(81d) Influence of Zeolite Framework on Cu Ion Mobility and the Kinetics of Selective Catalytic Reduction of Nitrogen Oxides with Ammonia
Isolated copper ions exchanged onto zeolites catalyze the selective reduction (SCR) of nitrogen oxides (NOx) with ammonia in the presence of dioxygen. At low temperatures (<523 K), ammonia fully coordinates with copper ions and solvates them away from the zeolite lattice, forming most abundant surface intermediates that are cationic copper-amine complexes bonded ionically to anionic aluminum centers in zeolite frameworks. Ionic tethering of mononuclear metal ion complexes provides a mechanism for the dynamic and reversible formation of binuclear metal complexes to activate dioxygen during oxidation half-cycles. Studying this phenomenon on different zeolite frameworks enables understanding the influence of pore connectivity on Cu ion mobility and low temperature NOx SCR kinetics. We use a combined experimental and computational approach focused on in operando interrogation of the catalyst under widely varying operating conditions, well beyond those typical of commercial operation. The effects of Cu ion mobility become preeminent for low temperature NOx SCR catalytic reactivity, and result in differences in kinetic behavior among Cu-zeolites of different topology.