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

Authors: 
Gao, F., Pacific Northwest National Laboratory
Cui, Y., Pacific Northwest National Laboratory
Wang, Y., Pacific Northwest National Laboratory
Walter, E. D., Pacific Northwest National Laboratory
Mei, D., Pacific Northwest National Laboratory
Szanyi, J., Pacific Northwest National Laboratory
Wang, Y., Washington State University

Meng Gao, Feng 2 1 2019-03-12T22:20:00Z 2019-03-12T22:20:00Z 1 550 3136 PNNL 26 7 3679 15.00

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

mso-char-indent-count:1.0">

mso-char-indent-count:1.0">Yanran Cui, Yilin Wang, Eric D. Walter, Donghai
Mei, János Szanyi, Yong Wang, Feng Gao* 

0in;margin-left:0in;margin-bottom:.0001pt"> font-family:" times new roman minor-fareast>Institute
for Integrated Catalysis, " times new roman>Pacific Northwest
National Laboratory

mso-char-indent-count:1.0">Richland, WA 99352, USA

mso-char-indent-count:1.0">

*Corresponding author: " times new roman> feng.gao@pnnl.gov

normal">Abstract

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]. 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 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.  

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.

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