OH desorption energies for a palladium catalyst characterised by kinetic modelling and laser-induced fluorescence

A kinetic model for the H2/O2 reaction on a polycrystalline palladium catalyst has been constructed using CHEMKIN in order to understand the coverage-dependent OH desorption energy. Each adsorbed oxygen atom was modelled to cover four Pd surface sites. The yield of OH and the water production were measured with laser-induced fluorescence (LIF) and microcalorimetry respectively as a function of the relative hydrogen concentration, αH2. The temperature of the catalyst was 1300 K, the total pressure was 13 Pa and the flow was set to 100 SCCM. In fitting the model to the experimental data, the OH desorption energy EdOH was found to have a first-order coverage dependence according to: EdOH(θ)=EdOH(0)−Bθ, where B is a constant set to 92 kJ/mol. The desorption energy at zero coverage EdOH(0) was determined to be 226 kJ/mol. The model could also qualitatively and quantitatively reproduce the apparent desorption energy as a function of αH2; therefore it is believed that the coverage could be predicted by the model. The values for EdOH(θ) were calculated as a function of αH2. From the results of a sensitivity analysis and rate of production calculations, there are strong reasons to believe that the main water-forming reaction on Pd at 1300 K is the hydrogen addition reaction, H + OH ⇌ H2O. Enthalpy diagrams for the water-forming reactions are also presented.