Facile Synthesis of Highly Active Mesoporous PdCeOx Solid Solution

May 18, 2015 - Matteo Compagnoni , Simon A. Kondrat , Carine E. Chan-Thaw , David J. Morgan , Di Wang , Laura Prati , Alberto Villa , Nikolaos Dimitra...
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Facile Synthesis of Highly Active Mesoporous PdCeOx Solid Solution for Low-Temperature CO Oxidation Gengnan Li,† Liang Li,*,† and Dong Jiang‡ †

Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China ‡ Research Center of Analysis and Test, East China University of Science and Technology, Shanghai 200237, China S Supporting Information *

ABSTRACT: A series of mesoporous PdCeOx solid solution with high specific area was successfully fabricated through a facile coprecipitation and low-temperature calcination strategy. The resulting materials possessed excellent catalytic activities for CO oxidation. The complete CO conversion could be achieved at as low as 20 °C. The doping of palladium increased the concentration of structural oxygen vacancy which is beneficial for the CO oxidation reaction process. The CO oxidation reaction mechanism over PdCeOx solid solution was proved through in situ DRIFTS analysis.

1. INTRODUCTION

2. EXPERIMENTAL SECTION 2.1. Synthesis of PdCeOx Solid Solution. PdCeOx solid solution was fabricated by a facile coprecipitation and lowtemperature calcination protocol. Briefly, 5 mmol Ce(NO3)3· 6H2O and a certain amount of Na2PdCl4 were dissolved and then added slowly into 100 mL of sodium hydroxide solution. After being heated at 60 °C under vigorous stirring for 30 min, the resulting mixture was filtered. The solid was washed thoroughly with deionized water and dried at 80 °C for 12 h. PdCeOx solid solution was obtained after calcinating at a certain temperature for 4 h. 2.2. Materials Characterization. X-ray diffraction (XRD) data of powder sample were collected using Cu Kα radiation (λ = 0.154 05 nm) through a Bruker D8 Focus powder diffractometer operated at 40 kV and 40 mA. N2 adsorption− desorption isotherms were performed at 77 K using a Micromeritics ASAP 2020 M analyzer. All the samples were outgassed at 423 K in vacuum for 6 h before the test. X-ray photoelectron spectroscopy (XPS) was performed on a VG Micro MK II instrument. The spectra were calibrated using the reference value of C (1s) at 284.6 eV. Morphology and structure of the materials were observed on a field emission JEM-2100 (JEOL) electron microscope (TEM) operated at 300 kV. Raman spectra for the solid solution samples were taken on a LabRAM HR-800 spectrometer at a wavelength of λ = 532 nm. H2 temperature-programmed reduction (H2-TPR) analysis was performed on a Micromeritics Chemisorb 2750

Because of the remarkable oxygen storage/release properties, cerium oxide becomes one of the most important catalysts or catalyst supports in many catalytic systems especially for the oxidation process.1−9 Recently, much attention has been focused on CeO2-based CO oxidation catalysts for the purification of the automotive exhausts and other industrial applications.10−17 It has been proved that Pd/CeO2 materials are promising catalysts for CO oxidation, whether PdO or Pd was used as active component.6−9 Besides, there are also some reports recently using PdCeOx solid solution directly as catalyst for CO oxidation.10−12 Priolkar et al. heated the mixture solution of Ce and Pd precursor first at 350 °C and then burned it with a flame (∼1000 °C) to form PdCeOx solid solution. The substitution of Pd2+ for Ce4+ led to the high dispersion and chemical stability of Pd2+ and resulted in much higher catalytic activity for CO oxidation.11 The CO could be completely converted at about 175 °C for 1 at. % Pd/CeO2. However, the formation of PdCeOx solid solution thus far always contains multisteps and needs high calcination temperature (>1000 °C), which may cause the grain growth and decreased specific surface area and thus the reduced catalytic activity. In this paper, the palladium was totally doped into CeO2 structure through coprecipitation and low-temperature calcination protocol. The as-prepared PdCeOx solid solution possessed high surface area and showed much higher catalytic activity for low-temperature CO oxidation. The reaction mechanism over PdCeOx solid solution was also discussed in detail. © 2015 American Chemical Society

Received: March 31, 2015 Revised: May 10, 2015 Published: May 18, 2015 12502

DOI: 10.1021/acs.jpcc.5b03061 J. Phys. Chem. C 2015, 119, 12502−12507

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The Journal of Physical Chemistry C apparatus equipped with a thermal conductivity detector (TCD). H2-TPR data were collected from 25 to 200 °C under 10 vol % H2/Ar flow (25 mL/min) at a heating rate of 10 °C/min. A Nicolet iS10 instrument equipped with a MCT detector was used for in situ diffuse reflectance infrared Fourier transform spectroscopy analysis (DRIFTS). A chamber fitted with KBr windows was utilized as reactor for in situ measurements. The typical gas mixture was 0.5 vol % CO, 20.0 vol % O2 balanced with He. 2.3. Catalytic Properties. The catalytic activity for CO oxidation was performed in a continuous fixed-bed quartz tubular reactor (i.d. = 8 mm). The resulting PdCeOx material was used as catalyst directly without any pretreatment. The conversion of CO was analyzed by a gas chromatograph (GC). The feed gas consisting of 1.0 vol % CO, 20.0 vol % O2 balanced with N2 was introduced to the catalyst (200 mg) at a flow rate of 50 mL min−1, equivalent to a space velocity of 15 000 mL h−1 gcat−1.

Table 1. Lattice Parameter and H2 Consumption of the Catalysts sample CeO2 0.85 wt % PdCeOx 2.5 wt % PdCeOx 4.4 wt % PdCeOx 5.9 wt % PdCeOx 7.2 wt % PdCeOx

crystal lattice parameters (Å)a

fwhm (cm−1)b

H2 consumption (μmol/g)c

5.400 5.402

16.9 33.1

− 68

5.410

40.2

72

5.415

52.1

102

5.417

65.7

112

5.435

56

114

a Calculated from the XRD analysis. bCalculated from the Raman analysis. cCalculated from the H2-TPR analysis

All the samples possess the type IV isotherms corresponding to the pore diameter within the mesoporous range. The same H3 hysteresis loops suggest the formation of slitlike pores. Interestingly, the specific areas of the synthesized PdCeOx solid solution are close to that of pure mesoporous cerium oxide. Meanwhile, the average pore sizes are all at about 6 nm (Table 2), indicating the doping of palladium hardly affects the meso-structure of the final materials. The crystalline morphology of the CeO2 and 5.9 wt % PdCeOx were further characterized by transmission electron microscopy (TEM) observations, as shown in Figure 3. All the images clearly prove the porous structure with pore network intersectioned by the cerium oxide nanocrystals. Almost the same well-defined diffraction rings could be found from the selected-area-electron diffraction (SAED) patterns. There is not any diffraction ring for Pd or PdO for PdCeOx material indicating the formation of solid solution or existence as amorphous. The energy dispersive X-ray spectroscopy (EDS) mapping analysis was further carried out on PdCeOx material. The colored spots correspond to the presence of the elements Pd, Ce, and O, respectively (Supporting Information). It clearly indicates that the Pd species have been homogeneously deposited into the polycrystalline CeO2 structure. 3.3. Surface Properties. The surface chemical states of the PdCeOx solid solution were investigated by XPS analysis. Figure 4B depicts the Ce 3d3/2,5/2 spectra of the materials. It clearly shows the coexistence of Ce3+ and Ce4+. The satellite peak at 916.9 eV associated with the Ce 3d3/2 is the characteristic binding energy of tetravalent Ce4+ in Ce compounds. The lower binding energy states are respectively located at 901.3, 882.8, 907.7, 889.1, and 898.4 eV. The signals located at about 903.5, 885.4, 898.9, and 880.9 eV, respectively, indicate the existence of Ce3+,9−13 which is attributed to the interaction between ceria and the surrounding atoms and may also be used as a sign for the formation of oxygen vacancies. It is noteworthy that the contents of Ce3+ are slightly changed after Pd doping (Table 3). The high content of Ce3+ and Pd2+ species with low valence means a large amount of oxygen vacancies, which has been proved to be beneficial for CO oxidation process.13−15 The O 1s spectra recorded from pure CeO2 and PdCeOx solid solution are also presented in Figure 4C. For the pure CeO2, the binding energy located at 529.3 eV, denoted as Olat, is characteristic of the lattice oxygen (O2−). When doped with Pd, the position of this lattice O shifts slightly to 529.5 eV. Two additional weak peaks located at 531.4 and 532.6 eV,

3. RESULTS AND DISCUSSION A series of PdCeOx solid solution was obtained through coprecipitation and calcination at 500 °C. The doping amounts of Pd in the resulting PdCeOx materials were analyzed by inductively coupled plasma spectrometry (ICP-AES) to be 0.85, 2.5, 4.4, 5.9, and 7.2 wt %, respectively. 3.1. X-ray Powder Diffraction. Figure 1 shows the X-ray diffraction (XRD) patterns of CeO2 and Pd-doped PdCeOx

Figure 1. XRD patterns of CeO2 (a) and PdCeOx solid solutions containing 0.85 wt % Pd (b), 2.5 wt % Pd (c), 4.4 wt % Pd (d), 5.9 wt % Pd (e), and 7.2 wt % Pd (f).

materials. All the diffraction peaks of the synthesized compounds can be well indexed to a cubic structure of CeO2 (PDF card no. 65-5923). Besides, no visible peaks for Pd or PdO crystal phase are observed even when the doping amount of Pd reached 7.2 wt %. Due to the very similar ionic radii of Pd2+ (86 pm) and Ce4+ (97 pm) as well as the molecular-level homogeneously mixed Pd2+ and Ce4+ ions in their precursor solution, the palladium could be easily doped into the CeO2 lattice structure.10,11 The gradually increased lattice constant (Table 1) clearly indicates the substitution of Ce4+ with Pd2+ in CeO2 crystal structure and the formation of PdCeOx solid solution. Thus, the type of −O2−−Ce4+−O2−−Pd2+−O2−− linkages may exist in the crystal structure of CeO2 and generate a large amount of oxygen vacancies, which is beneficial for the CO oxidation process. 3.2. Structure Analysis. N2 adsorption−desorption isotherms of the resulting materials are presented in Figure 2. 12503

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Figure 2. N2 adsorption−desorption isotherms of CeO2 (a) and PdCeOx solid solutions containing 0.85 wt % Pd (b), 2.5 wt % Pd (c), 4.4 wt % Pd (d), 5.9 wt % Pd (e), and 7.2 wt % Pd (f).

Table 2. Physicochemical Properties of the Catalysts with Different Pd Doping Amount sample CeO2 0.85 wt % PdCeOx 2.5 wt % PdCeOx 4.4 wt % PdCeOx 5.9 wt % PdCeOx 7.2 wt % PdCeOx

BET surface area (m2/g)

average pore diameter (nm)

pore volume (cm3/g)

101 111

7.6 7.6

0.19 0.22

110

6.1

0.17

111

6.2

0.17

108

6.1

0.17

104

6.5

0.17

Figure 3. TEM images of CeO2 (A) and 5.9 wt % PdCeOx (B).

Figure 4. XPS survey (A) and narrow XPS spectra of Ce 3d (B), O 1s (C), and Pd 3d (D) of CeO2 (a) and PdCeOx solid solutions containing 0.85 wt % Pd (b), 2.5 wt % Pd (c), 4.4 wt % Pd (d), 5.9 wt % Pd (e), and 7.2 wt % Pd (f).

respectively, denoted as Oad, are contributed by the surface adsorbed hydroxide or carbonate species.13 The ratio of oxygen species is almost unchanged before and after Pd doping (Table 3).

In addition, the Pd 3d peaks are observed at 337.6 and 342.9 eV for PdCeOx catalysts (Figure 4D). It agrees well with those of Pd2+ in PdCl2 clearly indicating the presence of Pd2+.10,11 Generally, the binding energies of Pd 3d5/2 in Pd0 and PdO are at 335.4 and 336.8 eV, respectively.10,11 The Pd core levels in 12504

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redox behavior of the catalyst (Figure 6). The bare CeO2 does not show any reduction behavior at temperatures lower than

Table 3. Surface Composition of the Samples with Different Pd Doping Amount Ce sample CeO2 0.85 wt % PdCeOx 2.5 wt % PdCeOx 4.4 wt % PdCeOx 5.9 wt % PdCeOx 7.2 wt % PdCeOx

O

Ce3+ (%)

Ce4+ (%)

lattice O (%)

absorbed O (%)

Pd2+ (%)

13.3 12.0

19.1 19.4

59.3 58.8

8.3 9.4

− 0.4

9.7 9.4 11.3 9.5

21.4 20.7 18.5 19.6

59.2 59.0 57.9 57.5

8.9 9.4 9.8 10.3

0.8 1.5 2.5 3.1

the PdCeOx catalysts are shifted by 2.2 eV with respect to Pd0, whereas the Pd 3d shift is 0.8 eV in comparison with PdO. Thus, the palladium does not exist as Pd0 or independent PdO in PdCeOx materials. All these results suggest that Pd ions are more likely incorporated in CeO2 in the form of a solid solution than PdO supported on the CeO2 matrix even when the doping amount reached 7.2 wt %, well consistent with above XRD analysis. 3.4. Raman Analysis. The as-synthesized PdCeOx solid solutions possess similar Raman spectra with CeO2 in 300−600 cm−1 range, as shown in Figure 5. The main band centered at

Figure 6. H2-TPR profiles of CeO2 (a) and PdCeOx solid solutions containing 0.85 wt % Pd (b), 2.5 wt % Pd (c), 4.4 wt % Pd (d), 5.9 wt % Pd (e), and 7.2 wt % Pd (f).

200 °C. Comparatively, there is a distinctive reduction peak in all the profiles for PdCeOx solid solution, and the position of the reduced peak gradually shifts to lower temperature with increasing Pd doping amount from 0.85 to 5.9 wt %. However, as Pd doping amount further increased to 7.2 wt %, the reduction temperature showed slight increase. The easier reduction may be related to the increased mobility of oxygen species. To reduce pure CeO2, O2− in the bulk should diffuse toward the surface and be reduced by H2. Because the diffusion of O2− was rate determining for the reduction of CeO2, H2 uptake was observed at high temperature (>200 °C). When doped with Pd, the diffusion of O2− toward the surface was promoted.21 Table 1 shows the amount of the hydrogen consumption during the TPR experiments. These values are higher than the nominal H2 consumption amount for Pd reduction process, suggesting a coreduction of CeO2 resulting from a typical H2 spillover effect.20 The relative larger deviation between the actual and nominal H2 consumption implies an enhanced redox capability of the catalysts. 3.6. CO Catalytic Activity and Stability. Figure 7A shows the catalytic activities of the pure CeO2 and PdCeOx solid

Figure 5. Raman spectra of CeO2 (a) and PdCeOx solid solutions containing 0.85 wt % Pd (b), 2.5 wt % Pd (c), 4.4 wt % Pd (d), 5.9 wt % Pd (e), and 7.2 wt % Pd (f).

460 cm−1 can be assigned to the vibrational mode of CeO2 (fluorite-structured materials).17,18 When doped with palladium, the intensity of the band decreases gradually, as the dark colored PdCeOx catalysts strongly absorbs excitation laser and scattering light, thus weakening the signal. In general, the intensity of the Raman peaks is related to various parameters of the physicochemical structure of the materials, while the full width at half-maximum (fwhm) is associated with the crystallite size and/or the amount of oxygen vacancies.19,20 A larger fwhm means a low crystallite size and/or a high amount of oxygen vacancies in the CeO2 structure. For PdCeOx materials, the fwhm value increased with the increase of Pd doping amount, and the crystallite sizes calculated from the results of XRD are almost unchanged, indicating the increased concentration of oxygen vacancies. As proved in the previous research, oxygen vacancy plays an important role during the oxidation process: the high amount of oxygen vacancies and the enhanced CO oxidation catalytic activities.13−15 3.5. H2-Temperature-Programmed Reduction. The H2TPR was performed in the experiment to further explore the

Figure 7. Catalytic activities of PdCeOx solid solutions (A) and the catalytic stability of 5.9 wt % PdCeOx at 25 °C (B).

solution for CO oxidation. Only 30% conversion could be achieved over the pure CeO2 at 120 °C. When doped with palladium, the catalytic activity is greatly enhanced, conforming the essential role of Pd species in CO oxidation process. The doping amount of Pd significantly affects the catalytic activity of the materials. The CO complete conversion temperature 12505

DOI: 10.1021/acs.jpcc.5b03061 J. Phys. Chem. C 2015, 119, 12502−12507

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The Journal of Physical Chemistry C decreases from 120 to 20 °C as the Pd doping amount increases from 0.85 wt % to 5.9 wt %. However, the conversion temperature does not decrease additionally when the Pd content further increases to 7.2 wt %. When the Pd content reached 7.2 wt %, the catalytic performance showed a slight decrease. The catalytic activities for CO oxidation over PdCeOx solid solution are consistent with the above TPR analysis. Compared with other Pd/CeO2 catalysts, all the samples show relatively high catalytic activities.22−25 On the other hand, the catalytic stability is also essential for CO oxidation in practical applications. The PdCeOx solid solution exhibits highly catalytic durability for CO oxidation. The activity has not any apparently changed within 250 min reaction when 5.9 wt % PdCeOx was used as catalyst (Figure 7B). 3.7. In Situ DRIFT Spectra Analysis. To explain the significantly enhanced catalytic activity for CO oxidation over PdCeOx solid solution, in situ DRIFT analysis under different atmosphere was performed in the experiment. Figure 8A shows

crystal structure and the abundant amount of surface hydroxyl on the catalyst could promote the CO oxidation process.

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ASSOCIATED CONTENT

* Supporting Information S

Effect of reactant concentration and space velocity on the lowtemperature CO oxidation; XRD patterns, structural analysis, and EDX mapping of the samples calcinated at different temperature and after reaction under different conditions. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b03061.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-21-64252599. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by National Basic Research Program of China 2013CB933201.

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REFERENCES

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Figure 8. In situ DRIFT spectra of PdCeOx solid solution (5.9 wt %) under 0.5 vol % CO-He (A) and 0.5 vol % CO-20.0 vol % O2-He (B) at 25 °C.

in situ DRIFT spectra of 5.9 wt % PdCeOx material exposed to 0.5 vol % CO-He at 25 °C. The bands at 1536 and 1334 cm−1 on PdCeOx catalyst are ascribed to υas (OCO) and υs (OCO) of (COOH)s.26−31 The other peaks at 1648 and 1866 cm−1 can be assigned to the OH group and the adsorption of CO on Pd, respectively.26−31 When PdCeOx catalyst was exposed to 0.5 vol % CO-He, the (COOH)s species was formed immediately. This result indicates that CO could directly react with surface hydroxyl on the catalyst to form (COOH)s intermediates and then decompose to CO2. Therefore, the intensity of OH group (1648 cm−1) decreases with the increase of contact time. When O2 was introduced into the feed gas, the intensity of Pd−CO changes slightly weakened (Figure 8B) due to the competitive absorption on the palladium between CO and O2. The existence of O2 prevents the adsorption of CO to a certain extent and promotes the oxidation process. These results are consistent with the XPS analysis.

4. CONCLUSION A facile coprecipitation and low temperature calcination strategy has been employed to fabricate the mesoporous PdCeOx solid solution. The resulting material owns high specific area and possesses excellent catalytic activity and stability for CO oxidation. The complete CO conversion could be achieved at as low as 20 °C for 5.9 wt % Pd-doped PdCeOx solid solution. The increased concentration of structural oxygen vacancies generated by the doping of palladium into CeO2 12506

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DOI: 10.1021/acs.jpcc.5b03061 J. Phys. Chem. C 2015, 119, 12502−12507