Cleaner car exhausts using ion-exchanged zeolites: Insights from atomistic simulations
Misbah Sarwar, Johnson Matthey Technology Centre, Sonning Common, Reading, UK
Zeolites are widely used as catalysts in a variety of reactions including the selective catalytic reduction of nitrous oxides which are a major component of diesel vehicle emissions. A number of studies recently have reported the high activity, stability and selectivity of Cu-SSZ-13  which has the CHA framework topology. However, the nature of the active site and its location within the framework is subject to much debate in the literature [2,3,4]. In addition, diffusion is a key step in the catalytic cycle within zeolites, as reactants must diffuse to the active site and the products formed diffuse out. An understanding of the nature and location of the active site within the framework, along with its accessibility will help guide the design of future catalysts with improved performance and efficiency.
In this talk an overview of a combined theoretical and experimental approach to determine the ion location and accessibility of adsorbates such as NH3 to the active site, will be presented. QM/MM calculations are used to assess the stability of the ion at different sites within the framework. The interaction of NO with the ion at each site is also modelled using the QM/MM approach to study how NO interacts with the ion and changes in ion geometry as a result. Vibrational frequencies are computed and compare well to values measured by DRIFTS.
Following on from this the accessibility of the active site is assessed using the diffusion of NH3 in the CHA framework as a test case. Molecular Dynamics (MD) simulations have been widely used to study the diffusion of molecules within zeolites. There are a number of complimentary experimental techniques, such as Pulse Field Gradient (PFG)-NMR and Quasi Elastic Neutron Scattering (QENS) that can probe the same length and time-scales as MD simulations and therefore provide a direct comparison. Using both QENS and MD simulations, we investigate the effect of the Cu2+ counter-ion on the diffusion of NH3 by comparing to the brønsted acid form of the zeolite.