Catalytic Mechanisms in Ammonia Oxidation Systems

Ammonia oxidation is central to the industrial production of nitric acid, employing platinum group metal catalysts to convert NH₃ into NO with high selectivity at temperatures exceeding 1,000 K. The reaction pathway involves adsorption of ammonia on active metal sites, sequential dehydrogenation, oxygen insertion and desorption of intermediates. Catalyst efficacy is governed by alloy composition, surface morphology and microstructure of knitted gauzes or wire meshes, commonly comprising Pt–Pd–Rh formulations. The strongly exothermic nature of the process induces thermal gradients and hot spots, driving oxide layer growth, metal diffusion and morphological evolution that influence catalyst lifetime. Recent advances in in situ spectroscopy, high-resolution electron microscopy and computational modelling have deepened understanding of active site distribution, reaction kinetics and heat- and mass-transfer interplay. Insights from these studies underpin optimisation of catalyst design, operating conditions and regeneration strategies, with profound implications for sustainable fertiliser manufacture and emission abatement.

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Technical terms

Heterogeneous catalysis: Catalysis in which the reactants and catalyst occupy different phases, typically involving gases reacting on a solid surface.

Catalytic gauze: A woven or knitted metal mesh, often comprising platinum group alloys, providing a fixed bed for high-temperature gas-phase reactions.

Dehydrogenation: The removal of hydrogen atoms from an organic or inorganic molecule during a catalytic transformation.

Grain reconstruction: Structural reorganisation of metallic grains under thermal, chemical or mechanical stress, affecting catalyst morphology and activity.

Pt-catchment: A downstream alloy or layer designed to trap volatilised platinum species escaping from the primary catalyst, preserving precious metal.

References

1. Catalytic Etching and Oxidation of Platinum Group Metals: Morphology, Chemical Composition and Structure of Pt, Pd and Rh Foils in the O2 Atmosphere and during NH3 Oxidation with Air at 1133 K. Catalysts (2023).
2. Chemical Composition of the Surface and Subsurface of Pt–Pd–Rh–Ru Catalytic Gauzes Used in the NH3 Oxidation with Air at 1133 K. Catalysts (2022).
3. Pt-Catchment Using Pd/Au Alloys: Effect of Enhanced Diffusion. Industrial & Engineering Chemistry Research (2023).