Measuring those mixed materials
Returning to metallic copper, while a very energy-efficient catalyst for CO2 reduction, copper is non-selective and will generate as many as 32 products in a mixture. This then creates an extreme engineering puzzle to separate and purify those products. Dr Morris’ molecular approach enables her group to select a single product despite current low turnover numbers.
One of the challenges across the field of catalysis is the difficulty of characterizing surface chemistry during the catalytic reaction. The reaction happens at the catalyst’s surface and may involve several intermediates before the product is formed. Few scientific techniques are available to probe molecular chemistry at the surface, and the surface is a complex environment. The surface of the copper metal is heterogenous: the atoms forming the surface will be bonded to the bulk of the copper in different ways, meaning they have differing availability for catalysis and potentially offer differing catalytic pathways. At an atomic level, researchers need to fully understand how many heterogenous catalysts work, including copper, for CO2 reduction. This means new catalyst formulations must be trialed and cannot simply be designed on paper.
Heterogenous catalysts consist of catalyst nanoparticles, often metallic, on a support, usually an oxide. Since all catalysis occurs at surfaces, nanoparticles, due to their small size, present an ideal surface-to-volume ratio and can be used to maximize the surface area of the catalyst component. The support keeps the nanoparticles in place, contributing to high turnovers. The metal-organic frameworks (M.O.F.s) used by Dr Morris’ group are the molecular equivalent of the nanoparticle-support structure. The copper nanoparticles are replaced by nodes of copper atoms connected by organic links (ligands), which act as the support. These catalytic ligands form extended 3D solid-state polymer structures or synthetic handles that improve catalytic effectiveness and energetics. Plus, and very importantly, the M.O.F.s can be separated from a solution in a centrifuge. The presence of these M.O.F.s increases the impact of copper as a molecular catalyst, offering a combination of stability and selectivity.