English

• 0.   Introduction

  Nanoparticles (NPs) surface faceting has profound effect for chemical transformations, such as heterogeneous catalysis^[1-3], hydrogen storage^[4-5], and fuel cells^[6]. Recently, a growing number of reports from academia and industry demonstrate that noble metals' facet plays crucial roles in hydrogen dissociation for catalytic hydrogenation^[7-11]. For example, in alkyne hydrogenation, which is a selective reaction used in the food industry, palladium (111)-octahedra have higher catalytic activity than (100)-cubes^[12]. Similarly, surface faceting has been reported to control hydrogen sensors. For example, in TiO₂ nanocrystal, H₂ tend to be adsorbed and dissociated on the (002) and (101) surface, leading to high sensitivity and short response time^[13].

  The nature of hydrogen dissociation on Pd surface faceting for the reaction is a longstanding scientific question. Up to now, it remains a major challenge to discover the consensus of dominant catalytic facet for the hydrogenation by Pd nanocatalysts. The previous study of the Kim group reported that the Pd(100) has easier decomposition of hydrogen than the Pd(111) contributed to high performance for selective hydrogenation of acetylene^[14]. On the other hand, Yarulin et al. thought that the Pd(111) is more active than the Pd(100) ^[15]. Moreover, DFT (density functional theory) calculations suggested that the styrene hydrogenation activity of the clean Pd(111), Pd(100), and Pd(110) surfaces decrease in the order of Pd(111) > Pd(100) > Pd (110) ^[16]. Yang et al. revealed that performance for selective hydrogenation of acetylene to ethylene on several Pd surfaces is Pd(211) > Pd(111) > Pd(100)^[17]. In addition, to our best knowledge, the discrimination of the hydrogen dissociation for each Pd surface is rarely studied from experimental observations.

  Most studies comparing particle morphology are performed over an ensemble of NPs with varied size and shape^[18-19]. While NPs synthesis have different shape with facet distributions, the bigger sizes lead to lower atom efficiency^[20-22]. On the other hand, single crystal particle studies can identify facet-specific activity and give better insight in the role of hydrogen dissociation on facets.

  In this work, we use the hydrogenation of single crystal Pd NPs to investigate the hydrogen dissociation on three low-index facets. Single crystal Pd NPs with different proportions of Pd(111), Pd(100), and Pd(110) facets were prepared at temperatures of 10, 15, 25, 30, and 35 ℃. The Pd NPs were then loaded onto activated carbon and labeled Pd/C-x, where x denoted the temperature value at which the Pd NPs were prepared. The Pd/C-x catalysts were characterized by performing transmission electron microscopy (TEM), X-ray diffraction (XRD), N₂ adsorption-desorption, inductively coupled plasma-optical emission spectroscopy (ICP-OES), hydrogen oxygen pulse titration (H₂-O₂), and H₂ temperature programmed desorption (H₂-TPD) analysis. Finally, we evaluated these catalysts for their styrene, cyclohexene, and p-nitrotoluene hydrogenation activities. We confirmed that Pd(111) facet proportion was linear with the hydrogenation activity of these Pd/C-x catalysts. These call for better understanding on improvement of hydrogenation activity by increasing the Pd(111) facet proportion, aiming to guide the rational design and facet optimization of the Pd-based catalyst.

  1.   Experimental

  1.1   Materials

  Tris-(dibenzylideneacetone) dipalladium(0) (Pd₂(dba)₃, AR) was purchased from Sigma-Aldrich Co., Ltd. Propylene carbonate (PC, AR) was purchased from Dongguan Youte environmental protection materials Co., Ltd. Active carbon (AR) was brought from Shanghai Lvqiang New Material Co., Ltd. Styrene (C₈H₈, AR), cyclohexene (C₆H₁₂, AR), and p-nitrotoluene (C₇H₇O₂N, AR) were purchased from Shanghai Aladdin Reagent Co., Ltd.

  1.2   Preparation of Pd nanoparticles

  Pd nanoparticles with different Pd(111) proportion were synthesized by the methods in different temperatures. Specifically, a measured amount of Pd₂(dba)₃ as a precursor and 100 mL PC were added into a 250 mL stainless steel stirred reactor. The reactor was initially purged with H₂ for 6 times, then slowly heated until the desired reaction temperature of 10, 15, 25, 30, and 35 ℃. After pressurized to 4.0 MPa with H₂, the reaction was started with a stirring rate of 500 r·min^-1 for 3 h. Then the prepared black Pd reactant was adsorbed by quantitative activated carbon for 24 h until the solu-tion was colorless and transparent after filtration. The samples were washed by ethanol and acetone, then nat-ural dried for 24 h. All these materials were defined as Pd/C-10, Pd/C-15, Pd/C-25, Pd/C-30, and Pd/C-35, respectively.

  1.3   Catalysts characterization

  TEM was taken on a JEOL JEM-1200EX with an accelerating voltage of 200 kV. Before being transferred into the TEM chamber, the samples dispersed in ethanol were deposited onto holey carbon films supported on Cu grids.

  Fast Fourier transformation (FFT) was performed on Digital-Micrograph software. The selected area of the high resolution TEM (HRTEM) images was treated by FFT, thus the reciprocal lattices corresponding to the reciprocal space were obtained. Then the distance from different lattices to the origin of reciprocity was measured, and the countdown of the distance was the actual interplanar distance. Referring to PDF card data, the specific crystal plane of corresponding substance was gained.

  The XRD patterns of the Pd/C-x were performed on a Rigaku D/Max-2500 X-ray diffractometer, which used a Cu Kα radiation (λ =0.154 nm) in the 2θ scan range (40 kV and 100 mA) from 10° to 80° with a step of 0.05°.

  The Pd content of the prepared Pd/C-x catalysts was determined by ICP-OES. The experiments were done by Aglient 720ES.

  N₂ adsorption at -196 ℃ was measured using a Micromeritics ASAP 2010 system, the samples were degassed at 200 ℃ for 6 h under high vacuum. The surface area was calculated by using the Brunauer-Emmett-Teller (BET) method. The total pore volume was determined by nitrogen adsorption at a relative pressure of 0.99, and the pore size distributions were calculated from the nitrogen adsorption isotherms by the Barrett-Joyner-Hallenda (BJH) method.

  The H₂-O₂ and H₂-TPD experiments were done by Micromeritics Autochem 2920 with a TCD detector. The principle of H₂-O₂ was as follows, the routine of "pre-reduction (adsorption of hydrogen) → titrated oxygen → titrated hydrogen → titrated oxygen → titrated hydrogen"was measured sequentially. As shown in Eq. 1 ~3, titrating a single palladium atom requires three hydrogen atoms. Specifically, loop ring (a quantitative loop, the volume was 0.5 mL) titration was performed with 5% H₂/Ar by injection, until the peak height remained constant, indicating that hydrogen adsorption on the Pt surface had reached saturation, hydrogen titration operation was completed. The adsorbed hydrogen volume on the Pd/C-x was calculated by Formula 4, where A_H2, V_r, V_m, and m represent quantity of adsorbed H₂, H₂ titration volume, molar volume of gas (22.4 L·mol^-1), and quality of sample, respectively.

  $ {{\rm{Pd}} + 1/2{{\rm{H}}_2} \to {\rm{PdH}}} $

  (1)

  $ {{\rm{PdH}} + 3/4{{\rm{O}}_2} \to {\rm{PdO}} + 1/2{{\rm{H}}_2}{\rm{O}}} $

  (2)

  $ {{\rm{PdO}} + 3/2{{\rm{H}}_2} \to {\rm{PdH}} + {{\rm{H}}_2}{\rm{O}}} $

  (3)

  $ {{A_{{{\rm{H}}_2}}} = \frac{{2{V_{\rm{r}}}}}{{3{V_{\rm{m}}}m}}} $

  (4)

  1.4   Catalytic test

  In each experiment, the autoclaves were purged 6 times with H₂ to remove air. After a fixed reaction time, the autoclaves were cooled down to room temperature and H₂ pressure was carefully released. In the hydroge-nation process, stirring speed was kept at 1 200 r· min^-1 to avoid mass transfer limitations. The H₂ pres-sure changes of the 250 mL gas tank was recorded automatically with a pressure sensor, which connected to the autoclaves.

  The hydrogenation reaction rates were computed based on calculated H₂ consumption per unit time (r) using the equation given by Formula 5. The t₂-t₁ represents the time period when hydrogenation reaction is stable. The n₂-n₁ represents variable quantity in amount of substance of H₂. The amount of substance of H₂ were calculated by Redlich-Kwong Eq. 6~8, where P, V, T, R, P_c, and T_c represent the H₂ pressure in storage tank, H₂ molar volume, H₂ temperature, thermody-namic constant (8.314 J·mol^-1·K^-1), critical condition pressure and temperature, respectively.

  $ {r = \frac{{{n_2} - {n_1}}}{{{t_2} - {t_1}}}} $

  (5)

  $ {P = \frac{{RT}}{{V - b}} - \frac{a}{{{T^{1/2}}V(V + b)}}} $

  (6)

  $ {a = 0.42748\frac{{{R^2}T_{\rm{c}}^{2.5}}}{{{P_{\rm{c}}}}}} $

  (7)

  $ {b = 0.08664\frac{{R{T_{\rm{c}}}}}{{{P_{\rm{c}}}}}} $

  (8)

  2.   Results and discussion

  2.1   Characterization results

  We investigated the Pd NPs of Pd/C-10, Pd/C-15, Pd/C-25, Pd/C-30, and Pd/C-35 by TEM, and the results are shown in Fig. 1. Spherical Pd nanocrystals were observed in each image. The size of the Pd NPs was approximately 4.3 nm. As shown in Fig. 2, three typical Pd NPs were magnified by HRTEM, which were characterized by eight triangular (111) facet, six square (100) facet, and dodecahedron (110) facet, respectively. For each sample, 50 Pd crystals chosen randomly from several HRTEM images were examined and classified into three categories: Pd crystals exposed only (111), (100) and (110) facets (Fig.S1~S5). Based on statistical analysis, the proportion of Pd(111) facet in the Pd/C-10, Pd/C-15, Pd/C-25, Pd/C-30, and Pd/C-35 catalyst were 84%, 75%, 63%, 55%, and 43%, respectively. This suggests that the reaction temperature influences the formation of Pd crystals exposed by only the (111) facet.

  Figure 1

  

  Figure 1.  TEM images and derived particle size distributions of Pd/C-x samples: (a) Pd/C-10, (b) Pd/C-15, (c) Pd/C-25, (d) Pd/C-30 and (e) Pd/C-35

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  Figure 2

  

  Figure 2.  HRTEM and FFT images of single Pd NPs

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  Fig. 3 shows the XRD patterns of Pd/C-x catalysts. In each XRD pattern, three diffraction peaks were observed at 2θ=40.1°, 46.7°, and 68.1°, which are assigned to (111), (200) and (220) facet of face-centered cubic Pd, respectively; this suggests the formation of metallic Pd. The ratios of peak intensity of the (111) facet to that of the (220) facet for Pd/C-10, Pd/C-15, Pd/C-25, Pd/C-30, and Pd/C-35 were 16.7, 11.2, 9.7, 6.5, and 4.3, respectively. This suggested that Pd NPs had a higher proportion of Pd(111) facet synthesized in lower temperature. Meanwhile, the particle size of Pd NPs of different catalysts, which calculated from FWHM of diffraction peaks according to Scherrer equation ^[23], are listed in Table 1, which is consistent with the TEM results. There is little difference in crystallinity between catalysts. Above results eliminated the possi-bility of a particle size effect and difference of crystallinity, allow us to directly compare their catalytic performance^[24].

  Figure 3

  

  Figure 3.  XRD patterns of Pd/C-x catalysts

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  Table 1

  

  Table 1.  XRD analysis of the catalysts

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  Catalyst	Da/nm	Crystallinity/%	I(111)/I(220)
  Pd/C-10	5.9	26.05	16.7
  Pd/C-15	5.9	24.87	11.2
  Pd/C-25	6.2	27.66	9.7
  Pd/C-30	6.8	21.27	6.5
  Pd/C-35	7.1	23.36	4.3
  a Calculated by Scherrer equation.

  It is known that the surface of crystals can be easily controlled via adjusting supersaturation of crystal growth units during the crystal growth process^[25]. Xie et al. extensively proposed that the faster reduction rate results in the higher surface energy of crystallites^[26]. On the other hand, the surface energy on Pd single crystals has been reported to increase in the order of Pd(111) < Pd(100) < Pd(110) ^[27]. The change in the temperature can exponentially influence reduction rate of metal precursor which explains the slower reduction rate lead to higher Pd(111) proportion in lower reaction temperature.

  The texture properties of different samples are measured by N₂ adsorption and desorption experiment and the results are summarized in Table 2. Compared to carbon, the mesoporous volume and mesporous area of different catalysts slightly decrease, which is attributed to Pd NPs clogged the pores of active support carbon during catalyst preparation process^[28]. However, the external surface area of all catalysts substantially remains unchanged. Based on the total amount of Pd in the impregnation solution, the theoretical Pd loading (mass fraction) was 1.00% of that in Pd/C-x catalysts. The Pd loading of all catalysts varied from 0.90% to 0.95% (within the range of test errors).

  Table 2

  

  Table 2.  Properties of the catalysts

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  Catalyst	Surface area/(m2·g-1)	Pore volume/(cm3·g-1)	Pore size/nm	Mass fraction of Pda/%
  Pd/C-10	1 133	0.52	4.70	0.93
  Pd/C-15	1 154	0.49	4.73	0.91
  Pd/C-25	1 178	0.48	4.72	0.90
  Pd/C-30	1 135	0.49	4.69	0.91
  Pd/C-35	1 167	0.48	4.70	0.95
  C	1 138	0.63	5.03	0
  a Obtained by ICP-OES analysis.

  To explore effect of (111) facet proportion on H₂ adsorption capacity, H₂-O₂ titration was performed for the Pd/C-x catalysts. The adsorbed hydrogen volume of the Pd/C-x was calculated by integral quantity of stable peak area (Fig. 4a). Fig. 4b shows that the quantity of adsorbed H₂ was plotted against the Pd(111) proportion (%). The amount of adsorbed H₂ on Pd/C-10 was 39.46 μmol·g^-1, which was nearly 2.13 times greater than the amount of H₂ on Pd/C-35 (18.54 μmol g^-1). The Pd(111) proportion of Pd/C-10 was 1.95 times than that of Pd/C-35. The amounts of adsorbed H₂ on Pd/C-15, Pd/C-25, and Pd/C-30 were 34.92, 25.47, and 22.84 μmol·g^-1, respectively. It is clear that a linear relationship between the quantity of adsorbed H₂ and the Pd(111) proportion of each Pd/C-x catalyst. The linear curve in Fig. 4b had a high correlation coefficient (R²) of 0.98. It should be pointed out that the line through the origin point, indicating nonoccurrence H₂ dissociation with absence of Pd(111) facet. On the other hand, the volume of adsorbed H₂ was not positively correlated with the proportions of the Pd(100) and Pd(110) facets (Fig. S6). The results confirms that Pd(111) facet plays a central role in hydrogen dissociation.

  Figure 4

  

  Figure 4.  Results of H₂-O₂ titration of absorbed hydrogen for Pd/C-x catalysts: (a) H₂-O₂ titration peak map; (b) relationship between Pd(111) proportion and the quantity of adsorbed H₂

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  H₂-TPD was used to detect the metal properties of the catalysts with Pd NPs of different (111) facet proportions, which is shown in Fig. 5. Generally, the hydrogen adsorbed on the Pd surface can be assigned to two kinds of hydrogen species, including the surface hydrogen adsorbed on the Pd surface and subsurface hydrogen adsorbed on the subsurface or the bulk of Pd^[29]. As shown in Fig. 4, the desorption peak centered at 65 ℃ can be assigned to the desorption of H₂ molecules from Pd surface^[30-31], while the peak centred at 380 ℃ can be assigned to the desorption of H₂ molecules from the active support carbon^[32]. The dissociation adsorption capacity of Pd for H₂ of Pd/C-35 was too weak, whereas that of Pd/C-10 was too strong, which suggests that the high Pd(111) proportion has stronger ability to activate H₂.

  Figure 5

  

  Figure 5.  H₂-TPD desorption profiles of Pd/C-x

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  2.2   Catalytic activity of Pd/C-x catalysts for hydrogenation

  Generally, the facet of Pd NPs may affect product conversion and selectivity using defined experiments and DFT simulations^[33]. Therefore, it is imperative to study the (111) facet proportion of Pd influence the hydrogenation activity. The performance of styrene, cyclohexene, and p-nitrotoluene hydrogenation were evaluated for the different catalysts prepared with Pd NPs of different (111) facet proportion. Fig.S7 presents the lines of hydrogen consumption curves for three hydrogenation reactions, suggests the first order reaction for styrene, cyclohexene, and p-nitrotoluene hydrogena-tion reactions^[34-36]. The curves in the initial time was not linear, due to the instability of system when the reaction started^[37]. As the Pd(111) proportion increased the hydrogen consumption gradually increased for all catalysts due to hydrogenation active sites on Pd(111) facet. Moreover, it can be found that the hydrogen consumption rate over different catalysts follows the Pd/C-10 > Pd/C-15 > Pd/C-25 > Pd/C-30 > Pd/C-35, in consistent with the results of H₂-O₂.

  Table 3 shows the hydrogen consumption rate for three reactions in Pd/C-x catalysts with different Pd(111) proportions. All Pd/C-10 catalyst exhibited higher hydrogenation activity than other catalyst in every hydrogenation reaction. At styrene hydrogenation, the hydrogen consumption rate in Pd/C-x catalysts were 9.17, 8.11, 7.30, 5.68, and 4.59 mmol_H2·min^-1 for Pd(111) proportion of 84%, 75%, 63%, 55%, and 43%, respectively. The hydrogenation activity of Pd/C-10 catalyst was 2.00 times that of the Pd/C-35 catalyst, in consistent with the 1.95 times of that Pd(111) ratios. The H₂ consumption rate in Pd/C-x catalysts for cyclohexene hydrogenation were 0.59, 0.54, 0.47, 0.40, and 0.34 mmol_H2·min^-1 for Pd(111) proportion of 84%, 75%, 63%, 55%, and 43%, respectively, whereas 2.00, 1.79, 1.60, 1.38, and 1.17 mmol _H2·min^-1 of that for p-nitrotoluene hydrogenation. The data shown in Fig. 6 clearly showed a linear relationship between the proportion of Pd(111) facet in Pd/C-x catalysts and their H₂ consumption rate in every hydrogenation. Interestingly, each curve passed through the original point, and had a high R² of 0.99. This suggests that no hydrogenation occurred in the absence of Pd(111) under ideal conditions. In contrast, the proportion of Pd(100) and Pd(110) facets in Pd/C-x and their H₂ consumption rates were not positively correlated (Fig. S8). For the linear correlation between H₂ consumption rate and Pd(111) proportion, the dissociation adsorption capaci-ty of Pd(111) for H₂ were further proved from hydroge-nation aspects, suggesting that the hydrogenation active site originated from Pd(111) facet.

  Table 3

  

  Table 3.  Catalytic hydrogenation performance over different catalysts

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  Catalyst	Facet proportiona/%				H2 consumption rate /(mmolH2·min-1)
  	Pd(111)	Pd(100)	Pd(110)		Styrene	Cyclohexene	p-nitrotoluene
  Pd/C-10	84	12	4		9.17	0.59	2.00
  Pd/C-15	75	20	5		8.11	0.54	1.79
  Pd/C-25	63	31	6		7.30	0.47	1.60
  Pd/C-30	55	38	7		5.68	0.40	1.38
  Pd/C-35	43	47	10		4.59	0.34	1.17
  a Data were measured by FFTs (Fig.S1~S5).

  Figure 6

  

  Figure 6.  Relationship between the Pd(111) proportion of the catalysts and H₂ consumption rates in three hydrogenation reactions

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  The reusability study was conducted with Pd/C-10 catalyst for the p-nitrotoluene hydrogenation. As shown in H₂ consumption curves (Fig. S9), the test was performed up to 10 successive cycles for the reactions. The catalyst stayed active and showed consistent performance (Fig. 7). Interestingly, the catalyst was able to retain the activity after successive reuse.

  Figure 7

  

  Figure 7.  Recyclability test of Pd/C-10 catalyst for p-nitrotoluene hydrogenation

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  Furthermore, XRD patterns of both fresh and recycled catalysts for the p-nitrotoluene hydrogenation indicated that there was no change in phase purity and the crystalline structure remained stable after ten recycles (Fig. 8). In addition, FFT measurement of HRTEM images for recycled catalysts was performed (Fig.S10). The result indicated that the Pd(111) facet proportion could be substantially unchanged. About 82% of Pd (111) facet proportion in recycled Pd/C-10 was consistent with the result of 84% of that in fresh Pd/C-10.

  Figure 8

  

  Figure 8.  XRD patterns of fresh and recycled Pd/C-10 catalyst

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  3.   Conclusions

  In summary, we have described a method for the synthesis of different (111) facet proportions of Pd loaded active carbon catalysts with small size in well dispersion. Through systematic results of H₂-O₂, H₂-TPD and three typical hydrogenation reactions, Pd NPs with high Pd(111) proportion were found to be remarkably active for catalyzing hydrogen. Therefore, we propose that H₂ molecules prior to adsorb on the Pd(111) facet and dissociate into individual H atoms, which then participate in hydrogenation reactions. This concept of hydrogenation active sites on Pd(111) unlocks the pos-sibility for future nanocrystal catalyst design where the critical facet role can be optimized for a given catalytic reaction.

  
  Acknowledgements: The authors gratefully acknowledge the National Key Research and D & P of China (Grant No. 2017YFC0210900). Supporting information is available at http://www.wjhxxb.cn
  
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  Figure 1  TEM images and derived particle size distributions of Pd/C-x samples: (a) Pd/C-10, (b) Pd/C-15, (c) Pd/C-25, (d) Pd/C-30 and (e) Pd/C-35

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  Figure 2  HRTEM and FFT images of single Pd NPs

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  Figure 3  XRD patterns of Pd/C-x catalysts

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  Figure 4  Results of H₂-O₂ titration of absorbed hydrogen for Pd/C-x catalysts: (a) H₂-O₂ titration peak map; (b) relationship between Pd(111) proportion and the quantity of adsorbed H₂

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  Figure 5  H₂-TPD desorption profiles of Pd/C-x

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  Figure 6  Relationship between the Pd(111) proportion of the catalysts and H₂ consumption rates in three hydrogenation reactions

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  Figure 7  Recyclability test of Pd/C-10 catalyst for p-nitrotoluene hydrogenation

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  Figure 8  XRD patterns of fresh and recycled Pd/C-10 catalyst

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  Table 1.  XRD analysis of the catalysts

  Catalyst	Da/nm	Crystallinity/%	I(111)/I(220)
  Pd/C-10	5.9	26.05	16.7
  Pd/C-15	5.9	24.87	11.2
  Pd/C-25	6.2	27.66	9.7
  Pd/C-30	6.8	21.27	6.5
  Pd/C-35	7.1	23.36	4.3
  a Calculated by Scherrer equation.

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  Table 2.  Properties of the catalysts

  Catalyst	Surface area/(m2·g-1)	Pore volume/(cm3·g-1)	Pore size/nm	Mass fraction of Pda/%
  Pd/C-10	1 133	0.52	4.70	0.93
  Pd/C-15	1 154	0.49	4.73	0.91
  Pd/C-25	1 178	0.48	4.72	0.90
  Pd/C-30	1 135	0.49	4.69	0.91
  Pd/C-35	1 167	0.48	4.70	0.95
  C	1 138	0.63	5.03	0
  a Obtained by ICP-OES analysis.

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  Table 3.  Catalytic hydrogenation performance over different catalysts

  Catalyst	Facet proportiona/%				H2 consumption rate /(mmolH2·min-1)
  	Pd(111)	Pd(100)	Pd(110)		Styrene	Cyclohexene	p-nitrotoluene
  Pd/C-10	84	12	4		9.17	0.59	2.00
  Pd/C-15	75	20	5		8.11	0.54	1.79
  Pd/C-25	63	31	6		7.30	0.47	1.60
  Pd/C-30	55	38	7		5.68	0.40	1.38
  Pd/C-35	43	47	10		4.59	0.34	1.17
  a Data were measured by FFTs (Fig.S1~S5).

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