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 NOx 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 NOx 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 CuOx 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 . 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 NOx 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.
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