Three sets of experiments were conducted at three different temperatures: 200°C (runs 1–36), 70°C (runs 37–59) and 22°C (runs 60–83). The choice of reactors was based on experimental temperature: 15 mL passivated HIP^® Titanium 64 tube reactors (200°C); 20 mL I-Chem^® borosilicate vials with PTFE/Si septum caps (70°C) and 15 mL BD HDPE Falcon^® tubes (22°C). Reactors were kept at constant temperature in a heated water bath (20, 70°C) or an Isotemp^® oven (200°C). All experiments lasted 24 hours and were conducted in the absence of headspace (e.g., no gas phase). No additional pressure other than that of expanding liquid was imposed on the vessels (~400 psi/27 bars with Ni⁰ to ~800 psi/55 bars with Fe⁰ at 200°C).

All reacting solutions were prepared from freshly drawn UV/UF deionized water (henceforth DI) either by purging with a UHP gas of interest (e.g., Ar, N₂, H₂/N₂) and/or by dissolving appropriate amounts of reagents. NH₄⁺ content of all unreacted reagent solutions (e.g., blanks) was below the detection limit of ion chromatography (<0.1 μmol.kg^-1, reported by the manufacturer). DI purged with N₂ gas under ambient conditions results in equilibrium N₂(aq) concentration of 0.59 mmol.kg^-1. The effect of O₂ was, however, also determined (runs 39, 41, 45, 47, 50, 52, 56, 58, 62, 64, 68, 70, 74, 76, 80, 82) in experiments with DI equilibrated with present-day atmosphere (e.g., no N₂ purging).

To ensure clean and fresh mineral surfaces (e.g., free of oxidation products and/or atmospheric sorbed gases), all metal/alloys were ultrasonically cleaned for 1 hour in 0.06 M HCl immediately preceding the experiments. Subsequently they were washed three times with the designated reacting solution and loaded into reactors in the form of slurry. This "wet loading" procedure eliminated sorption of gases from the atmosphere onto freshly cleaned metal surfaces which was especially important in blank experiments.

Background NH₄⁺ production (e.g., release from reactants, reaction vessels, etc.) was assessed in blank experiments with Ar-saturated solutions and no added N source (runs 1, 9, 17, 25, 37, 43, 49, 54, 60, 66, 72, 78). NO₃^- and NO₂^- solutions were prepared by dissolution of ACS reagent grade KNO₃ and KNO₂, respectively. The pH was not buffered and was allowed to change as a result of solution-metal/alloy interactions and was recorded before and after the experiment. After pH measurements, all samples were acidified with 0.2 M HCl to ensure the conversion of NH₃ to NH₄⁺ and to prevent the formation of Fe precipitates. The samples were stored at 1°C and analyzed within 24 – 48 hours. The summary of all experimental conditions is presented in Tab. 1.

Metals and alloys representing an fcc solid solution of Ni in Fe were purchased from Alfa Aesar^® and Goodfellow. All starting and selected reacted solids were characterized by X-Ray Diffractometry, X-Ray Photoelectron Spectroscopy, Scanning Electron Microscopy, B.E.T. surface analysis, and Electron Microprobe. The results of metal/alloy characterization are summarized in Tab. 2 and Fig. 2, 3.

X-Ray Diffraction (XRD) data were collected using a Scintag PAD X diffractometer under the following conditions: CuK_α1, 40 kV, 25 mA, 5° – 90° 2Θ, step 0.02° 2Θ and variable scan rates.

Scanning Electron Analysis (SEM) was performed on the LEO 1550 SFEG scanning electron microscope equipped with an EDAX energy dispersive X-ray spectrometer (EDS) using an accelerating voltage of 15 kV and a 30 μm aperture.

Oxidation state of Ni, Fe and the presence of N in alloys were determined by X-Ray Photoelectron Spectroscopy (XPS). The data were acquired with unmonochromatized Mg_Kα and Al_Kα radiation at 1253.6 eV and 1486.7 eV using a Physical Electronics source controller in a vacuum chamber with a base pressure of 1 × 10^-9 Torr. A VG Microtech hemispherical analyzer was used to obtain the energy distribution of the photoemitted electrons at pass energies of 50 and 75 eV. The binding energies were calibrated by fixing the Au 4p3/2 and 4f7/2 peaks (546.3 eV, 87.5 eV) from a gold standard, and the metallic Fe 2p3/2 and Ni 2p3/2 cores (707.0 eV, 852.3 eV). Selected particles were sputtered with Ar⁺ accelerated to 2 kV to expose their interior and check for the presence of nitrogen using a Physical Electronics ion gun controller.

B.E.T. surface analysis (BET) was performed using a Micromeritics ASAP 2010 analyzer with a 10-mm Hg transducer using UHP N₂ gas. The surface area was calculated from measurements at 5 different N₂(g) pressures (42.58196, 85.30770, 132.82384, 180.25029 and 227.66101 Torr).

Chemical composition of alloys/metals was determined by a Cameca Camebax Micro electron microprobe (EPMA) equipped with four wavelength dispersive spectrometers and a Kevex Analyst 8000 energy dispersive detector. During all analyses, the accelerating voltage and beam current used were 15 kV and 10 nA (nominal), respectively.

Molecular hydrogen was analyzed on a SRI 8610C single column gas chromatograph (GC) with a TCD detector, 6' Hayesep D column and N₂ carrier gas. The sample (~0.2 mL) was withdrawn from the reaction vessel into a gastight^® Hamilton™ syringe with a Mininert™ valve and immediately analyzed using a 4-point calibration curve. Gas mixtures (Matheson™) of known composition were used as calibration standards.

Total nitrogen content of metals/alloys was analyzed by IMR Test Labs (Lansing, NY) by inert gas fusion 35. During the analysis, N is released from the metal at 1900°C into the stream of He gas and analyzed in a thermal conductivity cell.

Experimental solutions were analyzed for NH₄⁺ using a Dionex DX-500 ion chromatograph (IC) with a 100 μL sample loop. NH₄⁺ and NO₃^-/NO₂^- were analyzed using a 4 mm Dionex IonPac^® CS-16 (40°C, 34 mN H₂SO₄ eluent) and IonPac^® AS4A-SC (22°C, 5 mmol.kg^-1 Na₂B₄O₇ eluent), respectively. Concentrations were calculated from a 4-point calibration curve with R² values above 0.99.

No other compounds were analyzed. It is expected, however, that other reaction products and/or intermediates (e.g., NO) may have formed during a complex sequence of electron transfer reactions.

Geochemical equilibrium modeling was performed with the Geochemist's Workbench 5 36</