Template Mediated Ni(II) Dispersion in Mesoporous SiO2 for

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Template Mediated Ni(II) Dispersion in Mesoporous SiO2 for Preparation of Highly Dispersed Ni Catalysts: Influence of Template Type Xin Ning, Yiyuan Lu, Heyun Fu, Haiqin Wan, Zhaoyi Xu, and Shourong Zheng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 12 May 2017 Downloaded from http://pubs.acs.org on May 15, 2017

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ACS Applied Materials & Interfaces

Template Mediated Ni(II) Dispersion in Mesoporous SiO2 for Preparation of Highly Dispersed Ni Catalysts: Influence of Template Type Xin Ning, Yiyuan Lu, Heyun Fu, Haiqin Wan, Zhaoyi Xu, Shourong Zheng* State Key Laboratory of Pollution Control and Resource Reuse, Jiangsu Key Laboratory of Vehicle Emissions Control, School of the Environment, Nanjing University, Nanjing 210023, PR China

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ABSTRACT Supported Ni catalysts on three mesoporous SiO2 supports (i.e., SBA-15, MCM-41 and HMS) were prepared using a solid state reaction between Ni(NO3)2 and organic template occluded mesoporous SiO2. For comparison, supported Ni catalysts on mesoporous SiO2 synthesized by the conventional impregnation method were also included. The catalysts were characterized by scanning electron microscopy, X-ray diffraction, UV-Vis diffuse reflectance spectroscopy, N2 adsorption, X-ray photoelectron spectroscopy, H2 temperature-programmed reduction, transmission electron microscopy, and transmission electron microscopy-energy dispersive X-ray. The catalytic properties of the catalysts were evaluated using gas phase catalytic hydrodechlorination of 1,2-dichloroethane. The results showed that upon grinding Ni(NO3)2 with template occluded mesoporous SiO2 strong coordination between Ni2+ and dodecylamine was identified in the Ni(NO3)2-HMS system. Additionally, the results of H2 temperature-programmed reduction revealed that NiO in calcined NiO/HMS was reduced at higher temperature than those in calcined NiO/SBA-15 and NiO/MCM-41, reflecting the presence of a strong interaction between NiO and mesoporous SiO2 in NiO/HMS. Consistently, the average particle sizes of metallic Ni were found to be 2.7, 3.4, and 9.6 nm in H2 reduced Ni/HMS, Ni/SBA-15 and Ni/MCM-41, respectively, indicative of a much higher Ni dispersion in Ni/HMS. For the catalytic hydrodechlorination of 1,2-dichloroethane, Ni/MCM-41 synthesized by the solid state reaction method exhibited a similar catalytic activity to that prepared by the impregnation method, while higher catalytic activities were observed on Ni/HMS and Ni/SBA-15 than their counterparts prepared by the impregnation method. Furthermore, a higher conversion was identified on Ni/HMS than on Ni/SBA-15 and Ni/MCM-41, highlighting the

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importance of template type for the preparation of high dispersed metal catalysts on mesoporous SiO2. KEYWORDS: supported Ni catalysts; solid state reaction; template occluded mesoporous SiO2; gas phase catalytic hydrodechlorination; 1,2-dichloroethane

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INTRODUCTION Supported

Ni

catalysts

have

found

wide

applications

in

catalytic

hydrogenation,

hydrodechlorination, hydrodeoxygenation, CO2 methanation, H2 generation and CO2 reforming of methane.1-6 Mesoporous SiO2 is usually used as the support for Ni based catalysts, which is related to its high surface area, stable structure and ordered pores. Accordingly, a variety of methods have been explored to prepare Ni catalysts supported on mesoporous SiO2. In principle, it is essential to prepare highly dispersed metal precursors (e.g. metal salts) on supports in order to eventually obtain supported metal catalysts with high metal dispersions. However, supported Ni catalysts synthesized by the conventional impregnation method normally have large Ni particle sizes and low metal dispersions.7 Thus, numerous preparation methods have been attempted to enhance the dispersions of metals and metal oxides in mesoporous SiO2. For example, Li et al.8 used chemical vapor deposition (CVD) with NiCP2 as the metal precursor to prepare highly dispersed Ni nanoparticles in the mesoporous supports. Kim et al.9 obtained mesoporous SiO2 supported Ni with precisely controlled Ni deposition amount using atomic layer deposition. Kaydouh et al.10 used two solvents method to prepared Ni/SBA-15 with high NiO dispersion. To further enhance the interaction between support and metal salts, Aziz et al.11 used amine functionalized mesoporous SiO2 as the support, and high NiO dispersion was obtained. Alternatively, other agents, such as organic surfactants and inorganic NH3·H2O were used to facilitate the dispersion of Ni(NO3)2 into the mesopores.12,13 The loading of metal oxides into mesoporous SiO2 was usually involved in multiple steps, making catalyst preparation energy and time consuming. To overcome the disadvantages, Wang et

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al.14 first developed a facile solid state grinding method to prepare supported metal oxides using template occluded mesoporous SBA-15 as the precursor, and they attributed the high dispersion of metal oxides to the strong adhesion energy of the metal salt to the silica wall. Very recently, Gaudin et. al.15 reported a well dispersed CuO into the mesoporous channels of SBA-15 via the solid state reaction. Consistently, other metal oxide catalysts supported on mesoporous SiO2 were successfully prepared using the solid state reaction methods.16 Notably, a variety of mesoporous SiO2 have so far been prepared using different organic templates. The interaction between organic templates and Ni2+ is expected to differ with template type, likely resulting in a marked impact on the dispersion of Ni(II) in mesoporous SiO2, and thus varied structural properties and catalytic performances of Ni catalysts supported on mesoporous SiO2. However, systematical studies about the influence of template type on NiO dispersion in mesoporous SiO2 have not been conducted thus far. In this study, three mesoporous SiO2 (i.e., SBA-15, MCM-41 and HMS) samples prepared using different organic templates were used as the supports, and Ni catalysts supported on three mesoporous SiO2 were prepared by solid state grinding of Ni(NO3)2 with template occluded mesoporous SiO2. For comparison purposes, we used the conventional impregnation method to synthesize mesoporous SiO2 supported nickel catalysts with similar Ni loading amounts. The catalysts were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), infrared (IR) spectroscopy, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), N2 adsorption, transmission electron microscopy-energy dispersive X-ray (TEM-EDX), and H2 temperature-programmed reduction (H2-TPR). Volatile chlorinated organic compounds (e.g., 1,2-dichloroethane) with strong mutagenic and carcinogenic activities are

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generally considered to be highly toxic,17 and catalytic hydrodechlorination has been recognized as one of the most effective methods for the abatement of pollution from the volatile chlorinated organic compounds. The Ni based catalysts have been commonly used in the gas-phase hydrodechlorination due to their low cost compared to noble metal catalysts. Additionally, the catalytic hydrodechlorination of 1,2-dichloroethane was considered as a structure sensitive reaction, in which the catalytic performance of the catalysts could be well correlated to the structural properties of the catalysts.18,19 Hence, the gas phase catalytic hydrodechlorination of 1,2-dichloroethane was used as the model reaction to evaluate the catalytic performances of the catalysts. EXPERIMENTAL SECTION Synthesis of SBA-15. Mesoporous silica (SBA-15) was prepared using tetraethoxysilane (TEOS, 98%, Sinopharm Chemical Reagent Co., Ltd.) as the silica source and the triblock copolymer, EO20PO70EO20 (Pluronic P123, Aldrich) as the template.20 Specifically, 8.0 g of P123 was added to 300 mL of 1.6 M HCl at 40 oC until completely dissolved, and 18.0 g of TEOS was then added. The mixture was further stirred at 40 oC for 24 h, which was charged into a Teflon-lined stainless autoclave and heated at 100 oC for 48 h. After the resulted precipitate was collected by filtration, the cake was washed with ethanol and distilled water, and dried in an oven at 80 oC for 10 h. Synthesis of HMS. Hexagonal mesoporous silica (HMS) was synthesized using dodecylamine (DDA, 98%, Aladdin) as structure directing agent.21 Briefly, a mixture with a molar ratio of 1.0 tetraethoxysilane (TEOS): 0.27 DDA: 6.5 ethanol: 36 H2O was stirring at 40 oC for 24 h. The resultant product was separated and washed with distilled water until a neutral pH was obtained, and

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then dried at 50 oC overnight. Synthesis

of

MCM-41.

Mesostructured

silica

MCM-41

was

synthesized

using

cetyltrimethylammonium bromide (CTAB, Nanjing Chemical Co.) as the template.22 Typically, 12 g of CTBA, 600 mL of deionized water and 48 g of 25% NH3·H2O were mixed under stirring, to which 50 mL of TEOS was added dropwise. The mixture was subsequently stirred vigorously at 25 o

C for 12 h, and the solid product was isolated by filtration, followed by washing thoroughly with

ethanol and distilled water, and drying at 80 oC for 12 h. Synthesis of Ni/SiO2 Catalysts. Supported Ni catalysts were prepared using a direct solid state reaction between Ni(NO3)2 and mesoporous SiO2 occluded with template. 0.168 g of Ni(NO3)2⋅6H2O and 1 g of template occluded SBA-15 or 0.125 g of Ni(NO3)2⋅6H2O and 1 g of template occluded HMS/MCM-41 was repeatedly ground in an agate mortar at room temperature for 30 min. The mixture was then calcined at 550 oC for 6 h in air, with a ramping rate of 1 oC min-1. The ground mixture without calcination is denoted as Ni/T-SBA-15, Ni/T-HMS, or Ni/T-MCM-41, and the calcined samples are denoted as NiO/SBA-15, NiO/HMS, and NiO/MCM-41. For comparison, the supported nickel catalysts were also prepared by the conventional impregnation method using desired amount of Ni(NO3)2 solution to impregnate calcined mesoporous SiO2. After drying at 120 oC for 2 h, the samples were calcined in air at 550 oC for 6 h. The impregnated catalysts are referred to as im-NiO/SBA-15, im-NiO/HMS, or im-NiO/MCM-41. Before characterization of the reduced catalysts, the catalyst samples were treated by passivation after H2 reduction. Briefly, the calcined sample was loaded in a quartz tube, heated in a H2 flow (40 mL min-1) to 500 oC at ramping rate of 10 oC min-1 and the temperature was held at 500 oC for 3 h.

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After H2 reduction, the sample was purged with N2 (40 mL min-1) for 1 h, cooled to room temperature, and passivated in an 2vol% O2/N2 flow (100 mL min-1) for 1 h. The reduced samples are denoted as Ni/SBA-15, Ni/HMS or Ni/MCM-41. Material Characterizations. The X-ray diffraction (XRD) patterns of the samples were obtained on a Rigaku D/max-RA powder diffraction-meter (Rigaku, Tokyo, Japan). The morphologies of mesoporous SiO2 were observed using scanning electron microscope (S-3400N II,

Hitachi, Japan). The infrared (IR) spectra of the catalysts were collected on a Nexus 870 Spectrometer (Nicolet, USA) with a resolution of 4 cm-1. Ni contents in mesostructured silica were measured by inductively coupled plasma-atomic emission spectrometry (ICP-AES, J-A1100, Jarrell-Ash, USA). TEM measurements of the passivated samples were conducted with a JEM-2100 transmission electron microscope (JEOL, Japan). The elemental distributions of the passivated samples were characterized by transmission electron microscopy-energy dispersive X-ray (TEM-EDX, FEI Tecnai F20, USA). Surface analysis of the passivated catalysts was conducted with an X-ray photoelectron spectrometer (ESCALAB 250, ThermoScientific, USA) equipped with a monochromatized Al Ka X-ray source (hv = 1486.6 eV). N2 adsorption-desorption isotherms of the catalysts were carried out on a Micromeritics ASAP 2200 instrument (Micromeritics Instrument Co., Norcross, GA). The H2-TPR of the calcined samples was carried out on AMI-300 (Altamira Instruments, USA). Prior to analysis, 100 mg of the sample was pressed into wafers, sieved to particle sizes of 100-200 um and loaded into a U-tube reactor. The sample was heated for 30 min at 550 oC in a pure Ar stream (30 mL min-1), followed by cooling to 50 oC, and then heating to 800 oC at a ramping rate of 10 oC min-1 in 10 vol% H2 in Ar (30 mL min-1). The H2 consumption amount

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was determined using an on-line GC equipped with a thermal conduction detector (TCD), and the H2 consumption peaks were normalized to the respective sample mass. CO adsorption was followed using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) on a TENSOR 27 Fourier transform infrared (FT-IR) spectrometer (Bruker, US) equipped with a MCT detector cooled by liquid nitrogen. The DRIFTS cell (Praying Mantis, Harrick) had a ZnSe window and a heating cartridge. About 30 mg of fine catalyst was placed in a sample holder and pre-reduced in-situ at 500 oC for 2 h using a H2 stream (40 mL min-1). After cooling to room temperature, a background spectrum was recorded and the sample was saturated with a 10 vol.% CO/He stream for 30 min. Then the sample was purged with a He stream for 30 min to sweep the physical adsorption, and IR spectra were recorded by scan of 32 with a resolution of 4 cm-1. Gas Phase Catalytic HDC of 1,2-dichloroethane. The hydrodechlorination (HDC) of 1,2-dichloroethane was carried out at atomospheric pressure in a continuous fixed-bed tubular reactor. In brief, 100 mg of catalyst sieved to 20-40 mesh was loaded between two layers of quartz wool. Prior to the catalytic reaction, the catalyst was activated in a H2 flow (50 mL min-1) at 500 oC for 3 h. After gradually cooling to 300 oC, 1,2-dichloroethane was injected at 0.059 mL h-1 into a mixed gas flow (42 mL min-1) with an infusion pump controlled by a microprocessor. The reaction gas was composed of 36800 ppm H2, 7300 ppm 1,2-dichloroethane, and a balance of He. An online GC with a flame-ionization detector (FID) was used to quantitatively determine the reaction products. RESULTS AND DISCUSSIONS Structural and Chemical Characterizations. The morphologies of template included

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mesoporous SiO2 were observed using SEM, and Figure S1 (Supporting information) shows the SEM images of the samples. The three samples were µm-sized particles. SBA-15 consisted of ropelike domains of about 1.0 µm in length. The HMS sample had spherical particles of 0.5-1.5 µm. MCM-41 displayed aggregated particles of 1.0 µm. In order to characterize the mesoporous structure, the small-angle XRD patterns of the ground samples consisting of template occluded mesoporous SiO2 and Ni(NO3)2, and their calcined counterparts are displayed in Figure 1a. For comparison, the small-angle XRD patterns of the calcined supports are also included. For Ni/T-SBA-15, three strong diffraction peaks were observed with 2θ at 0.89°, 1.55°, and 1.77°, indexed as (100), (110) and (200) diffractions of SBA-15, reflecting the presence of typical well-ordered two dimensional hexagonal mesostructured phase.13 After calcinations at 550 oC, the diffraction peaks characteristic of the hexagonal mesostructure of SBA-15 were visible, but shifted to slightly larger 2θ angles due to the structural shrinkage during the calcination process.23,24 As for Ni/T-HMS, a distinct peak was observed with 2θ at 2 o, indexed as (100) diffraction of HMS, indicative of the presence of typical wormhole motif mesostructured HMS phase.25 Additionally, calcinations at 550 oC led to slight shift of 2θ to 2.12 o. In parallel, the XRD pattern of Ni/T-MCM-41 gave diffraction peaks at 2.18°, 3.78°, and 4.44°, while the diffraction peaks of calcined NiO/MCM-41 were identified at 2.34°, 4.04°, and 4.7°, larger than those of the ground mixture.26 Additionally, the XRD patterns of the calcined samples were found to be almost identical to their calcined supports, indicating that the mesoporous structures of the supports remained intact during NiO loading and calcination processes. Figure 2 displays the wide-angle XRD patterns of the calcined and passivated samples. For all

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samples, broad peaks with 2θ around 22o were identified, characteristic of amorphous SiO2.27 For NiO/MCM-41, diffraction peaks centered at 37.2º, 43.3°, and 62.9° were clearly observed, assigned to cubic NiO.28,29 Consistently, in the XRD pattern of passivated Ni/MCM-41 diffraction peaks characteristic of metallic Ni were observed at 44.4, 51.9 and 76.4o,30,31 reflecting the low dispersions of NiO and metallic Ni in NiO/MCM-41 and passivated Ni/MCM-41. As for calcined NiO/SBA-15, very weak diffraction peaks which are assigned to NiO phase were observed at 43.3 and 62.9 o. In parallel, weak diffraction peaks assigned to metallic Ni were identified in the passivated sample. In the XRD pattern of calcined NiO/HMS, diffraction peaks characteristic of NiO were not detected, but weak diffraction peaks attributed to metallic Ni were observed in the passivated Ni/HMS, reflecting very high dispersions of NiO and metallic Ni in the calcined and passivated Ni catalysts supported on HMS. Figure 3 and Figure S2 (Supporting information) show the IR spectra of the ground and calcined samples. For Ni/T-HMS, the IR bands around 3000 cm-1 were observed, attributed to asymmetric and symmetric stretching vibrations of C-H bonds, reflecting the occlusion of organic template in mesoporous SiO2. The IR peaks around 3500 and 1640 cm-1 was characteristic of hydrated silanol groups and the bending vibration of surface hydroxide, respectively. The band at 950 and 460 cm-1 are assigned to Si-O stretching and O-Si-O bending vibration mode of surface silanol groups. The bands at 1080 and 800 cm-1 are characteristic of Si-O asymmetric and symmetric stretching vibration from SiO2 framework. The band of asymmetric stretching vibration of N-O from nitrate at 1380 cm-1 was indicative of the presence of Ni(NO3)2. Additionally, a small IR band was identified at 1500 cm-1 from Ni/T-HMS, ascribed to the bending vibration of N-H from DDA. As for calcined

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NiO/HMS, the IR bands from SiO2 framework and surface hydroxides were still visible. However, the IR bands around 3000 cm-1 from organic template and 1380 cm-1 from nitrate disappeared, indicative of the effective decomposition of both organic template and Ni(NO3)2. Similar results were also obtained from the IR spectra of Ni/T-SBA-15 and Ni/T-CM-41 (see Figure S2). The XPS spectra of the passivated samples are presented in Figure S3 (Supporting information). The samples gave similar XPS spectra, in which the binding energies of Ni core line 2p3/2 were observed at 856.5 eV with a satellite at 862 eV, characteristic of oxidized Ni. The absence of metallic Ni in the samples was likely due to oxidation of Ni during the passivation process. N2 adsorption-desorption isotherms and pore size distributions of the ground and calcined catalysts are compiled in Figure S4 (Supporting Information). For comparison, calcined mesoporous SiO2 samples are also included. In comparison with calcined mesoporous SiO2 samples, all ground samples displayed much lower N2 adsorption amounts, reflecting the occlusion of organic templates in the mesopores of the samples. Upon calcination of the ground samples in air, typical capillary condensation was identified at relative pressure around 0.25-0.35, 0.3-0.58, and 0.44-0.9 for NiO/MCM-41, NiO/HMS, and NiO/SBA-15, respectively, suggesting the presence of characteristic mesopores in the samples. Additionally, NiO/MCM-41, NiO/HMS, and NiO/SBA-15 exhibited narrow pore size distributions with the most probable pore diameters centered at 2.8, 2.8, and 5.7 nm (see Figure S4b), respectively, which were nearly identical to those of calcined MCM-41, HMS, and SBA-15, reflecting that NiO loading did not markedly affect the structures of mesoporous SiO2 samples. Figure 4 presents the UV-Vis diffuse reflection spectra of the ground samples. To study the

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possible interaction between Ni2+ and organic template, UV-Dis spectrum of Ni(NO3)2 was also determined (see Figure S5a). Three very strong absorbance peaks were observed at 292, 388 and 678 nm on Ni(NO3)2, attributed to the charge transfer from nitrate and the d-d transition of octahedral Ni2+ species, respectively.32-35 For Ni/T-SBA-15 and Ni/T-MCM-41, similar UV-Vis spectra were observed with very weak absorbance peaks at 388, and around 600-800 nm, assigned to the absorbance from Ni(NO3)2. As for Ni/T-HMS, however, besides weak absorbance peaks around 388 and 600-800 nm, a very strong absorbance peak was identified at 309 nm. Notably, the strong absorbance peak appeared in higher wavenumber as compared with that of nitrate. To further exclude the strong absorbance from nitrate, UV-Vis spectra of the ground mixture consisting of NiCl2 and template occluded HMS is presented in Figure S5b (Supporting Information), in which the very strong absorbance peak at 309 nm was again clearly visible. Notably, the UV absorbance around 300 nm was absent for NiCl2 (see Figure S5a in the Supporting Information). Cattaneo et al.36 tested the structure of Ni-based catalyst in the presence of ethylenediamine and attributed the strong UV absorbance at 300-400 nm to Ni-amine complexes. In parallel, similar UV absorbance was also observed from other Ni-amine complexes.37-39 Hence, the strong absorbance at 309 nm in Ni/T-HMS could be assigned to the coordination between Ni2+ and amino-groups from DDA template. Accordingly, the strong coordination interaction may significantly impact the dispersion of NiO in HMS during the calcinations process. The UV-Vis spectra of the calcined samples are compared in Figure 5a, in which strong bands in the region of 240-280 nm from NiO particles were clearly observed. However, the absorbance threshold of NiO varied with the samples. The band gap (Eg) of NiO could be further calculated as follows,40

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(Ahv)2 = B(hv − Eg)

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(1)

where A is the absorbance intensity at light frequency of v, B is the absorbance constant, and Eg is the band gap energy, respectively. Figure 5b displays the plot of (Ahν)2 versus hν for the calcined samples. By extrapolating the linear curve to hv axis, the band gap was calculated to be 3.76, 4.22, and 4.27 eV for NiO/MCM-41, NiO/SBA-15 and NiO/HMS, respectively, indicative of an order of NiO/MCM-41< NiO/SBA-15 < NiO/HMS. The higher Eg of NiO in NiO/HMS was likely due to its more marked quantum size effect, resulting from the effective dispersion of Ni(II) due to the strong interaction between Ni(II) and organic template, and thus high NiO dispersion in NiO/HMS.32,41 The results can be verified by TEM observation. Figure 6 shows the TEM images and size distributions of Ni particles supported on the mesoporous SiO2. For all samples, the characteristic mesostructures of SBA-15, MCM-41 and HMS were visible, and metallic Ni particles could be clearly differentiated. As for Ni/HMS and Ni/SBA-15, fine Ni particle sizes were identified, ranging from 1.0 to 8.0 nm, while Ni particles in Ni/MCM-41 were found to be 5-14 nm, much larger than those of Ni/SBA-15 and Ni/HMS, confirming the higher Ni dispersions in Ni/SBA-15 and Ni/HMS than that in Ni/MCM-41. According to surface area weighted average diameter, the Ni particle sizes of the samples were further calculated. 42

d s = ∑ ni di3 / ∑ ni d i2

(2)

Where ni is the number of counted Ni particles of diameter di with the total number of particles exceeds 150 (∑ni > 150). The results are given in Table 1. The average particle sizes of Ni were calculated to be 2.7, 3.4

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and 9.6 nm for Ni(4.4)/HMS, Ni(4.7)/SBA-15 and Ni(4.6)/MCM-41, respectively, again confirming a higher Ni dispersion in Ni(4.4)/HMS. The metal dispersions of Ni in the passivated catalysts were further characterized using TEM-EDX and the images of Ni and Si mapping are compiled in Figure 7. For Ni/SBA-15 and Ni/HMS, nickel elements were evenly distributed, perfectly coordinating to the distribution of Si elements, reflecting that Ni species were homogeneously located inside SiO2 mesopores. In contrast, Ni element in Ni/MCM-41 was in the form of agglomerates, significantly differing from those in Ni/SBA-15 and Ni/HMS. The possible interaction between NiO and mesoporous SiO2 in the calcined samples could be further verified by H2-TPR of the calcined catalysts, and the results are displayed in Figure 8. The TPR profile of NiO gave a sharp H2 consumption peak at 320 oC, ascribed to the H2 reduction of crystalline NiO.43 Similarly, a strong H2 consumption peak at 320 oC was observed on calcined NiO/MCM-41, suggesting the presence of crystalline NiO without strong interaction with SiO2 wall in NiO/MCM-41. In contrast, besides a strong H2 consumption peak at 320 oC a weak and broad peak was observed in a temperature range of 400-700 oC in the TPR profile of calcined NiO/SBA-15, attributed to the reduction of NiO having a strong interaction with SBA-15.44,45 The ratio of the reduction peak at high temperature to that at 320 oC was calculated to be 1:3, suggesting that only a small portion of NiO had a strong interaction with SiO2 wall in calcined NiO/SBA-15. As for NiO/HMS, a small peak at 320 oC and a strong and broad peak in the high temperature range of 400-700 oC were observed. The ratio of NiO at high reduction temperature to that at 320 oC was 5.5:1, indicative of the dominance of NiO having a strong interaction with SiO2 wall in NiO/HMS.

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The DRIFTS of CO adsorption on reduced samples are presented in Figure S6 (Supporting Information). For reduced Ni/MCM-41, no evident IR band was observed after He purging, likely due to the very low CO absorption on large metallic Ni particles.46 On the contrary, clear IR bands at 2055 and 2065 cm-1 were shown on Ni/SBA-15 and Ni/HMS, attributed to linearly bonded CO adsorption on metallic Ni.46,47,48 Additionally, much stronger IR bands were identified on Ni/SBA-15 than on Ni/HMS, which could be ascribed to the higher content of metallic Ni of Ni/SBA-15. 47 The more effective reduction of NiO on Ni/SBA-15 under identical reduction conditions was also in good agreement with the H2-TPR results. The very different Ni dispersions of the catalysts suggested that effective dispersion of Ni(NO3)2 into the mesopores is strongly related to the interaction between Ni(NO3)2 and organic template. For Ni/T-HMS, Ni(II) was capable of forming Ni(II)-DDA complex, displaying a very strong interaction with DDA template. Hence, Ni(II) could effectively insert into the confined space between template and SiO2 wall, eventually leading to the formation NiO species with a strong interaction with SiO2 wall during the calcination process, as reflected by the results of UV-Dis spectra and H2-TPR. In parallel, the weak intermolecular interaction between Ni(II) and organic template P123 in Ni/T-SBA-15 also favored the effective dispersion of Ni(II) into the interspace between organic template and SiO2 wall, leading to an effective Ni dispersion. On the contrary, strong repulsive interaction between Ni(II) and organic template was expected in Ni/T-MCM-41 due to the anion exchange nature of CTBA template, which strongly suppressed the dispersion of Ni(II) into the interspace of MCM-41 and thus resulted in a very low Ni dispersion. It may be argued that the very different properties of NiO were also caused by the pore structures. Notably, MCM-41 had nearly

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identical pore diameter to that of HMS, but much larger NiO particles and weak interaction between NiO and SiO2 wall were identified in NiO/MCM-41, confirming the crucial role of DDA for effective Ni2+ dispersion in HMS. Gas Phase Catalytic HDC of 1,2-dichloroethane. Figure 9 shows the hydrodechlorination of 1,2-dichloroethane on the catalysts as a function of time on stream (TOS). Besides ethylene and chloroethylene, trace ethane and other dechlorinated products (e.g. methane and chloroethane) were also detected. The catalysts exhibited very high ethylene selectivities (higher than 95%) but varied 1,2-dichloroethane conversions. The almost identical selectivities from the three different catalysts suggested that very high ethylene selectivity could be achieved on Ni catalysts supported on mesoporous SiO2 irrespective of metallic Ni with different structural properties. Additionally, the catalysts had very high catalytic stability, implying that Ni/SiO2 could be used as stable and selective catalysts for the HDC of 1,2-dichloroethane. For the catalysts synthesized by the impregnation method, the conversions of 1,2-dichloroethane at TOS of 10 h were found to be 39.2%, 40.2% and 38.1% on im-Ni(4.9)/HMS, im-Ni(4.9)/MCM-41 and im-Ni(4.6)/SBA-15, respectively (see Figure S7), indicative of nearly identical catalytic activities of the catalysts despite of different supports with varied mesopore structures. On the contrary, the activities of the catalysts prepared by the solid state reaction method differed with the supports. At 10 h of TOS, the conversions of 1,2-dichloroethane were 73%, 62% and 42% on Ni/HMS, Ni/SBA-15, and Ni/MCM-41, respectively. More clear comparison could be acquired by calculating turnover frequency (TOF) values of the catalysts, and the TOF values of the catalysts were determined to be 12.0, 9.6, and 6.6 h-1, for Ni/HMS, Ni/SBA-15 and Ni/MCM-41, respectively,

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reflecting a catalytic activity order of Ni/HMS>Ni/SBA-15>Ni/MCM-41. The much different catalytic performances for the catalytic HDC of 1,2-dichloroethane can be explained in terms of metal-support interaction and Ni dispersion in supported Ni catalysts. In comparison with Ni/SBA-15 and Ni/MCM-41, a higher Ni dispersion and smaller Ni particle size is achieved on Ni/HMS due to effective dispersion of Ni(II) mediated by template DDA, likely giving rise to a larger number of active sites than those of Ni/SBA-15 and Ni/MCM-41. Furthermore, previous studies have shown that the spiltover hydrogen plays a critical role in the catalytic HDC process on supported Ni catalysts.49,50 In principle, spiltover hydrogen is formed via H2 dissociation on metallic Ni surface into atomic H, followed by spillover to the support surface.51 Hence, the formation of spiltover hydrogen occurs on the metal-support interface, which closely correlates to the metal-support interaction.52,53 Compared with large Ni particles, highly dispersed fine Ni particles with strong metal-support interaction are capable of making intimate contact with the support surface, favoring hydrogen spillover and consequently resulting in higher catalytic activity. In parallel, Chen et al.54 observed the high catalytic activity of the supported Ni catalyst with a strong metal-support interaction, facilitating the formation of reactive spiltover hydrogen. Consistently, Wojcieszak et al.55 reported that the intensity of the spillover effect increased with the decrease of metal particle. The H2-TPR results demonstrate that NiO with strong metal-support interactions dominates in NiO/HMS, resulting in small metallic Ni particles in Ni/HMS. Hence, a higher catalytic activity is obtained on Ni/HMS than those on Ni/SBA-15 and Ni/MCM-41. The used catalysts were characterized and Figure S8 shows the results. In comparison with fresh catalysts, very clear diffraction peaks assigned to NiO were identified in the used catalysts, likely

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due to the growth of Ni particles during the catalytic hydrodechlorination process. The results were also supported by TEM images of the used catalysts (results presented in Figure S9). As shown in the histograms of Ni particle size distributions of the used catalysts, the contents of large sized Ni particles increased as compared with the fresh catalysts (see Figure 6). As a result, the average Ni particle sizes were 4.9, 2.8, and 12.9 nm for Ni/SBA-15, Ni/HMS and Ni/MCM-41, respectively, larger than those of the fresh catalysts. In parallel, Ni growth was previously observed during the gas-phase catalytic hydrodechlorination.56,57 Accordingly, slow deactivation was identified on the three catalysts due to the growth of Ni particles with TOS of the catalytic hydrodechlorination reaction. CONCLUSIONS In the present study, Ni catalysts supported on HMS, SBA-15, and MCM-41 were prepared by solid state reactions between template occluded mesoporous SiO2 and Ni(NO3)2, and the hydrodechlorination of 1,2-dichloroethane was used to evaluate the catalytic performances of the catalysts. For Ni/MCM-41, a strong electrostatic expulsive interaction occurs between Ni(II) and CTBA, leading to suppressed dispersion of Ni(NO3)2 into the mesopores of MCM-41 and to the formation of aggregated Ni particles upon calcination. As for Ni/SBA-15, a weak interaction presents between template P123 and Ni(II), favoring the effective dispersion of Ni(NO3)2 into the mesopores. Accordingly, a portion of NiO exhibits strong interactions with SiO2 wall. On the contrary, DDA is capable of strongly coordinating with Ni(II), which effectively facilitates the dispersion of Ni(NO3)2 into the pores of HMS. As a result, highly dispersed NiO particles with strong interactions with SiO2 wall dominate in the calcined NiO/HMS sample. In comparison with

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supported Ni catalysts synthesized by the impregnation method, a similar catalytic activity is obtained on NiO/MCM-41 due to the presence of aggregated and large Ni particles, whereas much higher catalytic activities are achieved on Ni/HMS owning to its higher Ni dispersion and formation of spiltover hydrogen. The findings in the present study highlight the importance of template type for the preparation of mesoporous SiO2 supported Ni catalysts with high Ni dispersions. ASSOCIATED CONTENT Supporting Information. SEM of template included mesoporous silica oxide, IR spectra of ground and calcined samples of Ni/T-SBA-15, and Ni/T-MCM-41; XPS spectra of the passivated samples; N2 adsorption-desorption isotherms and pore size distributions of the ground and calcined sample; UV-Dis spectra of Ni(NO3)2, NiCl2 and the ground mixture consisting of NiCl2 and template occluded mesoporous SiO2; IR spectra of CO adsorption on the reduced samples; HDC of 1,2-dichloroethane over impregnated catalysts as a function of time on stream; Wide-angle XRD patterns of the used catalysts; TEM images and histograms of metallic Ni particle size distributions of the used catalysts. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ACKNOWLEDGEMENTS The financial supports from National Key Basic Research Program of China (2014CB441103), the Natural Science Foundation (21577056 and 21277066) and the Natural Science Foundation of Jiangsu Province (BK20150568 and BK20151381) is gratefully acknowledged.

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hydrodechlorination of 1,2-dichloroethane. Appl. Catal., B 2014, 147, 208-220.

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Table 1. Structural properties of the samples

Ni content Catalyst (wt.%)

a

Pore

Pore

volume

(nm)

diameter Ni

BET surface 2

-1

area (m g )

particle

sizes (nm)b

(cm3 g-1)

SBA-15

-

688.8

0.91

6.01

-

HMS

-

1138.7

1.05

3.01

-

MCM-41

-

1381.4

0.96

2.82

-

Ni/T-SBA-15

-

320.4

0.58

-

-

Ni/T-HMS

-

36.8

0.06

-

-

Ni/T-MCM-41

-

2.6

0.02

-

-

Ni/SBA-15

4.7

682.8

0.83

5.61

3.4

Ni/HMS

4.4

1075.1

0.98

3.01

2.7

Ni/MCM-41

4.6

1260.4

0.93

2.92

9.6

a

Determined by ICP.

b

Calculated from TEM.

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Figure captions: Figure 1: The small-angle XRD patterns of (a) the ground, calcined samples and (b) calcined supports. Figure 2: The wide-angle XRD patterns of the ground, calcined and H2 reduced samples supported on (a) SBA-15, (b) HMS, and (c) MCM-41. Figure 3: The IR spectra of the Ni/T-HMS and NiO/HMS. Figure 4: The UV-Vis diffuse reflection spectra of the ground sample consisting of Ni(NO3)2 and template occluded mesoporous SiO2. Figure 5: (a) The UV-Dis spectra of the calcined samples, (b) The plot of (Ahν)2 versus hν for the calcined samples. Figure 6: TEM images and histograms of metallic Ni particle size distributions of the passivated samples. Figure 7: EDX images of Si (red) and Ni (green) mapping in (a) Ni/SBA-15 (b) Ni/HMS and (c) Ni/MCM-41 (selected area). Figure 8: H2-TPR of the calcined samples. Figure 9: The HDC of 1,2-dichloroethane over (a) Ni/SBA-15 (b) Ni/HMS (c) Ni/MCM-41 as a function of time on stream. (●) Ethylene; (■) ethane; (◆) methane; (x) chloroethylene; (▲) others; and (△) conversion.

29



Figure 1: (a)

Intensity (a.u.)

Ni/T-SBA-15

NiO/SBA-15 Ni/T-HMS NiO/HMS Ni/T-MCM-41 NiO/MCM-41 1

2

3

4

5

2θ (degree)

(b)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

SBA-15 HMS

MCM-41

1

2

3 2θ (degree)

4

30


5

Page 30 of 40

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Figure 2:

(a)

Intensity (a.u.)

Ni/T-SBA-15

NiO/SBA-15

Ni/SBA-15

10

20

30

40

50

60

70

80

2θ (degree)

(b)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Ni/T-HMS

NiO/HMS

Ni/HMS

10

20

30

40

50

60

70

80

2θ (degree)

31



(c)

Ni/T-MCM-41

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

NiO/MCM-41

Ni/MCM-41

10

20

30

40

50

60

70

80

2θ (degree)

32


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Figure 3:

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


HMS

Ni/T-HMS

NiO/HMS

4000 3500 3000 2500 2000 1500 1000

500

Wavelength (cm-1)

33



Figure 4:

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Ni/T-HMS Ni/T-MCM-41 Ni/T-SBA-15 300

400

500

600

700

800

Wavelength (nm)

34


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Figure 5:

Absorbance (a.u.)

(a)

NiO/MCM-41 NiO/SBA-15 NiO/HMS

300

400

500

600

700

800

Wavelength (nm)

(b)

(Ahv)2(eV)2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


NiO/MCM-41 NiO/HMS NiO/SBA-15

1.5

2.0

2.5

3.0

3.5

4.0

4.5

hv (eV)

35


5.0


Figure 6:

80

Frequency (%)

Frequency (%)

80 d=3.4nm 60 40 20 0

d=2.7nm 60 40 20 0

1 2 3 4 5 6 7 8 9 10 Particle Diameter (nm)

1 2 3 4 5 6 7 8 9 10 Particle Diameter (nm)

Ni/SBA-15

Ni/HMS

80

Frequency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

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d=9.6nm 60 40 20 0 4

6 8 10 12 14 16 Particle Diameter (nm)

Ni/MCM-41

36


Page 37 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7:

a

Ni

b

O c

37



Figure 8:

NiO

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

NiO/MCM-41 NiO/SBA-15

NiO/HMS

200

300

400

500

600

700

800

Temperature (℃)

38


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Figure 9:

100

100

80

80

60

60

40

40

20

20

0

0 0

1

2

3

4

5

6

7

8

9

Conversion (%)

Selectivity (%)

(a)

10 11

Time (h)

100

100

80

80

60

60

40

40

20

20

0

0 0

1

2

3

4

5 6 7 Time (h)

8

39


9

10 11

Conversion（%）

(b)

Selectivity (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



100

100

80

80

60

60

40

40

20

20

0

0 0

1

2

3

4

5

6

7

8

Time (h)

40


9

10 11

Conversion (%)

(c)

Selectivity (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Template Mediated Ni(II) Dispersion in Mesoporous SiO2 for

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