Proposed importance of HOCO chemistry: Inefficient formation of CO$_2$ from CO and OH reactions on ice dust

TL;DR

This study experimentally shows that the reaction between CO and OH on ice dust predominantly produces the HOCO radical rather than CO$_2$, emphasizing the complex chemistry and competition between reaction pathways in interstellar ice formation.

Contribution

It provides new experimental evidence that HOCO formation dominates over CO$_2$ in CO + OH reactions on ice dust, challenging previous assumptions.

Findings

HOCO is the major product of CO + OH reactions on ice dust.

HOCO can convert into CO$_2$ via H-abstraction, but competing reactions lead to carboxylic acids.

Energy dissipation influences the reaction pathway and product distribution.

Abstract

With the advent of JWST ice observations, dedicated studies on the formation reactions of detected molecules are becoming increasingly important. One of the most interesting molecules in interstellar ice is CO$_2$. Despite its simplicity, the main formation reaction considered, CO + OH -> CO$_2$ + H through the energetic HOCO* intermediate on ice dust, is subject to uncertainty because it directly competes with the stabilization of HOCO as a final product which is formed through energy dissipation of HOCO* to the water ice. When energy dissipation to the surface is effective during reaction, HOCO can be a dominant product. In this study, we experimentally demonstrate that the major product of the reaction is indeed not CO$_2$, but rather the highly reactive radical HOCO. The HOCO radical can later evolve into CO$_2$ through H-abstraction reactions, but these reactions compete with addition reactions, leading to the formation of carboxylic acids (R-COOH). Our results highlight the importance of HOCO chemistry and encourage further exploration of the chemistry of this radical.