Ambient NO₂ Removal by Transition Metal Ion-Exchanged Small-Pore Zeolites

Abstract

Atmospheric NO₂ abatement is challenging due to the ineffectiveness of the current techniques (e.g., SCR, SNCR, LNT, TWC, etc.) at low temperature (< 80 ^oC). Adsorption is utilized as an alternative approach to remove NO₂ in ambient condition in this study. Transition metal ion-exchanged zeolites are developed as NO₂ adsorbents based on the π-complexation interaction between transition metal ion and the molecules with π-bond (i.e., NO₂). Efforts are contributed to prepare adsorbents with high NO₂ selectivity and adsorption capacity. The high NO₂ selectivity, related to the high interaction strength of Pi-complexation, is achieved by altering the type and the valence state of the cation as well as the topology of the zeolites. The high NO₂ capacity is obtained by increasing the cation density in the zeolite through changing the zeolite with low Si/Al ratio, which enables a higher amount of adsorption sites for NO₂.

SSZ-13 (Si/Al=12) is initially selected as the support due to the low charge density (2.77 Al atom in each unit cell), which enables the existence of isolated cation in each unit cell. This simple interaction environment allows us to study the interaction mechanism between the adsorption sites and NO₂.

Various transition metal ions (Cu²⁺, Co²⁺, Ni²⁺, Zn²⁺, In³⁺, Cr³⁺, Ce³⁺ and Ag⁺) are introduced separately into SSZ-13 (Si/Al=12) as extra-framework cation through ion exchange to study their differential affinity to NO₂ molecule. Ni²⁺SSZ-13-12-L performs the best in terms of the highest NO₂ capacity (1.74 mmol/g) and low NO releasing amount (1.22 %) followed by Co²⁺- and Cu²⁺SSZ-13-12-L. The NO₂ capacity of the samples is found closely associated with the NO₂ uptake per cation, indicating the transition metal ions act as the dominant NO₂ adsorption sites. The NO₂ uptake on each cation is found correlated with the valance state and the effective nuclear charge of the cation. Proton can also act as adsorption sites for NO₂ in the samples undergoing incomplete ion exchange. NO₂ dissociation occurs on both extra-framework cation and the oxygen vacancies. The low level of NO releasing suggests the NO₂ adsorption on the as-prepared material is dominantly non-reactive adsorption.

Except for the type of cation, we also verified the NO₂ adsorption performance on the H₂-reduced Cu²⁺SSZ-13 (Si/Al=12) to understand the effect of the valence states on the formation and strength of Pi-complexation interaction. The Cun+SSZ-13 samples are prepared by reducing Cu²⁺SSZ-13 under H₂ at various temperatures (100, 150, 170, 190, 250, 390, 490 and 750 ^oC). Generally, at temperature below 190 ^oC, the NO₂ adsorption capacity increases with the increasing reduction temperature, which is attributed to the formation of higher proportion of Cu⁺ under higher temperature. 190 ^oC was determined to be the optimum temperature to prepare Cun+SSZ-13-R-A to achieve the highest NO₂ uptake (1.79 mmol/g), which is 19.4 % higher than that of Cu²⁺SSZ-13 (without H₂ pretreatment). This improvement is attributed to the improved proportion of Cu⁺ and the absence of Cu⁰ in the obtained sample. Apart from improving the NO₂ capacity, the formation of Cu⁺ also improves the affinity of the adsorbent to NO₂ simultaneously.

Co²⁺, Ni²⁺ and Cuⁿ⁺ ion-exchanged AEI-11, SSZ-13-6 and CHA-2 zeolites are prepared to understand the effects of topology and cation density on the NO₂ adsorption behavior. AEI-11 materials show a higher NO₂ adsorption capacity than SSZ-13-12, which is attributed to the differential location of extra-framework cations in these two supports, i.e., divalent cations preferring locating at 6-ring in SSZ-13-12 and at 8-ring in AEI-11. For Co²⁺ and Ni²⁺ ion-exchanged CHA zeolites with different Si/Al ratio, i.e., SSZ-13-12, SSZ-13-6, and CHA-2, the NO₂ adsorption capacity is proportional to the cation density, indicating the transition metal ions dominate as the adsorption sites for NO₂. The NO₂ adsorption on H₂ reduced CHA and AEI zeolites at 190 ^oC shows inconsistent behavior, suggesting the optimum reduction temperature varies among samples, which highly depends on the interaction strength between Cu²⁺ and the lattice oxygen atoms. Co²⁺CHA-2 shows outstanding NO₂ adsorption capacity of 4.65 mmol/g (unsaturated capacity); although it decreases to 3.54 mmol/g in the second cycle, it remains quite stable in the afterward multiple cycles. The NO releasing amount becomes negligible in the cyclic NO₂ adsorption, suggesting the oxygen vacancies in the fresh sample rendering the NO₂ dissociation, which can be healed by NO₂. The double-layer adsorber loaded with Co²⁺CHA-2 (in column 1) and Co²⁺SSZ-13-6 (in column 2) significantly reduces the NO releasing amount, which provides a route for designing effective NO₂ adsorber in practical application.

Date of Award	1 Sept 2020
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Jin SHANG (Supervisor)