English

• 1. Introduction

  Catalyst plays an important role in the development of human society, from the chemicals production to energy utilization and environmental problem solving [1]. Among them, platinum group metals (PGMs), including Pt, Pd and Rh, are utilized broadly in catalytic fields, arising from their significant catalytic activity, high thermal stability and chemical toxicity resistance [2,3]. The catalysts based on the PGMs are therefore widely used in the fields containing the automobile catalytic converters, pharmaceutical industry, hydrogen production, oil refining, and fine chemical synthesis. However, the high price of the PGMs restricts their further applications [2,4,5]. The ever-increasing industry led to explosion of new catalysts with excellent performance, low price and high stability.

  Complex metal oxide (AB_mO_n) presents stable properties in catalysis reactions reasoning from their skeleton structure connected by metal oxygen polyhedron [6]. As a representative complex metal oxide, perovskite-type oxide presents flexible crystal structures, and therefore provides an opportunity to introduce foreign atoms into the initial structure for stability and catalytic performance enhancement [1,6]. The previous reports confirmed the higher activity of doped Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ and La_0.2Sr_0.8CoO_3-δ catalysts in oxygen evolution reaction (OER) [7,8] and NO_x conversion reactions than that of the most advanced Pt catalysts [9].

  Mullite-type oxide SmMn₂O₅, a typical complex metal oxide, was first reported in 2012 as a diesel engine oxidation catalyst with superior performance in NO conversion [10]. This catalyst with a mixed phase of (SrCeMn₅O_9.83)(SmMn₂O₅) exhibits a ~45% increasement over commercial Pt catalyst in NO catalytic oxidation reaction at 300 ℃ arising from the high chemical activity of SmMn₂O₅. In oxygen reduction reaction (ORR), the overpotential of SmMn₂O₅ reaches to 0.78 V, higher than that of Pt catalyst (0.7 V), and shows better cycle stability than that of Pt [11]. SmMn₂O₅ exhibits excellent catalysis performance and provides a new option of highly active, stable, and low-cost heterogeneous catalyst; however, to our knowledge, there is no review article covering the comprehensive information of SmMn₂O₅ and its applications.

  Here, we review the latest advances in the physiochemical properties of mullite SmMn₂O₅, its significant catalytic performance, and its applications (Fig. 1). We begin with a brief discussion of the crystal structure and key physical properties of SmMn₂O₅, then discuss its catalytic applications, the catalytic activity sources and reaction mechanisms. We follow with a summary of the recent advances in modification methods that improve the catalytic ability of SmMn₂O₅. Finally, we present a perspective on further innovations for catalytic applications.

  Figure 1

  

  Figure 1.  Structure features, physiochemical properties and catalytic applications of SmMn₂O₅ discussed in this review.

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  2. Structure and properties

  2.1 Crystal structure

  SmMn₂O₅ crystallizes in an orthorhombic three-dimensional crystal structure with a space group Pbam. The lattice parameters are a = 7.43 Å, b = 8.59 Å, c = 5.70 Å with α = β = γ = 90° [12–15]. The structure consists of two main units in structure of MnO₆ octahedron and MnO₅ square-pyramid (Fig. 2a), where the MnO₆ octahedra shares edges via O² and O³ oxygens to form infinite chains along the c axis while the MnO₅ pyramid shares edges via O¹ to form a dimer unit Mn₂O₈. The chains are linked together by MnO₅ units in the a-b plane through O³ and O⁴ oxygens, forming a stable crystal structure [12,13].

  Figure 2

  

  Figure 2.  (a) Crystal structure of mullite SmMn₂O₅. The dashed circles denote two O sites with the lowest oxygen vacancy formation energy, named O³ and O², respectively. Reproduced with permission [16]. Copyright 2017, Wiley-VCH. (b) Schematic view of the conduction mechanism of bridging oxygen atoms (O_bri) in SmMn₂O₅. Reproduced with permission [17]. Copyright 2005, Wiley-VCH.

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  In this specifical crystal structure, there are two different Mn atoms: Mn⁴⁺ locates at the 4f Wyckoff site in the MnO₆ distorted octahedron and the Mn³⁺ occupies the 4h Wyckoff site in the MnO₅ distorted square-pyramid [12,13,16]. Despite the onefold Sm³⁺, four type oxygen atoms exist in the SmMn₂O₅ bulk whereat the denoted O¹, O², and O⁴ atoms connect two Mn ions to form Mn-O-Mn structure while an O³ connects a Mn³⁺ ion and two Mn⁴⁺ ions [6,12,13,16]. The presented double crystal units and the polyvalency of the Mn enable the excellent catalytic activity of SmMn₂O₅.

  The rich coordination environment between Mn and O atoms endows abundant oxygen vacancy in the crystal structure. The density functional theory (DFT) calculation reveals that the low formation energies of O³ and O² vacancies (E_Vo = 2.85 eV and 3.86 eV, respectively) enable rich oxygen vacancies appearing along the c-axis direction [6]. The oxygen ion conduction therefore can happen through the hopping of the oxygen from the site of the bridging oxygen O_bri in MnO₅ to the oxygen vacancy in the octahedral chain (Fig. 2b) [17]. Furthermore, the SmMn₂O₅ has a small oxygen migration barrier of 0.97–2.81 eV based on its rich oxygen vacancies, revealing the excellent oxygen storage and transmission capability of SmMn₂O₅ [6]. The polyvalency of Mn atoms and excellent oxygen conductivity of SmMn₂O₅ lay the foundation for further applications in the catalysis such as gas oxidation, water electrolysis, and ORR.

  2.2 Physical and chemical properties

  In the past decades, the Mn-based mullite as a dielectric material has been widely investigated in magnetic and ferroelectric properties [18–24]. The excellent electrical and thermodynamic behaviors determine the further application in catalysis.

  2.2.1   Ferroelectric and semiconductor properties

  RMn₂O₅ (R = Sm-Lu, Y, or Bi) exhibits ferroelectric property at low temperature (25–40 K) [18]. The intrinsic difference of polar molecule between pristine positively and negatively poled ferroelectric surfaces in the adsorption and reaction processes provide the potential to manipulate catalytic reactions and create ferroelectric chemical sensors [25,26]. However, SmMn₂O₅ is intrinsically pyroelectric that causes a weak spontaneous polarization at room temperature [25]. The change of dielectric constant and loss tangent with frequency of SmMn₂O₅ confirm the above property (Fig. 3a) [27].

  Figure 3

  

  Figure 3.  (a) Variation of dielectric constant (left) and loss tangent (right) as a function of frequency at various temperatures. Reproduced with permission [27]. Copyright 2020, IOP Science. (b) Change of resistivity as a function of temperature. Reproduced with permission [27]. Copyright 2020, IOP Science. (c) Variation of conductivity as a function of frequency. Reproduced with permission [27]. Copyright 2020, IOP Science. (d) Temperature dependence of the oxygen partial pressure for the thermal dissociation of SmMn₂O₅. Reproduced with permission [31]. Copyright 2007, Springer. (e) Thermal stability diagram of SmMn₂O₅ compounds. Reproduced with permission [33]. Copyright 2016, IOP Science. (f) XRD profiles of the SmMn₂O₅ (SMO) samples. Reproduced with permission [34]. Copyright 2016, Royal Society of Chemistry.

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  SmMn₂O₅ is also a p-type indirect gap semiconductor material with a band gap of ~1.0 eV (Fig. 3b) [28–30]. This p-type characteristics of SmMn₂O₅ is conducive to the adsorption of oxygen, benefiting for its oxidation reaction. The further temperature dependent impedance spectroscopy confirms the charge carriers hopping conduction in this material (Fig. 3c), which is benefit for the high chemical reactivity of SmMn₂O₅ [27]. The above rich electrical properties ensure the good activities of SmMn₂O₅ in chemical reactions.

  The thermodynamic property of SmMn₂O₅ closely relates to temperature and partial pressure of oxygen, and exhibits follow thermal dissociation process [31,32]:

  (1)

  (2)

  (3)

  Yankin et al. studied the phase equilibria of SmMn₂O₅ over the temperature ranging from 973 K to 1123 K while pressure ranging from 10⁻³ Pa to10⁻¹⁶ Pa, respectively [31]. They found that the thermal decomposition temperature of SmMn₂O₅ increases with higher oxygen partial pressure (Fig. 3d). The high temperature (point a, 1418 K) of thermal dissociation in air confirms the high thermal stability of SmMn₂O₅. Li et al. also obtained a similar decomposition temperature of 1373 K for SmMn₂O₅ by employing the first principles simulations (Fig. 3e), which is in good agreement with the value (1439 K) obtained from thermogravimetric analysis and differential scanning calorimetry measurements [33]. The unchanged phase structure of SmMn₂O₅ at 1323 K for 8 h further confirmed its high thermal stability (Fig. 3f) [34]. The proved good thermal stability enables the wide reaction temperature of SmMn₂O₅.

  3. Origin of catalytic activity and reaction mechanism

  In order to clarify the origin of the catalytic properties, including active centers, electronic structures, and metal-oxygen bond strength, a descriptor is defined to explain the catalytic activity, and a discussion on the fundamental mechanisms of catalytic reactions is further presented.

  3.1 Metal-oxygen bond strength

  The catalytic activity of SmMn₂O₅ is mainly derived from Mn-Mn dimeric structure [10]. This distinctive active site is more reactive than single Mn active site based on the cooperative electron transfer between the two Mn atoms in the dimer [30]. Here, the filled e_g-orbital and the hybridization between O-2p and Mn-3d determine the high catalytic activity of the SmMn₂O₅, which is different to precious metals that only the half-filled d-band states determine the activity [35].

  Li and coworkers defined a general descriptor for AMn₂O₅ to clarify the origin the catalytic property in NO oxidation reaction by employing soft X-ray absorption spectroscopy and DFT calculations [36]. They showed that the ground-state high-spin configured Mn⁴⁺ (Mn_oct) and Mn³⁺ (Mn_pyr) can be regarded as t_2g³ and (d_xy¹, d_xz¹, d_yz¹, d_z²) respectively; d_z² orbital of Mn_pyr closing to Fermi level is occupied by a single electron (Fig. 4a) [29]. Compared with t_2g orbital of Mn⁴⁺, the d_z² orbital of Mn³⁺ has strong bonding ability with O-2p orbital. The occupy states of d_z²-band therefore determines the interaction between Mn and adsorbed O atom but cannot be a catalytic descriptor because of similar d-band shapes and d-band filling of AMn₂O₅ with different A atoms [36]. The O-K edge absorption spectra showed that the varied pd hybridization caused by A atom between the Mn-3d and the bulk O-2p orbital exhibits linear relationship with NO conversion efficiency (Fig. 4b), revealing that the pd hybridization is the appropriate catalytic descriptor for AMn₂O₅ catalysts [36,37]. In detail, the weak hybridization between Mn_oct-3d and O_bulk-2p leads to the energy lifting of the d_z² orbital of Mn_pyr and strongly interaction with O^*-2p orbitals, while the strong pd hybridization can produce lower d-band position and weak combination with O^* (Fig. 4c) [11,36,38,39].

  Figure 4

  

  Figure 4.  (a) The Mn-O octahedral and pyramidal crystalline fields, and the corresponding the d-orbital splitting configurations. Reproduced with permission [29]. Copyright 2018, Elsevier. (b) The pd hybridization strength versus the NO conversion rate. Reproduced with permission [36]. Copyright 2016, Royal Society of Chemistry. (c) Schematic diagram of the connection between the Mn_oct-3d and O_bulk-2p hybridization and the desorption of O^*. The energy of the d_z² orbital could be modulated by the hybridization between Mn_oct-3d and O_bulk-2p, which consequently results in tunable bonding strength between Mn_pyr and O^*. Reproduced with permission [36]. Copyright 2016, Royal Society of Chemistry.

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  3.2 Reaction mechanisms

  3.2.1   Oxidation reaction mechanisms

  There are three typical reaction mechanisms for transition metal oxides (TMOs) in oxidation reaction: Langmuir-Hinshel-wood mechanism (LH) [40,41], Eley-Rideal mechanism (ER) [42,43] and Mars-van Krevelen mechanism (MvK) [44,45]. In the LH mechanism, the TMOs serve as a substrate where the reactants simultaneously adsorb and then react with each other to form new molecules (Fig. 5a), while, in the ER mechanism, only one reactant adsorbs on the substrate and reacts with another reactant appearing in gas phase (Fig. 5b). In the MvK mechanism, the surface oxygen atom of TMOs participates in the reaction. Taking the NO oxidation reaction for example, there are mainly two reaction steps listed in MvK (Fig. 5c): (1) The adsorbed NO on substrate is oxidized to NO₂ when combing with a lattice surface oxygen; (2) The reduced surface is re-oxidized to the original state through oxygen adsorption and dissociation [6,37,45–47]. In general, multiple reaction routes combining with different mechanisms can be found in one oxidation reaction [48,49].

  Figure 5

  

  Figure 5.  Schematic of NO oxidation reaction pathways with mechanisms of (a) L-H, (b) ER, (c) classic MvK, (d) cooperative MvK, Reproduced with permission [50]. Copyright 2021, Wiley-VCH. and (e) coexistence of MvK and ER. The green balls, red balls, and red dashed circles represent N atoms, O atoms, and oxygen vacancies, respectively.

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  Among various oxidation reactions of SmMn₂O₅, the NO oxidation is the most popular reaction and can be used to explore the reaction mechanism. Based on DFT calculations, Wang et al. [10] proposed that NO oxidation reaction followed ER mechanism via the Mn dimer active sites on the SmMn₂O₅ (110) surface and displayed the general NO oxidation reaction route as shown in Fig. 5b that: (1) O₂ prefers adsorbing on the Mn-Mn sites and decomposes into oxygen atoms; (2) The incident NO gas adsorbs on the O site to form monodentate Mn-nitrite (^*NO₂), and then transforms into bidentate Mn-nitrate (^*NO₃) when reacts with the other absorbing oxygen; (3) The ^*NO₃ reacts with an oxygen vacancy on the surface and convert to dissociation NO₂. In contrast, Zheng et al. [37] believed that NO oxidation was a MvK reaction process through the cooperation of multiple lattice oxygens (denoted O_α, O_β and O_γ) on SmMn₂O₅ (010) surface (Fig. 5d). They believed that: (1) The O_β serves as a reversible anionic redox center where NO adsorbs on the surface when accepting electrons from the catalyst; (2) O₂ molecules fill the O_α vacancy (v_Oα) to stabilize the critical rotation step to produce much more monodentate nitrites; (3) NO₂ desorbs on the surface when giving out electrons; (4) O_α serves as an oxidant to help turning NO into NO₂ mediated by O_β and O₂ gas.

  Despite the above explanation of NO oxidation on SmMn₂O₅, there are still some disputes over the real reaction process and mechanism. Chen et al. [6] reported a comprehensive explanation about reaction mechanism of SmMn₂O₅ by studying the oxygen chemistry in bulk phases as well as on the four low index surfaces-(110), (100), (010) and (001) by the DFT calculations. The results shown that the (001)⁴⁺ surface and (010)⁴⁺ surface significantly promote the total oxidation rate through the synergistic ER and MvK mechanism (Fig. 5e). Significantly, the rate-limiting step of this reaction is the desorption of NO₂ on the surface regardless of the NO oxidation [6,10].

  3.2.2   Reduction reaction mechanisms

  SmMn₂O₅ is also mostly used as an efficient catalyst in metal-air batteries based on the ORR. The common ORR mechanism can be divided into two categories: adsorbate evolution mechanism (AEM) [50–54] and lattice oxygen-mediated mechanism (LOM) [53,54]. In AEM, a 4e⁻ reaction process coupling with protons happens on the active metallic sites to achieve an ORR without participation of the lattice oxygen. As shown in Fig. 6a, O₂ first adsorbs on the electrode surface that is occupied by ^*OH, and then form to ^*OO with releasing a OH⁻. It is further reduced to ^*OOH by binding protons in solution. Finally, the O-O bond in the ^*OOH breaks to form ^*O and OH⁻. The ^*O is further protonated to form ^*OH in the solution [38]. In LOM, the lattice oxygen directly participates in the reaction and does not involve the formation of ^*OOH (Fig. 6b). The nature of LOM is mainly related to the covalency of the metal-oxygen (M-O) bond in metal oxides; the stronger covalency of the M-O bonds achieves the higher ability to achieve ORR. Theoretically, the AEM or LOM occurs in a reaction can be adjusted by changing the strength of the M-O bonds that depends on the relative position of the metal d and oxygen p bands in the metal oxides [55]. If the d band of the metal locates at higher energy than that of oxygen p band, the AEM will be prioritized for ORR; conversely, if the energy of oxygen p band is higher than that of the metal d band, electrons will be easier to transfer from the oxygen p band to the metal d band along with the formation of ligand holes, enhancing the redox activity of the lattice oxygen in an ORR with changing from AEM to LOM [55,56].

  Figure 6

  

  Figure 6.  ORR mechanism diagrams with a comparison between (a) AEM, (b) LOM, and (c) LAM. The atoms marked in red represent the original atoms of the system, while the adsorbed substances are labeled in black. Reproduced with permission [38]. Copyright 2021, Royal Society of Chemistry.

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  However, SmMn₂O₅ with multiple ligand units can hold more variable oxygen environment on the terminated surfaces, causing these two mechanisms may fail to precisely describe the ORR process. Wang et al. [38] reported the third ORR mechanism for SmMn₂O₅ that named labile oxygen participant adsorbate evolving mechanism (LAM). This mechanism can be described that: proton H firstly adsorbs on labile oxygen to form ^*OH, then O₂ replaces ^*OH to form adsorbed ^*OO (Fig. 6c), and then is reduced to intermediate ^*OOH by binding protons in solution, and finally, the O-O bond in ^*OOH is broken to form ^*O and ^*OH to complete the 4e⁻ process. Based on the DFT calculations, the obtained overpotential of ORR is 0.410 V based on the LAM, which is in good agreement with the experimental result of 0.413 V. In contrast, the overpotential values of ORR based on the AEM and LOM are obviously higher than the experimental results. The above results confirm the rationality of the LAM on the explication of ORR for SmMn₂O₅ [38].

  4. Application in catalysis

  The variable-valence element Mn, good oxygen conductivity and excellent thermal stability enhance the catalysis performance. In this section, we provide a brief review on the catalytic applications of SmMn₂O₅ on both catalytic oxidation reaction and catalytic reduction reaction.

  4.1 Automobile catalytic converters

  Vehicle exhaust mainly includes nitrogen oxides (NO_x) (> 90% NO), carbon monoxide (CO), hydrocarbons (HCs) and particulate matter produced by incomplete combustion (mainly soot), which are harmful to the environment and human health [57–61]. One of the most reliable ways to reduce the toxicity of vehicle exhaust gasses and remove harmful substances is the complete oxidation of exhaust components using catalytic compositions on metal or ceramic block carriers. Comparing to the commercial PGM catalysts (such as Pt/Al₂O₃), SmMn₂O₅ displays comparable catalytic activity on automobile exhaust, such as comparable or lower light-off temperature T₅₀ (the temperature at 50% conversion) (Figs. 7a–e, middle) and higher conversion for all five oxidation reactions (Figs. 7a–e, bottom). For example, Wang et al. [10] found that SmMn₂O₅ achieved a maximum conversion efficiency of 79% compared with 62% for Pt/Al₂O₃ in NO oxidation (Fig. 7a, bottom). Thampy et al. [62] further confirmed that SmMn₂O₅ has a maximum NO conversion efficiency of 52% at lower temperature (200 ℃) and Li et al. [36] reported that SmMn₂O₅ can achieve the maximum NO conversion of 75% at 300 ℃. Similarly, Zhao et al. [33] employed SmMn₂O₅ in CO oxidation and found that the T₅₀ of SmMn₂O₅ is 130 ℃ while the highest conversion efficiency of 98% is achieved at 180 ℃, showing comparable performance with Pt/Al₂O₃ (Fig. 7b, bottom). For the catalysis on soot, SmMn₂O₅ also shows excellent catalytic activity, especially in NO_x-assisted soot combustion [63–66]. Feng et al. [64] showed that both the light-off temperature and activation energy in soot combustion based on SmMn₂O₅ are greatly accelerated under NO + O₂ atmosphere (Fig. 7c, top) comparing to only under O₂ (Fig. 7c, bottom), —368 ℃ to 518 ℃ and 65 kJ/mol to 135 kJ/mol, respectively. Moreover, SmMn₂O₅ catalyst possesses excellent soot oxidation efficiency of 90% at 420 ℃, lower than that of Pt/Al₂O₃ (480 ℃) [64]. SmMn₂O₅ also displays better ammonia selective catalytic oxidation performance than 1.2%-Pt/γ-Al₂O₃ samples when used for NH₃ removing from the exhaust gas [67]. The other previous reports further confirmed that SmMn₂O₅ exists superior activity on VOCs (such as ethanol [30], toluene [68] and benzene [69]) with high hydrothermal stability and durability. For instance, the ethanol complete oxidation temperature is 175 ℃ for SmMn₂O₅, which is much lower than those for the other catalysts (Fig. 7d, bottom) [30]. SmMn₂O₅ achieves more than 90% conversion efficiency for methane [70], propane [71,72], propylene [73] and other hydrocarbons at 300–600 ℃ (Fig. 7e). These results confirm that SmMn₂O₅ is an effective catalyst for automotive exhaust oxidation. The good catalytic activities of SmMn₂O₅ are attributed to the rich active sites on the surface of SmMn₂O₅ and the unique Mn-Mn dimer structure.

  Figure 7

  

  Figure 7.  (a) NO, (b) CO, (c) soot (d) VOC and (e) HC oxidation reactions catalyzed by SmMn₂O₅ oxide. Top: Schematics of various oxidation reactions in (a–e). The green, black, white, and red balls represent N, C, H, and O atoms, respectively. Middle: Comparison of light-off temperature (T₅₀) for SmMn₂O₅ (red) and commercial Pt/Al₂O₃ (blue) for the oxidation reactions in (a–e). Data adapted from Refs. [10,34,64,30,71]. Bottom: Catalytic activities of SmMn₂O₅ for the oxidation reactions in (a–e). Reproduced with permission [10,34,64,30,71]. Copyright 2012, American Association for the Advancement of Science; Copyright 2016, Royal Society of Chemistry; Copyright 2017, Royal Society of Chemistry; Copyright 2020, Royal Society of Chemistry; Copyright 2021, Elsevier.

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  4.2 OER and supercapacitor

  SmMn₂O₅ is also used in OER and energy storage field due to its abundant lattice oxygen and good oxygen conductivity. Rani synthesized SmMn₂O₅ hollow long nano-cuboids (HLNCs) and nanorods, and used them for OER in water electrolysis and electrochemical supercapacitor applications [74,75]. The results exhibited that the SmMn₂O₅ has a high specific capacitance of 256 F/g at 5 mV/s scan rate and high OER activity of 0.37 mA/g at 10 mV/s scan rate with low Tafel slope value of 64 mV/dec (Fig. 8). The internal resistance and the charge transfer resistance were respectively estimated as 13 and 355 Ω from EIS spectra (Fig. 8b), which providing the clear view of fast charge transfer and electronic mobility. Furthermore, the fabricated electrode showed a high stability of 100% retention for 3 h in water oxidization reaction (Fig. 8f) [74]. As shown in Fig. 8c, the specific capacitance calculated from GCD curve is 141 F/g at 1 A/g and acts as a good achievement of fabricated SmMn₂O₅ HLNCs electrode in supercapacitor [74]. These results indicate that SmMn₂O₅ has unique oxygen storage, and both good charge transfer capacity and discharge capacity, indicating the significant potential applications in the field of energy storage.

  Figure 8

  

  Figure 8.  (a) CV, (b) EIS, (c) GCD, (d) LSV study, (e) Tafel slope and (f) CA study of mullite SmMn₂O₅ Hollow long cuboids. Reproduced with permission [74]. Copyright 2019, Elsevier.

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  4.3 Potentiometric gas sensor

  As a p-type semiconductor, the d_z² state with a single electron occupies the orbitals near the Fermi level, resulting in a lower sensing reaction barrier of SmMn₂O₅ and promoting the oxidation reaction of the gas molecular on the surface. The above excellent electronic structure therefore induces a rapid response behavior of SmMn₂O₅ as a gas sensor. Zhu et al. [76] employed SmMn₂O₅ nanofibers (SNFs), nanotube-in-tubes (NT@NTs), and porous nanotubes (PNTs) as sensors for acetone detection. They found that the SmMn₂O₅ SNFs sensor exhibited rapid response and recovery times as short as 3 s and 8 s at 300 ℃, respectively (Figs. 9a and b). When used as a sensor for methane detection, Yang et al. [77] found that SmMn₂O₅ exhibited rapid response and recovery rates of methane with different concentrations at 350–500 ℃. The response and recovery times were significantly shortened to 27 s and 33 s for 400 ppm at 400 ℃ (Fig. 9c). Despite the above less reported applications, SmMn₂O₅ is also a potential semiconductor material for gas detections directly, and it can be further combined with other semiconductors to form heterojunction structures for rapid detection of more hazardous gasses [78–80].

  Figure 9

  

  Figure 9.  (a) Schematic diagram of the prepared sensor. Reproduced with permission [77]. Copyright 2020, Elsevier. (b) Response and recovery time of SmMn₂O₅ sample to 100 ppm acetone at 300 ℃. Reproduced with permission [76]. Copyright 2019, Elsevier. (c) long-term stability test of the sensor attached with SmMn₂O₅ sensing electrode. Reproduced with permission [77]. Copyright 2020, Elsevier.

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  4.4 Metal-air batteries

  Similar to catalytic oxidation, the catalytic reduction reaction also involves the breaking of an oxygen molecule into two O atoms, which dependents on the bonding strength of metal and oxygen in the catalyst. SmMn₂O₅ with moderated Mn-O^* bond has great application potential in catalytic reduction due to the volcano curve relationship between Mn-O^* bond strength and catalytic performance [11,36,38,39]. In 2016, Wang et al. [11] first explored the catalytic activity of SmMn₂O₅ for ORR. They employed SmMn₂O₅ as an electrocatalyst in Mg-air battery and found that it exhibits similar ORR catalytic activity to Pt/C catalyst in 1 mol/L NaCl neutral solutions but has superior stability than Pt/C (Fig. 10a). In addition, they further investigated the electrocatalytic activity of SmMn₂O₅ in ORR under alkaline conditions (0.1 mol/L KOH) [39]. The catalyst exhibits an overpotential of 0.413 V and revealed the superior stability with only ~5% decay in activity over 20,000 s, exceeding 15% of Pt/C catalyst (Figs. 10b and c). Moreover, SmMn₂O₅ was also confirmed excellent electrochemical performance as an electrode of Al-air battery [81] and Zn-air battery [82]. Zn-air batteries constructed with SmMn₂O₅ nanorods exhibit not only higher peak power density (217 mW/cm²) over commercial Pt/C catalyst (190 mW/cm²) but also excellent cycling stability in long-term charging-discharging test over 170 h (Figs. 10d and e). In general, SmMn₂O₅ is a promising cathode catalyst and provides new options for advanced energy conversion and storage.

  Figure 10

  

  Figure 10.  (a) Schematic of a primary Mg-air battery using mixed-mullite catalyst in oxygen cathode, Mg anode, and aqueous 1 mol/L NaCl electrolyte. Reproduced with permission [11]. Copyright 2016, Elsevier. (b) LSV curves of oxygen reduction on pure SmMn₂O₅, carbon, MnO_x/C, SmMn₂O₅-NRs/C and Pt/C in O₂-saturated 0.1 M KOH at a rotating speed of 1600 rpm with a scan rate of 5 mV/s. Reproduced with permission [39]. Copyright 2017, Royal Society of Chemistry. (c) Chronoamperometric curves of SmMn₂O₅-NRs/C and Pt/C at a constant potential of 0.6 V vs. the RHE. Reproduced with permission [39]. Copyright 2017, Royal Society of Chemistry. (d) Typical polarization curves and the corresponding power density plots of the batteries made with SmMn₂O₅-NRs and Pt/C. Reproduced with permission [82]. Copyright 2018, Elsevier. (e) Cycling test of the ZAB made with SmMn₂O₅-NRs at different charging-discharging current density. Reproduced with permission [82]. Copyright 2018, Elsevier.

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  5. Strategies for improving the catalytic performance of SmMn₂O₅

  Despite the significant catalytic activity and stability on various chemical reactions, some strategies, including morphology regulation [71,82–85], metal ion doping [29,86,87], oxygen vacancies introduction [16,28,88,89], and composition with other materials [69,63,73,90–95] have been performed to enhance the properties of low-temperature catalysis, gas adsorption strength and electronic conductivity of SmMn₂O₅.

  5.1 Morphology regulation

  The common synthetic methods of mullite oxide are co-precipitation and hydrothermal method. The former method relies heavily on calcination conditions and precipitation pH while the latter is greatly affected by the pH of the solution and the type of precursor. We therefore can obtain larger specific surface area (SSA) and more catalytic active sites (Mn³⁺ and Mn⁴⁺) of SmMn₂O₅ by modulating the morphology of SmMn₂O₅ based on different reaction conditions.

  By changing the pH of precursor solution, Thampy et al. [83] reported a linear relationship between the SSA of SmMn₂O₅ and pH while Ma et al. [84] obtained single crystal SmMn₂O₅ nanorods and nanoparticles. Similarly, by utilizing Mn precursors with different anions, Wang et al. [85] synthetized different SmMn₂O₅ nanostructures with different morphologies (Fig. 11a), such as granular, flower-like, and mixed structures. In addition, Yang et al. [73] proposed a simple selective dissolution method to modify the surface of SmMn₂O₅ oxide. They found that part of Sm on the surface can be removed after acid treatment, therefore increasing the exposure of Mn⁴⁺ and the oxygen vacancy concentration.

  Figure 11

  

  Figure 11.  (a) SEM images of SmMn₂O₅ samples. Reproduced with permission [64,71,84,85]. Copyright 2017, Royal Society of Chemistry; Copyright 2021, Elsevier; Copyright 2018, Elsevier; Copyright 2021, Elsevier. (b) The schematic diagram of the interface effect: mullite (SmMn₂O₅)/O-deficient perovskite (BaMnO_2.83). Reproduced with permission [29]. Copyright 2018, Elsevier. (c) Specific and mass activities at the potential of 0.9 V (vs. RHE). Reproduced with permission [29]. Copyright 2018, Elsevier. (d) Energetic routes of O₂ adsorption and dissociation on the Mn₂ dimer and Mn-X heterodimers at the interface of Pt₂₀/SMO. The corresponding barrier energies of O₂ dissociation have been labeled. Reproduced with permission [92]. Copyright 2019, Royal Society of Chemistry. (e) Specific activities (left) and mass activities (right) as a function of Mn valence state and δ value in SMO_5-δ at 0.63 V (vs. RHE). Reproduced with permission [89]. Copyright 2020, Elsevier.

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  In addition to the above methods, the researchers have developed new preparation processes for different application scenarios and achieved the regulation of morphology. SmMn₂O₅ with unique interconnected macroporous structure was synthesized by the combustion of ethylene glycol and methanol solutions methods, and was successfully applied in NO_x-assisted soot combustion (Fig. 11a) [64]. Three-dimensionally ordered macroporous SmMn₂O₅ catalyst with large pore sizes was also designed and synthesized successfully by applying a carboxyl-modified colloidal crystal templates method. This synthesized SmMn₂O₅ nanoparticle exhibits high catalytic activity and anti-sintering performance for soot combustion [65]. Furthermore, leaf-like SmMn₂O₅ nanosheet with large SSA was applied in the combustion catalysis of C₃H₈ at 243 ℃ and exhibited an over 50% conversion, which is higher than that of 1 wt% Pd/Al₂O₃ catalyst [71]. Therefore, we can see that the innovation of preparation method can not only improve the catalytic performance of SmMn₂O₅ by regulating the morphology, but also improve the production efficiency of materials to a certain extent.

  5.2 Metal ions doping

  The strategy of ion doping is also an important way to enhance the catalytic activity by regulating the electronic structure of materials [96–99]. In SmMn₂O₅, replacing Sm atom by other rare earth elements benefits for the catalytic activity, thermal stability and oxygen storage capacity. Feng et al. [86] characterized a series of La_xSm_1-xMn₂O_5-δ (x = 0, 0.1, 0.3, 0.5) and found that the introduction of La atom increased the oxygen adsorption and Mn⁴⁺ content, benefiting for the decomposition of surface nitrate/nitrite and the dissociation of NO₂. The DFT calculations revealed that the Ba doping destabilizes the nitrite (^*NO) species and promotes NO oxidation performance, while the doping of Sr and La enhances the catalytic reactivity of the catalysts in a wide temperature range arising from the stronger ability of O₂ (^*O₂) dissociation [87].

  Beside the replacement of Sm atom, substitution of the Mn atoms with TMs can also change the local electronic structure, surface chemical coordination environment and therefore the catalytic activity of SmMn₂O₅. Liu et al. [92] doped SmMn₂O₅ by 3d transition metal dopants and found that the catalyst exhibits enhanced low-temperature oxidation activity for CO when doping by Fe. They reasoned this property to the reduced dissociation barrier of O₂ inducing by the appearance of Mn-Fe heterodimer at the interface (Fig. 11d).

  The mixed-phase mullite oxide by SmMn₂O₅ and perovskite AMnO_3-δ, such as A_xSm_1-xMn₂O_5-δ (A = Ca/Sr/Ba), can be seen as another strategy of ions doping. Zhao et al. [29] found that the atomic bonding interfaces between the active phase AMnO_3-δ and SmMn₂O₅ exhibited smaller work functions benefiting for the charge transfer and catalytic activity (Figs. 11b and c).

  5.3 Oxygen vacancy introduction

  Compared to element doping, the introduction of oxygen vacancy is considered to be a simpler and more effective way to improve the activity of catalyst, which can adjust the valence state of Mn and improve the oxygen conductivity and adsorption activity of O₂. The highly dispersed SmMn₂O₅ nanorods with rich oxygen vacancies were used for water electrolysis [75]; the electrochemical characterization displayed that the nanorods presented a high specific capacitance value of 352 F/g at 10 mV/s and a high current density of 178 mA/g with a lower internal resistance of 1.5 Ω, indicating the good electrical conductivity. Dong et al. [16] and Zhao et al. [89] synthesized a series of amorphous mullite SmMn₂O_5-δ by laser irradiation. The further ORR of the amorphous mullite SmMn₂O_5-δ revealed that the increased laser pulse induced higher oxygen vacancy concentration, and therefore induced the variation of ratio of Mn³⁺ and Mn⁴⁺ based on the charge balance principle. The volcano curve formed by ORR catalytic activity and the concentration of oxygen vacancies δ proved that the modest oxygen-defective sample with δ = 0.14 and valence of Mn as 3.36 possessed a superior catalytic activity among the reported family of Mn-based oxides, which was comparable to that of Pt/C (Fig. 11e) [16].

  5.4 Composited with other materials

  Collaborating with other materials to compensate for the inherent performance shortcomings of a material is a smart and popular strategy. As for SmMn₂O₅, different composite materials have different emphasis on the performance improvement, such as catalytic oxidation performance at low temperature, selectivity and electronic conductivity (Fig. 12a). The previous reports confirm that the excellent catalytic of the pure SmMn₂O₅ occurs mainly at medium and high temperature ranging of 300–600 ℃. The strategies of supporting metal nanoparticles on SmMn₂O₅ or building composite catalysts with SmMn₂O₅ can improve the low-temperature catalytic activity of SmMn₂O₅. For example, the platinum sub-nanoclusters were tightly anchored onto SmMn₂O₅ (Pt/SMO) via atomic layer deposition to form a novel composite catalyst with a bifunctional interfacial structure, resulting in a significantly lowered light-off temperature and high performance on CO oxidation [90,92]. As shown in Fig. 12b, Pt/SMO exhibits excellent low temperature activity for CO oxidation with a T₅₀ at ~86 ℃, which is much lower than that of pure SMO (171 ℃) [90]. The Pt_IWI/SMO (synthesized by incipient wetness impregnation method) and Pt/Al₂O₃ exhibited poorer activity than Pt/SMO, implying the key role of the interface structure and inherent active sites in SmMn₂O₅. The SmMn₂O₅ modified with Ag and Pd also showed the similar property to that of Pt [73]. Similarly, loading non-precious metal oxides MO_x (M = Mn, Fe, Co, Ni, Cu) onto SmMn₂O₅ mullite (MO_x/SMO) is an effective approach to enhance its catalytic performance of CO oxidation [91]. The values of activation energy deducting the contribution of SmMn₂O₅ for MO_x/SMO (M = Cu, Ni, Co, Mn) are 43.5, 67.8, 81 and 74.3 kJ/mol respectively, which are close to those values listed in Fig. 12c, indicating that the activity at low temperature can be mainly attributed to the MO_x modification over SmMn₂O₅ but not the SmMn₂O₅ [91]. The improvement of the low-temperature activity of SmMn₂O₅ composited with other catalyst is mainly attributed to the new formed interfaces, which can decrease the dissociation barrier of O₂, improve the reduction of the adsorbed oxygen, and increase the oxidation state of the interfacial Mn ion [63].

  Figure 12

  

  Figure 12.  (a) A comparison chart of the performance improvement of different materials composited with SmMn₂O₅ (SMO). PGMs, MO_x, C and AB represent platinum group metals, metal oxides, the highly conductive material, and acidic or basic materials, respectively. (b) CO conversion of Pt/SMO, Pt_IWI/SMO, Pt/Al₂O₃ and SMO as a function of reaction temperature. Reproduced with permission [90]. Copyright 2018, Royal Society of Chemistry. (c) Apparent activation energy E_a of the MO_x/SMO samples. Reproduced with permission [91]. Copyright 2018, Royal Society of Chemistry. (d) Soot combustion with NO_x-assisted. Reproduced with permission [93]. Copyright 2020, Elsevier. (e) Half-wave potential and Tafel’s slope of SMO, NrGO, SMO/NrGO-2, SMO@rGO-2, SMO@NrGO-2, and Pt/C. Reproduced with permission [95]. Copyright 2019, American Chemical Society. (f) Differential scanning calorimetry (DSC) curves for SmMn₂O₅ nanocrystals and their composites. Reproduced with permission [94]. Copyright 2020, Elsevier.

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  Beside compositing with other catalysts, the surface acidity and alkalinity of SmMn₂O₅ can also be modulated by compositing with other acidic or basic materials, benefiting for the effective adsorption of gas molecules. Chen et al. [94] increased the surface alkalinity of the SmMn₂O₅ by complexing appropriate amounts of K-based oxide (K₂Mn₄O₈) for NO_x adsorption and further reaction. This enhanced adsorption capacity helps ^*NO₂ to participate in the oxidation directly and accelerates the reaction thoroughly. As shown in Fig. 12d, 5% KMO-SMO sample has showed the best activity for soot combustion, with T₅₀ of 336 ℃. Similarly, the modification of the amorphous acid oxide Nb₂O₅ on the SmMn₂O₅ catalyst increases the number of -NH groups on the surface, which is suitable for the catalytic oxidation of the basic gas NH₃ and further improves its reaction selectivity to N₂ [69]. In conclusion, the acid-base complementary design of the catalyst according to the acidity and alkalinity of different gasses is conducive to improve the inherent activity of the catalyst for specific reactions and release the maximum catalytic ability of SmMn₂O₅.

  On the other hand, the semiconductor properties and poor electronic conductivity of SmMn₂O₅ also limit its catalytic ability. Compounding SmMn₂O₅ with a highly conductive substrate is one of the effective ways to improve its conductivity. SmMn₂O₅ nanoparticles was successful synthesized on nitrogen-doped reduced graphene oxide (SMO@NrGO) for highly efficient ORR [95]. A comparable half-wave potential of 0.84 V to that of the Pt/C was achieved by efficiently storing and compensating electrons at the interfacial Mn-N(C) bonds of SMO@NrGO (Fig. 12e). The improvement of electronic conductivity can enhance interactions between intermediates and active sites in SMO@NrGO for better catalytic performance. Beside, Chen et al. [94] found that the good thermal conductivity of carbon nanotubes (CNTs) and graphene oxide (GO) could improve the catalytic activity of SmMn₂O₅ for NO oxidation over a wider temperature range, attributing to the higher heat release of SmMn₂O₅/CNTs (200–280 ℃) and SmMn₂O₅/GO (250–300 ℃) in different temperature ranges (Fig. 12f). We can see that SmMn₂O₅ can obtain the better charge carrier transmission with the help of other highly conductive materials to achieve superior catalytic performance.

  6. Conclusion and outlook

  We herein presented an overview of the basic physiochemical properties, catalytic activity origin, reaction mechanism, catalytic applications and modification methods of the mullite-type oxide SmMn₂O₅. Due to the existence of two crystal fields, MnO₆ octahedral and MnO₅ pyramid, as well as the unique structure of Mn-Mn dimer, SmMn₂O₅ exhibits good thermal stability, ferroelectric and semiconductor properties, and good oxygen conductivity, providing the basis for its extensive catalytic applications such as automobile catalytic converters, potentiometric gas sensor, oxygen evolution reaction, supercapacitor and metal-air batteries. In order to meet the needs of various catalytic fields, the catalytic performance of SmMn₂O₅ oxide was further explored by regulating morphology, doping metal ions, introducing oxygen vacancies, and compounding with other materials.

  Despite several years of extensive research on the catalysis and properties enhancement, much room remains to further advance the concepts reviewed herein. Although the SmMn₂O₅ was confirmed sensitive to many gasses, expanding the catalytic application field of SmMn₂O₅ is also an interesting work: (1) SmMn₂O₅ as a catalyst can be tried for more reactions, such as the conversion and utilization of CO₂, and the catalytic reforming of methane, combining with current development needs or the latest policies. It can also be used as an electrode in various energy storage and conversion devices, such as solid oxide fuel cells and solid oxide electrolysis cell, arising from its significant catalytic activity at high temperature; (2) In order to explore the greater catalytic ability of SmMn₂O₅, new synthesis methods and processing conditions can be developed for the structure and morphology modulation. (3) The following strategies may be of great importance for further properties study: synchrotron radiation devices and in-situ test to study the chemical and structural changes on the surface under catalytic transients, which can provide new insights for revealing surface chemistry and catalytic mechanisms. In addition, it is more practical to introduce real reaction conditions such as temperature and pressure into theoretical simulations to study the reaction kinetics on the surface of the catalyst. We believe that a comprehensive understanding of the crystal and electronic structure, basic properties, and reaction mechanisms of mullite-type oxide SmMn₂O₅ will help to develop SmMn₂O₅ catalysts with high activity, high selectivity and long-term stability to satisfy energy and environmental needs.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgments

  This research was financially supported by the National Natural Science Foundation of China (Nos. 52072134, U1910209, 51972128, 52272205), and Hubei Province (Nos. 2021BCA149, 2021CFA072, 2022BAA087).

  
  
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  Figure 1  Structure features, physiochemical properties and catalytic applications of SmMn₂O₅ discussed in this review.

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  Figure 2  (a) Crystal structure of mullite SmMn₂O₅. The dashed circles denote two O sites with the lowest oxygen vacancy formation energy, named O³ and O², respectively. Reproduced with permission [16]. Copyright 2017, Wiley-VCH. (b) Schematic view of the conduction mechanism of bridging oxygen atoms (O_bri) in SmMn₂O₅. Reproduced with permission [17]. Copyright 2005, Wiley-VCH.

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  Figure 3  (a) Variation of dielectric constant (left) and loss tangent (right) as a function of frequency at various temperatures. Reproduced with permission [27]. Copyright 2020, IOP Science. (b) Change of resistivity as a function of temperature. Reproduced with permission [27]. Copyright 2020, IOP Science. (c) Variation of conductivity as a function of frequency. Reproduced with permission [27]. Copyright 2020, IOP Science. (d) Temperature dependence of the oxygen partial pressure for the thermal dissociation of SmMn₂O₅. Reproduced with permission [31]. Copyright 2007, Springer. (e) Thermal stability diagram of SmMn₂O₅ compounds. Reproduced with permission [33]. Copyright 2016, IOP Science. (f) XRD profiles of the SmMn₂O₅ (SMO) samples. Reproduced with permission [34]. Copyright 2016, Royal Society of Chemistry.

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  Figure 4  (a) The Mn-O octahedral and pyramidal crystalline fields, and the corresponding the d-orbital splitting configurations. Reproduced with permission [29]. Copyright 2018, Elsevier. (b) The pd hybridization strength versus the NO conversion rate. Reproduced with permission [36]. Copyright 2016, Royal Society of Chemistry. (c) Schematic diagram of the connection between the Mn_oct-3d and O_bulk-2p hybridization and the desorption of O^*. The energy of the d_z² orbital could be modulated by the hybridization between Mn_oct-3d and O_bulk-2p, which consequently results in tunable bonding strength between Mn_pyr and O^*. Reproduced with permission [36]. Copyright 2016, Royal Society of Chemistry.

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  Figure 5  Schematic of NO oxidation reaction pathways with mechanisms of (a) L-H, (b) ER, (c) classic MvK, (d) cooperative MvK, Reproduced with permission [50]. Copyright 2021, Wiley-VCH. and (e) coexistence of MvK and ER. The green balls, red balls, and red dashed circles represent N atoms, O atoms, and oxygen vacancies, respectively.

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  Figure 6  ORR mechanism diagrams with a comparison between (a) AEM, (b) LOM, and (c) LAM. The atoms marked in red represent the original atoms of the system, while the adsorbed substances are labeled in black. Reproduced with permission [38]. Copyright 2021, Royal Society of Chemistry.

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  Figure 7  (a) NO, (b) CO, (c) soot (d) VOC and (e) HC oxidation reactions catalyzed by SmMn₂O₅ oxide. Top: Schematics of various oxidation reactions in (a–e). The green, black, white, and red balls represent N, C, H, and O atoms, respectively. Middle: Comparison of light-off temperature (T₅₀) for SmMn₂O₅ (red) and commercial Pt/Al₂O₃ (blue) for the oxidation reactions in (a–e). Data adapted from Refs. [10,34,64,30,71]. Bottom: Catalytic activities of SmMn₂O₅ for the oxidation reactions in (a–e). Reproduced with permission [10,34,64,30,71]. Copyright 2012, American Association for the Advancement of Science; Copyright 2016, Royal Society of Chemistry; Copyright 2017, Royal Society of Chemistry; Copyright 2020, Royal Society of Chemistry; Copyright 2021, Elsevier.

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  Figure 8  (a) CV, (b) EIS, (c) GCD, (d) LSV study, (e) Tafel slope and (f) CA study of mullite SmMn₂O₅ Hollow long cuboids. Reproduced with permission [74]. Copyright 2019, Elsevier.

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  Figure 9  (a) Schematic diagram of the prepared sensor. Reproduced with permission [77]. Copyright 2020, Elsevier. (b) Response and recovery time of SmMn₂O₅ sample to 100 ppm acetone at 300 ℃. Reproduced with permission [76]. Copyright 2019, Elsevier. (c) long-term stability test of the sensor attached with SmMn₂O₅ sensing electrode. Reproduced with permission [77]. Copyright 2020, Elsevier.

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  Figure 10  (a) Schematic of a primary Mg-air battery using mixed-mullite catalyst in oxygen cathode, Mg anode, and aqueous 1 mol/L NaCl electrolyte. Reproduced with permission [11]. Copyright 2016, Elsevier. (b) LSV curves of oxygen reduction on pure SmMn₂O₅, carbon, MnO_x/C, SmMn₂O₅-NRs/C and Pt/C in O₂-saturated 0.1 M KOH at a rotating speed of 1600 rpm with a scan rate of 5 mV/s. Reproduced with permission [39]. Copyright 2017, Royal Society of Chemistry. (c) Chronoamperometric curves of SmMn₂O₅-NRs/C and Pt/C at a constant potential of 0.6 V vs. the RHE. Reproduced with permission [39]. Copyright 2017, Royal Society of Chemistry. (d) Typical polarization curves and the corresponding power density plots of the batteries made with SmMn₂O₅-NRs and Pt/C. Reproduced with permission [82]. Copyright 2018, Elsevier. (e) Cycling test of the ZAB made with SmMn₂O₅-NRs at different charging-discharging current density. Reproduced with permission [82]. Copyright 2018, Elsevier.

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  Figure 11  (a) SEM images of SmMn₂O₅ samples. Reproduced with permission [64,71,84,85]. Copyright 2017, Royal Society of Chemistry; Copyright 2021, Elsevier; Copyright 2018, Elsevier; Copyright 2021, Elsevier. (b) The schematic diagram of the interface effect: mullite (SmMn₂O₅)/O-deficient perovskite (BaMnO_2.83). Reproduced with permission [29]. Copyright 2018, Elsevier. (c) Specific and mass activities at the potential of 0.9 V (vs. RHE). Reproduced with permission [29]. Copyright 2018, Elsevier. (d) Energetic routes of O₂ adsorption and dissociation on the Mn₂ dimer and Mn-X heterodimers at the interface of Pt₂₀/SMO. The corresponding barrier energies of O₂ dissociation have been labeled. Reproduced with permission [92]. Copyright 2019, Royal Society of Chemistry. (e) Specific activities (left) and mass activities (right) as a function of Mn valence state and δ value in SMO_5-δ at 0.63 V (vs. RHE). Reproduced with permission [89]. Copyright 2020, Elsevier.

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  Figure 12  (a) A comparison chart of the performance improvement of different materials composited with SmMn₂O₅ (SMO). PGMs, MO_x, C and AB represent platinum group metals, metal oxides, the highly conductive material, and acidic or basic materials, respectively. (b) CO conversion of Pt/SMO, Pt_IWI/SMO, Pt/Al₂O₃ and SMO as a function of reaction temperature. Reproduced with permission [90]. Copyright 2018, Royal Society of Chemistry. (c) Apparent activation energy E_a of the MO_x/SMO samples. Reproduced with permission [91]. Copyright 2018, Royal Society of Chemistry. (d) Soot combustion with NO_x-assisted. Reproduced with permission [93]. Copyright 2020, Elsevier. (e) Half-wave potential and Tafel’s slope of SMO, NrGO, SMO/NrGO-2, SMO@rGO-2, SMO@NrGO-2, and Pt/C. Reproduced with permission [95]. Copyright 2019, American Chemical Society. (f) Differential scanning calorimetry (DSC) curves for SmMn₂O₅ nanocrystals and their composites. Reproduced with permission [94]. Copyright 2020, Elsevier.

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