Effects of Treatment Conditions on Pd Speciation in CHA and Beta Zeolites for Passive NO_x Adsorption

Abstract

The structure and evolution of Pd species in Pd-exchanged zeolite materials intended for use as passive NO x adsorbers were examined under various pretreatment conditions. Using in situ CO-diffuse reflectance infrared spectroscopy, Pd structures were characterized after 500 °C pretreatments in inert (Ar), water (1–2% H2O in Ar), oxidizing (air), and reducing (H2, CO) atmospheres. Two zeolites of similar Si/Al ratios but different framework topologies (Beta, CHA) were found to show different distributions of Pd species, depending on the reducing agent used. Reduction in H2 (500 °C; 10% H2 in Ar) followed by re-oxidation (500 °C; air) led to higher amounts of single-site Pd ions on Pd-CHA than Pd-Beta, whereas high-temperature reduction in CO (500 °C; 1000 ppm CO in Ar) followed by re-oxidation (500 °C; air) led to significant loss of ionic Pd on both Pd-CHA and Pd-Beta, albeit H2 temperature-programmed reduction and XPS experiments suggest that this phenomena may be limited to surface Pd. High-temperature treatments with water (500 °C; 1–2% H2O in Ar) are shown to form either Pd metal or PdO particles, with Pd-Beta being more susceptible to these effects than Pd-CHA. This work suggests that the effects of CO are especially problematic with respect to the durability of these materials in passive NO x adsorption applications, especially in the case of Beta zeolite.

Experimental and Theoretical Studies of Pd Cation Reduction and Oxidation During NO Adsorption on and Desorption from Pd/H–CHA

“…Figure b shows a spectrum of CO adsorbed on Pd/H–CHA. The band at 2130 cm –1 is attributable to CO adsorbed on either Pd + or Pd 2+ . ,, The absence of this band after NO adsorption or after coadsorption of CO and NO is consistent with the stronger adsorption of NO than CO (see more on this topic below) and the displacement of adsorbed CO by NO illustrated in Figure b. This result matches the aforementioned TPD experiments that demonstrate that there is no formation of a NO/CO coadsorbed complex.…”

Molecular Mechanism of Low-Temperature Passive NOx Adsorption (PNA) on Palladium-Loaded FER Zeolite

“…Such treatment results in an almost atomic dispersion of palladium in the zeolite pores. 23,30,33,34 Depending on pretreatment conditions, palladium is observed in oxidation states of Pd 0 , Pd I , and Pd II , and even Pd IV has been suggested. Many studies revealed the presence of Pd 2+ cations and [Pd II PdO, and PdO 2 have been proposed to contribute to the NO x storage capacity.…”

“…Zeolites investigated thus far in PNA research have MFI, − BEA, ,,− CHA, ,,− or FER − framework topology. The investigations revealed that the storage mechanism of NO x on these Pd-loaded zeolites is complicated.…”

“…The highest reported reversible NO x adsorption capacities have been achieved on Pd-loaded zeolites that underwent oxidative pretreatment at temperatures exceeding 700 °C under oxygen gas or air with or without water vapor. Such treatment results in an almost atomic dispersion of palladium in the zeolite pores. ,,, Depending on pretreatment conditions, palladium is observed in oxidation states of Pd 0 , Pd I , and Pd II , and even Pd IV has been suggested. Many studies revealed the presence of Pd 2+ cations and [Pd II OH] + complexes.…”

A Study of CeSnOx and Pd/CeSnOx as Low-Temperature NOx Adsorbers with Excellent Hydrothermal Stability

“…3,6 Recently, passive NO x adsorbers (PNAs) have been considered an attractive technology for removing NO x from the cold-start exhaust of diesel vehicles, directly adsorbing NO x gases at low temperatures and releasing them at a temperature where SCR systems can operate efficiently. 7 Noble metals supported on both zeolites and oxides have been studied as PNA materials. 5,7 Among those, Pd-based zeolites (such as Pd/Beta, Pd/SSZ-13 and Pd/ZSM-5) are of particular interest because of their excellent resistance to hydrothermal aging.…”