Splitting water with a laser to produce hydrogen and hydrogen peroxide

Splitting water with solar-powered lasers is a practical solution and a viable method (Shutterstock)

Hydrogen (H2) and hydrogen peroxide (H2O2) are important raw materials and energy carriers for industry. However, traditional methods for producing these two substances rely on complex catalysts and multiple intermediate steps. They are also expensive, have low efficiency, and result in environmental pollution.

A new study presented a direct, highly efficient and environmentally friendly method to split water with a laser for the purpose of generating H2 and H2O2 at the same time, under normal ambient conditions, and without the use of any catalysts.

The study was conducted by a group of Chinese researchers with support from the National Natural Science Foundation of China and the State Key Laboratory of Optoelectronic Materials and Technologies, and was published on the website of the Journal of the National Academy of Sciences (PNAS) on February 23.

The method of work

In this method, the laser is focused below the surface of pure water, where a high-energy pulsed laser is used in the form of ultra-fast pulses. The water splitting process takes place in a specially designed closed cylindrical quartz reactor containing 250 ml of pure water. When the pulsed laser is irradiated in water, liquid water molecules violently decompose into active molecules, resulting in the appearance of small nanobubbles.

These laser-induced bubbles are characterized by a very high temperature that can reach 10⁴K. These laser-induced bubbles generate an ideal microenvironment for water splitting reactions without any catalysts due to their very high temperature. From a thermodynamic standpoint, molecules can Water decomposes quickly at high temperatures.

Drawing of laser water splitting: (A) splitting device, (B) image of laser focused on water, (C) diagram of resulting bubbles, (D) histogram of laser pulse, (E) time stages of H2 and H2O2 preparation corresponding to laser pulse in (D) (National Academy of Sciences)

After this significant increase in temperature, the bubbles are rapidly cooled at a high rate of about 10¹⁰ K/s by the surrounding liquid water. This rapid cooling quickly stops the reactions and prevents adverse reactions. H2 and H2O2 can easily return to the water in the absence of rapid cooling. Finally, when the bubbles collapse, H2 is released while H2O2 remains dissolved in the water.

It is noted here that the transient high temperature and rapid cooling are the essence of this technology.

For pulsed lasers, very short intermittent pulses are used, with each laser pulse having a duration of 10 nanoseconds and a frequency of 10 Hz. This method is an intermittent process of dividing water, similar to the “batch-by-batch” method.

The efficiency of converting laser light energy depends on the purpose of splitting water. depends on the laser used, and thus depends on the progress in laser science and technology, as high laser pulse energy and laser pulse frequency can effectively increase the water splitting efficiency. It is believed that with the development of laser science and technology, the cost of lasers can decrease rapidly, and the price of lasers has decreased significantly over the past few decades.

The laser used in this study works with electrical energy. But electricity is not the only way to operate lasers. Many previous studies have shown that solar light can be efficiently converted into lasers. Therefore, splitting water using solar-powered lasers is considered a practical solution and an applicable method.

(Clip of a laser water splitting process in a specially designed closed cylindrical quartz reactor containing 250 ml of pure water)

Efficiency of the method

This direct laser water splitting method achieves amazing efficiency in converting light energy into chemical energy, with high H2 and H2O2 production rates in a limited reaction zone within a very short reaction time. All this happens under conditions of room temperature and normal atmospheric pressure, without the use of any catalysts, and without any harmful environmental effects.

(Clip showing bubbles containing gaseous products being generated around the laser focus point during the laser-induced water splitting process)

Traditional methods are currently used

Hydrogen is the most abundant chemical element in the universe, but producing it in pure form for use in a range of industrial processes is energy intensive, resulting in significant carbon emissions that pollute the environment.

Obtaining hydrogen in traditional methods depends on renewable and non-renewable energy sources, and is usually derived from fossil fuels. Hydrogen is produced by reforming methane gas with steam, by heating the gas to high temperatures in the presence of steam and a catalyst, which causes the disintegration of molecules. Methane and the formation of carbon monoxide and hydrogen.

Although this method is the most widespread and least expensive, because of the carbon emissions it produces, it is not desirable in light of the global shift to the production of clean, non-polluting energy. Electrocatalysis and photocatalysis are also common methods used to produce hydrogen from water, but these methods usually require complex catalysts and tedious catalytic processes.

As for the traditional methods of producing hydrogen peroxide, electrolysis, anthraquinone spontaneous oxidation, isopropanol oxidation, and electrochemical oxygen reduction are commonly used in hydrogen peroxide production.

Although the method of producing hydrogen peroxide via the spontaneous oxidation of anthraquinone is a low-cost method and represents more than 95% of the global production of hydrogen peroxide, it is an energy-intensive method that causes environmental pollution.

Hydrogen peroxide is used to purify wastewater, especially those containing cyanide (Shutterstock)

H2 and H2O2... their importance and uses

The importance of hydrogen and hydrogen peroxide has increased in industrial production and daily life as green, non-polluting energy. The importance of hydrogen lies in its being a clean, renewable energy with high energy density and calorific value, and its use does not produce carbon dioxide. Hydrogen is considered a necessary resource to solve the current energy and environmental crisis, but hydrogen in itself is not an energy source, as it is not a primary energy that exists freely in nature, but hydrogen is a secondary form of energy that must be manufactured like electricity, as it is an energy carrier.

Although hydrogen is the smallest molecule in the universe, it has enormous potential, as it is used in industrial processes starting from the production of industrial fuels and petrochemicals to the manufacture of semiconductors. It is a gas that can be burned inside engines, and it can also be used in fuel cells to operate electric vehicles, or produce Electricity or heat generation, and it can be a primary raw material in other chemical products such as ammonia, which is one of the most important basic chemicals in the production of nitrogen fertilizers, and methanol, which is used in the production of plastics. Hydrogen and its derivatives can also be stored in tanks or salt domes, and the advantage of stored energy is that it can be used in the long term. According to the International Energy Agency, demand for hydrogen has more than tripled since 1975.

As for hydrogen peroxide, it is an oxidizing raw material and is used as a disinfectant in the fields of medicine, health care, and pharmaceutical industries. It is also used in many other applications such as bleaching and oxidation processes. It is also environmentally friendly, so there has been an increase in the industrial use of hydrogen peroxide over the past 15 years.

In the textile industry, hydrogen peroxide is used to bleach cotton, linen, polyester, and polyurethane fibres. It is also used for bleaching in the wood and paper industry, and can be used to bleach human hair.

In the chemical industry, it is used in the production of several chemicals. It is also used as a cleaning and etching chemical, and is used in the production of semiconductor chips.

As for environmental applications, hydrogen peroxide is used to purify wastewater and wastewater containing cyanide, and it can also be used in relatively small concentrations as a disinfectant chemical in swimming pools.

Due to the importance of hydrogen and hydrogen peroxide and their multiple uses, there is an urgent need for green, non-polluting methods for their production. Compared with traditional catalytic production methods, laser water splitting technology is simple, clean and can be operated at room temperature and conventional pressure without any catalysts.

Source: Al Jazeera + websites