Computational Study on Electrochemical Reduction of CO2 using Transition Metal- p Block Catalyst Compositions

Over the past century, greenhouse gases such as carbon dioxide emissions have increased due to over-dependence on fossil fuels as energy sources. There is a greater chance that these emissions will lead to disastrous climate changes, such as extensive heating of the atmosphere or thermal expansion of ocean water that would be irreversible. An important challenge the world is facing today would be guaranteeing proper energy supply for future generations without increasing the level of greenhouse gases in the atmosphere. Hence, it is important to reduce/recycle the amount of CO₂ from the atmosphere to meet the needs of green energy development. Electrochemical reduction of CO₂ using heterogeneous catalysis is one of the solutions to convert CO₂ to value-added hydrocarbons such as methane, methanol, ethanol which can be used as transportation fuels and commodity chemicals using renewable energy as an input. Recent literature studies indicate transition metal-p block catalysts such as metal oxides and metal sulfides show improved catalyst activity and desired product selectivity for this reduction reaction. But, the design principle and reaction mechanisms are barely explored. In this work, we present a detailed computational study on electrochemical CO₂ reduction reaction (CO₂RR) to methane and methanol over different transition metal-p block catalysts using Density Functional Theory (DFT) calculations.

We performed plane wave DFT calculations on VASP 5.4x installed in the Materials Design MEDEA environment to calculate electronic structure properties. Throughout this work, all the electronic structure calculations are performed using Van Der Waals, opt –PBE functional. A Fermi smearing of 0.2 eV is used and calculations are performed with gamma centered k-points mesh of 2x2x1 with convergence of ground state energies less than 0.05eV/mole-unit cell with respect to k-point sampling. A vacuum space of 12 Å is defined to minimize the interactions between repeated slabs.

In this work, we investigated the catalyst activity and product selectivity of various transition metal oxide and sulfide catalysts for the CO₂ reduction reaction. We predicted the lowest energy pathway for CH₃OH and CH₄ formation from free energy diagrams (FEDs) and computed their corresponding reducing potentials and overpotentials . We inferred that both methane and methanol share common intermediate species in their mechanisms and these species either bind through carbon atom (via CO) or oxygen atom (via OH). Therefore, we developed scaling relations between all the active C bound intermediate species with 𝛥G (CO*) and O bound intermediate species with 𝛥G (OH*) to determine and rank the activity of different catalyst materials. We built thermodynamic volcano plots using these scaling relations which could further benefit in capturing the activity trends in CO₂RR using transition metal/p-block catalyst materials. In this presentation, we will also show how electronic and geometric changes would affect catalyst activity and product selectivity for CO₂ electroreduction process which could help in designing/developing an ideal catalyst for CO₂RR in the near future.