Selective catalytic reduction of NOx with NH3 over Ce-Mn oxide and Cu-SSZ-13 composite catalysts-Low temperature enhancement

A composite catalyst comprised of ceria-manganese oxide and Cu-SSZ-13 components represents a potential solution for improving selective catalytic reduction (SCR) of NOx by ammonia. Here, clarity is provided to the pathway of promotion on these catalysts at low temperatures (< 200 degrees C), and it is shown that improved performance comes solely from interaction of the oxide phase with Bronsted acid bound NH3. We demonstrate that at low temperatures, the catalytic components interact via oxide-derived nitrites/nitrite-precursors (e.g., HONO, N2O3) reacting with Bronsted acid bound NH3 from the zeolite. Then, only at elevated temperatures is the influence of the oxide phase on Cu redox in the Cu-SSZ-13 observed. Furthermore, we show how the oxide component has the great potential to circumvent NH4NO3 inhibition on Cu-SSZ-13 in the composite catalysts at low temperatures by bypassing the formation of nitrates from NO2 disproportionation, thus providing a practical solution to improving low temperature SCR reactivity.