PdCoNi nanoparticles supported on nitrogen-doped porous carbon nanosheets for room temperature dehydrogenation of formic acid

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PdCoNi nanoparticles supported on nitrogendoped porous carbon nanosheets for room temperature dehydrogenation of formic acid Zhong Dong a, Feiyu Li a, Qiang He a, Xuezhang Xiao a,*, Man Chen a, Chuntao Wang b,**, Xiulin Fan a,c,***, Lixin Chen a,d,* a

State Key Laboratory of Silicon Materials and School of Materials Science and Engineering, Zhejiang University, Hangzhou, 310027, PR China b College of Naval Architecture and Mechanical-electrical Engineering, Zhejiang Ocean University, Zhoushan, 316000, PR China c Department of Chemical and Biomolecular Engineering, University of Maryland, College Park, MD, 20742, USA d Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province, Hangzhou, 310013, PR China

article info

abstract

Article history:

The development of cost-effective heterogeneous catalysts for the dehydrogenation of

Received 22 January 2019

formic acid (FA) is the key challenge for the commercialization of FA as a hydrogen-storage

Received in revised form

medium. Herein, PdCoNi nanoparticles (NPs) with different element ratios supported on N-

16 March 2019

doped carbon nanosheets (N-CN) were designed, which exhibit excellent catalytic dehy-

Accepted 19 March 2019

drogenation performance for FA. Compared with PdCoNi NPs loaded on the carbon

Available online 11 April 2019

nanosheets (CN), the introduction of pyrrolic N to CN induces the formation of ultrafine, monodispersed and amorphous Pd0.6Co0.2Ni0.2 NPs with a size of 1.60 nm, which signifi-

Keywords:

cantly increases the number of active sites and the instant contact between FA and cata-

Heterogeneous catalysis

lysts. The as-prepared Pd0.6Co0.2Ni0.2/N-CN catalyst shows more than 99% conversion and

Formic acid

100% H2 selectivity at room temperature, with a record-high initial turnover frequency

Carbon nanosheets

(TOFinitial) of 1249.0 h
1 among non-noble containing Pd-based catalysts, which demon-

Pd0.6Co0.2Ni0.2

strates the high potential of Pd0.6Co0.2Ni0.2/N-CN as a practical catalyst for the hydrogen

Pyrrolic N

generation from FA. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

* Corresponding authors. Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province, Hangzhou, 310013, PR China. ** Corresponding author. *** Corresponding author. State Key Laboratory of Silicon Materials and School of Materials Science and Engineering, Zhejiang University, Hangzhou, 310027, PR China. E-mail addresses: [email protected] (X. Xiao), [email protected] (C. Wang), [email protected] (X. Fan), [email protected] (L. Chen). https://doi.org/10.1016/j.ijhydene.2019.03.155 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

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Introduction In recent decades, fossil fuels have been considered as the most important resource for our society, but the use of fossil fuels has leaded to a series of environmental issues at the meantime [1]. Hydrogen is one of the renewable resources which could replace fossil fuels in the near future [2,3]. However, budget and controlled storage/release of H2 is still one of the biggest problems which hinder society from using hydrogen energy [4,5]. FA (HCOOH) is regarded as an excellent hydrogen carrier due to the following advantages: (a) high hydrogen content and stability, (b) nontoxicity and (c) simple synthesis [6e9]. There are two methods to decompose FA, dehydrogenation (HCOOH / CO2 þ H2) or dehydration (HCOOH / CO þ H2O) [10]. For practical applications, the latter should be avoided. So far, many homogeneous catalysts have been reported to be highly efficient for dehydrogenation of FA [11e13]. However, these catalysts have significant drawbacks such as separation issues, poisonous organic solvents and additives, which prevent them from being utilized in fuel cells. Therefore, heterogeneous catalysts, which need relatively simple catalytic conditions, are more suitable for the application of FA dehydrogenation [14]. Until now, progress has been made in the effective dehydrogenation of FA with heterogeneous catalysts at near room temperatures [14e16]. Among the reported heterogeneous catalysts, many methods have been employed to promote the possibility of practical applications: (1) Adding additional metal elements, such as Au [17], Ag [18], Co [19], Ni [20], MnOx [21] and rare earth elements [22]. (2) Seeking special ingredient or structure supports, like kinds of carbon supports [23e26], TiO2 [27], SiO2 [28], MOFs [29] and so on. (3) Devising new morphology, for instance, core-shell [30], hollow [31] and alloy [32] structure. (4) Using extra additives (HCOONa, NR3, LiBF4, etc.) [33,34]. Although pure Pd catalysts possess extremely superior catalytic performances [35,36], the cost of metal remains high inevitably. Recently, the catalysts [37,38], containing~40 at% of non-noble metals, have attracted much interest, which could reduce the cost of noble metal successfully. However, most of these high active catalysts still adopted graphene or N-doped graphene as supports which dramatically increased the cost due to the complicated synthesis route [38e43]. Metals supported on other supports, such as TiO2, SiO2, MOFs or activated carbon, can't bear comparison with the performance of the former. Theoretically, cost-effective CN or N-CN could be employed as an ideal support to replace graphene or N-doped graphene. In this work, we have combined CN and (3-Aminopropyl) triethoxysilane (APTS) to form N-CN, which successfully increases the content of pyrrolic N in the support. And then we successfully anchored Pd0.6Co0.2Ni0.2 NPs on N-CN as an excellent catalyst for FA, which exhibits high catalytic activity (TOFinitial ¼ 1249.0 h
1) and hydrogen selectivity (CO＜10 ppm) at room temperature. On the other hand, thanks to the facile and easy scaling-up synthesis method, the Pd0.6Co0.2Ni0.2/NCN is much cheaper than most catalysts ever reported [16]. Compared with Pd0.6Co0.2Ni0.2/CN, we designed Pd0.6Co0.2Ni0.2/ N-CN catalyst with much smaller and uniformly dispersed

Pd0.6Co0.2Ni0.2 NPs shows more than 6 times higher catalytic activity.

Experimental Material Anhydrous ethanol (C2H5OH, >99%), urea (CH4N2O, >99%), glucose (C6H12O6$H2O, >99%), sodium borohydride (NaBH4, >96%) and cobaltous chloride hexahydrate (CoCl2$6H2O, >99%) were obtained from Sinopharm Chemical Reagent Co., Ltd. Sodium formate (CHNaO2, 99.5%), APTS (C9H23NO3Si, 99%), npropyltriethoxysilane (NPTS, C9H23O3Si, 98%), FA (HCOOH, >98%), sodium tetrachloropalladate (Na2PdCl4, 98%) and palladium on carbon (Pd/C, 5 wt% Pd basis) were supplied from Shanghai Aladdin Bio-Chem Technology Co., Ltd. Nickel chloride hexahydrate (NiCl2$6H2O, 98%) was purchased from Alfa Aesar Chemical Co., Ltd. All reagents were used without further purification.

Synthesis of g-C3N4 g-C3N4 was prepared according to the reported procedure [44]. In brief, a covered crucible, which contained 20 g of ground urea powders, was transferred to a muffle furnace, heated to 823 K at a ramp rate of 5 K min
1 and kept this temperature for 4 h. After that, the crucible was cooled to room temperature to obtain g-C3N4.

Synthesis of CN The preparation of CN was based on a reported method [45]. In detail, 1.5 g g-C3N4 was mixed with 40 mL deionized water containing 1.8 g C6H12O6$H2O. After sonicating and stirring for 6 h, the uniformly dispersed suspension was transferred into a Teflon-lined stainless-steel autoclave, and then heated to 433 K and kept for 15 h in an oven. After that, the g-C3N4@Glucose was washed with deionized water and ethanol for four times, and dried under vacuum at 333 K for 24 h. Subsequently, CN was obtained by heating g-C3N4@Glucose at 1173 k for 1 h under the pure nitrogen flow.

Synthesis of PdCoNi/N-CN 0.05 g CN was ultrasonically dispersed in 10 mL deionized water for 30 min to obtain the well-dispersed CN suspension. Then 0.2 mL APTS was added into the suspension with magnetic stirring. In an hour, NiCl2 (0.05 M, 0.4 mL), CoCl2 (0.05 M, 0.4 mL) and Na2PdCl4 (0.05 M, 1.2 mL) were added into the above suspension. After stirring overnight, NaBH4 (1.5 M, 1.0 mL) was injected into the suspension quickly with magnetic stirring for 2 h. Finally, the Pd0.6Co0.2Ni0.2/N-CN was centrifuged and washed with deionized water and ethanol for four times, and then, dried under vacuum at 333 K. By the way, other samples with different Pd:Co:Ni molar ratios were synthesized in the same way. Particularly, Pd0.6Co0.2Ni0.2/CN and Pd0.6Co0.2Ni0.2/g-C3N4 were prepared without the addition of APTS, and Pd0.6Co0.2Ni0.2/Si-CN was prepared with NPTS instead of APTS.

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Catalytic activities for dehydrogenation of FA Before the test, Pd0.6Co0.2Ni0.2/N-CN (nmetal ¼ 0.1 mmol) and 8 mL water were magnetically stirred in a two-necked flask and preheated to the setting temperature. The temperature remained constant during the test. One of the necks was connected to a gas burette to measure the volume of gas, and the other sealed by a rubber tube was used to inject the FA/ SF(FA:SF ¼ 1:1, 2.5 M, 2 mL) solution into the flask.

Characterizations The microstructure of samples was investigated by transmission electron microscopy (TEM, FEI Tecnai G2 F20 (HR)). Fourier transform infrared spectroscopy (FTIR, Burker Tensor 27) and X-ray photoelectron spectrometry (XPS, VG ESCALAB MARK II, Mg Ka) were applied for chemical composition analyses of samples. The binding energy values were calibrated by C 1s (284.6 eV). X-ray diffraction (XRD, PANalytical the Netherlands) was used to characterize the phase composition and crystallinity of samples. Specific surface areas and pore size distributions were determined by Brunauer-EmmetTeller (BET) method on a Quantachrome NOVA 1000e adsorption analyser. Inductively coupled plasma-atomic emission spectroscopy (ICP-AES, Thermo Fisher Scientific iCAP6300) was used to analyze the actual chemical compositions of metals in samples. CO2 and CO were identified by gas chromatography (GS, SP-6890) with thermal conductivity detector and flame ionization detector.

Results and discussion As shown in Fig. S1, the g-C3N4 support possesses a sheet-like structure with distinct porosity. After hydrothermal treatment and calcination, the as-formed CN support also shows an analogous sheet-like structure which is even thinner than g-C3N4. Besides, the BET specific surface area of CN is 1377.9 m2/g (Fig. S2b), which is about 10 times that of g-C3N4 (122.3 m2/g, Fig. S2a). The pore size distribution of g-C3N4 (Fig. S2c) and CN (Fig. S2d) are around 2.9 nm and 5.0 nm, respectively. In addition, after hydrothermal treatment and calcination steps, the pore volume of sample is also increased from 0.202 to 5.93 cm3 g
1. Thus it can be seen that the hydrothermal-calcining process during synthesis exerts a

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great influence on the specific surface areas and pore structures, and results in the formation of a planar twodimensional material CN finally. Therefore, CN should be an ideal support to prepare the N-modified support to load different ratios of Pd:Co:Ni. The synthesis procedure of the PdCoNi/N-CN is schematically illustrated in Scheme 1. Firstly, N-CN was synthesized using APTS and CN; after that, PdCoNi NPs were coprecipitated and in situ anchored on N-CN, forming the composite of the PdCoNi/N-CN. Fig. 1 shows the TEM images of the as-prepared Pd0.6Co0.2Ni0.2/g-C3N4, Pd0.6Co0.2Ni0.2/CN and Pd0.6Co0.2Ni0.2/N-CN. It is obvious that the as-prepared Pd0.6Co0.2Ni0.2 NPs are dispersed on g-C3N4 with an average particle size of about 3.37 nm (Fig. S3a), while the NPs supported on CN are uniformly dispersed with an average particle size of about 2.60 nm (Fig. S3b). Besides, high-resolution TEM (HRTEM) image of Pd0.6Co0.2Ni0.2/CN (Fig. S4a) shows the crystalline nature of NPs, and the lattice spacing is measured to be 0.220 nm, which is between the (111) planes of face-centered cubic Pd (0.224 nm) [38], Co (0.204 nm) [46]and Ni (0.203 nm) [38]. Meanwhile, the selected area electron diffraction (SAED) pattern of Pd0.6Co0.2Ni0.2/CN shows two diffraction rings corresponding to the (111) and (311) of Pd (Fig. S4b). After adding APTS, the size of Pd0.6Co0.2Ni0.2 is further decreased to 1.60 nm (Fig. 1e and f), the thickness of Pd0.6Co0.2Ni0.2 is greatly thinned and the uniformity of Pd0.6Co0.2Ni0.2 is highly improved concurrently. Due to the high defects, the lattice spacing can't be detected in PdCoNi NPs (Fig. 1e), and only a broadened diffuse diffraction ring can be observed in Fig. 1g, which means the significant amorphous feature of PdCoNi NPs. As shown in Fig. 1h, the presence of Si indicates that APTS not only affects size and morphology of PdCoNi NPs, but also combines with the CN successfully. And the distribution of Pd, Co and Ni further proves the NPs have excellent dispersion. The energy dispersive X-ray Spectroscopy (EDS) analysis of Pd0.6Co0.2Ni0.2/N-CN (Fig. S5) shows a pattern containing element of Pd, Co and Ni, with a relative molar content of 58.9%, 17.1% and 23.9%, respectively, which agrees well with the design ratio and ICP-AES result (Table S1). Based on the above results, it can be reasonably believed that, after N doping, the N-CN highly increases the dispersion and bidimensionality of PdCoNi, decreases the crystalline nature of PdCoNi, and meanwhile promotes the transformation of PdCoNi from ~2.60 nm to ~1.60 nm, which

Scheme 1 e Schematic illustration for the preparation procedure of PdCoNi/N-CN catalyst.

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Fig. 1 e TEM images of: (a) Pd0.6Co0.2Ni0.2/g-C3N4, (b) Pd0.6Co0.2Ni0.2/CN and (c,d) Pd0.6Co0.2Ni0.2/N-CN samples. HRTEM image (e), size distributions(f), SEAD image (g) and high-angle annular dark-field scanning transmission electron microscopy image and elemental distribution (h) of Pd0.6Co0.2Ni0.2/N-CN sample.

may significantly improve the catalytic activity of Pd0.6Co0.2Ni0.2. The XRD patterns of Pd0.6Co0.2Ni0.2/g-C3N4, Pd0.6Co0.2Ni0.2/ CN and Pd0.6Co0.2Ni0.2/N-CN samples are shown in Fig. 2a. Compared with the XRD patterns of g-C3N4, CN and N-CN (Fig. S6), there is no change in the peaks belonging to the supports. The peaks located at 13.0 and 27.4 are ascribed to (100) and (002) crystal planes of g-C3N4 [44]. And the CN and NCN have the broad peaks at around 24.0 , which belong to (002) crystal plane of C. As shown in Fig. 3a, besides the diffraction peaks of the supports, the XRD patterns of Pd0.6Co0.2Ni0.2/gC3N4 and Pd0.6Co0.2Ni0.2/CN show the peaks are approximately consistent with the diffraction peaks of Pd (JCPDS:65-2867) [38], which slightly traverse towards Co (JCPDS:15-0806) [46] and Ni (JCPDS:65-2865) [38]. On the other hand, the XRD pattern of Pd0.6Co0.2Ni0.2/N-CN shows no Bragg peak corresponding to Pd, Co and Ni, which agrees well with the HRTEM and SAED images of Pd0.6Co0.2Ni0.2/N-CN (Fig. 1e and f). The results of XRD and HRTEM indicate the poor crystallinity of Pd0.6Co0.2Ni0.2 NPs supported on N-CN. And the XRD pattern of Pd0.6Co0.2Ni0.2/N-CN after heat treatment further confirms the amorphous structure of the Pd0.6Co0.2Ni0.2 NPs (Fig. S7), in which all the diffraction peaks are located between metallic Pd, Ni and Co.

FTIR spectra for Pd0.6Co0.2Ni0.2/g-C3N4, Pd0.6Co0.2Ni0.2/CN and Pd0.6Co0.2Ni0.2/N-CN are demonstrated in Fig. 2b. For the Pd0.6Co0.2Ni0.2/g-C3N4, the vibration at 810 cm
1 is origin from s-triazine ring units, and the vibrations at the region of 1100e1600 cm
1 are attributed to the stretching vibrations of CN heterocycles [45]. Moreover, the vibrations at 881, 1637, 3178 and 3253 cm
1 are assigned to amine (-NH2) group [45]. The FTIR spectrum exhibits several obvious variations after using CN as a support instead of g-C3N4. The First one is the disappearance of the variations which belong to -NH2 group. Secondly, the vibrations of CN heterocycles and s-triazine ring units are replaced by the C]C bonds and C-N bonds at 1203, 1560 cm
1, respectively [47]. Compared with g-C3N4, these variations indicate that the decrease of nitrogen content in CN. The third is the appearance of the adsorption state of CO2 [48]and H2O [49] vibrations at 2354 and 3423 cm
1, which is due to the high specific surface area of CN. For Pd0.6Co0.2Ni0.2/N-CN, compared with Pd0.6Co0.2Ni0.2/CN, the appearance of Si-O, Si-C and -CH2- [50], suggests the existence of APTS on the CN to form N-CN, which agrees very well with the EDS image of Pd0.6Co0.2Ni0.2/N-CN. In order to study influence of APTS to CN in Pd0.6Co0.2Ni0.2/ N-CN, high-resolution XPS analyses were applied. The N 1s

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Fig. 2 e XRD patterns (a) and FITR spectra (b) of Pd0.6Co0.2Ni0.2/g-C3N4, Pd0.6Co0.2Ni0.2/CN and Pd0.6Co0.2Ni0.2/N-CN samples.

Fig. 3 e High resolution XPS spectra of N 1s (a) in Pd0.6Co0.2Ni0.2/CN and Pd0.6Co0.2Ni0.2/N-CN samples. High resolution Pd 3d (b), Co 2p (c) and Ni 2p (d) XPS spectra with Pd0.6Co0.2Ni0.2/CN and Pd0.6Co0.2Ni0.2/N-CN, Pd/N-CN, Co/N-CN and Ni/N-CN specimens.

peak of CN (Fig. 3a) can be resolved into two individual peaks centered at 398.1 and 400.7 eV, which are related to pyridinic N and graphitic N, respectively [51]. After adding APTS to CN, a new strong peak centered at 399.4 eV related to the pyrrolic N could be identified [51], indicating the successful doping of N in CN. Notably, the pyrrolic N content, which was origin from APTS entirely, increased from 0.0% to 43.5% by calculating the peak areas, while the pyridinic N content decreased from 25.4% to 14.1% and graphitic N content decreased from 74.6%

to 42.4% (Table 1). Taking into account both FTIR and XPS results, N element in Pd0.6Co0.2Ni0.2/N-CN is located as doping atoms in CN rather than -NH2 group. Therefore, the specimen is named as Pd0.6Co0.2Ni0.2/N-CN. Because pyridinic N and graphitic N have much better affinity to water molecules than pyrrolic N [52] (Fig. S8), the increased content of pyrrolic N increases the opportunity of sufficient contact for the FA with catalysts, which could further improve the catalytic activity of Pd0.6Co0.2Ni0.2.

Table 1 e The content of different type of nitrogen species in the samples from XPS. Catalyst Pd0.6Co0.2Ni0.2/CN Pd0.6Co0.2Ni0.2/N-CN

Pyridinic N (at.%)

Graphitic N (at.%)

Pyrrolic N (at.%)

Pyridinic N:Graphitic N:Pyrrolic N

25.4 14.1

74.6 42.4

0 43.5

1:2.94:0 1:3.01:3.08

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The charge transfer tendency among Pd, Co and Ni in Pd0.6Co0.2Ni0.2/N-CN sample is closely related to the catalytic activity of Pd0.6Co0.2Ni0.2. Pd 3d, Co 2p and Ni 2p core levels XPS spectra of Pd0.6Co0.2Ni0.2/N-CN, Pd/N-CN, Co/N-CN, Ni/N-CN and Pd0.6Co0.2Ni0.2/CN are given in Fig. 3. It can be seen that Pd, Co and Ni in the above samples are mostly at their zerovalence state [33], and the observation of oxide signals can be attributed to the surface oxidation of Pd, Co and Ni during samples preparing for XPS. The binding energies for Pd 3d, Co 2p and Ni 2p in Pd0.6Co0.2Ni0.2/N-CN are unchanged compared with those in Pd0.6Co0.2Ni0.2/CN, which means that no electrons are transferred from doping N atoms substrate to the Pd0.6Co0.2Ni0.2. However, compared with Pd/N-CN, the binding energy for Pd 3d in Pd0.6Co0.2Ni0.2/N-CN is shifted to the lower values, while the binding energies for Co 2p and Ni 2p are shifted to the high values in Co/N-CN and Ni/N-CN. These shifts demonstrate that some electrons are transferred from Co and Ni to Pd atoms due to their electronegativity differences (Pd (PE) ¼ 2.20; Co (PE) ¼ 1.88; Ni (PE) ¼ 1.91) [53]. Combined with the result of XRD pattern of Pd0.6Co0.2Ni0.2/N-CN after heat treatment (Fig. S7), the electron transfer indicates the formation of solid solution structure of Pd0.6Co0.2Ni0.2 NPs. Meanwhile, the alloy of Ni, Co and Pd increases the electron density of Pd, which enhances the stability of non-noble metals and improves the catalytic activity of Pd active sites simultaneously.

The catalytic activity of Pd0.6Co0.2Ni0.2/N-CN (6.71 wt% Pd) together with Pd0.6Co0.2Ni0.2/g-C3N4 (9.52 wt% Pd), Pd0.6Co0.2Ni0.2/CN (9.31 wt% Pd) (Table S2) and commercial Pd/C (5.0 wt% Pd) for H2 generation from FA decomposition at 298 K in the presence of SF is shown in Fig. 4a to study the effects of different support materials and pyrrolic N on the catalytic activity. In terms of supports, although the catalytic activity of Pd0.6Co0.2Ni0.2/g-C3N4 is inferior to Pd/C, the catalytic activity of Pd0.6Co0.2Ni0.2/CN is much better than that of Pd/C, which illustrates the importance of a suitable support for the catalyst. In addition, the doping of pyrrole N to CN further promotes the catalytic activity. Undoubtedly, the catalytic performance of Pd0.6Co0.2Ni0.2 immobilized on N-CN is the best among the four catalysts, which releases 243 mL of gas in only 10 min and achieves 99% conversion. The TOFinitial (Eq. S2) is measured to 1249.0 h
1 with additive SF at 298 K, which is a relatively high value among the noble catalysts and a record-high value among the nonnoble containing Pd-based catalysts (Table S3). The generated gas was identified by GC (Fig. S9), and no CO was detected (detection limit: CO＜10 ppm), indicating the excellent H2 selectivity for FA dehydrogenation by the assynthesized Pd0.6Co0.2Ni0.2/N-CN. In contrast, Pd0.6Co0.2Ni0.2/CN can release 240 mL gas in 30 min (TOFinitial ¼ 206.1 h
1), while Pd0.6Co0.2Ni0.2/g-C3N4 and Pd/C only release 32 mL gas and 99 mL in 40 min, respectively. Comparing

Fig. 4 e Gas generation by decomposition of FA/SF solution (0.5 M, 10 mL, nFA/nSF ¼ 1:1) with (a) commercial catalyst Pd/C and Pd0.6Co0.2Ni0.2 (nmetal/nFA ¼ 0.02) loaded on different supports, and (b) different Pd:Co:Ni (nmetal/nFA ¼ 0.02) molar ratios loaded on N-CN. (c) Gas generation by decomposition of FA/SF solution (0.5 M, 10 mL, nFA/nSF ¼ 1:1) with Pd0.6Co0.2Ni0.2/NCN (nmetal/nFA ¼ 0.02) at 298 K, 303 K, 308 K, 313 K and 318 K. (d) Arrhenius plot for the FA/SF decomposition by Pd0.6Co0.2Ni0.2/N-CN.

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Pd0.6Co0.2Ni0.2/CN, Pd0.6Co0.2Ni0.2/N-CN and Pd0.6Co0.2Ni0.2/ Si-CN (Fig. S10), the fundamental reason for such a dramatic change in catalytic activity comes from the introduction of pyrrole N, not from Si. Due to the presence of pyrrole N, Pd0.6Co0.2Ni0.2/N-CN has smaller particle size, better particle dispersion and higher degree of amorphization than Pd0.6Co0.2Ni0.2/CN, and shows more than 6 times higher catalytic activity than Pd0.6Co0.2Ni0.2/CN, suggesting that pyrrolic N plays a significant role in behavior of the catalyst for FA dehydrogenation. These results highlight the key fact of the synergistic effect between Pd0.6Co0.2Ni0.2 NPs, CN and APTS. The catalytic activity of PdCoNi/N-CN with different Pd:Co:Ni molar ratios is shown in Fig. 4b to study the composition effect among Pd, Co and Ni. The results of ICP-AES (Table S1), prove that the actual molar ratios of Pd:Co:Ni in the samples are consistent to nominal molar ratios. Except Co/N-CN and Ni/N-CN, the prepared PdCoNi/ N-CN shows the excellent catalytic activity, which depends on the composition of PdCoNi. As shown in Fig. 4b, both high catalytic activity and low cost are obtained, when Co and Ni addition comes to 40 at%. Monometallic Pd/N-CN only shows lower performance with the TOFinitial of 886.4 h
1. Although the catalytic activity of Pd0.8Co0.1Ni0.1/N-CN is little higher (TOFinitial ¼ 1322.4 h
1) than that of Pd0.6Co0.2Ni0.2/N-CN, the cost of the catalyst rises too much. When the proportion of Co and Ni exceeds 40 at%, the activity of trimetallic system deceased sharply, with the TOFinitial of 705.9 h
1 and 375.0 h
1 for Pd0.4Co0.3Ni0.3/N-CN and Pd0.2Co0.4Ni0.4/N-CN. The catalytic activity of Pd0.6Co0.2Ni0.2/CN is not comparable to Pd0.2Co0.4Ni0.4/N-CN yet, which further indicates the importance of pyrrole N. In addition, it can be confirmed that the catalytic activity of bimetallic catalysts is inferior to that of trimetallic catalysts (Fig. S11). These results indicate that alloying appropriate content of Co and Ni to Pd can significantly improve the catalytic activity of Pd, which increases from 886.4 h
1 to 1249.0 h
1 or even higher. To further research the influence of temperature on dehydrogenation of FA catalyzed by Pd0.6Co0.2Ni0.2/N-CN, the dehydrogenation reactions were tested with different temperature from 298 K to 318 K. The Arrhenius plot of ln k vs. 1/T for the catalyst is plotted Fig. 4d, and the value of apparent activation energy (Ea) is calculated to be approximately 20.04 kJ mol
1. After the completion of hydrogen generation, re-adding another equivalent of FA (5 mmol) to the reaction flask to test the catalytic durability of catalyst. Slight decrease in the catalytic activity of Pd0.6Co0.2Ni0.2/N-CN between the first cycle and the second cycle can be detected (Fig. S12). ICP-AES analyses of the recovered Pd0.6Co0.2Ni0.2/N-CN catalyst after the first cycle gave a little different Pd, Co and Ni ratio with that of the fresh catalyst, which was similar to the value of the recovered Pd0.6Co0.2Ni0.2/N-CN catalyst after the fourth cycle. Since the size of NPs has no obvious change during the cycle test (Fig. S13), the reduction of non-noble metals in catalysts, which decreases the electron density of Pd (Fig. S14), is the key reason for the decrease in the catalytic activity between the first cycle and the second cycle, and the slight decrease in the catalytic activity between the second and fourth cycles. Based on the above, the reusable catalyst still can release 232 mL of

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gas in 10 min and achieves 94% conversion in the 4th catalytic reuse, which is better than Pd0.8Co0.1Ni0.1/N-CN (Fig. S15). Therefore, we consider that Pd0.6Co0.2Ni0.2/N-CN is the optimal catalyst for this paper.

Conclusions In this work, PdCoNi NPs anchored on N-CN were successfully synthesized by co-reduction of CN, APTS and the metal precursors. The introduction of pyrrole N significantly decreases the metal particle size to less than 2.0 nm. Thus also reduces the crystallinity of the metal NPs and provides the sufficient contact for FA with catalysts, resulting in the improvement of catalytic activity successfully. Besides, taking into account both cost and catalytic activity, the resultant Pd0.6Co0.2Ni0.2/NCN is the optimum catalyst to facilitate the liberation of COfree H2 from FA/SF aqueous solution at room temperature with a highest TOFinitial of 1249.0 h
1. The reduction of the noble metals ratio and the replacement of graphene support may promote the practical application of FA as a favorable H2 storage/generation material.

Acknowledgements The authors gratefully acknowledge the financial supports for this research from the National Basic Research Program of China (2018YFB1502104), the National Natural Science Foundation of China (51671173 and 51571179), and the Program for Innovative Research Team in University of Ministry of Education of China (IRT13037).

Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.03.155.

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