Sonochemical Production Of Hydrogen Research Articles

Critical Analysis of Hydrogen Production by Aqueous Methanol Sonolysis.

Recently, several experimental and theoretical studies have demonstrated the feasibility of enhancing the sonochemical production of hydrogen via methanol pyrolysis within acoustic cavitation bubbles(i.e. sonolysis of aqueous methanol solution). This review includes both the experimental and theoretical achievements in the field of hydrogen production by methanol sonolysis. Additionally, the limits of the process's applicability and plausible solutions are highlighted. The impact of different parameters influencing the process performance is discussed. Finally, the effects of methanol concentration on the size distribution of active cavitationbubbles are analyzed.

Microscopic Analysis of Hydrogen Production from Methane Sono-Pyrolysis

The sonolysis of certain substrates in water has proved its effectiveness for the enhancement of the sonochemical production of hydrogen. In this study, the sonolysis of methane has been investigated for the first time in a single acoustic bubble (microreactor) over a frequency from 140 to 515 kHz. The obtained findings have been compared to those available in the literature. Independently of the methane dose (inside the bubble), the yield of H2 was improved especially with the decrease in wave frequency (from 515 to 140 kHz). For the driving frequencies 140, 213, 355, and 515 kHz, the production of hydrogen was maximized at 20, 15, 10, and 10% CH4, respectively. For 213 kHz, and the presence of 10% methane, the yield of hydrogen goes up by 111 fold compared to the case where the gas atmosphere is saturated only by argon. On the other hand, the highest methane conversions (~100% for 2, 5 and 7% CH4) were retrieved at 140 and 213 kHz. In terms of hydrogen formation and methane decay, the use of 140 kHz was found to be the best choice, whereas for a multi-bubble system, the number of acoustic bubbles should be taken into account for an optimal choice of frequency. Interestingly, it was observed that at 140 and 213 kHz and for methane mole fractions lower than or equal to 30 and 10%, respectively, a maximal formation of H2 and a relatively important production of •OH could result simultaneously.

Methanol sono-pyrolysis for hydrogen recovery: Effect of methanol concentration under an argon atmosphere

A number of theoretical and experimental works showed the possibility of increasing the sonochemical production of hydrogen through the pyrolysis of methanol within the acoustic cavitation (i.e. sonolysis of aqueous methanol solution). In this study, numerical simulations have been conducted in order to reveal, for the first time, the effect of methanol concentration (in the bulk liquid) on the maximal sonochemical efficiency for hydrogen yielding, methanol conversion and the broadness of active bubbles range. The current adopted model is built on a set of ordinary differential equations that account for non-equilibrium evaporation and condensation of water vapor and methanol at the bubble wall, thermal conduction both within and outside the bubble and heat of chemical reactions. All numerical simulations were carried out for a single bubble oscillating in O2 and/or argon-saturated water with varying concentration of methanol. It was found that the variation of methanol concentration (0–100% (v/v)) affects slightly the broadness of active bubbles ranges for methanol consumption. Conversely, for hydrogen production, a gradual decrease of the active bubbles ranges is observed for methanol concentration greater than 20%. The maximal sonochemical efficiency for hydrogen yielding, methanol conversion and the broadness of active bubbles ranges (for H2 production and CH3OH conversion) is obtained at 80% of argon and for methanol concentration between 7% and 20%. The yield of hydrogen increased by 26.16 and 22.4 times at 10 and 20% (v/v) of methanol in bulk solution, respectively, whereas CH3OH conversion decreased by 13.84 and 22.3%, respectively.

Clean hydrogen production by ultrasound (sonochemistry): The effect of noble gases

Power ultrasonic (>100 ​kHz) splits water into free radicals and hydrogen. As a result, water sonochemistry is considered as an alternative clean and fossil-fuel-free hydrogen production technique. In this research work, the impact of rare gases (Xe, Ar, and He) on the sonochemical production of hydrogen as well as the population of active bubbles has been investigated computationally for various sonicated frequencies (213–515 ​kHz) and intensities (1–2 ​W/cm2). It has been found that both the H2 yielding and the bubble population size for H2 yielding are in the order Xe ​> ​Ar ​> ​He, whatever the imposed sonolytic parameters (i.e., frequency and power). These findings were principally ascribed to the thermal conductivity of the saturating gases, which is in the reverse order (He ​> ​Ar ​> ​Xe). Besides, the difference between Ar and Xe is condensed in comparison with the He gas. For wave frequencies larger than 213 ​kHz, however, all saturating gases (Xe, Ar, and He) behave identically, with the influence of thermal conductivity of these gases on the optimal radius muted. At 213 ​kHz, however, this impact is plainly visible (Ropt (Ar and Xe)>Ropt (He)). As per the results obtained, helium's inefficiency as a saturating gas for hydrogen production is verified, but xenon's maximal efficacy is reached when water is saturated with it. These results support the fewer experimental data reported in this emerging branch of sonochemistry, while the discussed results in the present paper (i.e., noble gases effect on sono-hydrogen production) are treated for the first time. Consequently, our work is considered as a guideline for increasing the efficacy of hydrogen production in a sonochemical reactor.

Low carbon ultrasonic production of alternate fuel: Operational and mechanistic concerns of the sonochemical process of hydrogen generation under various scenarios

Sonochemistry is considered as one of the cleaner pathways for hydrogen production. The present paper investigates the potential of this technique based upon mass, mass to energy and energy conversion metrics, using modelling and experimental approaches. Four scenarios are examined assuming four saturating gases, namely O2, air, N2 and Ar, four acoustic frequencies, i.e., 20, 210, 326 and 488 kHz, and considering common acoustic intensities then common net electric power. The study revealed that Ar is the best fitting saturating gas for the sonochemical production of hydrogen. With a common acoustic intensity of 0.48 W/cm2, an optimum ratio of H2 molar yield to acoustic energy intensity is retrieved at 210 kHz, while with a common net electric power of 87 W, the highest ratio of hydrogen yield to electric energy was observed at 20 kHz. Results were interpreted based upon emitter surface, energy conversion and distinction of calorimetric and cavitational energies.

How do dissolved gases affect the sonochemical process of hydrogen production? An overview of thermodynamic and mechanistic effects – On the “hot spot theory”

Although most of researchers agree on the elementary reactions behind the sonolytic formation of molecular hydrogen (H2) from water, namely the radical attack of H2O and H2O2 and the free radicals recombination, several recent papers ignore the intervention of the dissolved gas molecules in the kinetic pathways of free radicals, and hence may wrongly assess the effect of dissolved gases on the sonochemical production of hydrogen. One may fairly ask to which extent is it acceptable to ignore the role of the dissolved gas and its eventual decomposition inside the acoustic cavitation bubble? The present opinion paper discusses numerically the ways in which the nature of dissolved gas, i.e., N2, O2, Ar and air, may influence the kinetics of sonochemical hydrogen formation. The model evaluates the extent of direct physical effects, i.e., dynamics of bubble oscillation and collapse events if any, against indirect chemical effects, i.e., the chemical reactions of free radicals formation and consequently hydrogen emergence, it demonstrates the improvement in the sonochemical hydrogen production under argon and sheds light on several misinterpretations reported in earlier works, due to wrong assumptions mainly related to initial conditions. The paper also highlights the role of dissolved gases in the nature of created cavitation and hence the eventual bubble population phenomena that may prevent the achievement of the sonochemical activity. This is particularly demonstrated experimentally using a 20 kHz Sinaptec transducer and a Photron SA 5 high speed camera, in the case of CO2-saturated water where degassing bubbles are formed instead of transient cavitation.

Sonochemical production of hydrogen: A numerical model applied to the recovery of aqueous methanol waste under oxygen‐argon atmosphere

AbstractThe present paper consists of a numerical simulation that investigates the opportunity to treat wastewater contaminated with methanol by sonolysis. The technique offers a double benefit in regard to two axes: methanol degradation and hydrogen emergence. The adopted model based on the simultaneous evolutions of bubble dynamics, physical and chemical kinetics as well as thermodynamics parameters inside the bubble, resulted in very interesting trends of methanol degradation efficiency and percentage of recovery to hydrogen. On one hand, the sonolysis of wastewater containing 6 to 40% (vol/vol) of methanol conducts to more than 90% of elimination efficiency under an atmosphere of 40% molar of argon. On the other hand, hydrogen production attains an optimum within a solution of 40% (vol/vol) of methanol under 50% molar of argon. The best combination of both axes is retrieved in the case of wastewater containing 20% (vol/vol) of methanol and saturated with 70% molar of argon; it leads to the elimination of 99.9% of the alcohol, and the conversion of the equivalent of 59% of the molar yield of degraded methanol to hydrogen.

Sonochemical Hydrogen Production as a Potential Interference in Light-Driven Hydrogen Evolution Catalysis.

The use of sonication to dissolve or disperse solids in solvents is a common practice in catalytic studies. However, the frequency and power of commercial ultrasonic baths are within the right range to trigger unwanted sonochemical reactions that can be a source of interference. Based on our own experience, we have noted that sonication used to disperse heterogeneous photocatalysts in water–alcohol mixtures as the first step in light-driven hydrogen evolution can lead to hydrogen evolution, which is not related to any photochemical or photocatalytic process. Furthermore, “dark” hydrogen evolution continues even when sonication is stopped, which can cause significant problems for time-dependent studies. To the best of our knowledge, this phenomenon has not been described as a potential issue for light-driven hydrogen evolution studies, and it should, therefore, be brought to the attention of the energy conversion research community to avoid errors in ongoing and future hydrogen evolution studies.

An investigation of ultrasonic based hydrogen production

A comprehensive numerical study is performed to establish a link between the primary effect of the acoustic cavitation bubble activity and the consequent effect of the chemical kinetics mechanism associated with the sonochemical process. In this work, we studied a possible reaction kinetics mechanism for the sonochemical hydrogen production, which we called the sonohydrogen process. The reaction kinetics mechanism consists of 19 reversible reactions taking place inside the acoustic cavitation micro-bubble at different conditions. The simulation of the reaction kinetics is validated and utilized to quantify the amount of hydrogen produced by a single bubble that is initially saturated with water vapor/oxygen. The results from the bubble dynamics model and the chemical kinetics model are compared with two different experiments available in the literature. Furthermore, the work evaluated the energy efficiency of this technology to produce 1 μmol of hydrogen in kWh. The present results revealed that the bubble temperature governs the chemical reaction mechanism of the water vapor dissociation. The minimum bubble temperature required for a slight production of hydrogen is 3000 K, the hydrogen is in the range of 5.46E-06 – 8.59E-06 μmol/h. At the following conditions of an ultrasonic frequency of 20 kHz, an acoustic power of 30 W, and a bubble temperature of 6000 K, the amount of hydrogen produced is 1.05E-03 μmol/h. The higher the bubble temperature, the faster the chemical reaction rate, which will lead to a higher hydrogen production rate. In terms of performance, the sonohydrogen process produces 2.22E-02 μmol/kWh.

Sonochemical production of hydrogen: Enhancement by summed harmonics excitation

A particular case of harmonics excitation using ultrasonic waves with the frequencies of 200, 300 and 500 kHz at an acoustic amplitude of 2 atm is numerically investigated in the present paper, as a new method aiming to improve the sonochemical production of hydrogen within active cavitation bubble. The modification of the conventional sine signal, travelling across an aqueous medium saturated with oxygen, into evolutionary number of harmonics tending to square signal proved to be of a notable efficiency regarding the hydrogen emergence kinetics. The productivity was revealed to be the highest under 200 kHz, while the gain due to the use of a sum of six successive harmonics instead of sine wave was the most important under 500 kHz. Moreover, the sum of six harmonics proved to be more efficient than the square wave in terms of hydrogen production.

Correlations Between the Sonochemical Production Rate of Hydrogen and the Maximum Temperature and Pressure Reached in Acoustic Bubbles

Water sonolysis generates hydrogen through acoustic cavitation. In this work, based on a model for a reactive acoustic bubble, correlations between the sonochemical production of hydrogen and the maximum temperature and pressure reached in the bubble at the violent collapse have been made. The computational analysis has been performed for more than 800 points obtained by combining various cavitation parameters, i.e., frequency, acoustic intensity, liquid temperature, and ambient bubble radius. The simulation results showed that hydrogen production rate progressed linearly with the bubble temperature and pressure rise up to plateaus, which begin at 3500 ± 200 K and 100 ± 10 atm. Analyzing the progress of \(\text {H}^{{\cdot }}\) and \(^{{\cdot }} \)OH (\(\text {H}_{2}\) precursors) as function of bubble temperature and pressure showed very similar evolutions as those obtained for \(\text {H}_{2}\) with the same optimums at 3500 ± 200 K and 100 ± 10 atm. Consequently, in addition to the quench of hydrogen formation at very high bubble temperatures through the reaction \(\text {H}_{2}+\,^{{\cdot }}{\hbox {OH}}\rightarrow \text {H}_{2}\hbox {O}+\text {H}^{{\cdot }}\), the existing optimum temperature and pressure for \(\text {H}_{2}\) production may also be due the hard consumption of their precursors (\(^{{\cdot }}\)OH and \(\text {H}^{{\cdot }})\) above 3500 K and 100 atm.

Numerical estimation of ultrasonic production of hydrogen: Effect of ideal and real gas based models

Based on two different assumptions regarding the equation describing the state of the gases within an acoustic cavitation bubble, this paper studies the sonochemical production of hydrogen, through two numerical models treating the evolution of a chemical mechanism within a single bubble saturated with oxygen during an oscillation cycle in water. The first approach is built on an ideal gas model, while the second one is founded on Van der Waals equation, and the main objective was to analyze the effect of the considered state equation on the ultrasonic hydrogen production retrieved by simulation under various operating conditions. The obtained results show that even when the second approach gives higher values of temperature, pressure and total free radicals production, yield of hydrogen does not follow the same trend. When comparing the results released by both models regarding hydrogen production, it was noticed that the ratio of the molar amount of hydrogen is frequency and acoustic amplitude dependent. The use of Van der Waals equation leads to higher quantities of hydrogen under low acoustic amplitude and high frequencies, while employing ideal gas law based model gains the upper hand regarding hydrogen production at low frequencies and high acoustic amplitudes.

Mechanism of the sonochemical production of hydrogen

It has been long recognized that propagation of an ultrasonic wave in water results in hydrogen production. The chemical effects of ultrasound (sonochemistry) originate from acoustic cavitation, that is, the formation, growth and implosive collapse of microscopic bubbles in liquid irradiated by ultrasound wave. Enormous temperatures and pressures are generated within the bubbles at the collapse, making each bubble as a microreactor within which typical flame reactions occur. The combustion in the cavitation bubbles yield species such as OH, H, O, HO2 and others. Although H2 is the most molecular product of water sonolysis, the mechanism of its production is until now not understood and the most reported suggestions are controversial. In this paper, a comprehensive numerical work was carried out, for the first time, to explain the mechanism of ultrasound induced generation of H2 in water. Computer simulations of chemical reactions occurring inside a bubble oscillating in water irradiated by an ultrasonic wave have been performed for different conditions. A kinetics mechanism of 25 reversible chemical reactions was proposed for studying the internal bubble-chemistry. The numerical simulations have evidenced the formation of H2 as well as other products such as O2, H2O2, OH, H, HO2 and O in the bubble during implosion. In all cases, H2 was the main product formed in the bubbles at appreciable amount. Basing on the simulation results and using material balance for hydrogen in the gas and liquid phases, the production rate of H2 in each phase has been quantified. The conclusion was that the main source of H2 production during water sonolysis is the gas phase of the bubbles through the reaction H + OH ↔ H2 + O.

Sonochemical Hydrogen Production Efficiently Catalyzed by Au/TiO2

Au/TiO2 was used as a highly efficient sonocatalyst to produce H2 from water or aqueous solutions. Au/TiO2 significantly increased the yields of H• and •OH radicals in the sonolysis of water. Product analysis and isotope evidence indicated that hydrogen molecules derived from methanol/water solutions were formed by three pathways: (1) recombination of two H• atoms from the cleavage of water molecules, (2) H-abstraction from methanol by H• generated by water cleavage, and (3) thermal reforming of methanol. The relative importance of each pathway was assessed by carefully analyzing the hydrogen-isotope composition of the evolved hydrogen gas using a modified gas chromatograph. The source of hydrogen in the H2 evolved from methanol/water solutions during sonolysis was also addressed. Data showed that, although the addition of methanol in the presence of Au/TiO2 resulted in a 12-fold increase in the rate of H2 evolution, nearly half of the hydrogen atoms were nevertheless derived from water molecules. Control...