Revisiting effects of alkali metal and alkaline earth co-cation additives to Cu/SSZ-13 selective catalytic reduction catalysts

Abstract

Cu,M/SSZ-13 (M = Na+, K+ and Ca2+) selective catalytic reduction (SCR) catalysts with Si/Al = 6 and M/Cu ratios varying from 0.1 to 1.0 are prepared by solution ion exchange, followed by slurry drying and hydrothermal aging at 800 °C. The catalysts are characterized with X-ray diffraction and N2 adsorption isotherms to probe their textural properties; 27Al and 29Si nuclear magnetic resonance and NH3 temperature-programmed desorption to probe their acidity properties and levels of dealumination; and electron paramagnetic resonance and H2 temperature-programmed reduction to quantitatively explore the nature of Cu moieties in the catalysts. These studies are followed by density functional theory calculations to elucidate Cu and co-cation interactions at an atomic level and SCR reaction tests to reveal correlations between the chemical and physical properties of the catalysts and their SCR performance. Through these comprehensive investigations, it is discovered that alkali metal co-cation addition can be used for the synthesis of highly stable, and highly active and selective Cu/SSZ-13 catalysts with relatively high Al content. In catalysts with optimal alkali incorporation, dealumination of the SSZ-13 substrate is largely inhibited, allowing high concentrations of SCR-active isolated Cu ions. At the same time, repulsive interactions between Cu ions and alkali metal co-cations preclude excessively high isolated Cu-ion loadings. Interplay between and optimization of these two factors is considered the underlying origin for this successful catalyst synthesis strategy. On the other hand, the alkaline earth co-cation Ca2+ fails to demonstrate any beneficial effects, since it destabilizes isolated Cu ions via site competition.