Properties of Hydrogen Peroxide Encapsulated in Silica Hydrogels

Oct 26, 2015 - Chemical Engineering Department, University of Massachusetts Lowell, 1 University Avenue, Lowell, Massachusetts 01854, United. States...
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Properties of Hydrogen Peroxide Encapsulated in Silica Hydrogels and Xerogels Fulya Sudur and Nese Orbey* Chemical Engineering Department, University of Massachusetts Lowell, 1 University Avenue, Lowell, Massachusetts 01854, United States ABSTRACT: Hydrogen peroxide (H2O2) was entrapped in silica hydrogels using the sol−gel approach, and its transition to xerogel was studied by monitoring H2O2 retention as a function of weight loss at ambient temperature. The transition took place in different drying regimes, and the maximum H2O2 concentration was obtained at 80% weight loss of the initial hydrogel. The stability of H2O2 at a higher initial concentration of 33.2 wt % was studied and found to be comparable to the values obtained with hydrogels having lower initial concentrations (10−20 wt %). The release rate of entrapped H2O2 from silica gel was studied as a function of the initial H2O2 concentration, the sodium content of the gel, the pH of the aqueous medium, and the form of the gel (hydrogel/xerogel). The release occurred via a biphasic process, with an initial fast liberation during the first 10 min where 60−70% of the H2O2 was released, followed by a much slower release rate from 10 to 60 min. The release rate was independent of the initial H2O2 concentration but was affected significantly by the other parameters. Thermal stability of the silica gels was studied using thermogravimetric analysis, and was found to depend strongly on the sodium content. The form of the gel (hydrogel/xerogel) did not have an appreciable effect on the thermal stability. Substituting sodium partially with magnesium decreased the gelation time and increased the stability of entrapped H2O2.

1. INTRODUCTION Hydrogen peroxide (H2O2) is an environmentally friendly oxidant whose current applications are limited due to its instability according to the following reaction. 1 H 2O2(aq) → H 2O(l) + O2(g) (1) 2 Efforts have been made to store H2O2 in a stable form for long periods of time to extend its use in industry. In particular, there has been an increasing demand for solid forms of H2O2 which provide better safety during storage and handling.1 Urea−H2O2 complex is a well-known crystalline solid that has been used in many oxidation processes.2 However, undesired contamination by urea, decomposition products, and other urea side reactions limit its use as a source of H 2 O 2 . 3 Polyvinylpyrrolidone has been used as an alternative matrix to urea−H2O2 due to its eco-friendly nature. The resulting solid matrix serves as an efficient oxidant for direct iodination of activated aromatic compounds.4 However, both these matrixes are water-soluble and change the pH of the surrounding aqueous environment. Moreover, their presence along with H2O2 might be considered as an impurity in some applications. In addition, there is no controlled release mechanism of H2O2 from these matrixes and the stability of H2O2 in the aqueous environment is very poor.1,5 Another promising approach to H2O2 storage is microencapsulating the molecule to improve its stability and to provide release for certain applications. However, microencapsulation of H2O2 is a challenge due to the low molecular weight and high hydrophilicity of the molecule. H 2 O 2 encapsulated in various polymer matrixes, such as polylacticco-glycolic acid,6 polylactic acid,7 and, more recently, poly(methyl methacrylate),8 results in poor encapsulation efficiency due to the high diffusion coefficient of H2O2. The use of © 2015 American Chemical Society

organic solvents in these syntheses also accelerates the decomposition reaction of H2O2. Microcapsules need to be stored at low temperatures to avoid the decomposition and evaporation of H2O2. Sol−gel chemistry offers new possibilities for entrapping any molecule that is capable of making hydrogen bonds with silica hydrogels at room temperature (RT), and for controlling the release kinetics from the gel matrix.9 The sol−gel method is inexpensive, versatile, and simple, and the resulting silica hydrogels are nontoxic and biocompatible.10 Physical and chemical properties of silica hydrogels are affected by the synthesis conditions, such as the reagents used, drying and storage temperatures, and pH of the silica sol.11 Silica hydrogels are formed via sol−gel chemistry by using an aqueous sodium silicate solution with ion exchange. The sol−gel transition point is reached when a strong increase in the viscosity is observed, resulting in a “wet gel” or “hydrogel”. Upon drying by evaporation, the gel network contracts, expels liquid from the pores, and forms a xerogel. Sol−gel technology offers improved delivery of encapsulated material and enables the use of unstable biodegradable molecules to avoid decomposition in large-scale applications.12 Ż egliński et al.13 reported the synthesis of H2O2-containing xerogels. The antibacterial activity of xerogels containing 3.8− 68 wt % H2O2 was tested, and the stability of a xerogel that containing 3.8 wt % H2O2 was monitored for 63 days. Retention of H2O2 in the xerogel was 71% or as high as 94% when the xerogel was stored at RT or 3 °C, respectively. The Received: Revised: Accepted: Published: 11251

September 9, 2015 October 19, 2015 October 26, 2015 October 26, 2015 DOI: 10.1021/acs.iecr.5b03373 Ind. Eng. Chem. Res. 2015, 54, 11251−11257

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reagent), hydrogen peroxide (contains inhibitor, 50 wt % in H2O, ACS reagent), orthophosphoric acid (85 wt % in H2O, 99.99% trace metals basis), potassium permanganate (ACS reagent >99%), ammonium hydroxide solution (ACS reagent, 28−30% NH3 basis), and sulfuric acid (99.999%), were purchased from Sigma-Aldrich Chemical Co. and magnesium chloride (ACS reagent, > 99%) was purchased from J.T. Baker. All chemicals were used without further purification. 2.2. Hydrogel Synthesis. Details of the hydrogel synthesis procedure are described in our earlier work.9 Briefly, as-received sodium silicate solution was diluted with deionized water and combined with 30 wt % H2O2 solution and 7 μL of phosphoric acid (H3PO4). The sodium content of the sodium silicate solution was controlled by adding different amounts of ionexchange resin (Amberlite IR 120, hydrogen form) to the sol (0−3.2 g). The mixture was stirred with an overhead mixer for 4 min, the sol was separated from the resin, and the sodium content was measured using a flame atomic absorption spectrometer (240 AA, Agilent Technologies). This method was found to give better control and reproducibility of sodium ions in the system compared to using an ion-exchange column. The sol was then transferred to a glass Petri dish (D = 100 mm, h = 15 mm) and placed in a BlueM gravity oven at 40 °C until gelation was observed. In this work, 50 wt % H2O2 was used in the experiments at a constant sodium content following a similar procedure as described above. In the experiments where the effect of magnesium ion was studied, magnesium chloride was dissolved in deionized water and added dropwise to the sodium silicate solution to prevent precipitation. The molar ratio of Na to Mg was adjusted to be 1:0.2. Once a homogeneous solution was obtained, 30 wt % H2O2 and 7 μL of H3PO4 were added. The ion content of the sodium/magnesium silicate solution was controlled by adding ion-exchange resin as explained above. The same procedure was used for sol to hydrogel transition. 2.3. Characterization. The effect of sodium content on the H2O2 stability was studied by maintaining a constant H2O2 concentration of 19.9 wt % while changing the sodium concentration between 0.027 and 0.557 wt %. The effect of the initial H2O2 concentration on the H2O2 stability was studied by varying the initial H2O2 concentration between 9.9 and 33.2 wt % while keeping the sodium concentration constant at 0.027 wt %. The 30 wt % H2O2 was used to obtain initial H2O2 concentrations of 9.9 and 19.9 wt %, and 50 wt % H2O2 was used to obtain an initial H2O2 concentration of 33.2 wt %. The H2O2 content of the hydrogels was measured by potassium permanganate (KMnO4) titration immediately after gelation and remeasured daily to assess H2O2 retention in the hydrogel. The hydrogels are stored in covered glass Petri dishes. Three repeated titrations were performed for each gel sample. Hydrogels gradually turned into xerogels during storage at ambient temperature, due to the loss of aqueous content (H2O2 and water) and decomposition of H2O2. Weight loss and H2O2 retention were monitored during this transition. Seven samples were prepared to determine the amount of H2O2 released at different times (0−60 min). Samples were agitated at RT and 100 rpm in a KJ201BD orbital shaker (Chang Bioscience Inc.). The supernatants were filtered, and the amount of the released H2O2 was determined by KMnO4 titration. The pH of the aqueous media was changed by adding sulfuric acid (H2SO4) and ammonium hydroxide (NH4OH)

entire H2O2 content of the xerogel was released into water within 10 min. Bednarz et al.3 entrapped H2O2 within silica xerogels, which were used as oxidants in organic synthesis. When xerogels were stored at 5 °C, only half of the initial H2O2 loading of 8 wt % was retained. However, no information regarding the stability of H2O2 at RT or the H2O2 release rate profile was reported. More recently, Wolanov et al.1 studied aluminate and alumina xerogels, determining the stability of H2O2 in these xerogels and comparing their properties with those of silica xerogels. The H2O2 content of the xerogels after drying was not reported. Aluminate and alumina xerogels retained 5−10 and 45−50% of their H 2 O 2 contents, respectively, when they were stored at RT or 5 °C for 35 days. The high pH and impurities present in the aluminate xerogels were considered to explain their higher decomposition rates.1 Xerogels are easy to handle and contain high amounts of H2O2. The drying protocol used to form xerogels varies between studies, which aim to obtain xerogels with the highest H2O2 loading for an intended application. The transition between hydrogels and xerogels needs to be well understood to find the relationship between H2O2 loading and weight loss. Properties of hydrogels can then be tailored for different end uses by adjusting these variables. Owing to the release rate of H2O2, the use of sol−gels to release H2O2 from an inert matrix has high potential to be an effective method for many applications, such as water treatment, sterilization, drug delivery, and wound healing. When H2O2 is released from silica hydrogels, the only byproducts are silica and water. The stability of H2O2 is not affected by its release, because there is no undesired contamination or pH change of the aqueous environment. However, the release kinetics of H2O2 from silica hydrogels into different aqueous environments needs to be determined for different applications. Sudur et al.9 entrapped H2O2 in silica hydrogels and found out that the amount of sodium plays a significant role in the structure of hydrogels and affects the stability of H2O2. Higher retention and better stability of H2O2 at RT was obtained because the gelation conditions were optimized by varying the sodium content and initial H2O2 concentration. Mesoporous hydrogels (3−24 nm) with high surface area (1000−1400 m2/ g) were obtained at low sodium content. This paper is a continuation of our earlier work9 to understand the H2O2−silica hydrogel interactions for possible future applications. Hydrogel−xerogel transition was studied in more detail by monitoring H2O2 retention as a function of weight loss in the hydrogel matrix. The stability at a higher initial H2O2 concentration (33.2 wt %) and its consequent effects were studied and compared with previously reported data. Effects of the sodium content, H2O2 concentration, pH of the surrounding medium, and form of the gel (hydrogel/ xerogel) on the release rate of H2O2 from the hydrogel matrix were investigated. Thermal stability of the hydrogels was studied by thermogravimetric analysis (TGA). The effect of charge of the ion in the starting silicate solution on H2O2 stability was also studied by partially substituting sodium with magnesium ions.

2. EXPERIMENTAL PROCEDURE 2.1. Materials. Sodium silicate solution (reagent grade, 10.6% Na2O, 26.5% SiO2), Amberlite IR120 (hydrogen form), hydrogen peroxide (contains inhibitor, 30 wt % in H2O, ACS 11252

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Industrial & Engineering Chemistry Research solutions. Experiments were repeated five times to check reproducibility. The results were consistent and reproducible within 2.8%. Thermal stability of the silica hydrogels was studied by TGA on a Q50 (TA Instruments) in the range 25−500 °C with a heating rate of 20 °C/min. Hydrogel morphology was studied by scanning electron microscopy (SEM) with a JEOL JSM 7401F instrument. All samples were coated with gold by using a vacuum sputter coater and imaged with an accelerating voltage of 10−15.0 kV.

30 to 66 wt % (region II), with weight loss mostly being due to water evaporation. Owing to its double protons, H2O2 forms stronger hydrogen bonds with the silanol groups than does water. Consequently, entrapped H2O2 molecules are more stable and their loss is much lower than that of the water molecules, which accounts for the sharp increase in H2O2 concentration in this region. The maximum H2O2 concentration of 66 wt % was obtained when 80% of the hydrogel weight (point B) was lost, consistent with our previously reported data.9 Water in the silica gel can stabilize H2O2 by allowing the formation of a less strained structure and improving the cluster geometry of H2O2.13,14 In region III, the amount of water left in the xerogel was not enough to stabilize the H2O2; therefore it began decomposing after it reached maximum (point B). After most of the water and H2O2 were removed, the Si−O−Si linkages formed faster, the number of Si−OH groups decreased, and the hydrogen bonding capacity was reduced, resulting in further loss of H2O2 from the matrix (region IV). The morphology of the xerogel obtained at 80% mass loss (point B in Figure 1) was studied by SEM and compared to the morphology of the initial hydrogel (point A in Figure 1). Figure 2 shows images obtained at three different parts of the xerogel

3. RESULTS AND DISCUSSION 3.1. Hydrogel−Xerogel Transformation. Formation of a xerogel from a hydrogel is highly dependent on the time and temperature regime used during drying. Previous researchers have emphasized obtaining a xerogel with the highest H2O2 loading for the intended application and used various drying conditions to achieve a subjective weight loss range of 85−90%, in order to minimize decomposition and maximize stability of H2O2 during xerogel formulation.3,13 Understanding the relationship between the H2O2 content and the total weight loss during the hydrogel-to-xerogel transition will be useful in many applications requiring knowledge of the precise concentration of entrapped H2O2. This would allow the synthesis to be customized and design gels with high or low aqueous content determining the form of the silica matrix (gel or powder), depending on the application. Figure 1 shows the hydrogel−xerogel transition (containing 0.027 wt % Na and 19.9 wt % H2O2) where the H2O2 content

Figure 2. SEM images: (a−c) xerogel and (d) hydrogel.

(Figure 2a−c) and an SEM image of the initial hydrogel (Figure 2d). It is clear from these scans that the morphology of the xerogel was not uniform. Removal of the aqueous content during drying left sites available for hydrogen bonding. Molecules that have hydrogen bonding affinity such as H2O2, H2O, and other Si−OH groups can attach to the silanol groups to form different structures. Although not unexpected, the inhomogeneity in the structure may be a drawback to using xerogels rather than hydrogels depending on the application. 3.2. Room Temperature Stability of H2O2 at Higher Concentration. To our knowledge, in the literature the stability of H2O2 entrapped in silica hydrogels has been studied with a maximum initial concentration of 20% H2O2. We examined the entrapment capability of hydrogels and the longterm stability of H2O2 at a higher initial H2O2 concentration of 33.2 wt % and a sodium content of 0.027 wt % (Figure 3). Under these conditions, more than 70% of the H2O2 was retained after 40 days of storage at RT. This promising result

Figure 1. Hydrogel−xerogel transition (Na, 0.027 wt %; initial H2O2 concentration, 19.9 wt %).

of this sample was monitored as a function of weight loss as it dried at ambient temperature. During the transition from sol to hydrogel, the amount of hydrogen peroxide initially present in the sol was decreased by 3%, and it was decreased by 40% during the transition from hydrogel to xerogel. Data point A corresponds to the hydrogel immediately after the sol−gel transition point was reached. The results obtained indicate that the transition from hydrogel to xerogel takes place in four different regimes. Initially, the hydrogel exhibited a gradual increase in H2O2 concentration (region I), with a weight loss up to 40−50% that was mainly due to the evaporation/ dissociation of non-entrapped H2O2 and water on the hydrogel surface. Next, the H2O2 concentration sharply increased from 11253

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stronger hydrogen bonds as also observed by Ż egliński et al.13 In addition, hydrogen peroxide will form a higher number of hydrogen bonds compared to water, resulting in faster gelation. 3.3. Release Rate Study. Understanding the release rate of entrapped H2O2 from the inert silica matrix into an aqueous medium is important for applications requiring long-term H2O2 exposure, such as teeth whitening and disinfection systems. The release rate is defined as the ratio of the amount released to the initial amount of H2O2 over time.13 Entrapped H2O2 is released through a combined process of diffusion and matrix erosion. Specifically, the aqueous surrounding medium diffuses into the pores during H2O2 release and causes physicochemical changes in the silica matrix which, in turn, affect the release behavior of H2O2.11,13,15 To analyze this process, we examined the effects of the sodium content, H2O2 concentration, pH of the surrounding medium, and form of the gel on the release rate (parts a, b, c, and d, respectively, of Figure 4). In all experiments, H2O2 was released via a two-step biphasic process, with an initial fast liberation from open pores near the bulk surface during the first 10 min, followed by a much slower release via diffusion of the remaining liquid from the matrix from 10 to 60 min. The initial burst of H2O2 released during the first 10 min is believed to be due to the high solubility and diffusivity of H2O2, which is weakly bonded to the silica surface. More than 60% of the H2O2 content of the gels was released during this period. After the first 10 min, the release rate plateaued and, thereafter, proceeded at a much slower rate. We believe that this deceleration is because of H2O2 entrapment through strong hydrogen bonds with the silanol groups. This mechanism is analogous to that observed during the hydrogel-to-xerogel transition.

Figure 3. Stability of H2O2 as a function of storage time at various H2O2 concentrations (Na, 0.027 wt %).

indicates that silica hydrogels can be used to store high concentrations of H2O2 in a stable solid form. Using the same sodium content, we previously found an H2O2 retention value of 89% after storage for 70 days at RT using initial H2O2 concentrations of 3.3−9.9 wt %.9 At higher initial H2O2 concentrations, the decomposition was faster; only 70% of the H2O2 was retained after 40 days at RT when the initial H2O2 concentration was 19.9 wt % (Figure 3). The stability levels of H 2 O 2 were very similar at initial concentrations of 19.9 and 33.2 wt %. In our previous work we observed that gel time decreased as the H2O2 concentration increased.9 A shorter gelation time was observed with an initial H2O2 concentration of 33.2 wt %, consistent with this previous observation. This may be because H2O2 can adjust better to the adsorption centers of silica gel than the water molecules, resulting in formation of shorter and

Figure 4. Release rates of H2O2 for (a) hydrogel as a function of sodium content (initial H2O2 concentration, 19.9 wt %), (b) hydrogel as a function of initial H2O2 concentration (Na, 0.027 wt %), (c) hydrogel as a function of pH (Na, 0.027 wt %; initial H2O2 concentration, 19.9 wt %), and (d) xerogel (Na, 0.027 wt %; initial H2O2 concentration, 19.9 wt %). 11254

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Figure 5. TGA of hydrogels with different sodium contents (initial H2O2 concentration, 19.9 wt %). Parts b and c are the zoomed-in versions of (a).

The release rate was studied for 60 min, in accordance with work reported in the literature. However, in contrast to results obtained by other researchers who studied release rates from xerogels,13 we found that not all of the H2O2 was released by 60 min. These findings indicate a more effective entrapment of H2O2 in the present work because the gelation conditions were optimized during hydrogel and xerogel formation as explained above.9 We also measured the release rate of H2O2 after 8 h for only one sample (0.027 wt % Na and 19.9 wt % H2O2) and found that 74% of the hydrogen peroxide was released. In the present work, neither the hydrogels nor the xerogels dissolved in aqueous solutions for a period of 24 h. 3.4. TGA. As hydrogels and xerogels can be used in hightemperature applications, we studied their thermal behavior and stability by TGA. The thermal behavior of hydrogels and xerogels depends on the loss of aqueous content and the rate of H2O2 decomposition. We examined the weight loss of hydrogels with different sodium contents as a function of temperature (Figure 5a). Hydrogels with the highest sodium content (0.0557 wt %) used in this work had lost 20% of their original weight at 52 °C, compared to 84 °C for hydrogels with the lowest sodium content (0.027 wt %) (Figure 5b). This result was expected because the average pore diameter was larger (24.3 nm) at a higher sodium content. We determined the highest operating temperatures at which the gels retained their aqueous content and beyond which only silica remained (Figure 5c). At the highest and lowest sodium concentrations, the aqueous contents of the gels were completely lost by 117 and 162 °C, respectively. This finding indicates that the presence of sodium negatively affected the thermal stability of the silica hydrogels. A comparison of the thermal stabilities of hydrogels and xerogels with the same sodium content (0.027 wt %) revealed a slightly higher thermal stability of the xerogel compared to the hydrogel (Figure 6). The xerogel had lost all of its aqueous content at 177 °C, compared to 162 °C for the hydrogel.

Figure 4a shows the effect of sodium content on the release rate of H2O2. The amount of sodium has a major effect on the structure of the silica hydrogels: the average pore diameter changes from 3.4 to 24.3 nm as the sodium content increases from 0.027 to 0.557 wt %.9 This in turn causes a higher release rate of the H2O2. The results indicated a slight dependence of the release rate on the initial H2O2 content, in agreement with findings reported in the literature (Figure 4b).13 The release behavior of H2O2 from silica hydrogels was studied when the pH of the aqueous media was acidic (pH 0.9), basic (pH 12.1), or neutral (pH 6.3). Higher release rates were obtained in both acidic and basic media compared to neutral medium as shown in Figure 4c. The presence of hydroxyl ions in the basic medium and hydrogen ions in the acidic medium is thought to disrupt the hydrogen bonding between silanol groups and peroxide molecules and increase the driving force for peroxide molecules to be released to the surrounding medium. The lower release rate in the acidic medium may be due to the weak acidity of H2O2. The pH of the medium is important in industrial applications, and this effect should be studied further. The release rate of H2O2 from xerogels was studied to understand whether the form of the silica matrix affects the release rate. Xerogels were obtained using one hydrogel formulation that was optimized in our earlier work by tailoring the pH, the H2O2 stability, the sodium content, and the time− temperature protocol used during the sol to hydrogel transition.9 Faster release rates were observed from the xerogel matrix compared to the hydrogel, with 80% of the initial H2O2 being released from the xerogel after 60 min compared to 70% from the hydrogel. The decreased density of silanol groups, caused by the drying-induced condensation and formation of Si−O−Si linkages,16 results in decreased hydrogen bonding capacity and faster H2O2 release from the matrix (Figure 4d). 11255

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4. CONCLUSIONS Hydrogel−xerogel transition of hydrogen peroxide entrapped in a silica matrix was studied at ambient temperatures. The transition took place in different drying regimes, and a maximum H2O2 concentration of 66 wt % was obtained corresponding to 80% weight loss of the initial hydrogel. Stability of higher initial H2O2 concentration (33.2 wt %) was studied and found to be similar to the stability of hydrogels containing 10−19.9 wt % H2O2. This indicates that high concentrations of peroxide can be stored successfully in silica hydrogels. Release rate studies of H2O2 from the silica matrix showed a biphasic trend, with an initial fast liberation followed by a diffusion-controlled slow process. The sodium content, the pH of the aqueous medium, and the form of the gel were found to affect the release rate of H2O2. The release rate was found to be slightly dependent on the initial H2O2 concentration. The presence of sodium affected adversely the high temperature stability of hydrogels. Maximum operating temperatures were determined to be 117 and 162 °C at the highest and lowest sodium contents, respectively. Faster gelation and higher H2O2 stability were achieved when sodium was partially substituted by magnesium.

Figure 6. TGA of hydrogel and xerogel (Na, 0.027 wt %; initial H2O2 concentration, 19.9 wt %).

3.5. Hydrogels Containing Magnesium Ion. When silica hydrogels were obtained using sodium silicate as the starting material, the results showed that the amount of sodium significantly affected the properties of hydrogels and the stability of the entrapped H2O2.9 Therefore, we wanted to study what effect the charge of the ions in the starting silicate solution would have on the stability of entrapped H2O2. Magnesium was selected to partially exchange the sodium in the silicate because it is smaller in size than sodium and is divalent. Next, the silicate that was partially substituted with magnesium ions was subjected to ion exchange using Amberlite IR120 (hydrogen form) to reduce the total ion content of the sol. The amount of ion-exchange resin was determined based on the previous work that resulted in the lowest sodium content (0.027 wt %).9 Hydrogels obtained with and without ion exchange were studied for H2O2 stability. The results are presented in Figure 7, where previously obtained data with sodium silicate are also included for comparison. In both cases (Figure 7), the stability of entrapped H2O2 was improved by the presence of magnesium ions. Substituting sodium with magnesium ions reduced the gelation time by 80 and 50% in samples with and without ion exchange, respectively. We believe that faster gelation and higher H2O2 stability are obtained because magnesium is a divalent ion and therefore more sites will be available for hydrogen bonding between the silanol groups and peroxide molecules during the conversion from sol to hydrogel. However, a detailed study needs to be carried out to understand these effects better and to further improve H2O2 stability.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors want to acknowledge the startup funds from University of Massachusetts Lowell. The authors would like to thank Ms. Naigambi Patience Namusuubo for her help with experiments.

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REFERENCES

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Figure 7. Room temperature stability of H2O2 as a function of storage time (a) without ion exchange and (b) with ion exchange (initial H2O2 concentration, 19.9 wt %). 11256

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