The carbon footprint of the chemical industry is huge; in the UK alone direct emissions from the sector accounted for 11.2m tonnes of CO2 - 10m tonnes from burning fuel and 1.2m tonnes from chemical processes, producing greenhouse gases as a by-product.
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An important chemical feedstock is a synthetic gas, also known as syngas. The combustible fuel consists of carbon dioxide and hydrogen and is generated via a process known as methane dry reforming. However, the method is not environmentally friendly; it uses carbon dioxide and methane, two powerful and problematic greenhouse gases.
Syngas is a product of coal gasification, a process that converts carbon-rich materials – coal, biomass, natural gas - into carbon monoxide and hydrogen. It is used to make fuels, fertilizers, lubricants, and chemicals in an industry estimated to be worth $46 billion in 2017.
Syngas can be produced in many ways. One method utilizes steam and a catalyst to uncouple the hydrocarbon raw material; the catalyst is critical to the gasification process, as it speeds up the reaction. The result is that hydrogen atoms pair up to form hydrogen gas, while the carbon atoms combine with oxygen to yield carbon monoxide.
In methane dry reforming, steam is replaced by carbon dioxide but requires much higher temperatures and energy input than steam-based methods. Methane dry reforming also suffers from coke-induced instability, a build-up of surface carbon which eventually stops the catalyst from working.
Engineers from Rice University in Texas, US, have invented light-powered nanoparticles that could help reduce the carbon footprint, resulting from the methane dry reforming process by lowering the energy and temperatures required. Together with scientists from the University of California Los Angeles (UCLA) and the University of California Santa Barbara (UCSB), the researchers have developed tiny spheres of copper dotted with ruthenium, a rare transition metal.
It builds on previous research by Naomi Halas, director of Rice University’s Laboratory for Nanophotonics (LANP); the engineer and chemist have spent over 25 years pioneering the use of light-activated nanomaterials. Her team combines plasmonic nanoparticles with traditional catalysts to yield high light absorption and efficient reactions, which are highly customizable for applications.
Halas and her team showed in 2011 that light-activated nanoparticles could boost the amount of short-lived, high-energy electrons called “hot carriers” that are created when light strikes metal. Five years later they explored the idea of “antenna reactors” that use hot carriers to drive catalysis, specifically copper and ruthenium for making hydrogen from ammonia.
This work formed the basis of a 2018 paper in Science, where copper acts as an antenna, and ruthenium acts as an atomic ‘reactor’ site on the nanoparticle surface. In each antenna, a copper sphere around 5-10 nanometers in diameter is dotted with ruthenium islands. For the ammonia catalysts, each island contained a few dozen atoms of ruthenium, but this had to be shrunk to a single atom for the dry reforming catalyst.
However, the catalysts were prone to coking, so researchers worked to isolate the active ruthenium sites where carbon was separated from hydrogen. The aim is to reduce the chance of it reacting with more carbon, thus increasing the likelihood of it reacting with oxygen to form carbon monoxide.
Linan Zhou, a postdoctoral researcher at LANP said the team’s experimental and theoretical investigations suggest hot carriers drive hydrogen away from the reactor surface.
“When hydrogen leaves the surface quickly, it is more likely to form molecular hydrogen,” he said. “It also decreases the possibility of a reaction between hydrogen and oxygen and leaves the oxygen to react with carbon. That is how you can control with the hot electron to make sure it does not form coke.”
The light-excited hot carriers, together with single-atom active sites, improve the performance of the reaction and a reduction in a barrier for methane activation. The new catalyst is ideal for light-driven, methane dry reforming, and can provide high light energy efficiently, even when illuminated at room temperature.
The engineers’ recent work, published in Nature Energy, could pave the way for sustainable light-driven, low-temperature, methane-reforming reactions to produce hydrogen on demand. It could also be useful in designing energy-efficient catalysts for green applications, thus reducing the industry’s carbon footprint.
References and further reading
Gasification goes green
Nanoparticles powered by light aim to reduce chemical industry's carbon footprint
Light-driven methane dry reforming with single atomic site antenna-reactor plasmonic photocatalysts
Heterometallic antenna−reactor complexes for photocatalysis
How nanoparticles that harvest light could curb climate emissions
Chemicals Sector |Joint Industry – Government | Industrial Decarbonisation and Energy Efficiency Roadmap | Action Plan