A Discussion on Hydrogen and Photo-Catalytic Water-Splitting

A crucial discussion in our current times is the lack of sustainability in the energy sources we use, and the pollution that come hand in hand with them. It is an agreed consensus amongst scientists that unrenewable, carbon-based energy sources are both disappearing from our planet, and destroying it.  In the industrial world, many of the products and commodities that we consider ‘fundamental’ or elementary resources require multiple processes of manufacture. The lightest of all the elements, and, ironically, the most abundant element in the universe, is Hydrogen. It is very difficult to isolate due to its high reactivity, and is currently produced in an unrenewable, unsustainable way. However, hydrogen is a useful element and research into low-cost, low-energy isolation of hydrogen is requisite.

Hydrogen has many potential and current uses in the developed world. The Haber Process is how bulk ammonia is made, using the reaction . Amonia is most commonly used as a fertilizer and is vital in food production. Over 159 million tons of ammonia are produced worldwide annually. Although ammonia is associated with a variety of concerns, it is indisputable that ammonia is fundamental in harvests, and without its production it would be very difficult to produce crops at the quantity and in the time-range they are needed. Moreover, whilst the Haber process requires substantial energy to maintain high pressures and temperatures, ground-breaking research is taking place to reduce this by mimicking natural enzymes which can catalyse the breaking of the stable nitrogen triple bond at low pressures and temperatures. Additionally, ammonia is widely used in the pharmaceutical industry both as a base for things like vitamins, and to produce other key constituents of medicine like nitric acid. Ammonia is also a component of the radioactive diagnostic injection Ammonia N13, which is used to detect myocardial profusion in patients with coronary artery disease. Hydrogen also has lots of potential as a zero-emission fuel.

Currently, the rife method of obtaining bulk hydrogen for industrial use is via the steam reforming of natural gas, equating to roughly 95% of world production. Steam reforming works by using high temperature steam (700-1000⁰C) to isolate hydrogen from a methane source (commonly natural gas). This produces the desired hydrogen, but also carbon monoxide and minimal amounts of carbon dioxide. The reaction is as follows: .

Although relatively efficient, the process of steam reforming demands copious amounts of energy both to complete the reaction, and to heat steam up to high temperature. Furthermore, obtaining the natural gas needed is also very energy-consuming. The procedure is harmful to the environment and non-renewable.

One suggestion from scientists in the field of hydrogen production is photocatalytic water-splitting. Water is an abundant, renewable source with the molecular formula , therefore it is a promising means of obtaining pure Hydrogen. Water-splitting is a naturally occurring phenomenon; it is involved in photosynthesis, where photon energy (from sunlight) is absorbed and converted to chemical energy via a complex biological pathway. Photocatalysis is using light (or specifically the energy contained in photons) to accelerate chemical reactions. Photocatalytic water-splitting entails the use of nano-sized  technology to absorb photons of light, thus ‘exciting’ the electrons and causing them to jump to conductive bands, reducing water molecules into – this creates ‘holes’ in the valence band (a positive charge) of the Ti which can oxidise another molecule of . Theoretically, infrared light possesses enough energy to spilt water into its components, however the catalytic activity of using infrared light is minimal. Frankly, only ultra-violet light has sufficient energy to trigger photocatalysis to an effective rate, making it a difficult method to use on an industrial scale.  Currently, the issue with photocatalytic water-splitting lies with the low production of hydrogen.  has a relatively positive conductive band, which means there is little driving force for Hydrogen production. Consequently, co-catalysts such as platinum are used to increase the yield of production. Furthermore, revolutionary research is being made into making visible light a sufficient photocatalyst for water-splitting. The process is very much exploratory; however, its potential is vast.

To conclude, hydrogen is one of the plentiful so called ‘ingredients’ of many processes and manufactures the industrial world demands. The uses of hydrogen and the products it can be part of creating are prodigious, and used ubiquitously in sectors such as healthcare, agriculture and many more. It is also presently being researched as a zero-emission fuel source. Hydrogen can combust in atmospheric air and release energy which, in an electrochemical cell, can be utilized very efficiently. The problem lies with the fact that the bulk production of hydrogen is pollutive and unrenewable and uses more energy than is later released when it is used as a fuel. However, experimental research in the field of photocatalytic water-splitting has great prospects for a renewable and clean synthesis of hydrogen gas, and a eco-friendlier future.

Bibliography:

Ammonia. (n.d.). Retrieved from Drugs : https://en.wikipedia.org/wiki/Ammonia_production

Ammonia Prodcution . (2017, December 06). Retrieved from Wikapedia : https://en.wikipedia.org/wiki/Ammonia_production

Hydogen Fuel. (n.d.). Retrieved from US Department of Energy : https://www.afdc.energy.gov/fuels/hydrogen_basics.html

MengNi, M. K. (2017, 01 25 ). Photolytic Water-Splitting . Retrieved from Science Direct : https://www.sciencedirect.com/science/article/pii/S1364032105000420

Photocatalytic Water-splitting . (2018, January 2018). Retrieved from Wikapedia : https://en.wikipedia.org/wiki/Photocatalytic_water_splitting

Sato, K. (2018, Janurary 25). Kentaro Sato. Retrieved from UTokyo Research : http://www.u-tokyo.ac.jp/en/utokyo-research/feature-stories/the-world-of-titanium-dioxide/index.html

 

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