The Operating Cycle of NO Adsorption and Desorption in Pd–Chabazite for Passive NO_x Adsorbers

Abstract

Pd-doped chabazite (Pd/CHA) offers unique opportunities to adsorb and desorb NO x in the target temperature range for application as a passive NO x adsorber (PNA). The ability of Pd/CHA to trap NO x emissions at low temperatures (<200 °C) is facilitated by the binding of NO x species at various Pd sites available in the CHA framework. Density functional theory (DFT) simulations are performed to understand Pd speciation in CHA and the interaction of NO with Pd/CHA to explain the mechanisms of NO adsorption, oxidation, and desorption processes. The calculations are used to elucidate the important role of Pd1+ cationic species, anchored at 6MR-3NN, in providing a strong (E b = −272 kJ/mol) NO adsorption site in Pd/CHA. For NO release, the redox transformation of Pd species comes into play and Pd1+ species are suggested to transform into cationic Pd2+, [PdOH]+, or [Pd–O–Pd]2+ species, all of which show significantly reduced NO binding (−116, −153, and −117 kJ/mol, respectively) as compared to Pd1+. This enables NO desorption at the operating temperature of a downstream catalyst for subsequent catalytic reduction.

Theoretical Study of Small Molecules Adsorption on Pristine and Transition Metal Doped GeSe Monolayer for Gas Sensing Application

“…To further improve the sensing behavior of pure nanomaterials, the doping of elements are the most common strategies. − Transition metal elements are commonly applied in surface modification and performance optimization, − and previous research has proved that the sensing properties of materials are improved significantly after substitutional doping with transition metal elements. , For example, substitutional doping of WSe 2 monolayer with transition metal elements (Pd, Ag, Au, Pt) have been studied for its adsorption performance toward CO 2 , NO 2 , and SO 2 . It is found that, compared with pristine WSe 2 monolayer, doped WSe 2 has a great effect on the adsorption of these gas molecules.…”

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

“…Previous theoretical studies on Pd zeolites used both semilocal and hybrid functionals. [62][63][64][65][66][67][68][69][70][71]84 Mei et al 67 obtained similar energies of NO adsorption in Pd-HBEA using Perdew− Burke−Ernzerhof (PBE) 85 and Heyd−Scuseria−Ernzerhof (HSE06) 86 functionals. However, Mandal et al 64 found significant dependence of NO adsorption energies on the type of exchange−correlation (XC) functional, with more exothermic NO adsorption energies for the PBE functional compared to the HSE06 functional.…”

“…Theoretical methods have been utilized only in a few studies of Pd zeolites with the focus on PNA. − Khivantsev et al showed that Pd ions in Pd-SSZ-13 are present as super electrophilic Pd 2+ species. Mandal et al concluded that Pd II preferentially populates 2Al sites in the six-membered rings (6MRs), while the formation of ZPd II OH in 1Al sites is energetically prohibitive.…”

Complete Mechanism for NOx Adsorption and Release on Atomically Dispersed Palladium in Pd-CHA

“…11,12,26,27,38 The assumption of dimeric sites allows formulating mechanisms in which Pd takes formal oxidation states of +2 and +1 and avoids the complete reduction to metallic Pd. The complete mechanisms with elementary steps reported so far involve the formation of dimeric sites 27,39,40 while the reported mechanism on monomeric Pd sites by Mei et al does not explain the full catalytic cycle. 38 These mechanisms are inconsistent with the molecular chemistry (involving 1/2 O 2 ) but also with characterization of Pd-exchanged CHA showing only atomically dispersed Pd atoms in the zeolite framework in their study.…”