Researchers have made significant strides in understanding the complex interactions between sulfur dioxide (SO2) and copper-chabazite (Cu-CHA) catalysts, crucial for the selective catalytic reduction (SCR) of nitrogen oxides (NOx) by ammonia (NH3) in diesel exhaust systems. This study, led by Dr. Ton Janssens from Umicore Denmark ApS and Dr. Kirill Lomachenko from the European Synchrotron Radiation Facility, was conducted by Anastasia Molokova, Dr. Olivier Mathon, and Dr. Kirill Lomachenko from the European Synchrotron Radiation Facility; Reza Abasabadi, Dr. Elisa Borfecchia, Professor Silvia Bordiga, and Professor Gloria Berlier from the University of Turin; and Dr. Fei Wen from Umicore AG & Co. It sheds light on the mechanisms of SO2 poisoning in Cu-CHA catalysts using advanced X-ray absorption spectroscopy (XAS). The research is published in the journal Chemical Science.
Cu-CHA catalysts are favored in diesel exhaust systems due to their high activity at low temperatures and stability at high temperatures. However, their performance is severely hampered by the presence of SO2, necessitating the use of ultra-low sulfur diesel fuel. The primary challenge has been the deactivation of Cu-CHA catalysts at low temperatures due to SO2 exposure. This study aims to elucidate the reaction mechanisms involved in SO2 uptake and its impact on catalyst performance.
The researchers employed X-ray absorption spectroscopy (XAS) at the Cu K-edge and S K-edge, along with X-ray emission spectroscopy (XES), to investigate the interaction between SO2 and Cu-CHA catalysts with varying copper content. They discovered that SO2 reacts with the [Cu2II(NH3)4O2]2+ complex formed during the NH3-SCR cycle, leading to the formation of CuI species and a sulfated CuII complex that accumulates in the zeolite pores. The study found that the SO2 uptake mechanism is consistent across catalysts with different Cu loadings.
An important finding was the increased SO2 uptake in the presence of oxygen, which facilitates the re-oxidation of CuI species into the [Cu2II(NH3)4O2]2+ complex, making them available for further reaction with SO2. This was evidenced by X-ray adsorbate quantification (XAQ) and temperature-programmed desorption of SO2 (SO2-TPD).
Dr. Lomachenko remarked, “Our study reveals that the uptake of sulfur is actually facilitated by the presence of oxygen via the reoxidation of the CuI species. This is an important insight necessary to devise methods to mitigate the effects of SO2.”
The research team used a combination of multivariate curve resolution-alternating least squares (MCR-ALS) method and linear combination fitting (LCF) to analyze the experimental spectra. They identified the formation of a sulfated component, predominantly existing as SO42- species, in the Cu-CHA catalysts exposed to SO2. The S K-edge XANES and Kα XES spectra confirmed the oxidation state of sulfur and its local environment, aligning with the presence of sulfated species.
Dr. Janssens emphasized, “Understanding the interaction between SO2 and Cu-CHA catalysts is crucial for developing more efficient SCR catalysts that can withstand sulfur poisoning, ensuring the long-term viability of diesel exhaust systems.”
The study’s findings offer valuable insights into the mechanisms of SO2 poisoning and pave the way for the development of more robust Cu-CHA catalysts capable of maintaining high activity in the presence of sulfur compounds. Dr. Janssens, Dr. Lomachenko, and their colleagues plan to focus future research on optimizing the catalyst composition and reaction conditions to further enhance their resistance to SO2 deactivation.
Journal Reference
Molokova, A. Y., Abasabadi, R. K., Borfecchia, E., Mathon, O., Bordiga, S., Wen, F., Berlier, G., Janssens, T. V. W., & Lomachenko, K. A. (2023). Elucidating the reaction mechanism of SO2 with Cu-CHA catalysts for NH3-SCR by X-ray absorption spectroscopy. Chemical Science, 14, 11521–11531. DOI: https://doi.org/10.1039/D3SC03924B
About The Authors
Anastasia Molokova is a post-doctoral researcher specializing in X-ray Absorption Spectroscopy (XAS) for chemistry applications. She works at the BM23 and ID24 XAS beamlines of the European Synchrotron Radiation Facility (ESRF) in France. Anastasia completed her Ph.D. in chemical and materials sciences in 2023 under the joint supervision of Prof. Gloria Berlier from the University of Turin in Italy, Dr. Kirill A. Lomachenko from the European Synchrotron Radiation Facility in France, and Dr. Ton V.W. Janssens from Umicore in Denmark. Her primary research focus lies in utilizing XAS spectroscopy to establish structure-property relationships of novel materials.
Ton V.W. Janssens is a senior scientist at Umicore Denmark ApS. His main research interest is the elucidation of catalytic reactions and catalyst deactivation. After his PhD in surface science from Eindhoven University of Technology (1993), he became postdoc at University of California, Riverside, and the Fritz-Haber Institute, Berlin, working with surface reactions. In 1998, he joined Haldor Topsøe (Topsoe) in Lyngby, Denmark, and worked in the area of hydrogen catalysts, methanol-to-gasoline, and later automotive catalysis. Since 2017, he is a part of the Automotive Catalysts department of Umicore in Denmark, with focus on SCR catalysts.
Kirill A. Lomachenko is responsible for the chemistry research programme at BM23 and ID24 XAS beamlines of the European Synchrotron Radiation Facility (France). He completed his PhD under the joint supervision of Profs. Carlo Lamberti (University of Turin, Italy) and Alexander Soldatov (Southern Federal University, Russia) in the intersection of physics, chemistry, and materials science. His main field of research is the application of XAS and XES spectroscopies and related characterization techniques to establish the structure-property relationships of novel materials for catalysis, energy storage, and gas sorption.
