A sizable French - Japanese - Spanish - Chinese team report a novel material for the chemical recycling of carbon dioxide with light and copper (DOI). Copper nanoclusters have a well-known catalytic activity and the metal itself is cheap but they suffer from oxidation and aggregation. The encapsulation of nanoclusters in so-called metal-organic frameworks (MOFs) is an active research field but the authors explain that compared to metals such as gold and palladium, copper behaves shifty especially with cluster dimensions as small as 2 nanometer.
The article describes a so-called bottle-around-the-ship strategy and given the complexity of the final molecular cluster the synthesis is surprisingly simple. Copper nanoclusters were synthesised by reduction of copper nitrate with L-ascorbic acid in water. A MOF precursor was synthesised by reacting zirconium chloride with acetic acid and isopropanol. This Zr oxocluster was then mixed with the copper NC solution and stirred with fumaric acid. Secret of the success is the low acidity of this solution. All components are also soluble in water and the solid endproduct slowly separates out. The ultimate particles are 200 nm in size with a 3% copper payload.
The diameter of a copper NC (2 nanometer) is much larger than a Zr oxocluster pore (0.5 nm) and the overall configuration according to the authors is considered core-shell. If the Ncs would block the pores instead, the porosity would suffer and experimentally porosity was found to remain intact. Combining copper NC’s with a preformed Zr-MOF also did not result in a material with any detectable amount of copper.
Then the intended application. The authors mention that carbon dioxide emission is considered "a hazard nowadays" which seems like a tremendous understatement. The report describes a room-temperature photoreduction with UV irradiation and triethanolamine as a sacrificial reducing agent. Sure enough after 18 hours carbon dioxide was found to have converted to carbon monoxide (23%) and formic acid (65%) with evolution rates respectively 30 and 100 micromol per hour per gram. To put these numbers in perspective: if a petrol car exhausts 120 grams of carbon dioxide per kilometer, getting rid of that amount would require 27000 hours. Battling climate change however is not what the authors have in mind with the invention, instead they hope this work will stimulate new research into heterogeneous catalysis.