Nitrogen oxides (NO_x) emitted from vehicles are a major contribution to air pollution because of their toxicity. Selective catalytic reduction (SCR) with NH₃ is regarded as one of the most efficient approaches to remove NO_x, transforming them into non-toxic N₂ and H₂O. This approach is widely applied as a denitration (DeNO_x) process [1-3]. Various catalysts have been developed and tested [4-6]. For example, vanadium– tungsten–titania-based catalysts have been commercially used for more than three decades [7, 8]. However, these catalysts fail to convert NO_x under lean-burn conditions with high air/fuel ratios.

Zeolites have also been widely studied for SCR with NH₃, including ZSM-5 with the MFI structure, beta zeolite, SAPO-34, and SSZ-13 with the CHA structure [9-11]. The Cu (or Fe) ion- exchanged ZSM-5 exhibited better performance than a commercial vanadia–titania catalyst [12, 13]. However, their stability remains an issue because of the sintering of copper species and disruption of zeolitic crystallinity and porosity under harsh reaction conditions [14, 15]. In comparison, Cu/Fe ion- exchanged beta catalysts exhibited better durability [1, 16]. CHA- type zeolite with smaller pores and stronger acidity, especially SSZ-13 zeolite upon exchange with Cu²⁺, has been shown with better NH₃-SCR activity and selectivity than those of beta catalysts and ZSM-5 as well as higher hydrothermal stability [17-22]. However, these zeolite-based catalysts still undergo deactivation above 550 ℃. In real applications, the temperature can reach beyond 800 ℃, which frequently degrades the durability of the catalyst. Therefore, it is desirable to develop catalysts which can be applied at a wider temperature window beyond 550 ℃ or even higher.

Silicon carbide (SiC) is a chemically inert and mechanically stable material with a thermal conductivity that is two orders of magnitude higher than that of SiO₂ and three times higher than that of alumina [23-25]. With these interesting properties, we explore the use of SiC as the support for a zeolite-based SCR catalyst in an attempt to strengthen its anti-thermal shock and improve its high-temperature stability. Gu et al. [23] reported that a Mo-ZSM-5/porous SiC catalyst exhibited clearly improved activity in the methane dehydroaromatization reaction. In addition, Elamin and coworkers [24] reported that a SAPO-34/SiC composite with a foam structure exhibited excellent selectivity and stability in methanol dehydration to a dimethyl ether. In this study, we demonstrate that SSZ-13 can be grown directly on the surface of SiC using a hydrothermal method, as confirmed by powder X-raydiffraction (XRD), scanning electron microscopy (SEM), and nitrogen adsorption– desorption results. The use of the SiC support enhances the catalytic activity of Cu/SSZ in the NH₃-SCR reaction compared with that of unsupported Cu/SSZ-13.

NaOH, N, N, N-trimethyl-1-ammonium adamantane (TMAdaOH), Al(OH)₃, and fine SiO₂ powder were purchased from Sinopharm Chemical Reagent Co., Ltd., InnoCHEM, Tianjin Kemel Chemical Reagent Co., Ltd., and Shenyang Chemical Industry Co., Ltd., respectively. All the chemicals were directly used as received without further purification.

The pure SSZ-13 was synthesized using a hydrothermal method adapted from that reported by Shishkin et al. [26]. Typically, 4 g H₂O was added to 3 g NaOH aqueous solution (1 mol/L), followed by the addition of 4 g TMAdaOH. After stirring for 30 min, 0.1 g Al(OH)₃ and 1.2 g SiO₂ were added to the mixture. The resulting suspension was then transferred into a 50 mL Teflon-lined stainless-steel autoclave. The autoclave was sealed and maintained at 160 ℃ for 2 d in a rotary oven (0.7 r/min) and subsequently cooled to room temperature. The resulting white powder was washed with ethanol and deionized water three times, sequentially, using filtration, followed by drying in air at 100 ℃ overnight. Finally, the powder was calcined for 5 h at 550 ℃.

SiC samples were provided by BASF. The SSZ-13 was grown on SiC using a hydrothermal synthesis method. First, the mother liquor was prepared following the same procedure as that for the unsupported SSZ-13. Then, SiC with dimensions of 0.5 cm × 0.5 cm × 1 cm was added into the mother liquor in an autoclave with a capacity of 50 mL. After reaction at 160 ℃ in a rotary oven (0.7 r/min) for varying periods, the composites were collected and ultrasonically washed with deionized water in a beaker and dried in air at 100 ℃ overnight. SSZ-13@SiC was finally obtained following calcination for 5 h at 550 ℃.

The SSZ-13@SiC was subjected to ion exchange using 0.5 mol/L Cu(NO₃)₂ aqueous solution at 80 ℃ for different durations, followed by calcination for 5 h at 550 ℃. The resulting catalyst was named Cu(X)/SSZ-13@SiC, where X represents the Cu loading (in mass percentage). For comparison, unsupported Cu/SSZ-13 was also prepared following the same method.

Powder XRD was performed on a Panalytical X'Pert Empyrean-100 diffractometer using a Cu K_α source (λ = 1.5418 ) at 40 kV and 40 mA. The patterns were recorded in the range of 2θ = 5°–50° using a step of 0.19°/s. SEM was performed on a FEI Quanta 200 F microscope. The Cu loadings were measured using inductively coupled plasma optical emission spectrometry (ICP-OES; PerkinElmer 7300 DV). N₂ adsorption–desorption isotherms were measured at −196 ℃ using a Quantachrome QUADRASORB SI system. The specific surface areas of the samples were calculated using the Brunauer-Emmett-Teller (BET) equation.

The catalyst (0.18 g) with a size of 40–60 mesh was loaded into a fixed-bed microreactor made of quartz with an inner diameter of 6 mm. The reaction was performed under the following conditions: 500 ppm NH₃, 500 ppm NO, 10 vol% O₂, 5 vol% H₂O, balance N₂, 400 mL/min total gas flow, and 80000 h⁻¹ gas hourly space velocity (GHSV). The concentration of NO in the effluent was analyzed using an ECOTCH ML9841AS analyzer. The NO conversion was calculated using the following equation: NO conversion = (C_{NO, in} – C_{NO, out})/C_{NO, in} × 100%.

Fig. 1 presents XRD patterns of the SiC support, pure SSZ-13, and SSZ-13@SiC. The XRD pattern for the SiC support (Fig. 1(a)) contains typical diffraction peaks for hexagonal SiC at 34.1°, 35.6°, 38.1°, and 41.4° corresponding to the (101), (006), (103), and (104) planes (PDF #49-1428), respectively. In addition, the peak at 21.6° is attributed to cubic SiO₂ (111) (PDF #27-0605), and those at 28.4° and 47.3° are indexed as the cubic Si (111) and (220) planes (PDF #27-1402), indicating that the SiC support contained SiO₂ and Si impurities. The XRD pattern of SSZ-13 showed a well-crystalized CHA structure without impurities. Fig. 1(c) reveals the coexistence of SSZ-13 and SiC, suggesting that the SSZ-13 layer was successfully grown in the presence of SiC. Furthermore, the structure of SiC was not damaged because all the characteristic diffraction peaks were retained. However, the SiO₂ and Si diffraction peaks initially present in the SiC support disappeared, which indicates that SiO₂ and Si were consumed as Si sources for the growth of SSZ-13.

Fig. 2(a) shows that the fresh SiC is almost black. Following hydrothermal synthesis for 5 d in the rotating oven, it became pale white (Fig. 2(d)), indicating that the SiC surface was successfully covered with a layer of SSZ-13. No obvious white powder peeled off when the SSZ-13@SiC was repeatedly rubbed on a piece of black cloth, which indicates that the SSZ-13 layer was rather strongly attached to the SiC support, likely via chemical bonding at the interface. This bonding occurred because SiC contains SiO₂ and Si, which act as Si sources for the nucleation and crystallization of SSZ-13 during the hydrothermal synthesis. Comparison between the images in Fig. 2(b) and (e) reveals that the disordered holes of the SiC support were filled with SSZ-13, and thus, the surface became much smoother. Closer inspection of Fig. 2(f) and its inset reveals the characteristic cubic morphology of SSZ-13. However, the particles were not very homogeneous, which was likely induced by the inhomogeneity of the SiC surface (Fig. 2(c)).

Considering that the alkalinity of the pregnant solution has an important effect on the growth of zeolite, we varied the amount of NaOH aqueous solution during hydrothermal synthesis. Fig. 3(a) shows that the resulting SSZ-13 exhibited a relatively low crystallinity for SiO₂/NaOH = 0.2. With increasing amount of NaOH, the diffraction peaks of the impurity become weaker and finally disappear for SiO₂/NaOH = 0.08. When the amount of NaOH was further increased, only the pure SSZ-13 phase was detected with a high crystallinity. This was further validated by SEM, as shown in Fig. 3(b)–(f). Many mussy and elliptical blocks were generated at low NaOH concentration (SiO₂/NaOH = 0.2), and only a few scattered small SSZ-13 cubes were observed. When the ratio of SiO₂/NaOH was decreased to 0.1, increasingly more cubes emerged, and the impurities with irregular shapes gradually disappeared (Fig. 3(e) and (f)). The size of these SSZ-13@SiC cubes varied from 2 to 7 μm.

The XRD patterns in Fig. 4(a) hardly show the characteristic diffraction of SSZ-13 on SSZ-13@SiC synthesized in only 1 d. Fig. 4(b) reveals the appearance of some sporadic cubic crystals on the surface of SiC, and the SiC surface was still exposed on the first day. Obviously, 1 d was not sufficient to grow a full layer of SSZ-13 crystals on the surface of SiC. With the hydrothermal synthesis time extended beyond 1 d, the crystallinity improved, as reflected by the larger and more angular crystals in the SEM images. Furthermore, the SiC surface was fully covered by the SSZ-13 grains after 2 d and no SiC was exposed. The samples prepared for 2–5 d all consisted of the pure phase of SSZ-13, and no other impurity phases were detected in the XRD patterns.

The nitrogen adsorption-desorption curves of different samples are displayed in Fig. 5(a), and their surface areas and textural properties are summarized in Table 1. Pure SSZ-13 exhibited a type-Ⅰ isotherm, which is characteristic of microporous materials. Its BET surface area was 567.7 m²/g, and its total pore volume and microporous pore volume were 0.31 and 0.30 cm³/g, respectively. In comparison, the SiC support exhibited a negligible pore volume and external surface area. The surface area increased from 22.0 to 201.3 m²/g after growth of SSZ-13 on the SiC support with increasing synthesis time from 1 to 3 d. However, beyond 3 d, the specific surface area did not change much further (Fig. 5(b)). Because SiC exhibits an insignificant adsorption of N₂ and specific surface area, it can be concluded that the loading of SSZ-13 on the SiC surface increased with synthesis time. It is interesting that the growth almost stopped after the third day. Note that the BET surface area and microporous pore volume of SSZ-13@SiC were lower than those of pure SSZ-13 because the SiC itself only contributed weight but not pores. The composite obtained for the 5 d synthesis (SSZ-13@SiC-5d) exhibited a relatively high BET surface area (196.8 m²/g) and micropore volume (0.10 cm³/g).

The Cu/SSZ-13@SiC catalyst was obtained through an ion-exchange process using the SSZ-13@SiC composites synthesized in 5 d. Fig. 6(a) shows that the Cu loading did not change the crystal phase of the SSZ-13@SiC-5d composite. No diffraction peaks related to Cu were detected, indicating well- distributed Cu species. In addition, the zeolite crystals remained cubic upon ion exchange and were homogeneous in size (Fig. 6(b) and (c)). Furthermore, Cu/SSZ-13@SiC exhibited a specific surface area of 193.0 m²/g, which was similar to that of SSZ-13@SiC-5d.

Fig. 7 shows the NH₃-SCR performance of the Cu/SSZ-13 and Cu/SSZ-13@SiC catalysts with different Cu loadings. The NO conversion increased with the Cu loading on Cu/SSZ-13@SiC in the range of 0.37–1.71 wt%. The catalyst with a loading of 0.37% (denoted Cu(0.37)/SSZ-13@SiC) exhibited the lowest activity. Hardly any conversion of NO to N₂ was observed at low temperatures. The highest NO conversion was only 44% at 350 ℃. Cu/SSZ-13@SiC with a Cu loading of 1.02% (Cu(1.02)/SSZ-13@SiC) performed obviously better, and the NO conversion reached 90% around 240 ℃ and remained above 90% until reaching 350 ℃. More interestingly, the use of Cu(1.71)/SSZ-13@SiC extended the application temperature even further because the NO conversion reached over 90% at temperatures as low as 200 ℃ and remained at 95% until reaching 400 ℃. It dropped below 90% only around 450 ℃ and still retained over 70% at 500 ℃. In comparison, the unsupported Cu(4.03)/SSZ-13 prepared using the same method exhibited similar catalytic performance as that reported for Cu(4.3)/SSZ-13 and Cu/SSZ-13 by Kwak et al. [27, 28] with an 80% ion-exchange degree. The NO conversion over Cu(1.71)/SSZ-13@SiC was practically the same as that over Cu(4.03)/SSZ-13 below 250 ℃. However, the NO conversion was much more stable at a higher temperature than that of the unsupported Cu(4.03)/SSZ-13. The latter gradually deactivated with increasing temperature, with NO conversion dropping below 90% at 400 ℃, which is much lower than that of Cu(1.71)/SSZ-13@SiC. This result clearly demonstrates the effects of SiC in enhancing the activity and stabilizing Cu/SSZ-13 at high temperatures.

SSZ-13 zeolite was successfully grown on the surface of SiC, a material that exhibits good thermal conductivity, wear resistance, and chemical inertness. The Si and SiO₂ impurities in the SiC support acted as silicon sources for the growth of SSZ-13, likely leading to the formation of a chemically bonded SSZ-13 layer on SiC. The effects of different synthesis parameters on the structure and morphology of SSZ-13@SiC were investigated, and the results indicated that the use of SiC as a support for Cu/SSZ-13@SiC enhanced its activity, particularly for the catalyst Cu(1.71)/SSZ-13@SiC. The NO conversion reached above 90% close to 200 ℃ and was retained at this level beyond 400 ℃. The use of the SiC support extended the application temperature window more broadly than that of the unsupported Cu(4.03)/SSZ-13 and enhanced the catalytic activity of Cu/SSZ-13 in NH₃-SCR, although the latter catalyst exhibited a much higher Cu loading.

Acknowledgments

The authors acknowledge the assistance from Prof. Chuan Shi and Ms. Qi Zhao during the NH₃-SCR activity tests. This work is supported by the INCOEmission project coordinated by BASF SE, Germany. Qing Yuan acknowledges the support from the Fundamental Research Funds for the Central Universities (grant no. DC201502080409).