Determination of adsorption and desorption parameters from ignition temperature measurements in catalytic combustion systems

Abstract

When a cold catalyst is exposed to a fuel-oxygen mixture, the surface gets covered with the more effectively adsorbing species. When the temperature is increased, this species is desorbed and the ignition temperature is determined by the rate of desorption. Based on the equations for the heat balance, expressions were derived for the calculation of ignition temperature from the parameters of the experimental setup, the preexponential factor Ad and activation energy E-d of desorption, the ratio of sticking coefficients, and the ratio of adsorption orders of fuel and oxygen. Published experimental data for the catalytic ignition of CO, H-2, and CH4 were reinterpreted using the expressions obtained, and the following parameters were determined for polycrystalline platinum catalyst: E-d(H-2/Pt) = 43.3+/-5.2 kJ/mol, E-d(CO/Pt) = 107.2+/-12.7 kJ/mol, E-d(O-2/Pt) = 190 34 kJ/mol, S-H2,S-0/S-O2,S-0 = 36.7+/-9.6, S-CO,S-0/S-O2,S-0 = 41.2+/-8.5, S-O2,S-0/SCH4,0 = 5.9+/-0.3. Error limits refer to a confidence level of 0.95. The activation energy of desorption for CO and O-2 and the ratio of zero coverage sticking coefficients of O-2 and CH4 are the first experimentally based determinations of these parameters. Experimental ignition temperatures could be reproduced assuming second-order ' adsorption of CO, H-2, and O-2 on the Pt surface. These reaction orders have been debated in the literature.