Regulation of the copper-oxide cluster structure and DeNOx activity of Cu-ZSM-5 catalysts by variation of OH/Cu

The present work studies RedOx properties and reactivity in N₂O decomposition and selective catalytic reduction of NO (NO-SCR) by propane of Cu-modified ZSM-5 catalysts prepared by hydrolytic polycondensation (HPC). Their characteristics are compared with properties of Cu-ZSM-5 samples having the same copper content (1wt.%) and produced by ion exchange (IEX) with water–ammonia solutions of copper chloride at NH₄OH/Cu²⁺ equal to 6–30. The HPC method consists in the treatment of the copper-impregnated zeolite by water–ammonia solution for polycondensation of hydrolyzed copper ions in the zeolite pores. It is shown that varying the NH₄OH/Cu²⁺ ratio allows a regulation of the size and structure of copper active complexes. The copper-impregnated zeolite contains copper as CuO-like particles after calcination. At NH₄OH/Cu²⁺ equal to 1–3, active sites in dried samples are disordered polynuclear copper-hydroxo complexes, which are characterized by symmetric ESR signal with g₀=2.18. Increase of NH₄OH/Cu²⁺ to 6–30 lead to the orbital ordering of Cu²⁺ ions in the copper–ammoniate complexes, observed as the rhombic ESR signals (gz ᵉˣ,¹=2.18, gy ᵉˣ,¹=2.08, gₓ ᵉˣ,¹=2.03 and g∥ ᵉˣ,²=2.24, g⊥ ᵉˣ,²=2.03), and to dispersion of CuO-like particles during following thermal treatment at 500°C. The dried 1%Cu(30)ZSM-5-HPC sample prepared at NH₄OH/Cu²⁺=30 has a higher amount of isolated tetraammine–copper complex and a lower number of weak associates of Cu²⁺ ions among samples. The addition of ammonia solution to the copper-impregnated zeolite for hydrolytic polycondensation has a favorable impact on the Cu-ZSM-5 activity in DeNOx processes. In N₂O decomposition, the ion-exchanged CuZSM-5 catalysts were more reactive in the low-temperature region, while the ex-polycondensation catalysts provided a high N₂O conversion at high temperatures. In NO-SCR by propane, the more reactive were the ion-exchanged catalysts, and NO reduction rate increased with NH₄OH/Cu²⁺ grew for 10. It is suggested that the ordered associated Cu²⁺ ions of ex-polycondensation and ion-exchanged catalysts have higher reactivity and selectivity in both DeNOx processes as compared to the isolated copper ions.