Enhanced Catalytic Effect of Ti2CTx-MXene on Thermal Decomposition Behavior of Ammonium Perchlorate

Transition metal carbonitrides (MXenes) are promising catalysts due to their special structures. Recently, many studies have shown that MXenes have a catalytic effect on the thermal decomposition of ammonium perchlorate (AP). However, the catalytic effects have not been extensively investigated. Therefore, it is important to illustrate the catalytic mechanisms of pure MXene in AP thermal decomposition. Herein, the catalytic properties of Ti2CTx for ammonium perchlorate (AP) thermal decomposition were investigated by numerous catalytic experiments. The results showed that the high-temperature decomposition (HTD) decreased by 83 °C, and the decomposition heat of AP mixed with Ti2CTx increased by 1897.3 J/g. Moreover, the mass spectrum (MS) data showed that the NH3, H2O, O2, N2O, NO, HCl, and NO2 were formed. In addition, according to the X-ray diffraction (XRD), Raman spectrum, high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), and X-ray photoelectron spectra (XPS) results, the Ti2CTx nanosheets can adsorb the gaseous products and react with them in-situ, generating anatase-TiO2 and carbon layers. The Ti2CTx, as-resulted anatase-TiO2, and carbon can synergize and further catalyze the thermal decomposition of AP when both electron and proton transfers are accelerated during AP decomposition.


Introduction
MXenes, a new group of two-dimensional (2D) early transition metal carbonitrides, are synthesized from the etching of the A layers from M n+1 AX n phases. In this formula, M stands for an early transition metal; A stands for A-group elements; X stands for carbonitrides; and n is 1, 2, or 3 [1,2]. Notably, with the termination of O, OH, and/or F groups on the surface of MXene, M n+1 X n T x becomes more accurate to describe the MXene, where T x stands for the surface terminations [2]. Due to its good electrical conductivity, large inner surface area, abundant surface terminations, and graphene-like morphology, MXene has been increasingly investigated as a potential material for sensors [3], lithium-ion batteries [4,5], supercapacitors [6,7], metal adsorption materials [8], catalysts [9,10], etc.
Ammonium perchlorate (NH 4 ClO 4 , AP) is one of the important oxidative and energetic materials in solid rocket propellants, which provides a powerful thrust to rockets by releasing a large amount of heat and gases [11]. Therefore, to improve the performance of AP thermal decomposition, various catalysts including metals [12], metal oxides [13,14], carbon or carbon-based materials [15,16], and so on, have been exploited for many decades.
Recently, due to their large surface area, special 2D layered structure, and excellent electrical conductivity, MXenes also have been used directly as catalysts or ideal substrates to support nanoparticles in boosting the thermal decomposition or burning of AP [17][18][19][20][21]. For instance, by high-temperature heating, Gao and his co-workers prepared MXene-Cu 2 O composites [18]. Afterward, the as-resulted hybrids were found to greatly influence the thermal

Materials and Methods
The preparation of Ti 2 CT x powders has been reported in detail in our previous work [32]. Briefly, the precursor phase Ti 2 AlC was exposed to aqueous HF (40 wt.%) for 2.5 h at room temperature; then, the as-prepared suspension was cleaned by DI water several times; at last, purified samples were freeze-dried to obtain Ti 2 CT x flakes. The AP particles were brought from Guoyao and preheated to 60 • C for 24 h in a vacuum oven to remove moisture. Then, a series of weight ratios of Ti 2 CT x (0, 5 wt.%, 10 wt.%, 20 wt.%, 30 wt.%) were added into AP, accordingly, and ground in an agate mortar for 15 min.
The catalytic effect of the prepared samples on the thermal decomposition of AP was studied by using an STA449F TG-DSC at heating rates of 5, 10, 20 • C/min, and between 40 and 500 • C. The purge gas used was 99.995% pure Argon. The overall purge flow rate was maintained at 20 mL/min. Simultaneously, a Netzsch (Aeolos QMS403, Selb, Germany) mass spectrometer was used to study the decomposition products.

Thermal Decomposition of Pure AP
The morphologies and structures of AP and Ti 2 CT x were characterized by SEM and XRD, as shown in Figure 1. Figure 1a shows that the AP particles were relatively coarse with various sizes, and in Figure 1b, all diffraction peaks were ascribed to AP, confirming its purity. Figure 1c shows the typical 2D stacked multilayered structure of Ti 2 CT x . Moreover, the XRD pattern in Figure 1d shows that the diffraction peaks were located at around 9 • , which also confirmed the formation of Ti 2 CT x [33]. PHI 5000 VersaProbe, Ulvac Phi, Japan, Al-Kα radiation), and Raman spectroscopy (JY HR800, JY, France, λ = 514 nm). The catalytic effect of the prepared samples on the thermal decomposition of AP wa studied by using an STA449F TG-DSC at heating rates of 5, 10, 20 °C /min, and between 4 and 500 °C . The purge gas used was 99.995% pure Argon. The overall purge flow rate wa maintained at 20 mL/min. Simultaneously, a Netzsch (Aeolos QMS403, Selb, Germany mass spectrometer was used to study the decomposition products.

Thermal Decomposition of Pure AP
The morphologies and structures of AP and Ti2CTx were characterized by SEM an XRD, as shown in Figure 1. Figure 1a shows that the AP particles were relatively coars with various sizes, and in Figure 1b, all diffraction peaks were ascribed to AP, confirmin its purity. Figure 1c shows the typical 2D stacked multilayered structure of Ti2CTx. More over, the XRD pattern in Figure 1d shows that the diffraction peaks were located at aroun 9°, which also confirmed the formation of Ti2CTx [33].  Figure 1e shows the TG and DSC curves of pure AP, which reveal that the thermal decomposition of pure AP occurred in three stages. In the first stage, the endothermic peak appeared at 242 • C without weight loss, which is due to the transition from orthorhombic to cubic form [34]. In the second stage (named low-temperature decomposition, LTD), the exothermic peak at 311 • C was assigned to the partial decomposition of AP with 17.5 wt.% weight loss, and some intermediates were formed by dissociation and sublimation. The third peak (high-temperature decomposition, HTD) appeared at a relatively higher temperature of 447 • C, indicating the complete decomposition of the intermediate to volatile products.
For describing the release of volatiles, MS was used to follow the representative mass fragments of the substances produced by AP. As shown in Figure 1f , and NO 2 + (m/z = 46) along with temperatures were presented with 10 • C min −1 by MS. The curves above show that the main decomposition products in LTD were NH 3 , H 2 O, O 2, and N 2 O, indicating that AP was first decomposed into NH 3 and HClO 4 , and HClO 4 was further decomposed into oxidizing substances, and then these oxidizing substances reacted with NH 3 , forming H 2 O, O 2, and N 2 O. Compared with the LTD stage, the ion current intensity in the HTD stage was higher, suggesting that the decomposition of AP was mainly concentrated in this stage. The broad peak type of each ion flow indicates that the decomposition reaction of pure AP proceeded slowly, in which the NH 3 , H 2 O, O 2 , N 2 O, NO, HCl, and NO 2 were mainly examined. In addition, the formation temperature of HCl complied with that of H 2 O, but later than other products, which illustrates that HCl is a secondary product. Figure 2 shows that the catalytic performance of various ratios' addition of Ti 2 CT x on thermal decomposition of AP was tested by DSC and TG. Figure 2a shows the DSC curves of AP in the presence of 0, 5 wt.%, 10 wt.%, 20 wt.%, and 30 wt.% Ti 2 CT x . As shown, the results indicated that Ti 2 CT x did not affect the crystal transformation. The changes in peak decomposition temperature on DSC curves and initial decomposition temperature on TG curves (in Figure 2b) show the positively catalytic role of Ti 2 CT x in the thermal decomposition of AP. With increasing Ti 2 CT x content, the LTD peak rarely changed, but two exothermic peaks appeared in the HTD stage, and both exothermic peaks moved towards the temperature reduction direction, and finally became a sharp exothermic peak when the additional amount of Ti 2 CT x was up to 30 wt.%. The HTD temperatures of AP-x wt.% Ti 2 CT x (x = 5, 10, 20, and 30) mixtures were 435, 415, 385, and 359 • C, respectively, and decreased by 12, 32, 63, and 88 • C compared with pure AP (447 • C), respectively. In addition, the TG curves show that the addition of Ti 2 CT x affected the initial decomposition temperature of AP, decreasing it by 15 • C when adding 5~20 wt.% Ti 2 CT x and 35 • C when adding 30 wt.% Ti 2 CT x , respectively. These results suggest that Ti 2 CT x has a catalytic effect on the decomposition of both LTD and HTD.

The Catalytic Activity of Ti 2 CT x
In Figure 2c, the released energies of the mixtures were 1103, 1332, 2087, and 2527 J/g corresponding to AP-x wt.% Ti 2 CT x (x = 5, 10, 20, 30) mixtures, enhanced by 473.3, 702.3, 1457.3, and 1897.3 J/g compared to 629.7 J/g of pure AP. However, it is obvious that the growth rates of released energies were not always positive, which is probably due to the excessive addition of Ti 2 CT x -MXene. In addition, as shown in Table 1, MXene-based composites exhibited better catalytic activity when the mixture ratio of the catalyst was low.   Figure 3 demonstrates the MS results of AP in the presence of Ti2CTx (0, 10, and 30 wt.%). AP was completely decomposed at lower temperatures and shorter times. In addition, the results show that the intensity of the ion currents of NO, NO2, H2O, and HCl of AP mixed with Ti2CTx was lower than that of pure AP, suggesting that the layered Ti2CTx easily absorbed the gaseous products. Notably, the intensity of the ion currents of O2 and N2O of Ap-10 wt.% Ti2CTx was higher than that of pure AP, which indicates that the addition of Ti2CTx promoted the formation of the two gases. However, when 30 wt.% Ti2CTx was added, the change trend of the MS results of O2 and N2O complied with that of other ions. This also complies with the previous report [22,24] that Ti2CTx with special structures can adsorb gases and be easily oxidized by oxidizing gases. Therefore, it can be inferred that Ti2CTx could absorb O2 and N2O and react with the two gases. Usually, this phenomenon occurs in the thermal decomposition process of AP catalyzed by transition metals, and the generated transition metal oxides will synergistically catalyze the thermal decomposition of AP [37].   Figure 3 demonstrates the MS results of AP in the presence of Ti 2 CT x (0, 10, and 30 wt.%). AP was completely decomposed at lower temperatures and shorter times. In addition, the results show that the intensity of the ion currents of NO, NO 2 , H 2 O, and HCl of AP mixed with Ti 2 CT x was lower than that of pure AP, suggesting that the layered Ti 2 CT x easily absorbed the gaseous products. Notably, the intensity of the ion currents of O 2 and N 2 O of Ap-10 wt.% Ti 2 CT x was higher than that of pure AP, which indicates that the addition of Ti 2 CT x promoted the formation of the two gases. However, when 30 wt.% Ti 2 CT x was added, the change trend of the MS results of O 2 and N 2 O complied with that of other ions. This also complies with the previous report [22,24] that Ti 2 CT x with special structures can adsorb gases and be easily oxidized by oxidizing gases. Therefore, it can be inferred that Ti 2 CT x could absorb O 2 and N 2 O and react with the two gases. Usually, this phenomenon occurs in the thermal decomposition process of AP catalyzed by transition metals, and the generated transition metal oxides will synergistically catalyze the thermal decomposition of AP [37]. Materials 2023, 16, x FOR PEER REVIEW 6 of 14

Catalytic Mechanism of Ti2CTx
To reveal the thermal decomposition mechanism of AP catalyzed by MXene, the Ti2CTx after catalysis was characterized by the SEM, XRD, Raman Spectroscope, XPS, TEM, and HRTEM. Figure 4a shows the XRD results of the Ti2CTx after the catalytic thermal decomposition of AP. After catalyzing the thermal decomposition of AP from room temperature to 500 °C , peak centers at 25.3°, 36.9°, 37.8°, 48°, 53.9°, 55°, 62.7°, 68.8°, 70°, 75°, and 76° appeared in all the samples, which can respectively be attributed to (101)

Catalytic Mechanism of Ti 2 CT x
To reveal the thermal decomposition mechanism of AP catalyzed by MXene, the Ti 2 CT x after catalysis was characterized by the SEM, XRD, Raman Spectroscope, XPS, TEM, and HRTEM. Figure 4a shows the XRD results of the Ti 2 CT x after the catalytic thermal decomposition of AP. After catalyzing the thermal decomposition of AP from room temperature to 500 • C, peak centers at 25. Notably, the typical peak of Ti 2 CT x shifted to a lower angle. Based on these results, A-TiO 2 particles were formed during the catalytic process, which paralleled heating MXenes in the air or other atmospheres [38,39]. The formation of A-TiO 2 was further explained by the Raman results (shown in Figure 4b). After the thermal decomposition catalyzed by AP, four peaks centered at 144, 399, 519, and 639 cm −1 appeared, which responded well to the vibrational modes E g(1) , B 1g(1) , A 1g , and E g(3) of anatase (A-TiO 2 ), respectively [40]. Meanwhile, relatively weak D-and G-bands were observed (shown in the inset of Figure 4b), indicating that only a small amount of carbon was formed on the surface of Ti 2 CT x nanosheets with the formation of A-TiO 2 .
Materials 2023, 16, x FOR PEER REVIEW 7 of 14 particles were formed during the catalytic process, which paralleled heating MXenes in the air or other atmospheres [38,39]. The formation of A-TiO2 was further explained by the Raman results (shown in Figure 4b). After the thermal decomposition catalyzed by AP, four peaks centered at 144, 399, 519, and 639 cm −1 appeared, which responded well to the vibrational modes Eg(1), B1g(1), A1g, and Eg(3) of anatase (A-TiO2), respectively [40]. Meanwhile, relatively weak D-and G-bands were observed (shown in the inset of Figure 4b), indicating that only a small amount of carbon was formed on the surface of Ti2CTx nanosheets with the formation of A-TiO2.   Table 2. On the sample surfaces, almost 79% of the Ti 2p region belonged to TiO2, and 38% of the O 1s region belonged to TiO2. However, after 10 s Ar sputtering, the percentage of the Ti 2p region and O 1s region decreased to 3% and 7%, respectively. This suggested that the Ti atoms on the surface of the Ti2CTx nanosheets were oxidized to TiO2 during the process of accelerating the exothermic decomposition of AP. The interior structures of the Ti2CTx were still maintained. These results demonstrated that the C-Ti bonds were broken, forming TiO2 particles and carbon layers. Additionally, there were still numerous C-Ti-Tx bonds [41], and the carbon layers stuck the TiO2 particles on the surface of Ti2CTx like "glue", which was helpful to maintain the stability of Ti2CTx.
The SEM image (Figure 6a,b) shows that the interlamellar spacing of Ti2CTx became larger after the catalytic thermal decomposition of AP. This amplification was partially due to A-TiO2 formation on the external surface, and presumably also between the layers. However, no typical A-TiO2 particles were observed from the SEM image. Therefore, further confirmation was conducted by the TEM analysis. The TEM image (Figure 6c) and HRTEM image (Figure 6d) demonstrated that some Ti2CTx nanosheets still retained their planar structure, while others transformed into small crystalline particles. The calculated d-spacing values of the latter phase agreed with values for anatase in the JCPDS card . The indexed SAED pattern (shown in Figure 6e), taken from the square region, combined two phases: the original Ti2CTx and anatase-TiO2. Herein, it is also clear that Ti2CTx underwent surface oxidation when the catalytic exothermic decomposition of AP happened.   Table 2. On the sample surfaces, almost 79% of the Ti 2p region belonged to TiO 2 , and 38% of the O 1s region belonged to TiO 2 . However, after 10 s Ar sputtering, the percentage of the Ti 2p region and O 1s region decreased to 3% and 7%, respectively. This suggested that the Ti atoms on the surface of the Ti 2 CT x nanosheets were oxidized to TiO 2 during the process of accelerating the exothermic decomposition of AP. The interior structures of the Ti 2 CT x were still maintained. These results demonstrated that the C-Ti bonds were broken, forming TiO 2 particles and carbon layers. Additionally, there were still numerous C-Ti-T x bonds [41], and the carbon layers stuck the TiO 2 particles on the surface of Ti 2 CT x like "glue", which was helpful to maintain the stability of Ti 2 CT x .
The SEM image (Figure 6a,b) shows that the interlamellar spacing of Ti 2 CT x became larger after the catalytic thermal decomposition of AP. This amplification was partially due to A-TiO 2 formation on the external surface, and presumably also between the layers. However, no typical A-TiO 2 particles were observed from the SEM image. Therefore, further confirmation was conducted by the TEM analysis. The TEM image (Figure 6c) and HRTEM image (Figure 6d) demonstrated that some Ti 2 CT x nanosheets still retained their planar structure, while others transformed into small crystalline particles. The calculated d-spacing values of the latter phase agreed with values for anatase in the JCPDS card . The indexed SAED pattern (shown in Figure 6e), taken from the square region, combined two phases: the original Ti 2 CT x and anatase-TiO 2 . Herein, it is also clear that Ti 2 CT x underwent surface oxidation when the catalytic exothermic decomposition of AP happened.      Generally, there are two main viewpoints on the mechanism of the thermal decomposition of AP (NH4ClO4)-electron transfer and proton transfer-and the process is usually as follows: (1) electrons (e − ) transfer from ClO4 − to NH4 + : (2) protons (H + ) transfer from NH4 + to ClO4 − : In the actual thermal decomposition process of AP, both of these mechanisms may occur, but the occurrence and degree of occurrence will vary under different induced environments. According to the previous results, it can be considered that the mechanism of Ti2CTx catalyzed the thermal decomposition of AP in the following aspects. Generally, there are two main viewpoints on the mechanism of the thermal decomposition of AP (NH 4 ClO 4 )-electron transfer and proton transfer-and the process is usually as follows: (1) electrons (e − ) transfer from ClO 4 − to NH 4 + : (2) protons (H + ) transfer from NH 4 + to ClO 4 − : In the actual thermal decomposition process of AP, both of these mechanisms may occur, but the occurrence and degree of occurrence will vary under different induced environments. According to the previous results, it can be considered that the mechanism of Ti 2 CT x catalyzed the thermal decomposition of AP in the following aspects.
First, XPS results show that the Ti element in Ti 2 CT x presented multiple valence states. In Rudloff's report [51], the catalytic effect of transition metal oxides on the thermal decomposition of AP may be attributed to their multivalent change, which can result in the acceleration of electrons' migration from ClO 4 − to NH 4 + during the redox process, thus boosting the entire decomposition reaction. Here, Ti atoms on the surface of Ti 2 CT x are transition metals, and their 3d orbits are electron-unsaturated, where the empty obits could provide convenient bridges for electron transfer. In addition, Ti 3+ ions possess separate d electrons in the valence band, which could provide more convenient tunnels for electron movement. Herein, the interaction between Ti 2 CT x and AP could contribute to thermal decomposition, forming NH 3 and HClO 4 .
Second, the special layered structure endows Ti 2 CT x with good gas adsorption performance. Based on the first-principle calculation, Yu and co-workers [22] found that the Ti 2 CO 2 (T x = O 2 ) can selectively adsorb NH 3 (g), and the process is reversible, wherein the NH 3 (g) could be desorbed after the strain of Ti 2 CO 2 releasing. Accordingly, when the NH 3 (g) is formed during the decomposition of AP, it can easily be adsorbed by Ti 2 CT x nanosheets. Thus, the MS results confirmed that no NH 3 (g) signal was detected when Ti 2 CT x nanosheets were added. Accordingly, the adsorption behavior of Ti 2 CT x nanosheets can reduce the concentration of decomposition products, promote thermal decomposition, and provide a place for the oxidation of NH 3 . Moreover, the MS result further confirmed the adsorption ability of Ti 2 CT x , showing that the concentration of the resulting H 2 O, O 2, and N 2 O gases with Ti 2 CT x addition was lower than that of pure AP, which is important to catalyze thermal decomposition.
Third, as can be seen from the XRD and TEM results above, TiO 2 was formed when oxidizing gases were emitted by AP decomposition, as follows: In addition, the formation of TiO 2 could reduce the concentration of gaseous products. Generally, carbon layers and Ti 3+ ions are formed with the formation of anatase-TiO 2 particles. Meanwhile, these TiO 2 particles can prevent Ti 2 CT x from further oxidation and participate in the catalytic reaction as transition metal oxides [52].
Fourth, some electrons would be generated from the semiconductor Ti 2 CT x (T x = OH and O) and the resulting anatase-TiO 2 when they are excited by thermal energy. Furthermore, MXenes have excellent thermal conductivity [56]; therefore, the heat conduction rate of the whole reaction system would be significantly improved by the Ti 2 CT x addition.
Based on the abovementioned results and analysis, as Figure 7 shows, the thermal decomposition mechanism of AP catalyzed by Ti 2 CT x is described as follows: above all, Ti 2 CT x , TiO 2 , and carbon layers with excellent electron conductivity could boost the transfer of electrons (e − ) from ClO 4 − to NH 4 + , promoting decomposition of NH 4 ClO 4 and the formation of HClO 4 and NH 3 ; meanwhile, the Ti 2 CT x nanosheets can adsorb gaseous products, reduce the concentration of products, and increase the contact chance of HClO 4 and NH 3 , and shorten the reaction path as well; the formation of TiO 2 could also reduce the concentration of gaseous products, accelerating the decomposition; then, excited electrons (e − ) generated from the semiconductor Ti 2 CT x (x = -OH and = O) and resulting TiO 2 when they are heated, would be transmitted to HClO 4 , the formation of superoxide O 2 − , which could oxidize the NH 3 to produce the final products and release significant amounts of heat; finally, MXene has good thermal conductivity, which can conduct heat quickly and promote a reaction. transfer of electrons (e − ) from ClO4 − to NH4 + , promoting decomposition of NH4ClO4 and the formation of HClO4 and NH3; meanwhile, the Ti2CTx nanosheets can adsorb gaseous products, reduce the concentration of products, and increase the contact chance of HClO4 and NH3, and shorten the reaction path as well; the formation of TiO2 could also reduce the concentration of gaseous products, accelerating the decomposition; then, excited electrons (e − ) generated from the semiconductor Ti2CTx (x = -OH and = O) and resulting TiO2 when they are heated, would be transmitted to HClO4, the formation of superoxide O2 − , which could oxidize the NH3 to produce the final products and release significant amounts of heat; finally, MXene has good thermal conductivity, which can conduct heat quickly and promote a reaction.

Conclusions
In conclusion, the Ti2CTx showed an enhanced catalytic effect on AP thermal decomposition. The thermal decomposition temperature of the composite decreased by 15, 28, 65, and 83 °C corresponding to 5, 10, 20, and 30 wt.% additives, respectively. X-ray diffraction (XRD) and Raman spectroscopy results showed anatase-TiO2 formation during the exothermic decomposition process. The high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), and XPS results demonstrated that anatase-TiO2 were uniformly distributed on the surface of Ti2CTx. Based on these results, a possible catalytic mechanism for the thermal decomposition of AP catalyzed by Ti2CTx was proposed, which would be beneficial for understanding the catalytic mechanism of MXenes and developing efficient catalysts.

Conclusions
In conclusion, the Ti 2 CT x showed an enhanced catalytic effect on AP thermal decomposition. The thermal decomposition temperature of the composite decreased by 15, 28, 65, and 83 • C corresponding to 5, 10, 20, and 30 wt.% additives, respectively. X-ray diffraction (XRD) and Raman spectroscopy results showed anatase-TiO 2 formation during the exothermic decomposition process. The high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), and XPS results demonstrated that anatase-TiO 2 were uniformly distributed on the surface of Ti 2 CT x . Based on these results, a possible catalytic mechanism for the thermal decomposition of AP catalyzed by Ti 2 CT x was proposed, which would be beneficial for understanding the catalytic mechanism of MXenes and developing efficient catalysts.