Migration, reactivity, and sulfur tolerance of copper species in SAPO-34 zeolite toward NOx reduction with ammonia

Abstract

Cu/SAPO-34 catalysts were prepared by wet impregnation and ion-exchange methods, and both the catalysts were hydrothermally treated at 750 °C in 10 vol% H₂O/air for 24 h. Subsequently, the as-received and hydrothermally treated catalysts were exposed to a sulfur poisoning treatment at 350 °C in 100 ppm SO₂/10 vol% H₂O/air for 24 h and examined for NO_x conversion. Sulfur poisoning considerably decreased the NO_x conversion efficiency of the catalysts at low temperatures. In contrast, it improved the high-temperature selective catalytic reduction (SCR) activities of the as-received catalysts. The ion-exchange-prepared catalysts displayed higher sulfur tolerance than the impregnation-prepared catalysts at 150–350 °C. The electron paramagnetic resonance, X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy and H₂-temperature-programmed reduction results showed that sulfur poisoning significantly influenced the migration of the copper species and thereby the amount of active isolated Cu²⁺. More dispersed CuSO₄ was produced on the ion-exchange-prepared catalysts than on the impregnation-prepared catalysts after sulfur poisoning. The dispersed CuSO₄ showed considerably higher SCR activity than the crystalline CuSO₄. Both the dispersed CuSO₄ and remaining isolated Cu²⁺ determined the low-temperature SCR behavior of the sulfur-poisoned catalysts.