The chemical nature of SO₂ poisoning of Cu-CHA-based SCR catalysts for NOx removal in diesel exhausts

Abstract

This study addresses the impact of SO₂ exposure on the catalytic performance of a Cu-chabazite-based SCR catalyst, as used in diesel exhausts, to reduce the emission of NOx through the NH₃-SCR reaction. The SCR activity is determined by a reaction of NO with the [Cu₂^II(NH₃)₄O₂]²⁺ intermediate. The same intermediate is also the most reactive Cu-species towards SO₂. We demonstrate here that the reaction with NO at 200 °C is limited after exposure of the [Cu₂^II(NH₃)₄O₂]²⁺ complex to SO₂ or SO₂/O₂. Heating the catalyst to 300 °C in NO restores the reaction, albeit at a significantly lower rate. The lower reactivity towards NO indicates that exposure of [Cu₂^II(NH₃)₄O₂]²⁺ to SO₂ induces changes in the chemistry of Cu in the catalyst. This implies that poisoning of Cu-chabazite catalysts by SO₂ is, at least in part, of the chemical nature, and may be not limited to the physical pore blocking.

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Introduction

Cu-chabazite (CHA) zeolites are efficient catalysts for the selective catalytic reduction of NOx using ammonia (NH₃-SCR) from the exhaust of diesel vehicles, due to their high activity below 300 °C and good hydrothermal stability.^1,2 However, the activity of Cu-CHA below 300 °C, which is in the normal operation range for SCR catalysts, is significantly reduced in the presence of SO₂, and therefore, the application of Cu-CHA in diesel exhaust after-treatment systems requires ultra-low-sulfur diesel fuel.^3,4

To understand why SO₂ affects the performance of Cu-CHA, it is necessary to understand how SO₂ affects the NH₃-SCR reaction cycle. The NH₃-SCR reaction is a redox cycle, proceeding via a number of Cu-complexes formed by alternating oxidation (Cu^I → Cu^II) and reduction (Cu^II → Cu^I) steps of Cu, which further involve adsorption and reaction of NO, NH₃ and O₂ as ligands on the Cu-ions in the CHA zeolite.^5–15 Many available studies on the reaction of SO₂ with Cu-CHA show that the temperature of the reaction, gas composition and the oxidation state and local environment of Cu play a role in the effect of SO₂ on the performance of Cu-CHA.^4,16–25 To identify the mechanism of the deactivation of Cu-CHA by SO₂, we have shown earlier that the [Cu₂^II(NH₃)₄O₂]²⁺ dimeric species are sensitive to SO₂.²⁴ The same species are crucial for the performance of the NH₃-SCR cycle,^11,13,14,26 indicating that the sensitivity of these species to SO₂ plays a key role in the deactivation mechanism.

The identification of the mechanism of the reaction between [Cu₂^II(NH₃)₄O₂]²⁺ and SO₂ is the next necessary step in the study of the SO₂ effect on the catalytic performance of Cu-CHA in NH₃-SCR. In our recent work,²³ we proposed a possible mechanism of the interaction of [Cu₂^II(NH₃)₄O₂]²⁺ complexes with SO₂ and the structure of the sulfated Cu species forming as a product of the oxidation of SO₂ by the [Cu₂^II(NH₃)₄O₂]²⁺ complex. We also found that the presence of O₂ enhances the formation of sulfated species, which corresponds to the observed increase in the uptake of SO₂ by the Cu-CHA catalyst.

According to the theoretical work of Bjerregaard et al.,¹⁷ the main reason for the deactivation is the formation of ammonium bisulfate in the zeolite cages, which is formed together with ammonium sulphate. The latter has been observed²⁷ together with sulfuric acid²⁸ and copper and aluminum sulphates.²⁹ The important part of the low-temperature NH₃-SCR cycle in Cu-CHA is the activation of a pair of mobile [Cu^I(NH₃)₂]⁺ complexes by O₂ to form the active dimeric [Cu₂(NH₃)₄O₂]²⁺ complexes.^6,7,11,26 In the model presented by Bjerregaard et al.,¹⁷ ammonium bisulfate accumulates in the zeolite and limits the mobility of the [Cu^I(NH₃)₂]⁺ complexes, which hinders the formation of new [Cu₂^II(NH₃)₄O₂]²⁺ complexes. If [Cu₂^II(NH₃)₄O₂]²⁺ complexes are not formed, the NH₃-SCR redox cycle stops. According to this model, deactivation is a consequence of a lower amount of [Cu₂^II(NH₃)₄O₂]²⁺ complexes in the catalyst.

In this article, we shed light on whether the observed deactivation of Cu-CHA is entirely due to the lower number of active [Cu₂^II(NH₃)₄O₂]²⁺ complexes or a change in the chemistry of the system also plays a role. The interaction of [Cu₂^II(NH₃)₄O₂]²⁺ in Cu-CHA with NO is an essential part of the SCR cycle, and the understanding of the effect of SO₂ on the performance of Cu-CHA is not possible without revealing how SO₂ affects this essential step. The goal of this work is to clarify how the exposure to SO₂ alters the reaction between [Cu₂^II(NH₃)₄O₂]²⁺ and NO.

Experimental

We apply temperature-programmed reaction with NO, and combine in situ X-ray absorption spectroscopy (XAS) simultaneously with diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) to follow the reaction between [Cu₂^II(NH₃)₄O₂]²⁺ complexes in a Cu-CHA NH₃-SCR catalyst and NO before and after exposing it to SO₂. For the measurements reported here, a Cu-CHA catalyst with a Si/Al ratio of 6.7, containing 3.2 wt% Cu (Cu/Al = 0.24) has been used. To determine the impact of SO₂, we first prepare the [Cu₂^II(NH₃)₄O₂]²⁺ complexes, expose these to SO₂, and then to NO to follow the reaction. The results are then compared to those obtained in a similar measurement without the exposure to SO₂.

In the NO temperature-programmed reaction, the consumption of NO is followed during heating of the [Cu₂^II(NH₃)₄O₂]²⁺ complexes, in NO/N₂ from 50 °C to 550 °C to follow the difference in the NO consumption with and without exposure to SO₂. The combined XAS + DRIFTS experiment was performed on the BM23 beamline of the European Synchrotron Radiation Facility (ESRF). The [Cu₂^II(NH₃)₄O₂]²⁺ complexes, with or without prior treatment in SO₂ + O₂, were exposed to NO at 200 °C to follow the changes in the oxidation state and local environment of Cu with XAS and redistribution of NH₃/NH₄⁺ with DRIFTS. For the sample exposed to SO₂ + O₂, this was followed by heating to 300 °C in NO.

The experimental details are presented in the ESI.†

Results and discussion

Fig. 1 shows the results for the temperature-programmed reaction between NO and the [Cu₂^II(NH₃)₄O₂]²⁺ species with and without prior exposure to SO₂. In temperature-programmed reaction with NO, the impact of SO₂ on the reaction of NO and the [Cu₂^II(NH₃)₄O₂]²⁺ complex becomes visible as a change in the NO consumption as a function of temperature. When the [Cu₂^II(NH₃)₄O₂]²⁺ complex is exposed to NO, the NO consumption takes place at around 120 °C. After exposure of the [Cu₂^II(NH₃)₄O₂]²⁺ complex to SO₂, the NO consumption peak shifts to around 250 °C. The shift indicates that the chemistry of the reaction of NO has changed upon exposure to SO₂, such that a higher reaction temperature is required.

Fig. 1 Temperature-programmed reaction with NO, and NH₃ desorption from [Cu₂^II(NH₃)₄O₂]²⁺ with (orange) and without (grey) prior exposure to SO₂.

We also monitored the desorption of NH₃ in this experiment. The desorption peaks for both protocols start at around 300 °C, possibly corresponding to the desorption of NH₃ from the Brønsted sites and/or decomposition of the [Cu^I(NH₃)₂]⁺ complex.³⁰ The amount of NH₃ desorbed is slightly lower for the “exposed to SO₂” experiment, indicating that some NH₃ was probably lost during exposure to SO₂.

The integrated amounts are presented in the ESI.†

The light-blue curves in Fig. 2a and c correspond to the exposure of the [Cu₂^II(NH₃)₄O₂]²⁺ complex to NO at 200 °C. The weak 1s–3d transition at 8977.3 eV characteristic of Cu^II species disappears, while the 1s–4p transition at 8982.5 eV grows, indicating the reduction to Cu^I, in agreement with previous results.^11,31,32 This reduction is accompanied by the decomposition of the [Cu₂^II(NH₃)₄O₂]²⁺ complexes, as indicated by the lower intensity of the first shell in the EXAFS part. The peak in the second shell broadens, which may reveal the presence of a mixture of different Cu species.

Fig. 2 Cu K-edge XANES (a and b) and FT-EXAFS (c and d) spectra collected in situ during the exposure of the [Cu₂^II(NH₃)₄O₂]²⁺ species to NO at 200 °C (“not exposed to the SO₂” protocol); exposure of [Cu₂^II(NH₃)₄O₂]²⁺ to SO₂ + O₂, then to NO at 200 °C, and at 300 °C (“exposed to the SO₂” protocol).

After exposure of the [Cu₂^II(NH₃)₄O₂]²⁺ complex to SO₂ + O₂, a sulfated Cu^II species is present (brown line in Fig. 2b and d). This species has been rationalized in our previous work as Cu^IISO₄Z, where Z may comprise O from the SO₄⁻ group, framework O or NH₃.²³ Exposure of the catalyst in this state to NO at 200 °C (red line) leads to some reduction of Cu, according to the XANES results, but to a lesser extent as compared to the [Cu₂^II(NH₃)₄O₂]²⁺ complexes not exposed to SO₂. Both, the increase of the 1s–4p transition at 8982.5 eV and the decrease in the intensity of the first coordination shell in the EXAFS part are weaker, as compared to the fresh [Cu₂^II(NH₃)₄O₂]²⁺ complexes, indicating that the reaction with NO is less pronounced. When the catalyst is heated to 300 °C, both XANES and EXAFS features develop further, and the final result becomes similar to the light-blue spectrum of the [Cu₂^II(NH₃)₄O₂]²⁺ complex exposed to NO at 200 °C without exposure to SO₂.

To quantify the effect of SO₂ on the reaction between NO and the [Cu₂^II(NH₃)₄O₂]²⁺ complex, we apply a combination of multivariate curve resolution – alternating least squares (MCR-ALS) and linear combination fitting (LCF). This combination is a powerful tool that has been successfully applied to the in situ XANES datasets of Cu-CHA before.²³ This approach allows us to resolve the spectra of the unknown Cu-CHA species at different stages of the protocol using multivariate curve resolution (Fig. 3a) and to calculate the concentration profiles of the Cu species from linear combination fitting (Fig. 3b and c). The choice of the references for LCF and the related discussion of errors are explained in the ESI.†

Fig. 3 (a) Spectra used as references in the linear combination fit. The spectra of fw-Cu^II, [Cu^I(NH₃)₂]⁺, and [Cu₂^II(NH₃)₄O₂]²⁺ are experimental spectra. The spectra of fw-Cu^I and the sulfated component were obtained with MCR-ALS. (b and c) Linear combination fit results of the experimental procedures. (b) Exposure of [Cu₂^II(NH₃)₄O₂]²⁺ to NO at 200 °C (“not exposed to the SO₂” protocol); (c) exposure of [Cu₂^II(NH₃)₄O₂]²⁺ to SO₂ + O₂, then to NO at 200 °C, and then heating in NO to 300 °C (“exposed to the SO₂” protocol). Upper panels – R-factors of the fit as a function of time and lower panels – concentration profiles of the reference spectra.

When the [Cu₂^II(NH₃)₄O₂]²⁺ complexes are exposed to NO, without exposure to SO₂, we see a fast reduction of Cu and the formation of [Cu^I(NH₃)₂]⁺ and framework-coordinated Cu^I (fw-Cu^I) species, in agreement with previous reports.¹¹ When we introduce NO at 200 °C after the exposure of [Cu₂^II(NH₃)₄O₂]²⁺ to SO₂ + O₂, (Fig. 3c), we observe a decrease of the concentration of sulfated Cu^II species (yellow curve) and [Cu₂^II(NH₃)₄O₂]²⁺ complexes (green curve).

The concentration of fw-Cu^II species (red curve), however, remains unchanged. The concentrations of fw-Cu^I and [Cu^I(NH₃)₂]⁺ (blue and purple curves) increase, but the reaction rate is noticeably lower compared to the rates observed without exposure to SO₂.

Upon increasing the temperature to 300 °C, the reaction accelerates and we find a similar fw-Cu^I and [Cu^I(NH₃)₂]⁺ complex as in the absence of SO₂. However, the ratio of the fw-Cu^I/[Cu^I(NH₃)₂]⁺ complex changes from 60/35 in the absence of SO₂ to 73/23 after exposure to SO₂ and heating to 300 °C. This explains the difference between the shapes of the blue spectrum in Fig. 2a and the yellow spectrum in Fig. 2b. The concentration of [Cu^I(NH₃)₂]⁺ is lower (23% instead of 35% in the not exposed to SO₂), which can be caused by loss of some NH₃ from the [Cu₂^II(NH₃)₄O₂]²⁺ complexes during the exposure to SO₂ + O₂ in the previous stage of the protocol, in agreement with the temperature-programmed reaction results in Fig. 1.

The gas composition in the outlet of the cell during the in situ XAS + DRIFTS experiment, measured with a mass spectrometer, corroborates the slower reaction of NO with the [Cu₂^II(NH₃)₄O₂]²⁺ complex after SO₂ exposure (Fig. 4). When NO reacts with the fresh [Cu₂^II(NH₃)₄O₂]²⁺ complexes without SO₂, NO is consumed, whereas N₂ and H₂O form, in agreement with previous reports.¹¹ When the [Cu₂^II(NH₃)₄O₂]²⁺ complex is exposed to SO₂ + O₂ first, the formation of N₂ at 200 °C upon exposure to NO is reduced down to 0.36 of the one of the fresh samples, according to the ratio of the corresponding N₂ peaks. The main peaks for NO consumption and production of N₂ and H₂O appear around 250 °C, in good agreement with the observed shift in the temperature-programmed reaction with NO (Fig. 1). Furthermore, we also observe desorption of SO₂ at 250 °C, corresponding to the decomposition of the sulfated species, according to the linear combination fit of the XAS data.

Fig. 4 Mass spectrometer data during exposure of the [Cu₂^II(NH₃)₄O₂]²⁺ complexes “not exposed to SO₂” (top panel) and “exposed to SO₂” (bottom panel) to NO. Masses: H₂O: 18, NO: 30, N₂: 28, and SO₂: 48.

Fig. 5a shows in situ DRIFTS spectra after the Kubelka–Munk (KM) transformation, that were obtained simultaneously with the XAS and MS data presented above. The spectrum of the [Cu₂^II(NH₃)₄O₂]²⁺ complex (Fig. S4, ESI†) is subtracted from the presented spectra to better highlight the changes upon exposure to NO. In the absence of SO₂, exposure of the [Cu₂^II(NH₃)₄O₂]²⁺ complexes to NO at 200 °C (light blue line in Fig. 5a) results in the growth of the OH stretching mode (νOH) of Brønsted sites (3590 cm⁻¹),^33,34 and the decrease of the broad and complex absorption band related to the νNH of NH₃ and NH₄⁺ (2500–3500 cm⁻¹).^33,35,36 This indicates some consumption of NH₃ from [Cu₂^II(NH₃)₄O₂]²⁺ complexes and/or NH₄⁺, leading to restoration of the Brønsted H⁺ sites.

Fig. 5 (a) In situ difference DRIFTS spectra of the Cu-CHA catalyst during exposure of the [Cu₂^II(NH₃)₄O₂]²⁺ complexes to NO at 200 °C (blue), to a mixture of SO₂ + O₂ (brown) and then to NO at 200 °C (red) and after heating in NO to 300 °C (yellow). The initial spectrum of [Cu₂^II(NH₃)₄O₂]²⁺ is subtracted. The presented curves are averages of the last 10 curves of the protocol step, equivalent to a 5-min acquisition time. (b and c) Time evolution of the intensities corresponding to 3590 cm⁻¹ (signal of Brønsted sites) and 3320 cm⁻¹ (signal of NH₃/NH₄) during: (b) exposure of [Cu₂^II(NH₃)₄O₂]²⁺ to NO at 200 ° and (c) exposure of [Cu₂^II(NH₃)₄O₂]²⁺ after prior SO₂ + O₂ treatment at 200 °C, to NO at 200 °C and 200–300 °C.

After exposure of the [Cu₂^II(NH₃)₄O₂]²⁺ complexes to SO₂ + O₂ (red line in Fig. 5a), the band of the Brønsted sites (3590 cm⁻¹) remains unchanged, while the bands in the range 2500–3500 cm⁻¹, corresponding to the νNH of NH₃ and NH₄⁺ increase. This could indicate a redistribution of the NH₃ ligands in the zeolite, with the formation of NH₄⁺ ions that could exchange fw-Cu sites and/or form ammonium sulphate or bisulfate, as proposed in the literature.^{17,29,37–39} In this state, exposure to NO at 200 °C does not cause significant changes in the spectrum profile (brown line in Fig. 5a), indicating no reaction, or at most a slow reaction with NO, in agreement with the results from temperature-programmed reaction, XAS, and mass spectrometry as presented above. The subsequent heating to 300 °C in NO (yellow line in Fig. 5a) then results in the release of NH₃ from Cu sites or stored on the Brønsted sites. Interestingly, the final state in this case shows a clearly higher intensity for the Brønsted sites (3590 cm⁻¹) and lower intensity for the νNH of NH₃ and NH₄⁺ (2500–3500 cm⁻¹), as compared to the not exposed to the SO₂ state (blue line). This is in line with the loss of some amount of NH₃ from the sample after exposure to SO₂ + O₂ as evidenced by the XANES linear combination fitting and temperature programmed reaction.

Fig. 5(b and c) presents time evolutions of the intensities in two points: 3590 cm⁻¹ corresponding to the OH stretching mode (νOH) of Brønsted sites and 3320 cm⁻¹ corresponding to the νNH of NH₃ and NH₄⁺. By plotting the trends of these two intensities, we can follow the dynamics of the NH₃ and NH₄⁺ species in the sample described above during the experimental protocol. The evolutions of the corresponding raw DRIFTS spectra and KM transformed spectra are presented in Fig. S5 and S6 of the ESI.† According to these data, the reaction of NO with the [Cu₂^II(NH₃)₄O₂]²⁺ complex is accompanied by an increase in intensity for Brønsted sites, and a decrease in intensity for the νNH of NH₃ and NH₄⁺, both without exposure to SO₂ and after exposure to SO₂. Without exposure to SO₂, the reaction is fast at 200 °C. After exposure to SO₂, this reaction is very slow at 200 °C and accelerates only after heating to 300 °C.

From the presented results, a reaction of [Cu₂^II(NH₃)₄O₂]²⁺ with NO takes place at 150–200 °C, without exposure to SO₂. In this reaction, N₂ and H₂O are formed. Simultaneously, a reduction of Cu^II to Cu^I, accompanied by the consumption of NH₃ from both Cu and Brønsted sites, consumption of NO and production of N₂ and H₂O takes place. The reaction stops when the reduction of Cu is complete.

When [Cu₂^II(NH₃)₄O₂]²⁺ is exposed to SO₂, the reaction with NO at 200 °C is hindered and requires a higher temperature (250–300 °C). The same is true for a reduction of Cu^II to Cu^I, consumption of NH₃ from Cu and Brønsted sites, consumption of NO and production of N₂ and H₂O. Furthermore, some desorption of SO₂ is observed at around 250 °C. This reaction is also limited by the number of Cu^II species being able to participate in the reaction. This means that the exposure of the [Cu₂^II(NH₃)₄O₂]²⁺ complex to SO₂ changes the reaction with NO, as the rate of the observed reaction is lower, compared to the case without SO₂ exposure. This is visible on the time evolution curves of the IR bands (Fig. 5b and c) and the concentration profiles resulting from multivariate curve resolution (Fig. 3b and c). Without exposure to SO₂, the reaction completes in ∼15 minutes. After exposure to SO₂, however, the full transition takes ∼50 minutes, even though the system is clearly above 200 °C the entire time. This indicates that a SO₂-induced change in the chemical properties of Cu in Cu-CHA catalysts contributes to the poisoning effect of SO₂ on the reaction between Cu-CHA and NO. Because the latter is an important stage in the NH₃-SCR reaction cycle, this effect also contributes to the deactivation of Cu-CHA catalysts for NH₃-SCR by SO₂.

In both procedures, exposed and not exposed to SO₂, desorption of NH₃ occurs above 300 °C (see NH₃ desorption curves in Fig. 1). It is noteworthy that after exposure to SO₂, the NH₃ desorption peak is slightly smaller due to NH₃ loss during SO₂ exposure. The linear combination fits of the XAS data are consistent with a loss of NH₃ after SO₂-exposure, revealing a larger fraction of the Cu bound to the zeolite framework and the lower concentration of the [Cu^I(NH₃)₂]⁺ species in the end of the protocol.

Conclusions

In this work, we monitored the reaction of NO with the [Cu₂^II(NH₃)₄O₂]²⁺ complex in a Cu-CHA catalyst for NH₃-SCR without and with exposure to SO₂, using temperature-programmed reaction with NO and simultaneous in situ X-ray absorption spectroscopy (XAS), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and mass spectrometry. Since this reaction step occurs in the NH₃-SCR reaction cycle after the activation of O₂, the effect of SO₂ on this reaction reflects the deactivation of Cu-CHA catalysts for NH₃-SCR.

The reaction of NO with the [Cu₂^II(NH₃)₄O₂]²⁺ complex results in the formation of N₂ and H₂O, a loss of some NH₃, formation of some Brønsted acid sites, and a reduction of Cu^II to Cu^I. Without exposure of the [Cu₂^II(NH₃)₄O₂]²⁺ complex to SO₂, the reaction with NO takes place around 120 °C.

When the [Cu₂^II(NH₃)₄O₂]²⁺ complex is exposed to SO₂, prior to the reaction with NO, the required reaction temperature increases to 250–300 °C. Some desorption of SO₂ takes place around 250 °C, while the reaction products and other observed effects are similar to the case without SO₂ exposure. However, the fraction of Cu bound to the zeolite framework becomes higher after SO₂ exposure, and the formation of Brønsted acid sites, loss of NH₃, and the reduction of Cu^II become significantly slower. From the NH₃ desorption profile, less NH₃ is stored on Cu, while NH₃ stored on Brønsted acid sites is mostly unaffected, in agreement with our previous observation that the SO₄⁻ ligands may take the place of some of the NH₃ in the coordination sphere of Cu. Our results indicate that SO₂ induces changes in the chemical properties of Cu in Cu-CHA catalysts for NH₃-SCR, which are, at least in part, responsible for the poisoning of CuCHA catalysts by SO₂.

Data availability

ESRF data DOI for the CH-6662 beamtime: https://doi.org/10.15151/ESRF-ES-1049152918.

Author contributions

Anastasia Yu. Molokova: investigation, conceptualisation, formal analysis, software, visualization, and writing – original draft; Davide Salusso: investigation and writing – review & editing; Elisa Borfecchia: supervision, conceptualization, and writing – review & editing; Fei Wen: supervision and writing – review & editing; Stefano Magliocco: investigation; Silvia Bordiga: supervision, project administration, and funding acquisition; Ton V. W. Janssens: writing – review & editing, supervision, project administration, and funding acquisition; Kirill A. Lomachenko: investigation, supervision, project administration, and funding acquisition; Gloria Berlier: supervision, project administration, and funding acquisition.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

ESRF is kindly acknowledged for the provision of beamtime at the BM23 beamline (experiment CH-6662). This project has received funding from the European Union's Horizon 2020 research and innovation Programme under the Marie Skłodowska-Curie grant agreement No. 847439 (InnovaXN). This research acknowledges support from the Project CH4.0 under the MIUR program ‘Dipartimenti di Eccellenza 2023-2027’ (CUP: D13C22003520001).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4cy00792a

‡ Present address: Dipartimento di Ingegneria Elettronica, Chimica ed Ingegneria Industriale, University of Messina, V.le F. Stagno D'Alcontres 31, 98166 Messina, Italy.

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