Schematic of the Cu nanoparticle/graphene catalyst and its high selectivity for ethylene at -0.9 V
Schematic of the Cu nanoparticle/graphene catalyst and its high selectivity for ethylene at -0.9 V

A new type of composite catalyst made up of copper nanoparticles deposited on graphene could enable the transformation of carbon dioxide into a widely used industrial chemical, say chemists [Li et al., Nano Energy 24 (2016) 1].

Conventional means of converting carbon dioxide (CO2) into reusable hydrocarbons like ethylene (C2H4) require high temperatures and pressures. Electrochemical reduction reactions are a potential alternative to thermal catalysis, enabling the conversion to take place under ambient conditions. In practice, however, it is very difficult to compel the reaction to take the right pathway to produce the desired product.

Now Shouheng Sun and his team at Brown University and the University at Buffalo have made important progress in the drive for an electrochemical catalyst with the necessary activity and selectivity to reduce CO2 into useable hydrocarbons.

Cu is already known to be a promising catalyst for the conversion of CO2 into hydrocarbons, particularly in the form of nanoparticles. But when Sun and his team combined Cu nanoparticles with a nitrogen-doped graphene support, the researchers found both good catalytic activity and selectivity for C2H4.

The best results were achieved with 7 nm polycrystalline Cu nanoparticles on pyridinic-nitrogen rich graphene (or pNG). The combination produced 19% C2H4 at a potential of -0.9 V compared with just a few per cent or less of other products. The team believe that this enhanced activity and C2H4 selectivity of 79% can be put down to the way in which the two components act together.

‘‘The reaction is likely controlled by a synergistic effect between the doped graphene and Cu nanoparticles,’’ says Sun. ‘‘Our experiments indicate that the presence of pyridinic nitrogen in the graphene network may help to anchor the Cu nanoparticles down and to attract more CO2/protons to the Cu to facilitate the reaction.’’

The exact mechanism is not yet clear, Sun cautions, and further investigation is required to clarify the roles of each component. Nevertheless, the researchers believe that the results indicate a promising new approach to enhancing the catalytic activity and selectivity of Cu nanoparticles in general and may represent a new class of catalysts for the electrochemical reduction of CO2 into useful hydrocarbons.

Feng Jiao of the University of Delaware agrees. ‘‘The catalyst discovered by Sun et al. exhibits remarkable ethylene selectivity, which, in my opinion, represents a major breakthrough in CO2 electrocatalysis research,’’ he says. ‘‘This work... may open opportunities to design new processes that convert the greenhouse gas CO2 into something with high value.’’

The ability to convert excess CO2 into C2H4, which is the raw material for many widely used plastics including polyethylene terephthalate (PET), polyvinyl chloride (PVC) and polystyrene (PS), could make innumerable products from packaging to adhesives more sustainable.

‘‘We hope that this new catalyst could be a step toward a greener way to produce ethylene,’’ says Sun. ‘‘There is much more work to be done to bring such a process to an industrial scale, but this is a start.’’

This article was originally published in Nano Today (2016), doi:10.1016/j.nantod.2016.05.004