Carbon Monoxide Hydrogenation on Ice Surfaces

Abstract

We have performed density functional calculations to investigate the carbon monoxide hydrogenation reaction (H+CO→HCO), which is important in interstellar clouds. We found that the activation energy of the reaction on amorphous ice is lower than that on crystalline ice. In the course of this study, we demonstrated that it is roughly possible to use the excitation energy of the reactant molecule (CO) in place of the activation energy. This relationship holds also for small water clusters at the CCSD level of calculation and the two-layer-level ONIOM (CCSD : X3LYP) calculation. Generally, since it is computationally demanding to estimate activation energies of chemical reactions in a circumstance of many water molecules, this relationship enables one to determine the activation energy of this reaction on ice surfaces from the knowledge of the excitation energy of CO only. Incorporating quantum-tunneling effects, we discuss the reaction rate on ice surfaces. Our estimate that the reaction rate on amorphous ice is almost twice as large as that on crystalline ice is qualitatively consistent with the experimental evidence reported by Hidaka et al. [Chem. Phys. Lett., 2008, 456, 36.].

Single-atom catalysis in space: Computational exploration of Fischer–Tropsch reactions in astrophysical environments

“…The simplest organic compounds, formaldehyde (CH 2 O) and methanol (CH 3 OH), have been repeatedly observed from starless cores to star-forming regions (Van Gelder et al 2020;McGuire 2018;Bianchi et al 2019;López-Sepulcre et al 2015;Vasyunina et al 2014); the widely accepted scenario is that they form by the hydrogenation of CO by the successive addition of H atoms occurring on icy grains. Experimental (Hiraoka et al 2002;Watanabe & Kouchi 2002;Watanabe et al 2003;Hidaka et al 2004;Fuchs et al 2009;Pirim et al 2010;Minissale et al 2016;Ohno et al 2022) and theoretical (Rimola et al 2014;Korchagina et al 2017;Song & Kästner 2017;Kuwahata & Ohno 2018;Pantaleone et al 2020;Enrique-Romero et al 2021;Tieppo et al 2023) evidence support this reaction formation mechanism.…”