Rice engineers have created a new low-temperature photocatalyst to reduce the carbon footprint of the chemical industry.
The light-powered nanoparticle – tiny spheres of copper dotted with single atoms of ruthenium – is the key component in a green process for making synthesis gas (syngas).
“Syngas can be made in many ways, but one of those, methane dry reforming, is increasingly important because the chemical inputs are methane and carbon dioxide, two potent and problematic greenhouse gases,” says Rice chemist and engineer Naomi Halas.
In dry reforming, the oxygen atoms come from carbon dioxide rather than steam. But dry reforming hasn’t been attractive to industry because it typically requires even higher temperatures and more energy than steam-based methods, says study first author Linan Zhou, a postdoc at Rice’s Laboratory for Nanophotonics (LANP).
Halas, who directs LANP, has worked for years to create light-activated nanoparticles that insert energy into chemical reactions with surgical precision.
In 2016, Halas’ team unveiled the first of several “antenna reactors” that use hot carriers to drive catalysis.
One of these, a copper and ruthenium antenna reactor for making hydrogen from ammonia, was the subject of a 2018 Science paper by Halas, Zhou and colleagues.
According to Zhou, the syngas catalyst uses a similar design. In each, a copper sphere about 5-10 nanometers in diameter is dotted with ruthenium islands.
For the ammonia catalysts, each island contained a few dozen atoms of ruthenium, but Zhou had to shrink these to a single atom for the dry reforming catalyst.
Most gasification catalysts are prone to “coking,” a buildup of surface carbon that eventually renders them useless.
By isolating the active ruthenium sites where carbon is dissociated from hydrogen, Zhou reduced the chances of carbon atoms reacting with one another to form coke and increased the likelihood of them reacting with oxygen to form carbon monoxide.
“But single-atom islands are not enough,” he said. “For stability, you need both single atoms and hot electrons.”
Zhou says the team’s experimental and theoretical investigations point to hot carriers driving hydrogen away from the reactor surface.
“When hydrogen leaves the surface quickly, it’s 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’s how you can control with the hot electron to make sure it doesn’t form coke.”
Halas contends that their research could pave the way “for sustainable, light-driven, low-temperature, methane-reforming reactions for production of hydrogen on demand.”
Image and content: Jeff Fitlow/Rice University