The Molecular Nature of the Cu Active Sites Under NOx Selective Catalytic Reduction

The Molecular Nature of the Cu Active Sites Under NO_x Selective Catalytic Reduction

The selective catalytic reduction (SCR) of NO_x in excess oxygen is a key challenge in meeting increasingly stringent emission regulations. The metal-exchanged zeolite Cu-SSZ-13 in particular exhibits sufficient activity and hydrothermal stability for practical applications.  A primary obstacle for further improvement of SCR catalysts is a lack of molecular-level understanding of the active sites and mechanism.  Previous work [1] identified isolated Cu ions in the six-membered ring of Cu-SSZ-13 as an active site for the standard NH₃-SCR reaction:

4 NH₃ + 4 NO + O₂  →4 N₂ + 6 H₂O

Here, using density functional theory (DFT) calculations, ab-initio molecular dynamics (AIMD), and ab-initio thermodynamic modeling we:

1. Identify two forms of single Cu species, [Cu]²⁺ and [CuOH]⁺ as the precursors to the active Cu sites under SCR.  We show there is a strong thermodynamic driving force to preferentially form [Cu]²⁺ over [CuOH]⁺  and predict the resulting Cu speciation between the two as a function of the zeolite's Silicon to Aluminum (Si:Al) and Copper to Aluminum (Cu:Al) composition.

2. Predict the response and resulting molecular structures of these different species to non-catalytic treatments including exposure to atmosphere at 298 K and oxidizing/reducing environments at 673 K.  We validate these molecular level predictions with X-Ray Absorption Spectroscopy (XAS) experiments.
3. Identify the coordination these Cu species take under SCR and compare with operando XAS.  We then produce, and evaluate using DFT, a mechanism consistent with the previous results for SCR at 473 K on both sites.

We show that SCR occurs through a redox mechanism at all isolated Cu sites regardless of their speciation, and identify key reaction steps in the reduction and oxidation half cycles [2].  We then extend these ideas to other zeolite supports.

References

1.     Bates, S.A., Verma, A.A., Paolucci, C., Parekh, A.A., Anggara, T., Yezerets, A., Schneider, W.F., Miller, J.T., Delgass, W.N., and Ribeiro, F.H., J. Catal. 312, 87 (2014)

2.     Paolucci, C., Verma, A.A., Bates, S.A., Kispersky, V.F., Miller, J.T., Gounder, R., Delgass, W.N., Ribeiro, F.H., and Schneider, W.F., Angew. Chem. Int. Ed. 53, 44 (2014)