Black gold swallows CO2

Nanogold converts CO2 into methane and desalinates seawater - only with the help of sunlight

Nanogold-filled beads not only absorb sunlight extremely effectively, they could also catalyze the conversion of CO2 into methane. © Royal Society of Chemistry / Chemical Science
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Gold can convert CO2 into the fuel methane - only with the help of water and sunlight, as an experiment shows. This is possible because gold nanoparticles of a certain size absorb all visible and near-infrared radiation from the sun. This in turn creates localized heat zones and electric fields on the gold surface that make the material an effective catalyst and heating element. A desalination of sea water by evaporation is possible with gold, as researchers report.

Gold is not only a coveted precious metal, it is also a special element from a chemical-physical point of view. Its atomic properties give it shine and consistency, but also make it a valuable helper in chemistry, medicine and physics.

When the nanogold colloidosomes are irradiated with light, they form plasmonic hotspots and high-energy electrons. © Royal Society of Chemistry / Chemical Science

Porous spheres with nanogold support

This is especially true for nanoscale gold particles: when you crush gold into nano-sized beads and position them at a certain distance, they become extremely effective radiation absorbers: they absorb both the visible sunlight and its near-infrared fraction. As a result, the otherwise shiny gold suddenly appears deep black.

The fact that this nanogold is also suitable for the production of environmentally friendly fuels and other useful chemicals has now been proven by Mahak Dhiman from the Tata Institute for Basic Research in Mumbai and his team. They developed a method to deposit gold nanoparticles on porous microspheres of a branched silicate substrate. These colloidosomes can be prepared with different distances and sizes of gold nanoparticles.

Enormous heating effect

The trick here: through targeted testing, the researchers identified a variant of these nanogold colloidosomes, which optimally absorbs sunlight. When these deep-black microspheres were dissolved in water, they would heat the water up to 88 degrees after one hour of exposure to sunlight, according to Dhiman and his team. By way of comparison, a solution of the same size and the same number of silicate beads in the water warmed this to only about 38 degrees. display

"We even used the material as a nano-heater to convert seawater into drinking water by vaporizing it - with excellent efficiency, " the researchers report. "The nanogold colloidosomes produced water vapor from water with an efficiency of 78 percent."

Further analysis revealed that this heating effect of the nanogold colloidosomes is due to a physical resonance effect: on their surface, local zones form, in which both heat and electromagnetic fields are concentrated, In the case of Goldk gelchen this so-called plasmonic resonance is strong enough to heat up the entire environment significantly.

Methane from CO2 only with light

Even more interesting, however, might be the ability of nanogold colloidosomes to function as effective photocatalysts in chemical reactions, as further experiments by Dhiman and his team have shown. For example, thanks to the plasmonic resonance, the gold beads provide the energy to oxidize alcohols and initiate other reactions of organic molecules with the help of sunlight alone.

But the exciting thing is that with the aid of gold balls, carbon dioxide can also be converted into the high-energy fuel methane. In the test, the researchers used nanoparticle colloidosomes in water with isopropanol and passed CO2 through it. Then they irradiated the whole thing with a sunlight lamp. The result: It formed between 1.5 and 2 micromol of methane gas per gram of gel, as reported by Dhiman and his team.

Fuel production of the future?

"At this stage, the production rate of methane is still low, " the researchers say. "But in the years to come, we could optimize the method and then convert CO2 to fuel on a commercial scale - under atmospheric conditions and with the help of sunlight." This would allow nanogold -Colloidosomes become one of the methods that produce high-energy fuels from the atmospheric CO2 by means of "sun-to-liquid" technology.

However, whether or not nanogold can one day actually help to process CO2 on a larger scale and generate more environmentally friendly fuel has yet to be seen. (Chemical Science, 2019; doi: 10.1039 / C9SC02369K)

Source: Tata Institute of Fundamental Research

- Nadja Podbregar