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. 2026 May 20;65(27):e8855639. doi: 10.1002/anie.8855639

Suppression of Ammonia Slip by Smart Reductants in Selective Catalytic Reduction of Nitrogen Oxides

Xiaoya Xue 1, Aling Chen 1, Jun Liu 1, Mzamo Shozi 2, Xiaonan Hu 1, Fuli Wang 1, Ming Xie 3, Wenqiang Qu 4, Jiang Deng 1,✉, Dengsong Zhang 1,✉
PMCID: PMC13327621  PMID: 42159887

ABSTRACT

Ammonia (NH3)‐based selective catalytic reduction (SCR) is a mainstream flue gas denitration technology for stationary and mobile sources, but NH3 slip remains a critical challenge from mismatches between stoichiometric reaction requirements and fluctuating nitrogen oxides (NOx) concentrations. Herein, we show organic amines (e.g., n‐butylamine, n‐B) act as smart reductants for SCR systems. Unlike NH3, n‐B reacts stoichiometrically with NOx and undergoes NO‐accelerated self‐elimination when overdosed over TiO2‐supported VOx catalysts. Mechanistic studies reveal Ti─OH sites on the catalyst facilitate n‐B adsorption and activation, with the *NH3 intermediate driving NOx reduction. *NH2 from excess n‐B oxidizes to nitrites and NO2 under NO+O2, which further react with *NH3 via an internal SCR pathway, avoiding free NH3 formation and slip. We propose a hybrid reductants system (NH3 + n‐B) enabling efficient NH3 slip suppression over a reductant/NOx molar ratio of 1.2, providing a promising strategy without modifying existing SCR hardware or structures.

Keywords: air pollution control, hybrid reductants, NOx elimination, SCR, suppression of NH3 slip

Organic ammonia acts as a smart denitration reductant with the newly discovered ability to effectively mitigate NH3 slip, a property arising from the newly uncovered mechanism of NO‐accelerated self‐elimination of its excess fraction, which avoids undesirable ammonia emissions.

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1. Introduction

Nitrogen oxides (NOx) are hazardous atmospheric pollutants emitted from industrial flue gases and vehicle exhausts. Ammonia‐based selective catalytic reduction (NH3‐SCR) is a mature flue gas denitration (DeNOx) aftertreatment technology for stationary and mobile sources. The NH3‐SCR process is governed by two key reactions: the standard SCR reaction (4NH3 + 4NO + O2 → 6H2O + 4N2) and the fast SCR reaction (2NH3 + NO + NO2 → 3H2O + 2N2) [1]. Both reactions demand a stoichiometric NH3/NOx ratio. However, flue gas NOx concentration is unstable and varies with operating conditions, especially in mobile sources, where road conditions cause highly volatile operation. Precise NH3 injection is required to maintain the stoichiometric ratio [2]. Still, NH3 slip is unavoidable due mainly to NOx detection lags. Excess NH3 is often injected deliberately to achieve high NOx removal efficiency, worsening NH3 slip as a persistent challenge.

NH3 contributes far more significantly to PM2.5 formation than NOx [3]. An NH3 oxidation module is typically installed downstream of SCR systems to mitigate residual NH3 slip [4, 5, 6]. Retrofitting existing equipment for this module raises capital and operational costs and consumes valuable space for other critical components. CO and H2, studied as potential NOx reductants, are highly sensitive to flue‐gas oxygen [7, 8, 9, 10]. Avoiding NH3 slip without modifying existing SCR systems remains challenging.

Herein, we identified that organic amines (OA) such as n‐butylamine (n‐B) can act as smart reductants for NOx elimination. Unlike NH3, these amines not only react stoichiometrically with NOx during SCR but also undergo NO‐accelerated self‐elimination when dosed in excess, thereby avoiding slip. Using the TiO2‐supported VOx catalyst as a model, we found that moderate oxidation of n‐B via Ti─OH is essential for the production of the *NH3 intermediate, which serves as the reductant for NOx via a Langmuir–Hinshelwood (L–H) reaction mechanism. At an n‐B/NOx molar ratio >1 (overdose), *NH2 from excess n‐B oxidizes to nitrite and NO2, which react with *NH3 intermediates via internal SCR (i‐SCR), preventing free NH3 and eliminating slip. We further propose a mixed NH3–OA reductant system that suppresses NH3 slip efficiently at reductant/NOx molar ratios of 1.10–1.20. This work offers a promising strategy to mitigate NH3 slip in SCR aftertreatment systems without hardware or structural modifications.

2. Results and Discussion

V‐based catalysts were investigated for organic amine assistant SCR (OA‐SCR) with n‐B as probe. All catalysts (V/SiO2, V/CeO2, V/Al2O3, and V/TiO2) possess highly dispersed VOx species, confirmed by x‐ray diffraction and energy dispersive spectroscopy (Figures S1 and S2). OA‐SCR evaluation at n‐B/NOx = 2 (Figure 1a,b) reveals rising NOx removal efficiency with temperature for V/TiO2, which maintains 90% n‐B conversion across all temperatures. NOx conversion exceeds 90% at ≥300°C. Above 240°C, n‐B conversion over V/Al2O3 matches V/TiO2, but their NOx conversion is far lower. V/CeO2 and V/SiO2 show negligible NOx conversion and only high n‐B conversion above 330°C. V/TiO2 also excels in OA‐SCR selectivity. Product distribution (Figure 1c,d) shows >85% N2 selectivity for V/TiO2 across temperatures, superior to the other three catalysts. Minor N2 selectivity loss at high temperature stems from HCN (∼10 ppm) and NH3 slip (∼10 ppm), not N2O. V/TiO2 displays nearly no NH3 slip at n‐B/NOx = 2 across temperatures, unlike other catalysts. Zengel et al. noted toxic HCN formation in carbon‐containing SCR emissions [11, 12]. V/TiO2 generates HCN below 10 ppm, while V/CeO2, V/Al2O3, and V/SiO2 produce far more. CO byproducts also appear in all the catalysts (Figure S3). OA‐SCR with n‐B as reductant achieves high NOx conversion and negligible NH3 slip over V/TiO2. V/TiO2 stability testing at 300°C (Figure S4) confirms stable OA‐SCR efficiency and excellent N2 selectivity over 500 min.

FIGURE 1.

FIGURE 1

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Plots of (a) NOx conversion and (b) n‐B conversion as a function of reaction temperature over different catalysts. Column chart of (c) N2 selectivity (curve chart) and NH3 slip concentration (bar graph) at different temperatures. (d) HCN concentration at different temperatures. Reaction conditions for OA‐SCR: 200 ppm n‐B, 100 ppm NO, 5 vol.% O2, N2 as equilibrium gas, and SV of 50 000 mL∙h−1∙g−1.

Conversely, at NH3/NO = 2 for NH3‐SCR, counterparts show higher NOx conversion but severe NH3 slip (Figure S5). The synergy is confirmed by n‐B oxidation tests over four catalysts with/without NO (Figure S6). Space velocity (SV) effects on OA‐SCR were examined (Figure S7). Elevated SV lowered NOx but not n‐B conversion and increased HCN. UV–vis and Raman results show distinct polymeric species across catalysts (Figures S8 and S9); Hydrogen temperature‐programmed desorption (H2‐TPR) and ammonia temperature‐programmed desorption (NH3‐TPD) define redox and surface acidity differences (Figures S10 and S11). OA‐SCR performance is dominated by catalyst intrinsic properties. Abundant acidic sites and high reducibility of V/TiO2 and V/Al2O3 are highly beneficial.

In situ transition state diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) results were analyzed to elucidate the mechanism. As shown in Figure 2a, the V/TiO2 underwent the adsorption of n‐B for 30 min after pretreatment of O2 and the intensity of absorption bands assigned to ─OH [13], *NH2 [14], NH3 adsorbed on Lewis acidic site (L‐NH3) [15], ─C = O [16], ─NO2 − [17], and ─CH3 [18] groups varies with the n‐B atmosphere (Figure 2a). Specifically, the intensity of bands corresponding to ─NO2 −, *NH2, L‐NH3, and ─C = O increases as ─OH (3635 cm−1) groups are continuously consumed. In particular, the emergence of ─NO2 − species demonstrates that ─OH sites catalyze the initial oxidation of n‐B, a phenomenon not observed during NH3 adsorption (Figure S12). The conclusion was further verified by NO+O2 adsorption on V/TiO2 pretreatment with n‐B or O2 in Figure 2b, with the dynamic evolution of the corresponding spectra provided in Figure S13. Most of the bands in the spectra collected after adsorption of NO+O2 are assigned to nitrate (1510 and 1182 cm−1), nitrite (1373∼1363 and 1313∼1303 cm−1), and NO2 (1616 cm−1) [19, 20]. It should be noted that the intensity of the characteristic bonds corresponding to nitrate, nitrite, and NO2 in n‐B pretreated V/TiO2 catalysts was more obvious than that of the O2‐treated counterpart. NO could also inhibit the harmful HCN (Figure S14). To clarify whether the increased bond intensity of nitrate, nitrite, and NO2 stems from the enhanced activation of NO or the oxidation of N‐containing species derived from n‐B. 15NO+O2 was introduced after pretreatment with n‐B. The newly observed bands at 1595 and 1345 cm−1 are assigned to 15NO2 and 15NO2 −, which are attributed to NO only. The remaining bands undoubtedly derived from the oxidation of N‐containing species derived from n‐B. Combined with these observations, it can be concluded that the L‐NH3 and B‐NH4 + derived from n‐B dissociation were oxidized into nitrite and NO2 in the presence of NO+O2.

FIGURE 2.

FIGURE 2

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(a) In situ DRIFTS under n‐B atmosphere at 240°C, the blue and yellow lines respectively represent the initial and final states in this atmosphere. (b) In situ DRIFTS of NO+O2 adsorption over V/TiO2 catalysts after different treatments at 240°C. (c) Variation trends of typical intermediates and products as a function of time under different atmospheres collected from operando DRIFTS spectra at 300°C.

Specifically, the spectra collected at steady state upon n‐B adsorption after pretreatment with NO+O2 displayed weaker intensity compared to that of O2 (Figure S15). This demonstrated that NO+O2 pretreatment yields stronger oxidizing ability than O2 alone [21], as reflected in the high n‐B conversion observed across the entire temperature range (Figure S16). Additionally, the NH3 slip during the OA‐SCR process is significantly lower than that in the oxidation of n‐B by sole O2. The above findings indicate that NO and n‐B achieve mutual promotion of purification, n‐B provides ammonia sources for NO to undergo reduction, while NO facilitates the formation of nitrite and NO2 species that subsequently react with L‐NH3/B‐NH4 + through the i‐SCR mechanism, thereby reducing NH3 slip.

Operando transition state DRIFTS with the reaction atmosphere was designed to further clarify the reaction pathway (Figures S17 and S18). We repeated the treatment of sole n‐B to obtain similar intermediates, and the introduction of oxidants reduces the concentration of these intermediates and regenerates the ─OH active sites. This was followed by investigating the kinetics of the oxidation process via different oxidants (Figure 2c). With the introduction of n‐B, the band intensities of the *NH2, ─NO2 −, L‐NH3, and ─C═O groups increased continuously. In contrast, while no CO2 signal was detected in the mass spectroscopy (MS) results, a distinct NH3 signal was observed. This phenomenon recurred consistently in subsequent n‐B introduction experiments, further confirming that partial oxidation occurs during the adsorption of n‐B. During this oxidation process, the consumption rate of intermediates varied with the type of oxidant, in the order: NO+O2> NO > O2. Notably, the amount of CO2 released also followed this identical sequence. Notably, the intensity of the NH3 MS signal is similar whenever the oxidant is O2 or NO, while a much lower intensity of NH3 is observed when both NO and O2 are introduced. It demonstrated the consumption of NH3 mainly related to the SCR process, which needs NO, O2, and the adsorbed NH3 derived from n‐B.

In situ DRIFTS showed distinct reaction pathways over different catalysts (Figure S19). V/CeO2’s strong oxidizing capacity rapidly converts adsorbed n‐B into abundant nitrate and amide species prone to further oxidation, insufficiently supplying reductants for NO reduction, causing low denitration efficiency. V/SiO2, deficient in acidity and redox capability, adsorbs n‐B weakly and cannot efficiently reduce NO. While V/Al2O3 mainly adsorbs n‐B as amino species, its much weaker signal intensity than V/TiO2 corresponds to poorer denitration performance.

Considering the reductive properties of n‐B compared to O2 and the application of V/TiO2 in DeNOx [22, 23], n‐B might alter the status of the active sites, mainly the VOx species, which might also influence the reaction mechanism. To explore the combined effect of n‐B and NO over the VOx species, in situ Raman spectroscopy of the V/TiO2 catalyst was further conducted (Figure S20). Figure 3a exhibited the Raman spectra collected during different atmospheres, and the intensity of the Raman peak assigned to the terminal V═O [24, 25] stretching is tracked (Figure 3b). The intensity of the V═O vibration intensified with a blue shift under the atmosphere of NO+O2, indicating increased V═O site abundance and enhanced V─O bonding strength. Strengthened V─O bonding implies weakened electron‐donating ability from active sites to reactants [26, 27], which reveals the oxidation of NO+O2 on the V site. In contrast, the V═O signal increased initially, then decreased with a red shift within n‐B, suggesting an expanded V─O distance and the adsorption of intermediates. When switching back to NO+O2, the intensity of V═O bonds recovered while inducing a further blue shift, and the phenomenon is reversed vice versa when the reaction gases switch from NO+O2 to n‐B. The above analysis of in situ Raman and DRIFTS results demonstrates that V═O sites play a crucial role in activating n‐B into nitrite and NO2.

FIGURE 3.

FIGURE 3

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(a) In situ Raman spectra under different atmospheres. (b) Normalized intensity variation trends of V═O signals derived from Raman spectra under different atmospheres. (c) Binding energy of K for V═O, V─OH and Ti─OH sites. The red, dark green, blue, purple, and silver balls represent O, V, Ti, K, and H atoms. (d) Plots of the n‐B (solid line) and NOx (dashed line) conversion as a function of reaction temperature over different catalysts under OA‐SCR. Reaction conditions of OA‐SCR: 200 ppm n‐B, 100 ppm NO, 5 vol.% O2, N2 as equilibrium gas, and SV of 50 000 mL∙h−1∙g−1.

V═O mainly participates in the redox cycle and is crucial for NO activation, as confirmed [28, 29]. The critical role of V═O oxidation for L‐NH3 and B‐NH4+ in NO+O2 is also verified. However, poor performance of V/CeO2 and V/SiO2 highlights the importance of supports. Given ─OH significance in n‐B activation and uncertainty over ─OH origin on V/TiO2 during n‐B‐mediated NOx reduction, identifying the main ─OH source is essential. Inspired by K poisoning of V‐based catalysts [30, 31, 32, 33], where K+ preferentially interacts with V active sites before other sites upon full V deactivation, a sequential K poisoning experiment was designed. Low K loading strongly inhibited NOx conversion but not n‐B conversion (Figure S21). 1 wt.% and 5 wt.% K+ were loaded onto V/TiO2 to form 1K‐V/TiO2 and 5K‐V/TiO2. Figure 3c shows K binding energies of −4.80, −2.88, and −2.05 eV for K adsorbed at V═O, V─OH, and Ti─OH sites, respectively, confirming preferential K occupation of V sites. Pyridine‐Infrared (Py‐IR) results (Figure S22) show V─OH/V═O occupation in 1K‐V/TiO2 and combined V─OH/V═O and Ti─OH occupation in 5K‐V/TiO2. Stepwise K occupation was further confirmed by x‐ray photoelectron spectroscopy and chemisorption (Figures S23–S26). OA‐SCR performance of 1K‐V/TiO2 and 5K‐V/TiO2 was evaluated (Figures 3d and S27). 1K‐V/TiO2, with blocked V─OH/V═O, lost nearly all SCR activity despite retained n‐B conversion (>80%). 5K‐V/TiO2, with blocked Ti─OH and V─OH/V═O, lost SCR activity and showed severely suppressed n‐B conversion. These results indicate Ti─OH provides initial n‐B adsorption and activation sites, with generated NH3 reacting with NO via standard SCR. Performance analysis confirms OA‐SCR mainly follows the L‐H mechanism, supported by Ti─OH and V═O trends under varied reaction atmospheres at 300°C (Figure S28).

We explored OA‐SCR performance of additional TiO2‐supported catalysts (Figures S29–S36). All catalysts showed OA‐SCR activity but differed in NOx removal efficiency. Catalysts with highly reducible metal oxides (CeO2, CuO) exhibited superior NOx elimination. Fe2O3, Nb2O5, and WO3 inhibited NOx removal, requiring further investigation. V/TiO2 generated substantial CO during OA‐SCR, an environmentally unfavorable byproduct. This issue is mitigated via catalyst optimization: Cu/TiO2 produced almost no CO. Commercial DeNOx catalysts commonly contain WO3, but TiO2‐supported WO3 failed to drive NOx conversion in OA‐SCR (Figure S37). WO3 addition enhanced low‐temperature NOx conversion but increased HCN formation (Figure S38). Excess WO3 lowered OA‐SCR activity and promoted byproduct generation. Cu was doped into V/TiO2 (denoted VCu/TiO2) to suppress byproducts. The modified catalyst raised NOx conversion from 46.5% to 79.6% at 240°C and strongly inhibited harmful byproducts (Figure S38). These results support the development of highly efficient tailored catalysts for this reaction.

The possible OA‐SCR reaction mechanism on V/TiO2 is proposed in Figure 4. (i) n‐B was first adsorbed on Ti─OH and underwent initially oxidative decomposition giving rise to carbonyl species and *NH3. (ii) The *NH3 would be activated by adjacent V═O, where the NO will be incorporated to produce the nitrosamine intermediates. The intermediates would decompose into N2 and H2O, while the as‐obtained V─OH would be reoxidized into V═O. (iii) The carbonyl species was transferred onto V═O, which was then oxidized by O2 and gave rise to the CO2 and H2O. (iv) The released V─OH was then oxidized into V═O. NO serves as the promoter and accelerates the oxidation process, and hence promotes the oxidation of carbonyl species and *NH2. The as‐formed nitrite and NO2 would react with *NH3 to give rise to N2 and H2O. As a whole, two situations can happen during OA‐SCR, which are summarized in Figure S39. When OA and NOx are in equal amounts, the denitration process follows the L‐H mechanism. When OA is in excess, it will follow the i‐SCR mechanism to undergo oxidation reactions, effectively preventing NH3 slip.

FIGURE 4.

FIGURE 4

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Possible reaction mechanism of OA‐SCR on V/TiO2 catalyst.

3. Conclusion

Considering the relatively weaker reduction ability for NO and the higher cost of OA, we proposed a hybrid reductant strategy by mixing NH3 and OA. For the purpose of preliminary exploratory tests, the NH3/NOx molar ratio was provisionally fixed at 0.8, without subsequent optimization of this parameter. The extra reductants were supplemented by OA. The catalyst exhibits excellent NOx elimination performance with much less NH3 slip, and this superior performance is achieved over a wide reductant/NOx molar ratio range (1.10–1.20) over V/TiO2 and Cu‐SSZ‐13 catalysts, as shown in Figure 5. Cu‐SSZ‐13 exhibits a better ability to inhibit the formation of byproducts than V/TiO2 catalysts (Figure S40). By contrast, the conventional NH3‐SCR technology still suffers from significant NH3 slip. This hybrid reductant strategy provides a practical approach to make full use of current post‐treatment equipment and sufficiently suppress the NH3 slip. Nevertheless, the optimized ratio of the mixed reductants and the corresponding high‐performance catalysts still need further exploration.

FIGURE 5.

FIGURE 5

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Comparison of NOx conversion (curve) and NH3 slip (column chart) when using NH3 mix n‐B and pure NH3 as reductants on (a) V/TiO2 catalysts at 300°C and (b) Cu‐SSZ‐13 catalysts at 330°C. Reaction conditions: 500 ppm NO, 5 vol.% O2, when pure NH3 as reductants, the ratio of 1.10, 1.15, and 1.20 represent, respectively, 550, 575, and 600 ppm NH3, when hybrid ammonia as reductants, the ratio of 1.10, 1.15, and 1.20 represent 400 ppm NH3 mix respectively 150, 175, and 200 ppm n‐B, N2 as equilibrium gas and SV of 50 000 mL·h−1·g−1.

Author Contributions

Xiaoya Xue: investigation, formal analysis, writing – original draft. Aling Chen: investigation, validation, data curation, writing – review and editing. Jun Liu: investigation, formal analysis, writing – review and editing. Mzamo Shozi: investigation, writing – review and editing. Xiaonan Hu: investigation, validation, writing – review and editing, formal analysis. Fuli Wang: investigation, validation, writing – review and editing. Ming Xie: investigation, writing – review and editing. Wenqiang Qu: investigation, writing – review and editing, formal analysis. Jiang Deng: investigation, conceptualization, methodology, writing – review and editing. Dengsong Zhang: conceptualization, methodology, software, investigation, formal analysis, supervision, resources, project administration, visualization, funding acquisition, writing – review and editing.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File: anie72820‐sup‐0001‐SuppMat.docx.

ANIE-65-e8855639-s001.docx (6.1MB, docx)

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (22125604, 22476121, 22436003, and 22306117), the Science & Technology Commission of Shanghai Municipality (23230713700 and 24230711600), and Shanghai Pujiang Programme (24PJD033). This work was also supported by the Shanghai Technical Service Center of Science and Engineering Computing, Shanghai University.

Contributor Information

Jiang Deng, Email: jiangdeng@shu.edu.cn.

Dengsong Zhang, Email: dszhang@shu.edu.cn.

Data Availability Statement

The data that supports the findings of this study are available in the Supporting Information of this article.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

Supporting File: anie72820‐sup‐0001‐SuppMat.docx.

ANIE-65-e8855639-s001.docx (6.1MB, docx)

Data Availability Statement

The data that supports the findings of this study are available in the Supporting Information of this article.

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