(36e) Revisiting Effects of Alkali and Alkaline Earth Co-Cation Additives to Cu/SSZ-13 Standard Selective Catalytic Reduction Catalysts

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

Yanran Cui, Yilin Wang, Eric D. Walter, Donghai
Mei, János Szanyi, Yong Wang, Feng Gao* 

Institute
for Integrated Catalysis, Pacific Northwest
National Laboratory

Richland, WA 99352, USA

*Corresponding author: feng.gao@pnnl.gov

Abstract

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]. 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 [2]. 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, 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.