The Zr Modiﬁed γ -Al 2 O 3 Catalysts for Stable Hydrolytic Decomposition of CF 4 at Low Temperature

: CF 4 , one of the Perﬂuorocompounds (PFCs), also known as a greenhouse gas with high global warming potential. In this study, Zr/ γ -Al 2 O 3 catalysts were developed for CF 4 decomposition. The addition of Zr onto γ -Al 2 O 3 achieves a high CF 4 conversion efﬁciency of 85% at 650 ◦ C and maintain its activity for more than 60 h, which is obviously higher than that of bare γ -Al 2 O 3 (50%). The mechanism involved in CF 4 decomposition over the Zr/ γ -Al 2 O 3 are clariﬁed that the surface Lewis acidity sites are the main active center for CF 4 directly adsorbing and decomposing. The results of NH 3 -TPD and FT-IR analyses suggest that the amount of Lewis acidity sites on catalyst surface increases signiﬁcantly after the introduction of Zr, thereby enhancing the activity of catalyst for CF 4 decomposition. The results of XPS analyses conﬁrms the electrons transfer from Zr to Al, which contribute to the increase in Lewis acidity sites. The results of this work will help the development of more effective catalysts for CF 4 decomposition.


Introduction
Perfluorocompounds (PFCs), such as CF 4 , have a high global warming potential (GWP), which is about 6500 times higher than that of CO 2 over a 100-year time scale. Moreover, the lifetime of PFCs can reach up to 50,000 years [1]. CF 4 is an extremely stable molecule because of its symmetry structure. The bond energy of C-F in CF 4 molecule is 543 kJ mol −1 , as known as the strongest bond in organic chemistry [2,3]. The non-ferrous smelting and semiconductor industry are generally regarded as the main sources of PFCs emissions [4]. Therefore, it is of great importance to remove PFCs presented in the exhaust flue gas from non-ferrous smelting and semiconductor industry.
Several technologies have been developed for CF 4 emission control, such as plasma, incineration and catalytic hydrolysis [5][6][7]. Among them the catalytic hydrolysis has been suggested as the most economical choice to decompose CF 4 . Aluminum-based catalysts have displayed excellent performance in CF 4 decomposition due to its high specific surface area and great amount of Lewis acid [8,9]. However, Al 2 O 3 is only active for CF 4 decomposition at high temperature, and will fast lose its activity with the prolonging of reaction time. Recently, various methods have been developed to improve the activity of aluminum-based catalyst. El-Bahy et al. [10] researched the CF 4 decomposition over Ga-Al oxide catalyst. They revealed that pretreatment with sulfuric acid enhanced the Lewis acid sites, and then improved the activity and lifetime of catalyst. The above results suggest that the amount of Lewis acid sites represent the key property of catalysts for C-F activation.
Some mixed oxides exhibit strong surface acidity due to the generation of excess negative or positive charge in the model structure of the binary oxides [11]. Reddy et al. [12] found that Zr-Al had strong acidic, since the exposed Al 3+ and Zr 4+ ions could act as Lewis acid sites. Luo et al. [13] reported an efficient Zr-Si oxide catalyst for the synthesis of furanic diesel precursor, and suggested that the addition of Zr species could enhance the formation of Lewis acid sites. Although there are varied Lewis acid type catalysts can be applied in CF 4 decomposition, but they often require a high reaction temperature (over 700 • C) to decompose the strong C-F bond. Takita et al. [14] researched the catalytic decomposition of CF 4 over metal phosphate-based catalysts including phosphates of Ca, B, Fe, Zn, Bi, and Ni, and found that none of them were active at temperature lower than 700 • C. Xu et al. [15] studied the decomposition of CF 4 over γ-Al 2 O 3 modified by several metal elements, and found that the addition of Zn and Ni could enhance the stability of catalysts, but such catalysts were only effective at high temperature (750 • C).
In view of the above reasons, a serious of Zr/Al 2 O 3 catalysts were developed to decompose CF 4 at lower temperature, i.e., under 650 • C. The aim of this work is to investigate the precise role of Zr/Al 2 O 3 in CF 4 decomposition. The physicochemical properties of catalysts were characterized by different techniques. This study provided in-sights in catalytically decomposing CF 4 over Zr/Al 2 O 3 .

Characterization of Catalysts as Prepared
The elemental composition and distribution of fresh catalysts is measured by SEM-EDS, as Figure 1 and Table 1. The microscopic images of bare γ-Al 2 O 3 ( Figure 1a) exhibits the disorder structure consist of irregular blocky particles with rough surface. All the samples containing Zr (Figure 1b-d) present morphology similar to that of alumina. It can be intuitively understood from the EDS element mapping that the existence and uniform distribution of Al and Zr.  To investigate the crystal structure of the modified and bare γ-Al 2 O 3 , the X-ray diffraction is performed. The results are shown in Figure 2, that all catalysts contain γ-Al 2 O 3 (JCPDS:10-0425) [16,17]. There are four different polytypes of zirconia (i.e., monoclinic (m-ZrO 2 ), tetragonal (t-ZrO 2 ), cubic (c-ZrO 2 ), and orthorhombic (o-ZrO 2 )) depend on the temperature and pressure [18]. As for the prepared Zr/γ-Al 2 O 3 catalysts, the peak of t-ZrO 2 (JCPDS:80-0965) is observed in XRD patterns [19], indicating the formation of zirconium oxide. The BET surface areas of the samples are listed in Table 2. The specific surface area and pore diameter of catalysts increases after the addition of Zr. The (16%) Zr/Al 2 O 3 shows the largest surface area (132 m 2 g −1 ) and pore diameter (4.1 nm). However, the surface area and pore diameter of Zr/γ-Al 2 O 3 decreases when further increase the amount of Zr to 35%. The BET results are consistent with previous reported, that the low content of Zr highly dispersed on the surface of aluminum oxide, the small particles on the surface provided a large surface area [20,21]. However, only a few Zr ions replaced the Al ion sites during the calcination, because the Zr ion is larger than the Al. As the content of Zr further increase, small particles will agglomerate together to form larger clusters and block the original pore, results in the surface area and pore diameter decrease. It can be anticipated that the catalyst was consist of aluminum oxide and zirconium oxide, they crystallized at the interface during the calcination [22].  The NH 3 -TPD experiments are conducted to study the acidic properties of catalysts, as shown in Table 2 and Figure 3. The desorbed ammonia is observed at 50-800 • C, according to the different thermal stability of NH 3 species, three kinds of acid sites are considered depending on the desorption temperature: weak acidic sites (T < 250 • C), moderate acidic sites (250 • C < T < 400 • C), and strong acidic sites (T > 400 • C) acid sites [23]. These indicate the presence of different acidic sites. The results show that the amounts of three kinds of acidic sites increased with the addition of Zr. The (16%) Zr/γ-Al 2 O 3 has the most amounts of three kinds of acidic sites especially the strong and total acidic sites, which are 2 times larger than that of bare γ-Al 2 O 3 . The (9%) Zr/γ-Al 2 O 3 has the second most amounts of weak, strong and total acidic sites. However, the increments of three kinds of acidic sites in (35%) Zr/γ-Al 2 O 3 are very slight. These finding indicates that the addition of Zr mainly increase the amount of strong and total acidic sites, but excess Zr leads to the agglomeration on surface of catalyst, which decreases the surface area and number of acidic sites. It should be noted that, the amounts of acid sites per surface unit have similar trend except the (35%) Zr/γ-Al 2 O 3 , but it can be predicted that the catalytic performance of it is limited by its small specific surface area. The above results suggest that the addition of Zr strongly enhances the surface acidity of aluminum oxide. With the addition of Zr to aluminum oxide, it generates excess negative or positive charge in the binary oxides [24]. The exposed Zr 4+ and Al 3+ can act as Lewis acids, meanwhile, they absorb electrons to generate more Lewis acid centers on the surface of catalysts. Moreover, previous research has reported that the increase of Zr in the Al 2 O 3 catalysts favored the formation of tetraand, mainly, penta-coordinated Al species, which are considered as potential Lewis acid sites [25], therefore, increase the acidity of catalysts. In order to gain more information of acidic properties of catalysts, the FT-IR spectra of pyridine adsorption experiments are conducted, as shown in Figure 4. Spectra are recorded over a frequency range of 1400-1700 cm −1 , where characteristic vibration modes of adsorbed pyridine will appear. The characteristic peaks at 1450 cm −1 , 1491 cm −1 , 1577 cm −1 and 1611 cm −1 can be assigned to the Lewis acid sites, whereas no peak of Bronsted acid sites 1545 cm −1 is observed [15,25]. The modified γ-Al 2 O 3 exhibited more Lewis acid sites than the bare γ-Al 2 O 3 , especially (16%) Zr/γ-Al 2 O 3 , shows the most peak intensity.

Catalytic Performance of CF 4 Decomposition
The conversion reaction of CF 4 over catalysts are conducted at 650 • C for 300 min, as shown in Figure 5. The initial conversion over (16%) Zr/γ-Al 2 O 3 reaches 85%, which is much higher than that over bare γ-Al 2 O 3 (50%). Furthermore, the initial conversion over (9%) Zr/γ-Al 2 O 3 and (36%) Zr/γ-Al 2 O 3 are 64% and 48%, respectively. The bare ZrO 2 exhibits no catalytic activity. Therefore, (16%) Zr/γ-Al 2 O 3 is chosen as the representative sample, and then the effects of temperature on CF 4 conversion over bare γ-Al 2 O 3 and (16%) Zr/γ-Al 2 O 3 are investigated, as shown in Figure 6, the (16%) Zr/γ-Al 2 O 3 catalyst exhibits much better catalytic performance.  The long-term tests are conducted to investigate the stability of catalysts, as shown in Figure 7. The (16%) Zr/γ-Al 2 O 3 catalyst exhibits excellent catalytic activity and stability compares to other works listed in Table 3. With time increasing, the catalytic efficiency of bare γ-Al 2 O 3 decreases rapidly from 30 h (42%) to 60 h (8%) while the efficiency of (16%) Zr/γ-Al 2 O 3 still remains 76%. The large difference between the conversion performances and stabilities in CF 4 decomposition are due to the modification by Zr element, which provides more Lewis acidic sites.  The correction between CF 4 conversion and amounts of different kind of acidic sites is shown in Figure 8. The CF 4 conversion consist with the total and strong acidic sites, since the amount of strong acidic sites dominating over the others, we conclude that the strong acidic sites are the main factor that promotes the activity of the catalysts. Similarly, it has been reported that the Lewis acid sites and total acid sites played a promoting role in the decomposition of hydrofluorocarbons [29][30][31]. Thus, the NH 3 -TPD and FT-IR results imply that the amount of acid sites on the γ-Al 2 O 3 could be increased by modified with species Zr and then improved the activity and stability of catalysts for CF 4 hydrolytic decomposition.

Mechanism Analysis of Hydrolytic Decomposition of CF 4
The close correlation between amount of Lewis acid sites and the conversion of CF 4 strongly suggest the direct participation of the acid sites in CF 4 decomposition, probably a step for subtracting fluorine atom from CF 4 molecule by Lewis acid sites. To understand the mechanisms more detailly, we investigate the physicochemical properties of fresh and used catalysts. The XRD patterns of used catalysts are given in Figure 9. All the used catalysts had the characteristic peaks of α-Al 2 O 3 and AlF 3 (JCPDS:84-1672). The peak intensity of α-Al 2 O 3 decreases with the increasing of Zr addition. Moreover, the AlF 3 peak intensity of used 16% Zr/γ-Al 2 O 3 is the weakest. SEM-EDS analysis was conducted on the used catalysts as the Table 4. The results showed that the modifier Zr could inhibit the formation of AlF 3 on the surface of catalysts, the amount of F on used 16% Zr/Al 2 O 3 catalyst is the lowest in all used catalysts, which showed a good agreement with the stability test. It can be speculated that ZrO 2 on the γ-Al 2 O 3 surface prevent the formation of α-Al 2 O 3 and AlF 3 to some extent during the CF 4 decomposition reaction.  In order to acquire insights into the surface chemical composition of catalysts, X-ray photoelectron spectroscopy (XPS) analyses of catalysts are carried out and shown in Figure 10. The analysis of the Zr 3d core level spectra ( Figure 10A) shows that two major binding energy values are located at range 182.9 to 183.1 eV and 185.2 to 185.5 eV, attributing to Zr 3d 5/2 and Zr 3d 3/2 , respectively. These binding energies are higher than that of the bulk ZrO 2 due to the higher electronegativity of Al in Zr-O-Al bond, an Al-O-bond around Zr is expected to withdraw more electron density than a Zr-O-bond environment, lead to more positively charges on Zr site [32]. This will increase the amount and strength of Lewis acid sites on catalysts, which shows good agreement with the results of NH 3 -TPD and FTIR [33,34]. As showed in Figure 10B, for the fresh catalysts, the Al 2p core level spectra exhibit peaks located at 74.3 eV, which can be ascribed to Al-O bond [35,36]. As showed in Figure 10C, for used catalysts, another set of peaks are located in the range of 75.9 to 76.4 eV, which can be attributed to Al-F bond [35][36][37]. Moreover, all the peak intensities of Al-O bond are remarkably decreased with the appearance of peak ascribed to Al-F bond, except 16% Zr/Al 2 O 3 [35][36][37]. It indicates the formation of AlF 3 by the reaction between Al 2 O 3 and decomposition product HF, but in the case of 16% Zr/Al 2 O 3 catalyst, species Zr alleviate the formation of AlF 3 from Al 2 O 3 .

Catalyst Preparation
The Zr/γ-Al 2 O 3 catalysts were prepared by the homogeneous co-precipitation method. An aqueous solution containing the required amount of zirconium oxychloride (ZrOCl 2 ·8H 2 O) and Aluminum nitrate (Al(NO 3 ) 3 ·9H 2 O) were prepared. The solutions were hydrolyzed with dilute ammonium hydroxide under stirring until the pH of the solutions reached to 9, and then subjected to hydrothermal treatment at 90 • C for 24 h. The resulting precipitate was filtered with deionized water several times and dried for 12 h at 100 • C. Finally, the catalysts were calcined at 800 • C for 4 h in N 2 atmosphere.

Catalytic Activity Test
In the aluminum production, the CF 4 generated in electrolytic cell during the anode effect, the CF 4 content in the flue gas is about 1%. Therefore, we simulate the industrial condition to conduct experiments. The hydrolytic decomposition of CF 4 was carried out in a fixed-bed reactor. The temperature was maintained at 650 • C. The gas flow consists of l% CF 4 , 35% H 2 O, and balance by Ar. The water was pre-heated at 150 • C and then constantly introduced into the reaction system by using a syringe pump. The effluent gas was washed by aqueous potassium hydroxide to remove the produced HF, and then passed through a cold trap to remove water. Finally, the gas was analyzed by an online gas chromatography (GC-9790 II) equipped with a thermal conductivity detector (TCD) detector.

Catalyst Characterization
The morphology of the samples was examined by scanning electron microscope (SEM, JSM-IT300LA, JEOL, Japan) with energy dispersive X-ray (EDX) analysis. The X-ray diffraction analysis was performed on a TTR III diffractometer (XRD, Rigaku, Japan). The chemical composition and state of the elements on catalysts surface were investigated by X-ray photoelectron spectroscopy (XPS, EscaLab 250Xi, Thermo Fisher Scientific, USA). The specific surface areas were measured with the N 2 adsorption method on an ASAP analyzer (BET, ASAP2020, Micromeritics, USA). Ammonia temperature programmed desorption (NH 3 -TPD, AutoChem II 2920, Micromeritics, USA) analyses were performed using the following procedures. A 100 mg sample was pretreated at 550 • C, with helium flow of 30 mL/min for 1h, and then cooled to 50 • C. Ammonia (10% NH 3 /He) was introduced to the catalyst for 1 h at 50 • C. After that, the sample was flushed with helium of 50 mL/min for 1 h to remove absorbed NH 3 , then the temperature was programmed to increase to 900 • C at a rate of 10 • C/min. The amount of ammonia desorbed from the catalyst was detected using TCD. Fourier transform infrared spectra (FT-IR, Nicolet iS50, Thermo Fisher, USA) of pyridine absorption was conducted using the following procedures. The sample was pressed and put into an IR cell, and then it was degassed at 400 • C under vacuum for 2 h to dehydrate. Next, the cell was cooled to room temperature, and the background signal was recorded. After that, pyridine vapor was introduced to the system until reaching adsorption equilibrium. The sample was evacuated out at 150 • C for 30 min follow by cooling down to 50 • C, and then spectral acquisition was performed.

Conclusions
We modified the alumina-based catalysts with Zr to develop an efficient catalyst for CF 4 hydrolytic decomposition at 650 • C. The addition of Zr increase the Lewis acid sites on the surface of catalyst. The effects of modification were confirmed by using NH 3 -TPD and FT-IR of pyridine adsorption methods. We achieved an excellent catalytic CF 4 conversion (85%) over modified catalyst. Furthermore, the decomposition efficiency of modified catalyst maintains at 76% after 60 h reaction. These results indicate that the acidity properties of catalysts are strongly improved by the addition of Zr. We also investigated the physicochemical properties of fresh and used catalysts, the results suggested that addition of Zr could inhibit the formation of AlF 3 .
Author Contributions: All authors contributed to the study conception and design, X.Z. and K.X. developed and designed the methodology of this experiment. X.Z. and F.S. prepared the original draft. H.L. supervised the project and had leadership responsibility for the research. All authors have read and agreed to the published version of the manuscript.

Conflicts of Interest:
The authors declare no competing interests.