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.