Deactivation of Cu/SSZ-13 NH3-SCR Catalyst by Exposure to CO, H2, and C3H6

Abstract

Lean nitric oxide (NOx)-trap (LNT) and selective catalytic reduction (SCR) are efficient systems for the abatement of NOx. The combination of LNT and SCR catalysts improves overall NOx removal, but there is a risk that the SCR catalyst will be exposed to high temperatures and rich exhaust during the LNTs sulfur regeneration. Therefore, the effect of exposure to various rich conditions and temperatures on the subsequent SCR activity of a Cu-exchanged chabazite catalyst was studied. CO, H2, C3H6, and the combination of CO + H2 were used to simulate rich conditions. Aging was performed at 800 °C, 700 °C, and, in the case of CO, 600 °C, in a plug-flow reactor. Investigation of the nature of Cu sites was performed with NH3-temperature-programed desorption (TPD) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) of probe molecules (NH3 and NO). The combination of CO and H2 was especially detrimental to SCR activity and to NH3 oxidation. Rich aging with low reductant concentrations resulted in a significantly larger deactivation compared to lean conditions. Aging in CO at 800 °C caused SCR deactivation but promoted high-temperature NH3 oxidation. Rich conditions greatly enhanced the loss of Brønsted and Lewis acid sites at 800 °C, indicating dealumination and Cu migration. However, at 700 °C, mainly Brønsted sites disappeared during aging. DRIFT spectroscopy analysis revealed that CO aging modified the Cu2+/CuOH+ ratio in favor of the monovalent CuOH+ species, as opposed to lean aging. To summarize, we propose that the reason for the increased deactivation observed for mild rich conditions is the transformation of the Cu species from Z2Cu to ZCuOH, possibly in combination with the formation of Cu clusters.

Comparative Study of SO2 and SO2/SO3 Poisoning and Regeneration of Cu/BEA and Cu/SSZ-13 for NH3 SCR

“…High temperature in presence of water vapor is known to degrade the crystallinity of the zeolite structure, and at very high temperatures, the structure collapses [2,31], which urges the development of highly stable zeolites. The detrimental effect of reducing treatment on the copper dispersion and the subsequent SCR activity has been revealed by Auvray et al [6,32]. It was shown that exchanged copper ions tend to agglomerate in larger particles under the action of hydrogen.…”

Urealess NOx Reduction by Carbon Monoxide in Simulated Lean-Burn Exhausts

“…Both catalysts show poor NOx reduction activity up to 175 °C; however, the aged IrRu/Al 2 O 3 reveals higher NOx conversion by CO between 200 and 250 °C than the aged Cu/CHA. As the reaction temperature increases beyond this, the NOx reduction activity of a-IrRu/Al 2 O 3 by CO starts to decrease, while a-Cu/CHA showed higher activity although NH 3 can be oxidized by O 2 . Note that low temperature lean NOx reduction has been commonly regarded as a distinct area of urea (NH 3 )/SCR technology in the automotive aftertreatment system to date.…”

Editorial Catalysts: Catalysis for the Removal of Gas-Phase Pollutants

“…Olsson et al studied the deactivation of the Cu/SSZ-13 catalyst that is commonly used for NH 3 -SCR by exposing it to CO, H 2, and C 3 H 6 [18], and they proposed that the reason for the increased deactivation observed in mild rich conditions was the transformation of the Cu species. These centres evolved from active isolated Cu 2+ , balancing two negative charges of Al atoms in the zeolite framework, to inactive [CuOH] + connected to one Al atom together with the formation of Cu clusters.…”