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Reaction Pathway Investigation on the Selective Catalytic Reduction of NO with NH<sub>3</sub> over Cu/SSZ-13 at Low Temperatures
The mechanism of the selective catalytic
reduction of NO with NH<sub>3</sub> was studied using Cu/SSZ-13. The
adspecies of NO and NH<sub>3</sub> as well as the active intermediates
were investigated using
in situ diffuse reflectance infrared Fourier transform spectroscopy
and temperature-programmed surface reaction. The results revealed
that three reactions were possible between adsorbed NH<sub>3</sub> and NO<sub><i>x</i></sub>. NO<sub>2</sub><sup>β</sup> could be generated by direct formation or NO<sub>3</sub><sup>β</sup> reduction via NO. In a standard selective catalytic reduction (SCR)
reaction, NO<sub>3</sub><sup>β</sup> was hard to form, because
NO<sub>2</sub><sup>β</sup> was consumed by ammonia before it
could be further oxidized to nitrates. Additionally, adsorbed NH<sub>3</sub> on the Lewis acid site was more active than NH<sub>4</sub><sup>+</sup>. Thus, SCR mainly followed the reaction between Lewis
acid site-adsorbed NH<sub>3</sub> and directly formed NO<sub>2</sub><sup>β</sup>. Higher Cu loading could favor the formation
of active Cu-NH<sub>3</sub>, Cu-NO<sub>2</sub><sup>β</sup>,
and Cu-NO<sub>3</sub><sup>β</sup>, improving the SCR activity
at low temperature