts were synthesized by adding g-C3 N4 to the solution mixture with different molar ratios of Ag/Pd at 298 K, followed by reduction with NaBH4 at 273 K. The molar ratio of Ag/Pd in the catalysts could be changed by changing the composition of AgNO3 and K2 PdCl4 . The Ag/Pd ratios (measured by ICP-AES) were found to be 1:0, 5:95, 10:90, 15:85, 20:80, and 0:1 for Ag/g-C3 N4 , Ag4 Pd96 /g-C3 N4 , Ag9 Pd91 /g-C3 N4 , Ag16 Pd84 /gC3 N4 , Ag21 Pd79 /g-C3 N4, and Pd/g-C3 N4, respectively (Table S1). The

activities of the as-prepared catalysts toward the dehydrogenation of FA/SF at 323 K were evaluated by measuring the volume of gas produced from the reaction and the TOF values (Fig. 1, Table S2). The catalytic performance was influenced by the composition of the AgPd NPs. The Ag/g-C3 N4 catalyst showed almost no activity for the dehydrogenation. Among the various bimetallic catalysts, Ag9 Pd91 /g-C3 N4 showed a TOF of 480 h−1 at 323 K, which is superior to that of most of the reported catalysts under the same conditions (Table 1). However, the TOF Ag9 Pd91 /g-C3 N4 is lower than that of Ag10 Pd90 /0.2CND/SBA-15, The reason is ascribed to the low specific surface (Table S2). As the palladium content in AgPd NPs increased to 100% (pure Pd/g-C3 N4 ) the catalytic performance degraded, suggesting the existence of a bi-functional effects between AgPd alloy and g-C3 N4 for the dehydrogenation of FA, consistent with the literature [16,37–39]. For composition analysis, the gas was treated with a 10 M NaOH solution trap, which completely absorbed CO2 from the gas. The amount of gas changed into one-half the original value after treatment with NaOH, as shown in Fig. S1, indicating that this reaction produces only CO2 and H2 without generating CO. In addition, GC analyses confirmed the presence of CO2 without the existence of CO, as shown in Figs. S2 and S3 . It was also found that the Ag9 Pd91 /g-C3 N4 catalyst exhibited excellent H2 selectivity in FA dehydrogenation. To measure the activation energy (Ea ) for the dehydrogenation of FA/SF over Ag9 Pd91 /g-C3 N4 , the catalytic reactions were investigated at different temperatures ranging from 313 K to 353 K. The values of the rate constant k at different temperatures were calculated by the linear slope of the dehydrogenation plot in Fig. 2(a). Fig. 2(b) shows the Arrhenius plot of ln k vs. 1/T for the Ag9 Pd91 /g-C3 N4 ; from the plot, Ea was calculated to be approximately 29.5 kJ mol−1 , which is lower than that of most reported heterogeneous catalysts, especially AgPd catalysts (Table 1). In addition, we investigated the recyclability of Ag9 Pd91 /g-C3 N4 catalyst for the dehydrogenation of FA/SF at 353 K (Fig. S4, ESI). It was revealed that the catalyst activity decreased slightly after the third run, while the H2 selectivity remained stable. The decline of the catalytic capability perhaps ascribe to the agglomeration of metal particles in the catalyst (Fig. S5, ESI), which is in accordance with the literature reported [37,55]. Fig. 3 shows the X-ray diffraction (XRD) pattern of the gC3 N4 , Ag/g-C3 N4 , Pd/g-C3 N4 , Ag4 Pd96 /g-C3 N4 , Ag9 Pd91 /g-C3 N4 , Ag16 Pd84 /g-C3 N4 , and Ag21 Pd79 /g-C3 N4 catalysts. The XRD patterns of the as prepared Ag/g-C3 N4 and Pd/g-C3 N4 (Fig. 3b and c) match

608


Table 1 Comparison performance of different catalysts for hydrogen released from FA/SF. Catalyst

T (K)

TOF(h−1 )

Ea (kJ/mol)

Refs.

Ag9 Pd91 /g-C3 N4 Ag10 Pd90 /0.2CND/SBA-15 Au/C Monodisperse Ag42 Pd58 /C Ag24Pd58/C Ag24 Pd58 /C Ag42 Pd58 /C Ni18 Ag24 Pd58 /C Pd/C Monodisperse Au41 Pd59 /C PdAu/C-CeO2 AuPd@ED-MIL-101 Ag@Pd Ag/Pd alloy

323 323 323 323

480 893 80 382

29.5 43.2

This work [53] [44] [43]

323 323 323 365 363 293 293

85 30 230 113.5 106 125 144

20.5

22

28

30.0 30

[39] [44] [44] [54] [19] [46] [46]

Fig. 4. XPS spectra of Ag9 Pd91 /g-C3 N4 .

with the literature data (JCPDS, File no. 04-0783 for Ag and 46–1043 for Pd) suggesting the existence of pure Pd and Ag NPs only. As displayed, the characteristic diffraction peaks of the as-prepared Ag/g-C3 N4 sample (Fig. 3(b)) were observed at approximately 38.1◦ , 44.3◦ , 64.4◦ , 77.5◦ , and 81.5◦ , which can be attributed to the (1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) planes of face-centered cubic (fcc) Ag (JCPDS No. 04-0783). For Pd/g-C3 N4 (Fig. 3(c)), the characteristic peaks were observed at 40.1◦ , 46.7◦ , 68.1◦ , and 82.1◦ and can be indexed to the (1 1 1), (2 0 0), (2 2 0), and (3 1 1) planes of fcc Pd (JCPDS No. 46-1043). The characteristic peaks of pure Pd and Ag in the XRD patterns for AgPd/g-C3 N4 catalysts were absent

in Fig. 3d–g, these X-ray diffraction (XRD) patterns show that the (111) peak of AgPd is located between that of Ag(111) (2␪ = 38.1◦ ) and Pd (111) (2␪ = 40.1◦ ), confirmed the presence of a bimetallic phase rather than a blend of monometalline Pd and Ag NPs. The XRD patterns for AgPd/g-C3 N4 catalysts with different Ag/Pd molar ratios (Fig. 3) depict diffraction peaks between those of pure Ag (JCPDS: 04-0783) and Pd (JCPDS: 46-1043), which clearly indicates the existence of an alloyed structure [16,37–39,54]. Furthermore, one sharp peak is observed at 2␪ = 27.5◦ for all the catalysts, which can be ascribed to the (1 1 0) plane of g-C3 N4 . [51]. Based on the above results, we can conclude that an alloy structure of Ag9 Pd91


609

Fig. 5. (a,b) TEM images of Ag9 Pd91 /g-C3 N4 with different magnification, and (c) Ag9 Pd91 nanoparticle size distribution of Ag9 Pd91 /g-C3 N4 , Mean size = 3.0 nm.

Fig. 6. (a) STEM images and elemental mapping of elements Pd, Ag, C and N of Ag9 Pd91 /g-C3 N4 catalysts, (b) distribution of Ag and (c) distribution of Pd in the AgPd NPs obtained by the line-scan analysis using STEM-EDX along the red arrow on the HAADFSTEM image of Ag9 Pd91 /g-C3 N4 . (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

NPs deposited on g-C3 N4 has been successfully prepared via our synthetic process. The chemical components and the elemental chemical status of Ag9 Pd91 /g-C3 N4 was investigated using X-ray photoelectron spectroscopy (XPS). Fig. 4 shows high-resolution XPS spectra of C (1s), N (1s), Ag (3d), and Pd (3d) observed in Ag9 Pd91 /g-C3 N4 . Two peaks at 284.6 and 287.8 eV can be observed in the C 1s spectrum, which can be ascribed to the adventitious carbon impurities and the sp2 -hybridized carbon atoms in the aromatic rings of g-C3 N4 ,

respectively. The peak of N 1s spectra centered at around 398.1 and 399.8 eV, which can be attributed to the sp2 -hybridized nitrogen atoms involved in the triazine (C N C) units and the amino groups (C N H), respectively. These results are consistent with the literature data for g-C3 N4 [51]. Further, the Pd 3d XPS spectrum of the catalyst exhibits two peaks at 335.1 eV and 340.3 eV, corresponding to the Pd 3d5/2 and Pd 3d 3/2 Co 2p3/2 spin orbit, which prove the presence of metallic Pd0 . In addition, the presence of Ag0 can be confirmed by the Ag 3d XPS peak at 367.3 eV and 373.3 eV. Com-

610


pared to the core-level pure Ag and Pd NPs, the binding energy for Ag 3d and Pd 3d in Ag9 Pd91 /g-C3 N4 changed obviously. The binding energy shifts of the core electrons indicate that quite a few electrons are transformed from Ag to Pd, which leads to transformation in the bulk charge around the atomic sites of Ag9 Pd91 /g-C3 N4 . These result confirm the alloy structure of Ag9 Pd91 NPs in Ag9 Pd91 /gC3 N4 , which is consistent with some literatures revealed [39,45]. The morphologies of the as-synthesized Ag9 Pd91 /g-C3 N4 catalysts were studied by TEM. It can be observed in Fig. 5(a–c) that the AgPd NPs display a limited size distribution as well a mean particle size of 3.0 ± 0.2 nm. The crystalline nature of the Ag9 Pd91 NPs was characterized by the high-resolution TEM (HRTEM) image (Fig. 5b), and the lattice spacing is evaluated to be 0.230 nm, which is between the (111) plane of face-centered cubic (fcc) Ag (0.235 nm) and fcc Pd (0.224 nm), thus indicating the presence of an alloy architacture of AgPd NPs [16,37–39,54]. STEM images and elemental mapping of Pd, Ag, C, and N in the Ag9 Pd91 /g-C3 N4 catalyst are shown in Fig. 6(a). It is evident that C and N are dispersed uniformly in the scan region. Moreover, the strengthening of the Pd signal in the scan region is accompanied by the strengthening of the Ag signal, as shown in Fig. 6(a), which also confirms the existence of an alloy structure of Ag9 Pd91 NPs in Ag9 Pd91 /g-C3 N4 . Furthermore, when the distribution of the elements in the chosen AgPd NPs was characterized by utilizing the line scanning analysis in the STEMEDX mode, as shown in Fig. 6(b), (c), overlapping of the Ag and Pd signals was observed. The results of XRD, HRTEM, STEM-mapping, and HAADF-STEM-line analyses confirm that AgPd alloy NPs were deposited on the surface of g-C3 N4 . 4. Conclusion In conclusion, we synthesized bimetallic AgPd nanoparticles loaded on graphitic carbon nitride (g-C3 N4 ) via a facile co-reduction approach, and used it for the dehydrogenation of FA/SF. The synergistic effect of AgPd/g-C3 N4 was investigated for the dehydrogenation of FA. The Ag9 Pd91 /g-C3 N4 catalyst demonstrated remarkable superior activity over its monometallic and bimetallic counterparts, suggesting an obvious molecular-scale synergistic effect of the Ag–Pd alloy. The as-synthesized Ag9 Pd91 /g-C3 N4 exhibited 100% H2 selectivity and exceedingly high catalytic capability and it has an initial TOF of 480 h−1 at 323 K. Compared with AgPd/mCND/SBA-15, AgPd-supported carbon nitride with high specific surface may exhibit excellent catalytic performance, Thus, the investigated results may paves a new approach for designing the catalyst for the dehydrogenation of FA, which is significance for large scale utilization of FA as a H2 storage material. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (21376005), Research Fund for Young Teachers of Anhui University of Technology (QZ201610), Open Fund of Shaanxi Key Laboratory of Energy Chemical Process Intensification (SXECPI201601), The key project of Scientific Research Foundation sponsored by the Education Department of Anhui Province (KJ2017A045), Anhui Provincial Natural Science Foundation (1608085QF156) and the Scientific Research Foundation of Graduate School of Anhui University of Technology (2016012,2016017). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc.2017.07. 193.

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