M A T T E R A T E X T R E M E S

S C I E N T I F I C H I G H L I G H T S

2 4 H I G H L I G H T S 2 0 2 2 I

evidence for the synthesis of a covalent compound with AsN stoichiometry, in which each of the two elements is single-bonded to three atoms of the other and hosts an electron lone pair, in a tetrahedral anisotropic coordination (Figure 13).

From a chemical point of view, the experimental results of this study mark a significant advancement about reactivity and bond theory in pnictogens, providing insights into the chemical interaction between As and N

and highlighting the key role played by the electron lone pairs in the high-pressure chemistry of group 15 elements [1,3,4]. Furthermore, the discovery and structural characterisation of AsN, the first crystalline nitride of As, not only experimentally confirm theoretical predictions indicating the existence of novel nitrides, but also open new perspectives for the design and high-pressure synthesis of advanced As- and N-based inorganic materials of energetic and technological relevance, potentially recoverable at ambient conditions.

PRINCIPAL PUBLICATION AND AUTHORS

Single-bonded cubic AsN from high pressure and high temperature chemical reactivity of arsenic and nitrogen, M. Ceppatelli (a,b), D. Scelta (a,b), M. Serrano-Ruiz (b), K. Dziubek (a,b), M. Morana (c), V. Svitlyk (d), G. Garbarino (d), T. Poręba (d), M. Mezouar (d), M. Peruzzini (b), R. Bini (a,b,e), Angew. Chem. Int. Ed. 61, e202114191 (2022); https:/doi.org/10.1002/anie.202114191 (a) LENS, European Laboratory for Non-linear Spectroscopy, Firenze (Italy) (b) ICCOM-CNR, Institute of Chemistry of OrganoMetallic Compounds, National Research Council of Italy, Firenze (Italy) (c) Department of Chemistry and INSTM, University of Pavia (Italy) (d) ESRF (e) Dipartimento di Chimica Ugo Schiff dell Università degli Studi di Firenze (Italy)

REFERENCES

[1] M. Ceppatelli et al., Nat. Commun. 11, 6125 (2020). [2] M. Ceppatelli et al., Inorg. Chem. 61, 12165-12180 (2022). [3] D. Scelta et al., Angew. Chem. Int. Ed. 56, 14135 (2017). [4] D. Scelta et al., Chem. Commun. 54, 10554 (2018).

Understanding the SO2 poisoning of Cu-CHA deNOX catalysts by X-ray absorption spectroscopy

Cu-exchanged chabazites (Cu-CHA) are effective catalysts for removing harmful nitrogen oxides (NOx) from the exhaust gas of diesel vehicles. X-ray absorption spectroscopy data has helped to shed light on the details of SO2 poisoning of the catalyst. This information is crucial for the further improvement of catalyst performance.

The emission of nitrogen oxides (NOx) from diesel vehicles is a global environmental challenge [1,2]. State-of-the-art exhaust gas after-treatment 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 chabazite zeolites (Cu-CHA) are efficient catalysts for NH3-SCR due to their unsurpassed low-temperature activity and stability under operation conditions.

In practice, the application of Cu-CHA catalysts for NH3- SCR is restricted to ultralow-sulfur diesel fuels because the activity at low temperatures is very sensitive to the presence of SO2 in the exhaust gas [3,4]. To identify the Cu-species that are sensitive to SO2, well-defined framework-bound

and NH3-solvated CuI and CuII species were prepared. The SO2 uptake and interaction of SO2 with the Cu species were monitored by X-ray spectroscopy and temperature- programmed desorption.

X-ray absorption spectroscopy (XAS) was applied to follow the Cu oxidation state and local structure in situ during SO2 uptake by Cu-CHA samples. The measurements were carried out at beamline BM23. The different CuI or CuII species with diverse ligands were prepared by known pre- treatment procedures. The changes due to SO2 uptake were followed by using an in-situ cell, allowing for precise control of gas atmosphere and sample temperature. Figures 14a and b show the evolution of Cu K-edge X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra during the exposure of the pre-treated Cu-CHA catalyst to SO2. For all CuI species and framework-coordinated CuII species (fw-CuII), only minor changes were observed upon SO2 exposure, indicating that these species are not very reactive towards SO2. In contrast, for CuII-species in the presence of NH3, significant changes were observed in the spectra.

X-ray adsorbate quantification (XAQ) [5] and temperature- programmed desorption of SO2 (SO2-TPD) allowed to calculate the SO2 uptake for each of the species (Figure 14c). These methods confirmed that the CuII species with NH3 show the highest SO2 uptake and thus are more reactive towards SO2.