J Mater Sci COMPOSITES Composites

Highly efficient metal–organic-framework catalysts for electrochemical synthesis of ammonia from N2 (air) and water at low temperature and ambient pressure Xinran Zhao1,3, Fengxiang Yin1,2,3,4,*, Ning Liu3,4, Guoru Li4, Tianxi Fan3, and Biaohua Chen3 1

State Key Laboratory of Organic–Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China 2 Advanced Catalysis and Green Manufacturing Collaborative Innovation Center, Changzhou University, Changzhou 213164, Jiangsu, People’s Republic of China 3 College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China 4 Changzhou Institute of Advanced Materials, Beijing University of Chemical Technology, Changzhou 213164, Jiangsu, People’s Republic of China

Received: 30 December 2016

ABSTRACT

Accepted: 4 May 2017

Metal–organic-frameworks (MOFs) (i.e., MOF(Fe), MOF(Co) and MOF(Cu)) were synthesized by a hydrothermal process. The prepared MOFs were characterized using X-ray diffraction, Fourier transform infrared spectroscopy and N2 adsorption–desorption. The catalytic activities of the MOFs for the electrochemical synthesis of ammonia were evaluated when using N2 (air) and water as raw materials at low temperature and ambient pressure. The results indicated that the prepared MOFs have fine crystalline structures, abundant micropores, and large specific surface areas. The prepared MOFs showed excellent catalytic activity for the electrochemical synthesis of ammonia at low temperature and ambient pressure. Among these MOFs, the MOF(Fe) displayed the best catalytic activity, and the highest ammonia formation rate and the highest current efficiency reached 2.12 9 10-9 mol s-1 cm-2 and 1.43%, respectively, at 1.2 V and 90 °C, when using pure N2 and water as raw materials. The prepared MOFs in this work showed remarkable catalytic activities for the electrochemical synthesis of ammonia at low temperature and ambient pressure among the nonnoble metal catalysts. It was the first exploration to apply MOFs as the electrocatalysts for the electrochemical synthesis of ammonia at low temperature and ambient pressure.

Ó

Springer Science+Business

Media New York 2017

Address correspondence to E-mail: [email protected]

DOI 10.1007/s10853-017-1176-5

J Mater Sci

Introduction Ammonia is an essential chemical terminal product and industrial intermediate, traditionally synthesized through the Haber–Bosch process on the commercial Fe-based catalysts at high temperature (400–600 °C) and high pressure (20–40 MPa) from H2 and N2 [1, 2]. In this process, the one-way conversion rate of hydrogen is relatively low (10–15%) because of the thermodynamic limitations [3]. Moreover, the massive fossil resources, such as coal and natural gas, are used to maintain such high temperature and produce H2, thus unavoidably consuming a large amount of energy and emitting much CO2 [4–6]. In order to increase the hydrogen conversion rate and break the link with fossil resources, many alternative methods to synthesize ammonia have been proposed in recent years, such as photochemical synthesis [7], enzyme-catalyzed synthesis [8] and electrochemical synthesis [9–13] for ammonia. Among these methods, the electrochemical synthesis of ammonia has been regarded as the most potential one because the one-way hydrogen conversion rate on theory at anode can reach 100% without thermodynamic restrictions [14]. In terms of the operating temperature, the electrochemical synthesis of ammonia can be mainly separated into the high temperature (over 500 °C) [15–20], medium temperature (200–500 °C) [21] and low temperature (below 100 °C) [22, 23] synthesises. Although the relatively high ammonia formation rate can be obtained at high temperature [24], there are still some disadvantages. Due to the kinetics of the ammonia formation process, NH3 will decompose at high temperatures. In order to meet this challenge, a few literatures tried to find a solution at the medium temperature zone (200–500 °C). Cui et al. [21] synthesized ammonia using N2 and water as raw materials on activated carbon-supported Fe-based catalysts. The maximum ammonia formation rate reached 8.27 9 10-9 mol s-1 cm-2 at 250 °C and 1.15 V. However, ammonia will decompose into H2 and N2 when it is over 175 °C [25], thus leading to the decrease in the ammonia formation rate at medium temperature. In addition, the massive energy is used to keep the high temperature, which greatly increases the ammonia synthesis cost. The electrochemical synthesis of ammonia at low temperature has been developed rapidly since Rod et al. [26] and Ko¨leli

et al. [27] demonstrated the theoretical and experimental possibilities of the electrochemical synthesis of ammonia at low temperature, respectively. Although the electrochemically synthesizing ammonia at low temperature can overcome the defects above, H2 is always utilized as the raw material. At present, H2 is industrially produced via the water splitting and natural gas reforming, which consumes massive energy and discharges a great deal of CO2. More recently, water, as an easily available hydrogen source, has been directly utilized as the raw material to electrochemically synthesize ammonia at low temperature [28]. For example, Lan et al. [5] synthesized ammonia using N2 and water as the raw materials. The ammonia formation rate reached 9.37 9 10-10 mol s-1 cm-2 at 80 °C and 1.2 V on Pt/ C catalyst. Nevertheless, the electrochemical ammonia formation rate at low temperature was relatively low because of the fact that the reaction temperature could not satisfy the kinetics. Great efforts have been made to enhance the ammonia formation rate at low temperature in recent years. On one hand, many highly efficient proton exchange materials, such as Nafion membrane [29, 30], Li?/H? hybrid membrane [5] and strontia-ceria-ytterbia (SCY) layer [3], have been applied to facilitate H? conduction and thus improve the ammonia formation rate. Lan et al. [29] applied the Nafion-211 membrane to the electrochemical synthesis of ammonia directly from air and water at ambient temperature. In particular, a mixed NH4?/H? conducting Nafion-211 membrane formed during the reaction, and it was proved to own excellent proton conduction. On the other hand, the highly efficient catalysts have been developed to reduce the activation energy of the ammonia formation and thus speed up the entire reaction [31, 32]. Up to now, the noble metal catalysts, such as Pd, Ru and Pt, have shown the best catalytic activity for the electrochemical synthesis of ammonia at low temperature [33, 34]. However, the limited reserve and high price of these noble metals encourages us to develop non-noble metal catalysts for the electrochemical synthesis of ammonia [35]. Renner et al. [36] synthesized ammonia on a Fe-based catalyst using N2 and water as raw materials at ambient temperature. The ammonia formation rate reached 3.8 9 10-12 mol s-1 cm-2 at 50 °C and 1.2 V. Despite the great progress made in the non-noble metal catalysts for the electrochemical synthesis of ammonia at low

J Mater Sci

temperature, there is still a large gap in the catalytic activity to catch up with the noble metal catalysts. Metal–organic-frameworks are a class of porous materials assembled with metal ion centers/clusters and organic ligands. MOFs own fantastically high specific surface areas, easily tunable structures and functions. In particular, the functional groups and the metal ion centers in MOFs can be directly designed as the catalytic active sites in the designing stage. These advantages permit MOFs to be widely applied in many fields, such as gas storage/separation [37], chemical sensor [38], catalysis [39] and sewage disposal [40]. More recently, MOFs have attracted plenty of attention in the electrochemical catalysis [41]. However, to the best of our knowledge, there is no report that MOFs have been used as the cathode catalysts in the electrochemical synthesis of ammonia at low temperature. Herein, a series of MOFs (i.e., MOF(Fe), MOF(Co) and MOF(Cu)) were synthesized by a hydrothermal process, and their catalytic activities for the electrochemical synthesis of ammonia were investigated using water and N2 (or air) as raw materials at low temperature and ambient pressure. Among the prepared MOF catalysts, MOF(Fe) displayed the best catalytic activity, and its highest ammonia formation rate reached 2.12 9 10-9 mol s-1 cm-2 at 1.2 V and 90 °C using H2O and pure N2 as raw materials. In addition, ammonia was also synthesized using air and water as raw materials on the MOF(Fe) catalyst. This work may open up a new direction to develop highly efficient MOFs-based catalysts with low cost for the electrochemical synthesis of ammonia at low temperature and ambient pressure.

temperature. After stirring for 3 h, the obtained mixture was transferred into a 100 mL Teflon-lined stainless steel autoclave. The autoclave was heated in the oven from room temperature to 150 °C and maintained at this temperature for 72 h. After cooling naturally to room temperature, the obtained product was filtered and washed using deionized water under 80 °C for 6 h, and then washed by ethanol at 60 °C for 6 h, and eventually the MOF(Fe) was obtained by drying at 60 °C in air for 5 h. A 7 mmol Co(CH3COO)24H2O was dissolved into 20 mL methanol, and 70 mmol 2-methylimidazole was dissolved into 20 mL methanol under drastic magnetic stirring, respectively. After stirring for 1 h, the former pink solution containing Co(CH3COO)2 was dropwise added into the latter clear solution containing 2-methylimidazole under drastic stirring. After stirring for 2 h, the resulting mixture was transferred into a 100 mL Teflon-lined stainless steel autoclave. It was heated in the oven from room temperature to 120 °C and kept at this temperature for 24 h, then naturally cooling to room temperature. The obtained product was filtered and washed by deionized water at 80 °C, and then washed by ethanol at 60 °C for several times. The MOF(Co) was eventually obtained by drying at 60 °C in air for 5 h. A 3.6 mmol Cu(NO3)23H2O was dissolved into 20 mL methanol, and 2 mmol 1,3,5-BTC was dissolved into 20 mL methanol under drastic magnetic stirring, respectively. After stirring for 2 h, the former blue solution containing Cu(NO3) was dropped into the latter clear solution containing 1,3,5-BTC under stirring, and the following treatment was similar to that of the MOF(Co).

Experimental

Characterizations of MOFs

Preparation of MOFs

X-ray diffraction (XRD) patterns of the MOFs were obtained on the D/max-2500/PC X-ray powder diffractometer (Rigaku, Japan) with the Cu-Ka radi˚ ) over a 2h range of 5°–80° with a ation (k = 1.54056 A step size of 0.02°. Fourier transform infrared spectra (FTIR) of the MOFs were collected on the Prote´ge´460 Fourier infrared spectrometer (Nicolet, USA) within the wave range of *400 to 4000 cm-1. The N2 absorption–desorption isotherms of the prepared MOFs were collected by the ASAP 2020 automatic analyzer (Micromeritics, USA). The specific surface areas were calculated using the Brunauer-EmmettTeller (BET) method [41].

Analytically pure Fe(NO3)39H2O, Co(CH3COO)2 4H2O, Cu(NO3)23H2O and hydrofluoric acid (HF, C40 wt%) were purchased from Shanghai Xinbao Chemical Reagent Co., Ltd., 1,3,5-benzenetricarboxylic acid (1,3,5-BTC, 98%) and 2-methylimidazole (98%) were purchased from Aladdin Reagent (China) Co., Ltd. All the chemicals were utilized without further purification. A 7 mmol Fe(NO3)39H2O, 4.6 mmol 1,3,5-BTC and 14 mmol HF were dissolved into 35 mL deionized water under drastic magnetic stirring at room

J Mater Sci

Ammonia synthesis and measurements A 17.5 mg MOF (MOF(Fe), MOF(Co) or MOF(Cu)), 7.5 mg Super P (SP, Timcal, Switzerland) and 100 lL Nafion solution were dispersed in 1 mL absolute ethanol, then a homogeneous suspension ink was obtained after ultrasonic oscillation for 30 min. The catalysts were dispersed uniformly in it. The suspension ink was dropped onto a 1.0 9 3.0 cm2 hydrophobic carbon paper (TGP-H-060, Toray, Japan). The working electrode was obtained after drying at 60 °C for 10 h. The electrochemical synthesis of ammonia was carried out in a three-electrode cell using 2 M KOH aqueous solution as the electrolyte. As shown in Fig. 1, the working electrode and a brass wire mesh were pressed together to form the cathode, and a Nafion-117 membrane (Alfa Aesar, USA) was used as the proton conductor. A platinum wire was used as the anode, and a KCl-saturated Ag/ AgCl electrode was used as the reference electrode. The direct current (DC) voltage was applied to the cell by a computer-controlled CHI 760E electrochemical workstation (CH Instruments, China). The cell’s temperature was controlled by the water-bath heating. A N2 (or air) stream was imported into the cathode chamber at a flow rate of 15 mL min-1. The produced ammonia was collected with 25 mL H2SO4 solution (0.001 M). One milliliter Nessler’s reagent was dropped into the ammonia solution, and the uniform mixture was kept in darkness for 10 min to

Figure 1 The schematic representation of the electrolysis cell.

obtain a yellow solution. The absorbance of the solution was analyzed by the UV 5100B spectrophotometer (Metash, Shanghai), and the NH4? ion concentration was calculated by the equation of Lambert–Beer Law [42]: ½NHþ 4 ¼

A ab

ð1Þ

where [NH4?] represents the concentration of NH4? ion (mol L-1), A is the absorbance of the solution, a and b are constants determined by the standardcurve method reported previously [42]. The ammonia formation rate R(NH3) was calculated by the following equation [25]: R(NH3 Þ¼

½NHþ 4V tS

ð2Þ

where V stands for the volume of the dilute H2SO4 for ammonia collection (L), t is the absorbing time (s) and S is the effective area of the cathode (cm2).

Results and discussion XRD, FTIR and N2 adsorption–desorption characterizations Figure 2a shows the XRD patterns of the prepared MOFs. There are main diffraction peaks at (2h=) 3.40°, 6.12°, 6.80°, 10.10°, 10.86°, 14.14°, 17.64°, 18.46° and 19.92° in the MOF(Fe). The positions of these peaks are in agreement with the main diffraction peaks of MIL-100(Fe) [40], indicating that the MOF(Fe) has been successfully synthesized. There are main diffraction peaks located at (2h=) 7.28°, 10.30°, 12.64°, 17.96°, 22.06°, 24.42°, 25.56°, 26.58° in the MOF(Co). These peaks’ positions are in good agreement with those of zeolite imidazole framework-67 (ZIF-67) [43], demonstrating the MOF(Co) has the standard structure of ZIF-67. As for the MOF(Cu), the main diffraction peaks located at (2h=) 6.66°, 9.46°, 11.58°, 13.38°, 16.42°, 17.42°, 18.98°, 20.18°, 25.90°, 29.28° are in consistent with the main diffraction peaks of Cu-BTC (HKUST-1) [38], revealing that the MOF(Cu) has been synthesized well. Figure 2b displays the FTIR patterns of the prepared MOFs. For the MOF(Fe) and MOF(Cu) samples, the similar FTIR patterns are observed due to the same organic ligand 1,3,5-BTC. The peaks near 3000 and 3500 cm-1 are assigned to C–H in 1,3,5-BTC and O–H in water, respectively. The peaks near

J Mater Sci

30

40

50

Quantity adsorbed (cm3/g)

745 685 430 492

761 709

1386

1629

1456 1573 1413 1303 1113 1587 1453 1370

1630

3500 3000 2500 2000 1500 1000 500

60

Wavenumber (cm-1)

2 Theta (degree)

600

1709 1632 1578 1452

MOF(Fe) 20

2975

3430

MOF(Fe) 2980

MOF(Co)

MOF(Co) 3420

Transmittance (%)

MOF(Cu)

10

(b)

MOF(Cu)

448

(a)

Intensity (a.u.)

Figure 2 XRD patterns (a), FTIR patterns (b) and N2 adsorption–desorption isotherms (c) of the MOF(Fe), MOF(Co) and MOF(Cu).

(c)

500 400 300

MOF(Fe) ads MOF(Fe) des MOF(Co) ads MOF(Co) des MOF(Cu) ads MOF(Cu) des

200 100 0

0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure (P/P0)

1700 cm-1 indicate that the carboxylate groups in 1,3,5-BTC are deprotonated. The peaks around 1600 cm-1 and those around 1400 cm-1 are related to asymmetric and symmetric vibrations of COO in 1,3,5-BTC, respectively. The values of Dv(vas(COO)vs(COO)) in the MOF(Fe) are 180 and 192 cm-1, and those in the MOF(Cu) are 173 and 160 cm-1, indicating the carboxylate groups in 1,3,5-BTC are coordinated to Fe3? or Cu2? ions in a bidentate bridging mode [38, 40]. Besides, the peak at 448 cm-1 in the MOF(Fe) and the one at 430 cm-1 in the MOF(Cu) are because of Fe–O and Cu–O bonding tensile vibrations, respectively. In terms of the MOF(Co), the wide peak at 3420 cm-1 is due to O–H bonding in water. The peaks at 1300–1500 cm-1 represent the stretching mode of the whole aromatic ring in 2-methylimidazole. The peaks in the range of 900–1350 cm-1, 500–800 cm-1 are assigned to the in-plane bending and out-of-plane bending of the aromatic ring, respectively. There is no obvious peak in the range of

1800–2600 cm-1 (the feature of N–HN hydrogen bond and N–H bond in 2-methylimidazole). The result indicates that 2-methylimidazole is entirely deprotonated and coordinated with Co2? in a bridging mode [37]. The Co–N bonding vibration is obviously observed at 492 cm-1. The N2 adsorption–desorption isotherms of the MOFs are shown in Fig. 2c. The curve of the prepared MOF(Fe) displays the type I isotherm, the isotherm of micropore, based on International Union of Pure and Applied Chemistry (IUPAC) classification. With the relative pressure p/p0 increasing from 0 to 0.15, the amount of adsorbed N2 increases rapidly, then increases slowly with p/p0 further increasing. The similar trend can be observed in the prepared MOF(Co) and MOF(Cu), but their N2 adsorbed quantities increase more rapidly than the MOF(Fe) when p/p0 increases from 0 to 0.025, and their further increases are more gentle than the MOF(Fe). In the case of the MOF(Co) and MOF(Cu), the former initially reaches higher and then displays a slower

J Mater Sci

1.4 V 1.3 V 1.2 V 1.1 V 1.0 V

-2

Current density (mA cm )

70

60

50

40

30

0

3000

6000

9000

12000

Time (s)

Figure 3 Current density-time curves on the MOF(Fe) at different applied voltages at 80 °C.

increasing trend at higher relative pressures till it finally ends at the same value as the latter. These results indicate the prepared MOFs possess abundant micropores. The MOFs have extremely large specific surface areas and pore volumes. The specific surface areas and pore volumes are 1491 m2 g-1, 0.342 cm3 g-1 for the MOF(Fe), 2011 m2 g-1, 0.671 cm3 g-1 for the MOF(Co), and 1373 m2 g-1, 0.402 cm3 g-1 for the MOF(Cu), respectively.

Ammonia synthesis at different applied voltages Figure 3 shows the current density changing against time for the electrolytic cell during the ammonia synthesis process on the MOF(Fe) at different applied voltages at 80 °C. The current densities at different voltages rapidly drop at the beginning of the reaction. This phenomenon is assigned to the ‘blocking effects’ [25, 44]. In theory, H?, OH- and K? ions in electrolyte are free mobile in the absence of the electric field. When a direct current voltage is applied to the cell, the cations tend to move to the cathode. Due to the proton permselectivity of Nafion membrane, K? ions cannot transfer through the membrane and consequently form a positively charged layer at the interface between KOH electrolyte and the Nafion membrane. Protons are partially blocked due to the same charge repel. These factors result in the decrease in the current densities. At 1.0–1.1 V, the current densities quickly tend to stability with the reaction continuing. However, it is noted that the current densities at 1.2–1.4 V start to increase after the initial decrease, which is because of the activation

effect that can be commonly observed in the electrolytic cell at high voltage [44]. At about 3000 s, the current densities at different voltages reach the maximums, and then decrease gradually. With a higher applied voltage, the blocking effect of K? becomes more obvious, which causes the further decreases in current densities [15, 35]. The overall current density of the MOF(Fe) increases with the applied voltage increasing from 1.0 to 1.4 V. In the ammonia synthesis process, the reaction occurring at the anode is: H2 O ! 2Hþ þ 1=2O2 þ 2e

ð3Þ

in this reaction, when the applied voltage increases, water splits more quickly and more protons are produced at the anode. More protons transfer through the Nafion membrane to the cathode, resulting in the increase in the current density [45]. Figure 4a shows the ammonia formation rates on the prepared MOF(Fe), MOF(Co) and MOF(Cu) at different applied voltages at 80 °C, using water and N2 (or air) as raw materials. The amount of ammonia absorbed in the Nafion-117 membrane during the ammonia synthesis was added to the whole amount of ammonia for calculating the ammonia formation rate. The detailed calculation is according to the report by Lan et al. [29]: it has been estimated that the maximum water uptake of Nafion-117 membrane was 20.64% by volume and the density of saturated ammonia solution is 0.88 g/cm3 with a solubility of 31% by weight. Considering the membrane’s volume of 7.5 9 10-3 cm3, the estimated absorbed ammonia in the membrane is 2.41 9 10-6 mol. The ammonia formation rates of these MOFs increase with the applied voltages increasing from 1.0 to 1.2 V, then decrease with the applied voltages increasing from 1.2 to 1.4 V. When the applied voltage increases, the H? concentration at the anode increases according to reaction (3), resulting in more H? transferring through the Nafion membrane to the cathode. Thus, the ammonia formation rate increases with H? concentration increasing at the cathode. Nevertheless, when the applied voltage increases further, there will be oversaturated H? at the cathode. When joining with two electrons, the surplus H? ions form massive hydrogen as a by-product at the cathode. The hydrogen produced at the cathode also consumes the current density owing to the circulating of H2 across the cell, causing the decrease in the ammonia formation rate [44]. All the prepared MOFs show the

2.0 1.8

MOF(Fe) MOF(Co) MOF(Cu) MOF(Fe)-air

1.4 1.2 1.0

MOF(Fe) MOF(Co) MOF(Cu) MOF(Fe)-air

1.4 1.2 1.0 0.8 0.6

0.2

0.6 1.0

1.1

1.2

highest ammonia formation rates at 1.2 V. The highest ammonia formation rates at 80 °C for the MOF(Fe), MOF(Co) and MOF(Cu) are 1.87 9 10-9, 1.53 9 10-9 and 1.13 9 10-9 mol s-1 cm-2, respectively. Ammonia was also successfully synthesized using air and water as raw materials on the prepared MOF(Fe) at different applied voltages at 80 °C. As shown in Fig. 4a, the ammonia formation rate when using air and water as raw materials shows the same changing tendency as that using pure N2 and water as raw materials. The highest ammonia formation rate is 1.50 9 10-9 mol s-1 cm-2 at 1.2 V and 80 °C, which is lower than that when using pure N2 and water as raw materials, but can be comparable to that on the MOF(Co) in pure N2. The N2 content is about 78% in air. The reducing in the partial pressure of N2 leads to the decrease of N2 concentration on the catalyst surface, which reduces the ammonia formation rate. Besides, O2 in air might oxidize the ammonia at cathode during the ammonia synthesis [25]. The current efficiencies (CEs) of the MOF(Fe), MOF(Co) and MOF(Cu) at different applied voltages at 80 °C are calculated by the following equation [33]: 3FR I

(b)

0.4

0.8

Voltages (V)

CE =

1.8 1.6

1.6

0.9

2.0

(a) Current efficiency (%)

-2

-1

2.2

-9

Figure 4 Ammonia formation rates (a) and current efficiencies (b) on the MOFs at different applied voltages at 80 °C.

Ammonia formation rate (10 mol s cm )

J Mater Sci

ð4Þ

where F represents the Faraday’s constant (C mol-1), R represents the ammonia formation rate (mol s-1 cm-2), I stands for the average current density at a specific applied voltage (mA cm-2). As shown in Fig. 4b, the CEs of the prepared MOFs first increase with the applied voltages increasing from 1.0 to 1.2 V and reach the highest values at 1.2 V, then decrease with the applied voltages increasing further. The highest CEs at 80 °C for the MOF(Fe), MOF(Co) and

1.3

1.4

1.5

0.0 0.9

1.0

1.1

1.2

1.3

1.4

1.5

Voltages (V)

MOF(Cu) are 1.41, 0.93 and 0.95%, respectively. When ammonia is synthesized using air and water as raw materials on the prepared MOF(Fe), the CE increases with the applied voltage increasing from 1.0 to 1.2 V, then decreases with the applied voltage increasing further. The CE reaches the highest value of 0.87% at 1.2 V, which is lower than that on the MOF(Fe) when using pure N2 and water as raw materials, but also comparable to those on the MOF(Co) and MOF(Cu) using pure N2 and water as raw materials.

Ammonia synthesis at different temperatures Figure 5 shows the current density changing against time for the electrolytic cell during the ammonia synthesis process on the prepared MOF(Fe) at different temperatures at 1.2 V. The current densities rapidly drop at the beginning of the ammonia synthesis because of the blocking effect from K? in the electrolyte [25, 44]. After the initial decrease of the current densities, the current densities start to increase due to the activation effect in the electrolytic cell [44]. This increase is more evident when the temperature is higher. At 50–80 °C, the current densities increase till about 3000 s and then tend to be stable. It should be emphasized that the current density at 90 °C keeps decreasing after about 3000 s, which is due to the decrease in the proton conductivity of the Nafion membrane with the temperature increasing [30]. The overall current density increases with the temperature increasing from 50 to 90 °C. On one hand, when the temperature is higher, H? is

J Mater Sci

90 80 70 60 50

-2

50

C C C C C

40

30

20

0

3000

6000

9000

12000

Time (s)

Figure 5 Current density-time curves on the MOF(Fe) at different temperatures at 1.2 V.

2.0

-2 -1 -9

Figure 6 Ammonia formation rates (a) and current efficiencies (b) on the MOFs at different temperatures at 1.2 V.

Ammonia formation rate (10 mol s cm )

produced more quickly according to reaction (3). On the other hand, the proton conductivity of the Nafion membrane is improved when the temperature increases [5]. These factors lead to the increase in the current density. Figure 6a shows the ammonia formation rates on the prepared MOF(Fe), MOF(Co) and MOF(Cu) at different temperatures at 1.2 V. The ammonia formation rates of these MOFs increase rapidly with the temperatures increasing from 50 to 90 °C. It should be noted that the increase extent in the ammonia formation rate decreases when the temperature is higher than 80 °C. It is believed that the morphological reorganizations happen to the Nafion membrane when temperature is too high, which weakens the membrane’s proton conductivity and hinders the increase in the H? concentration at the cathode [30]. Thus, the ammonia formation rate increases slowly

when the temperature is higher than 80 °C. The highest ammonia formation rates are obtained on the prepared MOFs at 90 °C and 1.2 V, which are 2.12 9 10-9 mol s-1 cm-2 for the MOF(Fe), 1.64 9 10-9 mol s-1 cm-2 for the MOF(Co) and 1.24 9 10-9 mol s-1 cm-2 for the MOF(Cu), respectively. As shown in Fig. 6a, when using air and water as raw materials, the ammonia formation rate on the prepared MOF(Fe) increases quickly and reaches the highest value of 1.52 9 10-9 mol s-1 cm-2 at 90 °C. It is lower than that on the prepared MOF(Fe) using pure N2 and water as raw materials because the partial pressure of N2 in air is lower than pure N2 and the produced ammonia might be oxidized by the O2 in air as discussed above [25]. However, it is comparable to that on the MOF(Co) in pure N2. The CEs of the MOF(Fe), MOF(Co) and MOF(Cu) at different temperatures at 1.2 V are shown in Fig. 6b. The CEs of the prepared MOFs increase with the temperatures increasing, and reach the highest values at 90 °C. The highest CEs values of the prepared MOF(Fe), MOF(Co) and MOF(Cu) are 1.43, 1.06 and 0.96%, respectively. When ammonia is synthesized on the MOF(Fe) using air and water as raw materials, the CEs also increase with the temperatures increasing from 50 to 90 °C and reach the highest value of 0.88% at 90 °C, which is lower than that on the prepared MOF(Fe) using pure N2 and water as raw materials, but also comparable to those on the MOF(Co) and MOF(Cu) when using pure N2 and water as raw materials. A similar trend can be observed from Figs. 4 and 6 that the ammonia formation rates for the samples from high to low is: MOF(Fe), MOF(Co) and MOF(Fe)-air, MOF(Cu). Nevertheless, the CEs on the

2.2

(a)

MOF(Fe) MOF(Co) MOF(Cu) MOF(Fe)-air

2.0 1.8

1.6

1.6 1.4 1.2 1.0

(b)

MOF(Fe) MOF(Co) MOF(Cu) MOF(Fe)-air

1.8

Current efficiency (%)

Current density (mA cm )

60

1.4 1.2 1.0 0.8 0.6 0.4

0.8

0.2

0.6 50

60

70

80

Temperature ( C)

90

0.0

50

60

70

80

Temperature ( C)

90

J Mater Sci

Table 1 The ammonia formation rates and CEs at various experimental conditions Catalyst

Raw materials

T (°C)

V (V)

Ammonia formation rate (mol s-1 cm-2)

MOF(Fe) Ru 30 wt% Pt/C Fe-based catalyst La0.8Cs0.2Fe0.8Ni0.2O3-d Pr0.6Ba0.4Fe0.8Cu0.2O3-d

N2, H2O N2, H2O Air, H2O N2, H2O N2, H2O Air, H2O

90 90 Room temperature 250 600 400

1.2 1.02 1.6 1.55 1.4 1.4

2.12 2.12 1.14 8.27 1.23 1.07

MOF(Co), MOF(Cu) and MOF(Fe)-air are almost at the same level and lower than that on the MOF(Fe). The low CEs are dominantly caused by the current consumption by the side reaction—H2 formation process. The highest ammonia formation rate (2.12 9 10-9 mol s-1 cm-2) and CE (1.43%) in this work are obtained on the prepared MOF(Fe) at 90 °C and 1.2 V using pure N2 and water as raw materials. As shown in Table 1, the ammonia formation rate and the CE in this work are much higher than those reported previously at low temperature, also comparable to those reported at medium and high temperatures. Consequently, the prepared MOFs are highly effective non-noble metal catalysts for the electrochemical synthesis of ammonia at low temperature. The electrochemical ammonia formation rate at low temperature is dominantly controlled by the adsorption and dissociation of N2 on the cathode [47–49]. The energy barrier for breaking the N:N bond is about 1000 kJ/mol at 25 °C [46]. The prepared MOFs have abundant micropores and large specific surface areas, which offer a large amount of adsorption sites for N2 and facilitate the adsorption of N2 on the surface of the prepared MOFs. Fe3?, Co3? and Cu2? ions have been proved to possess brilliant catalytic activity for ammonia synthesis [50]. The uniformly distributed transition metal ions (Fe3?, Co3?, and Cu2?) in the prepared MOFs own empty d-orbitals, naturally providing electron accepting sites, therefore, the prepared MOFs can be classified as Lewis acids, which have been widely applied in the heterogeneous catalysis to weaken many species’ p-bonds [51, 52]. In the catalytic process for the electrochemical synthesis of ammonia, MOFs with massive uniformly distributed metal ions (behave as Lewis acids) withdraw electrons from N2, weaken the N:N p-bond and eventually decrease the activation

9 9 9 9 9 9

10-9 10-11 10-9 10-9 10-10 10-10

CE (%)

Reference

1.43 0.92 0.52 4.91 0.55 4.7

This work [46] [29] [21] [44] [25]

energy of the N2-dissociation, which is beneficial for the electrochemical synthesis of ammonia.

Conclusions A series of MOFs(M) (M = Fe, Co, Cu) were synthesized by a hydrothermal process. The prepared MOFs had good crystalline structures, abundant micropores and high specific surface areas. The prepared MOFs were applied to the electrochemical synthesis of ammonia at low temperature and ambient pressure. Among the catalysts, the MOF(Fe) displayed the best catalytic activity. The highest ammonia formation rate and CE value on the MOF(Fe) were 2.12 9 10-9 mol s-1 cm-2 and 1.43%, respectively, at 1.2 V and 90 °C when using pure N2 and water as raw materials. Ammonia was also synthesized directly using air and water as raw materials. The highest ammonia formation rate and the CE value on the MOF(Fe) were 1.52 9 10-9 mol s-1 cm-2 and 0.88%, respectively, at 1.2 V and 90 °C. This preliminary exploration indicated that the prepared MOFs had commendable catalytic activities for the electrochemical synthesis of ammonia at low temperature. It directed a new route to develop highly efficient MOFs-based catalysts for the electrochemical synthesis of ammonia at low temperature and ambient pressure.

Acknowledgements We gratefully acknowledge the Natural Science Foundation of China (21276018), the Natural Science Foundation of Jiangsu Province of China (BK20140268 and BK20161200), Fundamental Research Funds for the Central Universities

J Mater Sci

(buctrc201526), Changzhou Sci & Tech Program (CJ20159006 and CJ20160007), and the Advanced Catalysis and Green Manufacturing Collaborative Innovation Centre of Changzhou University (ACGM2016-06-02, ACGM2016-06-03).

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