Preparation of Palladium Nanoparticles Decorated Polyethyleneimine / Polycaprolactone Composite Fibers Constructed by Electrospinning with Highly E ﬃ cient and Recyclable Catalytic Performances

: Nano-sized palladium nanoparticles showed high catalytic activity with severe limitations in catalytic ﬁeld due to the tendency to aggregate. A solid substrate with large speciﬁc surface area is an ideal carrier for palladium nanoparticles. In present work, polyethyleneimine / polycaprolactone / Pd nanoparticles (PEI / PCL@PdNPs) composite catalysts were successfully designed and prepared by electrospinning and reduction methods using PEI / PCL elexctrospun ﬁber as carrier. The added PEI component e ﬀ ectively regulated the microscopic morphology of the PEI / PCL ﬁbers, following a large number of pit structures which increased the speciﬁc surface area of the electrospun ﬁbers and provided active sites for loading of the palladium particles. The obtained PEI / PCL@PdNPs catalysts for reductions of 4-nitrophenol (4-NP) and 2-nitroaniline (2-NA) exhibited extremely e ﬃ cient, stable, and reusable catalytic performance. It was worth mentioning that the reaction rate constant of catalytic reduction of 4-NP was as high as 0.16597 s − 1 . Therefore, we have developed a highly e ﬃ cient catalyst with potential applications in the ﬁeld of catalysis and water treatment.


Introduction
In recent years, there has been an increasing shortage of water sources available in the context of rapid technological and economic development, and widespread attention has been paid to the protection and purification of water sources. Harmful organic waste is a main source of water pollution. That conversion of harmful organic chemicals into harmless or low toxicity compounds under mild conditions has become an extremely important area of research. It is well known that 4-nitrophenol (4-NP) is widely used in the synthesis of dyes and medicines, and is one of the common chemical products used in the chemical industry [1]. However, it is prone to contaminate soil and water [2]. Methods for purifying such compounds are photocatalytic degradation [3], adsorption [4], microbial degradation [5], and nitro reduction [6], etc. Among these methods, the nitro reduction is one of the most commonly used methods and has the characteristics of simple process, mild conditions and environmental protection [7]. Therefore, it is important to develop effective catalysts to convert hazardous organics into reusable products. Figure 1 illustrated the experimental process of electrospinning, chemical modification and catalytic reduction in the preparation and application of PEI/PCL@PdNPs composite catalyst. Firstly, the granular PCL and the liquid PEI were dissolved in a good organic solvent, chloroform, to obtain a homogeneous electrospinning precursor solution which was magnetically stirred at room temperature overnight. The precursor solution, placed in a 10 mL syringe with a stainless steel needle (20G), was electrospun under a high voltage driving force to obtain a PEI/PCL composite fiber. Secondly, NaBH 4 as a good reducing agent reduced aqueous PdCl 2 to palladium to obtain a solution containing palladium nanoparticles. The prepared PEI/PCL fibers were immersed in the palladium nanoparticles solution for one hour to complete chemical modification and obtained PEI/PCL@PdNPs composites. Finally, the prepared PEI/PCL@PdNPs was used as catalyst to catalytically reduce 4-NP and 2-NA to explore its catalytic activity. Due to the large number of functional groups present in PEI, it was possible that some NH 2 groups reacted with the carbonyl ester bond, and other functional amine groups remained free on the surface, allowing them to bind to other molecules such as palladium [34].  Figure 2 depicted the scanning electron microscope (SEM) surface morphologies of PEI/PCL fibers (Samples 1-6) at different PEI contents. It was clear that the structures of the PEI/PCL electrospun fibers were related to the mixing ratio of PEI and PCL. PEI/PCL fibers of different mixing ratios exhibited inhomogeneous thickness structure. As the PEI content increased, the smooth PEI/PCL composite fibers gradually appeared pits on the fiber surface. The larger the mixing ratio of PEI to PCL, the more and deeper pits on the fiber surface. It was well known that parameters such as polymer viscosity and concentration were key factors influencing electrospinning [22]. The pit structure of the PEI/PCL electrospun fiber was attributed to the difference of PEI component contained in the polymer fibers. As shown in Table 1, the specific surface area of PEI/PCL fibers improved from 8.34 to 12.93 m 2 g −1 with the increment of PEI component in composite fibers. The electrospun fibers with pit-like surface structures further increased the specific surface area of the fibers, which increased the particles attachment sites and helpful to next modification of palladium nanoparticles on surface of the synthesized composite fiber.  Preparation process of polyethyleneimine/polycaprolactone/Pd nanoparticles (PEI/PCL@PdNPs) composite and its catalytic performance. Figure 2 depicted the scanning electron microscope (SEM) surface morphologies of PEI/PCL fibers (Samples 1-6) at different PEI contents. It was clear that the structures of the PEI/PCL electrospun fibers were related to the mixing ratio of PEI and PCL. PEI/PCL fibers of different mixing ratios exhibited inhomogeneous thickness structure. As the PEI content increased, the smooth PEI/PCL composite fibers gradually appeared pits on the fiber surface. The larger the mixing ratio of PEI to PCL, the more and deeper pits on the fiber surface. It was well known that parameters such as polymer viscosity and concentration were key factors influencing electrospinning [22]. The pit structure of the PEI/PCL electrospun fiber was attributed to the difference of PEI component contained in the polymer fibers. As shown in Table 1, the specific surface area of PEI/PCL fibers improved from 8.34 to 12.93 m 2 g −1 with the increment of PEI component in composite fibers. The electrospun fibers with pit-like surface structures further increased the specific surface area of the fibers, which increased the particles attachment sites and helpful to next modification of palladium nanoparticles on surface of the synthesized composite fiber.  Figure 2 depicted the scanning electron microscope (SEM) surface morphologies of PEI/PCL fibers (Samples 1-6) at different PEI contents. It was clear that the structures of the PEI/PCL electrospun fibers were related to the mixing ratio of PEI and PCL. PEI/PCL fibers of different mixing ratios exhibited inhomogeneous thickness structure. As the PEI content increased, the smooth PEI/PCL composite fibers gradually appeared pits on the fiber surface. The larger the mixing ratio of PEI to PCL, the more and deeper pits on the fiber surface. It was well known that parameters such as polymer viscosity and concentration were key factors influencing electrospinning [22]. The pit structure of the PEI/PCL electrospun fiber was attributed to the difference of PEI component contained in the polymer fibers. As shown in Table 1, the specific surface area of PEI/PCL fibers improved from 8.34 to 12.93 m 2 g −1 with the increment of PEI component in composite fibers. The electrospun fibers with pit-like surface structures further increased the specific surface area of the fibers, which increased the particles attachment sites and helpful to next modification of palladium nanoparticles on surface of the synthesized composite fiber.   In order to further observe the structure of as-prepared PEI/PCL@PdNPs composite, the transmission electron microscopy (TEM) morphologies of the obtained fibers were examined, as shown in Figure 3. It was obvious that the surface of PEI/PCL fiber (sample 6, w/w, 35:65) was uneven, as shown in Figure 3b. The rugged structure increased the specific surface area helpful to next chemical modification of the fibers. It could be seen in Figure 3c that the PdNPs modified on the surface of PEI/PCL fibers showed the mean size range of 5-10 nm. As shown in Figure 3d, the interplanar spacing of the palladium nanoparticles was 0.2243 nm, which well matched the (111) crystal plane of Pd [35]. Furthermore, the Pd element mapping shown in Figure 3e was good evidence of the presence and distribution of palladium nanoparticles on the surface of the prepared PEI/PCL fibers. It was reasonably speculated that hydrophilic PdNPs were successfully anchored on the surface of PEI/PCL composite fibers via weak intermolecular interactions, then the composites could be expected good catalytic activity in next catalytic process [36,37].  In order to further observe the structure of as-prepared PEI/PCL@PdNPs composite, the transmission electron microscopy (TEM) morphologies of the obtained fibers were examined, as shown in Figure 3. It was obvious that the surface of PEI/PCL fiber (sample 6, w/w, 35:65) was uneven, as shown in Figure 3b. The rugged structure increased the specific surface area helpful to next chemical modification of the fibers. It could be seen in Figure 3c that the PdNPs modified on the surface of PEI/PCL fibers showed the mean size range of 5-10 nm. As shown in Figure 3d, the interplanar spacing of the palladium nanoparticles was 0.2243 nm, which well matched the (111) crystal plane of Pd [35]. Furthermore, the Pd element mapping shown in Figure 3e was good evidence of the presence and distribution of palladium nanoparticles on the surface of the prepared PEI/PCL fibers. It was reasonably speculated that hydrophilic PdNPs were successfully anchored on the surface of PEI/PCL composite fibers via weak intermolecular interactions, then the composites could be expected good catalytic activity in next catalytic process [36,37].   In addition, the morphology of PEI/PCL@PdNPs was characterized by scanning electron microscope (SEM), as shown in Figure 4. Figure 4a depicted the SEM structure of PEI/PCL@PdNPs, and it was apparent that deposits were present on the surface of the PEI/PCL fibers. The energy dispersive spectroscopy (EDS) image of the obtained PEI/PCL@PdNPs shown in Figure 4b-d clearly showed the presence of carbon, oxygen, nitrogen, and palladium elements. Due to the shielding effect, only a part of the nitrogen element in the PEI could be displayed, as shown in Figure 4d. The presence of small amounts of gold and copper was due to the coated-gold treatment and copper substrate used during the measurement. From EDS data, the detailed atomic percentages were found to be C = 75.37%, O = 12.61%, N = 7.88%, and Pd = 1.23%. The above results indicated that the palladium nanoparticles were successfully modified on the surface of the PEI/PCL fibers. In addition, the morphology of PEI/PCL@PdNPs was characterized by scanning electron microscope (SEM), as shown in Figure 4. Figure 4a depicted the SEM structure of PEI/PCL@PdNPs, and it was apparent that deposits were present on the surface of the PEI/PCL fibers. The energy dispersive spectroscopy (EDS) image of the obtained PEI/PCL@PdNPs shown in Figure 4b-d clearly showed the presence of carbon, oxygen, nitrogen, and palladium elements. Due to the shielding effect, only a part of the nitrogen element in the PEI could be displayed, as shown in Figure 4d. The presence of small amounts of gold and copper was due to the coated-gold treatment and copper substrate used during the measurement. From EDS data, the detailed atomic percentages were found to be C = 75.37%, O = 12.61%, N = 7.88%, and Pd = 1.23%. The above results indicated that the palladium nanoparticles were successfully modified on the surface of the PEI/PCL fibers. The prepared PEI/PCL fibers and PEI/PCL@PdNPs composite were further characterized by Xray diffraction spectroscopy, as shown in Figure 5. It was obvious from curves (a) and (b) that both the PEI/PCL composite fibers with different weight ratios showed characteristic diffraction peaks at 2θ = 21.4° and 23.8°. The PEI/PCL fiber with low PEI component showed strong characteristic peaks, while the binary fiber with high PEI component showed the weaker intensity of the characteristic peak. The difference in the intensity of the characteristic peaks of PEI/PCL fibers with different blend ratios demonstrated that the higher the PEI component, the lower the crystallinity of the composite fibers. From the X-ray diffraction spectrum of the PEI/PCL@PdNPs composite, it could be seen that a characteristic peak appeared at 2θ = 40.1°, corresponding to the (111) crystal plane of the palladium particles, which further proved that the palladium nanoparticles successfully modified the PEI/PCL composite fibers [38,39]. In addition, the characteristic peak intensity of the PEI/PCL@PdNPs composite increased as the palladium particles were successfully modified on the surface of the PEI/PCL composite fibers.
To investigate the thermal stability of the prepared PEI/PCL@PdNPs composite, the thermogravimetric characterization of the obtained PCL fibers, PEI/PCL fibers, and PEI/PCL@PdNPs composite were measured under argon gas atmosphere, as shown in Figure 6. TG analysis of the three composite fiber materials showed that the weight loss was slower when the temperature was lower than 120 °C due to the volatilization of residual moisture in the samples [40]. In addition, when the temperature was higher than 500 °C, the quality of the three species of samples tended to be stable. Between 120 °C and 450 °C, the weight of above-mentioned three samples was firstly slow loss, and then the weight of these samples was sharply lost at 400 °C, which was due to the thermal decomposition of various chemical groups and alkyl chains contained in these samples at high The prepared PEI/PCL fibers and PEI/PCL@PdNPs composite were further characterized by X-ray diffraction spectroscopy, as shown in Figure 5. It was obvious from curves (a) and (b) that both the PEI/PCL composite fibers with different weight ratios showed characteristic diffraction peaks at 2θ = 21.4 • and 23.8 • . The PEI/PCL fiber with low PEI component showed strong characteristic peaks, while the binary fiber with high PEI component showed the weaker intensity of the characteristic peak. The difference in the intensity of the characteristic peaks of PEI/PCL fibers with different blend ratios demonstrated that the higher the PEI component, the lower the crystallinity of the composite fibers. From the X-ray diffraction spectrum of the PEI/PCL@PdNPs composite, it could be seen that a characteristic peak appeared at 2θ = 40.1 • , corresponding to the (111) crystal plane of the palladium particles, which further proved that the palladium nanoparticles successfully modified the PEI/PCL composite fibers [38,39]. In addition, the characteristic peak intensity of the PEI/PCL@PdNPs composite increased as the palladium particles were successfully modified on the surface of the PEI/PCL composite fibers.
To investigate the thermal stability of the prepared PEI/PCL@PdNPs composite, the thermogravimetric characterization of the obtained PCL fibers, PEI/PCL fibers, and PEI/PCL@PdNPs composite were measured under argon gas atmosphere, as shown in Figure 6. TG analysis of the three composite fiber materials showed that the weight loss was slower when the temperature was lower than 120 • C due to the volatilization of residual moisture in the samples [40]. In addition, when the temperature was higher than 500 • C, the quality of the three species of samples tended to be stable. Between 120 • C and 450 • C, the weight of above-mentioned three samples was firstly slow loss, and then the weight of these samples was sharply lost at 400 • C, which was due to the thermal decomposition of various chemical groups and alkyl chains contained in these samples at high temperature [41]. The tempertature points with mass loss 5 wt.% showed the values of 353, 311, and 236 • C for PCL fibers, PEI/PCL fibers, and PEI/PCL@PdNPs composite fibers, respectively, indicating that the addition of the PEI component and PdNPs reduced the thermal stability of the composite fibers. The above significant difference in the thermal stability could reasonably be attributed to the inclusion of palladium nanoparticles on the surface of the composite fibers.
Catalysts 2019, 9, x FOR PEER REVIEW 6 of 15 temperature [41]. The tempertature points with mass loss 5 wt.% showed the values of 353, 311, and 236 °C for PCL fibers, PEI/PCL fibers, and PEI/PCL@PdNPs composite fibers, respectively, indicating that the addition of the PEI component and PdNPs reduced the thermal stability of the composite fibers. The above significant difference in the thermal stability could reasonably be attributed to the inclusion of palladium nanoparticles on the surface of the composite fibers.  Next, X-ray photoelectron spectroscopy (XPS) was measured to investigate the interface element composition and the reduction level of Pd 2+ of the obtained PEI/PCL@PdNPs composite. The results of XPS spectroscopy of PEI/PCL fiber and PEI/PCL@PdNPs composite were shown in Figure 7a. The signals of C1s, N1s and O1s appeared in the XPS full spectrum of PEI/PCL fibers, while the new Pd3d and Si2p signals appeared in the XPS full spectrum of the prepared PEI/PCL@PdNPs. The Si2p signal is due to the contact of the sample with the silicon plate during the examining process. From surface analysis data, the detailed atomic percentages were found to be C = 71.44%, O = 16.08%, N = 6.63%, and Pd = 0.88%. Figure 7b was the deconvolution of the Pd3d peak in the prepared PEI/PCL@PdNPs composite. The XPS spectrum of Pd3d showed double peaks with binding energies at 334.5 eV and 339.7 eV, corresponding to the Pd3d5/2 and Pd3d3/2 components of the metal Pd(0) state, respectively, which confirmed the presence of metal Pd [20,[42][43][44]. The above result indicated that excess NaBH4 reduced all Pd 2+ in PdCl2 to Pd 0 without any residual Pd 2+ ions. The mentioned XPS results further demonstrated the successful preparation of PEI/PCL@PdNPs composites.    Next, X-ray photoelectron spectroscopy (XPS) was measured to investigate the interface element composition and the reduction level of Pd 2+ of the obtained PEI/PCL@PdNPs composite. The results of XPS spectroscopy of PEI/PCL fiber and PEI/PCL@PdNPs composite were shown in Figure 7a. The signals of C1s, N1s and O1s appeared in the XPS full spectrum of PEI/PCL fibers, while the new Pd3d and Si2p signals appeared in the XPS full spectrum of the prepared PEI/PCL@PdNPs. The Si2p signal is due to the contact of the sample with the silicon plate during the examining process. From surface analysis data, the detailed atomic percentages were found to be C = 71.44%, O = 16.08%, N = 6.63%, and Pd = 0.88%. Figure 7b was the deconvolution of the Pd3d peak in the prepared PEI/PCL@PdNPs composite. The XPS spectrum of Pd3d showed double peaks with binding energies at 334.5 eV and 339.7 eV, corresponding to the Pd3d5/2 and Pd3d3/2 components of the metal Pd(0) state, respectively, which confirmed the presence of metal Pd [20,[42][43][44]. The above result indicated that excess NaBH4 reduced all Pd 2+ in PdCl2 to Pd 0 without any residual Pd 2+ ions. The mentioned XPS results further demonstrated the successful preparation of PEI/PCL@PdNPs composites. Next, X-ray photoelectron spectroscopy (XPS) was measured to investigate the interface element composition and the reduction level of Pd 2+ of the obtained PEI/PCL@PdNPs composite. The results of XPS spectroscopy of PEI/PCL fiber and PEI/PCL@PdNPs composite were shown in Figure 7a. The signals of C1s, N1s and O1s appeared in the XPS full spectrum of PEI/PCL fibers, while the new Pd3d and Si2p signals appeared in the XPS full spectrum of the prepared PEI/PCL@PdNPs. The Si2p signal is due to the contact of the sample with the silicon plate during the examining process. From surface analysis data, the detailed atomic percentages were found to be C = 71.44%, O = 16.08%, N = 6.63%, and Pd = 0.88%. Figure 7b was the deconvolution of the Pd3d peak in the prepared PEI/PCL@PdNPs composite. The XPS spectrum of Pd3d showed double peaks with binding energies at 334.5 eV and 339.7 eV, corresponding to the Pd3d 5/2 and Pd3d 3/2 components of the metal Pd(0) state, respectively,

Catalytic Performance of PEI/PCL@PdNPs Composites
Free palladium nanoparticles exhibit high catalytic activity, while the tendency to aggregate during the catalytic process reduced their catalytic performance. Various palladium composite catalysts reported previously exhibited different catalytic performance at 25 C, as shown in Table 2 [17-20, [45][46][47]. Therefore, the prepared PEI/PCL@PdNPs composite catalyst was applied to catalytic reduction of 4-NP and 2-NA to explore its catalytic performance. The catalytic activity of PEI/PCL@PdNPs containing active functional groups such as C=O and -NH2 was attributed to the interaction of PdNPs with the 4-NP substrate adjacent to the catalytic site to promote the reduction of the nitro group. It could be reasonably speculated that the PEI/PCL@PdNPs played a leading role during the catalytic reduction process. When the PEI/PCL@PdNPs catalyst was added to the solution system of the oxidant 4-NP and the reducing agent NaBH4, the electrons were transferred from the BH4 − donor to the 4-NP receptor, thereby producing amino derivative [17]. The 4-nitrophenolate and BH4 − were adsorbed on the surface of the PEI/PCL@PdNPs catalyst by weak electrostatic interaction. When electrons transferred to Pd NPs, the active hydrogen groups formed and reduced 4-nitrophenol molecules [17]. This electron transfer induced 4-NP hydrogenation spontaneously occurred on the surface of the metal catalyst. Finally, 4-nitroaniline (4-AP) was desorbed from the catalyst surface.
The catalytic performance of PEI/PCL@PdNPs catalyst for 4-NP and 2-NA was observed by UVvisible spectroscopy at room temperature. As shown in Figure 8a, it was clear that the maximum absorption peak at 317 nm of 4-NP aqueous solution shifted to 400 nm owing to the addition of freshly prepared reducing agent NaBH4, indicating the formation of 4-nitrophenol ions [48]. At the same time, the color of the mixed solution system changed from light yellow to bright yellow. In addition, 10 mg of PEI/PCL@PdNPs catalyst (PEI:PCL, w/w, 35:65) was added to the system of 4-NP (10 mL, 0.005 M) and NaBH4 (20 mL, 0.1 M). As shown in Figure 8b, after several seconds, the intensity of the absorption peak at 400 nm significantly decreased and a new absorption peak appeared at 300 nm. Referring to previous literature, the emerging absorption peak corresponded to 4-AP [20]. After 20 seconds, the intensity of the absorption peak at 400 nm of the 4-NP and NaBH4 mixed system decreased to zero, and the color of the solution became transparent (inset photo in Figure 8b), indicating that all of the 4-NP was reduced to 4-AP. The PEI/PCL@PdNPs catalyst showed high catalytic activity for the reduction of 4-NP, which is attributed to the active sites of PEI/PCL substrate fibers for palladium nanoparticles. In addition, the prepared PEI/PCL@PdNPs composite with weight ratios (w/w, 10:90) catalyst was applied to catalytic reduction of 4-NP to explore its catalytic performance, as shown Figure S1. The PEI/PCL@PdNPs catalyst at 10:90 weight ratio completely reduced 4-NP to 4-AP in 100 s, and the rate constant k of catalytic reduction reaction was 0.00061 s −1 . At the same time, The loading of Pd in the catalyst was found to be Pd = 0.63% in the detailed atomic percentages of PEI/PCL@PdNPs (w/w, 10:90) material. Compared with the PEI/PCL@PdNPs

Catalytic Performance of PEI/PCL@PdNPs Composites
Free palladium nanoparticles exhibit high catalytic activity, while the tendency to aggregate during the catalytic process reduced their catalytic performance. Various palladium composite catalysts reported previously exhibited different catalytic performance at 25 • C, as shown in Table 2 [17][18][19][20][45][46][47]. Therefore, the prepared PEI/PCL@PdNPs composite catalyst was applied to catalytic reduction of 4-NP and 2-NA to explore its catalytic performance. The catalytic activity of PEI/PCL@PdNPs containing active functional groups such as C=O and -NH 2 was attributed to the interaction of PdNPs with the 4-NP substrate adjacent to the catalytic site to promote the reduction of the nitro group. It could be reasonably speculated that the PEI/PCL@PdNPs played a leading role during the catalytic reduction process. When the PEI/PCL@PdNPs catalyst was added to the solution system of the oxidant 4-NP and the reducing agent NaBH 4 , the electrons were transferred from the BH 4 − donor to the 4-NP receptor, thereby producing amino derivative [17]. The 4-nitrophenolate and BH 4 − were adsorbed on the surface of the PEI/PCL@PdNPs catalyst by weak electrostatic interaction. When electrons transferred to Pd NPs, the active hydrogen groups formed and reduced 4-nitrophenol molecules [17]. This electron transfer induced 4-NP hydrogenation spontaneously occurred on the surface of the metal catalyst. Finally, 4-nitroaniline (4-AP) was desorbed from the catalyst surface. The catalytic performance of PEI/PCL@PdNPs catalyst for 4-NP and 2-NA was observed by UV-visible spectroscopy at room temperature. As shown in Figure 8a, it was clear that the maximum absorption peak at 317 nm of 4-NP aqueous solution shifted to 400 nm owing to the addition of freshly prepared reducing agent NaBH 4 , indicating the formation of 4-nitrophenol ions [48]. At the same time, the color of the mixed solution system changed from light yellow to bright yellow. In addition, 10 mg of PEI/PCL@PdNPs catalyst (PEI:PCL, w/w, 35:65) was added to the system of 4-NP (10 mL, 0.005 M) and NaBH 4 (20 mL, 0.1 M). As shown in Figure 8b, after several seconds, the intensity of the absorption peak at 400 nm significantly decreased and a new absorption peak appeared at 300 nm. Referring to previous literature, the emerging absorption peak corresponded to 4-AP [20]. After 20 seconds, the intensity of the absorption peak at 400 nm of the 4-NP and NaBH 4 mixed system decreased to zero, and the color of the solution became transparent (inset photo in Figure 8b), indicating that all of the 4-NP was reduced to 4-AP. The PEI/PCL@PdNPs catalyst showed high catalytic activity for the reduction of 4-NP, which is attributed to the active sites of PEI/PCL substrate fibers for palladium nanoparticles. In addition, the prepared PEI/PCL@PdNPs composite with weight ratios (w/w, 10:90) catalyst was applied to catalytic reduction of 4-NP to explore its catalytic performance, as shown Figure S1. The PEI/PCL@PdNPs catalyst at 10:90 weight ratio completely reduced 4-NP to 4-AP in 100 s, and the rate constant k of catalytic reduction reaction was 0.00061 s −1 . At the same time, The loading of Pd in the catalyst was found to be Pd = 0.63% in the detailed atomic percentages of PEI/PCL@PdNPs (w/w, 10:90) material. Compared with the PEI/PCL@PdNPs composite at 35:65 weight ratio, it could be reasonably speculated that the improved catalytic performance was owing to more palladium nanoparticles anchored on the surface of PEI/PCL ((w/w), 35:65) fibers with the increment of PEI component in fiber structures. For comparison, the synthesized palladium nanoparticle in solution catalytically reduced the same solution system of 4-NP (10 mL, 0.005 M) and NaBH 4 (20 mL, 0.1 M), and completely reduced 4-NP after 10 minutes, The kinetic constant k of the reduction reaction was 0.006 s −1 as shown in Figure S2. The catalytic performance of PEI/PCL@PdNPs catalyst was significantly better than that of palladium nanoparticles in solution, because the PEI/PCL substrate improved the dispersibility of palladium nanoparticles. At the same time, 10 mg of PEI/PCL fibers without Pd nanoparticles was added to the system of 4-NP (10 mL, 0.005 M) and NaBH 4 (20 mL, 0.1 M). After 20 min, no color changes in the solution system had occurred. Moreover, the catalytic properties of PEI/PCL@PdNPs catalyst for the reduction of 2-NA were investigated by the same method. As shown in Figure 8d,e, it was worth mentioning that there was no significant change in the absorption peak of the mixed solution after adding fresh NaBH 4 to the 2-NA solution. After 9 min of addition of the PEI/PCL@PdNPs catalyst, the 2-NA was completely reduced to o-phenylenediamine (OPD), as reported in the literature [49]. The prepared PEI/PCL@PdNPs exhibited different catalytic capabilities for 4-NP and 2-NA. Apparently, PEI/PCL@PdNPs exhibited higher catalytic efficiency for 4-NP. Furthermore, the component amont of palladium nanoparticles on fibers surface depended on the modification times. Compared with appropriate and enough modification time like 1 hour, palladium nanoparticles were hardly found on the surface of PEI/PCL composite fibers at 10 minutes modification with poor catalytic performance. by PEI/PCL@PdNPs, respectively, reflecting the rate of catalytic reduction of 4-NP and 2-NA by PEI/PCL@PdNPs. The kinetic constant k of the above catalytic reduction reaction was calculated to be 0.16597s −1 and 0.00401 s −1 , respectively. The kinetic constant k could intuitively reflect the speed of the catalytic reduction reaction [58][59][60][61][62][63][64][65]. Obviously, PEI/PCL@PdNPs catalytically reduced 4-NP faster.
dCt/dt = −kt or ln(Ct/C0) = ln(At/A0) = −kt (1) The stability and recyclability of the catalyst was another aspect of evaluating the catalyst. Therefore, it was important and necessary to explore the repeated catalytic ability of PEI/PCL@PdNPs catalyst for 4-NP and 2-NA, as shown in Figure 9. The catalytic efficiency of the PEI/PCL@PdNPs catalyst after 8 cycles of repeated catalytic reduction of fresh 4-NP and 2-NA systems were 95% and 92%, respectively. The above result indicated the high catalytic activity, stability and recyclability of the PEI/PCL@PdNPs catalyst. Referring to previous literature, the few decrement in catalytic efficiency was reanonably attributed to the attachment of organics on the surface of catalyst and the loss of palladium particles on the catalyst surface during the washing of PEI/PCL@PdNPs with ethanol and ultrapure water [36]. Since the concentration of excess NaBH 4 in the catalytic reaction was much higher than that of 4-NP or 2-NA, the concentration of NaBH 4 in the mixed solution system could be regarded as a constant. Therefore, the concentration of 4-NP or 2-NA was a physical quantity that changed with time t, and the catalytic reduction reaction was a pseudo-first-order reaction [50][51][52][53][54][55][56][57]. The rate of the catalytic reaction (containing adsorption process) was evaluated according to Lambert-Beer's law and pseudo-first-order reaction kinetics (Equation (1)), where A 0 , A t and C 0 , C t were the absorbance and concentration of the mixed solution at the initial and time t, respectively. Figure 8c and f were the linear relationships between the reaction time t and ln(C t /C 0 ) of catalytic reduction of 4-NP and 2-NA by PEI/PCL@PdNPs, respectively, reflecting the rate of catalytic reduction of 4-NP and 2-NA by PEI/PCL@PdNPs. The kinetic constant k of the above catalytic reduction reaction was calculated to be 0.16597s −1 and 0.00401 s −1 , respectively. The kinetic constant k could intuitively reflect the speed of the catalytic reduction reaction [58][59][60][61][62][63][64][65]. Obviously, PEI/PCL@PdNPs catalytically reduced 4-NP faster.
The stability and recyclability of the catalyst was another aspect of evaluating the catalyst. Therefore, it was important and necessary to explore the repeated catalytic ability of PEI/PCL@PdNPs catalyst for 4-NP and 2-NA, as shown in Figure 9. The catalytic efficiency of the PEI/PCL@PdNPs catalyst after 8 cycles of repeated catalytic reduction of fresh 4-NP and 2-NA systems were 95% and 92%, respectively. The above result indicated the high catalytic activity, stability and recyclability of the PEI/PCL@PdNPs catalyst. Referring to previous literature, the few decrement in catalytic efficiency was reanonably attributed to the attachment of organics on the surface of catalyst and the loss of palladium particles on the catalyst surface during the washing of PEI/PCL@PdNPs with ethanol and ultrapure water [36]. Catalysts 2019, 9, x FOR PEER REVIEW 10 of 15

Preparation of PEI/PCL@PdNPs Composites
11% (w/v) polycaprolactone polymer solution was prepared by dissolving PCL in chloroform referring to previous literature [66,67]. Secondly, according to the different blend ratios of PEI:PCL (10:90, 15:85, 20:80, 25:75, 30:70, 35:65 (w/w)), PEI were added to the abovementioned 11% polycaprolactone solution to obtained different concentration gradients of PEI/PCL mixed solutions. The PEI/PCL mixed solutions of different blend ratios were magnetically stirred overnight at room temperature to obtain uniform electrospinning precursor solutions. The precursor solutions were sequentially electrospun under the parameters of 15 kV voltage, 1 mL/h flow rate, and 25 cm distance between the needle and the receiving plate to obtain PEI/PCL electrospun composite fibers (samples 1-6) of different blend ratios. 0.003 M PdCl2 aqueous solution and 0.01 M NaBH4 solution were disposed in 50 mL volumetric bottles respectively. The above-mentioned PdCl2 aqueous solution (20 mL) in a clean beaker was stirred vigorously and subsequently added 0.01 M NaBH4 solution drop by drop until the color of the solution changed from khaki to dark brown, indicating the reduction of Pd 2+ to Pd 0 and the formation of Pd particles [68]. Then two species of 10 mg PEI/PCL composite fibers (sample 1 and 6) were added to the above-mentioned Pd particles solution and continued stirring vigorously to avoid the Pd particles agglomeration. After an hour, the PEI/PCL fibers were taken out and washed with deionized water three times before drying. Thus, the PEI/PCL@PdNPs composite materials (sample 7 and 8) were obtained and completed the chemical modification of Pd particles.

Catalytic Performance of PEI/PCL@PdNPs Composite
According to the previous literature, the prepared PEI/PCL@PdNPs composite was used as a catalyst to reduce 4-NP and 2-NA. The entire catalytic experiment was carried out at 25 C. Fresh NaBH4 solution (20 mL, 0.1 M) was thoroughly mixed with 4-NP solution (10 mL, 0.005 M), and the UV-vis spectrum of the mixed solution was measured as the absorbance value at the initial time (0

Preparation of PEI/PCL@PdNPs Composites
11% (w/v) polycaprolactone polymer solution was prepared by dissolving PCL in chloroform referring to previous literature [66,67]. Secondly, according to the different blend ratios of PEI:PCL (10:90, 15:85, 20:80, 25:75, 30:70, 35:65 (w/w)), PEI were added to the abovementioned 11% polycaprolactone solution to obtained different concentration gradients of PEI/PCL mixed solutions. The PEI/PCL mixed solutions of different blend ratios were magnetically stirred overnight at room temperature to obtain uniform electrospinning precursor solutions. The precursor solutions were sequentially electrospun under the parameters of 15 kV voltage, 1 mL/h flow rate, and 25 cm distance between the needle and the receiving plate to obtain PEI/PCL electrospun composite fibers (samples 1-6) of different blend ratios. 0.003 M PdCl 2 aqueous solution and 0.01 M NaBH 4 solution were disposed in 50 mL volumetric bottles respectively. The above-mentioned PdCl 2 aqueous solution (20 mL) in a clean beaker was stirred vigorously and subsequently added 0.01 M NaBH 4 solution drop by drop until the color of the solution changed from khaki to dark brown, indicating the reduction of Pd 2+ to Pd 0 and the formation of Pd particles [68]. Then two species of 10 mg PEI/PCL composite fibers (sample 1 and 6) were added to the above-mentioned Pd particles solution and continued stirring vigorously to avoid the Pd particles agglomeration. After an hour, the PEI/PCL fibers were taken out and washed with deionized water three times before drying. Thus, the PEI/PCL@PdNPs composite materials (sample 7 and 8) were obtained and completed the chemical modification of Pd particles.

Catalytic Performance of PEI/PCL@PdNPs Composite
According to the previous literature, the prepared PEI/PCL@PdNPs composite was used as a catalyst to reduce 4-NP and 2-NA. The entire catalytic experiment was carried out at 25 • C. Fresh NaBH 4 solution (20 mL, 0.1 M) was thoroughly mixed with 4-NP solution (10 mL, 0.005 M), and the UV-vis spectrum of the mixed solution was measured as the absorbance value at the initial time (0 s). Subsequently, 10 mg of the prepared PEI/PCL@PdNPs composite was quickly added to the mixed solution. The absorbance of the mixed solution was measured by UV-vis spectroscopy at intervals (5 s, 15 s, 20 s) until it was reduced to zero, and the catalytic curve of PEI/PCL@PdNPs catalyst for 4-NP was obtained. The same method was used to investigate the catalytic performance of PEI/PCL@PdNPs catalyst for 2-NA solution. NaBH 4 acted as a reducing agent during the catalytic reduction process. At the same time, according to the above experimental method, PEI/PCL@PdNPs catalyst was carried out repeated catalytic experiments on fresh 4-NP and NaBH 4 mixed solutions. After the catalytic reduction reaction is completed, the PEI/PCL@PdNPs catalyst was removed and the washed several times with ethanol and ultrapure water until the color of the washing liquid did not change. The washed PEI/PCL@PdNPs were then dried for the next catalytic experiment. Catalytic reduction of fresh 4-NP and 2-NA solutions was carried out 8 times in succession.

Characterization
Scanning electron microscope (SEM, FEI Corporate, Hillsboro, OR, USA) with a field emission gun FEI QUANTA FEG 250 were measured to obtain the morphologies of prepared electrospun fibers. Before measurement, the samples were coated with AuNPs by sputtering in vacuum for 30 seconds. In addition, energy dispersive spectroscopy (EDS) were utilized to acquire the chemical elements analysis of the as-obtained fiber samples. The morphology of the samples was further characterized via Transmission electron microscopy (TEM, HT7700, High-Technologies Corp., Ibaraki, Japan).The microstructure was observed by High-resolution transmission electron microscopy (HRTEM, Tecnai-G2 F30 S-TWIN, Philips, Netherlands). X-ray diffraction patterns were conducted on an X-ray diffractometer (SMART LAB, Rigaku, Akishima, Japan) with a CuKα X-ray radiation source and a Bragg diffraction apparatus. Thermogravimetry-differential scanning calorimetry (TG-DSC) analysis was carried out by NETZSCH STA 409 PC Luxxsi thermal analyzer (Netzsch Instruments Manufacturing Co., Ltd., Seligenstadt, Germany) under argon gas atmosphere. X-ray photoelectron spectroscopy (XPS) was investigated on Thermo Scientific ESCALab 250Xi (Netzsch Instrument Manufacturing GmbH, Seligenstadt, Germany) equipped with 200 W of monochromatic AlKα radiation. UV-vis absorption spectra were measured with UV-visible spectroscopy (UV-2550, Shimadzu Corporation, Tokyo, Japan). BET measurements (NOVA 4200-P, Quantachrome Instruments, Boynton Beach, FL, USA) were obtained to characterize the specific surface areas and pore diameter distribution. The surface areas were calculated by the Brunauer-Emmett-Teller (BET) method.

Conclusions
In summary, we have successfully designed and prepared PEI/PCL@PdNPs composite fiber catalyst with remarkable catalytic activity and stability. After the microscopic morphology of the PEI/PCL fibers was regulated, the plenty of pits structures appeared on the surface of the fibers increased the specific surface area of solid substrate fibers and provided abundant active attachment sites and sufficient space for the chemical modification of the palladium nanoparticles, which favored the adhesion of the palladium particles. The catalytic reduction of 4-nitrophenol and 2-nitroaniline by PEI/PCL@PdNPs catalyst exhibited fast, stable and reusable catalytic performance. This work provided new research clues for the preparation of composites loaded with PdNPs and ideal metal particle-based carriers.