NO oxidation on zeolite supported Cu catalysts: Formation and reactivity of surface nitrates

The comparative activities of a small-pore Cu-CHA and a large-pore Cu-BEA catalyst for the selective catalytic reduction (SCR) of NOx with NH₃, and for the oxidation of NO to NO₂ and the subsequent formation of surface nitrates were investigated. Although both catalysts are highly active in SCR reactions, they exhibit very low NO oxidation activity. Furthermore, Cu-CHA is even less active than Cu-BEA in catalyzing NO oxidation but is clearly more active for SCR reactions. Temperature-programed desorption (TPD) experiments following the adsorption of (NO₂ + NO + O₂) with different NO₂:NO ratios reveal that the poor NO oxidation activity of the two catalysts is not due to the formation of stable surface nitrates. On the contrary, NO is found to reduce and decompose the surface nitrates on both catalysts. To monitor the reaction pathways, isotope exchange experiments were conducted by using ¹⁵NO to react with ¹⁴N-nitrate covered catalyst surfaces. The evolution of FTIR spectra during the isotope exchange process demonstrates that ¹⁴N-nitrates are simply displaced with no formation of ¹⁵N-nitrates on the Cu-CHA sample, which is clearly different from that observed on the Cu-BEA sample where formation of ¹⁵N-nitrates is apparent. The results suggest that the formal oxidation state of N during the NO oxidation on Cu-CHA mainly proceeds from its original +2 to a +3 oxidation state, whereas reaching a higher oxidation state for N, such as +4 or +5, is possible on Cu-BEA.