## 2 Materials and Methods  
In the experiments, a pre-reduced iron ammonia synthesis catalyst was applied. Samples for the experiments were obtained by fusing magnetite with promoter oxides and then activating them according to the procedure described elsewhere [59; 60; 61; 79; 89]. Chemical composition of the catalyst samples was measured using the inductively coupled plasma method (ICP-OES, spectrometer Perkin Elmer, type Optima 5300 DV, Perkin Elmer, Woodbridge, Ontario, Canada). The catalyst studied contained, apart from metallic iron, the following promoters: 3.3 wt.% Al\({}_{2}\)O\({}_{3}\), 2.8 wt.% CaO, and 0.7 wt.% K\({}_{2}\)O.  
Studied processes (reduction of a catalyst passive layer, annealing the catalyst under hydrogen atmosphere, and nitriding with gaseous ammonia) were conducted using a differential tubular reactor, which was designed and built at our university [81; 90] and then certified by the external authorities). The software controlling this apparatus and recording of the measurement results is also our product, written in Turbo Pascal by the person who built the apparatus. The results recorded in this way were further calculated and visualized using MS Excel(tm) 2016 software. This apparatus enabled us to perform thermogravimetric measurements (accuracy of 5\(\cdot\)10\({}^{-5}\) g) as well as examine hydrogen concentration in a gas phase (using a katharometric analyzer; accuracy of 1 vol.%) [58]. Gas samples were taken from the immediate vicinity of the catalyst. Then, ammonia concentration in the gas phase was calculated making use of the mass balance of the reactor. Flow rates of gaseous reactants were regulated by electronic mass flow controllers. A catalyst sample of ca. 1 g (including 0.840 g Fe) was placed in the form of a single layer of grains (grain size in the range of 1.0-1.2 mm) in a glass basket connected with the arm of a balance. Conditions for the chemical processes conducted in the kinetic region were ensured.  
Reduction of a passive layer of the catalyst was carried out at 500 \({}^{\circ}\)C (the rate of temperature increase was 10 \({}^{\circ}\)C min\({}^{-1}\)) with a flow of hydrogen 0.009 mol min\({}^{-1}\) up to the stabilization of the catalyst mass.  
Then, the reduced and activated catalyst sample was nitridted at 350 \({}^{\circ}\)C with gaseous ammonia (0.009 mol min\({}^{-1}\); reactor inlet: 100% of ammonia). After this process, the nitridted sample was reduced at the same temperature. After that, the specific surface area was determined making use of a volumetric method (by means of the Brunnauer-Emmet-Teller (BET) equation; apparatus AutoChem II 2920, Micromeritrics, Norcross, GA, USA).  
Additionally, the applied reactor was equipped with a system enabling the in situ measurements of magnetic permeability of the solid samples. Details of the measuring system were presented elsewhere [90].  
Samples of identical iron catalyst were also examined using transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray diffraction (XRD) methods. Examples of representative results for samples of the catalyst in the present work are given, e.g., in the works [81; 85; 88; 89].