Highly Active and Stable Pt-Loaded Ce0.75Zr0.25O2 YolkâShell

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Highly active and stable Pt loaded Ce0.75Zr0.25O2 yolk-shell catalyst for water-gas shift reaction Jae-Oh Shim, Young Jun Hong, Hyun-Suk Na, Won-Jun Jang, Yun Chan Kang, and Hyun-Seog Roh ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b03915 • Publication Date (Web): 17 Jun 2016 Downloaded from http://pubs.acs.org on June 22, 2016

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ACS Applied Materials & Interfaces

Highly active and stable Pt loaded Ce0.75Zr0.25O2 yolk-shell catalyst for water-gas shift reaction Jae-Oh Shima,‡, Young Jun Hongb,‡, Hyun-Suk Naa, Won-Jun Janga, Yun Chan Kangb,* and Hyun-Seog Roha,*

Addresses: aDepartment of Environmental Engineering, Yonsei University, 1 Yonseidae-gil, Wonju, Gangwon 220-710, Republic of Korea; bDepartment of Materials Science and Engineering, Korea University, Anam-Dong, Seongbuk-Gu, Seoul 136-713, Republic of Korea ‡These authors contributed equally to this work.

*Corresponding authors. E-mail: [email protected] (Yun Chan Kang, Fax: (+82) 2-9283584), [email protected] (Hyun-Seog Roh, Fax: (+82) 33-760-2571)

Keywords: Yolk-shell; Spray pyrolysis; Water-gas shift; Pt loading; Reducibility; Sintering resistance

1

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ABSTRACT Multi-shelled, Pt-loaded Ce0.75Zr0.25O2 yolk-shell microspheres were prepared by a simple spray pyrolysis process for use in the water-gas shift (WGS) reaction. The Pt loading

was

optimized,

obtaining

highly

active

Pt/Ce0.75Zr0.25O2

yolk-shell

nanostructures for the WGS. Of the prepared catalysts, a 2% Pt loading of the Ce0.75Zr0.25O2 yolk-shell microspheres showed the highest CO conversion. The high catalytic activity of the 2% Pt/Ce0.75Zr0.2O2 catalyst was mainly due to its easier reducibility and the maintenance of active catalytic Pt species. The Pt-loaded Ce0.75Zr0.25O2 catalyst microspheres were highly resistant to Pt sintering because of their unique yolk-shell structure. Spray pyrolysis was found to be highly efficient for the production of precious-metal-loaded, multicomponent metal oxide yolk-shell microspheres for catalytic applications.

2


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INTRODUCTION The water-gas shift (WGS) reaction has come to the fore in recent years.1−5 The WGS reaction is a critical step in catalytic process to produce pure H2, which can be utilized in H2 fuel cells.6−9 Recently, bifunctional catalysts have been used for the WGS reaction due to their improved catalytic activities and stabilities. In particular, the combination of noble metals (Pt, Ru, and Au) and rare earth oxides (ZrO2, TiO2, and CeO2) into a single nanostructure yields an attractive material because of the synergism between components during catalytic reactions, in contrast to single-component materials.10 Among these catalysts, a Pt/CeO2 catalyst is considered to be effective for the WGS reaction.7,11,12 Recently, many researchers have tried to enhance the catalytic performance of Pt/CeO2 catalysts for the WGS reaction, and various strategies have been applied. For example, (1) changing the sizes of Pt particles and the Pt loadings, (2) employing mesoporous CeO2, (3) controlling the morphologies of CeO2 nanoparticles to be cubes, rods, or octahedra, and (4) the introduction of ZrO2 into the CeO2 lattice.7,13−15 However, there remain a few goals that have not been achieved; i.e., enhancement of the catalytic performance of Pt/CeO2 catalysts for the WGS reaction. Furthermore, several problems still exist. Firstly, ceria supported Pt catalysts are easily sintered under WGS reaction conditions, leading to a loss of catalytic activity.16 Secondly, Pt/CeO2 catalysts prepared by conventional methods, such as the incipient wetness impregnation method, are limited by the diffusion rate of the material because Pt particles can be buried deeply in the support matrix.17 Therefore, it is necessary to develop advanced catalysts that do not suffer from aggregation of Pt particles and that overcome the irregular diffusion rates of traditional materials. Precious-metal-loaded yolk-shell microspheres have attracted great attention because they effectively inhibit the sintering of the noble metal components, and also, because a synergistic effect occurs between the core and the shell.18−21 Furthermore, the yolk-shell 3



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structure allows active sites of the metal core inside the shell to be in contact with the reactants during the catalytic process.17 Catalytic microspheres with a yolk-shell structure are mainly prepared by cost ineffective, multistep solution processes; therefore, for efficient, cost-effective, and large scale production of catalytic yolk-shell microspheres, a new scalable process must be developed. In this study, precious-metal-loaded multicomponent metal oxide microspheres with yolkshell structures were prepared by a facile and scalable spray pyrolysis process, which is an efficient gas phase reaction process. Pt-loaded CeO2-ZrO2 yolk-shell nanostructures with varying Pt loadings were prepared by spray pyrolysis method. The effectiveness of the yolkshell nanostructures was studied by carrying out WGS at a high gas hourly space velocity (GHSV) of 18,193 h−1.

RESULTS AND DISCUSSION

Scheme 1. Schematic diagram of the formation mechanism of structured Pt-doped Ce0.75Zr0.25O2 powders with yolk-shell structure.

The formation mechanism of the multishelled, Pt-loaded Ce0.75Zr0.25O2 yolk-shell microspheres is shown in Scheme 1. The droplets, several microns in size, containing 4


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sucrose, Ce, Zr, and Pt components were formed by ultrasonic nebulization. Drying of the droplets led to formation of the composite microspheres, which had a gas-impermeable, dense structure consisting of sucrose and metal salts of Ce, Zr, and Pt. The gelation of sucrose minimized the phase separation of the Ce, Zr, and Pt components during droplet drying. The repeated combustion and contraction processes of the composite microspheres led to formation of the multishelled Pt-loaded Ce0.75Zr0.25O2 yolk-shell microspheres.22−24 Microspheres, composed of a solid solution of Ce0.75Zr0.25O2, were formed, even at the short residence times (25 s) experienced by the microspheres inside the reactor maintained at 800

°C, due to heat evolution caused by the combustion of sucrose. The homogeneous composition of the dried microspheres enabled the formation of a Ce0.75Zr0.25O2 solidsolution microsphere by spray pyrolysis. The Pt catalyst loading over the Ce0.75Zr0.25O2 yolkshell microspheres was easily controlled by changing the concentration of the Pt component dissolved in the spray solution, as shown in Scheme 1. The morphologies of the Ce0.75Zr0.25O2 microspheres with various Pt loading amounts are shown in Figure S1. The SEM images shown in Figure S1 revealed the formation of Ce0.75Zr0.25O2 yolk-shell microspheres by spray pyrolysis irrespective of the Pt catalyst loading. The detailed morphologies of the Ce0.75Zr0.25O2 microspheres containing 0.5, 2, and 4% Pt are shown in Figures S2, 1, and S3, respectively. The TEM images shown in Figures 1a, S2a, and S3a reveal the formation of multi-shelled Pt-loaded Ce0.75Zr0.25O2 microspheres, irrespective of their size. In this study, each Pt-loaded Ce0.75Zr0.25O2 yolk-shell microsphere was formed from one droplet by repeated combustion and contraction gas phase reactions within the tubular reactor. Therefore, the sizes of the Pt-loaded Ce0.75Zr0.25O2 yolk-shell microspheres were dependent on the sizes of the droplets. The size distribution of droplets formed by ultrasonic nebulization resulted in Pt-loaded Ce0.75Zr0.25O2 yolk-shell microspheres 5



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Figure 1. TEM and elemental mapping images of the yolk-shell structured 2% Pt-doped Ce0.75Zr0.25O2 powders: (a,b) TEM images, (c) high resolution TEM image, (d) SAED pattern, and (e) elemental mapping images.

with a broad size distribution. The TEM images shown in Figures 1b, S2b, and S3b show the three-shelled, thin yolk-shell. The high resolution TEM images shown in Figures 1c, S2c, and S3c show ultrafine nanocrystals with lattice fringes separated by 0.31 nm, which correspond to the (111) crystal plane of the cubic Ce0.75Zr0.25O2 phase. The selected area electron 6


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diffraction (SAED) patterns shown in Figures 1d, S2d, and S3d reveal that a phase pure Ce0.75Zr0.25O2 solid-solution was formed by the one-pot spray pyrolysis process. The XRD patterns shown in Figure S4 also confirm the formation of a Ce0.75Zr0.25O2 solid-solution by spray pyrolysis, irrespective of the loading amount of Pt catalyst.25−27 In addition, the broad XRD peaks confirm the formation of microspheres with ultrafine nanocrystals. The elemental mapping images shown in Figures 1e, S2e, and S3e show the uniform distribution of Pt, Ce, and Zr components over the whole multi-shelled yolk-shell microsphere. Therefore, ultrafine Pt

nanocatalysts

were

uniformly

distributed

throughout

the

yolk-shell-structured

Ce0.75Zr0.25O2 solid-solution microsphere.

Table 1. Characteristics of yolk-shell structured Pt/Ce0.75Zr0.25O2 catalysts with varying Pt loading. Catalyst

Pt dispersion

Pt S.A. (m /g)

0.5% Pt/Ce0.75Zr0.25O2

15.15

37.42

7.48

23.41

2% Pt/Ce0.75Zr0.25O2

15.21

37.56

7.45

30.32

4% Pt/Ce0.75Zr0.25O2

12.58

31.07

9.00

9.48

Estimated from CO-chemisorption

b

Estimated from N2 adsorption at −196 oC

a

size (nm)

a

BET S.A.

(%)

a

2

Pt crystallite

a

(m2/g)b

Table 1 summarizes the characteristics of Pt/Ce0.75Zr0.25O2 catalysts with varying Pt loadings. The isotherms of all Pt/Ce0.75Zr0.25O2 catalysts could be categorized as type II according to the IUPAC classification (Figure S5). The BET surface area increases with Pt loading from 0.5 to 2%. In contrast, a further increase in Pt loading results in a decrease in the BET surface area. Therefore, the 2% Pt/Ce0.75Zr0.25O2 catalyst had the largest BET surface area (30.32 m2/g). Similarly, the CO chemisorption results showed a similar trend to 7



that of the BET results. Among the prepared catalysts, the 2% Pt/Ce0.75Zr0.25O2 catalyst has the highest Pt dispersion, 15.2%. The average Pt crystallite size was found to decrease in the following order: 4% Pt/Ce0.75Zr0.25O2 > 0.5% Pt/Ce0.75Zr0.25O2 > 2% Pt/Ce0.75Zr0.25O2. As a result, 2% Pt/Ce0.75Zr0.25O2 catalyst had the highest BET surface area and Pt dispersion among the prepared catalysts.

98 144 211

H2 consumption (a.u.)

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360

4% Pt/Ce0.75Zr0.25O2

163 215 326

2% Pt/Ce0.75Zr0.25O2

258 315

100

200

300

0.5% Pt/Ce0.75Zr0.25O2 400

500

600

700

o

Temperature ( C) Figure 2. TPR patterns of yolk-shell structured Pt/Ce0.75Zr0.25O2 catalysts with varying Pt loading.

Figure 2 shows the TPR patterns of the Pt/Ce0.75Zr0.25O2 catalysts with varying Pt loading. The reduction profile of the 0.5% Pt/Ce0.75Zr0.25O2 catalyst contains two main peaks, centered at 258 and 315 °C. The former peak is assigned to the reduction of PtO species that interact with the support, which are active species for the WGS reaction.28 The latter peak is due to the reduction of surface CeO2 from the spillover of H2 from Pt.29,30 For the 2% 8


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Pt/Ce0.75Zr0.25O2 catalyst, three peaks can be seen in the TPR profile. The first peak at 163 °C was attributed to reduction of PtO species interacting with the support. The second peak at 215 °C was assigned to surface CeO2 in close contact with Pt, while the last peak was due to surface CeO2 not in close contact with Pt.31 In the case of the 4% Pt/Ce0.75Zr0.25O2 catalyst, four reduction peaks can be seen at 98, 144, 211, and 360 °C. The first peak was observed at 98 °C, and this is attributed to the reduction of surface PtOx species with no interaction between Pt and the Ce0.75Zr0.25O2 support.30,31 The small peak at 144 °C was ascribed to the reduction of PtO species interacting with the Ce0.75Zr0.25O2 support. The reduction peaks at 211 and 360 °C were due to surface CeO2 contact with Pt and surface CeO2, respectively. The reduction peak arising from reduction of PtO species interacting with the Ce0.75Zr0.25O2 support shifted to lower temperature with increasing Pt loading. This indicates that the Pt loading amounts can significantly affect the reducibility of the catalyst. However, when 4% Pt was loaded onto the Ce0.75Zr0.25O2 support, the active Pt species transformed into nonactive Pt species. As a result, the main peak in the TPR pattern of the 2% Pt/Ce0.75Zr0.25O2 catalyst was dominated by the reduction of active Pt species, whereas those of the 4% Pt/Ce0.75Zr0.25O2 catalyst were divided into the reduction of active Pt species and non-active Pt species. This is well supported by calculated results for H2 consumption. In the case of the 4% Pt/Ce0.75Zr0.25O2 catalyst, the H2 consumption value for non-active Pt species was 4.0 cm3/g and that for active Pt species was 3.1 cm3/g. On the other hand, the H2 consumption value of active Pt species in the 2% Pt/Ce0.75Zr0.25O2 catalyst was 3.3 times higher than that of the 4% Pt/Ce0.75Zr0.25O2 catalyst. H2 consumption of active Pt species decreased in the following order: 2% Pt/Ce0.75Zr0.25O2 (10.2 cm3/g) > 0.5% Pt/Ce0.75Zr0.25O2 (9.1 cm3/g) > 4% Pt/Ce0.75Zr0.25O2 (3.1 cm3/g). Although, the reduction properties of active Pt species in the 4% Pt/Ce0.75Zr0.25O2 catalyst were slightly better than those of the 2% Pt/Ce0.75Zr0.25O2 catalyst, 9



0

2+

Pt

0

Pt

Pt

Pt

2+

2% Pt Ce0.75 Zr0.25O2

Intensity (a.u.)

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4% Pt Ce0.75 Zr0.25O2

70

72

74

76

78

Binding energy (eV) Figure 3. Pt 4f XPS spectra of yolk-shell structured Pt/Ce0.75Zr0.25O2 catalysts with varying Pt loading.

the amount of active Pt species in 2% Pt/Ce0.75Zr0.25O2 catalyst was significantly larger than that of the 4% Pt/Ce0.75Zr0.25O2 catalyst. Thus, we expected that the 2% Pt/Ce0.75Zr0.25O2 catalyst would show high activity for the WGS. Additionally, the Pt 4f XPS spectra of the 2% Pt/Ce0.75Zr0.25O2 and 4% Pt/Ce0.75Zr0.25O2 catalysts were collected to identify the chemical states of the Pt components, as shown in Figure 3.32−35 The survey XPS spectra (Figure S6) indicate the presence of Pt, Ce, Zr, and O in the both catalysts. Both catalysts contained Pt in its divalent form, Pt2+, as shown in Figure 3. In the Pt 4f spectra (Figure 3), the peaks at around 72.6 and 75.9 eV correspond to Pt 4f7/2 and Pt 4f5/2, respectively, which are characteristic of the PtO phase. Pt2+ peaks in XPS spectra are originated from the platinum oxide (PtO) layer formed over the surface of the Pt nanoparticles. This result agrees well with the results of TPR measurements. 10


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100

CO conversion (%)

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


80

0.5% Pt/Ce0.75Zr0.25O2

Error < ±0.7%

2% Pt/Ce0.75Zr0.25O2 4% Pt/Ce0.75Zr0.25O2

60 40 20 0 200

250

300

350 o

Temperature ( C) Figure 4. CO conversion with reaction temperature over yolk-shell structured Pt/Ce0.75Zr0.25O2 catalysts with varying Pt loading (H2O/(CH4+CO+CO2) = 2.0; Reduction condition = 5% H2/N2, 400 oC for 1 h, Error < ±0.7%).

Figure 4 shows CO conversion over Pt/Ce0.75Zr0.2O2 catalysts with various Pt loadings. At all reaction temperatures, the 0.5% Pt/Ce0.75Zr0.2O2 catalyst showed the lowest amount of CO conversion. At Pt loadings between 0.5 and 2%, CO conversion increased with increasing Pt loading. This result correlates with the trend in reducibility observed in TPR measurements, because the reducibility has a strong effect on CO conversion. However, in the case of the 4% Pt/Ce0.75Zr0.2O2 catalyst, CO conversion decreased compared to that of the 2% Pt/Ce0.75Zr0.2O2 catalyst. The reducibility of the 4% Pt/Ce0.75Zr0.2O2 catalyst was greater than that of the 2% Pt/Ce0.75Zr0.2O2 catalyst, but the active Pt species in the 4% Pt/Ce0.75Zr0.2O2 catalyst were transformed into non-active Pt species. In other words, the decrease in catalytic activity observed in the 4% Pt/Ce0.75Zr0.2O2 catalyst was due to the loss of active Pt species. 11



Zhai et al. reported that, to obtain stable catalytic ability for the WGS, it is important to stabilize the active Pt species.36,37

100

CO conversion (%)

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80 60 40

0.5% Pt/Ce0.75Zr0.25O2

20

2% Pt/Ce0.75Zr0.25O2 4% Pt/Ce0.75Zr0.25O2

0 0

5

10

15

20

Time on stream (h) Figure 5. CO conversion with time on stream over yolk-shell structured Pt/Ce0.75Zr0.25O2 catalysts with varying Pt loading (H2O/(CH4+CO+CO2) = 2.0; T = 320 oC; Reduction condition = 5% H2/N2, 400 oC for 1 h).

To evaluate the stability of Pt-loaded Ce0.75Zr0.25O2 yolk-shell catalysts with time on stream (TOS), CO conversion data were collected at 320 °C for 20 h (Figure 5). Interestingly, all of the Pt-loaded Ce0.75Zr0.25O2 yolk-shell catalysts exhibited very stable catalytic performance, without detectable catalyst deactivation. Therefore, the Ce0.75Zr0.25O2 yolkshell-structure prevents Pt sintering, thereby maintaining a high catalyst activity. In another words, this nanoarchitecture protects the Pt nanoparticle from migration and sintering, making catalysts more stable. Although the initial degree of CO conversion for each catalyst 12


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was different, the stability of the catalysts were very similar. This result indicates that the differences in initial CO conversion originated from the reducibility of the catalysts and active Pt species whereas their similar stabilities originated from the yolk-shell structure. As a result, we determined that 2% Pt loading was the optimum for Pt/Ce0.75Zr0.25O2 catalysts for the WGS reaction.

Table 2. Comparison of reported data and our results. Catalyst description

2% Pt/Ce0.75Zr0.25O2

1% Pt/Ce0.8Zr0.2O2 1% Pt/Ce0.75Zr0.25O2 1% Pt/Ce0.6Zr0.4O2

Preparation method

One-pot spray pyrolysis

Pt: Impregnation Sup.: Co-precipitation Pt: Impregnation Sup.: Co-precipitation Pt: Impregnation Sup.: Urea precipitation

CO conversion

Deactivation rate (%)

(Temp.) 89% (320 oC)

82% (320 oC)

This

(0%)

work

82% → 73% (22 h)

63% → 53% (20 h) (↓15.9%)

82% (300 oC)

No.

89% → 89% (20 h)

(↓11.0%) 63% (300 oC)

Ref.

82% → 75% (20 h) (↓8.5%)

26 38 39

A cycling test was additionally conducted to check the recyclability of 2% Pt/Ce0.75Zr0.25O2 catalyst (Figure S7). To test the recyclability of optimized catalyst, the supply of feed gas was stopped after reaction and the reactor was cool-down to room temperature. The same process was repeated for five times. The result clearly shows that the yolk-shell structured 2% Pt/Ce0.75Zr0.25O2 catalyst is stable under simulated startup/shutdown condition. Table 2 provides comparison of reported data on values of CO conversion and deactivation rate for WGS reaction using Ce-ZrO2 supported Pt catalysts. Based on Table 2, developed catalyst shows higher catalytic activity although reaction condition and Pt loading are different from each other. Especially, it has superior stability compared with reported catalysts. 13



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CONCLUSIONS Pt-loaded Ce0.75Zr0.25O2 catalyst microspheres with a unique yolk-shell structure were prepared for use in highly efficient water-gas shift reactions. The loading amount of Pt catalyst over the Ce0.75Zr0.25O2 yolk-shell microspheres was controlled by changing the concentration of Pt component dissolved in the spray solution. 2% Pt/Ce0.75Zr0.25O2 catalyst microspheres had the greatest Pt dispersion over the multi-shelled yolk-shell structure and showed the greatest CO conversion within a temperature range of 200 to 360 °C. The unique yolk-shell structure prevented Pt sintering, thereby maintaining the high activity of the Ptloaded Ce0.75Zr0.25O2 catalyst microspheres for the WGS reaction. The facile one-pot spray pyrolysis process used in this study could also be applied to the preparation of uniquely structured catalyst microspheres with wide applications including the water-gas shift reaction.

14


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EXPERIMENTAL SECTION Synthesis of catalysts Pt-loaded Ce0.75Zr0.25O2 yolk-shell microspheres with multiple shells were prepared by a spray pyrolysis process. The spray pyrolysis system applied in this study is shown in Figure S8.22,23 The reactor temperature during the spray pyrolysis process was fixed at 800 °C. Chloroplatinic acid hexahydrate (H2PtCl6·6H2O, Sigma-Aldrich, USA), cerium (III) nitrate hexahydrate

(Ce(NO3)3·6H2O,

Samchun,

Korea),

and

zirconyl

nitrate

hydrate

(ZrO(NO3)2·xH2O, Sigma-Aldrich, USA) were used as the source materials for Pt, Ce, and Zr components, respectively. The concentrations of chloroplatinic acid hexahydrate, cerium (III) nitrate hexahydrate, and zirconyl nitrate hydrate dissolved in distilled water at 2% for the Pt/Ce0.75Zr0.25O2 catalyst were 0.016, 0.15, and 0.05 M, respectively. The concentration of sucrose was fixed at 0.5 M. Characterization The Brunauer-Emmett-Teller (BET) surface area of the catalysts was determined by the adsorption and desorption isotherms of nitrogen at –196

o

C using an ASAP 2010

(Micromeritics) instrument. A degassing procedure was performed at 110 oC under vacuum (pressure less than 0.5 mmHg) for 12 h prior to BET analysis to remove the adsorbed moisture from the catalyst surfaces. Hydrogen temperature-programmed reduction (H2–TPR) measurement in the range of 50 ~ 750 oC has been performed in an Autochem 2920 (Micromeritics) using 10% H2/Ar mixture gas at a heating slope of 10 oC/min and a mass sample of 100 mg placed in a U-shaped quartz reactor. X-ray photoelectron spectra (XPS) were recorded with an AXIS-NOVA spectrometer (Kratos Analytical Ltd.) equipped with Al Kα monochromatic (1486.6 eV) X-rays. An analyzer pass energy of 160 eV was used for the survey scan and 40 eV for the narrow scan. The binding energies were referenced to the carbon peak at 284.6 eV. CO chemisorption was performed in an Autochem 2920 (Micromeritics). In this experiment, 100 mg of the sample was reduced at 400 °C for 1 h using 10% H2/Ar flow. After that, the sample was cooled to 50 oC, CO was added to the catalysts until CO saturation occurred. From the amount of chemisorbed CO, Pt dispersion and crystallite size were calculated, assuming the CO:Pt stoichiometry to be unity. The crystal structures of the Pt-loaded Ce0.75Zr0.25O2 yolk-shell microspheres were measured using X-ray diffractometry (XRD, Rigaku DMAX-33). The morphologies of the powders 15



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were characterized using scanning electron microscopy (SEM, TESCAN VEGA3-SB) and high-resolution transmission electron microscopy (TEM, JEOL, JEM-2100F). Catalytic reaction The WGS reaction was carried out in a fixed-bed quartz reactor. The reaction temperature ranged from 200 to 360 °C. A 48 mg of prepared catalyst was positioned inside the reactor. Before the catalytic measurements, the catalysts were reduced at 400 °C for 1 h in a 5% H2/N2 flow. The gas feed was a premixed 1.01 vol.% CH4, 9.99 vol.% CO2, 9.02 vol.% CO, 60.08 vol.% H2, and 19.90 vol.% N2, and the flow rate was controlled by Brooks 5850E mass flow controllers. Total gas flow was 14.5 ml/min (STP), which corresponds to GHSV of 18,193 h−1. Water (steam/carbon = 2) was injected by means of a syringe pump and vaporized in a heating coil before entering the reactor. The reactor effluent was analysed online by two separation columns of a micro-gas chromatograph (Agilent 3000). A molecular sieve column was used to analyse H2, CH4, and CO; a PLOT U column was used to analyse CO2.

■ ASSOCIATED CONTENT Supporting Information. SEM images of the yolk-shell structured Pt-doped Ce0.75Zr0.25O2 powders with different doping concentration of Pt, TEM and elemental mapping images of the yolk-shell structured 0.5% Pt-doped Ce0.75Zr0.25O2 powders, TEM and elemental mapping images of the yolk-shell structured 4% Pt-doped Ce0.75Zr0.25O2 powders, XRD patterns of the yolk-shell structured Ptdoped Ce0.75Zr0.25O2 powders with different doping concentration of Pt, adsorptiondesorption isotherms of the yolk-shell structured Pt-doped Ce0.75Zr0.25O2 catalysts with varying Pt loading, the survey XPS spectra of yolk-shell structured Pt/Ce0.75Zr0.25O2 catalysts with varying Pt loading, recyclability test of 2% Pt/Ce0.75Zr0.25O2 catalyst, schematic diagram of the spray pyrolysis process. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION 16


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Corresponding Authors E-mail: [email protected] (Yun Chan Kang) Fax: (+82) 2-928-3584 E-mail: [email protected] (Hyun-Seog Roh) Fax: (+82) 33-760-2571

ACKNOWLEDGMENTS This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2013R1A1A1A05007370).

REFERENCES (1) Trimm, D. L.; Önsan, Z. I. Onboard Fuel Conversion for Hydrogen-Fuel-Cell-Driven Vehicles. Catal. Rev. Sci. Eng. 2001, 43, 31−84. (2) Pati, R. K.; Lee, I. C.; Hou, S.; Akhuemonkhan, O.; Gaskell, K. J.; Wang, Q.; Frenkel, A. I.; Chu, D.; Salamanca-Riba, L. G.; Ehrman, S. H. Flame Synthesis of Nanosized Cu-Ce-O, Ni-Ce-O, and Fe-Ce-O Catalysts for the Water-Gas Shift (WGS) Reaction. ACS Appl. Mater.

Interfaces 2009, 1, 2624−2635. (3) Jeong, D. W.; Na, H. S.; Shim, J. O.; Jang, W. J.; Roh, H. S. A Crucial Role for the CeO2–ZrO2 Support for the Low Temperature Water Gas Shift Reaction over Cu–CeO2–ZrO2 Catalysts. Catal. Sci. Technol. 2015, 5, 3706−3713. (4) Jha, A.; Jeong, D. W.; Shim, J. O.; Jang, W. J.; Lee, Y. L.; Rode, C. V.; Roh, H. S. Hydrogen Production by the Water Gas Shift Reaction using CuNi/Fe2O3 Catalyst. Catal. Sci.

Technol. 2015, 5, 2752−2760. (5) Reina, T. R.; Ivanova, S.; Delgado, J. J.; Ivanov, I.; Idakiev, V.; Tabakova, T.; Centeno, M. A.; Odriozola, J. A. Viability of Au/CeO2–ZnO/Al2O3 Catalysts for Pure Hydrogen Production by the Water–Gas Shift Reaction. ChemCatChem 2014, 6, 1401−1409. (6) Fu, Q.; Saltsburg, H.; Flytzani-Stephanopoulos, M. Active Nonmetallic Au and Pt Species on Ceria-Based Water Gas Shift Catalysts. Science 2003, 301, 935−938.

17



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

(7) Jeong, D. W.; Potdar, H. S.; Shim, J. O.; Jang, W. J.; Roh, H. S. H2 Production from a Single Stage Water Gas Shift Reaction over Pt/CeO2, Pt/ZrO2, and Pt/Ce(1-x)Zr(x)O2 Catalysts.

Int. J. Hydrogen Energy 2013, 38, 4502−4507. (8) Gamboa-Rosales, N. K.; Ayastuy, J. L.; Iglesias-Gonzalez, A.; Gonzalez-Marcos, M. P.; Gutierrez-Ortiz, M. A. Oxygen-Enhanced WGS over Ceria-Supported Au–Co3O4 Bimetallic Catalysts. Chem. Eng. J. 2012, 207−208, 49−56. (9) Mei, Z.; Li, Y.; Fan, M.; Zhao, L.; Zhao, J. Effect of the Interactions Between Pt Species and Ceria on Pt/Ceria Catalysts for Water Gas Shift: The XPS Studies. Chem. Eng. J. 2012, 259, 293−302. (10) Li, G.; Tang, Z. Noble Metal Nanoparticle@metal Oxide Core/Yolk-Shell Nanostructures as catalysts: Recent Progress and Perspective. Nanoscale 2014, 6, 3995−4011. (11) Panagiotopoulou, P.; Kondarides, D. I. Effect of the Nature of the Support on the Catalytic Performance of Noble Metal Catalysts for the Water-Gas Shift Reaction. Catal.

Today 2006, 112, 49−52. (12) Roh, H. S.; Jeong, D. W.; Kim, K. S.; Eum, I. H.; Koo, K. Y.; Yoon, W. L. Single Stage Water Gas Shift Reaction over Supported Pt Catalysts. Catal. Lett. 2011, 141, 95−99. (13) Kalamaras, C. M.; Dionysiou, D. D.; Efstathiou, A. M. Mechanistic Studies of the Water−Gas Shift Reaction over Pt/CexZr1−xO2 Catalysts: The Effect of Pt Particle Size and Zr Dopant. ACS Catal. 2012, 2, 2729−2742. (14) Wen, C.; Zhu, Y.; Ye, Y.; Zhang, S.; Cheng, F.; Liu, Y.; Wang, P.; Tao, F. Water-Gas Shift Reaction on Metal Nanoclusters Encapsulated in Mesoporous Ceria Studied with Ambient-Pressure X-ray Photoelectron Spectroscopy. ACS Nano 2012, 6, 9305−9313. (15) Lee, H. J. Utilization of Shape-Controlled Nanoparticles as Catalysts with Enhanced Activity and Selectivity. RSC Adv. 2014, 4, 41017−41027. (16) Wieder, N. L.; Cargnello, M.; Bakhmutsky, K.; Montini, T.; Fornasiero, P.; Gorte, R. J. Study of the Water-Gas Shift Reaction on Pd@CeO2/Al2O3 Core-Shell Catalysts. J. Phys.

Chem. C 2011, 115, 915−919. (17) Park, J. C.; Song, H. Metal@Silica Yolk-Shell Nanostructures as Versatile Bifunctional Nanocatalysts. Nano Res. 2011, 4, 33−49. (18) Wang, Z.; Li, L.; Han, D.; Gu, F. CO Oxidation on Au@CeO2 Yolk-Shell Nanoparticles with High Catalytic Stability. Mater. Lett. 2014, 137, 188−191.

18


Page 18 of 21

Page 19 of 21

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


(19) Li, Z.; Li, M.; Bian, Z.; Kathiraser, Y.; Kawi, S. Design of Highly Stable and Selective Core/Yolk–Shell Nanocatalysts-A Review. Appl. Catal. B: Environ. 2016, 188, 324−341. (20) Lee, I.; Joo, J. B.; Yin, Y.; Zaera, F. A Yolk@Shell Nanoarchitecture for Au/TiO2 Catalysts. Angew. Chem. Int. Ed. 2011, 50, 10208−10211. (21) Lee, I.; Albiter, M. A.; Zhang, Q.; Ge, J.; Yin, Y.; Zaera, F. New Nanostructured Heterogeneous Catalysts with Increased Selectivity and Stability. Phys. Chem. Chem. Phys. 2011, 13, 2449−2456. (22) Hong, Y. J.; Son, M. Y.; Kang, Y. C. One-Pot Facile Synthesis of Double-Shelled SnO2 Yolk-Shell-Structured Powders by Continuous Process as Anode Materials for Li-ion Batteries. Adv. Mater. 2013, 25, 2279−2283. (23) Hong, Y. J.; Son, M. Y.; Park, B. K.; Kang, Y. C. One-Pot Synthesis of Yolk–Shell Materials with Single, Binary, Ternary, Quaternary, and Quinary Systems. Small 2013, 9, 2224−2227. (24) Hong, Y. J.; Yoon, J. W.; Lee, J. H.; Kang, Y. C. One-Pot Synthesis of Pd-Loaded SnO2 Yolk–Shell Nanostructures for Ultraselective Methyl Benzene Sensors. Chem. Eur. J. 2014, 20, 2737−2741. (25) Shim, J. O.; Jeong, D. W.; Jang, W. J.; Jeon, K. W.; Jeon, B. H.; Cho, S. Y.; Roh, H. S.; Na, J. G.; Ko, C. H.; Oh, Y. K.; Han, S. S. Deoxygenation of Oleic Acid over Ce(1–x)Zr(x)O2 Catalysts in Hydrogen Environment. Renewable Energy 2014, 65, 36−40. (26) Jeong, D. W.; Potdar, H. S.; Roh, H. S. Comparative Study on Nano-Sized 1wt.% Pt/Ce0.8Zr0.2O2 and 1wt.% Pt/Ce0.2Zr0.8O2 Catalysts for a Single Stage Water Gas Shift Reaction. Catal. Lett. 2012, 142, 439−444. (27) Jang, W. J.; Jeong, D. W.; Shim, J. O.; Roh, H. S.; Son, I. H.; Lee, S. J. H2 and CO Production over a Stable Ni-MgO-Ce0.8Zr0.2O2 Catalyst from CO2 Reforming of CH4, Int. J.

Hydrogen Energy 2013, 38, 4508−4512. (28) Pierre, D.; Deng, W.; Flytzani-Stephanopoulos, M. The Importance of Strongly Bound Pt-CeOx Species for the Water-Gas Shift Reaction: Catalyst Activity and Stability Evaluation.

Top. Catal. 2007, 46, 363−373. (29) Hwang, K. R.; Ihm, S. K.; Park, J. S. Enhanced CeO2-Supported Pt Catalyst for WaterGas Shift Reaction. Fuel Process. Technol. 2010, 91, 729−736.

19



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

(30) Jeong, D. W.; Jang, W. J.; Shim, J. O.; Han, W. B.; Kim, H. M.; Lee, Y. L.; Bae, J. W.; Roh, H. S. Optimization of a Highly Active Nano-Sized Pt/CeO2 Catalyst via Ce(OH)CO3 for the Water-Gas Shift Reaction. Renewable Energy 2015, 79, 78−84. (31) Bi, Y. D.; Zhang, W.; Xu, H. Y.; Li, W. Z. Nanocrystalline CeO2 in SBA-15: Performance of Pt/CeO2/SBA-15 Catalyst for Water-Gas Shift Reaction. Catal. Lett. 2007, 119, 126−133. (32) Parkinson, C. R.; Walker, M.; McConville, C. F. Reaction of Atomic Oxygen with a Pt(1 1 1) Surface: Chemical and Structural Determination Using XPS, CAICISS and LEED. Surf.

Sci. 2003, 545, 19−33. (33) Jin, J.; Li, C.; Tsang, C. W.; Xu, B.; Liang, C. Catalytic Combustion of Methane over Pt−Ce Oxides under Scarce Oxygen Condition. Ind. Eng. Chem. Res. 2016, 55, 2293−2301. (34) Neitzel, A.; Lykhach, Y.; Skala, T.; Tsud, N.; Vorokhta, M.; Mazur, D.; Prince, K. C.; Matolin, V.; Libuda, J. Surface Sites on Pt–CeO2 Mixed Oxide Catalysts Probed by CO Adsorption: A Synchrotron Radiation Photoelectron Spectroscopy Study. Phys. Chem. Chem.

Phys. 2014, 16, 24747−24754. (35) Sui, X. L.; Wang, Z. B.; Yang, M.; Huo, L.; Gu, D. M.; Yin, G. P. Investigation on CTiO2 Nanotubes Composite as Pt Catalyst Support for Methanol Electrooxidation. J. Power

Sources 2014, 255, 43−51. (36) Zhai, Y.; Pierre, D.; Si, R.; Deng, W.; Ferrin, P.; Nilekar, A. U.; Peng, G.; Herron, J. A.; Bell, D. C.; Saltsburg, H.; Mavrikakis, M.; Flytzani-Stephanopoulos, M. Alkali-Stabilized PtOHx Species Catalyze Low-Temperature Water-Gas Shift Reactions. Science 2010, 329, 1633−1636. (37) Aranifard, S.; Ammal, S. C.; Heyden, A. On the Importance of Metal-Oxide Interface Sites for the Water-Gas Shift Reaction over Pt/CeO2 Catalysts. J. Catal. 2014, 309, 314−324. (38) Duarte de Farias, A. M.; Nguyen-Thanh, D.; Fraga, M. A. Discussing the Use of Modified Ceria as Support for Pt Catalysts on Water–Gas Shift Reaction. Appl. Catal., B 2010, 93, 250−258. (39) Kim, Y. T.; You, S. J.; Park, E. D. Water-Gas Shift Reaction over Pt and Pt-CeOx Supported on CexZr1−xO2. Int. J. Hydrogen Energy 2012, 37, 1465−1474.

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Highly Active and Stable Pt-Loaded Ce0.75Zr0.25O2 YolkâShell

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