Promoting Effects of Hydrothermal Treatment on the Activity and

Sep 11, 2017 - Ceria-supported Pd nanoparticles are known to be efficient catalysts for vehicle exhaust purification, especially diesel oxidation. The...
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Promoting Effects of Hydrothermal Treatment on the Activity and Durability of Pd/CeO2 Catalysts for CO Oxidation Hojin Jeong,† Junemin Bae,† Jeong Woo Han,‡ and Hyunjoo Lee*,† †

Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, Daejeon 34141, South Korea ‡ Department of Chemical Engineering, University of Seoul, Seoul 02504, South Korea S Supporting Information *

ABSTRACT: Ceria-supported Pd nanoparticles are known to be efficient catalysts for vehicle exhaust purification, especially diesel oxidation. The exhaust often undergoes harsh conditions, suffering from high temperature up to ∼750 °C. These conditions cause Pd nanoparticles to sinter, losing the catalytic active sites. In addition, carbonate and sulfate species might be formed on the catalyst surface, blocking the active sites with degraded activity. Hydrothermal treatment on Pd/CeO2 affects the catalyst structure, resulting in enhanced catalytic activity and durability for CO oxidation. CO conversion approached 100% at temperatures lower than 150 °C even in the presence of propylene or SO2. The high activity for CO conversion changed little for longer reaction times and even for temperature fluctuations up to 850 °C. A promoting effect was obtained due to Pd redispersion and surface hydroxyl groups formed after the hydrothermal treatment. The redispersion was confirmed by TEM, EXAFS, XRD, in situ DRIFT, and CO chemisorption, and the suppression of surface-poisoning species was investigated using in situ DRIFT and TPD techniques. KEYWORDS: Pd/CeO2, CO oxidation, hydrothermal treatment, redispersion, poisoning cycle.15−18 Among various precious metals, Pd could be finely dispersed on a ceria surface with the highest activity for CO oxidation.16,19 Pd/CeO2 has been actively studied as a potential catalyst with high low-temperature activity.20−24 However, the Pd/CeO2 catalysts often suffer from severe deactivation by sintering at high temperature and poisoning in the presence of sulfur.25−27 Redispersion, by which the size of nanoparticles decreases somewhat after certain treatments, has been reported.28 Nagai et al. showed that Pt nanoparticles supported on ceria were redispersed under an oxidative atmosphere.29,30 Wu et al. reported that Pt nanoparticles on cubic ceria were redispersed by repeating oxidative and reductive condition alternatively.31 Jones et al. showed that Pt was atomically dispersed on ceria by atom trapping.32 Lambrou et al. showed that Pd nanoparticles on CeO2−Al2O3 were redispersed by oxychlorination.33 Lira et al. reported that Pt nanoparticles on γ-Al2O3 were redispersed by oxidation under NO.34 Most of the redispersed metal catalysts showed enhanced activity for CO oxidation. Surface poisoning should be suppressed to improve the activity and durability of the catalyst. For CO oxidation, carbonate species are easily formed on the catalyst surface, resulting in degraded activity.11,12,35,36 Schumacher et al. reported that Au/TiO2 catalyst is deactivated by accumulated carbonate species on the surface, not by sintering of Au

1. INTRODUCTION There is a growing demand for an exhaust catalyst possessing low-temperature activity to meet high fuel efficiency and stringent environmental regulations.1 A state of the art engine with high fuel efficiency shows lower temperatures in the exhaust, often requiring the use of additional fuel to make the exhaust catalysts work. This necessity of using additional fuels would obviously make the overall fuel efficiency poor. An exhaust catalyst that works at temperatures lower than 150 °C is highly desired for the development of diesel vehicles with high fuel efficiency and low pollutant emission.2 An exhaust treatment such as diesel oxidation uses preciousmetal catalysts of Pt, Pd, or Rh deposited on metal oxide supports.3,4 The smaller metal nanoparticles typically present higher activity at lower temperature.5−7 However, the catalysts in the vehicle aftertreatment system often experience very high temperatures up to ∼750 °C, in which small nanoparticles are heavily sintered into larger particles, losing their original high activity.8−10 It is important to prepare small metal nanoparticles and preserve the small sizes even under harsh reaction conditions. Additionally, the exhaust catalysts often suffer from surface poisoning by residual sulfur in the fuel. Even the fuel itself can form carbonates at the catalyst surface, hindering further reaction.11−14 High resistance to the poisoning species of sulfates and carbonates is required to guarantee the high activity and durability of the exhaust catalyst. Ceria recently has received much attention due to high oxygen storage capacity, strong interaction with deposited metals, and redox properties obtained by the Ce4+/Ce3+ © XXXX American Chemical Society

Received: June 2, 2017 Revised: August 31, 2017 Published: September 11, 2017 7097

DOI: 10.1021/acscatal.7b01810 ACS Catal. 2017, 7, 7097−7105

Research Article

ACS Catalysis nanoparticles.37 Ntho et al. showed that bicarbonates are formed on Au/TiO2 nanotubes competitively during CO oxidation, and this is responsible for the catalyst deactivation.38 In addition, resistance to sulfur poisoning is very important because most fuels contain a small amount of sulfur, at the 20− 30 ppm level.13,14 In particular, the Pd/CeO2 is deactivated severely in the presence of sulfur.39,40 Only a few studies have been reported to enhance the sulfur resistance. Monai et al. showed that Pd@ZrO2/Si-Al2O3 has enhanced resistance against SO2 poisoning because sulfates were formed on ZrO2 rather than on Pd.27 In this work, Pd/CeO2 catalysts were hydrothermally treated at 750 °C and the effect on the catalyst structure was investigated. Redispersion of Pd nanoparticles was observed. The activity and durability of the hydrothermally treated Pd/ CeO2 catalyst were tested for CO oxidation. The activity for CO oxidation was also tested even in the presence of SO2 or propylene. The effect of the hydrothermal treatment on the surface poisoning by carbonates or sulfates was investigated using various techniques.

for CO oxidation. The effect of the presence of propylene was tested using 100 sccm of feed gas consisting of 1% CO, 0.2% C3H6, and 10% O2 in He balance. The effect of sulfur poisoning was evaluated by a priori flowing 100 sccm of 100 ppm of SO2/ He flow at 300 °C for 2 h on the Pd/CeO2 catalysts. In some cases, water was flown together with CO and O2. The water content was adjusted by bubbling the inlet flow in the water bath at various temperatures. The reactor was heated to the target reaction temperature with a ramping rate of 5 °C/min and was held at that temperature for 18 min to reach a steady state. The product gases were monitored with an online gas chromatography (GC, Younglin GC 6500 system) equipped with a packed bed carboxen 1000 column (75035, SUPELCO, 15 ft × 1/8 in. × 2.1 mm), a thermal conductivity detector (TCD), a capillary column (GS-GASPRO, Agilent Technologies, 30 m × 0.32 mm), and a flame ionization detector (FID). The CO oxidation was also monitored by in situ diffuse reflectance Fourier transform infrared spectroscopy (DRIFT) using a Nicolet iS-50 instrument (Thermo Scientific). The catalyst was mixed with KBr and ground. The mixed powder was put into a sample cup and set inside a DRIFT cell. The cell was purged with an Ar flow at 100 °C for 1 h to remove water and cooled to room temperature. The cell was heated to 85 °C and kept at the same temperature. A 2% CO gas with Ar balance was charged for 10 min, and the gas flow was stopped. The DRIFT spectra were collected under vacuum. Then, 2% O2 gas with Ar balance was flown and the DRIFT spectra were collected simultaneously. Finally, the O2 flow was stopped and additional spectra were collected under vacuum. 2.3. Characterizations. Transmission electron microscopy (TEM) images were recorded on a Tecnai G2 F30 S-Twin (FEI) instrument operated at 300 kV. The high angle annular dark field scanning TEM (HAADF-STEM) images and energydispersive X-ray spectroscopy (EDS) mapping images were obtained using a Titan cubed G2 60-300 instrument (FEI) with accelerating voltages of 300 and 200 kV, respectively. Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) analyses were performed on an OPTIMA 7300 DV instrument to determine the actual amount of Pd in the Pd/CeO2 catalysts. The crystalline structure of the catalysts was observed by powder X-ray diffraction (XRD) patterns (Rigaku, Cu Kα radiation). The metallic state of Pd was investigated by X-ray photoelectron spectroscopy (XPS, K-Alpha, Thermo VG Scientific). Binding energies were calculated using the maximum intensity of the advantageous C 1s signal at 284.8 eV as a reference. Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) measurements were conducted at the 10C Wide XAFS beamline of the Pohang Light Source (PLS). Pd dispersion was determined by a pulsed CO adsorption technique which was modified from the method of Takeguchi et al.41 First, the Pd/CeO2 catalyst (50 mg) was heated under 3.5% O2/He gas at 300 °C for 10 min and subsequently cooled to 50 °C ,where the sample was purged with He gas for 5 min. Next, the sample was heated under a 4.9% H2/Ar flow to 200 °C. After it was cooled to 50 °C, the sample was exposed to the gas flows in the following sequence: (i) He (5 min); (ii) 3.5% O2/He (5 min); (iii) CO2 (10 min); (iv) He (20 min); (v) 5% H2/Ar (5 min). Finally, CO was pulsed every 1 min under a stream of He until the adsorption of CO onto the sample was saturated. CO2 was injected to form carbonates on the ceria surface. Otherwise, Pd dispersion might be overestimated

2. EXPERIMENTAL SECTION 2.1. Catalyst Synthesis. Ceria was prepared by a simple precipitation method. Ce(NO3)3·6H2O (99.99%, Kanto Chemical; 1 g) was dissolved in 23 mL of deionized water in a 250 mL round-bottom flask. Then, ammonia−water (30%, Duksan) was added dropwise with stirring at 500 rpm until the pH of the solution reached 8.5. The solution was stirred for an additional 1 h until it turned yellow. The resulting yellow slurry was decanted, filtered, and washed several times with deionized water to make it free of anionic impurities. The precipitates were dried in a convection oven at 80 °C for 12 h, and the obtained cake was crushed in a ceramic mortar to a fine powder. The powder was calcined at 500 °C for 5 h with a ramping rate of 5 °C/min in static air. Pd/CeO2 catalysts were synthesized by a conventional deposition−precipitation method. The synthesized CeO2 powder was dispersed in deionized water with stirring at 800 rpm in a 20 mL vial. H2PdCl4 solution was prepared with a 2:1 molar ratio of HCl to PdCl2 (99%, Sigma-Aldrich) in deionized water. The H2PdCl4 solution was briefly sonicated and kept shaken until a homogeneous solution was formed. Na2CO3 solution was also prepared by mixing 530 mg of Na2CO3 (99.5%, Sigma-Aldrich) and 20 mL of deionized water. Then, H2PdCl4 and Na2CO3 aqueous solutions were simultaneously introduced dropwise into the ceria solution with stirring at 800 rpm, making the pH of the final solution around 9. After it was stirred for an additional 2 h and aged for 2 h, the solution was filtered and washed with deionized water several times and dried at 80 °C for 5 h. Pd weight percentages of 0.1−5.0 wt % Pd/CeO2 catalysts were synthesized. 2.2. Chemical Reactions. CO oxidation was performed in a U-shaped quartz glass fixed-bed reactor at atmospheric pressure. The catalyst (50 mg) was charged inside the reactor. The as-made Pd/CeO2 catalyst, thermally treated Pd/CeO2 catalyst, and hydrothermally treated Pd/CeO2 catalyst were compared. The thermally treated catalyst was prepared by flowing 144.5 sccm of dry air at 750 °C for 25 h, and the hydrothermally treated catalyst was prepared by flowing 144.5 sccm of air with 10% H2O at 750 °C for 25 h. The reactor was purged with 100 sccm of He gas at 100 °C for 1 h and cooled to room temperature under the He flow. A total 100 sccm of feed gas was introduced with 1% CO and 1% O2 in He balance 7098

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ACS Catalysis

catalyst was hydrothermally treated in wet air with 10% H2O at 750 °C for 25 h, however, the Pd domain was hardly observed. EDS mapping images of the hydrothermally treated sample shown in Figure 1d clearly indicate that Pd was uniformly distributed on the ceria, but Pd nanoparticles were not distinguishable by HAADF-STEM. The EDS mapping images of the as-made and thermally treated samples showed that the Pd formed distinct domains, as shown in Figure S2 in the Supporting Information. High-resolution TEM images in Figures S3−S5 in the Supporting Information indicate that the average size of Pd domains is 1.5 nm for 2 wt % as-made Pd/CeO2, 2.2 nm for the thermally treated sample, and 0.9 nm for the hydrothermally treated sample. This also supports that the Pd domain became larger after thermal treatment while the Pd domain became smaller after hydrothermal treatment. The decrease in Pd domain size after post-treatment, which is called a “redispersion” phenomenon, could be also confirmed by EXAFS and CO chemisorption. Pd K edge EXAFS data were compared for 2 wt % as-made Pd/CeO2, thermally treated Pd/CeO2, hydrothermally treated Pd/CeO2, PdO, and Pd foil, as shown in Figure 2, and their fitting results are shown in

because CO could also be adsorbed on the ceria surface, forming carbonates. The surface carbonates formed at the Pd/CeO2 catalysts were measured by temperature-programmed desorption (TPD) on a BELCAT-B (BEL, Japan) equipped with mass spectrometer (MS). Each catalyst (50 mg) was loaded into a U-shaped quartz cell, pretreated under 100 sccm of He flow at 100 °C for 1 h, and cooled to room temperature under He flow. The 1% CO and 1% O2 balanced with He gas was introduced into the cell at room temperature for 20 min. After physically adsorbed gas molecules were purged with 100 sccm of He flow for 1 h, the catalysts were immediately charged to a TPD-quartz cell. The TPD-quartz cell was heated to 600 °C with a ramping rate of 10 °C/min under 50 sccm of He flow. Carbon dioxide with m/z 44 in the outlet gas was analyzed by the MS detector. The availability of lattice oxygen was also measured by TPDMS by monitoring oxygen. The samples were pretreated with 50 sccm of 3.5% O2/He at 300 °C for 1 h and cooled to room temperature under 50 sccm of He flow. The temperature was increased to 800 °C with a ramping rate of 10 °C/min under He flow. Oxygen with m/z 32 in the outlet gas was analyzed by the MS detector.

3. RESULTS AND DISCUSSION 3.1. Change in Catalyst Structures upon Hydrothermal Treatment. Electron microscopy images of the Pd/ ceria catalysts for as-made, thermally treated, and hydrothermally treated samples are shown in Figure 1. The approximate size of ceria was around 15−30 nm, as shown in Figure S1 in the Supporting Information. When 2 wt % of Pd was deposited on the ceria surface, very small Pd nanoparticles were observed, as noted by white circles in Figure 1a. When the as-made catalyst was thermally treated in dry air at 750 °C for 25 h, the Pd domain became notably larger. When the as-made

Figure 2. (a) Pd K edge k3-weighted Fourier transformed EXAFS spectra of the various Pd samples: 2 wt % as-made, thermally treated (T), and hydrothermally treated (HT) Pd/CeO2, PdO, and Pd foil. Solid lines indicate experimental data, and dashed lines are fitted results. The magnitudes of FT-EXAFS spectra of PdO and Pd foil were multiplied by one-third and one-fourth, respectively, for easier comparison. (b) Overlapping of the FT-EXAFS spectra of 2 wt % as-made, thermally treated, and hydrothermally treated Pd/CeO2.

Table 1. The coordination number for Pd−O−Pd increased after thermal treatment from 3.5 in the as-made sample to 5.2 in the thermally treated sample. However, the coordination number for Pd−O−Pd decreased after hydrothermal treatment from 3.5 to 0.6 in the hydrothermally treated sample, and the coordination number for Pd−O−Ce increased from 0.3 to 1.7. The hydrothermal treatment reduced the Pd domain size while strengthening the Pd−O−Ce interaction. The Pd dispersion was measured by following the method explained in detail in the Experimental Section, and the size of the Pd domain was also estimated from the dispersion as shown in Table 2. The Pd size decreased from 1.5 to 0.9 nm for 2 wt % Pd/CeO2 after

Figure 1. HAADF-STEM images of (a) as-made, (b) thermally treated, and (c) hydrothermally treated 2 wt % Pd/CeO2 catalysts. White circles indicate Pd species on CeO2 surface. (d) EDS mapping images of hydrothermally treated 2 wt % Pd/CeO2 catalyst (red, Pd; green, Ce). 7099

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ACS Catalysis Table 1. Best Fit Values for the EXAFS Analysis of Various Pd Samples sample

path

coordination no.

interatomic distance (Å)

Debye−Waller factor σ2 (Å2)

R factor (%)

as made

Pd−O Pd−Pd Pd−Cl Pd−O−Ce Pd−O−Pd Pd−O Pd−Pd Pd−O−Ce Pd−O−Pd Pd−O Pd−Pd Pd−O−Ce Pd−O−Pd Pd−O Pd−O−Pd Pd−O−Pd Pd−Pd

2.236 0.113 1.872 0.321 3.491 4.571 0.453 0.429 5.236 4.575 0.122 1.678 0.585 4.0 4.0 8.0 12.0

1.979 2.681 2.290 3.214 3.354 2.003 2.739 3.400 3.360 1.998 2.705 3.215 3.407 2.024 3.060 3.443 2.742

0.001 0.003a 0.006 0.003a 0.015 0.003 0.003a 0.001 0.001 0.003 0.003a 0.004 0.003a 0.004 0.007 0.005 0.006

0.015

2.0 T

2.0 HT

PdO

Pd foil a

0.018

0.026

0.005

0.003

This factor was fixed during the EXAFS fitting.

Table 2. Changes in BET Surface Area, Pd Dispersion and Estimated Diameter, Pd Oxidation State, and T100 for As-Made, Thermally Treated, and Hydrothermally Treated Pd/CeO2 Catalysts

a

catalyst

SBET (m2/g)

ceria thermally treated hydrothermally treated as-made 2 wt % Pd/CeO2 thermally treated hydrothermally treated

58.7 4.6 4.9 57.4 38.5 39.9

DPd (%)a

59.1 36.7 97.8

dPd (nm)b

1.5 2.4 0.9

Pd0/(Pd0 + Pd2+)c

T100 (°C)d

0.40 0.22 0.14

270 300 290 125 145 75

Pd dispersion was measured by pulsed CO chemisorption. bPd size was estimated from the Pd dispersion. cEstimated from Pd 3d XPS spectra. Temperature at which CO conversion reached 100%.

d

In situ infrared (IR) peaks for CO adsorption on the metal sites can provide valuable information about the structure. The CO molecules adsorbed onto single Pd sites show a peak at 2000−2200 cm−1, while the CO adsorbed onto ensemble Pd sites with a bridge mode or a 3-fold hollow mode show a peak at 1900−2000 or 1800−1900 cm−1, respectively.43 Figure 3

hydrothermal treatment, whereas the size increased from 1.5 to 2.4 nm after thermal treatment, which is consistent with the estimation from HR-TEM images. The effect of post-treatment on physicochemical properties of the Pd/CeO2 catalysts was evaluated more by measuring the change in BET surface area and Pd oxidation state as shown in Table 2. It is well-known that the surface area of CeO2 decreases significantly upon high-temperature treatment.42 The BET surface area dropped from 58.7 to 4.9 m2/g after hydrothermal treatment for bare ceria, but the decrease was alleviated from 57.4 to 39.9 m2/g for 2 wt % Pd/CeO2. The Pd addition certainly suppressed the ceria aggregation.20,42 The change in Pd oxidation state after post-treatments was estimated using XANES and XPS as shown in Figures S6 and S7 in the Supporting Information, respectively. The XANES data showed that a white line peak of hydrothermally treated Pd/CeO2 was shifted to higher energy in comparison to the asmade and thermally treated samples, indicating a highly oxidized feature of Pd species in the hydrothermally treated sample. The Pd was oxidized further after hydrothermal treatment. The ratio of Pd0 to Pd0 + Pd2+, which was estimated from XPS data, decreased significantly after the hydrothermal treatment. The change in the crystalline structure was also observed by XRD, as shown in Figure S8 in the Supporting Information. The XRD peaks for PdO were observed for asmade 2 wt % Pd/CeO2, but the PdO peaks disappeared after hydrothermal treatment, also supporting redispersion of the Pd domain.

Figure 3. Change in CO adsorption mode after thermal (T) or hydrothermal (HT) treatment observed by in situ DRIFT measurements over 2 wt % Pd/CeO2 catalyst.

shows the IR peaks of CO adsorbed on as-made, thermally, and hydrothermally treated Pd/CeO2 samples. The thermally treated sample exhibited a larger hollow CO peak due to Pd aggregation. However, the bridge or hollow CO peaks almost disappeared and the linear CO increased significantly over the hydrothermally treated sample, supporting the Pd redispersion. HR-TEM and STEM images, EXAFS data, Pd dispersion 7100

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ACS Catalysis estimated from CO chemisorption, XRD, and in situ IR data for CO adsorption indicate that the Pd domain on Pd/CeO2 catalysts was redispersed upon hydrothermal treatment. A change in the size of the Pd domain over hydrothermal treatment time was observed, as shown in Table S1 in the Supporting Information. The Pd domain becomes larger from 1.5 to 2 nm until 4 h and then becomes smaller. The hydrothermal treatment may provide surface hydroxyl groups, forming Pd2+−OH. The amount of surface hydroxyl groups is insufficient up to 4 h, but longer hydrothermal treatment would provide sufficient amounts of hydroxyl groups, promoting Pd redispersion. The Pd2+−OH species may move along the ceria surface with stronger interaction between Pd and CeO2. The redispersion was observed for various sizes of initial Pd domain. As shown in Table S2 in the Supporting Information, the initial sizes were varied from 1.5 to 17.8 nm by annealing 2 wt % asmade Pd/CeO2 at various temperatures, then hydrothermal treatment was performed. The Pd nanoparticles with sizes of 1.5, 1.6, 2.3 nm were successfully redispersed with nearly 100% dispersion, but the Pd nanoparticles with larger sizes were not fully redispersed, although the Pd domain sizes still decreased significantly. The redispersion could occur repeatedly, as shown in Table S3 in the Supporting Information. The redispersed Pd sample was intentionally sintered by thermal annealing and then hydrothermally treated again. The redispersion occurred reversibly up to 13 times without any increase in Pd domain size after the hydrothermal treatment. 3.2. Catalytic Activity and Durability for CO Oxidation. The catalytic activity of Pd/CeO2 catalysts with various Pd contents was evaluated for CO oxidation, as shown in Figure S9 in the Supporting Information. While bare ceria showed little activity up to 200 °C, only 0.1 wt % Pd addition could considerably enhance the activity. The catalytic activity increased as the Pd content of as-made samples increased up to 5 wt %. In the case of hydrothermally treated samples, the activity increased up to 2 wt %, and then a further increase in the Pd contents did not make a difference. When the Pd contents were varied, the sizes of as-made Pd nanoparticles were changed, as shown in Table S4 in the Supporting Information. The Pd nanoparticles with larger sizes (3.0 nm for 4 wt % and 3.8 nm for 5 wt %) presented less redispersion, as we mentioned in Table S2 in the Supporting Information. Full redispersion was observed up to 2 wt %, and higher Pd contents were not fully redispersed after the hydrothermal treatment. The redispersed Pd species increased up to 2 wt % with higher activity for CO oxidation, while the redispersed Pd species was not increased further for higher Pd contents. The effect of post-treatments was investigated in detail for 2 wt % Pd/CeO2 catalysts, as shown in Figure 4. The temperatures where CO conversion reached 100%, which will be denoted as T100 below, were 125, 145, and 75 °C for asmade, thermally treated, and hydrothermally treated 2 wt % Pd/CeO2, respectively. The hydrothermally treated sample showed enhanced activity in spite of the decrease in surface area, resulting from Pd redispersion. The activity of the thermally treated sample was poorer due to Pd sintering. The conversion for CO oxidation reached 100% at a much lower temperature of 40 °C for the hydrothermally treated sample when the space velocity was lower and O2 content was higher, as shown in Figure S10 in the Supporting Information. In situ DRIFT analyses were performed to elucidate the actual active sites for CO oxidation at low temperatures. Figure 5 clearly shows that Pd2+ sites cause CO oxidation at lower

Figure 4. CO oxidation results for as-made, thermally treated (T), and hydrothermally treated (HT) 2 wt % Pd/CeO2 samples. The reactions were performed using 50 mg of catalyst and 100 sccm of total feed flow (1% CO and 1% O2 balanced with He) at 120000 mL/(h gcat). “2.0 H2O” indicates the reaction results when as-made 2 wt % Pd/ CeO2 was used with a feed containing 3.2% of H2O in addition to CO and O2.

Figure 5. In situ DRIFT results using hydrothermally treated 2 wt % Pd/CeO2 catalyst for CO oxidation. Analysis procedures are as follows: 2% CO with Ar balance was flown for 20 min at 25 °C, the residual CO gas was purged out under vacuum, then 2% O2 with Ar balance was flown for 10 min while IR spectra were obtained from 25 to 125 °C.

temperatures. The peaks at 2074 and 2106 cm−1 can be assigned as CO linearly adsorbed on Pd0 sites. The peak at 2152 cm−1 can be assigned as CO linearly adsorbed on Pd2+ sites.44,45 The CO−Pd2+ peak disappeared first from 25 to 75 °C, and then the CO−Pd0 peaks disappeared gradually from 65 to 125 °C. This indicates that the CO oxidation activity at low temperature mainly results from Pd2+ sites. When water was introduced together with CO and O2, the activity of the as-made 2 wt % Pd/CeO2 catalyst was enhanced, but not as much as for the hydrothermally treated sample. It was previously reported that the activity of supported metal catalysts could be significantly enhanced by the addition of moisture in the feed flow.46−50 The water content in the feed flow was varied up to 10%, as shown in Figure S11 in the Supporting Information. A water content of 3.2% showed the highest enhancement, and the activity somewhat decreased as the water content increased more. The addition of water would increase the amount of surface hydroxyl groups at the catalyst, which enhances CO oxidation.48,51 The best activity of the hydrothermally treated catalyst results from both Pd redispersion and surface hydroxyl groups. The aging time for the hydrothermal treatment was varied, as shown in Figure S12a in the Supporting Information. When 12, 25, and 50 h were tested, 25 h showed the best activity. The BET surface areas after the hydrothermal aging were 47.2, 39.9, and 13.3 m2/g, respectively. Twelve hours might not be enough for Pd redispersion, and the surface area decreased too much after 50 h. The CO oxidation was repeated five times for the 7101

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ACS Catalysis hydrothermally treated catalyst, as shown in Figure S12b. No change in the activity was observed. The effect of Pd redispersion and surface hydroxyl groups on CO oxidation was separately observed by preparing a Pd/CeO2 catalyst that has the same Pd domain size but has more surface hydroxyl groups in comparison to the as-made sample. When the as-made sample was annealed at 570 °C under He flow for 2 h first and then hydrothermally treated at 750 °C for 25 h, denoted as 570-HT, the Pd domain size was 1.5 nm, which is the same size as for the as-made sample, but this catalyst would contain more surface hydroxyl groups. Figure S13 in the Supporting Information shows CO oxidation results for this catalyst. The activity was better than that of the as-made sample but worse than that of the hydrothermally treated sample, indicating that both Pd dispersion and surface hydroxyl groups play roles in enhancing CO oxidation activity. When the activation energy was estimated, the values were 57.0, 53.8, and 53.5 kJ/mol for as-made, 570-HT, and hydrothermally treated samples, respectively. The surface hydroxyl groups might play a more important role in decreasing the activation energy. It was observed whether the surface hydroxyl groups participate in CO oxidation by isotope experiments.52,53 The hydrothermal treatment was performed with H218O. The formation of −18OH surface hydroxyl groups was confirmed by TPD, as shown in Figure S14a in the Supporting Information. Then CO oxidation was performed on this catalyst, as shown in Figure S14b. Only C16O2 was observed without any C16O18O or C18O2, indicating that the oxygen on surface hydroxyl groups does not participate in CO oxidation. In order to evaluate the potential of the hydrothermally treated sample as a diesel oxidation catalyst, CO and C3H6 were introduced into the reactor simultaneously and the reaction results are shown in Figure 6a. T100 increased from 75 to 145 °C in the presence of C3H6. This shift of CO oxidation to higher temperature in the presence of propylene also had been observed previously.54−56 Nevertheless, both CO and C3H6 were completely oxidized at 155 °C, which almost meets a DOE target for a low emission vehicle program, LEV III.2 Because sulfur species can contaminate the catalyst surface resulting in degraded activity, resistance to sulfur poisoning is very important for a diesel oxidation catalyst. Ceria is especially known to suffer severely from sulfur poisoning.39 The 2 wt % Pd/CeO2 catalysts were intentionally poisoned with 100 ppm of SO2 a priori to CO oxidation, and the CO conversions on the poisoned catalysts are shown in Figure 6b. T100 increased from 125 to 235 °C for the as-made sample, from 145 to 285 °C for the thermally treated sample, and from 75 to 145 °C for the hydrothermally treated sample. For the hydrothermally treated sample, the temperature increase was the smallest, and CO was completely oxidized below 150 °C even in the presence of sulfur. The hydrothermal treatment enhanced the sulfur resistance of the Pd/CeO2 catalyst significantly. Long-term durability of the 2 wt % Pd/CeO2 catalysts was tested by oxidizing CO at 150 °C for a prolonged period, and the reaction results are shown in Figure 7a. While the activity of as-made and thermally treated samples started to degrade after 27 and 8 h of reaction, the hydrothermally treated sample retained 100% CO conversion for 34 h. The activity of the thermally treated sample decreased more significantly, and the as-made and hydrothermally treated samples followed similar trends up to 60 h; then the activity of the hydrothermally treated sample dropped. The surface hydroxyl group might be mostly removed at this stage. When CO oxidation occurred in

Figure 6. (a) Results for simultaneous oxidation of CO and C3H6 using as-made and hydrothermally treated (HT) 2 wt % Pd/CeO2 catalysts. The reactions were performed using 50 mg of catalyst and 100 sccm of total feed flow (1% CO, 0.2% C3H6, and 10% O2 balanced with He) at 120000 mL/(h gcat). (b) CO oxidation results after sulfur poisoning for as-made, thermally treated (T), and hydrothermally treated (HT) 2 wt % Pd/CeO2 catalysts. The sulfur poisoning was performed using 100 ppm of SO2 balanced with He with a flow rate of 100 sccm at 300 °C for 2 h. The reactions were performed using 50 mg of catalyst and 100 sccm of total feed flow (1% CO and 1% O2 balanced with He) at 120000 mL/(h gcat).

Figure 7. (a) CO oxidation results over long-term durability tests using as-made, thermally treated (T), and hydrothermally treated (HT) 2 wt % Pd/CeO2. The reactions were performed using 50 mg of catalyst and 100 sccm of total feed flow (1% CO and 1% O2 balanced with He) at 120000 mL/(h gcat) and 150 °C. “2.0 HT with 3.2% H2O” indicates the results when the HT 2 wt % Pd/CeO2 was used with a feed containing 3.2% of H2O in addition to CO and O2. (b) CO oxidation results in the presence of 100 ppm of SO2 flow.

the presence of 3.2% water, however, the durability of the hydrothermally treated sample was enhanced without a sudden activity drop. When 100 ppm SO2 was flown together with CO 7102

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ACS Catalysis

Figure 8. In situ DRIFT analysis results over 2 wt % Pd/CeO2 showing the formation of carbonates for (a) as-made and (b) hydrothermally treated samples and the presence of surface hydroxyl groups for (c) as-made and (d) hydrothermally treated samples during CO oxidation. A 2% CO with balance Ar gas was flown over 10 min, then IR spectra were obtained for 10 min under vacuum, and 2% O2 with balance Ar was flown over 10 min while IR spectra were obtained, and IR spectra were additionally obtained for 10 min under vacuum.

specifically, the hydroxyl groups and surface carbonates were formed at the CeO2 surface. When the in situ DRIFT analysis was performed on the bare CeO2 as shown in Figure S16 in the Supporting Information, the same trend as for Pd/CeO2 was observed. The surface carbonates formed on the catalysts during CO oxidation were further investigated using CO2 temperatureprogrammed desorption (TPD) analysis, as shown in Figure S17 in the Supporting Information. The reactant gases of CO and O2 were introduced together under the same conditions as for CO oxidation, and physisorbed gases were removed by purging with He; then the temperature was increased while the desorbed CO2 was detected by mass spectroscopy. The asmade Pd/CeO2 catalyst showed desorption peaks at 250, 410, and 550 °C, which were not observed for the hydrothermally treated sample. The hydrothermal treatment surely suppressed the formation of surface carbonates. The sulfur can deactivate supported Pd catalysts severely by forming sulfates on the surface.13,14,27 The as-made or hydrothermally treated Pd/CeO2 catalysts were intentionally poisoned with 100 ppm of SO2, and then the formed surface sulfates were observed using DRIFT, as shown in Figure S18 in the Supporting Information. The intensity of sulfate IR peaks definitely decreased after the hydrothermal treatment. When the as-made or hydrothermally treated bare CeO2 catalysts were also poisoned with 100 ppm of SO2, the intensity of sulfate IR peaks likewise decreased significantly after the hydrothermal treatment. When as-made 2 wt % Pd/Al2O3 was poisoned, no sulfate peak was observed. These results indicate that the sulfates were mainly formed at the CeO2 surface, and the sulfate formation on the CeO2 surface was suppressed significantly after the hydrothermal treatment. The surface hydroxyl group and the surface carbonates can affect the oxygen transfer. Figure 9 shows O2-TPD results in the absence and presence of surface carbonates, respectively, for the bare ceria and 2 wt % Pd/CeO2 catalysts. All of the samples

and O2 as shown in Figure 7b, 100% CO conversion was maintained up to 14 h for the hydrothermally treated sample, and then fluctuation in CO conversion was observed, although the high CO conversion above 80% was maintained up to 60 h. When the as-made sample was tested for CO oxidation in the presence of SO2, the activity dropped significantly at the early stage, and the CO conversion became 0% after 35 h. The hydrothermal treatment certainly helped maintaining high activity even in the presence of sulfur. Additionally, the durability against temperature fluctuation was tested on the hydrothermally treated 2 wt % Pd/CeO2. In Figure S15 in the Supporting Information, CO oxidation was repeated up to various target temperatures in the range of 75− 850 °C. CO oxidation was performed up to one target temperature, and the reactor was cooled to room temperature; then CO oxidation started again up to the next target temperature. The hydrothermally treated sample showed high durability during the fluctuation with little activity change in CO oxidation. 3.3. Surface Carbonates and Lattice Oxygen. It is wellknown that surface carbonates are formed during CO oxidation and the carbonates often behave as surface poisons blocking further reaction.12,36 The extent of carbonate formation was compared for as-made and hydrothermally treated Pd/CeO2 catalysts using in situ DRIFT analysis as shown in Figure 8a,b. The carbonate species were formed on the catalyst surface as bidentate carbonate (1585 and 1299 cm−1) or monodentate carbonate (1482 and 1416 cm−1).57 The intensity of carbonate IR peaks decreased dramatically after the hydrothermal treatment. Figure 8c,d shows that the hydrothermally treated sample contained many more surface hydroxyl groups than the as-made sample during CO oxidation. The surface hydroxyl groups possibly hindered the formation of surface carbonates.51,58 The hydrothermal treatment produced more surface hydroxyl groups on the Pd/CeO2 catalyst, and the hydroxyl groups prevent the formation of surface carbonates. More 7103

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velocity of 120000 mL/(h gcat). In the presence of C3H6, the CO conversion reached 100% at 145 °C and the C3H6 conversion reached 100% at 155 °C. Even when the catalyst was poisoned with SO2, the CO conversion reached 100% at 145 °C. The activity was significantly improved in comparison to the as-made or thermally treated samples. The hydrothermally treated catalyst exhibited good durability even in the presence of SO2, and it also showed little degradation in CO conversion when the temperature was varied harshly. All of the results of TEM images, EXAFS, CO chemisorption, XRD, and in situ IR data indicate that the Pd domain on Pd/CeO2 catalysts was redispersed upon the hydrothermal treatment. Additionally, the hydrothermal treatment produced surface hydroxyl groups, which hindered the formation of surface carbonates during CO oxidation. The surface hydroxyl group also could suppress the formation of surface sulfates. When the desorption of surface oxygen was compared before and after the carbonate formation, the hydrothermally treated catalyst was hardly affected, whereas the surface oxygen could not be taken out from the as-made sample after the carbonate formation. The smaller size of the Pd domain and resistance to surface poisoning species enabled high activity and durability for the hydrothermally treated Pd/CeO2 catalyst.

Figure 9. Temperature-programmed desorption of oxygen (m/z 32) monitored by mass spectroscopy over bare ceria and 2 wt % Pd/CeO2 catalyst (a) before and (b) after the formation of surface carbonates. The as-made and hydrothermally treated (HT) samples were compared.

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ASSOCIATED CONTENT

S Supporting Information *

were pretreated under an O2 flow at 300 °C for 1 h. In comparison to the bare ceria, the addition of Pd facilitated oxygen transfer with larger O2 peaks. For the Pd/CeO2 catalyst, the hydrothermal treatment, which produced surface hydroxyl groups at the catalyst surface, desorbed the surface lattice oxygen or bulk lattice oxygen at lower temperatures. The asmade sample exhibited the desorption peak of surface lattice oxygen at 300 °C and bulk lattice oxygen at 550 °C, whereas the hydrothermally treated sample showed the peaks at 260 and 530 °C, respectively. The surface carbonates were formed by flowing CO and O2 at room temperature, and then O2-TPD was performed. The as-made sample showed no O2 desorption peak, whereas the hydrothermally treated sample still showed large O2 peaks for surface lattice oxygen at 270 °C and bulk lattice oxygen at 520 °C. While the oxygen transfer was hindered by the surface carbonates at the as-made sample, the hydrothermally treated sample was hardly affected. TPD analysis was also performed under an CO/He flow, as shown in Figure S19 in the Supporting Information. The hydrothermally treated Pd/CeO2 showed a large peak at 83 °C, irrelevant to the carbonate formation. However, the as-made sample showed a CO2 peak at 129 °C for the clean surface catalyst only, and the as-made sample at which the carbonates were formed showed few CO2 peaks. The amounts of desorbed CO2 were estimated as shown in Table S5 in the Supporting Information. The hydrothermally treated catalysts produced a large amount of CO2, but the amount of desorbed CO2 decreased significantly after the carbonate formation over the as-made sample. The hydrothermal treatment promoted the lattice oxygen transfer and prohibited the carbonate formation, resulting in high activity and durability for CO oxidation.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.7b01810. Tables and figures as described in the text giving additional data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*H.L.: e-mail, [email protected]; tel, +82-42-350-3922. ORCID

Jeong Woo Han: 0000-0001-5676-5844 Hyunjoo Lee: 0000-0002-4538-9086 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Research Foundation of Korea (NRF-2015R1A2A2A01004467; NRF2016R1A5A1009592). The experiments at PLS were supported in part by MSIP and POSTECH.

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REFERENCES

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