Hydrogenation of aqueous nitrate and nitrite with ruthenium catalysts

[Display omitted] •Supported ruthenium materials catalyze reduction of aqueous nitrate and nitrite.•Thermal pretreatment activates Ru by removing surface residues and exposing labile Ru oxides.•Nitrate reduced selectively to ammonium, but nitrite reduced to a mixture of N2 and ammonium.•Density Functional Theory calculations support proposed surface reaction mechanism. Historically, development of catalysts for treatment of nitrate-contaminated water has focused on supported Pd-based catalysts, but high costs of the Pd present a barrier to commercialization. As part of an effort to develop lower cost hydrogenation catalysts for water treatment applications, we investigated catalysts incorporating Ru with lower cost. Pseudo-first-order rate constants and turnover frequencies were determined for carbon- and alumina-supported Ru and demonstrated Ru’s high activity for hydrogenation of nitrate at ambient temperature and H2 pressure. Ex situ gas pretreatment of the catalysts was found to enhance nitrate reduction activity by removing catalyst surface contaminants and exposing highly reducible surface Ru oxides. Ru reduces nitrate selectively to ammonium, and no aqueous nitrite intermediate is observed during reactions. In contrast, reactions initiated with nitrite yield a mixture of two endproducts, with selectivity shifting from ammonium towards N2 at increasing initial aqueous nitrite concentrations. Experimental observation and Density Functional Theory calculations together support a reaction mechanism wherein sequential hydrogenation of nitrate to nitrite and NO is followed by parallel pathways involving the adsorbed NO: (1) sequential hydrogenation to ammonium, and (2) N–N coupling with aqueous nitrite followed by hydrogenation to the detected N2O intermediate and N2 endproduct. These findings open the door to development of alternative catalysts for purifying and recovering nutrients from nitrate-contaminated water sources, and insights into the controlling surface reaction mechanisms can guide rational design efforts aimed at increasing activity and tuning endproduct selectivity.