Ternary N , S , and P-Doped Hollow Carbon Spheres Derived from Polyphosphazene as Pd Supports for Ethanol Oxidation Reaction

Ethanol oxidation reaction (EOR) is an important electrode reaction in ethanol fuel cells. However, there are many problems with commercial ethanol oxidation electrocatalysts today, such as poor durability, poor anti-CO poisoning ability, and low selectivity for C–C bond cleavage. Therefore, it is very meaningful to develop a high-performance EOR catalyst. Herein, we designed ternary N, S, and P-doped hollow carbon spheres (C–N,P,S) from polyphosphazene (PCCP) as Pd supports for EOR. Using SiO2 spheres as the templates, the PCCP was first coated on the surfaces of SiO2 spheres by in situ polymerization. Through high-temperature pyrolysis and hydrofluoric acid-etching, the hollow PCCP has a large surface area and porous structure. After loading Pd nanoparticles (NPs), the Pd/C–N, P, S catalysts with Pd NPs decorated on the surfaces of C–N, P, S can achieve a high mass peak current density of 1686 mA mgPd, which was 2.8 times greater than that of Pd/C. Meanwhile, the Pd/C–N, P, S catalyst also shows a better stability than that of Pd/C after a durability test of 3600s.


Introduction
Recently, direct ethanol fuel cells (DEFCs) have become the ideal solution to the energy crisis and environmental issues and, therefore, have attracted enormous interest.Specifically, DEFCs have many advantages such as high efficiency, high energy density, low pollution, and excellent electrochemical stability.Besides, the biomass fuel ethanol is widely used because of its many advantages, such as easy to store, non-toxic, simple to synthesize [1][2][3][4].The ethanol oxidation reaction (EOR) is a crucial reaction in DEFCs, which is the most important part of determining the performance of the entire fuel cell.Platinum (Pt) is the most widely used precious metal for EOR in acidic media because of its attractive characteristics of high electrochemical activity and low over potential.Unfortunately, high cost and poor anti-poisoning have inhibited Pt-based catalysts application in DEFCs [5,6].Because of this, many researchers have turned their attention to another metal.Some studies have found that Palladium (Pd) is a more suitable alternative than Pt in DEFC.Although the Pd-based catalyst activity in the acidic medium is not as good as that of the Pt-based catalyst, the Pd-based catalyst performs better than the Pt-based catalyst in the alkaline medium.Regarding price, Pd is cheaper than Pt, which significantly reduces the cost of experimentation and research [7][8][9][10][11].
As we all know, the activity and stability of an electrocatalyst depends significantly on the composition and structure of the supported catalysts and metallic nanoparticles.Therefore, the construction of a new type of Pd-incorporated supported catalyst can play an important role in DEFCs.Carbon-based materials often serve as catalyst supports because of their good electrical conductivity and stability.So far, graphene, carbon microspheres, carbon nanotubes and carbon black have widely been employed as supporting materials of Pd [12][13][14][15].As is well-known, catalyst supports need not only a large specific surface area but also good electrical conductivity, so that the noble metal particles have a high dispersion state while exposing more active sites.Many studies have shown that materials with hollow structures and mesopores can serve as good supports for electrocatalysts.Material such as mesoporous hollow carbon hemispheres [16], spherical carbon capsules [17], hollow graphitized carbon nanocage [18] were used as supports of noble metals electrocatalysts due to their large specific surface area and high stability.Moreover, core-shell structures are widely used in the preparation of catalysts, such as Pd@porous SiO 2 yolk-shell nanostructure [19], PdO/ZnO@mSiO 2 [20], Cu@Cu 2 O core-shell nanocatalyst [21].
On the other hand, heteroatom-doped carbon materials are considered to be better catalyst supports than pristine carbon-based supports.Studies have indicated that heteroatom doping can effectively improve catalyst activity and make more active sites exposed [22][23][24].In particular, when two or more elements, such as N, S, etc. are doped in the structure of carbon, the catalyst activity can improve obviously.That is because N and S as dopants can create synergistic effects that make them easier to modulate for some conjugated bonds and electron distribution, and then making catalysis more efficient [25].Furthermore, P atom doping could also improve the active sites of the carbon materials [26][27][28].
In summary, N, S, and P-ternary hollow carbon sphere materials can be used as good precious metal carriers.In this work, N, S, and P-ternary hollow carbon sphere materials (C-N,P,S) were fabricated by a one-step procedure with high-temperature pyrolysis of polyphosphazene coated SiO 2 spheres template composites, followed by removing SiO 2 spheres template via HF solution etching.Finally, Pd nanoparticles were grown on the surfaces of C-N,P,S via the immersion reduction method.The as-prepared Pd/C-N,P,S catalysts exhibited superior electrocatalytic performance toward EOR than that of Pd/C.We have reason to believe that the Pd/C-N,P,S catalyst can be a commercial Pd/C alternative in practical applications towards EOR in the future.

Results and Discussion
The morphologies and microstructures of the precursors and Pd/C-N,P,S catalysts were first investigated by scanning electron microscope (SEM) and transmission electron microscope (TEM) images.As shown in Figure 1a, the SEM image of pure SiO 2 microsphere templates exhibits uniform with the diameters ranged from 300 to 500 nm.Through in situ polymerization, the polyphosphazene (PCCP) was coated on the surfaces of SiO 2 microspheres.From the Figure 1b, the obtained PCCP-coated SiO 2 microspheres (PCCP@SiO 2 ) show the core-shell structures.From the edge, the thickness of the PCCP layer is about 140 nm.Afterward, the PCCP@SiO 2 was pyrolyzed at 900 • C. From the SEM image (Figure 1c), the pyrolyzed PCCP@SiO 2 still remained spherical with a certain fusion.After etching by HF solution, the SiO 2 templates were removed and C-N,P,S were formed.Finally, using C-N,P,S as the supports, the Pd/C-N,P,S was prepared by using in situ reduction.As observed in Figure 1d, Pd/C-N,P,S shows the distinct uniform hollow struture with Pd NPs decorated on the surface.In the EDS elemental mapping (Figure 1e-i), the C, N, P, S, Pd elements are distributed on the surfaces of Pd/C-N,P,S, demonstrating su18ccessful fabrication of Pd/C-N,P,S catalysts.Furthermore, from the high-resolution TEM (HRTEM) image of the Pd/C-N,P,S (Figure 2a), the metallic Pd NPs with distinct lattice fringe were close contact with the surface of C-N,P,S which benefits the synergistic effect between the Pd and C-N,P,S.The diameter of Pd NPs is ~ 5 nm.The lattice fringe spacing of d = 0.22 nm is attributed to the (111) planes of metallic Pd. Figure 2b shows the X-ray diffraction (XRD) patterns of as-prepared Pd/C and Pd/C-N,P,S.The characteristic peak at 2θ = 25° is corresponded to the C (002) crystal plane.The Pd/C and Pd/C-N,P,S all show characteristic peaks at 2θ values of 39.1°, 45.1°, 66.1°, and 79.7°, which were assigned to (111), ( 200), ( 220) and (311) crystal planes of face-centered cubic (fcc) Pd (JCPDS 65-6174), respectively.This agrees with the analysis of the HRTEM image.Figure 3 is the X-ray photoelectron spectroscopy (XPS) spectra of the Pd/C-N, P, S catalyst.The coexistences of C, O, N, P, S, and Pd elements are all in the structure of Pd/C-N,P,S (Figure 3a).The presence of O element may come from the surface hydroxyl groups.In Figure 3b, the high-resolution C 1s XPS spectra were deconvoluted into three distinct peaks which are located at 284.6, 285.5, and 286.7 eV, respectively.The first peak is the classical peak of the sp 2- hybridized C-C bond, while the second peak contributes to the C-N or C-S backbone.Besides, the third peak is determined as the C-O groups [23,29].Furthermore, we also carried out high-resolution XPS analysis of N 1s, P 2p, S 2p and Pd 3d.As shown in Figure 3c, there are two peaks at 398.4 (pyridinic-N) and 402.1 eV (pyridinic-oxide-N) in the deconvoluted N 1s peak [30,31].The P 2p peak at 133.8 eV can be assigned as the P-N bond (Figure 3d) [26,32].From the S 2p spectra (Figure 3e), the two peaks at 164.4 and 168.5 eV are contributed to the -S-C-S-and -C-S (O) X -C-groups, which prove that the doped S atoms are tightly bound to the adjacent C atoms [32].Meanwhile, the high- Furthermore, from the high-resolution TEM (HRTEM) image of the Pd/C-N,P,S (Figure 2a), the metallic Pd NPs with distinct lattice fringe were close contact with the surface of C-N,P,S which benefits the synergistic effect between the Pd and C-N,P,S.The diameter of Pd NPs is ~5 nm.The lattice fringe spacing of d = 0.22 nm is attributed to the (111) planes of metallic Pd. Figure 2b 200), ( 220) and (311) crystal planes of face-centered cubic (fcc) Pd (JCPDS 65-6174), respectively.This agrees with the analysis of the HRTEM image.Figure 3 is the X-ray photoelectron spectroscopy (XPS) spectra of the Pd/C-N, P, S catalyst.The coexistences of C, O, N, P, S, and Pd elements are all in the structure of Pd/C-N,P,S (Figure 3a).The presence of O element may come from the surface hydroxyl groups.In Figure 3b, the high-resolution C 1s XPS spectra were deconvoluted into three distinct peaks which are located at 284.6, 285.5, and 286.7 eV, respectively.The first peak is the classical peak of the sp 2− hybridized C-C bond, while the second peak contributes to the C-N or C-S backbone.Besides, the third peak is determined as the C-O groups [23,29].Furthermore, we also carried out high-resolution XPS analysis of N 1s, P 2p, S 2p and Pd 3d.As shown in Figure 3c, there are two peaks at 398.4 (pyridinic-N) and 402.1 eV (pyridinic-oxide-N) in the deconvoluted N 1s peak [30,31].The P 2p peak at 133.8 eV can be assigned as the P-N bond (Figure 3d) [26,32].From the S 2p spectra (Figure 3e), the two peaks at 164.4 and 168.5 eV are contributed to the -S-C-S-and -C-S (O) X -C-groups, which prove that the doped S atoms are tightly bound to the adjacent C atoms [32].Meanwhile, the high-resolution Pd 3p XPS spectra shown in Figure 3f were deconvoluted into two groups peaks: the peaks at 335.7 and 341 eV are contributed to the metallic Pd.In addition, the other XPS peaks at 337.0 and 342.5 eV could be ascribed to Pd oxide species [33,34].resolution Pd 3p XPS spectra shown in Figure 3f were deconvoluted into two groups peaks: the peaks at 335.7 and 341 eV are contributed to the metallic Pd.In addition, the other XPS peaks at 337.0 and 342.5 eV could be ascribed to Pd oxide species [33,34].resolution Pd 3p XPS spectra shown in Figure 3f were deconvoluted into two groups peaks: the peaks at 335.7 and 341 eV are contributed to the metallic Pd.In addition, the other XPS peaks at 337.0 and 342.5 eV could be ascribed to Pd oxide species [33,34].classification, the C-N,P,S, and Pd/C-N,P,S both exhibit type-IV isotherms, and their hysteresis loops are clear H4 type hysteresis loops.The BET specific surface areas of C-N,P,S, and Pd/C-N,P,S are 801 and 212 m 2 g −1 ,respectively.Furthermore, from the inset BJH pore size and pore size distributions, the pore sizes of the C-N,P,S, and Pd/C-N,P,S are about ~4 nm.The large specific surface area of C-N,P,S supports with porous structure can facilitate the infiltration of electrolyte and charge transport in EOR, thereby enhancing the electrochemical activity.As can be seen from Figure 4c, in the Fourier transform-infrared (FT-IR) spectra of SiO 2 , PCCP@SiO 2 and pyrolytic PCCP@ SiO 2 composites, the characteristic peaks at 464, 795, and 1092cm −1 are attributed to the bending and symmetrical stretching vibrations of Si-O-Si, respectively.Besides, the peak at 952 cm −1 is attributed to the specific vibration of Si-O.The peak at 3453 cm −1 is assigned to the vibration of hydroxyl groups on the surfaces of SiO 2 microspheres.As templates, the PCCP@SiO 2 with PCCP is coated on the surfaces of SiO 2 microspheres.Toward PCCP@SiO 2 , the absorption peaks at 1588 and 1490 cm −1 originate from the benzene ring in PCCP.The peaks at 1295 and 1154 cm −1 are ascribed to the sulfone in PCCP.Furthermore, the peaks at 1187 and 887 cm −1 originate from the vibrations of P=N and P-N in the structure of PCCP.More importantly, the characteristic peak at 949 cm −1 implied the formation of P-O-benzene ring.Therefore, the PCCP can be grafted onto the surfaces of SiO 2 microspheres via in situ polymerization.As for sample pyrolytic PCCP@SiO 2 , the characteristic peaks of PCCP disappeared.The peaks at 470 and 800 cm −1 are corresponded to the symmetrical stretching vibration of Si-O.It demonstrates that the layer of PCCP in the PCCP@ SiO 2 has already changed into porous carbon structure.Figure 4d  As can be seen from Figure 4c, in the Fourier transform-infrared (FT-IR) spectra of SiO2, PCCP@SiO2 and pyrolytic PCCP@ SiO2 composites, the characteristic peaks at 464, 795, and 1092cm −1 are attributed to the bending and symmetrical stretching vibrations of Si-O-Si, respectively.Besides, the peak at 952 cm −1 is attributed to the specific vibration of Si-O.The peak at 3453 cm −1 is assigned to the vibration of hydroxyl groups on the surfaces of SiO2 microspheres.As templates, the PCCP@SiO2 with PCCP is coated on the surfaces of SiO2 microspheres.Toward PCCP@SiO2, the absorption peaks at 1588 and 1490 cm −1 originate from the benzene ring in PCCP.The peaks at 1295 and 1154 cm −1 are ascribed to the sulfone in PCCP.Furthermore, the peaks at 1187 and 887 cm −1 originate from the vibrations of P=N and P-N in the structure of PCCP.More importantly, the characteristic peak at 949 cm −1 implied the formation of P-O-benzene ring.Therefore, the PCCP can be grafted onto the surfaces of SiO2 microspheres via in situ polymerization.As for sample pyrolytic PCCP@SiO2, the characteristic peaks of PCCP disappeared.The peaks at 470 and 800 cm -1 are corresponded to the symmetrical stretching vibration of Si-O.It demonstrates that the layer of PCCP in the PCCP@ SiO2 has already changed into porous carbon structure.Figure 4d shows the Raman spectra for the samples of C-N,P,S and Pd/C-N,P,S composites.The D and G bands of carbon were observed at ~1335 and ~1595 cm −1 [35].The value of ID/IG for the C-N,P,S (≈1.1) was lower than that of the Pd/C-N,P,S (≈1.18), which could be due to increased defects in C atom for the Pd/C-N,P,S after Pd NPs decoration.The cyclic voltammetry (CV) curves of Pd/C and Pd/C-N,P,S catalysts were measured in 1 mol/L sodium hydroxide water solution.As shown in Figure 5a, the hydrogen adsorption-desorption peak of Pd/C-N,P,S catalyst shows a higher current density between -1.0 and -0.7 V vs. SCE.Besides, the The cyclic voltammetry (CV) curves of Pd/C and Pd/C-N,P,S catalysts were measured in 1 mol/L sodium hydroxide water solution.As shown in Figure 5a, the hydrogen adsorption-desorption peak of Pd/C-N,P,S catalyst shows a higher current density between −1.0 and −0.7 V vs. SCE.Besides, the peaks which is located at around −0.35 V is the reduction peak of PdO x .It is noting that there is a negative shift of ~2 mV for the Pd/C-N,P,S catalyst in comparison with Pd/C, indicating that the decreased oxophilicity on the Pd surfaces can reduce chemisorptions of these oxygen species, which benefits the EOR in the alkaline condition.Furthermore, the CV curves of Pd/C-N,P,S and Pd/C in the sodium hydroxide and ethanol mixture solution were shown in Figure 5b.Obviously, the mass peak current density for Pd/C-N,P,S catalyst can reach ~1686 mA mg −1 , which is 2.8 times higher than that of Pd/C.Also, the mass peak current density of Pd/C-N,P,S catalyst is higher than previously reported Pd-based EOR catalysts (Table S1), such as Pd-Ag nanoparticles, Pd 7 /Ru 1 ,etc.[36][37][38][39][40][41][42].Considering the structure of the Pd/C-N,P,S catalyst, the doped N, P, and S atoms can significantly tune the electronic structure of the carbon shell of C-N,P,S.On the one hand, the C-N,P,S with large surface area are conductive to the diffusion of electrolyte and charge transportation; on the other hand, the electrons can transfer from Pd to C-N,P,S due to the existences of heteroatoms.The synergic effects between Pd and C-N,P,S contributed the high electrochemical activity.More importanly, the C-N,P,S supports also can faciliate the CO ad intermediates.
Catalysts 2019, 9, x FOR PEER REVIEW 6 of 11 peaks which is located at around -0.35 V is the reduction peak of PdOx.It is noting that there is a negative shift of ~ 2 mV for the Pd/C-N,P,S catalyst in comparison with Pd/C, indicating that the decreased oxophilicity on the Pd surfaces can reduce chemisorptions of these oxygen species, which benefits the EOR in the alkaline condition.Furthermore, the CV curves of Pd/C-N,P,S and Pd/C in the sodium hydroxide and ethanol mixture solution were shown in Figure 5b.Obviously, the mass peak current density for Pd/C-N,P,S catalyst can reach ~1686 mA mg -1 , which is 2.8 times higher than that of Pd/C.Also, the mass peak current density of Pd/C-N,P,S catalyst is higher than previously reported Pd-based EOR catalysts (Table S1), such as Pd-Ag nanoparticles, Pd7/Ru1,etc [36][37][38][39][40][41][42].
Considering the structure of the Pd/C-N,P,S catalyst, the doped N, P, and S atoms can significantly tune the electronic structure of the carbon shell of C-N,P,S.On the one hand, the C-N,P,S with large surface area are conductive to the diffusion of electrolyte and charge transportation; on the other hand, the electrons can transfer from Pd to C-N,P,S due to the existences of heteroatoms.The synergic effects between Pd and C-N,P,S contributed the high electrochemical activity.More importanly, the C-N,P,S supports also can faciliate the COad intermediates.The CO monolayer stripping curves of the Pd/C and Pd/C-N,P,S catalysts were recorded in 0.5 mol/L of sulfuric acid.As shown in Figure 5c, the onset potential of CO oxidation on Pd/C-N,P,S catalyst is 0.665 V.In comparison with Pd/C, there is a negative shift of 2.1 mV, indicating the COads can be oxidized at a lower potential on Pd/C-N,P,S catalyst.From CO-stripping CV, the electrochemical surface areas (ECSA) can be calculated by CO desorption [43].Considering the 420 μC cm −2 is the charge required to oxidize a monolayer of COads on Pd surface [44].Therefore, the ECSA of the Pd/C-N,P,S catalyst is 76.31 m 2 g −1 , which was 2.76 times higher than that of the Pd/C.This indicated that the synergistic effects between the ternary doped hollow carbon sphere and those uniformly dispersed Pd particles which ensure the Pd/C-N,P,S catalyst with large ECSA and better anti-CO poisoning ability.Generally, the stability of an electrocatalyst is a very important factor for further practical application.The chronoamperometric measurement was carried out in the sodium hydroxide and ethanol mixture solution at a constant potential of − 0.2 V.As shown in Figure 5c, the The CO monolayer stripping curves of the Pd/C and Pd/C-N,P,S catalysts were recorded in 0.5 mol/L of sulfuric acid.As shown in Figure 5c, the onset potential of CO oxidation on Pd/C-N,P,S catalyst is 0.665 V.In comparison with Pd/C, there is a negative shift of 2.1 mV, indicating the CO ads can be oxidized at a lower potential on Pd/C-N,P,S catalyst.From CO-stripping CV, the electrochemical surface areas (ECSA) can be calculated by CO desorption [43].Considering the 420 µC cm −2 is the charge required to oxidize a monolayer of CO ads on Pd surface [44].Therefore, the ECSA of the Pd/C-N,P,S catalyst is 76.31 m 2 g −1 , which was 2.76 times higher than that of the Pd/C.This indicated that the synergistic effects between the ternary doped hollow carbon sphere and those uniformly dispersed Pd particles which ensure the Pd/C-N,P,S catalyst with large ECSA and better anti-CO poisoning ability.Generally, the stability of an electrocatalyst is a very important factor for further practical application.The chronoamperometric measurement was carried out in the sodium hydroxide and ethanol mixture solution at a constant potential of −0.2 V.As shown in Figure 5c, the Pd/C-N,P,S catalyst always exhibits the higher oxidation current density than that of the Pd/C after different periods.After a repetitive 200 potential cycling tests, the peak current density of Pd/C-N, P, S has decreased by ~4.2%, and the peak current density of Pd/C has decreased by ~18.6%.Obviously, the Pd/C-N, P, S shows higher structural stability than that of Pd/C.From Figure 5d, towards the ratio (i 3600 /i 10 ) of the peak current density after 3600 s to that after 10 s, the i 3600 /i 10 of Pd/C-N, P, S is 8.6% higher than that of Pd/C (6.7%).

Synthesis of C-N,P,S
Figure 6 shows the schematic illustration of fabrication for C-N,P,S hollow microsphere.Uniform SiO 2 microsphere templates were fabricated by a reported method [45].First, 0.2 g of SiO 2 was dispersed into 200 mL of acetonitrile.Then, a certain amount of HCCP, BPS and TEA were added to the above SiO 2 /acetonitrile dispersion with continuous stirring.After 6 h of stirring, the products of PCCP@SiO 2 was separated by centrifuge washing with water and ethanol, and then vacuum dried at 50 • C for 12 h.Then, PCCP@SiO 2 was subjected to pyrolysis under a nitrogen atmosphere.The pyrolysis condition is 600 • C for 1 h, and then 900 • C for 3 h (2 • C/min).After being pyrolyzed, the C-N, P, S@SiO 2 was collected.IThen, the above C-N, P, S@SiO 2 was etched in HF to remove SiO 2 microsphere templates.Finally, the C-N, P, S was then washed by lots of water and vacuum dried overnight at 50 • C. Pd/C-N,P,S catalyst always exhibits the higher oxidation current density than that of the Pd/C after different periods.After a repetitive 200 potential cycling tests, the peak current density of Pd/C-N, P, S has decreased by ~ 4.2%, and the peak current density of Pd/C has decreased by ~18.6%.Obviously, the Pd/C-N, P, S shows higher structural stability than that of Pd/C.From Figure 5d, towards the ratio(i3600/i10)of the peak current density after 3600 s to that after 10 s, the i3600/i10 of Pd/C-N, P, S is 8.6% higher than that of Pd/C (6.7%).

Synthesis of C-N,P,S
Figure 6 shows the schematic illustration of fabrication for C-N,P,S hollow microsphere.Uniform SiO2 microsphere templates were fabricated by a reported method [45].First, 0.2 g of SiO2 was dispersed into 200 mL of acetonitrile.Then, a certain amount of HCCP, BPS and TEA were added to the above SiO2/acetonitrile dispersion with continuous stirring.After 6 h of stirring, the products of PCCP@SiO2 was separated by centrifuge washing with water and ethanol, and then vacuum dried at 50 °C for 12 h.Then, PCCP@SiO2 was subjected to pyrolysis under a nitrogen atmosphere.The pyrolysis condition is 600 °C for 1 h, and then 900 o C for 3 h (2 °C / min).After being pyrolyzed, the C-N, P, S@SiO2 was collected.IThen, the above C-N, P, S@SiO2 was etched in HF to remove SiO2 microsphere templates.Finally, the C-N, P, S was then washed by lots of water and vacuum dried overnight at 50 °C.

Preparation of Pd/C-N,P,S Catalysts
The Pd/C-N,P,S was prepared by a simple and efficient chemical reduction impregnation method.50 mg of C-N,P,S was first dispersed into 30 mL of H2O with bath sonication of 30 min.Then, 50 mg of K2PdCl4 and 50 mg of EDTA were added into a C-N,P,S/water dispersion with continuous stirring.In the next stage, the pH value of above mixture solution was adjusted to about 10 by concentrated ammonia water.After that, 0.1 g of Na2CO3 and 0.1 g of NaBH4 were dissolved in 20 mL of H2O as reductant solution.The reductant solution was slowly added dropwise to the previous dispersion.After reaction of 4 h with stirring, the Pd/C-N,P,S was collected by centrifugation.After washing by water and ethanol, the Pd/C-N,P,S was vacuum dried at 70 °C for 12 h.

Preparation of Pd/C-N,P,S Catalysts
The Pd/C-N,P,S was prepared by a simple and efficient chemical reduction impregnation method.50 mg of C-N,P,S was first dispersed into 30 mL of H 2 O with bath sonication of 30 min.Then, 50 mg of K 2 PdCl 4 and 50 mg of EDTA were added into a C-N,P,S/water dispersion with continuous stirring.In the next stage, the pH value of above mixture solution was adjusted to about 10 by concentrated ammonia water.After that, 0.1 g of Na 2 CO 3 and 0.1 g of NaBH 4 were dissolved in 20 mL of H 2 O as reductant solution.The reductant solution was slowly added dropwise to the previous dispersion.After reaction of 4 h with stirring, the Pd/C-N,P,S was collected by centrifugation.After washing by water and ethanol, the Pd/C-N,P,S was vacuum dried at 70 • C for 12 h.

Electrocatalytic Activity Test
A CHI 660E electrochemistry workstation (Shanghai, China) was used for testing the electrocatalytic performance of all catalysts with a conventional three-electrode method.The graphite rod electrode was employed as counter electrode.A saturated calomel electrode (SCE) was the reference electrode.The Pd/C-N,P,S and Pd/C modified glassy carbon electrodes (Φ = 3 mm) were used as the working electrodes.CO stripping experiment was peformed in diluted sulfuric acid solution.CV tests and chronoamperometry were undertaken in the sodium hydroxide and ethanol mixture solution.

Conclusions
In summary, the Pd/C-N,P,S with Pd NPs decorated on the surfaces of ternary N, S, and P-doped hollow carbon microspheres was successfully prepared using the SiO 2 microspheres and PCCP as templates and carbon source.As EOR catalyst, the Pd/C-N,P,S exhibits high electrochemical activity with a mass peak current density of 1686 mA mg -1 which is 2.8 times higher than that of Pd/C.Remarkably, the ECSA of the Pd/C-N,P,S catalyst can reach ~76.31 m 2 g −1 which is 2.76 times higher that of the Pd/C.In addition, the Pd/C-N,P,S catalyst also shows good stability.The high performance of Pd/C-N,P,S catalysts may be mainly due to the following reasons: (1) the porous C-N,P,S with large surface area are conductive to the diffusion of electrolyte and charge transportation, and (2) the electrons can transfer from Pd to C-N,P,S due to the existences of heteroatoms.Also, the C-N,P,S supports can also faciliate the CO ad intermediates.The synergic effects between Pd and C-N,P,S contributed to the high electrochemical activity.Therefore, the Pd/C-N,P,S catalyst has promising prospects for high-performance EOR catalysts.

Figure 1 .
Figure 1.Scanning electron microscope (SEM) images of (a) SiO2, transmission electron microscope (TEM) image of (b) PCCP@SiO2, (c) SEM image of C-N,P,S, (d) TEM image of Pd/C-N,P,S and electrospray ionization (EDS) elemental mapping of (e-i) C, N, P, S, and Pd elements for Pd/C-N,P,S catalysts.

Figure 1 .
Figure 1.Scanning electron microscope (SEM) images of (a) SiO 2 , transmission electron microscope (TEM) image of (b) PCCP@SiO 2 , (c) SEM image of C-N,P,S, (d) TEM image of Pd/C-N,P,S and electrospray ionization (EDS) elemental mapping of (e-i) C, N, P, S, and Pd elements for Pd/C-N,P,S catalysts.
shows the X-ray diffraction (XRD) patterns of as-prepared Pd/C and Pd/C-N,P,S.The characteristic peak at 2θ = 25 • is corresponded to the C (002) crystal plane.The Pd/C and Pd/C-N,P,S all show characteristic peaks at 2θ values of 39.1 • , 45.1 • , 66.1 • , and 79.7 • , which were assigned to (111), (

Figure 2 .
Figure 2. (a) The high-resolution TEM (HRTEM) image of the sample Pd/C-N,P,S, (b) X-ray diffraction (XRD) patterns of C-N,P,S, Pd/C and Pd/C-N,P,S.

Figure
Figure 4a,b show the N2 adsorption-desorption isothermals over typical samples of C-N,P,S, and Pd/C-N,P,S.According to the original International Union of Pure and Applied Chemistry

Figure 2 .
Figure 2. (a) The high-resolution TEM (HRTEM) image of the sample Pd/C-N,P,S, (b) X-ray diffraction (XRD) patterns of C-N,P,S, Pd/C and Pd/C-N,P,S.

Figure 2 .
Figure 2. (a) The high-resolution TEM (HRTEM) image of the sample Pd/C-N,P,S, (b) X-ray diffraction (XRD) patterns of C-N,P,S, Pd/C and Pd/C-N,P,S.

Figure
Figure 4a,b show the N2 adsorption-desorption isothermals over typical samples of C-N,P,S, and Pd/C-N,P,S.According to the original International Union of Pure and Applied Chemistry

Figure
Figure4a,b show the N 2 adsorption-desorption isothermals over typical samples of C-N,P,S, and Pd/C-N,P,S.According to the original International Union of Pure and Applied Chemistry (IUPAC) classification, the C-N,P,S, and Pd/C-N,P,S both exhibit type-IV isotherms, and their hysteresis loops are clear H4 type hysteresis loops.The BET specific surface areas of C-N,P,S, and Pd/C-N,P,S are 801 and 212 m 2 g −1 ,respectively.Furthermore, from the inset BJH pore size and pore size distributions, the pore sizes of the C-N,P,S, and Pd/C-N,P,S are about ~4 nm.The large specific surface area of C-N,P,S supports with porous structure can facilitate the infiltration of electrolyte and charge transport in EOR, thereby enhancing the electrochemical activity.As can be seen from Figure4c, in the Fourier transform-infrared (FT-IR) spectra of SiO 2 , PCCP@SiO 2 and pyrolytic PCCP@ SiO 2 composites, the characteristic peaks at 464, 795, and 1092cm −1 are attributed to the bending and symmetrical stretching vibrations of Si-O-Si, respectively.Besides, the peak at 952 cm −1 is attributed to the specific vibration of Si-O.The peak at 3453 cm −1 is assigned to the vibration of hydroxyl groups on the surfaces of SiO 2 microspheres.As templates, the PCCP@SiO 2 with PCCP is coated on the surfaces of SiO 2 microspheres.Toward PCCP@SiO 2 , the absorption peaks at 1588 and 1490 cm −1 originate from the benzene ring in PCCP.The peaks at 1295 and 1154 cm −1 are ascribed to the sulfone in PCCP.Furthermore, the peaks at 1187 and 887 cm −1 originate from the vibrations of P=N and P-N in the structure of PCCP.More importantly, the characteristic peak at 949 cm −1 implied the formation of P-O-benzene ring.Therefore, the PCCP can be grafted onto the surfaces of SiO 2 microspheres via in situ polymerization.As for sample pyrolytic PCCP@SiO 2 , the characteristic peaks of PCCP disappeared.The peaks at 470 and 800 cm −1 are corresponded to the symmetrical stretching vibration of Si-O.It demonstrates that the layer of PCCP in the PCCP@ SiO 2 has already changed into porous carbon structure.Figure4dshows the Raman spectra for the samples of C-N,P,S and Pd/C-N,P,S composites.The D and G bands of carbon were observed at ~1335 and ~1595 cm −1[35].The value of I D /I G for the C-N,P,S (≈1.1) was lower than that of the Pd/C-N,P,S (≈1.18), which could be due to increased defects in C atom for the Pd/C-N,P,S after Pd NPs decoration.

11 (
Figure4a,b show the N 2 adsorption-desorption isothermals over typical samples of C-N,P,S, and Pd/C-N,P,S.According to the original International Union of Pure and Applied Chemistry (IUPAC) classification, the C-N,P,S, and Pd/C-N,P,S both exhibit type-IV isotherms, and their hysteresis loops are clear H4 type hysteresis loops.The BET specific surface areas of C-N,P,S, and Pd/C-N,P,S are 801 and 212 m 2 g −1 ,respectively.Furthermore, from the inset BJH pore size and pore size distributions, the pore sizes of the C-N,P,S, and Pd/C-N,P,S are about ~4 nm.The large specific surface area of C-N,P,S supports with porous structure can facilitate the infiltration of electrolyte and charge transport in EOR, thereby enhancing the electrochemical activity.As can be seen from Figure4c, in the Fourier transform-infrared (FT-IR) spectra of SiO 2 , PCCP@SiO 2 and pyrolytic PCCP@ SiO 2 composites, the characteristic peaks at 464, 795, and 1092cm −1 are attributed to the bending and symmetrical stretching vibrations of Si-O-Si, respectively.Besides, the peak at 952 cm −1 is attributed to the specific vibration of Si-O.The peak at 3453 cm −1 is assigned to the vibration of hydroxyl groups on the surfaces of SiO 2 microspheres.As templates, the PCCP@SiO 2 with PCCP is coated on the surfaces of SiO 2 microspheres.Toward PCCP@SiO 2 , the absorption peaks at 1588 and 1490 cm −1 originate from the benzene ring in PCCP.The peaks at 1295 and 1154 cm −1 are ascribed to the sulfone in PCCP.Furthermore, the peaks at 1187 and 887 cm −1 originate from the vibrations of P=N and P-N in the structure of PCCP.More importantly, the characteristic peak at 949 cm −1 implied the formation of P-O-benzene ring.Therefore, the PCCP can be grafted onto the surfaces of SiO 2 microspheres via in situ polymerization.As for sample pyrolytic PCCP@SiO 2 , the characteristic peaks of PCCP disappeared.The peaks at 470 and 800 cm −1 are corresponded to the symmetrical stretching vibration of Si-O.It demonstrates that the layer of PCCP in the PCCP@ SiO 2 has already changed into porous carbon structure.Figure4dshows the Raman spectra for the samples of C-N,P,S and Pd/C-N,P,S composites.The D and G bands of carbon were observed at ~1335 and ~1595 cm −1[35].The value of I D /I G for the C-N,P,S (≈1.1) was lower than that of the Pd/C-N,P,S (≈1.18), which could be due to increased defects in C atom for the Pd/C-N,P,S after Pd NPs decoration.Catalysts 2019, 9, x FOR PEER REVIEW 5 of 11 (IUPAC) classification, the C-N,P,S, and Pd/C-N,P,S both exhibit type-IV isotherms, and their hysteresis loops are clear H4 type hysteresis loops.The BET specific surface areas of C-N,P,S, and Pd/C-N,P,S are 801 and 212 m 2 g −1 ,respectively.Furthermore, from the inset BJH pore size and pore size distributions, the pore sizes of the C-N,P,S, and Pd/C-N,P,S are about ~ 4 nm.The large specific surface area of C-N,P,S supports with porous structure can facilitate the infiltration of electrolyte and charge transport in EOR, thereby enhancing the electrochemical activity.As can be seen from Figure4c, in the Fourier transform-infrared (FT-IR) spectra of SiO2, PCCP@SiO2 and pyrolytic PCCP@ SiO2 composites, the characteristic peaks at 464, 795, and 1092cm −1 are attributed to the bending and symmetrical stretching vibrations of Si-O-Si, respectively.Besides, the peak at 952 cm −1 is attributed to the specific vibration of Si-O.The peak at 3453 cm −1 is assigned to the vibration of hydroxyl groups on the surfaces of SiO2 microspheres.As templates, the PCCP@SiO2 with PCCP is coated on the surfaces of SiO2 microspheres.Toward PCCP@SiO2, the absorption peaks at 1588 and 1490 cm −1 originate from the benzene ring in PCCP.The peaks at 1295 and 1154 cm −1 are ascribed to the sulfone in PCCP.Furthermore, the peaks at 1187 and 887 cm −1 originate from the vibrations of P=N and P-N in the structure of PCCP.More importantly, the characteristic peak at 949 cm −1 implied the formation of P-O-benzene ring.Therefore, the PCCP can be grafted onto the surfaces of SiO2 microspheres via in situ polymerization.As for sample pyrolytic PCCP@SiO2, the characteristic peaks of PCCP disappeared.The peaks at 470 and 800 cm -1 are corresponded to the symmetrical stretching vibration of Si-O.It demonstrates that the layer of PCCP in the PCCP@ SiO2 has already changed into porous carbon structure.Figure4dshows the Raman spectra for the samples of C-N,P,S and Pd/C-N,P,S composites.The D and G bands of carbon were observed at ~1335 and ~1595 cm −1[35].The value of ID/IG for the C-N,P,S (≈1.1) was lower than that of the Pd/C-N,P,S (≈1.18), which could be due to increased defects in C atom for the Pd/C-N,P,S after Pd NPs decoration.

Figure 5 .
Figure 5. (a) Cyclic voltammetry (CV) of Pd/C and Pd/C-N,P,S catalysts in sodium hydroxide (1 mol/L), (b) CVs of Pd/C and Pd/C-N,P,S catalysts in sodium hydroxide (1 mol/L) and ethanol (1 mol/L) mixture solution, (c) CO monolayer stripping voltammograms of the Pd/C and Pd/C-N,P,S catalysts in diluted sulfuric acid solution (0.5 mol/L), (d) chronoamperometric curves for the the Pd/C and Pd/C-N,P,S catalysts in sodium hydroxide (1 mol/L) and ethanol (1 mol/L) mixture solution at −0.20 V. Scan rate: 50 mV/s.

Figure 5 .
Figure 5. (a) Cyclic voltammetry (CV) of Pd/C and Pd/C-N,P,S catalysts in sodium hydroxide (1 mol/L), (b) CVs of Pd/C and Pd/C-N,P,S catalysts in sodium hydroxide (1 mol/L) and ethanol (1 mol/L) mixture solution, (c) CO monolayer stripping voltammograms of the Pd/C and Pd/C-N,P,S catalysts in diluted sulfuric acid solution (0.5 mol/L), (d) chronoamperometric curves for the the Pd/C and Pd/C-N,P,S catalysts in sodium hydroxide (1 mol/L) and ethanol (1 mol/L) mixture solution at −0.20 V. Scan rate: 50 mV/s.

Figure 6 .
Figure 6.Schematic illustration for fabrication of hollow C-N,P,S sphere.

Figure 6 .
Figure 6.Schematic illustration for fabrication of hollow C-N,P,S sphere.

Figure S1 :
CVs of Pd/C-N,P,S catalysts in sodium hydroxide (1 mol/L) at different scan rates; Figure S2: The calibration plot of oxidation and reduction peaks currents vs. scan rate; Figure S3: CVs of Pd/C-N,P,S and Pd/C after 200 repetitive potential cycling tests, Table