Selective Reduction of Nitrate by the Local Cell Catalyst Composed of

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Selective Reduction of Nitrate by the Local Cell Catalyst Composed of Metal-Doped Covalent Triazine Frameworks Kazuhide Kamiya, Tomomi Tatebe, Shuhei Yamamura, Kazuyuki Iwase, Takashi Harada, and Shuji Nakanishi ACS Catal., Just Accepted Manuscript • Publication Date (Web): 20 Feb 2018 Downloaded from http://pubs.acs.org on February 20, 2018

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Selective Reduction of Nitrate by the Local Cell Catalyst Composed of Metal-Doped Covalent Triazine Frameworks Kazuhide Kamiya, ‡, §, ¶ Tomomi Tatebe, § Shuhei Yamamura, § Kazuyuki Iwase, † Takashi Harada‡ and Shuji Nakanishi ‡, §,* ‡

Research Center for Solar Energy Chemistry, Osaka University, 1-3 Machikaneyama,

Toyonaka, Osaka 560-8531, Japan. §

Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka,

Osaka 560-8531, Japan. ¶

Japan Science and Technology Agency (JST) PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama

332-0012, Japan †

Department of Applied Chemistry, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo

113-8656, Japan.

KEYWORDS: covalent triazine frameworks, local cell reaction, electrocatalysts, nitrate reduction, single atom catalyst

ACS Paragon Plus Environment

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ABSTRACT; So-called local cells resulting from the coupling of oxidation and reduction reactions on the same conductive substrate represent a well-known cause of metallic corrosion. In the present study, we attempted to demonstrate that catalytic systems based on the principle of local cell reactions can be successfully fabricated using metal-doped covalent triazine frameworks as catalytic units. A conductive substrate carrying platinum- and copper-doped covalent triazine frameworks as catalysts for the oxidation and reduction processes, respectively, was developed to fabricate a local cell catalytic unit for the concurrent reduction of nitrate to nitrous oxide and oxidation of hydrogen.


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1. Introduction The development of redox catalysts that exhibit both high activity and selectivity is indispensable to attaining the goal of a sustainable society. All redox reactions can be divided in two half-cell reactions, consisting of oxidation and reduction. Specific redox reactions are typically achieved by coupling these half-cell oxidation/reduction reactions via an electrochemical circuit incorporating an anode and a cathode in conjunction with appropriate catalysts. However, a two-electrode system is not necessarily required for this purpose, since a redox reaction can also proceed based on spatially-separated oxidation/reduction half-cell reaction sites on a single conductive substrate. Such redox systems are called local cells, which are known as the mechanism of corrosion. If a material can form a local cell that promotes a useful reaction, such as the production of valuable chemicals or the decomposition of environmental pollutants, in conjunction with a suitable counter reaction on a conductive substrate, a redox system (hereafter referred to as a local cell system) can be obtained. However, such systems require the fabrication of catalytic units integrating the appropriate catalysts for the half-cell reactions on a conductive substrate. Metal-doped covalent triazine frameworks (CTFs) have shown promise as components of catalytic units because of their unique properties.1-20 The most significant advantage of these compounds is that various catalytic functions can be obtained simply by changing the metal species. Our own work has demonstrated that a platinum-doped CTF deposited on conductive carbon particles (Pt-CTF/CP) can serve as an electrocatalyst for hydrogen evolution and oxidation16 and oxygen reduction reactions,18 whereas copper-doped CTF/CP (Cu-CTF/CP) has been shown to catalyze oxygen19 and nitrate reduction reactions.20 Importantly, these metaldoped CTF/CPs exhibited unique selectivity originating from the single-atom nature and/or


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unsaturated coordination of the catalytic metal centers. As an example, Pt-CTF/CP showed methanol-tolerant oxygen reduction activity,18 while Cu-CTF/CP catalyzed nitrate (NO3-) reduction to selectively produce nitrous oxide (N2O).20 The selective reduction of NO3- to N2O has applications in water purification, as N2O is an intermediate to dinitrogen (N2). The high selectivity obtained with this new material has never been observed using conventional metal nanoclusters, and is an important feature of metal-doped CTF/CPs with regard to local cell fabrication. We report herein that a conductive carbon substrate loaded with both Pt-CTF/CP and Cu-CTF/CP functions as a local cell system for NO3- reduction by H2. The Pt-CTF/CP promotes the hydrogen oxidation reaction (HOR) but is inactive for the NO3- reduction reaction (NRR). Conversely, the Cu-CTF/CP exhibits selective NO3- reduction to N2O via nitric oxide (NO) but shows no HOR activity. Accordingly, it was expected that the selective reduction of NO3- to N2O using H2 would proceed, based on the local cell principle, subsequent to electrochemically connecting the CTF-based catalysts promoting the two half-cell reactions via a conductive plate. This general concept is illustrated in Figure 1.


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Figure 1. Schematics showing (a) the local cell catalyst composed of Cu-CTF/CP and PtCTF/CP, (b) the Cu-CTF/CP and Pt-CTF/CP structures, and (c) the reaction pathways from H2 to NH4+, N2O and N2.

2. Experimental Methods 2.1 Catalyst synthesis


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The CTF was polymerized in the presence of carbon nanoparticle (CP) to add electron conductivity using a previously reported method.18,19 Briefly, CTF/CP was obtained by heating a mixture of ZnCl2 (3.41 g, Wako), 2,6-dicyanopyridine (64.5 mg, Aldrich) and Ketjen Black EC600JD (64.5 mg, Lion Corp., 34 nm) in a vacuum-sealed glass tube at 400 °C for 40 h. The product was washed with distilled water, tetrahydrofuran and 0.1 M HCl to obtain CTF/CP. The detail characterization of the CTF/CP has been reported in our previous paper.18 Cu or Pt atoms were added via impregnation using 1 mM CuCl2 or K2PtCl4 (Wako) solutions for 4 h at 60 °C, respectively, and then, we separately obtained Pt-CTF/CP and Cu-CTF/CP particles. 2.2 Electrochemical measurements A three-electrode system operating at room temperature was used to evaluate half reaction activities, employing Ag/AgCl/KCl (sat.) and a titanium wire as the reference and counter electrodes, respectively. A catalyst ink was prepared by dispersing 5 mg Cu-CTF/CP or PtCTF/CP in a mixture of 200 µL ethanol and 100 µL Nafion solution (5 wt%; Aldrich) using a homogenizer, and was subsequently dropped onto a glassy carbon (GC, 0.196 cm2) electrode. Then, the solvent of catalyst ink was evaporated at room temperature. The catalyst loading was held constant at approximately 0.42 mg cm-2. For the half-cell test of HOR activity, the working electrode was rotated with a rotation speed of 2,500 r.p.m. A two-electrode experiment (battery test) was also conducted using a double-chamber electrochemical cell in which the two chambers were separated by a Nafion membrane. In this trial, Pt-CTF/CP and Cu-CTF/CP were deposited on large GC electrodes (1.3 cm2, loading amount: 0.63 mg cm-2) to form the anode and the cathode electrodes, respectively. 2.3 Local cell unit testing


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The NO3- reduction activities of the local cell systems were evaluated in a 0.1 M HClO4 solution (pH 1), using

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N-labeled sodium nitrate (Na15NO3; Cambridge Isotope Laboratories,

Inc.) as the nitrate source to exclude the possibility of detecting N-containing reaction products from the degradation of CTF. Firstly, Pt-CTF/CP and/or Cu-CTF/CP were weighed as listed in Table S1. Then, the weighed catalyst powders, 200 µL ethanol and 100 µL Nafion solution (5 wt%; Aldrich) were just physically mixed using a homogenizer to obtain the catalyst inks. The ink (50 ml) was subsequently dropped onto the GC plate (4.9 cm-2), and the solvent was evaporated at room temperature. The GC plate was subsequently placed at the base of a reactor (gas volume: 27 mL) containing 10 mL of a 0.1 M Na15NO3 solution in 0.1 M HClO4 (pH 1). The reaction was initiated by adding H2 to the reactor while magnetically stirring the reaction solution at 700 rpm. Note that the local-cell testing was conducted at 25 °C without any external electricity or applied potential. Gaseous reaction products were quantitatively analyzed by gas chromatography-mass spectrometry (GC-MS; GCMS-QP 2010 Plus, Shimadzu, Japan), while the ammonia concentration was determined colorimetrically by the indophenol blue method.21,22 The HNO2 reduction reaction was also conducted by Pt-CTF/CP and Pt/C in the same method, except for the use of 1 mM Na15NO2 (Cambridge Isotope Laboratories, Inc.) as a substrate. An experiment assessing N2 production was also performed, using iridium metal particles (Nilaco Corporation, 99.9 %, loading amount: 6.3 mg cm-2) deposited on the ceiling of the reactor via a 5 wt % Nafion ionomer solution. 2.4 Physical characterizations Surface inspection was carried out with a high-resolution scanning electron microscope (SEM; JEOL, JSM-7600F.) and transmission electron microscope (TEM; Hitachi, H9000NAR.). X-ray absorption fine structure (XAFS) measurements were conducted by a


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transmission method using the hard X-ray beam line BL01B01 of SPring-8, Japan and fluorescence X-rays were detected using a double-crystal Si (111) monochromator and a 19element germanium solid state detector (SSD). All obtained spectra were analyzed using ATHENA software. X-ray photoelectron spectra (Axis Ultra, Kratos Analytical Co.) were measured with monochromatic Al Kα X-rays of hν = 1486.6 eV.

3. Results and discussion 3.1.Electrochemical half-cell measurements The detail characterization of Pt-CTF/CP and Cu-CTF/CP has been reported in our previous papers.16,18-20 (see also Figure S1 for the TEM images) The voltammograms obtained from the Pt-CTF/CP in a HClO4 electrolyte with and without dissolved H2 are shown in Figure 2a. A comparison of the two plots in this figure demonstrates that the Pt-CTF/CP efficiently catalyzed the HOR. Figure 2b presents the voltammograms acquired in a HClO4 electrolyte with and without 0.1 M NaNO3. There is no evident difference between these curves, indicating that the NRR catalytic activity of the Pt-CTF/CP is negligible. Results of similar experiments using the Cu-CTF/CP are shown in Figures 2c and d. In contrast to the Pt-CTF/CP, the Cu-CTF/CP exhibits no HOR activity but shows excellent NRR catalytic activity. Thus, the Pt-CTF/CP and Cu-CTF/CP exhibited unique selectivity for the HOR and NRR, respectively, due to the singleatom nature of the catalytically active sites.16,18,20 It should be noted that some of the above results have already been reported in our previous papers, 16,18-20 but are presented here to assist in understanding the discussion of the present work.


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3.2. Battery tests The possibility of a spontaneous battery unit based on the HOR and NRR was verified using a two-electrode system incorporating the Pt-CTF/CP and Cu-CTF/CP as electrocatalysts for the two reactions (Figure 2e). The power curves obtained from this setup are provided in Figure 2f. A larger battery output was obtained when NO3- ions were included in the cathode compartment, confirming that an electromotive force was obtained from this system. It should be noted that a small output was observed even in the absence of NO3- due to the cathodic reduction of oxygen (which was unavoidably introduced into the system). It is also noteworthy that the battery output did not change when an ion exchange membrane separating the anode and the cathode compartment was inserted (blue curve in Figure 2f), as a result of the exceptional selectivities of the Pt-CTF/CP and Cu-CTF/CP for the HOR and NRR, respectively. This finding is in contrast to the results obtained when employing a commercial Pt nanocluster-loaded carbon as the anode catalyst (Figure S2).


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Figure 2. Polarization curves obtained from (a) Pt-CTF/CP and (c) Cu-CTF/CP in 0.1 M HClO4 with (solid curve) and without (dashed curve) dissolved H2 (scan rate: 2 mV/s), and (b) PtCTF/CP and (d) Cu-CTF/CP in 0.1 M HClO4 with (solid) and without (dashed) 0.1 M NaNO3 (scan rate: 10 mV/s), (e) a schematic of the two-electrode battery system with Pt-CTF/CP and Cu-CTF/CP as the anode and cathode catalysts, respectively, and (f) power curves obtained from systems with (solid) and without (dashed) NO3-, and (dotted) with NO3- but inserting an ionexchange membrane between the anode and cathode compartments


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3.3. Local cell tests We subsequently prepared local cell devices incorporating Pt-CTF/CP and Cu-CTF/CP to ascertain whether or not these materials were able to catalyze the NRR using H2 (Figures 3a and S3a). Figures 3b, 3c and S4a present scanning electron microscopy (SEM) images of the catalyst layer containing both the Pt-CTF/CP and Cu-CTF/CP (sample 3 in Table 1). The particle size of 20-80 nm, which corresponded to the size of CPs (KetchenBlack EC600JD), revealed that CTF was well mixed with CPs.18 The Pt-CTF/CP and Cu-CTF/CP formed the porous catalyst layer (porous size; 10~ 500 nm) which is similar with an electrode of polymer electrolyte fuel cells.23,24 The Pt and Cu distributions determined by energy dispersive spectroscopy (EDS) for the SEM image in Figure 3b are shown in Figures 3d and e, respectively. The Pt:Cu ratio in this sample was close to 1:1 regardless of the measurement location, indicating that the Pt-CTF/CP and Cu-CTF/CP were loaded on the carbon plate without localization. In addition, the crosssectional SEM image (Figure S4b) revealed that the thickness of the catalyst layer was about 4 µm. The XPS elemental analyses and other detail physical characterizations (XPS, XANES and EXAFS) of the resulting material (sample 3 in Table 1) were shown in Table S2 and Figure S5 – S7. Based on the well-known NRR pathway in Figure 1c, the quantities of N2O, N2 and ammonium ions (NH4+) generated were determined, with the results summarized in Figure 4. For comparison purposes, we also prepared samples containing only Cu-CTF/CP (sample 1, Table 1) and only Pt-CTF/CP (sample 2). The trial with sample 1 showed the production of only extremely small quantities of the nitrogen compounds. Conversely, sample 2 generated a small amount of product primarily consisting of NH4+. In the case of the specimen containing both the Pt-CTF/CP and Cu-CTF/CP (sample 3), the production of N2O was greatly increased. The


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quantity of N2O produced for the sample 3 was also confirmed to increase with increasing reaction time, as shown in Figure S8. These results indicated that a local cell process resulting from the coupling of the HOR and NRR took place on this device, in good agreement with the electrochemical data in Figure 2. Based on the above data, the NRR activity of each sample can be discussed. Sample 1, containing only the Cu-CTF/CP, was unable to promote the NRR despite the catalytic activity of the Cu-CTF/CP, because the HOR could not occur due to the lack of the appropriate catalyst, and therefore no reducing equivalents are provided to the Cu-CTF/CP. In the case of sample 2, carrying only the Pt-CTF/CP, the amount of reaction product was greater than that obtained from sample 1 and the main component of the product was NH4+. Although the Pt-CTF/CP catalyzed the HOR, it did not promote the NRR, because the NRR requires Pt assemble sites.20,25-29 Therefore, the local cell process did not proceed on this sample. However, it is known that, although most of the Pt on the Pt-CTF/CP are atomically dispersed, some will form aggregates (Ptagg).16 It is therefore possible that both the NRR and HOR proceeded on this trace amount of Ptagg such that a local battery was formed on sample 2. The factor that determines whether the main product of the NRR is N2O or NH4+ is the NO adsorption energy on the catalytic surface.20,25,26,29-32 N2O is produced by the reaction between adsorbed and solvated NO.20,25,30,31 We have previously found that NO has a moderate adsorption energy (140.1 kJ/mol) on Cu-CTF/CP suitable for pathway A or B.20 Thus, nitrate was reduced to NO at Cu-CTF/CP, and a part of generated NO diffused into the bulk electrolyte. The solvated and surface-bound forms of NO react together on Cu-CTF/CP to form N2O (pathway A in Figure 1c). In addition, we have confirmed using HNO2 reduction reaction that the dimerization of NO to N2O occurs on even Pt-CTF/CP. (see Figure S9 and S10 in the


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Supporting Information for details). Thus, nitrate was reduced to NO at Cu-CTF/CP, and then N2O formation proceeded at even Pt-CTF/CP using the solvated NO (pathway B). In contrast, the NO adsorption energy on Ptagg is large (250.6 kJ /mol), and thus the desorption of NO generated by nitrate reduction does not occur.20 Consequently, NH4+ is preferentially produced via pathway C.29-32 Lowering the loading of either the Pt-CTF/CP or Cu-CTF/CP in sample 3 to 20% changed the reaction products in a manner consistent with the above interpretation. That is, reducing the amount of Cu-CTF/CP decreased and increased the quantities of N2O and NH4+ in the product, respectively (sample 4 in Figure 4). Conversely, the proportions of N2O and NH4+ increased and decreased, respectively, when the amount of Pt-CTF/CP was reduced (sample 5). The data also confirmed that significantly less of the NRR product was obtained from sample 5 compared to sample 4, suggesting that the HOR occurring on the Pt is the rate limiting step of the local cell catalysis system. The reducing power can be, in principle, transferred from Pt to Cu sites in our catalysts by the hydrogen spillover instead of the electron transfer. We measured the working potential of the local-cell catalyst to verify whether the hydrogen spillover served in the local-cell catalysts (Figure S11). The working potential of sample 3 during the NRR by hydrogen was about 0.05 V vs RHE, at which the Pt-CTF/CP efficiently catalyzed the HOR as shown in Figure 2a. Importantly, it has been reported that hydrogen spillover from Pt sites to carbon-based supports does not occur during the HOR in electrolytes (see Figure S11 for details)33-36, meaning that the electron transfer is major reaction pathway in our catalysts. In addition, we also measured the NRR activity of the catalyst composed of CTFs hybridized with non-conductive SiO2 (CTF/SiO2) instead of CPs to examine whether the electron transfer is essential for our catalysts


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(Figure S12-S13 and Table S3 in the Supporting Information). The NRR activity of the local-cell catalysts composed of metal-modified CTF/SiO2 was negligible compared with that of CTF/CPs, also suggesting that the electron transfer dominates over the hydrogen spillover in our catalysts.

Figure 3. (a) A diagram showing the preparation of the local cell catalyst unit and a photographic image of the unit, and (b, c) representative SEM images and (d, e) EDX maps of the local cell catalyst composed of Cu-CTF/CP and Pt-CTF/CP (sample 3).


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Figure 4. NRR yields 48 h after the initiation of the reaction in 0.1 M HClO4 with 0.1 M Na15NO3 under H2 gas. Table 1. The Cu-CTF/CP and/or Pt-CTF/CP loadings of the samples 1-6. sample

1

2

3

4

5

Cu-CTF

0.83

×

0.83

0.17

0.83

0.83 (mg)

Pt-CTF

×

0.83

0.83

0.83

0.17

0.83 (mg)

Ir

×

×

×

×

×

6

○

3.4.N2 formation Although bulk Pt-group metals can reduce N2O to N2, the activity for N2O reduction on single Pt sites is almost negligible (see Figure S14 in the Supporting Information for details).37,38 Thus, finally, we attempted to reduce NO3- to N2 using sample 3 (incorporating both the PtCTF/CP and Cu/CTF CP), based on previous reports that N2O is an intermediate in the reduction of NO3- to N2 (pathway B) and that iridium metal (Ir) catalyzes the gas phase reduction of N2O to N2.38-41 Because N2O is a gas at ambient temperature, it moved spontaneously from the liquid phase to the gas phase in the reactor. Furthermore, some portion of the H2 introduced into the system was also present in the gas phase. Therefore, it was thought that fixing Ir particles in the


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reactor headspace (as shown in Figure S3b) would, in principle, result in a reduction pathway from NO3- to N2. The products obtained from such a system were quantitatively analyzed and are summarized in Figure 4 (sample 6), from which it is evident that the amount of N2 generated was drastically increased, as expected (see also Figure S15). 4. Conclusion The present work demonstrated that the conductive carbon substrate carrying Pt- and Cu-CTF selectively reduce nitrate to N2O based on the local cell principle. Furthermore, the highly selective reduction of NO3- to N2 was achieved with a hybrid system combining a solution-based local cell unit and a gaseous catalytic reaction, which could have important applications to the purification of contaminated water. To the best of our knowledge, this is the first demonstration of the bottom-up fabrication of a local cell process designed from half-cell oxidation/reduction reactions for a specific target reaction. We anticipate that the synthesis strategy reported here can be applied to construct novel redox catalysts using existing half-cell catalysts.

ASSOCIATED CONTENT Supporting Information. Physical and morphological characterizations, power curve, image of reaction cell, HNO2 reduction reaction N2 and N2O generation data, electrochemical N2O reduction reaction (PDF).

AUTHOR INFORMATION


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Corresponding Author [email protected] (S.N.).

Notes The authors declare no competing financial.

ACKNOWLEDGMENT This research was supported by the PRESTO Program of the Japan Science and Technology Agency (JST) (JPMJPR1415) and a Grant-in-Aid for Young Scientists (A) (17H04798). Synchrotron radiation experiments were performed at the BL01B1 and BL14B2 beamlines of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI; Proposal nos. 2016B1098, 2016B1696, 2017A1790 and 2017B1171). TEM measurement was carried out by using a facility in the Research Center for Ultra-High Voltage Electron Microscopy, Osaka University and TEM observation was supported by Dr Takao Sakata.

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Selective Reduction of Nitrate by the Local Cell Catalyst Composed of

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