Facile synthesis and synergistically acting catalytic performance of supported bimetallic PdNi nanoparticle catalysts for selective hydrogenation of citral

Molecular Catalysis 436 (2017) 237–247

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Research paper

Facile synthesis and synergistically acting catalytic performance of supported bimetallic PdNi nanoparticle catalysts for selective hydrogenation of citral Chunling Liu a , Chunshi Nan a , Guoli Fan a , Lan Yang a , Feng Li a,b,∗ a b

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing 100029, China

a r t i c l e

i n f o

Article history: Received 3 January 2017 Received in revised form 10 March 2017 Accepted 21 April 2017 Keywords: Bimetallic nanoparticles Synergistic effect Layered double hydroxide Selective hydrogenation Citral

a b s t r a c t Because of sustainable development in the field of heterogeneous catalysis, it is of paramount importance to the design and synthesis of high-performance supported metal nanoparticle catalyst systems for various catalytic reactions. Herein, new carbon-supported bimetallic PdNi nanoparticles (NPs) were facilely synthesized using our developed “carbonization-in situ self reduction” strategy based on hybrid composite precursors of PdCl4 2− -containing Mg-Ni-Al layered double hydroxide and amorphous carbon (LDH-C) and employed for selective hydrogenation of citral. The incorporation of PdCl4 2− complex ions into the LDH structure facilitated the dispersion of Pd species, and subsequently highly dispersed bimetallic PdNi NPs could be formed on the support surface. The bimetallic PdNi nanocatalyst with the Pd/Ni mass ratio of 67:33 showed higher catalytic activity than supported monometallic counterparts prepared by the same method, which could be ascribed to a synergistic effect between Pd and Ni in PdNi NPs. Furthermore, as-formed supported bimetallic PdNi nanocatalyst possessed excellent structural stability, owing to strong metal-support interactions stemming from the perfectible hybrid nanostructure of LDH-C composite precursors. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Currently, bimetallic nanoparticles (NPs) as high-performance catalysts are intensively investigated in a wide range of heterogeneous catalytic reactions [1–7], because of their superior properties to corresponding monometallic counterparts attributable to synergistic effects between two metals [8,9]. Meanwhile, incorporating a base metal into precious-metal-based bimetallic NPs can greatly lower the cost of catalysts [10]. For instance, dendrimer-stabilized bimetallic RhFe NPs were reported to show a more inspiring performance in the nitroarene hydrogenation with the introduction of Fe [9]. Especially, the dendrimer matrices supplied a favorable reaction interface, thus preventing the aggregation of bimetallic NPs. Also, Li and co-workers reported that the composition of bimetallic RhNi NPs played an important role in governing catalytic performance in the hydrogenation of nitroarenes [11,12]. Selective hydrogenation of ␣,␤-unsaturated aldehydes to produce industrially important chemicals is of particular importance

∗ Corresponding author at: Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing 100029, China. E-mail addresses: [email protected], lifeng [email protected] (F. Li). http://dx.doi.org/10.1016/j.mcat.2017.04.025 2468-8231/© 2017 Elsevier B.V. All rights reserved.

in terms of several advantages of environmental friendliness, low cost, and economic sustainability [13–17]. At present, precious metal catalysts (e.g., Pt [18,19], Pd [13,20], and Rh [21]) have been explored extensively. Generally, these catalysts are known to be highly selective toward the C C bond using hydrogen as a reducing agent, leading to the formation of saturated aldehydes [22]. Typically, citronellal (CAL), which is an industrially important intermediate in the production of fine chemicals used in the food, fragrance, and pharmaceutical industries, can be produced by hydrogenating the C C bond in citral with a conjugated C C double bond, a carbonyl group and an isolated C C double bond [23–26]. In contrast, geraniol/nerol isomers may be formed by hydrogenating the conjugated C O bond in citral. In this regard, palladium especially appears to be selective for the hydrogenation of the C C bond [20,27–31]. Thus, it is important to design stable, low-cost and highly efficient Pd-based catalysts for the sake of the production of desired CAL product in terms of economic and sustainable development. On the other side, layered double hydroxides (LDHs) with the formula [M1−x 2+ Mx 3+ (OH)2 ]x+ [Ax/n ]n− ·mH2 O, well known as a class of two-dimensional layered clay materials [32], act widely as catalyst precursors owing to their structural versatility and flexibility. Specially, the host structure of LDHs can accommodate different

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divalent or trivalent metal cations on the brucite-like layers or various metal complex anions within the interlayer to achieve highly dispersed and stable metal NPs through structural transformation, which makes LDHs extremely attractive in the synthesis of a variety of robust supported catalyst systems. Recently, we explored the generation of supported Ni or Pd NPs based on LDH-carbon composite precursors (LDH-C) [33,34]. In comparison to traditional impregnation and coprecipitation methods for the synthesis of supported catalysts, the LDH-C precursor route is of great advantage especially in terms of the significantly high metal dispersion and the formation of strong metal-support interactions (SMSI). In spite of a variety of successful synthesis of unsupported precious monometallic and bimetallic NPs [35–39], controlling the dispersion of NPs in supported bimetallic catalysts remains challenging. Herein, we report the synthesis of new carbon-supported bimetallic PdNi NPs with different Pd/Ni mass ratios by our developed “carbonization-in situ self reduction” strategy, which involves the assembly of PdCl4 2− -containing ternary MgNiAl-LDHs and amorphous carbon composite precursors through the carbonization of glucose, followed by an in situ self-reduction process using the carbon component as reducing agent. The dilution of costly Pd by inexpensive Ni is economical. The results indicate that as-formed bimetallic PdNi nanocatalyst with the Pd/Ni mass ratio of 67:33 shows higher catalytic activity in the selective hydrogenation of citral than monometallic counterparts prepared by the same method, attributable to the synergy between Pd and Ni in PdNi NPs. Furthermore, the present supported PdNi nanocatalyst also possesses excellent structural stability, due to the presence of SMSI originating from the perfectible hybrid nanostructure of LDH-C composite precursors 2. Experimental 2.1. Synthesis of supported catalysts MgNiAl-LDH precursors containing PdCl4 2− anion were prepared through coprecipitation method. Typically, Mg(NO3 )2 ·6H2 O, Ni(NO3 )2 ·6H2 O, Al(NO3 )3 ·9H2 O and desired amount of H2 PdCl4 were dissolved in decarbonated deionized water, where the (Mg2+ + Ni2+ )/Al3+ molar ratio is 2:1 and the mass percentage of Pd in total amount of Pd and Ni is x% (x = 80, 67, 55). Subsequently, the above salt solution was titrated by NaOH solution (0.2 M) under vigorous agitation at N2 atmosphere at room temperature, until the pH value reaches 10.0. The resulting suspension was aged at 70 ◦ C overnight, centrifuged and washed with deionized water for several times. Then, the obtained precipitant was transferred into Teflonlined autoclave, mixed with 60 mL of glucose solution (5[Mg2+ +Ni2+ + Al3+ ] = 2 [C6 H12 O6 ]), and kept at 150 ◦ C for 12 h to obtain LDH-C composite (denoted as PdNi-LDH-x-C). At last, PdNi-LDH-xC was heated to 600 ◦ C at a rate of 5 ◦ C min−1 under N2 atmosphere in a tube furnace reactor and held for 2 h to obtain supported samples (denoted as PdNi-x/C). Similarly, Pd-LDH-C and Ni-LDH-C with the same metal loading of about 3.0 wt% also were prepared using to the above identical procedure in the presence of H2 PdCl4 or Ni(NO3 )2 ·6H2 O. Subsequently, obtained composites were calcined at 600 ◦ C for Pd-LDH-C and 700 ◦ C for Ni-LDH-C under N2 atmosphere for 2 to obtain Pd/C and Ni/C samples. In addition, pure carbon support was prepared through the hydrothermal treatment using glucose solution (0.15 M) and following calcination at 600 ◦ C for 2 h under N2 atmosphere. 2.2. Characterization Powder X-ray diffraction (XRD) patterns of samples were collected using Shimadzu XRD-6000 diffractometer using Cu K␣ radiation with  = 0.15418 nm.

Elemental analysis was carried out on Shimadzu ICPS-7500 inductively coupled plasma atomic emission spectroscopy (ICPAES). Thermogravimetric analysis and mass spectrometry (TG-MS) of samples were carried out using PerkinElmer Diamond TG/DTA instrument connected to a Pfeiffer ThermoStar MS analyzer under argon flow (300 mL min−1 ) at a heating rate of 5 ◦ C min−1 . Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were observed using JEOL JEM-2010 electron microscope with an accelerating voltage of 200 kV. Scanning transmission electron microscopy (STEM) experiments were carried out using a HITACHI S-5500 STEM equipped with an Oxford INCA-450 energy dispersive X-ray (EDX) spectrometer detector. Low temperature N2 adsorption-desorption experiments of samples were performed using Micromeritics ASAP 2020 sorptometer. The specific surface area was determined based on the multipoint Brunauer–Emmett–Teller (BET) method. X-ray photoelectron spectroscopy (XPS) data was collected on a Thermo VG ESCALAB250 X-ray photoelectron spectrometer with a monochromatic Al K␣ X-ray radiation of 1486.6 eV photons. H2 temperature-programmed reduction (H2 -TPR) and temperature programmed desorption of hydrogen (H2 -TPD) experiments were conducted over reduced samples on a Micrometric ChemiSorb 2920 chemisorption instrument equipped with a thermal conductivity detector (TCD). First of all, the reduced sample (0.1 g) was degassed under a flow of Ar (40 mL min−1 ) at 500 ◦ C for 1 h. For TPR, the reduced sample was further treated in a H2 /Ar flow (1:9, v/v; 40 mL min−1 ) at a heating rate of 10 ◦ C min−1 from 500 to 1000 ◦ C. The reduction degree of Ni species in reduced samples was calculated by the percentage of the difference value to the theoretical hydrogen consumption for the reduction of Ni2+ species. Herein, the difference value was the theoretical hydrogen consumption for the reduction of Ni2+ species minus the hydrogen consumption for the reduction of unreduced Ni2+ species in the TPR from 500 to 1000 ◦ C, considering that the amount of hydrogen consumption for the reduction of unreduced Pd2+ species in reduced samples could be estimated by XPS results of Pd 3d core level. For H2 -TPD, the reduced sample was treated in a flow of a H2 /Ar flow (1:9, v/v; 40 mL min−1 ) at room temperature and held for 2 h. At last, chemisorbed H2 was desorbed from room temperature to 700 ◦ C in a flow of Ar at a rate of 10 ◦ C min−1 . The dispersion degree of metal NPs was estimated by H2 –O2 titration using Micromeritics ChemiSorb 2720. First, LDH-C composite sample (100 mg) underwent a reduction in a N2 flow (40 mL min−1 ) from 50 ◦ C to 600 ◦ C for Pd-containing samples or to 700 ◦ C for Ni-LDH-C sample at a heating rate of 5 ◦ C and held for 2 h, which was the same condition as that for preparing reduced samples. After the sample was cooled to 120 ◦ C, the sample was switched to a H2 /Ar mixture flow (1:9, v/v; 40 mL min−1 ) and maintained for 1 h, and then purged by an Ar flow (40 mL min−1 ) for 1 h to ensure the desorption of physically adsorbed hydrogen on its surface. Subsequently, pulses of oxygen were introduced into the sample until the signal area was constant. Finally, the chemisorbed oxygen was titrated by pulses of H2 . The dispersion degree of metal (Dis, %) on reduced samples was determined according to the following equation [40]: Dis(%) =

2 × VTH × 10−3 3 × 22.4 × W × (

PPd ×RPd MPd

+

PNi ×RNi ) MNi

× 100

(1)

where MPd and MNi are the formula weight of Pd (MPd = 106.42 g mol−1 ) and Ni (MNi = 58.69 g mol−1 ), respectively; VT H is the volume of H2 used for the titration of O2 (mL); W is the mass of catalyst (g); PPd and PNi are the weight percentage of

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Fig. 1. TG-MS profiles of the representative PdNi-LDH-67-C composite precursor.

Pd and Ni species in the LDH-C composite sample as determined by ICP-AES, respectively; RPd is the reduction degree of Pd species determined by the XPS results of Pd 3d core level; RNi is the reduction degree of Ni species determined by the H2 -TPR results of reduced samples. Temperature-programmed desorption (TPD) experiments were carried out on Chemisorb 2720 instrument with a TCD. First, the sample (100 mg) was first heated at 200 ◦ C in a He flow (40 mL/min) and held for 1 h. After cooling to 50 ◦ C, pure CO2 (or NH3 ) was introduced for 1 h, and the sample tube was purged with a He flow for 1 h. At last, Chemisorbed CO2 (or NH3 ) was desorbed with the increased temperature to 700 ◦ C at a rate of 5 ◦ C min−1 . 2.3. Catalytic hydrogenation test The selective hydrogenation of citral was performed in a 200 mL stainless steel autoclave, where citral (5.85 mmol), the catalyst with the citral/(Pd + Ni) mass ratio of 68.4, and 100 mL isopropanol was charged. Afterward, the reactor was flushed by the nitrogen to exclude the air, sealed and then introduced by the H2 of 2.0 MPa for ten times. The reactor was placed into an oil bath and preheated to certain reaction. H2 was fed into the reactor to certain pressure and then the hydrogenation of citral was carried out under stirring at a speed of 900 rpm. After reaction, the reactor was cooled with an ice-water bath, and depressurized carefully. Finally, the products were quantitatively analyzed by gas chromatography (Agilent GC-7890B) equipped with a flame ionization detector and a DB-WAX capillary column (30.0 m × 250 ␮m × 0.25 ␮m). The conversions and selectivities were obtained based on at least 3 parallel experiments with experimental errors of less than 3%. 3. Results and discussion 3.1. Structural characterization The thermal evolution of the representative PdNi-LDH-67-C composite precursor was analyzed by TG-MS technique. As shown in Fig. 1, there are three weight loss steps in the temperature range of 80–700 ◦ C. The first step at ca. 80–275 ◦ C is correlated with the release of physisorbed water and CO2 on the layers and the desorption of interlayer water in the structure of LDH component, as clearly reflected by two H2 O peaks at about 92 ◦ C and 250 ◦ C and a CO2 peak at 204 ◦ C in the corresponding MS profiles. The second

Fig. 2. The release profiles of CO generated in the TG-MS tests over different LDH-C precursors: Pd-LDH-C (a), PdNi-LDH-80-C (b), PdNi-LDH-67-C (c), PdNi-55-LDH-C (d), Ni-LDH-C (e).

one in the range of 275–400 ◦ C is assigned to the dehydroxylation of the LDH lattices (360 ◦ C) and the release of CO2 (345 ◦ C) due to decomposition of interlayer carbonate anions. The third one above 400 ◦ C involves the release of CO (413, 460 and 552 ◦ C) and CO2 (462 ◦ C) due to the reduction of metal oxide phases containing Ni and Pd by the carbon component, which corresponds to the following two reactions: MO + C→CO + M and CO + O→CO2 , as clearly observed by the appearance of CO and CO2 peaks in the MS profiles. Accordingly, in combination with the above TG-MS results, the reduction temperatures of LDH-C composites under inert atmosphere should be in accordance with the peak temperatures for the release of CO gas in the TG-MS profiles. Fig. 2 shows the release profiles of CO generated in the TG-MS tests over different LDH-C precursors. According to the release of CO generated, the reduction temperatures for two Pd-LDH-C and Ni-LDH-C precursors appear in the temperature ranges of 300–475 ◦ C and 500–700 ◦ C, respectively. The reduction temperatures for other composite precursors containing Pd and Ni, which are lower than that for Ni-LDH C but higher than that for Pd-LDH-C, increase gradually with the decreasing Pd/Ni mass ratio in samples, probably due to the hydrogen spillover from the reduced Pd thereby easily reducing nickel oxide species [41]. The aforementioned results imply the presence of the interactions between Pd and Ni species in composites, thus easily forming bimetallic PdNi NPs. In addition, the reduction of partial Pd2+ species by glucose during the hydrothermal step for the formation of LDH-C composites can occur. Correspondingly, the presence of reduced Pd00 species in LDH-C composites as growth centers may facilitate the formation of bimetallic PdNi NPs upon heating under N2 atmosphere. The morphology and microstructure of supported monometallic and representative bimetallic PdNi samples were examined by TEM measurements (Fig. 3). In each case, uniform and spheroidal NPs are well dispersed over the support. The average crystallite size of NPs determined from more than 200 individual metal particles is in the range of 5.5–6.0 nm (Table 1), indicating a slight difference in the particle size of monometallic and bimetallic NPs regardless of the compositions of metallic NPs and the Pd loadings. Furthermore, a typical HRTEM image of the representative

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Fig. 3. TEM and HRTEM images of Pd/C (a,b), PdNi-67/C (c,d) and Ni/C (e,f).

PdNi-67/C reveals that the lattice image of a single NP shows an interplanar spacing of about 0.215 nm. Such value of interplanar spacing is smaller than that for the (111) plane of metallic Pd phase

(0.225 nm) and larger than that for the (111) plane of metallic Ni phase (0.203 nm), strongly mirroring the formation of bimetallic PdNi NPs in the PdNi-67/C sample.

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Table 1 Analytical, structural and catalytic data of different supported reduced samples. Samples Pd/C PdNi-80/C PdNi-67/C PdNi-55/C Ni/C

Pd a (wt%)

Ni a (wt%)

2.9 2.2 2.0 1.6 0

0 0.6 1.0 1.5 3.2

RPd

b

(%)

90.3 84.1 89.8 90.7 –

RNi

c

(%)

– 90.6 89.5 87.2 83.3

SBET

d

(m2 /g)

D e (nm)

Dis.f (%)

SBg (mmol/g)

SAh (mmol/g)

Ri (mol gPdNi −1 s−1 )

TOF j (s−1 )

5.5 5.9 6.0 5.8 5.7

33.1 36.7 37.3 35.4 27.9

0.183 0.176 0.167 0.158 0.141

0.031 0.025 0.019 0.024 0.012

0.167 0.196 0.244 0.174 0.104

0.417 0.438 0.541 0.344 0.306

221 224 229 232 218

a

determined by ICP-AES. Reduction degree of Pd species determined by XPS analysis. Reduction degree of Ni species determined by H2 -TPR. d BET surface area. e Particle size of metal NPs obtained by TEM analysis. f Degree of metal dispersion determined by H2 -O2 titration. g The density of surface basic sites determined by CO2 -TPD. h The density of surface acidic sites determined by NH3 -TPD. i Initial hydrogenation reaction rate of citral hydrogenation. j Turnover frequency (TOF) of citral hydrogenation, which was given as the overall rate of citral conversion normalized by the number of surface active Pd and Ni sites within the initial 15 min. b c

Fig. 4. XRD patterns of Pd/C (a), PdNi-80/C(b), PdNi-67/C (c), PdNi-55/C, (d) Ni/C(e) and XRD card of metallic Pd (JCPDS no. 88–2335) and Ni (JCPDS no. 04-0850).

Further, XRD characterizations were carried out. Fig. 4 shows the XRD patterns of different supported samples. In the case of supported monometallic Pd and Ni samples, the XRD lines indicate the characteristic (111), (200), and (220) diffractions for metallic Pd (JCPDS 88-2335) and metallic Ni (JCPDS 04-0850), respectively, except for MgO phase and amorphous carbon. Interestingly, in the cases of supported bimetallic PdNi NPs, their (111), (200), and (220) planes appear between those of metallic Pd and Ni peaks and their positions gradually shift to the higher values of 2␪ with the decreasing Pd/Ni mass ratios. The results demonstrate the formation of bimetallic PdNi phase.. To determine the elemental distributions of Pd and Ni over the support, STEM characterization was further examined. Fig. 5 shows STEM images and resulting Pd and Ni elemental mapping in the representative PdNi-67/C sample. Noticeably, two Pd and Ni elements are distributed uniformly over an individual NP. The

elemental analysis by ICP-AES shows that the Pd/Ni mass ratios are well consistent with the designed compositions of bimetallic PdNi NPs (Table 1). As a result, above TG-MS, HRTEM, XRD and STEM results strongly confirm the formation bimetallic PdNi NPs in supported samples. In order to determine surface electronic structures of active metal species in supported samples, XPS measurements were conducted. Due to quite low content of Ni species (0.6–1.5 wt%), however, no discernible Ni 2p signals can be detected in all Pdand Ni-containing samples. As shown in Fig. 6, the signals centered at about 335.5 and 340.8 eV for Pd/C sample are ascribed to Pd 3d 5/2 and Pd 3d3/2 core levels, respectively, reflecting the character of metallic Pd0 [42]. Besides, there are another two very small peaks centered at about 336.7 eV and 342.0 eV, respectively, which is the character of unreduced Pd(II) species in samples. Clearly, the small Pd(II) to Pd(0) peak area ratio reveals that most of Pd species should be reduced upon heating under the present inert atmosphere. It is interesting to note that the binding energy (BE) value of Pd 3d 5/2 region is higher than that (335.0 eV) reported in the literature for Pd0 [43,44], due to the presence of the electronic interaction between the metallic Pd and the support, thus leading to the transfer of partial charge from the Pd to the support. With the introduction of Ni, the Pd 3d 5/2 peak for Pd0 shows a gradual shift toward the higher BE value of ∼336.1 eV for PdNi55/C, indicative of the presence of interactions between Pd and Ni in bimetallic samples. Commonly, it is believed that the BE shifts for metal species can be affected by both the initial and final state effects. Due to higher electronegativity of Pd atom (2.2) than that of Ni atom (1.8) [45], palladium atoms in bimetallic PdNi NPs should be negatively polarized and thus negatively charged according to the initial state effect. Based on the final state effect, the filling of the valence d-band in Pd atoms can cause a different screening of the core hole [46], thereby causing the shift of the Pd 3d levels to higher BE values. In addition, the dispersion degree of metal NPs can be determined by using H2 -O2 titration technique, considering that the reduction degree of Pd species or Ni species in reduced samples may be estimated by XPS or H2 -TPR results for reduced samples. As shown in Table 1, the dispersion degree of metal NPs is slightly enhanced with the increasing Ni/(Pd + Ni) mass ratio from 0 to 0.33. However, further introduction of Ni results in the slight decline in the dispersion of metallic NPs. Among all reduced sample, Ni/C presents a smallest dispersion degree of metal NPs (Table 1), which is probably associated with the presence of more surface unreduced NiO species, thus leading to the lower reduction degree of Ni species.

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Fig. 5. STEM images (A,D) of representative PdNi-67/C sample, the elemental mapping of Pd−K (B) and Ni−K (C), and EDX spectrum (E) of the sample. The inset in (E) shows EDX lines of Pd-K and Ni-K along the yellow line in (D) over an individual NP. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Scheme 1. Reaction pathways for the selective hydrogenation of citral.

3.2. Liquid phase hydrogenation of citral As shown in Scheme 1, first, either the C O group or the conjugated C C group can be hydrogenated to generate geraniol/nerol isomers or CAL, respectively, in the course of citral hydrogenation. In addition, in some cases, isolated C C group in citral may be hydrogenated to 3,7-dimethyl-2-octenal. Subsequently, geraniol/nerol isomers and CAL are consecutively hydrogenated to citronellol (COL). Finally, formed COL is further hydrogenated

to 3,7-dimethyloctanol. In our case, undesired acetalization byproducts cannot be found. Fig. 7 shows the change in the compositions of main products (geraniol/nerol, CAL, and COL) with reaction time over different supported catalysts in the course of citral hydrogenation. In the presence of pure carbon support, no citral conversion is observed, indicating that the support is inactive for the hydrogenation of citral. Obviously, CAL generated is major product under present reaction conditions, except for COL and allylic alcohols (geraniol and nerol). No 3,7-dimethyloctanol and 3,7-dimethyl-2-octenal are

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Fig. 6. Pd 3d XPS of different supported samples: Pd/C (a), PdNi-80/C (b), PdNi-67/C (c), and PdNi-55/C (d).

formed in all cases, demonstrating that only the C O and conjugated C C double bond can be hydrogenated over all catalysts. It implies that the adsorption of C C group on the catalyst surface should be more favorable in comparison with that of the carbonyl bond in citral, thereby resulting in easier activation of the C C bond. In addition, it is found that the amount of citral almost linearly drops at the initial period of reaction. Especially, over the PdNi-67/C, the concentration of citral declines dramatically and a complete conversion is achieved within 2 h. According to the calculated initial hydrogenation rates (Table 1), the catalytic activity of catalysts increases gradually in the following order: Ni/C < Pd/C < PdNi-55/C < PdNi-80/C < PdNi-67/C. In addition, the reaction was carried out conducted over the PdNi-67/C under the same reaction conditions and stopped after reaction for 30 min. After the catalyst was removed quickly, the reaction continued to proceed for another 2 h. However, no further citral conversion is found, reflecting the heterogeneity of the present reaction. Fig. 8 shows the comparison of conversions and CAL selectivities over different catalysts at a reaction time of 30 min. Clearly, a pronounced synergistic effect of between Pd and Ni atoms in catalysts is observed. Bimetallic PdNi catalysts are definitely more active than monometallic Pd/C and Ni/C catalysts. With the introduction of Ni into catalysts, the catalytic activity of bimetallic PdNi catalysts is improved gradually. Especially, the combination of Pd and Ni metals in the bimetallic PdNi-67/C catalyst leads to a highest CAL yield of 64% under the conditions used. Specially, the initial hydrogenation rate over the PdNi-67/C is almost 1.5 times as that over the Pd/C (Table 1). The further increase in the Ni content results in a decline in the activity in the case of PdNi-55/C. In contrast, Ni/C catalyst yields a lowest yield of CAL. It reflects that the appropriate amount of introduction of Ni into catalysts facilitates the formation of CAL. However, the evolution of catalytic activity within the series is not associated with the number of accessible catalytically active metallic Pd and/or Ni atoms. In the cases of the present Pd-containing catalysts, the concentration of CAL rapidly increase at the initial period of reaction and finally remains high over Pd/C, PdNi-80/C and PdNi-67/C ( > 75%) at high conversion levels. Although the C O group in citral can be hydrogenated to produce geraniol and nerol, the final selectivity to allylic alcohols is very low (< 10%). Meanwhile, CAL and allylic alcohols can be further converted to COL by hydrogenating the C O bond within the whole reaction time, in spite of relatively

243

small amount of COL. Here, owing to the presence of the electrostatic interaction between catalysts and reactants, the C C C O structure in citral may be delocalized as the C␦+ –C␦– –C␦+ –O␦– one. Correspondingly, the adsorption of reactants on the catalyst surface is weakened gradually in the following order: citral > CAL » allylic alcohols, thereby resulting in the easier hydrogenation of CAL than that of geraniol and nerol. Therefore, the hydrogenation of the conjugated C C group is always dominant in the course of reaction process, regardless of the difference in the activity of catalysts with the increasing Ni content. Correspondingly, the CAL selectivity is dramatically increased at the intital reaction period and changed a little at the late stage of the reaction, whereas the COL selectivity is slowly increased with the whole reaction process. The above results signify the preferential hydrogenation of the conjugated C C group in citral, because hydrogenating the C C bond is thermodynamically favorable over that of the C O bond by about 35 kJ mol−1 [47]. As for Ni/C catalyst, citral is selectively hydrogenated to CAL at the initial reaction period. With the reaction time, CAL produced is further hydrogenated to produce COL, and correspondingly, the concentration COL is gradually enhanced and reaches 48% at about 90% conversion. In addition, it was reported that surface basic sites may interact with the ␲* acceptor orbital of carbonyl group [48], probably favoring the adsorption of substrate molecules containing carbonyl groups. As shown in Table 1, the density of surface basic sites for reduced samples determined by CO2 -TPD slightly decreases with the increasing Ni content, due to the slight decrease in the amount of MgO phase in samples. In addition, surface acidity of all samples determined by NH3 -TPD is found to be weak (Table 1), and the difference in the density of surface acidic sites is not significant. It suggests that a small amount of Cl-containing species may be present over the support surface. Therefore, in our case, it is likely that surface acid-base property of catalysts is not the critical factor in the higher citral conversion over the PdNi-67/C sample than those over other samples, while the weak acid-base property of all samples can have a role in suppressing side reaction, namely acetalization. On the other side, as shown in Table 1, the initial rate of citral hydrogenation seems to increase gradually with the increasing dispersion degree of metal NPs. As for metal-catalyzed hydrogenation reactions, the metal dispersion usually can play an important role in governing the catalytic performance. As a result, the higher dispersion of metallic species, the more amount of dissociative H2 generated on catalytically active metallic Pd0 and Ni0 sites in the course of hydrogenation and thus the higher rate of citral hydrogenation [49], in spite of the difference in the capability of the dissociation of H2 to atomic H between metallic Pd0 and Ni0 species. According to the XPS and H2 -TPR results for reduced catalysts, metallic Pd0 and Ni0 species exist as major components on the surface of bimetallic or monometallic NPs. Noticeably, although the metal dispersion degree of PdNi-67/C is about 12.6% higher than that of Pd/C, the initial rate of citral hydrogenation is increased by about 46.0%. The above results indicate that the dispersion of metal NPs is not the only important factor in improving the catalytic performance of catalysts. In order to gain the intrinsic activity of metallic Pd and/or Ni sites on supported metal catalysts, the turnover frequency (TOF) was calculated according to the moles of converted citral per mole surface metallic Pd and Ni in the initial 15 min. Note that from Fig. 9 that the TOF values present a volcano-like shape with the Ni/(Pd + Ni) mass ratio, well consistent with the change in the initial hydrogenation reaction rate. That is, the TOF value increases gradually with the introduction of Ni and reaches the maximum in the case of PdNi-67/C (0.541 s−1 ), and then begins to drop as the Ni content further increases. Therefore, all Pd-containing catalysts exhibit higher activity than Ni/C catalyst, demonstrating that the

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Fig. 7. Catalytic performance in citral hydrogenation as a function of reaction time over different catalysts: Pd/C (A), PdNi-80/C (B), PdNi-67/C (C), PdNi-55/C (D) and Ni/C (E). Reaction conditions: H2 pressure, 1.0 MPa; 100 ◦ C.

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Fig. 10. H2 -TPD profiles of representative Pd/C (a), PdNi-67/C (b) and Ni/C samples. Fig. 8. Comparison of conversions and CAL selectivities over catalysts with different Ni/(Ni + Pd) mass ratios. Reaction conditions: H2 pressure, 1 MPa; 100 ◦ C; 30 min.

Fig. 9. Initial citral conversion rate (mol gPdNi −1 s−1 ) and TOF (s−1 ) value of citral converted catalyzed by supported catalysts with different Ni/(Ni + Pd) mass ratio. Reaction condition: H2 pressure, 1 MPa; 100 ◦ C.

introduction of Pd can improve the activity of bimetallic PdNi catalysts in comparison to Ni/C catalyst. In particular, the activities of PdNi-67/C and PdNi-80/C catalysts are even higher than that of monometallic Pd/C catalyst. The above result evidently confirms a synergistic effect between Pd and Ni on enhancing the citral conversion, and the importance of having an appropriate Pd:Ni mass ratio, especially 67:33 in catalysts, in order to achieve the desired catalytic activity in the selective hydrogenation of citral. We believe that the proper electronic and geometric structure of PdNi NPs in the PdNi-67/C catalyst may be more beneficial for the progress of the reaction. More importantly, despite different reaction parameters (i.e., hydrogen pressure, reaction temperature), the activity of the present PdNi-67/C catalyst is comparable to those of other some supported Ni- and Pd-based catalysts reported previously and even much better, such as Pd/C/TiO2 (TOF: 0.1 s−1 )[20], Ni-Sn/Al2 O3 [27], Ni/multi-walled carbon nanotubes [50], Ni/graphite (reaction rate: 0.187 mmol gNi −1 s−1 ) [51], Pd/(SiO2 /AlPO4 ) (reaction rate: 17.8 mmol gPd −1 s−1 )[52], Pd/Fe7 Co3 /carbon [53], Pd/polyketones (TOF: 0.343 s−1 )[54], Pd/C [55], and Pd/porous glass [56].

In order to illuminate the nature of the interactions between the catalyst surface and hydrogen species and between metals and the support, H2 -TPD experiments of representative supported samples were carried out. As shown in Fig. 10, Pd/C sample presents a strong hydrogen desorption in the temperature range of 70–350 ◦ C, with the desorption peak of about 196 ◦ C, which is due to the release of chemisorbed H2 on the surface of Pd atoms. Hydrogen desorption over the Ni/C sample lies in a broad temperature range of 250–700 ◦ C and can be fitted to two peaks centered at 435 and 540 ◦ C, respectively, which are associated with the desorption of H2 bound to the Ni0 atoms and hydrogen spillover on the support [57,58]. Interestingly, in the case of bimetallic PdNi-67/C sample, there are two fitted hydrogen desorption peaks centered at 167 ◦ C and 288 ◦ C, respectively. Compared with those for Pd/C and Ni/C samples, low-temperature desorption and high temperature desorption peaks of hydrogen obviously shift to lower temperatures, demonstrating more easier desorption of hydrogen from surface metal atoms due to the presence of the electronic interaction between Pd and Ni in the PdNi-67/C sample. Traditionally, compared with oxides supported Pd catalysts, supported Ni ones are less active for the hydrogenation of ␣,␤unsaturated aldehydes regardless of the selectivities to products. Previously, it was reported that the electronic modifications by a second metal upon alloying Pd could greatly affect the adsorption of alkenes in the hydrogenation reactions [59], and the geometrical (or ensemble) effect on bimetallic alloy surfaces could significantly determine the adsorption properties of carbon monoxide, oxygen and nitrogen [60]. Based on the positively shifted BE value as evidenced by the above XPS results of Pd 3d regions for the present bimetallic PdNi catalyst system, the strength of electronic interaction between Pd and Ni presents an increasing trend with the increase in the Ni content. In addition, it is generally accepted that an optimum strength of adsorption for reactants, neither too weak nor too strong, can enhance the catalytic activity. Correspondingly, an enhanced catalytic activity of PdNi-67/C catalyst probably suggests a more favorable adsorption of citral molecule on the surface of bimetallic PdNi catalysts. On the other side, it also was found that quasi-equilibrium for dissociative hydrogen adsorption, competitive adsorption between hydrogen and citral, and the addition of the first H atom could be considered as the rate determining steps for the selective hydrogenation of citral over supported Pt-based catalysts [49]. As a result, the enhanced ability for the dissociation of H2 and appropriate adsorption strength for reactants stemming from the electrostatic and ensemble effects in present bimetallic

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efficient and stable supported bimetallic PdNi nanocatalyst system offers new and promising potential for the hydrogenation of ␣,␤unsaturated aldehydes in terms of sustainable chemistry. Acknowledgement This study was funded through National Natural Science Foundation of China and Fundamental Research Funds for the Central Universities (buctrc201528). References

Fig. 11. The reusability of PdNi-67/C catalyst in the hydrogenation of citral. Reaction conditions: H2 pressure, 1 MPa; 100 ◦ C; 1 h.

PdNi-67/C catalyst may account for its enhanced hydrogenation activity to a large extent. As for supported metal catalysts, the stability of catalysts in the heterogeneous reaction processes is of vital importance for their potential applications. Therefore, the reusability of the PdNi-67/C catalyst was further studied. After each catalytic reaction, the used catalyst was washed with ethanol and deionized water for several times, and then dried at 70 ◦ C for 12 h. As shown in Fig. 11, both the high conversion and the high CAL selectivity still can be achieved over the PdNi-67/C catalyst after six consecutive cycles, indicative of no significant decrease in the catalytic performance. Elemental analysis by ICP-AES indicates that the amount of Pd leaching in the reactant mixture is negligible. The above results demonstrate the excellent stability of the PdNi-67/C catalyst, due to strong interactions between metal and support stemming from the perfectible hybrid nanostructure of LDH-C precursor, thus preventing the leaching, aggregation and growth of PdNi NPs during the reaction. In general, the present LDH-C precursor route for the synthesis of supported bimetallic PdNi nanocatalysts is an economical and effective strategy for finely adjusting the compositions of bimetallic catalysts and thus reasonably constructing favorable synergistic effects between Pd and Ni atoms to facilitate the adsorption of reactants. Moreover, in view of catalytic applications, the carbon matrix is well-suited to stabilize metal NPs through SMSI, and simultaneously high surface area of catalysts guarantees the accessibility of catalytically active sites of metal NPs. 4. Conclusion In summary, we reported new carbon-supported bimetallic PdNi nanocatalysts via a carbonization-in situ self reduction route based on LDH-C composite precursors and their synergistically acting catalyst system in liquid-phase hydrogenation of citral. A series of characterizations revealed that the introduction of Ni into catalysts effectively modifies the structures of PdNi NPs, thus greatly improving the adsorption of C C bond in citral and the dissociation of H2 in the hydrogenation. The bimetallic PdNi catalyst with the Pd/Ni mass ratio of 67:33 showed much higher catalytic activity than supported monometallic Pd and Ni catalysts. The excellent catalytic efficiency of bimetallic PdNi catalyst was ascribed to a synergistic effect between Pd and Ni atoms in bimetallic NPs. Moreover, the carbon support ideally combined high stability and good access of reactants to active sites on PdNi NPs. Such flexible, high-

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