Structure and surface states
A series of Mn-substituted cobalt oxides (MnCoOx-z with varied Mn/Co ratios (z) of 02.0) were successfully prepared by chemical reduction method. The obtained MnCoOx catalysts present as hierarchical nanospheres with an average diameter of 250-500nm, 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).
a SEM image of MnCoOx-0.5. b TEM image of MnCoOx-0.5 with an insert showing a corresponding electron diffraction (SAED) pattern. c Schematic illustration of the grain boundary of MnCoOx with varied Mn/Co ratio. d Raman spectra. (Yellow shading area: the vibrational bonds of Co species in MnCoOx; 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 MnCoOx catalysts was examined. Figure1d presents the Raman spectra of MnCoOx catalysts. Note that, the Raman spectra of MnCoOx-0.1 is similar to that of Co3O4 reference. While, the main peak of octahedrally coordinated Co sites (CoO6: 670cm-1) gradually shifted to lower wavenumber and merged with the shoulder peak (604cm-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 NixCo3-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) ions32,33,34. The induced coordination environmental change further initiates the occurrence of structural defects and lattice distortion on the developed MnCoOx, which in turn benefits the formation of oxygen vacancies. Besides, the peak position of tetrahedrally coordinated Co (CoO4: 191cm-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 MnCoOx catalysts, suggesting that the Mn ions are highly dispersed and/or exist as solid solution in Co3O4. The bulk structure of MnCoOx was further studied by power X-ray diffraction (XRD) (Supplementary Fig.3, Supplementary Table1). 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 Table2) 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 MnCoOx catalysts. Notably, the MnCoOx presented a high Co2+/Co3+ ratio of 0.4~0.6 compared to Co3O4 (Co2+/Co3+=0.35, Supplementary Table3), 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 MnCoOx catalysts35,36,37. 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 process38,39,40. From Mn 2p3/2 spectra, we can notice that the MnCoOx-0.1 catalyst showed the highest Mn3+/Mn4+ ratio, indicating that more Ov are created to maintain the electrostatic balance of the system (4Mn4++O2-2Mn4++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 MnxCo3-xO4 substrate. The average oxidation state (AOS) of Mn 3s 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 MnxCo3-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 1s spectra were fitted into three peaks, which attributed to lattice oxygen (O), surface adsorbed oxygen (or defects, O), and chemisorbed water (O) with B.E.s of 530.1, 531.3, and 532.7eV, respectively44,45 .The O species account about 7585% over MnCoOx 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 MnCoOx catalysts. It was found that the O2 desorption peak in the range of 300600C (Region II) obviously shifted towards low temperature on MnCoOx 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 Ov on MnCoOx-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 MnCoOx catalysts with Mn/Co ratio of 0.10.5 (Supplementary Fig.5b). Peak(I) appearing at 100200C belongs to the surface adsorbed O39. Noted that the peak (I) accounts for 20% of all the consumed H2 on the MnCoOx-0.1 catalyst, while this value decreased to 10% once more Mn was introduced (Supplementary Tables2 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 MnCoOx. Also, it is noticeable that the reduction peak (III) shifted towards the lower temperature region (355375C) compared to the bulk Co3O4 (387C), perhaps due to the facile H2 transfer from MnO2-MnxCo3-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 Table4). 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 MnCoOx 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 MnCoOx-0.5>MnCoOx-0.2>MnCoOx-0.1>MnCoOx-1.0.
To determine the influence of Mn addition, all the synthesized MnCoOx catalysts were employed for ethane combustion (Fig.2a, Supplementary Table5). Taking the temperature at 50% ethane conversion (T50) as an indicator, we found that the oxidation activities decreased in the order of MnCoOx-0.5 (205C)>MnCoOx-0.2 (215C)>MnCoOx-0.1 (219C)>MnCoOx-1.0 (260C)>MnCoOx-2.0 (282C)>Co3O4 (325C)>MnO2 (348C), 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 MnCoOx 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, Tables67). It is well-known that the initial H abstraction of short-chain alkanes is often regarded as the key elementary step7,46. 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 (1st C-H bond strength: CH4 (465kJmol-1)>C2H6 (442kJmol-1)>C3H8 (427kJmol-1))47. For comparison, the catalytic activity of Co-based oxides for low-chain alkane (C1-C3) combustion were summarized in Supplementary Table8. Clearly, the prepared MnCoOx-0.5 in this work revealed a better catalytic activity than many of the reported catalysts in the literature.
a Light-off curves of the as-prepared catalysts (reaction conditions: ca. 200mg catalyst, [C2H6]=3000 ppm, Q=200mLmin-1 and WHSV=60,000h-1). b corresponding Arrhenius plots. c specific ethane conversion rate at 200C. d a correlation of the fitted peak area of oxygen species from O2-TPD analysis with C2H6 oxidation rate at 200C. e comparation of ethane conversion rate at T50 with other catalysts reported in the literature (see TableS9 for details). f cyclic thermal stability test of MnCoOx0.5 catalyst. g cyclic hydrothermal stability test of MnCoOx-0.5 catalyst (reaction conditions: ca. 200mg catalyst, [C2H6]=3000 ppm, Q=600mLmin-1, WHSV=180,000h-1 w/o and w/ 5vol% H2O, respectively). h long-term scale up stability tests by 1wt% MnCoOx-0.5 coated on micro-monolith substrate for 1000h (reaction conditions: 350C, [C2H6]=1300 ppm, Q=10,000mLmin-1, and GHSV=6000h-1, w/ and w/o 5vol% of H2O). (Source Data are provided as a Source Data file).
To further evaluate the catalytic performance of MnCoOx catalysts, a kinetic study was completed. Figure2b presents the Arrhenius plots of MnCoOx catalysts for ethane combustion based on the normalized reaction rates at ethane conversion in the range of 5-10%. The obtained apparent activation energy (Ea) of MnCoOx is in the range of 80-116kJmol-1, exhibiting a volcano-typed trend with increased Mn content. Also, the calculated Ea is strongly correlated with the reactivity of MnCoOx catalysts. Note that, the Ea value of MnCoOx-0.5 catalyst (Ea=81.83.2kJmol1) is the lowest, indicating an easier oxidation of C2H6. Also, the turn-over frequency (TOF) of MnCoOx-0.5 catalyst for ethane oxidation is 3.9310-2s1 at 200C, which is significantly higher than other MnCoOx samples. A good correlation was built between TOF and the initial H2 consumption rate for MnCoOx catalysts, as shown in Supplementary Fig.7b. These results suggest that the MnCoOx-0.5 catalyst with low Ea (81.83.2kJmol1) and high TOF (3.9310-2s-1) is more effective for ethane oxidation on per site basis. Moreover, the effect of space velocity on catalytic activity of MnCoOx-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 MnCoOx catalysts, the areal rates normalized by the specific surface area (Supplementary Fig.10, Supplementary Table9) of synthesized catalysts (expressed in the unit of mol m-2 s-1) were calculated and plotted in Fig.2c. MnCoOx-0.5 catalyst showed the highest areal rate (6.3 10-3 mol 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 Ea. 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 MnCoOx-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 Table10).
Moreover, the cyclic stability tests were performed both under dry and humid conditions at a relatively high WHSV of 180,000h-1 (Fig.2f, g, Supplementary Fig.11). As shown in Fig.2f, the MnCoOx-0.5 catalyst was able to be completely oxidized at 295C, and showed no attenuation on ethane conversion (Xethane<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 280C for all cycles over MnCoOx-0.5 catalyst (Fig.2g). Also, the activity almost recovered after H2O removal, which suggests the reversible deactivation of MnCoOx-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 MnCoOx-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 350C (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 1000h time-on-stream (TOS) measurement, which demonstrates the superior water-resistance of monolith MnCoOx-0.5 catalyst.
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 MnCoOx catalysts (Fig.3, Supplementary Figs.1315). An enlarged image on these nanosheets yields a periodic lattice fringe of 0.48nm, 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. 45nm for MnCoOx-0.5. The measured lattice spacing is about 0.24, 0.21, and 0.31nm, 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-MnxCo3-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.3eg). Also, the EELS area scanning images give a direct view on the close contact between Co and Mn (Fig.3hk). 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 MnCoOx catalyst. Supplementary Fig.16 presents the depth profile of 55Mn+ and 59Co+ elements, which again confirms the enrichment of Mn on the surface of MnxCo3-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).
a, b HRTEM images of MnCoOx-0.5 catalyst at selected interfacial areas. c HRTEM image of MnCoOx-2.0 catalyst. d schematic illustration of MnCoOx-0.5 at interface. e high-angle annular dark-field (HAADF) images of MnCoOx-0.5 catalyst at MnO2-MnxCo3-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)). hk EELS elemental maps of Mn, Co, and the corresponding Mn-Co overlap of MnCoOx-0.5 catalyst. Scale bar, 5nm; Red: Mn, Green: Co. (Source Data are provided as a Source Data file).
After studying the microstructure of MnCoOx 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 1wt% of Mn loading was synthesized. Firstly, C2H6-TPSR was carried out to study the properties and reactivity of involved O on MnCoOx-0.5 (Fig.4a). Both CO2 (m/z=44) and H2O (m/z=18) were detected in the tested temperature range (50500C). 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 400C (as indicated in the pink box) is assigned to the bulk MnCo2O4 substrate. Besides, a relatively weak CO2 peak appeared at 347C (as indicated in the blue box), suggestive of the existence of a small portion of aggregated MnO2. Comparatively, the C2H6-TPSR result of MnCoOx-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 (OLatt) 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 MnCoOx-0.5 catalyst without gas-phase O2 supply, indicating the participation of lattice O at 250C (Supplementary Fig.19). The transient DRIFT-MS analysis showed that the transition period for O2-depletion follows the trend of MnCoOx-0.5 (t=210s)>MnO2/MnCo2O4 (t=106s)>MnO2 (t=90s) (Supplementary Fig.20), which is in accordance with the isotherm experiments.
a C2H6-TPSR-MS profile. b 18O isotopic labeling experiment in the temperature programmed oxidation of ethane over MnCoOx-0.5 catalyst. ch Temporal analysis of products (TAP) of ethane oxidation over MnCoOx-0.5 as a function of temperature from 200 to 400C (the insert of d represents the TAP analysis of MnO2 reference at 200C, T1 stands for the maximum temperature of generated C16O2, T2 stands for the maximum temperature of produced C16/18O2). (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 65C, indicating the active nature of Olatt on MnCoOx-0.5 (Fig.4b). Noted that, C16O2 doublet peak appeared (158 and 237C), which represent two types of lattice O. As the reaction proceeds, the formation of C16/18O2 occurs (163C) 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 (200C) due to the depletion of surface lattice 16O and the 18O2 replenishment. The obtained isotope results emphasized the effectiveness of lattice O in MnCoOx-0.5. For MnO2 reference, the lattice 16O could also participate in oxidation, but with a higher onset temperature (189C), an indicator of the low activity of Olatt (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 MnCoOx-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 MnCoOx-0.5 and MnO2 reference as a function of temperature (Fig.4ch, Supplementary Fig.22). During each test, a small quantity of reactant mixture (5ml, C2H6+18O2+He) was injected in the temperature range of 200400C 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 C16/18O2, accounts for >95%) significantly exceed that of C18O2 at 200 oC on the MnCoOx-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 MnCoOx-0.5 is significantly higher than that of bulk MnO2 (insert of Fig.4d). At 250C, the surface lattice 16O is quickly consumed as indicated by the increase of C16/18O2 and C18O2. However, once the temperature is above 300C, the amount of 16CO2 slightly increased due to the enhanced bulk phase O migration/diffusion to refill the surface Ov at high temperature. Thereby, we can infer that the replenishment of Ov 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 TableS11) showed that the ratio of Co2+/Co3+ quickly increased from 0.55 (fresh sample at RT) to 0.64 (C2H6 at 200C) with no more change above 200C, 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+/Mn4+ ratio from 0.94 (fresh sample at RT) to 3.18 (C2H6 at 400C) accompanied by the shifting of Mn+ 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+/Mn4+ ratio slightly increased from 3.18 (C2H6 at 400C) to 2.11 (C2H6/O2 at 400C), 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 (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 species gradually consumed during C2H6 reduction from 85.4% (fresh catalyst at RT) to 74.8% (C2H6 at 400C). This observation further confirmed the participation of lattice O species over the MnCoOx-0.5 catalyst during C2H6 oxidation, which is consistent with the isotopic labeling experiments.
a in-situ XPS analysis of MnCoOx-0.5 catalyst under different gas atmospheres. b DRIFT spectrum of ethane adsorption over MnCoOx-0.5. c The correlation between ethane conversion rate (or temperature at constant rate of 0.21 mol gcat1 s1) and Mn4+/Mn3+ ratio (or C2H6 adsorption capacity). (Source Data are provided as a Source Data file).
Next, the adsorption of C2H6 over MnCoOx catalysts was investigated. As shown in the time-resolved DRIFT spectra (Fig.5b), the intensity of ethane adsorption bands (3000cm-1) gradually increased with time-on-stream operation to reach a steady-state level. The MnCoOx-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 MnCoOx-0.5 (11.0min)>MnCo2O4 (7.5min)>MnO2 (4.0min), indicating that more C2H6 are adsorbed/activated over MnCoOx-0.5 catalyst. Again, CO2 was detected at 23002400cm-1, which indicates the participation of lattice O. Moreover, C2H6-TPD was employed to address the chemisorption behavior of C2H6 over MnCoOx. 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 MnCoOx-0.5>MnCo2O4>MnO2, suggesting that more ethane was preserved over MnCoOx-0.5 catalyst.
Furthermore, the ethane oxidation activity of MnCoOx-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 MnCoOx-0.5 (1.35 10-2 mol m-2 s-1, 220C) is close to that of MnO2/MnCo2O4 (1.1410-2 mol m-2 s-1, 220C), indicating that the high conversion of MnCoOx-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=311C). 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 MnCoOx 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-FeOx/TiO2 catalysts25,49. From XPS analysis, we know that there are more high valence Mn species appeared on the surface of the air calcined MnCoOx-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 MnxCo3-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 MnCoOx-0.2-Air has a strong C2H6 storage capacity compared to that of MnCoOx-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).
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, 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.3031). Figure6a showed the adsorption energy of C2H6 and O2 as well as the oxygen vacancy formation energy (EOv) 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 (-1.25eV) is negatively higher than the corresponding bulk MnO2-111-C (-0.73eV), indicating the preferential adsorption of C2H6 on the former catalyst. Also, the adsorption of O2 at the interface of MnCo2O4/MnO2-111-C model (1.01eV) is negatively less than that of C2H6 (-1.25eV), 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).
a The calculated adsorption energy of C2H6, O2, and the Ov formation energy on the bulk MnO2111-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 -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 MnCoOx0.5 catalyst ( C2H6 adsorption; Initiated 1st H abstraction; Continuous H abstraction; CO2 and H2O desorption; Refilling Ov 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=12) 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 (EOv=0.65eV) 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) is larger than that of MnO2-111 (1.83), 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 MnCoOx-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 (ETS) of C-H bond cleavage on the O sites of MnO2 cluster (ETS: 0.89eV) is lower than that on the MnCo2O4 substrate (ETS: 1.74eV), 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 1.00eV, whereas the OH group formation on the MnCo2O4 substrate is endothermic by 1.05eV. 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 (1.27eV) is higher than that obtained on the MnCo2O4/MnO2-111-C interfacial model, inferring an interfacial engineering of MnO2-MnxCo3-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 2.14eV. 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 1.8eV. 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 0.89eV. Based on the above analyses, we can reasonably conclude that the simultaneous enhancement on ethane adsorption/activation and lattice O mobility of MnCoOx-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.
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Redox-induced controllable engineering of MnO2-MnxCo3-xO4 interface to boost catalytic oxidation of ethane - Nature.com
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