Elucidation of Nitrate Reduction Mechanisms on a Pd-In Bimetallic Catalyst using Isotope Labeled Nitrogen Species

To understand nitrate reduction pathway and to improve selectivity towards dinitrogen (N₂) over toxic ammonia species (NH₄⁺, NH₃), aqueous reduction experiments with an Al₂O₃-supported Pd-In bimetallic catalyst were conducted by using isotope-labeled nitrite (¹⁵NO₂^-). Nitrite is the first reduction intermediate of nitrate. Experiments were performed using nitrite alone and in combination with unlabeled proposed reduction intermediates (N₂O, NO), and using only N₂O and NO alone, each as a starting reactant. Use of ¹⁵N-labeled species eliminates interference from ambient N₂ when assessing mass balances and product distributions. Simultaneous catalytic reduction of ¹⁵NO₂^- and ¹⁴N₂O shows no isotope mixing in the final N₂ product, demonstrating that N₂O does not react with other NO₂^- reduction intermediates; N₂O reduction alone yielded only N₂. In contrast, simultaneous catalytic reduction of ¹⁵NO₂^- and ¹⁴NO yielded mixed-labeled ^15/14N₂ (MW: 29), whereas reduction of ¹⁵NO alone yields a mixture N₂ and NH₄⁺, the ratio of which varies with initial ¹⁵NO concentration. These findings, along with those from a new kinetic model we propose, indicate that highly reactive adsorbed NO (NO*), or other unspecified adsorbed N species (N_ads), is a key intermediate involved in determining final product selectivity.