Third-Body Stabilization of Supercritical CO2 in CO Oxidation: Development and Application of a ReaxFF Force Field for the CO/O/CO2 System

TL;DR

This study develops a reactive force field for CO/O/CO2 systems to understand supercritical CO2's role in stabilizing reaction intermediates, revealing how dense scCO2 acts as an efficient third body in CO oxidation.

Contribution

A novel ReaxFF force field calibrated with ab initio methods enables detailed atomistic simulations of CO oxidation in supercritical CO2, capturing reaction dynamics and stabilization mechanisms.

Findings

The force field accurately reproduces experimental and ab initio properties of CO2 and supercritical CO2.

In dense scCO2, CO2 formation is stabilized by molecular collisions, reducing dissociation.

Excess energy from reactions dissipates mainly into internal vibrational and rotational modes.

Abstract

Supercritical CO2 (scCO2) plays a crucial role as a solvent in separation processes, advanced power cycles, and materials processing. Nonetheless, the atomistic comprehension of how the dense scCO2 matrix influences the fundamental reaction of carbon monoxide (CO) is still insufficiently explored. Experimental studies and molecular dynamics (MD) simulations frequently fail to detect the highly reactive, transient intermediates, such as atomic oxygen (O), that drive these reactions. To address this issue, we have developed a novel ReaxFF reactive force field for the CO2/CO/O system. The force field parameters were calibrated using density functional theory and second-order Moller-Plesset calculations to model CO2 crystal properties, intermolecular interactions, bond dissociation curves, and reaction energy barriers. The force field reproduces the cohesive energy of the CO2 crystal, the pressure characteristics of bulk scCO2, the equation-of-state behavior over a wide pressure-density range, the pressure dependence of the C-O bond length under compression, and the structural properties of liquid and scCO2, as documented by experiments, ab-initio MD, and prominent non-reactive models. The force field was subsequently applied to study the CO + O -> CO2 reaction. In a dilute environment, the reaction is inefficient as the newly formed CO2 rapidly dissociates due to excess kinetic and potential energy acquired from the exothermic reaction. Conversely, in a dense scCO2 environment, the surrounding matrix acts as an efficient third body, stabilizing the emerging CO2 product via molecular collisions. Statistical analysis confirms an average excess energy dissipation of 133.9 +/- 3.6 kcal/mol over 112.4 +/- 17.9 ps. Kinetic energy decomposition reveals that approximately 92% of the excess kinetic energy is stored in internal (rotational and vibrational) degrees of freedom.