The selective catalytic reduction (SCR) of NOx in excess oxygen is a key challenge in meeting increasingly stringent emissions regulations. The current catalysts are small pore metal-exchanged zeolites, which exhibit high activity and stability over the large temperature window needed for mobile applications. Cu-SSZ-13 is one such catalyst, and previous studies have identified the reaction as redox and Cu as the active species in Standard NH3 SCR: 4 NH3 + 4 NO + O2 -&gt; 4 N2 + 6 H2O A primary obstacle for further improvement is a lack of molecular-level understanding of how the active site manifests under reaction conditions and its dynamics. This information is critical for formulating the elementary steps involved in the catalytic cycle. Here we use density functional theory (DFT) and ab-initio molecular dynamics (AIMD) to: (1) Construct a DFT based thermodynamic phase diagram (see figure) that allows us to predict the relative free energies of formation for species with variable HxNyOz stoichiometry on Cu-SSZ-13 as a function of the local Al distribution, external gas pressures, and temperature. (2) Use AIMD to simulate the dynamics of the CuI/II species predicted in (1) under low (200 C) and high (400 C) temperature SCR conditions. This data is compared with X-Ray absorption spectroscopy on Cu-SSZ-13 under similar conditions. (3) Metadynamics AIMD simulations are used to construct temperature dependent free energies of activation for the reduction and oxidation steps of the SCR cycle on the species arrived at in (2). We show that both temperature and external gas conditions play a critical role in Cu mobility and the elementary reaction steps of Std. SCR.

Cu coordination and dynamics under reaction conditions for NOx selective catalytic reduction

251st National Meeting (2016)

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