The small-pore Cu-CHA zeolite is today the object of intensive research efforts to rationalize its outstanding performance in the NH3-assisted selective catalytic reduction (SCR) of harmful nitrogen oxides and to unveil the SCR mechanism. Herein we exploit operando X-ray spectroscopies to monitor the Cu-CHA catalyst in action during NH3-SCR in the 150-400 °C range, targeting Cu oxidation state, mobility, and preferential N or O ligation as a function of reaction temperature. By combining operando XANES, EXAFS, and vtc-XES, we unambiguously identify two distinct regimes for the atomic-scale behavior of Cu active-sites. Low-temperature SCR, up to ∼200 °C, is characterized by balanced populations of Cu(I)/Cu(II) sites and dominated by mobile NH3-solvated Cu-species. From 250 °C upward, in correspondence to the steep increase in catalytic activity, the largely dominant Cu-species are framework-coordinated Cu(II) sites, likely representing the active sites for high-temperature SCR.

The NH 3 -mediated selective catalytic reduction (NH 3 -SCR) of NOx over Cu-ion-exchanged chabazite (Cu-CHA) catalysts is the basis of the technology for abatement of NOx from diesel vehicles. A crucial step in this reaction is the activation of oxygen. Under conditions for low-temperature NH 3 -SCR, oxygen only reacts with Cu I ions, which are present as mobile Cu I diamine complexes [Cu I (NH 3 ) 2 ] + . To determine the structure and reactivity of the species formed by oxidation of these Cu I diamine complexes with oxygen at 200 °C, we have followed this reaction, using a Cu-CHA catalyst with a Si/Al ratio of 15 and 2.6 wt% Cu, by Xray absorption spectroscopies (XANES and EXAFS) and diffuse reflectance UV-Vis spectroscopy, with the support of DFT calculations and advanced EXAFS wavelet transform analysis. The results provide unprecedented direct evidence for the formation of a [Cu 2 (NH 3 ) 4 O 2 ] 2+ mobile complex with a side-on μ-η 2 ,η 2 -peroxo diamino dicopper(II) structure, accounting for 80−90% of the total Cu content. These [Cu 2 (NH 3 ) 4 O 2 ] 2+ are completely reduced to [Cu I (NH 3 ) 2 ] + at 200 °C in a mixture of NO and NH 3 . Some N 2 is formed as well, which suggests the role of the dimeric complexes in the low-temperature NH 3 -SCR reaction. The reaction of [Cu 2 (NH 3 ) 4 O 2 ] 2+ complexes with NH 3 leads to a partial reduction of the Cu without any formation of N 2 . The reaction with NO results in an almost complete reduction to Cu I , under the formation of N 2 . This indicates that the lowtemperature NH 3 -SCR reaction proceeds via a reaction of these complexes with NO.

Multivariate XAS analysis and in situ FTIR enable an unprecedented quantitative understanding of the composition impact on temperature-dependent Cu-speciation and reducibility in Cu-CHA zeolite catalysts.

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In this work, we show the potentiality of operando FTIR spectroscopy to follow the formation of Cu -(N,O) species on Cu exchanged chabazite zeolites (Cu-CHA), active for the selective catalytic reduction of NO with NH (NH -SCR). In particular, we investigated the reaction of NO and O at low temperature (200 and 50 °C) on a series of Cu-CHA zeolites with different composition (Si/Al and Cu/Al ratios), to investigate the nature of the formed copper nitrates, which have been proposed to be key intermediates in the oxidation part of the SCR cycle. Our results show that chelating bidentate nitrates are the main structures formed at 200 °C. At lower temperature a mixture of chelating and monodentate nitrates are formed, together with the nitrosonium ion NO , whose amount was found to be proportional to the zeolite Brønsted site concentration. Nitrates were found to mainly form with Cu ions stabilized by one negative framework charge (Z), Z-[Cu(OH] or Z-[Cu(O ] , without involvement of Z -Cu ones. This evidence, together with the absence of bridging nitrates in samples with high probability for Cu-Cu pairs, indicate that the nitrate ligands are not able to mobilize copper ions, at variance with what recently reported for NH . Finally, water was found to replace preformed chelating copper nitrates and deplete NO (though with different kinetics) at both temperatures, while favouring the presence of monodentate ones.

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In situ XAS and UV-vis–NIR spectroscopy shed light on Cu-speciation during NH3 temperature-programmed desorption and surface reaction (TPSR) over a commercial Cu-chabazite deNOx catalyst, expanding the fundamental knowledge required to unravel the NH3-SCR mechanism across the whole operation-relevant temperature range.

An innovative approach in EXAFS analysis and fitting using wavelet transforms reveals local structure and nuclearity of Cu-species in zeolites.