Porous Silica

Article pubs.acs.org/Langmuir

Effect of a Swelling Agent on the Performance of Ni/Porous Silica Catalyst for CH4−CO2 Reforming Mohamad Hassan Amin,† Putla Sudarsanam,†,§ Matthew R. Field,‡ Jim Patel,∥ and Suresh K. Bhargava*,† †

Centre for Advanced Materials & Industrial Chemistry, School of Science, and ‡RMIT Microscopy & Microanalysis Facility, RMIT University, Melbourne, VIC 3001, Australia § Leibniz-Institut für Katalyse, Universität Rostock, Albert-Einstein Straße 29A, 18059 Rostock, Germany ∥ CSIRO Energy, Private Bag 10, Clayton South, Melbourne, VIC 3168, Australia S Supporting Information *

ABSTRACT: Hierarchical porous materials are of great interest in various industrial applications because of their potential to overcome the mass transport limitations typically encountered for single-mode porous materials. This report describes the synthesis of a hierarchical trimodal porous silica-based material using a 7.5 molar ratio of a relatively inexpensive nonionic surfactant template, triblock copolymer P123, EO20PO70EO20. The pore size distribution curve shows the presence of three types of pores with average diameters of ∼8, 25, and 89 nm. Electron microscope images confirm the existence of smaller ordered mesopores (first mode), larger ordered mesopores (second mode), and macropores (third mode). Ni nanoparticles dispersed on this trimodal porous silica produce a material that exhibited excellent catalytic performance for the CO2 reforming of CH4. This research provides new insights that will facilitate the development of trimodal porous silica (TMS) materials for a variety of applications. The results demonstrated that the presence of large pores (second and third mode pores) in TMS material increased the number of accessible active Ni sites, which led to the high activity observed for Ni/TMS catalyst.

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INTRODUCTION Because of increasing carbon dioxide (CO2) emission problems caused by extreme consumption of fossil fuels (natural gas, coal, and oil), the utilization of CO2 as a feedstock for the production of valuable products has become a topic of significant importance.1 Among the various strategies available currently, the catalytic CO2 (dry) reforming of methane (DRM) is a promising process that could potentially be used to convert CO2, along with another problematic greenhouse gas, CH4, into a valuable product, syngas (CO + H2), as shown in eq 1.2−5 CO2 + CH4 ⇌ 2CO + 2H 2

methane. Among them, nickel-based catalysts are highly preferred because of their high catalytic performance, low cost, and extensive availability compared to noble metals.6,9 However, the rapid deactivation of the Ni-based catalysts caused by metal oxidation, particle sintering, and coke deposition is a major drawback in DRM process. A variety of catalyst formulation modifications have been made to tackle the above problems, including the use of porous metal oxide supports and basic metal oxide supports, doping of Ni catalyst with alkali metals, and the addition of secondary metals to Ni.6 Among them, the immobilization of Ni nanoparticles within the structured pores of oxide supports is an effective strategy for dispersing and controlling the Ni particle size as well as for inhibiting the coke formation.10−12 Especially, mesoporous silica is a preferred support for Ni catalysts because of its high thermal stability and high surface area.13,14 However, the limited diffusion of reactants and products through confined porous structures is a major drawback for single-mode porous materials like mesoporous silica from the point of view of

(ΔH ° = 247.4 kJ/mol) (1)

The DRM process not only reduces the emissions of greenhouse gases (CO2 and CH4) but also provides an effective approach to utilizing natural gas resources.6 The syngas (CO + H2) can be used for the production of synthetic fuels by Fischer−Tropsch process, oxo synthesis, and also for the methanol synthesis, which can be eventually transformed into ether, olefins, aromatics, etc.7,8 Several noble-metal-based catalysts (Ru, Rh, and Pt) and nickel-based catalysts have been studied for CO2 reforming of © 2017 American Chemical Society

Received: August 8, 2017 Revised: September 12, 2017 Published: September 20, 2017 10632

DOI: 10.1021/acs.langmuir.7b02753 Langmuir 2017, 33, 10632−10644

Article

Langmuir practical applications.15,16 Another drawback is that the sintering resistance of the Ni/mesoporous SiO2 catalysts is not so obvious due to the weak Ni−SiO2 interaction.13,14 A variety of strategies have been developed to enhance the catalytic performance of Ni/mesoporous SiO2 catalysts for CO2 reforming of CH4, including the use of different Ni precursor,17 varying the preparation method,18,19 modifying the morphology of Ni/SiO2 catalyst,14 and doping of cerium into the mesoporous silica.20 In recent years, significant efforts have been directed toward the development of hierarchical porous materials incorporating different pore size regimes (micro, meso, and macro). Such materials have the potential for enhanced properties compared with analogous single-mode porous materials.15,21−28 The advantage of hierarchical porous materials in heterogeneous catalytic reactions is that micro/ mesopores can offer active sites for reactants, whereas macropores can provide a flexible route for reactants accessing the active sites, resulting in high yields of the products.24,29−33 In addition, the incorporation of hierarchical combinations of multiple-scale pores can enhance the surface area and pore volume of silica, which can enable functionalization of pore wall and strong interaction with various species.34 As a result, hierarchical porous silica materials have been applied in various heterogeneous catalytic applications, such as catalytic dibenzothiophene hydrodesulfurization,16 selective hydrogenation of styrene and 3-hexyn-1-ol,35 Fischer−Tropsch synthesis,36 oxidative removal of 4,6-dimethyldibenzothiophene,37 biodiesel synthesis,38 etc.39 In recent years, hierarchical porous materials have been employed as active catalyst supports in the CO2 reforming of CH4 to produce valuable syngas chemical feedstock.40,41 In CO2 reforming of CH4 reaction, both reactants (CO2 and CH4) and the syngas product (H2/CO) are gases. Hence, the accessibility of active sites to reactant species and the egress of the products can be governed by hierarchical porous SiO2. It is therefore possible that the application of high surface area hierarchical porous silica as a catalyst support not only controls the Ni particle size and minimizes the Ni0 sintering during the reaction but also controls the diffusion of reactants/products, hence improving the performance of Ni catalysts in the CO2 reforming of CH4. The main objective of this work is therefore to synthesize hierarchical porous silica material and testing its application as a catalyst support for the preparation of Ni catalyst. In this work, 1,3,5-trimethylbenzene (swelling agent), triblock copolymer P123 (template), and tetraethyl orthosilicate (silica precursor) are used for synthesizing hierarchical porous silica. It has been reported that the pore size of mesoporous silica can be expanded from 10 to 30 nm by varying the mass ratio of 1,3,5-trimethylbenzene/P123 from 0 to 2, respectively.21 In addition, the phase transition from hexagonal to mesocellular foam occurred at a 1,3,5-trimethylbenzene/P123 mass ratio of 0.2−0.3.42 However, hierarchical porous silica having multiple-scale pores is not yet synthesized using 1,3,5-trimethylbenzene and P123, which may be possible by varying their composition. Based on this concept, a thorough study has been undertaken by varying the molar ratio of 1,3,5-trimethylbenzene/P123 from 2.5 to 10.0 and investigated the consequent effect in modifying the porous structure of the silica. Hierarchical trimodal porous silica was obtained at 7.5 molar ratio of 1,3,5-trimethylbenzene/triblock copolymer P123, and hence, the catalytic performance Ni/ trimodal porous silica was tested for DRM reaction. The catalytic performance of Ni catalysts supported on single-mode

and non-ordered porous SiO2 materials in DRM were also compared with that of a Ni catalyst supported on hierarchical tri-modal porous silica. The morphology and porous structure of trimodal porous silica, as well as the physicochemical and textural properties of Ni/SiO2 catalysts, were investigated using small-angle X-ray scattering, N2 adsorption−desorption, highresolution TEM, TEM-EDX, and SEM techniques. Our experimental findings provide new insights into understanding the structure of hierarchical trimodal porous silica and its effect on the performance of Ni/SiO2 catalyst in the CO2 reforming of CH4.

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EXPERIMENTAL SECTION

Synthesis of Trimodal Porous Silica. A trimodal porous silica was synthesized using the triblock copolymer P123 (Aldrich, EO20PO70EO20, average molecular weight = 5800 g/mol) as a template, 1,3,5-trimethylbenzene (TMB, Aldrich, C9H12, molecular weight = 120.19 g/mol) as a swelling agent, and tetraethyl orthosilicate (TEOS, Aldrich, C8H20O4Si, molecular weight = 208.33 g/mol) as the silica source. The molar composition of the starting mixture was 0.0075 TMB:0.001 P123:8.5 deionized water and hydrochloric acid (pH ∼ 1.5−2). The molar ratio of TMB/P123 was 7.5. The mixture was magnetically stirred at room temperature for 2 h. 0.072 mol of TEOS was slowly added to the TMB/P123 mixture with vigorous stirring at 40 °C for 24 h. The resulting gel was transferred into a Teflon-lined autoclave. Hydrothermal treatment was performed at 100 °C for 24 h. The solid product was filtered off, washed with deionized water, dried at room temperature, and then calcined at 600 °C for 6 h exposed to air to remove the organic materials. Similarly, four more silica materials were prepared by using pure P123 and by varying the molar ratio of TMB/P123 from 2.5, 5.0, and 10.0 following an identical synthesis procedure to that reported above (only the amount of TMB was changed while using a fixed amount of P123 and TEOS for all the materials). Synthesis of Ni Catalyst Supported on Trimodal Porous Silica. A simple wet impregnation method was used to synthesize Ni catalysts supported on trimodal porous silica. In brief, the required amount of Ni(NO3)2·6H2O was dissolved in Milli-Q water with stirring. Finely powdered trimodal porous silica was then added to the above suspension. The excess water was removed by evaporation on a hot plate at ∼100 °C. The sample obtained was oven-dried at 100 °C for 12 h and finally calcined over a 4 h period in the air with heating to 600 °C with a ramp rate of 5 °C/min. The same synthetic procedure was used for the synthesis of Ni catalysts supported on silica materials synthesized using pure P123 as well as 2.5, 5.0, and 10.0 molar ratio of TMB/P123. The catalysts obtained are denoted as Ni/TMB (0), Ni/ TMB (2.5), Ni/TMB (5), and Ni/TMB (10), respectively. Characterization Techniques. Nitrogen adsorption−desorption isotherms and BET surface areas of the samples were determined on a Micromeritics ASAP 2020 instrument at −196 °C. Prior to analysis, the sample was degassed under vacuum for 30 min at ambient temperature followed by fast-mode degassing at 280 °C for 12 h. Small-angle X-ray scattering (SAXS) patterns were recorded using a Bruker D8 Advance X-ray diffractometer using Cu Kα1 radiation (λ = 0.15406 nm) with a voltage of 40 kV and a current of 40 mA. The diffractometer was equipped with a Nanostar Specifications Hi Star 2D area detector. Elemental analysis of the catalysts was carried out using an X-ray fluorescence Bruker S4 and the energy dispersive X-ray spectrometer (EDX) attached to a JEOL 2010 TEM operating at 200 kV. SEM studies were performed using an extreme high-resolution FEI Verios 460L field-emission scanning electron microscope (FESEM). TEM studies were performed using a JEM-2100F instrument equipped with a Gatan Orius SC1000 CCD camera. Measurements were taken with the TEM operating at an accelerating voltage of 200 kV. TGA analysis was conducted on a PerkinElmer Pyris1instrument. Catalytic Activity Tests. The catalysts were prereduced in situ in a mixed flow of H2 and He (10:40 mL min−1) at 700 °C for 2 h. The 10633


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Figure 1. (A) Pore size distribution curve, (B) N2 adsorption−desorption isotherm, and (C) small-angle X-ray scattering (SAXS) pattern of trimodal porous silica material (inset: radial intensity plot). catalytic test conditions used in this study are 700 °C, feed flow rate (CO2:CH4:He = 1:1:2 mL min−1, and gas hourly space velocity of 52000 mL g−1 h−1), catalyst particle size (within the particle size range of 500−710 μm), and catalyst mass (100 mg) and were similar to our previous studies,9,43−45 which have been shown to avoid any internal or external mass transfer limitations (based on the calculation of the Weisz criterion46). The product gas mixture was analyzed by an online gas chromatograph (PerkinElmer Clarus 580 GC) equipped with a silica-packed column and a TCD. Catalysts were also subjected to a reduction process to simulate that used in the in situ reductions of Ni/SiO2 catalysts before use in catalytic CO2 reforming, which is henceforward referred to as reduced catalysts. This process involved exposing catalysts to a mixed flow of 40:10 mL min−1 He and H2 for 2 h at 700 °C.

Table 1. Textural Properties of Silica Materials Synthesized by Varying the Molar Ratio of TMB/P123

RESULTS AND DISCUSSION Textural Characterization. Nitrogen Adsorption−Desorption. Figure 1A shows the pore size distribution of the silica material synthesized using a 7.5 molar ratio of TMB/ P123. Three distinctive peaks can be seen from the pore size distribution graph. This indicates the presence of three types of pores in the silica material (hereafter denoted as trimodal porous silica, TMS). The first peak appeared between 2.4 and 20.8 nm and had an average pore size of 8 nm (first mode), while the second peak appeared between 21.6 and 38.2 nm with an average pore size of 25 nm (second mode). The last peak was broad and started from 70.1 nm and ended at 124.9 nm, with an average pore size of 89 nm (third mode). Careful inspection of the peak areas in Figure 1A revealed that the TMS sample had an abundant first mode, followed by third mode and second mode pores. As shown in the Supporting Information (Figure S1), the silica materials synthesized using pure P123 as well 2.5 and 5.0 molar ratios of TMB/P123 exhibited only a single peak in the first mode region. Another interesting observation was that the broadness of the peaks present in the first mode region increased up to 7.5 molar ratio of TMB/P123. On the other hand, very low-intensity peaks were found for silica material synthesized using a 10.0 molar ratio of TMB/P123. The specific surface area, average pore size, and pore volume of the TMS material were found to be ∼599 m2 g−1, 22.66 nm, and 1.56 cm3 g−1, respectively (Table 1). All silica materials

exhibited type IV isotherms which were typical for mesoporous adsorbents, but the position and shape of the hysteresis loop were highly dependent on the molar ratio of TMB/P123 (Figure 1B and Figure S2). As can be seen in Figure S2, the capillary condensation step shifted to higher pressures up to a molar ratio of TMB/P123 of 7.5 and then shifted to lower pressures for silica materials synthesized using a molar ratio of TMB/P123 of 10.0. The shifting of the capillary condensation step to higher pressures indicated the formation of the second mode in the silica material.47 The silica materials synthesized using pure P123 and 2.5 molar ratio of TMB/P123 exhibited an H1 hysteresis loop, which is typical of a material with independent pores.48 These isotherms indicated the presence of uniformly sized mesopores, in line with the pore size distribution results (Figure S1).49 On the other hand, the silica material synthesized using 10.0 molar ratio of TMB/P123 exhibited an H4 hysteresis loop, which is often associated with narrow slit-like pores.48 The silica material synthesized using a 5.0 molar ratio of TMB/P123 exhibited H3 and H2 hysteresis loops at an intermediate relative pressure and high P/P0, respectively. This isotherm can be associated with the intrananoparticle mesopores.15,48 Contrary to the findings above, the TMS material exhibited three distinct adsorption and desorption steps, resulting in the formation of three well-defined hysteresis loops (Figure 1B). The first adsorption step at a relative pressure (P/P0) of 0.5 with an H3 hysteresis loop indicated capillary condensation of

molar ratio TMB/P123 = 0 (pure P123) TMB/P123 = 2.5 TMB/P123 = 5.0 TMB/P123 = 7.5 TMB/P123 = 10.0

BET surface areaa (m2/g)

average pore sizeb (nm)

pore volumec (cm3/g)

659

9.69

1.60

630 621 599 225

12.53 17.49 22.66 19.30

1.59 1.58 1.56 1.3

a

BET surface area. bDesorption average pore diameter (4V/A by BET). cSingle point desorption.

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Figure 2. XHR-SEM images of TMB/P123 (7.5) material.

Figure 3. High-resolution TEM images of TMS material.

N2 molecules in the cagelike mesopores of the silica.27 This observation supports the formation of a relatively narrow peak in the first mode range as shown in Figure 1A. The second adsorption step at P/P0 = 0.81 with an H2 hysteresis loop can be assigned to secondary interstitial multicellular pores, which resulted in the formation of second mode pores centered at 28

nm (Figure 1A). The third adsorption step at P/P0 = 0.87 with an H4 hysteresis loop corresponds to third mode pores of the silica material, as can be seen in the pore size distribution curve (Figure 1A). Interestingly, there was a correlation between the pore size distributions (Figure 1A) and hysteresis loops of trimodal porous silica (Figure 1B). These results are in good 10635


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Figure 4. Schematic representation of the formation of Ni/TMB (0), Ni/TMS, and Ni/TMB (10).

agreement with earlier reports,15,50 the pore size distribution results (Figure S1), and SEM (Figure 2) and TEM results (Figure 3). Small-Angle X-ray Scattering Characterization. The radial intensity plot of the small-angle X-ray scattering (SAXS) and SAXS pattern of the TMS material are shown in Figure 1C. The silica materials synthesized using pure P123 as well as those with a molar ratio of TMB/P123 of 2.5, 5.0, and 7.5 showed an XRD peak in the 2θ range of 0.8°−1.0° (Figure 1C and Figure S3). The observed 2θ values centered at ∼0.86°, 1.54°, and 1.79° can be indexed to (100), (110), and (200) planes of a 2D hexagonal space group (p6mm) of mesoporous silica, respectively.21 It was found that the intensity of the (100) peak decreased with increasing TMB concentration up to a molar ratio of TMB/P123 of 7.5 (Figure S3). This observation indicated that the addition of TMB led to a disordered hexagonal mesoporous structure of silica.16 It is important to note that the (100) peak shifted to lower angles with an increase of the TMB concentration up to a molar ratio of TMB/P123 of 7.5 (Figure S3). This observation confirms the formation of second mode pores in the silica. This finding supported the observations obtained from the pore size distribution (Figure S1) and N2 adsorption− desorption isotherm studies (Figure S2) of the silica materials. On the contrary, no characteristic peaks of 2D hexagonal mesoporous symmetry were found for the silica material synthesized using a molar ratio of TMB/P123 of 10.0 (Figure S3). Ordered rings can be seen in the SAXS pattern for the silica material synthesized using pure P123 (Figure S4). This confirmed that the mesoporous walls of these materials have well-defined pore sizes. Conversely, no such order was found for the silica material synthesized using a molar ratio of TMB/ P123 of 10.0 (Figure S4). This was either due to the poor distribution of the porous structure or due to the absence of porous structure as suggested by the pore size distribution curve (Figure S1).

Scanning Electron Microscopy. Evidence for the formation of TMS was further provided by XHR-SEM (extreme highresolution scanning electron microscopy) and high-resolution transmission electron microscopy (HRTEM) analyses. Figure 2 shows XHR-SEM images of TMS material. As can be seen in Figure 2A−C, the large dark parts revealed the existence of third mode pores that ranged from 55 to 210 nm. In Figure 2A,B, the TMS material presented second mode pores in the range of 32−45 nm. The first mode structures were also found in Figure 2B−D. A noteworthy observation from Figure 2C,D was that second mode pores were incorporated into first mode structures. Therefore, it can be seen from SEM images that the TMS material had a hierarchical porous structure with three different types of pores: first mode (smaller mesopores), second mode (larger mesopores), and third mode (macropores). This result was in line with the pore size distribution curve (Figure 1A). Furthermore, each type of pore was open to the external surface of the TMS material (Figure 2). It should be noted that the open hierarchical porous structure observed in the TMS material is desirable for catalytic applications as it can significantly increase the accessibility of active sites to reactant species and offer a more flexible route for the egress of the products and hence facilitate increased reaction rates. Transmission Electron Microscopy. Figure 3 shows highresolution TEM images of the TMS material. An ordered 2D hexagonal mesoporous structure with a pore size of ∼7.3 nm can be seen in Figure 3A,B. Figure 3C shows the transformation of a first mode structure (∼10.3 nm) into a second mode structure (∼37.6 nm). Both mesoporous structures showed parallel fringes corresponding to side-on projections of the mesostructure. This indicated that the TMS material had two types of 2D hexagonal mesoporous structures. Interestingly, weakly ordered porous patterns with the combination of first mode and second mode can also be seen in Figure 3C. The formation of disordered porous structures was also evident in the SAXS pattern of the TMS material (Figure 1C). In 10636


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The results showed that the introduction of TMB (below the threshold value) into the mesoporous structure could expand the sizes of some pores. However, when the TMB/P123 ratio reaches as high as 7.5, the critical value of the ordered mesoporous structure is reached. Therefore, when the molar ratio of TMB/P123 is increased to 7.5, not only does the transformation of first mode pores into second mode pores occur, but the formation of third mode pores takes place, as is evidenced by SEM (Figure 2) and TEM studies (Figure 3). Hence, a TMB/P123 molar ratio of 7.5 could be considered as the optimum combination among all of the tested ratios. It must be noted here that TMB molecules also have the ability to aggregate by self-assembly. As a result, a foamy structure of silica was formed at higher concentrations of TMB (TMB/ P123 > 7.5) because the surplus TMB molecules can aggregate by self-assembly and diffuse into the hydrophobic core of the P123 micelles. This observation is similar to the work reported by Schmidt-Winkel and co-workers. They found that the addition of small amounts of TMB (TMB/P123 < 0.2 (w/w)) to the aqueous Pluronic P123 solution led to the formation of SBA-15 type silicas,53 and the addition of more TMB (TMB/ P123 ≥ 0.2 (w/w) or ≥9.6 (mol/mol)) led to the formation of mesocellular siliceous foams (MCFs).54 The phase transition from highly ordered p6mm mesostructure of SBA-15-type mesoporous silica to trimodal porous structure and MCF phase depends on the synthesis parameters such as the temperature, pH, aging temperature, and mixing conditions.52−55 There is a marked difference between this work, with pH ∼1.5−2 and the conventional SBA-15 synthesis process with very strong acidic conditions (pH < 1). It can be considered that a less acidic media, but with pH below the aqueous isoelectric point of silica, impedes the production of the high amount of protonated silicate species in comparison to conventional SBA-15 synthesis processes.21 The well-controlled amount of positively charged silicate species provide mild electrostatic and hydrogen-bonding interactions which slows down the condensation of silicate species in silicate/P123 hybrid aggregates. For this reason, loosely packed silicate/P123 hybrid aggregates can be formed which can generate interconnectivity between ordered mesopores and thus formation of the hierarchical porous structure. This assumption is in agreement with earlier reports.59,60 A simplified proposed scheme of this process is presented in Figure 4. It can be concluded that a 7.5 molar ratio of TMB/ P123 is the optimum composition to obtain hierarchical porous silica, resulting in ordered multiple scale mesopores and abundant macropores. Application of Trimodal Porous Silica in the Catalytic CO2 Reforming of CH4. Catalytic Tests. In this study, the efficacy of TMS as a support material for dispersing Ni nanoparticles was studied, and the resultant catalysts were tested for catalytic efficiency in the CO2 reforming of CH4. A key reason for selecting this reaction was that both reactants (CO2 and CH4) and the syngas product (H2/CO) are gases; hence, the accessibility of active sites to reactant species and the egress of the products are governed by hierarchical porous materials. On the other hand, Ni-based materials are wellstudied catalysts for CO2 reforming of CH4 due to their high catalytic activity, low cost, and abundance.9,44,61 The ability of the Ni/SiO2 materials to catalyze the CO2 reforming of methane was investigated at 700 °C using a gas hourly space velocity (GHSV) of 5.2 × 104 mL g−1 h−1 for 43 h. The results, including the H2/CO ratio and CH4 and CO2

addition, a defective mesoporous structure was apparent between the first mode and second mode structures as shown in Figure 3D. It is clear from inspection of Figure 3E,F that differently shaped macropores with various sizes are present. It was interesting that the first mode pores form a shell structure on a third mode porous core. It was therefore clear from the TEM analysis that three types of pores were presented in the TMS material, thus confirming the observations obtained from the SEM images (Figure 2) and pore size distribution curve (Figure 1A) studies. Proposed Mechanism of Pores Formation. It is wellestablished that triblock copolymer, P123, acts as the structuredirecting agent for the synthesis of 2D ordered hexagonal mesoporous silica materials.51 Hence, a well-defined 2Dordered hexagonal mesoporous structure was formed of the silica material synthesized using pure P123, as demonstrated by N2 adsorption−desorption (Figures S1 and S2) and SAXS (Figures S3 and S4) studies. In contrast, a 2D hexagonal mesostructure was not formed in silica materials when a high amount of TMB (TMB/P123 molar ratio of 10.0) was used (Figures S3 and S4). These results confirmed the necessity of P123 polymer for the synthesis of 2D hexagonal mesoporous silica materials. In this study, TMB was used as the swelling agent. An attempt was made to elucidate the parameters responsible for the expansion of pore size as well as for the introduction of macropores in silica after the addition of TMB into the P123/ TEOS system. Control over the pore size in mesoporous silica materials can be achieved by adding a sufficient amount of a hydrophobic organic cosolvent or swelling agent at a pH below the aqueous isoelectric point (IEP) of silica (pH ∼2).52−55 Various co-organic solvents, such as benzene,56 cyclohexane,57 1,3,5-triisopropylbenzene,57 trimethylbenzene,42 2,2,4-trimethylpentane,49 and n-butanol,16 have been introduced into the P123/TEOS system to modify the morphology and structure of mesoporous silica. Owing to the hydrophobic nature, these coorganic solvents can diffuse into the hydrophobic core of the P123 micelles, resulting in swelling of the P123 micelles, and therefore the formation of second mode and/or third mode pores.25 It was found from the pore size distribution curves (Figure S1) that with an increase in the molar ratio of TMB/ P123 from 0 to 5.0 the pore size of the silica material expanded from 6.77 to 8.12 nm, respectively (Figure 4). The pore size expansion was also apparent from the N2 adsorption− desorption isotherms (Figure S2) and SAXS studies (Figure S3). The condensation step of the hysteresis loop was shifted to higher pressures, and the characteristic (100) peak of the hexagonal mesoporous structure shifted to lower angles with an increase in the TMB/P123 ratio from 0 to 5.0, respectively. All these observations confirmed the swelling effect of TMB in the silica pore size expansion. TMB molecules can penetrate into the hydrophobic cores of the micelle structure and can eventually expand them, resulting in an increased pore size of the materials.52 As noticed from the pore size distribution curves (Figure S1), only one type of pore was formed up to the molar ratio of TMB/P123 of 5.0. There is no doubt that the surplus TMB molecules can play a crucial role in the formation of different types of pores. As expected, the average pore size of the materials is increased by increasing the TMB concentration, as a pore expanding agent,5 while the BET surface area of the materials is decreased due to the interconnection of the pores58 (Table 1). 10637


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Figure 5. Conversion of (a) CH4, (b) CO2, and (c) H2/CO ratio over Ni/SiO2 catalysts (reaction conditions: temperature = 700 °C; GHSV = 5.2 × 104 mLg−1 h−1; CO2:CH4 = 1.0, TOS = 43 h).

Table 2. Effect of TMB/P123 Molar Ratio on H2/CO Ratio and CH4 and CO2 Conversions over Ni/SiO2 Catalysts at 700 °C and at GHSV of 52 000 mL h−1 g−1 conversion % (1 h) catalyst Ni/TMB Ni/TMB Ni/TMB Ni/TMS Ni/TMB

(0) (2.5) (5) (10)

% conversion (43 h)

% decrease in conversion

CH4

CO2

H2/CO ratio (1 h)

CH4

CO2

H2/CO ratio (43 h)

CH4

CO2

85.1 86.2 87.6 91.4 82.3

92.4 93.7 94.5 97.5 89.0

0.97 0.95 0.96 0.99 0.98

66.2 67.3 73.8 88.2 78.8

73.9 75.2 83.6 94.3 85.5

0.74 0.85 0.90 0.98 0.97

22.2 21.9 15.8 3.5 4.3

20.0 19.7 11.5 3.3 4.0

conversion % and was stable up to 43 h under these reaction conditions. Of the catalysts tested, the Ni/TMS catalyst exhibited the highest activity reaching, at 700 °C. CH4 conversions of 91.4% approached the thermodynamic maximum (91.5%) and CO2 conversions (97.5%) for 1 h (entry 4, Table 2). Also, the lowest % decrease in the CH4 and CO2 conversions was found with the Ni/TMS catalyst after a reaction time of 43 h (entry 4, Table 2). Afterward, the CH4 and CO2 conversions decreased for the Ni/SiO2 catalyst synthesized using Ni/TMB (10) (entry 5, Table 2). It can be seen from Figure 5c and Table 2 that the H2/CO ratios were not 1.0, as predicted based on the stoichiometry of the overall reaction (CO2 + CH4 → 2CO + 2H2). In addition, the CO2 conversions were higher than expected (∼66.3% at thermodynamic equilibrium, i.e., CH4/CO2 = 1, atmospheric pressure and 700 °C).62 The reasons for the differences were fully explained earlier by the occurrence of the reverse water gas shift (RWGS) reaction.9,44 The H2/CO ratios that were obtained show a similar trend to the CH4 and CO2 conversions observed for the different Ni catalysts supported on silica materials (Table 2). A low H2/CO

conversions, are presented in Figure 5. A summary of the results is also shown in Table 2. From the results presented in Figures 5a,b, it can be seen that all catalysts show a reasonable catalytic efficiency for the CO2 reforming of methane. The catalytic activity of the Ni/SiO2 catalysts is increased with increasing amounts of TMB until the TMB/P123 ratio reached a critical collapse value. This result was not surprising because the introduction of TMB into a mesoporous structure below the threshold value could expand the sizes of some pores and therefore provide more accessible Ni active centers for CH4 and CO2 as discussed later. However, a considerable decrease in the catalytic activity was observed when the amount of TMB in the material surpassed the threshold value (TMB/P123 > 7.5). This is most likely due to the breakdown of the ordered mesoporous structure as discussed in the previous section. As can be seen, the performance of the Ni/SiO2 catalyst with higher TMB/P123 ratios; i.e., TMB/P123 = 10 exhibited a gradual deactivation during the reaction time. In contrast, the conversions over catalysts with lower TMB/P123 ratios such as Ni/TMB (0) and Ni/TMB (2.5) catalysts decreased sharply during the initial reaction time followed by a period of steady performance. The Ni/TMS catalyst showed the highest 10638


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Langmuir ratio (0.95) was found for the Ni/TMB (0) catalyst, which suggests that the RWGS reaction was promoted on this catalyst. This could be due to the presence of more unreacted CO2 with low catalytic activity. Of the catalysts tested, the Ni/TMS catalyst exhibits the highest H2/CO ratio of 0.99 (entry 4, Table 2). This high H2/CO ratio was most likely due to reduced RWGS reaction which may be due to the rapid conversion of CO2 on Ni/TMS catalyst. On the whole, the Ni/ TMS catalyst exhibited a good catalytic performance and better lifetime for CO2 reforming of CH4 to produce a high H2/CO ratio compared with its counterparts having single-mode and nonporous structures. The textural properties of the reduced Ni/SiO2 catalysts before and after 43 h reaction were examined using N2 physisorption, and a summary of the results are shown in Table 3. The surface areas and pore volumes of the materials Table 3. Textural Properties of the Reduced Catalysts before and after 43 h Reaction

catalyst Ni/TMB (0) Ni/TMB (2.5) Ni/TMB (5) Ni/TMS Ni/TMB (10)

BET surface areaa (m2/g)

average pore sizeb (nm)

pore volumec (cm3/g)

fresh catalyst

spent catalyst

fresh catalyst

spent catalyst

fresh catalyst

spent catalyst

459.4

405.1

7.7

6.8

0.99

0.52

455.8

401.4

7.9

7.1

0.96

0.48

429.4

398.8

8.8

7.9

0.75

0.43

406.9 145.4

379.1 144.6

9.5 10.5

7.8 11.2

0.66 0.38

0.41 0.33

Figure 6. Transmission electron micrographs of spent catalysts (a) Ni/TMB (0), (b) Ni/TMB (2.5), (c) Ni/TMB (5), (d) Ni/TMS, and (e) Ni/TMB (10). The inset in (a) shows a HRTEM image of Ni inside the small pores covered by carbon.

a

BET surface area. bDesorption average pore diameter (4V/A by BET). cSingle point desorption.

methane.9,63 In the case of second and third modes, the blockage of pores was not the main reason for the catalyst deactivation. From the image presented in Figure 6e, it can be concluded that the deactivation of the Ni/TMB (10) catalyst is mainly due to the coverage of the Ni active centers by amorphous carbon.

having a higher portion of small pores such as Ni/TMB (0) and Ni/TMB (2.5) catalysts were found to decrease after 43 h reaction. The obvious decrease in the surface areas and pore volumes of these materials could be attributed to the blockage of the small pores (first mode) by the coke formed during the CO2 reforming of methane. This was confirmed by TEM images of spent catalysts (Figure 6). It can be noted from TEM images of the spent catalysts (Figure 6) that the graphene nucleated on the surface of Ni present inside of the small pores (first mode) is partly blocking the active sites (Figure 6a and its inset). Therefore, it is reasonable to expect that the conversions over catalysts having a higher portion of small pores such as Ni/TMB (0) and Ni/ TMB (2.5) catalysts were decreased drastically in the initial time on stream (see Figure 5). Blockage of small pores in these materials is in agreement with the decrease of the specific surface area of the catalyst (see Table 3). From these TEM images (Figure 6), it could be seen that carbon nanotubes were the primary form of carbon deposition on nickel particles present outside of the small pores, where the catalyst still shows activity because the Ni particles are present outside of the pores (Figure 6a,b). In the case of Ni/TMB (5) and Ni/TMB (7.5) catalysts, a mixture of amorphous carbon and carbon nanotubes was formed on the Ni particles (Figure 6c,d). The carbon nanotubes mostly grew away from the catalyst surface and did not cover the active sites. Therefore, not only do they show excellent resistance toward catalyst deactivation, but they can also enhance the Ni-catalyzed carbon dioxide reforming of

Table 4. Quantitative Calculation of Carbon Deposition on Spent Catalysts catalyst weight lossa (%) a

Ni/TMB (0)

Ni/TMB (2.5)

Ni/TMB (5)

Ni/ TMS

Ni/TMB (10)

6.95

7.21

7.59

8.78

13.54

Determined by TGA, weight loss between 100 and 800 °C.

The amount of carbon deposition measured from the TGA profiles (Figure S5) is shown in Table 4. The obtained TGA data show that the carbon deposition rate was lowest when using the Ni/TMB (0) catalyst and highest when using the Ni/ TMB (10) catalyst. A trend in the increasing amount of carbon deposits was observed as the TMB/P123 ratio increased. However, only a slight increase in the carbon deposition was found before the TMB/P123 ratio reached the collapsed value, while a substantial amount of carbon deposition was found on the Ni/TMB (10) catalyst. In addition, this result shows a clear relation between the atom % of Ni on the surface of the reduced catalysts (Table 5), the activity level (Table 2), and the 10639


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reforming of CH4 as macropores favor mass transfer and reduce transport limitations, while smaller pores provide active sites for the adsorption of CO2 and CH4.64−66 It was found that shaping the mesoporous silica using a micelle swelling agent led to a significant increase in catalytic activity and stability of the Ni/SiO2 catalysts and the production of H2 and CO at a ratio close to unity (∼0.99) (Table 2). Of the catalysts tested, the Ni/TMS catalyst showed good catalytic performance for the CO2 reforming of CH4 with excellent stability up to 43 h. The significant improvement in activity and stability of the Ni catalysts supported on mesoporous silica materials was most likely due to a combination of factors. Previous studies on dry reforming of methane catalyzed by Ni-based catalysts have suggested numerous possible factors, including gas transportability, metal particle size, and the accessible amount of nickel on the catalyst surface that can contribute to increased activity and stability.9,40,41,44,68 The results obtained for catalytic CO2 reforming of CH4 (Table 2) indicatethat both CO2 and CH4 conversions increase with an increase in the pore size of the silica support (Table 1). Thus, it can be concluded that the high activity observed on Ni/TMS could be due to the improved accessibility of CO2 and CH4 molecules to Ni-active centers in this material. The bulk and surface compositions of the reduced Ni/SiO2 catalysts, with respect to Ni, were determined using XRF and XPS, respectively. The results are summarized in Table 5. As shown in the fourth row, the atom % of Ni on the surface of ordered porous silica materials having only first mode porosity is less than on the silica materials which possess second and third modes. This is most likely due to the embedding of metal nanoparticles inside the small pores (first mode) of the ordered porous material. This result is in agreement with the EDSmapping results presented in Figure 7 which shows selected STEM electron images and their corresponding fluorescence signals of Ni (the blue region) in the Ni/TMB (0), Ni/TMS, and Ni/TMB (10) catalysts. XRF and XPS data confirmed that

Table 5. Distribution and Composition of Ni Particles on and in Silica Materials Synthesized with Different Molar Ratios of TMB/P123 catalyst average Ni particle size on the fresh catalystsa (nm) average Ni particle size on the spent catalystsa (nm) wt % of nickelb atom % of nickelc

Ni/ TMB (0)

Ni/ TMB (2.5)

Ni/ TMB (5)

Ni/ TMS

Ni/ TMB (10)

9.0

9.4

9.1

9.05

15.2

10.4

11.1

13.6

13.7

15.75

8.23 1.14

8.48 1.53

8.44 1.85

8.32 1.94

8.27 2.42

a Measured by ImageJ using HRTEM images. bWt % of Ni in the bulk of reduced catalysts measured by XRF. cAtom % of Ni on the surface of reduced catalysts measured by XPS.

amount of carbon deposition (Table 4). This finding is not surprising because the encapsulation of nickel nanoparticles inside the small pores could effectively hinder the accessibility of Ni-active sites to the reactants, resulting in decreased catalytic activity and thus a decreased amount of carbon deposition. Role of Trimodal Porous Silica. Compared to single-mode porous materials, hierarchically porous materials provide novel properties and have the potential for application in various industrial processes. The concept of a hierarchical porous structure is also critical for catalytic CO2 reforming of CH4. It is a well-established fact in heterogeneous catalysis that a high surface area support, usually consisting of fine micropores or mesopores, significantly improves the dispersion of the active phase, thus resulting in the availability of a larger number of catalytic active sites and hence enhanced the efficiency of the catalyst. The micropores and/or mesopores present in high surface area supports will adversely influence the diffusion of reactants and products,64−67 resulting in the poor efficiency of the catalyst. In this context, hierarchical porous materials having multiple scale pores show a favorable effect in the CO2

Figure 7. Selected STEM electron images and the corresponding fluorescence signals of Ni (the blue region) in (a) Ni/TMB (0), (b) Ni/TMS, and (c) Ni/TMB (10). 10640


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Figure 8. Ni particle size distribution in the fresh and spent Ni/SiO2 catalysts.

the TEM images using ImageJ software, and the obtained data are presented in Figure 8 and Table 5. It is evident from Figure 8 that the Ni particles in the fresh Ni/SiO2 catalysts synthesized using a small amount of TMB were more uniform in comparison with the Ni particles in the catalysts that were prepared using a higher amount of TMB. On the other hand, the size of the Ni particles in the spent catalysts having first mode porosity remained unchanged during 43 h of on stream reaction at 700 °C. The uniform distribution and thermal sintering resistance of the Ni nanoparticles in these materials

while the total amount of Ni among all catalysts was similar (see Table 5, row 3), the amount of surface Ni (see Table 5, row 4) is strongly correlated with the average pore sizes of these materials (see Table 1). Therefore, it can be concluded that the presence of large pores (second and third mode pores) had a significant contribution in increasing the number of accessible active Ni sites, which led to the higher activity observed for the Ni/TMS catalyst (Table 2 and Figure 5). The particle size distribution of Ni in the fresh and spent Ni/ SiO2 catalysts was estimated by measuring ca. 200 particles in 10641


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University’s Microscopy & Microanalysis Facility, a linked laboratory of the Australian Microscopy & Microanalysis Research Facility. The authors thank the Australia−India Strategic Research Fund (AISRF) for financial support of the Mini DME Grand Challenge Project (fund code GCF020009).

under severe calcination, reduction, and reaction conditions could be attributed to the “confinement effect” of their uniform rigid channels which control the Ni particle size.19,69 However, the distribution of the Ni particle size in the spent Ni/TMB (10) catalyst was broad to some extent, and about 5% of particles had aggregated and sintered to a size more than 30 nm. Based on the above results and discussion, the high catalytic performance of the Ni/TMS material was attributed to the hierarchical porous structure of the silica support. On one hand, the third mode pores provided more exposed Ni active sites for CO2 and CH4 adsorption and enhanced the mass transport properties of the reactants and products. On the other hand, the mesoporous matrix stabilized the Ni nanoparticles by the confinement effect.19,69 This improved their long-term catalytic stability by preventing the deactivation of the Ni catalyst caused by sintering and agglomeration of the Ni nanoparticles under severe reduction and reaction conditions.70

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(1) Zarei, M.; Meshkani, F.; Rezaei, M. Preparation of mesoporous nanocrystalline Ni-MgAl2O4 catalysts by sol-gel combustion method and its applications in dry reforming reaction. Adv. Powder Technol. 2016, 27 (5), 1963−1970. (2) Qian, L.; Cai, W.; Zhang, L.; Ye, L.; Li, J.; Tang, M.; Yue, B.; He, H. The promotion effect of hydrogen spillover on CH4 reforming with CO2 over Rh/MCF catalysts. Appl. Catal., B 2015, 164, 168−175. (3) Yu, M.; Zhu, Y.-A.; Lu, Y.; Tong, G.; Zhu, K.; Zhou, X. The promoting role of Ag in Ni-CeO2 catalyzed CH4-CO2 dry reforming reaction. Appl. Catal., B 2015, 165, 43−56. (4) Aw, M. S.; Zorko, M.; Djinović, P.; Pintar, A. Insights into durable NiCo catalysts on β-SiC/CeZrO2 and γ-Al2O3/CeZrO2 advanced supports prepared from facile methods for CH4−CO2 dry reforming. Appl. Catal., B 2015, 164, 100−112. (5) Donphai, W.; Faungnawakij, K.; Chareonpanich, M.; Limtrakul, J. Effect of Ni-CNTs/mesocellular silica composite catalysts on carbon dioxide reforming of methane. Appl. Catal., A 2014, 475, 16−26. (6) Cao, Y.; Li, H.; Zhang, J.; Shi, L.; Zhang, D. Promotional effects of rare earth elements (Sc, Y, Ce, and Pr) on NiMgAl catalysts for dry reforming of methane. RSC Adv. 2016, 6 (113), 112215−112225. (7) Xie, X.; Otremba, T.; Littlewood, P.; Schomäcker, R.; Thomas, A. One-Pot Synthesis of Supported, Nanocrystalline Nickel Manganese Oxide for Dry Reforming of Methane. ACS Catal. 2013, 3 (2), 224− 229. (8) Zhang, S.; Muratsugu, S.; Ishiguro, N.; Tada, M. Ceria-Doped Ni/SBA-16 Catalysts for Dry Reforming of Methane. ACS Catal. 2013, 3 (8), 1855−1864. (9) Amin, M. H.; Mantri, K.; Newnham, J.; Tardio, J.; Bhargava, S. K. Highly stable ytterbium promoted Ni/γ-Al2O3 catalysts for carbon dioxide reforming of methane. Appl. Catal., B 2012, 119−120, 217− 226. (10) Zhao, X.; Cao, Y.; Li, H.; Zhang, J.; Shi, L.; Zhang, D. Sc promoted and aerogel confined Ni catalysts for coking-resistant dry reforming of methane. RSC Adv. 2017, 7 (8), 4735−4745. (11) Liu, Z.; Zhou, J.; Cao, K.; Yang, W.; Gao, H.; Wang, Y.; Li, H. Highly dispersed nickel loaded on mesoporous silica: One-spot synthesis strategy and high performance as catalysts for methane reforming with carbon dioxide. Appl. Catal., B 2012, 125, 324−330. (12) Quek, X.-Y.; Liu, D.; Cheo, W. N. E.; Wang, H.; Chen, Y.; Yang, Y. Nickel-grafted TUD-1 mesoporous catalysts for carbon dioxide reforming of methane. Appl. Catal., B 2010, 95 (3−4), 374−382. (13) Du, X.; Zhang, D.; Gao, R.; Huang, L.; Shi, L.; Zhang, J. Design of modular catalysts derived from NiMgAl-LDH@m-SiO2 with dual confinement effects for dry reforming of methane. Chem. Commun. 2013, 49 (60), 6770−6772. (14) Zhao, X.; Li, H.; Zhang, J.; Shi, L.; Zhang, D. Design and synthesis of NiCe@m-SiO2 yolk-shell framework catalysts with improved coke- and sintering-resistance in dry reforming of methane. Int. J. Hydrogen Energy 2016, 41 (4), 2447−2456. (15) Morales, J. M.; Latorre, J.; Guillem, C.; Beltrán-Porter, A.; Beltrán-Porter, D.; Amorós, P. Scale-up low-cost synthesis of bimodal mesoporous silicas. Solid State Sci. 2005, 7 (4), 415−421. (16) Zhou, X.; Duan, A.; Zhao, Z.; Gong, Y.; Wu, H.; Li, J.; Wei, Y.; Jiang, G.; Liu, J.; Zhang, Y. Synthesis of hierarchically porous silicas with mesophase transformations in a four-component microemulsiontype system and the catalytic performance for dibenzothiophene hydrodesulfurization. J. Mater. Chem. A 2014, 2 (19), 6823−6833. (17) Baktash, E.; Littlewood, P.; Pfrommer, J.; Schomäcker, R.; Driess, M.; Thomas, A. Controlled Formation of Nickel Oxide Nanoparticles on Mesoporous Silica using Molecular Ni4O4 Clusters

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CONCLUSIONS In summary, a hierarchical porous silica material with trimodal porosity was developed using a hydrothermal assisted sol−gel method. This silica material had three types of pores with average diameters of ∼8, 25, and 89 nm. High-resolution microscopic images further confirmed the formation of trimodal porous silica with the integration of second mode into first mode structures and abundant third mode pores. It was found that Ni nanoparticles supported on the trimodal porous silica exhibited a better performance in the CO2 reforming of CH4 to produce valuable syngas feedstock with a high H2/CO ratio (0.99) compared with Ni nanoparticles supported on single-mode and nonporous SiO2 materials. This study showed that the presence of large pores (second and third modes) in TMS material increased the number of accessible active Ni sites, which led to the high activity observed for Ni/TMS catalyst.

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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.langmuir.7b02753. Figures S1−S5 (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel +61 3 99253365; Fax +61 399253747 (S.K.B.). ORCID

Mohamad Hassan Amin: 0000-0001-5842-2511 Suresh K. Bhargava: 0000-0002-9298-5112 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Frank Antolasic and Dr. Samuel Ippolito for their help in automating the reactor system. We also thank Moein Amin and Azin Amin for their help acquiring the data as well Dr. Blake Plowman and Dr. Selvakannan Periasamy for their helpful discussions. The authors duly acknowledge the facilities and the scientific and technical assistance of the RMIT 10642


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