New "photosynthesis" turns carbon dioxide into methane

A new catalyst increases the hope of using renewable energy to produce methane, which is the main component of natural gas used for heating and power generation. Image source: MEHMETCAN

Researchers have long tried to simulate photosynthesis and use the energy of the sun to produce chemical fuel. Now, a research team is closer to this goal than ever-they have developed a new copper and iron-based catalyst that can use light to convert carbon dioxide into methane, the main component of natural gas. If further improved, the new catalyst will help reduce people's dependence on fossil fuels.

This new study is "exciting progress." Edward Sargent, a chemist and solar fuel expert at the University of Toronto, Canada, who did not participate in the study, said: "The benefit of generating methane is that the infrastructure for storing, distributing and using this fuel already exists."

In the United States, methane has recently surpassed coal as the main fuel for power generation. When methane burns, it will decompose into carbon dioxide and water, releasing heat for power generation. The process of using sunlight to produce methane is reversed, starting with carbon dioxide and water, plus electricity to recast the chemical bonds of methane.

However, achieving this transformation is not easy. Eight electrons and four protons must be added to a carbon dioxide molecule to form a methane molecule. Each addition of electrons and protons requires energy to drive conversion. Metal catalysts can help promote these reactions. They seize each reaction molecule "partner", making the reaction more likely to occur and reducing energy consumption.

A few years ago, scientists discovered that when combined with light-absorbing materials, copper particles showed initial potential in converting carbon dioxide into more energy-rich compounds, but the efficiency and speed were still low. Therefore, the researchers tried to combine copper with other metals. They placed two kinds of metal particles on tiny, hair-like nanowires. These nanowires are designed like miniature solar cells that can absorb sunlight and convert it into electrical energy to provide electrons for the catalyst's reaction.

In 2016, researchers reported that catalysts containing copper and gold on silicon nanowires help convert carbon dioxide to carbon monoxide.

In March 2019, Zetian Mi, an electrical engineer at the University of Michigan, Ann Arbor, and colleagues discovered that in a light-absorbing gallium nitride (GaN) nanowire array, a catalyst based on ruthenium and zirconium can effectively convert carbon dioxide to alpha Acid salt (an industrially useful compound). But none of these efforts have produced fuel that can be widely used.

Now, Mi and colleagues have found a solution to this problem. They started with GaN nanowires grown on commercial silicon wafers, and then used a standard technique called electrodeposition to add 5-10 nanometer wide particles of copper and iron mixed into it. In the presence of carbon dioxide and water, the device can convert 51% of the energy in the light into methane when it is illuminated, and the speed is very fast.

Other researchers have previously achieved higher solar methane generation efficiency, but the working speed is too slow and impractical. The Proceedings of the National Academy of Sciences published this month reported that this new type of catalyst, as a light-driven catalyst for converting carbon dioxide to methane, has the highest efficiency and output in history. Computer simulations show that the two metals in the catalyst combine with carbon dioxide molecules, causing them to bend, making them more likely to react and absorb electrons. "It lowers the energy barrier of critical steps." Mi said.

Compared with many other light absorbers and catalysts, all components of this method are cheap and abundant, and have been used in industry. Sargent pointed out that the next step is to increase the efficiency and speed of methane production, which is a necessary condition for making the current system feasible. Once implemented, the new method will provide a way to make fuel using sunlight. (Hu Xuanzi Fu Rong)