The Effects of Platinum Dispersion and Pt State on Catalytic Properties of Pt/Al2O3 in NH3 Oxidation

Dependence of NH₃ oxidation on the state and dispersion of Pt species in Pt/γ‐Al₂O₃ catalysts was investigated. Prereduced Pt/γ‐Al₂O₃ catalysts containing Pt⁰ nanoparticles exhibited significantly higher activity than preoxidized ones with the same Pt dispersion. The most significant improvement of the catalytic activity (TOF increased by 30 times) was observed when the size of Pt⁰ particles increased from ∼1 to ∼8 nm. N₂ selectivity was found to be mainly determined by the reaction temperature, with a minor influence of Pt particle size. Preoxidized catalysts containing ionic Pt were activated by the reaction medium, while partial deactivation was observed for the prereduced ones. The activity improvement was associated with the presence of Pt⁴⁺/Pt²⁺ species on the surface of preoxidized catalysts. The activity decrease of the prereduced catalysts was due to the partial oxidation and subsequent redispersion of Pt particles. Introduction of H₂O and CO₂ to the reaction mixture only moderately influenced NH₃ oxidation activity shifting NH₃ conversion curves by about +15 °C.