The reactions of hydrogen and carbon monoxide with surface-bound oxides on carbon

The stability of surface oxides formed on exposure of Spherocarb to oxygen has been investigated in various atmospheres (Ar, CO, and H₂) through gravimetry and mass spectrometry. Oxide complexes, formed in oxygen (2.67 Pa) at 973 or 1073 K, were exposed to 2.67 Pa of the bath gas in the temperature range 923 to 1073 K. Changes in the population of surface complexes were identified through analysis of gas evolution (principally CO) profiles during temperature-programmed desorption (TPD) of the oxides in an inert atmosphere. Oxides formed in this work display a distribution of activation energies for their decomposition, E_des, from 300 to 420 kJ mol⁻¹. Soaking of these oxides in Ar showed the frequency factor for their thermal decomposition to be 10^{14.3 ± 0.3} s⁻¹, independent of E_des. The effect of soaking in CO was essentially the same as that of Ar, and it is concluded that no significant reaction occurs under these conditions between surface oxides and CO. However, on soaking in H₂, H₂O was evolved as a result of reaction between hydrogen and the surface oxide complex

Abstract

The kinetics of the reaction between hydrogen and surface complexes are dependent on the reaction temperature and on the thermal stability of the reacting surface oxide. Less stable oxides react more readily than do more stable oxides. If all the reaction rate variation between complexes of differing thermal stability is attributed to changes in the activation energy of the reaction, the activation energy for the reaction of H₂ with complexes may be expressed as E_H = 0.5E_des − 50 kJ mol⁻¹, 348 < E_des < 408 kJ mol⁻¹. Comparison of the apparent preexponential factor for the reaction with the expected frequency factor for gas–surface-complex collision under the experimental conditions suggests that direct reaction of hydrogen with the oxygen complex may be the limiting step in the overall reaction.