Effects of synthesis conditions on structure and surface properties of SmMn2O5 mullite-type oxide

doi:10.1016/j.apsusc.2016.05.151

Applied Surface Science

Volume 385, 1 November 2016, Pages 490-497

https://doi.org/10.1016/j.apsusc.2016.05.151 Get rights and content

Highlights

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Investigate the effects of calcination temperature and precipitation pH on crystallinity, phase purity, particle size, surface composition, and NO adsorption capacity of SmMn₂O₅.
•
High calcination temperature increases mullite phase purity but decreases specific surface area (SSA).
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Mullite phase purity is independent of pH while SSA monotonically increases.
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SSA and surface Mn/Sm ratio determine NO uptake.

Abstract

A mixed-phase compound that contains SmMn₂O₅ mullite-type oxides has been reported to display excellent catalytic activity for nitric oxide (NO) oxidation. Here we investigate the effects of calcination temperature and precipitation pH on structural, physical, chemical, and surface properties of SmMn₂O₅. As the calcination temperature increases from 750 °C to 1000 °C, mullite phase purity increases from 74% to 100%, while specific surface area (SSA) decreases from 23.6 m²/g to 5.1 m²/g with particle size increases correspondingly. Mullite phase purity (87%) is independent of pH between 8.5–10.4, whereas SSA monotonically increases from 12.5 m²/g at pH 8.1 to 27.4 m²/g at pH 13. X-ray photoelectron spectroscopy (XPS) studies reveal that the surface Mn/Sm ratio is similar to the bulk value and is unaffected by calcination temperature and pH values up to 10.4, whereas sample precipitated at pH 13 is surface-rich in Sm. NO chemisorption studies show that the SSA and surface Mn/Sm ratio determine NO uptake by SmMn₂O₅ mullite oxides.

Graphical abstract

Introduction

Mullite-type mixed valence oxides based on transition metals (TM) with general formula RM₂O₅ (R = rare-earth, Y, Bi, and M = TM) have been the subject of extensive research due to interesting properties arising from their unique crystal structure [1], [2]. Mullite oxides have orthorhombic crystal structure in which M exists in two oxidation state: M⁴⁺ and M³⁺. Each M⁴⁺ ion is coordinated to six oxygen atoms forming a chain of edge-sharing M⁴⁺O₆ octahedra parallel to c-axis with neighboring chains linked by the distorted square M³⁺O₅ pyramids in the a–b plane [2], [3], [4]. Almost all studies on these oxides have been on their magnetic and multiferroic properties [1], [2], [5] or to develop materials with new multiferroic properties by partial substitution of Mn with another transition metal [6], [7]. However, in recent years mullite oxides are also shown to be a promising alternative to binary TM oxide catalysts for energy and environmental applications [8], [9]. In particular, Wang et al. [9] showed that a mixed-phase mullite oxide containing SmMn₂O₅ as the active phase display superior catalytic activity for NO oxidation, and have identified Mn⁴⁺-Mn⁴⁺ dimers as the active sites [9], [10]. Moreover, mullite oxides can form energetically stable crystal structures due to the strongly connected backbones of metal-oxygen polyhedra (MO₆ units). In contrast, when binary TM oxides alter its TM oxidation state during catalytic reactions, they necessarily change their crystalline phase, TM d-orbital configuration, and catalytic activity [11]. While Wang’s seminal work showed potentials for mullite oxides as highly active and stable NO oxidation catalysts, fundamental studies on the synthesis-structure-property relations of these materials with the objectives to optimize their surface properties for such an application have not yet been engaged.

This work focuses on the characterization of SmMn₂O₅ prepared by coprecipitation method followed by calcination, with emphasis on the effects of synthesis conditions on the structural and surface properties for NO chemisorption. While several methods, e.g. solid state reaction, citrate gel, and hydrothermal deposition [12], have been used to synthesize complex TM oxides, coprecipitation is the most widely used as this method offers several advantages, including low cost, easy control of composition, and possibility for industrial-scale production. For atomic input ratio of Mn/Sm = 2, calcination temperature, time, and precipitation pH are systematically varied to elucidate their effects on resulting mullite phase purity, specific surface area (SSA), and surface composition. To test surface properties with the eye on NO oxidation catalysis application, quantitative NO chemisorption provides additional probe on gas-surface interaction. The insight gained through this fundamental study will advance our understanding of the relationship between synthesis, structure, and surface properties of mullite-type materials.

Section snippets

Synthesis

All chemicals purchased were of analytical grade and used as received. SmMn₂O₅ with atomic ratio of Mn/Sm = 2 was prepared according to the method published by Wang et al. [9]. Appropriate amounts of Pluronic F127 (Spectrum) and poly(ethylene glycol) 600 (Alfa Aesar) were dissolved in deionized (DI) water. Next, manganese acetate tetrahydrate (Alfa Aesar) was added, followed by samarium nitrate hexahydrate (Alfa Aesar). The color of the solution turned yellow with a pH of 5–6 (Fig. 1a). The pH of

Results and discussion

While pure-phase SmMn₂O₅ was successfully synthesized at 1000 °C, XRD spectra of samples calcined below 1000 °C all showed an additional peak indicating the presence of a secondary phase. It was reported that the pure-phase mullite oxides can be obtained only when the thermal treatments are carried out under high oxygen pressure [7], [15]. For example. Alonso et al. reported that RMnO₃ perovskite phase in their RMn₂O₅ (R = Pr, Nd, Sm, Eu) samples prepared by a citrate technique could be eliminated

Conclusions

In summary, we have demonstrated that the calcination conditions and pH have strong effects on the structural and surface properties of SmMn₂O₅. In particular, calcination temperature is found to control both the mullite phase purity and SSA of SmMn₂O₅ samples. A weak dependence between pH and mullite phase purity is found, whereas SSA increases monotonically as the pH is increased. While surface Mn/Sm atomic ratios measured by XPS are similar to the bulk values for samples precipitated under

Acknowledgements

We thank Luis E. Reyes and Julia Y. Chan for helpful discussion on XRD results. This research was funded in part by a grant (AT-1843) from the Welch Foundation and a funding from Global Frontier R&D Program on Center for Multiscale Energy System (National Research Foundation for Korea). V.I. acknowledges the George A. Jeffrey NanoExplorers program at the University of Texas at Dallas. J.W.P.H acknowledges the Texas Instruments Distinguished Chair in Nanoelectronics.

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Effects of synthesis conditions on structure and surface properties of SmMn2O5 mullite-type oxide

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Synthesis

Results and discussion

Conclusions

Acknowledgements

J. Solid State Chem.

Vib. Spectrosc.

J. Solid State Chem.

Appl. Catal. A

J. Solid State Chem.

Mater. Res. Bull.

Appl. Surf. Sci.

J. Electron Spectrosc. Relat. Phenom.

A structural study from neutron diffraction data and magnetic properties of RMn2O5 (R = La rare earth)

J. Phys. Condens. Matter

The mullite-type family of crystal structures

Crystal chemistry aspects of the magnetically induced ferroelectricity in TbMn2O5 and BiMn2O5

J. Phys. Condens. Matter

Effects of synthesis conditions on structure and surface properties of SmMn₂O₅ mullite-type oxide

A structural study from neutron diffraction data and magnetic properties of RMn₂O₅ (R = La rare earth)

Crystal chemistry aspects of the magnetically induced ferroelectricity in TbMn₂O₅ and BiMn₂O₅