Producing hydrogen from plastic waste

Moritz Kuehnel

A PROCESS has been developed to convert waste plastic into hydrogen which could be used as fuel.

Researchers at the UK’s Swansea University and the University of Cambridge have developed a method that uses sunlight and a photocatalyst to convert plastic waste into hydrogen and other chemicals. Hydrogen is currently produced mainly through steam reforming of fossil fuels. It is used in the agricultural, chemical, and pharmaceutical industries and has the potential to be used as a clean fuel. Pyrolysis of waste plastics has been suggested as a fossil fuel-free method to produce hydrogen. However this still requires high temperatures of 500–800^oC and it emits around 12 kg of CO₂ per 1 kg of H₂ produced.

The new process adds an inexpensive cadmium sulfide quantum dot photocatalyst to the plastic and places it in an alkaline solution. The plastic is then exposed to sunlight and produces hydrogen and organic products such as formate, acetate, and pyruvate. The solar-driven reforming process takes place at room temperature, unlike pyrolysis.  

The researchers tested photoreforming on different types of plastic: polylactic acid (PLA), polyethylene terephthalate (PET), and polyurethane (PUR). Photoreforming can produce CO₂, although it depends on the type of plastic and how far the reaction is driven.

“In the case of PLA, we would produce 11 kg CO₂ per 1 kg H₂, but only if we fully oxidise the PLA to CO₂,” said Moritz Kuehnel, lecturer at Swansea’s chemistry department. “We do not actually observe formation of any CO₂, because we only convert PLA to pyruvic acid and H₂, and the pyruvic acid is not further converted to CO₂. For the other plastics, PET for example, we produce 8.8 kg CO₂ per kg H₂. We haven't done a lifecycle analysis, so this information is based only on the reaction principle.”

They also found that the plastic does not have to be cleaned first.

“Even if you do recycle [PET], it needs to be very pure – so only PET, nothing else mixed in with it... and it has to be clean, no grease, no oil,” said Kuehnel. “Potentially you need to wash it which is very expensive, and even if you do all of that, the plastic you get isn't always as nice as virgin material. The beauty of this process is that it's not very picky. It can degrade all sorts of waste. Even if there is food or a bit of grease from a margarine tub, it doesn't stop the reaction, it makes it better.”

The new process offers a solution to two global problems at the same time – plastic waste and clean hydrogen production. Future work will focus on improving the process by identifying catalysts and methods that will enhance the activity.

Energy & Environmental Science http://doi.org/cvdx