A Novel Four-Way Plasma-Catalytic Approach for The After-Treatment

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A Novel Four-Way Plasma-Catalytic Approach for The Aftertreatment of Diesel Engine Exhausts Shuiliang Yao, Huanhuan Zhang, Xing Shen, Jingyi Han, Zuliang Wu, Xiujuan Tang, Hao Lu, Boqiong Jiang, Tomohiro Nozaki, and Xuming Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04166 • Publication Date (Web): 05 Jan 2018 Downloaded from http://pubs.acs.org on January 5, 2018

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Industrial & Engineering Chemistry Research

A Novel Four-Way Plasma-Catalytic Approach for The Aftertreatment of Diesel Engine Exhausts Shuiliang Yao1, Huanhuan Zhang1, Xing Shen2, Jingyi Han1, Zuliang Wu1, Xiujuan Tang1, Hao Lu1, Boqiong Jiang1, Tomohiro Nozaki3, Xuming Zhang1* 1. School of Environmental Science and Engineering, Zhejiang Gongshang University, Hangzhou, Zhejiang 310018, China. 2. Chilwee Group, Changxing, Zhejiang 313100, China. 3. Department of Mechanical Engineering, School of Engineering, Tokyo Institute of Technology, 2-12-1 O-okayama, Tokyo 152-8550, Japan. * Corresponding author. E-mail address: Zhang. [email protected] (X. Zhang)

Abstract: This study proposes a novel four-way plasma-catalytic removal of particulate matter, nitrogen oxides, hydrocarbons and carbon monoxide without external heating using a pulsed dielectric barrier discharge reactor combined with Au/CaSO4/γ-Al2O3 catalyst balls. With the optimized amount of Au and CaSO4, over 90% of particulate matter, 40% of nitrogen oxides and nearly 100% of hydrocarbons were removed from the diesel engine exhaust, while the increase of carbon monoxide concentration due to the oxidation of particulate matter and hydrocarbons, and the decomposition of carbon dioxide was minimized. Importantly, the performance of the plasma-catalytic reactor was maintained without decline for the duration of the experiment (7 h) with an attractive cost/performance ratio. These findings offer a new approach to the simultaneous treatment of diesel exhausts and provide a novel concept for the design of a practical and compact aftertreatment device for the diesel engine exhaust gases.

Keywords: non-thermal plasma, particulate matter, low temperature, catalyst, nitrogen oxides

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Abbreviations DPM DPF NTP DBD SCDBD MCDBD FWHM TEM FID XPS HCs DOC SCR

Diesel particulate matter Diesel particulate filter Non-thermal plasma Dielectric barrier discharge Single-cell dielectric barrier discharge Multi-cell dielectric barrier discharge Full width at half maximum Transmission electron microscopy Flame ionization detector X-ray photoelectron spectroscopy Hydrocarbons Diesel oxidation catalyst Selective catalytic reduction

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1. INTRODUCTION Diesel particulate matter (DPM), typically having a size range between 5 nm and 300 nm, is mainly composed by elemental carbon, metal sulfates, hydrocarbons from the incomplete combustion of fuel and lubricant oil.1 Once it is released into the atmosphere, it actively contributes to haze formation, and can give rise to serious environmental and health problems.2–4 For this reason, the emission standard of DPM has become stricter globally. For example, Ministry of Environment Protection of China requires all vehicles to satisfy China V emission standard by the beginning of July 2017.5 The DPM are conventionally removed by a diesel particulate filter (DPF) with or without catalysts. The DPF requires a periodic regeneration to avoid an excessive increase in the back-pressure in the exhaust line that causes decrease in engine efficiency.6 Thermal catalytic or non-catalytic regeneration process, however, causes a fuel penalty (typically 6 – 11% 7) for the light-duty diesel engine because the required DPM combustion/oxidation temperature is at least 240 oC in the presence of NOx,8–10 while the temperature of light-duty diesel exhaust gas is typically around 200 oC. The response time to reach the activation temperature also limits the thermal regeneration process in vehicle applications that require rapid start-up from cold conditions. For this reason, techniques based on non-thermal plasma (NTP) technology have been proposed.11–18 One technique is to use NTP to generate oxidative radicals for the regeneration of DPFs by the low-temperature oxidation of the accumulated DPM in the DPF.19–21 The combination of heterogeneous catalysts into NTP regeneration process can improve the oxidation in terms of energy efficiency and products selectivity. For example, Liu et al. 3



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reported that the oxidation efficiency of simulated DPM on MnOx (5 wt%)/CeO2

catalysts is as high as 85.2% under conditions of 20 oC gas temperature, 18.0 W of discharge power, and 30 ml/min of air flowrate. Burachaloo et al.21 found that in the presence of transition metal oxides, the DPM oxidation efficiency and CO2 selectivity can be enhanced by 77% and 29%, respectively, as compared to the plasma alone process. Meanwhile, some researchers used NTP as an ion source to remove DPM by electrostatic precipitation.22 Such electrostatic precipitation technique offers following advantages over DPFs based technique: moderate energy consumption, low maintenance costs, and no interference in diesel engine operation. However, water washing is required to continuously clean electrodes by washing down the precipitated DPM from electrodes, where the water washing process needs a water supply system which may prevent the development of compact system for vehicle applications. With the intent to develop a commercially feasible NTP based process for the diesel vehicle application, we investigated the on-board removal of DPM. To this end, we developed a pulsed dielectric barrier discharge (DBD) reactor that could take full advantage of NTP by utilizing plasma produced ions and oxidative radicals in a single treatment process.23–25 The DPM removal was proposed due to two steps24: (1) the electrostatic precipitation of charged DPM (through ion attachment), and (2) the oxidation of the precipitated DPM into CO and CO2. By optimizing the reactor configuration, relative high DPM removal (~80%) and CO2 selectivity (~60%) with a very competitive fuel penalty (3%) have been achieved for light-duty diesel vehicles.7 Further optimization of DPM removal and CO2 selectivity requires the combination of heterogeneous catalysts with the pulsed DBD reactor. 4


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In addition to DPM, diesel engines emit nitrogen oxides (NOx) and hydrocarbons (HCs) which are photochemically active substances, which are responsible for haze formation.26 Conventionally, HCs are oxidized to CO2 over a diesel oxidation catalyst (DOC),

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and NOx is reduced with ammonia by means of selective catalytic reduction

(SCR).28 Recent studies have shown that the simultaneous removal of HCs and NOx can be achieved by using HCs as a reductant to reduce NOx in a plasma-catalytic reactor.29 By taking these observations into account, it is possibly to simultaneously remove DPM, CO, HCs, and NOx in a plasma-catalytic reactor by using a proper heterogeneous catalyst and reactor configuration. Herein, we report a novel four-way plasma-catalytic removal of DPM, CO, HCs, and NOx without external heating for the first time using a pulsed DBD reactor combined with Au/CaSO4/γ-Al2O3 catalyst balls. γ-Al2O3 balls were used because of their large surface area, good mechanical and chemical stability, and excellent nano-particle trap ability.30 CaSO4 and Au were used as catalysts because CaSO4 has a good ability for the plasma-catalytic oxidation of DPM,25 while Au has a high activity for the simultaneous removal of NOx, HCs and CO at low-temperature.31 The activity of plasma-catalytic oxidation was evaluated by DPM oxidation efficiency because the DPM is the most important emission that needs to be removed from diesel exhaust. A screened catalyst was then used in a diesel engine (China IV standard diesel oil fueled) test, in which the removal of several pollutants was investigated. In addition, we compared the operation cost increase due to the use of NTP reactor and that due to the use of China V standard diesel oil instead of China IV standard diesel oil.

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2. EXPERIMENTAL SECTION A single-cell DBD (SCDBD) reactor was used to evaluate the activity of plasma-catalytic oxidation as shown in Fig. 1(a). This SCDBD reactor consisted of two alumina plates (115 × 115 × 1.0 mm3, 96% Al2O3), two stainless steel plates (95 × 95 × 0.1 mm3) and two alumina spacers (10 × 115 × 1.60 mm3). The discharge gap was filled with ca. 8 g catalyst balls having diameter between 1.0 and 1.5 mm. The SCDBD reactor was heated to a target temperature and maintained at a constant temperature using a feedback-controlled electrical furnace (CX-GW-1, Oubo). Each flowrate of the supplied gases, nitrogen and oxygen, were controlled by mass follow controllers (D07 series, Sevenstar), and the total flowrate was fixed at 1 L/min. The activity of plasma-catalytic oxidation was evaluated by investigating the oxidation efficiency of model DPM, where the model DPM was Printex U (Degussa). The Printex U is considered as a reliable model DPM and is commonly used in exploratory studies,9, 20 because it had similar reactivity to the actual DPM.32 The model DPM (40 mg) was dispersed in 2 ml liquid ethanol (99.7%, Kelong) by vigorous stirring. The catalyst balls were (ca. 8 g) then added in the mixture which contained liquid ethanol and model DPM under stirring to make the model DPM uniformly loading to the surface of the balls, and were then dried at 100 oC for 1 h. As for diesel engine test (Fig. 1(b)), a multi-cell DBD (MCDBD) reactor, which was designed for practical application, was employed. The MCDBD reactor had 20 discharge cells connected in parallel. The number of discharge cells was optimized on the basis of trade-off between energy efficiency and reactor cost.33 The configuration of 6


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each discharge cell was the same with that of the above mentioned SCDBD reactor. All discharge cells were carefully assembled to maximize the discharge uniformity. The MCDBD reactor was installed at the downstream (1.2 m) of a diesel engine (YT6800T-ATS, Yiten, specification of this diesel engine is listed in Table S1 of Supporting Information) that was fueled with China IV standard diesel oil and used two electrical heaters (HC2038S, Aimeite) as a load. The temperature at the inlet and outlet of the MCDBD reactor were monitored using thermocouples (K-type, Hengfeng). The support used for catalyst was γ-Al2O3 balls with a diameter between 1.0 and 1.5 mm. Before use, the support was pretreated by washing with distilled water (to remove water soluble contamination, mainly inorganic compounds) and calcining at 500 o

C for 2 h in air. The CaSO4 was deposited onto the pretreated γ-Al2O3 balls by the

impregnation method using CaSO4 (AR, Kelong) solution followed by drying at 110 oC for 5 h and calcining at 500 oC for 3 h. The resulting CaSO4/γ-Al2O3 was then used to prepare the Au/CaSO4/γ-Al2O3 catalyst by impregnation using AuCl3·HCl·4H2O (Au ≥ 47.8%, Zhanyun) as the precursor. The prepared catalyst was dried at 110 oC for 5 h, followed by calcinations at 500 oC for 3 h. The DBD reactors were energized by a pulsed power source (HV-10-08, Ketai) with 70 µs in full width at half maximum (FWHM), 30 µs in rise time, and a typical range of pulse repetition rates of 400 – 500 Hz and applied peak voltages of 5 – 6 kV. The information about instruments for catalyst characterization, electrical measurement and gas composition measurement could be found in S1 of Supporting Information. The physical properties of the samples are listed in Table 1 which shows that the surface area and pore volume of γ-Al2O3 was not obviously changed with the deposition of CaSO4 7



and/or Au in the present study. The definitions of DPM oxidation efficiency, DPM removal, NOx removal, HCs removal were described in S2 of Supporting Information. We note that each experiment was repeated at least three times. Typical relative standard deviation for the presented data is CaSO4 (0.05 wt.%)/γ-Al2O3 > Au (0.005 wt.%)/γ-Al2O3 > pure γ-Al2O3 balls. Regarding to CO2/CO ratio (Fig. 5(b)), it decreased in the order of Au (0.005 wt.%)/CaSO4 (0.05 wt.%)/γ-Al2O3 > Au (0.005 wt.%)/γ-Al2O3 > CaSO4 (0.05 wt.%)/γ-Al2O3 > pure γ-Al2O3 balls. These results indicate that the CaSO4 is more beneficial to the enhancement of DPM oxidation efficiency, while Au is more useful for the improvement of CO2/CO ratio. The maximum oxidation efficiency (3.2 g/kWh) and CO2/CO ratio (4.7) were observed at T=250 oC with the Au (0.005 wt%)/CaSO4 (0.05 wt%)/γ-Al2O3 catalyst. Au/CaSO4//γ-Al2O3 catalyst is considered as a suitable catalyst because it shows high activity for DPM oxidation in the tested temperature range. We also investigated model DPM oxidation in a lower oxygen concentration (15%) gas condition (Fig. 5(a) and Fig. 5(b)), which represents the condition of exhaust gas from a higher load diesel engine. With or without catalyst, the lower oxygen concentration plasma processing decreased model DPM oxidation efficiency as well as CO2/CO ratio as compared to a higher oxygen concentration (20%) plasma processing. In NTP, plasma produced oxidative reactive species, such as O atom and O3, are responsible for the DPM oxidation at a relatively low temperature (Eqs. (1) – (3)).24 DPM + O → CO, CO2

(1)

DPM + O3 → CO, CO2

(2)

CO + O → CO2

(3)

where O atom is produced from the electron disassociation of O2 (O2 + e → O + O + e) 10


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and O3 is produced from the combination of O atom and O2 (O + O2 → O3). Here, we note that the direct contribution of O3 to model DPM oxidation might be insignificant. It was reported that the reaction rate of DPM-atomic O reaction is about ten times higher than that of DPM-O3 reaction in the temperature range between 100 and 320 oC.16 In this regard, the difference in model DPM oxidation with different catalysts might be due to the difference in the O atom utilization rather than O3 utilization in the model DPM oxidation process. This is supported by the evidence that the O3 concentration was almost the same (~ 7 ppm) for the different catalysts (Fig. 5 (c)). In addition, the model DPM oxidation was decreased in the relative low concentration oxygen discharge because the oxidative reactive species (O and O3) decreased with the oxygen concentration. Since the applied peak voltage (5 kV) and the discharge gap distance (1.6 mm) was kept constant in the experiment, the temperature increase resulted in the elevated reduced field intensity (V/dn, where V is the applied voltage, d is the discharge gap distance and n is the gas number density) and corresponding average electron energy.35, 36

This led to the enhanced electron-impact reaction, and the higher O atom

production.35, 36 In addition, at the higher temperature condition, ozone was decomposed (O3 → O + O2) which further enriched O atoms in the discharge space. Thus, as shown in Fig. 5 (a), the model DPM oxidation efficiency increased with the gas temperature. When a catalyst was used, it could promote the DPM oxidation through Eq. (4) and increase the CO2/CO ratio through Eq. (5):37 DPM + CAT(O) → CO, CO2 + CAT

(4)

CO + CAT(O) → CO2 + CAT

(5)

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where CAT(O) denotes the active catalyst, acting as a source of activated oxygen, and CAT denotes the reduced catalyst. According to our experimental result, CaSO4 is more beneficial to the enhancement of DPM oxidation efficiency, while Au is more useful for the improvement of CO2/CO ratio. The reduced catalysts could be regenerated by oxygen refilling reactions using O atom and O3 through Eqs. (6) and (7), which are considered as the main reason for the synergetic of plasma and catalyst.37 CAT + O →CAT(O)

(6)

CAT + O3 →CAT(O) + O2

(7)

3.2 Diesel engine test In previous section, we optimized the catalyst to enhance the DPM oxidation. The combination of 0.05 wt.% CaSO4 and 0.005wt.% Au was most effective to the DPM oxidation. In this section, the optimized catalyst was added in MCDBD reactor for the diesel engine test at fixed plasma discharge power (Pdis≈42 W). Table 2 summarizes the gas temperature, oxygen concentration and major pollutant composition of the exhaust gas for the tested engine power (0.8 kW, 1.2 kW and 2.0 kW) after 0.5 h warming up of the diesel engine. With the increase of engine power, because more oxygen was used in the combustion process, the oxygen concentration decreased, while the gas temperature increased. Regarding to pollutants, at higher engine power, more DPM and NOx, but less HCs were produced. We note that the NOx includes NO and NO2, and the HCs include non-aromatic hydrocarbons and aromatic hydrocarbons, where the non-aromatic hydrocarbons include ethene, 1,3-butadiene, and n-pentane, etc., and the aromatic hydrocarbons include benzene, toluene, and ethylbenzene, etc.38 The 12


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flow rate was maintained at around 1.2 m3/min in the tested range of engine power, resulting in the discharge energy density of around 2.1 J/L. Figure 6 shows the time-resolved DPM total number concentration and CO concentration with different catalysts packed in the MCDBD reactor at fixed diesel engine power (Pengine=1.2 kW). The reference time, t=0 h, was a time when diesel engine and plasma were switched on. The stabilization of the diesel engine requires ~0.6 h, after which the number concentration of DPM and the concentration of CO became stable. Note that the DPM number difference between the inlet and the outlet of the MCDBD reactor was around 10%. As shown in Fig. 6(a), in the case of purely thermal reaction, the total number concentration of DPM was around 5.3×107 #/cm3. When the DBD was switched on, the total number concentration of DPM was significantly reduced regardless of a catalyst was deposited. It decreased in the order of γ-Al2O3 > CaSO4/γ-Al2O3 ≈ Au/CaSO4/γ-Al2O3. When the DBD was turned off, the concentration of DPM rapidly returned (within 0.4 h) to a value which was the same to a value that without plasma (Figure S3 of Supporting Information). This indicates that the DPM removal was due to the use of DBD. It is worth mentioning that the carbon deposition on the tested catalysts is considered to be insignificant, which could be indicated by the fact that the DPM removal did not obviously change with the plasma processing time for the tested catalysts.25 For the DPM removal that without catalyst (but with discharge), it decreased with processing time due to the accumulation of DPM on the surface of dielectric barrier as reported in our previous work. 25 In our MCDBD reactor, the removal of DPM is resulted from the plasma 13



oxidation and the electrostatic precipitation of the DPM.7 Since we used the same physical configuration of reactor (size of Al2O3, discharge gap) and the same applied voltage, the effectiveness of electrostatic precipitation on the DPM removal for the three catalysts, Al2O3, CaSO4/Al2O3 and Au/CaSO4/Al2O3, packed MCDBD reactor should be the same. For this reason, the difference in the DPM removal for these catalysts is attributed to their different oxidation ability. Figure 6(b) shows that DBD resulted in more CO in the diesel exhaust gas which is due to the oxidation of DPM and HCs, and/or the decomposition of CO2 (CO2 + e → CO + O + e). Interestingly, the CO concentration with the Au/CaSO4/γ-Al2O3 catalyst is the same as with the purely thermal reaction, but clearly lower than that using γ-Al2O3 and CaSO4/γ-Al2O3. The deposition of Au successfully inhibited the production CO although it did not significantly improve the DPM removal in the diesel engine test. Figure 7 shows the typical size distribution of DPM with different catalysts. Without DBD, the DPM size distribution had a mode at 94.5 nm. For the cases that with DBD, the size of DPM tended to become smaller. The reduction in the DPM size with plasma discharge was also observed in our previous work, which removes DPM using an empty MCDBD reactor (without packed balls).39 We propose that this reduction could be attributed to the incomplete oxidation of DPM. This hypothesis is supported by the evidence that the higher the oxidation ability of MCDBD reactor, the smaller the size of DPM; the DPM number size distribution for γ-Al2O3, CaSO4/γ-Al2O3, Au/ CaSO4/γ-Al2O3 had a mode at 85.5 nm, 77.6 nm and 77.6 nm, respectively. In addition to the DPM removal, the use of DBD could reduce the emission of NOx and HCs. After 1 h processing, the NOx concentration was ca. 270 ppm in the case 14


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without DBD (Fig. 8(a)). When the plasma was used, the NOx concentration decreased to ca. 240 ppm, 250 ppm and 180 ppm, for γ-Al2O3, CaSO4/Al2O3 and Au/ CaSO4/γ-Al2O3, respectively. The deposition of CaSO4 had negative effect on the NOx removal, while the deposition of Au/CaSO4 promoted the NOx removal. Regarding to the plasma processing of HCs (Fig. 8(b)), it reduced by ca. 6 ppm and 21 ppm when γ-Al2O3 and CaSO4/γ-Al2O3 were used, respectively. Notably, no HCs could be detected when Au/CaSO4/γ-Al2O3 was used as catalyst. This indicates that the deposition of Au significantly enhanced the HCs removal. The removal of DPM, NOx and HCs with different diesel engine power were summarized in Fig. 9. The DPM removal (Fig. 9(a)) decreased in the order of Pengine=1.2 kW > Pengine=2.0 kW > Pengine=0.8 kW. The deposition of CaSO4 or Au/CaSO4 had positive effect on the DPM removal. At the engine power of 1.2 and 2.0 kW, the DPM removal for the deposition of Au/CaSO4 reached as high as ~90%. For the NOx removal (Fig. 9 (b)), it decreased in the order of Pengine=0.8 kW > Pengine=1.2 kW > Pengine=2.0 kW. The deposition of CaSO4 did not increase the NOx removal, but the deposition of Au/CaSO4 enhanced the NOx removal by 3 – 6 times depending on the diesel engine power. Regarding to the HCs removal (Fig. 9(c)), it also decreased in the order of Pengine=0.8 kW > Pengine=1.2 kW > Pengine=2.0 kW, and it was promoted by the deposition of CaSO4 or Au/CaSO4. Significantly, the HC removal for the Au/CaSO4/γ-Al2O3 catalyst always remained at 100% when the power of the diesel engine was varied. The removal of DPM in the diesel engine test has at least two reaction routes: one is the reaction of active oxidative species (e.g. O and OH) with DPM as shown in Eq. 15



(1), and the other is the reaction of NO2 with DPM to produce CO and NO (Eq. (8)). The latter reaction could take place at temperatures higher than 80 oC.40 DPM + NO2 → NO + CO

(8)

where NO2 is from exhaust gas or from the conversion of NO by active oxidative species. Note that the exhaust gas containing NO2 was at the concentration of 25 ppm and this concentration did not change with the diesel engine power. The mechanism of DPM oxidation in plasma was proposed by Yao,7 who suggested that the decomposition of O2 by electric discharges plays an important role in reducing DPM oxidation temperature. The synergetic effect of plasma and catalyst is possibly because Au has an ability to keep reactive oxygen species (such as O atoms, OH radicals, and O3) on its surface; the reactive oxygen species are responsible for the DPM oxidation. Regarding to the effect of CaSO4, it is proposed that the CaSO4 could accelerate the decomposition of surface oxygen complexes to facilitate the DPM oxidation, where the surface oxygen complexes are the intermediate products of DPM oxidation.9,25 However, the detailed mechanisms of the positive effect of Au and CaSO4 on the DPM oxidation is still unknown. We will elaborate the mechanism using various techniques such as infrared spectrometer (IR) and X-Ray absorption fine structure (EXAFS) in future work. Because the concentration of DPM increased and the concentration of O2 (the active oxidation species) decreased with increasing engine power, the decrease of DPM removal at higher engine power could be understood, although the exhaust gas temperature was increased. According to previous studies, following routes are possibly responsible for the NOx removal:40, 41 (1) the conversion of NO to HNO2 by OH radical, (2) the conversion 16


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of NO2 to HNO3 by OH radical, (3) the conversion of NO2 to nitrites, and (4) the conversion of NO to N2 by CxHyOz, where the CxHyOz is produced from the partial oxidation of HCs. In this study, the production of OH would be the same for the different catalysts as the discharge power was the same.42 The beneficial effect of CaSO4 and Au/CaSO4 on the NOx removal was the increased CxHyOz production as the HCs removal was increased by these catalysts. However, the CaSO4 could also inhibit NOx removal by the prevention of NO2 to nitrites conversion; previous study has proposed that the acidic nature of sulfates inhibits the adsorption of NO2 on the alumina support.9 This might be responsible for the decrease in the NOx removal with the deposition of CaSO4. The removal of HCs is initiated by the reaction of HCs with active oxidative species and/or NO2. It reduces at higher engine power because the concentration of O2 was decreased. The deposition of CaSO4 obviously promoted the HC removal. The promotion of HC removal with the deposition of sulfates was also reported in the thermal-catalytic oxidation of propane.43 It was proposed that the sulfates may act as active sites for the cleavage of C-H bond. The addition of Au further increased the HCs removal to 100% as Au could provide activated oxygen for the HCs removal. After the initial step of HCs removal, the intermediate products, CxHyOz, were produced which then contribute to the NOx removal. Regarding byproducts, due to the low discharge energy density, nitrogen oxides, such as NOx and N2O, would not be produced from the electrical discharge in air. This was supported by the fact that no nitrogen oxides could be detected in the air DBD regardless of catalysts were used. However, as aforementioned, the byproduct CxHyOz 17



from the partial oxidation of HCs could be produced. The use of CaSO4 and Au could promote the formation of CxHyOz because the HCs removal were increased. Fortunately, the existence of CxHyOz is beneficial for the NOx removal (via NO-CxHyOz reaction). The detailed reactions for the formation and consumption of CxHyOz is our ongoing work, the result of which will be reported in the near future. 3.3 Operation cost of plasma-catalytic reactor We have shown that over 90% of DPM, 40% of NOx and 100% of HCs could be removed from the diesel exhaust gas, satisfying China national V emission standard, using the combination of MCDBD and Au/CaSO4/γ-Al2O3 catalyst; as required by the China national V emission standard, the emission of DPM, NOx and HCs should be decreased by 82%, 28%, and 23%, respectively, as compared to the China national IV emission standard.44,

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Taking 2 kW diesel engine power as an example, 1 h of

operation consumed about 0.4 L of China IV standard diesel fuel. The required plasma energy per consumed volume of diesel fuel was around 1 h × 42 W/ (1000× 0.4 L)=0.1 kWh/L. The increased operation cost due to the use of MCDBD was therefore around 0.1 kWh/L × 0.5 RMB/kWh=0.05 RMB/L, where 0.5 RMB/kWh was the price per kWh for electricity in China. However, the increased operation cost required for the replacement of China IV standard diesel fuel into China V standard diesel fuel would be around 0.15 RMB/L, which is three times higher than the use of MCDBD reactor. Pressure drop is an important parameter for the practical application. At the gas flow rate of 1.2 m3/h, typical pressure drop for the MCDBD reactor was 11 kPa, while the pressure drop limit for the wall-flow DPF is 20 kPa.46 Notably, the pressure drop for the MCDBD reactor could easily be decreased by optimizing the reactor configuration 18


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as reported in our previous work.33 It is also worth mentioning that at ~200 oC (typical temperature for the light-duty diesel exhaust gas), the removal of NOx in this study (~40%) was higher than that for the commercial Cu-zeolite catalyst based NH3-SCR (~30%) at a similar NO/NOx ratio (NO/NOx ratio is close to 1),47 and the removal of HCs in this study (100%) was the same to that for the commercial DOC device (100%).48 Note that the most attractive point of the plasma based device is the rapid start-up from cold condition and the simultaneous removal of DPM, NOx and HCs in a single device at low temperature. In terms of economical point of view, the MCDBD reactor has great application potential for the aftertreatment of exhaust gas from light-duty diesel vehicle. 4. Concluding Remarks

In this study, we report a novel Au/CaSO4/γ-Al2O3 catalyst packed dielectric barrier discharge reactor for the after treatment of a diesel engine exhaust without external heating. The deposition of a small amount of Au/CaSO4 on γ-Al2O3 not only increased particulate matter removal and CO2/CO ratio, but also enhanced NOx and HCs removal at all tested diesel engine powers. Within the tested plasma processing time, i.e. 7 h, a high level of reactor performance was maintained. Significantly, the emission of diesel engine exhaust satisfied China national V emission standard, where the diesel engine was fueled with China IV standard diesel fuel. The increased operation cost due to the use of newly developed reactor is only one third of the operation cost required for the replacement of China IV standard diesel fuel into China V standard diesel fuel. In terms of economical point of view, the proposed reactor has great 19



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application potential for the aftertreatment of exhaust gas from light-duty diesel vehicle.

Supporting Information Contents S1: The instrument for catalyst characterization, electrical measurement, and gas composition measurement………………………………………………………….. S2 S2:

DPM

oxidation

efficiency,

DPM

removal,

HCs

removal,

NOx

removal……………………………………………………………………………... S3 Table S1. Specification of diesel engine in this study………………………….........S4 Figure S1. The effect of Au loading on model DPM oxidation…………………….. S4 Figure S2. High-resolution Au(4f) spectra……………………………….…………….... S5 Figure S3. Temporal variation of DPM number……………………………………. S5 Reference…………………………………………………………………………….S5

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (No. 11575159 & No.11775189) and the Program for Zhejiang Leading Team of S&T Innovation (No. 2013TD07).

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Page 27 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure captions Figure 1. A schematic of experimental setup for the (a) activity of plasma-catalytic oxidation, and (b) diesel engine test. Figure 2. The effect of CaSO4 loading on model DPM oxidation and CO2/CO ratio. Figure 3. The effect of Au loading on model DPM oxidation and CO2/CO ratio value. Figure 4. TEM images (a) γ-Al2O3, (b) 1wt.%Au/γ-Al2O3, (c) 2wt.%Au/γ-Al2O3, and (d) 5wt.%Au/γ-Al2O3. Figure 5. The effect of temperature on (a) model DPM oxidation, (b) CO2/CO ratio and (c) O3 concentration. Figure 6. Time resolved (a) DPM number concentration and (b) CO concentration with different catalysts (1.2 kW diesel engine power). Figure 7. Typical size distribution of DPM with different catalysts (1.2 kW diesel engine power). Figure 8. Time-resolved (a) NOx concentration and (b) HCs concentration with different catalysts (1.2 kW diesel engine power). Figure 9. The effect of engine power on (a) DPM, (b) NOx and (c) HCs removal with different catalysts.

27



Page 28 of 38

Table 1. BET surface area and pore volume of each catalyst Catalyst

SBET (m2/g)

Pore volume (cm3/g)

γ-Al2O3

178

0.488

0.001wt.%CaSO4/γ-Al2O3

177

0.492


175

0.490


174

0.484


176

0.498


182

0.506

0.001wt.%Au-0.05wt.%CaSO4/γ-Al2O3

176

0.487


177

0.502


178

0.500


176

0.495

Table 2. Summarization of the gas temperature, oxygen concentration and major pollutant composition of exhaust gas Engine power Temperature O2 Total DPM NOx HCs (%) (#/cm3) (kW) ( oC ) (ppm) (ppm) 7 0.8 180 16 226 76 3.7×10 270 74 1.2 200 15 5.3×107 7 2.0 230 13 410 68 6.8×10 Note: temperature was recorded at a time when the temperature of gas inlet and gas outlet were the same.

28


Page 29 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. A schematic of experimental setup for the (a) activity of plasma-catalytic oxidation, and (b) diesel engine test.

29


Page 30 of 38

8

3

7 6

2

5

Experiment: Pdis=4.5 W, t=1 h, Q=1 L/min, [O2]=20%,T=250 °C

4

1

CO2/CO ratio

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Model DPM oxidation efficiency (g/kWh)


3 2

0 0.0

0.1

0.2

0.3

0.4

0.5

CaSO4 loading (wt%) Figure 2. The effect of CaSO4 loading on model DPM oxidation and CO2/CO ratio.

30


8

4 Experiment: Pdis=4.5 W, t=1 h, Q=1 L/min, [O2]=20%,T=250 °C, [CaSO4]=0.05 wt%

3

7 6 5

2

4

CO2/CO ratio

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



Page 31 of 38

1 3 2

0 0.00

0.02

0.04

0.06

0.08

0.10

Au loading (wt%) Figure 3. The effect of Au loading on model DPM oxidation and CO2/CO ratio value.

31



Figure 4. TEM images (a) γ-Al2O3, (b) 1wt.%Au/γ-Al2O3, (c) 2wt.%Au/γ-Al2O3, and (d) 5wt.%Au/γ-Al2O3.

32


Page 32 of 38

5

(a) 4

3

Experiment: Pdis=4.5 W, t=1 h, Q=1 L/min

[O2]=20%: [O2]=15%:

γ-Al2O3

Au/γ-Al2O3

CaSO4/γ-Al2O3

Au/CaSO4/γ-Al2O3

Au/CaSO4/γ-Al2O3

2

1

0

150 °C

200 °C

250 °C

8

(b)

CO2/CO ratio

6


[O2]=20%: [O2]=15%:

γ-Al2O3

Αu/γ-Al2O3

CaSO4/γ-Al2O3

Au/CaSO4/γ-Al2O3

Au/CaSO4/γ-Al2O3

4

2

0

150 °C

200 °C

250 °C

10

(c) O3 concentration (ppm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



Page 33 of 38

8


6

Al2O3

Al2O3+Au

Al2O3+CaSO4

Al2O3+Au+CaSO4

4

2

0

150 °C

200 °C

250 °C

Figure 5. The effect of temperature on (a) model DPM oxidation, (b) CO2/CO ratio and (c) O3 concentration. 33



3

DPM number concentration (#/cm )

8

1.0x10

(a)

Without discharge

With discharge γ-Al2O3

7

8.0x10

CaSO4/γ-Al2O3 Au/CaSO4/γ-Al2O3

7

6.0x10

7

4.0x10

Experiment: Pdis=42 W, Pengine=1.2 kW

7

2.0x10

0.0 1.0

(b) 0.8

CO concentration(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 38

0.6

0.4

0.2

0.0 0

2

4

Time (h)

6

8

Figure 6. Time resolved (a) DPM number concentration and (b) CO concentration with different catalysts (1.2 kW diesel engine power).

34


Page 35 of 38

6

8.0x10

Without discharge

6

4.0x10

6

2.0x10

With discharge Al2O3 Al2O3+CaSO4

Experiment:

6

6.0x10

Normalized number concentration (a.u.)

3

DPM number concentration (#/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Pdis=42 W, Pengine=1.2 kW

Al2O3+Au+CaSO4

1.2

77.6 nm 85.5 nm 94.5 nm 1.0

0.8 0.6 0.4 20

40

60

80

100 120 140 160

Particle Diameter (nm)

0.0 10

100

Particle Diameter (nm)

Figure 7. Typical size distribution of DPM with different catalysts (1.2 kW diesel engine power).

35



NOx concentration(ppm)

300

(a)

200

With discharge

Without discharge 100

Experiment:

γ-Al2O3

Pdis=42 W, Pengine=1.2 kW

Au/CaSO4/γ-Al2O3

CaSO4/γ-Al2O3

0 100

HCs concentration(ppm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 38

(b)

80

60

40

20

0 0

2

Time (h)

4

6

Figure 8. Time-resolved (a) NOx concentration and (b) HCs concentration with different catalysts (1.2 kW diesel engine power).

36


Page 37 of 38

100

DPM, NOx & HCs removal (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a)

DPM

(b)

(c)

NOx

HCs

γ-Al2O3

80

CaSO4/γ-Al2O3 Au/CaSO4/γ-Al2O3

60

Experiment: Pdis=42±2 W 40

20

0 0.8 kW

1.2 kW

2.0 kW

0.8 kW

1.2 kW

2.0 kW

0.8 kW

1.2 kW

2.0 kW

Figure 9. The effect of engine power on (a) DPM, (b) NOx and (c) HCs removal with different catalysts.

37



~

~

Table of Contents

Diesel Engine

To ground

To high voltage DPM, NOx & HCs removal (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 38

100

DPM

HCs

NOx

80

γ-Al2O3

CaSO4/γ-Al2O3

60

Au/CaSO4/γ-Al2O3 40

20

0

0.8 kW

1.2 kW

2.0 kW

0.8 kW

1.2 kW

2.0 kW

0.8 kW

38


1.2 kW

2.0 kW

A Novel Four-Way Plasma-Catalytic Approach for The After-Treatment

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