Palladium Supported on Titanium Carbide: A Highly Efficient, Durable, and Recyclable Bifunctional Catalyst for the Transformation of 4-Chlorophenol and 4-Nitrophenol

Developing highly efficient and recyclable catalysts for the transformation of toxic organic contaminates still remains a challenge. Herein, Titanium Carbide (Ti3C2) MXene modified by alkali treatment process was selected as a support (designated as alk-Ti3C2X2, where X represents the surface terminations) for the synthesis of Pd/alk-Ti3C2X2. Results show that the alkali treatment leads to the increase of surface area and surface oxygen-containing groups of Ti3C2X2, thereby facilitating the dispersion and stabilization of Pd species on the surface of alk-Ti3C2X2. The Pd/alk-Ti3C2X2 catalyst shows excellent catalytic activity for the hydrodechlorination of 4-chlorophenol and the hydrogenation of 4-nitrophenol in aqueous solution at 25 °C and hydrogen balloon pressure. High initial reaction rates of 216.6 and 126.3 min−1·gpd−1 are observed for the hydrodechlorination of 4-chlorophenol and hydrogenation of 4-nitrophenol, respectively. Most importantly, Pd/alk-Ti3C2X2 exhibits excellent stability and recyclability in both reactions without any promoters. The superior property of Pd/alk-Ti3C2X2 makes it as a potential material for practical applications.


Introduction
Noble metal nanoparticles have been intensively investigated as catalysts for a variety of organic catalytic reactions. It is well known that the structure and surface properties of the supports significantly affect the catalytic activity, chemical and environmental stability, and recyclability of the catalysts. Owing to its abundant surface chemistry, two-dimensional (2D) graphene has captured considerable attention as a promising matrix for nanoparticle stabilization of metal in many organic catalytic reactions [1][2][3][4]. Although graphene-supported catalysts have shown superior properties, the preparation procedures of graphene are tedious and time-consuming, which impede production scale-up. For practical applications, it is highly desirable to synthesize efficient catalysts by using carriers that can be conveniently prepared on a large scale. Graphene-like 2D transition metal carbides, designated as MXenes, are such materials, with the advantages of excellent electrical conductivity and good structural and chemical stability [5]. As an essential member of the MXenes group, Ti 3 C 2 with its tailorable physicochemical properties and high physical and chemical stability can be easily synthesized on the large scale by etching the Al layers of Ti 3 AlC 2 in hydrofluoric acid (HF) solution at for HDC of 4-CP reaction may attribute to the strong interaction between the metal and the support. In addition, this catalyst also showed a very high activity toward the hydrogenation of 4-NP with hydrogen balloon pressure without additional strong reducing agents such as NaBH 4 , which is a cost-effective and green process. Finally, the Pd/alk-Ti 3 C 2 X 2 catalyst is quite stable and can be recycled at least seven times in the absence of additives. The high stability and recyclability of the Pd/alk-Ti 3 C 2 X 2 catalyst, especially for HDC of 4-CP, have never been reported so far. The high catalytic performance and recycling efficiency of the Pd/alk-Ti 3 C 2 X 2 catalyst make it as a promising bi-functional material in HDC and hydrogenation reactions.

Materials
Sodium tetrachloropalladate (Na 2 PdCl 4 ·6H 2 O) was purchased from the Kunming Institute of Precious Metals, Kunming, China. Ethylene glycol (EG, >99%), absolute ethanol, concentrated HF (>49%), and sodium hydroxide (NaOH) were bought from Aladdin Industrial Inc., Shanghai, China. Hydrogen gas with a purity of 99.999% was supplied by Chengdu Taiyu Gas Co. Ltd., Chengdu, China. All reagents were used directly as received and without further purification. Ultrapure deionized water (18.2 MΩ) was used in all experiments.

Characterization
Brunauer-Emmett-Teller (BET) surface areas were measured by nitrogen adsorption at 77 K on an ASAP-2020 adsorption apparatus. Scanning electron microscopy (SEM) was carried out using a JSM-6510 (Rigaku, Osaka, Japan) operating at an accelerating voltage of 20 kV. Morphology and microstructure of Pd/alk-Ti 3 C 2 X 2 were characterized by transmission electron microscopy (TEM, Tecnai G20, FEI, Hillsboro, OR, USA) operating at an accelerating voltage of 200 kV. X-ray diffraction (XRD) characterization data were collected on a PANalytical X'eprt diffractometer (Egham, Surrey, UK) with Cu Kα radiation (40 kV, 40 mA) from 5 • to 50 • . X-ray photoemission spectroscopy (XPS) was performed on a Thermo ESCALAB 250 Axis Ultra spectrometer (Thermo, Waltham, MA, USA) using a monochromatic Al Kα (hν = 1486.6 eV). ICP-OES analysis was carried out on SPECTRO ARCOS spectrometer (SPECTRO, Kleve, Germany). The removal efficiency of the catalyst for 4-CP was analyzed by gas chromatography (GC, Agilent 7890A, Agilent Technologies, Santa Clara, CA, USA) with a flame ionization detector (FID) detector and PEG-20 M capillary column (30 m × 0.25 mm, 0.25 µm film) and nitrogen was used as a carrier gas.
2.3. Synthesis of Ti 3 C 2 X 2 , alk-Ti 3 C 2 X 2 , and Pd/alk-Ti 3 C 2 X 2 Ti 3 C 2 X 2 was synthesized through the etching of the Al layers of Ti 3 AlC 2 in a concentrated HF solution at 25 • C. Typically, Ti 3 AlC 2 (4.0 g) was slowly added into a 250-mL beaker containing a concentrated HF solution (HF: 49%, 40 mL). Then, the mixture was continuously stirred at 25 • C for 24 h to remove the Al layers of Ti 3 AlC 2 . The suspension was treated by centrifugation and the resulting black solid was washed with water and absolute ethanol several times. Finally, the product was dried under a vacuum at 60 • C for 12 h. Alk-Ti 3 C 2 X 2 was prepared according to the following procedures. The as-prepared Ti 3 C 2 X 2 (0.3 g) was transferred into a 500-mL two-necked round-bottomed flask containing NaOH (15.0 g) and distilled water (300 mL). The mixture was stirred at 25 • C for 6 h and then separated by centrifugation. The resulting solid was treated by several cycles of washing with water and ethanol. Finally, the collected product was dried in a vacuum at 60 • C for 12 h.
The Pd/alk-Ti 3 C 2 X 2 catalyst was synthesized as follows: alk-Ti 3 C 2 X 2 (0.3 g), NaOH (0.1 g), and Na 2 PdCl 4 ·6H 2 O (0.0438 g) were added into 20 mL of EG. After being treated by sonication for 1 h, the mixture was heated to 140 • C and maintained at this temperature for 4 h. The obtained precipitate was separated by centrifugation and washed several times with ethanol. Finally, the resultant catalyst was dried in a vacuum at 60 • C for 12 h. The Pd loading, determined by inductively coupled plasma (ICP), was around 4.4 wt %.

Activity Tests
The catalytic activity of Pd/alk-Ti 3 C 2 X 2 was investigated for the HDC of 4-CP at 25 • C and hydrogen balloon pressure. In a typical reaction process, 5.0 mg of Pd/alk-Ti 3 C 2 X 2 and 5 mL of aqueous solution of 4-CP (4-CP concentration: 2.5 g/L) were transferred into a 25-mL two-necked round-bottomed flask equipped with a hydrogen balloon. The flask was treated in a vacuum and then flushed with pure hydrogen several times. The reaction temperature was controlled at 25 • C by a water bath. During the reaction process, samples were withdrawn by a syringe at an interval of 10 min. The catalytic process for 4-NP hydrogenation was similar with 4-CP HDC except that 4-NP was used as the substrate. To test the recyclability, the Pd/alk-Ti 3 C 2 X 2 catalyst was separated by centrifugation after each recycling run. The collected solid was washed with water and ethanol repeatedly and directly used for the next run. In all tests, the conversions for 4-CP and 4-NP catalyzed by Pd/alk-Ti 3 C 2 X 2 were analyzed by GC (Agilent 7890A, Agilent Technologies, Santa Clara, CA, USA).

Characterization of the Catalyst
As shown in Scheme 1, the facile synthesis of Pd/alk-Ti 3 C 2 X 2 was realized by using alk-Ti 3 C 2 X 2 as a carrier and Na 2 PdCl 4 as a precursor. First, the pristine Ti 3 AlC 2 was soaked in concentrated HF at room temperature to produce Ti 3 C 2 X 2 . The resulting Ti 3 C 2 X 2 was delaminated by sonication in aqueous solution to separate the thick-layered Ti 3 C 2 X 2 . Ti 3 C 2 X 2 was further delaminated to alk-Ti 3 C 2 X 2 through the alkalization treatment of Ti 3 C 2 X 2 in NaOH aqueous solution. Subsequently, Pd ions were attached on the alk-Ti 3 C 2 X 2 surface through coordination with oxygen-containing groups such as hydroxyl groups. Pd nanoparticles were anchored on the Ti 3 C 2 X 2 surface through the reduction of Pd ions using ethylene glycol as the reducing agent.

Activity Tests
The catalytic activity of Pd/alk-Ti3C2X2 was investigated for the HDC of 4-CP at 25 °C and hydrogen balloon pressure. In a typical reaction process, 5.0 mg of Pd/alk-Ti3C2X2 and 5 mL of aqueous solution of 4-CP (4-CP concentration: 2.5 g/L) were transferred into a 25-mL two-necked round-bottomed flask equipped with a hydrogen balloon. The flask was treated in a vacuum and then flushed with pure hydrogen several times. The reaction temperature was controlled at 25 °C by a water bath. During the reaction process, samples were withdrawn by a syringe at an interval of 10 min. The catalytic process for 4-NP hydrogenation was similar with 4-CP HDC except that 4-NP was used as the substrate. To test the recyclability, the Pd/alk-Ti3C2X2 catalyst was separated by centrifugation after each recycling run. The collected solid was washed with water and ethanol repeatedly and directly used for the next run. In all tests, the conversions for 4-CP and 4-NP catalyzed by Pd/alk-Ti3C2X2 were analyzed by GC (Agilent 7890A, Agilent Technologies, Santa Clara, CA, USA).

Characterization of the Catalyst
As shown in Scheme 1, the facile synthesis of Pd/alk-Ti3C2X2 was realized by using alk-Ti3C2X2 as a carrier and Na2PdCl4 as a precursor. First, the pristine Ti3AlC2 was soaked in concentrated HF at room temperature to produce Ti3C2X2. The resulting Ti3C2X2 was delaminated by sonication in aqueous solution to separate the thick-layered Ti3C2X2. Ti3C2X2 was further delaminated to alk-Ti3C2X2 through the alkalization treatment of Ti3C2X2 in NaOH aqueous solution. Subsequently, Pd ions were attached on the alk-Ti3C2X2 surface through coordination with oxygen-containing groups such as hydroxyl groups. Pd nanoparticles were anchored on the Ti3C2X2 surface through the reduction of Pd ions using ethylene glycol as the reducing agent. The specific surface areas of Ti3C2X2, alk-Ti3C2X2, and Pd/alk-Ti3C2X2 were determined by BET measurement and the results are shown in Figure 1. It is noteworthy that Ti3C2X2 has a surface area of 6.1 m 2 ·g −1 because of its stacked structure ( Figure 1a). Interestingly, the surface area of alk-Ti3C2X2 (11.5 m 2 ·g −1 ) is about twice that of the pristine Ti3C2X2 after the alkalization process (Figure 1b), which is probably attributed to the increase of interlayer spacing because of the intercalation of Na ions [5]. The surface area of Pd/alk-Ti3C2X2 further increased to 24.6 m 2 ·g −1 because of the attachment of Pd nanoparticles on the layered Ti3C2X2 surface (Figure 1c).
To identify the etching of Al layers in Ti3AlC2 using HF as a reducing agent, the morphology of the pristine Ti3AlC2 and Ti3C2X2 was first examined by SEM. As illustrated in Figure 2a, the pristine Ti3AlC2 displays a closely aligned layered structure. As shown in Figure 2b, the successful exfoliation of Ti3AlC2 was authenticated through observing the detriment of the closely aligned layered structure in a concentrated HF solution. The morphology of Pd on alk-Ti3C2X2 surface was characterized by the TEM technique and the results are shown in Figure 2c,d. Pd nanoparticles with a worm-like Scheme 1. The procedure for the synthesis of Pd/alk-Ti 3 C 2 X 2 .
The specific surface areas of Ti 3 C 2 X 2 , alk-Ti 3 C 2 X 2 , and Pd/alk-Ti 3 C 2 X 2 were determined by BET measurement and the results are shown in Figure 1. It is noteworthy that Ti 3 C 2 X 2 has a surface area of 6.1 m 2 ·g −1 because of its stacked structure ( Figure 1a). Interestingly, the surface area of alk-Ti 3 C 2 X 2 (11.5 m 2 ·g −1 ) is about twice that of the pristine Ti 3 C 2 X 2 after the alkalization process (Figure 1b), which is probably attributed to the increase of interlayer spacing because of the intercalation of Na ions [5]. The surface area of Pd/alk-Ti 3 C 2 X 2 further increased to 24.6 m 2 ·g −1 because of the attachment of Pd nanoparticles on the layered Ti 3 C 2 X 2 surface (Figure 1c). morphology are observed on the surface of alk-Ti3C2X2 when using Na2PdCl4 as a precursor and EG as a solvent and reducing agent. The high-resolution TEM (HRTEM) image shows a typical crystal plane with lattice spacing of 0.224 nm, which corresponds to the (111) plane of Pd [32].  XRD patterns of Ti3AlC2, Ti3C2X2, alk-Ti3C2X2, and Pd/alk-Ti3C2X2 were recorded to investigate the structure of the selected materials. As shown in Figure 3a, the strong and sharp diffraction peaks match well with the standard card of Ti3AlC2 (JCPDS no. 52-0875), indicating the high purity of Ti3AlC2. The characteristic peaks of Ti3AlC2 disappear when Ti3AlC2 is treated with a concentrated HF aqueous solution. Consequently, several new peaks at 2θ = 9.0°, 18.5°, and 27.5° are observed because of the insertion of OH and F functional groups after the removal of Al atoms [6]. The observation of weak and broad peaks in Ti3C2X2 is attributed to the introduction of oxygen-containing functional groups on the surface of Ti3C2X2 after HF etching, indicating successful synthesis. It is noteworthy that the (002) diffraction peak of Ti3C2X2 shifts to a low angle after the alkalization process, owing to To identify the etching of Al layers in Ti 3 AlC 2 using HF as a reducing agent, the morphology of the pristine Ti 3 AlC 2 and Ti 3 C 2 X 2 was first examined by SEM. As illustrated in Figure 2a, the pristine Ti 3 AlC 2 displays a closely aligned layered structure. As shown in Figure 2b, the successful exfoliation of Ti 3 AlC 2 was authenticated through observing the detriment of the closely aligned layered structure in a concentrated HF solution. The morphology of Pd on alk-Ti 3 C 2 X 2 surface was characterized by the TEM technique and the results are shown in Figure 2c,d. Pd nanoparticles with a worm-like morphology are observed on the surface of alk-Ti 3 C 2 X 2 when using Na 2 PdCl 4 as a precursor and EG as a solvent and reducing agent. The high-resolution TEM (HRTEM) image shows a typical crystal plane with lattice spacing of 0.224 nm, which corresponds to the (111) plane of Pd [32].   XRD patterns of Ti3AlC2, Ti3C2X2, alk-Ti3C2X2, and Pd/alk-Ti3C2X2 were recorded to investigate the structure of the selected materials. As shown in Figure 3a, the strong and sharp diffraction peaks match well with the standard card of Ti3AlC2 (JCPDS no. 52-0875), indicating the high purity of Ti3AlC2. The characteristic peaks of Ti3AlC2 disappear when Ti3AlC2 is treated with a concentrated HF aqueous solution. Consequently, several new peaks at 2θ = 9.0°, 18.5°, and 27.5° are observed because  XRD patterns of Ti 3 AlC 2 , Ti 3 C 2 X 2 , alk-Ti 3 C 2 X 2 , and Pd/alk-Ti 3 C 2 X 2 were recorded to investigate the structure of the selected materials. As shown in Figure 3a, the strong and sharp diffraction peaks match well with the standard card of Ti 3 AlC 2 (JCPDS no. 52-0875), indicating the high purity of Ti 3 AlC 2 . The characteristic peaks of Ti 3 AlC 2 disappear when Ti 3 AlC 2 is treated with a concentrated HF aqueous solution. Consequently, several new peaks at 2θ = 9.0 • , 18.5 • , and 27.5 • are observed because of the insertion of OH and F functional groups after the removal of Al atoms [6]. The observation of weak and broad peaks in Ti 3 C 2 X 2 is attributed to the introduction of oxygen-containing functional groups on the surface of Ti 3 C 2 X 2 after HF etching, indicating successful synthesis. It is noteworthy that the (002) diffraction peak of Ti 3 C 2 X 2 shifts to a low angle after the alkalization process, owing to the occurrence of intercalation of Na ions [5]. In addition, no peaks corresponding to Pd or Pd oxide are detected by XRD characterization because of the low loading of Pd on the surface of alk-Ti 3 C 2 X 2 . It should be pointed out that the (002) peak position of Pd/alk-Ti 3 C 2 X 2 further shifts toward a low angle, implying that Pd intercalation occurs. Compared with the pristine Ti 3 AlC 2 , the disappearance of the broad and strong diffraction peaks and the observation of new broad and weak peaks together with their slight shifts to a low angle verify the successful synthesis of Pd/alk-Ti 3 C 2 X 2 . Additionally, the XRD results also suggest that the crystal structure of alk-Ti 3 C 2 X 2 does not change obviously during the reduction process. the occurrence of intercalation of Na ions [5]. In addition, no peaks corresponding to Pd or Pd oxide are detected by XRD characterization because of the low loading of Pd on the surface of alk-Ti3C2X2.
It should be pointed out that the (002) peak position of Pd/alk-Ti3C2X2 further shifts toward a low angle, implying that Pd intercalation occurs. Compared with the pristine Ti3AlC2, the disappearance of the broad and strong diffraction peaks and the observation of new broad and weak peaks together with their slight shifts to a low angle verify the successful synthesis of Pd/alk-Ti3C2X2. Additionally, the XRD results also suggest that the crystal structure of alk-Ti3C2X2 does not change obviously during the reduction process. The surface compositions of Ti3C2X2 and Pd/alk-Ti3C2X2 were measured by XPS surface survey scans. As shown in Figure 3b, signals corresponding to titanium, carbon, aluminum, oxygen, and fluorine are distinctively observed. A characteristic peak corresponding to Pd is detected in the Pd/alk-Ti3C2X2 sample, showing that Pd has been successfully anchored on the surface of alk-Ti3C2X2 with a surface atomic ratio of 1.85%. It should be pointed out that the detection of high fluorine content confirmed the insertion of F into the layers of Ti3C2 via the reaction of Ti3C2 with HF (Ti3C2 + 2HF = Ti3C2F2 + H2) when the Al layers were removed [33]. Significantly, the F groups could be replaced by OH groups and resulted in a decrease of F content after alkalization treatment. These results show that the alkalization of Ti3C2X2 benefited from the transformation of F functional groups to hydroxyl groups during the alkalization process [5,34]. The oxygen-containing groups favor the stabilization of Pd nanoparticles on the surface of alk-Ti3C2X2, thereby improving the hydrophilicity Figure 3. (a) XRD patterns of Ti 3 AlC 2 , Ti 3 C 2 X 2 , alk-Ti 3 C 2 X 2 , and Pd/alk-Ti 3 C 2 X 2 ; (b) XPS survey scans of Ti 3 C 2 X 2 and Pd/alk-Ti 3 C 2 X 2 (Inset is the atomic ratio of the detected elements); (c,d) High-resolution XPS spectra of C1s and Pd 3d for Pd/alk-Ti 3 C 2 X 2 .
The surface compositions of Ti 3 C 2 X 2 and Pd/alk-Ti 3 C 2 X 2 were measured by XPS surface survey scans. As shown in Figure 3b, signals corresponding to titanium, carbon, aluminum, oxygen, and fluorine are distinctively observed. A characteristic peak corresponding to Pd is detected in the Pd/alk-Ti 3 C 2 X 2 sample, showing that Pd has been successfully anchored on the surface of alk-Ti 3 C 2 X 2 with a surface atomic ratio of 1.85%. It should be pointed out that the detection of high fluorine content confirmed the insertion of F into the layers of Ti 3 C 2 via the reaction of Ti 3 C 2 with HF (Ti 3 C 2 + 2HF = Ti 3 C 2 F 2 + H 2 ) when the Al layers were removed [33]. Significantly, the F groups could be replaced by OH groups and resulted in a decrease of F content after alkalization treatment. These results show that the alkalization of Ti 3 C 2 X 2 benefited from the transformation of F functional groups to hydroxyl groups during the alkalization process [5,34]. The oxygen-containing groups favor the stabilization of Pd nanoparticles on the surface of alk-Ti 3 C 2 X 2 , thereby improving the hydrophilicity of Pd/alk-Ti 3 C 2 X 2 . The electronic states of C and Pd on alk-Ti 3 C 2 X 2 surface were further analyzed by high-resolution XPS measurement. As shown in Figure 3c, the spectrum of C1s in Pd/alk-Ti 3 C 2 X 2 sample can be fitted into four peaks with binding energies of 281.5, 284.8, 286.4, and 288.7 eV, which are attributed to various types of carbon atoms including Ti-C, C-C, C-OH, and HO-C=O, respectively [35]. These results verify the presence of oxygen-containing functional groups on the Ti 3 C 2 X 2 surface, which is coincident with XRD results. The binding energy of Pd3d level was analyzed after the immobilization of Pd particles on the alk-Ti 3 C 2 X 2 surface (Figure 3d). The two peaks with the binding energies of 335.2 and 340.3 eV are assigned to 3d 5/2 and 3d 3/2 of metallic Pd • . Notably, the Pd3d peak also presents two small peaks with binding energies of 337.1 and 342.3 eV, which correspond to the 3d5/2 and 3d3/2 of Pd n+ species.

Catalytic HDC of 4-CP
The catalytic performance of the as-synthesized Pd/alk-Ti 3 C 2 X 2 catalyst was first studied for the HDC of 4-CP in a base-free medium at 25 • C and hydrogen balloon pressure. Regarding the reaction pathway shown in Scheme 2(1), hydrogen is absorbed and activated by the active sites of Pd on the surface of alk-Ti 3 C 2 X 2 . Secondly, the activated hydrogen provides two active hydrogen atoms by the cleavage of the H-H bonds. Lastly, the C-Cl bond in the adsorbed 4-CP is attacked by the active hydrogen atom on the surface of the catalyst, yielding the product phenol and byproduct HCl [36]. Figure 4a shows the time evolution of the yield of phenol and conversion of 4-CP over the catalyst Pd/alk-Ti 3 C 2 X 2 . It can be seen that 4-CP is completely converted to phenol within 70 min. The yield of phenol gradually increases with the increase in reaction time. With respect to product distribution, phenol is detected as the sole product in the HDC process because of the low reaction temperature and catalyst concentration [37]. As reported previously, phenol could be further hydrogenated to cyclohexanone and/or cyclohexanol with a high catalyst concentration and high reaction temperature [14,20]. With regards to the reaction kinetics, the HDC of 4-CP with Pd-based catalysts could be assumed to be a pseudo-first-order reaction considering the excess amount of H 2 gas [36]. To this end, the reaction rate constant was determined depending on the slope of the fitting line, where ln (C t /C 0 ) was plotted against reaction time (Figure 4b). The observation of a good linear correlation shows that the HDC of 4-CP catalyzed by Pd/alk-Ti 3 C 2 X 2 is a pseudo-first-order reaction. According to the slope of the fitting line, the reaction rate constant k was calculated to be 5.42 × 10 −2 min −1 for the catalytic HDC of 4-CP with the Pd/alk-Ti 3 C 2 X 2 catalyst. Considering the different dosages of supported Pd catalysts, the reaction rate constant k', defined as the ratio of the rate constant to the quality of the active sites, was calculated to compare the catalytic activity with the reported Pd catalyst for the HDC of 4-CP. Even though no base additives was employed, the Pd/alk-Ti 3 C 2 X 2 catalyst exhibited a high HDC rate of 216.6 min −1 ·g −1 pd . This value is higher than that of the reported Pd-based catalysts, such as Pd/Al 2 O 3 [38] and Pd/DMSNs [37], with kinetic reaction rates (k Pd ) of 3.33 and 13.2 min −1 ·g −1 pd , respectively.

Catalytic HDC of 4-CP
The catalytic performance of the as-synthesized Pd/alk-Ti3C2X2 catalyst was first studied for the HDC of 4-CP in a base-free medium at 25 °C and hydrogen balloon pressure. Regarding the reaction pathway shown in Scheme 2(1), hydrogen is absorbed and activated by the active sites of Pd on the surface of alk-Ti3C2X2. Secondly, the activated hydrogen provides two active hydrogen atoms by the cleavage of the H-H bonds. Lastly, the C-Cl bond in the adsorbed 4-CP is attacked by the active hydrogen atom on the surface of the catalyst, yielding the product phenol and byproduct HCl [36]. Figure 4a shows the time evolution of the yield of phenol and conversion of 4-CP over the catalyst Pd/alk-Ti3C2X2. It can be seen that 4-CP is completely converted to phenol within 70 min. The yield of phenol gradually increases with the increase in reaction time. With respect to product distribution, phenol is detected as the sole product in the HDC process because of the low reaction temperature and catalyst concentration [37]. As reported previously, phenol could be further hydrogenated to cyclohexanone and/or cyclohexanol with a high catalyst concentration and high reaction temperature [14,20]. With regards to the reaction kinetics, the HDC of 4-CP with Pd-based catalysts could be assumed to be a pseudo-first-order reaction considering the excess amount of H2 gas [36]. To this end, the reaction rate constant was determined depending on the slope of the fitting line, where ln (Ct/C0) was plotted against reaction time (Figure 4b). The observation of a good linear correlation shows that the HDC of 4-CP catalyzed by Pd/alk-Ti3C2X2 is a pseudo-first-order reaction. According to the slope of the fitting line, the reaction rate constant k was calculated to be 5.42 × 10 −2 min −1 for the catalytic HDC of 4-CP with the Pd/alk-Ti3C2X2 catalyst. Considering the different dosages of supported Pd catalysts, the reaction rate constant k', defined as the ratio of the rate constant to the quality of the active sites, was calculated to compare the catalytic activity with the reported Pd catalyst for the HDC of 4-CP. Even though no base additives was employed, the Pd/alk-Ti3C2X2 catalyst exhibited a high HDC rate of 216.6 . This value is higher than that of the reported Pd-based catalysts, such as Pd/Al2O3 [38] and Pd/DMSNs [37], with kinetic reaction rates (kPd) of 3.33 and 13.2 To kinetically study the HDC of 4-CP catalyzed by Pd/alk-Ti3C2X2, a set of control experiments was carried out at temperatures from 20 °C to 35 °C while keeping the other reaction conditions unchanged. As shown in Figure 4c, temperature has a positive influence on the catalytic HDC of 4-CP. The time for complete conversion of 4-CP to phenol decreases from 90 min to 35 min with the elevation of the temperature. It is noteworthy that the selectivity to phenol is still higher than 99% in process, respectively [27,[41][42][43][44]. According to the proposed dual mechanism, zero-valent Pd chemisorbed and homolytically dissociated hydrogen into the covalent adatom Pd-H [43,44]. Electrodeficient Pd n+ species were the centers for C-Cl bond activation via nucleophilic chloride anion abstraction [27,41]. In the present work, the presence of different Pd species including Pd° and Pd n+ was responsible for the high catalytic activity of Pd/alk-Ti3C2X2 for HDC reaction. These results confirm the findings related to the proposed dual mechanism of the HDC reaction.

Catalytic Hydrogenation of 4-NP
Taking into account the high catalytic activity toward the HDC of 4-CP, Pd/alk-Ti3C2X2 was also selected as a catalyst for the hydrogenation of 4-NP in an aqueous solution at 25 °C and balloon hydrogen pressure. Similarly, the reaction pathway for the hydrogenation of 4-NP to 4-AP can be described in Scheme 2 (2). Specifically, the absorbed hydrogen is activated by the active sites of Pd dispersed on the alk-Ti3C2X2 surface. Two active hydrogen atoms are formed through the scission of the H-H bonds in the activated hydrogen. Finally, the -NO2 groups in the adsorbed 4-NP is attacked by the active hydrogen atom on the surface of the catalyst, yielding the product 4-AP. The conversion of 4-NP was monitored by GC. 4-AP was detected as the sole product; no other species were observed during the reaction process. Figure 5a shows the time-dependent conversion of 4-NP and the yield of 4-AP for the hydrogenation of 4-NP by Pd/alk-Ti3C2X2. Results indicate that Pd/alk-Ti3C2X2 also exhibits excellent catalytic activity for the hydrogenation of 4-NP to 4-AP in the presence of hydrogen gas. The complete hydrogenation of 4-NP to 4-AP was finished in 70 min. The yield of 4-NP gradually increased with the reaction time, and a 99.9% yield of 4-AP was detected. During the hydrogenation process, large amount of H2 was applied as compared with the concentration of 4-NP. Thus, the concentration of H2 can be assumed as a constant and the hydrogenation reaction is assumed to be a pseudo first-order reaction with respect to the 4-NP concentration. To this end, the reaction kinetic is expressed as ln(Ct/C0) = −kt, in which k represents the apparent rate constant, and t is the reaction time. Figure 5b shows the plots of ln(Ct/C0) versus reaction time t. The fitting straight line indicates To kinetically study the HDC of 4-CP catalyzed by Pd/alk-Ti 3 C 2 X 2 , a set of control experiments was carried out at temperatures from 20 • C to 35 • C while keeping the other reaction conditions unchanged. As shown in Figure 4c, temperature has a positive influence on the catalytic HDC of 4-CP. The time for complete conversion of 4-CP to phenol decreases from 90 min to 35 min with the elevation of the temperature. It is noteworthy that the selectivity to phenol is still higher than 99% in all tests, indicating that the increase of reaction temperature only increases the reaction rate without changing the selectivity. The rate constant at different temperatures was calculated from the slope of the linear part of ln(C t /C 0 ) versus time plots. Based on the slope of the Arrhenius plots in Figure 4d, the activation energy (E a ) for 4-CP HDC over Pd/alk-Ti 3 C 2 X 2 was calculated to be 51.2 kJ/mol. This value is lower than that of Pd on activated carbon (58 kJ·mol −1 ) [39], Pd/DMSNs catalyst (61.5 kJ·mol −1 ) [37], and Pd/Al pillared clays (85.8 kJ·mol −1 ) [40], corresponding to the high catalytic activity of Pd/alk-Ti 3 C 2 X 2 for the HDC of 4-CP without any additives. The XPS results show the presence of two Pd species, Pd • and Pd n+ , in the Pd/alk-Ti 3 C 2 X 2 catalyst. Previous results have shown that electron-deficient and zero-valent Pd species are essential for increasing the catalytic activity of Pd-based catalysts for HDC reaction. They play different roles, activating the C-Cl bond and H 2 in the HDC process, respectively [27,[41][42][43][44]. According to the proposed dual mechanism, zero-valent Pd chemisorbed and homolytically dissociated hydrogen into the covalent adatom Pd-H [43,44]. Electrodeficient Pd n+ species were the centers for C-Cl bond activation via nucleophilic chloride anion abstraction [27,41]. In the present work, the presence of different Pd species including Pd • and Pd n+ was responsible for the high catalytic activity of Pd/alk-Ti 3 C 2 X 2 for HDC reaction. These results confirm the findings related to the proposed dual mechanism of the HDC reaction.

Catalytic Hydrogenation of 4-NP
Taking into account the high catalytic activity toward the HDC of 4-CP, Pd/alk-Ti 3 C 2 X 2 was also selected as a catalyst for the hydrogenation of 4-NP in an aqueous solution at 25 • C and balloon hydrogen pressure. Similarly, the reaction pathway for the hydrogenation of 4-NP to 4-AP can be described in Scheme 2 (2). Specifically, the absorbed hydrogen is activated by the active sites of Pd dispersed on the alk-Ti 3 C 2 X 2 surface. Two active hydrogen atoms are formed through the scission of the H-H bonds in the activated hydrogen. Finally, the -NO 2 groups in the adsorbed 4-NP is attacked by the active hydrogen atom on the surface of the catalyst, yielding the product 4-AP. The conversion of 4-NP was monitored by GC. 4-AP was detected as the sole product; no other species were observed during the reaction process. Figure 5a shows the time-dependent conversion of 4-NP and the yield of 4-AP for the hydrogenation of 4-NP by Pd/alk-Ti 3 C 2 X 2 . Results indicate that Pd/alk-Ti 3 C 2 X 2 also exhibits excellent catalytic activity for the hydrogenation of 4-NP to 4-AP in the presence of hydrogen gas. The complete hydrogenation of 4-NP to 4-AP was finished in 70 min. The yield of 4-NP gradually increased with the reaction time, and a 99.9% yield of 4-AP was detected. During the hydrogenation process, large amount of H 2 was applied as compared with the concentration of 4-NP. Thus, the concentration of H 2 can be assumed as a constant and the hydrogenation reaction is assumed to be a pseudo first-order reaction with respect to the 4-NP concentration. To this end, the reaction kinetic is expressed as ln(C t /C 0 ) = −kt, in which k represents the apparent rate constant, and t is the reaction time. Figure 5b shows the plots of ln(C t /C 0 ) versus reaction time t. The fitting straight line indicates that the hydrogenation of 4-NP is a pseudo-first-order reaction by using hydrogen gas as the reducing agent. According to the slope of the fitting line, the reaction rate constant k for the catalytic reduction of 4-NP by Pd/alk-Ti 3 C 2 X 2 is 3.16 × 10 −2 min −1 , corresponding to a high initial reaction rate of 126.3 min −1 ·g −1 pd . Figure 5c shows the hydrogenation of 4-NP over Pd/alk-Ti 3 C 2 X 2 catalyst at different temperatures. An increase of 3.6-fold times in the reaction rate constant was observed with the elevation of temperature from 20 • C to 35 • C. The rate constant k at different temperatures was evaluated from the slope of the linear part of each plot from Figure 5c. The Arrhenius plots of lnk versus 1/T for the catalyst are shown in Figure 5d. The activation energy was calculated to be 67.5 kJ/mol. . Figure 5c shows the hydrogenation of 4-NP over Pd/alk-Ti3C2X2 catalyst at different temperatures. An increase of 3.6-fold times in the reaction rate constant was observed with the elevation of temperature from 20 °C to 35 °C. The rate constant k at different temperatures was evaluated from the slope of the linear part of each plot from Figure 5c. The Arrhenius plots of lnk versus 1/T for the catalyst are shown in Figure 5d. The activation energy was calculated to be 67.5 kJ/mol. Stability and recyclability of heterogeneous catalyst are of great significance in terms of practical applications. Therefore, the stability and recyclability of catalyst Pd/alk-Ti3C2X2 for the HDC of 4-CP and the hydrogenation of 4-NP were studied, respectively. After the completion of the first run, the solid catalyst was recovered from the mixture by centrifugation, and the supernatant liquid was collected for detecting the conversion efficiency by GC. The recovered black solid was treated with successive washing cycles with water and ethanol and transferred into the reactor for the next run with the introduction of fresh 4-CP and 4-NP. Results shown in Figure 6 indicate that the Pd/alk-Ti3C2X2 catalyst is quite stable and can be recycled seven times without significant loss of removal efficiency for the hydrogenation of NP. With regard to the recyclability of Pd/alk-Ti3C2X2 for the HDC of 4-CP, the catalyst can be recycled five times without the loss of any catalytic activity. These results show that Pd/alk-Ti3C2X2 has high durability toward deactivation during the recycling process, which has never been reported before.  Stability and recyclability of heterogeneous catalyst are of great significance in terms of practical applications. Therefore, the stability and recyclability of catalyst Pd/alk-Ti 3 C 2 X 2 for the HDC of 4-CP and the hydrogenation of 4-NP were studied, respectively. After the completion of the first run, the solid catalyst was recovered from the mixture by centrifugation, and the supernatant liquid was collected for detecting the conversion efficiency by GC. The recovered black solid was treated with successive washing cycles with water and ethanol and transferred into the reactor for the next run with the introduction of fresh 4-CP and 4-NP. Results shown in Figure 6 indicate that the Pd/alk-Ti 3 C 2 X 2 catalyst is quite stable and can be recycled seven times without significant loss of removal efficiency for the hydrogenation of NP. With regard to the recyclability of Pd/alk-Ti 3 C 2 X 2 for the HDC of 4-CP, the catalyst can be recycled five times without the loss of any catalytic activity. These results show that Pd/alk-Ti 3 C 2 X 2 has high durability toward deactivation during the recycling process, which has never been reported before.

Conclusions
In summary, we have developed a simple method to prepare a Pd/alk-Ti3C2X2 catalyst with large surface area and abundant surface terminations through the alkalization of the support. These merits endow the as-prepared Pd/alk-Ti3C2X2 with superior catalytic activities toward the HDC of 4-CP and the hydrogenation of 4-NP under mild conditions. The Pd/alk-Ti3C2X2 catalyst showed very high initial reaction rates of 216.6 and 126.3 for the HDC of 4-CP without alkali additive and hydrogenation of 4-NP without the reducing agent, respectively. Moreover, the Pd/alk-Ti3C2X2 catalyst was quite stable and could be recycled five or seven times without significant loss of catalytic efficiency in the HDC of 4-CP and the hydrogenation of 4-NP, respectively. The high catalytic activity and reusability of Pd/alk-Ti3C2X2 make it a promising candidate for various catalytic applications.