Oxyreactivity of carbon surface oxides

The behavior of carbon surface oxides formed in O-18(2), and exposed subsequently to O-16(2) has been studied by mass spectrometric determination of the gaseous products which include all the possible isotopic forms of CO and CO2. In addition, the oxides remaining on the surface of the carbon have been characterized by temperature-programmed desorption (TPD) to yield CO. The experiments were conducted at a total pressure of 2.7 Pa at temperatures of 873, 973, and 1073 K. Both the transient kinetic experiments and the TPD studies point to the occurrence of a reaction between surface-bound oxides and gas-phase oxygen to gasify carbon and at the same time create new surface oxide. The overall reaction proceeds as C(O-18) + O-16(2) --> C(O-16) + (CO)-O-18, (COO)-O-16-O-18 but the precise stoichiometry remains unknown. The kinetics of reaction R3 have been studied as a function of the characteristic energy for decomposition of the surface complexes (E-des), in the range 280 < E-des < 420 kJ mol(-l). The pseudo-first-order rate constant for the reaction in 2.7 Pa O-2 is found to be essentially independent of E-des. The activation energy for the reaction over the temperature range 873 < T < 1053 is 34 kJ mol(-l) The contribution of reaction R3 to the overall rate of gasification of the carbon under the experimental conditions ranges from 45% at 873 to 4% at 1073 K.