Facile Ammonia Synthesis from Electrocatalytic N2 Reduction under

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Facile Ammonia Synthesis from Electrocatalytic N2 Reduction under Ambient Conditions on N-Doped Porous Carbon Yanming Liu, Yan Su, Xie Quan, Xinfei Fan, Shuo Chen, Hongtao Yu, Huimin Zhao, Yaobin Zhang, and Jijun Zhao ACS Catal., Just Accepted Manuscript • Publication Date (Web): 02 Jan 2018 Downloaded from http://pubs.acs.org on January 2, 2018

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Facile Ammonia Synthesis from Electrocatalytic N2 Reduction under Ambient Conditions on NDoped Porous Carbon Yanming Liu,† Yan Su,‡ Xie Quan,*,† Xinfei Fan,† Shuo Chen,† Hongtao Yu,† Huimin Zhao,† Yaobin Zhang,† and Jijun Zhao‡ †

Key Laboratory of Industrial Ecology and Environmental Engineering (Ministry of Education),

School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, China. ‡

Key Laboratory of Materials Modification by Laser, Ion and Electron Beams (Ministry of

Education), School of Physics and Opto-Electronic Technology, Dalian University of Technology, Dalian 116024, China.

ACS Paragon Plus Environment

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ABSTRACT: Ammonia has been using in important areas such as agriculture and clean energy. Its synthesis from electrochemical reduction of N2 is an attractive alternative to the industrial method that requires high temperature and pressure. Currently, electrochemical N2 fixation suffered from slow kinetics due to the difficulty of N2 adsorption and N≡N cleavage. Here, Ndoped porous carbon (NPC) was reported as a cost-effective electrocatalyst for ammonia synthesis from electrocatalytic N2 reduction at ambient conditions, where its N content and species were tuned to enhance N2 chemical adsorption and N≡N cleavage. The resulted NPC was effective for fixing N2 to ammonia with high ammonia production rate (1.40 mmol g-1 h-1 at -0.9 V vs RHE). Experiments combined with density functional theory calculation revealed pyridinic and pyrrolic N were active sites for ammonia synthesis and their contents were crucial for promoting ammonia production on NPC. The energy favorable pathway for ammonia synthesis was *N≡N → *NH=NH → *NH2-NH2 → 2NH3. KEYWORDS: nitrogen fixation, ammonia synthesis, electrocatalysis, N-doped porous carbon, Heterogeneous catalysis TOC:

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INTRODUCTION Converting earth-abundant N2 into ammonia is highly desirable because ammonia is one of the most widely produced chemicals, which can be used to produce agricultural fertilizer, pharmaceutical and so on. It is also a carbon-free energy carrier that can be easily condensed to liquid and overcomes the storage limitation of H2. However, N2 fixation is extremely difficult due to high energy barrier for N≡N bond cleavage. In industry, ammonia is synthesized from N2 and H2 with Fe-based catalyst at high temperature and high pressure (400-600 °C, 20-40 MPa), which uses H2 as reductant (from steam reforming of natural gas), consumes over 1% of the world’s energy supply and produces large amount of fossil fuel derived CO2 (eg. about 570 million tons in 2015 from NH3 synthesis).1 To synthesize ammonia under mild conditions and reduce its energy consumption, numerous efforts have been dedicated to develop alternative routes for fixing N2 to ammonia,1-4 which include nitrogenase catalysis,5-6 homogeneous catalysis with organic or reductive agents,7-9 photocatalysis10-13 and electrocatalysis.14-17 Biological reduction of N2 to ammonia can occur on FeMo protein of nitrogenases at ambient conditions,6 which has led to considerable efforts to mimic their functions using molybdenum and/or iron complexes7-9 in homogeneous system containing organic and/or reductive agents. Similar to the mechanism of nitrogenase catalysis, electrochemical reduction of N2 to ammonia proceeds upon addition of protons and electrons (N2 reacting with H+/H2O instead of H2).18-19 It is an attractive alternative that can be powered by renewable energy and avoids the use of costly agents, which is preferable for sustainable energy economy. The challenges of electrochemical N2 fixation are N2 adsorption and N≡N cleavage, which strongly depends on the composition and structure of electrocatalyst. Only a few electrocatalysts including Pt/C,14 Ru,19 Au15 and Fe/CNT16 have been proven to be effective for


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reducing N2 to ammonia under mild conditions. While having achieved significant progress, electrocatalysts for N2 fixation under mild conditions still suffer from slow kinetics due to their low N2 adsorption and reduction activity. N-doped porous carbon (NPC) has attracted great interests in electrochemical reduction.20-21 Its high surface area and abundant pores can provide plenty of exposed active sites and fast mass transfer for N2 adsorption and N≡N cleavage. N-doping mediates the electronic structure of carbon nanomaterials,22 induces the formation of defects and charge polarization, leading to enhanced adsorption and electrocatalytic activity for O2 and CO2 reduction.23-24 Particularly, NPC derived from zeolite imidazolate framework (ZIF) pyrolysis has high N contents and tunable N species,25-26 which are expected to promote chemical adsorption of N2 and facilitate the dissociation of adsorbed N2, and thereby enhance the kinetics of ammonia synthesis from electrocatalytic N2 reduction. In addition, the sp3-C and defects introduced to NPC by ZIF pyrolysis under H2 may act as active sites27-28 for ammonia synthesis. However, the performance of NPC for electrochemical N2 fixation remains unexplored as well as the impacts of its N content and N species on ammonia synthesis. In this work, ZIF-8 derived NPC was proposed for efficient ammonia synthesis from electrocatalytic N2 reduction at ambient temperature and pressure. The effects of N content and N species on ammonia synthesis along with the reaction mechanism were elucidated.

RESULTS AND DISCUSSION Preparation and characterization of NPCs. NPC was prepared by a two-step method (Figure 1), which includes synthesis of ZIF-8 precursor using zinc nitrate and methylimidazole and subsequent carbonization of ZIF-8 at 750 °C, 850 °C or 950 °C (denoted as NPC-750, NPC850 and NPC-950). The scanning electron microscopy (SEM) images (Figure 2a-c) show NPC-


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750, NPC-850 and NPC-950 consist of polyhedral crystals with particle sizes around 100-150 nm and porous structure is clearly observed on these NPC crystals, which is consistent with results of transmission electron microscopy (TEM, Figure 2d-f). Both SEM and TEM images reveal NPC-750, NPC-850 and NPC-950 exhibit similar crystal shape and particle size, implying carbonization of ZIF-8 under 750-950 °C leads to no obvious difference in morphology.

Zn2+

10 h

methylimidazole

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NPC

Figure 1. Schematic illustration of NPC preparation.

aa

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Figure 2. SEM images of (a) NPC-750, (b) NPC-850 and (c) NPC-950; TEM images of (d) NPC-750, (e) NPC-850 and (f) NPC-950. Figure 3a shows the N2 adsorption-desorption isotherms of NPC-750, NPC-850 and NPC-950. Their IV-type curves indicate the presence of mesopores and micropores on the three NPCs,


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which was confirmed by their pore size distribution curves (Figure S1). The surface area of NPC-750 is 896.0 cm3 g-1, smaller than those of NPC-850 (1034.4 cm3 g-1) and NPC-950 (1084.5 cm3 g-1). Two peaks around 1320 cm-1 (D band) and 1585 cm-1 (G band) are observed on their Raman spectra (Figure 3b). The D band can be ascribed to defects or sp3-C, whereas G band is characteristic peak of graphitic carbon. The intensity ratio of D and G band (ID/IG) is 1.04, 1.05 and 1.07 for NPC-750, NPC-850 and NPC-950, respectively. Their high ID/IG values suggest many defects or sp3-C bonds are introduced to NPCs, which may be beneficial for electrocatalytic N2 reduction. X-ray photoelectron spectroscopy (XPS) analysis shows NPC-750 has the highest N content (13.6 at.%), followed by NPC-850 (5.5 at.%) and NPC-950 (2.1 at.%), and Zn impurity (1015-1050 eV) is undetectable on NPCs (Figure S2). 1000 3

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NPC-750 NPC-850 NPC-950

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Figure 3. (a) N2 adsorption-desorption isotherms and (b) Raman spectra of NPC-750, NPC-850 and NPC-950. Electrocatalytic reduction of N2 to ammonia on NPCs. To evaluate electrocatalytic activity of NPCs for N2 reduction, their linear sweep voltammograms were measured in Ar or N2 saturated 0.05 M H2SO4 solution (Figure 4). When potential is more negative than -0.38 V (vs RHE), current density of NPC-750 under N2 increases obviously compared with that under Ar, demonstrating electrocatalytic N2 reduction occurs on NPC-750 with onset potential of -0.38 V.


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As expected, NPC-850 and NPC-950 are also active for electrochemical reduction of N2 with onset potential of -0.41 V (NPC-850) and -0.52 V (NPC-950), both of which are much more negative than that of NPC-750. Meanwhile, the net current density of NPC-750 for N2 reduction (jN2-jAr) is higher than that of either NPC-850 or NPC-950 at the same potential. These results suggest NPC-750 is more active than NPC-850 and NPC-950 for electrochemical reduction of N2. Based on these results, electrocatalytic activity of NPC-700 for N2 reduction was further examined. However, NPC-700 presents much lower activity than NPC-750 for N2 reduction (data not presented) due to its low conductivity, which is not considered in the following experiments. Besides, the hydrogen evolution potential of NPC-750 is more negative than those of NPC-850 and NPC-950, which is advantageous for N2 reduction. NPC-750

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NPC-850

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N2 -2

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Figure 4. Linear sweep voltammograms of NPC-750, NPC-850 and NPC-950 in Ar (dashed line) or N2 (solid line) saturated 0.05 M H2SO4 solution with scan rate of 50 mV s-1. The performance of NPCs for electrocatalytic N2 reduction was further evaluated by electrolyzing N2 at constant potential (under N2) and measuring the concentrations of produced ammonia. The average ammonia production rates (Figure 5a) reveal NPC-750 can rapidly reduce N2 to ammonia at potential of -0.5 V to -1.1 V, and the ammonia production rate increases as


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potential negatively shifted. Interestingly, the ammonia production rates of NPC-750 under N2 (1.00-1.40 mmol g-1 h-1 or 0.60-0.84 µmol cm-2 h-1 at -0.7 to -0.9 V) are one order of magnitude higher than those of Ru, Au and Fe/CNT electrocatalysts (0.012-0.097 µmol cm-2 h-1),15-16,19 and higher than those of Pt/C electrocatalyst (0.76 mmol g-1 h-1)14 under similar conditions. In addition, the ammonia production rates of NPC-750 is also faster than that of Au/Si photoelectrocatalyst (0.13 µmol cm-2 h-1).3 To verify ammonia generated from N2 reduction, electrolysis was also conducted on NPC-750 under Ar at -0.7 V and -0.9 V. Ammonia is undetectable after electrolysis under Ar for 2 h, demonstrating ammonia is generated from N2 reduction and N atoms of NPC is not the nitrogen source of ammonia. These results highlight the good performance of NPC-750 for converting N2 to ammonia. The performances of NPC-850 and NPC-950 for ammonia electrosynthesis are also assessed under N2 (Figure 5b). At -0.9 V, NPC-850 exhibits an ammonia production rate of 0.92 mmol g-1 h-1, which is higher than NPC950 (0.43 mmol g-1 h-1) but lower than NPC-750. The similar trend is observed at -0.7 V, suggesting ammonia production activity of NPC is enhanced by increasing its N content. Achieving high energy efficiency is one of the major challenges for fixing N2 to ammonia. The current efficiency of NPC-750 for electrochemical reduction of N2 to ammonia was investigated, which increases initially (-0.5 V to -0.9 V) and then starts to decrease as potential negatively shifted to -1.1 V (Figure 5c) due to that more energy is consumed for H2 evolution (Figure S3). At -0.9 V, a maximum current efficiency of 1.42% is obtained for ammonia synthesis on NPC-750, which is 2.8-10.1 times as high as those of recently reported electrocatalysts (0.14-0.51%)14,16,19 under similar conditions. The current density for electrochemical reduction of N2 to ammonia is stable (Figure S4). Figure 5d shows both ammonia production rate and current efficiency of NPC-750 presents no obvious changes during


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10 consecutive cycles of N2 reduction at -0.9 V, indicating the high stability of NPC-750 for electrochemical N2 fixation. 1.6

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Figure 5. Ammonia production rates of (a) NPC-750 at -0.5 ~ -1.1 V and (b) NPC-750, NPC850 and NPC-950 at -0.7 V and -0.9 V; (c) current efficiency of NPC-750; (d) ammonia production rates and current efficiency of NPC-750 during 10 consecutive cycles at -0.9 V (All data obtained under N2). Direct synthesis of ammonia from air can considerably reduce the cost for N2 fixation. Here, electrosynthesis of ammonia from air was performed on NPC-750. As expected, ammonia is effectively synthesized from air by NPC-750. Its production rate under air also increases as potential is more negative (Figure 6a), which reaches 1.02 mmol g-1 h-1 (or 0.61 µmol cm-2 h-1) at -0.9 V. Although ammonia production rate under air is slightly lower than that under N2, it is still higher than the reported values under N2 mentioned above. These results imply ammonia can be directly synthesized from air on NPC electrocatalyst at ambient conditions.


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Analysis of electrocatalytic activity. To probe factors contributed to the superior performance of NPC for ammonia synthesis, effects of Zn impurity (trace amount of Zn may exist on NPCs which are obtained from ZIF carbonization and HCl washing) and surface area on ammonia synthesis were first examined. Inductively coupled plasma atomic emission spectroscopy analysis shows Zn contents of NPCs are lower than 0.08 wt%. To clarify whether Zn impurity contributes to ammonia synthesis, NPC-750 with Zn contents of 0.38 wt% and 0.99 wt% were prepared by reducing HCl washing time to 10 h and 20 h (denoted as NPC-750-10h and NPC750-20h). At -0.9 V, their ammonia production rates present no obvious difference from that of NPC-750 (Figure S5), suggesting the trace Zn impurity of NPCs (< 0.08 wt%) has negligible effect on ammonia synthesis. The double layer capacitances of NPCs, which are linearly proportional to their electrochemical surface areas, were estimated from cyclic voltammograms. The double layer capacitance of NPC-750 is 1.16 mF cm-2, while it is 1.64 mF cm-2 for NPC-850 and 1.84 mF cm-2 for NPC-950 (Figure S6), illustrating that electrochemical surface areas of NPCs is in the order of NPC-750 < NPC-850 < NPC-950 (consistent with BET results). While NPC-750 has a lower electrochemical surface area relative to NPC-850 and NPC-950, it exhibits the highest kinetics for electrochemical reduction of N2 to ammonia. Thus, the high electrocatalytic performance of NPC-750 may benefit from its intrinsic catalytic activity rather than surface area. N2 adsorption is generally considered as one of important factors influencing N2 fixation kinetics. Since N-doping can reduce the energy barrier for O2 adsorption on carbon materials,29-30 temperature-programmed desorption of N2 (N2-TPD) was employed to assess the correlation between N content and N2 adsorption capability of NPCs. The N2 desorption peak appears at 90~185 °C on the N2-TPD curves of NPC-750, NPC-850 and NPC-950 (Figure 6b), which


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manifests N2 can be chemically adsorbed on NPCs. It is worthy noted that NPC-750 exhibits the largest N2 desorption peak area, followed by NPC-850 and NPC-950, implying N2 adsorption on NPC is facilitated by increasing its N content.

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0.9

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Figure 6. (a) ammonia production rates of NPC-750 in air; (b) N2-TPD profiles of NPCs; (c) contents of pyridinic, pyrrolic and graphitic N in NPCs; (d) ammonia production rate of NPC850 and NPC-1 under N2 at -0.9 V. To get insight into the much higher electrocatalytic activity of NPC-750, N species of NPCs were analyzed by XPS spectra (Figure S7). Pyridinic N (398.5 eV), pyrrolic N (400.1 eV) and graphitic N (401.2 eV) are identified from NPCs. The main N species of NPC-750 and NPC-850 is pyridinic N (Figure 6c), whereas the major N species of NPC-950 is graphitic N. Due to the higher total N content of NPC-750, its pyridinic (6.2 at.%), pyrrolic (5.3 at.%) and graphitic N contents (2.1 at.%) are the highest among three NPCs. To explore the influence of N species on ammonia synthesis, preparation of NPCs with similar total N content but different N species


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content was tried by tuning its preparation conditions. As presented in Figure 6c, pyridinic N content of NPC-1 (2.8 at.%) is higher than that of NPC-850 (2.1 at.%), whereas both of its pyrrolic and graphitic N contents are lower than those of NPC-850. At -0.9 V, ammonia production rate of NPC-1 is 1.05 mmol g-1 h-1 (Figure 6d). Although NPC-1 has slightly lower total N content (5.2 at.%) than NPC-850 (5.5 at.%), its ammonia production rate is 14% higher when its pyridinic N content is 0.7 at.% higher. These results imply increasing pyridinic N content of NPC can promote its ammonia production, which is further confirmed by DFT calculation. (The effects of pyrrolic and graphitic N on ammonia synthesis were unable to be investigated by experiment due to the difficulty of tuning their contents of NPC while keeping similar total N content). The role of different N species towards electrochemical reduction of N2 to ammonia was further explored by DFT calculation using 7×7 carbon doped with N, where pyridinic, pyrrolic and graphitic N are considered based on others’ works.23,31 Possible pathway2,32 for ammonia synthesis (Figure S8 and S9) on NPC was investigated by DFT calculation. Pyridinic and pyrrolic N are identified as key sites for N2 adsorption and dissociation on NPC (Figure 7), while it is difficult for N2 to adsorb on graphitic N doped carbon. The preferable pathway for ammonia synthesis on NPC is *N≡N →*NH=NH → *NH2-NH2 → 2NH3. Figure 7 shows the free energy diagram of the lowest energy pathways for N2 reduction on pyridinic or pyrrolic N doped carbon. At U=0 V, the maximum reaction free energy for elementary reaction is 0.45 eV for pyridinic N, similar to that of pyrrolic N (0.56 eV). Although the reaction free energy is positive for some elementary reaction at U= 0 V, converting N2 to ammonia can be a downhill process at potential more negative than -0.7 V. The results of DFT computation combined with experiments indicate the high pyridinic and pyrrolic N contents of NPC contributed to its superior activity. In addition,


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the free energy for H2 evolution reaction (Figure S10) confirms the lost energy during ammonia synthesis is used for H2 production. Therefore, the good performance of NPC could be attributed to the following factors: NPC has high N2 adsorption capability. Its porous structure is favorable for trapping N2 and stabilizing the N2 reduction intermediate (NHx and N2Hx) inside, which can produce high partial pressure and promote the further reactions33-34, leading to enhanced ammonia production. Meanwhile, N-doping can induce active sites for N2 adsorption and activation, and thereby facilitate ammonia synthesis. Besides, NPC has relatively high overpotential for H2 evolution, which is favorable for improving ammonia synthesis efficiency. a4

U= 0 V

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Reaction coordinate

-2

Reaction coordinate

Figure 7. The free energy diagram for ammonia synthesis on NPC with (a) pyridinic and (b) pyrrolic N.

CONCLUSION In summary, cost-effective ammonia synthesis from electrochemical reduction of N2 has been achieved on NPC at ambient conditions. It was highly active for ammonia synthesis with high production rate of 1.40 mmol g-1 h-1 (-0.9 V) and enhanced current efficiency relative to electrocatalysts reported. Its high contents of pyridinic and pyrrolic N were responsible for promoting ammonia synthesis. The preferable pathway for ammonia synthesis revealed by DFT


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calculation was *N≡N → *NH=NH → *NH2-NH2 → 2NH3. This study opens up a new avenue for exploring cost-effective methods to convert atmospheric N2 to ammonia under ambient condition.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Experimental methods, pore size distribution curves, XPS spectra, H2 production efficiency, current density, ammonia production rates, double layer capacitances, N 1s XPS spectra, possible pathways for ammonia synthesis, free energy for ammonia synthesis and H2 evolution. (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail for X.Q.: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China (NO.21437001 and NO. 21590813), the Programme of Introducing Talents of Discipline to Universities (B13012) and the Fundamental Research Funds for the Central Universities of China (No. DUT16RC(3)073).


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