Hydroxylamine intermediate governs selectivity in nitrite hydrogenation on Pd-based catalysts for sustainable water treatment

Abstract

Catalytic hydrogenation of nitrate (NO₃⁻) and nitrite (NO₂⁻) is a promising route for drinking water purification and rebalancing the global nitrogen cycle. Recently, hydroxylamine (NH₂OH) was detected as a persistent reaction intermediate although ammonium (NH₄⁺) and dinitrogen (N₂) were assumed to be the only significant reaction products for several decades. In this work, we systematically investigate NO₂⁻ hydrogenation over Pd/Al₂O₃ and SnPd/Al₂O₃ while explicitly quantifying NH₂OH under various conditions, including changes in H₂ partial pressure, the initial NO₂⁻ concentration, and reaction temperature, and through co-feeding of NH₂OH. We reveal 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 lower overall hydrogenation activity. 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 a 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.