The induction period of MXene for the ODH of ethane
In Fig. 1a, we found an interesting phenomenon about the reaction behavior of Ti3C2Tx MXene material for the ODH of ethane, which was expressed with the parameters of C2H6 conversion and product distribution along with the reaction time at 600 °C. In the initial stage of the ODH reaction, the ethane conversion increases and the carbon balance is above 100%, which could result from the oxidation of C-containing species from Ti3C2Tx MXene. This result implies the instability of MXene material in high-temperature oxidation condition that results in the evolution of catalytic performance. Nevertheless, when the reaction lasts over 1 h, the ethane conversion and product selectivity tend to be stable. At that time, the carbon balance realizes near 100%. It can be seen that the color of the catalyst changes from initially black to finally gray (Fig. S1), which may be due to the combustion of C-containing species and the transformation of MXene to TiO2 phase (denoted as M-TiO2) that would be confirmed in the following (Fig. 1d).
Remarkable catalytic performance of the evolved M-TiO2
The resulted M-TiO2 was then tested in the ODH of ethane from 550 to 600 °C. The commercial P25, anatase TiO2 (A-TiO2), and rutile TiO2 (R-TiO2) were also investigated for comparison. As shown in Fig. 1b and S2, M-TiO2 catalyst exhibits obviously higher performance than P25 TiO2, R-TiO2, and A-TiO2. Particularly, the yield of ethylene (Fig. 1b) is 29% on M-TiO2 (41% conversion with 70% selectivity, Fig. S2) at 590 °C, which is above four times higher than that of 6.6% on P25 (12% conversion with 55% selectivity). Besides, the total selectivity to C2H4, CO, and CH4 is more than 96%, along with to the remaining CO2 below 4% on M-TiO2 (Fig. S3). These results show that C2H6 can be also effectively converted to C2H4 over a specific structure TiO2, an oxide that is conventionally recognized as modifier or support in the ODH of ethane.
The intrinsically high performance of M-TiO2 is further demonstrated under different feed gas concentration. As shown in Fig. S4, the productivity of C2H4 increases obviously with the C2H6 concentration at 600 °C, which implies that our catalyst can work well under the practical high reactant concentration. The C2H4 productivity reaches as high as 15.4 gC2H4 gcat−1 h−1 on M-TiO2 (Table S1). This value is much higher than the previously reported catalysts as shown in Fig. 1c, even containing the noble metal and BN catalysts20, 21. It also surpasses the minimum value of 1 gC2H4 gcat−1 h−1 that need for potentially practical application22.
Furthermore, the stability of the resulted M-TiO2 catalyst was investigated for the potential practical application. As shown in Fig. S5, the ethane conversion, the ethylene selectivity and the yield keep rather stable values during 50 h time-on-stream run. These results indicate the good stability of M-TiO2 for the ODH reaction. Moreover, it also suggests the total evolution of MXene precursor to TiO2 phase in this high-temperature oxidative atmosphere.
Catalyst structure with Ti and oxygen vacancy defects on M-TiO2
The initial induction period with carbon balance much higher than 100% implies the structure evolution of MXene during the ODH reaction. The in situ XRD patterns (Fig. 1d) show the diffraction peaks of MXene that are all attributed to Ti3C2Tx phase originally. After the reaction lasting for 20 min, sharp reduced peak strength of MXene while increase of TiO2 related phases are observed, suggesting the appearance of evolved TiO2 species with the combustion of carbon species in MXene. Finally, total formation of TiO2 is presented after a certain run time (1 h) of the ODH reaction. In detail, the resulted M-TiO2 exhibits mixed phases of anatase and rutile TiO2, the composition of which is similar to commercial P25 (Fig. S6). Thermogravimetric analysis (TGA) under air atmosphere with an on-line mass spectrometer (MS) detection shows negligible weight loss on M-TiO2 and no production of CO2 (Fig. S7), confirming no presence of residual MXene on M-TiO2. On the basis of the above observations, the structure evolution of MXene to M-TiO2 during reaction is suggested (Fig. 1e). Various charcterizations and experiments were then performed to identify the special property of M-TiO2 accounting for the outstanding performance.
Scanning electron microscopy (SEM) characterization exhibits that a typical layered structure still remains on M-TiO2 due to the property of MXene precursor (Fig. 2a and 2b).18 In comparison, the commercial P25 (Fig. 2c) shows spherical morphology with smaller particle size. From the HRTEM images (Fig. 2d and 2e), both M-TiO2 and P25 show the lattice fringes of anatase and rutile TiO2, consistent with XRD results. Nevertheless, there are obviously twisted lattice fringes and the disappeared diffraction spots (red circle) on M-TiO2 (Fig. 2d1 and S8), which may originate from the presence of obvious Ti and oxygen vacancy defects on M-TiO2 evolved from the intrinsic MXene material23, 24. Comparatively, this phenomenon is not observed on P25 TiO2, suggesting the existence of lower concentration defects than the layered M-TiO2.
Various characterizations were used to demonstrate the defects in M-TiO2. Raman Spectroscopy (Fig. 3a) shows that both P25 and M-TiO2 have the typical Raman bands of TiO225. However, the peaks on M-TiO2 are broader and shifted to higher wavenumber compared with P25, which was suggested to result from the presence of Ti and oxygen defects.25 The EPR spectra results (Fig. 3b) show the signals with g value of 1.97 and 2.002 on M-TiO2, which can be ascribed to oxygen vacancy and the accompanying Ti3+ cation, respectively26, 27. In contrast, almost no such signals are observed on P25. The electronic states of Ti were then investigated and compared by XPS spectra. As shown in Fig. 3c, the binding energy of Ti 2p3/2 is 458.9 eV on P25, which can be ascribed to Ti4+ species28, while this value is a little lower (458.5 eV) on M-TiO2, suggesting the presence of Ti3+ species29, 30. Besides, an emerging peak located at 531.0 eV appears on M-TiO2 while not on P25 in the O 1s spectra (Fig. 3d), which could be ascribed to OH group at Ti vacancy defect (Tiv-OH)31. These characterizations demonstrate that M-TiO2 has abundant Ti and oxygen vacancy defects, which are supposed to be closely related to the catalytic performance.
Elucidation of structure-function relationship on M-TiO2
The role of Ti and oxygen vacancy defects on M-TiO2 in the ODH process was investigated. As shown in quasi in situ EPR results of Fig. 4a, the signals of Ti3+ and oxygen vacancy are enhanced obviously when M-TiO2 is treated by C2H6. It suggests that these defects can increase the reducibility of lattice oxygen and promote it to react with C2H6. After the further treatment by O2, this signal intensity becomes weak, indicating the facile adsorption of O2 on oxygen vacancy sites, which can complement lattice oxygen. The following treatment of C2H6 results in the recurrence of strong Ti3+ and oxygen vacancy signals. These processes demonstrate the facile cycle of oxygen species on the defects to participate in the ODH reaction. In comparison, the recycled Ti3+ and oxygen vacancy signals are much weaker on P25, Fig. 4b. There is less active lattice oxygen on P25 that is feasible to participate in the ODH reaction. The H2-TPR results also demonstrate the presence of more reducible lattice oxygen on M-TiO2 than on P25. As shown in Fig. S9, the hydrogen consumption on M-TiO2 is 320 μmol gcat−1, which is much higher than that (55 μmol gcat−1) on P25. Therefore, the defects on M-TiO2 can increase the reducibility of lattice oxygen to participate in the reaction while oxygen vacancy and Ti3+ species can promote the activation of O2 to complement the lattice oxygen species.
It should be noted that our M-TiO2 is different from the reduced TiO2 (H-TiO2) which is known for the presence of oxygen vacancy and Ti3+ cation defects27, 32. The H-TiO2 catalyst was obtained by the reduction of P25 with H2 at 600 °C for 6 h. As shown in Fig. 4c, the EPR signals show the oxygen vacancy accompanied by Ti3+ cations on H-TiO2, which are much higher than P25. However, the catalytic performance on H-TiO2 is not improved compared with P25, and still much lower than that on M-TiO2, Table S2. The lower performance can be attributed to the easy disappearance of the only oxygen vacancy and Ti3+ cation on H-TiO2 after ODH reaction at high-temperature, Fig. 4c. In comparison, the MXene precursor is synthesized by the etching of Al layers from Ti3AlC2 MAX phase using hydrofluoric acid (HF). Some of the edge Ti atoms can be etched out during the corrosion, which results in the presence of abundant Ti vacancy defects23, 33. These defects can be reserved under reaction atmosphere during the evolution to M-TiO2 (Fig. 1d), which possess well stability in the ODH of ethane. Therefore, the Ti vacancy defect can not only increase the reducibility of M-TiO2 but also stabilize the defective structure to improve the catalytic performance. Based on the above research, a schematic diagram of structure-function relationship on different catalysts is displayed in Fig. 4d. Conventional TiO2 is not preferred as active component for the ODH of light alkanes due to the limited ability of lattice oxygen to participat in the reaction. Eventhough the H-TiO2 possesses oxygen vacancy and Ti3+ cation defects, these defects will disappear at reaction conditions and do not have the positive effect to improve performance. Only the M-TiO2 evolved from MXene precusor can exhibit outstanding performance as the facile utilization of oxygen species via the stable Ti and oxygen vacancy defects.
For the high-temperature and oxidative atomosphere during the ODH of light alkane, the structure evolution can endow the unanticipated catalytic properties and promote the researchers to discriminate the real active centers under in situ reaction condition34-36. In this work, the structure of MXene is evidently instable under harsh condition in the ODH of ethane. This material is evolved to the TiO2 phase with similar composition and structure as traditional P25 but with more Ti and oxygen vacancy defects. These defects make the conventionally recognized limited-reducibility TiO2 as a remarkable performance catalyst with good activity and stability for the ODH of ethane. To gain further insights on the effects of these defects, density functional theory (DFT) calculations about the activation of ethane were carried out. Two catalyst models with or without the Ti and oxygen vacancy defects on the anatase TiO2(101) surface were constructed, Fig. S10, which can represent P25 and M-TiO2, respectively. It has been reported that the breakage of the first C–H bond of ethane is the rate-determining step in the ODH of ethane11, 37. The lattice oxygen on pure crystal face can capture the H atom from C2H6 to generate C2H5* intermediate and HO* (C2H6 + O à C2H5* + HO*). As shown in Fig. 5a, this reaction has a high energy barrier of 1.21 eV on P25. When there is presence of only oxygen vacancy, this reaction barrier is not decreased but slightly increases (Fig. S11), which agrees well with the catalytic performance of H-TiO2 (Table S2). In contrast, with the presence of both Ti and oxygen vacancy defects (Fig. 5b), the reaction barrier decreases significantly to 0.38 eV, which can promote the activation of ethane and lead to the high reactivity on M-TiO2. The DFT calculations well explain the different reactivty as shown in Fig. 1b. The special TiO2 through the defect engineering can play as an efficient catalyst in the ODH of ethane, which highlights the importance of both Ti and oxygen vacancy defects to enhance catalytic performance.