Mechanistic insights into the cobalt promotion on low-temperature NH3-SCR reactivity of Cu/SSZ-13

Abstract

The ever-increasing demand for nitrogen oxides (NOx) emission control from diesel engines during cold start period has provided a strong impetus to improve the low-temperature NOx removal efficiency of Cu/SSZ-13 catalyst. In this work, a series of bimetallic catalysts CuCo/SSZ-13 with varying cobalt contents were prepared, and attained the significant performance enhancement in NH3 selective catalytic reduction (NH3-SCR) reaction. The promotion mechanism of these catalysts at low temperatures (< 200 °C) was proposed: (1) The introduced Co(II) ions could tune the distributions of copper species and acid sites, resulting in a higher ratio of ZCuOH/Z2Cu (Z represents a negatively charged site on zeolite framework) and much more Lewis acid sites compared with original Cu/SSZ-13. (2) The fast reaction of reactive bridging nitrates with NH3 adsorbed on Lewis acid sites via Langmuir-Hinshelwood (L-H) mechanism decreased the activation energy. However, the addition of high cobalt content facilitated the formation of inactive CuOx species, which was not conducive to maintaining high-temperature (> 500 °C) activity.