Reduction of N2 by Fe2+ via Homogeneous and Heterogeneous Reactions

Abstract

Because of their high kinetic barrier and thermodynamic favorability under moderately reducing atmospheric composition photochemical approaches appear to be ideally suited to the direct reduction of solvated nitrogen gas. Ferrous iron has been investigated as an electron donor, as it has a highly tunable redox character and is environmentally ubiquitous. Recent advances in mineralogy connected to the field of environmental remediation have led to the identification of an important class of rusts which have reductive potentials comparable to Fe(0). These materials possess redox couples that are, potentially, capable of overcoming the kinetic barrier to the production of NH(3). In this study, we attempted to produce ammonia from N(2) by oxidizing white rust both photochemically and in a dark reaction. All results indicated the reaction was inhibited by competing reactions; primarily the reduction of H(2)O to H(2). However, the dark reactions showed limited potential for reduction up to 1.4 mM. As a result, we turned to the question of closure temperature; the minimum temperature of rapid reaction based on a choice of reductant, which we demonstrate a model for its estimation. Due to the high thermodynamic energy of the N2-* intermediate, we conclude that aqueous photochemical reduction under the conditions studied here is an unlikely prebiotic source for reactive, i.e. reduced, nitrogen.

Abiotic ammonium formation in the presence of Ni-Fe metals and alloys and its implications for the Hadean nitrogen cycle

“…Magnetite and even goethite can also act as reductants for NO 2 - /NO 3 - , provided cations such as Fe 2+ , Cu 2+ , Fe 3+ , Al 3+ are present in the system [e.g., [ 53 ]] [ 56 , 79 ]. Equilibrium thermodynamic calculations, however, predict very little reactivity of magnetite, goethite, or ferrihydrite alone towards N 2 and anaerobic experiments in the presence of green rust at ambient T, p conditions corroborate these predictions [ 80 ].…”