Effects of alcohols and dissolved gases on sonochemical generation of hydrogen in a 300 kHz sonoreactor

Highlights

• •Sonochemical H₂ generation was investigated under Ar in a 300 kHz sonoreactor.
• •Five alcohols were used at various concentrations (0 – 100 % v/v) in water.
• •The highest H₂ generation was obtained using methanol (5 % v/v).
• •As the oxygen content increased, the H₂ generation decreased significantly.
• •The H₂ generation rates from this and previous researches were summarized.

Abstract

The sonochemical generation of hydrogen (H₂) was investigated using various water/alcohol solutions under argon (Ar) 100 % in a 300 kHz sonoreactor. Five types of alcohols–methanol, ethanol, isopropanol, n-propanol, and n-butanol–were used at various concentrations (0 – 100 % v/v). The H₂ generation rate in water was 0.31 μmol/min in the absence of alcohols. The H₂ generation rate increased, peaked, and then decreased as the alcohol concentration increased. The concentrations used for the peak H₂ generation were 5 %, 1 %, 0.5 %, 0.5 %, and 0.1 % for methanol, ethanol, isopropanol, n-propanol, and n-butanol, respectively. The highest generation rate (5.46 μmol/min) was obtained for methanol 5 % among all conditions in this study, and no H₂ was detected for 100 % alcohol concentrations. The reason for the enhancement of the sonochemical H₂ generation by the addition of alcohols might be due to strong scavenging effect of alcohols for sonochemically generated oxidizing radicals and vigorous reactions of alcohol molecules and their derivatives with H radicals. No significant correlations were found between the H₂ generation rates and physicochemical properties of the alcohols in any of the data in this study. As alcohol concentration increased, the calorimetric power decreased. This indicates that the calorimetric power does not represent the degree of sonochemical reactions in the water/alcohol mixtures. The effect of oxygen (O₂) content in the dissolved gases on the generation of H₂O₂ (representing sonochemical oxidation activity) and H₂ (representing sonochemical reduction activity) was investigated using Ar/O₂ mixtures for water, methanol 5 % and n-propanol 0.5 %. In water, the highest H₂O₂ generation was obtained for Ar/O₂ (50:50), which is similar to previous research results. However, the H₂O₂ generation increased as the O₂ content increased. In addition, H₂ generation decreased as the O₂ content increased under all liquid conditions (water, methanol, and n-propanol).

1. Introduction

Cutting-edge ultrasonic technology creates extremely concentrated energy in cavitation bubbles, and various sonochemical and sonophysical effects have proven to be effective in chemical, material, energy, and environmental engineering [1], [2], [3], [4], [5]. It has been reported that sonophysical effects are effective in the low-frequency range (20–40 kHz) [6], [7], [8] and sonochemical effects are advantageous in the range of several hundred kilohertz [1], [9], [10]. Recently, geometric effects have been studied to design more suitable and effective sonoreactors considering the frequency and power conditions [4], [11], [12], [13], [14], [15], [16], [17].

Many researchers have focused on the sonochemical effects of sonochemical oxidation reactions, particularly on the development of advanced oxidation processes (AOPs) for the removal of conventional and emerging pollutants [18], [19]. It has been reported that ultrasonic technologies exhibit high synergistic effects with other AOPs [20], [21], [22], [23] and superior sonochemical oxidation effects are obtained under dissolved gas mixtures of Ar and O₂ [1], [9], [10]. Some researchers have reported the application and optimization of sonochemical reduction reactions for material synthesis [2], [24], [25], pollutant removal [26], [27], [28], [29], and H₂ production [1], [3], [30]. They used mainly Ar 100 % as the dissolved gas to suppress the availability of oxidizing radicals [31].

Recent research on sonochemical H₂ generation that can significantly advance Sustainable Development Goals (SDGs) is summarized in Table 1 [31], [32], [33], [34], [35], [36], [37]. Recent studies have shown the potential of novel ultrasonic methods for sonochemical H₂ generation and suggested important topics for future research: Ar 100 % as a dissolved gas is essential; higher yields of H₂ can be obtained in water/alcohol solutions rather than in pure water (alcohols are widely used for H₂ generation in electrochemistry because they are considered renewable [38]); solid catalysts can enhance H₂ generation, and no significant synergistic effect was observed during photolysis.

Table 1.

Example of some research on sonochemical H₂ generation that can advance the SDGs.

The focus of the study	Key points	SDGs and targets	Countries of affiliation (authors)	Ref.
Sonophotocatalytic generation of H2 using TiO2 in water	- Sonolytic, sonophotolytic, sonocatalytic, and sonophotocatalytic processes were compared. - The highest generation of H2 was obtained in sonolytic process.	SDG- 7: Ensure access to affordable, reliable, sustainable and modern energy for all. Target 7.3 SDG-12 Ensure sustainable consumption and production patterns. Target 12.2	Japan	[32]
Sonophotocatalytic generation of H2 using rare earth catalysts in water/ethanol mixture	- Rare earth catalysts (La0.5Ga0.5InO3 La0.8Ga0.2InO3, S:La0.8Ga0.2InO3) were synthesized and used. - The highest generation of H2 was obtained in sonophotocatalytic process.	SDG- 7: Ensure access to affordable, reliable, sustainable and modern energy for all. Target 7.3 SDG- 9: Build resilient infrastructure, promote inclusive and sustainable industrialization and foster innovation. Target 9.4 SDG-12 Ensure sustainable consumption and production patterns. Target 12.5	Italy	[33]
Sonolytic generation of H2 using Au/TiO2 in water/alcohol mixtures	- TiO2 and Au/TiO2 were compared for the H2 generation under various water/alcohol mixtures. - The highest generation of H2 was obtained in sonocatalytic process using methanol and Au/TiO2.	SDG- 7: Ensure access to affordable, reliable, sustainable and modern energy for all. Target 7.3 SDG-12 Ensure sustainable consumption and production patterns. Target 12.2	China, Australia	[34]
Sonophotocatalytic generation of H2 using rare earth catalysts in water/ethanol mixture	- Rare earth catalysts using La, Gd, Y, and Yb were synthesized and used. - The highest generation of H2 was obtained in sonophotocatalytic process using S:Y0.8Ga0.2InO3.	SDG- 7: Ensure access to affordable, reliable, sustainable and modern energy for all. Target 7.3 SDG- 9: Build resilient infrastructure, promote inclusive and sustainable industrialization and foster innovation. Target 9.4 SDG-12 Ensure sustainable consumption and production patterns. Target 12.5	Italy	[35]
Sonocatalytic generation of H2 using metal oxides in water	- Macro and nano sized metal oxides (ThO2, ZrO2, and TiO2) were synthesized and used. - The highest generation of H2 was obtained in sonocatalytic process using TiO2.	SDG- 7: Ensure access to affordable, reliable, sustainable and modern energy for all. Target 7.3 SDG-12 Ensure sustainable consumption and production patterns. Target 12.2	France	[36]
Sonolytic generation of H2 in water/alcohol mixtures	- The highest generation of H2 was obtained in sonolytic process using water/methanol mixture. - Additional H2 generation was observed after the end of ultrasound irradiation.	SDG- 7: Ensure access to affordable, reliable, sustainable and modern energy for all. Target 7.3 SDG-12 Ensure sustainable consumption and production patterns. Target 12.2	Germany	[37]
Sonolytic generation of H2 in water	- The highest H2 generation was obtained in sonolytic process at 20 kHz. - The sonochemical H2 generation was simulated under various conditions (four gases and four frequencies).	SDG- 7: Ensure access to affordable, reliable, sustainable and modern energy for all. Target 7.3 SDG-12 Ensure sustainable consumption and production patterns. Target 12.2	Algeria, Saudi Arabia, Norway	[31]

Even though some important researches on sonochemical H₂ generation have been conducted as mentioned above, systematic investigation using various gaseous or liquid organic compounds are still required for industrial application. In this study, the effect of alcohols (methanol, ethanol, isopropanol, n-propanol, and n-butanol) in a water/alcohol solution on sonochemical H₂ generation was systematically investigated in a 300 kHz sonoreactor. Various alcohol concentrations (0 – 100 % v/v) were applied under Ar 100 % and the 1st-order generation rates for the H₂ generation were compared. To compare the sonochemical oxidation and reduction, the generation rates of H₂O₂ and H₂ were obtained for methanol and n-propanol under Ar/O₂ conditions. Moreover, H₂ generation in previous studies was summarized with the results of this study and analyzed in terms of concentration and mass per time.

2. Experimental methods

2.1. Chemicals

Methanol (CH₄O), ethanol (C₂H₆O), isopropanol (C₃H₈O), n-propanol (C₃H₈O), n-butanol (C₄H₁₀O), hydrogen peroxide (H₂O₂) and sodium hydroxide (NaOH) were obtained from Samchun Pure Chemical Co. Ltd. (KOR). Potassium biphthalate (C₈H₅KO₄) was obtained from Daejung Chemical &Metals Co., Ltd. (KOR). Ammonium molybdate [(NH₄)₂MoO₄] was purchased from Junsei Chemical Co., Ltd. (JPN). All chemicals were used as received.

2.2. Sonoreactor and gas supply

A circular glass vessel with a cooling jacket for temperature control and a sealing cover was used in this study, equipped with a 300 kHz transducer module (Mirae Ultrasonic Tech., Bucheon, KOR) that was placed at the bottom, as shown in Fig. 1. The inner diameter and height of the sonoreactor were 105 and 100 mm, respectively. The liquid height is determined using the following equation:

$$\lambda =\frac{c}{f}$$ (1)

where λ is the wavelength, c is the speed of sound in water (1500 m/s), and f is the applied frequency (28 kHz). The liquid height and volume was 5.0 λ (25 mm) and 220 mL, respectively. The headspace volume of the reactor is 865.9 mL. The inside of the reactor double wall was connected to a water chiller to enable the circulation of cooling water and the temperature in the liquid body was maintained at 20℃. The working electrical power was measured using a power meter (HPM-300A; ADpower, KOR), and the ultrasonic power (also called calorimetric power) was measured using the following equation:

$$P_{\mathit{cal}}=\frac{\mathit{dT}}{\mathit{dt}}{C}_{P}M$$ (2)

where P_cal is the ultrasonic/calorimetric energy, dT/dt is the rate of increase in the liquid temperature, C_p is the specific heat capacity of the liquid (water: 4.19 J/(g·K); methanol 2.51 J/(g·K); n-propanol: 2.40 J/(g·K)), and M is the mass of the liquid. The alcohol content in the water/alcohol solution was also considered when measuring the calorimetric power.

Fig. 1.

Schematic of the 300 kHz sonoreactor with the gas supply system and DO monitoring system.

Saturation gas was delivered into the liquid body using a gas supply system including gas cylinders of argon (Ar), oxygen (O₂), and nitrogen (N₂), mass flow controllers (HFC-D-302B; Teledyne Hastings Instruments, USA), a static mixer (Gasline, Seongnam, KOR), a mass flow meter (HFM-D-300B; Teledyne Hastings Instruments, USA), a gas flow monitoring controller, and a microporous glass sparger (pore size: 20–––30 μm) equipped with a glass pipe. A sparger was placed at 1 cm above the bottom of the reactor. The gas flow rate at saturation was set to 3 L/min. The DO concentration was continuously measured at 5 s intervals using an optical DO meter (OXY-1 SMA oxygen meter; PreSens Precision Sensing GmbH, Germany). The sparger is removed after gas saturation is achieved in the liquid phase.

The sonochemical production of H₂ and H₂O₂ was investigated using water and water/alcohol mixtures containing various concentrations (0.5, 1.0, 5.0, 10, and 20 % v/v) of aliphatic alcohols including methanol, ethanol, isopropanol, n-propanol, and n-butanol. The physicochemical properties of water and the five alcohols are summarized in Table 2. The irradiation time was determined as 60 min considering the detection limit and reliability of the generated H₂ concentration for the lowest H₂ productions. All experiments were repeated three times.

Table 2.

Physicochemical properties and reaction rates with H and OH radicals of water, methanol, ethanol, isopropanol, n-propanol, and n-butanol [39], [40].

Compound	Water	Methanol	Ethanol	Isopropanol	n-Propanol	n-Butanol
Formula	H2O	CH4O	C2H6O	C3H8O	C3H8O	C4H10O
Molecular weight	18.02	32.04	46.07	60.10	60.10	74.12
Density (g/L)	997	791	789	781	800	810
Vapor pressure (kPa)	3.17	16.9	7.87	6.02	2.76	0.86
Water solubility (mg/L)		miscible	miscible	miscible	miscible	79
Henry’s law constant (atm/mol/L)		6.74 × 10-3	7.43 × 10-3	1.10 × 10-3	7.94 × 10-3	8.76 × 10-3
Octanol-water partition coefficient		0.18	0.50	1.12	1.78	6.92
Reaction rate with H radical*	1.00 × 101	0.26 × 107	1.70 × 107	7.40 × 107	2.40 × 107	3.50 × 107
Reaction rate with OH radical**		0.97 × 109	1.90 × 109	1.90 × 109	2.80 × 109	4.20 × 109

*Reaction rate for the reaction of alcohol with H radicals (H₂ generation).

**Reaction rate for the reaction of alcohol with OH radical (H₂O generation).

2.3. Quantification of H₂ production

The gas phase sample was taken from the headspace of the reactor to quantify sonochemical production of H₂ using a 500 μL gas-tight syringe (Trajan Scientific and Medical, AUS). The concentration of H₂ was measured using a gas chromatograph (8890 GC system, Agilent, USA) equipped with a thermal conductivity detector (TCD). The amount of sonochemically generated H₂ was calculated using the concentration of H₂ and headspace volume.

2.4. Quantification of H₂O₂ production

The concentration of sonochemically generated H₂O₂ was spectrophotometrically analyzed using solution A (0.10 M potassium biphthalate), solution B (0.4 M KI, 0.06 M sodium hydroxide, and 10^-4 M ammonium molybdate), and a UV–vis spectrophotometer (SPECORD 40; Analytic Jena AG, Jena, DEU).

3. Results and discussion

During ultrasonic irradiation of water, H₂ can be produced using sonochemically generated H radicals, as shown by the following reactions [31]:

$$H·+H·\to H_2$$ (3)

$$H·+H{O}_2·\to O_2+H_2$$ (4)

$$H·+H_{2}O\to ·OH+H_2$$ (5)

$$H·+H_2{O}_2\to H_2+H{O}_2·$$ (6)

In water, the amount of H₂ produced depends mainly on dissolved gases such as Ar, O₂, and N₂ which can either enhance or diminish the amount of H₂ produced [1], [31]. Ar has a higher specific heat capacity ratio than O₂ or N₂, which induces extreme conditions inside cavitation bubbles and more effective sonochemical reactions [10]. During acoustic cavitation events, a larger number of H radicals can be generated under Ar than under O₂ or N₂ [41]. Reactive oxygen species (ROS) such as OH, HO₂, and O radicals, which easily scavenge H radicals, can be generated under O₂. [1], [31]. In addition, O₂ molecules themselves scavenge H radicals ($O_2+H·\to O+·OH$, $O_2+H·\to H{O}_2·$) [42]. Kohno et al. reported that no significant amount of H radicals were detected under O₂ (1,650 kHz) [41]. Büttner et al. (1 MHz) and Hart and Henglein (300 kHz) reported that no H₂ was detected in water under O₂ [43], [44]. N₂ can act as an ROS scavenger by forming nitrogen oxides, such as NO₂^– and NO₃^– ions [45], thereby increasing the availability of H radicals [31], [41]. Therefore, previous researchers have preferred to use Ar as a saturating gas for the sonochemical production of H₂ under various ultrasonic conditions [3], [46].

In this study, the sonochemical production of H₂ in water was investigated using various gases including Ar (100 %), N₂ (100 %), O₂ (100 %), Ar/N₂ (50:50), and Ar/O₂ (50:50) was quantified using the 300 kHz sonoreactor. As shown in Table 3, the order of the H₂ production rate was Ar 100 % > Ar/N₂ (50:50) > N₂ 100 %. No significant amounts of H₂ were detected with O₂ or Ar/O₂ (50:50), as discussed above.

Table 3.

Sonochemical production rate of H₂ in water under Ar 100%, N₂ 100%, O₂ 100%, Ar/N₂ (50:50), and Ar/O₂ (50:50).

	Ar 100 %	N2 100 %	O2 100 %	Ar/N2 (50:50)	Ar/O2 (50:50)
H2 production rate (μmol/min)	0.650	0.064	0	0.243	0

The effects of aliphatic alcohols on the sonochemical production of H₂ were investigated using methanol, ethanol, isopropanol, n-propanol, and n-butanol at various alcohol concentrations (0.1, 0.5, 1.0, 5.0, 10, and 20 % v/v). Previous studies added various organic and inorganic compounds to water and investigated the sonochemical production of H₂ under various ultrasonic conditions [34], [35], [37], [47], [48], [49]. As shown in Fig. 2 (expressed in molar concentration) and 1S (expressed in volume percentage concentration with error bars), the addition of alcohol significantly enhances the sonochemical production of H₂ within a certain range of alcohols. The highest H₂ generation rate for each alcohol condition was 5.46, 3.98, 3.45, 4.76, and 2.73 μmol/min for methanol (5 %, 1.23 M), ethanol (1 %, 0.17 M), isopropanol (0.5 %, 0.06 M), n-propanol (0.5 %, 0.07 M), and n-butanol (0.1 %, 0.01 M), respectively. Additional tests for n-butanol in lower concentrations were conducted and the H₂ generation rates were 2.08 and 2.44 μmol/min for 0.01 % (0.0011 M) and 0.05 % (0.0055 M), respectively. For all the cases in this study, the H₂ generation rate increased, peaked, and then decreased as the alcohol concentration increased.

Fig. 2.

Sonochemical generation rate of H₂ under Ar 100 % at each alcohol concentration of methanol, ethanol, isopropanol, n-propanol, and n-butanol. The same results were expressed in volume percentage concentration in Fig. 1S.

The reason for the enhancement of the sonochemical H₂ generation by the addition of alcohols can be explained as follows: First, the alcohol molecules can easily volatilize into cavitation bubbles, undergo extreme pyrolysis, and act as an additional source of H radicals, which are essential for the production of H₂ [42], [50]. A higher alcohol concentration resulted in greater pyrolysis of the alcohols inside the bubbles. Secondly, alcohol molecules can effectively scavenge ROS, which can easily scavenge H radicals. As the alcohol concentration increases, the scavenging effect increases because more alcohol molecules are present around the cavitation bubbles [2], [25]. Third, alcohol molecules and their derivatives can react with H radicals to generate more H₂. Under Ar, the generation of H₂ in the presence of methanol in water is suggested by the following reactions, in addition to reactions (3), (4), (5), (6) [42]:

$$C{H}_{3}OH\to ·C{H}_{2}OH+H·$$ (7)

$$C{H}_{3}OH+H·\to C{H}_{2}OH+H_2$$ (8)

$$C{H}_{2}O+H·\to HCO·+H_2$$ (9)

$$·C{H}_{2}OH+H·\to C{H}_{2}O+H_2$$ (10)

Despite the advantages of alcohol addition, high concentrations of alcohol can inhibit the sonochemical generation of H₂. As shown in Fig. 2, Fig. 1S, peaks of H₂ generation were observed at relatively low concentrations, and H₂ generation decreased significantly at higher concentrations. This inhibition may be due to a reduction in the intensity of the cavitation phenomenon. When the alcohol concentration is high, it can lead to an increased intrusion of alcohol molecules into the cavitation bubbles and decrease the bubble collapse temperature [2], [43], [51], [52]. Yusof et al. reported a lower intensity of multibubble sonoluminescence (MBSL) in 5 mM alcohol solutions than in water (intensity order: water > methanol > ethanol > n-propanol > n-butanol) [53]. In addition, it has been reported that organic liquids (100 %) have a lower cavitation intensity than water (water: 100, methanol: 52, ethanol: 46, isopropanol: 38, n-propanol: 42, and n-butanol: 43) [54].

Hansen investigated sonochemical reduction for nanoparticle synthesis using various alcohol solutions, including methanol, ethanol, n-butanol, and isopropanol. They found that longer-chain alcohols showed better scavenging efficiency at lower alcohol concentrations for oxidizing radicals, such as OH radicals. The alcohol concentrations for 90 % scavenging efficiency were 100 mM, 10 mM, 1 mM, 0.3 mM for methanol, ethanol, isopropanol, and n-butanol, respectively in their study [2]. In this study, the alcohol concentration for the highest H₂ generation was higher than the 90 % scavenging concentration, as shown in Fig. 2. This indicates that oxidizing radicals should be completely removed to maximize H₂ generation and that lower alcohol concentrations are required for longer-chain alcohols. Caruso et al. also reported higher amounts of sonochemically reduced products at lower alcohol concentrations when longer-chain alcohols [25]. Previous studies have reported optimal alcohol concentrations; Büttner et al. obtained the highest H₂ generation at 10 % (v/v) concentration in the 0–80 % methanol concentration range [43]. Rassokhin achieved the highest H₂ generation at 1.0 M in the methanol concentration range of 0–––24.7 M [55]. Tauber et al. observed the highest H₂ generation with t-butanol 0.01 M in the 0–––0.5 M range [56]. Wang et al. reported the highest H₂ generation at 1.24 M out of the methanol range of 0–––16.6 M in the presence of AuTiO₂ [34].

As shown in the inset of Fig. 2, no H₂ was generated at a 100 % alcohol concentration. This can be attributed to the extremely low cavitation intensity, as discussed above, and the absence of water. Previous researchers predicted no significant amount of H₂ at 100 % formic acid or methanol concentrations by extrapolating their results [43], [57]. Rassokhin et al. reported that H₂ generation for 100 % methanol was observed only at the temperatures below 10℃ even though the highest H₂ generation for 100 % methanol (24.7 M) at −7℃ was only 2 % of the highest H₂ generation in 1.0 M methanol solution [55]. Therefore, it appears that the sonochemical generation of H₂ is a function of alcohol concentration, temperature, and presence of water, and a mixture of water and alcohol with relatively low alcohol concentrations at moderate temperatures is desirable for the generation of H₂ using ultrasound technologies.

The relationship between the physicochemical properties of alcohols, including the vapor pressure, Henry’s law constant, and octanol–water partition coefficient, reaction rates with H and OH radicals, and sonochemical H₂ generation, was investigated, as shown in Fig. 3 [52]. The H₂ generation rates were obtained for three alcohol concentrations, namely 0.02 M, 0.2 M, and 2.0 M, using extrapolation and interpolation of the results in Fig. 2. Nanzai et al. reported the correlations between the sonochemical degradation rates of 12 aromatic compounds (initial concentration: 0.1 mM) and their physicochemical properties in certain ranges. Of the five properties, the strongest correlation was found for the octanol–water partition coefficient [58]. Nanzai et al. also reported that the sonoluminescence intensity in 100 % organic liquids decreased as the vapor pressure of organic solvents increased [52]. In this study, no significant correlations were observed between any of the results. However, considerable correlations were found for certain concentration conditions, even though there were not enough data to estimate reliably (only five alcohols with three concentrations). For example, as the vapor pressure of the alcohol increased, more H₂ was produced with a 2.0 M concentration of alcohol and less H₂ was produced at a concentration of 0.02 M. In addition, less H₂ was generated as the rate constant for H radical reaction increased for 0.2 M and 2.0 M.

Fig. 3.

Correlation between the sonochemical H₂ generation rate and physicochemical properties and reaction rates with H and OH radicals for five alcohols with three concentrations (0.02 M, 0.2 M, and 2.0 M).

The calorimetric powers of methanol and n-propanol were measured to investigate the relationship between the measured ultrasonic energy and H₂ generation, as shown in Fig. 4. The working electrical power was 77.2 ± 9.7 W. The specific heat capacities for water, methanol, and n-propanol were 4.19, 2.51, and 2.40 J/(g·K), respectively. The calorimetric energy should increase as the alcohol concentration increases. However, the calorimetric power decreases for both methanol and n-propanol as the alcohol concentration increases. The highest calorimetric power was obtained using water (0 % alcohol) in this study [59]. Even though the H₂ generation rate decreased above certain concentrations, as shown in Fig. 2, and a lower calorimetric power was obtained for higher alcohol concentrations, it was difficult to relate the H₂ generation rate to the calorimetric power considering the entire set of conditions and results. Koda et al. pointed out that the quantification of ultrasonic energy using a calorimetric method could be inappropriate because the wasted heat energy in the liquid is measured using this method [60]. It has been reported that the calorimetric energy can vary significantly under the same input power depending on the geometric conditions [11], [12], [13], [14], [15]. Few studies have been reported on the calorimetric energy of organic liquids and water/organic liquid mixtures [59].

Fig. 4.

Calorimetric powers at each alcohol concentration of methanol and n-propanol.

Most previous researchers have used Ar 100 % as the saturation gas for the sonochemical generation of H₂ as mentioned above, whereas Dehane et al. suggested that a mixture of Ar/O₂ (80:20) and a methanol concentration of 7 % (v/v) were the best conditions for H₂ generation at 355 kHz using their numerical simulation [42]. In this study, the sonochemical generation of H₂ representing reduction activity, and H₂O₂ representing oxidation activity, was investigated under various Ar/O₂ mixtures in water, methanol 5 % and n-propanol 0.5 % n-propanol. The concentration at which the highest H₂ generation was achieved was used for each alcohol. As shown in Fig. 5, as the oxygen content in the gas mixture increases, the generation of H₂ decreases because O₂ and ROS, which are strong scavengers of H radicals, increase [1], [31], [61]. Only 100 % Ar induced H₂ generation in water, and no H₂ was detected under O₂ 100 % for both alcohol. Therefore, the presence of O₂ could inhibit the sonochemical generation of H₂ significantly.

Fig. 5.

Sonochemical generation of H₂ and H₂O₂ under Ar/O₂ mixture conditions for methanol and n-propanol.

The highest H₂O₂ generation was observed under water and Ar/O₂ (50:50) conditions. It was found that an Ar/O₂ mixture (O₂: 20–40 %) could result in high sonochemical oxidizing activity [1], [9], [10], [62], [63], [64]. The absence of alcohols likely minimizes the ROS scavenging effect [2]. Relatively low H₂O₂ generation was obtained for Ar 100 % and O₂ 100 % in water because Ar 100 % or O₂ 100 % was not advantageous for sonochemical oxidizing activity in terms of oxidizing radical availability and cavitation event severity, respectively [10], [65].

Despite the presence of alcohols, the sonochemical generation of H₂O₂ increased as the oxygen content increased, in contrast to H₂ generation. This indicates that generating more ROS from O₂ is more important for enhancing the sonochemical oxidizing activity (generation of H₂O₂) than increasing the intensity of cavitation events under excessive Ar in the presence of a high concentration of oxidizing radical scavengers.

The sonochemical generation of H₂ in previous studies since 1990 is summarized in Table 4 with the highest value for each experimental condition. The obtained values are shown in Fig. 6 in terms of mM/h (concentration/time) and mmol/h (mass/time) by dividing them into four categories: water/Ar (water saturated with Ar 100 %), water + chemical/Ar (water/liquid chemical mixture saturated with Ar 100 %), water/Ar + hydrocarbon (water saturated with a mixture of Ar and hydrocarbon gas), and water + chemical + catalyst/Ar (water/liquid chemical mixture saturated with Ar 100 % in the presence of solid catalyst). As some researchers did not provide data on H₂ generation in both units, some results cannot be included in Fig. 6(a) and 6(b). It was determined that it would be more appropriate to compare the H₂ generation results in terms of mass/time, as shown in Fig. 6(b), regardless of the sonoreactor conditions, including the headspace volume differences. It should be noted that only seven studies (#8, #10, #11, #13, #16, #17, and #18 in Table 4) out of the 18 eligible studies were aimed directly at the sonochemical production of H₂ and the enhancement of production. Except for seven studies, H₂ was considered as one of the by-products under various conditions in previous studies. More detailed information on previous studies since 1985 is provided in Table 1S.

Table 4.

Sonochemical H₂ generation rates in previous research since 1990. The highest values for each experimental condition were summarized in the unit of μM/hr and μmol/hr. Some numerical data including concentrations, masses and their corresponding irradiation durations were extracted in the graphs using PlotDigitizer (https://plotdigitizer.com/).

		Experimental conditions and H2 generation rate	References
#1	Hart et al. (1990)	− 300 kHz/water/Ar: 1,608 μM/hr (128.6 μmol/hr) − 300 kHz/water/Ar + C2H6 (10 % v/v): 7,873 μM/hr (629.9 μmol/hr)	[67]
#2	Hart et al. (1990)	− 1,000 kHz/water/Ar: 2,548 μM/hr (57.3 μmol/hr) − 1,000 kHz/water/Ar + C2H2 (1.5 % v/v): 6,402 μM/hr (144.0 μmol/hr)	[66]
#3	Büttner et al. (1991)	− 1,000 kHz/water/Ar: 1,464 μM/hr − 1,000 kHz/water + methanol (10 % v/v)/Ar: 9,127 μM/hr	[43]
#4	Gutierrez et al. (1991)	− 1,000 kHz/water/Ar: 2,520 μM/hr	[68]
#5	Rassokhin et al. (1995)	− 724 kHz/water/Ar: 677 μM/hr (35.2 μmol/hr) − 724 kHz/water + methanol (1 M)/Ar: 11,972 μM/hr (622.6 μmol/hr)	[55]
#6	Harada (1998)	− 200 kHz/water/Ar + CO2 (2.9 % v/v): 17.1 μmol/hr	[69]
#7	Tauber et al. (1999)	− 321 kHz/water/Ar: 693 μM/hr (6.9 μmol/hr) − 321 kHz/water + t-buthanol (0.01 M)/Ar: 8,612 μM/hr (86.1 μmol/hr)	[56]
#8	Harada (2001)	− 200 kHz/water/Ar: 48.5 μmol/hr − 200 kHz/Light/water/Ar: 41.8 μmol/hr − 200 kHz/water + TiO2/Ar: 42.1 μmol/hr − 200 kHz/Light/water + TiO2/Ar: 35.6 μmol/hr	[32]
#9	Harada and Sogawa (2004)	− 200 kHz/water + NaHCO3 (0.04 M)/Ar: 112 μM/hr (27.9 μmol/hr)	[70]
#10	Gentili et al. (2009)	− 38 kHz/water + ethanol (10 % v/v) + rare earth catalyst/Ar: 70.8 μmol/hr − 38 kHz/light/water + ethanol (10 % v/v) + rare earth catalyst/Ar: 82.6 μmol/hr - light/water + ethanol (10 % v/v) + rare earth catalyst/Ar: 2.15 μmol/hr	[33]
#11	Wang et al. (2010)	− 40 kHz/water/Ar: 4 μM/hr (0.4 μmol/hr) − 40 kHz/water + methanol (4 % v/v)/Ar: 34 μM/hr (3.4 μmol/hr) − 40 kHz/water + ethanol (1.9 % v/v)/Ar: 8 μM/hr (0.8 μmol/hr) − 40 kHz/water + glycerol (0.69 % v/v)/Ar: 10 μM/hr (1.0 μmol/hr) − 40 kHz/water + DMSO (0.69 % v/v)/Ar: 28 μM/hr (2.8 μmol/hr) − 40 kHz/water + methanol (4 % v/v) + Au/TiO2/Ar: 2,823 μM/hr (282.3 μmol/hr)	[34]
#12	Navarro et al. (2011)	− 20 kHz/water/Ar (sparging, 90 mL/min): 8.6 μmol/hr − 20 kHz/water + formic acid (0.1 M)/Ar (sparging, 90 mL/min): 17.3 μmol/hr − 200 kHz/water/Ar (sparging, 90 mL/min): 45.4 μmol/hr − 607 kHz/water/Ar (sparging, 90 mL/min): 114.7 μmol/hr	[47]
#13	Penconi et al. (2015)	− 38 kHz/water/Ar: 80.0 μmol/hr − 38 kHz/water + ethanol (20 % v/v)/Ar: 112.0 μmol/hr − 38 kHz/light/water + ethanol (20 % v/v) + rare earth catalyst/Ar: 125 μmol/hr	[35]
#14	Pflieger et al. (2015)	− 20 kHz/water + t-butanol (0.5 M)/Ar (sparging, 100 mL/min): 434 μM/hr − 362 kHz/water + t-butanol (0.02 M)/Ar (sparging, 100 mL/min): 1,146 μM/hr	[48]
#15	Harada and Ono (2015)	− 2,400 kHz/water + KI (0.1 M)/Ar: 142 μM/hr (5.7 μmol/hr)	[49]
#16	Morosini et al. (2016)	− 20 kHz/water/Ar (sparging, 90 mL/min): 2.8 μmol/hr − 20 kHz/water + ThO2/Ar (sparging, 90 mL/min): 8.4 μmol/hr − 362 kHz/water/Ar (sparging, 90 mL/min): 75.0 μmol/hr − 362 kHz/water + TiO2/Ar (sparging, 90 mL/min): 103.8 μmol/hr	[36]
#17	Roger et al. (2020)	[5 min Sonication] − 45 kHz/water + methanol (20 % v/v)/Ar: 1,633 μM/hr (17.1 μmol/hr) − 45 kHz/water + ethanol (20 % v/v)/Ar: 1,491 μM/hr (15.7 μmol/hr) − 45 kHz/water + isopropanol (20 % v/v)/Ar: 602 μM/hr (6.3 μmol/hr) − 45 kHz/water + ethylene glycol (20 % v/v)/Ar: 144 μM/hr (1.5 μmol/hr) − 45 kHz/water + glycerol (20 % v/v)/Ar: 116 μM/hr (1.2 μmol/hr) [5 hrs after 5 min sonication] − 45 kHz/water + methanol (20 % v/v)/Ar: 2,847 μM/hr (29.9 μmol/hr) − 45 kHz/water + ethanol (20 % v/v)/Ar: 1,579 μM/hr (16.6 μmol/hr) − 45 kHz/water + isopropanol (20 % v/v)/Ar: 930 μM/hr (9.8 μmol/hr) − 45 kHz/water + ethylene glycol (20 % v/v)/Ar: 307 μM/hr (3.2 μmol/hr) − 45 kHz/water + glycerol (20 % v/v)/Ar: 153 μM/hr (1.6 μmol/hr)	[37]
#18	Kerboua et al. (2021)	− 20 kHz/water/Ar: 57 μM/hr (33.9 μmol/hr) − 210 kHz/water/Ar: 95 μM/hr (11.6 μmol/hr) − 326 kHz/water/Ar: 85 μM/hr (10.4 μmol/hr) − 488 kHz/water/Ar: 46 μM/hr (5.6 μmol/hr)	[31]
#19	This study	− 300 kHz/water/Ar: 21 μM/hr (18.4 μmol/hr) − 300 kHz/water + methanol (5 % v/v)/Ar: 378 μM/hr (327.3 μmol/hr) − 300 kHz/water + ethanol (1 % v/v)/Ar: 275 μM/hr (238.6 μmol/hr) − 300 kHz/water + isopropanol (0.5 % v/v)/Ar: 239 μM/hr (207.2 μmol/hr) − 300 kHz/water + n-propanol (0.5 % v/v)/Ar: 329 μM/hr (285.3 μmol/hr) − 300 kHz/water + n-butanol (0.5 % v/v)/Ar: 172 μM/hr (149.2 μmol/hr)

Fig. 6.

Sonochemical generation of H₂ in previous research in terms of mM/hr (a) and mmol/hr (b). The data (the highest value for each experimental condition) was divided into four categories: water/Ar, water + chemical/Ar, water/Ar + hydrocarbon, and water + chemical + catalyst/Ar.

As shown in Fig. 6(b), the H₂ generation range was 0.4–––128.6, 0.8–––622.6, 17.1–––629.9, and 8.4–––282.3 μmol/hr for water/Ar, water + chemical/Ar, water/Ar + hydrocarbon, and water + chemical + catalyst/Ar, respectively. In pure water, 0.4 – 80.0, 5.6–––128.6, and 57.3 μmol/hr of H₂ were generated for the frequency (f) ranges of < 100 kHz, 100 ≤ f < 1,000 kHz, and f ≥ 1,000 kHz, respectively. The addition of organic chemicals including methanol, ethanol, isopropanol, n-propanol, t-butanol, glycerol, DMSO, formic acid, and ethylene glycol in water enhanced the sonochemical generation of H₂: 0.8–––112.0 μmol/hr for f < 100 kHz; 27.9–––622.6 μmol/hr for 100 ≤ f < 1,000 kHz, and 5.7 μmol/hr for f ≥ 1,000 kHz. Based on the results of studies where both water alone and water with chemicals were provided, 1.4 to 17.7 times higher H₂ generation was reported with the addition of chemicals [34], [35], [43], [47], [55], [56], [57]. In this study, we observed 17.8 times higher generation of H₂ with the addition of methanol (5 %). In addition, Roger et al. reported the need for degassing before measuring H₂ generation to ensure that H₂ generation could not be underestimated [37].

For the cases with the addition of hydrocarbon gases, including methane, ethane, ethylene, acetylene, propane, and butane, under an Ar atmosphere, 2.5 to 4.9 times larger amount of H₂ were obtained compared to the cases with only Ar [66], [67]. The addition of C₄H₁₀ resulted in the highest H₂ generation among all the cases shown in Table 4 [67]. In addition, a very small amount of H₂ was generated with a C₂H₆ content of >40 % [67]. This indicates that a high hydrocarbon content can significantly inhibit the sonochemical generation of H₂, similar to the high concentration of alcohols in this study. Some researchers have used solid catalysts, including TiO₂, TiO₂/Au, ThO₂, and rare-earth catalysts. No significant enhancement was reported when using TiO₂ for 200 and 362 kHz [32], [36]. However, large enhancements were observed using TiO₂/Au (83 times higher H₂ generation compared to the case of no catalyst in methanol solution (4 % v/v) [34] and ThO₂ (3.0 times higher H₂ generation compared to the case of no catalyst in water) [36] at 40 and 20 kHz, respectively. Some researchers compared sonocatalysis and sonophotocatalysis and obtained small enhancement (1.1 – 1.2 times) in H₂ generation [33], [35].

4. Conclusion

The sonochemical generation of H₂ was investigated using various alcohol/water solutions (methanol, ethanol, isopropanol, n-propanol, and n-butanol) under Ar 100 % in a 300 kHz sonoreactor in this study. The addition of alcohol to water enhanced the H₂ generation significantly. For each alcohol, the concentration of the peak of H₂ generation varied significantly because of the different physicochemical properties of the alcohols, including their ROS scavenging ability. However, no meaningful relationship between the H₂ generation rates and the physicochemical properties of the alcohols was observed for the entire dataset in this study because the mechanism for the sonochemical generation of H₂ was complicated. In the alcohol/water mixtures, the sonochemical oxidation activity (H₂O₂ generation) increased, and the sonochemical reduction activity decreased (H₂ generation) as the O₂ content increased in Ar/O₂ mixtures. From a summary of previous research and this study on sonochemical H₂ generation, it was found that the addition of organic chemicals (liquid or gas) and solid catalysts could enhance the H₂ generation significantly. However, it could be argued that the sonochemical H₂ generation requires a lot of energy and chemicals, which makes it an unsustainable and costly process for the environment. Therefore, further systematic research is required to suggest more applicable and efficient H₂ generation methods.

CRediT authorship contribution statement

Jongbok Choi: Data curation, Formal analysis, Investigation, Methodology, Validation, Writing – original draft. Seokho Yoon: Data curation, Formal analysis, Investigation, Methodology, Validation. Younggyu Son: Conceptualization, Formal analysis, Investigation, Methodology, Project administration, Resources, Supervision, Validation, Visualization, Writing – original draft, Writing – review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Appendix A. Supplementary data

The following are the Supplementary data to this article:

Data availability

No data was used for the research described in the article.