Article pubs.acs.org/JPCC
Oscillatory Kinetics in the Process of Reduction of Nanocrystalline Iron Nitride γ′-Fe4N Katarzyna Skulmowska, Rafał Pelka,* and Walerian Arabczyk West Pomeranian University of Technology, Szczecin, Institute of Chemical and Environment Engineering, 10 Pułaskiego Street, 70-322 Szczecin, Poland ABSTRACT: The kinetics of the reduction of nanocrystalline γ′-iron nitride to α-iron in mixtures of nitrogen and hydrogen of different hydrogen concentration was studied. Kinetic measurements were carried out in a differential chemical reactor with a thermogravimetric system for measuring the weight of a sample and an analyzer of hydrogen at 400 °C and at atmospheric pressure. The occurrence of cyclic changes in the rate of chemical reaction in a solid phase has been observed. Such an oscillatory phenomenon was not observed so far in the reaction between gases and a solid with phase transition in a solid phase, which results in a solid products. The mechanism of the observed phenomenon is explained, taking into account processes occurring in the volume of the solid phase. The oscillatory rate changes of the described processes prove a nonmonotonic coverage degree of the surface with nitrogen, which is caused by a step change in the free enthalpy of segregation of nitrogen in the reduction of γ′-Fe4N nitride. On the basis of model calculations the changes of free enthalpy of segregation during reduction of nitrides have been specified, and modification of the Fowler−Guggenheim equation has been proposed.
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INTRODUCTION The occurrence of oscillatory phenomena in the systems with chemical reaction has been observed since the first half of the 19th century.1−4 The first reports on the oscillatory kinetics of heterogeneous catalytic reactions appeared in the 1970s.5 The oxidation reaction of CO with oxygen is one of the most intensively studied processes involving heterogeneous catalysts.6−9 Ertl,5,10−12 when researching surface reactions, observed oscillations in the system of CO + O2/Pt and NO + CO/Pt. He concluded that the oscillatory state is caused by periodic change of coverage degree, viz. rhythmic blocking and unblocking the catalyst surface, depending on the high or low rate of formation of CO2. Oscillatory states were also observed in the oxidation of propane over nickel foil,13−15 methane oxidation over nickel,16 hydrogen combustion at low pressures,17 and N2O decomposition over Cu-ZSM5 catalyst.18,19 The occurrence of these phenomena was associated with surface processes or with a change in the chemistry or morphology of near-surface layers of the materials studied (a few nanometers in depth). Recently, Saraev et al.20 have explained that the oscillations observed during the methane oxidation process over nickel foil are due to the reversible bulk oxidation of metallic nickel to nickel oxide. However, the above-mentioned systems are connected to surface reactions from a gas phase, and a cyclic activation and deactivation of catalysts’ surface takes place. On the basis of the study of the kinetics of nitriding of an iron catalyst for ammonia synthesis, KM1R, containing nanocrystalline iron, with ammonia,21−28 it has been found that the rate of this reaction is limited by the rate of disassociate © 2017 American Chemical Society
ammonia adsorption on the surface of iron. This process is a limiting step in the nitriding process until the critical concentration of nitrogen in a volume of iron nanocrystallite is reached. Because of the small size of nanocrystallites, diffusion processes are faster than the other stages of the nitriding process. Therefore, in the volume of nanocrystallite there is no concentration gradient. Consequently, each nanocrystallite undergoes a phase transition of α-iron to γ′-nitride in its entire volume. Phase change itself as well as the following processes are relatively fast compared with the stage where the critical nitrogen concentration in iron nanocrystallite is reached, and they do not limit the overall rate of the nitriding process. As a result of analysis of the obtained results, a reaction model in the adsorption range has been developed.29 In this model, at the nitriding potential, P = pNH3/pH23/2, far greater than the minimum nitriding potential, which is necessary to start the phase transition of α-iron to γ′-nitride, the phase transition of nanocrystallites occurs in order from the smallest to the largest ones. It has been, therefore, concluded30 that the rate of the nitridation reaction of a nanocrystalline iron with ammonia is also a function of nanocrystallite size distribution in the sample by the active surface area. As a result of the works carried out on the chemical equilibrium in the system of iron/ammonia/ hydrogen, there was developed dependence of the phase composition on the nitriding potential of ammonia−hydrogen Received: May 9, 2017 Revised: June 13, 2017 Published: June 14, 2017 14712
DOI: 10.1021/acs.jpcc.7b04418 J. Phys. Chem. C 2017, 121, 14712−14716
Article
The Journal of Physical Chemistry C gas mixtures and temperature−Lehrer diagram.31−33 In the nitriding process of nanocrystalline industrial iron catalyst, KM1R, and reduction of the obtained nanocrystalline iron nitrides at constant nitriding potentials and at constant temperature, there the occurrence of the steady states was observed.34−39 At steady state, chemical equilibrium is established between the gas-phase nitriding potential and a solid phase. Under these conditions, two phases were observed, α + γ′ or γ′ + ε, in the nitriding process,38 and in the reduction of nanocrystalline iron nitride, three solid phases were found, α-Fe(N) + γ′-Fe4N + ε-Fe3−2N, within wide ranges of nitriding potential.36,37 This phenomenon cannot be explained on the basis of the Lehrer diagram. In the system under study we found the phenomenon of hysteresis for the dependence of nitriding degree on nitriding potential in the process of nitriding and reducing of nitrides.34−37 It has been found37,39 that along with an increase in nitriding potential at stationary states smaller and smaller iron nanocrystallites undergo the phase transition to nitride; namely, in thermodynamic studies nanocrystallites undergo a phase transition in the reverse order compared with the kinetic studies. In this study the kinetics of the reduction of nanocrystalline iron nitrides in a mixture of hydrogen−nitrogen, viz. reaction in a solid phase, was investigated, and the occurrence of cyclical changes in the rate of a chemical reaction has been observed. For the first time, the presence of oscillatory phenomena in chemical reaction in the solid phase was linked with the changes taking place in a volume of solid phase. Additionally, a modification of the Fowler−Guggenheim equation has been proposed.
Investigation of the nanocrystalline iron nitrides reduction was carried out isothermally at 400 °C at atmospheric pressure and the different composition of the reducing mixture. Hydrogen partial pressure at the reactor inlet was from 1.00 to 0.09 bar. To verify the reproducibility of the obtained results, five-fold studies of the series of nanocrystalline iron nitriding in the atmosphere of ammonia and reduction of iron nitrides in the atmosphere of hydrogen were performed (Figure 1).
Figure 1. Changes in the nitrogen concentration in the sample and the partial pressure of hydrogen at the reactor outlet during the nitriding process of the nanocrystalline iron with ammonia and reduction of nanocrystalline iron nitride with hydrogen at 400 °C.
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RESULTS AND DISCUSSION Figure 2 shows the dependence of nitrogen concentration within the sample, XN, and partial pressure of hydrogen, pH2, on
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EXPERIMENTAL SECTION In the study the industrial prereduced iron ammonia synthesis catalyst, KM1R (nanocrystalline iron promoted with hardly reducible oxide of aluminum, calcium, and potassium), was applied. The average size of iron nanocrystallites, as determined by X-ray diffraction (XRD) method, was 45 nm. The test catalyst consists of a set of nanocrystallites described by their size distribution, GSD (grain size distribution).40,41 Specific surface area of the catalyst, as determined by thermal desorption, was 12 m2/g. The surface is partially wetted by oxide of promoters; thereby, active surface area is smaller than the specific surface area, and its value depends on the temperature.42,43 Studies have been carried out in the differential reactor equipped with means for thermogravimetric measuring of the weight of a sample and a hydrogen analyzer. A sample of catalyst was placed as a single layer of grains (with diameter 1.0 to 1.2 mm) in a platinum basket of the reactor suspended on a shoulder of thermobalance. The flow rate of gases (ammonia, hydrogen, and nitrogen) was determined using automatic electronic controllers. Gas samples were taken for analysis in the immediate vicinity of the sample. Weight change of the sample, temperature, and partial pressure of hydrogen at the outlet of the reactor were recorded digitally. Before the start of the nitriding process, passivated catalyst was reduced with hydrogen at 500 °C. Nanocrystalline iron was nitrided with ammonia (12 dm3/(h g)) at 400 °C at atmospheric pressure. Iron ε iron nitrides were obtained. The samples were characterized by XRD. After reaching a steady state in the nitriding process, nanocrystalline iron nitride reduction with hydrogen (H2 + N2) was carried out (phase transition of γ′ iron nitride to α-iron).
Figure 2. Dependence of nitrogen concentration in the sample and partial pressure of hydrogen on time during the reduction process of nanocrystalline iron nitrides using different partial pressures of hydrogen in the mixture of H2/N2 at the inlet to the reactor (temperature 400 °C). Vertical line indicates an area of constant partial pressure of hydrogen.
time (TG curves) during the reduction process of nanocrystalline iron nitrides using different partial pressures of hydrogen in the mixture of H2/N2. Nitrogen concentration in the sample (XN) is a sum of nitrogen concentration in the volume of nanocrystallite, Xb, and the concentration of nitrogen adsorbed on the nanocrystallite’s surface, Xs. On the basis of the TG curves, reaction rates of the reduction processes were determined. One exemplary result of a dependence 14713
DOI: 10.1021/acs.jpcc.7b04418 J. Phys. Chem. C 2017, 121, 14712−14716
Article
The Journal of Physical Chemistry C
reduction process rate of the nanocrystalline γ′ nitride each time undergoes the same oscillatory changes. Vertical solid lines indicate the concentration of nitrogen in the sample at which the nitriding process takes place at a maximum rate in each oscillatory cycle. To explain the observed phenomenon a numerical modeling of the reduction process rate of γ′-Fe4N, r, was carried out. The measured reaction rate is the sum of the reaction rates, ri, specified for the individual ith nanocrystallites in the sample
r=
∑ ri
(2)
i
Considering the single nanocrystallite, the slowest step in the process is a process of growth in the number of defects in iron sublattice due to lowering the nitrogen concentrations. The time required for a recrystallization of single nanocrystallite depends on the time in which in a volume of nanocrystallite, the critical concentration of nitrogen in the reduction process, Xbcri, is reached. The critical concentration of nitrogen in iron nanocrystallite necessary to start the phase transition is dependent on the active surface of nanocrystallite, Sact,i, [m2/m3], which is the ratio of the surface area of nanocrystallite on which the surface reaction takes place to its volume and is reached faster in small nanocrystallites than in large ones.44 Equation 1 for single ith nanocrystallite takes the form
Figure 3. Dependence of the reduction process rate of the nanocrystalline iron nitride, γ′-Fe4N, under an atmosphere of nitrogen−hydrogen mixture on time.
of the reduction process rate of nanocrystalline iron nitride on time (DTG curve), for pH2 = 0.29 bar, is presented in Figure 3. Reduction of γ′-Fe4N nitride by gas mixture of the hydrogen partial pressure in nitrogen−hydrogen mixture
