Accepted Manuscript Carbon fiber paper supported nano-Pt electrode with high electrocatalytic activity for concentrated nitric acid reduction
Qingfa Wang, Kun Yue, Xiangwen Zhang, Li Wang, Michael D. Guiver, Junfeng Zhang PII: DOI: Reference:
S1572-6657(17)30226-6 doi: 10.1016/j.jelechem.2017.03.043 JEAC 3208
To appear in:
Journal of Electroanalytical Chemistry
Received date: Revised date: Accepted date:
19 January 2017 3 March 2017 26 March 2017
Please cite this article as: Qingfa Wang, Kun Yue, Xiangwen Zhang, Li Wang, Michael D. Guiver, Junfeng Zhang , Carbon fiber paper supported nano-Pt electrode with high electrocatalytic activity for concentrated nitric acid reduction. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Jeac(2017), doi: 10.1016/j.jelechem.2017.03.043
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ACCEPTED MANUSCRIPT
Carbon fiber paper supported nano-Pt electrode with high electrocatalytic activity for
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concentrated nitric acid reduction
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a
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Zhanga,b,*
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Qingfa Wanga,c, Kun Yuea, Xiangwen Zhanga,c, Li Wanga,c, Michael D. Guiverb,c,*, Junfeng
Key Laboratory of Green Chemical Technology of Ministry of Education, School of
State Key Laboratory of Engines, School of Mechanical Engineering, Tianjin University,
Tianjin 300072, China.
Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin
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c
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b
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Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China.
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University, Tianjin 300072, China
* Corresponding Authors * E-mail :
[email protected] (J. Z.). * E-mail:
[email protected] (M. D. G.).
ACCEPTED MANUSCRIPT Abstract: A high-performance electrode for the electrocatalytic reduction of concentrated nitric acid is developed from an electrode based on carbon fiber paper supported Pt nanoparticles (nano-Pt/CFP). The nano-Pt/CFP electrode exhibits remarkable electrocatalytic reactivity at room temperature (488.6 mA mg-1 for nanopolyhedral Pt, and 384.9 mA mg-1 for nanothorn Pt) with a significant reduction in Pt usage compared with conventional Pt
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electrodes (0.0190 mA mg-1 for Pt sheet or 0.3667 mA mg-1 Pt mesh) and commercial Pt/C
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(288.5 mA mg-1). The mechanism is discussed based on the electrochemical voltammograms
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and the composition of gaseous products, indicating a homogeneous autocatalysis process
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with NO and N2O as the main products.
Keywords: concentrated nitric acid; electrocatalytic reduction; Pt nanoparticles; carbon fiber
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paper; high-performance electrode
ACCEPTED MANUSCRIPT 1.
Introduction Nitrate is a common and ubiquitous contaminant in water resources and is recognized as a
serious threat to human health and the nitrogen cycle balance [1-5]. Industrial wastewater containing nitric acid and nitrates needs to undergo denitrification before being discharged
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into the environment. Major sources of concentrated nitric acid effluent (2.0 ~ 7.0 M) are from the metallurgical industry [6] and high-level nuclear waste [7]. The electrochemical
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reduction of nitric acid is considered to be the most promising and practical pathway to
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degrade nitrate [8-11], especially since this process also beneficially generates many useful compounds (NH3, N2O, NO2, etc.) and high-value chemicals (NH2OH) [12-15]. Furthermore,
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research on the concentrated nitric acid reduction electrochemical process is helpful in
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understanding the corrosion rate of various materials used in nuclear fuel reprocessing plants, such as stainless steels [16], titanium [17], zirconium [18], and in the design of high-
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performance electrodes used in strongly acidic environments [19].
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Electrode materials, such as Pt [20], Pd [21], Tl [22], Cu [23], alloy [24] and modified electrodes [25] have been studied in previous work at low nitrate concentrations (< 0.1 M),
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suggesting that Pt has a high reactivity for nitrate reduction [12]. Because of significant structure sensitivity, Pt has been the subject of a detailed investigation [13]. Extensive
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research has been devoted to the electrocatalytic reduction of nitrate on platinum electrodes primarily at low nitrate concentrations (< 0.1 M), including Pt mesh and single-crystal Pt [2628]. Recently, Pt nanoparticles have also been studied as electrocatalysts for nitrate reduction, which show higher reactivity compared with conventional Pt electrodes [29, 30]. In our previous study, a nano-Pt/GC electrode prepared by electrodeposition was used to investigate the reduction mechanism of Pt nanoparticles, indicating that nitrate reduction occurs preferentially on the Pt (100) surface, while the Pt (110) surface has little activity [30].
ACCEPTED MANUSCRIPT Due to the harshly corrosive conditions of a strongly acidic environment (>1.0 M nitric acid), reduction electrodes with high chemical stability and reactivity are desired. Prior research on Ti, Ta, Zr, Au and Pt indicates that electrode materials have a strong influence on the nitric acid reduction kinetics [31]. Electrocatalytic reduction on Pt is considered to be an autocatalytic process, and the electroactive species (HNO2) plays an important role in the
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reduction [31-37]. Until now, Pt is still recognized as the most appealing cathode material
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candidate for concentrated nitric acid reduction, due to its high reactivity compared with
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other materials [31]. For the reduction of high concentrations of nitric acid in industrial effluents, a more reactive electrode based on Pt is required, allowing a simultaneous
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reduction in Pt usage. Carbon fiber paper (CFP) has high mechanical strength, a large surface area, excellent gas permeability and good electrical conductivity [38, 39], making it an
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attractive alternative to particulate carbon for electrode substrates. It is anticipated that carbon fiber paper-supported nano-Pt electrode will provide a practical electrode for high-efficiency
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electrocatalytic reduction of concentrated nitric acid as well as reducing Pt usage. To the best
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of our knowledge, there is no report exploring high-performance electrodes for the reduction of concentrated nitric acid. The aim of the present study is to address this gap, and discuss the
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electro-reduction mechanism. Electrodes composed of Pt nanoparticles supported on 3D CFP were prepared by the electrodeposition method. The effect of nitric acid concentration
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between 1.0 M and 4.0 M on the reduction process was investigated.
2.1. Reagents Perchloric acid (Alfa Aesar, 48-50 wt%), sulfuric acid (98 wt%), nitric acid (65-68 wt%), dihydrogen hexachloroplatinate hexahydrate (Alfa Aesar, 99.9 wt%), sodium nitrate (Alfa Aesar, 99.0 wt%) and sodium nitrite (Alfa Aesar, 98 wt%) were used as received.
ACCEPTED MANUSCRIPT 2.2. Electrocatalyst Preparation and Catalysis Investigation The electrodeposition of Pt nanoparticles onto a carbon fiber paper (Toray TGP-H) was accomplished using 2 mM H2PtCl6 + 0.5 M H2SO4 according to the method described in previous research [30, 40]. The catalysis experiments were conducted in a standard three-
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electrode glass cell with a platinum mesh and a saturated calomel electrode (SCE, 0.244 V vs standard hydrogen electrode) as the counter electrode and the reference electrode,
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respectively. The electrochemical experiments were conducted at room temperature with
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ultrapure water (Millipore Milli-Q quality, 18.2 MΩ cm). Voltage potential was controlled using an Autolab 302N potentiostat/galvanostat. The cyclic voltammetry experiment was
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performed between 0.4 V and -0.25 V vs SCE at 50 mV s-1 for 50 cycles to synthesize Pt
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nanopolyhedral (denoted as Pt-NP), while Pt nanothorn (denoted as Pt-NT) was obtained by square wave potential between -0.2 V and 0.8 V vs SCE at 10 Hz for 20 min.
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2.3. Instrumentation and Measurements
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The nitric acid reduction experiments were conducted in a standard three-electrode glass cell, while the reference electrode was a Hg/Hg2SO4/K2SO4 (Sat.) electrode (0.640 V vs
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standard hydrogen electrode). The current interrupt method was used to measure the uncompensated resistance, and 90 % IR compensation was applied in the polarization and
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controlled potential electrolysis experiments to account for the voltage drop between the reference and working electrodes. Field emission scanning electron microscopy (FE-SEM) images and transmission electron microscopy (TEM) images were obtained on JSM-6700F FE-SEM (JEOL, Japan) and FEI Tecnai G2 F20 TEM instruments, respectively. Photomicrographs and the geometric area of CFP were measured by a digital microscope VHX-5000 (KEYENCE, Japan). The mass of Pt nanoparticles deposited on the CFP was weighed by an analytical balance (XSE205, METTLER TOLEDO). The reduction
ACCEPTED MANUSCRIPT experiments were carried out in various different concentrations of HNO3 (1.0 ~ 4.0 M) and other specific mixed solutions (HNO3 + NaNO2, NaNO3 or HClO4). To better understand the mechanisms, a specific reduction process was conducted in an H-type electrolytic cell (50 ml) connected by glass sand core, and the resulting evolved gases
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were detected by online mass spectrometry (OmniStar GSD320, Pfeiffer) that is a selfpriming instrument, and the gas flow rate is 2 ml min-1. The time delay between the gaseous
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sampling and the faradic-current is about several hundred milliseconds. Linear sweep
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voltammetry and cyclic voltammetry were used to explore the reaction mechanisms. The mass signals for m/z = 30, 32, 44 and 46 assigned to nitric oxide, oxygen, nitrous oxide and
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nitrogen dioxide, respectively, were monitored [10, 24, 34]. Prior to the measurement, argon
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at 20 mL min-1 was purged to remove the air and maintain the system at steady state. Results and discussion
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The photomicrographs of CFP before and after electrodeposition are shown in Figure 1.
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As shown in Figure 1A, the CFP shows metallic luster in the digital microscope image, indicating a clean CFP surface was obtained by pretreatment with 0.1 M H2SO4 for 3 h. After
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electrodeposition, the Pt nanoparticles were grown on the carbon fibers, and dispersed homogeneously on the whole surface of the carbon fiber paper (Figure 1B). The insets in
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Figure 1(B) also shows a uniform nano-Pt/CFP electrode. The typical SEM images of the two kinds of Pt nanoparticles (Pt nanothorn and Pt nanopolyhedra) on CFP are shown in Figure 1C-E. The morphology of Pt nanopolyhedra (Pt-NP) and Pt nanothorn (Pt-Nt) can be distinguished more clearly in Figure S1. It is observed that both Pt-NT and Pt-NP nanoparticles are homogenously dispersed on the carbon fiber. The lattice spacing of Pt-NP and Pt-NT is 0.2296 nm and 0.2171 nm, respectively, which indicates that growth direction is along [111] (Figure S1) [40]. However, typical cyclic voltammograms of nano-Pt/CFP in 0.1
ACCEPTED MANUSCRIPT M HClO4 (Figure S2) indicate that the exposed active surface of the Pt nanoparticles is predominantly Pt (110) and Pt (100). The surface of Pt-NP consists of equivalent (110) and (100) facets, while Pt-NT is predominantly the (100) facet. These observations are consistent
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with previous literature [30].
ACCEPTED MANUSCRIPT Figure 1. Photomicrograph of CFP (A) and nano-Pt/CFP (B) and typical SEM images of Pt nanoparticles: Pt nanothorn (C, D) and Pt nanopolyhedral (E, F). The insets show the entirety of blank CFP (A) and nano-Pt/CFP electrode (B). Table 1. The normalized current density of different Pt electrodes in 4 M HNO3. P4
Shape mA mg-1
mA cm-2geometric
Pt-NP
71.92
488.6
34.53
Pt-NT
75.39
384.9
Pt/C a
8.614
288.6
Pt mesh b
19.66
0.3667
Pt sheet c
1.997
0.0190
0.147
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mg cm-2geometric
234.6
34.17
174.5
0.196
2.083
69.78
0.0298
0.9874
0.0184
53.62
0.2117
0.0020
105.1
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mA mg-1
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mA cm-2geometric
Pt loading
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P3
Commercial Pt/C (20 wt.%) was purchased from Johnson Matthey.
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The diameter of Pt wire in the Pt mesh (52 mesh) is 0.1 mm with a 10 mm (L) × 10 mm (W).
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The thickness of Pt sheet is 0.1 mm and the length and width are 10 mm. The density of Pt
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is 21.45 g cm-3 at room temperature.
In order to investigate the performance of nano-Pt/CFP electrodes, the experiments were conducted in different concentrations of HNO3 (1.0 to 4.0 M). Pt sheet, Pt mesh and commercial Pt/C were also used as control samples for 4 M HNO3 reduction (shown in Figure S4) and the normalized current density are summarized in Table 1. Figure 2 shows the cyclic voltammograms with the current density normalized by the geometric surface area. As the HNO3 concentration increased from 1.0 M to 3.0 M, three reduction peaks (P1 at around 0.58 VSSE, P2 at around -0.55 VSSE, and P3’ at around 0.15 VSSE) were observed. The
ACCEPTED MANUSCRIPT reduction peaks at around -0.6 VSSE (P1 and P2) are ascribed to the reduction of NO3- to NH3 [30]. The cathodic peak P3’ suggests the adsorption of NO3- [41]. Moreover, the current density of the peaks around -0.6 VSSE increases slightly for both Pt-NP/CFP and Pt-NT/CFP samples (Figures 2 A, B and C), but no obvious change occurs for the peak P3’, as the HNO3 concentration increases from 1.0 M to 3.0 M. When the concentration of HNO3 is increased
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to 4.0 M, the current density of cathodic peak (abbreviated as P3 for 4.0 M HNO3 to indicate
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the different implication) increased dramatically to 488.6 mA mg-1, which was about 1.69
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times higher than commercial Pt/C and far better than Pt mesh and Pt sheet. In the subsequent anodic scan, a reduction peak (denoted P4) is also observed, which is further discussed in the
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paragraph following equation 4. The higher current density of P3 and P4 indicates that the nano-Pt/CFP electrode has remarkable activity towards the reduction of concentrated nitric
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acid compared to conventional Pt electrodes at room temperature, as shown in Table 1, which can be attributed to the size effect of nano-Pt as well as the microstructure of the CFP
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substrate. The 3D structure of nano-Pt/ CFP not only exposes more electro-active area to the
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electrolyte but also promotes the mass transport of gases and liquids. Furthermore, the Pt loading on Pt-NT/CFP and Pt-NP/CFP electrode is much lower (about 0.20 mg cm-2 geometric for Pt-NP/CFP) compared to the 53.6 mg cm-2
of Pt mesh and 105 mg cm-2 geometric of Pt sheet.
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geometric
geometric
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for Pt-NT/CFP and 0.15 mg cm-2
Table 2. Comparison of nitrate reduction mechanism.
Heterogeneous (Vetter)
Homogeneous (Schimd)
NO2 + e- → NO2-
NO+ + e- → NO
NO2- + H+ ⇌ HNO2
HNO2 + H+ ⇌ NO+ + H2O
HNO2 + HNO3 ⇌ 2NO2 + H2O
HNO2 + HNO3 ⇌ N2O4 + H2O
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N2O4 + 2NO + 2H2O ⇌ 4HNO2
The apparent increase in the current intensity of P3 in 4.0 M HNO3 indicates the occurrence of an autocatalytic reduction pathway: heterogeneous or homogenous
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autocatalysis as shown in Table 2, which is the reduction of NO2 or NO+ arising from the decomposition of high concentration nitric acid [31-36]. From Figure 2D, the reduction
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behavior of 4.0 M HNO3 can be explained by this autocatalysis mechanism. Also, 4.0 M is
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the critical HNO3 concentration at room temperature. Generally, nitric acid is subject to
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thermal decomposition and can also undergo an autoprotolysis reaction due to its acidic and basic properties. Therefore, it is reasonable to infer that a chemical equilibrium and a
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decomposition process occur in 4.0 M concentrated HNO3 (see equations 1 and 2). Nitric acid in the concentration range of 1.0 to 3.0 M tends to dissociate into H+ and NO3- due to the
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observed nitrate reduction peak at around -0.6 VSSE, while 4.0 M HNO3 is prone to
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decompose to HNO2. A detailed investigation was conducted on Pt-NT/CFP by controlling the solution composition.
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H+ + NO3- ⇌ HNO3
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HNO3 → HNO2 + ½ O2
(1) (2)
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Figure 2. Cyclic voltammograms of Pt-NP/CFP and Pt-NT/CFP electrode in (A) 1.0 M HNO3, (B) 2.0 M HNO3, (C) 3.0 M HNO3 and (D) 4.0 M HNO3. The insets show the partial
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enlargement between -0.8 and -0.5 V. Scan rate: 5 mV s-1.
To confirm the presence of a small fraction of HNO2 in the high concentration nitric acid, 50 mM NaNO2 was added into 2.0 M and 4.0 M HNO3 solution, respectively. Figure 3 shows the cyclic voltammograms of nitric acid reduction in the presence and absence of nitrite. Two reduction peaks between 0.0 and 0.2 VSSE appear in both the cathodic and anodic scan in 2.0 M HNO3 + 50 mM NaNO2 in Figure 3A, while no obvious peaks are observed in 2.0 M HNO3 at the same potential range, indicating the occurrence of nitrite reduction. The gaseous products were also detected (see Figure S5). No gaseous reduction products can be detected
ACCEPTED MANUSCRIPT for 2.0 M HNO3, while the reduction peaks at around -0.6 V vs SSE indicates the reduction mechanism of 2.0 M HNO3 is similar with that of low concentration (0.1 M) [30]. For 2.0 M HNO3 + 50 mM NaNO2, the molecular ion fragments of NO (m/z = 30) and N2O (m/z = 44) show periodicity with the variation of reduction current density for both positive- and negative-going scan, indicating that the decomposition of 2.0 M nitric acid to generate HNO2
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or NO2- hardly processes at room temperature. However, as shown in Figure 3B, the 4.0 M
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HNO3 solution shows two significant reduction peaks between 0.1 ~ 0.2 VSSE. When 50 mM
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NaNO2 was added into 4.0 M HNO3 solution, the current density of these two reduction peaks became stronger without the appearance of new peaks. These results suggest that a
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small amount of nitrous acid is present in 4.0 M HNO3 solution and autocatalysis occurs,
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which is in agreement with previous literature [34-36].
The HNO2 formed in the cathodic scan is suspected to enhance the initiation of the autocatalytic reaction in the next anodic scan, resulting in a higher current density. To avoid
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this influence of products generated in negative scans, linear scan voltammetry (LSV) was
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performed in combination with online gas spectroscopy.
Figure 3. Cyclic voltammograms of (A) 2.0 M HNO3, 2.0 M HNO3 + 50 mM NaNO2 and (B) 4.0 M HNO3, 4.0 M HNO3 + 50 mM NaNO2. Scan rate: 5 mV s-1.
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Figure 4. Simultaneously recorded LSVs (A, B) and mass signal m/z = 30, 32, 44, 46 (C, D)
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on Pt-NT/CFP electrode in 4.0 M HNO3. Scan rate: 5 mV s-1.
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The independent anodic or cathodic LSV was performed on the Pt-NT/CFP electrode in an H-type cell between -0.65 and 0.6 VSSE in 4.0 M HNO3 solution (Figure 4A, B). Prior to
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the measurement, an argon purge in the system was used to remove oxygen. The m/z signals for the reduction products are shown in Figure 4C and 4D. In the anodic scan, the fragments m/z = 30 and 44 corresponding to the production of NO and N2O are observed with the increase in potential from 0.0 to 0.4 VSSE. Moreover, the maximum ion abundance appears at the potential of the reduction peak. The current density of the peak near 0.15 VSSE increases up to 4.9 mA cm-2geometric, while the m/z = 46 has no obvious fluctuation, indicating that no NO2 evolution occurrs in the solution. Meanwhile, O2 (m/z = 32) was detected in the cathode
ACCEPTED MANUSCRIPT chamber and its ion current has no obvious fluctuation, indicating no release of gaseous oxygen on the cathode. Therefore, homogeneous autocatalysis appears to be the dominant pathway. For the cathodic scan, an obvious reduction peak appears near 0.15 VSSE and the current density
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increases up to 75 mA cm-2geometric, which is about 15 times larger than in the anodic scan. The abundances of m/z = 30, 44 increase with the increase of the current density (Figure 4D).
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These LSV observations (peak position and current density) are identical with the CV results
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in Figure 3B, suggesting an independent reduction reaction in either the anodic or cathodic scan. It should be noted that the onset of production of m/z = 44 is behind the onset of m/z =
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30, indicating that N2O production is related and subsequent to NO generation. The
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autocatalytic cycle at more negative potentials terminates due to the further reduction of NO by a two-step mechanism, finally resulting in N2O generation. According to the literature [12,
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NOads + NO + H+ + e- → HN2O2, ads
(4)
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HN2O2, ads + H+ + e- → N2O + H2O
(3)
NO would continue to produce N2O as long as NO exists in solution. Based on the
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analyses of LSV and online mass spectrometry, nitrous acid reduction occurs independently in the anodic and cathodic scan, resulting in the formation of NO and N2O. The lower current density of the reduction peak in the anodic scan might result from the adsorption of ions (e.g. NO3-) on the Pt plane, probably Pt (100), which inhibits the reduction of HNO2. In other words, the cathodic reduction peak at 0.15 VSSE is the combined nitrous acid reduction on Pt (110) and Pt (100) planes.
ACCEPTED MANUSCRIPT According to the assumed eq. 1 and eq. 2, addition of HClO4 and NaNO3 as the source of H+ and NO3- respectively is anticipated to shift the equilibrium toward HNO2 production. Thus, 1.0 M HClO4 and 1.0 M NaNO3 were added into the 4.0 M HNO3, respectively. Figure S6 shows the CVs in 4.0 M HNO3, 4.0 M HNO3 + 1.0 M NaNO3, and 4.0 M HNO3 + 1.0 M HClO4 on Pt-NT/CFP electrodes. The current density of P3 and P4 increase with the addition
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of 1.0 M HClO4 and 1.0 M NaNO3 (Table S2), indicating that more nitrous acid is generated
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from the nitric acid decomposition. Furthermore, the current increase of the reduction peak in 4.0 M HNO3 + 1.0 M HClO4 is more apparent. As studied previously and the results listed in
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Table 2, H+ is also involved in the autocatalysis process, resulting in the formation of NO+
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that can be further reduced on Pt-NT/CFP electrodes.
Figure 5. Simultaneously recorded current density and mass ion fragments m/z = 30, 44, 46 on Pt-NT/CFP electrode in 4.0 M HNO3 for cyclic voltammetry experiment. Scan rate: 5 mV s-1.
In order to investigate the stability of electro-reduction of nitric acid on the Pt-NP/CFP electrode, the cyclic voltammetry experiment was conducted in 4.0 M HNO3 between -0.65 ~
ACCEPTED MANUSCRIPT 0.6 VSSH at 5 mV s-1 in an H-type electrolytic cell. Figure 5 shows the time dependence of the current density and online mass spectroscopy signal (ion current). The molecular ion fragments of NO (m/z = 30) and N2O (m/z = 44) show clear periodicity with the variation of reduction current density, indicating a stable reduction process on the Pt-NP/CFP electrode.
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In summary, nitrate is predominately reduced on nano-Pt/CFP to generate NH3 in the HNO3 from 1.0 to 3.0 M, while in 4.0 M HNO3, nitric acid reduction progresses mainly via
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homogenous autocatalysis, with HNO2 acting as the active intermediate to generate NO
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product. The nano-Pt/CFP electrodes show remarkable reactivity for the electrocatalytic reduction of concentrated nitric acid in 4.0 M HNO3 due to the presence of a small fraction of
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in the electrocatalytic reduction process.
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active intermediate HNO2. The composition of the nitric acid solution plays an important role
Conclusions
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Nano-Pt/CFP electrodes have been studied for the effective electrocatalytic reduction of
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concentrated nitric acid. The nano-Pt/CFP electrodes showed higher reactivity compared with conventional Pt mesh, Pt sheet electrode and commercial Pt/C, which greatly reduces the Pt
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usage. The reduction mechanism on this electrode in concentrated nitric acid solution (≥ 4.0 M) involves a homogeneous autocatalysis process, with NO and N2O as the main products. A
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chemical equilibrium and a decomposition process occur in concentrated nitric acid. Thus, the electrocatalytic reduction process is significantly influenced by the composition of electrolyte. The findings of this work are anticipated to provide a high-performance electrode for concentrated nitrate wastewater treatment needed for emerging industrial applications, particularly metallurgical processes and nuclear waste. Acknowledgments
ACCEPTED MANUSCRIPT This work was supported by the National Natural Science Foundation of China (No. 21203137). Appendix A. Supplementary data Supplementary materials associated with this article can be found, in the online version, at http: //xxx.xxx.xxx.
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A high-performance nano-Pt/CFP electrode was prepared by electrodeposition.
The nano-Pt/CFP shows remarkable reactivity for electro-reduction of 4 M HNO3 at room temperature.
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The mechanism indicates a homogeneous autocatalysis process on this electrode.
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