Supported Nickel–Cobalt Bimetallic Catalysts Derived from Layered

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Supported Nickel−Cobalt Bimetallic Catalysts Derived from Layered Double Hydroxide Precursors for Selective Hydrogenation of Pyrolysis Gasoline Jianfeng Xiang, Xin Wen, and Fazhi Zhang* State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China ABSTRACT: Generation of bimetallic sites on supported catalysts has been proved to be capable of enhancing catalytic performance when compared to those of the corresponding monometallic ones. Herein we report the preparation of novel Al2O3-supported NiCo bimetallic catalysts (NiCo/Al2O3) derived from Ni2+Co2+Al3+-containing layered double hydroxides (NiCoAl-LDHs) precursors, which were in situ grown on the surface and in the pore canals of microspherical γ-Al2O3, and their catalytic performance for selective hydrogenation of pyrolysis gasoline. Bimetallic NiCo/Al2O3 derived from LDHs precursor exhibited the better catalytic performance than the sample prepared by impregnation. The appropriate amounts of substitution of nickel for cobalt improved the catalytic activity and stability dramatically, indicating the synergistic effects of nickel and cobalt in terms of the hydrogenation reactivity. The enhanced reducibility and smaller particles for NiCo/Al2O3 samples derived from LDHs precursor were proposed to suppress the coke formation during the hydrogenation reaction, leading to improve catalytic activity and stability.

1. INTRODUCTION Pyrolysis gasoline (PyGas) is a valuable byproduct during ethylene and propylene production from the high-temperature naphtha pyrolysis. It has high potential for use both as a gasoline blending stock due to its generally high octane value and a feedstock for extraction of aromatics owing to its large amount of aromatic hydrocarbons. There are typically up to 15 wt % of gum-forming agents, mostly styrene and diolens, existing in PyGas. Thus, catalytic hydrogenation of PyGas is in great demand for stabilization and optimal utilization of PyGas.1−4 Alumina-supported nickel4−6 or palladium1−4,7,8 catalysts are frequently used for PyGas hydrogenation at moderate pressure and temperature conditions. Much effort has been devoted to enhancing the performance of Ni or Pd catalysts for this process, such as adopting novel catalyst support with unique pore structure,4,9 e.g., the monolithic macropores and the hierarchical structure; introducing a new presulfidation process;6 controlling the egg-shell distribution of the catalytically active component;1,2,10−12 and combining of the target active component with other metal elements.13−15 Among these strategies, the generation of bimetallic sites on supported catalysts has been proved to be capable of enhancing catalytic performance when compared to those of the corresponding monometallic ones. The bimetallic catalyst material can combine the main advantages of the finite size effect and the alloying effect, showing excellent physical and chemical properties.16−19 Especially, the catalytic properties of the bimetallic catalysts could be easily tuned by adjusting the components composition and the atomic ordering.20−23 Herein we report the preparation of novel Al2O3-supported NiCo bimetallic catalysts (NiCo/Al2O3) derived from Ni2+Co2+Al3+containing layered double hydroxides (NiCoAl-LDHs) precursors, which were in situ grown on the surface and in the pore canals of microspherical γ-Al2O3 spheres. © 2014 American Chemical Society

LDHs are a family of anionic clays which can be expressed by the general formula [M2+1−xM3+x(OH)2]x+[An−]x/n·yH2O, where M2+ and M3+ are divalent and trivalent metal cations, respectively. The value of x is equal to the molar ratio of M3+/ (M2+ + M3+), which usually is between 0.20 and 0.33, and inorganic or organic anion An− is located in the hydrated interlayer galleries with charge n.24−28 LDHs containing three or even more cations in the layers can also be prepared. As a result of their high chemical versatility associated with a tunable layer composition and anionic exchange capacity, LDHs represent an interesting opportunity for developing new catalysts, catalyst precursors, and catalyst supports with tailored structure and properties.24,25 In recent works our group has prepared Al2O3-supported monometallic Ni catalysts (Ni/ Al2O3) derived from NiAl-LDHs precursor.12,29,30 Al2O3 sphere was used not only as the catalyst support but also the sole source of Al3+ for in situ growth of NiAl-LDHs precursor. The resulting Ni/Al2O3 catalysts have delivered superior catalytic performance for hydrodechlorination of chlorobenzene,29 CO2 methanation,30 and selective hydrogenation of PyGas,12 in comparison to the Ni/Al2O3 catalyst sample prepared by conventional impregnation of Al2O3 with an aqueous solution of Ni2+ ions. The objective of this work is to extend this research to the bimetallic NiCo/Al2O3 catalysts derived from NiCoAl-LDHs precursors. The investigation on the synergy between Ni and Co in bimetallic catalysts has attracted considerable attention recently because the NiCo bimetallic system could improve catalytic activity and anticoking property very significantly.31−34 We prepare a series of NiCo/Al2O3 bimetallic catalysts with modulating Ni:Co mass ratio through Received: Revised: Accepted: Published: 15600

July 8, 2014 September 13, 2014 September 16, 2014 September 16, 2014 dx.doi.org/10.1021/ie502721p | Ind. Eng. Chem. Res. 2014, 53, 15600−15610

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as LP-3Ni1Co/Al2O3, LP-2Ni1Co/Al2O3, and LP-1Ni1Co/ Al2O3 according to the different Ni:Co mass ratios. Other experiments were undertaken in which the total Ni and Co loadings of 6 and 8 wt % were employed with the Ni:Co mass ratio of 2:1. For comparison, NiCo/Al2O3 catalyst was obtained by a conventional wet impregnation method with total Ni and Co loading of 12 wt % and the Ni:Co mass ratio of 2:1. The sample was prepared by co-impregnating 5 g of γAl2O3 in Ni(NO3)2·6H2O and Co(NO3)2·6H2O solutions at room temperature for 24 h and then dried at 70 °C for 12 h. Then sample was calcined in air at 450 °C for 4 h with a ramping rate of 5 °C min−1 for obtaining the NiCo mixed oxide sample (denoted IM-2Ni1CoO/Al2O3; IM is expressed as impregnation method). Reduction was carried out in the microflow reactor, and the resulting sample is denoted IM2Ni1Co/Al2O3. Moreover, LP-Ni/Al2O3 and LP-Co/Al2O3 catalysts derived from NiAl-LDHs and CoAl-LDHs precursors, with metal Ni or Co loadings of 8 and 12 wt %, were obtained under the identical preparation and treatment conditions of calcination and reduction pretreatments for the LP-NiCo/ Al2O3 sample. For the sake of convenience, LP catalysts without further specification were referred to the samples synthesized with the total Ni and Co metal loading of 12 wt %. 2.3. Catalyst Characterization. XRD patterns were recorded on a Shimadzu XRD-6000 diffractometer using Cu Kα radiation (λ = 0.15406 nm) operated at 40 kV and 30 mA, with a scan step of 2θ 0.02° and scan speed of 10° min−1.The elemental analysis for carbon was carried out using a Vario EL cube elemental analyzer (ElementarAnalysensysteme GmbH). The morphology and structure of the samples were investigated by using a Zeiss Supra 55 scanning electron microscope with the surface of the sample coated with a thin platinum layer to avoid a charging effect. HRTEM was carried out on a JEOL JEM-2100 instrument operated at an accelerating voltage of 200 kV. The sample was ultrasonically dispersed in ethanol, and then a small amount of this solution was dropped onto a carbon-coated copper grid. STEM-EDX was performed by Tecnai F20 TEM. The interested particle was detected by the focused beam in STEM mode with drift corrections for attaining sufficient counts. The low-temperature N2 adsorption−desorption experiments were carried out using a Quantachrome Autosorb-1 system. The Barrett−Joyner− Halenda (BJH) method was used to calculate pore volume and the pore size distribution. XPS was recorded using an ESCALAB250 X-ray photoelectron spectrometer equipped with monochromatized Mg Kα X-ray radiation (1253.6 eV photons). The depth profiling experiment was conducted with a high-energy Ar+ ions beam. Binding energies were calibrated based on the graphite C 1s peak at 285.0 eV. H2-TPR and H2-TPD were performed on a Micromeritics ChemiSorb 2720. About 100 mg of sample was loaded in a quartz reactor. The procedure was as follows: First, the sample was cleaned at 200 °C for 1 h in argon with a flow rate of 40 mL min−1 in order to remove any physisorbed molecules. Then, after cooling to room temperature, the sample was heated with a heating rate of 10 °C min−1 up to 1000 °C in the stream of 10% H2 in Ar with a total flow rate of 40 mL min−1. The outlet gas was passed through a cold trap to remove the moisture produced during reduction. And the adsorption of the reduced samples was finished by 10% H2 in Ar. Chemisorbed hydrogen was desorbed by programmed heating at a rate of 10 °C min−1 in the stream of Ar with the flow rate of 40 mL min−1. The amount of hydrogen consumed was recorded using

progressively blending the second metal Co with the initial Ni in the NiCoAl-LDHs precursor for selective hydrogenation of PyGas. For comparison, a NiCo/Al2O3 bimetallic catalyst sample was prepared by conventional co-impregnation method under identical treatment conditions of calcination and reduction pretreatment. For evaluating the catalytic performance of NiCo/Al2O3 bimetallic catalyst samples for selective hydrogenation of PyGas, styrene was chosen as the model reactant to be hydrogenated with an excess of toluene and nheptane for it is the main component in PyGas (5−10 wt %) and can form gum which should be removed. The physical− chemical properties of the two kinds of supported NiCo bimetallic catalysts, along with the monometallic Ni and Co samples, were investigated by several characterization techniques including powder X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), scanning transmission electron microscopy (STEM), low-temperature N2 adsorption− desorption, X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction of hydrogen (H2-TPR), and temperature-programmed desorption of hydrogen (H2-TPD). Our approach may hold promise for the design and fabrication of supported bimetallic catalyst for selectivity hydrogenation reaction with enhanced catalytic performance.

2. EXPERIMENTAL SECTION 2.1. Materials. Ni(NO3)2·6H2O, Co(NO3)2·6H2O, urea (CO(NH2)2), styrene, n-heptane, and toluene were all of A.R. grade and used without further purification. The porous γAl2O3 spheres with a purity of 99.5%, prepared through a reduction−magnetic separation process in our laboratory,35 were crushed into particles with an average particle size of 20− 40 mesh and calcined at 600 °C for 4 h before being used as support. The deionized water with a conductance below 10−6 S cm−1 is used in all synthesis and washing processes. 2.2. Preparation of Catalyst Samples. A series of NiCo/ Al 2O 3 bimetallic catalysts derived from NiCoAl-LDHs precursors with a total Ni and Co loading at 12 wt % and modulated Ni:Co mass ratio were fabricated as follows. First, Ni(NO3)2·6H2O, Co(NO3)2·6H2O, and urea with molar ratio of urea:(Ni2+ + Co2+) = 2:1 were dissolved in 5 mL of deionized water to obtain about 10 mL of mixed solution. The resulting solution was then added to a four-necked flask with 5 g of spherical γ-Al2O3 particles. After being impregnated for 1 h, the particles together with the residual solution were transferred to an autoclave and aged at 130 °C for 24 h and were then thoroughly washed with deionized water until the pH value of the washings reached 7. The resulting NiCoAl-LDHs/ Al2O3 precursor with different Ni:Co mass ratios of 3:1, 2:1, and 1:1 (denoted as 3Ni1CoAl-LDHs/Al2O3, 2Ni1CoAlLDHs/Al2O3, and 1Ni1CoAl-LDHs/Al2O3, respectively) was obtained after drying at 70 °C for 12 h. Subsequently, the LDHs/Al2O3 precursor samples were calcined in air at 450 °C for 4 h with a ramping rate of 5 °C min−1 for obtaining the NiCo mixed oxide sample (denoted as LP-3Ni1CoO/Al2O3, LP-2Ni1CoO/Al2O3, and LP-1Ni1CoO/Al2O3 according to the different Ni:Co mass ratios; LP is expressed as a layered precursor). Finally, the NiCo mixed oxide samples were reduced at 500 °C and 0.5 MPa for 2 h in flowing H2 (50 mL min−1) and N2 (30 mL min−1) mixture before lowering to the setting temperature for hydrogenation reaction in the microflow reactor. The obtained catalyst samples are denoted 15601

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a thermal conductivity detector. The recorded data were quantified using a calibration sample of Ag2O as a standard. 2.4. Catalytic Testing. Styrene was chosen as the model reactant to be hydrogenated with an excess of toluene and nheptane (10.0 wt % styrene and 35.0 wt % toluene in nheptane) for evaluating the catalytic performance of samples for selective hydrogenation of PyGas. All of the catalytic reactions were carried out in a microflow reactor for selective hydrogenation of styrene. The catalyst bed in a constanttemperature zone consisted of 1.1 g of catalyst sample diluted with 3.3 g of SiC (60−80 mesh) to prevent maldistribution of heat and mass in the reactor. The other part of the trickle-bed reactor was filled with quartz sand. Activated alumina (5 g) was placed on top of the catalyst bed for the adsorption of 4-tertbutylcatechol, usually used as an inhibitor to prevent gumming during storage in styrene. The hydrogenation reactions were performed at 60 °C with weight hourly space velocity (WHSV) of 30 h−1, H2 pressure of 2.0 MPa, total pressure of 3.0 MPa, and hydrogen to oil volume ratio of 80. Reactants and products were analyzed using a Shimadzu GC-2014C chromatographic instrument by means of a flame ionization detector and a 9006PONA capillary column. Catalytic activity and selectivity were calculated using the following simplified equation, based on chromatographic data by area normalization method, and nheptane was used as a standard sample to analyze other data:

Figure 1. XRD patterns of γ-Al2O3 support (a), NiAl-LDHs/Al2O3 precursor (b), a series of NiCoAl-LDHs/Al2O3 precursor samples with different Ni/Co mass ratios 3:1 (c), 2:1 (d), and 1:1 (e), and CoAlLDHs/Al2O3 precursor (f).

⎛ ⎞ A1F1 conversion/% = ⎜1 − ⎟ × 100 A1F1 + A 2 F2 + A3F3 ⎠ ⎝

selectivity/% =

A 2 F2 × 100 A 2 F2 + A3F3

where A1, A2, and A3 denote area percentages as determined by GC-2014C of styrene, ethylbenzene, and ethylcyclohexane, respectively. Likewise, F1, F2, and F3 denote the relative area rectifying factors of them.

3. RESULTS AND DISCUSSION 3.1. Characterization of NiCoO/Al2O3 Catalyst Samples and the Corresponding NiCoAl-LDHs/Al2O3 Precursors. A series of NiCoAl-LDHs/Al2O3 precursor samples with different Ni/Co mass ratio, along with NiAl-LDHs/Al2O3 and CoAl-LDHs/Al2O3 samples, were fabricated by in situ growth using microspherical γ-Al2O3 as not only the catalyst support but also the sole source of Al3+.12,29,30 Figure 1a shows the characteristic (311), (400), and (440) diffraction peaks relating to γ-Al2O3 (JCPDS No. 29-1486). The marked (00l) diffraction lines at low 2θ value and (110)/(113) reflection line at high 2θ value in Figure 1b−f, attributed to the typical layered structure, are similar to those reported in the literature for LDHs phase.24,25 Moreover, the characteristic peaks of five LDHs/Al2O3 precursor samples are a superposition of typical reflections of the γ-Al2O3 and LDHs phases, implying the growth of LDHs on the γ-Al2O3 support. Typical SEM images of surface and a cross-section of a representative precursor sample 2Ni1CoAl-LDHs/Al2O3 are shown in Figure 2a,b. The characteristic hexagonal plate-like morphology of LDHs crystallite is clearly observed, directly giving us the evidence for the formation of LDH microcrystallites by in situ growth. A well-developed two-dimensional network was grown homogeneously on the surface and in the pore canals of the Al2O3 support. The crystallite size of the ab-faces of LDHs platelets on the support surface is about 0.5−1.0 μm.

Figure 2. Typical SEM images of surface (a, c) and cross-section (b, d) of 2Ni1CoAl-LDHs/Al2O3 precursor (a, b) and the LP-2Ni1CoO/ Al2O3 sample (c, d).

Calcination of LDHs is known to be an effective method for the fabrication of a wide variety of mixed metal oxide (MMO) composite materials composed of a metal oxide phase and a spinel-like phase.24−26 For instance, Ni-based MMO materials derived from LDHs have exhibited excellent catalytic performance in many reactions such as the steam re-forming of methanol, the partial oxidation of methane to synthesis gas, the growth of carbon nanomaterials, and the liquid-phase oxidation of benzyl alcohol to benzaldehyde. The resulting LDHs/Al2O3 precursor samples were calcined in air at 450 °C for 4 h for obtaining the MMO samples. Figure 3 shows XRD patterns of bimetallic NiCoO/Al2O3 samples with different Ni/Co mass ratios, along with monometallic LP-NiO/Al2O3 and LP-CoO/ Al2O3 samples. The major diffraction peaks of the XRD profiles of the catalysts can be attributed to the formation of Co3O4 (JCPDS No. 43-1003) and/or CoAl2O4 (JCPDS No. 38-0814) and/or NiAl2O4 (JCPDS No. 10−0339) which yield similar overlapping reflections at 2θ 19.0°, 31.3°, 36.8°, 44.8°, 55.7°, 59.4°, and 65.2°.34 Besides three characteristic (311), (400), and (440) diffraction peaks relating to γ-Al2O3, the LP-NiO/ Al2O3 sample after calcination of the corresponding NiAlLDHs/Al2O3 precursor at 450 °C shows two obvious peaks at 15602

dx.doi.org/10.1021/ie502721p | Ind. Eng. Chem. Res. 2014, 53, 15600−15610

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composition of the bimetallic CoNi/Al2O3 catalysts was illustrated by XPS and TPR analysis. Parts c and d of Figure 2 show typical SEM images of the surface and cross-section of a representative 2Ni1CoO/Al2O3 sample, respectively. After subsequent calcination of 2Ni1CoAlLDHs/Al2O3 precursor at 450 °C, the sheet-like shape of LDHs crystallites and the network architecture remains unchanged, suggesting a strong interaction between the LDHs crystallites and Al2O3 support. Previous investigation of27Al magic-angle spinning nuclear magnetic resonance analysis by Feng et al. indicated that rather than being simply supported on the spherical Al2O3 support, the in situ grown NiAl-LDHs crystallites were chemically grafted via bonding of (Nicoordinated) O atoms with AlO4 tetrahedra, resulting in the formation of additional AlO6 octahedra via Al−O−Ni linkages.29 Recently, our group reported the fabrication of MMO films containing Ni2+ and Al3+ on a porous anodic alumina/ aluminum substrate by a simple process involving calcination of in situ grown LDHs film precursors at 300−600 °C for 4 h in air.36 SEM revealed that the resulting MMO films maintain the original nest-like morphology of the LDHs precursor films without any deformation of the microstructure during the thermal decomposition process. N2 adsorption−desorption isotherms and pore size distribution of the γ-Al2O3 support, the 2Ni1CoAl-LDHs/Al2O3 precursor, and the LP-2Ni1CoO/Al2O3 sample are shown in Figure 4. The corresponding textural properties of the samples

Figure 3. XRD patterns of LP-NiO/Al2O3 (a), a series of LP-NiCoO/ Al2O3 samples with different Ni/Co mass ratios of 3:1 (b), 2:1 (c), and 1:1 (d), and LP-CoO/Al2O3 (e). IM-2Ni1CoO/Al2O3 prepared by wet impregnation method with Ni:Co mass ratio of 2:1 is also included (f). Peaks marked with ● are characteristic of Al2O3, peaks marked with ▼ are characteristic of NiO, and those with ∇ are characteristic of Co3O4, CoAl2O4, and NiAl2O4.

2θ 43.3° and 62.9° which are attributed to the characteristic (200) and (220) reflection of cubic NiO crystallites (JCPDS No. 47-1049), respectively (Figure 3a). NiO particles can be formed on the support surface after calcination of NiAl-LDHs/ Al2O3 precursor. While, for the LP-CoO/Al2O3 sample derived from the CoAl-LDHs/Al2O3 precursor, the peaks at 2θ 19.0°, 31.3°, and 59.4° correspond to the characteristic (111), (220), and (511) reflections of cubic Co3O4 and/or CoAl2O4, respectively (Figure 3e). We can also find a relatively stronger reflection at 2θ 36.8°, which may be a superimposed diffraction peak from Co3O4 (311) and/or CoAl2O4 (311) and a γ-Al2O3 (311) reflection. In addition, the coexistence of characteristic diffraction peaks of NiO and/or NiAl2O4 and Co3O4 and/or CoAl2O4 phase can be seen for the three LP-NiCoO/Al2O3 samples, demonstrating that NiCoAl-LDHs/Al2O3 was transformed into NiO and/or NiAl2O4 and Co3O4 and/or CoAl2O4 crystallites after calcination at 450 °C (Figure 3b−d). It is obvious that the intensity of reflection peaks of Co3O4 and/or CoAl2O4 improves with increasing the Co content in the LPNiCoO/Al2O3 sample. While the intensity of reflection peaks of NiO shows an opposite trend. For comparison, XRD of the bimetallic NiCoO/Al2O3 sample prepared by wet impregnation method with Ni:Co mass ratio of 2:1, denoted as IM2Ni1CoO/Al2O3, is included as Figure 3f. New diffraction peaks at 2θ 44.8°, 55.7°, and 65.2°, corresponding to the characteristic, (400), (422), and (440) reflection of the cubic Co3O4 and/or CoAl2O4 and/or NiAl2O4, respectively, can be found. It is noted that the peak intensity of the Co3O4 and/or CoAl2O4 and/or NiAl2O4 characteristic reflections for the IM sample is relatively higher than that of the LP samples with the same total Ni and Co loading of 12 wt % and the Ni:Co mass ratio of 2:1 (Figure 3c), indicating a higher dispersion of Co3O4 and/or CoAl2O4 and/or NiAl2O4 on the support for the latter. However, although it is difficult to distinguish clearly the structural changes with the addition of Co content by means of XRD, but in terms of the assignments of the NiO/NiAl2O4 and Co3O4/CoAl2O4 phases, it should be mentioned that further insight regarding the influence of the Co on the structural

Figure 4. N2 adsorption−desorption isotherms of γ-Al2O3 support, 2Ni1CoAl-LDHs/Al2O3 precursor, and LP-2Ni1CoO/Al2O3 samples. Inset shows the pore size distribution curves of the samples.

are listed in Table 1. All of the isotherms of the samples are type IV with an obvious hysteresis loop. The shape of the hysteresis loop is a superposition of types H1 and H3. This is generally taken to indicate that samples have both tubular and parallel slit-shaped capillary pores which are caused by the gas escaping during calcination and the stacking of alumina and Table 1. Textural Properties of γ-Al2O3, 2Ni1CoAl-LDHs/ Al2O3 Precursor, and the LP-2Ni1CoO/Al2O3 Sample

15603

sample

specific surface area (m2/gcat)

total pore volume (cm3/g)

most probable pore size (nm)

γ-Al2O3 2Ni1CoAl-LDHs/Al2O3 LP-2Ni1CoO/Al2O3

185.5 221.9 208.6

0.85 0.66 0.56

21.19, 86.57 18.87, 48.39 18.50, 48.37

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CoO and the second one at 685 °C to the reduction of CoO to Co0, demonstrating a two-step reduction process of Co3O4 to metallic Co0 passing through the formation of CoO as an intermediate.40 In addition, the feature at about 923 °C may be attributed to the high-temperature reduction of CoAl2O4.38 After addition of Co in the NiO/Al2O3 sample, the main peak for the reduction of smaller NiO crystallites shifts toward a higher reduction temperature for the LP-3Ni1CoO/Al2O3 sample, indicating a strong interaction between Ni and Co (Figure 5b). The interaction between Ni and Co is gradually improved by increasing the Co content in the sample, indicated by a higher main peak for the reduction of smaller NiO crystallites. In addition, parts b−d of Figure 5 exhibit a weakly expressed shoulder at a relatively low reduction region that may be attributed to the formation of bulk-like species of NiO and Co3O4.32,38,41 Figure 5f shows a relatively stronger intensity of the single peak at 287 °C and a relatively weaker main peak at 516 °C for the IM-2Ni1CoO/Al2O3 bimetallic catalyst prepared by impregnation method, demonstrating a weaker interaction between the bulk-like species of NiO and Co3O4 and Al2O3 support compared with the LP bimetallic catalyst derived from LDHs precursor with the same Ni:Co ratio. The relatively stronger interaction between Ni (or Co) species and Al2O3 support for LP samples may be caused by the confinement effect of the “Ni(or Co)−O−Al” structure in the LDHs precursor.24−26 Recent works in our group about the thermal decomposition of ZnAl-LDHs for the synthesis of ZnO-based MMO nanocomposites revealed a stronger coupling interaction existed at the interface in the network of ZnO with homogeneously dispersed ZnAl2O4 nanoparticles, compared with the similar ZnO/ZnAl2O4 material fabricated by chemical co-precipitation or physical mixing method.42,43 Based on the preceding results, obviously, the addition of a small amount of Co in the Ni-based catalysts brings a distinct change for the main peak. In the meantime, the character of a single but broad main reduction peak of the catalyst LP-NiCo/ Al2O3 indicates the coexistence of Ni and Co in the same or similar homogeneous oxide structure. The simultaneous reduction of Ni and Co oxides would benefit the formation of a Ni/Co alloy phase. And the result of the formation of the NiCo alloy phase can facilitate the reduction process to occur.38 3.2. Characterization of Reduced NiCo/Al2O3 Catalyst Samples. The oxide-based mono- and bimetallic catalyst samples were reduced at 500 °C for 2 h in flowing H2 and N2 mixture before hydrogenation reaction in the microflow reactor. XPS technique was employed for analysis of the changes of the surface properties, including electrons and chemical composition, of the catalyst samples before and after H2 reduction. The binding energies (BE) obtained in the XPS analyses were corrected for specimen charging by referencing the C 1s peak to 285.0 eV. The Ni 2p XPS (Figure 6) are characterized by the main Ni 2p3/2 peak around 855.1−856.9 eV, characteristic of Ni2+ ions, along with the typical satellite peak close to 862.0 eV. In detail, the peaks located around 855.0 and 856.0 eV can be attributed to the presence of surface NiO and NiO particles interacting more strongly with the alumina support, i.e., Ni2+low and Ni2+-high, respectively.12,29,44 The quantified percentages of the different Ni species, identified by decomposition of Ni 2p3/2 XPS emission lines, are shown in Table 2. Compared with the monometallic NiO/Al2O3 sample, LP-2Ni1CoO/ Al2O3 exhibits a higher content of Ni2+-high species than that of Ni2+-low ones, indicating a stronger bonding between Ni and Al2O3 support for the sample with the addition of Co.

LDHs crystallites.12,29,37 As can be seen from Figure 4, the volume values of 2Ni1CoAl-LDHs/Al2O3 precursor and the LP-2Ni1CoO/Al2O3 sample move slightly lower at the same P/ P0, compared to those of the porous Al2O3 support, which may result from the deposition of LDHs crystallites, leading to the aggregate block for some pores in Al2O3. Table 1 shows that 2Ni1CoAl-LDHs/Al2O3 and LP-2Ni1CoO/Al2O3 samples have a lower total pore volume and a narrower pore size distribution compared with the γ-Al2O3 support. In addition, the increase of the specific surface area for the 2Ni1CoAl-LDHs/Al 2O3 precursor can be indicated in Table 1, owing to the deposition of LDHs crystallites on the surface and in the pore canals of microspherical γ-Al2O3, leading to formation of a uniform and microcrystalline developed two-dimensional pore structure. The specific surface area of LP-2Ni1CoO/Al2O3, as well as the values of the total pore volume and the most probable pore size, is further reduced in comparison to those of the 2Ni1CoAl-LDHs/Al2O3 precursor, which may be caused by the decrease in the size of the particles in the pore canals of microspherical Al2O3 after calcination. Thus, the resulting NiCoO/Al2O3 takes up less of the pore volume of Al2O3 than the NiCoAl-LDHs/Al2O3 precursor.29 The reduction performance of the oxide-based samples was characterized by TPR technique, and the results are illustrated in Figure 5. The LP-NiO/Al2O3 sample shows one broad peak

Figure 5. TPR profiles of LP-NiO/Al2O3 (a), LP-3Ni1CoO/Al2O3 (b), LP-2Ni1CoO/Al2O3 (c), LP-1Ni1CoO/Al2O3 (d), LP-CoO/ Al2O3 (e), and IM-2Ni1CoO/Al2O3 (f).

around 350−750 °C with a maximum at 616 °C and a shoulder at 421 °C (Figure 5a). According to the literature, the former can be attributed to the reduction of smaller NiO crystallites (the amorphous NiO or crystal NiO species) in a strong interaction with the exposed support surface and the latter to the reduction of bulk NiO clusters weakly bound to the alumina surface.38,39 Also, we can see a high-temperature feature at 805 °C which could be attributed to the reduction of NiAl2O4 spinel.38 It should be noted that the NiAl2O4 and bulk NiO reduction signals have a relatively weak intensity compared to the reduction feature at 616 °C, suggesting that the Ni oxide species exist mostly in the form of small NiO particles strongly interacting with the Al2O3 support. The TPR profile of the LP-CoO/Al2O3 sample displays two main reduction peaks at 395 and 685 °C (Figure 5e). The first peak at 395 °C can be ascribed to the reduction of Co3O4 to 15604

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Figure 6. XPS spectra of Ni 2p3/2 for mono- and bimetallic Ni-based catalyst samples before (left) and after (right) H2 reduction.

However, a further increment of the Co content results in a decrease of Ni2+-high species. In addition, the IM-2Ni1CoO/ Al2O3 sample shows a low Ni2+-high species compared to the LP-2Ni1CoO/Al2O3 sample derived from LDHs precursor, implying a weak interaction between the NiO and Co3O4 species and Al2O3 support revealed by the TPR results (Figure 5). After a H2 reduction treatment process at 500 °C, a new peak at low BE of 525.5 eV, which is attributed to Ni0 species,12,45 can be found for the resulting reduced Ni-based catalyst samples. Table 2 shows the monometallic LP-Ni/Al2O3 sample has a similar ratio of Ni2+-high/Ni2+-low to the corresponding oxide-based LP-NiO/Al2O3 sample. However, after addition of Co, we can see a decline of the Ni2+-low species for the two reduced LP-NiCo samples, while Ni2+-high species are hardly reduced with the employed experimental conditions. This indicates the prompted reduction of the Ni species, including surface NiO and NiO particles interacting more strongly with the Al2O3 support, by Co species during the reduction process. The Co 2p XPS (Figure 7) shows the characteristic sharp Co 2p3/2 peak of the Co3O4 spinel at 779.8−780.0 eV and a broad

Figure 7. XPS spectra of Co 2p3/2 for mono- and bimetallic Ni-based catalyst samples before (left) and after (right) H2 reduction.

satellite signal at 785.0−791.0 eV. The asymmetric shape of the dominant Co 2p3/2 signal suggests the presence of Co3O4 exhibiting two oxidation states, Co3+ ions in octahedral positions and Co2+ ions in tetrahedral positions of the facecentered cubic oxygen lattice.38,46 Compared with the Co monometallic sample, the LP-1Ni1CoO/Al2O3 sample has more Co3+ species after addition of Ni, which is similar to the IM NiCo sample (Table 2). It is noted that part of the Co species in LP-2Ni1CoO/Al2O3 with a large amount of Ni species shifted from a higher to a lower oxidation state, implying the protection of metal from oxidation during the calcination. After a H2 reduction treatment process at 500 °C, the main Co 2p3/2 peak that occurred at a BE of 779.8−780.0 eV in the oxide-based samples shifted to 780.7−781.9 eV in the reduced samples (Figure 7), which is more closely related to that observed for reference CoAl2O447 with BE at about 782.0 eV. The significant increase in BE (0.7−2.1 eV) indicates the

Table 2. XPS Analyses of Different Mono-Nickel (or Cobalt) and Bimetallic NiCo Catalyst Samples before and after H2 Reductiona Ni species (at. %)

Co species (at. %)

sample

Ni2+-low

Ni2+-high

LP-NiO/Al2O3 LP-CoO/Al2O3 LP-2Ni1CoO/Al2O3 LP-1Ni1CoO/Al2O3 IM-2Ni1CoO/Al2O3 LP-Ni/Al2O3 LP-Co/Al2O3 LP-2Ni1Co/Al2O3 LP-1Ni1Co/Al2O3 IM-2Ni1Co/Al2O3

49.0 (852.7)

51.0 (853.6)

1.04

31.5 54.4 50.0 37.8

68.5 45.6 50.0 40.6

(855.1) (855.3) (855.4) (854.9)

23.0 (855.1) 28.7 (855.2) 47.5 (856.2)

Ni0

Ni2+-high/Ni2+-low ratio

(855.9) (855.9) (856.1) (856.0)

21.6 (852.4)

2.18 0.84 1 1.07

67.9 (856.2) 57.4 (855.9) 42.0 (856.9)

9.1 (852.5) 13.9 (852.5) 10.5 (852.5)

2.95 2 0.88

Co3+

Co2+

32.7 28.2 59.2 61.2

(780.4) (780.0) (779.7) (779.6)

67.3 71.8 40.8 38.8

(781.5) (781.0) (781.2) (780.7)

29.2 19.0 12.9 30.4

(781.9) (781.0) (780.5) (781.2)

58.5 61.2 50.2 39.5

(782.2) (782.0) (781.5) (782.0)

Co0

Co3+/Co2+ ratio 0.49 0.39 1.45 1.58

12.3 19.8 36.9 30.1

(779.0) (778.4) (778.5) (778.6)

0.5 0.31 0.26 0.77

a

Binding energies obtained from curve-fitted values of experimental spectra (shown in the parentheses) and the percentage of peak intensities (shown outside the parentheses). 15605

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Figure 8. Representative HRTEM images of catalyst samples after reduction by H2: LP-Ni/Al2O3 (a, b), LP-2Ni1Co/Al2O3 (c, d), LP-1Ni1Co/ Al2O3 (e, f), and IM-2Ni1Co/Al2O3 (g, h). The particle size distributions are also included. Measurements are based on 150 particles per sample.

Figure 8 displays that histogram of the particle size distribution for the samples. Compared with the bimetallic NiCo sample by impregnation method, the monometallic Ni and two bimetallic NiCo samples derived from LDHs precursor present a narrow distribution of nanoparticles with the surface area-weighted diameter smaller than 8 nm. In detail, with the addition of Co, the portion of particles between 1 and 4 nm was increased from 53% to 63%. Also, we can see that the portion of the smaller particles was decreased with the further increase in the Co content, while 53% of particles in the IM sample are larger than 16 nm. Revealed by the preceding TPR and XPS results, the relatively stronger interaction between Ni (or Co) species and Al2O3 support for LP samples derived from LDHs precursor, caused by the confinement effect of the “Ni(or Co)−O−Al” structure in LDHs precursor, may produce the smaller particles during the calcination/reduction process. A similar result was reported for the preparation of NiCo MMO catalyst samples from CoNiMgAl-containing LDHs precursors: the addition of cobalt showed a “dilution effect” on the Ni surface, resulting in much smaller metal NiCo bimetallic ensembles.51 Furthermore, STEM-EDX was employed for detecting the distributions of Ni and Co to see if there is a NiCo alloy phase. Figure 9 shows a dark-field STEM micrograph and complementary mapping and lining of the LP-2Ni1Co/Al2O3 catalyst. As can be seen from Figure 9, the relevant EDX mapping results of characteristic Al and O signals reveal the homogeneous distributions of Al2O3. It is particularly note-

appearance of CoAl2O4 during H2 reduction. At the same time, the reduced Co-based samples show a new Co 2p3/2 peak at low BE of 778.4−779.0 eV, which is attributed to Co0 species.31 In addition, compared to those in the oxide-based samples, the Co 2p3/2 satellite peak was gradually enhance after reduction. According to the rule that compounds containing high-spin Co2+ ions have strong satellite peaks in Co 2p spectra of XPS, Co3+ is diamagnetic and has no satellite peak valence of cobalt that can be determined by the XPS spectroscopy.48,49 The large amounts of Co2+ species in the reduced samples can be demonstrated by the decrease of the value of the Co3+/Co2+ ratio in Table 2. Both reduced NiCo samples derived from LDHs precursor exhibit a lower value of Co3+/Co2+ ratio, compared to the reduced Co sample, implying the prompted reduction of Co species by Ni species during the reduction process. The morphology and size distribution of the metal particles in the reduced monometallic Ni and bimetallic NiCo catalyst samples were further investigated using HRTEM technique. It can be seen from Figure 8 that sphere-like small particles, potentially based on the minimization of surface energy, can be found in the HRTEM images of each reduced catalyst sample. Particles in the monometallic LP-Ni/Al2O3 samples exhibits lattice fringe at 2.03 Å (Figure 8b), which should be ascribed to (111) planes of the Ni0 crystallite.50 Because the lattice spacings of Ni and Co are very similar, it is not easy to distinguish between these phases for the reduced bimetallic NiCo samples. 15606

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Figure 10. Styrene conversion versus reaction time over different catalysts with a total Ni and Co metal loading of 12 wt %. (Reaction conditions: WHSV = 30 h−1; H2:PyGas = 80; T = 60 °C; P = 3.0 MPa; PH2 = 2.0 MPa.)

selectivity to ethylbenzene is slightly lower for the former, about 98.2% and 100.0%, respectively. It is noted that the monometallic LP-Co/Al2O3 sample lost its activity during the initial stage of the reaction. Only 10.6% styrene conversion is obtained after a period of 12 h time-on-stream. According to the literature, monometallic Co catalyst shows the lowest catalytic activity for the dry re-forming of methane reaction, in comparison to the bimetallic NiCo systems with different Ni/ Co ratios.31,52 The higher tolerance to oxidation of Co metal had been proposed to be a cause of deactivation, as observed in the case of Co/TiO2 by investigating the oxidation behavior of the reduced catalysts.31 In our case, the formed metallic Co particles might also be less active for the selective hydrogenation of styrene. The small amount of substitution of nickel for cobalt improved the catalytic activity and stability dramatically. From XPS spectra for the bimetallic NiCo systems, the reduction of the Ni species might be prompted by Co species during the reduction process (Figure 7). Also HRTEM results indicate the portion of the smaller particles decreased with the further increase of the Co content (Figure 8). The enhanced reducibility and smaller particles cause the two NiCo samples, LP-3Ni1Co/Al2O3 and LP-2Ni1Co/Al2O3 with small Co content, to show high catalytic activity. Further investigation of the catalytic test shows that the catalytic activity had hardly any change with the decrease of the total metal loading from 12 to 8 wt % for the LP-2Ni1Co/Al2O3 sample (Figure 11). However, the styrene conversion is significantly decreased for the LP-Ni/Al2O3 sample with 12 h time-onstream. After further decreasing of the total metal loading to 6 wt %, the LP-2Ni1Co/Al2O3 sample delivers a 64.3% styrene conversion after 12 h time-on-stream. However, further increase in the content of Co for the LP1Ni1Co/Al2O3 sample causes a decreased and fluctuated styrene conversion (Figure 10). It is more likely that the surface properties of active catalyst phases were modified by regulation of the cobalt content, exhibiting the synergistic effects of nickel and cobalt in terms of the reactivity for the selective hydrogenation of styrene. It is noted that the IM sample displays a lower catalytic activity than the LP sample with the same total metal loading and Ni/Co ratio upon the employed reaction conditions. In addition, with the time-onstream the former shows a significant decline of styrene conversion. Coke deposition is believed to be the main reason for catalyst deactivation during the PyGas hydrogenation reaction. As proved by earlier reports, larger Ni particles have

Figure 9. Dark-field STEM micrograph and complementary mapping of the LP-2Ni1Co/Al2O3 catalyst (a), Al EDS mapping (b), O EDS mapping (c), Ni EDS mapping (d), and Co EDS mapping (e) with the corresponding EDX lining results (f).

worthy that the distribution of the metals shown in the STEM image was confirmed by the EDS mapping and lining of Co and Ni in the same field of view (Figure 9d−f). These results aforementioned indicate that novel Al2O3-supported NiCo bimetallic catalysts (NiCo/Al2O3) derived from the LDHs precursors form an alloy phase. And alloying of metals can increase the selectivity in the catalytic reaction. Meanwhile the geometrical situation on the alloy surface can suppress graphitization of the carbonaceous deposits. Therefore, they become less dangerous for the activity.19 3.3. Selective Hydrogenation of Styrene. For evaluating the catalytic performance of the reduced mono- and bimetallic catalyst samples for selective hydrogenation of PyGas, styrene was chosen as the model reactant to be hydrogenated with an excess of toluene and n-heptane in a microflow reactor. The main product detected by gas chromatography was ethylbenzene in the catalytic tests with a small amount of ethylcyclohexane as byproduct. The final selectivity to ethylbenzene as calculated with the internal standard method was higher than 99.6% for the employed catalyst samples except for the LP-3Ni1Co/Al2 O3 sample which has ethylbenzene selectivity of 98.2%. Toluene had hardly any change at all. Figure 10 shows the results of styrene conversion as a function of time-on-stream obtained with the resulting monometallic Ni or Co and bimetallic NiCo catalyst samples. Compared with the monometallic LP-Ni/Al2O3 sample, the bimetallic LP3Ni1Co/Al2O3 and LP-2Ni1Co/Al2O3 samples exhibit an enhanced styrene conversion and a better stability with the addition of Co. Although LP-3Ni1Co/Al2O3 has a styrene conversion (about 99.0%) similar to LP-2Ni1Co/Al2O3, the 15607

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To obtain information about the kind and amount of adsorbed hydrogen available before and during the hydrogenation reaction for several supported nickel-based catalysts, H2-TPD was employed for LP-Ni/Al2O3, LP-2Ni1Co/Al2O3, and IM-2Ni1CoO/Al2O3 samples (Figure 13). It can be seen

Figure 11. Styrene conversion versus reaction time over LP-2Ni1Co/ Al2O3 and LP-Ni/Al2O3 samples with different metal loadings. (Reaction conditions: WHSV = 30 h−1; H2:PyGas = 80; T = 60 °C; P = 3.0 MPa; PH2 = 2.0 MPa.)

been disclosed to catalyze effectively the coke formation reaction during methane re-forming reaction.44,53 The “critical size” required for carbon formation was proposed in the range of 7−10 nm.54 In our case, the IM sample has a relatively broad distribution of particle size with 53% particles larger than 16 nm, revealed by the HRTEM results (Figure 8). The relatively stronger interaction between Ni (or Co) species and Al2O3 support for LP samples derived from LDHs precursor, caused by the confinement effect of the “Ni(or Co)−O−Al” structure in the LDHs precursor, may produce the smaller particles (1−8 nm) during the calcination/reduction process. The amounts of coke for the IM sample were determined by elemental analysis after hydrogenation reaction, showing the value of about 2.57 wt % after 12 h of reaction, while 1.65 wt % coke was obtained for the LP-2Ni1Co/Al2O3 sample and 2.43 wt % coke for LPNi/Al2O3. The stability of the LP-2Ni1Co/Al2O3 sample was further tested within a period of 117 h time-on-stream. Figure 12 shows that the sample has a high initial activity, attaining

Figure 13. H2-TPD patterns of typical mono- and bimetallic Ni-based catalyst samples.

from Figure 13 that all three samples show two hydrogen desorption peaks with different relative intensities. Obviously, both LP samples exhibit the relatively lower desorption peaks than the IM sample. It is worth noting that the desorption peaks of the Co-promoted LP sample, LP-2Ni1Co/Al2O3, shift to a lower temperature. As we know, the low temperature of the desorption peak can be used as an indication of a low level of interaction between the hydrogen molecules and the metal species. Therefore, this shift may be attributed to the decreased binding energy of the adsorbed hydrogen due to the Copromoter effects, which is very important for the hydrogenation reaction.55 Also, the total hydrogen desorption amounts were monitored and the values of 2.63, 5.79, and 1.52 mL g−1 STP are obtained for LP-Ni/Al2O3, LP-2Ni1Co/Al2O3, and IM2Ni1CoO/Al2O3, respectively. The LP-2Ni1Co/Al2O3 sample presents stronger desorption behaviors than LP-Ni/Al2O3 and IM-2Ni1CoO/Al2O3. Therefore, as discussed previously, considering the important role of the hydrogen supply for the hydrogenation reaction, these improvements in desorption behaviors could be able to help explain the excellent catalytic activity occurring in bimetallic LP-NiCo/Al2O3 cases and the optimal sample LP-2Ni1Co/Al2O3 exhibiting the highest catalytic activity.

Figure 12. Time-on-stream analysis for the selective hydrogenation of styrene over LP-2Ni1Co/Al2O3 catalyst. (Reaction conditions: WHSV = 30 h−1; H2:PyGas = 80; T = 60 °C; P = 3.0 MPa; PH2 = 2.0 MPa.)

4. CONCLUSION Novel supported bimetallic NiCo catalysts were fabricated from NiCoAl-LDHs precursors with γ-Al2O3 sphere as support. A series of NiCoAl-LDHs/Al2O3 precursor samples with different Ni/Co mass ratios were first fabricated by in situ growth using urea solution as precipitator with microspherical γ-Al2O3 not only as the catalyst support but also the sole source of Al3+. XRD and SEM confirmed the deposition of LDHs crystallites on the surface and in the pore canals of microspherical γ-Al2O3 by in situ growth, resulting in a uniform and microcrystalline

100.0% conversion of styrene. It exhibits a better stability, maintaining 99.3% conversion after attaining a steady state within a period of 24 h time-on-stream. The conversion of styrene is reached to 93.2% after 117 h. The selectivity to ethylbenzene is higher than 100.0% during the reaction time. Only 2.76 wt % carbon was determined after 117 h of reaction. The better anticarbon ability provides the LP-2Ni1Co/Al2O3 sample with a stable catalytic performance. 15608

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(5) Hoffer, B. W.; van Langeveld, A. D.; Janssens, J. P.; Bonné, R. L. C.; Lok, C. M.; Moulijn, J. A. Stability of highly dispersed Ni/Al2O3 catalysts: Effects of pretreatment. J. Catal. 2000, 192, 432−440. (6) Hoffer, B. W.; Bonné, R. L. C.; van Langeveld, A. D.; Griffiths, C.; Lok, C. M.; Moulijn, J. A. Enhancing the start-up of pyrolysis gasoline hydrogenation reactors by applying tailored ex situ presulfided Ni/ Al2O3 catalysts. Fuel 2004, 83, 1−8. (7) Gaspar, A. B.; dos Santos, G. R.; de SouzaCosta, R.; da Silva, M. A. P. Hydrogenation of synthetic PYGASEffects of zirconia on Pd/ Al2O3. Catal. Today 2008, 133, 400−405. (8) Su, W. B.; Chen, W. R.; Chang, J. R. Impact of iron deposition on Pd/δ-Al2O3 in selective hydrogenation of pyrolysis gasoline. Ind. Eng. Chem. Res. 2000, 39, 4063−4069. (9) Zeng, T. Y.; Zhou, Z. M.; Zhu, J.; Cheng, Z. M.; Yuan, P. Q.; Yuan, W. K. Palladium supported on hierarchically macro-mesoporous titania for styrene hydrogenation. Catal. Today 2009, 147, S41−S45. (10) Zhou, Z. M.; Zeng, T. Y.; Cheng, Z. M.; Yuan, W. K. Kinetics of selective hydrogenation of pyrolysis gasoline over an egg-shell catalyst. Chem. Eng. Sci. 2010, 65, 1832−1839. (11) Badano, J. M.; Betti, C.; Rintoul, I.; Vich-Berlanga, J.; Cagnola, E.; Torres, G.; Quiroga, M. New composite materials as support for selective hydrogenation; egg-shell catalysts. Appl. Catal., A 2010, 390, 166−174. (12) Wen, X.; Li, R. S.; Yang, Y. X.; Chen, J. L.; Zhang, F. Z. An eggshell type Ni/Al2O3 catalyst derived from layered double hydroxides precursor for selective hydrogenation of pyrolysis gasoline. Appl. Catal., A 2013, 468, 204−215. (13) Qian, Y.; Liang, S. Q.; Wang, T. H.; Wang, Z. B.; Xie, W.; Xu, X. L. Enhancement of pyrolysis gasoline hydrogenation over Zn- and Mopromoted Ni/γ-Al2O3 catalysts. Catal. Commun. 2011, 12, 851−853. (14) Betti, C.; Badano, J.; Maccarrone, M. J.; Mazzieri, V.; Vera, C.; Quiroga, M. Effect of the sequence of impregnation on the activity and sulfur resistance of Pt-Ni/γ-Al2O3 bimetallic catalysts for the selective hydrogenation of styrene. Appl. Catal., A 2012, 435, 181−186. (15) Liu, Z.; Li, Z. L.; Wang, F.; Liu, J. J.; Ji, J.; Park, K. C.; Endo, M. Electroless preparation and characterization of Ni-B nanoparticles supported on multi-walled carbon nanotubes and their catalytic activity towards hydrogenation of styrene. Mater. Res. Bull. 2012, 47, 338−343. (16) Rodriguez, J. A.; Goodman, D. W. The nature of the metal bond in bimetallic surfaces. Science 1992, 257, 897−903. (17) Greeley, J.; Mavrikakis, M. Alloy catalysts designed from first principles. Nat. Mater. 2004, 3, 810−815. (18) Wang, D. S.; Li, Y. D. Bimetallic nanocrystals: Liquid-phase synthesis and catalytic applications. Adv. Mater. 2011, 23, 1044−1060. (19) Ponec, V. Alloy catalysts: The concepts. Appl. Catal., A 2001, 222, 31−45. (20) Lonergan, W. W.; Vlachos, D. G.; Chen, J. G. Correlating extent of Pt-Ni bond formation with low-temperature hydrogenation of benzene and 1,3-butadiene over supported Pt/Ni bimetallic catalysts. J. Catal. 2010, 271, 239−250. (21) Braos-García, P.; García-Sancho, C.; Infantes-Molina, A.; Rodríguez-Castellón, E.; Jiménez-López, A. Bimetallic Ru/Ni supported catalysts for the gas phase hydrogenation of acetonitrile. Appl. Catal., A 2010, 381, 132−144. (22) Alayoglu, S.; Zavalij, P.; Eichhorn, B.; Wang, Q.; Frenkel, A. I.; Chupas, P. Structural and architectural evaluation of bimetallic nanoparticles: A case study of Pt-Ru core-shell and alloy nanoparticles. ACS Nano 2009, 3, 3127−3137. (23) Li, X. F.; Sun, Z. Y.; Chen, J. L.; Zhu, Y.; Zhang, F. Z. One-pot conversion of dimethyl terephthalate into 1,4-cyclohexanedimethanol with supported trimetallic RuPtSn catalysts. Ind. Eng. Chem. Res. 2014, 53, 619−625. (24) Cavani, F.; Trifirò, F.; Vaccari, A. Hydrotalcite-type anionic clays: Preparation, properties and applications. Catal. Today 1991, 11, 173−301. (25) Tichit, D.; Coq, B. Catalysis by hydrotalcites and related materials. CATTECH 2003, 7, 206−217.

developed two-dimensional pore structure. Compared with the similar NiCo/Al2O3 sample prepared by co-impregnating γAl2O3 in Ni(NO3)2·6H2O and Co(NO3)2·6H2O solution, NiCo/Al2O3 samples derived from LDHs precursor showed a relatively stronger interaction between Ni (or Co) species and Al2O3 support, revealed by TPR and HRTEM results, which may lead to a narrow distribution of particles with smaller particle diameter than 8 nm. Furthermore, STEM-EDS confirmed the formation of the alloy phase and the alloy phase can further increase the selectivity in the catalytic reaction and inhibit the formation of coke, thereby improving the stability of the catalyst. In addition, XPS indicated that the reduction of the Ni species was prompted by Co species during the reduction process. The NiCo/Al2O3 sample derived from LDHs precursor exhibited significantly better catalytic performance than the sample prepared by impregnation with the same total metal loading and Ni/Co ratio. The appropriate amounts of substitution of nickel for cobalt improved the catalytic activity and stability dramatically. The enhanced reducibility and smaller particles for NiCo samples derived from LDHs precursor, as well as the relatively stronger interaction between Ni (or Co) species and Al2O3 support, were proposed to suppress the coke formation reaction during the styrene hydrogenation. And H2-TPD showed that the LP-2Ni1Co/ Al2O3 presents stronger desorption behaviors than LP-Ni/ Al2O3 and IM-2Ni1CoO/Al2O3. Therefore, as discussed previously, considering the important role of hydrogen supply for the hydrogenation reaction, these improvements in desorption behaviors could help explain the excellent catalytic activity occurring in bimetallic LP-NiCo/Al2O3 cases and the optimal sample LP-2Ni1Co/Al2O3 exhibiting the highest catalytic activity.

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AUTHOR INFORMATION

Corresponding Author

*Fax: (+86) 10-6442-5385. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Professor Dianqing Li at Beijing University of Chemical Technology, who offered us the pristine alumina spheres. This work was supported by the National Natural Science Foundation of China (Grant Nos. 20976006 and 21376019), the Program for Changjiang Scholars and Innovative Research Team in University (Grant No. IRT1205), and the 973 Program (Grant No. 2011CBA00506).

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REFERENCES

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dx.doi.org/10.1021/ie502721p | Ind. Eng. Chem. Res. 2014, 53, 15600−15610