OXYREACTIVITY OF CARBON SURFACE OXIDES

摘要

The behavior of carbon surface oxides formed in ~(18)O_2 and exposed subsequently to ~(16)O_2 has been studied by mass spectrometric determination of the gaseous products which include all the possible isotopic forms of CO and CO_2. 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(~(18)O) + ~(16)O_2→ C(~(16)O) + C~(18)O, C~(16)O~(18)O (R3) 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~(-1). 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 < 1073 is 34 kJ mol~(-1). 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.