Hydrophilic amorphous Cr₂O₃ supported Co species toward efficient hydrogen production from ammonia borane under visible light irradiation

Abstract

Herein, we synthesized a hydrophilic support formed from amorphous Cr₂O₃ and KIT-6 containing P123 using a simple solvent-free thermal method for Co species. The obtained catalyst (denoted as Co/Cr₂O₃-F-X) exhibits high activity for hydrogen production from ammonia borane (NH₃BH₃). The optimal Co/Cr₂O₃-F-0.5 exhibits the highest catalytic performance with the turnover frequency (TOF) value of 111.1 min⁻¹. Advanced characterizations suggest the high catalytic performance is attributed to the synergistic effect of the strong interaction between amorphous Cr₂O₃ and KIT-6 containing P123 and improved wettability of the catalyst.

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1. Introduction

Hydrogen (H₂) as clean energy has attracted much attention owing to zero carbon emissions and high combustion value.^1–3 Ammonia borane (NH₃BH₃) as a promising hydrogen storage material with high hydrogen content (19.6 wt%) can present the solution for the challenges of safe transportation and storage of H₂.^4,5 The hydrolysis of NH₃BH₃ (NH₃BH₃ + 2H₂O → NH₄BO₂ + 3H₂) is considered a promising strategy to generate H₂ due to the mild reaction conditions.^6,7 Supported heterogeneous catalysts play an important role in accelerating the reaction rate.^8–10 Various metal based catalysts including precious and non-precious metals have been employed for hydrolysis of NH₃BH₃.^11–18 However, the high cost of precious metals and low activity of non-precious metals hinder the large-scale application of NH₃BH₃. Many efforts have been made to decrease the dosage of precious metals and enhance the activity of non-precious metal based catalysts. Regulating the electronic structure of active metals is a common strategy to improve the activity of catalysts. Recently, introducing light irradiation into reaction systems, especially non-precious metals working as catalysts, has been proven to be an effective method to boost the charge imbalance distribution and then catalytic performance.^19–22 Nevertheless, the activity of non-precious metal catalyst is still far from satisfaction. As a result, it is urgent to rationally design the structure of non-noble metal catalysts to obtain high activity level.

Earth-abundant Co catalysts have been utilized in NH₃BH₃ hydrolysis.^23–26 For example, Co-Co₃O₄ has been employed as excellent catalyst for H₂ generation from NH₃BH₃ due to electronic structures at the interface and abundant adsorption site.²⁷ Our group have also prepared high performance Co/Cr₂O₃ composed of electron-deficient Co species and mesoporous Cr₂O₃. Cr₂O₃ is selected as support due to several reasons. Firstly, Cr₂O₃ has been widely applied in photoinduced reaction.²⁸ Secondly, Cr₂O₃ with relatively high work function induces electron transfer from Co. Importantly, electron-deficient Co species can decrease H₂O activation and dissociation energy barriers (rate-determining step in NH₃BH₃ hydrolysis) and then accelerate the rate of reaction.²⁹ To date, the design of catalyst has mainly focused on tuning electronic structures of active metal, while neglects catalyst wettability. The appropriate wettability of a catalyst can significantly enhance the adsorption and transfer of reactants, resulting in enhancement activity.³⁰ For example, Zhao group reported the activity of NiFe/NiFe:Pi toward oxygen evolution is significantly enhanced by tuning the wettability of the catalyst.³¹ Xiao group reported the conversion of syngas to methane, dimethyl ether, and olefins can be regulated by controlling the catalyst hydrophobicity.³² Accordingly, regulating catalyst wettability is an effective strategy to further boost the rate of NH₃BH₃ hydrolysis. Introduction of hydrophilic SiO₂ into reaction system can alter H₂O adsorption behaviour.³³ Therefore, introduction of material containing hydrophilic SiO₂ is effective strategy to enhance the catalyst activity.

Herein, we prepared hydrophilic Co-based catalyst Co/Cr₂O₃-F composed with amorphous Cr₂O₃ and functionalized with KIT-6 containing P123 and Co species based our group previous report. The optimal Co/Cr₂O₃-F-0.5 exhibits a high catalytic performance with TOF value of 111.1 min⁻¹. Advanced characterizations reveal the synergistic effect of strong interaction at interface and improved wettability is responsible for the high catalytic performance.

2. Experimental

2.1 Materials

Cobalt chloride hexahydrate (CoCl₂·6H₂O), ammonia borane (NH₃BH₃), deuterium oxide (D₂O), potassium iodide (KI), tetraethyl orthosilicate (C₈H₂₀O₄Si) and sodium borohydride (NaBH₄) were obtained from Aladdin. Chromium(III) nitrate nonahydrate (Cr(NO₃)₃·6H₂O) was obtained from Macklin. Isopropyl alcohol (C₃H₈O) was purchased from Tianjin Yongda ChemicalReagent Co. Ltd, poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) was obtained from Sigma-Aldrich. Potassium dichromate (K₂Cr₂O₇) were obtained from Sinopharm Chemical Reagent Co., Ltd.

2.2 Synthesis and photocatalytic H₂ production

Preparation of Cr₂O₃-F-0.5: KIT-6-F was firstly synthesized according to reported literature with minor modification.³⁴ We didn't calcine the sample to maintain the p123 in pore and on the surface of KIT-6. Next, a 0.5 g KIT-6-F and 0.2 g Cr(NO₃)₃ were ground and transferred into a Teflon-lined stainless steel and maintained at 240 °C for 3.5 h. Meanwhile, we prepared five other samples denoted as Cr₂O₃-F-X (X represents the quality of KIT-6-F). The KIT-6-E was prepared by calcination KIT-6-F at 550 °C for 3 h. The Cr₂O₃-E-0.5 was synthesized by KIT-6-E with same procedure as Cr₂O₃-F-0.5 for comparison.

Preparation of Co/Cr₂O₃-F-0.5 and catalytic H₂ production: 16.0 mg Cr₂O₃-F-0.5 was dispersed in containing 0.034 mmol CoCl₂·6H₂O aqueous solution (0.9 mL) for in reactor 1 h. Next, 0.068 mmol NaBH₄ acted as reduction and 1.71 mmol NH₃BH₃ were dissolved in 1.5 mL H₂O and was injected into the reactor. Meanwhile, the H₂ generated under visible light irradiation and in the dark. Five other samples including Co/Cr₂O₃-F-0.1, Co/Cr₂O₃-F-0.3, Co/Cr₂O₃-E-0.5, Co/KIT-6-F and Co/KIT-6-E were prepared by same method.

2.3 Catalyst characterization

Powder X-ray diffraction (PXRD) on D8 Advance with Cu-Kα radiation was employed to obtain crystalline structures and component of samples. Transmission electron microscopy (TEM, JEM-2100F) were used to observe the morphologies of samples. X-ray photoelectron spectroscopy (XPS) was conducted on a Thermo Scientific K-Alpha with pass energies = 50 eV and the calibrated using the C 1s peak at 284.8 eV was reference. PerkinElmer Lambda 750 was employed to collect UV-vis spectra of sample. Contact angle measurements were carried out on Dataphysics OCA40. TriStar II Plus 3.03 was used to measure the surface area of samples. The transient photocurrent and electrochemical impedance spectroscopy (EIS) were conducted on an electrochemical station (CHI600E) in 0.5 M Na₂SO₄ electrolyte. Fourier transform infrared (FTIR) was performed on FTIR-7600 spectrometer.

3. Results and discussion

3.1 Chemical structure analysis

As described in Fig. 1a, the catalyst Co/Cr₂O₃-F-0.5 was prepared by two-step method. Firstly, Cr₂O₃-F-0.5 was obtained by solvent-free thermal method. TEM image in Fig. 1b, S1a and S1b† shows the compound inherits the morphology of KIT-6. However, no long range ordered structure can be observed from HRTEM image, further confirming the amorphous nature of Cr₂O₃-F-0.5 (Fig. 1c). Moreover, the SAED of Cr₂O₃-F-0.5 (inset in Fig. 1c) demonstrates rings composed of diffraction spots, further verifying the amorphous feature of Cr₂O₃-F-0.5. XRD pattern displays abroad peak indexed to KIT-6,³⁵ and no characteristic peak assigned to Cr₂O₃ was clearly observed suggesting the sample is composed with KIT-6-F and amorphous Cr₂O₃ (Fig. 2). Additionally, samples including KIT-6-F, Cr₂O₃-F-0.1, Cr₂O₃-F-0.3, and Cr₂O₃-F-0.6 were also tested to further confirm the amorphous nature of support (Fig. 2). The result shows the compound sample remains amorphous after adjusting the quality of KIT-6-F. Moreover, a closer look at the abroad peaks in the compound shifts to big larger 2θ angle with increasing KIT-6-F, suggesting the strong electronic interaction between KIT-6-F and Cr₂O₃. Similar phenomenon has been observed in metal alloys due to changed electronic structure.³⁶ Additionally, the image and structure of Cr₂O₃-E-0.5 also be tested for comparison. As shown in Fig. S2,† Cr₂O₃-E-0.5 displays the same morphology as KIT-6 and Cr₂O₃-F-0.5. XRD pattern shows an amorphous nature of Cr₂O₃-E-0.5 which is similar to the Cr₂O₃-F-0.5 (Fig. S3†). Therefore, the introduction of P123 cannot significantly affect the crystallinity of support. After loading Co species, TEM images from Fig. 1d and S4† show Co species are formed on the surface of Cr₂O₃-F-0.5. The long-range order also can't be observed in HRTEM image of Co/Cr₂O₃-F-0.5 in Fig. 1e, indicating the catalyst is composed of amorphous component. The SAED of Co/Cr₂O₃-F-0.5 (inset in Fig. 1e) further confirms the amorphous feature of catalyst. Additionally, the element mapping (Fig. 1f–k) and EDX further the Co species has supported on the surface of Cr₂O₃-F-0.5 (Fig. S5†). No obvious sharp peak assigned to Co is observed in XRD pattern of Co/Cr₂O₃-F-0.5 (Fig. S6†), suggesting the formation of amorphous Co species which is consistent with previous reports.³⁷ Additionally, the broad peak around 45° assigned to Co can further confirm the existence of amorphous Co.³⁸ BET surface area of Cr₂O₃-F-0.5 was calculated to be 246.2512 m² g⁻¹ which is smaller than Co/Cr₂O₃-F-0.5 (296.8048 m² g⁻¹) (Fig. S7 and S8†). The result suggests Co species mainly disperse on the surface of Cr₂O₃-F-0.5 rather than channel of KIT-6 maybe due to the interaction between Co²⁺ and amorphous Cr₂O₃.

Fig. 1 (a) The illustration preparation process of Co/Cr₂O₃-F-0.5, (b and c) TEM images of Cr₂O₃-F-0.5 with different magnifications, (d–k) TEM images of Co/Cr₂O₃-F-0.5 with different magnifications, and the corresponding dark field elemental mappings.

Fig. 2 XRD patterns of samples.

XPS was employed to confirm the component Cr₂O₃-F-0.5 and complete XPS spectra for KIT-6-F for comparison. The full spectra display Si and O signal suggesting the coexisting of two elements in KIT-6-F. In addition to the aforementioned elemental signals, a faint Cr signal is also present in Cr₂O₃-F-0.5 (Fig. S9†), suggesting the compound is composed of Si, O and Cr elements. As shown in Fig. 3, in the high resolution XPS spectrum of Si 2p, a peak located at 103.5 eV assigns to the Si–O–Si band for KIT-6-F and Cr₂O₃-F-0.5.³⁹ Notably, high resolution XPS spectrum of Si 2p in Cr₂O₃-F-0.5 shifts to high binding energy compared with KIT-6-F. Similar phenomenon is observed in O 1s, suggesting the strong interaction between Cr₂O₃ and KIT-6-F (Fig. S10†). Additionally, high resolution XPS spectrum of Si in KIT-6-E and Cr₂O₃-E-0.5 also is collected to the effect of P123 on interaction between Cr₂O₃ and KIT-6. Compared to KIT-6-E, the Si 2p spectra in Cr₂O₃-E-0.5 undergo a shift to lower binding energy (Fig. S11†). This phenomenon is different from Cr₂O₃-F-0.5 due to the absence of P123.

Fig. 3 XPS spectra of Si 2p in Cr₂O₃-F-0.5 and KIT-6-F.

FTIR spectra of KIT-6-F and Cr₂O₃-F-0.5 were collected to investigate the structure of samples and interaction. As shown in Fig. 4, the bands in two samples at around 1635, 1081, 960, 799 and 464 cm⁻¹correspond to absorbed water, symmetric stretching vibrations of Si–O–Si,–Si–OH vibration, asymmetric stretching vibrations of Si–O–Si, and bending vibrations of Si–O–Si, respectively.^40,41 Notably, compared with pristine KIT-6-F, the band at 1 635 960 and 464 cm⁻¹ in compound are red-shifted, indicating the strong interaction between pristine KIT-6-F and Cr₂O₃, which is facilitated to regulate electron density of catalyst and obtain high performance.⁴² Combined with the above characterization result, Cr₂O₃-F-0.5 strong interaction between amorphous Cr₂O₃ and KIT-6-F had been successfully synthesized. Additionally, The IR spectra of KIT-6-F were collected after five months (Fig. S12†). The result shows no obvious change in characteristic peak is observed, suggesting reserved P123 on the surface of KIT-6 does not leach out.

Fig. 4 IR spectra of Cr₂O₃-F-0.5 and KIT-6-F.

UV-vis spectra were conducted to investigate the optical characterization of samples. As shown in Fig. 5, KIT-6-F is not capable of absorbing visible light while after coupling with Cr₂O₃, visible light absorption capacity is significantly improved and exhibits two absorbing peaks which is similar that of Cr₂O₃. Absorption strength is increasing with decreasing content of KIT-6-F because large quantities of KIT-6-F can form the light-shield effect and Cr₂O₃-F-0.3 has the strongest visible light reaction absorption. Notably, Cr₂O₃-F-0.1 doesn't show the best visible light response. This result may be due to too few KIT-6-F not beneficial to the formation of Cr₂O₃. Additionally, compared with Cr₂O₃-F-0.5, the Cr₂O₃-E-0.5 prepared by calcined KIT-6-F displays weak absorption of visible light, indicating the reserved p123 can strengthen the interaction between KIT-6-F and Cr₂O_3.

Fig. 5 UV-vis spectra of samples.

Separation efficiency of photogenerated electrons and holes has important influence in catalytic performance of catalyst. Transient photocurrent density was conducted to test the effect of reserved P123 on photogenerated carrier separation efficiency of compound. Cr₂O₃-E-0.5 also was test for comparison. As shown in Fig. 6a, the photogenerative carrier of the Cr₂O₃-E-0.5 can be effectively separated under visible light irradiation. In contrast, Cr₂O₃-F-0.5 exhibits stronger signal, revealing that Cr₂O₃-F-0.5 is benefit to carrier separation and transfer due to the strong interaction between KIT-6 and Cr₂O₃ with assistance of P123. EIS was performed to confirm the effect of different structure of samples on electron transition. As shown in Fig. 6b, Cr₂O₃-F-0.5 shows a slightly smaller arcradius, suggesting relatively small resistance to electron transport.

Fig. 6 (a) Profiles of time versus transient photocurrent density of two supports, (b) EIS Nyquist plots of samples.

To investigate the effect of reserved P123 on wettability of support, contact angle measurements were conducted for Cr₂O₃-F-0.5, Cr₂O₃-E-0.5, Co/Cr₂O₃-F-0.5 and Co/Cr₂O₃-E-0.5. Generally speaking, the lower the contact angle is liable to better H₂O adsorption. As illustrated in Fig. 7, the Cr₂O₃-E-0.5 shows the water-droplet contact angle of 17.9° while contact angles of water droplet on the surface of the Cr₂O₃-F-0.5 decrease values with 11.2°, confirming reserved P123 can enhance the wettability of support and favour the charge transfer and accelerate the rate of NH₃BH₃ hydrolysis. The contact angles of water droplet on surface of Co/Cr₂O₃-E-0.5 (28.1°) is larger than Cr₂O₃-E-0.5. Conversely, the contact angles of water droplet on surface of Co/Cr₂O₃-F-0.5 (6.1°) is smaller than Cr₂O₃-F-0.5. The result suggests water can easily spread on the surface of Co/Cr₂O₃-F-0.5, leading to an increased H₂ evolution rate.

Fig. 7 Water contact angle measurements for (a) Cr₂O₃-F-0.5, (b) Cr₂O₃-E-0.5, (c) Co/Cr₂O₃-F-0.5 and (d) Co/Cr₂O₃-E-0.5.

3.2 Photocatalytic performance

The photocatalytic activity of Co/Cr₂O₃-F-0.5 was tested in a reactor under visible light irradiation. Additionally, control experiments including Co/KIT-6-F, Co/Cr₂O₃-F-0.1, Co/Cr₂O₃-F-0.3, Co/Cr₂O₃-F-0.5, Co/Cr₂O₃-F-0.6, Co/Cr₂O₃-E-0.5 and Co/KIT-6-E were also conducted to reveal the origin of activity of catalyst. As shown in Fig. 8, both Co/KIT-6-F and Co/KIT-6-E exhibits similar activity in dark at same condition (Fig. 8a and b). Importantly, the activity of two catalysts is further boosted under visible irradiation (Fig. 8c and d). The enhanced catalytic performance is mainly attributed to the electronic structure changed induced by interband electron transfer of Co. In sharp contrast, the activity of Co/Cr₂O₃-F-0.5 exhibits greatly enhanced under visible irradiation than that of Co/KIT-6-F, while slightly boosted in dark. The TOF value of Co/Cr₂O₃-F-0.5 is 111.1 min⁻¹ which is much higher than that of some tested non-precious metal based catalysts (Table 1). The enhanced performance is primarily attributed to the following aspects: firstly, introduction of Cr₂O₃ with high visible light utilization ratio induces the generation of photogenerative carriers to provide a greater drive for reaction progression. Secondly, the electron-deficiency degree of Co nanosheets is improved under visible irradiation, resulting in greatly decreased H₂O activation and dissociation energy barrier.²⁹ Thirdly, the increased NH₃BH₃ hydrolysis rate over Co/Cr₂O₃-E-0.5 under visible irradiation further conform the effect of Cr₂O₃ on activity of catalyst. Notably, the activity of Co/Cr₂O₃-E-0.5 with TOF 95.7 min⁻¹ is lower than Co/Cr₂O₃-F-0.5. Combined with the UV-vis, EIS, transient photocurrent density and water contact angle measurements experimental result, the reserved P123 on surface of KIT-6 not only construct strong interaction among multiple interfaces but also improve catalyst wettability, which is benefit to adsorption of reactant molecules and carrier transfer and accelerate the NH₃BH₃ hydrolysis rate. Additionally, we added a control experiment over Co supported on Cr₂O₃-F-0.5-M (physically mixed KIT-6-F and Cr₂O₃) to provide evidence for the effect of strong interaction between KIT-6-F and Cr₂O₃ on Co-based catalytic activity. As expected, the performance of Co/Cr₂O₃-F-0.5-M with TOF of 76.9 min⁻¹ is greatly reduced in comparison to Co/Cr₂O₃-F-0.5 (Fig. S13†). Therefore, the synergistic effect of high visible light utilization, strong interaction between KIT-6-F and Cr₂O₃ and improved wettability leads to a significant enhanced catalyst performance under visible irradiation.

Fig. 8 Plots of time versus volume of H₂ evolution from NH₃BH₃ over Co-based catalysts (a) in dark, (b) corresponding TOF value (c) under visible light irradiation, (d) corresponding TOF value.

Table 1 Activities of catalysts in the H₂ evolution from NH₃BH₃

Catalyst	TOF (min^−1)	References	Temperature (K)	Irradiation
Co/Cr2O3-F-0.3	101.4	This work	298	With
Co/Cr2O3-F-0.5	111.1	This work	298	With
Co/Cr2O3-F-0.6	103.4	This work	298	With
Co/Cr2O3	106.8	29	298	With
NiCo-NC	35.2	43	298	Without
Co3O4–SnO2	17.6	44	298	Without
Ni/Ni2P	68.3	4	298	Without
Co@N-C-700	5.6	23	298	With
NiCu/CNS	30.6	22	298	With
NiCu/SiO2	25.3	45	298	Without
Cu0.5Co0.5O-rGO	81.7	24	room	Without
Cu/MIL-101	28.22	46	298	With

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In order to reveal the influence of interface effect on catalyst performance, the control experiments were performed based on Co/Cr₂O₃-F-0.1, Co/Cr₂O₃-F-0.3, Co/Cr₂O₃-F-0.5, Co/Cr₂O₃-F-0.6. It can be clearly observed that the introduction of amorphous Cr₂O₃ in support can slightly boost the activity of Co-based catalyst in dark (Fig. 8c). However, the activity of catalyst containing KIT-6-F can significantly improve under visible irradiation. Moreover, the activity enhances with increased KIT-6-F mass and the catalyst Co/Cr₂O₃-F-0.5 exhibits highest performance. The activity decreases when the mass of KIT-6-F in support is continued to increase due to light-shield effect.

Photogenerated carriers can not only tune the electronic structure of catalyst but also induce the generation of radical intermediates such as hydroxyl radicals (˙OH), which have significantly effect on the performance of catalyst in light induced reaction. Therefore, we investigated the influence of ˙OH to the performance of Co/Cr₂O₃-F-0.5 using trapping experiment and EPR characterization. In capture experiment, electrons, holes and ˙OH was captured by K₂Cr₂O₇ (100 μM), KI(100 μM) and 2-propanol (IPA) (100 μL), respectively.⁴⁷ The result was shown in Fig. 9a and b, the adding of sacrificial agents in reaction system can decrease the hydrolysis rate of NH₃BH₃, suggesting the photogenerated carriers and radical intermediates can boost the activity of Co/Cr₂O₃-F-0.5. It is worth noting that the effect of photogenerated carriers and radical intermediates on enhanced activity is different based on different catalyst. ESR measurements was conducted to confirm the existing of ˙OH and superoxide anions (˙O₂⁻) originated from the reaction of dissolved O₂ and photogenerated electron was also measured which can improve the rate of NH₃BH₃ hydrolysis.⁴⁶ As shown in Fig. 9c, no characteristic signal of ˙OH is observed in the dark based on Cr₂O₃-F-0.5, but the signal with 1 : 2 : 2 : 1 strength appears under irradiation. The same phenomenon is observed in ˙O₂⁻ detection (Fig. 9d). These results further conform the existing of ˙OH and ˙O₂⁻ under light irradiation.

Fig. 9 (a) and (b) Role of hydroxyl radicals and superoxide anions in NH₃BH₃ hydrolysis underlight irradiation. (c) and (d) DMPO ESR spin-labeling for ˙O₂⁻ and ˙OH.

The cleavage of O–H bond in H₂O molecules is the rate-determining step in NH₃BH₃ hydrolysis.^48,49 To further confirm the conclusion, the kinetic isotope effect (KIE: KH/KD) is conducted. As shown in Fig. 10 the H₂ evolution rates greatly decreases using D₂O to replace H₂O both light irradiation and in dark. The value of KIE is 2.89 in darkness and is 2.13 under light irradiation. This result indicates the cleavage of O–H bond is the rate-determining step over Co/Cr₂O₃-F-0.5 toward the hydrolysis of NH₃BH_3.

Fig. 10 Plots of time versus volume of hydrogen evolution from NH₃BH₃ in H₂O or D₂O over Co/Cr₂O₃-F-0.5 under visible light irradiation and in dark.

According to characterization, experimental results and previous report, we propose a possible mechanism for NH₃BH₃ hydrolysis induced by visible light. Firstly, Co species and Cr₂O₃-F-0.5 absorbs visible light. Secondly, photoinduced electrons and holes are generated and transfer to the surface of Co species and Cr₂O₃-F-0.5. Meanwhile, photoinduced electrons transfer from active species to support then ˙OH and ˙O₂⁻ are formed in this process. Thirdly, the B–H bond in NH₃BH₃ and O–H bond in H₂O are broken and H₂ is generated. ˙OH and ˙O₂⁻ can accelerate the breaking of two kinds of bonds. Notably, the strong interaction among multicomponent and enhanced wettability can favor the transition of photoinduced carriers, ˙OH and ˙O₂⁻ and then improve the production rate of H₂.

4. Conclusions

We synthesized a hydrophilic support by simple solvent-free thermal method. The compound works as amazing support for Co species. The optimal Co/Cr₂O₃-F-0.5 exhibits a highest catalytic performance for hydrogen production from NH₃BH₃ with TOF value of 111.1 min⁻¹. XRD, IR and XPS results indicate strong interaction between KIT-6-F and Cr₂O₃-F. Contact angle measurement verifies Cr₂O₃-F-0.5 has improved wettability. The synergistic effect of strong interaction and improved wettability induces high catalytic performance of Co/Cr₂O₃-F-0.5.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (52161038), Young Talents of Science and Technology in Universities of Inner Mongolia Autonomous Region (NJYT23033), Natural Science Foundation of Inner Mongolia Autonomous Region (2022MS05012), and the Talent Development Fund Project of Inner Mongolia Autonomous Region ([2022]-110).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3ra07367j

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