Insights into Migration and Transformation of Cu Species during SO2Poisoning and Hydrothermal Aging of Cu/SAPO-34 NH3–SCR Catalysts

Cu/SAPO-34 catalysts have been widely used for the reduction of NOx from diesel exhaust by ammonia-selective catalytic reduction (SCR). Sulfur poisoning remains a challenge for Cu/SAPO-34 catalysts in NH3–SCR, restricting its application. In this study, Cu/SAPO-34 samples containing a mixture of Cu species comprising isolated Cu2+located in the 6MR, Cu(OH)+in 8MR, CuO clusters/nanoparticles, and Cu+species were prepared. The Cu speciation during SO2and hydrothermal aging of Cu/SAPO-34 SCR catalysts was subsequently studied. While hydrothermal aging had little effect on catalytic activity, the sulfur-aged sample exhibited deterioration in NH3–SCR activity over the temperature range of 100–575 °C. The breakage and collapse of the zeotype framework and chemical deactivation of the Cu/SAPO-34 catalyst after exposure to SO2and H2O took place. A combination of X-ray photoelectron spectroscopy (XPS) and TGA-DT-DSC revealed the presence of ammonium bisulfate, copper bisulfate/sulfate, aluminum sulfate, and sulfuric acid after SO2and H2O copoisoning under SCR conditions. Furthermore, Cu(OH)+tended to migrate to form isolated Cu2+and Cu+, as well as agglomerate to form CuO particles. Density functional theory (DFT) calculations revealed that SO2and O2could readily be absorbed by CuO particles to form stable copper bisulfate/sulfate species with the following order of preference: CuO > Cu+> Cu2+> Cu(OH)+. In contrast, Cu(OH)+was less prone to be poisoned by easily releasing SO3than other Cu species sites.