Determination of the adsorption and desorption parameters for ethene and propene from measurements of the heterogeneous ignition temperature

If a cold catalyst is exposed to a mixture of fuel + oxygen, the surface coverage of the catalyst can be dominated by either the fuel or the oxygen, depending on the actual catalyst and the composition of the gaseous mixture. If the temperature is increased, heterogeneous ignition occurs; the ignition temperature is influenced by the adsorption and desorption properties of both the fuel and the oxygen. Based on the equations for the heat balance, expressions have been derived for calculating the ignition temperature from the parameters of the experimental setup and the adsorption and desorption parameters of the fuel and the oxygen. These expressions can also be used to evaluate measured ignition temperatures to determine unknown adsorption and desorption parameters, such as the pre-exponential factor AD and activation energy ED for the desorption of the dominant surface species, the ratio of the sticking coefficients, and the ratio of the adsorption orders of the fuel and oxygen. This latter approach was used to evaluate measurements made by Cho and Law for the catalytic ignition of ethene and propene on polycrystalline platinum. The following parameters were determined by means of nonlinear least-squares fitting: ED(C2H4/Pt)=136±21kJ/mol, ED(C3H6/Pt)=161±53kJ/mol, SC2H4,0/SO2,0=15.6±1.9, SC3H6,0/SO2,0=11.9±1.7. Using a previously determined value for the sticking coefficient of O2, the values SC2H4,0=0.38±0.08 and SC3H6,0=0.29±0.06 were obtained. These error limits refer to a confidence level of 0.95. Experimental ignition temperatures could be reproduced assuming second order adsorption of ethene and propene on a surface of Pt.