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

Abstract To understand nitrate reduction pathway and to improve selectivity towards dinitrogen (N 2 ) over toxic ammonia species (NH 4 + , NH 3 ), aqueous reduction experiments with an Al 2 O 3 ‐supported Pd‐In bimetallic catalyst were conducted by using isotope‐labeled nitrite ( 15 NO 2 − ). Nitrite is the first reduction intermediate of nitrate. Experiments were performed using nitrite alone and in combination with unlabeled proposed reduction intermediates (N 2 O, NO), and using only N 2 O and NO alone, each as a starting reactant. Use of 15 N‐labeled species eliminates interference from ambient N 2 when assessing mass balances and product distributions. Simultaneous catalytic reduction of 15 NO 2 − and 14 N 2 O shows no isotope mixing in the final N 2 product, demonstrating that N 2 O does not react with other NO 2 − reduction intermediates; N 2 O reduction alone yielded only N 2 . In contrast, simultaneous catalytic reduction of 15 NO 2 − and 14 NO yielded mixed‐labeled 15/14 N 2 (MW: 29), whereas reduction of 15 NO alone yields a mixture N 2 and NH 4 + , the ratio of which varies with initial 15 NO 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.