Dependence of the effective rate constants for the hydrogenation of CO on the temperature and composition of the surface

Experiments on the hydrogenation of CO on pure CO and CO–H2O mixed ice have been performed at temperatures between 8 and 20 K. We obtained temperature and compositional dependence of the effective reaction rate constants. Results indicate that hydrogenation proceeds efficiently on pure solid CO and CO–H2O mixed ice at temperatures below 10 and 20 K, respectively. Rate constants for pure CO decreased significantly at 12 K compared to those obtained with CO–H2O mixed ice. Hydrogenation of CO at temperatures greater than 12 K were catalyzed by the H2O adjacent to the CO. The importance of the experimental results for some relevant astrophysical environments has also been outlined.