Redox-induced controllable engineering of MnO₂-Mn_xCo_3-xO₄ interface to boost catalytic oxidation of ethane

Abstract

Multicomponent oxides are intriguing materials in heterogeneous catalysis, and the interface between various components often plays an essential role in oxidations. However, the underlying principles of how the hetero-interface affects the catalytic process remain largely unexplored. Here we report a unique structure design of MnCoO_x catalysts by chemical reduction, specifically for ethane oxidation. Part of the Mn ions incorporates with Co oxides to form spinel Mn_xCo_3-xO₄, while the rests stay as MnO₂ domains to create the MnO₂-Mn_xCo_3-xO₄ interface. MnCoO_x with Mn/Co ratio of 0.5 exhibits an excellent activity and stability up to 1000 h under humid conditions. The synergistic effects between MnO₂ and Mn_xCo_3-xO₄ are elucidated, in which the C₂H₆ tends to be adsorbed on the interfacial Co sites and subsequently break the C-H bonds on the reactive lattice O of MnO₂ layer. Findings from this study provide valuable insights for the rational design of efficient catalysts for alkane combustion.

Introduction

Low-chain alkanes (C₁-C₃) that released from industrial processes, have gained considerable attention because of the growing concerns regarding air quality and human health^1,2. Of particular interest in ethane, a representative non-methane volatile organic compounds (NMVOCs), has become the focus of regulatory scrutiny due to stringent standards on flue gas emissions³. As a result, various technologies have been developed to mitigate their emission. Catalytic oxidation is shown to be an effective approach for eliminating alkanes and their derivatives⁴. However, the development of high-performance catalysts, particularly for low temperature application, remains challenging due to the inherently strong C-H bonds. Additionally, ethane that vastly exists in nature gas (1–9 mol%), must be taken into account during the catalytic nature gas combustion to examine its energetic performance on combustion process as well as its impact on the employed materials. Noble metal-based catalysts (such as Pt or Pd) typically exhibit excellent catalytic activity toward low-chain alkane combustion at low temperature⁵. However, the high cost and limited availability have driven people to find alternatives. With this goal in mind, a substantial amount of work was undertaken to develop efficient non-noble metal-based catalysts⁶.

Special attention has been given to transition metal spinel-type oxides (AB₂O₄) because of their remarkable activities and durability in oxidation reactions^7,8,9. In a typical AB₂O₄ structure, the tetrahedral and octahedral sites are occupied by A²⁺ and B³⁺ cations, respectively, offering a unique atomic arrangement that allows the facile tuning of the redox property^10,11. Additionally, the interaction between A and B in spinels was identified, which accelerates the generation of reactive oxygen¹². Moreover, the electron configuration of metal ions in AB₂O₄ spinels can be readily tuned by metal doping, thereby alternating the adsorption strength of reactants¹³. However, it should be noted that the obtained spinel oxides may not always present in an ideal AB₂O₄ structure, because various factors are involved in the synthesis process¹⁴. In some cases, certain amount of metal ions may become isolated from their parent spinel grains during crystal growing or post-synthesis process, resulting in the formation of multi-phase oxides. The properties of mixed oxides are more complex than that of the pure spinel because of the involved various interfaces and their coordination environments. Therefore, further investigation is required to elucidate the inherent properties of these interfacial sites as well as optimize the synthesis parameters, to achieve better control over the microstructure of synthesized catalysts.

Interfacial engineering has emerged as an important approach in the design of first catalytic materials, enabling the facilitation of diverse chemical reactions, such as dehydrogenation¹⁵, CO oxidation^16,17, and water-gas shift reaction^18,19. As stimulated by the growing interests in interface catalysis, extensive investigations have been conducted to understand the properties of active sites located within the heterojunction region of mixed oxides. Notably, it has been observed that the interface of multicomponent oxides plays a vital role in facilitating the mass transfer of oxygen. Zhu et al.²⁰ found that the proximity between MnO₂ and CeO₂ increased the mobility of both surface and lattice oxygen around the grain boundary of MnO₂-CeO₂ interface, resulting in an enhanced activity in HCHO removal (T₁₀₀ = 100 °C, GHSV = 90 L h⁻¹). Similarly, Zhang et al.²¹ optimized the structure of ZnCo₂O₄@CeO₂ catalyst, and discovered that the nanoscale contacts between ZnCo₂O₄ and CeO₂ introduce an enhanced oxygen storage capacity and lattice oxygen mobility. Shan et al.²² adopted the acid-etching approach to create MnO₂-CoMn₂O₄ interfacial system and unveiled that the lattice O that located at interfacial sites was activated due to the weakened Mn-O bonds as well as the altered coordination environments of O atoms. Also, Ren et al.²³ discovered that the concentration of oxygen vacancies of CoMn₂O₄ spinel significantly increased after HNO₃ treatment, therefore generating more active surface O species during O₂ activation. Likewise, the established CeO₂-Co₃O₄ interface in CeO₂@Co₃O₄ nanofiber catalysts has proved to be effective in propane oxidation²⁴. Moreover, the intimate contact between mixed oxides gives rise to a synergistic catalytic effect, allowing for the simultaneous activation of different reactants. Zhu et al.²⁵ investigated the dual interfacial effects between PtFe and FeO_x in each nanowire (NWs) as well as the interaction between NWs and TiO₂ support on the PtFe-FeO_x/TiO₂ catalyst, and discovered their interfacial synergy in CO oxidation. Liu et al.²⁶ designed a hierarchical MnO₂@NiCo₂O₄@Ni foam catalyst, and found that the three-dimensional core-shell structure maximizes the interaction between NiCo₂O₄ and MnO₂, consequently improving the performance of NH₃-SCR at low temperature. Zhang et al.²⁷ constructed AgO/CeSnO_x tandem catalysts and studied the synergistic effects between AgO and CeSnO_x dual sites for selective oxidation of NH₃. It is noticed that the electrons on CeSnO_x support were more easily transferred to AgO NPs, which accelerates the oxidation activity of AgO and the reduction performance of CeSnO_x support, thus achieving a good match between NH₃ oxidation and NO_x reduction. Also, the strong interaction between different metal oxides affects the dispersion and crystallinity of active centers^28,29. Furthermore, the electronic property at interfacial region could be flexibly altered by tuning the interaction between different components³⁰. These examples emphasize the crucial role of interfaces in multicomponent catalysts. Hence, it is imperative to find out how critical the formed interfaces dictate the performance of complex oxides and further obtain a fundamental understanding on the “property-activity” relationship.

Herein, we report our finding on the manipulation of the MnO₂-Mn_xCo_3-xO₄ interface through engineering the Mn/Co ratio of MnCoO_x catalysts or adjusting the annealing conditions. The resulting materials predominantly exhibit as Mn_xCo_3-xO₄ spinel oxides with MnO₂ thin layers decorated on the surface. The catalytic performance of the obtained MnCoO_x catalysts was evaluated in ethane oxidation to build the correlation between their structure and performance. Our results showed that the presence of MnO₂ and Mn_xCo_3-xO₄ at the grain boundary of the involved oxides synergistically enhanced both the ethane adsorption/activation and the lattice oxygen mobility. Notably, the strong interaction between MnO₂ and Mn_xCo_3-xO₄ induces a charge rearrangement between these components as supported by the in-situ X-ray Photoelectron Spectroscopy (XPS) analysis and Density Functional Theory (DFT) calculations. Elucidating the role of structural heterogeneity in multicomponent oxides enables us to selectively tune the interface properties and oxygen defects in a wide range of complex oxides.

Results

Structure and surface states

A series of Mn-substituted cobalt oxides (MnCoO_x-z with varied Mn/Co ratios (z) of 0–2.0) were successfully prepared by chemical reduction method. The obtained MnCoO_x catalysts present as hierarchical nanospheres with an average diameter of 250-500 nm, which is mainly composed by ultrathin nanosheets with the surface covered by thin layers (Fig. 1a, b; Supplementary Fig. 1). A schematic illustration is presented to show the formed grain boundary layers as a function of Mn/Co ratio (Fig. 1c).

Fig. 1: Structural analyses of the as-synthesized MnCoO_x catalysts.

a SEM image of MnCoO_x-0.5. b TEM image of MnCoO_x-0.5 with an insert showing a corresponding electron diffraction (SAED) pattern. c Schematic illustration of the grain boundary of MnCoO_x with varied Mn/Co ratio. d Raman spectra. (Yellow shading area: the vibrational bonds of Co species in MnCoO_x; blue shading area: the vibrational bonds of Mn species in MnO₂) e XPS spectra. f A correlation of cumulative area under H₂ reduction peaks (I & II & III) and O₂ desorption peaks (I & II) (Dash line: it was drawn to guide the readers’ eyes). (Source Data are provided as a Source Data file).

Firstly, the evolution of composition-dependent crystal structure of MnCoO_x catalysts was examined. Figure 1d presents the Raman spectra of MnCoO_x catalysts. Note that, the Raman spectra of MnCoO_x-0.1 is similar to that of Co₃O₄ reference. While, the main peak of octahedrally coordinated Co sites (CoO₆: 670 cm^-1) gradually shifted to lower wavenumber and merged with the shoulder peak (604 cm^-1) to form a broader peak when Mn/Co ratio is ≥ 0.2, implying the weakened vibration of Co-O bonds. Similar phenomenon was also observed in Ni_xCo_3-xO₄ spinel³¹. Also, the added Mn ions significant altered the symmetry of CoO₆, resulting from the lattice replacement induced inhomogeneous distribution of Mn(III) or Co(III/II) ions^32,33,34. The induced coordination environmental change further initiates the occurrence of structural defects and lattice distortion on the developed MnCoO_x, which in turn benefits the formation of oxygen vacancies. Besides, the peak position of tetrahedrally coordinated Co (CoO₄: 191 cm^-1) was invariant with varied Mn/Co ratio, but their intensity decreased at high Mn/Co ratio due to Mn substitution. Similar result was also obtained from FT-IR analyses (Supplementary Fig. 2). Meanwhile, no active Raman bands belong to Mn-O bonds (as indicated by the blue dash line in Fig. 1d) were observed in the prepared MnCoO_x catalysts, suggesting that the Mn ions are highly dispersed and/or exist as solid solution in Co₃O₄. The bulk structure of MnCoO_x was further studied by power X-ray diffraction (XRD) (Supplementary Fig. 3, Supplementary Table 1). The results indicate the incorporation of Mn ions into Co₃O₄ lattice, leading to the formation of MnCo₂O₄ spinel (PDF#23-1237). Also, the selected area electron diffraction (SAED) pattern (the insert of Fig. 1b) is indexed to the cubic lattice typical of MnCo₂O₄.

To get more insights of Mn species, X-ray photoelectron spectroscopy (XPS) measurements were performed to investigate the surface states of Mn-O-Co entity (Fig. 1e). Clearly, the surface atomic ratios of Mn/Co measured by XPS (Supplementary Table 2) were higher than that of the corresponding bulk Mn/Co ratio measured by ICP-OES, indicating that part of the Mn ions was dispersed on the surface of MnCoO_x catalysts. Notably, the MnCoO_x presented a high Co²⁺/Co³⁺ ratio of 0.4 ~ 0.6 compared to Co₃O₄ (Co²⁺/Co³⁺ = 0.35, Supplementary Table 3), an indicator of Mn substitution into the octahedral sites of Co³⁺. Also, the presence of satellite peaks suggests the partial reduction from Co³⁺ to Co²⁺, which demonstrates the coexistence of Co²⁺ and Co³⁺ on the prepared MnCoO_x catalysts^35,36,37. The Co²⁺/Co³⁺ ratio increases with increased Mn addition and levels off above Mn/Co of 0.5. The formed Mn³⁺ ions increase the anionic defects as Co or Ni does in other spinels, thus benefiting the catalytic oxidation process^38,39,40. From Mn 2p_3/2 spectra, we can notice that the MnCoO_x-0.1 catalyst showed the highest Mn³⁺/Mn⁴⁺ ratio, indicating that more O_v are created to maintain the electrostatic balance of the system (4Mn⁴⁺+O^2-→2Mn⁴⁺+2Mn³⁺+□+0.5O₂)^41,42,43. A gradual decrease of Mn³⁺/Mn⁴⁺ ratio appeared while increasing the Mn/Co ratio due to the diffusion of MnO₂ onto the surface of Mn_xCo_3-xO₄ substrate. The average oxidation state (AOS) of Mn 3 s increased with increasing the Mn/Co ratio, which is consistent with Mn 2p results (Supplementary Fig. 4). Therefore, we can infer that the added Mn mainly remain in two states, in which part of the Mn is incorporated into the bulk structure of Co₃O₄ to form Mn_xCo_3-xO₄ spinel, and the rest contributes to the formation of MnO₂ layer or aggregates as determined by the amount of added Mn.

Moreover, the O 1 s spectra were fitted into three peaks, which attributed to lattice oxygen (O_α), surface adsorbed oxygen (or defects, O_β), and chemisorbed water (O_γ) with B.E.s of 530.1, 531.3, and 532.7 eV, respectively^44,45 .The O_α species account about 75–85% over MnCoO_x catalysts, indicating their significant role in oxidation reaction. In addition, O₂-TPD (Supplementary Fig. 5a) was performed to study the type and mobility of oxygen that contained in the MnCoO_x catalysts. It was found that the O₂ desorption peak in the range of 300–600 °C (Region II) obviously shifted towards low temperature on MnCoO_x catalysts compared to MnO₂ and Co₃O₄ references, implying an improved oxygen mobility after Mn addition. However, the desorption amount in Region II dramatically decreased when Mn/Co ratio is above 0.5, perhaps due to the excessive accumulation of MnO₂ on the surface. This trend is consistent with what we observed from EPR analysis (Supplementary Fig. 6), indicating that there was more O_v on MnCoO_x-0.5.

To clarify the reducibility of involved oxides and the interaction of various species, the H₂ reduction peak was roughly divided into three individual peaks for MnCoO_x catalysts with Mn/Co ratio of 0.1–0.5 (Supplementary Fig. 5b). Peak(I) appearing at 100–200 °C belongs to the surface adsorbed O³⁹. Noted that the peak (I) accounts for 20% of all the consumed H₂ on the MnCoO_x-0.1 catalyst, while this value decreased to 10% once more Mn was introduced (Supplementary Tables 2 and 3). The relative amount of peak (II) increased with increased Mn/Co ratio (max.26%), indicating the appearance of MnO₂ on the surface of MnCoO_x. Also, it is noticeable that the reduction peak (III) shifted towards the lower temperature region (355–375 °C) compared to the bulk Co₃O₄ (387 °C), perhaps due to the facile H₂ transfer from MnO₂-Mn_xCo_3-xO₄ interface to the bulk materials. Similar phenomenon was also observed on Mn₂O₃@MnO₂ catalyst via MnO₂-Mn₂O₃ interface⁴⁵. Note that the total integrated area of peaks (I) and (II) in the O₂-TPD analysis exhibits a linear correlation with the cumulative area under H₂ reduction obtained from (I), (II), and (III) peaks (Fig. 1f, Supplementary Table 4). However, the excessive amount of Mn shifts peak (III) towards high temperature and even induces the formation of peak (IV), a suggestive of the strong interaction between Mn and Co oxides¹². To better understand the low-temperature reducibility of MnCoO_x catalysts, the initial H₂ consumption rate was calculated and plotted as a function of inversed temperature (1/T), as shown in Supplementary Fig. 7a. Clearly, the initial H₂ consumption rate decreased in the sequence of MnCoO_x-0.5 > MnCoO_x-0.2 > MnCoO_x-0.1 > MnCoO_x-1.0.

Catalytic performance evaluation

To determine the influence of Mn addition, all the synthesized MnCoO_x catalysts were employed for ethane combustion (Fig. 2a, Supplementary Table 5). Taking the temperature at 50% ethane conversion (T₅₀) as an indicator, we found that the oxidation activities decreased in the order of MnCoO_x-0.5 (205 °C) > MnCoO_x-0.2 (215 °C) > MnCoO_x-0.1 (219 °C) > MnCoO_x-1.0 (260 °C) > MnCoO_x-2.0 (282 °C) > Co₃O₄ (325 °C) > MnO₂ (348 °C), suggesting that a small amount of Mn can greatly enhance the activity. However, the activity is sluggish while adding too much Mn, perhaps due to the aggregation of MnO₂. To further evaluate the commercial potential of MnCoO_x catalysts, the catalytic activities of other low-chain alkanes (CH₄ and C₃H₈) were tested since they are also contained in the industrial emission (Supplementary Fig. 8, Tables 6–7). It is well-known that the initial H abstraction of short-chain alkanes is often regarded as the key elementary step^7,46. The strength of C-H bond is closely related to the chain length of alkanes, which in turn determines their reactivity in oxidation reactions. Specifically, the C-H bonds become weaker as the chain length increased (1^st C-H bond strength: CH₄ (465 kJ mol^-1) > C₂H₆ (442 kJ mol^-1) > C₃H₈ (427 kJ mol^-1))⁴⁷. For comparison, the catalytic activity of Co-based oxides for low-chain alkane (C₁-C₃) combustion were summarized in Supplementary Table 8. Clearly, the prepared MnCoO_x-0.5 in this work revealed a better catalytic activity than many of the reported catalysts in the literature.

Fig. 2: Catalytic performance of MnCoO_x-0.5 catalysts for ethane oxidation.

a Light-off curves of the as-prepared catalysts (reaction conditions: ca. 200 mg catalyst, [C₂H₆] = 3000 ppm, Q = 200 mL min^-1 and WHSV = 60,000 h^-1). b corresponding Arrhenius plots. c specific ethane conversion rate at 200 °C. d a correlation of the fitted peak area of oxygen species from O₂-TPD analysis with C₂H₆ oxidation rate at 200 °C. e comparation of ethane conversion rate at T₅₀ with other catalysts reported in the literature (see Table S9 for details). f cyclic thermal stability test of MnCoO_x−0.5 catalyst. g cyclic hydrothermal stability test of MnCoO_x-0.5 catalyst (reaction conditions: ca. 200 mg catalyst, [C₂H₆] = 3000 ppm, Q = 600 mL min^-1, WHSV = 180,000 h^-1 w/o and w/ 5 vol% H₂O, respectively). h long-term scale up stability tests by 1 wt% MnCoO_x-0.5 coated on micro-monolith substrate for 1000 h (reaction conditions: 350 °C, [C₂H₆] = 1300 ppm, Q = 10,000 mL min^-1, and GHSV = 6000 h^-1, w/ and w/o 5 vol% of H₂O). (Source Data are provided as a Source Data file).

To further evaluate the catalytic performance of MnCoO_x catalysts, a kinetic study was completed. Figure 2b presents the Arrhenius plots of MnCoO_x catalysts for ethane combustion based on the normalized reaction rates at ethane conversion in the range of 5-10%. The obtained apparent activation energy (E_a) of MnCoO_x is in the range of 80-116 kJ mol^-1, exhibiting a volcano-typed trend with increased Mn content. Also, the calculated E_a is strongly correlated with the reactivity of MnCoO_x catalysts. Note that, the E_a value of MnCoO_x-0.5 catalyst (E_a = 81.8 ± 3.2 kJ mol⁻¹) is the lowest, indicating an easier oxidation of C₂H₆. Also, the turn-over frequency (TOF) of MnCoO_x-0.5 catalyst for ethane oxidation is 3.93 × 10^-2s⁻¹ at 200 °C, which is significantly higher than other MnCoO_x samples. A good correlation was built between TOF and the initial H₂ consumption rate for MnCoO_x catalysts, as shown in Supplementary Fig. 7b. These results suggest that the MnCoO_x-0.5 catalyst with low E_a (81.8 ± 3.2 kJ mol⁻¹) and high TOF (3.93 × 10^-2s^-1) is more effective for ethane oxidation on per site basis. Moreover, the effect of space velocity on catalytic activity of MnCoO_x-0.5 catalyst was investigated as shown in Supplementary Fig. 9. Clearly, the ethane conversion decreased with the increased WHSV, as a result of shortening the contact time.

To study the intrinsic activity of MnCoO_x catalysts, the areal rates normalized by the specific surface area (Supplementary Fig. 10, Supplementary Table 9) of synthesized catalysts (expressed in the unit of µmol m^-2 s^-1) were calculated and plotted in Fig. 2c. MnCoO_x-0.5 catalyst showed the highest areal rate (6.3 ×10^-3 µmol m^-2 s^-1), which might be attributed to the strong chemical interaction between Mn and Co oxides, thus creating more effective interfacial sites and further changes the interaction between reactants and lattice O upon Mn substitution. The specific ethane oxidation rate either as per surface area or per mass of prepared catalysts exhibited a similar volcano-typed trend as a function of Mn/Co ratio. This trend is in good agreement with the calculated E_a. Also, a linear correlation was established between C₂H₆ oxidation rate and the amount of surface or subsurface lattice oxygen species, as calculated by the cumulative area of peak (II) in O₂-TPD results (Fig. 2d). Note that the prepared MnCoO_x-0.5 catalyst showed a superior catalytic performance in ethane oxidation compared to the reported non-noble metal catalysts so far, and even better than several reported noble-metal supported catalysts (Fig. 2e, Supplementary Table 10).

Moreover, the cyclic stability tests were performed both under dry and humid conditions at a relatively high WHSV of 180,000 h^-1 (Fig. 2f, g, Supplementary Fig. 11). As shown in Fig. 2f, the MnCoO_x-0.5 catalyst was able to be completely oxidized at 295 °C, and showed no attenuation on ethane conversion (ΔX_ethane < 1%) during thermal cyclic tests. In addition to this, the effect of water vapor was examined. No significant change is observed during the hydrothermal cyclic tests, and the T₉₀ value is about 280 °C for all cycles over MnCoO_x-0.5 catalyst (Fig. 2g). Also, the activity almost recovered after H₂O removal, which suggests the reversible deactivation of MnCoO_x-0.5 catalyst. This reversible deactivation can be substantiated by C₂H₆-O₂/O₂ + H₂O TPSR results as shown in Supplementary Fig. 12. Due to its superior performance in our lab scale tests, the MnCoO_x-0.5 powder was chosen and mixed with Al₂O₃ to prepare into a suspension for monolith washcoating. A similar preparation method was also used in one of our recently published work³¹. Afterwards, a long-term stability test was performed at 350 °C (Fig. 2h). The ethane conversion slightly dropped from ca. 76 to 68% at the initial stage of the reaction either with or without water addition. After that, no deactivation was observed up to 1000 h time-on-stream (TOS) measurement, which demonstrates the superior water-resistance of monolith MnCoO_x-0.5 catalyst.

Role of MnO₂-Mn_xCo_3-xO₄ interface

To gain a better understanding on the interfacial regions, the aberration-corrected STEM images and EELS analyses were performed to determine the structure and morphology of MnCoO_x catalysts (Fig. 3, Supplementary Figs. 13–15). An enlarged image on these nanosheets yields a periodic lattice fringe of 0.48 nm, corresponding to the (111) plane of MnCo₂O₄, which again confirmed the successful substitution of Mn into the lattice of cubic Co₃O₄ (Fig. 3a). Outside the microspheres, some ultra-thin layers were noticeable with an average thickness of ca. 4–5 nm for MnCoO_x-0.5. The measured lattice spacing is about 0.24, 0.21, and 0.31 nm, which can be indexed to the (101), (111), and (110) planes of MnO₂ (PDF#24-0735), respectively (Fig. 3b). Overall, the HRTEM images provide visual evidence for the formation of MnO₂-MnCo₂O₄ interface, as illustrated in Fig. 3c, d. To better understand the chemical environment of elemental Mn and Co at MnO₂-Mn_xCo_3-xO₄ interface, the EELS line-scanning was employed. The elemental distribution from electron energy loss spectra (EELS) clearly showed that Mn is evenly distributed on the shell of MnCo₂O₄ microsphere (Fig. 3e–g). Also, the EELS area scanning images give a direct view on the close contact between Co and Mn (Fig. 3h–k). Noted that, Mn prefers to stay on the edge of MnCo₂O₄ nanosheets. Next, we employed surface-sensitive technique TOF-SIMS to distinguish the chemical composition between surface and interior of MnCoO_x catalyst. Supplementary Fig. 16 presents the depth profile of ⁵⁵Mn⁺ and ⁵⁹Co⁺ elements, which again confirms the enrichment of Mn on the surface of Mn_xCo_3-xO₄ microspheres. Similar conclusion was also obtained on the depth profile of Mn⁴⁺/Mn³⁺ and Co²⁺/Co³⁺ atomic ratio from the XPS data (Supplementary Fig. 17).

Fig. 3: Microstructure characterizations of MnCoO_x catalyst.

a, b HRTEM images of MnCoO_x-0.5 catalyst at selected interfacial areas. c HRTEM image of MnCoO_x-2.0 catalyst. d schematic illustration of MnCoO_x-0.5 at interface. e high-angle annular dark-field (HAADF) images of MnCoO_x-0.5 catalyst at MnO₂-Mn_xCo_3-xO₄ interface with an insert showing the change of Mn/Co ratio along the yellow line from point (1) to (2), as indicated by the green arrow. f, g Mn-L2, 3-edge and Co-L2, 3 edge spectra as a function of line scanning distance (indicated by the green arrow on (e)). h–k EELS elemental maps of Mn, Co, and the corresponding Mn-Co overlap of MnCoO_x-0.5 catalyst. Scale bar, 5 nm; Red: Mn, Green: Co. (Source Data are provided as a Source Data file).

After studying the microstructure of MnCoO_x catalysts, the properties of MnO₂-MnCo₂O₄ interface were explored. To attain a deeper understanding on the reactivity of MnO₂-MnCo₂O₄ interface, a platform MnO₂/MnCo₂O₄ catalyst with 1 wt% of Mn loading was synthesized. Firstly, C₂H₆-TPSR was carried out to study the properties and reactivity of involved O on MnCoO_x-0.5 (Fig. 4a). Both CO₂ (m/z = 44) and H₂O (m/z = 18) were detected in the tested temperature range (50–500 °C). After studied the O reactivity of MnO₂, MnCo₂O₄, and MnO₂/MnCo₂O₄ references (Supplementary Fig. 18), we deduce that the evolved CO₂ peak below 250 ^oC (as indicated in the yellow box) can be ascribed to the oxygen that is located at or near MnO₂-MnCo₂O₄ interfacial region, while the high-temperature peak above 400 °C (as indicated in the pink box) is assigned to the bulk MnCo₂O₄ substrate. Besides, a relatively weak CO₂ peak appeared at 347 °C (as indicated in the blue box), suggestive of the existence of a small portion of aggregated MnO₂. Comparatively, the C₂H₆-TPSR result of MnCoO_x-0.5 catalyst indicates the reactive nature of surface lattice O that located at the interface of MnO₂-MnCo₂O₄. To get more insights into the activity of lattice oxygen (O_Latt) near MnO₂-MnCo₂O₄ interfacial areas, two DRIFT-MS experiments were designed. One was carried out in an O₂-free environment under isothermal conditions, and the other experiment was performed in transient state. Notably, CO₂ was detected on the MnCoO_x-0.5 catalyst without gas-phase O₂ supply, indicating the participation of lattice O at 250 °C (Supplementary Fig. 19). The transient DRIFT-MS analysis showed that the transition period for O₂-depletion follows the trend of MnCoO_x-0.5 (Δt = 210 s) > MnO₂/MnCo₂O₄ (Δt = 106 s) > MnO₂ (Δt = 90 s) (Supplementary Fig. 20), which is in accordance with the isotherm experiments.

Fig. 4: Role of MnO₂-MnCo₂O₄ interface for ethane oxidation.

a C₂H₆-TPSR-MS profile. b ¹⁸O isotopic labeling experiment in the temperature programmed oxidation of ethane over MnCoO_x-0.5 catalyst. c–h Temporal analysis of products (TAP) of ethane oxidation over MnCoO_x-0.5 as a function of temperature from 200 to 400 °C (the insert of d represents the TAP analysis of MnO₂ reference at 200 °C, T₁ stands for the maximum temperature of generated C¹⁶O₂, T₂ stands for the maximum temperature of produced C^16/18O₂). (Source Data are provided as a Source Data file).

Following this result¹⁸O₂ isotopic labeling experiments were performed to monitor how the lattice oxygen was involved in ethane oxidation. The formation of C¹⁶O₂ (m/z = 44) became noticeable above 65 °C, indicating the active nature of O_latt on MnCoO_x-0.5 (Fig. 4b). Noted that, C¹⁶O₂ doublet peak appeared (158 and 237 °C), which represent two types of lattice O. As the reaction proceeds, the formation of C^16/18O₂ occurs (163 °C) accompanied with the gradual decline of C¹⁶O₂, indicating that the oxygen exchange was taking place between gas phase ¹⁸O₂ and lattice ¹⁶O from the catalyst. Followed by this, the formation of C¹⁸O₂ is initiated (200 °C) due to the depletion of surface lattice ¹⁶O and the ¹⁸O₂ replenishment. The obtained isotope results emphasized the effectiveness of lattice O in MnCoO_x-0.5. For MnO₂ reference, the lattice ¹⁶O could also participate in oxidation, but with a higher onset temperature (189 °C), an indicator of the low activity of O_latt (Supplementary Fig. 21). Also, the presence of C¹⁶O₂ (or H₂¹⁶O) single peak suggested that there is only one type of lattice O participating in the reduction process, which is distinct from MnCoO_x-0.5. Overall, these isotopic O exchange studies suggests that the ethane oxidation is dominated by a surface Mars-van Krevelen (MVK) mechanism in both cases.

Subsequently, temporal analysis of products (TAP) was undertaken to unveil the dynamic surface change of MnCoO_x-0.5 and MnO₂ reference as a function of temperature (Fig. 4c–h, Supplementary Fig. 22). During each test, a small quantity of reactant mixture (5 ml, C₂H₆ + ¹⁸O₂ + He) was injected in the temperature range of 200–400 °C to facilitate the scrambling of ¹⁸O/¹⁶O atoms, thereby making it possible to capture the initial catalytic behavior of the material. Despite the quantitative difference in product distribution between steady-state and TAP experiments, the general selectivity trends were consistent. Note that the amounts of ¹⁶O-containing products (C¹⁶O₂ and C^16/18O₂, accounts for >95%) significantly exceed that of C¹⁸O₂ at 200 ^oC on the MnCoO_x-0.5 catalyst. Upon combining with the results obtained from C₂H₆-TPSR analysis, we can confidently verify that the majority of the participated O arises from the lattice O that resides at MnO₂-MnCo₂O₄ interface, exhibiting a remarkable reactivity in promoting oxidation reactions, particularly at relatively lower temperatures. Also, we found that the activity of lattice O on MnCoO_x-0.5 is significantly higher than that of bulk MnO₂ (insert of Fig. 4d). At 250 °C, the surface lattice ¹⁶O is quickly consumed as indicated by the increase of C^16/18O₂ and C¹⁸O₂. However, once the temperature is above 300 °C, the amount of ¹⁶CO₂ slightly increased due to the enhanced bulk phase O migration/diffusion to refill the surface O_v at high temperature. Thereby, we can infer that the replenishment of O_v originates from a conjugated effect both from the gaseous O₂ and bulk phase O migration/diffusion, in which the contribution from the latter could be enhanced at high temperature. Also, the results evidently conclude that the lattice O stayed at MnO₂-MnCo₂O₄ interfaces plays a crucial role for low-temperature ethane activation.

Aside from this, the in-situ XPS analyses (Fig. 5a, Supplementary Table S11) showed that the ratio of Co²⁺/Co³⁺ quickly increased from 0.55 (fresh sample at RT) to 0.64 (C₂H₆ at 200 °C) with no more change above 200 °C, perhaps due to the efficient electron transfer from the absorbed C₂H₆ to the positively charged Co ions. While from Mn 2p spectra, we observed the significant increase of Mn^δ+/Mn⁴⁺ ratio from 0.94 (fresh sample at RT) to 3.18 (C₂H₆ at 400 °C) accompanied by the shifting of Mn^δ+ peak towards lower B.E., suggesting the consumption of lattice O on MnO₂ domains during H abstraction, thereby resulting in a coordination change on Mn species. Once C₂H₆/O₂ mixture was introduced into the system, the Mn^δ+/Mn⁴⁺ ratio slightly increased from 3.18 (C₂H₆ at 400 °C) to 2.11 (C₂H₆/O₂ at 400 °C), while the Co²⁺/Co³⁺ ratio almost went back to its original states. This result again indicates the participation of lattice oxygen from MnO₂ layer. Also, the in-situ XPS results revealed that the O_α (lattice O) peak gradually shifts towards lower B.E. with increased C₂H₆ reduction temperature, indicating the weakened interaction between Co/Mn and O atoms, potentially resulting in an increase in oxygen vacancies⁴⁸. The O_α species gradually consumed during C₂H₆ reduction from 85.4% (fresh catalyst at RT) to 74.8% (C₂H₆ at 400 °C). This observation further confirmed the participation of lattice O species over the MnCoO_x-0.5 catalyst during C₂H₆ oxidation, which is consistent with the isotopic labeling experiments.

Fig. 5: Properties of MnO₂-MnCo₂O₄ interface for ethane oxidation.

a in-situ XPS analysis of MnCoO_x-0.5 catalyst under different gas atmospheres. b DRIFT spectrum of ethane adsorption over MnCoO_x-0.5. c The correlation between ethane conversion rate (or temperature at constant rate of 0.21 µmol g_cat⁻¹ s⁻¹) and Mn⁴⁺/Mn³⁺ ratio (or C₂H₆ adsorption capacity). (Source Data are provided as a Source Data file).

Next, the adsorption of C₂H₆ over MnCoO_x catalysts was investigated. As shown in the time-resolved DRIFT spectra (Fig. 5b), the intensity of ethane adsorption bands (3000 cm^-1) gradually increased with time-on-stream operation to reach a steady-state level. The MnCoO_x-0.5 exhibited the strongest ethane adsorption capacity compared to MnCo₂O₄ and MnO₂ references (Supplementary Fig. 23). Interestingly, the time it took to detect ethane follows the order of MnCoO_x-0.5 (11.0 min) > MnCo₂O₄ (7.5 min) > MnO₂ (4.0 min), indicating that more C₂H₆ are adsorbed/activated over MnCoO_x-0.5 catalyst. Again, CO₂ was detected at 2300–2400 cm^-1, which indicates the participation of lattice O. Moreover, C₂H₆-TPD was employed to address the chemisorption behavior of C₂H₆ over MnCoO_x. As shown in Supplementary Fig. 24, CO, CO₂, and H₂O as main products were detected due to the reduction of C₂H₆ from lattice O, but with different desorption temperatures. Also, the integrated peak area of produced C-related species followed a decreasing trend of MnCoO_x-0.5 > MnCo₂O₄ > MnO₂, suggesting that more ethane was preserved over MnCoO_x-0.5 catalyst.

Furthermore, the ethane oxidation activity of MnCoO_x-0.5 is compared to MnO₂/MnCo₂O₄, MnCo₂O₄, and MnO₂ references, to identify the catalytic contribution of MnO₂-MnCo₂O₄ interface (Supplementary Fig. 25). Clearly, the areal rate of MnCoO_x-0.5 (1.35 × 10^-2 μmol m^-2 s^-1, 220 °C) is close to that of MnO₂/MnCo₂O₄ (1.14 × 10^-2 μmol m^-2 s^-1, 220 °C), indicating that the high conversion of MnCoO_x-0.5 catalyst may result from the presence of MnO₂-MnCo₂O₄ interface. Noted that the temperature of T50 dramatically reduced to 304 C for the physically mixed MnO₂ and MnCo₂O₄ (referred to as Phy-MnCo₂O₄-MnO₂) catalyst compared to pure MnO₂. A similar performance was obtained on the layer-packed MnCo₂O₄-MnO₂ (refers to as LP_MnCo₂O₄-MnO₂, T₅₀ = 311 °C). However, the catalytic activity of MnCo₂O₄ and MnO₂ mixtures was lower than that of the MnO₂/MnCo₂O₄ model catalyst regardless of their mixing methods, indicating the significant role of interfacial sites due to the proximity between the two components. In this regard, it is imperative to study the correlation of MnO₂-MnCo₂O₄ interface with catalytic properties. Therefore, several control experiments were designed by annealing the MnCoO_x precipitates under N₂ and air, respectively. It was found that the number of MnO₂-MnCo₂O₄ interfacial sites can be altered based on the strong O₂ affinity of Mn, which is similar to the synthesis of core/shell Au/MnO and PtFe-FeO_x/TiO₂ catalysts^25,49. From XPS analysis, we know that there are more high valence Mn species appeared on the surface of the air calcined MnCoO_x-0.2 catalyst compared to the N₂-treated one, as evidenced by the high AOS value and Mn⁴⁺/Mn³⁺ ratio (Supplementary Fig. 26). Hence, it is reasonable to deduce that more Mn species diffuse out onto the Mn_xCo_3-xO₄ spinel surface forming MnO₂ domains due to the strong O₂ driving force and consequently, creating more MnO₂-MnCo₂O₄ interfaces. Also, the C₂H₆-TPD results showed that the MnCoO_x-0.2-Air has a strong C₂H₆ storage capacity compared to that of MnCoO_x-0.2-N₂ (Supplementary Fig. 27). Eventually, a positive correlation was established between ethane conversion rate and Mn⁴⁺/Mn³⁺ ratio, which proved the highly effective of MnO₂-MnCo₂O₄ interface in catalyzing ethane oxidation (Fig. 5c, Supplementary Fig. 28).

Surface mechanism

Density functional theory (DFT) calculations were carried out to further assess the effects of MnO₂-MnCo₂O₄ interface and provide information on how the constructed interface contributes to the catalytic behaviors, especially in terms of the interactions with reactants. Similar to one of our recent work³¹, we constructed the MnCo₂O₄ crystal structure by replacing part of the octahedral Co atoms of cubic Co₃O₄ with Mn. As shown in supplementary Fig. 29a, the Type (II) model was found to be the most stable structure in our calclation by substituting octahedral Co³⁺ with Mn³⁺, as demonstrated by the lowest relative energy per Mn atom in the proposed MnCo₂O₄ models. The obtained lattice parameter of MnCo₂O₄ spinel is enlarged from 8.07 to 8.14 Å, which is consistent with the XRD results. Meanwhile, the bulk MnO₂ models exposed with (111), (110), and (101) facets as well as the MnCo₂O₄ (111) facets (Supplementary Fig. 29b, c) were built to correlate with what we observed from the HRTEM images (Fig. 3). After analyzing the termination stability of MnO₂ and MnCo₂O₄, the optimized interfacial models of MnO₂-MnCo₂O₄ were established by taking MnCo₂O₄-111-A as the underlying substrate and intercepting a structural unit from MnO₂-111-C, MnO₂-110-B, and MnO₂-101-B as the upper cluster (named as MnCo₂O₄/MnO₂-111-C, MnCo₂O₄/MnO₂-110-B, and MnCo₂O₄/MnO₂-101-B, respectively, see details in Supplementary Figs. 30–31). Figure 6a showed the adsorption energy of C₂H₆ and O₂ as well as the oxygen vacancy formation energy (E_Ov) on the bulk MnO₂ and MnCo₂O₄/MnO₂ catalyst models. Taken MnCo₂O₄/MnO₂-111-C as an example, we can clearly see that the adsorption energy of C₂H₆ at the interface of MnCo₂O₄/MnO₂-111-C model (-1.25 eV) is negatively higher than the corresponding bulk MnO₂-111-C (-0.73 eV), indicating the preferential adsorption of C₂H₆ on the former catalyst. Also, the adsorption of O₂ at the interface of MnCo₂O₄/MnO₂-111-C model (−1.01 eV) is negatively less than that of C₂H₆ (-1.25 eV), indicating that O₂ cannot compete with C₂H₆ for the adsorption at MnO₂-MnCo₂O₄ interface (Supplementary Fig. 32a). Similar results were also obtained on other interfacial models (MnCo₂O₄/MnO₂-110-B and MnCo₂O₄/MnO₂-101-B, Supplementary Fig. 32b, c).

Fig. 6: Mechanistic study of ethane oxidation over the MnCoO_x-0.5 catalyst.

a The calculated adsorption energy of C₂H₆, O₂, and the O_v formation energy on the bulk MnO₂−111-C and MnCo₂O₄/MnO₂-111-C catalyst models (Note: The adsorption energy of C₂H₆ was obtained by adsorbing C₂H₆ at the interfacial region of MnCo₂O₄/MnO₂-111-C model; the O₂ adsorption energy was obtained by adsorbing O₂ on the upper MnO₂ cluster of MnCo₂O₄/MnO₂-111-C model). b the calculated differential charge density between O atom in the upper MnO₂ cluster and the interfacial Co atom of MnCo₂O₄/MnO₂-111-C. c the calculated projected density of states (PDOSs) of Co-3d, Mn-3d and O-2p orbital on the MnCo₂O₄/MnO₂-111-C (the Fermi level was set to zero and the isosurface value was set to 0.005 e Å^-3; the cyan and yellow regions represent positive and negative charges, respectively). d energy profiles for the dissociation of the first C-H bond of C₂H₆ over MnCo₂O₄/MnO₂-111-C model (red line: H abstraction of adsorbed C₂H₆ at the interfacial region of MnCo₂O₄/MnO₂ model catalyst; blue line: H abstraction of adsorbed C₂H₆ at the upper MnO₂ cluster of the MnCo₂O₄/MnO₂ model catalyst). e schematic illustration of the reaction mechanism of ethane oxidation over MnCoO_x−0.5 catalyst (① C₂H₆ adsorption; ② Initiated 1^st H abstraction; ③ Continuous H abstraction; ④ CO₂ and H₂O desorption; ⑤ Refilling O_v by O₂). f Energy diagram of the optimal reaction paths for ethane oxidation on MnCo₂O₄/MnO₂-111-C catalyst surface and the optimized structures of all species involved. (Source Data are provided as a Source Data file).

Interestingly, we found that the O₂ molecule is prone to be activated on the topmost MnO₂ domain of the MnCo₂O₄/MnO₂-111-C catalyst as evidenced by the partial electron transfer from MnCo₂O₄ sublayer to MnO₂ via the interfacial Co cations to O anions that located at the adjacent of MnO₂ cluster (1.37 |e |), as shown in Fig. 6b. In addition, the calculated projected density of states (PDOS) shows a upshift of O p-band near Fermi level, indicating a strong interaction between Co 3d and O 2p orbitals. The enhanced C₂H₆ adsorption can also be explained by the strong hybridization between O 2p and Co-3d/Mn-3d orbitals (Fig. 6c). To further confirm this, we carried out a crystal orbital Hamilton population (COHP) calculation to get a quantitative analysis of the interfacial O-Co bond interaction of MnCo₂O₄/MnO₂ interfacial models (Supplementary Fig. 33). The integral values below the Fermi level are -1.74 over MnCo₂O₄/MnO₂-111-C catalyst, which again demonstrated the significant hybridization between O and Co sites. Additionally, we calculated the adsorption energy of C₂H₆ on the xCo/MnO₂ (x = 1–2) model catalysts by varying the Co content on different MnO₂ planes to gain a better understanding of the Co-O-Mn sites and their effects on C₂H₆ activation (Supplementary Fig. 34). Noticeably, the adsorption strength of C₂H₆ increases with increasing the Co substitution contents, indicating a positive effect of Co sites on C₂H₆ adsorption, which aligns with the experimental results (Supplementary Fig. 35). Meanwhile, the lowest oxygen vacancy formation energy (E_Ov = 0.65 eV) was obtained on the uppermost MnO₂ domain of MnCo₂O₄/MnO₂-111-C model compared to other proposed catalyst models, which confers a better O₂ adsorption ability on this catalyst (Supplementary Fig. 32d). Also, the average Mn-O bond length of MnCo₂O₄/MnO₂-111-C (1.94 Å) is larger than that of MnO₂-111 (1.83 Å), which implied a high O mobility on the former model (Supplementary Fig. 36). Therefore, the significant influence from the underlying spinel MnCo₂O₄ was identified.

To understand the underlying mechanism of C₂H₆ oxidation over the MnCoO_x-0.5 catalyst, a detailed discussion of the first C-H bond dissociation of C₂H₆ was carried out on the MnCo₂O₄/MnO₂-111-C model, because this step was typically being regarded as the kinetically relevant step³¹. As shown in Fig. 6d, two reaction pathways were proposed based on the position of the abstracted H, either bind to the O sites of the upper MnO₂ cluster or to the underlying MnCo₂O₄ substrate, eventually forming OH groups. The obtained results showed that the energy barrier (ΔE_TS) of C-H bond cleavage on the O sites of MnO₂ cluster (ΔE_TS: 0.89 eV) is lower than that on the MnCo₂O₄ substrate (ΔE_TS: 1.74 eV), indicating that the former route is kinetically more favorable. Moreover, the formation of OH group from C₂H₆ dissociation on the upper MnO₂ clusters is thermodynamically more favorable by releasing energy of 1.00 eV, whereas the OH group formation on the MnCo₂O₄ substrate is endothermic by 1.05 eV. Therefore, the lattice oxygen species of MnO₂ domain plays a significant role in C₂H₆ oxidation, as evidenced by both experimental and DFT results. Similar trends were also obtained on the other two interfacial models (MnCo₂O₄/MnO₂-110-B and MnCo₂O₄/MnO₂-101-B, Supplementary Fig. 37). Compared to the MnCo₂O₄-111-A model without MnO₂ domain, the C-H bond dissociation barrier (1.27 eV) is higher than that obtained on the MnCo₂O₄/MnO₂-111-C interfacial model, inferring an interfacial engineering of MnO₂-Mn_xCo_3-xO₄ catalyst to boost ethane oxidation. Here, a schematic illustration of the reaction mechanism was proposed and illustrated in Fig. 6e. Subsequently, the energy diagram of elementary steps for ethane oxidation along the reaction pathways was calculated to gain a deeper understanding on the MnO₂-MnCo₂O₄ interfacial system, as illustrated in Fig. 6f. After dissociating the first C-H bond of C₂H₆, the generated ^*CH₃CH₂ species is prone to bond on Co sites that located at the interface of MnO₂ and MnCo₂O₄ substrate (Fig. 6f, b), which aligns with the C₂H₆-TPSR results. Then, the adsorbed ^*CH₃CH₂ changes its adsorption site from interfacial Co to the lattice O^* of upper MnO₂ cluster to form ^*CH₃CH₂O (Fig. 6f, c), which is proved to be thermodynamically favorable by releasing an energy of 2.14 eV. This calculation is in line with our in-situ XPS results, which implies that further dehydrogenation mostly occurs on the upper MnO₂ domains. After that, the produced ^*CH₃CH₂O entities undergo further dehydrogenation, resulting in the formation of ^*CH₃HCOO intermediates (Fig. 6f, d). These intermediates subsequently decompose into^*CH₃O and ^*HCOO by breaking the C-C bonds, releasing an energy of 1.8 eV. Finally, the continuous dehydrogenation of ^*CH₃O and ^*HCOO leads to the formation of ^*CH₂O^*,CHO, CO₂, and H₂O species, showing a downhill energy profile. Overall, DFT results are consistent with the in-situ DRIFT studies (Supplementary Fig. 39) and confirm that the first C-H bond cleavage of C₂H₆ is the rate-determining step in ethane combustion on the MnCo₂O₄/MnO₂ interfacial catalyst, which has a barrier of 0.89 eV. Based on the above analyses, we can reasonably conclude that the simultaneous enhancement on ethane adsorption/activation and lattice O mobility of MnCoO_x-0.5 catalyst is proved to be the main reason of achieving an excellent activity in ethane oxidation, which is ingeniously controlled by interfacial engineering.

Discussion

In summary, we have successfully developed the MnCoO_x catalyst by a facile chemical reduction synthesis method, which shows the highest specific reaction rate in ethane combustion beyond all the reported non-noble metal catalysts, as well as an excellent long-term stability up to 1000 h even under humid conditions. Mn with strong O affinity tends to diffuse out onto the spinel surface forming MnO₂ domains during an O₂-rich environment. The established interaction between MnO₂ and Mn_xCo_3-xO₄ triggers the construction of interfacial sites. Surprisingly, the Co sites on the established hierarchical interface of MnO₂-Mn_xCo_3-xO₄ exhibit a preferential adsorption on ethane; while, the MnO₂ layer displays a strong ability of doing H abstraction on their active lattice O, and further proceed the ethane oxidation through a redox pathway at interfacial regions. Revealing the essential role of interface provides an effective strategy of regulating the coordination environment of involved components as well as their electron transfer ability.

Methods

Materials

Potassium permanganate (VII) (KMnO₄ powder, ≥99.0 %), manganese (II) nitrate tetrahydrate (Mn(NO₃)₂·4H₂O, ≥97%) and cobalt (II) nitrate hexahydrate (Co(NO₃)₂·6H₂O, ≥97%) purchased from Sinopharm, were used as received without further purification.

Catalyst preparation. (1) Synthesis of MnCoO_x

(1) Synthesis of MnCoO_x: MnCoO_x were synthesized by a redox-controlled synthesis method (Mn⁷⁺ + 3Co²⁺ → Mn⁴⁺ + 3Co³⁺). In a typical synthesis process, the Mn (VII) solution was prepared by dissolving certain amounts of KMnO₄ into 1000 mL deionized water under magnetic stirring for 30 min at 70 °C. The Co (II) solution was prepared by dissolving specific amounts of Co(NO₃)₂·6H₂O into aqueous solution with certain amounts of potassium citrate under magnetic stirring. Subsequently, the prepared Co precursor solution was added dropwise into the KMnO₄ solution at a specific injection speed to control the reduction process. After completed the injection, the mixed solution was keeping stirring for another 2 h at 80 °C. Then, the mixed solution was maintained under ambient conditions. After aging for a few hours, the black precipitate was collected by filtration, and washed by deionized water and absolute ethanol three times before drying. After that, the precursor was subjected to an annealing treatment in static air at 350 °C for 2 h at a ramping rate of 1 °C min^-1. Finally, the resulting catalysts were washed by 1 M NH₄NO₃ solution for 2 h at room temperature under stirring to remove K ions prior to the catalytic tests. The obtained catalysts were denoted as MnCoO_x-z, where z represents the nominal molar ratio of Mn/Co. The synthesis parameters of MnCoO_x catalysts are given in Table S10. (2) Synthesis of Co₃O₄: A typical precipitation method was employed to prepare Co₃O₄ reference by adding ammonia (1 mol L^-1) into cobalt nitrate solution. After vigorous stirring for 2 h, the solution was filtered and washed three times by deionized water and ethanol. The obtained precipitate was dried at 70 °C for 12 h and followed by annealing in air at 400 °C for 3 h. (3) Synthesis of MnO₂: MnO₂ nanoparticles (NPs) were synthesized by using KMnO₄ solution as Mn precursor to take K effects into account. The precursor was prepared according to the procedure described elsewhere^[50,51]. Typically, oleic acid (10.0 mL) was added to KMnO₄ solution (0.0126 mol L^-1). After vigorous stirring for 30 min, the emulsion was washed by water and ethanol three times to remove residuals. Then, the product was dried in air at 80 °C overnight before calcinated in air. Finally, the obtained precursor was treated at 200 °C in air for 5 h. (4) Synthesis of MnCo₂O₄ and MnO₂/MnCo₂O₄: MnCo₂O₄ support was synthesized by a conventional precipitation method. Typically, ammonia (1 mol L^-1) was added dropwise to the solution of Mn(NO₃)₂·4H₂O (0.19 mol L^-1) and Co(NO₃)₂·6H₂O (0.38 mol L^-1). After vigorous stirring for 6 h, the mixture was filtered and dried. The obtained powder was calcinated at 350 °C for 4 h. The resulting sample was denoted as MnCo₂O₄. A supported 1%MnO₂/MnCo₂O₄ catalyst was prepared by chemically reducing Mn on the obtained MnCo₂O₄ support. Firstly, 1 g of MnCo₂O₄ support was dispersed in 25 mL deionized water. After that, 7.286×10^-5mol KMnO₄ was added into the MnCo₂O₄ dispersed solution and stirring for 30 min. Followed by this, 1.09 × 10^-4mol Mn(NO₃)₂ was added to reduce KMnO₄. The mixture was stirred for another 30 min before increasing the temperature to 70 °C for 2 h. The obtained precipitate was filtered and washed by water and ethanol three times. The obtained precursor was firstly dried at room temperature for 24 h, and then dried at 70 °C for another 12 h. Eventually, the as-prepared sample was calcined at 350 °C for 2 h.

Characterization

The specific surface area, pore volume, and averaged pore size were determined from N₂ adsorption-desorption isotherms measured at -196 °C using a Micromeritics ASAP 2020 analyzer. All samples were degassed at 200 °C (100 μm Hg) for 6 h. The specific surface area (S_BET) was calculated from the measured N₂ isotherm using the Brunauer-Emmett-Teller (BET) equation applied in a relative pressure range (P/P_o) of 0.01-0.35. The total pore volume (V_total) was obtained from N₂ uptake at a relative pressure of P/P_o = 0.99. The averaged pore size was calculated by 4V_total/S_BET. The elemental analysis was conducted by inductively coupled plasma-optical emission spectroscopy (ICP-OES). An acid digestion was conducted by aqua regia at 100 °C to dissolve all metals.

Powder X-ray diffraction (XRD) patterns were performed on a Rigaku SmartLab 9 kW diffractometer with Cu Kα (λ = 1.5406 Å) radiation operating at 45 kV and 200 mA to determine the bulk structure of the synthesized materials. The scanning speed was set at 10 s/step with 2θ in the range of 10^o to 70^o.

Raman scattering spectra were collected on a DRX Microscope instrument (Thermo Fisher Scientific) with an exciting wavelength of λ_ex. =  532 nm equipped with a charge coupled device (CCD) detector at ambient conditions. The scanning range was set at 100–1000 cm^-1 with resolution of 1.0 cm^-1. The attenuated total reflection Fourier Transform Infrared Spectroscopy (ATR FT-IR) spectrum were collected by Thermo Nicolet iS50 spectrometer from 400 to 4000 cm^-1. The electron paramagnetic resonance (EPR) spectra were obtained on Bruker EPR equipment (model a220-9.5/12) at room temperature by detecting the unpaired electron.

Field emission scanning electron microscopy (FESEM) images were obtained with a Hitachi SU8220 instrument with an acceleration voltage of 5 kV to reveal the morphology of MnCoO_x catalysts. The average particle size was calculated by accounting >100 particles/clusters and then fitting the measured size to a normal distribution. Energy dispersive X-ray (EDX) elemental analyses were used to reveal the elemental composition of MnCoO_x catalysts. The high-resolution transmission electron microscopy (HRTEM) images and electron energy loss spectroscopy (EELS) spectrum images were recorded on Thermis ETEM (thermos Scientific) operated at 300 kV in dual EELS mode with energy resolution of 1.3 eV. A time flight secondary ion mass spectrometer (TOF-SIMS; TESCAN Amber) was used in a dynamic mode to get a depth profile of Mn and Co elements in MnCoO_x sample.

X-ray photoelectron spectroscopy (XPS) was used to analysis the relative abundance and chemical state of the surface components of MnCoO_x catalysts (Thermo Fisher ESCALAB^TM xi⁺). The monochromatic Al Kα was used as the photo source (1486 eV). Binding energies were corrected for surface charging by referring to C 1 s peak at 284.8 eV. For depth profile analysis, Ar⁺ sputtering was performed with an acceleration voltage of 500 eV with an irradiation area of 2 mm×2 mm. Also, the in-situ XPS analysis was conducted to investigate the evolution of MnCoO_x catalyst during ethane oxidation. XPSPEAK41 software was used to conduct peak deconvolution. The experimental peaks were decomposed though mixing Gaussian-Lorentzian functions (80%-20%) after Shirley background subtraction. The relative ratio of each element with different valence states was calculated based on the peak areas.

Both hydrogen temperature-programmed reduction (H₂-TPR) and oxygen temperature-programmed desorption (O₂-TPD) were performed on a Micromeritics AutoChem II 2920 analyzer equipped with a TCD detector. For H₂-TPR measurement, about 100 mg sample was loaded and pretreated in He at 200 °C for 2 h. After cooling down, the analysis was conducted under 10%H₂/Ar flow from 50 to 700 °C with a ramping rate of 10 °C min^-1. Similar to H₂-TPR tests, O₂-TPD was conducted by firstly purging with He flow at 100 ^oC for 30 min to remove moisture and subsequently, switch to 10%O₂/He for 1 h. After that, the temperature was cooling down to room temperature in He flow to remove the physiosorbed O₂ and stabilize the detector baseline. Eventually, the temperature was programmed from 50 to 800 ^oC at a ramping rate of 10 ^oC min^-1 in He flow. Ethane temperature-programmed surface reduction (C₂H₆-TPSR) was carried out in a fixed-bed reactor with a mass spectrometer (MKS-Cirrus3, USA) to investigate the property of MnO₂/MnCo₂O₄ interface. The catalyst was firstly pretreated by 10%O₂/Ar flow (40 mL min^-1) at 300 °C for 1 h. After cooling down, the inlet gas was switched to 10 vol%C₂H₆/He (50 mL min^-1) for 30 min to stabilize the baseline. After that, a temperature-programmed reduction was conducted from 50 to 400 °C at a ramping rate of 10 °C min^-1. The MS signals of C₂H₆ (m/z = 30) and CO₂ (m/z = 44) were recorded accordingly. Ethane temperature-programmed desorption (C₂H₆-TPD) were performed on the same instrument. Typically, about 0.1 g of catalyst was pretreated under 10 vol%O₂/Ar (40 mL min^-1) flow at 300  ^oC for 1 h to remove the surface adsorbed water. After cooling down to 50 °C, the sample was exposed to 10 vol%C₂H₆ for 30 min. Next, the inlet gas was switched to He flushed for another 30 min. After being saturated with C₂H₆, the temperature was subsequently ramped from 50 to 650 °C at a rate of 10 ^oC min^-1 in He flow (50 ml min^-1). The desorbed C₂H₆ (m/z = 30), CO₂ (m/z = 44), CO (m/z = 28), and H₂O (m/z = 18) were monitored by the mass spectrometer. For C₂H₆-O₂-TPSR and C₂H₆-O₂ + H₂O-TPSR experiments, the pretreated catalysts (ca. 0.1 g) were flushed with 10%C₂H₆/He (50 mL min^-1) flow at 50 ^oC for 1 h, followed by He (50 mL min^-1) purging for 30 min. After that, the catalyst was heated from 50 to 400 ^oC under O₂ flow with and without H₂O addition (10 vol% O₂/N₂, 5 vol% H₂O, total flow rate 40 mL min^-1).

CO chemisorption was used to determine the number of active sites (#CO, μmol g^-1) on a Quantachrome ChemBETPulsar analyzer. Samples were purged with He at 100 °C for 30 min to remove moisture, and then reduced by 5%H₂/Ar flow at 250 °C for 1 h. Afterwards, CO titration was performed by using thermal conductivity detector (TCD) as detector. The turnover frequency (TOF) defined as the number of alkane (CH₄, C₂H₆, and C₃H₈) molecules converted per active site per second, was calculated based on the following equation.

$${TOF}=\frac{{F}_{{CnH}2n+2.{inlet}}\cdot {X}_{{CnH}2n+2}}{{{{{{\rm{\#}}}}}}{CO}\cdot M}$$

(1)

where F_{C(n)H(2n+2), inlet} represented the inlet flow rate (mol s^-1) of C_nH_2n+2, M represents the molecular weight (g mol^-1) of alkane.

In-situ diffuse reflectance infrared Fourier transform spectroscopy (in-situ DRIFTs) experiments were performed on a Thermo Nicolet iS50 spectrometer equipped with mercury cadmium telluride (MCT) detector. Prior to each experiment, the catalysts were pretreated at 300 °C in 10%O₂/N₂ (50 ml min^-1) flow for 30 min and quickly cooling down to room temperature. After that, the experiment was performed under 1%C₂H₆/10%O₂/89%N₂ mixture to observe the evolution of reactants/products and intermediates. All the spectra were collected at a resolution of 16 cm^-1 with 32 scans in the temperature range of 50–350 °C. In addition, we examined the adsorption behavior of ethane over MnCoO_x-0.5 catalyst and references at 25 °C using the same setup. To investigate the interfacial property of MnCoO_x catalyst, DRIFT was coupled with MS (DRIFT-MS) to perform ethane oxidation at isotherm conditions without O₂ feed (1%C₂H₆ balanced by He, 250 °C). The MS signals of products were collected as a function of time.

Steady-state isotopic labeling experiments were performed in a fixed-bed reactor. 0.1 g of catalyst was pretreated in air at 300 °C for 1 h with a gas flow rate of 50 mL min^-1 and then flushed by N₂ for 30 min to clean the adsorbed O₂. After cooling down to 50 °C, the mixed gas of 1 vol%¹⁸O₂ and C₂H₆ (1 ml min^-1) balanced by He was introduced with a total gas flow rate of 50 ml min^-1. After the baseline of MS signal was stabilized for 15-20 min, the reactor was heated from 50 to 350 °C at a ramping rate of 10 ^oC min^-1. During this process, the produced oxygen containing products (C¹⁸O₂ (m/z = 48), C^16/18O₂ (m/z = 46), C¹⁶O₂ (m/z = 44), H₂¹⁸O (m/z = 20), H₂¹⁶O (m/z = 18)) were monitored online by mass spectrometer. Transient mechanistic studies: Ethane oxidation was investigated in the temporal analysis of products (TAP) in pulse mode over MnCoO_x and bulk MnO₂. Similar to steady-state isotopic labeling experiments, an oxidation treatment was conducted prior to the pulse experiments¹⁸O₂:C₂H₆ = 1:1 mixture was pulsed in the temperature range of 200–400 ^oC with a stepwise of 50 °C for MnCoO_x and 100 °C for MnO₂.

Catalytic reaction

Low-chain alkane combustion was carried out in a continuous flow packed bed reactor (Φ = 8 mm) to assess the catalytic activities of the MnCoO_x catalysts. 200 mg catalyst (40–60 mesh) was used for each activity test. The temperature was controlled by a K-type thermocouple. The reactant gas contains 3000 ppm C₂H₆ (CH₄ or C₃H₈) balanced by air and N₂ (O₂:N₂ = 11:89) at a flow rate of 200 ml min^-1 (weight hourly space velocity (WHSV) = 60,000 h^-1), accurately controlled by a gas distribution system with electric mass flow controllers (Brooks 5850 TR). The effluent was analyzed on-line by MKS-MultiGas analyzer. The range of test temperature was set at 50 to 400 ^oC. Alkane conversion was calculated by the following equations:

$${X}_{{CnH}2n+2}\left(\%\right)=\frac{\left[{C}_{n}{H}_{2n+2}\right]{in}-\left[{C}_{n}{H}_{2n+2}\right]{out}}{\left[{C}_{n}{H}_{2n+2}\right]{in}}\times 100\%,\,n\ge 1$$

(2)

where [C_nH_2n+2]_in and [C_nH_2n+2]_out represented the inlet and outlet concentration of C_nH_2n+2, respectively; S_co2 stand for CO₂ selectivity; [CO], [CO₂], and [C_nH_2n] represented the concentration of CO, CO₂, and C₂H₄ (or C₃H₆), respectively. The reaction temperatures for 10%, 50%, and 90% conversion of C_nH_2n+2 to CO₂ were assigned to T₁₀, T₅₀, and T₉₀, respectively. Y_CO2 stands for CO₂ yield.

The surface area normalized rate (µmol_C2H6 m^-2 s^-1) was calculated by the following equation:

$${r}_{C2H6=\frac{{X}_{C2H6}\cdot {F}_{C2H6}}{{S}_{{BET}}\cdot {m}_{{cat}.}}}$$

(3)

where X_C2H6 (%) represents the conversion of C₂H₆, F_C2H6 (mol s^-1) is the mole flow rate of C₂H₆, S_BET (m² g^-1) is the surface area of tested materials, m_cat.(g) is the mass of the applied catalyst.

Computational details

All calculations performed in this work were within the framework of Density Functional Theory (DFT) by using the Vienna Ab initio simulation program (VASP) 6.1.0. The projector-augmented wave (PAW) pseudopotentials were used to describe the electron-ion interactions⁵². The generalized gradient approximation with the Perdew-Burke-Ernzerhof functional (GGA-PBE) was used to treat the electron exchange and correlation energy⁵³. Electron smearing was employed via Gaussian smearing method with a smearing width consistent to 0.05 eV. Valence electrons were described by a plane wave basis with an energy cutoff energy of 450 eV. Optimized structures were obtained by minimizing the forces on each atom using the conjugate gradient (CG) algorithm until <0.03 eV/Å. The energy convergence criteria were set to 10^-5eV. A correction for Coulomb and exchange interactions was employed by setting U_eff = 3.5 eV and 3.1 eV (U_eff = coulomb U − exchange J) for Co and Mn atoms, respectively, using the model proposed by Dudarev et al.⁵⁴. The D3 correction method (DFT-D3) was employed in order to include the van der Waals (vdW) interactions⁵⁵.

The formation energy of an oxygen vacancy (E_Ov) was calculated by the following equation:

$${{{{{{\rm{E}}}}}}}_{{{{{{\rm{ov}}}}}}}={{{{{{\rm{E}}}}}}}_{{{{{{\rm{slab}}}}}},{{{{{\rm{Ov}}}}}}}-{{{{{{\rm{E}}}}}}}_{{{{{{\rm{slab}}}}}}}+1/2\,{{{{{{\rm{E}}}}}}}_{{{{{{\rm{O}}}}}}2}$$

(4)

where E_slab,Ov is the energy of the defective MnCo₂O₄ slab surface, E_slab is the energy of the perfect slab surface, and E_O2 is the energy of the gaseous oxygen molecule.

The adsorption energy of oxygen (E_ads,O2) was calculated based on a perfect slab surface by the following equation:

$${{{{{{\rm{E}}}}}}}_{{{{{{\rm{ads}}}}}},{{{{{\rm{O}}}}}}2}={{{{{{\rm{E}}}}}}}_{{{{{{\rm{slab}}}}}},{{{{{\rm{O}}}}}}2}-{{{{{{\rm{E}}}}}}}_{{{{{{\rm{slab}}}}}}}-{{{{{{\rm{E}}}}}}}_{{{{{{\rm{O}}}}}}2}$$

(5)

where E_slab,O2 is the energy of MnCo₂O₄ slab surface covered by the oxygen molecule, E_slab is the energy of the clean slab surface, and E_O2 is the energy of the gaseous oxygen molecule.

The adsorption energy of ethane (E_ads,C2H6) was calculated based on a perfect slab surface by the following equation:

$${{{{{{\rm{E}}}}}}}_{{{{{{\rm{ads}}}}}},{{{{{\rm{C}}}}}}2{{{{{\rm{H}}}}}}6}={{{{{{\rm{E}}}}}}}_{{{{{{\rm{slab}}}}}},{{{{{\rm{C}}}}}}2{{{{{\rm{H}}}}}}6}-{{{{{{\rm{E}}}}}}}_{{{{{{\rm{slab}}}}}}}-{{{{{{\rm{E}}}}}}}_{{{{{{\rm{C}}}}}}2{{{{{\rm{H}}}}}}6}$$

(6)

where E_slab,C2H6 is the energy of MnCo₂O₄ slab surface covered by the ethane molecule, E_slab is the energy of the clean slab surface, and E_C2H6 is the energy of the gaseous ethane molecule.

The calculated equation for the surface energy (Ω) of the three crystal facets can be expressed as follows:

$$\varOmega=1/2A[{G}_{{slab}}-N(O)\mu (O)-N({{{{{\rm{Mn}}}}}})\mu ({{{{{\rm{Mn}}}}}})-N({{{{{\rm{Co}}}}}})\mu ({{{{{\rm{Co}}}}}})]$$

(7)

where G_slab is approximated as the total energy calculated by DFT and A is the surface area of the crystal facet terminations. N(O), N(Mn) and N(Co) are the numbers of O, Mn and Co atoms on the crystal facets, and μ(O), μ(Mn) and μ(Co) are the chemical potentials of O, Mn and Co atoms. Since the chemical potentials of O, Mn and Co are assumed to be in equilibrium with the bulk MnCo₂O₄, they are related through the following expression:

$$\mu ({{{{{\rm{Mn}}}}}}{{{{{{\rm{Co}}}}}}}_{2}{{{{{{\rm{O}}}}}}}_{4})=\mu ({{{{{\rm{Mn}}}}}})+2\mu ({{{{{\rm{Co}}}}}})+4\mu (O)$$

(8)

where μ(MnCo₂O₄) is the chemical potential of the bulk MnCo₂O₄, which is approximated by the total energy of bulk MnCo₂O₄ unitary.