Effect of Sulfur Poisoning on Various Configurations of Dimeric Cu Species for Cu-SSZ13.

Metal-exchange zeolites are microporous materials made up of alumina silicates possessing a charge deficiency compensated by metal cations, and they have been widely used in industrial applications such as catalysis and separation. To determine the optimum synthesis and operating conditions for these materials, a molecular-level understanding of their response to reaction conditions and external gas is necessary. We are interested in Cu-exchanged SSZ13 zeolites, a chabazite-type structure and is consisted of 4-,6-, and 8-membered rings, which are commercially used in diesel engines for selective catalytic reduction (SCR) of NOx due to their good catalytic performance and hydrothermal stability. This SCR reaction selectively reduces NOx into N2 and water using a reductant solution, typically ammonia or urea. A challenge to the catalytic performance of this zeolite is catalyst poisoning due to the presence of SO2 and SO3 in the diesel exhaust. In Cu-exchanged SSZ-13 zeolites, typical types of cationic species that can form are monomeric and dimeric Cu species. While the effect of sulfur poisoning on monoatomic active sites such as Cu2+ and [CuOH]+ in Cu/SSZ13 have been studied previously, its impact on Cu dimers ([Cu2O]2+ and [Cu2O2]2+) have not been extensively studied. The presence of Cu dimers in zeolite can be controlled by synthesis protocols; however, dimeric Cu sites and protonated forms ([Cu2OxHy]2+, x,y=1-2) can also form when solvated [CuOH]+ species are positioned close to each other at Al exchange sites. Here, we studied the thermodynamics of the SOz (z=2,3) reaction with dimeric Cu while varying plausible Al-Al pair configurations and reaction conditions through molecular dynamics simulations and density functional theory calculations. We found that Cu dimers in Al-Al pairs in the same 8-member rings have a lower temperature (600 – 700 K) for sulfur dissociation than dimers in bridged Al-Al pairs, which are configurations with Al positioned at different membered rings (760 – 870 K). We also observed that SO2/SO3 adsorption for dimeric Cu species was more exothermic than monomeric Cu species, which is reflected in higher temperatures for sulfur dissociation in Cu dimers. This suggests dimeric Cu species are more vulnerable to sulfur attack, consistent with some recently reported experimental studies.