Combining Kinetics and <i>Operando</i> Spectroscopy to Interrogate the Mechanism and Active Site Requirements of NO_<i>x</i> Selective Catalytic Reduction with NH₃ on Cu-Zeolites

Abstract

NO_<i>x</i> selective catalytic reduction (SCR) with NH₃ on Cu-zeolites is a commercial emissions control technology for diesel and lean-burn engines. Mitigating low-temperature emissions remains an outstanding challenge, motivating an improved understanding of the reaction mechanism, active site requirements, and rate-determining processes at low temperatures (<523 K). In this Perspective, we discuss how <i>operando</i> spectroscopy provides crucial information about how the structures, coordination environments, and oxidation states of Cu active sites depend on reaction conditions and sample composition; when combined with kinetic measurements, such <i>operando</i> data provide insights into the Cu site and spatial density requirements for reduction and oxidation steps relevant to the Cu(II)/Cu(I) SCR redox cycle. Isolated Cu ions coordinated to zeolite oxygen atoms <i>ex situ</i> become coordinated to NH₃ <i>in situ</i> and dynamically interconvert between mononuclear and binuclear NH₃-solvated Cu complexes to catalyze SCR turnovers. We conclude with future research directions that can benefit from combining quantitative kinetic measurements with <i>operando</i> spectroscopy.