Effect of vanadium pentoxide on gas‐phase combustion. I. The carbon monoxide‐sulphur dioxide‐oxygen system

Abstract

AbstractThe kinetics of oxidation processes in the system carbon monoxide‐sulphur dioxide‐oxygen in the presence of surfaces, mainly vanadium pentoxide (V) and equimolar V2O5 and K2SO4 (V‐K), have been investigated using a static system. The upper limit of the explosive combustion of ‘wet’ CO was the same with a V, V‐K, KC1 or silica surface, the overall activation energy governing ignition being about 35 kcal. mole−1. Ignition was promoted by the addition of water (or the use of a boric oxide surface) but inhibited by SO2 and packing the vessel. The lower limit pressure was higher with a V‐ than with a V‐K‐surface, but varied little with temperature, and was unaffected by varying the water concentration or by the addition of SO2. Increase in surface/volume ratio raised the limit. The rate of the normal slow oxidation of ‘wet’ CO at pressures above the upper limit was very sensitive to changes in the nature of the surface, the order being V‐K > silica > V > KC1. Addition of SO2 greatly decreased the rate with a V‐surface but increased it with a V‐K‐surface. Packing the vessel had the same effect and with a V‐K‐surface the activation energy fell to about 18 kcal. mole−1. The heterogeneous reaction of SO2 and O2 was much faster on a V‐K‐ than on a V‐surface, the activation energies being 31 and 27 kcal. mole−1 respectively.The elementary steps in the chain mechanism, occurring in the systems under moist conditions, are discussed. It is concluded that, at the lower limit, branching is due to excited CO2 and termination to surface destruction of oxygen atoms, while, at or above upper limit pressures, branching by a process involving O atoms and H2O and the relative rates of reaction of HO2 radicals in the gas phase and at the wall are important. The mixtures undergoing ‘inhibited’ slow oxidation probably contain carbon, suboxides which can terminate chains by reaction with O atoms. SO2 may react with O atoms in the gas phase or with O2 on the surface depending on the conditions.