Two Ways Found To Convert CO2 Into Fuel

Lawrence Berkeley National Laboratory have discovered that recycling carbon dioxide into valuable chemicals and fuels can be economical and efficient using a new a single copper catalyst.

And Linköping University researchers have now taken an important step towards a method to convert water and carbon dioxide to the renewable energy of the future, using the energy from the sun and graphene applied to the surface of cubic silicon carbide.

The first to publish Linköping results have been published in the scientific journals Carbon and Nano Letters.

Carbon, oxygen and hydrogen. These are the three elements you would get if you took apart molecules of carbon dioxide and water. The same elements are the building blocks of chemical substances that we use for fuel, such as ethanol and methane. The conversion of carbon dioxide and water to renewable fuel, if possible, would provide an alternative to fossil fuels, and contribute to reducing our emission of carbon dioxide to the atmosphere. Jianwu Sun, senior lecturer at Linköping University, is trying to find a way to do just that.

The first step is to develop the material they plan to use. The Linköping have previously developed a world-leading method to produce cubic silicon carbide, which consists of silicon and carbon. The cubic form has the ability to capture energy from the sun and create charge carriers. This is, however, not sufficient. Graphene, one of the thinnest materials ever produced, plays a key role in the project. The material comprises a single layer of carbon atoms bound to each other in a hexagonal lattice.

In recent years, the researchers have attempted to improve the process by which graphene grows on a surface in order to control the properties of the graphene. Their recent progress is described in an article in the scientific journal Carbon.

Jianwu Sun, of the Department of Physics, Chemistry and Biology at Linköping University said, “It is relatively easy to grow one layer of graphene on silicon carbide. But it’s a greater challenge to grow large-area uniform graphene that consists of several layers on top of each other. We have now shown that it is possible to grow uniform graphene that consists of up to four layers in a controlled manner.”

One of the difficulties posed by multilayer graphene is that the surface becomes uneven when different numbers of layers grow at different locations. The edge when one layer ends has the form of a tiny, nanoscale, staircase. For the researchers, who want large flat areas, these steps are a problem. It is particularly problematic when steps collect in one location, like a wrongly built staircase in which several steps have been united to form one large step. The researchers have now found a way to remove these united large steps by growing the graphene at a carefully controlled temperature. Furthermore, the researchers have shown that their method makes it possible to control how many layers the graphene will contain. This is the first key step in an ongoing research project whose goal is to make fuel from water and carbon dioxide.

In a closely related article in the journal Nano Letters, the researchers describe investigations into the electronic properties of multilayer graphene grown on cubic silicon carbide.

“We discovered that multilayer graphene has extremely promising electrical properties that enable the material to be used as a superconductor, a material that conducts electrical current with zero electrical resistance. This special property arises solely when the graphene layers are arranged in a special way relative to each other,” said Jianwu Sun.

Theoretical calculations had predicted that multilayer graphene would have superconductive properties, provided that the layers are arranged in a particular way. In the new study, the researchers demonstrate experimentally for the first time that this is the case.

This is a very interesting development worthy of increased research. Meanwhile just over a month later . . .

Researchers at Berkeley Lab and the Joint Center for Artificial Photosynthesis demonstrated that recycling carbon dioxide into valuable chemicals such as ethylene and propanol, and fuels such as ethanol, can be economical and efficient – all through product-specific ‘active sites’ on a single copper catalyst.

For decades, scientists have searched for effective ways to remove excess carbon dioxide emissions from the air, and recycle them into products such as renewable fuels. But the process of converting carbon dioxide into useful chemicals is tedious, expensive, and wasteful and thus not economically or environmentally viable.

This study paper has been published in the journal Nature Catalysis.

When you take a piece of copper metal, it may feel smooth to the touch, but at the microscopic level, the surface is actually bumpy – and these bumps are what scientists call “active sites,” said Joel Ager, a researcher at JCAP who led the study. Ager is a staff scientist in Berkeley Lab’s Materials Sciences Division and an adjunct professor in the Department of Materials Science and Engineering at UC Berkeley.

These active sites are where the magic of electrocatalysis takes place: electrons from the copper surface interact with carbon dioxide and water in a sequence of steps that transforms them into products like ethanol fuel; ethylene, the precursor to plastic bags; and propanol, an alcohol commonly used in the pharmaceutical industry.

Ever since the 1980s, when copper’s talent for converting carbon into various useful products was discovered, it was always assumed that its active sites weren’t product-specific – in other words, you could use copper as a catalyst for making ethanol, ethylene, propanol, or some other carbon-based chemical, but you would have to go through a lot of steps to separate unwanted, residual chemicals formed during the intermediate stages of a chemical reaction before arriving at your final destination – the chemical end-product.

“The goal of ‘green’ or sustainable chemistry is getting the product that you want during chemical synthesis,” said Ager. “You don’t want to separate things you don’t want from the desirable products, because that’s expensive and environmentally undesirable. And that expense and waste reduces the economic viability of carbon-based solar fuels.”

So when Ager and co-author Yanwei Lum, who was a UC Berkeley Ph.D. student in Ager’s lab at the time of the study, were investigating copper’s catalytic properties for a solar fuels project, they wondered, “What if, like photosynthesis in nature, we could use electrons from solar cells to drive specific active sites of a copper catalyst to make a pure product stream of a carbon-based fuel or chemical?” Ager said.

Previous studies had shown that “oxidized” or rusted copper is an excellent catalyst for making ethanol, ethylene, and propanol. The researchers theorized that if active sites in copper were actually product-specific, they could trace the chemicals’ origins through carbon isotopes, “much like a passport with stamps telling us what countries they visited,” Ager said.

“When we thought of the experiment, we thought that this is such an inobvious idea, that it would be crazy to try it,” Ager said. “But we couldn’t let it go, because we also thought it would work, as our previous research with isotopes had enabled us to discover new reaction pathways.”

So for the next few months, Lum and Ager ran a series of experiments using two isotopes of carbon, carbon-12 and carbon-13, as “passport stamps.” Carbon dioxide was labeled with carbon-12, and carbon monoxide – a key intermediate in the formation of carbon-carbon bonds – was labeled with carbon-13. According to their methodology, the researchers reasoned that the ratio of carbon-13 versus carbon-12 found in a product would determine from which active sites the chemical product originated.

After Lum ran dozens of experiments and used state-of-the-art mass spectrometry and NMR (nuclear magnetic resonance) spectroscopy at JCAP to analyze the results, they found that three of the products – ethylene, ethanol, and propanol – had different isotopic signatures showing that they came from different sites on the catalyst. “This discovery motivates future work to isolate and identify these different sites,” Lum said. “Putting these product-specific sites into a single catalyst could one day result in a very efficient and selective generation of chemical products.”

The researchers’ new methodology – what Ager describes as “straightforward chemistry with an environmental and economic twist” – is the beginning of what they hope could be a new beginning for green chemical manufacturing, where a solar cell could feed electrons to specific active sites within a copper catalyst to optimize the production of ethanol fuels.

“Perhaps one day this technology could make it possible to have something like an oil refinery, but powered by the sun, taking carbon dioxide out of the atmosphere and creating a stream of useful products,” he said.

While these two new developments are not immediately seen as scaleable technologies, they are both major steps forward into producing fuels from the CO2 currently sourced from very old carbon sequestration by nature that’s used and simply dumped into the atmosphere. While nature is busily recycling the CO2, technology looks closer to participating in a more full short term way. Perhaps someday the world energy economy will short term recycle a major share of the carbon currently getting relocked back into the very long term natural carbon system.