Ab initio X-ray absorption modeling of Cu-SAPO-34: Characterization of Cu exchange sites under different conditions

• The 6MR is the most energetically favorable site for a bare Cu+ ion in Cu-SAPO-34.
• The presence of adsorbates decreases the energetic preference of Cu ions in a 6MR.
• The calculated XANES capture the trends of the edge shift with oxidation state.
• For Cu in Cu-SSZ-13 and Cu-SAPO-34, the XANES is independent of zeolite type.

Copper-exchanged SAPO-34 (Cu-SAPO-34) provides excellent catalytic activity and hydrothermal stability in the selective catalytic reduction (SCR) of NOx by using NH3 as a reductant. We find that the 6-membered ring (6MR) site is the most energetically favorable for a Cu+ ion while the 8-membered ring (8MR) sites are less favorable by about 0.5 eV with respect to the 6MR site in Cu-SAPO-34. Upon adsorption of molecular species (H2O, O, OH, O2), the energy differences between Cu in the 8MR and 6MR sites decreases and almost disappears. Further, a thermodynamic phase diagram study shows that a Cu+ ion bound to a single H2O molecule is the most stable species at low oxygen potential values while a Cu2+ ion bound to 2 OH species is more stable when the oxygen chemical potential is sufficiently high. By comparing Cu K-edge XANES between Cu-SSZ-13 and Cu-SAPO-34 with Cu in different oxidation states, we conclude that it is difficult to differentiate the simulated XANES of Cu in these structures at a given oxidation state. By studying the Cu K-edge XANES of several favorable structures in Cu-SAPO-34 in the presence of adspecies, the simulated XANES results capture the real trend of the edge shift with oxidation state and gives new insights into the experimentally determined XANES of Cu-SAPO-34 obtained under standard SCR conditions.

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