Enhanced Catalytic Dechlorination of 1,2-Dichlorobenzene Using Ni/Pd Bimetallic Nanoparticles Prepared by a Pulsed Laser Ablation in Liquid

: Bimetallic nanoparticles (NPs) exhibit advantageous electrical, optical, and catalytic properties. Among the various NP synthesis methods, pulsed laser ablation in liquid (PLAL) is currently attracting much attention because of its simplicity and versatility. In this study, a pulsed laser was used to produce nickel/palladium (Ni/Pd) bimetallic NPs in methanol and deionized water. The morphological and optical properties of the resulting Ni/Pd bimetallic NPs were characterized. The synthesized Ni/Pd bimetallic NPs were used for the dechlorination of 1,2-dichlorobenzene (1,2-DCB) under various conditions. The dechlorination rates of 1,2-DCB while using single (Ni and Pd) and bimetallic (Ni powder/Pd and Ni/Pd) NPs were investigated. The results showed that the Ni/Pd bimetallic NPs with 19.16 wt.% Pd exhibited much enhanced degradation efﬁciency for 1,2-DCB (100% degradation after 30 min). Accordingly, the results of enhanced the degradation of 1,2-DCB provide plausible mechanism insights into the catalytic reaction. synthesized by mixing the PdCl 2 stock solution to the prepared Ni powder and Ni NP solution with constant stirring and sonication for 30 min. The prepared Ni powder/Pd NPs and Ni/Pd bimetallic NPs solution was washed with methanol, and the residues were centrifuged at a rate of 13,000 rpm for 10 min. Different amounts of PdCl 2 (0.51 × 10 − 4 (1.60 wt.%), 1.53 × 10 − 4


Introduction
Global industrialization and the resulting continuous changes to environmental quality have caused serious water pollution problems. Chlorinated organic compounds (COCs), including alkyl and aryl chlorinated compounds, are widely used as raw materials, intermediates, and organic solvents in the chemical, agricultural and electronics industries [1,2]. However, COCs are toxic to humans and create health risks, such as birth defects, developmental impairment, infertile immune suppression, and cancer [3]. These COCs are environmentally persistent and they are challenging to directly degrade. Therefore, the decomposition of COCs in aqueous solutions and soil has received special research consideration [4]. Among the various COCs, 1,2-dichlorobenzenes (1,2-DCBs) are ubiquitously used and can be found in all major ecosystems [5]. Hence, there is a desperate need for efficient 1,2-DCB dechlorination methods that are suitable for treating both industrial wastewater and contaminated groundwater.
Many processes have been reported for the removal of 1,2-DCB, including advanced oxidation processes, photocatalytic reactions, ozonolysis, and catalytic ozonolysis [6]. Recently, catalytic   Figure 2e shows the formation of Pd NPs on the surface of the Ni NPs. From the HRTEM images, the deposition of Pd NPs onto the spherical Ni surface gradually increases with respect to the Pd concentration, which confirms that Pd NPs adhere to the Ni NPs. Figure 2f,g display the inter-planar spacing of 0.206 and 0.222 nm attributed to Ni NPs in the (111) fcc plane [37] and the Pd NPs indexed to the (111) fcc plane [38], respectively. HRTEM mapping analysis was executed for the accurate identity and distribution of the Ni and Pd composition in the nanocomposites. Figure 2h displays the mapping image of Ni/Pd bimetallic NPs. The HRTEM mapping images for Ni (red) and Pd (green) are shown in Figure 2j,k, respectively. Through the mapping analysis, the Pd NPs were uniformly distributed onto the Ni NPs. Moreover, the elemental composition of the as-prepared sample used in this study (Ni, Pd) was determined while using EDS (Figure 2i). The EDS spectrum demonstrates the presence of Ni and Pd in the sample. In the spectrum, any impurities were detected, thus confirming the presence of prepared samples with the anticipated composition and high purity. The results are in good agreement with the XRD results ( Figure 1). Additional information regarding the FE-SEM,   Figure 2e shows the formation of Pd NPs on the surface of the Ni NPs. From the HRTEM images, the deposition of Pd NPs onto the spherical Ni surface gradually increases with respect to the Pd concentration, which confirms that Pd NPs adhere to the Ni NPs. Figure 2f,g display the inter-planar spacing of 0.206 and 0.222 nm attributed to Ni NPs in the (111) fcc plane [37] and the Pd NPs indexed to the (111) fcc plane [38], respectively. HRTEM mapping analysis was executed for the accurate identity and distribution of the Ni and Pd composition in the nanocomposites. Figure 2h displays the mapping image of Ni/Pd bimetallic NPs. The HRTEM mapping images for Ni (red) and Pd (green) are shown in Figure 2j,k, respectively. Through the mapping analysis, the Pd NPs were uniformly distributed onto the Ni NPs. Moreover, the elemental composition of the as-prepared sample used in this study (Ni, Pd) was determined while using EDS (Figure 2i). The EDS spectrum demonstrates the presence of Ni and Pd in the sample. In the spectrum, any impurities were detected, thus confirming the presence of prepared samples with the anticipated composition and high purity. The results are in good agreement with the XRD results ( Figure 1). Additional information regarding the FE-SEM, HRTEM images, and selected area electron diffraction (SAED) characterization of pure Ni can be found in Figure S1. HRTEM images, and selected area electron diffraction (SAED) characterization of pure Ni can be found in Figure S1.

Catalytic Dechlorination of 1,2-DCB Using Ni/Pd Bimetallic NPs
The dechlorination of 1,2-DCB, which is a typical environmental pollutant of soil and water sources, by using various NPs as catalysts was investigated. The 1,2-DCB dechlorination mechanism involves a substitution reaction of Cl to H on the Ni/Pd bimetallic NPs and it results in benzene as the final degradation product [39]. It is well known that Pd is a good hydrogen activation catalyst when H2 is present [40]. Thus, the dechlorination efficiency was verified by changing the Pd amounts

Catalytic Dechlorination of 1,2-DCB Using Ni/Pd Bimetallic NPs
The dechlorination of 1,2-DCB, which is a typical environmental pollutant of soil and water sources, by using various NPs as catalysts was investigated. The 1,2-DCB dechlorination mechanism involves a substitution reaction of Cl to H on the Ni/Pd bimetallic NPs and it results in benzene as the final degradation product [39]. It is well known that Pd is a good hydrogen activation catalyst when H 2 is present [40]. Thus, the dechlorination efficiency was verified by changing the Pd amounts in the Ni NPs. Figure 3a displays the catalytic 1,2-DCB dechlorination as a function of Pd wt.% over 1 h. The dechlorination efficiency of 1,2-DCB was increased as the wt.% of Pd reached 19.16 wt.%.
However, a further increase in Pd loading decreased the dechlorination efficiency. The additional Pd NPs (24.29 wt.%) aggregated and minimized the surface energies, thus causing the change in morphology of the Pd NPs in the Ni/Pd system. This additional Pd doping has caused the reduced surface area of Pd and the decrease in the 1,2-DCB degradation rate [41].
To quantitatively determine the dechlorination efficiency of the Ni/Pd bimetallic NPs, the kinetic rate constants of the 1,2-DCB dechlorination reaction were calculated while using the pseudo-first-order correlation: −ln(C t /C 0 ) = kt, where C t and C 0 are the concentration of 1,2-DCB at times t and 0, respectively, and k is the reaction rate constant (min −1 ). The plot of ln(C t /C 0 ) as a function of reaction time for the dechlorination reaction of 1,2-DCB by various wt.% of Pd loading onto Ni NPs showed a linear correlation ( Figure S2). The rate constants were determined as 0.049 × 10 −4 , 0.0278, 0.0990, and 0.0607 min −1 for the sample of 1.60, 6 [41].
To quantitatively determine the dechlorination efficiency of the Ni/Pd bimetallic NPs, the kinetic rate constants of the 1,2-DCB dechlorination reaction were calculated while using the pseudo-firstorder correlation: −ln(Ct/C0) = kt, where Ct and C0 are the concentration of 1,2-DCB at times t and 0, respectively, and k is the reaction rate constant (min −1 ). The plot of ln(Ct/C0) as a function of reaction time for the dechlorination reaction of 1,2-DCB by various wt.% of Pd loading onto Ni NPs showed a linear correlation ( Figure S2). The rate constants were determined as 0.049 × 10 −4 , 0.0278, 0.0990, and 0.0607 min −1 for the sample of 1.60, 6.19, 19.16, and 24.29% Pd/Ni NPs, respectively. Among the assynthesized Ni/Pd bimetallic NPs, 19.16% Pd/Ni NPs showed the highest catalytic efficiency.  Figure 3b shows the1,2-DCB degradation rate comparison of the four different NPs fabricated in this study. The degradation rates of 1,2-DCB were very low in the case of monometallic Ni NPs, and it was a little better for the monometallic Pd NPs. These results clearly indicate that 1,2-DCB degradation while using monometallic Ni and Pd NPs requires 5 h [42]. For the case of bimetallic Ni powder/Pd NPs, the degradation efficiency was also low. However, the degradation efficiency was high for the Ni/Pd bimetallic NPs, reaching 100% 1,2-DCB degradation after 30 min. Furthermore, the degradation efficiency is much higher than those of the previously reported bimetallic NPs synthesized by other methods [43,44].

Mechanism of 1,2-DCB Degradation on the Ni/Pd Bimetallic NPs
The reductive 1,2-DCB degradation reaction in the Ni/Pd bimetallic catalytic system can be described, as follows: 2H· + 1,2-DCB → Chlorobenzene + H + + Cl − 2H· + Chlorobenzene → Benzene + H + + Cl −  Figure 3b shows the1,2-DCB degradation rate comparison of the four different NPs fabricated in this study. The degradation rates of 1,2-DCB were very low in the case of monometallic Ni NPs, and it was a little better for the monometallic Pd NPs. These results clearly indicate that 1,2-DCB degradation while using monometallic Ni and Pd NPs requires 5 h [42]. For the case of bimetallic Ni powder/Pd NPs, the degradation efficiency was also low. However, the degradation efficiency was high for the Ni/Pd bimetallic NPs, reaching 100% 1,2-DCB degradation after 30 min. Furthermore, the degradation efficiency is much higher than those of the previously reported bimetallic NPs synthesized by other methods [43,44].

Mechanism of 1,2-DCB Degradation on the Ni/Pd Bimetallic NPs
The reductive 1,2-DCB degradation reaction in the Ni/Pd bimetallic catalytic system can be described, as follows: 2H· + 1,2-DCB → Chlorobenzene + H + + Cl − 2H· + Chlorobenzene → Benzene + H + + Cl − In the case of Ni/Pd bimetallic NPs, as Ni is oxidized, protons from methanol are reduced to form molecular hydrogen at the Ni surface (Equations (1) and (2)). On the Ni/Pd bimetallic NPs surface, a galvanic cell process is involved [45], as shown in Figure 4. Ni and Pd act as anode and cathode, respectively, and the electrons that are transferred from Ni reduce the protons to form highly active atomic hydrogen radicals (H·) and hydrogen gas (H 2 ) at the surface of Pd. The H 2 gas is then absorbed onto the Pd NPs and thus partially dissociated back to atomic hydrogen radicals (Equation (3)), which acts as the primary reactive species for hydro-dechlorination. It is noted that the remaining H 2 from the zero-valent Ni NPs synthesis process can also undertake the same processes and dechlorinate 1,2-DCB. When 1,2-DCB adsorbs onto the Pd surface, the cleavage of R-Cl bonds occurs via hydro-dechlorination (Equations (4) and (5)). Figure 4 shows the dechlorination mechanism of 1,2-DCB in the Ni/Pd bimetallic system. According to the results from prior studies, benzene is the ultimate 1,2-DCB dechlorination product [46,47]. Figure 5 shows the plot of degradation ratio of 1,2-DCB and the formation of chlorobenzene and benzene versus the reaction time for the dechlorination reaction of 1,2-DCB with the sample of 24.29 wt.% of Pd loading onto Ni NPs (for the GC chromatograms, see Figure S3) [2,39,40,[48][49][50]. In the case of Ni/Pd bimetallic NPs, as Ni is oxidized, protons from methanol are reduced to form molecular hydrogen at the Ni surface (Equations (1) and (2)). On the Ni/Pd bimetallic NPs surface, a galvanic cell process is involved [45], as shown in Figure 4. Ni and Pd act as anode and cathode, respectively, and the electrons that are transferred from Ni reduce the protons to form highly active atomic hydrogen radicals (H·) and hydrogen gas (H2) at the surface of Pd. The H2 gas is then absorbed onto the Pd NPs and thus partially dissociated back to atomic hydrogen radicals (Equation (3)), which acts as the primary reactive species for hydro-dechlorination. It is noted that the remaining H2 from the zero-valent Ni NPs synthesis process can also undertake the same processes and dechlorinate 1,2-DCB. When 1,2-DCB adsorbs onto the Pd surface, the cleavage of R-Cl bonds occurs via hydro-dechlorination (Equations (4) and (5)). Figure 4 shows the dechlorination mechanism of 1,2-DCB in the Ni/Pd bimetallic system. According to the results from prior studies, benzene is the ultimate 1,2-DCB dechlorination product [46,47]. Figure 5 shows the plot of degradation ratio of 1,2-DCB and the formation of chlorobenzene and benzene versus the reaction time for the dechlorination reaction of 1,2-DCB with the sample of 24.29 wt.% of Pd loading onto Ni NPs (for the GC chromatograms, see Figure S3) [2,39,40,[48][49][50].  In the case of Ni/Pd bimetallic NPs, as Ni is oxidized, protons from methanol are reduced to form molecular hydrogen at the Ni surface (Equations (1) and (2)). On the Ni/Pd bimetallic NPs surface, a galvanic cell process is involved [45], as shown in Figure 4. Ni and Pd act as anode and cathode, respectively, and the electrons that are transferred from Ni reduce the protons to form highly active atomic hydrogen radicals (H·) and hydrogen gas (H2) at the surface of Pd. The H2 gas is then absorbed onto the Pd NPs and thus partially dissociated back to atomic hydrogen radicals (Equation (3)), which acts as the primary reactive species for hydro-dechlorination. It is noted that the remaining H2 from the zero-valent Ni NPs synthesis process can also undertake the same processes and dechlorinate 1,2-DCB. When 1,2-DCB adsorbs onto the Pd surface, the cleavage of R-Cl bonds occurs via hydro-dechlorination (Equations (4) and (5)). Figure 4 shows the dechlorination mechanism of 1,2-DCB in the Ni/Pd bimetallic system. According to the results from prior studies, benzene is the ultimate 1,2-DCB dechlorination product [46,47]. Figure 5 shows the plot of degradation ratio of 1,2-DCB and the formation of chlorobenzene and benzene versus the reaction time for the dechlorination reaction of 1,2-DCB with the sample of 24.29 wt.% of Pd loading onto Ni NPs (for the GC chromatograms, see Figure S3) [2,39,40,[48][49][50].

PLAL Conditions
Illustrative experimental outlines are well described elsewhere [34]. In brief, the Ni plate was placed in a 20 mL Pyrex vial of vigorously stirred solvent (10 mL). A LabVIEW program (8.5, National Instruments, Austin, TX, USA) was used to move the Ni plate continuously to provide a fresh sample for the laser ablation. A typical PLAL sequence involves the use of a 30 mm focal lens to focus a pulsed laser onto the Ni plate (~1 mm spot). The laser ablation lasted for 20 min with a laser pulse energy of 80 mJ/pulse.

Preparation of Ni/Pd Bimetallic NPs
Monometallic Pd NPs were synthesized by mixing the PdCl 2 stock solution, prepared by dissolving PdCl 2 in methanol with 0.01 M ascorbic acid (as a reductant) solution, into the Ni colloidal solution with constant stirring at room temperature. Pd/Ni bimetallic NPs were fabricated by mixing the PdCl 2 stock solution into the Ni colloidal solution prepared by PLAL without the reductant [51,52]. When the PdCl 2 stock solution was added to the Ni colloidal solution, the color of the sample changed from dark brown to black, indicating the formation of Pd NPs. The Pd 2+ ions were easily reduced without a reductant by the electrons that were produced in the PLAL process of synthesizing the Ni NPs in methanol, which have been revealed by the previous studies [53]. The Ni powder/Pd NPs and Ni/Pd bimetallic NPs were synthesized by mixing the PdCl 2 stock solution to the prepared Ni powder and Ni NP solution with constant stirring and sonication for 30 min. The prepared Ni powder/Pd NPs and Ni/Pd bimetallic NPs solution was washed with methanol, and the residues were centrifuged at a rate of 13,000 rpm for 10 min. Different amounts of PdCl 2 (0.51 ×

Dechlorination of 1,2-DCB
All of the dechlorination experiments were performed with the sediments as catalysts (0.5 mg) and 1.6 mL of 1,2-DCB solution (1.0 × 10 −5 M) in methanol. The solutions were ultra-sonicated for 120 min in an ultrasonic water bath (38 kHz at 15-min intervals for each GC experiment). At 15-min intervals, 150 µL of the dechlorinated solution was mixed with 300 µL of hexane in an e-tube and centrifuged (13,000 rpm for 10 min). A 200 µL aliquot was removed from the e-tube and analyzed using gas chromatography.

Conclusions
In summary, we have presented a nanoscale Ni/Pd bimetallic complex by using the facile PLAL technique. The resulting NPs were efficient catalysts for the dechlorination reaction of COCs. The synthesized Ni, Pd, Ni powder/Pd, and Ni/Pd NPs were investigated for the degradation of 1,2-DCB. Among these, Ni/Pd bimetallic NPs showed the maximum degradation rate for 1,2-DCB. The surface chemistry of Ni/Pd bimetallic NPs was directly responsible for the degradation of 1,2-DCB. Furthermore, the optimum Pd loading onto Ni was found to be 19.16 wt.% (100% dechlorination efficiency after 30 min). The fast catalytic activity was attributed to the well-dispersed nature of Pd onto the Ni surface. The rapid dechlorination of 1,2-DCB suggests that the Ni/Pd bimetallic NPs can be used in the purification treatment of polychlorinated aromatic compounds in the environment.