Abstract:

The selective catalytic reduction (SCR) of nitrogen oxides (NOx) with ammonia using Cu-exchanged zeolites is a pollution abatement technology used commercially in diesel emissions control. At low temperatures (<523 K), Cu ion active sites become solvated by ammonia reactants to form homogeneous-like copper coordination complexes that are bonded ionically to anionic aluminum centers in zeolite lattices. The ionic tethering of metal active sites to the zeolite host support confers localized mobility, providing a mechanism for the dynamic and reversible interconversion of mononuclear and binuclear sites, merging attractive features of both heterogeneous and homogeneous catalysts. We combine experimental and computational approaches to interrogate catalysts in operando under widely varying operating conditions, including beyond those typical of commercial operation. We show that the effects of Cu ion mobility are preeminent for low-temperature NOx SCR reactivity and selectivity, leading to dramatic differences in performance among Cu-zeolites of different bulk and atomic structure.