Researchers at Rice University boosted the stability of their low-energy, copper-ruthenium syngas photocatalysts by shrinking the active sites to single atoms of ruthenium (blue). Image: John Mark Martirez/UCLA.
Researchers at Rice University boosted the stability of their low-energy, copper-ruthenium syngas photocatalysts by shrinking the active sites to single atoms of ruthenium (blue). Image: John Mark Martirez/UCLA.

Researchers at Rice University have created a light-powered nanoparticle that could shrink the carbon footprint of a major segment of the chemical industry.

The nanoparticle comprises tiny spheres of copper dotted with single atoms of ruthenium, and is the key component in a green process for making syngas, or synthesis gas, a valuable chemical feedstock for making fuels, fertilizer and many other products. Researchers from Rice University, the University of California, Los Angeles (UCLA) and the University of California, Santa Barbara (UCSB) describe the low-energy, low-temperature syngas production process in a paper in Nature Energy.

"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," said Rice chemist and engineer Naomi Halas, a co-corresponding author of the paper.

Syngas is a mix of carbon monoxide and hydrogen gas that can be made from coal, biomass, natural gas and other sources. It's produced by gasification at hundreds of plants worldwide, and is used to make fuels and chemicals worth more than $46 billion per year, according to a 2017 analysis by BCC Research.

Catalysts are materials that spur reactions between other chemicals. They are critical for the gasification processes that produce syngas, in which steam and catalysts break apart hydrocarbons. This results in the hydrogen atoms pairing up to form hydrogen gas, and the carbon atoms combining with oxygen in the steam to form carbon monoxide.

In methane dry reforming, by contrast, 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, said Linan Zhou, a postdoctoral researcher at Rice's Laboratory for Nanophotonics (LANP) and first author of the paper.

Halas, who directs LANP, has worked for years to create light-activated nanoparticles that insert energy into chemical reactions with surgical precision. In 2011, her team showed how it could boost the amount of short-lived, high-energy electrons called ‘hot carriers’ that are created when light strikes metal, and in 2016 they 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 paper in Science by Halas, Zhou and colleagues. According to Zhou, the new syngas catalyst uses a similar design. In each, a copper sphere about 5–10nm 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 down to a single atom for the dry reforming catalyst.

"High efficiency is important for this reaction, but stability is even more important," Zhou said. "If you tell a person in industry that you have a really efficient catalyst they are going to ask, 'How long can it last?'"

This question is particularly important for syngas producers, because most gasification catalysts are prone to ‘coking’, a build-up of surface carbon that eventually renders them useless. "They cannot change the catalyst every day," Zhou said. "They want something that can last."

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 said that 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 said this research could pave the way "for sustainable, light-driven, low-temperature, methane-reforming reactions for production of hydrogen on demand."

"Beyond syngas, the single-atom, antenna-reactor design could be useful in designing energy-efficient catalysts for other applications," she added.

The technology has been licensed by Syzygy Plasmonics, a Houston-based start-up whose co-founders include Halas and study co-author Peter Nordlander.

This story is adapted from material from Rice University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.