# SO2 Poisoning of Cu-CHA deNOx Catalyst: The Most Vulnerable Cu Species Identified by X-ray Absorption Spectroscopy

Anastasia Yu. Molokova, Elisa Borfecchia, Andrea Martini, Ilia A. Pankin, Cesare Atzori, Olivier Mathon, Silvia Bordiga, Fei Wen, Peter N. R. Vennestrøm, Gloria Berlier, et al.

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## To cite this version:

Anastasia Yu. Molokova, Elisa Borfecchia, Andrea Martini, Ilia A. Pankin, Cesare Atzori, et al. SO2 Poisoning of Cu-CHA deNOx Catalyst: The Most Vulnerable Cu Species Identified by X-ray Absorption Spectroscopy. JACS Au, 2022, 2 (4), pp.787-792. 10.1021/jacsau.2c00053. hal-05075973

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# SO₂ Poisoning of Cu-CHA deNOₓ Catalyst: The Most Vulnerable Cu Species Identified by X-ray Absorption Spectroscopy

Anastasia Yu. Molokova, Elisa Borfecchia, Andrea Martini, Ilia A. Pankin, Cesare Atzori, Olivier Mathon, Silvia Bordiga, Fei Wen, Peter N. R. Vennestrøm, Gloria Berlier, Ton V. W. Janssens,* and Kirill A. Lomachenko*

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Cite This: JACS Au 2022, 2, 787–792

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Supporting Information

**ABSTRACT:** Cu-exchanged chabazite zeolites (Cu-CHA) are effective catalysts for the NH₃-assisted selective catalytic reduction of NO (NH₃-SCR) for the abatement of NOₓ emission from diesel vehicles. However, the presence of a small amount of SO₂ in diesel exhaust gases leads to a severe reduction in the low-temperature activity of these catalysts. To shed light on the nature of such deactivation, we characterized a Cu-CHA catalyst under well-defined exposures to SO₂ using *in situ* X-ray absorption spectroscopy. By varying the pretreatment procedure prior to the SO₂ exposure, we have selectively prepared Cu¹ and Cu¹¹ species with different ligations, which are relevant for the NH₃-SCR reaction. The highest reactivity toward SO₂ was observed for Cu¹¹ species coordinated to both NH₃ and extraframework oxygen, in particular for [Cu¹¹₂(NH₃)₄O₂]²⁺ complexes. Cu species without either ammonia or extraframework oxygen

ligands were much less reactive, and the associated SO₂ uptake was significantly lower. These results explain why SO₂ mostly affects the low-temperature activity of Cu-CHA catalysts, since the dimeric complex [Cu¹¹₂(NH₃)₄O₂]²⁺ is a crucial intermediate in the low-temperature NH₃-SCR catalytic cycle.

**KEYWORDS:** selective catalytic reduction, Cu-CHA, deNOₓ catalysis, sulfur poisoning, X-ray absorption spectroscopy, X-ray adsorbate quantification, XAS, XAQ

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The emission of nitrogen oxides (NOₓ) from diesel vehicles is a global environmental challenge.¹,² State of the art exhaust gas aftertreatment systems contain catalysts for selective catalytic reduction of NOₓ by ammonia (NH₃-SCR), capable of reducing well over 90% of the NOₓ emitted by the engine. In the NH₃-SCR reaction, NO reacts with NH₃ in the presence of O₂ to form N₂ and H₂O. At present, Cu-exchanged chabazites (Cu-CHA) are the preferred catalysts for NH₃-SCR, due to their superior low-temperature activity (150–350 °C)³,⁴ and hydrothermal stability.⁵,⁶ The temperature dependence of the NH₃-SCR activity of Cu-CHA catalysts shows a minimum at around 350 °C, which indicates that the reaction mechanism at low temperatures is different from that at higher temperatures.⁷

The NH₃-SCR reaction cycle for the low-temperature activity is a redox cycle, consisting of a series of oxidation and reduction steps, in which the oxidation state of Cu changes between Cu¹ and Cu¹¹. The NO and NH₃ coordinate to Cu in the zeolite, giving rise to a variety of Cu species along the NH₃-SCR cycle.⁸⁻¹¹ The low-temperature activity of Cu-CHA catalysts originates from the ability to form mobile Cu¹(NH₃)₂ complexes under SCR conditions. Pairs of these species constitute the active Cu sites capable of O₂ activation via the

formation of [Cu¹¹₂(NH₃)₄O₂]²⁺ dimers around 200 °C, which is a crucial step in the NH₃-SCR reaction cycle.¹²,¹³

In practice, the application of Cu-CHA catalysts for the NH₃-SCR is restricted to ultralow-sulfur diesel fuels, due to the fact that a few ppm of SO₂ present in the exhaust gas drastically reduces the activity at low temperatures.³,⁴,¹⁴ Multiple studies show that SO₂ affects the Cu mobility, the amount of Cu active sites,¹⁴ and the redox behavior of the Cu in the NH₃-SCR cycle.⁹,¹⁵ Most studies have focused on the overall effect of SO₂ on the performance of the catalysts,¹⁴⁻²¹ while the chemistry behind SO₂ poisoning at the molecular level remains poorly understood. To determine a mechanism for SO₂ poisoning, one must identify the species in the Cu-CHA catalysts that interact with SO₂. To this end, we have selectively prepared well-defined Cu¹ and Cu¹¹ species with

Received: January 27, 2022

Revised: April 4, 2022

Accepted: April 5, 2022

Published: April 11, 2022

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<table><caption>Table 1. Pretreatment Procedures and Resulting Cu Species</caption><thead><tr><th>procedure</th><th>conditions</th><th>dominant Cu state</th><th>designation in the text and figures</th><th>ref</th></tr></thead><tbody><tr><td>1</td><td>1% H<sub>2</sub> at 400 °C; cooling to 200 °C in He</td><td>fw-Cu<sup>I</sup></td><td>fw-Cu<sup>I</sup></td><td>24</td></tr><tr><td>2</td><td>500 ppm of NO + 600 ppm of NH<sub>3</sub> at 200 °C</td><td>mobile [Cu<sup>I</sup>(NH<sub>3</sub>)<sub>2</sub>]<sup>+</sup></td><td>[Cu<sup>I</sup>(NH<sub>3</sub>)<sub>2</sub>]<sup>+</sup></td><td>24</td></tr><tr><td>3</td><td>500 ppm of NO + 600 ppm of NH<sub>3</sub> at 200 °C; heating to 550 °C in He; cooling back to 200 °C in He</td><td>fw-Cu<sup>I</sup> (after thermal treatment of [Cu<sup>I</sup>(NH<sub>3</sub>)<sub>2</sub>]<sup>+</sup>)</td><td>[Cu<sup>I</sup>(NH<sub>3</sub>)<sub>2</sub>]<sup>+</sup> + T</td><td>24</td></tr><tr><td>4</td><td>10% O<sub>2</sub> at 200 °C</td><td>fw-Cu<sup>II</sup></td><td>fw-Cu<sup>II</sup></td><td>25</td></tr><tr><td>5</td><td>500 ppm of NO + 600 ppm of NH<sub>3</sub> at 200 °C; He purge; 10% O<sub>2</sub> at 200 °C</td><td>mobile [Cu<sup>II</sup><sub>2</sub>(NH<sub>3</sub>)<sub>4</sub>O<sub>2</sub>]<sup>2+</sup> dimer</td><td>[Cu<sup>II</sup><sub>2</sub>(NH<sub>3</sub>)<sub>4</sub>O<sub>2</sub>]<sup>2+</sup></td><td>13</td></tr><tr><td>6</td><td>600 ppm of NH<sub>3</sub> at 200 °C</td><td>mixed<sup>da</sup></td><td>Cu<sup>II</sup> + NH<sub>3</sub></td><td>this work</td></tr></tbody></table>

<sup>a</sup>Procedure 6 results in a mixture of two NH<sub>3</sub>-coordinated Cu species, as discussed further in the text.

different ligands inside the pores of the Cu-CHA catalyst and exposed them to SO<sub>2</sub> under well-defined conditions. We monitored the changes in the Cu K-edge X-ray absorption spectra (XAS) during the absorption of SO<sub>2</sub>. This allowed us to determine the chemical state of the Cu that interacts with SO<sub>2</sub>. The results were corroborated by X-ray emission spectroscopy (XES) and measurements of the SO<sub>2</sub> uptake using temperature-programmed desorption (TPD) of SO<sub>2</sub>.

The Cu-CHA catalyst used in this study had a Si/Al ratio of 6.7 and a Cu loading of 3.2 wt % (Cu/Al = 0.24). The Cu K-edge XAS and Cu Kβ valence-to-core XES measurements were carried out at the BM23<sup>22</sup> and ID26<sup>23</sup> beamlines of the European Synchrotron Radiation Facility (ESRF), respectively. Sample treatment protocols consisted of three distinct steps. First, all samples were heated to 550 °C in a 10% O<sub>2</sub>/He flow, removing water and forming Cu<sup>II</sup> species bound to the framework of the zeolite (fw-Cu<sup>II</sup>). Then, the specific state of Cu was prepared, using one of the six different pretreatment procedures summarized in Table 1. Finally, the catalyst was exposed to 400 ppm SO<sub>2</sub>/He flow at 200 °C for 3 h until no visible changes in the spectra occurred. Further experimental details are given in the Supporting Information.

The Cu species formed with the pretreatments differ in three aspects: (1) the oxidation state of Cu (Cu<sup>I</sup> or Cu<sup>II</sup>), (2) the coordination of the Cu (NH<sub>3</sub> or/and O), and (3) the interaction of the Cu with the framework (fw-coordinated or mobile species).

Figure 1 shows the evolution of Cu K-edge XANES and EXAFS spectra during the exposure of the pretreated Cu-CHA catalyst to 400 ppm SO<sub>2</sub>/He flow at 200 °C. For all Cu<sup>I</sup> species and fw-Cu<sup>II</sup> species (procedures 1–4 in Table 1), only minor changes are observed upon SO<sub>2</sub> exposure, indicating that these species are not very reactive toward SO<sub>2</sub>. In contrast, for Cu<sup>II</sup> species in the presence of NH<sub>3</sub> (procedures 5 and 6) significant changes are observed in the spectra. In these cases, the exposure to SO<sub>2</sub> results in a pronounced increase of the XANES peak at 8983 eV, characteristic for linear Cu<sup>I</sup> complexes,<sup>9,26,27</sup> and a decrease in the intensity of the first shell in the EXAFS FT. This means that some of the Cu<sup>II</sup> species are reduced to Cu<sup>I</sup> upon interaction with SO<sub>2</sub>. The decrease in the first-shell intensity indicates a reduction of the coordination number for the Cu ions, which is also in line with the formation of a linear Cu<sup>I</sup> species.

The species obtained in procedure 5 are the oxygen-bridged diamine dicopper complexes [Cu<sup>II</sup><sub>2</sub>(NH<sub>3</sub>)<sub>4</sub>O<sub>2</sub>]<sup>2+</sup>, which are formed by the reaction of O<sub>2</sub> with a pair of [Cu<sup>I</sup>(NH<sub>3</sub>)<sub>2</sub>]<sup>+</sup> complexes.<sup>13</sup> In the reaction cycle for the low-temperature NH<sub>3</sub>-SCR reaction,<sup>11,13,28</sup> the [Cu<sup>II</sup><sub>2</sub>(NH<sub>3</sub>)<sub>4</sub>O<sub>2</sub>]<sup>2+</sup> complexes react with NO, which eventually leads to the production of N<sub>2</sub>

<figure><img src="image_10.png" /><figcaption>**Figure 1.** Cu K-edge XANES (a) and FT-EXAFS spectra (b) collected *in situ* during the exposure of Cu species obtained in procedures 1–6 to SO<sub>2</sub> at 200 °C.</figcaption></figure>

and H<sub>2</sub>O. The observation that the [Cu<sup>II</sup><sub>2</sub>(NH<sub>3</sub>)<sub>4</sub>O<sub>2</sub>]<sup>2+</sup> complexes are reactive toward SO<sub>2</sub> is therefore a good explanation for the SO<sub>2</sub>-induced deactivation of Cu-CHA catalysts for NH<sub>3</sub>-SCR: the reaction with SO<sub>2</sub> interrupts the NH<sub>3</sub>-SCR cycle, thereby decreasing the activity of the catalyst.

The other case where Cu reacts with SO<sub>2</sub> is obtained in procedure 6 by exposure of the fw-Cu<sup>II</sup> species to NH<sub>3</sub>. Previously, a similar pretreatment resulted in a mixture of linear [Cu<sup>I</sup>(NH<sub>3</sub>)<sub>2</sub>]<sup>+</sup> and either square-planar [Cu<sup>II</sup>(NH<sub>3</sub>)<sub>4</sub>]<sup>2+</sup> complexes or mixed-ligand [Cu<sup>II</sup>O<sub>x</sub>(NH<sub>3</sub>)<sub>y</sub>]<sup>2+</sup> moieties.<sup>9,24</sup> For the sample reported in this work, the mixed-ligand configuration is more likely. Indeed, linear combination fits
of the XANES data on the basis of references for the linear
$[Cu^I(NH_3)_2]^+$ complex and pure $Cu^{II}(NH_3)_4$ groups (aqueous
$[Cu^{II}(NH_3)_4]^{2+}$ or solid-state $[Cu^{II}(NH_3)_4]SO_4 \cdot H_2O$) resulted
in visible discrepancies with the data (Figure S7 in the
Supporting Information). A better agreement is obtained when
the spectrum of oxygen-bridged diamine dicopper complex
$[Cu^{II}_2(NH_3)_4O_2]^{2+}$ is used as a $Cu^{II}$ reference in combination
with $[Cu^I(NH_3)_2]^+$, with approximately equal weights for each
component (Figure 2). The necessary stock of available oxygen

<figure><img src="image_11.png" /><figcaption>**Figure 2.** Linear combination fit of Cu K-edge XANES spectra obtained in Cu-CHA after exposing fw-Cu<sup>II</sup> species to NH<sub>3</sub> at 200 °C (Cu<sup>II</sup> + NH<sub>3</sub> pretreatment).</figcaption></figure>

needed for the formation of the mixed-ligand species is
expected to be present in the sample, as a wavelet analysis of
the EXAFS collected after heating to 550 °C and cooling to
200 °C in 10% O₂/He flow reveals the presence of Cu–Cu
scattering usually attributed to the oxygen-containing
dimers<sup>29,30</sup> (Figure S8 in the Supporting Information), which
may be susceptible to form mixed-ligand species upon
exposure to NH<sub>3</sub>.

The evolution of XANES spectra upon interaction with SO₂ shows that the most susceptible species are CuII with mixed (NH₃)ₓOᵧ ligation, whereas CuI species or CuII in the absence of NH₃ are much less affected. These findings are supported by X-ray adsorbate quantification (XAQ) data,³¹ collected simultaneously with the XAS measurements during the exposure to SO₂, and a TPD analysis of a parallel set of catalyst samples, exposed to the same pretreatments used in XANES experiments (Figure 3a). We find the highest sulfur content (S/Cu ratio) for the [CuII₂(NH₃)₄O₂]²⁺ and CuII + NH₃ procedures. The sulfur uptake of the [CuI(NH₃)₂]⁺ and fw-CuII moieties was ca. 3 times lower, and for the bare fw-CuI species, it was ca. 6 times lower. These results show that the reaction between the [CuII₂(NH₃)₄O₂]²⁺ species and SO₂ contributes the most to the accumulation of SO₂ in the Cu-CHA catalyst.

Interestingly, the sulfur content in the CuII + NH3 sample
lies between those for samples with pure $[Cu^I(NH_3)_2]^+$ and
$[Cu^{II}_2(NH_3)_4O_2]^{2+}$ species, which in combination with the
linear combination fit shown in Figure 2 suggests that the

<figure><img src="image_12.png" /><figcaption>**Figure 3.** (a) S/Cu ratios in the samples after exposure to SO₂ obtained from SO₂-TPD and XAQ. (b) SO₂-TPD profiles collected after exposure of the species obtained in procedures 1–6 to SO₂ in comparison to a reference SO₂-TPD curve of a CHA zeolite without Cu impregnated with 20 wt % (NH₄)₂SO₄, downscaled ×10.</figcaption></figure>

reactivity of the $Cu^{II}(NH_3)_xO_y$ species obtained after CuII +
NH_3 treatment toward SO_2 is similar to that of
$[Cu^{II}_2(NH_3)_4O_2]^{2+}$.

By comparing the SO₂-TPD curves of Cu-CHA samples
with that of (NH₄)₂SO₄, adsorbed on Cu-free CHA (Figure
3b), we can also deduce that the elevated sulfur content in the
samples with the CuII(NH₃)ₓOᵧ species is due to the reactivity
toward SO₂ and not to the formation of (NH₄)₂SO₄ in a
reaction of SO₂ with NH₃ and NH₄⁺ groups stored in the
zeolite framework. For the adsorbed (NH₄)₂SO₄, we observe
SO₂ desorption at around 380, 530, and 1000 °C (gray curve
in Figure 3b). The desorption at 380 °C matches the known
thermal decomposition of (NH₄)₂SO₄,³² the other two peaks
are probably due to the interaction of either (NH₄)₂SO₄ or
products of its decomposition with the zeolite, their precise
interpretation being beyond the scope of the present argument.
For all three Cu-CHA samples containing NH₃ before
exposure to SO₂ ([$Cu^I(NH_3)_2]^+$, $[Cu^{II}_2(NH_3)_4O_2]^{2+}$, and
$(Cu^{II} + NH_3)$ procedures), we observe SO₂ desorption at
around 420 °C (Figure 3b). As this does not match any of the
observed desorption characteristics of (NH₄)₂SO₄ in Cu-free
Cu-CHA, the SO₂-TPD feature at 420 °C reflects an
interaction of Cu with SO₂. Interestingly, the SO₂-TPD
curve for the sample with the dominant fw-CuII species shows
a significant SO₂ desorption peak close to 1000 °C, which,
together with the lack of changes in Cu K-edge XANES upon
exposure to SO₂, indicates the formation of some sulfur
deposits not directly coordinated to Cu.

The presence of Cu–N and Cu–O bonds in the
$[Cu^{II}_2(NH_3)_4O_2]^{2+}$ complex has been independently con-
firmed by valence-to-core XES.<sup>27,33,34</sup> XES spectra at different
stages of pretreatment leading to the formation of
$[Cu^{II}_2(NH_3)_4O_2]^{2+}$ dimers are reported in Figure 4. The
origin of the Kβ'' satellite peak is the transition from the ligand
s orbitals to Cu 1s, which makes its position sensitive to the
species directly coordinated to Cu and allows it to discriminate
<figure><img src="image_13.png" /><figcaption>**Figure 4.** Background-subtracted Cu Kβ valence-to-core XES spectra for different stages of procedure 5 leading to the formation of the $[Cu^{II}_2(NH_3)_4O_2]^{2+}$ complex and its exposure to SO₂.</figcaption></figure>

among Cu–O, Cu–N, and Cu–S bonds.³⁵⁻³⁷ Figure 4 shows that after heating in O₂ Cu is predominantly coordinated by oxygens (as expected for the fw-Cu¹¹ species), whereas after exposure to NO + NH₃ N ligands are dominating, as expected for a $[Cu^{I}(NH_3)_2]^{+}$ linear complex. After subsequent exposure to O₂ and formation of $[Cu^{II}_2(NH_3)_4O_2]^{2+}$ dimers, the peak broadens, confirming the presence of both Cu–N and Cu–O bonds. These bonds remain after exposure to SO₂, while no significant contribution from Cu–S bonds³⁸ is observed, suggesting that the possible SO₂ binding to the Cu is carried out through an oxygen atom.

In conclusion, the *in situ* XAS and XES measurements of different Cu intermediates formed in a Cu-CHA catalyst exposed to SO₂ demonstrate that Cu¹¹ species with mixed NH₃ and O ligation of Cu are particularly reactive toward SO₂, whereas Cu¹ species and Cu¹¹ without NH₃ are much less affected by it. In particular, the $[Cu^{II}_2(NH_3)_4O_2]^{2+}$ complex, which is formed upon activation of O₂ in the NH₃-SCR cycle, shows a clear reaction with SO₂, resulting in a partial reduction of the Cu¹¹ and accumulation of sulfur in the zeolite. Therefore, we conclude that this reaction is responsible for the poisoning of Cu-CHA catalysts in NH₃-SCR by SO₂.

## ASSOCIATED CONTENT

**SI** Supporting Information

The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacsau.2c00053.

Experimental details, XANES linear combination fits before SO₂ exposure, wavelet transform analysis of the sample heated in O₂, and EXAFS fitting results (PDF)

## AUTHOR INFORMATION

### Corresponding Authors

Kirill A. Lomachenko — European Synchrotron Radiation Facility, 38043 Grenoble Cedex 9, France; ● orcid.org/0000-0003-0238-1719; Email: lomachenko@esrf.fr

Ton V. W. Janssens — Umicore Denmark ApS, 2970 Hørsholm, Denmark; ● orcid.org/0000-0002-1225-0942; Email: tonv.w.janssens@eu.omicore.com

### Authors

Anastasia Yu. Molokova — European Synchrotron Radiation Facility, 38043 Grenoble Cedex 9, France; Department of Chemistry and NIS Centre, University of Turin, 10125 Turin, Italy; ● orcid.org/0000-0003-2053-2031

Elisa Borfecchia — Department of Chemistry and NIS Centre, University of Turin, 10125 Turin, Italy; ● orcid.org/0000-0001-8374-8329

Andrea Martini — Department of Chemistry and NIS Centre, University of Turin, 10125 Turin, Italy; The Smart Materials Research Institute, Southern Federal University, 344090 Rostov-on-Don, Russia; ● orcid.org/0000-0001-8820-2157

Ilia A. Pankin — The Smart Materials Research Institute, Southern Federal University, 344090 Rostov-on-Don, Russia

Cesare Atzori — European Synchrotron Radiation Facility, 38043 Grenoble Cedex 9, France; ● orcid.org/0000-0002-3227-7421

Olivier Mathon — European Synchrotron Radiation Facility, 38043 Grenoble Cedex 9, France

Silvia Bordiga — Department of Chemistry and NIS Centre, University of Turin, 10125 Turin, Italy; ● orcid.org/0000-0003-2371-4156

Fei Wen — Umicore AG & Co, 63457 Hanau, Germany

Peter N. R. Vennestrom — Umicore Denmark ApS, 2970 Hørsholm, Denmark; ● orcid.org/0000-0002-6744-5640

Gloria Berlier — Department of Chemistry and NIS Centre, University of Turin, 10125 Turin, Italy; ● orcid.org/0000-0001-7720-3584

Complete contact information is available at:
https://pubs.acs.org/10.1021/jacsau.2c00053

### Funding

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.

### Notes

The authors declare no competing financial interest.

## ACKNOWLEDGMENTS

ESRF is kindly acknowledged for the provision of beamtime at the BM23 and ID26 beamlines. We thank N. Daffé and B. Detlefs for help during the XES experiment at ID26.

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