Stabilizing Cu²⁺ ions by solid solutions to promote CO₂ electroreduction to methane

Abstract

Copper is the only metal catalyst that can perform the electrocatalytic CO2 reduction reaction (CRR) to produce hydrocarbons and oxygenates. Its surface oxidation state determines the reaction pathway to various products. However, under the cathodic potential of CRR conditions, the chemical composition of most Cu-based catalysts inevitably undergoes electroreduction from Cu²⁺ to Cu⁰ or Cu¹⁺ species, which is generally coupled with phase reconstruction and the formation of new active sites. Since the initial Cu²⁺ active sites are hard to retain, there have been few studies about Cu²⁺ catalysts for CRR. Herein we propose a solid-solution strategy to stabilize Cu²⁺ ions by incorporating them into a CeO₂ matrix, which works as a self-sacrificing ingredient to protect Cu²⁺ active species. In situ spectroscopic characterization and density functional theory calculations reveal that compared with the conventionally derived Cu catalysts with Cu⁰ or Cu¹⁺ active sites, the Cu²⁺ species in the solid solution (Cu-Ce-Oₓ) can significantly strengthen adsorption of the *CO intermediate, facilitating its further hydrogenation to produce CH₄ instead of dimerization to give C₂ products. As a result, different from most of the other Cu-based catalysts, Cu-Ce-Oₓ delivered a high Faradaic efficiency of 67.8% for CH₄ and a low value of 3.6% for C₂H₄.