ReaxFF reactive molecular dynamics simulation of the hydration of Cu-SSZ-13 zeolite and the formation of Cu dimers

A new Cu/Si/Al/O/H reactive ReaxFF force field was developed and used in molecular dynamics simulations of the hydration of Cu-exchanged SSZ-13 catalyst. It was observed that at temperatures close to room temperarture, all Cu cations, including those at the faces of the double 6-member rings (6MRs), become fully hydrated and detach from the framework. The hydrated Cu cations can diffuse through the Cu-SSZ-13 zeolite pore windows. At higher temperatures, the formation of cationic OH-bridged Cu dimers Cu₂OH and Cu₂(OH)₂ within the cages was observed. The simulations show that the temperature at which these species are formed is lower for zeolites that stabilize [CuOH]⁺ ions, as would be expected for relatively high-Cu- and low-Al-content zeolites. The stable dimers bind preferentially to 8MRs beside the large cages in a configuration that blocks the openings of the pores. The consequences of the present results for selective catalytic reduction and NO oxidation catalysis in Cu-SSZ-13 are discussed.