Insight into the Mechanism of Selective Catalytic Reduction of NO by CO over a Bimetallic IrRu/ZSM-5 Catalyst in the Absence/Presence of O₂ by Isotopic C¹³O Tracing Methods

Abstract

The development of efficient catalysts for the selective catalytic reduction of NO by CO (CO-SCR) in the presence of O 2 is highly desirable for controlling the emission of toxic gases from tailpipes. Here, a bimetallic IrRu/ZSM-5 catalyst was prepared for the selective catalytic reduction of NO by CO in the presence of O 2 (5%) for the low-temperature treatment of exhaust gas. IrRu/ZSM-5 afforded 90% NO x conversion in the range of 225−250 °C and maintained 90% NO x conversion after 12 h of reaction. Ru addition inhibited agglomeration of the Ir particles during the reduction process and provided more active sites for NO adsorption. Isotopic C 13 O tracing and in situ diffuse reflectance infrared Fouriertransform spectroscopy experiments were used to elucidate the CO-SCR mechanism in the absence or presence of O 2 . NCO could easily form on the surface of catalysts in the absence of O 2 , whereas NCO formation has been inhibited owing to the quick consumption of CO in the presence of O 2 . Moreover, some byproducts such as N 2 O and NO 2 are generated in the presence of O 2 . Finally, a possible mechanism for CO-SCR under different conditions was proposed based on in situ experiments and physicochemical analyses.

Unraveling the Synergistic Mechanism of Ir Species with Various Electron Densities over an Ir/ZSM-5 Catalyst Enables High-Efficiency NO Reduction by CO

“…During the reaction process, the active Ir 0 site formed by H 2 pretreatment is oxidized by high-concentration O 2 , resulting in a decrease in catalyst activity. 27 As shown in Figure 1e, the addition of H 2 O resulted in a decrease of about 20% in the NO conversion of Ir/Z5-200H, and the catalyst returned to its original catalytic level after cutting out H 2 O. The CO conversion is not affected by H 2 O.…”

“…The study of the CO-SCR reaction mechanism on the Ir/Z5-200H catalyst is shown in Figure . As shown in Figure a, after NO adsorption at 250 °C, infrared vibrational peaks of N 2 O 4 (1689 cm –1 ) and ONNO (1722 cm –1 ) , can be observed and the diffraction peak at 1839 cm –1 belongs to the adsorption of NO by Ir 0 . On the basis of Figure c, the gas-phase product N 2 O was observed at the moment of NO introduction.…”

“…As shown in Figure 4a, after NO adsorption at 250 °C, infrared vibrational peaks of N 2 O 4 (1689 cm −1 ) 59 and ONNO (1722 cm −1 ) 50,59 can be observed and the diffraction peak at 1839 cm −1 belongs to the adsorption of NO by Ir 0 . 27 On the basis of Figure 4c, the gas-phase product N 2 O was observed at the moment of NO introduction. Therefore, a portion of NO is adsorbed on the catalyst surface as solidphase ONNO and N 2 O 4 , while a portion is desorbed in the form of gas-phase product N 2 O.…”

“…Noble-metal-based catalysts exhibit significant performance advantages under O 2 -containing conditions, especially Ir-based catalysts. Ir 0 nanoparticles are considered as the active sites because they can simultaneously adsorb NO and CO, promoting the dissociation of NO. − However, the Ir 0 site also exhibits strong adsorption of O 2 , which may lead to a decrease in catalytic activity of the catalyst under O 2 competitive adsorption. Considering that Ir δ+ and Ir 0 usually coexist, there may be Ir sites in different states in the catalysts that are also responsible for the significant performance advantages in the presence of O 2 .…”

Tailoring Iridium Valence States on ZSM-5 for Enhanced Catalytic Performance in CO Selective Catalytic Reduction of NO under Oxygen-Enriched Environments

“…The Ir 4f signals of the Ir loaded onto different carriers are shown in Figure 10 a. The binding energies (BEs) at 61.1 and 64.1 eV were attributed to Ir 0 , and those at 62.1 and 65.1 eV to electron-deficient iridium species (Ir δ+ ) [ 46 , 47 ], indicating that Ir 0 and oxidized Ir δ+ coexisted in samples. Peak fitting and area analysis revealed contributions of 12.8% Ir 0 and 87.2% Ir δ+ for Ir/SAPO-34 and 12.1% Ir 0 and 87.9% Ir δ+ for Ir/MCM-41 ( Table 3 ).…”