Laser-Induced Methanol Decomposition for Ultrafast Hydrogen Production

Methanol (CH3OH) is a liquid hydrogen (H2) source that effectively releases H2 and is convenient for transportation. Traditional thermocatalytic CH3OH reforming reaction is used to produce H2, but this process needs to undergo high reaction temperature (e.g., 200 °C) along with a catalyst and a large amount of carbon dioxide (CO2) emission. Although photocatalysis and photothermal catalysis under mild conditions are proposed to replace the traditional thermal catalysis to produce H2 from CH3OH, they still inevitably produce CO2 emissions that are detrimental to carbon neutrality. Here, we, for the first time, report an ultrafast and highly selective production of H2 without any catalysts and no CO2 emission from CH3OH by laser bubbling in liquid (LBL) at room temperature and atmospheric pressure. We demonstrate that a super high H2 yield rate of 33.41 mmol·h−1 with 94.26% selectivity is achieved upon the laser-driven process. This yield is 3 orders of magnitude higher than the best value reported for photocatalytic and photothermal catalytic H2 production from CH3OH to date. The energy conversion efficiency of laser light to H2 and CO can be up to 8.5%. We also establish that the far from thermodynamic equilibrium state with high temperature inside the laser-induced bubble and the kinetic process of rapid quenching of bubbles play crucial roles in H2 production upon LBL. Thermodynamically, the high temperature induced using laser in bubbles ensures fast and efficient release of H2 from CH3OH decomposition. Kinetically, rapidly quenching of laser-induced bubbles can inhibit reverse reaction and can keep the products in the initial stage, which guarantees high selectivity. This study presents a laser-driven ultrafast and highly selective production of H2 from CH3OH under normal conditions beyond catalytic chemistry.


Introduction
Hydrogen (H 2 ) energy, the cleanest energy source with a high energy density, is one of the foundations of the future energy system [1][2][3]. However, H 2 is flammable and explosive, as well as inconvenient for transport [4,5]. Therefore, in situ H 2 production is an effective way to solve this problem. Methanol (CH 3 OH) is a promising liquid H 2 carrier and is widely used for H 2 production [6,7]. Thermocatalytic CH 3 OH reforming is a traditional technique of releasing H 2 from CH 3 OH, but this process needs to be carried out at relatively high temperatures (usually 200 °C) and, at the same time, a large amount of carbon dioxide (CO 2 ) is emitted [8][9][10][11][12]. Therefore, scientists have been looking for mild and sustainable ways to produce H 2 from CH 3 OH. Photocatalysis and photothermal catalysis carried out at a normal temperature have attracted extensive attention [12][13][14][15][16]. However, for these methods above, to avoid carbon monoxide (CO) poisoning of a catalyst, CO 2 emission is inevitable [17][18][19]. This should be detrimental to carbon neutrality and exacerbate a greenhouse effect. In fact, CO can be used as fuel [20]. Thus, how to realize the efficient and rapid release of H 2 from CH 3 OH without CO 2 emission under mild conditions is indeed a challenge.
Here, we, for the first time, report an ultrafast and highly selective production of H 2 without any catalysts and no CO 2 emission from CH 3 OH by laser bubbling in liquid (LBL) at a normal temperature and pressure. LBL means that the pulsed laser is focused below the liquid level to produce small bubbles in liquid as shown in Fig. 1. Note that the temperature inside the laser-induced bubble can reach as high as about 10 4 K [21,22]. At such high temperatures, the ionization temperature of CH 3 OH has been exceeded, thus generating CH 3 , CH 3 O, and CH 2 OH radicals as well as CH 3 O − , OH − , and H + ions in the bubble. At the same time, these bubbles would quench at a very short time because of the confining and cooling effect of the surrounding liquid, which would cause that the primary products of high-temperature chemical reactions occurring inside bubbles are directly frozen and retained at room temperature [23][24][25]. Our measurements show that, in such a local extreme environment inside bubbles, CH 3 OH is directly to release H 2 (33.41 mmol·h −1 ) and CO (16.88 mmol·h −1 ). The production rate of H 2 is much higher than that of photocatalytic and photothermal catalysis. Meanwhile, the selectivities for H 2 and CO are up to 94.26% and 89.25%, respectively. At 700 mJ per pulse of laser energy, the conversion efficiency from laser light to H 2 and CO can reach 8.5%. Notably, there is no CO 2 emission. Additionally, we establish the mechanism of laserdriven CH 3 OH releasing H 2 , including the thermodynamics and kinetics of high-temperature reactions in the bubble upon the LBL process. In terms of thermodynamics, the extreme high temperature of the bubble caused using laser promotes the efficient reaction and ensures a high yield. From a kinetic point of view, the rapid quenching process of bubbles freezes the final product, and the high-energy barrier of the reverse reaction prevents the product from returning to CH 3 OH. Therefore, the synergistic effect of thermodynamics and kinetics ensures the high yield and selectivity of H 2 release from CH 3 OH. Therefore, these results show that the laser-driven direct decomposition of CH 3 OH to release H 2 has great potential in the industry.

Results and Discussion
A schematic of the experimental setup is shown in Fig. 1, and the whole LBL process was carried out at room temperature and atmospheric pressure. In this case, LBL is used to enable rapidly switching reactions between high temperature and room temperature to achieve the release of H 2 from CH 3 OH (Fig. 1A). When LBL is performed, the bubbles are generated instantaneously using the focused laser and then collapsed in several hundred microseconds. A series of high-temperature chemical reactions would occur before the collapse of bubbles, and then products would be released together with the collapse of bubbles. Note that after the experiment was completed, the solution temperature remained basically unchanged. Figure 1B shows our proposal for the laser-driven H 2 production from CH 3 OH in practical applications. When the laser energy is excess, the laser beam can be divided to make the energy utilization rate higher. Dividing laser into multiple reaction devices to assemble them into an array can enhance the production rate for large-scale operations. Evidently, this technique opens a new direction for the industrial realization of H 2 production from CH 3 OH.
A series of measurements of the samples were performed as shown in Fig. 2. The yields of the generated gases positively correlated with the laser energy ( Fig. 2A). The H 2 yield is close to 0 when the laser energy is below 100 mJ per pulse and then increases with the increase of laser energy. Therefore, the 100 mJ per pulse is the threshold of laser energy for the reaction to occur. When the laser energy is higher than 100 mJ per pulse, the reaction can be carried out and a large amount of H 2 can be produced. At 700 mJ per pulse, the yields of H 2 and CO can reach 33.41 and 16.88 mmol·h −1 , respectively. These results can be attributed to the increase in the temperature in bubbles caused by the increase in the laser energy, which leads to more efficient high-temperature chemical reactions. It is worth noting that the process has only trace amounts of CH 4 , C 2 H 4 , C 2 H 6 , and C 2 H 2 generation, which have rates that were only 0.56, 0.22, 0.073, and 1.18 mmol·h −1 (Fig. S1), respectively. More importantly, there is no CO 2 generation in the LBL process. In addition to adjusting the laser energy, we study the frequency effect of the pulsed laser. As can be seen from Fig.  2B, the maximum yield (33.41 mmol·h −1 ) of H 2 was reached at the frequency of 10 Hz. However, only 3.34 mmol·h −1 of H 2 and 1.76 mmol·h −1 of CO were measured at 1 Hz. Therefore, this result can be attributed to more bubble generation per unit of time with an increase in the frequency of pulsed laser. To investigate the effect of the reaction time on the yield rate, gas samples at various reaction times at 700 mJ per pulse of laser energy were collected to calculate the yield rate. As shown in Fig. 2C, the laser action time had no obvious effect on the yield rate at the same laser energy. This is mainly because the gas product is released as the bubble collapses, then the surrounding CH 3 OH solution will replenish this reaction area, and new bubbles will be generated again. Importantly, the laser-driven CH 3 OH decomposition releases H 2 and CO with a high selectivity, up to 94.26% and 89.25%, respectively, as shown in Fig.  2D. Therefore, these results show that H 2 yield does not change with laser action time and further elucidate that this method can sustainably release H 2 from CH 3 OH over a long period of time through a laser-driven process. In addition, we also study the energy conversion efficiency from laser light to H 2 and CO. Figure 2E shows that, with the increase of laser energy, the conversion efficiency is also higher, and the energy conversion efficiency can be as high as 8.5% at 700 mJ.
To evaluate advanced performance of LBL, we compared H 2 yield rates of LBL with that of photocatalytic and photothermal catalytic H 2 release from CH 3 OH as shown in Table. Considering the characteristic terms involved in photocatalytic and photothermal catalytic CH 3 OH releasing H 2 , such as solution and irradiation area, the yield rate was normalized into 1-mm space dimensions (e.g., irradiation area and the laser-focused spot). From Table, we can clearly see that the laser-driven H 2 production from CH 3 OH is 3 orders of magnitude higher than the best value of photocatalysis and photothermal catalysis to date. Accordingly, these experimental results above suggest that CH 3 OH can efficiently release a large amount of H 2 and CO upon the LBL process, and the yield rate can be flexibly regulated by changing the energy and frequency of the pulsed laser. Finally, the gas produced by the reaction is collected. (B) A laser-driven amplification scheme of H 2 release from CH 3 OH is proposed, which divides the beam into multiple beams of light, forms an array, and expands the scale of production.
Additionally, ultrafast and highly selective production of H 2 and CO without CO 2 emission from CH 3 OH can be achieved by LBL.
To have a deep understanding to CH 3 OH decomposition reaction under the LBL process, the theoretical research was taken on the basis of the density functional theory (DFT) calculation, aiming to explain the high-temperature chemical reactions occurring inside bubbles upon the LBL process. The pulsed laser would lead to the high-temperature environment, which promotes the CH 3 OH decomposition reaction. As shown in Fig. 3A, the 3 possible reaction paths are the (a) ion path, (b) radical path, and (c) direct decomposition path, respectively. In the ion paths, CH 3 OH can be decomposed into a series of ions species: CH 3  (166.0 kcal/mol), and CH 3 + /OH − (−16.2 kcal/mol). The ions species are located at high-activation free energy, which are less favorable for the CH 3 OH decomposition, while the radical paths are more favorable for the CH 3 OH decomposition. As shown in Fig. S3, the formation of radical species requires lower temperature than that of ion species. When the reaction temperature is high than about 4,000 K, the CH 3  Furthermore, the direct decomposition paths have been studied as well. However, the transition states for the direct decomposition paths are located at high-activation free energy: TS 1 (102.2 kcal/mol), TS 2 (−174.9 kcal/mol), and TS 3 (−155.5 kcal/mol).
Moreover, the ab initio molecular dynamics (MD) simulations were performed to explain the high-temperature chemical reaction of CH 3 OH in bubbles. The simulation temperatures are 298, 5,000, and 10,000 K, respectively, aiming to explore the temperature effect of pulsed laser with different energies. As the temperature rises from 298 to 5,000 K, the H 2 , CO, H 2 O, CH 2 O, and hydrocarbon products were observed in MD simulation ( Fig. 3B and C). When the temperature rises to 10,000 K, a large amount of H 2 and CO products were generated (Fig. 3D). Furthermore, the distributions of radial distribution functions (RDFs) exhibited the decomposition of the CH 3 OH. As the temperature rises to 10,000 K, the maximum distribution of hydrogen-hydrogen was shifted from 1.75 to 0.75 Å (Fig. 3E), while the maximum distribution of carbon-oxygen was changed from 1.45 to 1.25 Å (Fig. 3F). The hydrogenhydrogen and carbon-oxygen RDFs show the formation of the H 2 and CO molecules. In addition, the carbon-carbon RDF further suggested the formation of ethylene and acetylene (Fig. S5).
To further understand the mechanism of laser-driven H 2 release from CH 3 OH, we propose basic physical and chemical perspectives on the basis of the experimental measurements, DFT calculations, and MD simulations as shown in Fig. 4. First, when the pulsed laser is focused below the liquid level, many small bubbles form in the solution (Fig. 4A). A high-energy laser can induce high temperatures up to thousands of kelvin inside the bubble, and this extreme high temperature can exceed the ionization temperature of CH 3 OH in the bubble, which results in the formation of CH 3 , CH 3 O, and CH 2 OH radicals as well as CH 2 OH − , OH − , and H + ions in the bubble (Fig. 4B). Note that the state in the bubble is far from thermodynamic equilibrium. Therefore, these laser-induced bubbles provide a favorable thermodynamic environment for the CH 3 OH decomposition.
On the basis of Eyring's transition state theory, the temperature and the activation free energy are 2 key factors for the chemical reaction rate processes. By comparing with each reaction step in Fig. S6, the CH 3 OH decomposition is an exothermic reaction in the whole process. The reduction of activation free energy is mainly due to the entropy increase of the reactions. As shown in Fig. S6, the ΔS are 27.8 and 24.7 cal/mol for the formation of CH 2 O and CO reactions, respectively. In the LBL process, the bubble environment is located at the high temperature, which amplifies the entropy effect. Thus, these results show that the entropy effect plays an important role in the reaction process at a high temperature, which can directly and effectively promote the decomposition of CH 3 OH to release H 2 .
Kinetically, the pulsed laser results in an instantaneous formation of bubbles at the focal point inside of the CH 3 OH solution. According to previous studies, far from equilibrium bubbles would disintegrate within 100 to 400 μs [21], enabling ultrafast H 2 release from CH 3 OH. Because the temperature drops to room temperature instantaneously, during the rapid cooling process, H 2 and CO of the products formed by the high temperature can be retained, preventing the products from slowly decomposing and returning those to the initial state. Because of the rapid quenching of bubbles, the formation of free radical species is prevented. The activation free energy of the formation of CH 2 OH • /H • is 86.6 kcal/mol in 298.15 K. In addition, the CO and H 2 conversion reaction are carried out in 298.15 K as well (Fig. S7). The rate-determining step of CO conversion is the formation of CHOH, which undergoes the transition state TS 3 -iso. The activation free energy of this step is 81.0 kcal/mol. The temperature for a practical CO and H 2 conversion reaction (t 1/2 = 1 s to 1 h) is around 866.3 to 1038.9 K (Fig. S8). The results clearly suggested that the CO and H 2 conversion are almost nonoccurrence below 1038.9 K. Additionally, the MD simulations were taken to simulate the rapid cooling stage of the LBL process. The RDFs results suggest that the H 2 and CO species are retained in high proportion (Figs. S9 and S10). When the bubble induced using laser expands to the maximum volume in CH 3 OH, it is close to the vacuum state because the number of internal molecules does not change much. The external liquid pressure acts on the bubble, and the resulting pressure difference causes the bubble to collapse quickly. Therefore, rapid quenching of nonequilibrium ensures the formation of CO/H 2 products in laser-induced high-temperature chemical reactions in bubbles, as well as prevented the products from further reversion to initial states. As a result, large amounts of H 2 and CO are released as the bubble collapsed, and the process can achieve a high product yield (Fig. 4C). Therefore, the LBL process can be carried out at room temperature and atmospheric pressure, which is simple and has no CO 2 emissions. The nonequilibrium process of bubbles is similar to the microreactor induced using a laser, in which the high-temperature chemical reaction takes place, which can proceed the reaction more effectively. Moreover, the extremely short quenching time can keep the product at room temperature and inhibit the reverse reaction.

Conclusion
In summary, we developed an ultrafast and highly selective production of H 2 and CO without a catalyst as well as no CO 2 emission from CH 3 OH driven using laser under normal temperature and pressure. A super high H 2 yield rate of 33.41 mmol·h −1 with 94.26% selectivity can be produced upon the laser-driven process, and this yield is 3 orders of magnitude higher than the best value reported for photocatalytic and photothermal catalytic H 2 production from CH 3 OH to date. On the basis of DFT calculation and MD simulation, we clarified the basic physical and chemical processes of laser-driven ultrafast H 2 release from CH 3 OH, which confirm the experimental measurements of high temperature promoting CH 3 OH decomposition. We also establish that the thermodynamic and kinetic mechanism of high-temperature chemical reactions inside bubbles induced pulsed laser. Thermodynamically, the high temperature of bubbles amplifies the entropy effect during the CH 3 OH decomposition, which reduces the activation energy of the decomposition process without catalysts. Kinetically, the bubbles have a short quenching time, which makes it difficult for the products H 2 and CO to return to their initial states through a reverse reaction. Therefore, this synergy between thermodynamics and kinetics results in high yield and selectivity for H 2 and CO. Accordingly, we believe that LBL can be expected to be a disruptive technology to release H 2 from CH 3 OH without CO 2 emission at normal conditions beyond catalytic chemistry.

Synthesis and characterization
First, CH 3 OH (200 ml) was poured into a quartz reactor. Then, the second harmonics generated using a Q-switched Nd:YAG (neodymium-doped yttrium aluminum garnet crystal) laser device with a wavelength of 532 nm; a pulse width of 10 ns; repetition rates of 1, 5, and 10 Hz; and laser pulse energies of 100 to 700 mJ was focused inside the liquid, forming a spot size of about 1 mm. Moreover, not only the argon (Ar) gas is introduced into the solution, which can play the role of a protective gas, but also the gas produced during the reaction that flowed out together with the Ar. Before the experiment, the Ar gas was a bubble in CH 3 OH (0.4 l/min) for 60 min to ensure that no air was present in the water. Gases produced by the laser-driven CH 3 OH decomposition process were collected and characterized. The yields of gases (y CO , y H2 , y C2H2 , y C2H4 , y CH4 , and y C2H6 ) under diverse laser energy and time were quantified by gas chromatography (GC-2014C, Shimadzu) with a thermal conductivity detector (5Å molecular sieve column) and 2 flame ionization detectors (MC-3 column). The formula for calculating H 2 (S H2 ) and CO (S CO ) product selectivity was as follows: Considering the enthalpy change of CH 3 OH decomposing to H 2 and CO is as follows: The formula for calculating conversion of laser light to H 2 and CO product selectivity was as follows: where η is the energy conversion of laser light to H 2 and CO, ΔH is the enthalpy change from CH 3 OH to CO and H 2 , and n(methanol) is the amount of CH 3 OH that takes part in reaction.

Calculation and simulation
The mechanism study was taken by DFT calculation, aiming to explore the reaction path of the H 2 release of CH 3 OH driven by LBL. The ab initio MD simulations were taken to explore the decomposition of CH 3

Data Availability
The data that support the findings of this study are available within the article and the Supplementary Materials. Raw data are available from the corresponding authors on reasonable request.

Supplementary Materials
Calculation section Supplementary figures Fig. S1. The yield rate of CH 4 , C 2 H 4 , C 2 H 6 , and C 2 H 2 at different times under 700 mJ per pulse of laser energy. Fig. S2. GC chromatogram of H 2 and CO released from laser-induced CH 3 OH. Fig. S3. Gibbs free energy of the formation of ions and radicals in LBL-promoted conversion of CH 3 OH. Fig. S4. Mechanism for the formation of C 2 H 6 , C 2 H 4 , C 2 H 2 , and CH 4 in CH 3 OH decomposition reaction under pulsed laser. Fig. S5. Carbon-carbon radial distribution functions. Fig. S6. Each step equations of CH 3 OH decomposition under pulsed laser. Fig. S7. Mechanism and free energy profiles for CO and H 2 conversion reaction. Fig. S8. Half-life along with reaction temperature for the formation of CHOH step in CO and H 2 conversion reaction. Fig. S9. Hydrogen−hydrogen radial distribution functions of the cooling stage. Fig. S10. Carbon-oxygen radial distribution functions of the cooling stage. Fig. S11. Carbon-carbon radial distribution functions of the cooling stage. Fig. S12. Key structures for CH 3 OH decomposition under pulsed laser.