(7b) Effect of Reaction Conditions and SO2 Exposure on Cu Speciation in SSZ-13 Zeolites

A molecular level understanding of the active sites in metal-exchanged zeolites, and their reaction condition-dependent speciation is critical for fine-tuning operating conditions and synthesis techniques for prolonging catalyst longevity and regenerability. This is especially significant for Cu-exchanged SSZ-13 zeolites (Cu/SSZ-13) used for selective catalytic reduction (SCR) of NO in diesel engines where SO₂ and SO₃ readily poison these materials, deteriorating their catalytic performance. While the effect of SO₂ on the predominant monatomic active sites in Cu/SSZ13, Cu²⁺ and [CuOH]⁺ has been explored, SO₂ poisoning on Cu dimers ([Cu₂O]²⁺ and [Cu₂O₂]²⁺) is unknown. While synthesis protocols dictate the extent of Cu dimers initially present in the zeolite, solvated [CuOH]⁺ at proximal Al exchange sites form Cu dimers and their protonated forms ([Cu₂O_xH_y]²⁺, x,y=1-2) on dehydration. Here, we employed computational tools (AIMD simulations, density functional theory calculations) to develop thermodynamic models for the reaction of SOx with dimeric Cu motifs as a function of Al-Al proximity in SSZ13, and the reaction environment. Our results demonstrate that unlike Cu²⁺ and [CuOH]⁺, dimeric Cu species at the identified Al-Al configurations exhibit strong binding to SO_x at a wide range of conditions. We observed desulfation at high temperatures (> 900 K), with variations seen across the Al-Al configurations. The most stable sulfated species for Cu dimers exchanged at Al pairs in the same eight-membered ring was found to be [Cu₂SO₄]⁺ while formation of [Cu₂HSO₄]⁺ was favorable at the oxygen-bridged sites. Moreover, interconversion of solvated ZCuOH to mono- and multinuclear Cu species was also strongly dependent on the proximity of Al-Al pairs and reaction conditions. Our thermodynamic phase diagrams indicate that at low temperatures, the presence of predominantly monatomic Cu inhibits SO_x poisoning; with an increase in temperature, formation of Cu dimers becomes thermodynamically favorable rendering SSZ-13 zeolites more susceptible to SO_x poisoning.