Turning plastic waste into hydrogen and high-value carbons

The ever-increasing production and use of plastics over the last half century has created a huge environmental problem for the world.  Currently, most of the 4.9 billion tonnes of plastics ever produced will end up in landfills or the natural environment, and this number is expected to increase to around 12 billion tonnes by 2050. 

In collaboration with colleagues at universities in the UK, China and the Kingdom of Saudi Arabia, researchers in the Edwards / Xiao group at Oxford’s Department of Chemistry have developed a method of converting plastic waste into and hydrogen gas which can be used as a clean fuel, and high-value solid carbon.  This was achieved with a new type of catalysis developed by the Edwards group which uses microwaves on catalyst particles to effectively ’strip’ hydrogen from polymers. 

The findings detail how the researchers mixed mechanically-pulverised plastic particles with a microwave-susceptor catalyst of iron oxide and aluminium oxide.  The mixture was subjected to microwave treatment and yielded a large volume of hydrogen gas and a residue of carbonaceous materials, the bulk of which were identified as carbon nanotubes. 

This opens up an entirely new area of great potential in terms of selectivity and offers a potential route to the use of plastic waste Armageddon

This rapid one-step process for converting plastic to hydrogen and solid carbon significantly simplifies the usual processes of dealing with plastic waste and demonstrates that over 97% of hydrogen in plastic can be extracted in a very short time, in a low-cost method with no CO2 burden. 

The new method represents an attractive potential solution for the problem of plastic waste; instead of polluting our land and oceans, plastics could be used as a valuable feedstock for producing clean hydrogen fuel and value-added carbon products.

Professor Peter Edwards said: ’This is not good applied science, but rather good science, applied.  It opens up an entirely new area of great potential in terms of selectivity and offers a potential route to the use of plastic waste Armageddon, particularly in developing countries as one route to the hydrogen economy - effectively enabling them to leap-frog fossil fuels.  

’Perhaps above all else - it is absolutely critical for a deep understanding of the chemistry, physics and electronic engineering of the mesoscale regime in catalysis that underpins any important applied advance in our quest for sustainable energy advances.’

The idea for this very ’applied science’ advance has its origins in a deeply ’pure science’ project - the deep understanding of the science of the Size-Induced Metal to Insulator Transition (SIMIT), a topic that the Edwards group has studied for many years.  The idea is that if one fragments a piece of highly-conducting metal into smaller and smaller pieces, is there a stage (i.e. a critical size of particle), at which it stops becoming a metal? 

The researchers observed that when a metal enters the so-called Mesoscopic regime and traverses the SIMIT, the conductivity within a particle decreases by some 10 orders of magnitude, whilst at the same time the microwave absorption increases by some 10 orders of magnitude.  This means that small "metallic" particles below the SIMIT behave as "super microwave absorbers" - providing a highly effective route to heating catalyst particles, and effectively creating a system of tiny "hot spots" when exposed to microwave electromagnetic radiation. 

’Microwave-initiated catalytic deconstruction of plastic waste into hydrogen and high-value carbons’  in Nature Catalysis.