Reduction of CO₂ by Si at High Pressures

There is a long-standing question whether silicon and carbon can form mixed oxide compounds high pressure. At ambient pressure-temperature conditions, carbon forms a molecular gas with strong C=O double bonds in CO₂, whereas silicon forms covalent solids as crystalline SiO₂. These two distinct compounds do not react at ambient conditions; however, experimental efforts using diamond-anvil cells (DACs) as well as computational efforts have searched for stable Si-C-O compounds from possible reactions of CO₂ and zeolitic SiO₂ at high pressures. The reaction of CO₂ + SiO₂ has proven challenging, and clear evidence and structural characterization of possible crystalline Si-C mixed oxides is still needed. We propose a different pathway to forming Si-C mixed oxides through the reaction of CO₂ and pure Si. Our ab initio molecular dynamics simulations show that pure Si readily reduces CO₂ to elemental carbon at high pressure (20 GPa) and moderate temperatures above 2000 K. A chemical analysis of the Bader charges and the Crystal Orbital Hamilton Populations (COHP) of the reacted CO₂ + Si mixture reveal that Si donates its electrons mostly to the C atoms in the CO₂ molecules, creating unstable C-O bonds, and favorable Si-C, Si-O, and C-C covalent interactions. Finally, preliminary Raman spectroscopy measurements on the CO₂ + Si reaction using laser-heated DACs will be discussed along with future work on structural characterization.