Investigating the role of Cu-oxo species in Cu-nitrate formation over Cu-CHA catalysts

Abstract

The speciation of framework interacting CuII sites in Cu-chabazite zeolite catalysts active in the selective catalytic reduction of NOx with NH3 is studied, to investigate the influence of the Al...

Quantification of Adsorbates by X-ray Absorption Spectroscopy: Getting TGA-like Information for Free

“…The evolution of Cu K-edge XANES spectra agrees well with the literature data for Cu-CHA zeolites with similar Cu/Al and Si/Al ratios. 7 , 8 The usual way of presenting normalized XANES data, such as in Figure 1 a, however, does not allow to appreciate the change of the total absorption level due to desorption of water. However, in the raw data, the vertical shift of the spectra is clearly visible ( Figure 1 b).…”

Assessing the Influence of Zeolite Composition on Oxygen-Bridged Diamino Dicopper(II) Complexes in Cu-CHA DeNOx Catalysts by Machine Learning-Assisted X-ray Absorption Spectroscopy

“… 12 − 14 It is worthwhile to note that deviations from the random Al distribution, known to be promoted by specific synthesis approaches, 15 could additionally influence the Cu-speciation in dehydrated Cu-CHA. Nonetheless, previous studies 13 , 14 , 16 of the catalysts investigated herein point to a substantial agreement of Cu-speciation in the dehydrated state with the theoretical compositional phase diagram by Paolucci et al, 12 based on a random Al distribution in the zeolite framework subject to the Löwenstein’s rule. 17…”

“…These catalysts have been characterized previously in depth after pretreatment in O 2 at 400 °C. 14 We use the 0.5_15 catalyst as a reference, because it was previously investigated under the same redox conditions. 20 Additional details on the investigated catalysts, as well as on the experimental methods for in situ XAS data collection can be found in the SI, Section 1 .…”

“…For all three catalysts, the XANES after pretreatment in O 2 at 400 °C (dashed gray curves in Figure 1 ) shows the expected features for Z 2 Cu II and Z[Cu II OH]/Z x [Cu x II O y ] species, reflecting their different Cu/Al and Si/Al ratios. 12 − 14 At the end of the reduction step, for each catalyst, the XANES indicates the formation of mobile [Cu I (NH 3 ) 2 ] + for all the Cu ions present. For the 0.1_5 composition, the reduction to [Cu I (NH 3 ) 2 ] + of the dominant Z 2 Cu II species implies the formation of a new Brønsted acid site to maintain the charge balance in the zeolite.…”

“…After heating a Cu-CHA catalyst at temperatures above 400 °C and in the presence of O 2 , the Cu is present as a Cu II species, docked to the lattice oxygen atoms in correspondence of the Al exchange sites (Z) in the six- and eight-membered rings ( 6r and 8r ) of the CHA zeolite. Assuming a random Al distribution in the zeolite framework, the chemical identity and local structure are determined by the catalyst composition: low Si/Al and Cu/Al ratios favor bare Z 2 Cu II species in 6r , while at higher Si/Al and Cu/Al ratios an extra-ligand is required for charge compensation, resulting in Z­[Cu II OH] or oxygen-bridged multimeric Z x [Cu x II O y ] species preferentially hosted in 8r . − It is worthwhile to note that deviations from the random Al distribution, known to be promoted by specific synthesis approaches, could additionally influence the Cu-speciation in dehydrated Cu-CHA. Nonetheless, previous studies ,, of the catalysts investigated herein point to a substantial agreement of Cu-speciation in the dehydrated state with the theoretical compositional phase diagram by Paolucci et al, based on a random Al distribution in the zeolite framework subject to the Löwenstein’s rule…”