(465c) Sulfur Deactivation Pathways in Cu-SSZ-13 Determined through First-Principle Modeling and X-Ray Spectroscopy

SSZ-13 zeolites with exchanged copper ions are used commercially in diesel exhaust treatment for the selective catalytic reduction (SCR) of nitrogen oxides into nitrogen and water because of their high activity and stability [1]. Exposure to sulfur oxides, which are produced from the combustion of sulfur in diesel fuel, causes a decrease in catalytic activity. The sulfur-containing species will accumulate in the catalyst in the long term and cause worse deactivation over time. Understanding of the poisoning intermediates and reaction pathway at the molecular level is insufficient due to the lack of theoretical studies. In this work we use first-principle density functional theory (DFT) calculations and ab initio molecular dynamics (AIMD) simulations to identify poisoning intermediates and examine molecular configuration at finite temperatures. These simulations provide a molecular interpretation of ex situ and in situ X-ray adsorption spectroscopy (XAS) experiments performed on poisoned Cu-SSZ-13 samples.

The interaction between Cu active sites and sulfur-containing species with variable O/H/N stoichiometry (SO_xH_yN_z) was explored by thermodynamic phase diagrams. We used AIMD simulation to explore molecular configurations of the adsorbate and picked lowest energy snapshots for structure optimizations. The potential of mean force method was used to calculate adsorption entropies of SO_xH_yN_z on Cu sites. The phase diagrams show that ammonium sulfate and ammonium bisulfate are thermodynamically favorable below 600 K for a range of experimental conditions. Two Cu-SSZ-13 samples, containing either Z₂Cu or ZCuOH sites, were synthesized in lab [2] and exposed to dry SO₂ and O₂ at variable temperatures to simulate accumulated sulfur poisoning in actual catalytic converters. We performed ex situ and in situ X-ray spectroscopy on the sulfated samples and analyzed the near edge structure (XANES) region of the spectra. Oxidation states elucidated from both Cu K-edge XANES and S K-edge XANES were consistent with the conclusion from first-principle studies. The difference in poisoning species on the two types of Cu sites suggested the possibility of designing a more sulfur-resistant catalyst.

[1] Kumar, A., Smith, M., Kamasamudram, K., Currier, N.W., An, H., Yezerets, A.. Catal. Today. 231, 75 (2014).

[2] Paolucci, C., Parekh, A.A., Khurana, I., Di Iorio, J.R., Li, H., Albarracin Caballero, J.D., Shih, A.J., Anggara, T., Delgass, W.N., Miller, J.T., Ribeiro, F.H., J. Am. Chem. Soc. 138, 18 (2016).