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Article

Tailorable Formation of Hierarchical Structure Silica (HMS) and Its Application in Hydrogen Production

by
Luming Li
1,2,3,4,
Jie Deng
1,4,
Zhanglong Guo
2,4,
Wei Chu
4,* and
Yan Liu
2,*
1
School of Food and Biological Engineering, Chengdu University, Chengdu 610106, China
2
Institute of Sustainability for Chemicals, Energy and Environment, A*STAR, Singapore 627833, Singapore
3
Institute for Advanced Study, Chengdu University, Chengdu 610106, China
4
Department of Chemical Engineering, Sichuan University, Chengdu 610065, China
*
Authors to whom correspondence should be addressed.
Catalysts 2022, 12(9), 1061; https://doi.org/10.3390/catal12091061
Submission received: 24 August 2022 / Revised: 14 September 2022 / Accepted: 15 September 2022 / Published: 17 September 2022
(This article belongs to the Special Issue Advanced Catalysts for Achieving Hydrogen Economy from Liquids)
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Review Reports Versions Notes

Abstract

:
Relentless endeavors have been committed to seeking simple structure-directing agents for synthesizing hierarchical mesoporous silica (HMS) materials but remaining challenges. In this contribution, we offered an improved one-pot hydrothermal route to prepare HMS materials using a single non-ionic triblock copolymer (F127) structure-directing agent under a mild polycarboxylic (citric acid) mediated condition. Via studies of key synthetic parameters including acid concentration, crystallization temperature and aging time, it was found that citric acid medium presents an important bridging effect under the optimal concentration from 0.018 M (pH = 2.57) to 1.82 M (pH = 1.09), contributing to the self-assemblage of partially protonated non-ionic triblock copolymer and tetraethyl orthosilicate (TEOS) into a high-quality multistage structure of silica materials. The specific surface area (SSA) of HMS shows a volcanic trend and is closely associated with the concentration of citric acid while the highest SSA of 739.9 m2/g can be achieved at the citric concentration of 0.28 M. Moreover, the as-synthesized HMS-CTA supported Ni/CeO2 catalysts indicate an excellent production of hydrogen through dry reforming of methane (DRM) reaction over 172 h stability. The improved, facile synthesis strategy under polycarboxylic medium displays an expanded perspective for synthesizing other mesoporous materials in a wide range of applications such as catalytic material carriers and drug inhibitors.

Graphical Abstract

1. Introduction

Hierarchical mesoporous silica (HMS) materials are prestigious porous materials. Their interconnected periodic substructure, high specific surface area as well as high loading capacity are particularly attractive to the wide applications including catalysis, adsorption, separation, life science, and energy storage [1,2,3,4,5]. The most common synthesis route to HMS is implemented via the combinative self-assembly of nonionic block copolymers and silicate molecules [6,7]. For instance, ABC (i.e., PE-b-PEO-b-PCL) and ABA (i.e., PEO-a-PPO-b-PEO-a) type triblock copolymers have attracted great attention owing to their biodegradability, nontoxicity, and excellent interfacial stability. They have been widely applied as structure-directing agents to induce the formation of abundant spatial structure with various morphology (nanoparticles, fibers, and rods), such as cubic mesophase and hexagonal mesophase [8,9,10].
Generally, the secondary surfactants or additives are always engaged to construct these hierarchical mesoporous silica (HMS) materials [11]. Esquena et al. reported that silica materials characterized with a dual pore size distribution was prepared under highly concentrated co-templating emulsions conditions, where the co-templates consist of emulsion droplets (C12(EO)8/decane) and cubic liquid crystal of PEO-PPO-PEO block copolymer nonionic surfactant. The emulsion droplets would induce the formation of macropores whereas nonionic surfactant would template the formation of the mesopore structure [12]. Zhou et al. synthesized a series of micro/mesoporous silica materials using non-ionic block copolymer Pluronic (P123) and triethylamine acetate (PIL) as co-templates. The sub-structure of silica was closely related to the content of PIL and hydrothermal conditions [13]. Liu et al. prepared hierarchical mesoporous silicas using the triblock copolymer PEO106-PPO70-PEO106 (F127) and the diblock copolymer poly (ethylene oxide-b-ε-caprolactone) (PEO-b-PCL) as co-templates [14]. Moreover, Shi et al. found that hierarchically porous materials can be obtained from the polyelectrolyte-surfactant complex templating strategy [9]. The combination between surfactant templates and hard templates is another important approach in the formation of the hierarchical pore structure, as Stein et al. reviewed [15]. However, the secondary template would undoubtedly increase the complexity of preparation process, i.e., reaction conditions and formation mechanisms. Therefore, it is urgent to develop an efficient process to prepare hierarchical silica materials using only one type of surfactant to better meet the practical convenience.
Zhao et al. firstly reported that ABC amphiphilic triblock copolymer of poly(ethylene oxide)-block-poly(methyl methacrylate)-block-polystyrene (PEO-b-PMMA-b-PS) was of great importance for fabricating mesoporous materials [16]. However, the interconnected hydrophobic parts (PMMA and PS) were only capable to induce single hydrophobic domain and produce the mesoporous material with single pore size. Li et al. found that the hierarchical mesoporous silicas featured with two kinds of mesoporous size were synthesized using different hydrophobic segments (poly ethylene and poly ε-caprolactone), separately grafted at the chain end of the poly ethylene oxide [17]. It can be concluded that the location of hydrophobic segments plays an important role in strategically forming the hierarchical mesoporous morphology. These two kinds of hydrophobic groups were separately located at the end of the PEO, which showed an advantage in regulating the multistage spatial structure. However, it is rarely reported that these hierarchical silica structures can be prepared using only one ABA type of structure-directing agent (ABA, where the same hydrophobic groups are separately located at the end of the PEO) via a convenient evaporation-induced self-assembly (EISA) strategy [10]. In our previous study, we reported that polycarboxylic acids (citric acid) plays a key role in preparing high-quality mesoporous silica without the addition of mineral acid (HCl) [18]. In this contribution, we deeply investigate the interaction between the non-ionic triblock copolymer and polycarboxylic acid and their influence on the pore structure formation of silica materials. Surprisingly, hierarchical mesoporous silica (HMS) was formed with tunable dual pore structure. Via detailed characterization, we believe that the self-assemblage of partially protonated non-ionic triblock copolymer and tetraethyl orthosilicate (TEOS) plays a key role for the formation of high-quality multistage structure of silica materials.
Moreover, the hydrogen economy boom has drawn great attention because of the environmental and energy crisis [19,20,21,22] as well as the hydrogenation reaction [23]. Until now, hydrogen can be divided into three categories in view of the process of hydrogen production, namely green hydrogen (preparation hydrogen from renewable energy), purple hydrogen (preparation hydrogen from nuclear energy) and blue hydrogen (preparation hydrogen from coal gasification, natural gas and other derivatives of carbon) [24,25]. The route of hydrogen production via dry reforming of methane (DRM) reaction has become a hot scientific topic, covering the research on active compositions, catalysts carriers as well as their additives [26]. Catalyst support, as one of the core components, is vital to command the interaction strength between the metal and support, dispersion of the active species and the exposure of active center [27]. Undoubtedly, the mesoporous silica is propitious due to its large specific surface area, thermal stability and easy availability [27]. However, the weak interaction between active species and silica carrier causes the sintering of large metal nanoparticles, and formation of carbon deposition, resulting in less formation of hydrogen, especially on Ni-based catalysts [28]. It is usually advocated that the introduction of promoter favors the adjustment of the above drawbacks. For example, the CeO2 could not only improve the dispersion of Ni nanoparticles, but also activate carbon dioxide into mobile oxygen species and promote the carbon removal owing to its unique redox properties (), thermal resistance and enhancement of the metal-support interaction [29,30]. In our recent study, we found that the ceria loading interval ranges from 10 wt% to 30 wt% exhibited outstanding production of hydrogen with different carbon deposition resistance. The optimized ceria loading of 15 wt% (5Ni/15CeO2-SBA-15-CTA) catalyst yielded the best anti-coke performance with the lowest coke deposition rate of 13.5 wt% during 170 h DRM reaction [31]. These above interesting scientific phenomena inspire us to develop novel HMS and to support Ni/CeO2 catalysts for hydrogen formation via DRM strategy.

2. Results and Discussions

2.1. Formation Mechanism of HMS Materials

Figure 1 depicted that hierarchical mesoporous silica was synthesized using F127 as template under various citric acid concentrations from zero to 0.018 mol/L (pH = 2.57) and 1.82 mol/L (pH = 1.09). Without citric acid, the silica indicates the typical nitrogen adsorption–desorption behavior as mesoporous material [32]. While citric acid was added from 0.18 to 1.03 mol/L, their typical type-IV curves clearly highlights two overlapping hysteresis loops, indicating a hierarchical feature with two types of pores (Figure 1a). Figure 1b presented the pore size distribution curves of the prepared products, the adjustment of mesoporous size hinges on the concentration of citric acid (or pH). The high concentration (i.e., 1.82 mol/L) of acid or low pH would only cause a single mesoporous structure. On the contrary, the relatively low concentration of critic acid would facilitate the synthesis of HMS materials. Results suggest a controllable synthesis of silica materials with double mesoporous structure using a facile approach. Moreover, the specific surface area (SSA) of HMS was also closely associated with the concentration of citric acid. As displayed in Table 1, the SSA showed a volcanic trend with citric concentration for the HMS materials. The highest SSA of 739.9 m2/g (Entry 4) was achieved at the citric concentration of 0.28 mol/L. However, an even higher SSA can be obtained for a single mesoporous silica material at a high concentration of polycarboxylic acid at 1.82 mol/L, which is likely originated from the degree of well-ordered substructure.
It is well known in the literature that the hydrothermal temperature favors adjusting the pore substructure owing to the temperature sensitivity of polymerization between inorganic silicate and copolymer micelle [18,33]. By considering the hydrothermal temperature, a suitable temperature at 100 °C was found to be necessary for preparing the well-ordered double-porous materials. Either higher or lower hydrothermal temperature can cause fatal defects, resulting in the decrease in SSA (e.g., 60 °C, 443.2 m2/g, Entry 8; 100 °C, 739.9 m2/g, Entry 8; 140 °C, 565.3 m2/g, Entry 9), as Figure 2a,b and Table 1 depict.
Moreover, the hydrothermal time dependence on mesoporous silica materials prepared under 0.28 mol/L citric acid was performed, as shown in Figure 3. It was found that the hierarchical mesoporous silica with two typical hysteresis loops can only be formed while the duration time of hydrothermal was longer than 8 h (e.g., 16 h and 140 h) (Figure 3a). The hierarchical double pores were gradually formed and solidified during the extension of the hydrothermal treatment time (4 h–8 h–16 h–140 h–480 h). On the other hand, their pore size was positively increased along with the treatment time (Figure 3b and Table 1). Pore size ranges from 3.44 nm (Entry 10, 4 h) to 6.09 nm (Entry 15, 140 h). The variation trend of specific surface area presents a volcanic shape, peaked at 792.3 m2/g (Entry 13). It may suggest that the extended hydrothermal time (>48 h) would reduce the specific surface area of HMS materials with an enlarged pore size. Results are consistent with the report using the hyper-protonation of PEO block [33], where a stronger affinity to the silicate (TEOS) than that of the hydrophobic PPO block was found under the enduring reaction time, inducing the deep penetration of PEO chains into the frameworks of silicate micelles, and subsequent pore size expansion after calcination. Notably, the hierarchical merit with 629.7 m2/g SSA can be retained under the extended hydrothermal time of 480 h (20 day), indicating the hierarchical double pore structure is hydrothermally stable even with an excessive prolongation of the hydrothermal time up to 20 days (Figure 3a).
From the data of SSA and pore structure, it was inferred that the merit of double-pore structure mainly depends on the mediated dose of citric acid amount and the optimized hydrothermal time (≥16 h). Small-angle (2θ = 0–6°) XRD patterns were recorded on a Bruker D8 Advance X-ray diffraction meter, to study the crystal phase or periodic structure of synthesized HMS materials. As shown in Figure 4, no periodic structure was formed only with F127 as surfactants. The mesoporous structure can only be formed with the coordination of F127 and citric acid, clearly indicating the importance of the mediation from citric acid. It was found that the high concentration of citric acid could benefit the formation of well-ordered cubic structure, as the Figure 4 indicated. An apparent diffraction intensity appears with a large d spacing of 115 Å (110), and another two weak diffraction peaks are reflected in the range of 1–1.4° with interplanar d spacing of 72.9 Å (200) and 64.0 Å (211), which should correspond to a cubic structure (Im3m space group, SBA-16) [7,34]. Thus, it was worthwhile noting that the silica materials with a periodic pore structure would not be easily synthesized under the low concentration citric modulator.
In order to uncover the microstructure of the as-prepared HMS materials, we took the measurement of TEM observation of the typical HMS sample synthesized under the CTA concentration of 0.28 mol/L (pH = 1.82) calcined at 550 °C. The TEM and HRTEM (inset) images displayed in Figure 5 exhibit the well-ordered arrays and reveal an anticipant 3D cubic microstructure which corresponds well to the results of small angle XRD and BET measurements.
According to the above characterization results, the microstructure of mesoporous silica is closely related to the synthesis conditions such as concentration of citric acid, hydrothermal temperature as well as hydrothermal time. The proposed formation mechanism was briefly proposed in Scheme 1. Optimal acid conditions (0.018–1.82 mol/L), hydrothermal temperature (100 °C) and hydrothermal time (16 h) were necessary for the synthesis. However, it was noticed that the content of citric regulator is the key controlling parameter to synthesize HMS materials. Stronger acidity (above 1.82 mol/L) would promote a rapid hydrolysis/condensation of silicate species that would form a well-ordered cubic structure (SBA-16). The formation of HMS probably originated from the incomplete condensation of silicate under the mild acid condition (Scheme 1). Moreover, it was accepted that the aging process would enhance the silicate condensation and the hydrothermal stability of the HMS materials. In this contribution, prolonged hydrothermal treatment should not be advocated due to the minor changes in structure, accompanied by the enlargement of the pore size.

2.2. Catalytic Activity of Ni Based Catalysts in DRM Reaction

These porous silica materials possess high surface areas and tunable pore dimensions (2–50 nm), where the relatively large pores would enhance mass transfer, while the high surface area facilities the exposure of active centers per mass of catalysts, which are great catalyst carrier candidates for heterogeneous catalysis, such as DRM [27,35,36]. In our previous contributions, high-quality SBA-15-CTA (CTA signifies citric acid) materials with typical 2D hexagonal pore structure have been successfully prepared under moderate polycarboxylic citric acid, being utilized to synthesize Ni/SBA-15-CTA and CeO2 doped NiCeO2/SBA-15-CTA hybrid catalysts [18,31]. The catalytic activity results showed that the addition of CeO2 could significantly improve the sintering resistance properties of Ni and carbon elimination rate due to the strong interaction between Ni nanoparticles and CeO2 additives [31]. In this contribution, we will secondly explore the relationship between the strong interaction of metal support and its performance on the new hierarchical silica carrier, to evaluate their application prospects of our multi-porous silica materials. Figure 6a displayed the structural periodicity of the synthesized hierarchical materials loaded by the active species of Ni (5 wt%) and additive of CeO2 (15 wt%); it was found that the addition of active Ni nanoparticles and additive of CeO2 would not damage the spatial microstructure of HMS materials. However, the SSA of 429.1 m2/g showed a decreasing tendency compared to the SSA of 739.9 m2/g of HMS-CTA parent carrier as the channels are clogged with some nanoparticles (NiO and CeO2) to some extent (inserted adsorption–desorption curve of Figure 6a). Moreover, the first reduction peak (317 °C) of NiO nanoparticles and surface oxygen in ceria for the 5NiO15CeO2/HMS-CTA catalyst becomes narrower and shifted to the lower temperature (404 °C, Figure 6b), compared to the 5NiO15CeO2/SBA-15-CTA (464 °C), revealing that CeO2 could better promote the dispersion of NiO nanoparticles on HMS carriers more uniformly compared to that on 2D silica [31,37]. Nevertheless, the second reduction peaks of 404 °C and wide peaks of 450 °C should be attributed to the reduction of NiO species with strong interaction with CeO2 on HMS support, corresponding well to the previous study [38]. The fourth reduction peak at around 721 °C reflected the reduction of Ce4+ → Ce3+ species [39]. It is worth noting that CeO2 indeed improves the dispersion of NiO species and strengthens the interaction between metal and carriers of HMS materials.
The catalytic activity of 5NiO15CeO2/HMS-CTA catalyst was evaluated for 172 h at 700 °C in Figure 7. It was found that the initial conversion of CO2 and CH4 reached ca. 77.8% and 73.2%, with 93.2% H2 selectivity and H2/CO molar ratio of 0.91. Moreover, CO2 conversion, CH4 conversion, H2 selectivity and H2/CO molar ratio were maintained at ca. 71.5%, 64.1%, 91.8% and 0.90 after 172 h long term stability testing (Figure 7). The syngas production rate is substantially higher than that of reports on common amorphous silica supported Ni-based catalysts (<85%) [28,30]. Moreover, the weight loss curve on the spent 5NiO15CeO2/HMS-CTA catalyst was 26.2 wt% for 172 h with a DTA peak of 609 °C (right inserted image in Figure 7), probably further confirming that the graphitic carbon was successfully hindered and the formed coke was more reactive over the 5NiO15CeO2/HMS-CTA catalyst [40]. Hence, the synthesized HMS materials present a great potential in the heterogeneous catalysis field. In addition, it is noted that the CO2 conversion was slightly superior to that of CH4, which is mainly originated from the presence of side effects from reverse water gas shift reaction under high temperature conditions [41,42]. According to previous studies, the optimal loading of ceria promoter should be located at ca. 15 wt% for Ni-based catalysts supported on the mesoporous silica carriers such as SBA-15 and SBA-16 for DRM reaction [31,43]. The enhanced interaction between Ni nanoparticle and ceria over HMS could better improve the dispersion of active Ni species and achieve the high syngas formation rate. Moreover, the optimized ceria content of 15 wt% favors the formation of a high concentration of oxygen vacancy as well as Ce3+, serving the in situ coke removal and promoting the carbon resistance [31,37].

3. Materials and Methods

3.1. Materials

All chemicals, including anhydrous citric acid, tetraethyl orthosilicate (≥98%, TEOS), poly (ethylene glycol)-block-poly (propylene glycol)-block-poly (ethylene glycol) (P127, M = 12,600 g mol−1), anhydrous ethanol, cerium (III) nitrate hexahydrate and nickel nitrate hexahydrate were purchased from Sigma-Aldrich (Burlington, MA, USA).

3.2. Materials Preparation

HMS materials were synthesized via a self-assembly strategy in deionized water, using F127 as single template and TEOS as the silica precursor. Typically, TEOS (8.75 g) and a certain amount of citric acid were added to a premixed solution (125 mL water and 2.5 g copolymer template P127) and the mixture was stirred at 38 °C for 24 h. The resulting solution was poured into a Teflon autoclave and hydrothermal synthesized at various temperature (60 °C, 100 °C or 140 °C) for different time range from 4 h to 20 day (Table 1). After that, the crystallized precursor was filtered, washed, and dried at 100 °C for 24 h. Finally, the white powder product was calcined at 550 °C for 6 h to gain hierarchical mesoporous silica.
The above-mentioned HMS materials (2 g) were impregnated with an aqueous solution of Ce(NO)3 (0.073 mol/L) and Ni(NO)3 (2.5 × 10−4 mol/L), then dried at 100 °C for 12 h. After that, the materials were calcined at 750 °C for 2 h to obtain catalyst. CeO2 and Ni loadings were at 15 and 5 wt%, separately, which was labeled as 5NiO15CeO2/HMS-CTA.

3.3. Catalytic Activity Testing

DRM reaction was investigated in a fixed-bed quartz reactor (ID: 7 mm) at ambient pressure. Specifically, catalysts (150 mg, 45–60 mesh) were uploaded in the central of quartz reactor tube with the hold of quartz wool. Before activity testing, catalysts were pretreated with a flow of 10% H2/He (50 mL/min) at 700 °C with heating rate of 5 °C/min for 1 h. Then, the feed gas (CO2/CH4/Ar/N2 = 3:3:3:1) was introduced to the catalysts bed at a total flow rate of 50 mL/min (GHSV of 20,000 mL·gcat−1·h−1). The exhausted products were studied using online Gas Chromatograph (Agilent GC-6890, Santa Clara, CA, USA) equipped with a TCD detector (a Carbon-Plot coupled with a HP-Plot capillary column).

3.4. Materials Characterization

Small-angle (2θ: 0–6°) and normal X-ray diffraction (2θ: 5–80°) XRD patterns were measured on a Bruker D8 Advance X-ray diffractometer with a Cu Kα irradiation. N2 adsorption–desorption isotherms cures were analyzed on a Micromeritics ASAP 2420 automatic analyzer and the temperature of pretreatment was settled at 200 °C under vacuum for 12 h. A FEI Tecnai G2 F20 Transmission electron microscopy (TEM) was employed to test the pore structure of the HMS synthesized materials. Hydrogen temperature programmed reduction (H2-TPR) experiment was carried out to study the reduction properties of 5NiO15CeO2/HMS-CTA catalyst on a chemisorption (Thermo Scientific, Waltham, MA, USA, AutoChemII 2920) instrument. The weight loss curve of the spent 5Ni15CeO2/HSM was obtained on a Thermogravimetric analysis instrument (TGA Q500, New Castle, England).

4. Conclusions

In sum, we advocated an improved, facile one-pot hydrothermal route to prepare hierarchical mesoporous silica (HMS) materials using a single ABA type of non-ionic triblock copolymer structure-directing agent under a novel polycarboxylic acidic mediated condition. The process parameters such as citric acid concentration, hydrothermal temperature and crystallization time had been systematically studied. Results revealed that the spatial structure of HMS highly hinges on the concentration interval of citric acid ranges from 0.018 M to 1.82 M owing to the incomplete protonation and bridging effect of citric acid, while the hydrothermal temperature and aging time could more likely impact the pore distribution and size due to the condensation reactions. Moreover, as-synthesized HMS-CTA supported Ni/CeO2 catalysts displayed a long-term stability of 172 h during the DRM reaction. The improved synthesis route for HMS under citric acid medium exhibits great potential for other hierarchical mesoporous materials synthesis and applications in heterogenous catalysis, environmental governance and nanomedicine and so on.

Author Contributions

Experiment, L.L. and Z.G., writing—original draft, L.L. and J.D., characterization, L.L. and Y.L., writing—review and editing, L.L., J.D. and Y.L., project administration, Y.L. and W.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research receives no external funding.

Acknowledgments

We appreciate the Chinese Scholarship Council (CSC) and Institute of Sustainability for Chemicals, Energy and Environment (ISCE2, A*star, Singapore) for financial support. The authors feel grateful to Yeo Wen Cong and Wang Luo for experimental help, Melissa Prawirasatya for TEM characterization.

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 12 01061 g001 550
Figure 1. Nitrogen adsorption–desorption isotherms (a) and pore size distribution (b) profiles of the calcined HMS samples.
Figure 1. Nitrogen adsorption–desorption isotherms (a) and pore size distribution (b) profiles of the calcined HMS samples.
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Catalysts 12 01061 g002 550
Figure 2. N2-BET (a) and pore size distribution (b) curves of HMS materials (Entry No.4) synthesized under different hydrothermal temperature.
Figure 2. N2-BET (a) and pore size distribution (b) curves of HMS materials (Entry No.4) synthesized under different hydrothermal temperature.
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Catalysts 12 01061 g003 550
Figure 3. The curves of N2-BET (a) and pore size distribution (b) for HMS samples were selected as the function of reaction time ranges from 4 h to 480 h.
Figure 3. The curves of N2-BET (a) and pore size distribution (b) for HMS samples were selected as the function of reaction time ranges from 4 h to 480 h.
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Catalysts 12 01061 g004 550
Figure 4. Small-angle XRD patterns of the calcined HMS samples.
Figure 4. Small-angle XRD patterns of the calcined HMS samples.
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Catalysts 12 01061 g005 550
Figure 5. TEM images of HMS calcined at 550 °C.
Figure 5. TEM images of HMS calcined at 550 °C.
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Catalysts 12 01061 sch001 550
Scheme 1. Formation mechanism of an inorganic network of HMS with different pore sizes by applying single non-ionic triblock copolymer (F127) under citric acid condition.
Scheme 1. Formation mechanism of an inorganic network of HMS with different pore sizes by applying single non-ionic triblock copolymer (F127) under citric acid condition.
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Catalysts 12 01061 g006 550
Figure 6. Small-angle XRD pattern (a) of the 5NiO15CeO2/HMS-CTA catalyst and H2-TPR (b) profile of 5NiO15CeO2/HMS-CTA sample, the inserted image (left) correspond to its nitrogen adsorption–desorption isotherms curve.
Figure 6. Small-angle XRD pattern (a) of the 5NiO15CeO2/HMS-CTA catalyst and H2-TPR (b) profile of 5NiO15CeO2/HMS-CTA sample, the inserted image (left) correspond to its nitrogen adsorption–desorption isotherms curve.
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Catalysts 12 01061 g007 550
Figure 7. DRM activity results for the 5Ni15CeO2/HMS-CTA catalyst, the left inserted image stands for the reaction mechanism, the right inserted image presents the weight loss curve of the used catalyst.
Figure 7. DRM activity results for the 5Ni15CeO2/HMS-CTA catalyst, the left inserted image stands for the reaction mechanism, the right inserted image presents the weight loss curve of the used catalyst.
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Table
Table 1. Preparation conditions and physicochemical properties of the synthesized hierarchical mesoporous silica.
Table 1. Preparation conditions and physicochemical properties of the synthesized hierarchical mesoporous silica.
Entry No.F127 Mass
(g)		Citric Concentration (mol/L)Hydrothermal Temp./Time (°C, h)pH ValueSBET
(m2/g)		Micropore Area
(m2/g)		Volume
(cm3/g)		Pore Size
(nm)
12.50100, 246.80332.929.30.7810.84
22.50.018100, 242.57651.188.80.956.80
32.50.055100, 242.20690.486.90.996.87
42.50.28100, 241.82739.986.40.754.78
52.50.55100, 241.63725.869.60.785.21
62.51.03100, 241.40703.274.20.885.76
72.51.82100, 241.09782.4142.70.613.45
82.50.2860, 241.82443.2181.60.293.18
92.50.28140, 241.82565.326.01.389.37
102.50.28100, 41.82502.3167.10.363.44
112.50.28100, 81.82546.2160.20.464.12
122.50.28100, 161.82702.6104.20.694.67
132.50.28100, 481.82792.330.30.955.13
142.50.28100, 721.82729.718.20.965.39
152.50.28100, 1401.82745.234.51.176.09
162.50.28100, 4801.82629.755.71.168.28
1701.03100, 241.40586.3NA1.237.65
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Li, L.; Deng, J.; Guo, Z.; Chu, W.; Liu, Y. Tailorable Formation of Hierarchical Structure Silica (HMS) and Its Application in Hydrogen Production. Catalysts 2022, 12, 1061. https://doi.org/10.3390/catal12091061

AMA Style

Li L, Deng J, Guo Z, Chu W, Liu Y. Tailorable Formation of Hierarchical Structure Silica (HMS) and Its Application in Hydrogen Production. Catalysts. 2022; 12(9):1061. https://doi.org/10.3390/catal12091061

Chicago/Turabian Style

Li, Luming, Jie Deng, Zhanglong Guo, Wei Chu, and Yan Liu. 2022. "Tailorable Formation of Hierarchical Structure Silica (HMS) and Its Application in Hydrogen Production" Catalysts 12, no. 9: 1061. https://doi.org/10.3390/catal12091061

APA Style

Li, L., Deng, J., Guo, Z., Chu, W., & Liu, Y. (2022). Tailorable Formation of Hierarchical Structure Silica (HMS) and Its Application in Hydrogen Production. Catalysts, 12(9), 1061. https://doi.org/10.3390/catal12091061

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