Solar chemistry uses carbon dioxide and water to make raw materials for fuels and chemicals

Solar-powered syngas could recycle carbon dioxide into useful fuels and chemicals, an international team of researchers has shown.

“If we can generate syngas from carbon dioxide using only solar energy, we can use it as a precursor for methanol and other chemicals and fuels. This will significantly reduce global CO.2 said Zetian Mi, professor of electrical and computer engineering at the University of Michigan, who led the study published in the Proceedings of the National Academy of Science.

Composed mainly of hydrogen and carbon monoxide with a little methane, syngas is commonly produced from fossil fuels using electricity. Additionally, toxic chemicals are often added to make the process more efficient.

“Our new process is actually quite simple, but it’s exciting because it’s non-toxic, sustainable and very cost-effective,” said Roksana Rashid, first author of the study, who performed the experiments. as a PhD student in electrical and computer engineering at McGill University in Canada.

To create a process that uses only solar energy, Mi’s group overcame the difficulty of separating carbon dioxide molecules, which are among the most stable in the universe. To do this, they sprinkled a forest of semiconductor nanowires with nanoparticles. These nanoparticles, made of gold coated with chromium oxide, attract carbon dioxide molecules and deform them, weakening the bonds between carbon and oxygen.

The gallium nitride nanowires used light energy to release electrons and the positively charged spaces they leave behind, called holes. The holes split the water molecules, separating the protons (hydrogen) from the oxygen. Then, at the metal catalysts, electrons split carbon dioxide, producing carbon monoxide and sometimes attracting free hydrogen to produce methane. Processes are being developed to separate oxygen from other gases.

“Our technology sheds light on how to build distributed syngas production from air, water and sunlight,” said Baowen Zhou, co-corresponding author of the study with Mi and former postdoctoral researcher in Mi’s lab at McGill University and UM.

By altering the gold to chromium oxide ratio in the nanoparticles, Mi’s team was able to control the relative amounts of hydrogen and carbon monoxide produced during the reaction. This is important because the ratio of hydrogen to carbon monoxide affects how easily a type of fuel or chemical can be produced.

“What is surprising is the synergy between gold and chromium oxide to make CO2 efficient and adjustable syngas reduction. This was not possible with a single metal catalyst,” Mi said. “It opens up a lot of exciting opportunities that weren’t considered before.

Mi’s adjustable syngas configuration uses industry standard manufacturing processes and is scalable. While Rashid used distilled water in this experiment, seawater and other electrolyte solutions should also work, and Mi has used them in related water splitting studies.

“The semiconductor we use as a light absorber is based on silicon and gallium nitride, which are the most commonly produced semiconductors, and we use very little gallium nitride material. Each nanowire has about a micrometer thick,” Mi said.

Mi’s next goal is to increase the efficiency of the device, which currently sits at 0.89%. When 10% of light energy is converted into chemical energy, he hopes the technology could see the technology being adopted for renewable energy, similar to solar cells.

The project was supported by ERA Emission Reduction Alberta, based at McGill University in Canada, Mi’s former home. The co-authors all have current or past ties to McGill. Rashid is currently a postdoctoral researcher in electrical and computer engineering at the University of Waterloo in Canada. Zhou is currently an associate professor of mechanical engineering at Shanghai Jiao Tong University.

Portion of intellectual property related to this work has been licensed to NS Nanotech Inc. and NX Fuels Inc., both of which were co-founded by Mi. The University of Michigan and Mi have a financial interest in these companies.