A comparative study of high-temperature water formation and OH desorption on polycrystalline palladium and platinum catalysts

The water formation reaction has been studied on a hot polycrystalline palladium catalyst and a theoretical model has been derived with Chemkin in order to fit the experimental data. The results were compared with similar studies for platinum. The H2/O2 reaction was measured with LIF and microcalorimetry on palladium at a temperature of 1300 K, pressures of 13 and 26 Pa and flows of 100 and 200 SCCM. The experiments were performed in a stagnation-point flow field, which was also the geometry modelled with Chemkin. The maximum in water production as a function of αH2 (αH2,H2Omax), occurs at about 20% on platinum, while it occurs at about 40–45% on palladium; αH2 is the hydrogen mixing ratio. However, the maximum in OH desorption (αH2,OHmax), occurs at about the same αH2 for both palladium and platinum. In order that the theoretical model would match the experimental data for palladium it was necessary to assume a coverage dependent OH desorption energy, Ead,OH, according to Ead,OH(θ)=Ead,OH(0)−Bθ, where B is a constant equal to 100 kJ/mol. The apparent desorption energy for OH on palladium was also measured at αH2=17% to 200 kJ/mol and αH2=45% to 220 kJ/mol. The main water-forming reaction on platinum is thought to be the hydrogen addition reaction H* + OH* ⇔ H2O*. For palladium, there are difficulties to determine the main water formation route because the OH desorption energy is believed to be coverage dependent. The reverse of reaction H* + OH* ⇔ H2O* seems to have greater importance for the OH formation on palladium than on platinum. The initial sticking coefficient of hydrogen and oxygen is found to be greater on palladium than on platinum by a factor of about 10.