X-ray-Induced CO₂ Formation via CO Reaction with TiO₂ at Cryogenic Temperature

Cosmic rays, γ-rays, and X-ray-driven reactions on interstellar grains have been the main focus in understanding the extraterrestrial synthesis of complex organic molecules. Herein, using temperature-programmed desorption and X-ray photoelectron spectroscopy, we report for the first time X-ray-induced CO₂ production from a CO-covered rutile TiO₂(110) surface at 110 K, in addition to X-ray-induced CO desorption. The X-ray-induced CO₂ production was found to follow a Mars and van Krevelen (MvK) mechanism via the reaction between adsorbed CO and surface lattice oxygen of TiO₂. Defects of TiO₂ suppress the X-ray-induced CO₂ production but promote the X-ray-induced CO desorption. These findings suggest a novel X-ray-induced CO-to-CO₂ reaction on oxide surfaces at cryogenic temperature likely occurring in the interstellar medium and also warn us to keep in mind the possibility of X-rays inducing chemical reactions during structural characterizations of oxides with X-ray-involved techniques.