Abstract
While CO2 electrolysis is a highly useful technique allowing us to upconvert societies major greenhouse gas into a variety of useful chemicals, mechanistically the process is still not totally understood. One area of debate is whether the rate limiting step involves a proton, a water molecule, or is simply due to adsorption of the CO2 onto the surface. This work will show the benefits of using deuterated water to help resolve this mechanism.
By focusing on catalysts geared towards 2 electron transfer reactions to either formate or CO, we have a simple system to break down the mechanism into the reaction order in terms of protons and water molecules. Variations in pH from 3-7 allows us to show the influence of protons for CO or formate, with H2 evolution on a given catalysts acting as a benchmark. Using deuterated water allows us to denote the influence of water, again using H2 evolution as a benchmark catalyst. Combining both these influences we can mechanistically break down the reactions to denote the mechanism of the rate limiting step. (Since we have obtained relatively straight Tafel slopes over 3 orders of magnitude in current, this entails mass transfer is not significantly influencing our results.)
From our results we show that Au and Ag produce a self-consistent mechanism for CO2 to CO reduction, whereas Sn and In produce a self-consistent mechanism for formate evolution. Pd is unique in that it has the ability to solvate hydrogen and deuterium, thus the combined pH variations and deuterium experiments demonstrates how this effect influences our mechanistic understanding.