A large amount of energy goes into plastic production, and considerable quantities of carbon and hydrogen are bound up in the material. One engineer is using the power of sunlight to recover some of these resources and recycle the plastic into valuable and environmentally friendly fuel.
Around the world, 460 million tonnes of plastic are produced every year, with Australians consuming more than our fair share: 147 kg per person, a figure that is projected to grow to 260 kg each by 2050.
Engineers at the University of Adelaide, however, are eyeing this plastic waste as a source of feedstock to produce fuels – including zero-emission hydrogen.
Best of all, the process of transforming waste material into a valuable source of energy is facilitated by sunlight, taking place at close to room temperature.
“The reaction temperature is one of the major advantages of this process,” University of Adelaide PhD candidate Xiao Lu told create.
“Traditional recycling methods like pyrolysis that rely on thermal energy often operate at 500 to 800 degrees [Celsius], but photoreforming is thermodynamically favourable and mainly driven by light.
“It usually operates near room temperature, or under mild heating of 40 to 60 degrees.”
Solar-driven photoreforming, the process by which Lu transforms plastics into hydrogen and other valuable chemicals such as syngas, uses photocatalysts to spur on the sunlight-driven process.
READ: New recycling method aims to reduce plastic’s stranglehold on the planet
Shine a light
While the technique uses less energy than other plastic recycling methods, it also requires less energy than hydrogen production methods such as water electrolysis, since plastics are easier to oxidise.
“Its value comes from both, because plastic conversion and hydrogen production occur at the same time,” Lu said.
“The reaction proceeds through two coupled pathways. One side produces hydrogen gas, while the other breaks down the plastic and converts its carbon into valuable chemicals.
“Which benefit matters more will depend on the waste stream. Hydrogen can be produced from a wide range of plastics, while the carbon-containing products vary much more with plastic feedstock.”
Considering separation and storage costs, Lu predicts that hydrogen would be the most valuable product from the reaction using a feedstock of mixed-plastic waste, though other high-value chemicals could be effectively produced if the plastics are well sorted.
PET plastics, such as those used for soft drink bottles, have a structure that facilitates light absorption, aiding the process, while more complex plastics produce byproducts, including smaller hydrocarbons and other oxygenated compounds that could make it less efficient.
“In the photoreforming process, there are three key factors: light, catalysis and plastic,” Lu said.
“Photocatalysts, or what we can call semiconductor materials have a specific property called bandgap. When light hits the catalyst, the catalyst can absorb the energy and use it to overcome the bandgap. It excites an electron to a higher energy state, leaving behind a positively charged vacancy known as a hole.”
Then, the high-energy electron participates in a hydrogen reduction process, forming hydrogen gas, while the holes react with an oxidant to generate highly reactive chemicals named reactive oxygen species, or ROS, which break down the plastic’s chemical bonds.
The most effective catalysts, according to Lu, demonstrate “broad solar-light absorption, efficient charge carriers’ separation and transfer and long-term stability”.
“At laboratory scale, the catalytic performance can already be impressive,” she said.
“Some catalysts can achieve a very high hydrogen evolution rate, sustained for more than 100 hours. Some systems showed high product selectivity; they activated almost complete selectivity toward acetic acid or more than 85 per cent selectivity toward C15 to C18 diesel-range fuels.”
READ: Molecular recycling could be the future of sustainable plastic
Engineering for scale
While outside of the laboratory, researchers have demonstrated the process under natural sunlight, using a one-square-metre solar reactor, the hydrogen productivity in these trials has been lower.
According to Lu, the current challenge is to take this proof of concept and translate it into the kind of high-performance processes suitable for industrial-scale systems.
“There are still several challenges to scale up,” she said. “At large scale, sunlight becomes harder to distribute evenly, photon losses increase, and the mass transfer becomes more limiting.
“We also need continuous operation, durable catalysts, efficient product separation, and low-cost solvent and catalyst recovery. These factors determine whether the process can be economically competitive.”
Lu anticipates improved efficiency in outdoor experiments to show results by 2030, with continuous-flow operation coming after that, with broader integration into plastic recycling arriving by 2050.
