Molecular and Kinetic Modelling of the Ammonia Oxidation on Platinum

Abstract

The thesis contributes to the fundamental understanding of the chemistry of ammonia oxidation on platinum. The research concentrates on determining the kinetic and reaction mechanisms for the NH3(g) oxidation with O2(g) on flat and stepped platinum surfaces using first-principles methodologies. The research finds direct applications in the modelling of the NO(g) production from NH3(g), a significant key step in the HNO¬3 manufacture through the Ostwald process. This process is industrially very significant because nearly all of the world's HNO3 is made from NH3(g), and most of the world’s HNO3 production is dedicated to the production of fertilisers. The reactions studied in this work, however, are relevant to other chemical processes of interest such as the selective catalytic oxidation for the removal of NH3(g) from industrial waste streams, and the selective catalytic reduction of NOx. The work carried out seeks fundamental analysis of three main elements of the NH3(g) oxidation, i) the effect of Pt surface morphology towards the formation of N2O(g), ii) the existence of lower activated alternative pathways for NO(g) formation and iii) the effect of the surface coverage on the energetics of selected surface reactions and the adsorption/desorption process. A new thermodynamically-consistent microkinetic model for the NH3(g) oxidation over Pt has been proposed by using the DFT-calculated kinetic parameters on the Pt(211) surface as input parameters. The simulated profiles of NH3(g) conversion and product yields reproduce qualitatively the experimental data obtained previously in our group, for the NH3(g) oxidation over Pt using a micro–tubular reactor. However, sensitivity analysis of the input data suggests small changes to a selected set of values improve the quantitative agreement with experimental observations.