A New Application for Colloidal Silica Particles: Natural

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A new application for colloidal silica particles: natural, environmentally-friendly, low cost and reusable catalyst material for H2 production from NaBH4 methanolysis Nurettin Sahiner, and Alper O. Yasar Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03089 • Publication Date (Web): 13 Oct 2016 Downloaded from http://pubs.acs.org on October 19, 2016

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Industrial & Engineering Chemistry Research

A new application for colloidal silica particles: natural, environmentally-friendly, low cost and reusable catalyst material for H2 production from NaBH4 methanolysis Nurettin Sahiner†,‡,*, Alper O. Yasar‡ †

Nanoscience and Technology Research and Application Center (NANORAC), Canakkale, Turkey, 17100

‡

Faculty of Sciences and Arts, Chemistry Department, Canakkale Onsekiz Mart University, Terzioglu Campus, Canakkale 17100, Turkey

KEYWORDS: Silica particles; Silica particle catalyst; natural catalyst; environmentally-friendly catalyst; hydrogen production; methanolysis of NaBH4. ABSTRACT: Metal nanoparticles (NPs) and metal composites (MCs) are generally used as catalyst for the methanolysis reaction of NaBH4. Here, we report the direct use of silica (SiO2) particles as an alternative catalyst for green energy carriers in H2 generation from the methanolysis of NaBH4. The SiO2 particles of different sizes were synthesized using the very well-known Stöber method, and then treated with various acids such as hydrochloric acid (HCl), nitric acid (HNO3), sulfuric acid (H2SO4), phosphoric acid (H3PO4) and acetic acid (CH3COOH). The hydrodynamic diameter of prepared SiO2 particles ranged between 200-800 nm measured by Dynamic Light Scattering (DLS) with zeta potential values of about 46 and -37 mV. The BET surface area of SiO2 particles was measured between about 4-25 m2g-1. The catalytic performances of the prepared SiO2 particles in methanolysis of NaBH4 were tested in terms of the effect and the type of treatment agents, acid and base, the hydrodynamic diameter of SiO2 particles, the reusability of the particle as catalyst, the regeneration effects of the particles, the concentration of NaBH4, and temperature. It was found that SiO2 particles are better if not comparable catalysts with metal NPs in terms of high reusability for H2 production from the methanolysis reaction of NaBH4. The hydrogen generation rate (HGR) value from the methanolysis reaction of NaBH4 catalyzed by SiO2 particles treated with HCl was determined as 34000±840 mL H2 g-1min-1 using 20 mL 1000 mM NaBH4 and 50 mg SiO2 as catalyst. The true activity of SiO2 particles treated with HCl was calculated as approximately 60% even after the 10th use. The activation energy (Ea) value for the NaBH4 hydrolysis reaction catalyzed by SiO2 particles was also found to be comparable with the literature at 29.9 ± 0.4 kJ mol-1. According to the obtained results, the prepared SiO2 particles can be an alternative catalyst instead of metal nanoparticles.

INTRODUCTION Over the last few decades, silica (SiO2)-based materials have been a major focus of scientific and industrial research due to their being the most abundant material in nature and possessing excellent properties such as easy synthesis, chemical modifiability, tunable porosity and mechanical strength, environmentally friendly and biocompatible nature1–7. SiO2 particles can be commonly prepared via micelles forming from various surfactants 8–11 and the well-known Stöber process12. To prepare monodispersed SiO2 particles, the use of surfactants is preferable to the Stöber process, but the use of surfactant in large amounts compared to micelles is required8,11. And, the surfactant can cause impurity, and it is hard to remove from the solution containing SiO2 particles. In the well-known Stöber method, SiO2 particles are readily prepared by using just precursors such as tetraethoxysilane (TEOS) or tetramethoxysilane (TMOS) in a solution containing alcohol, ammonia and water in different proportions without any surfactants12,13.

Recently, renewable and clean energy resource research has gained prime importance due to harmful gas emissions such as COx, SOx and NOx arising from fossil fuel sources, and the depletion risk for fossil fuel in the near future14. Hydrogen (H2) energy is the one of most important subjects for renewable and benign energy generation and consumption systems as a renewable and clean energy resource with zero emissions 15,16. NaBH4 as a chemical hydride has high hydrogen storage capacity with 10.8% in mass theoretically. Therefore, NaBH4 is prevalently used in H2 production due to its stable and convenient properties described in the literature and the methanolysis reaction of NaBH4 is shown below17–19: NaBH4 + 4CH3OH NaB(OCH3)4 + 4H2

(1)

The methanolysis reaction of NaBH4 is commonly carried out in the presence of various metal catalysts such as Ru20, Co18, Ni21, Fe19 and so on. However, the time consuming preparation process, high cost and poor catalytic du-

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rability, and reusability of metal catalysts are some unfavorable properties22,23. Recently, metal-free catalysts have attracted great attention due to their unique properties such as being ecologically friendly, having low cost and highly active properties. Imidazolium-based polymeric ionic liquids (PILs) have been prepared as alternative to metal catalysts for H2 generation from methanolysis of NaBH4 24. In the present paper, SiO2 particles were prepared by using the modified Stöber process, and were used as catalyst in the methanolysis of NaBH4 to produce H2. The catalytic activity of SiO2 particles were systematically investigated after treatment with different acids and bases, such as HCl, HNO3, H2SO4, H3PO4 and CH3COOH, and NaOH, and their catalytic performance was compared with self-methanolysis of NaBH4. Additionally, H2 production from the methanolysis of NaBH4 catalyzed by these SiO2 particles was carried out to determine the optimum conditions, such as the effect of hydrodynamic diameter of SiO2 particles, reusability of SiO2 particles, the regeneration of SiO2 particles as catalyst, the concentration of NaBH4, and the reaction temperature of the methanolysis temperature (0-45 °C). EXPERIMENTAL SECTION Materials. To prepare SiO2 particles, tetraethoxysilane (TEOS) was used as silica source. The reaction solution was prepared with ammonia (NH4OH, 25% Aldrich) and ethanol (EtOH, pure Kimetsan). Hydrochloric acid (HCl, ACS reagent 37% Sigma-Aldrich), nitric acid (HNO3, ACS reagent 70% Sigma-Aldrich), sulfuric acid (H2SO4, ACS reagent 95-97% Sigma-Aldrich), phosphoric acid (H3PO4, 85% Merck), acetic acid (CH3COOH, 99.8-100.5% SigmaAldrich) and sodium hydroxide (NaOH, 98-100.5%, Sigma-Aldrich) were used as treatment agents. Sodium borohydride (NaBH4, 96% Merck) as H2 source and methanol (≥99.8, Aldrich) as solvent were used in the catalytic reactions. Distilled (DI) water with approximately 1.6 µS cm-1 (GFL 2108) was used to wash the prepared SiO2 particles. Preparation of SiO2 particles. SiO2 particles were prepared according to the modified Stöber method 12. In a typical process, 9.88 mmol TEOS was added into a solution containing 2.25 mL of 25% wt NH4OH and 15 mL ethanol, and this reaction solution was stirred at room temperature (RT) and 800 rpm. After 2 h, the reaction solution was centrifuged at 24680 g at 20 °C for 10 min. The prepared SiO2 particles were washed with DI water and ethanol twice each. Then, SiO2 particles were dried in an oven at 50 °C. The dried 0.5 g SiO2 particles were placed into 0.5 M 30 mL HCl, HNO3, H2SO4, H3PO4 or CH3COOH and mixed at 400 rpm for 1 h. Also, 0.5 M 30 mL NaOH solution was used for base treatment under the same conditions. Then, the treated SiO2 particles were centrifuged at 24680 g and 20 °C for 10 min and were

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washed with DI water and ethanol to remove excess acid or base on the surface of particles and centrifuged again. Finally, SiO2 particles were dried in an oven at 50 °C for characterization and catalysis reactions. To prepare SiO2 particles with different sizes, named as SiO2-4, SiO2-3 and SiO2-2, the same reaction conditions were used with various amounts of NH4OH such as 1, 1.5 and 1.75 mL, respectively. Therein, various amounts of NH4OH distinctly allowed to the preparation of SiO2 particles with various sizes. The prepared SiO2 particles: SiO2, SiO2-2, SiO2-3 and SiO2-4 with various hydrodynamic diameters (1 g each) were treated with 0.5 M 30 mL HCl solution, centrifuged for 10 min (24680 g and 20 °C) and washed with DI water and ethanol to remove excess acid, then dried in an oven at 50 °C for characterization and catalysis reactions. H2 generation tests. H2 generation experiments were conducted in a stirred thermostatic bath using a 50 mL flask. A certain amount of SiO2 particles, e.g., 50 mg, was used as catalyst for a certain amount of NaBH4 (125-1000 mM) in 20 mL methanol as H2 source in H2 generation reactions carried out at different reaction temperatures in the range of 0 to 45 °C at 1000 rpm. For H2 generation from NaBH4 using SiO2 particles as catalyst, the generated H2 volume (mL) was plotted as a function of reaction time. The H2 generation experiments were carried to determine the effects of various parameters, e.g., the effect of acid and base treatments, the types of acid as treatment agent, the hydrodynamic diameter of SiO2 particles, reuse of SiO2 particles, the effect of regeneration of SiO2 particles, the concentration of NaBH4 and the reaction temperature. The SiO2 particles that were used as catalyst in the methanolysis of NaBH4 were reutilized after applying a regeneration procedure. In the regeneration process, after use as a catalyst in the methanolysis reaction the SiO2 particles were centrifuged to remove reaction mixture, and were washed with DI water to remove adsorbed ions or impurities from the surface of SiO2 particles. Then, these SiO2 particles were treated with 0.5 M 30 mL HCl solution for 30 min, and re-centrifuged for 10 min (24680 g and 20 °C) to remove acid solution. Then, SiO2 particles were washed with DI water and ethanol and dried in an oven at 50 °C for reuse studies as catalyst in the methanolysis of NaBH4. Characterization. In the characterization studies, particle dimension, surface charge and surface area, structural and thermal properties were determined. The sizes of SiO2 particles were determined via dynamic light scattering (DLS) particle size analyzer (Brookhaven Ins. & Corp. 90 Plus) equipped with 35 mW solid-state laser detector at an operating wavelength of 658 nm. The DLS measurements were performed using 1-0.1 mg mL-1 (3 mL) SiO2 particles, dispersed in 10-3 M KCl aqueous solution at 25

2


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°C. Additionally, scanning electron microscopy (SEM) using an SEM (JEOL JSM-5600) with an operating voltage of 20 kV was used for visualization of SiO2 particles. The zeta potential analyzer BIC (Brookhaven Inst. Corp.) was used to determine surface charge using approximately 1 mL 1-0.1 mg mL-1 sample dispersed in 10-3 M KCl aqueous solution at 25 °C. The surface contaminants on SiO2 particles were removed by degassing at 80 °C for 8 h using a Flow Prep 060 degasser before surface area measurements. The Brunauer-Emmett-Teller (BET) method was used to determine surface area of SiO2 particles with various hydrodynamic diameters with a BET surface area measuring device (Micromeritics, TriStar II, Atlanta, GA). The structural characterization of SiO2 particles was carried out by Fourier Transform Infrared Spectroscopy (FTIR) (Thermo Nicolet iS10) with ATR technique. The FT-IR scans of SiO2 particles were performed in infrared in the region 650-4000 cm-1 at 4 cm-1 resolution using approximately 5 mg dried SiO2 particles. The thermal behavior of SiO2 particles was investigated by thermogravimetric (TG) analysis. TG analysis was performed from 50 to 900 °C with 10 °C min-1 heating rate under N2 flow of 20 mL min-1 using approximately 10 mg SiO2 particles. RESULTS AND DISCUSSION Characterization of SiO2 particles. SiO2 particles were prepared via modified Stöber method at different sizes, and their treatments with different acids and base scheme are presented in Figure 1(a). As depicted in Figure 1(a), -OH functional groups were generated on the surface of SiO2 particles by treatment with various acids such as HCl, HNO3, H2SO4, H3PO4 and CH3COOH.

Upon the treatment of SiO2 particles with NaOH, the surface gained negative charge (Si-Oˉ Na+). The SEM images of SiO2 particles (HCl-treatment) are given in Figure 1(b) with size of 811±45 nm SiO2 particles. As depicted in Figure 1(b), the SiO2 particles are monodispersed with hydrodynamic diameter of SiO2 approximately 700 nm. Figure 1(c) shows the FT-IR spectrum of SiO2 particles treated with HCl. It is obvious that the absorption bands between 3500-3200 cm-1 belong to O-H stretching. Also, the apparent intense characteristic absorption band at 1050 cm-1 is due to asymmetric vibration of Si-O25, and the asymmetric vibration of Si-OH at 943 cm-1, and symmetric vibration of Si-O at 795 cm-1 are in accordance with silica particle characteristic bands. Thermal behavior of the prepared SiO2 particles treated with HCl was determined by thermogravimetric (TG) analyzer.

Figure 1. (a) The preparation of SiO2 particles and their treatments with various acids and NaOH, (b) SEM image of SiO2 particles treated with HCl, (c) FT-IR spectrum of SiO2 particles, and (d) TG analysis of SiO2 particles.

The weight loss values of SiO2 particles were recorded against temperature under N2 atmosphere, and the obtained thermogram of SiO2 particles is shown in Figure 1(d). It is seen that the thermal degradation of SiO2 particles was observed in three stages. In the first step, the weight loss of SiO2 particles was approximately 6 wt% from 50 up to 190 °C. The second step of weight loss of SiO2 particles is in the range of 190-600 °C with weight loss of about 7wt%. The total weight loss of SiO2 particles was estimated as 13 wt% in the range of 50-600 °C. These weight losses between 50-600 °C can be attributed to condensation and hydrolysis of silica particles. Upon heating SiO2 particles up to 900 °C, little weight loss was observed in the third step, only about 1 wt%. Finally, SiO2 particles were degraded totally to approximately 14 wt% in the range of 50-900 °C. Size, surface charge and surface area of SiO2 particles. To determine hydrodynamic diameter and surface charge of prepared SiO2 particles, DLS and zeta potential measurements were conducted, and the results are shown in Table 1. It is obvious that hydrodynamic diameter of

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untreated SiO2 particles and SiO2 particles treated with HCl, HNO3, H2SO4, H3PO4, CH3COOH and NaOH was measured in range of 800-900 nm approximately. Table 1. The DLS and zeta potential measurements of SiO2 particles after treatment with various acids and NaOH. Particle with treatment agent

DLS* (nm)

Zeta Potential* (mV)

SiO2

811 ± 45

-43.6 ± 0.8

SiO2 (HCl)

823 ± 19

-37.7 ± 1.3

SiO2 (HNO3)

897 ± 32

-37.8 ± 1.4

SiO2 (H2SO4)

891 ± 56

-37.2 ± 2.6

SiO2 (H3PO4)

824 ± 19

-38.4 ± 1.3

SiO2 (CH3COOH)

851 ± 66

-42.3 ± 1.4

SiO2 (NaOH)

846 ± 33

-46.2 ± 1.2

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SiO2 particles was determined with Brunauer-EmmettTeller (BET) analysis. The BET experiments were carried out with nitrogen gas adsorption and desorption isotherms at 77 K. The adsorption-desorption isotherms obtained for the all SiO2 particles were categorized as Type III, that suggests non-porous structures 26. Additionally, BET surface area (m2g-1) of SiO2 particles with different hydrodynamic diameters were calculated from adsorption and desorption isotherms, and the obtained results are presented in Table 2. It is well known that the surface area of particles is related with size of the particles. Table 2. The result of DLS and BET measurements of SiO2 particles with different hydrodynamic diameters treated with HCl. 2

-1

Particle

DLS* (nm)

BET Surface area (m g )

SiO2

811 ± 45

4.29

*The all measurements were done in 10 M KCl at 25 C.

SiO2-2

394 ± 13

9.61

The hydrodynamic diameter of SiO2 particles measured by DLS measurements is a little bigger than the value visualized from SEM images. This could be due to the hydrophilic nature of SiO2 particles as on SEM images the particles are dried and have no molecular and hydrophilic interaction, whereas DLS measurements are carried out in in 10-3 M KCl aqueous solution. It is also apparent that the prepared SiO2 particles are monodispersed as a standard deviation of about 5% was obtained from DLS measurements. The zeta potential of SiO2 particles was measured as it is related surface functionalities, e.g., -OH groups on the surface of the particles. Table 1 shows zeta potential values of untreated SiO2 particles, and SiO2 particles treated with HCl, HNO3, H2SO4, H3PO4, CH3COOH and NaOH. As can be seen the measured values of zeta potential are in the range of -37 and -46 mV approximately. The zeta potential value of untreated SiO2 particles was determined as -43.6±0.8 mV. Upon the treatment of SiO2 particles with 0.5 M NaOH solution, the zeta potential value was slightly decreased to -46.2 ± 1.2. After treatment with various acids the prepared SiO2 particles had zeta potential values measured as -37.7±1.3 mV, -37.8±1.4 mV, 37.2±2.6 mV, -38.4±1.3 mV and, -42.3±1.4 mV for HCl, HNO3, H2SO4 H3PO4, and CH3COOH, respectively. It is apparent that when SiO2 particles are treated with various acids, their zeta potential values are increased very little in comparison to the zeta potential of SiO2 particles that are treated with a base or are not treated.

SiO2-3

264 ± 9

14.21

SiO2-4

186 ± 5

24.52

-3

o

The surface area of solid catalysts is an important parameter as the catalytic activity of the catalyst is directly related to the catalyst surface area. The surface area of the

-3

o

*The measurements were done in 10 M KCl at 25 C.

The BET surface areas were calculated as 24.52, 14.21, 9.61 and 4.29 m² g-1 for 186, 264, 394 and 811 nm SiO2 particles. It is apparent that BET surface area increased with the decrease in the hydrodynamic diameter of SiO2 particles as expected. Comparison of catalytic performance of acid- and base-treated SiO2 particles in methanolysis of NaBH4. To determine the catalytic performances of SiO2 particles before and after treatment with HCl and NaOH, the H2 production volume (mL) versus time graphs were constructed along with those for the self methanolysis of NaBH4 and the results are illustrated in Figure 2(a). The catalytic activity of bare SiO2 particles (811±45 nm) and SiO2 particles treated with HCl, and self methanolysis of NaBH4 produces H2 much faster than NaOH-treated SiO2 particles. The self-methanolysis of NaBH4 reaction was completed in 35 min, and all the H2 production experiments finished with 125 mM NaBH4 prepared in 20 mL methanol, 1000 rpm mixing rate, at 25 °C using 50 mg particles as catalyst with the exception of self methanolysis. The methanolysis reaction of non-treated SiO2 particles was completed in 23 min. After treatment of SiO2 particles with HCl, the hydrolysis reaction was completed in 14 min, keeping in mind that all hydrolysis reactions produced the same amount of H2, about 250 mL resulting

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Generated H2 volume (mL)

in 100% conversion. The SiO2 particles treated with NaOH did not show good catalytic performance in the methanolysis of NaBH4 due to their basic surface character. There is no H2 generation in 10 min, and then approximately 100 mL H2 was produced between 10-45 min in the methanolysis of NaBH4. Additionally, to check the effect of methanol, only 2o mL methanol mixed at 25 °C for 45 min, and there is no displacement or gas evolution in graded cylinder that is used in H2 measurement experiments was observed. It is clearly seen that the surface of SiO2 particles treated with HCl generated more -OH groups, in comparison to the negatively charged oxygen atoms (-O-Na+ as shown in Fig. 1(a)) on the surface of SiO2 particles after treatment with NaOH that resulted in slight catalytic activity as seen in Figure 2(a). It was reported that the catalysts with acidic character possess higher catalytic activity in the methanolysis of NaBH4 along with the suitable explanation of the catalytic mechanisms24,27,28. In this investigation, similar results were obtained, and SiO2 particles treated with HCl used as catalyst demonstrated better catalytic activity than SiO2 particles treated with NaOH in the methanolysis of NaBH4 reactions. (a)

250 200

Self-methanolysis Self methanolysis of NaBH4 NaBH4 Non-treated SiO2 SiO2-non-treatment SiO2 (NaOH) SiO2-NaOH SiO2 (HCl) SiO2-HCl Control

150 100 50 0 0

10

20

30

40

50


Time (min) 250

(b)

200 SiO2-HCl HCl SiO2-NO3 HNO3 SiO2-SO4 H2SO4 H3PO4 SiO2-PO4 CH3COOH SiO2-CH3COO

150 100 50 0 0

5

10

15 Time (min)

20

25

30

1500 HGR (mL H2 g-1 min-1)

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(c) 1200 900 600 300 0

Figure 2. (a) The effect of acid, and base treatments of SiO2 particles, (b) the effect of acid types used in the treatment of

SiO2 particles, and (c) HGR values of acid treated SiO2 particles in H2 production from the methanolysis of NaBH4 [Reaction conditions: 50 mg particle, 125 mM NaBH4 in 20 o mL methanol, 25 C, 1000 rpm].

Furthermore, the prepared SiO2 particles were treated with various acid solutions (0.5 M, 30 mL) such as HCl, HNO3, H2SO4, H3PO4 and CH3COOH, to understand their effects on catalytic activities for H2 production from the methanolysis of NaBH4. As demonstrated in Figure 2(b), about 250 mL H2 was produced from the methanolysis of NaBH4 with all the different acid-treated SiO2 particles as catalyst. H2 production in the presence of SiO2 particles treated with HCl, HNO3, H2SO4 and H3PO4 was completed in 14 min, whereas the H2 production using SiO2 particles treated with CH3COOH was completed in 26 min. This almost 2-fold decrease for the H2 production performance of SiO2 particles treated with CH3COOH can be attributed to its weak acid nature but still even this particle can complete 100% conversion in a longer time. The H2 generation is initially fast, then as time passes it slows down towards the end of reaction due to the depletion of NaBH4 in the reaction mixture. For this reason, hydrogen generation rate (HGR) values for H2 production from the methanolysis of NaBH4 using SiO2 particles as catalyst were calculated from the linear part of the volume of generated H2 as a function of time graphs (until about 200 mL H2). The HGR values of H2 production catalyzed by SiO2 particles treated with HCl, HNO3, H2SO4, H3PO4 and CH3COOH are compared in Figure 2(c). The HGR values for SiO2 particles treated with HCl, HNO3, H2SO4, H3PO4 and, CH3COOH were calculated as 1182±94, 1169±126, 1150±122, 1091±97, and 532±45 mL H2 g-1min-1. Therefore, it can be assumed that the SiO2 particles treated with HCl, HNO3, H2SO4, and H3PO4 show almost the same catalytic performance, whereas the catalytic performance of SiO2 particles treated with CH3COOH is much lower (almost 2-fold). As the catalytic activity of SiO2 particles treated with HCl is slightly better than the rest of the SiO2 particles treated with other acids, HCltreated SiO2 particles were used for the rest of this investigation. It is well known that the comparison of acid strengths of the acids used in this studies: HCl>H2SO4>HNO3>H3PO4>CH3COOH29. HGR value of methanolysis of NaBH4 catalyzed by SiO2 particles treated with HCl is slightly better than the other particles, and this result is in accordance with catalytic performance of these various acid treated SiO2 catalyst. The catalytic activity of SiO2 particles treated with HCl, H2SO4, HNO3 and H3PO4 is almost the same because their acid strength is almost the same to transfer the hydrogen atoms to the SiO2 particles surfaces. However, as CH3COOH is weaker acid than the other acids in this investigation, the HGR value of SiO2 particles treated with CH3COOH is lower in methanolysis of NaBH4 in comparison to the other acids treated SiO2 particles.

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Industrial & Engineering Chemistry Research The effect of hydrodynamic diameter of SiO2 particle catalyst in methanolysis of NaBH4. The catalytic performances of prepared SiO2 particles with different hydrodynamic diameters such as 811 nm (SiO2), 394 nm (SiO2-2), 264 nm (SiO2-3) and 186 nm (SiO2-4) treated with HCl were used in the methanolysis of NaBH4 and their H2 production rates are shown as a function of time in Figure 3(a). As can be seen in Figure 3(a), the methanolysis reactions of NaBH4 in the presence of SiO2 particles with different hydrodynamic diameters were completed in 14 min. Although hydrodynamic diameter of SiO2 particles decreased four fold, the completion times of the methanolysis reactions were almost the same.

each methanolysis reaction medium. The methanolysis reaction of NaBH4 was completed in approximately 14 min for each of the ten consecutive use. Therefore, it is obvious that as the methanolysis reaction times of NaBH4 are almost the same, the SiO2 particles as catalyst possess great potential for industrial applications as a reusable catalyst. The HGR values of the self-methanolysis of NaBH4, the methanolysis of NaBH4 catalyzed by HCl-SiO2, and the regenerated HCl-SiO2 particles are given in Figure 4(a). (a)

250

HGR (mL H2 min-1 g-1)


1500

(a)

200 ± 5 SiO2 nm SiO2 186 nm

150

± 9 SiO2 nm SiO2 264 nm

100 50

4

8 12 Time (min)

600 300

± 45SiO2 nm SiO2 811 nm

(b)

16

1

20

2000

(b) 1600 1200

HCl-SiO SiO2 (HCl) 2

Regenerated SiO2 HCl-SiO regeneration (HCl) 2

900

0 2

3

4 5 6 7 The number of usage

8

9

10

9

10

1000 HGR (mL H2 min-1 g-1)

0

Self-methanolysisof ofNaBH4 NaBH4 Self-methanolysis

1200

± 13SiO2 nm SiO2 394 nm 0

Hydrogen generation Rate (HGR) (mL H2 g-1 min-1)

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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SiO2 True (HCl) activity of HCl-SiO2 800

Regenerated True activityHCl-SiO2 of regenerated HCl-SiO2

600 400 200 0

800

1

2

400 0 5 9 13 811811 45 186186 nm±SiO2 264264 nm±SiO2 394394 nm±SiO2 nm±SiO2 The hydrodynamic diameter of SiO2 particles (nm)

Figure 3. (a) The effect of the hydrodynamic diameter of SiO2 particles treated with HCl in H2 production from the methanolysis of NaBH4 and (b) their HGR values [Reaction conditions: 50 mg particle, 125 mM NaBH4 in 20 mL methanol, 25 ºC, 1000 rpm].

The HGR values were calculated between 1180-1220 mL H2 g-1min-1 and are shown in Figure 3(b). The largest SiO2 particles (811 nm) prepared and treated with HCl were used as catalyst in the next experiments as they can be easily collected from the reaction media, and because of ease of centrifugation and handling. The reuse and regeneration of SiO2 particle catalyst for methanolysis of NaBH4. The reuse of HCl-treated SiO2 particles as catalyst was investigated for H2 generation experiments from the methanolysis of NaBH4. The experiments were successively proceeded ten times in the same reaction media by just adding NaBH4 (0.0965 g) for

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Figure 4. (a) The reuse of self-methanolysis of NaBH4, HClSiO2 particles, and the regenerated HCl-SiO2 particles as catalyst in H2 production from the methanolysis of NaBH4 (b) true activity values of NaBH4 methanolysis reactions catalyzed by HCl-SiO2 particles [Reaction conditions: 50 mg o particle, 125 mM NaBH4 in 20 mL methanol, at 25 C, 1000 rpm].

The regeneration was done by 0.5 mHCl treatments of the used SiO2 particles as catalyst. It is apparent that HGR values for the self-methanolysis of NaBH4 are almost the same at every use and they were calculated as approximately 400 mL H2 g-1min-1 at every usage. The methanolysis reactions of NaBH4 catalyzed by HCl-SiO2 and regenerated HCl-SiO2 particles were performed, and their HGR values were illustrated in Figure 4(a). As can be seen, the HGR value of 1st methanolysis reaction of NaBH4 catalyzed by HCl-SiO2 were found as 1182±94 mL H2 g-1min-1 and HGR values between 2nd-1oth methanolysis reactions were calculated in the range of approximately 850-1000 mL H2 g-1min-1. The decrease in the catalytic activity of HCl-SiO2 particles can arise from by-product formation due to

6


NaBH4 methanolysis such as sodium tetramethoxyborate (NaB(OCH3)4, and the ability of this by product to coat SiO2 particles surfaces resulting in deactivation. Therefore, after 10th use of HCl-SiO2 particles as catalyst for the methanolysis of NaBH4, they were washed with DI water and regenerated by treating with 0.5 M 30 mL HCl solution for 30 min to remove (NaB(OCH3)4 from the surface of SiO2 particles. Then, the catalytic activity of the regenerated HCl-SiO2 particles was tested ten times consecutively again under the same conditions in H2 generation experiments from the methanolysis of NaBH4 and the corresponding data is shown in Figure 4(a). All methanolysis reactions of NaBH4 catalyzed by HCl-SiO2 particles resulted in 100% conversion, and after regeneration of HCl-SiO2 particles, HGR values of methanolysis reactions were also determined in the range of 970-815 mL H2 g-1min-1. Although a slight decrease in HGR value of HClSiO2 particles was observed at the end of the 10th use, this is a promising result and fairly good catalytic activity performance in comparison to the some other catalysts reported in the literature21,30. Therefore, it is safe to say that HCl-SiO2 particles with ready regeneration ability is a feasible catalyst for H2 production from the methanolysis of NaBH4, and offers great potential as an alternative to toxic and expensive metal nanoparticles in real use. True activity values were calculated for methanolysis of NaBH4 catalyzed by HCl-SiO2 and regenerated HCl-SiO2 particles were done using equation (2), and their results is shown in Figure 4(b). True activity = activity over catalyst (HGR value) - activity without catalyst (HGR value) (2) True activity values of HCl-SiO2, and the regenerated HCl-SiO2 particle as HGR values for 1st use were found as 784±43 and 664±71 mL H2 g-1min-1, respectively. It is apparent that after regeneration of HCl-SiO2 particles as catalyst, the HGR value is decreased only about 15% under the same reaction conditions. It is shown that the prepared HCl-SiO2 particles as catalyst were demonstrated an efficient catalytic activity and great reusability. In the end of 10th use for HCl-SiO2, and regenerated HCl-SiO2 particle as catalyst in methanolysis reactions, HGR values were decreased to 468±37 and 463±47 mL H2 g-1min-1, respectively which about 60% reductions. Moreover, zeta potential values of HCl-SiO2 after NaBH4 methanolysis and regenerated HCl-SiO2 particle were measured as -73.0±0.6 and -49.7±1.0 mV. Additionally, it is known that zeta potential value of the prepared HCl-SiO2 particles is measured as -37.7±1.3 mV (Table 1). It is clearly seen that upon the use of HCl-SiO2 particles as catalyst for NaBH4 methanolysis, the zeta potential value was remarkably decreased to -73.0±0.6 mV from -37.7±1.3 mV. Similarly, it is apparent that HGR values were also decreased for NaBH4 methan0lysis. Therefore, HCl-SiO2

particles were regenerated by the treatment with HCl solution, and its zeta potential value were increased to 49.7±1.0 mV from -73.0±0.6 mV. Similarly, HGR value is also increased for the same particles in NaBH4 methan0lysis under same conditions. The effect of concentration of NaBH4 on the methanolysis of NaBH4 catalyzed by SiO2 particles. The H2 generation experiments from the methanolysis of NaBH4 catalyzed by HCl-SiO2 particles were tested for different NaBH4 concentrations such as 125, 250, 500 and 1000 mM in 20 mL methanol, and the obtained corresponding curves are given Figure 5(a). As can be seen, the H2 produced was approximately 254 mL for 125 mM NaBH4, 520 mL for 250 mM NaBH4, 1020 mL for 500 mM NaBH4 and 2010 mL for 1000 mM NaBH4 using 20 mL methanol as solvent. 125 125mM mMNaBH4 NaBH4 500mM mMNaBH4 NaBH4 500

Generated H2 (g) Volume (mL)

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


250 250mM mMNaBH4 NaBH4 1000mM mMNaBH4 NaBH4 1000

2000

(a) 1600 1200 800 400 0 0

4

8

12

16

20

Time (min)

Hydrogen generation Rate (mL H2 g-1 min-1)

Page 7 of 19

40000 35000 30000 25000 20000 15000 10000 5000 0

(b)

y = 38.431x - 5612.9 R² = 0.9808 0

250

500

750

1000

The concentration of NaBH4 (mM)

Figure 5. (a) The effect of NaBH4 concentration on H2 production from the methanolysis of NaBH4, and (b) their HGR values. [Reaction conditions: 50 mg particle, 125 mM NaBH4 in 20 mL methanol, 25 ºC, 1000 rpm].

It is well-known that 4 mole H2 is theoretically obtained from 1 mole of NaBH4 in the methanolysis reaction of NaBH4. Herein, experimentally generated volume of H2 is almost exactly the same as the theoretically calculated volume of H2. So, H2 generation from NaBH4 was calculated as 100% conversion for every concentration of NaBH4. As can be seen in Fig. 5 (b), the graph of HGR value versus the amount of NaBH4 (mM) shows a linear relationship. Therefore, according to the concentration of NaBH4

7


Industrial & Engineering Chemistry Research (mM), the methanolysis reaction of NaBH4 is first order as expected19. HGR of these methanolysis reactions of NaBH4 catalyzed by using the same amount of (50 mg) HCl-SiO2 particles was calculated with different NaBH4 concentrations, and HGR values were found as 1182±94 mL H2 g1 min-1 for 125 mM NaBH4, 353±98 mL H2 g-1min-1 for 250 mM NaBH4, 11147±470 mL H2 g-1min-1 for 500 mM NaBH4 and 34047±844 mL H2 g-1min-1 for 1000 mM NaBH4 (20 mL methanol as solvent). As can be seen, the HGR value reached a very high value in H2 production using 1000 mM NaBH4. This HGR value is greater than most of the studies reported in the literature. For example, 15000 mL H2 g-1min-1 for Ni2P-TOP(Ar) 21 and 18750 mL H2 g-1min-1 for Ni2P-TOP(H2) 21, 5487 mL H2 g-1min-1 for Fe-B NPs19, 5871 mL H2 g-1min-1 for p(C6VImBr)24, 4400 mL H2 g-1min-1 for Co/Al2O18. Therefore, because of the readily attainable value of HGR, along with ability to reuse a large number of times and regeneration ability, HCl-SiO2 catalyst offers great advantages over conventional MNP catalysts used for the same purpose. Effect of temperature on the methanolysis of NaBH4 catalyzed by HCl-SiO2 particles. In order to estimate the activation energy (Ea) of methanolysis reactions of NaBH4 catalyzed by HCl-SiO2 particles, the reactions were carried out in the presence of 50 mg SiO2 particles as catalyst in 125 mM 20 mL methanol under 1000 rpm mixing rate at various temperatures such as 0, 15, 25, 35 and 45 °C. About 250 mL H2 was generated in 42, 20, 14, 8 and 6 min for 0, 15, 25, 35 and 45 °C as shown in Figure 6(a). As expected, the methanolysis reactions of NaBH4 increased with the increase in the temperature. Generated H2 volume (mL)

250 (a) 200 0 °C

150

15 °C 100

25 °C

50

35 °C 45 °C

0 0

10

20

30

40

50

Time (min) -2

(b) 29.9 ± 0.4 kJ mol-1

-3

ln k

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

-4 -5

y = -3588.3x + 8.113 R² = 0.9681

-6 0.0031

0.0033

0.0035 1/T

0.0037

Figure 6. (a) The effect of temperature on the methanolysis NaBH4 catalyzed by SiO2 particles treated with HCl catalysts, and (b) the corresponding Arrhenius plot of ln k vs 1/T.

Page 8 of 19

The Ea value of the methanolysis reaction of NaBH4 catalyzed by HCl-SiO2 particles was calculated according to the Arrhenius equation. ln k = ln A - (Ea/RT)

(3)

In the Arrhenius equation, k is the reaction rate constant, A is the pre-exponential factor, Ea is activation energy, R is gas constant (8.314 J K-1 mol-1) and T is temperature (K). The Ea value was determined as 29.9 ± 0.4 kJ mol-1 from the slope of the Arrhenius plot, as illustrated in Figure 6(b). The Ea of value for the NaBH4 methanolysis reaction catalyzed by HCl-SiO2 particles is comparable with some the catalysts in the literature such as 62.99 kJ mol-1 for uncatalyzed18, 21.6 kJ mol-1 for P/boehmite30, 21.89 kJ mol-1 for Co/Al2O318, 25.21 and 29.07 kJ mol-1 for CoCl231. It is obvious that the prepared HCl-SiO2 particles showed good catalytic activity and low Ea value (29.9 ± 0.4 kJ mol1 ) for methanolysis reactions of NaBH4 and could be used in H2 power systems with ability to ensure the reaction occurs even at about 0 oC. CONCLUSION H2 generation from methanolysis of NaBH4 catalyzed by HCl-SiO2 particles offers a new, simple benign, environmentally friendly catalyst system with proven catalytic performances. The sizes of SiO2 particles can be tuned from about 200 – 800 nm resulting in almost the same catalytic activity as catalyst for methanolysis of NaBH4. More importantly, the reuse of HCl-SiO2 particles for H2 generation from methanolysis of NaBH4 is evidence that this silica particle has 100% conversion each time with up to 10 consecutive uses showing about 80% catalytic activity for each use after the first use. Moreover, after 10 repetitive uses of HCl-SiO2 particles for methanolysis of NaBH4, the particles were regenerated by simple HCl treatment and used in methanolysis of NaBH4 ten times more with catalytic performances comparable to the first 10 uses. The Ea value for methanolysis of NaBH4 catalyzed by HClSiO2 particles was determined as 29.9 ± 0.4 kJ mol-1 which is comparable with most of the metal catalysts used in the methanolysis of NaBH4 reported the literature. Therefore, in terms of low cost, high stability, simple preparation methods, reusability and low Ea in contrast to metal nanoparticles with expensive, long preparation process, low stability (easy deactivation, oxidation and aggregation, etc.) and complicated synthesis methods, lesser reusability and high Ea values, the HCl-SiO2 particles are favored materials for industrial applications. It is well known that silicon is the 8th most common element in the Milky Way Galaxy by mass, and SiO2 particles are biocompatible32,33, therefore HCl-SiO2 particles may be an environmentallyfriendly and reusable catalyst for H2 generation from methanolysis of NaBH4 and are expected to find use in real applications in industry.

8


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AUTHOR INFORMATION *Corresponding Author: E-mail: [email protected] (N. Sahiner) Tel: +90-2862180018-2041; Fax: +90-2862181948

ACKNOWLEDGMENTS Financial support from the Scientific and Technological Research Council of Turkey (TUBITAK-115M021) is greatly acknowledged.

REFERENCES (1) Huang, H. H.; Orler, B.; Wilkes, G. L. StructureProperty Behavior of New Hybrid Materials Incorporating Oligomeric Species into Sol-Gel Glasses. 3. Effect of Acid Content, Tetraethoxysilane Content, and Molecular Weight of Poly(dimethylsiloxane). Macromolecules 1987, 20, 1322–1330. (2) Li, H.; Xiao, H. G.; Yuan, J.; Ou, J. Microstructure of Cement Mortar with Nano-Particles. Compos. Part B Eng. 2004, 35, 185–189. (3) Naji Givi, A.; Abdul Rashid, S.; Aziz, F. N. A.; Salleh, M. A. M. Experimental Investigation of the Size Effects of SiO2 Nano-Particles on the Mechanical Properties of Binary Blended Concrete. Compos. Part B Eng. 2010, 41, 673–677. (4) Kokubo, T. Bioactive Glass Ceramics: Properties and Applications. Biomaterials 1991, 12, 155–163. (5) Yang, H.; Zhuang, Y.; Hu, H.; Du, X.; Zhang, C.; Shi, X.; Wu, H.; Yang, S. Silica-Coated Manganese Oxide Nanoparticles as a Platform for Targeted Magnetic Resonance and Fluorescence Imaging of Cancer Cells. Adv. Funct. Mater. 2010, 20, 1733–1741. (6) Anglin, E. J.; Cheng, L.; Freeman, W. R.; Sailor, M. J. Porous Silicon in Drug Delivery Devices and Materials. Adv. Drug Deliv. Rev. 2008, 60, 1266–1277. (7) Sahiner, N.; Yasar, A. O.; Aktas, N. Chemical Hydride Hydrolysis For H2 Production Via Co, Cu, Ni Metal Nanoparticles Prepared Within P(4-VP) Capsules. Curr. Nanomater. 2016, 1, 3–11. (8) Wu, X.; Tian, Y.; Cui, Y.; Wei, L.; Wang, Q.; Chen, Y. Raspberry-like Silica Hollow Spheres: Hierarchical Structures by Dual Latex - Surfactant Templating Route. J. Phys. Chem. C 2007, 111, 9704–9708. (9) Pileni, M.-P. No TitleThe Role of Soft Colloidal Templates in Controlling the Size and Shape of Inorganic Nanocrystals. Nat. Mater. 2003, 2, 145–150. (10) Cauvel, a; Brunel, D.; DiRenzo, F.; Garrone, E.; Fubini, B. Hydrophobic and Hydrophilic Behavior of Micelle-Templated Mesoporous Silica. Langmuir 1997, 13, 2773–2778. (11) Li, T.; Moon, J.; Morrone, A. A.; Mecholsky, J. J.; Talham, D. R.; Adair, J. H. Preparation of Ag/SiO2 Nanosize Composites by a Reverse Micelle and Sol-Gel Technique. Langmuir 1999, 15, 4328–4334. (12) Stöber, W.; Fink, A. Controlled Growth of Monodisperse Silica Spheres in the Micron Size Range. J. Colloid Interface Sci. 1968, 26, 62–69. (13) Wang, X. D.; Shen, Z. X.; Sang, T.; Cheng, X. B.; Li, M. F.; Chen, L. Y.; Wang, Z. S. Preparation of Spherical Silica Particles by St??ber Process with High Concentration of TetraEthyl-Orthosilicate. J. Colloid Interface Sci. 2010, 341, 23–29.

(14) Hadjipaschalis, I.; Poullikkas, A.; Efthimiou, V. Overview of Current and Future Energy Storage Technologies for Electric Power Applications. Renew. Sustain. Energy Rev. 2009, 13, 1513–1522. (15) Hosseini, S. E.; Wahid, M. A. Hydrogen Production from Renewable and Sustainable Energy Resources: Promising Green Energy Carrier for Clean Development. Renew. Sustain. Energy Rev. 2016, 57, 850–866. (16) Bhogilla, S. S.; Ito, H.; Kato, A.; Nakano, A. Research and Development of a Laboratory Scale Totalized Hydrogen Energy Utilization System. Int. J. Hydrogen Energy 2015, 41, 1224– 1236. (17) Fernandes, V. R.; Pinto, A. M. F. R.; Rangel, C. M. Hydrogen Production from Sodium Borohydride in MethanolWater Mixtures. Int. J. Hydrogen Energy 2010, 35, 9862–9868. (18) Xu, D.; Zhao, L.; Dai, P.; Ji, S. Hydrogen Generation from Methanolysis of Sodium Borohydride over Co/Al2O3 Catalyst. J. Nat. Gas Chem. 2012, 21, 488–494. (19) Ocon, J. D.; Tuan, T. N.; Yi, Y.; De Leon, R. L.; Lee, J. K.; Lee, J. Ultrafast and Stable Hydrogen Generation from Sodium Borohydride in Methanol and Water over Fe-B Nanoparticles. J. Power Sources 2013, 243, 444–450. (20) Su, C.-C.; Lu, M.-C.; Wang, S.-L.; Huang, Y.-H. Ruthenium Immobilized on Al2O3 Pellets as a Catalyst for Hydrogen Generation from Hydrolysis and Methanolysis of Sodium Borohydride. RSC Adv. 2012, 2, 2073–2079. (21) Yan, K.; Li, Y.; Zhang, X.; Yang, X.; Zhang, N.; Zheng, J.; Chen, B.; Smith, K. J. Effect of Preparation Method on Ni2P/SiO2 Catalytic Activity for NaBH4 Methanolysis and Phenol Hydrodeoxygenation. Int. J. Hydrogen Energy 2015, 40, 16137– 16146. (22) Shang, Y.; Chen, R. Hydrogen Storage via the Hydrolysis of NaBH4 Basic Solution: Optimization of NaBH4 Concentration. Energy & Fuels 2006, 20, 2142–2148. (23) Lu, L.; Zhang, H.; Zhang, S.; Li, F. A Family of HighEfficiency Hydrogen-Generation Catalysts Based on Ammonium Species. Angew. Chemie - Int. Ed. 2015, 54, 9328–9332. (24) Sahiner, N.; Yasar, A. O. Imidazolium Based Polymeric Ionic Liquid Microgels as an Alternative Catalyst to Metal Catalysts for H2 Generation from Methanolysis of NaBH4. Fuel Process. Technol. 2016, 152, 316–324. (25) Ou, D. L.; Seddon, A. B. Near- and Mid-Infrared Spectroscopy of Sol–gel Derived Ormosils: Vinyl and Phenyl Silicates. J. Non. Cryst. Solids 1997, 210, 187–203. (26) Sing, K. S. W. Reporting Physisorption Data for Gas/solid Systems with Special Reference to the Determination of Surface Area and Porosity (Recommendations 1984). Pure Appl. Chem. 1985, 57, 603–619. (27) Davis, R. E.; Gottbrath, J. A. Boron Hydrides. V. Methanolysis of Sodium Borohydride. J. Am. Chem. Soc. 1962, 84, 895–898. (28) Hannauer, J.; Demirci, U. B.; Pastor, G.; Geantet, C.; Herrmann, J. M.; Miele, P. Hydrogen Release through Catalyzed Methanolysis of Solid Sodium Borohydride. Energy Environ. Sci. 2010, 3, 1796–1803. (29) Housecroft, C.; Sharpe, A. G. Inorganic Chemistry, 2nd edition.; Pearson Educaiton Limited: Essex, UK, 2005. (30) Xu, D.; Zhang, Y.; Cheng, F.; Zhao, L. Enhanced Hydrogen Generation by Methanolysis of Sodium Borohydride in the Presence of Phosphorus Modified Boehmite. Fuel 2014, 134, 257–262.

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(31) Lo, C. T. F.; Karan, K.; Davis, B. R. Kinetic Assessment of Catalysts for the Methanolysis of Sodium Borohydride for Hydrogen Generation. Ind. Eng. Chem. Res. 2009, 48, 5177–5184. (32) Tang, F.; Li, L.; Chen, D. Mesoporous Silica Nanoparticles: Synthesis, Biocompatibility and Drug Delivery. Adv. Mater. 2012, 24, 1504–1534. (33) Asefa, T.; Tao, Z. Biocompatibility of Mesoporous Silica Nanoparticles. Chem. Res. Toxicol. 2012, 25, 2265–2284.

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Table of Contents

11



HO

(a) C2H5

O C2H5 O Si O C2H5 O C2H5

TEOS

Ethanol NH4OH

HO +

H 4N - O

0.5 M acid

HO

1 h, 25 C

HO

-

O NH4

HO

+

0.5 M NaOH

1 h, 25 C

OH OH OH

HO

SiO2 particle

Na

OH O-NH4+

OH

HO

OH

HO

2h 800 rpm

(b)

Na

O

Na O

O

Na

O

Na O

O

Na

O

Na

O

Na

Negative charged SiO2 particle SEM images of SiO2 particles

Acids: HCl, HNO3, H2SO4, H3PO4 and CH3COOH

T% (a.u.)

(c) O-H stretching 3500 – 3200 cm-1

4000

(d)

3500

Si-O symmetric vibration 795 cm-1 Si-OH asymmetric vibration 943 cm-1 Si-O asymmetric vibration 1050 cm-1

3000

2500 2000 1500 Wavenumbers (cm-1)

1000

650

100

TG %

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

Page 12 of 19

95 90 85 100 200 300 400 500 600 700 800 900 Temperature ( C)

Figure 1. (a) The preparation of SiO2 particles and their treatments with various acids and NaOH, (b) SEM image of SiO2 particles treated with HCl, (c) FT-IR spectrum of SiO2 particles, and (d) TG analysis of SiO2 particles.

1 ACS Paragon Plus Environment

(a)

250 200

Self-methanolysis Self methanolysis of of NaBH4 NaBH4 SiO2-non-treatment Non-treated SiO2 SiO2-NaOH SiO2 (NaOH) SiO2-HCl SiO2 (HCl) Control

150 100 50

0 0

10

20

30

40

50


Time (min) 250

(b)

200 SiO2-HCl HCl SiO2-NO3 HNO3 SiO2-SO4 H2SO4 SiO2-PO4 H3PO4 CH3COOH SiO2-CH3COO

150 100 50 0

0

5

10

15 Time (min)

20

25

30

1500 HGR (mL H2 g-1 min-1)

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



Page 13 of 19

(c) 1200

900 600 300

0

Figure 2. (a) The effect of acid and base treatments of SiO2 particles, (b) the effect of acid types used in the treatment of SiO2 particles, and (c) HGR values of acid treated SiO2 particles in H2 production from the methanolysis of NaBH4. [Reaction conditions: 50 mg particle, 125 mM NaBH4 in 20 mL methanol, 25 ºC, 1000 rpm]. 2 ACS Paragon Plus Environment

250

Page 14 of 19

(a)

200 150

186 nm5 SiO2 nm SiO2

100

264 nm9 SiO2 nm SiO2 394 nm13SiO2 nm SiO2

50

811 nm45SiO2 nm SiO2

0

0

Hydrogen generation Rate (HGR) (mL H2 g-1 min-1)

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



4

8 12 Time (min)

16

20

2000

(b) 1600 1200

800 400 0

5 9 13 811811 186186 nm SiO2 264264 nm SiO2 394394 nm SiO2 nm SiO2 45 The hydrodynamic diameter of SiO2 particles (nm)

Figure 3. (a) The effect of the hydrodynamic diameter of SiO2 particles treated with HCl in H2 production from the methanolysis of NaBH4 and (b) their HGR values [Reaction conditions: 50 mg particle, 125 mM NaBH4 in 20 mL methanol, 25 ºC, 1000 rpm].


Page 15 of 19

(a) HGR (mL H2 min-1 g-1)

1500

Self-methanolysis Self-methanolysis of ofNaBH4 NaBH4

SiO2 (HCl) HCl-SiO 2

regeneration (HCl) Regenerated SiO2 HCl-SiO 2

1200 900 600 300 0 1

2

3

(b)


8

9

10

9

10

1000 HGR (mL H2 min-1 g-1)

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


SiO2 True (HCl) activity of HCl-SiO2

800

Regenerated True activityHCl-SiO2 of regenerated HCl-SiO2

600 400 200

0 1

2

3


8

Figure 4. (a) The reuse of self-methanolysis of NaBH4, HCl-SiO2 particles, and the regenerated HCl-SiO2 particles as catalyst in H2 production from the methanolysis of NaBH4 (b) true activity values of NaBH4 methanolysis reactions catalyzed by HCl-SiO2 particles [Reaction conditions: o

50 mg particle, 125 mM NaBH4 in 20 mL methanol, at 25 C, 1000 rpm].



Generated H2 (g) Volume (mL)



2000

(a) 1600

1200 800 400 0

0

4

8

12

16

20

Time (min) Hydrogen generation Rate (mL H2 g-1 min-1)

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

Page 16 of 19

40000 35000 30000 25000 20000 15000 10000 5000 0

(b)

y = 38.431x - 5612.9 R² = 0.9808 0

250

500

750

1000

The concentration of NaBH4 (mM)

Figure 5. (a) The effect of NaBH4 concentration on H2 production from the methanolysis of NaBH4, and (b) their HGR values. [Reaction conditions: 50 mg particle, 125 mM NaBH4 in 20 mL methanol, 25 ºC, 1000 rpm].


250 (a) 200

0 °C

150

15 °C

100

25 °C

50

35 °C 45 °C

0 0

10

20

30

40

50

Time (min) -2

(b) 29.9

-3

ln k

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



Page 17 of 19

0.4 kJ mol-1

-4 -5

y = -3588.3x + 8.113 R² = 0.9681

-6 0.0031

0.0033

0.0035 1/T

0.0037

Figure 6. (a) The effect of temperature on the methanolysis NaBH4 catalyzed by SiO2 particles treated with HCl catalysts, and (b) the corresponding Arrhenius plot of ln k vs 1/T.



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

Page 18 of 19

Table 1. The DLS and zeta potential measurements of SiO2 particles after treatment with various acids and NaOH. Particle with treatment agent

DLS* (nm)

Zeta (mV)

Potential*

SiO2

811 ± 45

-43.6 ± 0.8

SiO2 (HCl)

823 ± 19

-37.7 ± 1.3

SiO2 (HNO3)

897 ± 32

-37.8 ± 1.4

SiO2 (H2SO4)

891 ± 56

-37.2 ± 2.6

SiO2 (H3PO4)

824 ± 19

-38.4 ± 1.3

SiO2 (CH3COOH)

851 ± 66

-42.3 ± 1.4

SiO2 (NaOH)

846 ± 33

-46.2 ± 1.2

*The all measurements were proceeded in 10-3 M KCl at 25oC.


Page 19 of 19

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


Table 2. The result of DLS and BET measurements of SiO2 particles with different hydrodynamic diameters treated with HCl. Particle

DLS* (nm)

BET Surface area (m2 g-1)

SiO2

811 ± 45

4.29

SiO2-2

394 ± 13

9.61

SiO2-3

264 ± 9

14.21

SiO2-4

186 ± 5

24.52 -3

o

*The measurements were proceeded in 10 M KCl at 25 C.


A New Application for Colloidal Silica Particles: Natural

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