Tuning the oxygen vacancy population of cerium oxide (CeO2−x, 0 < x &lt; 0.5) nanoparticles

doi:10.1016/j.apsusc.2017.06.101

Applied Surface Science

Volume 422, 15 November 2017, Pages 1102-1112

https://doi.org/10.1016/j.apsusc.2017.06.101 Get rights and content

Highlights

•
Rational design of catalyst by means of a CO reduction treatment.
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Very high Ce(III) fraction values obtained by CO reduction treatment.
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Surface hydrophobicity plays an important role in the CeO_2-x nanoparticles reduction.
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Low reduction temperatures values found for CeO_2-x nanoparticles.

Abstract

Oxygen vacancies are one of the most active defects existing at the surface of metal oxides. Engineering of defect chemistry, especially oxygen vacancies, represents the possibility of controlling the catalytic properties of nanoparticles. This work was dedicated to study the reduction properties of CeO_2-x nanoparticles prepared using different synthesis parameters when exposed to a CO reducing atmosphere. Near Ambient Pressure X-ray Photoelectron Spectroscopy (NAP-XPS) and in situ time-resolved X-Ray Absorption Near Edge Spectroscopy (XANES) techniques were used to probe in situ the electronic properties of these nanoparticles exposed to the CO atmosphere. Compared to typical results found for CeO_2-x nanoparticles, an improved reduction tendency is observed for these synthesized nanoparticles. It was possible to tune the oxygen vacancy population and the reduction temperature (T_R) by means of the combination of synthesis and CO reducing atmosphere. The T_R and Ce(III) fraction values reached can be as low as 108 °C and as high as 0.92, respectively. The influence of structural and electronic properties of the nanoparticles on the reduction kinetics of the CeO_2-x reduction is elucidated. The results provide a valuable route for the rational design of CeO_2-x nanoparticles with the desired oxygen vacancy population, and then preparing them in the best conditions for applications.

Graphical abstract

Introduction

Environmental pollution is a fast growing global problem. Carbon monoxide (CO), which is produced by the incomplete combustion of carbon-based substances and fuels, is one of the major air pollutants of the contemporary world [1]. A vast number of studies have been conducted linking the presence of a high CO concentrations in urban zones to bad fetal development and to health problems [2], [3], [4]. Catalytic oxidation reactions have been commonly used in industry as a strategy for controlling the CO emission, but more effort is still needed toward the search for new and more efficient catalysts that can be applied for this purpose.

Over the last decades, cerium oxide (CeO_2-x, 0 < x < 0.5) has been used as a support for metallic nanoparticles in several catalytic reactions such as the oxidation of methane, hydrocarbons and nitrogen oxide, as well as for the CO oxidation reaction [5]. The main characteristics of cerium oxide are its high surface ion mobility and its ability to store and release oxygen atoms [5]. These also make cerium oxide useful as a catalyst, instead of only as a support for metallic nanoparticles, when isolated cerium oxide nanoparticles are used [6]. The synthesis of CeO_2-x nanoparticles and their catalytic properties have been studied extensively in the last few years [6], [7], [8]. The literature reports that the temperature required for the CO oxidation reaction varies typically from 220 to 330 °C [9], [10], [11] when using isolated cerium oxide nanoparticles.

Several studies have been performed that aim to improve the catalytic activity of cerium oxide nanoparticles in the CO oxidation reaction. In order to do that, the presence of oxygen vacancies at the surface of cerium oxide is mandatory due to the fundamental role played by such defects in this reaction, as demonstrated previously [12], [13]. When creating an oxygen vacancy, energy is needed, which is directly related to the activity of the catalyst in this reaction. One can state that knowing the oxygen vacancy population at the surface of cerium oxide would allow making predictions about the catalytic activity of cerium oxide. Recently, Sayle et al. [13] showed by using non-equilibrium Molecular Dynamics simulations that there is an optimum oxygen vacancy concentration for the catalytic activity of cerium oxide in the CO oxidation reaction. For a surface with a higher oxygen vacancy population, the system needs to much energy to create an extra vacancy, and thus, the catalytic activity decreases. Therefore, tuning of the oxygen vacancy population allows the rational design of catalysts for the CO oxidation reaction.

A catalytic reaction is a sequence of several complex steps starting with the molecule adsorbing at a specific site on the surface of the catalyst and ending with the formation and the desorption of the final product. The detailed understanding of the events at the atomic level occurring during a catalytic reaction plays a fundamental role on the optimization and generation of advanced catalysts that can be applied in industry. However, the structure and surface chemistry of catalysts under reaction conditions are usually very different from those studied under high vacuum or at low temperatures [14]. Thus, in order to understand the catalytic reaction mechanisms at the molecular or atomic level, in situ techniques have been applied successfully over the last few years [14], [15]. One of the best techniques employed to study the surface of catalysts is X-Ray Photoelectron Spectroscopy (XPS). However, for a conventional XPS instrument, the samples must be maintained in an environment with a pressure lower than 10⁻⁷ mbar (high vacuum). In contrast, the Near Ambient Pressure X-Ray Photoelectron Spectroscopy (NAP-XPS) technique allows surface characterization of nanoparticles when exposed to conditions closer to the actual reaction environment (high temperature and pressure) [16]. In this way, important scientific issues such as the surface atomic population and electronic properties of the surface have been addressed in situ [17], [18], [19]. Moreover, the in situ X-Ray Absorption Spectroscopy (XAS) technique is widely employed to probe the oxidation state and the short-range order around a specific atom of the nanoparticle when submitted to a catalytic reaction [15], [20]. Combining both techniques is a powerful approach that scientists can use to elucidate the catalytic properties of nanoparticles.

In this work, a detailed study combining several in situ and ex situ experimental techniques was performed with cerium oxide nanoparticles synthesized with different electronic and structural properties and exposed to a CO atmosphere at high temperature. The main aim is to elucidate the influence of these properties on the reduction kinetics of cerium oxide nanoparticles. Moreover, by means of synthesis and thermal treatment under a CO atmosphere, it is aimed to tune the oxygen vacancy population (directly related to the Ce(III) fraction) of the nanoparticles, which then allows the rational design of smart nanoparticles in the near future.

Section snippets

Material and methods

The cerium oxide nanoparticles were synthesized according to a method described previously [21], with modifications. High purity ceric ammonium nitrate and ammonium carbonate were provided by Sigma-Aldrich (St. Louis, USA). The water was of “type 1” ultrapure grade, and all other solvents and reagents were of analytical grade and were used as received. Ceric ammonium nitrate (2 g) dissolved in water (6 mL) was added drop wise to a saturated aqueous solution (14 mL) of ammonium carbonate under

Results and discussion

A careful analysis of several SEM images (Fig. S1) shows a strong dependence of the morphology of the nanoparticles on the synthesis parameters used. Structures in the micrometer range were observed in all sample images. Nanoparticle agglomerates with more uniform morphology were obtained when only water was used as solvent (A and C series), compared to those synthesized in the presence of ethanol (B and D series). These results are probably related to the CeO_2-x precipitation rate for each

Conclusions

Considering the current interest in synthesizing more efficient nanoparticles that can be applied to the CO oxidation reaction, cerium oxides nanoparticles with different properties were synthesized and exposed to a CO atmosphere. The nanoparticles synthesized react with CO gas at lower temperatures than those typically found for CeO₂ nanoparticles in the literature. Consequently, the cerium oxide nanoparticles reached high concentration of oxygen vacancies at the end of the reaction. Moreover,

Funding

This work was supported by CNPq [grant number 487071/2013-1] and FAPERGS [grant number 131948-6].

Acknowledgements

This work was supported by CNPq (project number 487071/2013-1) and FAPERGS (project number 131948-6). F. B., G. B. D. M., A. S. T. and L. P. M. thank CNPq for the research grant. The authors thank the LNLS, CMM and the CNANO staff and Mr. Otelo for their assistance. The Leibniz Institut für Analytische Wissenschaften − ISAS − e.V. is funded by the Ministerium für Innovationen, Wissenschaft und Forschung des Landes Nordrhein-Westfalien, Germany and by the Bundesministerium für Bildung und

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Full Length ArticleTuning the oxygen vacancy population of cerium oxide (CeO2−x, 0 < x < 0.5) nanoparticles

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Material and methods

Results and discussion

Conclusions

Funding

Acknowledgements

Toxicology

Powder Technol.

Catal. Today

J. Catal.

J. Alloy Compd.

J. Catal.

Surf. Sci.

Surf. Sci.

Surf. Sci.

Atmospheric Chemistry and Physics: From Air Pollution to Climate Change

The effect of ambient carbon monoxide on low birth weight among children born in southern California between 1989 and 1993

Environ. Health Persp.

Association between ambient carbon monoxide levels and hospitalizations for congestive heart failure in the elderly in 10 Canadian cities

Epidemiology

Catalytic properties of ceria and CeO2-containing materials

Catal. Rev.

Nanostructured ceria-based materials: synthesis, properties, and applications

Environ. Environ. Sci.

Activity of CeOx and TiOx nanoparticles grown on Au(111) in the water-gas shift reaction

Science

Water-gas shift reaction on a highly active inverse CeOx/Cu(111) catalyst: unique role of ceria nanoparticles

Angew. Chem. Int. Ed.

Study of nano-structured ceria for catalytic CO oxidation

J. Mater. Chem.

Template-free synthesiscontrolled conversion, and CO oxidation properties of CeO2 nanorods, nanotubes, nanowires, and nanocubes

Eur. J. Inorg. Chem.

Oxygen vacancies and catalysis on ceria surfaces

Science

Structure-activity map of ceria nanoparticles, nanocubes and mesoporous architectures

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Evolution of structure and chemistry of bimetallic nanoparticle catalysts under reaction conditions

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Influence of the CeO2 support on the reduction properties of Cu/CeO2 and Ni/CeO2 nanoparticles

J. Phys. Chem. C

A differentially pumped electrostatic lens system for photoemission studies in the millibar range

Rev. Sci. Instrum.

Full Length Article
Tuning the oxygen vacancy population of cerium oxide (CeO_2−x, 0 < x < 0.5) nanoparticles

Catalytic properties of ceria and CeO₂-containing materials

Activity of CeO_x and TiO_x nanoparticles grown on Au(111) in the water-gas shift reaction

Water-gas shift reaction on a highly active inverse CeO_x/Cu(111) catalyst: unique role of ceria nanoparticles

Template-free synthesiscontrolled conversion, and CO oxidation properties of CeO₂ nanorods, nanotubes, nanowires, and nanocubes

Influence of the CeO₂ support on the reduction properties of Cu/CeO₂ and Ni/CeO₂ nanoparticles