Hydroxylamine Intermediate Governs Selectivity in Nitrite Hydrogenation on Pd-based Catalysts for Sustainable Water Treatment

Abstract

Catalytic hydrogenation of nitrate (NO₃⁻) and nitrite (NO₂⁻) on metal surfaces is a promising route for removing toxic oxy-nitrogen ions from water bodies. Recently, hydroxylamine (NH₂OH) has been identified as a persistent reaction intermediate in this process, breaking the long-standing assumption that ammonium ion (NH₄⁺) and dinitrogen (N₂) were the only significant reaction products. In this work, we systematically investigated NH₂OH evolution during the reduction of NO₂⁻ over Pd/Al₂O₃ and SnPd/Al₂O₃ catalysts as a function of H₂ partial pressure, initial NO₂⁻ concentration, reaction temperature, and co-feeding of NH₂OH. We revealed that NH₄⁺ selectivity depends strongly on the NO₂⁻ conversion level, reflecting shifts in surface coverages as the reaction progresses. Suppression of both NH₂OH and NH₄⁺ formation is only achievable under H₂-deficient conditions, though this comes at the expense of lowering the rate of reaction. Elevated temperatures enhance NH₂OH decomposition and thereby promote NH₄⁺ formation, while leaving N₂ selectivity largely unaffected. Co-feeding experiments further show that externally introduced NH₂OH does not influence the NO₂⁻ hydrogenation rate. We critically reviewed prior mechanistic studies on NO₂⁻ hydrogenation and propose a refined Langmuir–Hinshelwood scheme that explicitly incorporates NH₂OH as desorbed intermediate. This work highlights the importance of NH₂OH in the reaction network and underscores the need to include it in the assessment of the reaction selectivity.