**1. Introduction**

For health and environmental considerations, the current community strongly demands to improve air quality. Environmental protection agencies around the world have introduced more and more stringent motor vehicle exhaust emission regulations on CO, nitrogen oxides (NOx), unburned hydrocarbons and particulate matter [1]. The commercial technology of pollutants elimination is to adopt efficient catalytic converters. Cu/SSZ-13 (SSZ-13 is one framework type code of chabazite zeolite), one of the most excellent NO<sup>x</sup> removal catalysts, has been commercially used in the exhaust after-treatment system of medium and heavy diesel vehicles [2–6].

In the past decade, numerical theoretical and experimental studies have revealed the pore structure, NH3-SCR catalytic active sites, high-temperature hydrothermal stability, sulfur poisoning mechanism and alkali metal poisoning mechanism of Cu/SSZ-13 [7–20]. Alkali metals, such as sodium (Na) and potassium (K) are present in conventional diesel fuel and biodiesel fuel, and have been implicated in internal injector deposits [21,22]. Besides, alkali metals also exist in automotive urea, such as diesel exhaust fluid (DEF) or AdBlue. Previous reports have shown that alkali metal ions are volatilized under hightemperature conditions and diffuse to the surface of the catalyst, causing the catalyst to be poisoned [23,24]. Fundamental researches have revealed the alkali metal poisoning mechanism of Cu/CHA catalysts [25,26]. Researchers suggest that alkali metal ions can destroy the active center Cu2+ and degrade the zeolite framework, causing irreversible deactivation. However, how to improve the tolerance of Cu/zeolite catalysts to alkali metals is rarely reported. Yan et al. reported that highly dispersed Cu4AlOx mixed oxides have a good alkali metal resistance [27]. However, the multiple Cu complex cannot fit in

**Citation:** Chen, Z.; Shen, M.; Wang, C.; Wang, J.; Wang, J.; Shen, G. Improvement of Alkali Metal Resistance for NH3-SCR Catalyst Cu/SSZ-13: Tune the Crystal Size. *Catalysts* **2021**, *11*, 979. https:// doi.org/10.3390/catal11080979

Academic Editor: Anker Degn Jensen

Received: 21 July 2021 Accepted: 16 August 2021 Published: 16 August 2021

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the CHA's steric pore size and the inactive CuO clusters are easily formed in the CHA zeolites [28]. Du et al. compared the alkali resistance of conventional V2O5/WO3−TiO<sup>2</sup> catalysts and Fe2O3/HY zeolite catalysts and found that the HY zeolites played an alkalibuffer role to retain the catalytic activity [29]. Zha et al. developed a hollandite Mn–Ti oxide promoted Cu-SAPO-34 (SAPO-34 is one framework type code of chabazite zeolite) catalysts (HMT@Cu-S). HMT layer can trap alkali metal ions to protect Cu/SAPO-34 [30]. The similarity of previous research lies in the use of outer shell materials to protect the inner structure. However, the synthesis of core–shell materials often requires fine control, and it is difficult to achieve industrial low-cost production. To meet industrial needs, we tried to develop a simple preparation process to obtain core–shell-like molecular sieves with good alkali metal resistance. In our previous study [31], it was found that different crystal sizes of the Cu/SAPO-34 can affect the spatial distribution of acid sites, which inspired us to adjust the acid sites on the outer surface of the molecular sieve by tuning the crystal size.

In this work, we report that Cu/SSZ-13 catalysts with different crystal sizes (0.4–2.3 µm) present distinct sodium metal ions resistance. Due to the similar deactivation effect of Na and K, we only choose Na to study the improvement of alkali metal resistance [25]. Na-poisoned Cu/SSZ-13 with crystal size of 2.3 µm showed much higher NH3-SCR activity than ones with a size of 0.4 µm. The distribution of acidity and Cu ions is characterized by H2-TPR, UV–vis, diethylamine-TPD, and pyridine-DRIFTs. By enlarging the crystal size of SSZ-13, the active center Cu ions are allowed to locate the subsurface or inner core, so that the outer layer of the zeolites buffer the foreign alkali metal poisoning. Adjusting the distribution of acid sites by modifying the crystal size provides a simple technical route to improve alkali metal poisoning.

#### **2. Results and Discussions**

The crystallization process of SSZ-13 zeolites is mainly divided into induction period and crystallization period [32]. During the induction period, the raw materials in the gel gradually gather to form crystal nuclei, and the crystal grows rapidly after entering the crystallization period. The crystal size of SSZ-13 can be affected by the crystallization time, crystallization temperature, gel pH and added amount of crystal seed. Through a series of trials, we found that the crystal size of SSZ-13 is most sensitive to the amount of seed crystal added. The as-prepared catalysts are visualized by SEM. As shown in Figure 1, the average crystal size of each sample is obtained based on the statistical results of ~100 grains. All samples present cubic shape and uniform particle size. The results show that the crystal sizes of the synthesized SSZ-13 are 0.4, 0.8, and 2.3 µm.

1 **Figure 1.** SEM images of Cu/SSZ-13 catalysts with different crystal sizes.
