(560be) Electrochemical Reduction of CO2 on Transition Metal-P Block Catalyst Compositions

Among all the pollutants in the atmosphere, CO₂ has the highest impact on global warming and with the rising levels of this pollutant, studies on developing various technologies to convert CO₂ intocarbon neutral fuels and chemicals have become more valuable. Electrochemical reduction is one of the solutions to convert CO₂ to value added hydrocarbon fuels using non-precious, earth-abundant nanocatalysts making this process cost-effective. To understand the activity of catalysts for a particular reaction, we should be able to tailor the catalyst atom by atom. With the advances in computing power and quantum modeling tools, researchers are able to design and study different types of “in silico” materials. Previous experimental results indicate transition metal-p block catalysts such as oxides show improved catalyst activity and desired product selectivity. However, the design principle and reaction mechanism are poorly explored.

In this work, we present a detailed computational study of electrochemical reduction of CO₂ (CO₂RR) to methane and methanol over different transition metal-oxide and sulfide catalysts using Density Functional Theory calculations. In addition to the catalyst structure, we studied reaction mechanisms using free energy diagrams that explain the product selectivity with respect to the competing hydrogen evolution reaction. From these diagrams, we hypothesized that transition metal oxides and sulfides favor methanol over methane formation at lower overpotentials.

Furthermore, we developed scaling relations to find the key intermediate species for CO₂RR on these catalyst materials. We have found CO* as the descriptor (key species) from these relations and modifying the binding free energy of this species would modify the catalyst activity. We developed thermodynamic volcano plots for each product relating descriptor (CO*) binding energy to all other intermediate species binding energy to characterize and rank the activity of catalysts studied so far and determine the optimal binding energy region of the descriptor. This volcano plot will provide guidance to our future work on improving the activity of current transition metal-p block family of catalysts such as metal oxides and sulfides and develop new catalysts for this important reaction.

Table 1 below shows the reducing potentials for each product on different catalyst compositions.

Catalyst	Reducing potential (0V vs. SHE)
	CO	HCOOH	CH3OH	CH4
MoO3	-2.18	-3.44	-2.09	-2.09
WO3	-2.17	-3.11	-0.84	-0.65
ZnO-Wurtzite	-1.65	-2.63	-0.69	-0.69
ZnO-Zincblende	-1.66	-2.67	-1.28	-1.28
ZnS-Wurtzite	-1.35	-2.21	-0.44	-0.47