Revisiting Effects of Alkali and Alkaline Earth Co-cation Additives to Cu/SSZ-13 Standard Selective Catalytic Reduction Catalysts

Feng  Gao, PNNL

Cu/SSZ-13 selective catalytic reduction (SCR) catalysts have been widely studied for NO_x abatement in lean-burn engine exhausts, due to their superior activity, selectivity and long-term stabilities. It has been shown that the active sites in this catalyst are isolated Cu ions (Cu²⁺ and [Cu(OH)]⁺) in extra-framework exchange positions. In attempting to optimize Cu content of this catalyst, a dilemma has been recognized for some time: high Cu loading is beneficial to low-temperature NO_x conversion, but detrimental to catalyst hydrothermal stability. This dilemma stems from high SCR activity, but low hydrothermal stability of the [Cu(OH)]⁺ sites, which tend to convert to CuO_x clusters that destabilize the Cu/SSZ-13 catalysts [1, 2]. On the other hand, a low Cu-loaded catalyst is vulnerable to excessive dealumination, which is also detrimental to long-term stability. We have shown previously that for low Cu-loaded Cu/SSZ-13, alkali cocation addition was able to increase both low temperature activity and hydrothermal stability of the catalysts [3]. However, the mechanism is still not well understood, and catalyst composition optimization has not been attempted.

To obtain more insights into the cocation effects on Cu/SSZ-13 catalysts, a series of Cu/SSZ-13 samples with two Si/Al ratios (6 and 9) and various Cu loadings were prepared. Various amounts of Na⁺, K⁺ and Ca²⁺ cocations were added to probe their effects on low-temperature NO_x conversion and catalyst hydrothermal stability. Combined SCR reaction testing, and characterizations with electron paramagnetic resonance (EPR), H₂ temperature-programmed reduction (H₂-TPR) and ammonia temperature-programmed desorption (NH₃-TPD) demonstrate complex cocation effects as follows: (1) at low to intermediate Cu loadings, Na⁺ and K⁺ cocations show beneficial effects at low loadings in terms of catalyst activity and stability enhancement. However at high loadings these cocations are detrimental; (2) at high Cu loadings, Na⁺ and K⁺ cocations are detrimental; (3) Ca²⁺ cocations do not show beneficial effects at any loading. From isolated Cu ion quantification with EPR and from DFT simulations, Na⁺ and K⁺ cocations do not compete with Cu²⁺ for 6-membered ring cationic positions, but do promote [Cu(OH)]⁺ agglomeration, consistent with their complex loading dependent effects. In contrast, Ca²⁺ cocations compete favorably with Cu²⁺ for the most stable cationic sites and are thus detrimental to catalyst stability at all loadings. In summary, we demonstrate here that alkali cocation addition is indeed a feasible method to enhance activity, selectivity and durability of Cu/SSZ-13 SCR catalysts with compositions similar to commercial catalysts.

References:

[1] J. Song, Y.L. Wang, E.D. Walter, N.M. Washton, D.H. Mei, L. Kovarik, M.H. Engelhard, S. Prodinger, Y. Wang, C.H.F. Peden, F. Gao, Toward Rational Design of Cu/SSZ-13 Selective Catalytic Reduction Catalysts: Implications from Atomic-Level Understanding of Hydrothermal Stability, ACS Catal, 7 (2017) 8214-8227.

[2] F. Gao, J. Szanyi, On the hydrothermal stability of Cu/SSZ-13 SCR catalysts, Applied Catalysis A, General 560 (2018) 185–194.

[3] F. Gao, Y.L. Wang, N.M. Washton, M. Kollar, J. Szanyi, C.H.F. Peden, Effects of Alkali and Alkaline Earth Cocations on the Activity and Hydrothermal Stability of Cu/SSZ-13 NH₃-SCR Catalysts, ACS Catal, 5 (2015) 6780-6791.

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