Double-Confined Nickel Nanocatalyst Derived from Layered Double

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Double-Confined Nickel Nanocatalyst Derived from Layered Double Hydroxide Precursor: Atomic Scale Insight into Microstructure Evolution Qining Fan, Xuefeng Li, Zhixiang Yang, Jingjing Han, Sailong Xu, and Fazhi Zhang* State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China S Supporting Information *

ABSTRACT: Double-confined nickel nanocatalyst Ni/Ni(Al)Ox/AlOx, with metal Ni nanoparticles implanted in the weakly crystalline Ni(Al)Ox matrix and embedded in amorphous AlOx networks, was facilely fabricated by hydrogen reduction of the NiAl-LDH precursor at a controlled temperature. Direct structure imaging of Ni and Al species revealed that subnanometer Ni0 clusters nucleate initially in the Ni(Al)Ox matrix. Subsequent growth of Ni0 clusters proceeds at the expense of the surrounding Ni(Al)Ox, accompanied by consecutive transfer of Al3+ from the central part to the near-surface, suggesting a mechanism of ion reverse migration. The Ni(Al)Ox interfacial shell is proposed to provide a strong connection with the metallic Ni and the AlOx network, improving the hydrogen adsorption capacity of the double-confined Ni catalyst and consequently the catalytic activity for dimethyl terephthalate hydrogenation to dimethyl cyclohexane-1,4-dicarboxylate, a prominent modification reagent and intermediate in the polymer industry. The findings should be of great importance to both designing novel confined catalysts and understanding the structure−activity correlation.

1. INTRODUCTION It has been gradually recognized that support materials in heterogeneous catalysts can not only help the dispersion of supported metal particles but also affect the catalytic reaction process through the interaction with active metal centers; the local environment of metal particles, therefore, plays a critical role in determining the activity, selectivity, and stability of the heterogeneous metal catalysts.1−4 Recently, confined catalytic systems with novel nanostructured architectures have attracted an increasing interest and effective strategies were developed to quantitatively control the surface structure and electronic properties of the supported metal nanoparticles by means of modulating the intrinsic interaction between metal nanoparticles and their local chemical environment.5−18 For their unique pore structure characteristics, zeolites,5−8 mesoporous silicas,9,10 carbon nanotubes,11−14 and metal oxide nanotubes15,16 have been commonly used to create nanocomposite catalysts through encapsulating guest molecules, metal complexes, or nanoparticles into the well-defined channels, pores, or cages. Besides, many efforts are currently devoted to introducing active metal centers into a peculiar confined environment, such as metal/metal (hydr)oxide interfaces,17,18 silica matrixes,19 and interior wall cavities of the alumina nanotubes,20 for optimizing metal−support interaction and consequently enhancing catalytic performance. However, these attempted fabrication strategies usually suffer from problems of process complexities and/or microstructure controllabilities, and there are few studies that reported sufficient data for © XXXX American Chemical Society

elucidating microstructure evolution of metal centers during the synthesis process. In this paper, we report facile fabrication of a doubleconfined Ni metal catalyst Ni/Ni(Al)Ox/AlOx, of which Ni nanoparticles (about 4−6 nm in diameter) implanted in weakly crystalline Ni(Al)Ox oxide (i.e., Al3+-doped NiO structure) matrixes are embedded in amorphous AlOx networks, by H2 reduction of a single-source precursor of Ni2+Al3+-containing layered double hydroxide (NiAl-LDH) at a controlled temperature. LDHs are a class of clays with brucite-like cationic layers and charge-balancing anions inside the interlayers.21−23 By taking advantage of the structural characteristics of LDHs, such as uniformly distributed metal cations within the hydroxide layers, the flexible tunability of composition, and the facile exchangeability of intercalated anions, LDHs have shown potential as heterogeneous catalysts or catalyst supports for a variety of reactions.24−29 In particular, LDHs containing transition metal ions in the main layer, such as Ni2+, Co2+, and Fe3+ ions, have been used as precursors to produce metal particles well-dispersed in a mixed oxide matrix, upon calcination to generate a mixed metal oxide (MMO) material or subsequent H2 reduction.30−34 A detailed understanding of the microstructure evolution from LDH precursors to the supported metal nanoparticle catalysts is highly desirable for Received: June 23, 2016 Revised: August 6, 2016

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DOI: 10.1021/acs.chemmater.6b02553 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

deionized water ([Ni2+] + [Al3+] = 0.66 M) and an aqueous solution of NaOH (1.06 M) and Na2CO3 (0.44 M) in deionized water were simultaneously added dropwise into 100 mL of stirring deionized water at 25 °C in a beaker, during which time a green precipitate was formed. The pH of the slurry was controlled at 10.0 ± 0.1 by adding the two solutions alternately. After the two solutions were completely added, the slurry was transferred to an autoclave and aged at 120 °C for 24 h. The resulting slurry containing NiAl-LDH precursors was thoroughly washed with deionized water until the pH value of the washings reached 7.0. The actual Ni/Al molar ratio 2.0 in the NiAlLDH precursor was estimated by elemental analysis, which is in good accordance with the theoretical Ni/Al molar ratio in initial nitrate salt solutions. The above-synthesized LDH precursor was subsequently reduced in a H2 atmosphere at the target temperature for 5 h with a heating rate of 5 °C min−1 to obtain the reduced Ni-based catalyst samples. The result from elemental analysis demonstrates that the 600 °C reduced sample contains 63 wt % Ni. The reduced samples were protected in N2 atmospheres to prevent oxidation of Ni0 in air before characterization and hydrogenation reactions, and the exposure time is no longer than 1 min when carrying out the next procedure. 2.2. Characterization of Samples. XRD patterns were recorded on a Shimadzu XRD-6000 diffractometer using Cu Kα radiation (λ = 0.15406 nm) operated at 40 kV and 40 mA, with a scan step of 2θ 0.02° and scan speed of 10° min−1. In situ FTIR was carried out on a Nicolet 380 instrument equipped with a custom-built quartz cell. The background was collected without samples at room temperature in a 50 mL min−1 H2 flow. The NiAl-LDH precursor (1 mg) was combined with spectroscopic grade KBr (100 mg) and pressed into a disc, then placed in a reaction chamber. Spectra were collected at temperature points with a 50 mL min−1 H2 flow, using a heating rate of 5 °C min−1 and holding at temperature for 1 h before collection. All spectra were recorded by coaddition of 128 scans, with a nominal resolution of 4 cm−1. Elemental analysis was performed using a Shimadzu ICPS-7500 inductively coupled plasma emission spectrometer (ICPES). HRTEM was conducted on a JEOL JEM-2100F instrument with 0.19 nm point resolution. Conventional HAADF-STEM was performed on an FEI Tecnai G2 F20 with 0.20 nm resolution of HAADF-STEM imaging. Cs-corrected HAADF-STEM images were recorded by using a JEOL JEM-ARM200F (200 kV) equipped with a cold field emission gun and a spherical aberration corrector for probe correction. The resolution of this Cs-corrected HAADF-STEM is ≤0.08 nm. STEM-XEDS was also performed by a JEOL JEM-ARM200F, and the interested area was detected by the focused beam under the STEM mode with drift corrections for attaining sufficient counts. The sample was ultrasonically dispersed in ethanol, and then a small amount of this solution was dropped onto an ultrathin carbon film which is coated on a copper grid in electron microscopy experiments. XPS was recorded using an ESCALAB250 X-ray photoelectron spectrometer equipped with monochromatized Mg Kα X-ray radiation (1253.6 eV photons). Binding energy was calibrated based on the graphite C 1s peak at 284.5 eV. H2-TPR and H2-TPD were performed on a Micromeritics ChemiSorb 2920. The hydrogen consumption and desorption were then monitored by a thermal conductivity detector (TCD) and a computer data acquisition system. The TCD signals were calibrated using Ag2O as a standard. About 100 mg of the sample was loaded in a quartz reactor. The procedure for H2-TPR was as follows: The sample was cleaned at 100 °C for 0.5 h in an Ar (50 mL min−1) flow in order to remove any physical absorption. After cooling down to 50 °C, the sample was heated with recording the TCD signals and a heating rate of 5 °C min−1 up to 900 °C in a H2/Ar (50 mL min−1) flow. The outlet gas was passed through a cold trap to remove the moisture produced during reduction. H2-TPD experiments were carried out to evaluate H adsorption of catalysts. In the pretreatment, samples were heated with a heating rate of 5 °C min−1 up to reduction temperature and held for 0.5 h in a 10% H2/Ar (50 mL min−1) flow to eliminate surface oxide which is formed during transient exposure in air. Then, samples were held for 0.5 h in an Ar (50 mL min−1) flow and cooled down to room temperature. After pretreatment, the sample adsorbed H in a 10% H2/Ar (50 mL min−1) flow at room temperature for 0.5 h and then was swept by an Ar (50 mL min−1) flow for 0.5 h to remove

revealing deep insights into the structure−activity relationships of these important catalysts and for designing advanced catalysts with enhanced performance. Over the past few decades, remarkable advances have been made in the mechanism interpretation of transformation of the LDH precursor, including MgAl-, CoAl-, ZnAl-, and NiFe-LDH, into the corresponding MMO by thermal decomposition.35−39 Nevertheless, very little is known about the nucleation and growth mechanism of metal particles during the reduction process.31,32 The difficulties in the elucidation of metal particle formation may arise from the fact that there are almost few analytical techniques for the directly atomical view of the lowcrystalline initial metal phase from the surrounding lowcrystalline MMO matrix, both of which usually have a same face-centered cubic (fcc) structure, especially for the analysis of nucleation of very few metal atoms. Spherical aberrationcorrected scanning transmission electron microscopy with a high-angle annular dark-field detector (Cs-corrected HAADFSTEM), which can provide directly interpretable atomic number (Z) contrast information,40−42 has been proven to be an ideal means to probe atomic scale structural and chemical information on catalyst materials, such as metals, metal oxides, and metal sulfides catalysts.43−45 Herein, Cs-corrected HAADF-STEM and other complementary characterization, such as powder X-ray diffraction (XRD), in situ Fourier transformed infrared (FT-IR) spectroscopy, high-resolution transmission electron microscopy (HRTEM) combined with selected-area electron diffraction (SAED), conventional HAADF-STEM, X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction of hydrogen (H2-TPR), and temperature-programmed desorption of hydrogen (H2TPD), were performed to complete the emerging picture of the nucleation stage and the growth process of confined Ni metal nanoparticles during H2 reduction and to characterize physicochemical properties of the resulting Ni-based catalysts. To explore prospective applications in heterogeneous catalysis, we further evaluated the catalytic hydrogenation performance of the double-confined Ni metal catalyst for selective hydrogenation of dimethyl terephthalate (DMT) to dimethyl cyclohexane-1,4-dicarboxylate (DMCD).46 Industrially, DMCD is produced by selective hydrogenation of DMT using precious Pd catalysts under the H2 pressure range 30−48 MPa and reaction temperature range 160−180 °C. Recently, increasing demands for more economical processes in hydrogenation reactions have launched the extensive investigation on the new catalytic materials based on nonprecious metals. In the case of selective hydrogenation of DMT, various strategies, such as adopting multimetal cluster complexes and Pd nanoparticles as catalysts, have been developed for cutting the cost and maximizing the catalytic efficiency.47−49 Ni-based catalysts may be a suitable candidate for this reaction because Ni elements are abundant and those catalysts are still eagerly concerned in hydrogenation, especially in hydrogenating benzene to cyclohexane. The reaction results show that the double-confined Ni metal catalyst, Ni/Ni(Al)Ox/AlOx, exhibits an excellent catalytic activity for DMT hydrogenation.

2. EXPERIMENTAL SECTION 2.1. Preparation of Confined Ni Catalysts and the Corresponding NiAl-LDH Precursor. NiAl-LDH precursors with the Ni/Al molar ratio 2.0 were first prepared by a coprecipitation method with a stable pH value: An aqueous solution of Ni(NO3)2· 6H2O and Al(NO3)3·9H2O with the Ni/Al molar ratio of 2.0 in B


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°C, and XRD patterns of the resulting specimens were acquired to estimate the variation of crystalline phases (Figure 1). The

the physical absorption. The sample was heated with recording the TCD signals and a heating rate of 5 °C min−1 up to 850 °C in an Ar (50 mL min−1) flow. The percentage of reduced Ni was calculated based on H2 consumption from H2-TPR results where LDH precursors are considered as a totally unreduced sample, as in the following equation:

⎛ H Consumption of reduced sample ⎞ Reduced Ni % = ⎜1 − 2 ⎟ H 2 Consumption of LDH precursor ⎠ ⎝ × 100% 2.3. Selective Hydrogenation Tests. The catalytic tests were carried out in a batch reactor (a 250 mL stainless steel autoclave) equipped with a stirrer. DMT (1 g), isopropanol (80 mL), and catalyst samples (0.25 g) were rapidly placed into the reactor with a tight seal. Then, the sealed reactor was purged with N2 and H2 three times toward the pure H2 atmosphere at the target pressure. The reactor was heated up with a fixed 750 rpm stirring, and beginning to record time when the target temperature was reached. The pressure and temperature were monitored and adjusted during the reaction. In the recycling test, the catalyst and dissolved reactants were separated by centrifugation. The separated catalyst was washed with isopropanol for several times and then recycled into the next batch. The products were identified by using a gas chromatograph-mass spectrometer (GCMS-QP2010, Shimadzu), and quantified by a gas chromatograph (GC-2014C, Shimadzu) equipped with an Rtx-5 chromatographic column and a flame ionizing detector. The primary product is DMCD, and the main byproducts are methyl 4methylbenzoate (MMB), methyl 4-methyl-cyclohexane carboxylate (MMCHC), and 1,4-benzenedicarboxylic acid, 1-methyl 4-(1-methylethyl) ester (BDCAMMEE), which are probably derived from the conversion of branched chains, and excessive hydrogenation.

Figure 1. XRD patterns of the NiAl-LDH precursor and a series of reduced samples in flowing H2 at different temperatures ranging from 200 to 700 °C.

LDH precursor exhibits the characteristic feature of layered materials, with strong, symmetric lines at low 2θ values and weaker, less symmetric lines at high 2θ values. After reduction at 200 and 250 °C, the characteristic (003) and (006) reflection peaks of LDH shrink and shift to a higher 2θ value, indicating that the d-spacing between layers is compacted. The vanishment of LDH reflections after reduction at 300 °C demonstrates the total collapse of main layers. In situ FT-IR spectra of the reduced samples can further confirm decomposition of the LDH precursor by successive dehydration, dehydroxylation, and decarbonation after reduction at 300 °C (Figure S2). Meanwhile, the weak broad reflections located at ca. 37, 43, and 63° 2θ in the XRD pattern of the 300 °C reduced sample are adjacent to the (111), (200), and (220) facets of fcc NiO, respectively. This result demonstrates the formation of a weakly crystalline mixed oxide Ni(Al)Ox phase (i.e., Al3+-doped NiO structure). After reduction at 400 °C, we can observe three new peaks at 45, 52, and 76° 2θ, which can be assigned to reflections of the (111), (200), and (220) facets of fcc Ni, respectively. It should be noted that peak intensity of the Ni metal phase increases with raising the reduction temperature from 400 to 700 °C and, however, the Ni(Al)Ox phase shows a reverse trend. The phases distribution of the reduced samples was directly observed by Cs-corrected HAADF-STEM, which is atomically sensitive to Z-contrast. Images of the 350 °C reduced sample demonstrate that the abundant lattices of low-crystalline Ni(Al)Ox oxide partly align in a short-range order (Figure 2a and Figure S3a). The lattice fringes are assigned to the (111) planes of the fcc NiO phase. Notablely, the emerging extremely small Ni0 clusters, which are only 0.8 nm in diameter, are clearly recognizable as bright patches implanted inside the predominant less-bright Ni(Al)Ox matrix. Because Ni possesses the higher atomic number than Al and O, the metal Ni has high contrast in the HAADF-STEM image.50 Nevertheless, these metal Ni clusters are hardly visible in conventional HAADFSTEM images, owing to the resolution and sensitivity limitations (Figure S4a). We are likewise unable to determine visually the “birth” of metal Ni nuclei by HRTEM images, due

Catalytic activity and selectivity were calculated using the following simplified equations ⎞ ⎛ RDMTADMT Conversion = ⎜1 − ⎟ RDMTADMT + RDMCDADMCD + ∑ R iAi ⎠ ⎝ × 100%

⎛ ⎞ RDMCDADMCD Selectivity = ⎜ ⎟ × 100% ⎝ RDMCDADMCD + ∑ R iAi ⎠ where i refers to a certain byproduct, R signifies the relative area rectifying factor, and A denotes the area percentage as determined by GC.

3. RESULTS AND DISCUSSION TEM images of the as-synthesized NiAl-LDH precursor show a typical hexagonal nanoplatelet morphology with the edge length of about 30 nm (Figure S1 in the Supporting Information). The corresponding SAED pattern indicates that the LDH precursor is highly crystalline with a hexagonal structure (inset of Figure S1). The LDH precursor was reduced in flowing H2 at different temperatures ranging from 200 to 700 C


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Figure 2. Cs-corrected HAADF-STEM images of the reduced samples derived from NiAl-LDH precursors in flowing H2 at (a) 350, (b) 400, (c) 600, and (d) 700 °C. The scale bar is 2 nm. The insets show magnified images of the representative particles or areas of the samples. The yellow circles indicate the representative subnanometer Ni clusters consisting of only a few atoms, whereas the blue circles denote the Ni nanoparticles consisting of several nanometers. In addition, the amorphous AlOx phase around weakly crystalline species in the 600 and 700 °C reduced samples is shown as gray batches, and the boundary of the Ni(Al)Ox matrix is indicated by green circles.

diameter, are found in this sample. The Ni atoms of these larger particles align with lattice fringes assigned to (111) or (220) facets of fcc-Ni. STEM with X-ray energy-dispersive spectroscopy (XEDS) mapping can characterize the nature of Ni and Al spatial distribution. Figure 3a shows some small “lakes” of separate Ni elements surrounded with uniformly mixed Ni and Al elements in the 400 °C reduced sample, revealing that metal Ni clusters are segregated inside the Ni(Al)Ox matrix. In the 600 °C reduced sample, the enrichment of Ni at the center of particles, indicated as the brightest batches in HAADF images, increases the average size of Ni metal to 4−6 nm and simultaneously creates a Ni core/Ni(Al)Ox shell structure (Figure 2c and Figure S7). We can also observe an amorphous alumina phase as gray batches around the Ni/Ni(Al)Ox structure. STEM-XEDS mapping exhibits the enhancement of Ni signals at the center of the particle relative to the perimeter, but shows the opposite trend to Al signals (Figure 3b). Actually, Al signals are dispersed in a relatively larger area than Ni signals, indicating the surface enrichment of aluminum. It is proposed that reduction treatment may transfer Al3+ ions from the central part to the near-surface of Ni(Al)Ox, associated with the Ni0 nucleation. Increased migration of Al3+ expected at higher reduction temperatures (e.g., 600 °C) produces the amorphous AlOx outside Ni(Al)Ox, resulting in a greater degree of segregation of Ni/Ni(Al)Ox from each other. The above STEM characterization clearly reveals the formation of a double-confined Ni metal architecture, Ni/Ni(Al)Ox/AlOx, of which Ni nanoparticles confined in the weakly crystalline

to the superposition of mass-thickness and the diffraction contrast caused by the crystalline Ni(Al)Ox matrix and the small diameter of Ni clusters (Figure S5a). The emergence of these Ni clusters in the Al3+-doped NiO phase has long been proposed in literatures for preparation of Ni-based catalysts derived from the NiAl-LDH precursor by calcination and subsequent H2 reduction.31,32 To the best of our knowledge, this is the first time that subnanometer Ni clusters nucleated inside poorly crystallized oxide which is from the decomposed NiAl-LDH precursor were directly imaged and identified by using the atomic level HAADF-STEM technique. In addition, this sample presents clearly a disordered structure of Ni metal containing the jumbled Ni atoms. Stress due to the latticeparameter mismatch (3.52 and 4.18 Å for Ni and NiO, respectively) between Ni clusters and the Ni(Al)Ox matrix may lead to the lattice distortions of the confined Ni clusters.51 Moreover, transformation of the ordered fcc-NiO(111) phase to the disordered fcc-Ni(111) phases can be further figured out by fast Fourier transform (FFT) analysis of selected areas in the Cs-corrected HAADF-STEM image, which exhibited a series of intact six diffraction spots of a cubic lattice for Ni(Al)Ox, and only one pair for Ni (Figure S3b). The plate morphology of LDH crystals is hardly preserved and loose accumulation of spherical-like Ni(Al)Ox aggregates occurs during reduction at 400 °C (Figure 2b and Figure S6). The Ni clusters implanted in the less-bright Ni(Al)Ox matrix are observed, and they grow as large as 1−2 nm in diameter. In addition, some crystallites of metal Ni, with 3−4 nm in D


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Figure 3. High-resolution STEM-XEDS mappings of the reduced samples derived from NiAl-LDH precursors in flowing H2 at (a) 400, (b) 600, and (c) 700 °C. The scale bar is 5 nm.

Ni(Al)Ox internal shell are embedded in amorphous AlOx networks. In addition, further raising the reduction temperature to 700 °C causes the vanishment of the Ni(Al)Ox internal shell in Ni/Ni(Al)Ox/AlOx (Figure 2d and Figure S8). As a result, the Ni/AlOx structure was formed with Ni metal particles (6− 12 nm) confined in amorphous AlOx networks. The spatial distribution of the individual Ni particles in AlOx networks can be further visualized by the XEDS images (Figure 3c). It is noted that distinct Ni crystallites with clear particle boundaries in the 700 °C reduced Ni/AlOx sample can be discerned in the conventional HAADF-STEM (Figure S4d) and HRTEM images (Figure S5d). The X-ray photoelectron spectroscopy (XPS) technique was performed to obtain more information on the surface state of samples (Figure 4). The Ni 2p3/2 XPS spectrum of the LDH precursor shows binding energy of 856.2 eV and a corresponding shake-up satellite at 862.4 eV (Figure 4a). The sharp Ni 2p3/2 peak for the LDH sample indicates the uniform electron environment of Ni elements because of the highly ordered crystal structure. For the 350 °C reduced sample, the contribution of Ni2+ in XPS broadens and shifts peaks to lower binding energy (855.6 and 861.8 eV for the main peak and its satellite, respectively). The decomposition of LDH to weakly crystalline Ni(Al)Ox oxide after H2 reduction transforms the uniformly adjacent MO6 octahedra with close interaction into

Figure 4. Ni 2p3/2 (a) and Al 2p (b) XPS spectra of the NiAl-LDH precursor and the reduced samples in flowing H2 at different temperatures.

an Al3+-doped fcc NiO-like structure, increasing the electron density surrounding the Ni nuclei. Similarly, the binding energy of Al 2p shifts to lower values after reduction (from 74.1 to 73.6 eV; Figure 4b). Moreover, the Ni 2p3/2 spectra exhibit an additional tiny peak of Ni 0 metal around 852.0 eV, demonstrating the appearance of Ni clusters from the Ni(Al)Ox E


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Chemistry of Materials matrix. We can observe the apparent enhancement of the Ni0 peak for the 400 °C reduced sample. Moreover, raising the reduction temperature to 600 °C can lead to a sharp decrease of intensity of the Ni2+ peak, and meanwhile, the characteristic peak of Ni0 is predominant. Concomitantly, the intensity of the Al3+ peak increases obviously, indicating the transfer of dispersive Al from the central part to the near-surface of Ni(Al)Ox crystallites during the reduction process. Further raising the reduction temperature to 700 °C makes the Ni2+ peak almost vanish. On the basis of aforementioned results, we propose a transformation mechanism of the NiAl-LDH precursor to confined Ni catalysts during the H2 reduction process (Figure 5). The collapse of the main layers of LDH precursors occurs

temperature may be the crucial mechanism for constructing the double-confined architecture, Ni/Ni(Al)Ox/AlOx. In addition, further raising the reduction temperature to 700 °C causes the disappearance of the Ni(Al)Ox internal shell. Consequently, the Ni particles are enlarged with the coalescence/sintering of tiny Ni nanoparticles, generating the Ni/AlOx structure with Ni particles confined in the AlOx network. Catalytic properties of the resulting confined Ni catalysts were evaluated by hydrogenation of DMT to DMCD at the relatively low reaction temperature of 90 °C and suitable H2 pressure 6 MPa (Table 1). For all we know, the supported Ni Table 1. Physicochemical Properties and Catalytic Performances of the Confined Ni Catalysts Derived from LDH Precursorsa

sampleb NiAl-300 NiAl-350 NiAl-400 NiAl-500 NiAl-600 NiAl-700 Ni-Cf

average size of Ni metal (nm)c 0.8 2.4 4.8 7.9

reduced Ni (%)d 39 48 86 97 100

total amount of H2 desorption (mmol H/g)e

0.535 0.735 0.567

DMT conversion (%)

selectivity to DMCD (%)

0.7 24.8 44.9 72.6 99.9 73.2 32.2

27.3 91.3 91.6 93.1 93.3 94.3 92.9

a

Reaction conditions: catalyst 0.25 g, DMT 1.0 g, isopropanol 80 mL, reaction temperature 90 °C, H2 pressure 6 MPa, reaction time 4 h. Reaction was performed in a stainless steel autoclave. bThe number in the sample names indicates the reduction temperature in a hydrogen atmosphere. cAverage Ni metal size was determined by randomly measuring for ∼50 particles in Cs-corrected HAADF-STEM imaging. d Percentage of metallic nickel was calculated by TPR based on the H2 consumption. eTotal amount of H2 desorption was calculated by H2TPD. fCommercial Ni catalyst powder was purchased from Alfa Aesar (Product No.: 31276), containing about 66 ± 5 wt % Ni metal supported on SiO2-Al2O3.

Figure 5. A schematic representation of the proposed formation process of the double-confined Ni metal catalyst derived from the NiAl-LDH precursor during the H2 reduction process. Stage 1: Collapse of main layers causes decomposition of the LDH precursor, generating the Ni(Al)Ox matrix. Stage 2: “Birth” of Ni nuclei occurs inside the Ni(Al)Ox matrix. Stage 3: Growth of Ni clusters proceeds by the expense of the surrounding Ni(Al)Ox, accompanied by progressive solid-state migration of Al3+ from the inside to the outside, forming the double-confined Ni metal architecture Ni/ Ni(Al)Ox/AlOx. Stage 4: Coalescence/sintering of Ni nanoparticles leads to the larger Ni particles, generating the Ni/AlOx structure with Ni particles confined in the AlOx network.

catalysts have not been applied for DMT hydrogenation in the literature so far. The reduction temperature plays an important role in the catalytic activity of the LDH-derived Ni catalysts. The 300 °C reduced sample is almost inactive for DMT hydrogenation. The selectivity to the primary product DMCD is 27.3% with a main byproduct 1-methyl 4-(1-methylethyl) ester, produced by transesterification between DMT and solvent isopropanol. Raising the reduction temperature from 350 to 600 °C causes the catalyst samples a rapid increase in the DMT conversion, with a maximum DMT conversion of 99.9% for the NiAl-600 sample; however, further enhancing to 700 °C leads to a decrease of DMT conversion. The DMCD selectivity generally keeps between 91% and 95% with main byproducts, methyl 4-methylbenzoate and methyl 4-methylcyclohexane carboxylate, due to the hydrogenolysis of ester groups. As a reference sample, a commercial Ni catalyst containing about 66 ± 5% Ni metal supported on SiO2-Al2O3 was adopted for DMT hydrogenation, exhibiting a DMT conversion of only 32.2%. According to the literature, the amounts of metallic nickel, induced by the reduction temperature, may have a strong influence on the hydrogenation performance over the Ni-Al catalysts derived from the NiAl-LDH precursor.32 The percentage of metallic Ni in confined Ni catalysts can be calculated by TPR based on the H2 consumption. Figure 6a shows the H2-TPR profiles of the NiAl-LDH precursor and the

after reduction at 300 °C by successive dehydration, dehydroxylation, and decarbonation, generating the weakly crystalline mixed oxide Ni(Al)Ox matrix to potentially breed metal Ni nuclei. Stress due to the lattice mismatch of Ni metal and Ni(Al)Ox oxide may lead to the lattice distortions of initial Ni clusters confined in the matrix. The considerable degree of confusion within the Ni(Al)Ox crystal lattice after rupturing Ni−O bonds with dissociated hydrogen, possibly coupled with the lattice construction for further decreasing energy of the crystal lattice, is considered as the driving force to form Ni nuclei. Subsequently, metal Ni clusters grow at the expense of the surrounding Ni(Al)Ox phase, accompanied by progressive solid-state migration of Al3+ from the inside to the outside. Moreover, the increased diffusion of Al3+ expected at higher temperatures, 600 °C, produces the amorphous AlOx on the outside of Ni(Al)Ox, resulting in a greater degree of segregation of Ni/Ni(Al)Ox composite particles from each other. The reverse migration of Ni2+ and Al3+ ions at elevated reduction F


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for the NiAl-400 sample, demonstrating that the H2 is desorbed uniformly from the metal particles. Compared to the NiAl-600 sample, the NiAl-700 sample exhibits a decreased intensity of the broad asymmetric peak around 200 °C due to reducing Ni metal dispersion by coalescence/sintering of Ni nanoparticles. We note that a high temperature peak referred to spiltover species disappears for the NiAl-700 sample. Table 1 shows that a total amount of H2 desorption increases in accordance with the order of NiAl-400 < NiAl-700 < NiAl-600, which is in a good agreement with the catalytic activity of DMT hydrogenation. Some research work has revealed the potential positive impact of interfacial sites between the active metal and the support oxide on catalytic performance.54,55 Moreover, for promoting the catalytic activity in aromatic compounds hydrogenation, the strong metal−support interaction enhances the spillover of dissociated hydrogen from the metal surface to the support where the aromatic compounds are possibly adsorbed. As elucidated in Figures 2 and 3, the Ni(Al)Ox interfacial shell may provide a close connection with the metallic Ni and the amorphous alumina network, which is deemed to be necessary to induce the H spillover from Ni metal to the AlOx support and the partial spilt H strongly adsorbed on Ni(Al)Ox sites, improving the hydrogen adsorption capacity of the NiAl-600 catalyst. Moreover, Figure 6a shows that maximum peaks of TPR profiles gradually shift to a higher temperature if the reduction temperature raises to 600 °C, indicating that the remaining Ni2+ species in Ni(Al)Ox crystallites strongly interact with aluminum.32,56 Therefore, the NiAl-600 catalyst sample with enhanced amounts of chemisorbed hydrogen has an excellent catalytic activity for DMT hydrogenation. In addition, to check whether the catalyst sample can retain its hydrogenation activity and selectivity, NiAl-600 was repeatedly used for DMT hydrogenation for five times, and the experimental results are plotted in Figure S9. The results show the good reusability of this sample for keeping a high conversion and selectivity for DMT hydrogenation.

Figure 6. H2-TPR (a) and H2-TPD (b) profiles of the NiAl-LDH precursor and the reduced samples in flowing H2 at different temperatures.

reduced samples in flowing H2 at different temperatures. For the NiAl-LDH precursor, two tiny peaks near 270 and 310 °C are attributed to H2 consumption of reduction of intercalated CO32− and a trace of Ni2+ out of layers, respectively.31 A small reduction peak centered at 370 °C may be due to the Ni clusters with weak interaction with surrounding Ni(Al)Ox phases at the initial stage of the reduction process, and a broad peak ranging from 350 to 700 °C, including a maximum peak at 450 °C and a shoulder at 550 °C, is assigned to the presence of Ni cations in Ni(Al)Ox with different environment in terms of interaction with aluminum. For the series of Ni catalyst samples reduced at different temperatures, the maximum peak on a series of TPR profiles shifts to higher temperature as the reduction temperature raises. Simultaneously, the peak intensity decreases gradually and Ni2+ can be completely reduced at 700 °C. The percentage of metallic Ni of the Ni catalyst samples, determined by H2-TPR, increases with raising the reduction temperature (Table 1). Although the 350 °C reduced sample has a small average size of Ni clusters confined in the Ni(Al)Ox matrix, it delivers a very low DMT conversion of 24.8% since this sample was partly reduced. Likewise, reduction at 400 °C is not appropriate because the degree of metallic Ni is insufficient to attain high DMT conversion. As expected, raising the reduction temperature to 600 °C guarantees a sufficient amount of active metallic Ni sites for DMT hydrogenation. If the DMT hydrogenation activity is decided solely by the amounts of metallic nickel, the NiAl-700 and NiAl-600 samples should exhibit the similar activity. However, the former presents a descendent reaction activity, with a DMT conversion of 73.2%. Temperature-programmed desorption of hydrogen (H2TPD) test was carried out for explaining why the NiAl-600 sample possesses the optimal catalytic performance under the employed reaction conditions (Figure 6b). The NiAl-600 sample shows two domains of H2 desorption peaks. In the first domain, one broad asymmetric peak around 200 °C, accompanied by several subsidiary peaks at 180, 230, and 300 °C, is observed. The appearance of these low temperature peaks are generally ascribed to H atoms which are weakly and strongly linked to Ni metal particles and indicates the exposed fraction of Ni atoms.52 In the second domain, a small peak appeared at 700 °C is attributed to much more strongly adsorbed hydrogen, probably on the support as spillover species.53 Only a sharp symmetric peak around 200 °C is found

4. CONCLUSIONS In summary, we report a layered precursor technique to fabricate a double-confined nickel catalyst, Ni/Ni(Al)Ox/AlOx, with enhanced catalytic performance in hydrogenation of DMT to DMCD. Cs-corrected HAADF-STEM, along with other complementary characterization techniques, was used to investigate the microstructure evolution of the NiAl-LDH precursor toward confined Ni catalysts. Direct structure imaging of Ni and Al species revealed that subnanometer Ni0 clusters nucleate initially in the weakly crystalline Ni(Al)Ox matrix. Subsequently, Ni0 clusters grow at the expense of the surrounding Ni(Al)Ox, accompanied by consecutive transfer of Al3+ from the central part to the near-surface. A mechanism of reverse Ni2+ and Al3+ ions migration is proposed to interpret the transformation process of NiAl-LDH during H2 reduction. The Ni(Al)Ox interfacial shell is proposed to provide a strong connection with metallic Ni and the amorphous alumina network, improving the hydrogen adsorption capacity of the double-confined Ni catalyst and consequently the catalytic hydrogenation activity. The present exploration study of the transformation process of the LDH precursor to a systematic set of confined Ni metal catalysts during H2 reduction with a display of various catalytic hydrogenation activity shows a direct correlation between Ni metal structure and catalytic performance. G


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b02553. Experimental details and additional data (Figures S1− S9) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86 10 64425105. Fax: +86 10 64425385. E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 21376019), Fundamental Research Funds for the Central Universities (No. YS1406), and the Beijing Engineering Center for Hierarchical Catalysts.

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REFERENCES

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Double-Confined Nickel Nanocatalyst Derived from Layered Double

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