CeO2 Catalysts for a Low

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Cite This: Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Synergistic Effect of Cu/CeO2 and Pt−BaO/CeO2 Catalysts for a LowTemperature Lean NOx Trap Beom-Sik Kim,† Pyung Soon Kim,‡ Junemin Bae,† Hojin Jeong,† Chang Hwan Kim,‡ and Hyunjoo Lee*,† †

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Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, Daejeon 34141, Republic of Korea ‡ Advanced Catalysts and Emission-Control Research Lab, Hyundai Motor Group, Hwaseong, Gyeonggi 18280, Republic of Korea S Supporting Information *

ABSTRACT: A lean NOx trap (LNT) catalyst has been widely used for removing NOx exhaust from lean-burn engines. However, the operation range of LNT has been limited because of the poor activity of LNT catalysts at low temperatures (≤300 °C), especially in urban driving conditions. To increase NOx removal efficiency during lean−rich cycle operation, a Cu/CeO2 (CC) catalyst was added to a Pt−BaO/CeO2 (PBC) catalyst. In comparison to PBCor CC-only catalysts, the physical mixture of PBC and CC catalysts (PBC + CC) exhibited a significant synergy for both NOx storage and reduction efficiencies. In particular, low-temperature activity below 200 °C was greatly enhanced. A Pt−BaO−Cu/CeO2 (PBCC) catalyst, which was synthesized by depositing Pt and Cu together on a ceria support, showed poorer NOx removal efficiency. The origin of the synergistic effect over PBC + CC was investigated. Under lean conditions, the CC showed much better activity for NO oxidation, allowing for faster NOx storage on PBC. Under rich conditions, H2 was generated in situ on the CC by a water−gas shift reaction then accelerated the reduction of NOx, which had been stored on PBC, with a higher selectivity to N2. This simple modification in the catalyst can provide an important clue to enhance low-temperature activity of the commercial LNT system.

1. INTRODUCTION Lean-burn engines, such as a diesel engine, have attracted much attention as a result of their greater fuel efficiency and less CO2 emission compared to conventional Otto gasoline engines.1,2 However, NOx (NO and NO2) reduction to N2 is difficult under lean conditions because excess oxygen consumes nearly all of the reductants.3,4 A lean NOx trap (LNT), which is also called NOx storage and reduction (NSR), is proposed and considered as a promising NOx removal technology for light-duty diesel or lean-burn engines.5−9 Generally, LNT catalysts work under cyclic fuel-lean and fuel-rich atmospheres. During the lean period, incoming NO is oxidized to NO2 on noble metals and subsequently stored as nitrites or nitrates on a basic metal oxide (typically BaO). During the subsequent short rich period at which reductants are injected, trapped NOx is released and reduced to N2 over the noble metals. The efficiency of NOx removal relies on both NOx storage and NOx reduction activity of the LNT catalyst.8,10 With ever-tightening automotive emission standards, advanced combustion engine and powertrain technologies have been developed to lower emission and fuel consumption.11,12 These systems, however, paradoxically lead to lower exhaust gas temperatures, making aftertreatment of the exhaust gas more challenging because the catalysts would generally work better at higher temperatures.11,13 The lower temperatures can also be encountered during the cold startup or low© XXXX American Chemical Society

load operation. The automotive catalysts should have better low-temperature activities to meet stringent regulation requirements.14 The LNT catalysts have poor activity at lower temperatures because NOx reduction during the rich conditions became more difficult. Takahashi et al.10 reported that when the LNT operating temperature is below 400 °C, stored NOx was not fully reduced, even under rich conditions, hindering further NOx removal. It is known that H2 is the most active reductant for low-temperature NOx reduction among H2, CO, or hydrocarbons.14−16 However, the concentration of H2 in real diesel exhaust is very low. Because CO is the most abundant reductant in the exhaust gas and its concentration can be easily controlled by engine operation,15 producing H2 from CO by the water−gas shift reaction (WGSR, CO + H2O → CO2 + H2) can be a good strategy to enhance low-temperature activity in the LNT system. Cu/CeO2 catalysts have shown good activity for various CO-involving reactions, such as WGSR,16,17 CO oxidation,18,19 NO reduction by CO,20−22 and preferential CO oxidation in an excess H2 stream.23,24 Because Cu/CeO2 is a good catalyst for WGSR, it might be able to produce H2 at a low Received: September 21, 2018 Revised: February 6, 2019 Accepted: February 8, 2019

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DOI: 10.1021/acs.est.8b05329 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology

Ar balance was provided for 20 min, and then the gas flow was stopped. After Ar purging for 20 min under vacuum, the DRIFT spectra were collected. NOx temperature-programmed desorption (TPD) and NOx temperature-programmed reduction (TPR) were conducted in a fixed bed reactor using a mass spectrometer (BEL Mass) as a detector. Each sample was pretreated at 500 °C for 1 h under 1% H2/Ar flow and then cooled to the desired temperature in Ar flow. NOx adsorption was carried out at 200 °C by flowing 200 ppm of NO, 8% O2, and 5% H2O with Ar balance at 200 mL/min for 1 h. After the catalyst was purged for 30 min, the temperature increased up to 700 °C at a ramping rate of 10 °C/min in Ar flow. NOxTPR was conducted with a flow of 2% CO and 5% H2O (Ar balance) at a ramping rate of 10 °C/min after adsorbing NOx at 200 °C for 1 h. 2.3. Lean−Rich Cycling. NSR behaviors were investigated in a fixed bed quartz reactor under cyclic lean (200 ppm of NO, 8% O2, and 5% H2O in Ar for 12 min) and rich (200 ppm of NO, 2% CO, and 5% H2O in Ar for 2 min) conditions at a space velocity of 120 000 mL gcat−1 h−1. All reaction experiments were performed with 100 mg of the catalyst: 100 mg of CC for CC only, 100 mg of PBC for PBC only, 50 mg of CC and 50 mg of PBC for PBC + CC, and 100 mg of PBCC. The feed stream was connected to an electronically actuated four-way valve, which allows for periodic switching between lean and rich conditions. The total gas flow rate was 200 mL/min. Before each experiment, the samples were pretreated in 1% H2/Ar at 500 °C for 1 h. The composition of the effluent gases was monitored using a Nicolet iS10 FTIR spectrometer (Thermo Scientific) equipped with a cyclone C2 gas measurement cell (0.19 L of internal volume, Specac). The activity of the catalyst during lean and rich periods was evaluated with the following equation:

temperature for NOx reduction. In this work, we mixed a Cu/ CeO2 (CC) catalyst to a model LNT catalyst, Pt−BaO/CeO2 (PBC), to enhance low-temperature activity for NOx removal. PBC has been extensively studied for LNT catalysis.25−28 Pt is the active site for NO oxidation/reduction; BaO is the site for NOx storage; and CeO2 associates redox cycles at lower temperatures. The activity of the catalysts was estimated by periodically switching lean and rich conditions using CO as a reductant. The synergistic effect of CC and PBC catalysts was observed at low temperatures of 150−350 °C for NOx removal. The storage and reduction of the NOx species were investigated in lean and rich conditions, respectively, for CC only, PBC only, a physical mixture of CC and PBC (PBC + CC), and a Pt−BaO−Cu/CeO2 (PBCC) catalyst.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. First, CeO2 was synthesized by a precipitation method. A total of 2.17 g (10 mmol) of cerium nitrate [Ce(NO3)3·6H2O, Kanto Chemical] was dissolved in 50 mL of deionized water and stirred for 15 min. A total of 50 mL of NaOH solution (0.375 M) was added dropwise under vigorous stirring. After 30 min, the resulting precipitates were washed with deionized water and ethanol. The product was dried in a vacuum oven at 50 °C for 6 h. The obtained cake was crushed in a ceramic mortar to obtain a fine powder. The powder was calcined at 600 °C for 4 h in static air. CC and PBC catalysts were prepared by a conventional wet impregnation method. For CC, the prepared CeO2 powders were impregnated with copper acetate [Cu(CH3COO)2·H2O, Sigma-Aldrich] aqueous solution and stirred at 60 °C for 5 h; the target loading of Cu was 5 wt %. Then, the powders were dried at 80 °C in a convection oven for 12 h and calcined at 500 °C for 5 h in static air. For PBC, Pt and Ba were coimpregnated onto CeO2 with barium acetate [Ba(CH3COO)2, Sigma-Aldrich] and chloroplatinic acid (H2PtCl6·6H2O, Sigma-Aldrich) aqueous solution. The target loadings of Pt and Ba were 1 and 10 wt %, respectively. The impregnation procedure was the same as CC. PBCC was similarly prepared by impregnating all Pt, Ba, and Cu precursors simultaneously on the CeO2 support (1 wt % Pt, 10 wt % Ba, and 5 wt % Cu). PBC + CC was prepared by mixing 50 mg of PBC and 50 mg of CC. 2.2. Characterizations. The actual contents of metals in the prepared catalysts were determined by inductively coupled plasma optical emission spectroscopy (ICP−OES) on an OPTIMA 7300 DV instrument (PerkinElmer). Surface area and pore volume measurements were conducted by N2 adsorption at −196 °C with a Tristar 3000 chemisorption instrument (Micromeritics). The crystalline structure of the catalysts was measured by powder X-ray diffraction (XRD) using a SmartLab X-ray diffractometer (Cu Kα radiation, Rigaku). X-ray photoelectron spectroscopy (XPS) data were acquired using a Kα XPS spectrometer (Thermo VG Scientific). The binding energies were calibrated using an advantageous C 1s peak at 284.8 eV. Diffuse reflectance infrared Fourier transform (DRIFT) spectra were obtained with a Nicolet iS50 Fourier transform infrared (FTIR) spectrometer (Thermo Scientific). The catalyst powder was placed into a sample cup and set inside a cell with a ZnSe window. The sample was first pretreated in 1% H2/Ar at 500 °C for 1 h, followed by Ar purging at 500 °C for 30 min. After cooling to room temperature, the background signal was recorded under vacuum. A flow of 2% CO gas with

t + tR

NOx conversion =

∫0 L

(NOx in − NOx out )dt t + tR

∫0 L

(NOx in )dt

× 100%

2.4. NO Oxidation Capacity (NOC) and NOx Storage Capacity (NSC). NOC and NSC experiments were performed in a fixed bed quartz reactor using a FTIR spectrometer (Nicolet iS10, Thermo Scientific) as a detector. Prior to the measurements, the catalyst (100 mg) was pretreated at 500 °C under 1% H2/Ar flow for 1 h and then cooled to room temperature. NOC and NSC measurements were then conducted by exposing the catalyst to the gas flow containing 200 ppm of NO, 8% O2, 5% H2O, and balance Ar with a total flow rate of 200 mL/min. NOC was measured when a steady state had been achieved (typically requiring 2 h). The NOC and NSC were evaluated with the following equations: NOC =

NO2 out × 100% NOin t

NSC =

∫0 s (NOx in − NOx out )dt mcat

2.5. WGSR. WGSR experiments were conducted to quantify H2 formation by flowing 2.5% CO, 5% H2O, and 10% N2 with Ar balance. All of the catalysts were pretreated at 500 °C for 1 h in 1% H2/Ar flow before the experiments. The space velocity was 120 000 mL gcat−1 h−1 in all cases. The reactor temperature increased to a target temperature with a B


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Environmental Science & Technology ramping rate of 5 °C/min and was held at that temperature in a continuous feed stream for 15 min to obtain steady-state signals. The product gases were analyzed using online gas chromatography (GC 6500 system, Younglin) equipped with a packed bed carboxen 1000 column (Supelco) and a thermal conductivity detector. The N2 gas was used as an internal standard. H2 productivity of the catalysts under lean−rich cycling was also investigated by observing WGSR. The lean (8% O2 and 5% H2O in Ar) and rich (2% CO and 5% H2O in Ar) conditions were periodically switched every 5 min at 200 °C in the absence of NOx. The effluent gas was analyzed with an online mass spectrometer (BEL Mass).

were mixed; thus, the actual Pt amount in PBC + CC was a half of the cases of PBC or PBCC. Figure S1 of the Supporting Information shows XRD data of the catalysts. CeO2 peaks were mainly observed, and no distinct Cu peak was observed in CC, indicating a fine distribution of Cu species on the ceria support. NOx removal using the PBC, CC, PBCC, and PBC + CC catalysts was tested at various temperatures under lean−rich cycling conditions using CO as the reductant and in the presence of H2O. The overall NOx conversions were estimated at each temperature, and the results are shown in Figure 1. The NOx conversion significantly improved when PBC was physically mixed with CC, showing a synergistic effect. PBC + CC showed the highest NOx conversion, although only a half amount of Pt was used in comparison to PBC or PBCC. Especially, the NOx conversion increased more at lower temperatures: from 11.8% in PBC to 40.0% in PBC + CC at 150 °C and from 27.2% in PBC to 68.0% in PBC + CC at 200 °C. The PBCC catalyst, however, presented even poorer NOx conversion than PBC only. The outlet gas profiles recorded for NOx (NO + NO2), NH3, and N2O are shown during the cyclic operations at 200 °C in Figure 2. Unlike the PBC, CC, and PBCC catalysts, where significant NOx slip was observed, the PBC + CC catalyst showed that the NOx concentration was below the NO inlet concentration (200 ppm), even at fuel-rich conditions. While PBC produced N2O under rich conditions, the N2O peak was smaller in the PBC + CC catalyst. The corresponding overall NOx conversion, NOx storage efficiency (NSE), and NOx reduction efficiency (NRE) are listed in Table 1. NOx would be stored under lean conditions, and the NOx would be reduced under rich conditions. NSE and NRE were estimated using the equations denoted in the footnotes of Table 1 in lean and rich conditions, respectively. When NSE was compared for the PBC, CC, PBCC, and PBC + CC catalysts, PBC + CC presented much higher NSE at low temperatures. The NSE increased from 25.6% in PBC to 61.7% in PBC + CC at 150 °C and from 33.0% in PBC to 75.8% in

3. RESULTS AND DISCUSSION 3.1. Catalytic Performance in Lean−Rich Cyclings. The actual metal content measured by ICP−OES and textural

Figure 1. Overall NOx conversion under lean−rich cycling at various temperatures.

properties of PBC, CC, PBCC, and PBC + CC catalysts are shown in Table S1 of the Supporting Information. It should be noted that 100 mg of each catalyst was used for the reactions. In the case of PBC + CC, 50 mg of PBC and 50 mg of CC

Figure 2. Outlet gas profiles during lean−rich cycling at 200 °C (lean phase, 200 ppm of NO, 8% O2, 5% H2O, and Ar balance for 12 min; rich phase, 200 ppm of NO, 2% CO, 5% H2O, and Ar balance for 2 min). C


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Environmental Science & Technology Table 1. Comparison of NOx Storage and Release during Lean−Rich Cycling temperature (°C)

catalyst

150

PBC CC PBCC PBC + PBC CC PBCC PBC + PBC CC PBCC PBC + PBC CC PBCC PBC + PBC CC PBCC PBC +

200

250

300

350

CC

CC

CC

CC

CC

NOx conversion (%)

NOx storage efficiency (%)a

NOx reduction efficiency (%)b

11.8 15.9 17.8 40 27.2 21 27.8 68 61.8 38.7 37.9 82.1 68.5 49 35.1 88.2 74.9 42.4 33.1 91.3

25.6 40.6 47.6 61.7 33 43.6 52.4 75.8 64.5 59.3 62.7 85.7 76 59 63.2 92.9 86.9 53 61.6 93.5

32.4 30.8 32.4 59.4 53.9 42.8 46.9 85.4 88.8 68.4 55.7 93.6 91.2 75.5 51.3 94.1 90.1 70.9 49.2 96.4

a

NOx storage efficiency was estimated using the following equation: NSE =

NOx stored (NOin )lean − (NOx out )lean × 100% = (NOin )lean (NOin )lean

Figure 3. (a) NOC for the conversion of NO oxidation to NO2 and (b) NSC at 200 °C under lean conditions (200 ppm of NO, 8% O2, 5% H2O, and Ar balance).

× 100% b NOx reduction efficiency was estimated using the following equation:

NRE = =

NOx reduced NOx to be reduced

× 100%

[NOx stored + (NOin )rich ] − (NOx out )rich [NOx stored + (NOin )rich ]

× 100%

PBC + CC at 200 °C. The difference became smaller at high temperatures: from 86.9% in PBC to 93.5% in PBC + CC at 350 °C. Stored NOx was reduced under rich conditions using CO as a reductant. If the reduction is not efficient enough, stored NOx would not be removed, hindering further NOx storage in the subsequent cycle. When NRE was compared for the PBC, CC, PBCC, and PBC + CC catalysts, PBC + CC presented higher NRE at low temperatures. The NRE increased from 32.4% in PBC to 59.4% in PBC + CC at 150 °C and from 53.9% in PBC to 85.4% in PBC + CC at 200 °C. The PBCC catalyst, however, showed even lower NRE than the PBC catalyst. Figure S2 of the Supporting Information shows the effect of Cu loading in CC on the overall NOx conversion. The catalyst mass was identical to 50 mg of PBC and 50 mg of each CC. When 1, 3, 5, and 10 wt % Cu loadings were compared, the 5 wt % Cu showed the highest NOx conversion. In Figure S3 of the Supporting Information, the mass ratio of PBC to CC was varied: from 100 mg of PBC only to 75 mg of PBC + 25 mg of CC, 50 mg of PBC + 50 mg of CC, 25 mg of PBC + 75 mg of CC, and 100 mg of CC only. The combination of 50 mg of PBC + 50 mg of CC showed the highest NOx conversion. The effect of proximity between PBC and CC was also estimated by

Figure 4. O 1s XPS spectra of the catalysts.

Table 2. Deconvolution Results of O 1s XPS Peaks catalyst

Olatt (%)a

Oads (%)b

Ow (%)c

Oads/Olatt

PBC CC PBCC PBC + CC

54.6 45.8 52.9 48.7

31.5 40.2 34 37.9

13.9 14 13.1 13.4

0.58 0.88 0.64 0.78

a Lattice oxygen. bSurface oxygen adsorbed. cOxygen in adsorbed water.

loading the catalyst in a split-bed arrangement, as shown in Figure S4 of the Supporting Information. A total of 50 mg of CC was located on top of the silica sand, and 50 mg of PBC was located at the bottom of the silica sand. The feed flowed from the top to bottom. This sequential flow showed less NOx D


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Figure 7. In situ H2 production under lean−rich cycling on the various catalysts (lean conditions, 8% O2, 5% H2O, and Ar balance for 5 min; rich conditions, 2% CO, 5% H2O, and Ar balance for 5 min).

3.2. NO Oxidation and NOx Storage. Significant synergy was observed in both NOx storage and NOx reduction for the PBC + CC catalyst. To understand the synergistic effect better, the catalytic property for two different aspects was evaluated: (i) NOx storage under lean conditions and (ii) the reduction of stored NOx under rich conditions. NO oxidation is important for NOx storage because BaO can adsorb NO2 better than NO.6,8,29 Figure 3a shows the conversions of NO to NO2 at various temperatures. CC showed high conversion for NO oxidation, and PBC + CC also showed higher conversion than the PBC catalyst. Figure 3b shows the NSC at 200 °C as a function of the storage time under lean conditions. CC showed the poorest NSC value, and PBC showed the highest NSC value after 60 min; however, PBC + CC showed the highest NSC value at the initial stage. Because BaO is required for NOx storage, CC without BaO showed the lowest NSC value. Although the amount of BaO is a half in PBC + CC compared to PBC, PBC + CC showed the highest NSC at the initial stage because NO oxidation to NO2 was enhanced and NO2 adsorption onto BaO was accelerated in the PBC + CC catalyst. The surface oxygen species, which were known to play an important role in NO oxidation,30,31 were analyzed with XPS measurement. Figure 4 shows O 1s XPS spectra of the PBC, CC, PBCC, and PBC + CC catalysts. The O 1s peak was deconvoluted to lattice oxygen (Olatt) at ∼529.3 eV; surface oxygen adsorbed on the defect sites (Oads) at ∼531.2 eV; and oxygen in the molecular water adsorbed on the catalyst surface (Ow) at ∼533.2 eV.32−35 Table 2 shows the percentages of each oxygen species. When the ratio of Oads/Olatt was compared, CC showed the highest value of 0.88 and the values for PBC + CC and PBC were 0.78 and 0.58, respectively. The PBC + CC catalyst possesses abundant active surface oxygen, enhancing NO oxidation, and then formed NO2 could be easily stored on BaO, even at lower temperatures. The thermal stability of stored NOx was investigated by performing NOx-TPD experiments, as shown in Figure 5. It should be noted that the m/z 30 signal can result from the desorption of NO and/or NO2.27,31 In PBC, a shoulder peak appeared at 360 °C with no oxygen release (m/z 32), indicating weakly adsorbed nitrite species. A strong desorption peak was observed at 450 °C with O2 generation, indicating the decomposition of nitrates at Ba sites.31 In CC, NOx desorption peaks appeared at 200−300 °C, implying the

Figure 5. NOx-TPD profiles performed after NOx was adsorbed on each catalyst by flowing 200 ppm of NO, 8% O2, and Ar balance for 1 h at 200 °C.

Figure 6. (a) H2 yield and (b) Arrhenius plot for the WGSR (2.5% CO, 5% H2O, 10% N2, and Ar balance, with a space velocity of 120 000 mL gcat−1 h−1).

conversion than the physical mixture of 50 mg of PBC + 50 mg of CC, indicating that closer contact between PBC and CC is more advantageous for NOx removal. E


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Figure 8. NOx-TPR profiles in 2% CO and 5% H2O (Ar balance) after adsorbing NOx on the catalyst by flowing 200 ppm of NO, 8% O2, and 5% H2O (Ar balance) for 1 h at 200 °C.

WGSR activity. The formation of PtCu alloys was confirmed with XRD, as shown in Figure S5b of the Supporting Information. However, the Pt−CO peak (2097 cm−1) was not shifted on PBC + CC, indicating that the PtCu alloy was not formed upon the physical mixing of the PBC and CC catalysts and consequent reduction. Figure S5c of the Supporting Information also shows the XRD data obtained after the lean−rich cycling reaction, confirming the absence of the PtCu alloy in PBC + CC even after the reaction. In situ H2 production was confirmed under lean−rich cycling conditions, as shown in Figure 7. The lean−rich cycling was performed by flowing 8% O2 and 5% H2O in Ar balance for 5 min and then switching to 2% CO and 5% H2O in Ar balance for 5 min, in the absence of NOx. Surely, H2 was produced from CO and H2O flow by the WGSR, and CC produced H2 the most. The PBC + CC catalyst also produced much more H2 than PBC, while PBCC produced the least amount of H2. The reduction of stored NOx was investigated by TPR experiments, as shown in Figure 8. NOx was first adsorbed on the catalysts by flowing 200 ppm of NO with 8% O2 at 200 °C for 1 h, and then the catalysts were reduced while increasing the temperature in 2% CO and 5% H2O flow. A CO consumption peak (m/z 28) and NOx (m/z 30) or N2 (m/z 14) formation peaks were monitored with mass spectroscopy. In the PBC catalyst, CO consumption was observed at 150 °C and, simultaneously, the desorption of NOx was observed at the same temperature. Whereas thermal desorption of NOx required a much higher temperature of >300 °C, as shown in Figure 5, stored NOx was desorbed at a lower temperature in the presence of a reductant. The N2 formation peak was barely observed, indicating that NOx reduction did not occur much. In the PBC + CC catalyst, however, the CO consumption peak was much larger, the NOx formation peak was smaller, and the N2 formation peak was clearly observed. This difference indicates that the reduction of stored NOx occurred in the PBC + CC catalyst, probably as a result of in-situ-generated H2 from the WGSR.

poor thermal stability of adsorbed NOx. In PBCC, the NOx desorption peak appeared, even at a very high temperature of 645 °C, indicating that removing adsorbed NOx can be quite difficult. In PBC + CC, a broad NOx desorption peak was observed. The highest desorption peak appeared at 378 °C. The majority of NOx adsorbed on PBC + CC could be desorbed at lower temperatures than NOx adsorbed on PBC. 3.3. WGSR and NOx Reduction. NOx stored on the LNT catalyst under lean conditions should be reduced to N2 under rich conditions. It was previously found that H2 is the most effective reducing agent for reducing stored NOx among H2, CO, and C3H6, especially at low temperatures.8,10,36 H2 can be generated under rich conditions via WGSR (CO + H2O → CO2 + H2). CC is known as an excellent catalyst for the WGSR.37,38 The WGSR activity was investigated for the various catalysts, as shown in Figure 6. PBC shows relatively poor catalytic activity. H2 yields were only 3.9 and 9.7% at 150 and 200 °C, respectively. However, when the PBC + CC catalyst was used, the H2 yield were greatly enhanced, with 10.0 and 20.2% at 150 and 200 °C, respectively. The apparent activation energies were estimated at 39.2 and 26.8 kJ/mol for PBC and PBC + CC, respectively. PBCC exhibited inferior activity for H2 production in the WGSR. DRIFTS measurements were performed as shown in Figure S5a of the Supporting Information after CO chemisorption on the PBC, CC, PBCC, and PBC + CC catalysts to probe the Pt and Cu states. PBC showed an adsorption peak at 2097 cm−1, attributed to the CO species linearly adsorbed on Pt atoms. CC had a weak peak at 2103 cm−1, also attributed to the linear CO species on Cu atoms.39,40 In PBCC, there was a strong peak at 2077 cm −1 , with a shoulder at 2110 cm −1, corresponding to the linear CO species adsorbed on Pt and Cu atoms, respectively. The Pt−CO peak was shifted to a lower wavenumber, indicating that the interaction between Pt and CO became stronger in the PBCC catalyst.39,41 Too strong adsorption of CO molecules on the Pt atoms can lead to a decrease in catalytic activity.42 The PBCC catalyst would form PtCu alloys with stronger CO binding, resulting in poorer F



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ACKNOWLEDGMENTS This research was supported by the National Research Foundation of Korea (NRF, 2016R1A5A1009592 and 2018R1A2A2A05018849).

The hydrothermal stability was tested by treating the catalysts with 10% H2O in air at 750 °C for 25 h. The overall NOx conversion under lean−rich cycling was evaluated at 200 °C for PBC and PBC + CC, and the results are shown in Figure S6 of the Supporting Information. After the hydrothermal aging, the NOx conversion decreased significantly from 27.2% in PBC and 68.0% in PBC + CC to 10.9 and 18.7%, respectively. The synergistic effect of PBC and CC was still observed, even after the hydrothermal aging, but more durable structures should be studied further. In summary, the NOx removal efficiency over the PBC catalyst under lean−rich cycling was greatly enhanced by physically mixing it with CC. Particularly, the PBC + CC catalyst exhibited strong synergy at low temperatures below 200 °C. NOx storage and NOx reduction were separately evaluated, and the PBC + CC catalyst showed significant enhancement, particularly at lower temperatures. CC had more surface-adsorbed oxygen, enhancing NO oxidation to NO2 under lean conditions. This lead to higher NOx storage in the PBC catalyst. CC was also excellent in the WGSR, producing H2 from CO and H2O flow. Under rich conditions using CO as a reductant, in-situ-generated H2 enhanced the reduction of stored NOx in the PBC + CC catalyst. This simple method can provide a useful guideline to design better LNT catalysts working at lower temperatures.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.8b05329. Actual metal contents measured by ICP−OES and textural properties measured by Brunauer−Emmett− Teller (BET) (Table S1) and XRD patterns of the various catalysts (Figure S1), overall NOx conversion under lean−rich cycling obtained from physical mixtures of PBC and CC with various Cu loadings (Figure S2), overall NOx conversion under lean−rich cycling for physical mixtures of PBC and CC with various mixing ratios (Figure S3), overall NOx conversion under lean− rich cycling for a physically mixed PBC and CC catalyst and a sequential bed catalyst (Figure S4), DRIFT spectra of CO chemisorbed and XRD patterns on the PBC, CC, PBCC, and PBC + CC catalysts after pretreating the catalysts in 1% H2/Ar flow for 1 h at 500 °C and XRD data obtained after the lean−rich cycle reactions (Figure S5), and effect of hydrothermal aging on the overall NOx conversion under lean−rich cycling at 200 °C for the PBC and PBC + CC catalysts (Figure S6) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: +82-42-350-3922; e-mail: [email protected]. ORCID

Hyunjoo Lee: 0000-0002-4538-9086 Notes

The authors declare no competing financial interest. G


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CeO2 Catalysts for a Low

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