Drastic change in selectivity caused by addition of oxygen to the hydrogen stream for the hydrogenation of nitrite in water over a supported platinum catalyst

Abstract

In the present study, we investigated the influence of the addition of O₂ to the H₂ reactant stream during the hydrogenation of NO₂⁻ in water on the catalytic performances of Al₂O₃-supported precious metal catalysts including Pd, Pt, Ir, Rh, and Ru with 0.3 mmol g⁻¹ of the metal. Pd/Al₂O₃ showed high selectivity for N₂ irrespective of the presence and absence of O₂, and Rh and Ru/Al₂O₃ were inactive towards the hydrogenation of NO₂⁻ even in the absence of O₂. In contrast, while Pt/Al₂O₃ showed high selectivity for NH₃ (90%) in the absence of O₂ (P_H2 = 0.2 atm and P_O2 = 0 atm), the product drastically changed to N₂ with 93% selectivity when O₂ was added (P_H2 = 0.2 atm and P_O2 = 0.1 atm). Since Pt/Al₂O₃ was completely inactive towards the oxidation of NH₃ with O₂ in water under the reaction conditions, oxidative decomposition of the formed NH₃ was not the reason for the high selectivity for N₂ in the presence of O₂. Kinetic analysis of the reaction in the absence and presence of O₂ and studies on the effects of the Pt size suggested that hydrogen atoms activated on the Pt particles were mainly consumed by O₂ upon H₂O formation in the presence of O₂. We concluded that the inactivation of the Pt sites active for NH₃ formation and furthermore the change in the function of the sites to N₂ formation caused by the O₂ addition lead to the drastic change in the selectivity from NH₃ to N₂ in the presence of O₂.