Modeling Ammonia and Water Co-Adsorption in Cu^I-SSZ-13 Zeolite Using DFT Calculations

Abstract

Cu-SSZ-13 efficiently catalyzes the selective catalytic reduction (SCR) of NO by NH 3 but the structure of the active site and, particularly, the redox state of the copper (+I or +II) are still debated. This paper focuses on the possible contribution of Cu I species with a theoretical investigation of adsorption and co-adsorption of NH 3 and H 2 O on Cu I species using quantum chemistry and including dispersion forces. The calculations show that Cu I clearly migrates upon adsorption of NH 3. While Cu I is initially in close interaction with the zeolite framework, it preferentially forms coordination bonds with NH 3 and interacts with the zeolite in its second coordination sphere. The same tendency is calculated with H 2 O, even if the binding energy of H 2 O with Cu I is lower than with NH 3. All the Cu I complexes sit in the cage containing the 8 MR. The confined Cu I complexes interact with the zeolite framework through several H-bonds donated by the NH or OH bonds of the ligands. In the experimental temperature and pressure domain of SCR conditions, calculated phase diagrams show that coordination number of two is predicted for the co-adsorption of NH 3 and H 2 O on Cu I. For pure H 2 O, few stable domains for hydrated species containing Cu I are calculated in contrast with pure NH 3. Finally, the calculated phase diagrams on Cu I-SSZ-13 are discussed together with the more documented diagrams of Cu II-SSZ-13 and recent experimental characterizations, providing a wider picture of the real catalyst in SCR conditions. ORCID D.

UV–Vis and Photoluminescence Spectroscopy to Understand the Coordination of Cu Cations in the Zeolite SSZ-13

“…38,39 ■ PHASE DIAGRAMS Recent work indicates that the oxidation state of Cu will depend on the exact environment, that is, the presence of different gases such as H 2 O and O 2 and the temperature. 18,19,22 To calculate the phase diagram, we include Cu sites as extra framework (Cu-EF) atoms close to one Al atom and bound to the defect site with up to six adsorbed H 2 O molecules located in the periodic zeolite framework. While Cu in the Cu-EF-6R and Cu-EF-8R is exchanged for H + and binds to activated O atoms next to Al, we exchange one to three H atoms from the single silanol (Cu-d3H−Cu-d1H) defect and one H atom from the double silanol defect (Cu-d5H) for Cu to allow for a direct bond between O and Cu.…”

“…32−35 These either focused on DFT-based calculations, which in principle are not able to describe excited states correctly, 32,34 or used a small cluster model, 33 which neglects the complexity of the zeolite framework. Only in recent work, the rapid coordination changes of Cu cations in zeolites 11,17 have been included, 35 and so far coordination changes in the presence of water [18][19][20]22 have been neglected in spectroscopic assignments. Furthermore, optical spectra have never been correlated to theoretically calculated phase diagrams.…”

“…Initially, simple thermodynamic arguments based on static DFT calculations were used to identify the most stable Cu sites, ,, but soon it became clear that under ambient conditions, several other factors influence site speciation. On the one hand, a combination of molecular dynamics (MD) simulations and experimental results revealed that when Cu is directly bound to the zeolite framework, it shows a significant mobility and shifts between different local minima. ,, On the other hand, it was shown that molecules such as H 2 O and NH 3 bind very strongly to the Cu monomers. ,− This leads to the formation of mobile Cu-complexes inside the zeolite pore, which are only loosely bound to the framework . However, at increased temperatures or at lower gas concentrations, the coordination to the adsorbents is lost, and Cu moves back into its extra framework position. ,, …”

“…Optical spectroscopy is a method, which, after the pioneering work of Klier et al in the late 1970s and 1980s, has been used to characterize copper sites in different zeolite structures. , Today, optical spectroscopy is a routine characterization technique, − but few theoretical studies have been performed in this area to date. − These either focused on DFT-based calculations, which in principle are not able to describe excited states correctly, , or used a small cluster model, which neglects the complexity of the zeolite framework. Only in recent work, the rapid coordination changes of Cu cations in zeolites , have been included, and so far coordination changes in the presence of water − , have been neglected in spectroscopic assignments. Furthermore, optical spectra have never been correlated to theoretically calculated phase diagrams.…”

Thermodynamics Perspective on the Stepwise Conversion of Methane to Methanol over Cu-Exchanged SSZ-13

“…First-principles-based modeling can support and extend these experimental assignments by systematically considering an extensive set of possible active site structures and by calculating their relative thermodynamic stability. , This information can then be summarized in phase diagrams, which reveal the most stable phase under various experimental conditions. − Combining the information from phase diagrams with knowledge of the anchoring point distribution enables derivation of the site distribution present in the zeolite material under specific experimental conditions. , In the conversion of methane to methanol using Cu-exchanged zeolites, attempts so far have aimed at describing the relative stability of different Cu sites in mordenite, ZSM-5, or SSZ-13. , These studies, however, considered only few possible active sites, which does not describe the complex distribution of sites expected to be present in such a zeolite system.…”