Solar-powered chemistry uses carbon dioxide and water to make raw materials for fuels, chemicals
Diagram of a semiconductor nanowire made of indium, gallium, and nitrogen—decorated with gold and chromium oxide nanoparticles. When light hits the nanowire, it liberates electrons and positively charged “holes” that the electrons leave behind. In the nanowires themselves, the holes oxidize water to protons (hydrogen) and oxygen. Meanwhile, some of the electrons are drawn into the metal nanoparticles, where they break down the carbon dioxide. The molecules recombine into carbon monoxide, hydrogen and methane molecules to form syngas. Credit: Roksana Rashid, McGill University.
Solar-powered synthesis gas can recycle carbon dioxide into useful fuels and chemicals, an international research team has shown.
“If we can produce syngas from carbon dioxide using only solar energy, we can use this as a precursor for methanol and other chemicals and fuels. This will significantly reduce the overall CO.2 emissions,” said Zetian Mi, professor of electrical and computer engineering at the University of Michigan, who led the research published in the journal Proceedings of the National Academy of Science.
Composed primarily of hydrogen and carbon monoxide with a small amount of methane, syngas is generally derived from fossil fuels with the help of electricity. In addition, toxic chemicals are often added to make the process more efficient.
“Our new process is actually quite simple, but interesting because it is non-toxic, sustainable and very cost-effective,” said Roksana Rashid, the study’s first author, who conducted the experiment as a doctoral student in electrical and computer engineering at McGill University in Canada.
To create a process that uses only solar energy, the Mi group overcame the difficulty of breaking apart the carbon dioxide molecule, which is among the most stable in the universe. For this, they peppered the forest of semiconductor nanowires with nanoparticles. The nanoparticles, made of gold coated with chromium oxide, attract carbon dioxide molecules and bend them, weakening the bonds between carbon and oxygen.
Gallium nitride nanowires use light energy to free electrons and the positively charged space they leave behind, known as holes. The hole splits the water molecule, separating the proton (hydrogen) from the oxygen. Then, in metal catalysts, electrons split carbon dioxide, producing carbon monoxide and sometimes attracting free hydrogen to make methane. Processes are being developed to separate oxygen from other gases.
“Our technology explains how to establish distributed syngas production from air, water and sunlight,” said Baowen Zhou, co-author of the study with Mi and a former postdoctoral researcher in the Mi lab at McGill University and UM.
By changing the ratio of gold to chromium oxide in the nanoparticles, the Mi team was able to control the relative amounts of hydrogen and carbon monoxide produced in the reaction. This is important because the ratio of hydrogen to carbon monoxide affects how easy it is to produce a given type of fuel or chemical.
“What was surprising was the synergy between gold and chromium oxide to make CO2 reduction to efficient and tunable syngas. That’s not possible with single metal catalysts,” Mi said. “This opens up a lot of exciting opportunities that weren’t previously considered.”
Mi’s adjustable syngas setup uses standard industry manufacturing processes, and is scalable. While Rashid used distilled water in this experiment, seawater and other electrolyte solutions are also expected to work, and Mi has used them in related water separation 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 material for gallium nitride. Each nanowire is about one micrometer thick,” Mi said.
Mi’s next goal is to increase the efficiency of the device, which currently stands at 0.89%. When 10% of light energy is converted to chemical energy, he hopes the technology could see technology adopted for renewable energy, similar to solar cells.
‘Green methane’ from artificial photosynthesis can recycle CO2
Roksana Tonny Rashid et al, Generation of melodious green syngas from CO2 and H2O with sunlight as the only energy input, Proceedings of the National Academy of Sciences (2022). DOI: 10.1073/pnas.2121174119
Provided by the University of Michigan
Quote: Solar powered chemistry uses carbon dioxide and water to make feedstock for fuel, chemicals (2022, 06 July) retrieved 7 July 2022 from https://phys.org/news/2022-07-solar-powered-chemistry- carbon dioxide-feedstock.html
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