AuPd Nanoparticles Anchored on Nitrogen-Decorated Carbon

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AuPd Nanoparticles Anchored on Nitrogen-Decorated Carbon Nanosheets with Highly Efficient and Selective Catalysis for the Dehydrogenation of Formic Acid Yiqun Jiang, Xiulin Fan, Man Chen, Xuezhang Xiao, Yiwen Zhang, Chuntao Wang, and Lixin Chen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00082 • Publication Date (Web): 22 Feb 2018 Downloaded from http://pubs.acs.org on February 23, 2018

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The Journal of Physical Chemistry

AuPd Nanoparticles Anchored on Nitrogen-Decorated Carbon Nanosheets with Highly Efficient and Selective Catalysis for the Dehydrogenation of Formic Acid Yiqun Jiang †, Xiulin Fan ‡, Man Chen †, Xuezhang Xiao *,†, Yiwen Zhang †, Chuntao Wang *,ǁ, Lixin Chen *,†,§ †

State Key Laboratory of Silicon Materials and School of Materials Science and Engineering,

Zhejiang University, Hangzhou 310027, P.R. China. ‡

Department of Chemical and Biomolecular Engineering, University of Maryland, College Park,

MD 20742, USA. ǁ College of Naval Architecture and Mechanical-electrical Engineering, Zhejiang Ocean University,

Zhoushan 316000, P.R. China. §

Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province,

Hangzhou 310013, P.R. China. Fax: +86 571 87951152; Tel: +86 571 87951152 E-mail: [email protected] (X. Z. Xiao);[email protected] (C. T. Wang); [email protected] (L. X. Chen)

Abstract Formic acid (FA), a sustainable and safe hydrogen storage vector, has the advantages of nontoxicity, high hydrogen content (4.4 wt.%) and low cost. However, the dehydrogenation of formic acid at near room temperature remains a big challenge in terms of favorable hydrogen release rate and COabsence hydrogen production. Herein, a series of nitrogen-decorated carbon nanosheets (n-CNS) 1

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supported AuPd nanoparticles (NPs) were designed and employed as efficient catalysts to dehydrogenate FA for the first time. The catalyst AuPd NPs supported on n-CNS synthesized at hydrothermal temperature of 160 oC (AuPd/n-CNS-Th-160) exhibits excellent catalytic activity towards the dehydrogenation of FA compared with AuPd/g-carbon nitride (AuPd/g-C3N4) and commercial Pd/C catalysts, reaching an initial turnover frequency (TOF) of 527 h-1, 100% hydrogen generation and selectivity at room temperature (25 oC), while the TOF achieves even 1896 h-1 at 60 oC.

The enhanced catalytic performance can be attributed to the coordinated effect from Au-Pd

alloying and the doped nitrogen atoms on carbon nanosheets. It is the first time to systematically probe the promoting mechanism of nitrogen on FA dehydrogenation. It is also illustrated that the promoting mechanism of N atoms on carbon nanosheets results from its nitrogen-bonding configuration, specifically, the ratio between graphitic N and pyridinic N. The high ratio of graphitic N to pyridinic N can modify the distribution of electron density and minimize the size of the metal nanoparticles, thereby greatly enhances the catalytic effect. The present study, by varying the catalysts’ composition and regulating the active material to boost the catalytic performance, provides a general pathway to further enhance the efficiency of hydrogen generation strongly depending on the properties of the support.

1. Introduction With the progressive depletion of fossil fuel resources and the deterioration of the environmental issues, searching for sustainable and environmentally friendly energy supplies is urgently important. Hydrogen, an ideal candidate of renewable energy, has attracted considerable research interest as an alternative energy carrier owing to its advantages of high energy content and non-pollution intrinsically to the environment1, 2. However, the classical methods of hydrogen storage such as 2


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high-pressure and liquid containers, suffer from the problems of safety issues, low gravimetric and volumetric densities as well as high cost since its gas phase under ambient pressure.3 More and more attention has been devoted to hydrogen generation by hydrolysis of metals4-7, metal hybrids8-12 and chemical compound materials for their high theoretical hydrogen yield.

Chemical compound

materials, including hydrazine13, 14, borohydrides15, ammonia borane16, 17 and formic acid18, have been considered as the potentially alternative hydrogen storage materials providing significant advantages of easy transportation and recharging19, 20. Formic acid (FA, HCOOH), a liquid hydrogen carrier, is extensively studied by scientific community for its nontoxicity, regenerability and facility to handle. Suitable catalysts are necessary for FA decomposition via dehydrogenation way (HCOOH (l) →CO2 (g) + H2 (g)). However, the undesirable side reaction through dehydration way (HCOOH (l) →CO (g) + H2O (g)) must be restricted to exploit the hydrogen storage properties of FA efficiently21. Besides, the byproduct of CO is easy to make the catalysts poisoned resulting in the decrease of catalytic performance. Numerous efforts have been paid on both heterogeneous and homogeneous catalysts. Homogeneous catalysts generally show notable catalytic activities22. Nevertheless, these catalysts are composed of soluble organometallic complexes which lead to difficulties in separation issues for device fabrication23. For practical applications, the development of highly active heterogeneous catalysts for FA dehydrogenation under convenient conditions is of great importance. Tremendous works have been devoted to explore high catalytic properties of the heterogeneous catalysts including designing special nanostructures, such as core-shell and hollow structure24, 25, adding other nonnoble metals like Ag26, MnOx27, Cr28, Co29, Ni30, rare earth elements31, 32, and developing novel supports, for instance, basic resin33, SiO234, TiO235, metal-organic frameworks (MOFs)36-38 and 3



graphene39. Currently, synthesis and application of specific supports are attracting considerable interests. Doped carbon-based materials are carbon materials containing other elements such as phosphorus (P), boron (B) or nitrogen (N) besides carbon. Replacing individual carbon atoms with heteroatoms leads to the destruction of the sp2-hybridized carbon network, bringing about properties different from the pristine carbon material. Among these dopant elements, nitrogen is the most common one because of its similar size and one extra electron compared with carbon in the external shell40. They have emerged as very promising functional materials and been utilized widely in academic research. Dai et al. synthesized N-doped graphene chemical vapor deposition of methane in the presence of ammonia, and the obtained catalyst showed a stable methanol crossover effect, long-term operation durability for catalyzing the ORR in fuel cells41. Mullen et al. found that 3D Ndoped graphene aerogel supported Fe3O4 nanoparticles were efficient catalysts for ORR42. Dai and their group reported N, P-codoped carbon networks as efficient metal-free bifunctional catalysts for ORR and hydrogen evolution reactions (HER)43. Zhao et al. prepared N-doped graphene-hollow AuPd nanoparticle hybrid films for the highly efficient electrocatalytic reduction of H2O244. Yang et al. reported N-doped porous carbon derived from zeolitic imidazolate framework-8 (ZIF-8) and used it for hydrogenation of nitroarenes45. Li et al. developed a N-doped carbon matrix using MOFs as template to synthesize the Co@Pd catalyst with superior hydrogenation activities for substituted nitroarenes46. Despite intensive studies on the N-doped carbon materials, barely reports using Ndoped carbon materials for FA dehydrogenation have been published. Yan et al. reported the AuPdCeO2 nanocomposites and Au@Pd core-shell nanoclusters grown on N-doped reduced graphene oxide as catalyst for FA decomposition, respectively31, 47. Yoon and co-workers synthesized an electronically modified Pd nanocatalyst supported on nitrogen-doped carbon (Pd/N-C) for the 4


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dehydrogenation of formic acid and proposed that nitrogen atoms doped to carbon could enhance the metal-support interactions48. Their research group also demonstrated a Pd catalyst anchored on mesoporous graphite carbon nitride (Pd/mpg-C3N4)49. Recently, element-doped materials have attracted more and more research attentions in various kinds of catalytic reactions. To the best of our knowledge, there is no report to systematically research the promoting mechanism of N-doped carbon materials with controllable nitrogen content in terms of surface specific area, nitrogen doping density and nitrogen bonding configurations when they are applied as supports for metal catalyst on FA dehydrogenation. Besides, in this paper, n-CNS with different masses of nitrogen dopants are synthesized through a reliable method by altering the hydrothermal temperature, hydrothermal time and calcination temperature, and investigated thoroughly for FA dehydrogenation. Therefore, it is possible to further improve the activity of a heterogeneous catalyst by modulating the properties of supports for catalysts applied in FA dehydrogenation. Herein, we synthesized a set of highly efficient catalysts by reducing AuPd onto n-CNS with different nitrogen doping densities and specific surface areas. The n-CNS were synthesized by immobilizing carbonized glucose onto graphitic carbon to form a carbon nitride-glucose composite (g-C3N4@Glu). Afterwards, the g-C3N4@Glu composite was subjected to heat treatment leading to the production of n-CNS during which the g-C3N4 was decomposed and sacrificed as the nitrogen precursor and glucose as a carbon source. More interestingly, the catalytic performance of the resultant catalysts could be precisely tailored by simply manipulating the preparation procedure of n-CNS. The activity of AuPd/n-CNS-Th-160 exhibits a high turnover frequency (TOF) value of 459 h-1 and 1896 h-1 at 25 oC and 60 oC, respectively. The as-synthesized AuPd/n-CNS-Th-160 catalyst shows more than 6 times higher catalytic activity than that of the AuPd alloy nanocatalyst without 5



any support and noble metal benchmark (i.e. commercial Pd/C). Particularly, n-CNS with welldefined structure plays a significant role in understanding the impact of nitrogen dopants on property and behavior of the catalyst for FA dehydrogenation.

2. Experimental section 2.1. Materials

Urea (CH4N2O, >99%), glucose (C6H12O6·H2O, >99%), tetrachloroauric (III) acid hydrate (HAuCl4·4H2O, Au >47.8%), palladium(II) chloride (PdCl2, Pd >59%), hydrogen chloride (HCl, 36~38%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Commercial Pd/C (Pd >5%), Sodium borohydride (NaBH4, 98%), formic acid (FA, >98%) and sodium formate (CHNaO2, 99.5%) were purchased from Aladdin Reagent. All reagents were used without further purification. Deionized water was obtained by reverse osmosis followed by ion-exchange and filtration.

2.2. Preparation of H2PdCl4, g-C3N4, n-CNS and AuPd/n-CNS

A 0.05 M H2PdCl4 aqueous solution was prepared by adding 44.5 mg PdCl2 into 0.1 M 5 mL HCl staying for overnight to dissolve completely. The n-CNS was prepared according to Zhang’s work with a little modification50. g-C3N4 was prepared by heating urea from room temperature to 550 oC in a muffle furnace at a heating rate of 5 oC min-1 and kept for 4 hours until the crucible was cooled down to room temperature. For the preparation of n-CNS, 1 g g-C3N4 and 0.3 M glucose were dissolved in 40 mL water and ultrasonicated for 5 hours. Subsequently, the solution was transferred into a 100 mL Teflon-lined stainless-steel autoclave. The autoclave was sealed and heated to 160 oC and maintained for 10 hours in an oven. After the hydrothermal process, the obtained precursors was gathered through centrifugation and washed with deionized water for several times. The product 6


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g-C3N4@Glu was obtained after it was dried under vacuum at 60 oC overnight. Additionally, the product g-C3N4@Glu was further annealed at calcination temperature of 900 oC under a pure nitrogen flow for 1 hour. For comparison, other samples were treated using the same method except at higher hydrothermal temperature (Th= 180 oC, 200 oC and 220 oC) with same hydrothermal time of 10 hours or different hydrothermal time (t= 8 h, 12 h) at the same hydrothermal temperature of 160 oC. The above products were all annealed at calcination temperature (Tc) of 900 oC. In order to investigate the effect of Tc to the property of n-CNS, the hydrothermal temperature and hydrothermal time were maintained at 160 oC and 10 hours, respectively, while the Tc was set as 800 oC and 1000 oC. For simplicity, the samples are denoted as n-CNS-Th (n-CNS-Th-160, n-CNSTh-180, n-CNS-Th-200, n-CNS-Th-220), n-CNS-t (n-CNS-t-8, n-CNS-t-10, n-CNS-t-12) and nCNS-Tc (n-CNS-Tc-800, n-CNS-Tc-900, n-CNS-Tc-1000). To date, n-CNS-Th-160, n-CNS-t-10 and n-CNS-Tc-900 are the same sample. To prepare AuPd/n-CNS, 100 mg of n-CNS was dispersed in 10.0 mL of deionized water and sonicated to obtain the well dispersed n-CNS suspension. 1.0 mL of aqueous solution containing H2PdCl4 (0.05 M) and HAuCl4 (0.05 M) was poured into the above solution with magnetic stirring overnight. The fresh NaBH4 aqueous solution (1.0 mL, 1.0 M) was added into the above mixture with magnetic stirring. After 2.5 h, the product was washed with water for several times by centrifugation and dried in vacuum at 333 K. For comparison, other nanoalloy catalysts were also prepared with the same method.

2.3. Characterization

The structure, morphology and composition of as-synthesized catalyst were characterized by powder X-ray diffraction (XRD) patterns (PANalytical, the Netherlands), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), high-angle annular dark-field scanning 7



transmission electron microscopy (HAADF-STEM) images, element analysis mapping and energy dispersive X-ray spectroscopy (EDS) (FEI Tecnai F20 (HR), 200 kV) and X-ray photoelectron spectroscopy (XPS) (VG ESCALAB MARK II, Mg Kα). The XPS spectra were recorded with the pass energy of 50 eV after Ar+ sputtering of the surface for 15 min. All binding energy values were calibrated using the C 1s peak at 284.6 eV. Thermogravimetric (TG) measurements were conducted on a synchronous thermal analysis (Netzsch STA 449F3 analyzer) under flowing Ar2 conditions. Detailed analysis for the generated gas was performed on SP-6890 with thermal conductivity detector (TCD) and flame ionization detector (FID)-Methanator. Brunauer-Emmet-Teller (BET) method was employed to measure the specific surface areas of the samples.

2.4. Activity measurements

The catalytic activity of the as-prepared catalyst was determined by the rate of gas generation. The volume of released gas was measured by a gas burette system. Typically, 100 mg of catalyst and 10 mL of deionized water were transferred to a two-neck flask which was placed under magnetic stirring and preheated to the setting temperature before starting the catalytic activity test. The catalytic reaction was started when the FA/SF solution was injected into the flask.

3. Results and discussion The procedure for synthesizing the AuPd/n-CNS is schematically illustrated in Scheme 1. In a typical experiment, g-C3N4 template was coated with carbonized glucose via hydrothermal procedure to obtain g-C3N4@Glu. Afterward, g-C3N4@Glu was subjected to calcination inside the tube furnace at high temperature. The heat treatment of g-C3N4@Glu gave rise to the decomposition of g-C3N4 and thereby generated n-CNS. Subsequently, n-CNS was dispersed uniformly with 8


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ultrasonication, and then mixed with HAuCl4 and H2PdCl4 under magnetic stirring for 6 hours. At last, NaBH4 was added to reduce the metal ions, the obtained products, i.e. AuPd/n-CNS was fully characterized.

Scheme 1. Schematic illustration for the preparation procedure of AuPd/n-CNS nanocatalyst.

9



Figure 1. TEM (a, b), HRTEM (c), EDX spectra (d), HAADF-STEM (e) images and the corresponding elemental mappings for C (f), N (g), Au (h) and Pd (i) elements of AuPd/n-CNS-Th160. The inset of (b) and (c) show the corresponding particle size distribution and FFT image. The morphology and structure of n-CNS-Th-160 immobilized AuPd NPs are first investigated by TEM. It can be seen that the support has the shape of a wrinkled and folded morphology (Figure S1). Figure 1a and b show the typical TEM images of AuPd/n-CNS-Th-160 composite. It is clear that AuPd NPs are well dispersed on n-CNS-Th-160 and the corresponding histograms of particle size distribution are in the inset of Figure 1b with the mean particle diameter of about 1.82 nm. Detailed analyses for the AuPd/n-CNS-Th-160 catalyst are conducted by HRTEM and EDX measurements. A representative HRTEM image of an individual AuPd nanocrystal can be clearly distinguished in Figure 1c showing a lattice spacing of 0.234 and 0.202 nm, which is between the (111) and (200) d-spacing of standard fcc Pd and Au, respectively. The corresponding EDS spectrum 10


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(Figure 1d) on AuPd/n-CNS-Th-160 reveals the coexistence of N, Au and Pd elements in the AuPd/n-CNS-Th-160 sample. To better investigate the elemental distribution in AuPd/n-CNS-Th160, the element mapping analyses were performed using HAADF-STEM and STEM energydispersive X-ray spectroscopy (STEM-EDX) mapping (Figure 1e-i). The homogeneous distribution of N element on the carbon demonstrates the formation of nitrogen-decorated carbon nanosheets in Figure 1g. It is also found that the signals for Au and Pd are at the same position (Figure h and i), which strongly confirms the existence of AuPd alloy. It can be concluded that the AuPd NPs are successfully

formed

and

anchored

uniformly

on

the

nitrogen-decorated

carbon

nanosheets. Meanwhile, to probe the chemical composition and content of AuPd/n-CNS-Th-160, XPS measurements were carried out. X-ray photoelectron spectroscopy spectra strongly confirm that the sample is N doped (Figure S2b). The location of Pd 3d and Au 4f (Figure S2c and S2d) suggest the predominant state of the Pd and Au elements are metallic Pd and Au, further confirming the successful preparation of AuPd alloy NPs. The binding energies of pure Au and Pd metals are presented as vertical dotted lines. The binding energy of Pd 3d5/2 and Pd 3d3/2 in AuPd respectively shift to lower values by about 0.2 eV compared with those of pure Pd 3d (3d5/2 = 334.9 eV, 3d3/2 = 340.2 eV). The binding energy of Au 4f7/2 and Au 4f5/2 in AuPd respectively shift to lower values by about 0.3 eV compared with those of pure Au 4f (4f7/2 = 83.8 eV, 4f5/2 = 87.4 eV).These phenomena suggest that some electrons are transferred to Pd and Au.

11



 g-C3N4  AC  n-CNS-Th-160 

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Pd(JCPDS:87-0637) Au(JCPDS:89-3697)



AuPd/n-CNS-Th-160

 







AuPd/AC



AuPd/g-C3N4



n-CNS-Th-160



g-C3N4 10

20

30

40

50

60

70

80

90

2 (  )

Figure 2. XRD patterns of g-C3N4, n-CNS-Th-160, AuPd/g-C3N4, AuPd/Activated Carbon (AC) and AuPd/n-CNS-Th-160. The crystallinity of the obtained AuPd/n-CNS-Th-160 was also studied by powder XRD (Figure 2). As determined by power X-ray diffraction measurements, g-C3N4, which is connected by planar amino groups and stacks with a tri-s-triazine unit like graphite, features two diffraction peaks at 13.4o and 27.5o51. After the heat treatment step, the XRD pattern of one peak at 26.1o appears and the absence of peaks for g-C3N4 suggest that g-C3N4 template has completely decomposed. This is confirmed by thermogravimetric analysis (TGA) for g-C3N4 which shows that g-C3N4 decomposed completely at around 720 oC (Figure S3). A slight weight loss prior to 100 oC is assigned to the moisture loss. The weight loss (98%) started at around 550 oC is assigned to the decomposition of g-C3N4. After the introduction of noble metals, the XRD patterns at 13.4o and 27.5o, 26.1o are unchanged suggesting that AuPd nanoparticles are supported on g-C3N4 and n-CNSTh-160 without destruction of their solid-state packing. The diffraction patterns for the AuPd/g-C3N4, AuPd/AC and AuPd/n-CNS-Th-160 catalysts have five broad peaks at 38.3o, 44.6o, 64.8o, 77.7o and 81.9o, corresponding to (111), (200), (220), (311) and (222) planes of pure Pd (JCPDS: 87-0637) and Au (JCPDS: 89-3697) with a face-centered cubic (fcc) structure, respectively. The diffraction 12


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peaks locate between the characteristic peaks of Pd and Au demonstrating the formation of alloy structure. Based on the Hume-Rothery rule, Au and Pd can be incorporated into the lattice with each other to form the alloy structure as the same fcc structure and the slight difference in atomic radii between Pd (0.138 nm) and Au (0.144 nm) (4.2%) which is much lower than 15%29. Bragg equation is also used to calculate the lattice spacing of AuPd/n-CNS-Th-160 at 38.3o and 44.6o with the results of 0.2339 and 0.2024 nm, respectively, in good agreement with the TEM results. It is also known that Au and Pd can form the alloy no matter what the support is. g) f) e) d) c) b) a)

250

Gas Volume (mL)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


200 150 100 50 0 0

10

20

30

40

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60

Time (min)

Figure 3. The volume of gas (CO2+H2) versus time by decomposition of FA/SF (2.5 M/2.5 M, 2 mL) in the presence of different catalysts a) AuPd/n-CNS-Th-160, b) AuPd/AC, c) Pd/n-CNS-Th160, d) AuPd/g-C3N4, e) commercial Pd/C, f) AuPd/BP-2000 and g) AuPd at 25 oC under ambient atmosphere. The catalytic performance of AuPd/n-CNS-Th-160 composite for FA dehydrogenation is evaluated together with AuPd/AC, Pd/n-CNS-Th-160, AuPd/g-C3N4, commercial Pd/C, AuPd/BP2000 and AuPd as can be seen in Figure 3. Obviously, AuPd/n-CNS-Th-160 exhibits the highest activity among all the prepared catalysts with 215 mL gas released within 10 min at 298 K. On the contrary, the activities of other catalysts show much lower activity. The initial TOF over AuPd/n13



CNS-Th-160 is calculated to be 459 h-1 at 25 oC, which is almost 6.5, 6.1, 5.8, 2.8, 1.9 and 1.7 times than that of AuPd, AuPd/BP-2000, AuPd/g-C3N4, commercial Pd/C, Pd/n-CNS-Th-160 and AuPd/AC under the same conditions, respectively. To the best of our knowledge, this initial TOF value is much higher than those ever reported for FA decomposition and even at elevated temperatures (Table S1). Without the support of n-CNS-Th-160, pure AuPd NPs aggregate severely (Figure S4) and exhibit low catalytic activity, that is, the enhanced activity can be attributed to the formation of the ultrafine and uniformly distributed AuPd NPs on n-CNS-Th-160. On the other hand, the addition of Au accelerates the hydrogen release rate by comparing AuPd/n-CNS-Th-160 with Pd/n-CNS-Th-160, manifesting the significance of Pd alloying. Notably, Au has been proved to be no activity at all for FA dehydrogenation in our previous work and other researches25, 29, 34. Therefore, the improved catalytic activity comes from the alloying effect of Au and Pd not the addition of another active element. Similarly, to manifest the crucial role of n-CNS-Th-160 on high performance, we compared n-CNS-Th-160 with other conventional carbon supports (e.g. BP-2000 and AC). The catalytic activities of AuPd/BP-2000 and AuPd/AC are inferior to AuPd/n-CNS-Th-160 confirming the significance of nitrogen-doping carbon support for effective performance. From the experimental results, it is also known that the mass of nitrogen is not the more the better by comparing g-C3N4 and n-CNS-Th-160 with nitrogen content of 54% (Figure S5) and 6.25% (Table 1), respectively. Specifically, g-C3N4 is composed of graphitic N, pyridinic N and pyrrolic N with content of 11.83%, 17.6% and 24.57%, respectively. n-CNS-Th-160 mainly contains graphitic N and pyridinic N with content of 4% and 1.4%, respectively, which is much lower than that of gC3N4. However, g-C3N4 is composed of graphitic N, pyridinic N and pyrrolic N with atom content of 22.9%, 31.6% and 45.5% for the overall nitrogen, respectively, while n-CNS-Th-160 contains 14


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graphitic N, pyridinic N and pyridine-N-oxide with atom content of 64.4%, 22.5% and 13.1%, respectively. The ratio of graphitic N to pyridinic N (ratio= 2.86) in n-CNS-Th-160 is much higher than that of g-C3N4 (ratio= 1.38). Therefore, it is speculated initially that not the nitrogen content but the nitrogen-bonding configuration is the key factor for high catalytic activity of FA dehydrogenation. Mullen et al. proved that the overall nitrogen content does not directly affect the electrochemical performance52, 53. Details of this conclusion will be further discussed at a later time. The above results clearly indicate that the employment of n-CNS-Th-160 can significantly improve the performance of the AuPd/n-CNS-Th-160 composite toward the dehydrogenation of FA. In addition, to confirm that the generated hydrogen is not coming from g-C3N4 and n-CNS-Th-160, an extra experiment is carried out by employing g-C3N4 and n-CNS-Th-160 as catalysts (Figure S6). No gas is generated from g-C3N4 and n-CNS-Th-160 demonstrating that AuPd is the active site and g-C3N4 as well as n-CNS-Th-160 serves only as support for dehydrogenating reaction. 250

Volume of gas (mL)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


200 150

AuPd/n-CNS-Th-160 AuPd/n-CNS-Th-180

100

AuPd/n-CNS-Th-200 AuPd/n-CNS-Th-220

50 0 0

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15 20 Time (min)

25

30

Figure 4. The volume of gas (CO2+H2) versus time by decomposition of FA/SF (2.5 M/2.5 M, 2 mL) in the presence of AuPd/n-CNS-Th catalysts at 25 oC under ambient atmosphere. To further investigate the promoting mechanism of n-CNS, the catalytic performances of AuPd on different n-CNS-Th supports are tested for the dehydrogenation of FA/SF at 25 oC. It can be seen 15



that, the catalytic activity decreases with the increase of the hydrothermal temperature in Figure 4. In attempt to probe the inherent mechanism of the phenomenon, the specific surface areas for nCNS-Th are obtained, and the contents and chemical compositions of nitrogen are also carried out with XPS measurement.

Figure 5. N2 adsorption and desorption isotherms (a) and pore width distribution (b) of n-CNS-Th measured at 77 K. The N2 adsorption and desorption isotherms are conducted by N2 physisorption and all n-CNSTh samples have the type-IV curves with a hysteresis loop, indicating the presence of both microand mesopores (Figure 5). The specific surface areas (SBET) measured for n-CNS-Th-160, n-CNSTh-180, n-CNS-Th-200, and n-CNS-Th-220 are 1127, 822, 725, 624 m2 g-1, respectively. By contrast, g-C3N4 and g-C3N4@Glu have SBET of 122 and 59 m2 g-1, respectively (Figure S7). It can be seen that the SBET and pores of g-C3N4 decrease after hydrothermal procedure because glucose is coated on g-C3N4. The considerable increase in post-heat treatment SBET is ascribed to pores derived from g-C3N4 decomposition. Among the four n-CNS-Th samples, n-CNS-Th-160 has the highest surface area and the thinnest glucose coated on g-C3N4 of n-CNS that provided the most exposed surface during the pyrolysis/activation process. For further optimizing the catalytic activity of the catalyst, a hydrothermal temperature of 140

oC

is also carried out. However, because of the 16


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negligible thickness of carbonized glucose covered on g-C3N4, there is very low product yield after the thermal treatment leading to the abandonment of hydrothermal treatment of 140 oC. The order of SBET values for the n-CNS-Th samples exhibits the same trend with the order of catalytic performance. High SBET has the positive effect on catalytic activity.

Figure 6. XPS survey and N 1s XPS spectra of n-CNS-Th. XPS spectra of n-CNS-Th samples are shown in Figure 6. The contents of N in n-CNS-Th-220, nCNS-Th-200, n-CNS-Th-180 and n-CNS-Th-160 are 2.17, 3.33, 6.0 and 6.25%, respectively. N 1s XPS spectra for all n-CNS-Th samples include pyridinic N, graphitic N and pyridine N+-O- at binding energies of 398.2, 401.4 and ~402.5 eV, respectively. When the samples are pyrolyzed at temperatures of 800 oC and above, only pyridinic-N, graphitic-N and pyridine N+-O- exist in the Nmodified carbon nanosheets according to previous research54. The formation of pyridine N+-Oderives from the reaction between nitrogen-containing species and oxygen groups during the synthesis55. The graphitic N decreases with the increasing hydrothermal temperature, albeit with a decrease of overall nitrogen content (Table 1). The higher ratio of graphitic N to pyridinic N exhibits the better catalytic performance. The result is consistent with the above observation from the comparison between g-C3N4 and n-CNS-Th-160, which confirms the above mentioned speculation that the nitrogen-bonding configuration in n-CNS could be the intrinsic reason for high catalytic 17



performance. Therefore, such different nitrogen-bonding configurations in n-CNS samples are expected to play a crucial role in the catalytic performances. In the preceding, we have demonstrated that the excellent catalytic activity of AuPd/n-CNS-Th160 catalysts origins from high SBET and nitrogen dopant of carbon support, especially the higher ratio of graphitic N to pyridinic N can be the intrinsic reason. To verify the above conclusion, AuPd catalysts with n-CNS-t and n-CNS-Tc supports are synthesized. The assessments for catalytic performances of n-CNS-t and n-CNS-Tc catalysts are carried out (Figure 7). AuPd/n-CNS-t-10 displays the best catalytic performance comparing with other two catalysts from Figure 7a. The SBET for n-CNS-t-8, n-CNS-t-10 and n-CNS-t-12 are 1285, 1121 and 873 m2 g-1, respectively (Figure S8a and S8b). Even though n-CNS-t-8 has the highest SBET, it does not exhibit the best catalytic activity. The result manifests that SBET is not the key factor for catalytic performance. In association XPS survey (Figure S8c and S8d) described in Table S2 with the catalytic performance of AuPd/n-CNSt, it can be known that the content of overall nitrogen and the type of nitrogen on support do not play significant roles in the catalytic activity of FA dehydrogenation. In contrast, the catalytic performance depends on the ratio of graphitic N to pyridinic N. It is also clear that AuPd/n-CNSTc-900 shows the highest catalytic activity among the three catalysts. Likewise, XPS measurements are obtained for n-CNS-Tc-800, n-CNS-Tc-900 and n-CNS-Tc-1000 as shown in Figure S9 and detailed in Table S2. It can be found the tendency of AuPd/n-CNS-Tc is the same with AuPd/n-CNSt, that is, higher ratio of graphitic N to pyridinic N indicates better catalytic activity.

18


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Figure 7. The volume of gas (CO2+H2) versus time by decomposition of FA/SF (2.5 M/2.5 M, 2 mL) in the presence of AuPd/n-CNS-t (a) and AuPd/n-CNS-Tc (b) catalysts at 25 oC under ambient atmosphere. Table 1. The content of overall nitrogen and different type of nitrogen species in n-CNS-Th.

N content

Pyridinic N

Graphitic N

The ratio of graphitic N

Pyridine-N-oxide

(wt.%)

(at.%)

(at.%)

to pyridinic N

(at.%)

n-CNS-Th-160

6.25

22.5

64.4

2.86

13.1

n-CNS-Th-180

6.0

26.6

58.0

2.18

15.3

n-CNS-Th-200

3.33

30.0

54.5

1.82

15.2

n-CNS-Th-220

2.17

37.4

50.6

1.35

12.0

Sample

In the case of AuPd/n-CNS-Th, XPS measurement and TEM are further performed to probe the promoting mechanism of the higher ratio between graphitic N to pyridinic N.

19



Figure 8. Au 4f and Pd 3d XPS spectra of a) AuPd/n-CNS-Th-160, b) AuPd/n-CNS-Th-180, c) AuPd/n-CNS-Th-200 and d) AuPd/n-CNS-Th-220. XPS spectra of AuPd/n-CNS-Th are shown in Figure 8. It can be seen that after AuPd alloy supported on n-CNS-Th, the binding energies of Au 4f and Pd 3d in all AuPd/n-CNS-Th composites shift to lower values than those of the Au 4f and Pd 3d of pure Au and Pd, respectively. These shifts suggest that there are efficient electrons transfer from the support to Pd and Au. Generally, the lower value of binding energy of metal indicates the more electron transfer to the metal. Specifically, the binding energies of Au 4f and Pd 3d in AuPd/n-CNS-Th-160 shift to the lowest value which means the largest amount of electron transfer from n-CNS-Th-160 to Pd and Au than those of other catalysts. The electron transfer between the n-CNS-Th-160 substrate and AuPd alloy nanoparticles endows the product with enhanced catalytic activity for dehydrogenation of FA. XPS spectra were also performed to compare the Au and Pd metallic state between AuPd and AuPd/N-CNS-Th-160 to analyze the role of the n-CNS. From Figure S10, it can be seen that the comparison of Au 4f and Pd 3d between AuPd/n-CNS-Th-160 and AuPd reveals a electron transfer from n-CNS-Th-160 to Au and Pd.27

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Figure 9. TEM images and the corresponding size distributions of AuPd/n-CNS-Th-180 (a1, a2), AuPd/n-CNS-Th-200 (b1, b2) and AuPd/n-CNS-Th-220 (c1, c2). TEM was conducted to thoroughly investigate the promoting mechanism. TEM images and the corresponding particle size distributions of AuPd/n-CNS-Th-180, AuPd/n-CNS-Th-200 and AuPd/nCNS-Th-220 are shown in Figure 9. The average particle size is 2.31, 2.49 and 3.05 nm for AuPd/nCNS-Th-180, AuPd/n-CNS-Th-200 and AuPd/n-CNS-Th-220, respectively. The average particle size increases with the increasing hydrothermal temperature of support in consideration of the particle size of 1.82 nm for AuPd/n-CNS-Th-160. Moreover, the metal particles on higher hydrothermal temperature of support are distributed unevenly and inclined to aggregate. As can be seen, there are some metal agglomerations in AuPd/n-CNS-Th-200 and AuPd/n-CNS-Th-220. To further verify the above conclusion that higher ratio of graphitic N to pyridinic N indicates smaller average particle size, TEM images of AuPd/n-CNS-Tc-800 and AuPd/n-CNS-Tc-1000 were carried out in Figure S11. 21



The average particle size of AuPd/n-CNS-Tc-800 and AuPd/n-CNS-Tc-1000 is 2.98 and 2.02 nm， respectively. Because the ratio of graphitic N to pyridinic N in n-CNS-Tc-800 and n-CNS-Tc-1000 are lower than that of n-CNS-Tc-900, the average particle size of AuPd/n-CNS-Tc-800 and AuPd/nCNS-Tc-1000 are larger than that of AuPd/n-CNS-Tc-900, and the catalytic performance of AuPd/nCNS-Tc-800 and AuPd/n-CNS-Tc-1000 are inferior to AuPd/n-CNS-Tc-900. The high nitrogen content of n-CNS-Tc-800 even makes the metal NPs connect with each other. Specifically, even though the nitrogen content of AuPd/n-CNS-Tc-800 is the highest, AuPd/n-CNS-Tc-800 has the lowest ratio of graphitic N to pyridinic N and exhibits the worst catalytic performance for the decomposition of FA. The higher ratio graphitic N to pyridinic N can modify the distribution of electron density of metal and minimize the size of the metal nanoparticles. These should be the reason to explain why higher ratio graphitic N to pyridinic N shows better catalytic performance. To calculate the apparent activation energy for the dehydrogenation reaction catalyzed by AuPd/n-CNS-Th-160 catalyst, the time-dependent gas generation at different temperature is conducted (Figure S12). Arrhenius equation is applied to plot the logarithmic TOF vs. 1/T. The apparent activation energy is determined to be 28.7 kJ mol-1 and the initial TOF reaches as high as 1896 h-1 at 60 oC which is among the highest values in heterogeneous catalysts to our knowledge. The durability of the catalyst is crucial for the practical application. Therefore, we tested the stability of AuPd/n-CNS-Th-160 and the representative cycling curves are shown in Figure S13. As can be seen, the productivity of hydrogen remained almost unchanged after two cycles. TEM and XRD were employed to characterize the stability of the catalyst after 3 cycles. TEM images from Figure S14 show that the morphology and structure of the catalyst maintain almost the same after the catalytic test. From XRD pattern in Figure S15, we can know that there is no difference between 22


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the catalysts before and after the catalytic performance test demonstrating no aberration of metals. It is speculated that there was some losses of supported catalyst or degree of nanoparticles detachment from the support during the test. We also cannot rule out some by-products or intermediates were trapped in the porous catalyst, leading to the deactivation. Therefore, more detailed study is required. Catalyst poisoning caused by CO must be inhibited for its inherent damage to a number of catalytic reactions. The evolving gas mixture is characterized by gas chromatograph (GC) as shown in Figure S16. The image in the upper right corner is the enlarged image of the dashed rectangle. The composition of generated gas is H2 and CO2 without any CO detected which means that the released CO-free H2 can be used directly in fuel cell applications.

4. Conclusions In summary, AuPd alloy NPs uniformly dispersed on nitrogen-modified carbon nanosheets are prepared successfully via a simple wet impregnation process by using NaBH4 as the reductant. AuPd/n-CNS-Th-160 is capable of providing excellent catalytic activity reaching an initial TOF of 459 h-1 at 25 oC and a record value of 1896 h-1 at 60 oC among the heterogeneous catalysts along with 100% generation and selectivity to CO2 and H2 for the dehydrogenation of formic acid. The high catalytic performance derives from the coordinated Pd alloying and nitrogen-decorated carbon nanosheets. The mass of nitrogen dopant has no direct correlation with the catalytic activity, while the nitrogen bonding configurations, i.e. the higher ratio of graphitic N to pyridinic N can modify the electron structure distribution of metal and facilitate the interaction between AuPd metal NPs and support. Besides, the higher ratio of graphitic N to pyridinic N can minimize the size of the metal nanoparticle, thereby greatly enhanced the catalytic performance. The finding is of 23



significance for the application of N-decorated carbon materials on other catalytic systems and is helpful in improving the environmentally energy-related processes.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.XXX/acs.jpcc.XXXXX. Acknowledgements This work was supported by the National Natural Science Foundation of China (51671173, 51571179 and 21303161), and the Program for Innovative Research Team in University of Ministry of Education of China (IRT13037).

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AuPd Nanoparticles Anchored on Nitrogen-Decorated Carbon

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