Review
Unique role of Mössbauer spectroscopy in assessing structural features of heterogeneous catalysts

https://doi.org/10.1016/j.apcatb.2017.11.004Get rights and content

Highlights

  • Recent advances of applying Mössbauer technique in environmental and energy-related catalysis are thoroughly summarized.

  • The unique role of the Mössbauer spectroscopy for the investigation of catalytic mechanism is illustrated.

  • Insights into applying Mössbauer technique for Fe/Sn-based materials in their special catalysis fields are pointed out.

  • The potential future trends for the applications of Mössbauer spectroscopy are proposed.

Abstract

Their wide availability in nature, low cost, high reactivity, and low toxicity make Fe-based catalysts versatile in various catalysis fields, including photocatalysis, Fenton-like reaction, electrocatalysis, Li-ion batteries (LIBs), Fischer–Tropsch synthesis (FTS), biomass conversion, N2O decomposition and etc. Mössbauer spectroscopy, a powerful technique that is able to give account of structural features for all iron species taking part in the catalysis process, is considered to be a crucial technique for determining catalyst phase, identifying active site, and investigating correlations between catalytic behavior and the coordination structure of catalysts, which are highly desirable for clarifying the catalytic mechanisms. Each kind of Fe-based materials could be functionalized in the most suitable catalysis field, wherever Mössbauer technique may play a unique role. For instance, Fe-N-C based materials are extensively investigated as electrocatalysts for oxygen reduction reaction and Mössbauer spectroscopy application in this field has been utilized to identify the chemical nature of the active site on the Fe-N-C catalyst. Iron carbides are considered as the most active phase for FTS and Mössbauer technique is widely applied in determining the chemical phase of catalysts. Fe-based silicates, phosphates or polyanionic compounds are recognized as promising cathode materials for LIBs, for which Mössbauer technique has been mainly applied for tracking of the oxidation state and coordination environment change of Fe between charged and discharged states of the batteries. Similar phenomena can also be found in other catalysis fields. To give a clear understanding of which field is most suitable for a certain Fe-based catalyst and the best role of the Mössbauer technique in a certain catalysis field associated with the investigation of the mechanism, in this review, the recent advances of applying Mössbauer technique in catalysis are thoroughly summarized, including results from environmental catalysis and energy catalysis. Remarkable cases of study are highlighted and brief insight into applying Mössbauer technique for various Fe-based materials in their special catalysis field is presented. Finally, the trends for future potential applications of Mössbauer technique are discussed.

Introduction

Mössbauer spectroscopy, concerning the emission of γ-rays from exited nuclei and the absorption of these γ-rays by other nuclei of the same element, offers a powerful technique for the investigation of materials through measuring the hyperfine interactions arising from coupling the nuclear moments and electric and magnetic fields acting on the atomic nucleus. Isomer shift (δ; nomenclature for reporting Mössbauer Data from http://www.mossbauer.info/nomenclature.html; arises from Coulomb interaction between the nucleus and the s-electrons), quadrupole splitting (Δ; arises from interaction of the electric quadrupole moment of the nucleus with an electric field gradient) and magnetic hyperfine field (B; arises from interaction between the nuclear magnetic dipole moment and the magnetic field at the nucleus) are three main parameters in a Mössbauer spectrum, which gives information not only about the electron spin configuration and oxidation state, but also on the molecular symmetry and some clues about the magnetic structure involving the probe atom in a material under investigation [1]. Since 1959, Mössbauer spectroscopy has gradually become a routine method for catalysis characterizations. The further development of its in-situ application capability makes Mössbauer technique unique for uncovering the “black box” of catalysis [2]. Over the past five years, Mössbauer technique has been extensively applied for the characterization of catalysts in various application fields such as photocatalysis [3], [4], [5], [6], [7], [8], Fenton-like reaction [9], [10], [11], [12], [13], [14], electrocatalysis [15], [16], [17], [18], [19], [20], Li-ion batteries (LIBs) [21], [22], [23], [24], [25], [26], [27], Fischer–Tropsch synthesis (FTS) [28], [29], [30], [31], [32], [33], [34], [35], biomass conversion [36], [37], [38], [39], [40], [41], N2O decomposition [42], [43], [44], [45], [46], [47], [48] and etc. (Fig. 1A). More than six hundred publications have ever been published and the citation numbers have been exponentially increased for years (Fig. 1B), revealing the fast development on applying Mössbauer technique in catalysis.

The majority of Mössbauer spectroscopy investigations dealt with elements of iron (Fe) and tin (Sn). Investigations of other elements, such as iridium (Ir), platinum (Pt), ruthenium (Ru), or gold (Au), are limited in numbers although fruitful [2]. The applications of Mössbauer technique in catalysis research are mainly focused on: (1) identification of the active sites or active phases for the catalysis processes; (2) investigation of the correlations between the structure of catalysts and their catalytic performance; and (3) characterization of catalysts during activation and deactivation of the reaction under ex-situ (or in-situ) conditions.

The applications of Mössbauer spectroscopy in catalysis and its theoretical principles were previously summarized [1], [2], [49], including an article we published in Advances in Catalysis two years ago. These reviews mainly focused on the physical principle of Mössbauer spectroscopy instead of in its applications in catalysis. In addition, the cognition of which field is most suitable for a certain Fe-based catalyst and the acme role of Mössbauer technique in different catalysis field are still ambiguous. Considering the rapid development of Mössbauer technique in various catalysis fields over the past 5 years, this review mainly focuses on the application of Mössbauer technique for the investigation of: (1) the impact of Fe doping TiO2 to improve its photocatalytic performance; (2) the redox cycles and active sites of Fenton-like catalysts; (3) the active site of Fe-N-C and NiFe based materials as electrocatalysts; (4) the oxidation state and environment change of Fe in Fe-based silicates, phosphates and polyanionic compounds between charge and discharge states of LIBs; (5) the phase transformation of iron carbides as catalysts during or after FTS; (6) the crystallographic sites and structure of hexaaluminates as aerospace catalysts; and (7) the presence of Fe species for biomass conversion. Remarkable cases of study are highlighted and brief insights regarding the application of the Mössbauer technique for various Fe-based materials in their special catalysis field is pointed out. Finally, the potential future trends for the applications of Mössbauer technique are discussed.

Section snippets

Fe doped TiO2

Photocatalysis offers a promising way to cope with both the growing environmental and energy problems. The most widely investigated photocatalyst is TiO2, a cheap and non-toxic semiconductor sensitive to ultraviolet irradiation. However, the energy required for activating TiO2 is very high due to its wide band gap [50], [51], [52]. Fe is widely used as a dopant to TiO2, as it may act as electron traps and thus can narrow the band gap [53], [54], [55]. In addition, Fe is one of the few elements,

Fe-N-C based materials

Fuel cells (FCs) provide a clean and efficient technology to convert chemical energy into electrical energy. However, the requirement of platinum as catalysts for the oxygen reduction reaction (ORR), makes FCs currently cost-prohibitive [83], [84]. Since 1964, following the discovery of Co phthalocyanines as ORR catalysts [85], non-precious metal (NPM) Fe/Co-N based catalysts have attracted much attention due to the high activity and stability [86], [87], [88], [89]. However, the lack of

Applications of the Mössbauer technique in Li-ion batteries

Rechargeable Li-ion batteries (LIBs) are regarded as a promising technology for electric vehicles. The discovery of new materials with better performance and a clear insight into the intercalation electrode materials are both critical for ensuring a leap forward of LIBs [109], [110]. Fe-based silicates, phosphates or polyanionic compounds as promising cathode materials for LIBs have been widely investigated, wherever Mössbauer technique was usually applied for tracking the oxidation state and

Iron carbides

Fischer–Tropsch synthesis (FTS) has been considered as a crucial technology for the production of liquid fuel including unsustainable crude oil from carbon sources [131], [132], [133]. The wide availability, high adaptability to broad H2/CO ratios and resistance to poisoning make Fe-based catalysts ideal for converting H2-deficient syngas (CO and H2) from renewable biomass or coal. Iron carbides are generally acknowledged as the main active phase in FTS, among which, Hagg carbide (χ-Fe5C2) and

Applications of the Mössbauer technique in biomass conversion

Biomass is one of the most important renewable carbon resources, and it is regarded as the ideal feedstock to replace petroleum for the production of chemicals and fuels [147]. Over the past decades, great efforts have been dedicated to biomass conversion due to the fossil fuel depletion and environmental problems. Some new reaction pathways have been developed and some novel catalysts have been designed [148], [149]. Additionally, some sensitive characterization technologies have also been

Applications of the Mössbauer technique in N2O decomposition

The decomposition of N2O has received much attention in recent years, which provides a solution to mitigate nitrous oxide that is formed as a byproduct of ammonia oxidation over the Pt-Rh alloy gauzes in the nitric acid plants [166]. Various catalysts have been reported to show activity for the N2O abatement, such as zeolites [167], [168], [169], perovskites [170] and ex-hydrotalcites [171]. On the other hand, N2O as propellant is used in small satellite propulsion systems. The concentration of

Summary and concluding remarks

Mössbauer spectroscopy offers a powerful tool to characterize the local electronic structure of the probe-element in nano-structured or amorphous materials, crystalline and glass by determining the local coordination, bonding and oxidation state. It is beneficial for the identification of the redox processes and characterization of intermediate phases, making the study of catalytic mechanism become possible. Recent years have witnessed a blossoming interest in applying Mössbauer technique in

Acknowledgements

This work was supported by the National Natural Science Foundation of China (11079036, 21476232) and the Chinese Academy of Sciences Visiting Professorships for Senior International Scientists (2011T1G15) as well as the Chinese Academy of Sciences for “100 Talents” Project. This work was also partially supported by the China Ministry of Science and Technology under the Contract of 2016YFA0202804. Many thanks to the anonymous reviewers who have helped improve this paper.

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