(472f) Activity Descriptors for CO2 Reduction On Transition Metal Surfaces

Shi, C. - Presenter, Stanford University
Hansen, H. A., Stanford University
Nørskov, J., Stanford University and SUNCAT

The reversible electrochemical reduction of CO2 into fuels would allow for the sustainable use of hydrocarbon fuels and production of commodity chemicals all while maintaining current fossil fuel-based infrastructure.  A great deal of work has been done both experimentally and (in the last decade) theoretically on elucidating the mechanism of CO2 reduction on transition metal surfaces.  So far, however, the best catalyst, both heterogeneous and homogenous, found to reduce CO2 to hydrocarbons such as methane and ethylene is metallic copper and why this is the case has remained somewhat of a mystery until recently.  Unfortunately, copper performs this process at a reasonable (> 0.2 mA/cm2) current efficiency only at extremely high overpotentials (> 1V). 

The search for a better catalyst through usage of the Computational Hydrogen Electrode (CHE) using DFT calculations in previous works has shown that the reason why Cu is so good at reducing CO2 to methane is that it sits at the top of the CO2 reduction limiting potential volcano when the (211) stepped Cu surface is compared to the (211) surfaces of other transition metals primarily due to the slope of the *CO to *CHO step.  Our work extends this work on limiting potentials through the CHE model to first include the effects of higher *CO coverage on the strong-CO binding metal surfaces, Ni, Pt, Pd, and Rh, where we expect steady state *CO coverage in electrochemical reducing conditions to be significant.  Next, we also include limiting potential volcanoes for the (111) and (100) surfaces of FCC transition metals.  There, we find that for the strong-CO binding metals on the terrace surfaces it is the *CO to *COH step that is potential limiting, and that this limiting potential is actually lower than that of *CO to *CHO on Cu(211).  Finally, we compare the activity the hydrogen evolution reaction (HER) to CO2 reduction to methane on those surfaces, and find that all of the strong-CO binding surfaces are much better relative hydrogen evolution catalysts than the weak-CO binding surfaces, Cu, Ag, and Au, which helps explains their experimentally observed low activity for CO2 reduction.