Selective catalytic reduction of NO_x with NH₃ assisted by non-thermal plasma over CeMnZrO_x@TiO₂ core–shell catalyst

1.   Introduction

With the rapid development of the transportation industry, the emission of high-power marine diesel engine exhaust has aroused widespread concern from all over the world. The marine diesel engine mainly uses heavy fuel oil with high sulfur as fuel, and its major pollutants are NO_x and SO₂, which not only destroy the ecological environment but also pose great threats to human beings. The selective catalytic reduction of NO_x with ammonia (NH₃-SCR) is the most effective technology to remove NO_x from marine diesel engine exhaust, and has been successfully applied in the field of mobile source exhaust for several years [1–4]. V₂O₅–WO₃/TiO₂ has been commercially used in the exhaust treatment of marine diesel engines, but several issues have severely impeded its further application. The main obstacles of V-based catalysts are the narrow operating temperature window, high SO₂ oxidation activity and biological toxicity of V species. In addition, the exhaust temperature of a marine diesel engine is usually between 150 ℃ and 200 ℃ [5, 6]. Previous studies have reported that Ce- and Mn-based SCR catalysts boast remarkable low-temperature activity on account of their superior performance in the store–release of O and excellent redox ability. However, they are easily deactivated in the presence of SO₂ because (NH₄)₂SO₄, NH₄HSO₄ and metal sulfates can block and destroy the active sites [7, 8]. In these circumstances, constructing an SCR catalyst with core–shell structure could improve the SO₂ tolerance effectively [9–11]. Gan et al synthesized a MnO_x@CeO₂ core–shell catalyst using the chemical deposition method, and it exhibited excellent low-temperature activity and strong SO₂ tolerance. Further research showed that the high activity could be attributed to the rich Mn⁴⁺/Ce³⁺ species and highly crystalline α-MnO₂, while the strong tolerance to SO₂ was ascribed to the CeO₂ shell that protected the active α-MnO_x species from been poisoned [12].

Non-thermal plasma (NTP) has been confirmed to be a prospective technology for the removal of NO_x, which offers an unconventional way to initiate chemical reactions in gases and liquids but suffers from low selectivity when it is used alone [13, 14]. Currently, plasma catalysis has been a hot topic of research on account of its potential for applications in a wide range of chemical reaction, environmental protection and energy-related processes. Moreover, the coupling of plasma and catalysis can steer the reactions in the desired direction, thus providing improved selectivity and reducing unwanted by-products. Overall, plasma catalysis can improve energy efficiency and product selectivity, and sometimes it shows a synergistic effect, which promotes low-temperature activity while providing a high NO_x conversion in a wide operating window [15–18]. During the removal of NO_x, NTP generates plenty of reactive O species (ROS), such as ·O and ·OH, and NO can be oxidized to NO₂ by ROS. A fast SCR reaction (equation (2)) occurs when the amounts of NO and NO₂ are equimolar in an NTP system, and its reaction efficiency is faster than that of the standard SCR reaction (equation (1)) [19, 20].

There are two combinations of NTP and NH₃-SCR: in-plasma catalysis (IPC) and post-plasma catalysis (PPC). IPC is usually called a single-stage plasma-catalytic system, in which the SCR catalyst is placed in the discharge zone, and its advantage is that active species can react on the SCR catalyst surface. In contrast, PPC is called a two-stage configuration, which involves placing the SCR catalyst downstream from the NTP. The oxidation of NO to NO₂ in NTP increases SCR catalytic performance in the two-stage configuration because the NO₂ removal is better than NO removal near the SCR catalyst [21–24]. This work reports the deNO_x performance and catalytic stability of an NTP-SCR system to reveal the effect of influencing parameters on NO_x conversion via placing a CeMnZrO_x@TiO₂ core–shell catalyst downstream from NTP. It is of great significance to investigate the NO_x conversion of NH₃-SCR assisted by NTP, which can provide technical guidance and theoretical support to remove NO_x from marine diesel engine exhaust.

2.   Experimental

2.1   Catalyst preparation

There are two steps in the preparation of the CeMnZrO_x@TiO₂ catalyst. Firstly, for the CeMnZrO_x core a sol–gel method was adopted. The molar component loading of Ce: Mn: Zr was 7:2:1, and the corresponding Ce(NO₃)₂, Mn(NO₃)₂ and Zr(NO₃)₂ were dissolved in the deionized water. In addition, the C₆H₈O₇ was added at a 1:1 ratio, and the mixture was peptized by stirring for 6 h at room temperature to form a highly dispersed CeMnZrO_x sol. After that, it was dried into gel at 100 ℃ for 12 h, followed by calcination in the muffle furnace at 500 ℃ for 4 h to obtain a CeMnZrO_x catalyst. Secondly, 300 mg CeMnZrO_x catalyst was dispersed in 400 ml ethanol. Subsequently, 36 ml tetrabutyl titanate and 3 ml hydrous ammonia were added separately and stirred continuously for 4 h at 60 ℃. The sample was obtained by centrifugation, washed with ethanol and dried at 100 ℃ for 12 h, followed by calcination 500 ℃ for 4 h. Finally, all samples were pressed, crushed and sieved to 40–60 meshes for catalytic evaluation of NH₃-SCR assisted by NTP.

2.2   Catalyst characterization

The morphology analysis of the SCR catalysts was characterized by scanning electron microscopy (SEM, JEOL, Japan). The specific surface area of the SCR catalysts was determined by the Brunauer–Emmett–Teller (BET) method, and the pore volume and average pore diameter of the SCR catalysts were obtained from the desorption curve using the Barrett–Joyner–Halenda (BJH) method. Prior to the measurements, all samples were outgassed for 12 h at 200 ℃ in a vacuum. X-ray diffraction (XRD) measurements were carried out on an XRD–7000 S (Shimadzu Corporation, Japan), which was operated at 40 kV and 40 mA using Cu Kα radiation. The scanning angle (2θ) scans ranged from 10° to 80° with a step of 0.02°, and the scanning speed was 5° min^–1. H temperature-programmed reduction (H₂-TPR) measurements of the SCR catalysts were performed using a Micromeritics AutoChem Ⅱ 2920 chemisorption analyzer with a thermal conductivity detector (TCD). A 100 mg quantity of the catalyst was preprocessed at 200 ℃ for 1 h under Ar atmosphere and then cooled to room temperature. After that, the temperature was raised to 800 ℃ at 10 ℃ min^-1 with a flow of H₂ (10%)/Ar (50 ml min^-1). Temperature-programmed desorption by ammonia (NH₃-TPD) experiments were carried out on the Micromeritics AutoChem Ⅱ 2920. 100 mg catalyst was pretreated at 500 ℃ for 1 h in N₂ (50 ml min^-1) and cooled down to 120 ℃, then 10% NH₃/N₂ (50 ml min^-1) was passed through the catalyst bed for 1 h to obtain saturation. After purging with N₂ for 1 h, the temperature was increased to 800 ℃ at a rate of 10 ℃ min^-1.

2.3   Catalytic evaluation of NH₃-SCR assisted by NTP

The schematic diagram of NH₃-SCR assisted by NTP is shown in figure 1. The reaction gas mixture simulated the marine diesel engine exhaust, which consisted of 500 ppm NO, 500 ppm NH₃, 0–300 ppm C₃H₆ (when used), 100 ppm SO₂ (when used), and air as the balance gas. The total flow rate of feed gas was kept constant at 500 ml min^-1, corresponding to a gas hourly space velocity (GHSV) of 30 000 h^-1. The concentration of NO_x (NO_x =NO+NO₂), N₂O and NH₃ in the inlet and outlet gas was analyzed by a Fourier-transform infrared spectrometer (FT-IR, Shimadzu IRTracer–100). The NTP adopted DBD plasma reactor was powered by an alternating current (AC), and the output waveform was observed on an UNI–T UPO3254CS oscilloscope. A 1.0 ml quantity of SCR catalyst was employed, which was placed downstream from the NTP, and the reaction temperature was maintained at 150 ℃ in order to simulate the actual condition of marine diesel engine exhaust. The mathematical expressions for NO_x conversion and N₂ selectivity are as follows:

where ${\mathrm{NO}}_{x,\mathrm{in}}$ is the NO_x concentration at the inlet of the DBD reactor, ${\mathrm{NO}}_{x,\mathrm{out}}$ signifies the NO_x concentration at outlet of the DBD reactor, and ${\mathrm{N}}_2{\mathrm{O}}_{\mathrm{out}}$ represents the N₂O concentration at the outlet of the DBD reactor.

Figure  1.  Schematic diagram of NH₃-SCR assisted by NTP.

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The catalytic evaluations of the CeMnZrO_x@TiO₂ and CeMnZrO_x catalysts were tested three times, and if the experimental data fluctuated by more than 10%, it was discarded. Otherwise, it was averaged. The error bar was used as an index to evaluate the difference between the average and the measurement.

where $N_{\mathrm{a}}$ signifies the average of three experiment results. $N_1,$$N_2$ and $N_3$ represent the results of the experiment.

3.   Results and discussion

3.1   Characterizations of SCR catalysts

Figure 2 displays the SEM images of the CeMnZrO_x@TiO₂ and CeMnZrO_x catalysts. As depicted in figure 2(a), the CeMnZrO_x@TiO₂ core–shell structure catalyst presents a spherical particle and relatively smooth surface, and the outer surface is coated by a TiO₂ shell. By contrast, the CeMnZrO_x catalyst possesses a loose structure and rich porosity, which is beneficial for the high dispersion of metal oxides on the CeMnZrO_x surface [25]. The BET surface area, pore volume and average pore diameter results of the SCR catalysts are summarized in table 1. Visibly, the BET surface area of the CeMnZrO_x@TiO₂ catalyst is higher than that of CeMnZrO_x, and the pore volume and average pore diameter of CeMnZrO_x@TiO₂ are about 0.124 cm³ g^-1 and 1.98 nm, respectively.

Figure  2.  SEM images of SCR catalysts. (a) CeMnZrO_x@TiO₂ and (b) CeMnZrO_x.

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Table  1.  Physical characteristics of the CeMnZrO_x@TiO₂ and CeMnZrO_x catalysts.

SCR catalysts	BET surface area (m2 g-1)	Pore volume (cm3 g-1)	Average pore diameter (nm)
CeMnZrOx@TiO2	87±0.76	0.124±0.01	1.98±0.02
CeMnZrOx	82±0.83	0.139±0.02	2.16±0.01

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The XRD patterns of the CeMnZrO_x@TiO₂ and CeMnZrO_x catalysts are presented in figure 3. As illustrated in figure 3, the CeMnZrO_x@TiO₂ catalyst only exhibits CeO₂ crystalline phase, and the signals of MnO_x, ZrO₂ and anatase TiO₂ phases are not observed over CeMnZrO_x@TiO₂, which means that the MnO_x, ZrO₂ and TiO₂ species are homogeneously dispersed on the surface of CeMnZrO_x@TiO₂ as amorphous oxides or aggregated in minicrystals that are too small to be detected by XRD. Moreover, it is worth mentioning that the intensity of the diffraction peak attributed to CeO₂ crystalline phase over CeMnZrO_x@TiO₂ is reduced markedly compared to CeMnZrO_x, which may be attributed to the coating of the TiO₂ shell.

Figure  3.  XRD patterns of CeMnZrO_x@TiO₂ and CeMnZrO_x catalysts.

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Figure 4(a) illustrates the NH₃-TPD curves of the CeMnZrO_x@TiO₂ and CeMnZrO_x catalysts in the temperature range of 100 ℃–800 ℃. As plotted in figure 4(a), it is obvious that the CeMnZrO_x@TiO₂ catalyst demonstrates two NH₃ desorption peaks at 302 ℃ and 520 ℃, respectively. In addition, the NH₃ desorption signal of CeMnZrO_x catalyst occurs at 293 ℃, which exhibits a board peak ranging from 100 ℃ to 800 ℃. The H₂-TPR profiles of CeMnZrO_x@TiO₂ and CeMnZrO_x catalysts are presented in figure 4(b), and it can be seen that there are two reduction peaks at 210 ℃ and 435 ℃, respectively, which can be attributed to the reduction of CeO₂ and MnO_x. Furthermore, it can be observed that the reduction peak of the CeMnZrO_x@TiO₂ catalyst moves toward lower temperature, and the peak area of the CeMnZrO_x@TiO₂ catalyst is larger than that of CeMnZrO_x as elucidated in table 2, which indicates that there are more reductive sites in the surface of CeMnZrO_x@TiO₂.

Figure  4.  Chemical adsorption profiles of CeMnZrO_x@TiO₂ and CeMnZrO_x catalysts. (a) NH₃-TPD and (b) H₂-TPR.

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Table  2.  NH₃ and H₂ consumption of CeMnZrO_x@TiO₂ and CeMnZrO_x catalysts.

Samples	NH3 consumptiona (μmol g-1)	H2 consumptionb (μmol g-1)
CeMnZrOx@TiO2	95.4	1349
CeMnZrOx	73.6	1085
NH3 consumption derived from NH3-TPD data. H2 consumption calculated from H2-TPR data.

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3.2   Effect of specific energy input on NO_x conversion

The discharge power of the DBD reactor was determined by checking the charge–voltage (Q–U) Lissajous, and the specific energy input of the NTP system was calculated in the following equations (7) and (8).

P represents the discharge power (W). T is the period of the discharge cycle (s). u(t) is the high voltage applied between electrodes (kV). i(t) is the current change with time (A). f is the discharge frequency of the AC power signal (Hz). u_c(t) is the voltage at both ends of C_m (kV). C_m is the measuring capacitance, which is equal to 0.36 μF. A is the area of Q–U Lissajous graphics. SEI represents the specific energy input of the reaction system (J l^-1). Q signifies the total gas flow rate (l min^-1).

The effect of SEI on NO_x conversion and N₂ selectivity in the NTP-SCR system is shown in figure 5(a). Obviously, the NO_x conversion is very low if using NTP alone because NO is mostly converted into NO₂ in the NTP gas-phase reaction, and NO₂ does not react with NH₃ adequately to generate N₂ and H₂O without an SCR catalyst. The conclusion of NO being oxidized to NO₂ can be drawn from figure 5(b). In addition, the N₂ selectivity is also poor due to the reactions of (9) and (10). Furthermore, it can be seen that the NO_x conversions of CeMnZrO_x@TiO₂ and CeMnZrO_x are just 43.9% and 36.2% without NTP, respectively. In sharp contrast to this result, the NO_x conversion improves gradually with the increase of SEI when NTP is operated, and the NO_x conversion of CeMnZrO_x@TiO₂ reaches the maximum value of 73.96% at an SEI of 28 J l^-1. However, the NO_x conversion increases slowly when the SEI exceeds 28 J l^-1, which might be ascribed to the enhanced reverse reaction (equation (24)) due to the presence of a higher concentration of atomic O species.

Figure  5.  Effect of SEI on catalytic performance. (a) NO_x conversion and N₂ selectivity; (b) concentrations of NO, NO₂ and NO_x at the outlet of the DBD reactor.

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In the NTP-SCR system, NO can be oxidized by atomic O species generated by the NTP gas-phase reaction, which is beneficial to the SCR reaction to a certain degree. Zhao et al revealed that the rate constant of O₂ dissociation by electron collision reactions was almost two orders of magnitude higher than that of N₂ dissociation, and the concentration of atomic O species increases with increasing SEI. In this case, the rate constants of NO_x conversion decreased with increasing atomic O species concentration because the atomic O species were electronegative and thus reduced electron concentration [26]. The above experimental phenomenon is consistent with the research of Jiang et al [27], who also found that the heat effect caused by NTP had a negligible impact on enhanced NO_x conversion, while the intermediates generated in the NTP system might significantly contribute to the improved SCR catalytic performance of NO_x on the catalyst surface in the NH₃-SCR process. Moreover, the N₂ selectivity is maintained at 100%, which is almost independent of SEI. The by-products of N₂O and N₂O₅ are not detected in the outlet gas, which indicates that the NH₃-SCR assisted by NTP does not cause secondary pollution. The following chemical reactions occur in the NTP-SCR system according to the relevant reports [28–30].

where ${\mathrm{O}}_2*$ is an excited O₂ molecule, and ${\mathrm{N}}_2*$ is an excited N₂ molecule.

3.3   Effect of C₃H₆ on NO_x conversion

Previous studies have demonstrated that hydrocarbons can be attributed an important role in the removal of NO_x using NTP technology, and the reaction paths for NO_x removal can change significantly from those without hydrocarbon additives. C₃H₆ is a representative hydrocarbon on account of its superior efficiency of NO oxidation, which can significantly enhance NO to NO₂ conversion performance as an essential step for the subsequent NH₃-SCR reaction paralleled with economy in the input energy cost. Therefore, the effect of C₃H₆ on NO_x conversion over CeMnZrO_x@TiO₂ and CeMnZrO_x catalysts was investigated at 28 J l^-1, and figure 6 indicates the detailed results. Clearly, it can be seen that the NO_x conversion improves greatly when introducing the C₃H₆ additive to the NTP-SCR system due to the reactions between the hydrocarbons and NO_x. Meanwhile, the concentration of NO₂ increases significantly in the NTP gas-phase reaction as depicted in figure 3, which is beneficial for the fast SCR reaction in the subsequent device. The experimental result is similar to the research of Jiang et al [27], who reported that the NO_x conversion improved gradually with increasing C₃H₆ concentration, and that the addition of C₃H₆ not only promoted the conversion of NO into NO₂ in the NTP system but also enhanced the catalytic reduction of NO_x on the catalyst surface. Further research found that the oxidation of C₃H₆ with O atoms could generate oxygenates in first-stage plasma, and there might be two effects that could result in the enhanced reduction of NO_x. On one hand, the O atoms might be consumed by C₃H₆, which limits the reverse reaction equation (24). On the other hand, NO might react with oxygenates to produce N₂, as shown in equation (38). Guan et al also found that the NO_x removal efficiency was improved significantly at lower temperatures in the NH₃-SCR system because C₃H₆ could be decomposed into useful intermediates such as methyl, methoxy radicals, and partial oxidation products of C₃H₆ conversion, including peroxyl radicals and hydro-peroxy radicals [31]. In the presence of C₃H₆, specific pathways between NO and hydrocarbons could occur in the NTP-SCR system, such as the following chemical reactions [32–34]:

Figure  6.  Effect of C₃H₆ on catalytic performance at an SEI of 28 J l^-1. (a) NO_x conversion and N₂ selectivity; (b) concentrations of NO, NO₂ and NO_x at the outlet of the DBD reactor.

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3.4   Effect of H₂O on NO_x conversion

It is noted that in practical applications, H₂O is an inevitable combustion product in marine diesel engine exhaust and could generate a variety of strong oxidizing free radicals though NTP gas-phase reaction, which usually contributes to the oxidative removal of NO_x [35]. Numerous investigations have indicated that the conversion of NO to NO₂ is an important intermediate step in the reduction of NO_x to N₂, and the most efficient approach to achieve it is using NTP combined with an SCR catalyst. First of all, NO is oxidized to NO₂ in the presence of HC and H₂O using NTP technology. Then, the catalyst reduces NO₂ to N₂ by selective reduction using NH₃ as a reductant. However, the adsorbed H₂O molecules can consume high-energy electrons due to their electronegativity, and the specific energy input of the NTP system declines significantly when the H₂O content exceeds the limit [36–38]. Beyond that, H₂O can also poison and deactivate the catalyst by inhibition of the active sites. In light of the characteristics of marine diesel engine exhaust, the effect of H₂O on NO_x conversion at a specific energy input of 28 J l^-1 was investigated under different H₂O contents of 0, 3 vol.%, 5 vol.%, 7 vol.% and 10 vol.%. As can be seen from figure 7, the NO_x conversion over CeMnZrO_x@TiO₂ increases slightly from nearly 73.96% to about 82.67% when H₂O is added to the NTP-SCR system. However, the NO_x conversion over CeMnZrO_x increases first and then decreases when the H₂O concentration ranges from 0 to 10 vol.%. That is because H₂O can inhibit the removal of NO_x due to the competitive adsorption of NO, NH₃ and H₂O on the active sites, and it gives the conclusion that the TiO₂ shell of CeMnZrO_x@TiO₂ can prevent the adsorption of H₂O. Similarly, the concentration of NO₂ increases slightly when adding an appropriate amount of H₂O to the NTP-SCR system. Jiang et al reported that another reaction pathway for NO_x removal exists on the SCR catalyst surface in the presence of H₂O [27]. The related chemical reactions are involved when there is H₂O in the NTP system according to the relevant reports, which are as follows [39–42]:

Figure  7.  Effect of H₂O on catalytic performance at an SEI of 28 J l^-1. (a) NO_x conversion and N₂ selectivity; (b) concentrations of NO, NO₂ and NO_x at the outlet of the DBD reactor.

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3.5   Catalytic performance of SO₂ tolerance assisted by NTP

It is generally acknowledged that SO₂ easily forms unstable sulfite ions and further reacts with adsorbed O to generate metal sulfates, which seriously inhibit the catalytic activity of the SCR catalyst and eventually lead to SCR catalyst deactivation [43–46]. The SO₂ tolerance of the CeMnZrO_x@TiO₂ and CeMnZrO_x catalysts at a specific energy input of 28 J l^-1 was explored in the presence of 100 ppm SO₂, and the detailed results are presented in figure 8. As depicted in figure 8, the catalytic activity of CeMnZrO_x@TiO₂ decreased slightly after introducing the SO₂, and then remained almost constant. After that, the NO_x conversion recovered to the original level and remained stable for 6 h when the SO₂ was removed, which demonstrated that the addition of SO₂ had little effect on the SCR activity of CeMnZrO_x@TiO₂. By contrast, the NO_x conversion of CeMnZrO_x was severely suppressed when SO₂ was added to the NTP-SCR system, and the NO_x conversion of that was restored to a certain extent but was lower than the initial value when the SO₂ was cut off. Combined with the above observations, the CeMnZrO_x@TiO₂ core–shell catalyst possesses a stronger tolerance to SO₂ than CeMnZrO_x, and it can be deduced that the formation of (NH₄)₂SO₄, NH₄HSO₄ and metal sulfates may be restrained due to the presence of the TiO₂ shell, which can react with SO₂ and serve as a protective layer.

Figure  8.  Catalytic performance of SO₂ tolerance assisted by NTP in the presence of 100 ppm SO₂ at an SEI of 28 J l^-1.

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3.6   Catalytic stability of CeMnZrO_x@TiO₂ catalyst

The long-time stability is of vital importance for the SCR catalyst in the NTP-SCR system and can provide a valuable reference for practical applications. The catalytic stability of CeMnZrO_x@TiO₂ at a GHSV of 20 000 h^-1 was evaluated in this section. The stability test was carried out at an specific energy input of 28 J l^-1, and the tested concentrations of NO, NH₃, C₃H₆ and SO₂ were 500 ppm, 500 ppm, 300 ppm and 100 ppm, respectively. The H₂O concentration was kept at 10%, and the experimental result is depicted in figure 9. As can be seen from figure 9, the running time of the whole progress was 18 h without interruption, and the result of this continuous test illustrated that the activity of CeMnZrO_x@TiO₂ depressed little within 18 h of reaction. Remarkably, the NO_x conversion was maintained at 91.25% during the test period, which indicates that the CeMnZrO_x@TiO₂ catalyst exhibits superior catalytic stability over the entire range of the time frame investigated. The good stability of the CeMnZrO_x@TiO₂ catalyst in the NTP-SCR system provides the possibility for practical elimination of NO_x from marine diesel engine exhaust.

Figure  9.  Catalytic stability of the CeMnZrO_x@TiO₂ catalyst in the presence of 300 ppm C₃H₆, 5 vol.% H₂O and 100 ppm SO₂ at an SEI of 28 J l^-1.

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4.   Conclusions

This work has verified that the catalytic performance of NH₃-SCR assisted by NTP for NO_x removal is an effective approach, and the NO_x conversion of CeMnZrO_x@TiO₂ is enhanced significantly when assisted by NTP compared to the catalytic activity of NH₃-SCR without NTP, even in the presence of C₃H₆ and H₂O. The CeMnZrO_x@TiO₂ core–shell structure catalyst possesses excellent low-temperature activity and superior catalytic stability in the NTP-SCR system, and it exhibits strong tolerance to SO₂ poisoning because the TiO₂ shell prevents the generation of (NH₄)₂SO₄, NH₄HSO₄ and metal sulfates from blocking the active sites. It is expected that this work can provide a new strategy for the removal of NO_x from marine diesel engine exhaust and it has huge potential for industrial applications in the future.