A Review on Artificial Photosynthesis
We live in an age of electrification and green energy, where there is a global push to provide clean electricity to the world. Oil and other fossil fuels have been seen as a relic of the past that still harms us to this day in terms of greenhouse gas emissions. That said, we still need fuel. 40% of global transportation needs fuel to run, as only some level of fuel can provide the punch that moving jets or supply boats need. Electricity isn’t enough in this case to maintain the power we expect from our engines. The fuels that we have take a lengthy process to create, as the Earth’s pressure and time are needed to convert carbon in life forms to hydrocarbons.
Plants may provide the answer on how to get more fuel. Photosynthesis takes water and carbon dioxide with sunlight, and creates a carbohydrate of sugar. We can try to replicate this process, and the key lies in the catalyst of the reaction. We can’t just put carbon dioxide and water in a system and expect to get out a fuel without something breaking the reaction down to happen.
In this article we will analyze the different methods of artificial photosynthesis and how they can create a fuel to be used. This system can be used to create hydrogen, isopropanol, and other fuels that can change both our transportation system and our emissions drastically.
The Quantum Mechanics of Photosynthesis
Biology has often been seen as a hot, messy field, where perfection is all but a dream. Math and theoretical physics, although it can be used, seems as if it’s a far cry from ecosystems and biological mechanisms. This could not be farther from the truth. There are methods to map the field of biology using statistics, but core biological functions use advanced numerical theory and quantum physics. Photosynthesis, the only way that we have progressed to the point in life that we are, utilizes a principle of quantum mechanics called superposition. The basis of photosynthesis is in chlorophyll, which is found in an organelle known as the thylakoid in the chloroplasts.
The chlorophyll has a single Magnesium atom, which only has one electron that is fairly far away from the nucleus. Once the energy from sunlight enters the thylakoid, the electron is knocked away, leaving a positive ‘hole’ where the electron used to be. The combination of the positively charged hole and the negatively charged electron is known as the exciton, which can hold energy.
To extract that energy, the exciton must go towards a reaction center to remove the electron, making the magnesium more stable. Getting that exciton to the reaction center is a difficult process with just Newtonian physics. Through traditional models, it would likely just bounce around until it found the reaction center, but with that the excitons were more likely to bounce out of the environment instead of reaching the reaction center. This doesn’t make sense, as photosynthesis happens nearly perfectly every single time.
Here’s where the quantum mechanics comes in. A particle in the quantum world can exist in many different places with different probabilities. This particle must then be measured as a wave with different percentages of likelihood.
The more pathways, the higher the number of percentages, and since the particle is modeled as a wave it can take all possible pathways. In the case of photosynthesis, the positron takes all possible pathways to get to the reaction site, which is the only explanation we have of the near perfect efficiency that photosynthesis has.
This in part is a reason why photosynthesis is such a hard process to crack. Plants have complex system upon complex system layered in their mechanisms, and re engineering such a thing will take energy and time. The only incentive to continue with this process is if the outcome is enormous, which it is. If we are able to understand and replicate photosynthesis, then we have the best possible tool for carbon capture and energy creation. We solve two global problems in one blow.
Creating a Catalyst of -Lysis
In the process of Photosynthesis, individual atoms must be able to be formed off molecules to be rearranged into organic compounds. This is the process of photolysis. Plants do this in their chloroplasts fairly simply using light energy, but replicating this process in a lab is harder.
Breaking bonds requires energy, and that energy is found in the sun in the case of sunlight for traditional photosynthesis. To replicate the same process in a lab, all pathways of doing so are energy intensive. The main pathway to replicating this in a lab is in solar power, where solar powered electrolysis machines can be used to split water into hydrogen gas and oxygen gas. Since the concentration of hydrogen is very low in the atmosphere, all hydrogen must be created through an artificial process. Electrolysis, although promising, is still too expensive to become the main provider of hydrogen. Currently electrolysis consists of 4% of all hydrogen production, where natural gas production, oil production and coal production account for the lion’s share. If we are able to produce hydrogen efficiently and cleanly, we are then able to get some form of a fuel while conquering one step of the photosynthetic process.
The Pros and Cons of Hydrogen Gas
Hydrogen Gas has been used as a fuel since the early 20th century, and it has the potential to play a major role in a zero carbon future. The hydrogen economy has been continually growing quickly, and with the looming threat of excessive carbon dioxide creation it may be called upon to take a larger portion of the fuel economy. Hydrogen is a fantastic fuel, but only when it’s efficiently made. The gas is made through a number of ways, all of them involving fossil fuels in some way, and only a small amount being produced cleanly. It also isn’t an efficient, compact source of energy compared to hydrocarbons, as there are no carbon hydrogen bonds to take in. Being a gas, hydrogen can also leak out of fuel cells, possibly meaning explosions.
Storing the gas is another problem, as a gas can’t just be put in a massive barrel and kept away, but it must have a special environment that is expensive to create. Hydrogen Peroxide may be the solution to this.
Hydrogen Peroxide can be stored in a massive dark tank and not degrade quickly, meaning that transport will be easier and cheaper. That said, hydrogen is not the fuel for today’s problems. There are too many issues that are associated with the gas, and they all cannot be fixed right now. Our systems do not run on hydrogen due to a high chance of explosion and containment issues. We must look towards the full process of artificial photosynthesis in order to get a fuel we can use now.
Although not full on gasoline, syngas has been produced through artificial photosynthesis. Syngas is an industrial product which is used in medicines, agricultural products, plastics, and alternative fuels. Researchers at Cambridge developed a system to isolate oxygen gas and the ingredients for syngas in separate parts. The catalyst for this reaction is cobalt, which is then combined with two photolysis catalysts to create the product. Although this isn’t a direct fuel, the synthetic production of syngas shows a future that is strong for artificial photosynthesis.
Ethanol is best known as grain alcohol, but there are many other uses past medicine and as a psychoactive drug. The largest use of ethanol is as a fuel additive, due to its knocking resistance (octane ratings). Knocking resistance refers to how strong the compression of the piston, where the higher the resistance the higher the octane rating.
What an octane rating in the US looks like.
The reason methanol is mentioned here instead of ethanol is because methanol is an even simpler alcohol that has the potential to function as a fuel in the same manner as ethanol. Fuel is permitted to have a blend of 3% methanol in European countries, but the potential for the methanol economy doesn’t stop just there. Combustion of methanol still releases Carbon Dioxide and Water, but the advantage of methanol is that the production can be carbon-efficient.
The most traditional method of methanol synthesis is a reaction of carbon monoxide and hydrogen gas over a catalyst. This is an efficient process, but the issue here relates to Hydrogen gas, as its production is difficult.
There is a way to produce the alcohol from methane, however, and that involves bacteria. Certain enzymes known as methane monooxygenases can be in use, where methane can be converted into methanol with water as a byproduct. Methane itself is a fine fuel, but when there is a need for a liquid fuel, methanol must come in. The creation of this methane can come in the form of biohythane, which is a mixture of hydrogen and methane. Biohythane can be developed through the processing of organic wastes, which can then be converted into methanol. Waste can be converted into fuel, it’s just up to us to utilize it.
Finding the Perfect Catalyst
Every reaction here would heavily benefit from a catalyst, or in the case of a biological process every reaction would benefit from an enzyme. Without a catalyst, reactions happen at a much slower scale (the rate goes down by magnitudes), and in the case of biology we can’t sustain life. Organisms build catalysts through transcription from RNA to amino acids, and then through peptide bonds and folding the enzyme is made. That is certainly a possibility in many cases through synthetic biology, but it is by far the only solution to create such a catalyst. We have the potential to create our own catalysts through base elements on the periodic table to facilitate these reactions ourselves.
The platinum group of metals consist of 6 metals found in the middle of the periodic table.
These metals are non-reactive, durable, and used as transition metals. They have high melting and boiling points, where they are also high in conductivity. That said, these aren’t common elements. These elements are all incredibly rare and expensive, meaning that they cannot bear the sole brunt of energy transformation.
The solution here is through a heterobimetallic combination. This metal complex will have two different metals fused together in an alloy, where two cheaper metals can emulate a similar effect than to what the traditional platinum group of elements can provide. For example, an element like Titanium and Nickel can fuse together to create a heterobimetallic compound. Furthermore, bimetallic and polymetallic catalysts provide different reaction patterns than monometallic catalysts, which can be incredibly useful in biotech applications such as photosynthesis.
Artificial photosynthesis has the potential to change the world, but at the same time it is still a moonshot project, in that it may take 10+ years to come out of labs and into the real world. Good progress has been made and will continue to be made, but it is not a perfect solution right now. It is still expensive and inefficient compared to other forms of energy, and other forms of alternative energy must be used in some capacity. There is a reason that this technology is so massive, however, and that is because of the two pronged attack it brings to us. We get a platform of carbon capture and a source of fuel, meaning that we can tackle both energy storage and remove carbon dioxide from the atmosphere. The world is changing, and this technology may be the spearhead of that change.
Artificial photosynthesis takes Carbon Dioxide and converts it to a whole series of compounds, but it isn’t ready right now to be used into the world.
Hybrid Catalysts for Artificial Photosynthesis: Merging Approaches from Molecular, Materials, and…
Increasing demand for sustainable energy sources continues to motivate the development of new catalytic processes that…
Download Citations | ACS Publications
Pair your accounts. Export articles to Mendeley Get article recommendations from ACS based on references in your…