  The formation of CO2 in quiescent regions of molecular clouds is not yet
fully understood, despite CO2 having an abundance of around 10-34 % H2O. We
present a study of the formation of CO2 via the non-energetic route CO + OH on
non-porous H2O and amorphous silicate surfaces. Our results are in the form of
temperature-programmed desorption spectra of CO2 produced via two experimental
routes: O2 + CO + H and O3 + CO + H. The maximum yield of CO2 is around 8 %
with respect to the starting quantity of CO, suggesting a barrier to CO + OH.
The rate of reaction, based on modelling results, is 24 times slower than O2 +
H. Our model suggests that competition between CO2 formation via CO + OH and
other surface reactions of OH is a key factor in the low yields of CO2 obtained
experimentally, with relative reaction rates k(CO+H) \ll k(CO+OH) < k(H2O2+H) <
k(OH+H), k(O2+H). Astrophysically, the presence of CO2 in low AV regions of
molecular clouds could be explained by the reaction CO + OH occurring
concurrently with the formation of H2O via the route OH + H.
