In situ preparation and investigation of Pd/CeO2 catalysts for the low-temperature oxidation of CO

doi:10.1016/j.apcata.2012.06.045

Applied Catalysis A: General

Volumes 439–440, 10 October 2012, Pages 41-50

https://doi.org/10.1016/j.apcata.2012.06.045 Get rights and content

Abstract

The formation of Pd_0.05Ce_0.95O₂ catalysts for the low-temperature oxidation of CO by the thermal decomposition of Ce(NO₃)₃ and Pd(NO₃)₂ with oxygen was studied by X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) directly in the preparation chamber of a spectrometer. Palladium is represented by two species on the surface of the catalysts: solid solutions of Pd_xCe_1−xO_2−δ and palladium clusters. Pd clusters can be formed in an oxidized or reduced state depending on the reaction conditions. Treatment of the catalysts with hydrogen leads to a sharp increase in CO conversion because of the reduction of parts of the palladium accompanying the formation of the metallic clusters. The testing of “real” and model catalysts was conducted in a light-off mode. The correlation between the activity of the Pd/CeO₂ catalysts and the states of the palladium was proposed.

Graphical abstract

Highlights

► XPS and XAS were used in situ to study the formation of Pd/CeO₂ catalyst. ► Formation of mixed Pd_xCe_1−xO_2−δ surface phase occurs during the catalyst synthesis. ► Catalytic performance depends on Pd-ceria interaction. ► Treatment of the catalysts with hydrogen leads to a sharp increase in the activity.

Introduction

The Pd/CeO₂ catalytic system has been extensively investigated because it is an irreplaceable component of catalysts for the low-temperature oxidation of CO (LTO CO) [1], [2], [3], [4], [5], the water gas shift reaction (WGS) and selective methanation [6], [7], CH_x oxidation [8], [9] and other reactions. Despite a number of publications devoted to the investigation of the nature of these catalysts, ambiguous and contradictory data concerning the electronic and geometric structure of Pd/CeO₂ active centers persist. The electronic state of palladium in Pd/CeO₂ catalysts differs from that of Pd/Al₂O₃ catalysts. Palladium in Pd/Al₂O₃ catalysts exists in the form of metallic and PdO nanoparticles [10], [11] that have typical binding energies (BE) of the Pd3d_5/2 core level equal to 335.2 and 337.0 eV, respectively [10], [11], [12], [13], [14], [15], while the position of the Pd3d_5/2 level for Pd/CeO₂ catalysts is approximately 337.7–338.3 eV [4], [16], [17], [18], [19], [20], [21]. The elevated value of the BE for Pd3d_5/2 is approximately 337.7–338.3 eV because of the formation of palladium dioxide (PdO₂) [16], [18], fine particles of palladium oxide (PdO) [20], [22] and a solid solution of Pd_xCe_1−xO₂ [4], [5], [16], [21]. Previously, we showed that the main palladium state of the Pd/CeO₂ catalysts was characterized by a BE for Pd3d_5/2 of 338.0 eV, which corresponded to the substitutional solid solution of Pd_xCe_1−xO₂ at the surface and subsurface layers of the CeO₂ lattice [5], [23]. The formation of these structures in the Pd/CeO₂ catalysts was confirmed by quantum-chemical calculations [24], [25], [26]. In addition, the formation of Pd–O–Ce superstructures on the (1 1 0) surface of CeO₂ has been reported by Colussi et al. [27]. However, the determination of the valence state of the Pd atoms in solid solutions of Pd_xCe_1−xO₂ remains controversial. Therefore, there is a great interest in the formation and investigation of Pd_xCe_1−xO₂ structures that play a key role in LTO CO.

Here, we present the synthesis of Pd_xCe_1−xO₂ surface structures during the formation of the catalyst directly in the preparation chamber of a photoelectron spectrometer under UHV conditions. The synthesis of ceria by the thermal decomposition of Ce(NO₃)₃ has been demonstrated [28], [29], and we used this synthesis with nanoscale CeO₂ supports under clean conditions in the spectrometer preparation chamber. The derived model catalyst was compared to the real reference Pd/CeO₂ catalysts that had been prepared by incipient wetness impregnation (IWI) method [7], [30].

To determine the valence state of the elements during the synthesis, X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) were used. Because XPS and XAS have different surface sensitivity, these methods were selected to detect the bulk and surface localization of the Pd structures that occurred in the sample during the synthesis of the catalyst. Application of in situ XPS and XAS methods the investigation of the calcination process of palladium and cerium nitrate mixtures with oxygen directly in the preparation chambers of spectrometers helps us to investigate the processes of the impregnation of ceria with a Pd(NO₃)₂ solution, subsequent drying and calcination in detail.

Section snippets

Catalysts preparation

Model Pd_0.05Ce_0.95O₂-film catalyst was prepared for investigation by XPS and XAS by mixing aqueous solutions of Ce(NO₃)₃ and Pd(NO₃)₂, with a concentration of Ce³⁺ and Pd²⁺ ions of 0.95 M and 0.05 M, respectively, with 0.05 M nitric acid that was supported on fine Ni mesh. First, the solutions of Pd(NO₃)₂ and Ce(NO₃)₃ salts were mixed. Then, a droplet of the solution was placed onto the mesh and dried to form a thin crystal film. The solution of liquid film was dried under a flow of clean air at 60

XPS results

The XPS spectra of the Pd3d line for the initial nitrate mixture and after different pretreatments are shown in Fig. 1a. The electronic state of palladium in the initial nitrate mixture was uniform because the FWHM of the Pd3d line (Fig. 1a, sp. 1) was low, and the BE for Pd3d_5/2 was equal to 338.4 eV, which is typical for Pd²⁺ in nitrate and chloride complexes [37]. An increase in the calcination temperature to 200–450 °C under an oxygen atmosphere led to a significant shift of the main peak to

Conclusions

Two stages in the synthesis of the Pd_0.05Ce_0.95O₂ catalyst were detected by XPS and XAS. The “synthetic stage” (25–400 °C) consisted of cerium nitrate decomposition via oxidation of the Ce³⁺ ions by nitrate groups, which led to the formation of CeO₂ phase. The “relaxation stage” (400–500 °C) consisted of the complete removal of nitrogen, which led to structural relaxation and the formation of a more stable CeO_2−δ phase containing Ce³⁺ ions. Palladium is represented by two surface phases: a solid

Acknowledgements

This work was supported by BESSY II at the Helmholtz-Zentrum, Berlin (project No. 2010_1_91049 of the Russian German Lab) and the Ministry of Education and Science of the Russian Federation (project No. 14.740.11.0419). The authors are grateful to V.I. Zaikovskii and А.V. Ischenko for the TEM study and P.V. Plyusnin for the synthesis of the catalysts.

References (48)

H. Zhu et al.
J. Catal.
(2005)
A.I. Boronin et al.
Catal. Today
(2009)
J.-Y. Luo et al.
Appl. Catal. B: Environ.
(2009)
R. Ramírez-López et al.
Catal. Today
(2010)
A.K. Datye et al.
Appl. Catal. A: Gen.
(2000)
A.S. Ivanova et al.
Appl. Catal. B: Environ.
(2010)
L.S. Kibis et al.
Appl. Surf. Sci.
(2010)
K. Sun et al.
Appl. Catal. A: Gen.
(2004)
O. Pozdnyakova et al.
J. Catal.
(2006)
K. Otto et al.
Appl. Catal. B: Environ.
(1992)

P. Bera et al.

J. Catal.

(2000)

M.-F. Luo et al.

Appl. Catal. A: Gen.

(1999)

D. Zhang et al.

Powder Technol.

(2011)

F. Zhang et al.

Surf. Sci.

(2004)

R.I. Kvon et al.

Appl. Surf. Sci.

(1997)

N.I. Baklanova et al.

J. Eur. Ceram. Soc.

(2006)

S.A. Gorovikov et al.

Nucl. Instrum. Methods Phys. Res. Sect. A

(1998)

S.A. Gorovikov et al.

Nucl. Instrum. Methods Phys. Res. Sect. A

(2001)

S.I. Fedoseenko et al.

Nucl. Instrum. Methods Phys. Res. Sect. A

(2001)

F. Bozon-Verduraz et al.

J. Catal.

(1978)

L. Meng et al.

Appl. Catal. B: Environ.

(2012)

M. Jin et al.

Catal. Today

(2012)

B. Wang et al.

Appl. Surf. Sci.

(2011)

L.S. Kibis et al.

Appl. Surf. Sci.

(2009)

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