Electrochemical CO2 Reduction: A Classification Problem

In this work we propose four non-coupled binding energies of intermediates as descriptors, or 'genes', for predicting the product distribution in CO2 electroreduction. Simple reactions can be understood by the Sabatier principle (catalytic activity vs. one descriptor), while more complex reactions tend to give multiple very different products and consequently the product selectivity is a more complex property to understand. We approach this, as a logistical classification problem, by grouping metals according to their major experimental product from CO2 electroreduction: H2, CO, formic acid and beyond CO* (hydrocarbons or alcohols). We compare the groups in terms of multiple binding energies of intermediates calculated by density functional theory. Here we find three descriptors to explain the grouping: the adsorption energies of H*, COOH* and CO*. To further classify products beyond CO*, we carry out formaldehyde experiments on Cu, Ag and Au and combine these results with the literature to group and differentiate alcohol or hydrocarbon products. We find that the oxygen binding (adsorption energy of CH3O*) is an additional descriptor to explain the alcohol formation in reduction processes. Finally, the adsorption energy of the four intermediates, H*, COOH*, CO* and CH3O*, can be used to differentiate, group and explain products in electrochemical reduction processes involving CO2, CO and carbon-oxygen compounds.