Pure and Fe-doped CeO2 nanoparticles obtained by microwave assisted combustion synthesis: Physico-chemical properties ruling their catalytic activity towards CO oxidation and soot combustion

Tapas R Sahoo,Marco Armandi,Rossella Arletti,Marco Piumetti,Samir Bensaid,Maela Manzoli,Sirish R Panda,Barbara Bonelli

doi:10.1016/j.apcatb.2017.04.032

Abstract

A sample of pure CeO2 and two samples of Fe-doped CeO2 containing either 3 or 6 at.% Fe were obtained by microwave assisted combustion synthesis. The powders were extensively characterized by several techniques and tested as catalysts for both CO oxidation and soot combustion.As-synthesized CeO2 nanoparticles have a mostly squared shape and size well below 100nm; they are characterized by the presence of surface Ce3+ species likely due to the occurrence of very defective ceria crystalline phases, as revealed by HR-TEM. Oxidation at 400°C leads to the formation of a hydroxyls-rich surface, with several types of OH groups related to both Ce4+ and Ce3+ species; reduction in H2 at mild temperature (200°C) leads both to reduction of surface Ce4+ to Ce3+ and formation of new OH groups.With respect to CeO2 nanoparticles, Fe-doped ones have, as a whole, a larger size and less abundant surface OH species. A core-shell structure is inferred where Fe is mostly present in the shell, both in a secondary phase (CeFeO3) and as a dopant, finally lowering the band gap of the material. The presence of Fe improved samples reducibility, as shown by the lowering of the onset of temperature programmed reduction.Catalytic tests of CO oxidation showed that surface Fe species significantly improve the catalytic performance of the samples, by lowering the onset of CO conversion to CO2 especially at low Fe content (i.e. 3 at.%), whereas at 6 at.% Fe loading, the preferential formation of the secondary phase CeFeO3 occurs, finally lowering the CO conversion with respect to the sample containing 3 at.% Fe.Conversely, the soot combustion activity was higher for pure CeO2 nanoparticles, likely due to their smaller size, which increases the amount of solid–solid contact points between soot and the catalyst. However, a positive effect of the presence of surface Fe species on the catalytic activity towards soot combustion was observed, as well.

Highlights

CeO2 is one of the most studied metal oxides having environmental and/or energy applications: the CeO2-ZrO2 mixed oxide is a component of three-ways-catalyst used for automotive postcombustion [1,2,3], whereas CeO2 is studied, inter alia, as solid electrolyte for solid oxide fuel cells (SOFCs), especially in nanometric form [4,5]
The catalytic performance of the prepared materials was deeply affected by their physico-chemical properties, and in particular i) the extent of Fe doping within the ceria lattice, with respect to the formation of a secondary CeFeO3 phase, ii) the presence of defective ceria phases at the surface of larger agglomerates and iii) the nanoparticles size
The first property influences the presence of surface oxygen species (O ) that are responsible of the catalytic performance of the samples as catalysts for CO oxidation, whereas the other two parameters are crucial in the soot combustion

Summary

Introduction

CeO2 is one of the most studied metal oxides having environmental and/or energy applications: the CeO2-ZrO2 mixed oxide is a component of three-ways-catalyst used for automotive postcombustion [1,2,3], whereas (variously doped) CeO2 is studied, inter alia, as solid electrolyte for solid oxide fuel cells (SOFCs), especially in nanometric form [4,5]. The nanotechnologies era and the development of new synthetic routes allowed obtaining both pure and doped CeO2 nanoparticles (NPs) that bear some peculiar features, e.g. a higher oxygen mobility, with respect to those obtained by traditional methods [13,14]. Different morphologies are obtained by various methods, including addition of organic/inorganic templates or capping agents, variation of pH, etc. Those methods are, complicated and/or require the use of some costly and non-green reagent (e.g. a surfactant as organic template). Changes in morphology affect the surface properties of CeO2 NPs, the defects chemistry of the material and the related catalytic activity, especially as far as CeO2 reducibility is concerned [16]

Methods

Results

Conclusion