Catalytic ignition of hydrogen-oxygen on platinum

Catalytic ignition of hydrogen-oxygen on platinum at atmospheric pressure is studied experimentally. The hydrogen-oxygen mixtures are diluted with nitrogen to prevent the homogeneous ignition. The parameters are relative hydrogen concentration and dilution ratio, a mole fraction of nitrogen. The surface temperature is measured with an R-type thermocouple. The time histories of surface temperature show an inflection point, and the catalytic ignition temperature is defined as the temperature at the inflection point. As a result, the ignition temperature is increased with the dilution ratio and shows a minimum at a certain relative hydrogen concentration. The reciprocal ignition temperature is proportional to the natural log of total reactant mole fraction, and the proportional constants are almost the same in all relative hydrogen concentration.

If the catalytic ignition has occurred, an abrupt transition from a kinetically controlled system to one controlled by mass transport is produced. It is approximated that the surface conditions could be evaluated in a mass transport controlled system as the time needed to change the systems is very short and that the gaseous compositions are the same before the ignition has occurred as the surface reaction rate is much faster than the velocity of the mass transport. To analyze the relation between the ignition temperature and the reactant concentrations simply, an overall reaction model with an Arrhenius expression is used, and the orders of reaction of the adsorbed H and O are one in the expression. The expression includes the desorption of the excess of the adsorbed H.

The expression explained the dependence of the ignition temperature on the dilution ratio quantitatively and denoted a minimum ignition temperature at a certain relative hydrogen concentration as shown in the experimental results.