PNNL

Abstract

Commercial Cu/SAPO-34 selective catalytic reduction (SCR) catalysts have experienced unexpected and quite perplexing failure. Understanding the causes at an atomic level is vital for the synthesis of more robust Cu/SAPO-34 catalysts. Here we show, via application of model Cu/SAPO-34 catalysts with homogeneously dispersed, isolated Cu ions, that Cu transformations resulting from low- and high-temperature hydrothermal aging, and ambient temperature storage can be semi-quantitatively probed with 2-dimensional pulsed electron paramagnetic resonance (EPR). Coupled with SCR kinetics, additional material characterizations and DFT simulations, we propose the following catalyst deactivation steps: (1) detachment of Cu(II) ions from catalytically active cationic positions, likely in the form of Cu(OH)2; (2) irreversible hydrolysis of the SAPO-34 framework that forms terminal Al species; and (3) interaction between Cu(OH)2 and terminal Al species to form SCR inactive, Cu-aluminate like species. Especially significant is that these reactions are shown to be greatly facilitated by condensed water molecules under wet ambient conditions, causing the ‘mysterious’ low temperature failure of the commercial Cu/SAPO-34 catalysts. Furthermore, our discoveries also explain why the typical high-temperature hydrothermal aging conditions (e.g., 800 ?C in the presence of 10% water vapor) practiced by industries cannot be used to access these deactivation processes because of the lack of sufficient concentrations of adsorbed water within the zeolite under these conditions. The authors gratefully acknowledge the US Department of Energy (DOE), Energy Efficiency and Renewable Energy, Vehicle Technologies Office for the support of this work. The research described in this paper was performed in the Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by the DOE’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL). PNNL is operated for the US DOE by Battelle.