Elucidating the Role of CO in the NO Storage Mechanism on Pd/SSZ-13 with in Situ DRIFTS

Abstract

Pd-ion-exchanged zeolites have emerged as promising materials for the adsorption and oxidation of air pollutants. For lowtemperature vehicle exhaust, dispersed Pd ions can adsorb NO x even in H 2 O-rich exhaust containing carbon monoxide. To understand this phenomenon, changes in the Pd ligand environment have to be monitored in situ. Herein, we directly observe the activation of hydrated Pd ion shielded by H 2 O into a carbonyl−nitrosyl complex Pd 2+ (NO)-(CO) in SSZ-13 zeolite. The subsequent thermal desorption of ligands on the Pd 2+ (NO)(CO) complex proceeds to nitrosyl Pd 2+ rather than to carbonyl Pd 2+ under various conditions. Thus, CO molecules act as additional ligands to provide an alternative pathway with a lower energy barrier in accelerating NO adsorption on hydrated Pd 2+ ions through the Pd 2+ (NO)(CO) complex. We further demonstrate that hydration of Pd ions in the zeolite is a prerequisite for CO-induced reduction of Pd ions to metallic Pd. The reduction of Pd ions by CO is limited under dry conditions even at temperatures as high as 500 °C, while water makes it possible at near RT. However, the primary NO adsorption sites are Pd 2+ ions even in gases containing CO and water. These findings clarify additional mechanistic aspects of the passive NO x adsorption (PNA) process and will help extend the NO x adsorption chemistry in zeolite-based adsorbers to practical applications.

Interconversion of Atomically Dispersed Platinum Cations and Platinum Clusters in Zeolite ZSM-5 and Formation of Platinum gem-Dicarbonyls

“…Further structural information emerged from IR data showing changes in the region of framework T–O–T stretching vibrations (T is Si or Al) during the treatments described in the caption of Figure . The perturbation of T–O–T bonds by incorporation of metal ions in the zeolite causes T–O–T stretching frequencies to decrease from near 1100 to 850–1000 cm –1 . , The band locations have been used to identify the metals and to monitor changes in their coordination and oxidation states. ,,,− When calcined Pt/ZSM-5 was exposed to CO, a loss in intensity centered at 926 cm –1 was observed (Figure S24), signifying that bonding of CO to Pt 2+ caused a relaxation of the zeolite ring distortion, that is, platinum relaxed from the surface during formation of the platinum gem -dicarbonyl. During subsequent heating of this sample in flowing 10% H 2 in N 2 at 120 °C, a band appeared at 950 cm –1 , indicating that the conversion of Pt 2+ (CO) 2 to Pt δ+ (CO) was accompanied by the reappearance of a close interaction between platinum and the zeolite framework.…”

Single Ru(II) Ions on Ceria as a Highly Active Catalyst for Abatement of NO

“…Air pollution is one of the main issues to tackle in environmental science and catalysis. − Deteriorating air quality is directly related to toxic NO x emissions, the majority of which are produced by vehicles exhaust. There clearly exists the urgent need to decrease emissions from engines and develop catalytic materials for NO x abatement based on less expensive (noble) metals with improved atom economy. − NO oxidation is critical for environmental catalysis in diesel aftertreatment systems because NO 2 formation is important in lean NO x reduction. − Furthermore, NO 2 facilitates ammonia selective catalytic reduction (SCR) (so-called “fast” SCR, with an ideal 1:1 ratio between NO and NO 2 ) and lean NO x storage (in this case, NO must first be oxidized to NO 2 in order to be stored on LN traps materials). Best catalysts for NO oxidation typically contain a few wt % percent of expensive Pt and Pd. − Pt currently costs ∼1000 USD/ounce, whereas Pd costs ∼2600 USD/ounce (note that Pt is approximately ∼2 times heavier, so per molar basis, the price is only ∼1.3 higher for Pd than for Pt).…”

Computational Study of Noble Metal CHA Zeolites: NO Adsorption and Sulfur Resistance

“…Recently, Pd-exchanged zeolites were proposed as promising materials for this technology. , Highly dispersed Pd ions at exchange sites of a zeolite have been suggested as active sites for NO x storage at low temperatures . Subsequently, many research groups have studied NO x adsorption/desorption properties over Pd zeolites. − Small-pore Pd-SSZ-13 with chabazite (CHA) topology showed advantageous NO x release temperatures and higher hydrothermal stability compared to medium- and large-pore Pd zeolites. , …”