Comparing Copper-Exchanged SSZ-13 and SAPO-34 in Selective Catalytic Reduction

Copper-exchanged zeolites are vital in the selective catalytic reduction (SCR) of nitrogen oxides with ammonia, acting as catalysts in catalytic converters of diesel engines. Cu-SSZ-13 catalysts have performed successfully over the years, while Cu-SAPO-34 catalysts have failed commercially despite exhibiting higher stability during hydrothermal aging, an attribute that tends to be indicative of a stable catalyst. The purpose of our research is to compare Cu-SAPO-34 and Cu-SSZ-13 to understand their different behavior, specifically studying the conversion between ZCuOH and Z2Cu during SCR using computational methods. Cells with 36 T-sites were created for each structure of interest with ammonia-solvated copper complexes within the zeolite cage. SAPO-34 has an identical structure to SSZ-13, but with alternating phosphorus and aluminum atoms in place of silicon. To compensate for the positive charge of the Cu2+ ion, SSZ-13 has two aluminum substitutions for silicon and SAPO-34 has two silicon substitutions for phosphorus. Using density functional theory (DFT) calculations, we computed the energy of the conversion between ZCuOH and Z2Cu. We looked at different aluminum configurations in SSZ-13 and the analogous silicon configurations in SAPO-34 to study the effect of Al-Al or Si-Si distance on the reaction energy. To get sufficient sampling of each structure, we first ran ab initio molecular dynamics simulations at 573K on each structure for a minimum of 10 picoseconds and selected several lowest energy structures to optimize with DFT. Computing the reaction energy using the lowest energy structures, we found that for aluminum or silicon configurations within the same six-membered ring of the zeolite, the reaction is essentially isoenergetic. That is, the reaction energy is approximately zero for both Cu-SAPO-34 and Cu-SSZ-13. As Al-Al or Si-Si distance increases, the reaction becomes more exothermic for both zeolites. To verify these results, we are using a different atomistic simulation technique, metadynamics, to sample the free energy surface of these structures more thoroughly.