Preparation of supported core–shell structured Pd@Pd_xS_y/C catalysts for use in selective reductive alkylation reaction

Abstract

Supported noble-metal sulphide catalysts have attracted extensive scientific interest for their good selectivity in selective hydrogenation. However, the application of noble-metal catalysts is limited due to their lower activity, leading to harsh reaction conditions and poor conversion during hydrogenation reactions. In this study, Pd/C was sulphidized by H₂S to prepare a series of core–shell structured Pd@Pd_xS_y/C catalysts, which were characterized by BET, EDS, XPS, XRD and CO chemisorption to investigate the influences of sulphidation temperature, sulphidation time and sulphidation atmosphere on the structure of the resulting catalysts. The sulphidation of Pd/C at low temperatures resulted in a core–shell structured catalyst, Pd@Pd_xS_y/C; with increasing sulphidation temperature, the size of Pd⁰ as the core decreased, and the thickness of palladium sulphides as the shell increased correspondingly. When the sulphidation temperature reached 150 °C, the resulting catalyst transformed to a complete palladium sulphide catalyst, Pd_xS_y/C. The structure of Pd@Pd_xS_y/C sulphidized at 30 °C was independent of sulphidation time and sulphidation atmosphere. The sulphidized catalysts were applied to the reductive alkylation of PADPA and MIBK to DBPPD. The sulphidized catalysts presented a much higher selectivity for DBPPD compared with Pd/C, and Pd@Pd_xS_y/C showed higher activity than Pd_xS_y/C; moreover, the greater amount of Pd_xS_y content in the resulting catalyst led to a lower activity.

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1 Introduction

Supported noble-metal catalysts are among the most industrially applied heterogeneous catalysts in various reductive reactions and have attracted extensive scientific interest.^1–10 However, these catalysts are not qualified to catalyse the selective hydrogenation of organic compounds with multiple reducible functionalities because of their very high activity. Therefore, modifications of noble-metal catalysts for selective hydrogenation have been extensively applied in synthetic chemistry.^5–10 This is also an effective way to improve the selectivity for the use of noble-metal sulphide catalysts during the selective hydrogenation.^11–17 The supported noble-metal sulphide catalysts exhibit good performance in the reductive alkylation reaction of aldehydes/ketone and amine^11–14 and the selective hydrogenation of halogenated nitrobenzene to the corresponding halogenated aniline.^15–17 They are also suitable for the hydrogenation of sulfur-containing compounds.^18–23 Moreover, the noble-metal sulphide catalysts, which are well known for the catalytic hydrotreatment, are widely used in hydrodesulfurization and hydrodenitrogenation.^24–28

However, the activity of the noble-metal sulphide catalysts is considerably lower than the unmodified noble-metal catalysts; thus, the noble-metal sulphide catalysts are often used under harsh conditions during hydrogenation.^13–20 This severely limits their application. The key to solving this problem is improving the activity of noble-metal sulphide catalysts. Michele Breyssey et al. revealed that the activity of RuS₂ catalysts depended on both the S/Ru and high S/Ru ratios, corresponding to low activity during their study about the catalytic mechanism of RuS₂ catalysts.^29,30 In our previous study, we also found that the catalytic activity of several types of activated carbon-supported palladium sulphides in the hydrogenation of 4-nitrothioanisole were ranked in the following order: Pd₄S/C ≈ Pd₁₆S₇/C > Pd₃S/C ≫ PdS/C,²¹ which indicated that Pd atoms with a smaller number of the coordinated S atoms showed relatively higher activity in the hydrogenation of sulfur-containing compounds. It appears to be an efficient way to improve the activity of noble-metal sulphide catalysts by reducing the number of coordinated S atoms.

Core–shell catalysts as the core particles coated with a functional material increase the physical properties such as electrical, optical, magnetic and thermal properties of the combined particles and exhibit unique catalytic performance.^31–35 Recently, core–shell catalysts have been attracting increasing attention. Bao's group reported that Pt shells coated at the Cu core decorated with FeO patches used considerably less Pt but exhibited performance similar to that of only Pt nanoparticles covered with surface FeO patches in the catalytic oxidation of CO.³⁴ Wei and co-workers prepared a polyaniline (PANI)-decorated Pt/C@PANI core–shell catalyst, and they found that this novel catalyst presented high activity and stability in the oxygen-reduction reaction for not only the electron delocalization between the Pt d orbitals and the PANI π-conjugated ligand but also electron transfer from Pt to PANI.³⁵

N-(1,3-Dimethylbutyl)-N′-phenyl-p-phenylenediamine (DBPPD) as a rubber antioxidant is synthesized by the reductive alkylation of p-aminodiphenylamine (PADPA) and methyl isobutyl ketone (MIBK) over industrial copper or noble-metal catalysts.^36,37 In this reaction, PADPA and MIBK underwent a condensation reaction to form an imine compound, which hydrogenated in the presence of the catalyst to DBPPD (Scheme 1). PADPA, MIBK, imine and DBPPD are prone to undergo over-hydrogenation to byproducts using ordinary catalysts. Noble-metal sulphide catalysts, such as palladium sulphide and platinum sulphide, show high selectivity in the synthesis of DBPPD; however, their activity is insufficient.¹⁴

In this study, a series of activated carbon-supported Pd metal cores with Pd_xS_y shell catalysts were prepared by a simple method and characterized by CO chemisorption, BET, EDS, XRD and XPS. The catalysts were then used in the reductive alkylation of PADPA and MIBK to DBPPD.

2 Experimental

2.1 Catalyst preparation

10% Pd/C was prepared by an incipient wetness impregnation method using H₂PdCl₄ as a Pd precursor. The commercially activated carbon (Xinsen Chemical Industry Co. Ltd) and deionized water were mixed in a weight ratio of 1 to 50. Then, a desired volume of 0.05 g ml⁻¹ H₂PdCl₄ was added into the continuously stirred slurry solution at 80 °C, and the pH value of the sample was adjusted to 11 by adding 10% aqueous solution of NaOH. The precipitated Pd(OH)₂ was reduced by hydrazine hydrate, which was then washed with deionized water until the pH value reached to about 7 and dried under vacuum at 110 °C for 10 h.

A suspension of 1.0 g of 10% Pd/C and 50 ml deionized water was stirred in a three-necked flask, and a stream of gas containing H₂S was then passed into the suspension under different conditions, as listed in Table 1. The resulting catalyst was collected on a filter paper, washed with deionized water until the pH was determined to be 7, and then dried under vacuum at 110 °C for 10 h.

Table 1 The sulphidation conditions and properties of the sulphidized catalysts and fresh Pd/C

Catalyst	Sulphidation temperature/°C	Sulphidation atmosphere	Sulphidation time/h	Surface area/m^2 g^−1	Mean diameter of Pd particles/nm	CO chemisorption uptake/ml g^−1	Elemental composition/wt%			Mole ratio of S/Pd
							C	Pd	S
SPC 1	30	H2/H2S = 9/1	3	1508	5.9	0	81.79	9.73	1.45	0.50
SPC 2	60	H2/H2S = 9/1	3	1437	6.2	0	82.06	9.88	1.53	0.51
SPC 3	100	H2/H2S = 9/1	3	1552	6.5	0	82.61	9.82	1.55	0.52
SPC 4	150	H2/H2S = 9/1	3	1539	6.6	0	82.02	10.03	1.67	0.55
SPC 5	200	H2/H2S = 9/1	3	1477	7.0	0	82.31	9.68	1.72	0.59
SPC 6	30	H2/H2S = 9/1	5	1521	6.0	0	82.01	9.94	1.50	0.50
SPC 7	30	H2/H2S = 9/1	10	1570	6.1	0	81.33	9.67	1.49	0.51
SPC 8	30	N2/H2S = 9/1	3	1498	5.8	0	81.94	10.12	1.38	0.45
SPC 9	30	H2S	3	1463	5.5	0	82.65	9.75	1.48	0.50
SPC 10	30	H2/H2S = 4/1	3	1509	5.8	0	82.58	9.84	1.48	0.50
SPC 11	30	H2/H2S = 49/1	3	1526	5.9	0	82.11	9.88	1.46	0.49
Fresh Pd/C	—	—	—	1517	5.7	3.5	82.53	9.85	0	—

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2.2 Catalyst characterization

The morphologies and particle sizes of the catalysts were detected using a Philips-FEI Tecnai G2 F30 S-Twin transmission electron microscope operated at 300 kV.

X-ray diffraction (XRD) of the catalysts was carried out on a Thermo ARL X'TRA diffractometer using Cu K-α radiation. The tube voltage was 40 kV and the tube current was 45 mA. The samples were scanned at 4° min⁻¹.

X-ray photoelectron spectroscopy (XPS) analysis was performed with a Kratos AXIS Ultra DLD system. A monochromatized incident Al X-ray radiation (Al K-α = 1486.6 eV) with a fixed analyser pass energy of 80 eV was used for the excitation of the sample, and the binding energy values were referenced to the C 1s level (284.6 eV), resulting from surface contaminants.

The elemental content of the catalysts was detected using EDS (Thermo Vantage ESI).

The surface area of the catalysts was determined by nitrogen physical adsorption at 77 K. The samples were heated for 10 h to 473 K under vacuum conditions to remove the adsorbed species, and nitrogen adsorption isotherm was then obtained using a NOVA 1000e surface area analyzer (Quantachrome Instruments Corp.). The surface area of samples was calculated by the BET equation.

CO chemisorption uptake was measured by pulse chemisorption with a mass spectrometer (Omnistar TM) at ambient temperature and pressure.

2.3 Reductive alkylation reaction

Liquid-phase reductive alkylation was carried out at 200 °C and 3 MPa of hydrogen for 4 h in a 75 ml stainless-steel autoclave (Parr Instruments Company), which contained 3.7 g (0.02 mol) PADPA, 10 ml (0.08 mol) MIBK and 0.037 g catalyst. The final products were analyzed by a gas chromatograph (Agilent-6890) equipped with an HP-5 capillary column and an FID detector. The area normalization method was used for the quantitative determination of components.

3 Results and discussion

3.1 Catalyst preparation and characterization

Supported metal sulphide catalysts are generally sulphidized from metal or metal oxide as the precursor by H₂S or S as a sulphidation agent.^14,38 The sulphidation conditions, such as sulphidation temperature, sulphidation time and sulphidation atmosphere, always cause important influences on the structure of the resulting supported metal sulphide catalysts.

We first investigated the influence of sulphidation temperature on the structure of the palladium sulphide catalysts. 10 wt% Pd/C was sulphidized under an atmosphere of H₂/H₂S with a volume ratio of 9/1 for 3 h at the temperatures of 30, 60, 100, 150, and 200 °C, respectively, and the resulting catalysts, denoted as SPC 1–5, were characterized using N₂ physisorption for surface area, CO chemisorption, TEM, EDS, XPS, and XRD. The surface areas of SPC 1–5 (Table 1) were almost the same as that of fresh Pd/C, suggesting that the influence of the sulphidation temperature on the activated carbon structure was negligible. The morphology of Pd particles of fresh Pd/C and SPC 1–5 observed by TEM (Fig. 1) showed that the Pd particle sizes of the six catalysts were about 4–10 nm. The elemental content of SPC 1–5 catalysts, determined by EDS (Table 1), showed that all the five catalysts after sulphidation contained S, and its content increased from 1.45 wt% to 1.76 wt% with the sulphidation temperature from 30 °C to 200 °C, whereas the content of Pd in SPC 1–5 was nearly unchanged. CO chemisorption is used to determine the content of Pd⁰ on the surface of the supported palladium catalyst. In our experiment, the CO chemisorption uptakes of five catalysts (Table 1) after sulphidation at various temperatures were 0 ml g⁻¹, whereas the CO chemisorption uptake of the fresh Pd/C was 3.5 ml g⁻¹. This result indicates that there was no Pd⁰ on the surface of the catalysts after sulphidation at different temperatures. Because the content of Pd in SPC 1–5 did not decrease after sulphidation, we can infer that the Pd metal on the surface of these catalysts had been converted to palladium sulphide.

Fig. 1 TEM images of the sulphidized catalysts and fresh Pd/C.

The fresh Pd/C and SPC 1–5 catalysts were also investigated by XPS (Fig. 2 and Table 2). The fitted XPS spectra of Pd 3d_5/2 for the fresh Pd/C exhibit peaks at 335.3 eV and 336.7 eV, which can be attributed to Pd⁰ and Pd²⁺, respectively, because a part of palladium atoms on the surface of the Pd/C catalyst was oxidized by air. However, no peak was observed at about 335.3 eV for the catalysts SPC 1–5, which suggests that there was no Pd⁰ on the surface of the SPC 1–5 catalysts. The fitted XPS spectra of Pd 3d_5/2 of SPC 1–5 contain peaks at 336.4, 337.1 and 338.2 eV, meaning that all the Pd atoms on the surfaces of these catalysts had been changed to high-valency palladium ions as palladium sulphides. This is consistent with the results of CO chemisorption examinations. There are several types of palladium sulphide crystalline phases, such as Pd₄S, Pd₃S, Pd₁₆S₇, PdS and PdS₂, with different valencies of Pd elements. It is also found that the areas of the peaks at 337.1 and 338.2 eV increase with increasing sulphidation temperature, which indicates that there were more high-valency palladium ions in the catalyst sulphidized at higher temperatures. The XRD patterns of the fresh Pd/C and sulphidized catalysts are shown in Fig. 3. The broad diffraction peaks at about 2θ = 25° and 44° are ascribed to the activated carbon. The sharp diffraction peaks of Pd for the fresh Pd/C appear at 2θ = 40.1°, 46.6° and 68.1°. The XRD pattern of the catalyst SPC 1 sulphidized at 30 °C is similar to that of the fresh Pd/C, and no peaks of palladium sulphides are observed, indicating that the catalyst sulphidized at 30 °C did not form a palladium sulphide crystal, and that most of the palladium on the sulphidized catalyst still existed as Pd⁰. The results of CO chemisorption and XPS as previously described have confirmed that Pd atoms on the surface of the sulphidized catalyst had all been changed to palladium sulphides, and these palladium sulphides located just on the surface of the palladium particles and the core of palladium particles should be still Pd⁰. Therefore, the catalyst SPC 1 exhibits a structure with Pd⁰ as the core and the membrane of palladium sulphides as the shell (Pd@Pd_xS_y/C). With increasing sulphidation temperature, the peaks of Pd⁰ for the sulphidized catalysts become weak and broad. The palladium particle size observed from TEM did not change significantly after sulphidation, which means that the particle sizes of Pd⁰ for the resulting catalysts was decreased with increasing sulphidation temperature. When the sulphidation temperature was increased to 150 °C, the Pd⁰ diffraction peaks disappeared, which suggests that the Pd⁰ particle on this catalyst has been reduced to a range undetectable by XRD. We also observe that the diffraction peaks of Pd₄S appear on the XRD pattern of the catalyst sulphidized at 150 °C, and the intensity of the Pd₄S peaks is further enhanced when the sulphidation temperature is increased to 200 °C. This indicates that the content of Pd₄S was increased, and the particle of Pd₄S grew larger with an increase in sulphidation temperature. Therefore, the sulphidation temperature has a significant influence on the structures of the resulting catalysts. When Pd/C was sulphidized at a low temperature, the sulphidized catalyst was a core–shell structure, Pd@Pd_xS_y/C. By increasing the sulphidation temperature, the size of Pd⁰ as the core decreased, and the thickness of palladium sulphides as the shell increased correspondingly. Moreover, when the sulphidation temperature was increased to 150 °C, the resulting catalyst transformed to a complete palladium sulphide catalyst, Pd_xS_y/C.

Fig. 2 Pd 3d XPS spectra of Pd/C and SPC 1–5.

Table 2 The XPS data of Pd 3d_5/2 for the sulphidized catalysts and fresh Pd/C

Catalyst	Peak 1		Peak 2		Peak 3
	Binding energy/eV	Peak area/%	Binding energy/eV	Peak area/%	Binding energy/eV	Peak area/%
SPC 1	336.4	6.6	337.2	78.1	338.3	15.3
SPC 2	335.8	3.4	337.1	83.6	338.7	13.0
SPC 3	336.3	4.4	337.2	81.9	338.3	13.7
SPC 4	—	—	337.2	88.4	338.7	11.6
SPC 5	336.5	4.8	337.2	75.5	338.6	19.7
SPC 6	336.3	4.7	337.2	83.6	338.3	11.7
SPC 7	336.2	3.9	336.9	85.2	338.5	10.9
SPC 8	335.8	6.3	337.0	84.8	338.3	8.9
SPC 9	335.8	4.1	336.7	88.1	338.2	7.8
SPC 10	335.8	5.0	337.0	83.8	338.2	11.2
SPC 11	336.0	10.8	336.8	77.5	338.1	11.7
Fresh Pd/C	—	—	335.3	71.7	336.7	28.3

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Fig. 3 XRD patterns of Pd/C and SPC 1–5.

The influence of sulphidation time on Pd@Pd_xS_y/C catalysts was also investigated. 10 wt% Pd/C was sulphidized under an H₂/H₂S atmosphere with the volume ratio of 9/1 at 30 °C for 5 and 10 h, respectively, and the resulting catalysts were designated as SPC 6 and 7. The surface areas of this series of sulphidized catalysts ranged from 1508 to 1570 m² g⁻¹, similar to that of fresh Pd/C; this implies that the sulphidation time also had a little impact on the structure of the activated carbon. The TEM images (Fig. 1) also showed that the size of the Pd particles did not change significantly after sulphidation treatment for different times. The content of S in SPC 1, 6, and 7 detected by EDS were maintained in the range of 1.45–1.50 wt%, whereas the Pd content in these catalysts were maintained in the range of 9.67–9.94 wt%. The CO chemisorption uptakes for all three catalysts were at 0 ml g⁻¹. SPC 6 and 7 were also investigated using XPS and XRD. The XPS Pd 3d_5/2 spectra of these catalysts (Fig. 4 and Table 2) exhibited peaks at 336.4, 337.1 and 338.2 eV, and there no peak was observed at 335.3 eV. The results of CO chemisorption and XPS show that all the Pd atoms on the surfaces of the catalysts had been transformed to palladium sulphides. XRD patterns of SPC 6 and 7 (Fig. 5) exhibited only the diffraction peaks of Pd⁰ and activated carbon, and no peaks for palladium sulphides were detected. Furthermore, the intensity of the Pd⁰ peaks is similar to that of the fresh Pd/C. Therefore, after the sulphidation for 3, 5 and 10 h, the resulting catalysts still maintained Pd⁰ as the core. In addition, the particle size of the Pd⁰ core did not unambiguously decrease. All the catalysts sulphidized for different times were core–shell, Pd@Pd_xS_y/C; moreover, the structure of the Pd@Pd_xS_y/C catalysts is almost independent of sulphidation time.

Fig. 4 Pd 3d XPS spectra of SPC 1, 6, and 7.

Fig. 5 XRD patterns of Pd/C and SPC 6 and 7.

Recently, we reported that the formation of palladium sulphides supported on the activated carbon is highly dependent on the composition of sulphidation atmosphere.¹⁴ We then investigated the influence of the sulphidation atmosphere on the structure of the Pd_xS_y@Pd/C catalysts. 10 wt% Pd/C was sulphidized for 3 h at 30 °C under N₂/H₂S with the volume ratio of 9/1, in addition to pure H₂S and the H₂/H₂S atmosphere with volume ratios of 4/1 and 49/1, respectively, and the resulting catalysts were denoted as SPC 8–11. The surface areas of these catalysts were similar to that of fresh Pd/C. The content of S in SPC 11 sulphidized under the atmosphere of H₂/H₂S with a volume ratio of 49/1 was only 1.38 wt%, which was considerably lower than that of other sulphidized catalyst. This should be caused by the low content of H₂S in the atmosphere with a sulphidation time of only 3 hours, leading to incomplete sulphidation. When the content of H₂S in the atmosphere was increased to 4/1, and even when it was used as pure H₂S, the S content in the resulting catalyst increased negligibly. The same result was also found during the sulphidation of Pd/C with N₂/H₂S with a volume ratio of 9/1. The CO chemisorption uptakes of SPC 8–11 were all 0 ml g⁻¹, which indicated that all the five catalysts, including the incompletely sulphidized SPC 11, had no Pd⁰ on their surface. This result was also confirmed by the XPS spectra (Fig. 6 and Table 2) of these catalysts for their fitted Pd 3d_5/2 peaks only at 336.4, 337.1 and 338.2 eV, which meant that there were only high-valence palladium ions on the surface. Moreover, the area of peak at 336.4 eV for SPC 11 is considerably larger than that of other catalysts, which should be due to incomplete sulphidation. The XRD patterns of SPC 8–11 (Fig. 7) are similar to that of fresh Pd/C, and no palladium sulphide peaks are observed, suggesting that there was only Pd metal crystal as the core. Therefore, Pd/C sulphidized under different atmospheres also resulted in a core–shell catalyst, Pd@Pd_xS_y/C, and the influence of the sulphidation atmosphere was also not significant.

Fig. 6 Pd 3d XPS spectra of SPC 1 and 8–11.

Fig. 7 XRD patterns of Pd/C and SPC 8–11.

The sulphidation process of Pd/C with H₂S to Pd@Pd_xS_y/C and Pd_xS_y/C is shown in Fig. 8. First, H₂S was adsorbed on the surface of Pd atoms, and then H₂S was cracked and reacted with Pd to yield palladium sulphides, and H₂ was released simultaneously. Therefore, the Pd atoms on the surface of Pd/C were first sulphidized to palladium sulphides during the sulphidation process. According to the previously discussed results, we conclude that certain conditions such as high sulphidation temperature to sulphidize the Pd metal in the core of Pd particle to palladium sulphide were required. If the temperature was very low (for example, room temperature) the use of a simple method of increasing sulphidation time or changing sulphidation atmosphere was not able to transform all the Pd atoms to palladium sulphides, and just sulphidized the Pd atoms on the surface to form a core–shell catalyst, Pd@Pd_xS_y/C. The reason should be that the adsorption and cracking of H₂S became more difficult on palladium sulphides than on Pd metal, and obtaining a sufficient amount of sulfur from the palladium atoms in the core of Pd@Pd_xS_y to generate palladium sulphide was highly difficult. The thickness of Pd_xS_y on Pd@Pd_xS_y/C was dependent on the sulphidation temperature. After increasing the sulphidation temperature, H₂S was promoted to be cracked on the surface of the catalyst, and the shell of Pd_xS_y was then thickened. When the sulphidation temperature was increased to 150 °C, it was sufficient to completely sulphidize the entire Pd particle into palladium sulphide. Therefore, SPC 4 and SPC 5 had been completely sulphidized to Pd_xS_y/C, and the other sulphidized catalysts formed a core–shell structure, Pd@Pd_xS_y/C.

Fig. 8 The sulphidation process of Pd/C with H₂S.

3.2 Catalytic performances

The catalytic performances of Pd@Pd_xS_y/C, Pd_xS_y/C and Pd/C were tested for the reductive alkylation of MIBK and PADPD to DBPPD under the reaction conditions of 200 °C and 3 MPa for 4 h (Scheme 1), and the results are listed in Table 3. 100% conversion of PADPD was obtained over Pd/C catalyst, but most of PADPD was converted to by-products such as N-(1,3-dimethylbutyl)-N′-cyclohexane-p-phenylenediamine and aniline, and the selectivity of DBPPD was only 39.5%. In contrast, with the exception of SPC 11, all the sulphidized catalysts presented a considerably higher selectivity of DBPPD of up to 96.7%. The low selectivity for SPC 11 in the reductive alkylation should be caused by its incomplete sulphidation under the atmosphere of H₂/H₂S with a volume ratio of 49/1, leading to its content of Pd_xS_y on the surface being lower than that of other sulphidized catalysts. The sulphidized catalysts presented a lower activity than Pd/C for reductive alkylation. However, Pd@Pd_xS_y/C showed higher activity than Pd_xS_y/C for the conversions of PADPD for SPC 4 and SPC 5 at only 93.2% and 90.3%, whereas the conversions of PADDP for all the other sulphidized catalysts were more than 98.3%. In addition, we also observed that the activity of the sulphidized catalyst decreased with increasing sulphidation temperature, which meant that more content of Pd_xS_y in the resulting catalyst led to lower activity.

Scheme 1 Reaction pathways.

Table 3 Reductive alkylation over Pd@Pd_xS_y/C, Pd_xS_y/C and Pd/C catalysts

Catalyst	Conversion/%	Selectivity/%
		DBPPD	Imine	Others
SPC 1	100	98.2	0.2	1.6
SPC 2	99.4	97.5	0.4	2.1
SPC 3	98.3	97.4	1.1	1.5
SPC 4	93.2	97.0	1.5	1.5
SPC 5	90.3	96.7	1.9	1.4
SPC 6	99.9	97.9	0.1	2.0
SPC 7	99.8	97.9	0.2	1.9
SPC 8	100	97.9	0.1	2.0
SPC 9	99.8	97.6	0.3	2.1
SPC 10	99.8	97.5	0.2	2.3
SPC 11	100	90.1	0	9.9
Pd/C	100	39.5	0	60.5

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Researchers^29,30 had investigated the catalytic hydrogenation mechanism of RuS₂ catalyst, and they found that H₂ adsorbed on the surface of RuS₂ due to homolytic adsorption of dihydrogen and heterolytic dissociation on an Ru–S site, leading to Ru–H and SH groups. In addition, the activity was directly proportional to the amount of homolytic adsorption of dihydrogen, which adsorbed on the Ru centers in low-sulfur coordination. The mechanism for palladium sulphide catalyst should be similar to that of RuS₂ catalyst. Pd@Pd_xS_y/C is a catalyst with a membrane of palladium sulphide as the shell, thus it has high selectivity as a common Pd_xS_y/C catalyst during selective hydrogenation. In addition, its core of palladium metal causes the Pd ions on the surface to exhibit a lower sulfur coordination than that of common Pd_xS_y/C catalyst, which improves the activity. Thus, Pd@Pd_xS_y/C is a hydrogenation catalyst with a combination of high selectivity and good activity.

4 Conclusion

A simple and effective method for the preparation of a core–shell Pd@Pd_xS_y/C catalyst through sulphidation with H₂S is proposed. The sulphidation temperature played an important role in the structure of the sulphidized catalyst. The low sulphidation temperature resulted in a core–shell structured catalyst, Pd@Pd_xS_y/C. With increasing sulphidation temperature, the size of Pd⁰ as the core decreased, and the thickness of palladium sulphides as the shell increased correspondingly. Moreover, when the sulphidation temperature reached 150 °C, the resulting catalyst transformed to a complete palladium sulphide catalyst, Pd_xS_y/C. The structure of Pd@Pd_xS_y/C sulphidized at 30 °C was independent of sulphidation time and sulphidation atmosphere. The sulphidized catalysts presented considerably higher selectivity for DBPPD, compared with Pd/C during the reductive alkylation of PADPA and MIBK to DBPPD, and Pd@Pd_xS_y/C showed slightly higher activity than Pd_xS_y/C. Pd@Pd_xS_y/C is a promising catalyst with a combination of high selectivity and good activity for selective hydrogenation reactions.

Acknowledgements

This study was financially supported by the National Natural Science Foundation of China (NSFC-21406199 and 21476208) and the program from the Science and Technology Department of Zhejiang Province (2015C31042).

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