The NOx storage-and-reduction (NSR) catalyst 1Pt/20Ba/Co1Mg2Al1Ox combines a conventional BaO storage phase with a layered-double-oxide Co1Mg2Al1Ox support that itself adsorbs NOx, giving a NOx storage capacity of 1.38 mmol/g versus 0.56 mmol/g for the conventional Pt/Ba/gamma-Al2O3. The unresolved scientific problem is a quantitative, site-resolved account of this dual-site catalyst: the relative contributions of the BaO and Co1Mg2Al1Ox adsorption phases to total storage, and the identity of the stored nitrite and nitrate species and the kinetics of their formation during lean operation and removal during rich regeneration on each site class. The durability of the dual-site architecture under extended lean-rich cycling, hydrothermal aging, and sulfur exposure is likewise unestablished and is retained here as related context. Follow-up literature has resolved the mechanism for the Ba-free Pt/Co1Mg2Al1Ox system (hydrogen spillover, in-situ DRIFTS speciation, CO2/H2O/SO2 effects) and extended the composition family (K2CO3, Ni, Mn, Co-doped spinels), but no published study deconvolutes the two coexisting adsorption site classes in the dual-site catalyst itself.

Background

Lean-burn and diesel engines combust fuel with excess oxygen, so the exhaust is net-oxidizing and the three-way catalyst used on stoichiometric gasoline engines cannot reduce NOx (the sum of NO and NO2) there. NOx storage and reduction (NSR), also called the lean NOx trap (LNT), solves this by cycling the engine between long lean (oxygen-rich) phases and short rich (fuel-rich) phases. During the lean phase a storage component, typically barium oxide (BaO) on a platinum-bearing alumina catalyst, traps NOx as surface nitrites and nitrates; Pt oxidizes NO to NO2, which forms stable nitrates far more readily than NO. During the rich phase Pt activates the reductant (H2, CO, or hydrocarbons), and the stored nitrates/nitrites are released and reduced to N2. The storage capacity, regeneration rate and efficiency, and resistance to water, CO2, sulfur and thermal aging are the central performance metrics of this catalyst class.

A separate materials strand comes from layered double hydroxides (LDHs): clay-like compounds with brucite-type cation layers of mixed divalent (Mg2+, Co2+, Ni2+...) and trivalent (Al3+) cations charge-balanced by interlayer anions (here carbonate). Calcining an LDH collapses the layers into a high-surface-area mixed-metal oxide, a layered double oxide (LDO), whose surface basicity and, for cobalt-containing compositions, NO-oxidation activity make it an effective NOx adsorbent. Yu and co-workers (2007) first showed that such LDH-derived mixed oxides can decompose and store NOx, and Yang, Cui, O'Hare, Wang and co-workers (2017) systematically screened binary M2+Al3+ LDOs (M = Ca, Zn, Cu, Ni, Co, Mg) for naphtha-cracker flue-gas deNOx, finding the ternary CoMgAl composition optimal.

Against this background, Cui, Yan, Zhang, Qiu and Wang (2017) built the dual-site NSR catalyst 1Pt/20Ba/Co1Mg2Al1Ox: 1 wt% Pt and 20 wt% BaO co-deposited on the Co1Mg2Al1Ox LDO obtained from a Co1Mg2Al1-carbonate LDH. The support is not inert: it carries its own NOx adsorption sites, so the catalyst contains two distinct classes of NOx adsorption sites, and it stores 1.38 mmol NOx/g against 0.56 mmol/g for Pt/Ba/gamma-Al2O3, with good water resistance, stability over the reported short lean-rich cycling window, and about 82.2% NOx removal. Temperature-programmed desorption (TPD, measuring desorbed species while heating) distinguished the two site classes only qualitatively. The paper's schematic lean-rich mechanism left open the detailed chemistry.

Later work sharpened the picture on neighboring systems but not on the dual-site catalyst itself. A transient kinetic study of the Ba-free Pt/Co1Mg2Al1Ox-LDO system (Zhang, Damaskinos, Vasiliades, Liu, Jiang, Wang and Efstathiou, 2023) established, by in-situ DRIFTS (diffuse-reflectance infrared Fourier-transform spectroscopy under reaction conditions) and transient experiments in a swing reactor, that stored NOx on the LDO support is reduced by hydrogen spillover (migration of activated hydrogen from Pt to neighboring support sites), quantified the deleterious effects of CO2 and H2O and of SO2 pretreatment, and demonstrated stable cycling for 2 h at 350 C. Composition variants with K2CO3 instead of Ba, and with Ni or Mn instead of Co, mapped the composition space, and a 2024 study of Pt/Ba/Co-doped MgAl2O4 spinels addressed high-temperature (250-500 C) performance and short-term thermal stability. What no study provides is the quantitative decomposition of storage between the BaO and Co1Mg2Al1Ox phases when both coexist, or the site-resolved kinetics of storage and regeneration in that coupled two-site system; its behavior under long-term cycling and aging likewise remains untested. Understanding such multi-site cooperation matters because it determines whether the second adsorption phase genuinely adds usable storage or duplicates the BaO function, and it is the design principle behind a broad family of LDH-derived NSR materials reviewed by Xue, Gao, Mei, Cui and Wang (2018) and by Tan and co-workers (2024).

Problem Statement

For the dual-site NOx storage-and-reduction catalyst 1Pt/20Ba/Co1Mg2Al1Ox (1 wt% Pt and 20 wt% BaO on the layered double oxide Co1Mg2Al1Ox obtained from a Co1Mg2Al1-carbonate layered double hydroxide, in which the BaO phase and the support each provide NOx adsorption sites), determine quantitatively and mechanistically how the two adsorption site classes share the NSR function: (i) the relative contributions of the BaO and Co1Mg2Al1Ox sites to the total lean-phase NOx storage capacity under representative lean-burn conditions; and (ii) the chemical identity of the stored nitrite and nitrate species on each site class and the elementary steps and kinetics of their formation during the lean phase and their release and reduction to N2 during the rich phase, including any coupling between the two site classes (for example hydrogen spillover from Pt or transfer of stored NOx between BaO and support sites). These two components serve one objective: a quantitative, site-resolved account of the dual-site catalyst that establishes whether and how the support's own adsorption capacity adds to the BaO storage function under realistic exhaust conditions (temperature range, water and CO2 levels representative of lean-burn exhaust). The long-term durability of the dual-site architecture under extended lean-rich cycling, hydrothermal aging, and sulfur exposure, and how each site class degrades under such stress, have not been studied and remain related open practical concerns; they are context for, not required components of, the site-resolved account sought above.

References

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