Promoting C–F bond activation via proton donor for CF₄ decomposition

Significance

Tetrafluoromethane (CF₄), also known as carbon tetrafluoride, is a permanent potent greenhouse gas. It finds extensive use in semiconductor manufacturing and is the main by-product released during the smelting of electrolytic aluminum and rare earth metals. Due to the lack of effective treatment means, the concentration of CF₄ in the atmosphere has been increasing yearly. While thermal catalytic technology can decompose CF₄, the high reaction temperature and low activity restrict its practical applications. Here, we developed a novel and effective strategy for CF₄ decomposition, achieving efficient decomposition at a record low reaction temperature. Our findings hold significant promise in the context of global warming, offering practical and impactful solutions to combat the detrimental effects of greenhouse gases.

Abstract

Tetrafluoromethane (CF₄), the simplest perfluorocarbons, is a permanently potent greenhouse gas due to its powerful infrared radiation adsorption capacity. The highly symmetric and robust C–F bond structure makes its activation a great challenge. Herein, we presented an innovated approach that efficiently activates C–F bond utilizing protonated sulfate (–HSO₄) modified Al₂O₃@ZrO₂ (S-Al₂O₃@ZrO₂) catalyst, resulting in highly efficient CF₄ decomposition. By combining in situ infrared spectroscopy tests and density function theory simulations, we demonstrate that the introduced –HSO₄ proton donor has a stronger interaction on the C–F bond than the hydroxyl (–OH) proton donor, which can effectively stretch the C–F bond for its activation. Consequently, the obtained S-Al₂O₃@ZrO₂ catalyst achieved a stable 100% CF₄ decomposition at a record low temperature of 580 °C with a turnover frequency value of ~8.3 times higher than the Al₂O₃@ZrO₂ catalyst without –HSO₄ modification, outperforming the previously reported results. This work paves a new way for achieving efficient C–F bond activation to decompose CF₄ at a low temperature.

Global warming has emerged as one of the most pressing concerns in the 21st century (1–3). Perfluorocarbons (PFCs) have alarmed widespread attention due to their potent greenhouse effects (4–7). Notably, the Carbon Border Adjustment Mechanism issued by European Union has listed PFCs among the greenhouse gases to be accounted for (8). Among them, tetrafluoromethane (CF₄), being the simplest and maximum concentrated PFCs in the atmosphere, possesses a remarkably high global warming potential of 7,390 and extraordinarily long atmospheric lifetime of 50,000 y (9–14). Considering the doubling of its atmospheric concentration since the preindustrial era (15), the development of a cost-effective CF₄ decomposition method becomes crucially important and highly desirable for achieving a sustainable future.

Among various CF₄ decomposition methods, the thermocatalytic hydrolysis process stands out due to its improved decomposition rate, no toxic by-products, and large-scale applications potential (16–21). While the highly symmetrical single carbon structure and C–F bond with strong ionic character make CF₄ decomposition require a high temperature (22), the introduction of the catalyst can significantly promote the C–F bond activation and decrease the decomposition temperature. Takita et al. (16) first proposed that CF₄ could be hydrolyzed over Ce10%-AlPO₄ at 700 °C, well below its pyrolysis of 1,200 °C (23). El-Bahy et al. (19) found that the introduction of Ga promoted C–F bond activation by increasing Lewis (L) acid sites on γ-Al₂O₃ surface, achieving a 84% CF₄ decomposition at 630 °C. Takashi et al. (24) reported that the Zn-modified γ-Al₂O₃ catalyst with high density of strong L acid sites could completely hydrolyze CF₄ at 650 °C. Our previous study (21) revealed the relationship between the L acid strength of γ-Al₂O₃ surface site and its CF₄ decomposition ability. We found that the tricoordination Al (Al_III) sites with the strong L acidity were the main active site for CF₄ decomposition.

Despite the aforementioned progress in reducing temperature, the CF₄ hydrolysis at beyond 600 °C still falls short of the desired efficiency for energy utilization. In addition to metal L acid sites (25, 26), Glusker et al. (27) discovered that the proton donor was also capable of interacting with C–F bond to activate it. For example, the hydroxyl (–OH) groups could interact with C–F bond to promote fluorochemical decomposition (28–30). The interaction strength with C–F bond was affected by the type of the proton donor (27, 31–33). Thus, it could be predicted that the C–F bond of CF₄ could be further activated by modulating the proton donor on the catalyst surface.

Herein, we achieve stable CF₄ decomposition with an efficiency of 100% operating at 580 °C, under a significantly reduced temperature. Through density function theory (DFT) simulations, we unveil that the introduced protonated sulfate (–HSO₄) strongly stretches the C–F bond of adsorbed CF₄ and facilitates its activation, which is experimentally verified by in situ infrared spectroscopy (in situ IR) tests. To capitalize on this insight, we design and fabricate a unique catalyst, composed of sulfated Al₂O₃ dispersed on ZrO₂ nanosheet (S-Al₂O₃@ZrO₂). A combination of X-ray photoelectron spectroscopy (XPS) and pyridine-infrared (py-IR) tests confirm the introduction of –HSO₄ proton donors on the S-Al₂O₃@ZrO₂ surface, facilitating a record low temperature for CF₄ decomposition. This work provides a new strategy for efficient C–F bond activation and CF₄ decomposition at low temperature, opening new avenues for sustainable catalysis with environmental benefits and promising energy efficiency.

Results and Discussion

Synthesis and Characterization.

S-Al₂O₃@ZrO₂ was prepared by modifying Al₂O₃@ZrO₂ with sulfuric acid, and calcined at 650 °C for 24 h to remove the excess sulfate species (SI Appendix, Fig. S1 and see Materials and Methods for details). X-ray diffraction (XRD) patterns of S-Al₂O₃@ZrO₂ showed only the characteristic peaks of tetragonal-ZrO₂ (PDF #79-1767), which was the same as that of unmodified Al₂O₃@ZrO₂ (Fig. 1A), revealing that sulfuric acid modification did not change the crystal structure of the catalysts. Brunauer–Emmett–Teller (BET) measurements (SI Appendix, Fig. S3 and Table S1) showed that the surface area slightly decreased from 61.3 m² g⁻¹ (Al₂O₃@ZrO₂) to 49.8 m² g⁻¹ (S-Al2O₃@ZrO₂), further confirming no significant changes in its structure properties after sulfuric acid modification.

Fig. 1.

(A) XRD patterns of Al₂O₃@ZrO₂ and S-Al₂O₃@ZrO₂ catalysts. (B) TEM and (C) EDS mapping images of S-Al₂O₃@ZrO₂ catalyst. XPS spectra of (D) S 2p, (E) Al 2p, and (F) Zr 3d for Al₂O₃@ZrO₂ and S-Al₂O₃@ZrO₂ catalysts.

To detect the morphologies of the synthesized catalysts, transmission electron microscopy (TEM) was performed. The pristine ZrO₂ had a nanoparticle morphology with a size of ~25 nm (SI Appendix, Fig. S4A). Al₂O₃@ZrO₂ showed an identical nanoparticle morphology (SI Appendix, Fig. S4C) as the pristine ZrO₂, and its high-resolution TEM (HRTEM) image revealed the presence of Al₂O₃ (SI Appendix, Fig. S5D), indicating that Al₂O₃ was uniformly dispersed on ZrO₂ surface. After sulfuric acid modification, the morphology of S-Al₂O₃@ZrO₂ had no obvious change (Fig. 1B) compared with that of Al₂O₃@ZrO₂. In contrast, S-Al₂O₃ showed a significant agglomeration after sulfuric acid modification (SI Appendix, Fig. S4D), indicating that ZrO₂ ensured the uniform dispersion of Al₂O₃ during sulfuric acid modification process. Energy-dispersive spectroscopy (EDS) mapping images of S-Al₂O₃@ZrO₂ (Fig. 1C) further proved the uniform distributions of S and Al on ZrO₂ surface.

To investigate the surface chemical environments of the S, O, Al, and Zr in S-Al₂O₃@ZrO₂, XPS was carried out. The O 1s spectra (SI Appendix, Fig. S7) validated the presence of sulfate species on S-Al₂O₃@ZrO₂ after sulfuric acid modification. The sulfate species were further analyzed by S 2p spectra. The peaks at 169.0 eV and 170.3 eV of S-Al₂O₃@ZrO₂ could be attributed to –SO₄ and –HSO₄ (Fig. 1D), respectively (34), indicating the partial sulfate protonation on the catalyst surface. Meanwhile, the binding sites of –HSO₄ on S-Al₂O₃@ZrO₂ surface were investigated by Al 2p and Zr 3d spectra (Fig. 1 E and F). The Al–O peaks (35) of S-Al₂O₃@ZrO₂ were shifted to higher energy region compared to that of Al₂O₃@ZrO₂, which could be attributed to the electron-attracting effect of sulfate group to the surface Al site. However, no observable change could be found in Zr 3d region after sulfuric acid modification (Fig. 1F and SI Appendix, Fig. S10) (36), revealing the sulfate group was not bonded with Zr site. The coordination environment of the Zr sites in S-Al₂O₃@ZrO₂ was precisely characterized by synchrotron X-ray absorption spectroscopy (SI Appendix, Fig. S11A) to further determine the binding sites of –HSO₄. The fitting result of Zr k-edge extended X-ray absorption fine structure spectrum only found three scattering paths of Zr–O, Zr–Zr, and Zr–Al (SI Appendix, Fig. S11B and Table S3), demonstrating that the introduced –HSO₄ proton donor was mainly bonded with Al site.

Properties of –HSO₄ Proton Donor.

To determine the properties of –HSO₄ proton donor, the surface acidity of Al₂O₃@ZrO₂ and S-Al₂O₃@ZrO₂ was investigated. First, the acid content and strength of the catalysts were tested by NH₃ temperature-programmed desorption (NH₃-TPD) (Fig. 2A and SI Appendix, Table S4). Al₂O₃@ZrO₂ showed three NH₃ desorption peaks, in which two desorption peaks below 200 °C were attributed to weak acid site, and one desorption peak between 200 °C and 300 °C was assigned to medium acid site (37). The desorption peak intensity and desorption temperature of S-Al₂O₃@ZrO₂ were obviously increased compared with those of Al₂O₃@ZrO₂, indicating a significant increase in acid content and strength via sulfuric acid modification. In particular, not only the medium acid peak of S-Al₂O₃@ZrO₂ was shifted to 300 °C and 400 °C but also the medium acid amount increased from 3.63 μmol g⁻¹ (Al₂O₃@ZrO₂) to 12.11 μmol g⁻¹ (S-Al₂O₃@ZrO₂), demonstrating the S-Al₂O₃@ZrO₂ surface possessed more and stronger acid sites compared to that on Al₂O₃@ZrO₂. The increased strong acid sites were further analyzed by py-IR tests at different desorption temperature (Fig. 2 B–D). The observed bands at 1,444 cm⁻¹ and 1,544 cm⁻¹ can be assigned to pyridine adsorbed at L acid sites and Brønsted (B) acid sites, respectively (38). The results showed that Al₂O₃@ZrO₂ contained only L acid sites, and the B acid sites were introduced after sulfuric acid modification, which could be attributed to –HSO₄ (39, 40). The strong L acid sites were detected on both Al₂O₃@ZrO₂ and S-Al₂O₃@ZrO₂ catalysts upon desorption at 100 °C (Fig. 2C). Combined with the γ-Al₂O₃ (110) facet exposure observed in HRTEM results (SI Appendix, Fig. S5 D and E), it could be determined that the detected strong L acid sites were Al_III sites with strong CF₄ adsorption. With the desorption temperature rising to 200 °C (Fig. 2D), the band corresponding to L acid sites on Al₂O₃@ZrO₂ and S-Al₂O₃@ZrO₂ disappeared, and the band corresponding to B acid sites was observed only on S-Al₂O₃@ZrO₂, indicating the B acid sites in S-Al₂O₃@ZrO₂ had strong acidity. This result confirmed that the introduced –HSO₄ proton donor has a strong proton donating ability.

Fig. 2.

(A) NH₃-TPD profiles of the Al₂O₃@ZrO₂ and S-Al₂O₃@ZrO₂ catalysts. Py-IR spectra of the Al₂O₃@ZrO₂ and S-Al₂O₃@ZrO₂ at (B) 50 °C, (C) 100 °C, and (D) 200 °C desorption temperature.

Influence of –HSO₄ Proton Donor on C–F Bond Activation.

To investigate the influence of –HSO₄ proton donor on C–F bond activation, the in situ IR spectra of S-Al₂O₃@ZrO₂ at 580 °C were tested under CF₄ atmosphere (Fig. 3A). The bands of CF₄ decomposition product HF (3,741, 3,785, 3,832, 3,877, and 3,920 cm⁻¹) (41) were detected, indicating that the C–F bond could be directly activation on S-Al₂O₃@ZrO₂ surface. Meanwhile, the depletion of –HSO₄ (1,026 cm⁻¹) (42) and the production of –SO₄ (998, 1,072, and 1,142 cm⁻¹) (36, 43) revealed that the C–F bond was activated by –HSO₄ and generated –SO₄ and HF.

Fig. 3.

In situ IR spectra of S-Al₂O₃@ZrO₂ at 580 °C (A) with CF₄, or (B) with CF₄ and H₂O, and intermediate switch to with H₂O. (C) The calculation model of γ-Al₂O₃, γ-Al₂O₃–OH, and γ-Al₂O₃–HSO₄. (D) CF₄ adsorption energies and C–F bond length of γ-Al₂O₃, γ-Al₂O₃–OH, and γ-Al₂O₃–HSO₄ models. (E) CF₄-TPD profiles of the Al₂O₃@ZrO₂ and S-Al₂O₃@ZrO₂. (F) Schematic illustration of –HSO₄ proton donor promoting the C–F bond activation for CF₄ decomposition.

To determine the C–F bond activation with –HSO₄ during CF₄ hydrolysis, the in situ IR spectra of S-Al₂O₃@ZrO₂ at 580 °C were tested under CF₄ and H₂O atmosphere and only H₂O atmosphere, respectively (Fig. 3B). When feeding with CF₄ and H₂O simultaneously, the production of HF and –SO₄ as well as the depletion of –HSO₄ was observed, which was similar to the case with CF₄ only, indicating –HSO₄ directly involved in C–F bond activation with or without H₂O. After switching to H₂O only (Fig. 3B), it could be found that the –SO₄ bands gradually disappeared, while the regeneration of –HSO₄ was observed. The results demonstrated that the stable C–F bond activation could be achieved with –HSO₄ during CF₄ hydrolysis, and –HSO₄ could be regenerated by H₂O dissociation over S-Al₂O₃@ZrO₂.

The effect of proton donor on C–F bond activation was further investigated by DFT calculations. On the basis of γ-Al₂O₃ (110) facet containing Al_III site (SI Appendix, Fig. S17), the CF₄ adsorption energy at Al_III site (E_ads) and C–F bond length were calculated on three different models of γ-Al₂O₃, γ-Al₂O₃ with –OH (γ-Al₂O₃–OH), and γ-Al₂O₃ with –HSO₄ (γ-Al₂O₃–HSO₄), respectively (Fig. 3C). The calculation results showed that the E_ads of γ-Al₂O₃–HSO₄ was −0.64 eV, which was much stronger than those of γ-Al₂O₃–OH (−0.51 eV) and γ-Al₂O₃ (−0.44 eV) (Fig. 3D), indicating that the CF₄ adsorption was significantly enhanced by introducing –HSO₄. The CF₄-temperature programed desorption (CF₄-TPD) results of Al₂O₃@ZrO₂ and S-Al₂O₃@ZrO₂ (Fig. 3E) also proved this result, as the CF₄ desorption temperature of S-Al₂O₃@ZrO₂ was significantly increased after sulfuric acid modification. On the other hand, the C–F bond length on γ-Al₂O₃–HSO₄ increased from 1.36 Å to 1.45 Å, which was larger than that on γ-Al₂O₃ (1.42 Å) and γ-Al₂O₃–OH (1.43 Å) (Fig. 3D). The enhanced adsorption and molecular deformation of CF₄ proved that –HSO₄ can promote the C–F bond activation. Based on these results, a synergistic mechanism for C–F bond activation was proposed (Fig. 3F). The Al_III site stably securely CF₄ molecule, while the adjacent –HSO₄ proton donor generates the H···C–F interaction with the adsorbed CF₄. The synergistic stretching effect of Al_III-HSO₄ pair sites on CF₄ promotes the C–F bond activation.

CF₄ Hydrolysis Performance.

In order to determine the promotion of –HSO₄ for C–F bond activation, the CF₄ hydrolysis performances were tested under 440 to 660 °C (Fig. 4A and SI Appendix, Fig. S20). The S-Al₂O₃@ZrO₂ catalyst with abundant –HSO₄ showed excellent CF₄ hydrolysis activity and could stably achieve 100% CF₄ decomposition at the temperature (T₁₀₀) of 580 °C (Fig. 4B), which was much lower than that of Al₂O₃@ZrO₂ (660 °C) and other previously reported catalysts (SI Appendix, Table S5), proving the great promotion of C–F bond activation by the introduction of –HSO₄ proton donor. Similarly, the CF₄ hydrolysis test results on Al₂O₃ (640 °C) and S-Al₂O₃ (620 °C) also demonstrated this promoted effect. The S-Al₂O₃@ZrO₂ catalyst after stability test was then characterized (SI Appendix, Figs. S21−S24 and Tables S7 and S8). It could be found that the surface –HSO₄ proton donor content remained almost unchanged. The key effect of –HSO₄ in promoting C–F bond activation was further demonstrated.

Fig. 4.

(A) Plots of CF₄ decomposition versus temperature over ZrO₂, Al₂O₃, S-Al₂O₃, Al₂O₃@ZrO₂, and S-Al₂O₃@ZrO₂ catalysts. (B) The stability test of S-Al₂O₃@ZrO₂ catalyst at 580 °C. (C) The Arrhenius plots and (D) reaction rate of Al₂O₃, S-Al₂O₃, Al₂O₃@ZrO_2, and S-Al₂O₃@ZrO₂ catalysts for CF₄ decomposition. Reaction condition: 2,500 ppm of CF₄ and 10% of H₂O balanced with Ar, total flow rate of 33.3 mL min⁻¹, and weight hourly space velocity (WHSV) of 1,000 mL g⁻¹ h⁻¹.

Combined with the above DFT simulations and previous report (21), the Al_III site is the main active site for CF₄ decomposition. The turnover frequency (TOF) values at 500 °C were calculated according to the content of the surface Al_III site determined by the results of the surface acidity test (SI Appendix, Table S4). The results demonstrated the significantly enhanced of the catalyst intrinsic activity after introducing –HSO₄ proton donor, and the S-Al₂O₃@ZrO₂ catalyst presented the highest TOF of 3.91 × 10⁻³ s⁻¹, which was ~8.3 times as that of Al₂O₃@ZrO₂ (0.47 × 10⁻³ s⁻¹). Further, the activation energies (E_a) of CF₄ over different catalysts (Fig. 4C) was evaluated by the CF₄ hydrolysis reaction rates (Fig. 4D). The results showed that the E_a of both S-Al₂O₃ and S-Al₂O₃@ZrO₂ were significantly reduced compared with those before sulfuric acid modification, demonstrating the activation of CF₄ molecules by –HSO₄. Especially, the S-Al₂O₃@ZrO₂ had the lowest E_a of 86.5 kJ mol⁻¹, corresponding to its efficient CF₄ decomposition.

Conclusion

In summary, a strategy to activate the C–F bond for effective CF₄ decomposition has been proposed by enhancing the interaction between –HSO₄ proton donor and C–F bond. The S-Al₂O₃@ZrO₂ catalyst with abundant –HSO₄ exhibited an excellent CF₄ hydrolysis activity, in which CF₄ was completely decomposed at a record low temperature of 580 °C with a TOF value ~8.3 times that of the Al₂O₃@ZrO₂ catalyst with –HSO₄. It was demonstrated that the enhanced interaction between the introduced –HSO₄ proton donor and the C–F bond was decisive to enhance CF₄ hydrolysis activity, verified by in situ IR test and DFT simulation. –HSO₄ had a stronger interaction with C–F bond than that of –OH could effectively activate the C–F bond, thus promoting CF₄ decomposition. This proposed strategy paves a way for the development of efficient C–F bond activation and CF₄ decomposition catalyst. As global warming continues to be a pressing concern, our findings open new avenues for practical and impactful solutions to combat the detrimental effects of greenhouse gases.

Materials and Methods

Materials.

All chemicals were obtained commercially and used as received. Zirconium oxychloride octahydrate (ZrOCl₂⋅8H₂O, 99%) and aluminum nitrate nonahydrate (Al(NO₃)₃·9H₂O, 99%) were purchased from Aladdin. Aqueous ammonia (NH₃·H₂O) and sulfuric acid (H₂SO₄) were purchased from Sinopharm.

Preparation of Al₂O₃@ZrO₂.

Al₂O₃@ZrO₂ was synthesized using the hydrothermal method. First, 30 mM (9.7 g) ZrOCl₂·8H₂O and 3.6 mM (1.4 g) Al(NO₃)₃·9H₂O were dissolved in 100 mL of deionized water by stirring at room temperature, and NH₃·H₂O was added till pH~10. Then, the above solution was transferred into a 150-mL Teflon-lined stainless-steel autoclave and kept at 150 °C for 24 h. After cooling to room temperature, the precipitate was washed with deionized water, dried under 60 °C overnight, and calcined in air at 600 °C (a heating rate of 3 °C min⁻¹) for 5 h. The pristine Al₂O₃ and ZrO₂ samples were obtained by the same method with only adding aluminum source and zirconium source, respectively.

Preparation of S-Al₂O₃@ZrO₂.

First, 2.0 g Al₂O₃@ZrO₂ and 2.0 g Al₂O₃ samples were added to 10 mL of 1 M H₂SO₄ and dried overnight at 60 °C, and then calcined in air at 650 °C (a heating rate of 3 °C min⁻¹) for 24 h to obtain S-Al₂O₃@ZrO₂ and S-Al₂O₃, respectively.

Catalyst Characterization.

XRD patterns were obtained by using a STADIP automated transmission diffractometer, operated at 36 kV and 20 mA by using CuKa1 radiation. The XRD patterns were scanned in the 2θ range of 15 to 90°.

The TEM and EDS images were obtained by JEOL 3010 operated at 200 kV. The finely ground sample was dispersed in ethanol and then dropped onto a copper grid for TEM and EDX testing.

XPS was recorded on a Kratos Axis Ultra DLD X-ray photoelectron spectrometry, using a standard Al Ka X-ray source and an analyzer pass energy of 40 eV. All binding energies were referenced to the adventitious C 1s line at 284.6 eV.

The BET surface area and pore size distribution of the catalysts were determined by N₂ adsorption-desorption analysis using AUTOSORB IQ. Prior to measurements, the samples were degassed at 300 °C for 6 h, at a heating rate of 10 °C min⁻¹.

NH₃-TPD and CF₄-TPD were performed by using a PCA-1200 on a chemisorption analyzer equipped with a thermal conductivity detector (TCD). The chemisorption analyzer was carried out on the PCA-1200 from Beijing Builder electronic technology Co., Ltd. For each experiment, the weighed sample (100 mg) was pretreated at 600 °C (10 °C min⁻¹) for 2 h under Ar (30 mL min⁻¹) and cooled to room temperature. Then, the NH₃ gas (30 mL min⁻¹) or 20% CF₄/Ar gas (30 mL min⁻¹) was introduced instead of Ar at this temperature for 1 h to ensure the saturation adsorption. The sample was then purged with Ar for 1 h (30 mL min⁻¹) until the signal returned to the baseline as monitored by a TCD. The desorption curve of NH₃ or CF₄ was acquired by heating the sample from room temperature to 800 °C (10 °C min⁻¹) under Ar with the flow rate of 30 mL min⁻¹.

Py-IR spectra of samples were analyzed by a Thermo IS-50 Fourier Transform infrared (FTIR) spectrometer. The sample was heated at 500 °C for 5 h and cooled to room temperature. Then, vacuumized to 10⁻³ Torr, samples were exposed to pyridine vapour (3,000 Pa) at 100 °C for 1 h, followed by reevacuation for 1 h, and lower the temperature to take out our samples. After this step, the sample was analyzed by FTIR.

In situ IR spectra of the sample were also analyzed by a Thermo IS-50 FTIR spectrometer. Self-supported wafer was prepared from catalyst powder (ca. 10 mg). The wafer was loaded into an in situ IR thermal catalytic cell with CaF₂ windows and pretreated under Ar flow at 600 °C for 2 h. Then regulated to the target temperature to obtain a background spectrum which should be deducted from the sample spectra. As for the transient reactions between 1) CF₄ and water vapor and 2) water vapor, after the background spectra at appointed temperatures under Ar flow were obtained, the catalysts were exposed to 1) 1 mL min⁻¹ 20% CF₄/Ar + water vapor (50 mL Ar passing through water bottle) or 2) water vapor (50 mL Ar passing through water bottle) at 580 °C and meanwhile the reaction process was recorded as a function of time. For the adsorption of CF₄ studies, after the same pretreatment, the catalysts were exposed to a flow of 20% CF₄/Ar at 100 °C for 2 h. The desorption process then went on under a flow of Ar, and the temperature was gradually raised to 700 °C (a step of 50 °C) and recorded as a function of temperature.

Catalytic Activity Evaluation and Analytical Methods.

CF₄ hydrolysis was measured by using a continuous flow reaction system with a quartz fixed-bed reactor (i.d. 20 mm) under atmospheric pressure in a temperature range from 440 to 660 °C. A gas flow of 33.3 mL min⁻¹ (0.25% CF₄ in Ar) controlled by a mass flow controller, together with 0.2 mL h⁻¹ of water pumped by an injection pump, were passed over 2.0 g of catalyst. According to the following equations, the CF₄ decomposition is calculated:

$${\mathrm{C}\mathrm{F}}_4 \mathrm{d}\mathrm{e}\mathrm{c}\mathrm{o}\mathrm{m}\mathrm{p}\mathrm{o}\mathrm{s}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} (\%)=\frac{{\left[{\mathrm{C}\mathrm{F}}_4\right]}_{\mathrm{i}\mathrm{n}}-{\left[{\mathrm{C}\mathrm{F}}_4\right]}_{\mathrm{o}\mathrm{u}\mathrm{t}}}{{\left[{\mathrm{C}\mathrm{F}}_4\right]}_{\mathrm{i}\mathrm{n}}}\times 100\%,$$

where [CF₄]_in and [CF₄]_out indicate the input and output relative gas concentrations, respectively.

Theoretical Calculation Studies.

All our investigations in this study were based on density functional theory, as implemented in the Vienna ab initio simulation package (44, 45). The exchange-correlation potential is treated with the Perdew–Burke–Ernzerhof formula by using the projected augmented wave method within the generalized gradient approximation (46). The cut-off energy for all calculations was set to be 450 eV. All the positions of atoms are fully relaxed until the Hellmann–Feynman forces on each atom are less than 0.01 eV Å⁻¹. Meanwhile, a k-points Γ-centered mesh is generated for Brillouin zone samples. The DFT-D₃ method proposed by Grimme was adopted to describe the van der Waals interactions, which has been shown to accurately describe chemisorption and physisorption properties on layered material. In addition, a vacuum region of about 15 Å was used to decouple the periodic replicas.

Supplementary Material

Appendix 01 (PDF)

Data, Materials, and Software Availability

All study data are included in the article and/or SI Appendix.