Location of Cu²⁺in CHA zeolite investigated by X-ray diffraction using the Rietveld/maximum entropy method

Abstract

Accurate structural models of reaction centres in zeolite catalysts are a prerequisite for mechanistic studies and further improvements to the catalytic performance. The Rietveld/maximum entropy method is applied to synchrotron powder X-ray diffraction data on fully dehydrated CHA-type zeolites with and without loading of catalytically active Cu2+for the selective catalytic reduction of NOxwith NH3. The method identifies the known Cu2+sites in the six-membered ring and a not previously observed site in the eight-membered ring. The sum of the refined Cu occupancies for these two sites matches the chemical analysis and thus all the Cu is accounted for. It is furthermore shown that approximately 80% of the Cu2+is located in the new 8-ring site for an industrially relevant CHA zeolite with Si/Al = 15.5 and Cu/Al = 0.45. Density functional theory calculations are used to corroborate the positions and identity of the two Cu sites, leading to the most complete structural description of dehydrated silicoaluminate CHA loaded with catalytically active Cu2+cations.

Recent Progress in Atomic-Level Understanding of Cu/SSZ-13 Selective Catalytic Reduction Catalysts

“…From such quantifications, it is evident that [Cu(OH)] + -Z species primarily convert to Cu 2+ -2Z and perhaps a portion to CuO x clusters at aging temperatures ≤700 • C. The former transformation, as described by Reaction (3), is also found for the current state-of-the-art commercial Cu/SSZ-13 catalyst ( Figure 5) [18]. At aging temperatures of 750 • C and above, Cu 2+ -2Z stabilizes at~1.2 wt % indicating that this portion of Cu 2+ must be located in windows of 6MRs with paired Al sites; that is, at the energetically most stable locations for Cu 2+ -2Z [33]. The remaining Cu 2+ species are less stable, ultimately converting to CuO x at higher aging temperatures.…”

“…Unless Cu is present at levels exceeding the ion-exchange capacities of the particular SSZ-13 composition used, the current general consensus is that, in the freshly prepared specimens, Cu is present as isolated ions, including Cu 2+ balanced by two nearby framework negative charges (abbreviated as Cu 2+ -2Z, where Z represents a charged zeolite framework site) and a [Cu(OH)] + species balanced by one framework negative charge (as [Cu(OH)] + -Z) [17,[31][32][33]. It is also generally agreed, based on almost identical X-ray absorption near edge structure (XANES) spectra with model Cu salt solutions, that these ions are fully solvated as [Cu(H 2 O) 6 ] 2+ and [Cu(OH)(H 2 O) 5 ] + complexes in hydrated ambient samples [20,34,35].…”

A Consistent Reaction Scheme for the Selective Catalytic Reduction of Nitrogen Oxides with Ammonia

“…These two locations for the Cu ions are well known from structural analysis of Cu-CHA catalysts, although some controversy exists on which form actually constitutes the active site. 6,12,28,34,[39][40][41]44,52,72,73 However, when adsorbates are present, the Cu ion is lifted out of the 6-ring plane and the preference for the 6-ring location diminishes. 42,43,74 This is also observed for the formation of Cu 2+ −NO 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 (step 1), where the difference in energy for the Cu located in the 8-ring is only 0.08 eV lower compared to the Cu in the 6-ring, showing that there is no real preference for location of the Cu in the 6-ring or 8-ring in this case.…”

“…Recently, it has been shown that the Cu ions in a Cu-CHA zeolite are located in both the 6-and 8-rings of the chabazite framework structure. 28,29,34,41 Upon adsorption of NH 3 , NO or H 2 O on the Cu atoms in the 6-rings, the Cu atoms are lifted out from their original position into the larger cavities in the zeolite, [42][43][44] and therefore it seems that the SCR reaction actually takes place in the large cavities in Cu-CHA. However, only the Cu ions located in or close to the 6-rings seem to contribute to the SCR activity in Cu-CHA.…”

HERFD-XANES and XES as complementaryoperandotools for monitoring the structure of Cu-based zeolite catalysts during NOx-removal by ammonia SCR

“…For Cu-SSZ-13, there are several XAS and XRD studies combined with computational modelling that suggest Cu in positions close to six or eight membered rings in the zeolite pores [14,15]. Under reaction conditions though, reactive gases are expected to adsorb at the Cu sites and therefore change coordination number, geometry and oxidation state of these sites.…”