Effect of Surface Oxides on Pt-catalysed NH3 Oxidation Kinetics: Transient Response Measurements and Modelling

The correct description of the kinetics of NH3 oxidation over Pt is crucial for the optimal catalyst design. Despite a general agreement on the elementary steps, major discrepancies on the value of the activation energy are still present, with experimental studies showing activation energies > 100 kJ/mol, way above the activation energies predicted by DFT (typically around 50 kJ/mol ). In this work, we apply a novel kinetic analysis, reconciling experimentally and theoretically derived activation energies. We developed a Transient Response Measurement protocol which allowed to measure the intrinsic kinetics, observing an activation energy of 39 kJ/mol, close to theory, and then the following Pt oxide formation/decomposition. By combining the two phenomena, we obtained a model describing the experimental light-off curves and their high apparent activation energy (>100 kJ/mol). This is explained by the Pt-oxide decomposition evolving concurrently with the heating ramp, leading to a gradual enhancement of the catalytic activity beyond the increase related to the low intrinsic activation energy. Furthermore, our model explains previously observed effects like the NH3 conversion hysteresis during light-up/light-down, and the O2 inhibition on NH3 light-off. The work paves the way for a more fundamental understanding of a wide range of NH3 conversion/abatement catalytic systems.