Zeolite-based hydrocarbon (HC) traps can adsorb gasoline cold-start HC emissions and release them once the three-way catalyst is hot enough to oxidize them, but their use in close-coupled gasoline systems is blocked by two coupled obstacles: the zeolite framework collapses under 950–1000 °C hydrothermal aging, and part of the trapped HC desorbs unpurified before the catalyst reaches light-off. Endo et al. (SAE 2018-01-0947) solved the durability obstacle for BEA zeolite by zirconium-phosphate modification, yet their aged close-coupled trap system still released 23% of the adsorbed cold-start HC unpurified (purification ratio B/A = 77%), and the proposed durability mechanism (aluminum phosphate formation plus increased hydrophobicity) remained an indirect hypothesis. The literature on zeolite HC traps from 1997 to 2025, including the post-2018 'active trap' line (Cu+ adsorption sites inside the zeolite combined with external CuO nanoparticles for low-temperature oxidation) and system-level modeling, contains no material or system that simultaneously achieves wet aromatic-HC adsorption, essentially complete purification of the desorbed HC, and close-coupled-gasoline durability, and the most recent work (2025) still frames hydrothermal robustness at ≥800 °C as an open challenge. The problem of closing the purification gap while retaining, and mechanistically understanding, close-coupled durability remains open.

Background

A three-way catalyst (TWC) is the standard exhaust-cleaning device for gasoline engines: it oxidizes unburned hydrocarbons (HC) and carbon monoxide and reduces nitrogen oxides simultaneously, but only once it reaches its light-off temperature (roughly 250–300 °C). During the first minute after a cold engine start the exhaust is too cool for the TWC to work, and this cold-start window accounts for the majority of HC emissions over a regulatory test cycle. One mitigation is a hydrocarbon (HC) trap: a zeolite — a crystalline aluminosilicate with a regular network of sub-nanometer pores — washcoated on a ceramic monolith so that it adsorbs HC at low temperature and releases them as the exhaust warms, ideally after the downstream TWC has lit off. Zeolite frameworks are denoted by three-letter codes: BEA (beta zeolite, with large pores that admit aromatic molecules), MFI (ZSM-5) and CHA (both with smaller pores that restrict aromatics).

Two long-standing obstacles define the field. The first is hydrothermal durability: in the close-coupled position (mounted directly at the exhaust manifold, where it warms fastest) the catalyst must survive roughly 950–1000 °C in about 10% water vapor for the vehicle's life; under these conditions aluminum is expelled from the zeolite framework (dealumination) and the structure collapses, which is why zeolite traps were historically relegated to cooler positions on diesel systems. Stability improves at higher SiO2_2/Al2_2O3_3 ratios, but HC adsorption capacity — especially for unsaturated and aromatic HC that bind to the framework's acid sites — improves with more aluminum, an intrinsic tension. The second obstacle is desorption slip: the trap begins releasing stored HC at temperatures well below the TWC light-off, so a fraction of the stored HC escapes unpurified. Following Endo et al., performance is quantified by A, the fraction of cold-start HC trapped during the cycle, and B/A, the fraction of the trapped amount that is subsequently purified rather than released raw.

Endo et al. (SAE 2018-01-0947) attacked the durability obstacle by modifying BEA with zirconium phosphate (Zr/P-BEA): after hydrothermal aging at 1000 °C for 25 h in 10% water, the modified zeolite retained more than 80% of its specific surface area and its toluene (a model aromatic HC) adsorption capacity, whereas unmodified BEA became essentially amorphous. In engine-dynamometer tests with a close-coupled palladium/rhodium (Pd/Rh) TWC, an aged (950 °C, 50 h, stated as equivalent to 150,000 miles) Zr/P-BEA trap reduced cold HC emissions by up to 43% versus a TWC+TWC system without zeolite, under a transient cycle simulating the start of the LA4 driving cycle (a US federal test procedure). But only 77% of the adsorbed HC was ultimately purified (A = 56%, B/A = 77%): the remaining 23% desorbed before the TWC could oxidize it. The authors proposed, on the basis of 27^{27}Al/31^{31}P/29^{29}Si nuclear magnetic resonance spectroscopy and thermogravimetry–mass spectrometry, that the durability stems from phosphate species reacting with expelled aluminum to form aluminum phosphate (AlPO4_4) together with increased hydrophobicity, but explicitly treated this as a hypothesis requiring direct evidence. Their modification was also most effective on BEA: CHA and MFI retained surface area but their pores are too small for aromatic HC.

The surrounding literature pursued the two obstacles along separate lines. Materials studies established that cation exchange or metal impregnation can add adsorption selectivity or even low-temperature oxidation: Cu-exchanged ZSM-5 and BEA films as early traps (Navlani-García et al., 2013), Ag-Beta, on which water strongly inhibits ethylene uptake below 100 °C (Kang et al., 2017), La/Fe/Pd-doped Beta (Jonsson et al., 2020), and Cu-impregnated zeolites that combine Cu+^+ ions inside the pores (which adsorb propene preferentially even in wet exhaust) with a-few-nanometer CuO particles on the external surface that begin oxidizing HC from about 210 °C (Jang et al., 2021; Kim et al., 2022, 2023; Zeng et al., 2024, 2025). Modeling and engine-bench work mapped the coupled adsorption–desorption dynamics, including competition between water and HC for adsorption sites (Sharma and Shane, 2016; Peng et al., 2018; Feng et al., 2022). The durability of these active traps, however, has only been demonstrated at up to 800–900 °C — below close-coupled gasoline severity — and the most recent work in that line (Zeng et al., 2025) states that robust high-performance trapping at ≥800 °C 'remains a great challenge'.

The unresolved scientific problem is the conjunction: the trap that is durable at close-coupled gasoline temperatures does not purify its own load, and the traps that purify their load do not survive close-coupled temperatures.

Problem Statement

Close the purification gap of zeolite-based cold-start hydrocarbon traps without sacrificing close-coupled gasoline durability. Concretely: identify or design a zeolite-based trap material and trap–TWC system such that after full-life hydrothermal aging representative of close-coupled gasoline operation (about 950–1000 °C in roughly 10% water, 150,000-mile-equivalent) the system (i) still adsorbs a substantial fraction of cold-start HC — including aromatic species such as toluene, and olefins in the presence of copious water vapor — retaining at least the source system's aged cold-start trapping performance (A ≈ 56% of the cold-start HC), and (ii) purifies essentially all of the adsorbed HC, reducing the unpurified fraction released during desorption substantially toward zero — from the source's 23% of the adsorbed HC (B/A = 77%) to a small residual (B/A ≥ 95%, i.e. at most 5% of the adsorbed HC). A complete answer additionally provides direct mechanistic evidence — beyond the NMR/TG-MS based hypothesis of aluminum phosphate formation plus increased hydrophobicity — for whatever stabilization makes the material survive the aging, sufficient to guide rational design. These results jointly define one objective, a durable self-purifying gasoline HC trap: performance is meaningless without the aging severity, and durability without purification leaves the 23% slip intact. The performance must hold under realistic exhaust composition (water, CO, NOx) and cold-start transient cycle conditions of the kind in which the gap was measured (LA4-like cold-start cycle, aged close-coupled layout). The source's further system-level questions — behavior over broader real-world drive cycles and longer aging, and interactions of the trap with NOx and CO — define the operating regime rather than separate objectives.

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