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

摘要

Dependence of NH3 oxidation on the state and dispersion of Pt species in Pt/gamma-Al2O3 catalysts was investigated. Prereduced Pt/gamma-Al2O3 catalysts containing Pt-0 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 Pt-0 particles increased from similar to 1 to similar to 8 nm. N-2 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 degrees C.