Integrating RuO₂@TiO₂ catalyzed electrochemical chlorine evolution with a NO oxidation reaction for nitrate synthesis

Abstract

We report a cascade chlorine evolution–nitric oxide (NO) oxidation system for the homogeneous synthesis of nitrate, in which NO can be efficiently converted to nitrate by reacting with the active chlorine species. This highly efficient nitrate synthesis system was demonstrated by using a RuO₂ nanoparticle-decorated TiO₂ nanobelt array on a titanium plate (RuO₂@TiO₂/TP) under ambient conditions, capable of attaining a high nitrate yield of 95.22 mg cm⁻² h⁻¹ at 2.1 V versus the reversible hydrogen electrode in NO-saturated 0.5 M NaCl + 0.01 M HClO₄. Electrochemical in situ Raman spectroscopy studies reveal the adsorption of Cl–O species on the electrode surface and the key role of high-valence Ru^x+ (x > 4) at high potentials. The electron paramagnetic resonance results confirmed the existence of chlorine radicals and hydroxyl radicals generated by RuO₂@TiO₂/TP in the electrolyte.

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Nitrate, as one of the most crucial forms of oxidized nitrogen, which is essential for the production of fertilizers, explosives, and gunpowder, plays a significant role in our modern society.^1–3 The traditional route to nitrate production in industry involves catalytic oxidation of NH₃ through the Ostwald process, which results in a high rate of pollution to the environment.⁴ Moreover, NH₃ is mainly produced by the Haber–Bosch process operating at a high temperature and pressure, which is energy-intensive and emits large amounts of greenhouse gases.⁵ Electrocatalytic nitrogen oxidation is considered an attractive alternative for direct production of nitrate under ambient conditions but suffers from low nitrate yield and faradaic efficiency (FE).^6–8

Nitric oxide (NO), as one of the major air pollutants, has a lower bonding energy than N₂ (N=O: 607 kJ mol⁻¹ and N≡N: 941 kJ mol⁻¹ at 298 K),^9–15 suggesting its use as an alternative nitrogen source to N₂ for nitrate electrosynthesis via an NO oxidation reaction (NOOR). Wang et al. proposed a plasma-engraved commercial carbon cloth to synthesize nitrate by the electrochemical NOOR, which only occurs at the electrode surface resulting in low nitrate yield.¹⁰ The chlor-alkali process is one of the most critical chemical industry processes in the global economy,^16,17 and the generated Cl₂via an anodic chlorine evolution reaction (CER) is disproportionate in aqueous environments to form oxidative chlorine species.^18–21 Our recent work suggests that using active chlorine species generated by the CER can improve the efficiency of NO electro-oxidation²² and may be an alternative path to drive efficient and sustainable nitrate synthesis.

Here, we put forward a cascade catalytic system for efficient nitrate synthesis enabled by a RuO₂ nanoparticle-decorated TiO₂ nanobelt array on a titanium plate (RuO₂@TiO₂/TP) as a CER electrocatalyst producing active chlorine species to directly convert NO into nitrate under ambient conditions. Such a RuO₂@TiO₂/TP catalyst exhibits a low potential of 1.78 V to reach a current density of 20 mA cm⁻² and a high active chlorine yield of 83.3 mg cm⁻² h⁻¹ at 2.1 V versus the reversible hydrogen electrode (RHE) in 0.5 M NaCl + 0.01 M HClO₄. Owing to the generation of active chlorine, the reaction sites for the oxidation of NO to nitrate are not restricted to the electrode surface, but extend throughout the electrolyte, achieving homogeneous nitrate production. RuO₂@TiO₂/TP affords an outstanding nitrate yield of 95.22 mg cm⁻² h⁻¹ at 2.1 V vs. the RHE in NO-saturated 0.5 M NaCl + 0.01 M HClO₄. It does not show performance decay for at least 12 h of continuous operation, with a remarkable nitrate accumulation of 1222.44 mg. Electrochemical in situ Raman and quasi in situ electron paramagnetic resonance (EPR) experiments revealed the Ru^x+ (x > 4) active sites and Cl–O species adsorbed on the electrode surface as well as the presence of chlorine radicals and hydroxyl radicals in the electrolyte catalytically produced by RuO₂@TiO₂/TP.

The preparation of RuO₂@TiO₂/TP involves the hydrothermal growth of Na-titanate on the TP, a cation-exchange reaction between Na-titanate and Ru³⁺, and an air calcination treatment of Ru-titanate/TP (Fig. S1†). Comparison of the X-ray diffraction (XRD) patterns of Na-titanate/TP and Ru-titanate/TP reveals that Ru-titanate/TP has an almost similar crystal structure to that of Na-titanate/TP (Fig. S2a†). In addition, compared with Na-titanate/TP, the diffraction peak of Ru-titanate/TP at the (200) plane has a slight shift to a lower scattering angle (Fig. S2b†). The XRD patterns of both TiO₂/TP and RuO₂@TiO₂/TP (Fig. 1a) show diffraction peaks for anatase TiO₂ (JCPDS no. 43-1027) with mainly characteristic peaks at 25.28°, 38.57°, 48.04°, 53.89°, and 55.06° indexed to the (101), (112), (200), (105), and (211) planes, respectively. Meanwhile, a characteristic peak at ca. 28.01° is detected, which is well indexed to the (110) plane of RuO₂ (JCPDS no. 43-1027). The scanning electron microscopy (SEM) images reveal the full coverage of the TP substrate (Fig. S3†) by the Na-titanate nanobelt array (Fig. S4a†), whose morphology is well preserved following the subsequent cation-exchange reaction (Fig. S4b†). The SEM images of TiO₂/TP (Fig. 1b) and RuO₂@TiO₂/TP (Fig. 1c) show that the morphologies of both materials are nanobelt arrays. Energy dispersive X-ray spectroscopy (EDX) elemental mapping images acquired from RuO₂@TiO₂/TP (Fig. 1d) identified the co-existence of Ru, Ti, and O elements with a uniform distribution. More details of RuO₂@TiO₂ were observed by transmission electron microscopy (TEM). As exhibited in Fig. 1e, some small nanoparticles can be seen in the nanobelt. The high-resolution TEM (HRTEM) image of RuO₂@TiO₂ shows lattice fringes with distances of 0.318 and 0.352 nm attributed to the (110) and (101) planes of RuO₂ and TiO₂, respectively (Fig. 1f). The chemical state of RuO₂@TiO₂/TP can be revealed by X-ray photoelectron spectroscopy (XPS). As depicted in Fig. 1g, the two strong peaks of Ru 3d_3/2 at 284.9 eV and Ru 3d_5/2 at 280.9 eV are attributed to Ru⁴⁺.^23,24 The Ti 2p spectrum (Fig. 1h) shows two dominant peaks located at 464.2 eV (Ti 2p_1/2) and 458.4 eV (Ti 2p_3/2).²⁵ In the O 1s region (Fig. 1i), two peaks located at 531.9 eV and 530.1 eV are regarded as the absorbed water and lattice of oxides, respectively, while the other peak at 529.4 eV refers to the metal–O bond.^26,27

Fig. 1 (a) XRD patterns of RuO₂@TiO₂/TP and TiO₂/TP. SEM images of (b) TiO₂/TP and (c) RuO₂@TiO₂/TP. (d) The SEM image and corresponding EDX elemental mapping images of RuO₂@TiO₂/TP. (e) TEM and (f) HRTEM images of RuO₂@TiO₂. High-resolution XPS spectra of RuO₂@TiO₂/TP in (g) the Ru 3d, (h) the Ti 2p, and (i) the O 1s regions.

The electrocatalytic activities of RuO₂@TiO₂/TP for CER were tested in a gas-tight H-type cell. Fig. 2a shows the linear sweep voltammetry (LSV) curves of RuO₂@TiO₂/TP, the benchmark RuO₂ on TP (RuO₂/TP), TiO₂/TP, and the bare TP in 0.5 M NaCl + 0.01 M HClO₄. This clearly shows a higher current density for RuO₂@TiO₂/TP compared to RuO₂/TP, TiO₂/TP, and bare TP at the potential range of 1.7 V–2.1 V. RuO₂@TiO₂/TP shows a lower potential of 1.78 V compared to RuO₂/TP (2.0 V) at a current density of 20 mA cm⁻², which is superior to that of most recently reported CER electrocatalysts listed in Table S1.† RuO₂@TiO₂/TP also achieves a low Tafel slope of 169.1 mV dec⁻¹ (Fig. 2b), which is comparable to RuO₂/TP (157.5 mV dec⁻¹) and smaller than those of TiO₂/TP (300.4 mV dec⁻¹) and bare TP (328.4 mV dec⁻¹). Active chlorine was evaluated with colorimetric measurements using N,N-diethyl-p-phenylenediamine (DPD), and the ultraviolet-visible (UV-vis) calibration curves are shown in Fig. S5.† ²⁸ Additionally, the chronoamperometry curves and UV-vis absorption spectra are illustrated in Fig. S6.† The average FE of active chlorine for RuO₂@TiO₂/TP reaches a maximum value of 83.4% at 2.1 V. Of note, a high average active chlorine yield of 83.3 mg cm⁻² h⁻¹ was also delivered at the same potential, namely 2.1 V (Fig. 2c). To confirm the origin of the active chlorine in the CER, several control experiments were conducted. As shown in Fig. 2d and Fig. S7,† RuO₂@TiO₂/TP affords a high active chlorine yield of up to 83.3 mg cm⁻² h⁻¹ at 2.1 V, while only 1.11 mg cm⁻² h⁻¹ and 0.24 mg cm⁻² h⁻¹ are measured at the open circuit potential (OCP) and in a blank electrolyte, respectively, indicating that the facilities and medicines were less polluted. In addition, the yields of active chlorine generated by RuO₂@TiO₂/TP, TiO₂/TP, and bare TP during the CER process were compared at 2.1 V (Fig. 2d and Fig. S8†). It can be seen distinctly that TiO₂/TP and bare TP produce little active chlorine, suggesting that RuO₂ possesses a wonderful catalytic activity toward the CER. The stability of RuO₂@TiO₂/TP was examined by a long-term stability test at a constant current density of 50 mA cm⁻² in 0.5 M NaCl + 0.01 M HClO₄. As shown in Fig. 2e, the fluctuation of potential at a current density of 50 mA cm⁻² is almost negligible. The good electrochemical stability of our RuO₂@TiO₂/TP was further verified by the LSV curve after electrolysis, which shows a slight decline compared with the initial curve (Fig. S9†).

Fig. 2 (a) LSV curves of RuO₂@TiO₂/TP, RuO₂/TP, TiO₂/TP, and bare TP at a scan rate of 5 mV s⁻¹ in 0.5 M NaCl + 0.01 M HClO₄ and (b) the corresponding Tafel plots of different catalysts. (c) Active chlorine yields and FEs of RuO₂@TiO₂/TP at different potentials. (d) Comparison of the active chlorine yields for RuO₂@TiO₂/TP under different electrochemical conditions, and for TiO₂/TP and bare TP at 2.1 V in 0.5 M NaCl + 0.01 M HClO₄. (e) Long-term stability test of RuO₂@TiO₂/TP at a constant current density of 50 mA cm⁻² in 0.5 M NaCl + 0.01 M HClO₄.

In situ electrochemical Raman spectroscopy was employed to probe the dynamic evolution of the surface structure of RuO₂@TiO₂/TP during the CER (Fig. S10†). As shown in Fig. 3a, the peaks for the Ti–O and Ru–O bands appear at 399 cm⁻¹ and 524 cm⁻¹ in the potential window from the OCP to 2.015 V, respectively.^29,30 With gradually increasing the potential to 1.915 V, the typical Raman peaks at 820 cm⁻¹ and 933 cm⁻¹ appeared, which were attributed to the Ru^x+ (x > 4) and Cl–O stretching bands, respectively,^31–33 and the intensity gradually became larger when the potential was increased. These results illustrate that the surface of RuO₂ was oxidized into Ru^x+ (x > 4) and adsorbed by the Cl–O species. Electrochemical impedance spectroscopy is a classical method for probing the electrochemical behavior of the electrode/electrolyte interfaces.^34,35 A series of impedance spectra for RuO₂@TiO₂/TP are exhibited in Nyquist (Fig. 3b) and Bode (Fig. 3c) formats over a range of potentials (from the OCP to 2.26 V). These Nyquist plots were fitted with an equivalent circuit (Fig. S11†). Of note, the impedance responses of RuO₂@TiO₂/TP at the low-frequency region (below 10² Hz) were remarkable once the potentials were increased above 1.71 V. The impedance responses at these frequencies could be related to the metal oxyhydroxides on the RuO₂ electrode surface, and the newly formed metal oxyhydroxides at high potentials may be the active phase of the CER.^36,37 Inspired by the formation of various radicals in the oxidation system,²⁰ we speculated that there may be free radicals in the electrolyte. Quasi in situ EPR spectroscopy was applied to investigate the Cl-related intermediate. 5,5-Dimethyl-1-pyrroline N-oxide (DMPO) as a spin trapping agent was added into 0.5 M NaCl + 0.01 M HClO₄ to improve the lifetimes of the intermediate³⁸ and to facilitate the EPR tests (Fig. 3d). Notably, three peaks at 3494, 3509, and 3524 G indexed to DMPO-Cl³⁹ can be detected at a potential of 2.1 V in 0.5 M NaCl + 0.01 M HClO₄, implying the generation of chlorine radicals⁴⁰ catalyzed by RuO₂@TiO₂/TP. The quartet of peaks at 3487, 3502, 3517, and 3532 G with an intensity ratio of 1 : 2 : 2 : 1 corresponded to DMPO-OH. The control experiment showed that no relevant DMPO-Cl and DMPO-OH signals were detected in the blank electrolyte.

Fig. 3 (a) In situ Raman spectra recorded for RuO₂@TiO₂/TP, shown as a function of potential, during the CER in 6 M NaCl + 0.01 M HClO₄. (b) Nyquist plots and (c) Bode phase plots for RuO₂@TiO₂/TP at different applied potentials in 0.5 M NaCl + 0.01 M HClO₄. (d) Quasi in situ EPR trapping of the radicals.

Fig. 4a exhibits the LSV curves of RuO₂@TiO₂/TP in Ar- and NO-saturated 0.5 M NaCl + 0.01 M HClO₄ in a gas-tight H-cell under ambient conditions. As the potential exceeds 1.9 V, the oxidation current density of RuO₂@TiO₂/TP slightly decreases in the NO-saturated electrolyte, indicating that NO can reduce the current density of the CER. This observation is worthy of study, and we further discuss this point in the later product analysis part. The produced nitrate and nitrite are quantified based on the standard method by using UV-vis spectrophotometry (Fig. S12 and Fig. S13,† respectively). The influence of the different potentials on the yields of nitrate and nitrite was investigated (Fig. 4b and c, respectively). It can be seen that the nitrate yields increase with more positive potential, and achieve a high yield of 95.22 mg cm⁻² h⁻¹ at 2.1 V, considerably outperforming the previously reported electrocatalysts for NOR under ambient conditions (Table S2†). Of note, yields of the nitrite are essentially the same and negligible at each potential, with the highest value of 0.27 mg cm⁻² h⁻¹ at 1.9 V. The corresponding chronoamperometry curves and UV-vis absorption spectra are displayed in Fig. S14.† NO reacts with O₂ in air easily to produce NO₂ chemically, which can also easily dissolve in water to form nitrate.¹⁰ The control experiment of NO and O₂ co-saturated electrolyte without electricity was conducted. As shown in Fig. 4d and Fig. S15,† in 0.5 M NaCl + 0.01 M HClO₄, the nitrate yield from the reaction between NO and O₂ is very low relative to our system, being about 15.6 mg (Fig. 4d). Subsequently, we systematically investigated the NOOR performance of RuO₂@TiO₂/TP. It can be seen distinctly that RuO₂@TiO₂/TP produces trace nitrate, and the maximum nitrate yield of 2.76 mg cm⁻² h⁻¹ was achieved at 1.9 V in NO-saturated 0.5 M Na₂SO₄ + 0.01 M HClO₄ (Fig. 4e and Fig. S16†). Of note, RuO₂@TiO₂/TP produced much more nitrite than nitrate at each potential for the NOOR. A high nitrite yield of 14.31 mg cm⁻² h⁻¹ was obtained at 2.2 V, which was significantly higher than that when NO reacts with active chlorine generated by the CER at all potentials (Fig. 4f). The cascade catalytic system consisting of the CER and NO oxidation exhibiting an enhanced nitrate yield can be explained as follows. Due to the low solubility of NO in water, a conventional NOOR occurs at the electrode surface and is quickly limited by mass transport at high current densities, resulting in poor nitrate yields. In our cascade catalysis system, chloride is oxidized by RuO₂@TiO₂/TP to active chlorine species in the electrolyte, which can rapidly oxidize NO to nitrate. Chloride also acts as a redox mediator between the electrode and NO, and the active chlorine species donate electrons to NO in solution and return to the initial chloride state (Fig. 4g). Moreover, the total nitrate yields increase almost linearly from 78.01 mg at 1 h to 1222.44 mg at 12 h, which reveals the good stability of our RuO₂@TiO₂/TP catalyst in the cascade catalytic system with the CER and NO oxidation (Fig. 4h and Fig. S17†). Notably, the XRD pattern and SEM image of post-electrolysis RuO₂@TiO₂/TP confirm that the crystal structure and nanobelt array are maintained, respectively (Fig. S18 and S19†). Moreover, the Ru 3d region of post-electrolysis RuO₂@TiO₂/TP shows a high-valence Ru oxidation state, which is consistent with previous reports (Fig. S20†).^24,41

Fig. 4 (a) LSV curves of RuO₂@TiO₂/TP in Ar- and NO-saturated 0.5 M NaCl + 0.01 M HClO₄. (b) Nitrate and (c) nitrite yields over RuO₂@TiO₂/TP at each given potential in NO-saturated 0.5 M NaCl + 0.01 M HClO₄. (d) Nitrate yields of RuO₂@TiO₂/TP for 1 h of electrolysis in gas-tight (in an NO-saturated electrolyte) and open electrolytic cells (under an NO + O₂ atmosphere). (e) Nitrate and (f) nitrite yields over RuO₂@TiO₂/TP at each given potential in NO-saturated 0.5 M Na₂SO₄ + 0.01 M HClO₄. (g) Possible reaction pathways for nitrate production. (h) Long-term stability tests for continuous generation of nitrate over RuO₂@TiO₂/TP in NO-saturated 0.5 M NaCl + 0.01 M HClO₄.

In the present study, we demonstrate an efficient production process of nitrate under ambient conditions by using RuO₂@TiO₂/TP as the electrode to produce active chlorine to oxidize NO in aqueous solution. Such a RuO₂@TiO₂/TP catalyst exhibits a low potential of 1.78 V to reach a current density of 20 mA cm⁻² and a high active chlorine yield of 83.3 mg cm⁻² h⁻¹ at 2.1 V in 0.5 M NaCl + 0.01 M HClO₄. Owing to the high active chlorine yield, our system enables a superior nitrate yield of 95.22 mg cm⁻² h⁻¹ in NO-saturated 0.5 M NaCl + 0.01 M HClO₄ at 2.1 V, and trace amounts of unwanted nitrite (0.25 mg cm⁻² h⁻¹) are generated. Moreover, a 12 h stability test with steady nitrate build-up is performed. The electrochemical in situ Raman analysis revealed the active site role of Ru^x+ (x > 4) in catalyzing the CER and the interaction between the Cl atom and O atom at high potentials (1.915 V to 2.215 V). The quasi in situ EPR results confirmed the presence of chlorine radicals and hydroxyl radicals catalytically produced by RuO₂@TiO₂/TP in the electrolyte. Our work highlights a promising strategy for highly efficient conversion of NO into nitrate, and provides opportunities for the synthesis of high-value chemicals from other small molecules with low solubility via this cascade system.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 22072015).

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Footnote

† Electronic supplementary information (ESI) available: Experimental section and ESI figures. See DOI: https://doi.org/10.1039/d3qi00209h

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