Photocatalyst Promises Greener Syngas

March 4, 2020
Single-atom “antenna reactor” system could reduce carbon footprint

A light-powered copper nanoparticle dotted with single atoms of ruthenium is the key component in a green process for making syngas, say researchers from Rice University, Houston; University of California, Los Angeles (UCLA); and the University of California, Santa Barbara (UCSB).

“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,” explains Naomi Halas, director of Rice’s Laboratory for Nanophotonics (LANP), who worked on the project.

Dry reforming hasn’t been attractive to industry because it typically requires even higher temperatures and more energy than steam-based methods, adds Linan Zhou, a postdoctoral researcher at LANP.

To address this, the researchers have been working on using light-activated nanoparticles that insert energy into chemical reactions with surgical precision. This boosts the amount of short-lived, high-energy electrons called “hot carriers” created when light strikes metal. The team devised “antenna reactors” that use hot carriers to drive catalysis.

“This approach, using a precisely engineered photocatalyst and reactors based on modern efficient lighting sources is truly transformational, allowing one to run chemical reactions at hundreds of degrees lower than conventional catalysts,” says Zhou.

In addition, to increase stability and prevent coking, the team isolated the active ruthenium sites where carbon is dissociated from hydrogen. This reduces the chances of carbon atoms interreacting to form coke and increases oxygen interaction to form carbon monoxide.

“When hydrogen leaves the surface quickly, it’s more likely to form molecular hydrogen,” Zhou explains. “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.”

An article in Nature Energy provides further details on the process.

Halas says the 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, for example, ammonia decomposition, ammonia synthesis and Fischer Tropsch processes, note the researchers.

Under an exclusive license from Rice University, Syzygy Plasmonics, Houston, has developed and scaled up a similar photocatalyst for the steam methane reforming (SMR) reaction. It’s an active project to develop a commercial reactor system that uses photocatalytic SMR to produce H2 on demand, notes Suman Khatiwada, CTO of Syzygy Plasmonics, and Zhou, Syzygy Plasmonic’s soon-to-be chief scientist.

“Commercialization efforts at Syzygy Plasmonics already have led to scaled-up synthesis of the catalyst to 100s of grams in a small pilot batch. This effort was made to demonstrate scalability for commercial use,” they add.

The Syzygy scientists admit that the photocatalyst is poisoned by sulfur species just like most other catalyst and haven’t yet investigated its regenerability if it were to become coked up. “Having said that, we are highly confident that is regenerable by illuminating the photocatalyst with light in a CO2 atmosphere,” they say.

Syzygy Plasmonics plans to demonstrate a pilot reactor system in the next two years.

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