Carbon dioxide reduction mechanism on Ru-based electrocatalysts [Ru(bpy)2(CO)2]2+: insights from first-principles theory

Abstract

Solar fuel production through the so-called artificial photosynthesis has attracted a great deal of attention to the development of a new world energy matrix that is renewable and environmentally friendly. This process is characterized by light absorption with enough photon energy to generate conduction electrons, which drive the carbon dioxide reduction to produce organic fuels. It is also common to couple Ru-complex electrocatalysts to form a more efficient and selective hybrid system for this application. In this work, we have undertaken a thorough investigation of the redox reaction mechanism of Ru-based electrocatalysts by means of density functional theory (DFT) methods under the experimental conditions that have been previously reported. More specifically, we have studied the electrochemistry and catalytic activity of the [Ru(bpy)₂(CO)₂]²⁺ coordination complex. Our theoretical assessment supports the following catalytic cycle: (i) [Ru(bpy)₂(CO)₂]²⁺ is transformed into [Ru(bpy)₂(CO)]⁰ upon two-electron reduction and CO release; (ii) [Ru(bpy)₂(CO)]⁰ is protonated to form the [Ru(bpy)₂(CO)H]⁺ hydride complex; (iii) CO₂ is activated by the hydride complex through an electrophilic addition to form the [Ru(bpy)₂(CO)(OCHO)]⁺ intermediate; (iv) the resulting formic acid ligand is released in solution; and, finally, (v) the CO ligand is reattached to the complex to recover the initial [Ru(bpy)₂(CO)₂]²⁺ catalyst.