Reactive and Klnetlc Properties of Carbon Monoxide and Carbon Dioxide on a Graphite Surface

Temperature-programmed desorption (TPD) results after chemisorption of carbon monoxide (CO) and carbon dioxide (CO,) on polycrystalline graphite are presented. CO adsorbs onto graphite with a very low sticking coefficient. After CO Chemisorption, CO (mass 28 amu) desorbs in two temperature regions, between 400 and 700 K and between 1000 and 1300 K, and COz (mass 44 amu) desorbs below 950 K. The intensity of the COz signal is less than 1 order of magnitude lower than the CO intensity. After C02 adsorption the major desorption product is CO at high temperatures (1000 < T (K) < 1300), whereas a small amount of COz desorbs around 450 K. The adsorption of a CI6Oz and C'*Oz mixture leads to a nearly total oxygen scrambling of the C02 desorbed. A mechanism for CO and COz interconversion on the graphite surface is presented in terms of surface oxide species, mainly lactones and semiquinones, and their relative stability. Assignments of the TPD features are proposed accordingly. Reaction studies on the C02 gasification of clean graphite and the CO disproportionation (Boudouard reaction) have been performed. A good agreement is found between the activation energies obtained and the desorption energies calculated from the analysis of the TPD results.