(674f) Experimental and Computational Interrogation of Fast SCR Mechanism and Active Sites on H-Form SSZ-13

The performance of cation-exchanged zeolites for the selective catalytic reduction (SCR) of NOx is sensitive to the identity of the cations as well as the composition of gaseous feed. While the redox function of a metal-exchanged zeolites (most notably Cu-SSZ-13) is necessary for so-called â??standardâ? SCR (4 NO + 4 NH3 + O2 â?? 2 N2 + 6 H2O) activity, proton-exchanged zeolites, like H-SSZ-13, are comparable in performance to their metal-exchanged cousins for â??fastâ? SCR (2 NO + 2 NO2 + 4 NH3 â?? 2 N2 + 6 H2O) [1]. Further, residual exchanged protons (Brønsted acid sites) are present in all metal-exchanged zeolites, and these acid sites may have a role even in standard SCR [2]. For these reasons, it is important to understand the nature and the reactivity of exchanged proton under SCR conditions.

Here we report the results of an integrated density functional theory (DFT) and kinetics analysis of H-SSZ-13 under fast SCR conditions. Rates, rate orders, and apparent activation energies collected under differential conditions reveal distinct kinetic regimes suggestive of different intermediates and/or coverages. First-principles thermodynamics models based on ab initio dynamics distinguish correspondingly different NH3 coverage regimes, and in particular identify NH3 physisorbed to NH4+ as a key low-temperature intermediate. Further, we use non-equilibrium dynamics methods to determine activation energies for rate-limiting steps in the catalysis. Based on these results, we propose a kinetic model that explains the available experimental data, including in particular the changes across temperature regimes. The results highlight the importance of considering realistic reaction conditions in both measuring and modeling catalytic function.

Reference:

[1] Beale, A.M., Gao, Feng., Lezcano-Gonzalez, I., Peden, C.H.F., Szanyi, J., Chem. Soc. Rev., 2015,44, 7371-7405

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