Revealing the effect of electrochemical promotion on FeNC catalysts for electro-assisted NH₃-SCR

Abstract

We devised an electro-assisted method utilizing Fe_xN_yC catalysts for low-temperature NH₃-SCR. Compared to traditional heating methods, electric heating significantly reduced the T₉₀ of NO_x conversion for the Fe₃N_1.5C catalyst from 160 °C to 120 °C. Under electro-assisted conditions, the applied current facilitates the migration of bulk phase lattice oxygen to the catalyst surface, promoting the oxidation of NH₃ species adsorbed on the catalyst surface and thereby promoting the NH₃-SCR reaction.

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The emission of nitrogen oxides (NO_x) has led to environmental issues such as photochemical smog, haze, and acid rain, posing significant threats to both natural ecosystems and human health.² Among the various technologies for NO_x control, selective catalytic reduction with NH₃ (NH₃-SCR) stands out as the most effective method currently available.^3,4 Among NH₃-SCR catalysts, V₂O₅–WO₃/TiO₂ and copper-based molecular sieves exhibit superior performance and find widespread application.⁵ However, V₂O₅–WO₃/TiO₂ demonstrates optimal performance only within the range of 300–400 °C, while copper-based molecular sieves, with a broader temperature window, are effective primarily within flue gas conditions approximately ranging from 200–550 °C.^6–9 Consequently, they are inadequate for efficiently treating low-temperature flue gas generated by processes such as coke production, steel manufacturing, and automotive cold starts. Enhancing the denitrification performance of catalysts at temperatures below 200 °C has thus emerged as a pressing research challenge in the field.

To tackle the challenge of poor catalyst performance under low-temperature conditions, researchers have devised an electric heating approach. In this technique, the conductive material is electrically heated, ensuring that the catalyst, which is supported on its surface, attains the necessary reaction temperature. For instance, Li et al. devised an electrothermal alloy-embedded V₂O₅–WO₃/TiO₂ catalyst, employing internal electric heating to regulate catalyst temperature, thereby sustaining high NH₃-SCR performance over a wide gas temperature range of 100–400 °C.¹⁰ However, in the electric heating method, the contact between the conductive material and the active component of the catalyst is insufficient, preventing accurate heating of the reaction site. Additionally, since the catalyst itself is non-conductive, the current cannot be effectively utilized without passing through the catalyst.

To address the limitations of electric heating, researchers have further developed the method of electrical assistance. This approach leverages the electron-promoting effect to drive reactions, thus enhancing catalytic efficiency at low temperatures.¹¹ For example, Mei et al. introduced an electrification strategy aimed at reducing the ignition temperature required for 50% soot conversion to below 75 °C, employing conductive oxides as catalysts.¹² Presently, electro-assisted catalysis has been extensively explored in various applications such as soot oxidation, CO₂ methanation, CO oxidation and three-way catalysis, yet its application in NH₃-SCR remains relatively less studied.^11–14

Carbon materials are widely acknowledged for their suitability as catalyst supports in both electrochemistry and selective catalytic reduction (SCR) applications due to their excellent conductivity, cost-effectiveness, tunable structure, and favourable adsorption properties.^15–18 Hence, carbon-based catalysts are considered ideal for the electro-assisted NH₃-SCR reaction. In this study, we synthesized a series of Fe_xN_yC catalysts and investigated their NH₃-SCR performance under both electro-assisted and conventional conditions. Compared to conventional heating methods, the electron-promoting influence of electric heating notably reduced the T₉₀ of NO_x conversion for the Fe₃N_1.5C catalyst from 160 °C to 120 °C, effectively increasing the low-temperature activity of the catalyst. Our findings indicate that the applied current promotes the migration of bulk lattice oxygen from the catalyst to the surface, where it participates in the oxidation of adsorbed NH₃ species, thereby facilitating the NH₃-SCR reaction.

The catalytic activity was evaluated using a specialized fixed-bed reactor, as depicted in Fig. 1. The catalyst was loaded between two sintered copper sheets within a quartz tube. Each of these copper sheets was in direct contact with a copper rod and a copper wire, acting as the positive and negative electrodes, respectively, and were powered by a constant power supply. A K-type thermocouple was inserted through a small aperture in the quartz tube wall and positioned between the two copper plates to monitor the catalyst temperature. The Fe_xN_yC catalysts were synthesized using the milling–calcination method (see Fig. S1, SI). The catalyst was named Fe_xN_yC based on the proportion of the iron precursor and nitrogen precursor (see Fig. S2–S9, SI for details regarding catalyst preparation, reaction procedure and structural information).¹⁹

Fig. 1 Schematic diagram of a self-made reaction system for the ELEC test.

The NH₃-SCR performances of Fe_xN_yC are depicted in Fig. 2a. The dashed line segments represent the NO_x conversion rate under traditional NH₃-SCR conditions, recorded as TRA, while the solid line segments depict the NO_x conversion rate under electrical assistance conditions (where a specific current is applied to the catalyst through electrodes on both sides, heating the catalyst to the required temperature), recorded as ELEC. Fe₃CNT showed poor NO_x conversion, reaching only 26% under ELEC conditions at 200 °C. Increasing the nitrogen-doping amount significantly enhanced the ELEC performance of the catalysts.²⁰ Among the nitrogen-doped variants, Fe₃N_1.5C demonstrated the highest ELEC performance, achieving 92% NO_x conversion at 125 °C. Compared to TRA, ELEC reduced the T₉₀ of Fe₃N_1.5C in the NO_x conversion rate from 160 °C to 120 °C. Importantly, as shown in Fig. 2b, ELEC consistently outperformed TRA performance across all catalysts at equivalent temperatures. Furthermore, it is noteworthy that ELEC enhances the catalytic activity more significantly at low temperatures than at high temperatures, a trend consistent with reported results.¹⁴ These findings demonstrate the significant enhancement of NH₃-SCR performance by ELEC compared to traditional methods, particularly in the low-temperature range.

Fig. 2 (a) NO_x conversion of the catalysts under TRA and ELEC conditions; (b) NO_x conversion of the catalysts under TRA and ELEC conditions at 100 °C; (c) NO_x conversion of Fe₃N_1.5C at 50 °C and 75 °C under different electric power levels; (d) Arrhenius equation plots of Fe₃N_1.5C and Fe₃N₃C under ELEC and TRA.

To elucidate the influence of both Joule heating and electrochemical promotion effects of current on the NH₃-SCR reaction, the low-temperature SCR performance of Fe₃N_1.5C was tested at the same temperature but different power levels. As depicted in Fig. 2c, the ELEC performance of Fe₃N_1.5C at 50 °C and 75 °C exhibited significant enhancement with increasing input electric power. At 75 °C, the NO_x conversion rate increased from 59% to 77% with increasing input electric power. Notably, the relationship between input electric power and ELEC performance closely adhered to a linear trend, suggesting that the applied current passes through the active site, thereby stimulating the active site and facilitating the reaction rate. This finding suggests that the effect of the electric pair reaction surpasses the Joule heating effect on the carrier.²¹

To elucidate the enhancement mechanism of ELEC on catalytic activity, the reaction kinetics of Fe₃N_1.5C and Fe₃N₃C under ELEC and TRA conditions were compared (Fig. 2d).¹⁴ For both Fe₃N_1.5C and Fe₃N₃C, the apparent activation energy values under ELEC and TRA conditions are close to each other, both close to 22 kJ mol⁻¹, indicating that there is no significant difference in the reaction mechanism under the two conditions, which is consistent with the literature report.¹⁴ Under ELEC conditions, both catalysts exhibit a marked increase in the apparent pre-exponential factor compared to TRA conditions. This observed increase implies a greater availability of active sites, which leads to enhanced reaction efficiency. Concurrently, as established in the literature, electrical assistance accelerates the release of lattice oxygen from the bulk to the catalyst surface.^12,22 It is thus inferred that these surface lattice oxygen species constitute the active sites. The primary role of electrical assistance is, therefore, to augment the density of these sites via promoted oxygen migration, consequently accelerating the NH₃-SCR reaction.

Several studies have noted that lattice oxygen on the catalyst surface can engage in the NH₃-SCR reaction by transforming NO into NO₂ or NH₃ into NH₂,^1,23,24 while electricity can facilitate the release of lattice oxygen from the bulk phase to the surface.^12,22 Therefore, we propose the following mechanism for the electro-assisted promotion of NH₃-SCR: the enhancement of the reaction rate by electro-assistance may be attributed to the provision of energy for the migration of lattice oxygen to the catalyst surface, besides Joule heating. This process activates more active sites, facilitating the activation of adsorbed NH₃ species on the catalyst, consequently increasing the reaction rate.²¹

To validate this conjecture, O₂ on–off experiments were conducted on Fe₃N_1.5C to examine the impact of current on lattice oxygen and its relationship with catalyst performance enhancement.¹ As shown in Fig. 3(a–c), upon cessation of oxygen supply, the NO_x conversion decreased rapidly under both ELEC and TRA conditions, stabilizing after varying durations due to the balance of the gradual consumption of lattice oxygen on the catalyst surface and migration of bulk lattice oxygen. Fe₃N_1.5C required a longer time to reach reaction equilibrium under ELEC conditions compared to TRA, with the time difference diminishing as the temperature rose, attributed to the accelerated migration of bulk lattice oxygen to the surface under the influence of the current. Furthermore, as depicted in Fig. 3d, the comparison of NO_x conversion rates at different temperatures after oxygen supply cessation revealed that Fe₃N_1.5C exhibited higher NO_x conversion under ELEC conditions compared to TRA, with the difference amplifying with temperature escalation, contrary to the trend under aerobic conditions. This phenomenon may be attributed to the adsorption and activation of O₂ by the catalyst to form O anions, which can also activate the adsorbed NH₃/NO species, with ELEC having a limited impact on this reaction.²⁵ After the oxygen supply is cut off, the adsorbed species on the catalyst surface can only be activated by lattice oxygen, and ELEC can promote the migration of lattice oxygen, thus significantly impacting the NH₃-SCR reaction in the absence of O₂. In summary, under ELEC conditions, the accelerated migration of bulk lattice oxygen of Fe₃N_1.5C to the catalyst surface results in the creation of more reaction sites and heightened catalytic activity.

Fig. 3 O₂ on–off experiments of the Fe₃N_1.5C catalyst under TRA and ELEC conditions at (a) 100 °C and (b) 125 °C; (c) time required for Fe₃N_1.5C to reach reaction equilibrium, the equilibrium of conversion refers to dX/dt < −0.3 (X represents conversion and t represents the reaction time),¹ and (d) NO_x conversion of the Fe₃N_1.5C catalyst in the reaction equilibrium state after stopping oxygen supply.

To confirm the role of current in promoting lattice oxygen migration within the catalyst bulk phase, electric-assisted and electric-unassisted NH₃-SCR tests were conducted under O₂-free conditions (the two conditions are referred to as OF-ELEC and OF-TRA). OF-ELEC/OF-TRA conditions mirrored ELEC/TRA conditions, except that the 5% O₂ in N₂ was replaced with N₂ alone.¹² Fe₃N_1.5C catalysts underwent OF-ELEC, OF-TRA, and ELEC conditions for 3 hours at 200 °C. The Raman spectra in Fig. 4a shows that after OF-ELEC and OF-TRA treatment, the peaks corresponding to the Fe crystalline phase at 222 cm⁻¹ and 282 cm⁻¹ vanished, indicating the migration of lattice oxygen from the catalyst bulk phase to the surface, resulting in the disruption of the Fe crystalline phase on the catalyst surface. The XRD spectra in Fig. 4b reveal that after OF-ELEC and OF-TRA treatment, the peaks attributed to Fe⁰ and Fe₃C were weakened, while those of Fe₃O₄ and Fe₂O₃ were enhanced, consistent with the Raman results. Notably, compared to OF-TRA treatment, the catalyst peaks after OF-ELEC treatment underwent more significant changes, suggesting that the current could further promote lattice oxygen migration within the catalyst bulk phase, beyond providing Joule heat. Similar trends were observed in XPS and HRTEM, where the OF-ELEC-treated catalyst exhibited a higher iron species valence, more lattice oxygen (Fig. 4c and d), and more disordered lattice fringes (Fig. 4e and f).¹²

Fig. 4 Raman patterns (a), XRD spectra (b), deconvoluted Fe 2p XPS spectra (c), and O 1s XPS spectra (d) of the fresh Fe₃N_1.5C catalyst and Fe₃N_1.5C catalyst following ELEC, OF-TRA(O₂-free TRA), and OF-ELEC(O₂-free ELEC); (e and f) HRTEM images of the fresh Fe₃N_1.5C catalyst and Fe₃N_1.5C catalyst following OF-ELEC.

To further explore the effect of ELEC in promoting lattice oxygen migration on the subsequent reaction of NH₃-SCR, the reaction of NO + O₂ and NH₃ on the surface of the Fe₃N_1.5C catalyst was observed through in situ DRIFT experiments at 150 °C. Initially, the adsorption of NH₃ and NO + O₂ at 150 °C was scrutinized. Fig. 5a displays the DRIFT spectrum captured during NH₃ adsorption. Peaks corresponding to NH₂ species (1518, 1539 and 1558 cm⁻¹), NH₃ (1624 cm⁻¹), a modest quantity of NH₄⁺ species (1650 cm⁻¹), and weakly adsorbed NH₃ (965 930 cm⁻¹) emerged, indicating that the NH₃ species adsorbed on the catalyst surface can be rapidly oxidized to NH₂ by surface lattice oxygen. Fig. 5b shows the DRIFT spectrum captured during NO + O₂ adsorption. Peaks indicative of adsorbed NO₂ (1624 cm⁻¹) and monodentate nitrate (1521–1540 cm⁻¹) emerged, signifying the adsorption of NO on the catalyst surface and its subsequent oxidation to various NO₂ species.

Fig. 5 In situ DRIFT spectra of (a) NH₃ adsorption, (b) NO + O₂ adsorption, (c) NO + O₂ reacted with pre-adsorbed NH₃ species, and (d) NH₃ reacted with pre-adsorbed NO + O₂ species for Fe₃N_1.5C catalysts at 150 °C.

To further elucidate the reaction behaviour of the adsorbed species on the catalyst surface, in situ DRIFTS experiments were conducted to investigate the interaction of NO + O₂ with pre-adsorbed NH₃ and the reaction of NH₃ with pre-adsorbed NO + O₂. Fig. 5c presents the in situ DRIFTS spectra of the reaction between pre-adsorbed NH₃ species and NO + O₂ on the Fe₃N_1.5C catalyst. Upon introduction of NO + O₂, the NH₂ peak (1558 cm⁻¹) increased immediately, indicating the presence of the NH₂NO intermediate species.¹⁸ Subsequently, the emergence of NO₃⁻ (1384 cm⁻¹) and C–NO₂ (1360 cm⁻¹) with prolonged exposure indicated NO consumption of the pre-adsorbed NH₃ species, suggesting that the reaction of adsorbed NH₃ with NO follows the Eley–Rideal (E–R) mechanism. NH₃ is initially adsorbed and activated to NH₂, subsequently reacting with gaseous NO to form NH₂NO, which is then decomposed into N₂ and H₂O.^26,27Fig. 5d illustrates the in situ DRIFTS spectra of NH₃ reacting with pre-adsorbed NO + O₂ species. Upon introduction of NH₃, the peak intensities corresponding to monodentate nitrate, NO₃⁻, and C–NO₂ species gradually decreased, and a peak of weakly adsorbed NH₃ species emerged. However, no obvious peak of the reaction intermediate was observed, indicating that the Langmuir–Hinshelwood (L–H) mechanism may be present, but the E–R mechanism predominates.²⁸

Based on the results of in situ DRIFTS spectra, the potential reaction pathway in the NH₃-SCR reaction of the Fe₃N_1.5C catalyst is illustrated in Fig. S6. Initially, NH₃ is adsorbed onto the catalyst surface and subsequently activated by lattice oxygen to form NH₂. The resulting NH₂ then reacts with the adsorbed NO species to form an NH₂NO intermediate, which decomposes into N₂ and H₂O. Subsequently, surface lattice oxygen is replenished via oxygen migration from the gas phase or bulk lattice oxygen, enabling its continued participation in subsequent reaction cycles. According to the reaction kinetics results presented in Fig. 2d, electrical assistance does not alter the NH₃-SCR reaction pathway over the Fe_xN_yC catalyst. Therefore, the electrically assisted NH₃-SCR pathway for the Fe_xN_yC catalyst is consistent with that illustrated in Fig. 6. Furthermore, based on the O₂ on–off experiments (Fig. 3) and the results shown in Fig. 4, electrical assistance promotes the migration of bulk lattice oxygen to the catalyst surface in the Fe_xN_yC catalyst. This enhancement facilitates step 4-2 depicted in Fig. 6, thereby improving the overall NH₃-SCR performance. Collectively, these results demonstrate that during the electrically assisted NH₃-SCR process on the Fe_xN_yC catalyst, NH₃ is initially adsorbed and activated to NH₂ by surface lattice oxygen. The resulting NH₂ then reacts with NO to form an NH₂NO intermediate, ultimately decomposing into N₂ and H₂O. The key role of electrical assistance is to promote the migration of bulk lattice oxygen to the surface, which sustains the catalytic cycle and enhances the reaction rate.

Fig. 6 Reaction path in the electro-assisted NH₃-SCR reaction of the Fe₃N_1.5C catalyst.

In summary, this study investigated the effect of electron promotion on the NH₃-SCR reaction at low temperature. Compared to traditional NH₃-SCR, the T₉₀ of NO_x conversion of the Fe₃N_1.5C catalyst was reduced from 160 °C to 120 °C by electro-assistance. Detailed characterization revealed that the presence of current promotes the migration of bulk lattice oxygen to the surface, thereby facilitating the activation of NH₃ species to form NH₂, which then reacts with NO species, thus improving NH₃-SCR performance. This study provides valuable insights for enhancing the low-temperature NH₃-SCR performance.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI).

Supplementary information: additional experimental details, including catalyst preparation, catalytic activity evaluation, and characterization methods, N₂ adsorption–desorption isotherms, TEM, HRTEM, and EDS mapping images, XRD, XPS, and Raman spectra, H₂-TPR, NH₃-TPD, N₂ selectivity, NO_x conversion at different space velocities, NO_x conversion before and after SO₂ poisoning, and O₂ on–off experimental results. See DOI: https://doi.org/10.1039/d5cy00302d.

Acknowledgements

The authors acknowledge the National Natural Science Foundation of China (No. 22276133 and No. 22076136) and the National Key Research and Development Program of China (No. 2022YFB3504100 and No. 2022YFB3504102) for providing the financial support.

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