High NH₃-SCR reaction rate with low dependence on O₂ partial pressure over Al-rich Cu–*BEA zeolite

Abstract

Dependence of NH₃-SCR reaction rate on O₂ partial pressure was investigated at 473 K over Cu ion-exchanged MOR, MFI, CHA and *BEA zeolites with varying “Cu density in micropores”. Among the zeolites, Cu–*BEA zeolite demonstrated promising potential as an effective catalyst for NH₃-SCR over a wide range of O₂ partial pressure.

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Selective catalytic reduction of NO_x using NH₃ as a reducing agent (NH₃-SCR) is one of the most effective ways to remove NO_x from exhausts with O₂-rich compositions. A lot of catalysts for the reaction have been developed, such as V-based mixed oxides, Fe-zeolites, and Cu-zeolites.¹ Among the catalysts so far, Cu-zeolites exhibit a high reaction rate at a low NO₂ concentration (standard SCR) in a low temperature region (below 550 K).¹ Therefore, the current central trend of the study on the catalysts for NH₃-SCR is based on Cu-zeolites. Since the development of Cu/SSZ-13 zeolite catalyst with CHA topology, which exhibits a high reaction rate in wide temperature region, high selectivity of N₂, and a high durability under hydrothermal conditions,² a wide variety of zeolites were tested toward the application of NH₃-SCR.³

In the catalytic activity tests of NH₃-SCR, it is often the case that the O₂ concentration in the reaction feed is fixed at a certain value,^3,4 and the effect of O₂ partial pressure (P_O2) on the reaction rate has attracted little attention. The current application of NH₃-SCR is mainly the removal of NO_x emitted from diesel engine. The emission contains an excess amount of O₂ (typically 2–17%).⁵ A portion of the O₂ contained in the exhaust is steadily consumed over other catalysts for exhaust purification processes (e.g., diesel oxidation catalyst; DOC and diesel particulate filter; DPF).⁶ The DOC catalyst plays a role in oxidative removal of unburned hydrocarbon (HC) and carbon monoxide (CO) using O₂. Particulate matter (PM) is trapped on the DPF and eliminated by catalytic combustion using O₂ and NO_x. In the exhaust purification system of diesel engine, such DOC and DPF units are generally mounted at the upstream of the SCR catalyst. The state-of-the-art SCR system of diesel engines tends to be integrated to DPF to make the whole system compact.⁷ Moreover, the application of exhaust gas recirculation (EGR) system, which introduces a part of exhaust into engine cylinder to make the temperature of combustion decrease, resulting in the decrease of thermal NO_x, is in progress for the combustion process.⁸ These purification technologies will make the temperature of exhaust and the concentration of O₂ lower than they are.

On the other hand, a recent fundamental research⁹ has shown that the reaction rate for NH₃-SCR over Cu–SSZ-13 zeolite catalyst is greatly influenced by the P_O2 in a low P_O2 region with practical conditions (P_O2 < 18 kPa)⁵ at 473 K, where the overall rate of this reaction is largely affected by the oxidation of Cu ion on zeolites by O₂.¹⁰ It is shown in the literature⁹ that the SCR rate increases with increasing Cu volumetric density. However, the significant rate drop at the P_O2 below 15 kPa is a common behaviour of Cu–SSZ-13 zeolite regardless of its composition. Considering the recent trends on the composition of emission from practical diesel engines and the behaviour of Cu–SSZ-13 zeolite catalyst described above, it will be desired to widen the active window for exhaust composition at low reaction temperatures (∼473 K) to pass future regulations.¹¹

Herein, a comparative study is conducted regarding NH₃-SCR reaction rate dependence on the P_O2 at 473 K over Cu ion-exchanged MOR, MFI, CHA and *BEA zeolites from the viewpoint of “Cu density in micropores”¹² to understand the effect of zeolite topology on the dependence.

Details on the preparation of the catalysts and the measurement of the reaction rates have been written in our previous reports.^12,13 The reaction rate was calculated by determining the amount of NO converted to N₂ per second, which was divided by the amount of Cu in the catalyst. The O₂ concentration was kept at 5% during the pretreatment at 873 K, followed by cooling the temperature to 473 K. The cooling to 473 K was conducted under the feed of NH₃-SCR reactants and at least 45 min since the temperature was set to 473 K was ensured to reach stable temperature and steady-state NO conversion. Then, the O₂ concentration was altered from 1 to 15%, and more than 10 min was needed to reach initial steady-state NO conversion at each targeted O₂ concentration. Formed NO₂ was transformed to NO by a NO₂ converter catalyst unit attached to a chemical luminescence NO_x analyser (HORIBA VA-3000); therefore, the NO conversion detected by the analyser was regarded as the NO_x conversion and the effect of background NO₂ was eliminated.

First of all, the catalytic activity of a reference Cu–SSZ-13 catalyst¹⁴ with a similar composition to the state-of-the-art commercial catalyst for NH₃-SCR was measured. It is reported that the catalyst has the composition of Si/Al and Cu/Al ratios ∼9.5 and 0.3, respectively, corresponding 3.1 wt% Cu,¹⁵ whose Cu content is higher than that of any catalyst used in the report⁹ on dependence of NH₃-SCR rates on O₂ pressure.

Fig. 1a shows the rate dependence on P_O2 for NH₃-SCR per Cu ((mole NO to N₂) per (mole Cu) per s) by a kinetic measurement at 473 K over the reference Cu–SSZ-13 catalyst. Similar to “Langmuirian dependence” shown in the previous report by Jones et al.,⁹ a monotonic increase of SCR rate along with P_O2 was observed. Note that the dependence obtained in this work could be expressed by the Langmuir–Freundlich equation better than the Langmuir equation (Fig. S1†). The reaction order with respect to O₂ for NH₃-SCR per Cu was determined according to the following power law model equation.^4a

(NH₃-SCR rate per Cu) = A₀ × exp(−E_app/RT) × (P_O2)^α

Fig. 1 (a) The dependence of NH₃-SCR rate per Cu at 473 K on O₂ pressure and (b) the log–log plot for the calculation of apparent O₂ order over the reference Cu–SSZ-13 zeolite.

The α in this equation expresses the reaction order for O₂. Fig. 1a was re-plotted to log–log axises (Fig. 1b) to know the slope corresponding to α. As shown in Fig. 1b, the log–log plot did not follow a liner relationship. It is observed that the slope of the plot decreased with increase in P_O2 in the reaction flow. This result shows that the reaction order for O₂ decreases with increase in P_O2.

This phenomenon can be explained by the suggested redox mechanism between Cu⁺ and Cu²⁺ in the micropore of zeolites. The reduction of Cu²⁺ to Cu⁺ is thought to proceed by NH₃ + NO co-reductants,¹⁶ and the oxidation of Cu⁺ to Cu²⁺ is thought to proceed by O₂ oxidant.¹⁷ It has been observed by several operando analyses that both Cu⁺ and Cu²⁺ exist under a steady-state NH₃-SCR condition,¹⁸ although the ratio between two oxidation states depends both on the composition of Cu-zeolites and the reaction conditions. From these results, it has been suggested that the reaction rate is not solely limited by the rate for the Cu²⁺ reduction step (reduction half-cycle) nor Cu⁺ oxidation step (oxidation half-cycle).¹⁰ In other words, the reduction and oxidation half-cycles are kinetically relevant under the conditions below 523 K and at 5–20 kPa O₂ pressure over Cu–SSZ-13 zeolite.¹⁰

From the description above, the phenomenon observed in Fig. 1b can be understood as follows; the reaction is strongly influenced by the oxidation half-cycle under a low P_O2 reaction condition because the supply of oxidant is relatively insufficiency, and the oxidation half-cycle rate is improved with increasing the P_O2. The reaction order for O₂ could be calculated in the low P_O2 region (≤4 kPa) and high P_O2 region (5 ≤ P_O2 ≤ 15 kPa). The results are shown in Table 1. The reaction order for O₂ decreased with increasing P_O2, but did not reach to zero-order in the P_O2 region in this work. It is indicated from the results that the effect of oxidation half-cycle on the whole reaction rate remains in all the P_O2 region in this work, and the effect becomes stronger in a lower P_O2 region below 5 kPa than in the higher P_O2 region over this Cu–SSZ-13 catalyst. The apparent activation energy (E_app) for the reaction around 473 K calculated from the Arrhenius plots (Fig. S2†) was not changed obviously (Table 2) in the P_O2 region between 1 and 15 kPa. The value of the E_app was typical for the NH₃-SCR over Cu-zeolite catalysts.⁴ Therefore, it is confirmed that alteration of the reaction condition does not change the apparent E_app for kinetically relevant step(s).

Table 1 The reaction order for O₂ at 473 K over several P_O2 region

O2 partial pressure region/kPa	Reaction order for O2
1–4	0.46
5–15	0.14

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Table 2 The apparent activation energy around 473 K in several O₂ pressure reaction over the reference Cu–SSZ-13 catalyst

O2 partial pressure/kPa	Eapp/kJ mol^−1
1	49
5	44
15	44

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The same measurements were conducted over the Cu-zeolite catalysts with MOR, MFI, *BEA, and CHA topologies that have similar cation density in micropores of zeolites and several Cu density in micropores.^11,12 Cu-Zeolites with different topologies and cation density in micropores were applied in this study to minimize the contributions from factors other than the topology that can affect the NH₃-SCR rate.¹² Fig. 2 show the dependence of NH₃-SCR rate per Cu at 473 K on P_O2 over each topology. Cu density in micropores increases with light-to-dark shading (Table S1†). As shown in Fig. 2, monotonic increase of SCR rate along with P_O2 increase similar to shown in Fig. 1a was observed over all Cu-zeolites. However, the changes of SCR rate along with both P_O2 and Cu density strongly affected by zeolite topologies and Cu density in micropores.

Fig. 2 The dependence of NH₃-SCR rate per Cu at 473 K on O₂ pressure over (a) MOR, (b) MFI, (c) CHA, and (d) *BEA zeolite catalyst with several Cu density in micropores. Cu density in micropores increases with light-to-dark shading.

Cu–MOR zeolite applied in this study exhibited the lowest NH₃-SCR rate per Cu over all P_O2 region (Fig. 2a). Note that the scale of Y axis in Fig. 2a is as large as a quarter of Fig. 1a. A little increase was observed in both magnitudes and slopes of NH₃-SCR rate per Cu along with increasing Cu density in micropores. However, the rate was far smaller than that of the reference Cu–SSZ-13 zeolite catalyst, even with the higher Cu density.

Cu–MFI zeolite used in this study exhibited a higher NH₃-SCR rate per Cu over all P_O2 region than Cu–MOR zeolite (Fig. 2b). The scale of Y axis in Fig. 2b is as large as a half of that in Fig. 1a. In the case of the MFI zeolite, the increase was also observed in both magnitudes and slopes of NH₃-SCR rate per Cu with increasing Cu density in micropores. Both the reaction rate and its increase over Cu–MFI zeolite were larger than over Cu–MOR zeolite. However, the reaction rate over Cu–MFI zeolite was smaller than over the reference Cu–SSZ-13 zeolite catalyst regardless of Cu density in micropores. Even the Cu–MFI zeolite with Cu density in micropores at 7.4 (1000 ų)⁻¹ (approximately 3 times as large as the reference Cu–SSZ-13 zeolite) did not represent an exception.

In the case of Cu–CHA zeolite, the increase behaviour in NH₃-SCR rate per Cu along with P_O2 was largely affected by the Cu density in micropores (Fig. 2c). The catalyst with a low Cu density in micropores showed relatively steady increase in NH₃-SCR rate per Cu along with P_O2. On the other hand, the catalyst showed the rapid increase in NH₃-SCR rate per Cu in low P_O2 region, and the rate became constant in the higher P_O2 region (>6 kPa). Note that the slight decrease in NH₃-SCR rate per Cu of the Cu–CHA zeolite with the largest Cu density in micropores over 8 kPa P_O2 region is mainly caused by the increase in the formation of N₂O. Interestingly, the zeolites with Cu density in micropores at 1.6 and 3.4 (1000 ų)⁻¹ exhibited almost the same NH₃-SCR rate per Cu when P_O2 was at 15 kPa. This result suggests that they would reach the zero-order dependence on P_O2, which means that the oxidation half-cycle does not determine the overall rate in the P_O2 region regardless of Cu density in micropores.

Among the Cu-zeolite catalysts, Cu–*BEA zeolite employed in this study exhibited high NH₃-SCR rate per Cu with relatively low dependence of on P_O2 (Fig. 2d). Moreover, the effect of Cu density in micropores of the catalyst was small on the behaviour of the rate along with P_O2. Surprisingly, even the Cu–*BEA zeolite catalyst with Cu density in micropores at 0.76 (1000 ų)⁻¹ (The sample shown as hollow red square symbol in Fig. 2d and described as B12 in Table S1†) exhibited a higher NH₃-SCR rate per Cu than the reference Cu–SSZ-13 catalyst with Cu density in micropores at 2.7 (1000 ų)⁻¹ (Fig. S3a†). This difference was more obvious in lower temperature region (Fig. S3b†). In other words, the Cu–*BEA zeolite catalyst exhibited a high NH₃-SCR rate per Cu with low dependence on both Cu density in micropores and P_O2 in the temperature region below 473 K.

In the case of the zeolites other than Cu–CHA, the obvious deviation from a liner relationship following a Langmuir equation was observed in the dependence of SCR rate on P_O2 (Fig. S4†). To analyze the relationship, the Langmuir–Freundlich equation, which introduced the order on P_O2 as a correction factor to the Langmuir equation, was needed. From these results, it is suggested that the dependence of SCR rate on P_O2 over Cu-zeolites generally follows the Langmuir–Freundlich equation.

The reaction order for O₂ was calculated in a similar manner as the reference Cu–SSZ-13 catalyst in a low P_O2 region (≤5 kPa). The results were displayed as a function of Cu density in micropores (Fig. 3a). As shown in Fig. 3a, the reaction order for O₂ decreased with increase in the Cu density in micropores for all the zeolites investigated in this study. This result was consistent with the previous report on Cu–SSZ-13 zeolites with several Cu densities¹⁸ and can be understood by the increase in the rate for the oxidation half-cycle with increasing Cu density in micropores. When the reaction order for O₂ was compared among the Cu-zeolites with a similar Cu density in micropores, the tendency was observed that Cu-zeolite with a high reaction rate showed a low reaction order for O₂ (Fig. 3b). This result means that the effect of P_O2 on the reaction rate is small over a catalyst with a high reaction rate such as Cu–CHA with high Cu density in micropores or Cu–*BEA.

Fig. 3 The relationship between reaction order for O₂ in the O₂ pressure region (≤5 kPa) and the (a) Cu density in micropores or (b) SCR rate per Cu at 5 kPa O₂ over the MOR (▲), MFI (●), *BEA (■), and CHA (◆) zeolites. The cross symbol (+) shows the reference Cu–SSZ-13 zeolite.

Both the Cu density in micropores and P_O2 could play a role in the oxidation half-cycle in recently suggested mechanism of NH₃-SCR.¹⁷ Moreover, we have reported that the dependence of SCR rate against Cu density in micropores are related to the oxidation half-cycle in a previous report,¹² which investigated SCR rate at O₂ partial pressure of 5 kPa over the same catalysts tested in this study in detail. Thus, it can be assumed that the high NH₃-SCR rate of Cu–*BEA catalyst shown in this report is derived from the oxidation property for Cu⁺ ion by O₂ even insensitive to the P_O2. However, detailed analysis using operando spectroscopic techniques will be necessary to elucidate the origin. It will be reported and discussed in the closest future.

Dependence of NH₃-SCR rate on P_O2 was investigated at 473 K over Cu ion-exchanged MOR, MFI, CHA and *BEA zeolites with several “Cu density in micropores”. The reaction rate with respect to P_O2 was largely affected by the zeolite topology. Among the zeolites investigated here, Cu–*BEA zeolite catalyst exhibited a higher reaction rate regardless of the Cu density in micropores (or Cu loading) than a Cu–SSZ-13 reference catalyst in the whole range of Cu content tested in this study. The Cu–*BEA zeolite has a promising potential as the effective catalyst for NH₃-SCR in a wide range of P_O2.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by “Elements Strategy Initiative for Catalysts & Batteries (ESICB)” of MEXT; Ministry of Education, Culture, Sports, Science and Technology, Japan, Grant Number JPMXP0112101003.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1ra00943e

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