Catalytic Ammonia Decomposition during Nanocrystalline Iron

Article pubs.acs.org/JPCC

Catalytic Ammonia Decomposition during Nanocrystalline Iron Nitriding at 475 °C with NH3/H2 Mixtures of Different Nitriding Potentials Rafał Pelka,* Karolina Kiełbasa, and Walerian Arabczyk West Pomeranian University of Technology, Szczecin, Institute of Chemical and Environment Engineering, 10 Pułaskiego Str., 70-322 Szczecin, Poland ABSTRACT: Ammonia is now regarded as an effective source of hydrogen containing no CO. In this paper, we have studied the nitriding of nanocrystalline iron accompanied by catalytic ammonia decomposition reaction. The chemical processes were carried out in a tubular differential flow reactor at 475 °C. NH3/H2 gas mixtures of different compositions viz. different nitriding potentials were let into the reactor. At each nitriding potential the stationary states were observed, when mass of solid and rate of ammonia decomposition were constant. In the whole range of nitriding potentials three regions of different phase composition of solid were observed: α-iron, mixture of α-iron and Fe4N nitride, and Fe4N nitride. It occurred surprisingly that along with increase in nitriding potential the rate of ammonia decomposition decreased when Fe4N nitride phase was present. A model of the process in question was proposed to explain that phenomenon. The decrease in decomposition reaction rate, related with decrease in iron surface coverage degree, resulted from changes in segregation enthalpy of nitrogen. The model allowed the determination of such parameters as bulk concentration of nitrogen, enthalpy of segregation, and surface coverage degree.

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INTRODUCTION Nowadays, due to the concerns about still increasing environmental pollution, hydrogen fuel cells are attracting more and more attention as a new energy source.1,2 The difficulty in storage and transportation of pure hydrogen makes the most preferred technical solution to obtain it in place from hydrocarbons or ammonia. Reforming of hydrocarbons is now commonly used for the production of hydrogen. Hydrogen thus obtained must be purified from CO, since this compound poisons catalysts used in fuel cells.3−5 Hydrogen originating from ammonia decomposition does not contain COx species. The relatively easy possibility of condensation of the ammonia is also very important, making it easy to transport, as compared to hydrogen. Studies of both synthesis and decomposition of ammonia were conducted for the last ca. 100 years and an abundant amount of research works on this subject was elaborated, including comprehensive studies.6−18 As a result, several models of catalytic ammonia decomposition were proposed. Presence of two solid phases coexisting at a given nitriding potential when dealing with nanoiron, in contrary to Lehrer diagram, also was reported. One has to keep in mind that size distribution of nanocrystallites strongly affects chemical processes (e.g., ammonia synthesis or decomposition) on solid surface as well.13,19,20 However, the commonly known models did not fully explain the observed21 dependence of the decomposition reaction rate on the nitriding potential of the © 2014 American Chemical Society

gas phase during nitriding of nanocrystalline iron. Namely, the rate of decomposition decreased over the nitride phase and a character of that dependence in nonlinear. An empirical equation that describes just such a relationship was proposed.13 However, in the previous papers13,21 only phenomenological conclusions on ammonia decomposition reaction kinetics were stated. Change in reaction rates on iron and iron nitride depending on nitriding potential was observed and finally new empirical equations were proposed. In the present paper much more detailed investigations were carried out together with modeling of the observed processes. On the other hand, investigations on metal nitrides formation (reaction parallel to catalytic ammonia decomposition during nitriding process) and stability are still of great interest,22 despite abundant research material already gathered. Adsorption of at least one gaseous substrate is the initial stage of an appropriate reaction on the surface of the solid phase. For heterogeneous catalytic reactions involving two substrates, mechanisms by Langmuir−Hinshelwood and Eley− Rideal can be applied. According to the first of them, adsorption of two substrates precedes the appropriate reaction between them, and its rate is proportional to the product of the surface concentrations of substrates. According to the second Received: August 31, 2013 Revised: February 26, 2014 Published: February 26, 2014 6178

dx.doi.org/10.1021/jp4087283 | J. Phys. Chem. C 2014, 118, 6178−6185

The Journal of Physical Chemistry C

Article

rate is limited by the rate of adsorption of gaseous reactant upon the catalyst surface. The rate of chemical reactions occurring during the nitriding process of iron catalyst was measured in a tubular reactor (thermo-balance with the measuring range ±100 mg; resolution 0.1 mg; sample weight 1 g) making it possible both to thermogravimetrically measure the nitrogen content in the nitrided sample and to analyze the composition of the gas phase.21 Hydrogen concentration was measured. The gas flow rate at the inlet to the reactor was controlled by means of electronic flow-meters. Gas samples for analysis were collected in the direct vicinity of the catalyst and the hydrogen concentration was determined. It was found that the composition of the gas mixture over and below the catalyst bed is the same. Due to the lack of the gradient of gaseous reactants concentrations in the reaction zone of the reactor and the arrangement of the sample of catalyst the reactor can be considered as a differential one. The nitriding process was preceded by reduction of a passive layer of catalyst and annealing of samples. The catalyst was reduced with hydrogen (9 dm3 h−1 g−1) polythermally to a temperature 500 °C, at atmospheric pressure. After reaching a constant weight of the solid sample, it was heated at 500 °C under hydrogen flow for ca. 3 h. Then the temperature was changed to 475 °C and nitriding process was performed. The nitriding process was carried out at a temperature of 475 °C. The reactor was fed with mixtures containing different amounts of ammonia in relation to hydrogen (range 0−100% of ammonia) at atmospheric pressure. Gas mixture load was kept constant at 12 dm3 h−1 g−1. After each change of gas mixture concentration, the nitriding process was carried out until a steady state was achieved (mass of solid sample and hydrogen concentration were constant).Stable hydrogen concentration meant that catalytic reaction of ammonia decomposition occurred at a constant rate. Nitriding degree, α, was defined as the ratio of the number of moles of nitrogen attached to iron in a given moment to the number of moles of iron present in the solid sample. Ammonia conversion degree, αNH3, was calculated after measuring the hydrogen concentration at the inlet of the reactor according to the relationship

mechanism, only one of the reacting substances is absorbed and the creation of active complex involves molecules of another substrate that are coming to the surface of the catalyst from the gas or liquid phase. Under such conditions, the reaction rate is proportional to the product of the surface concentration of adsorbed substrate and concentration or pressure of another substrate in the bulk (gas or liquid) phase. Dependence of the amount of adsorbed substance on the adsorbate pressure at a given temperature can be described by adsorption isotherms. In the area of catalysis and nanomaterials the isotherm proposed by Langmuir,23 Freundlich,24 Temkin,25 or Marczewski and Jaroniec26 can be used.27 The aim of this study is to explain the unexpected phenomenon observed during the nitriding process, namely the reaction rate of ammonia decomposition decreased while nitriding potential of a gas phase was increasing. Furthermore, the dependence of the reaction rate on natural logarithm of P is nonlinear. In order to clarify the problem in question a modeling of the catalytic decomposition of ammonia on the surface of promoted nanocrystalline iron was performed. Combination of heterogeneous reaction model by Langmuir− Hinshelwood with adsorption isotherm by Langmuir was taken into consideration.

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EXPERIMENTAL SECTION Industrial prereduced iron ammonia synthesis catalyst, KM1R (Haldor Topsoe, Lyngby, Denmark), was used in this work. It is a fused catalyst obtained by fusing magnetite and promoters. Metallic iron content (as determined by manganometry) was 84 wt %. Analysis by Inductively Coupled Plasma (ICP-AES) enabled the determination of catalyst chemical composition (in weighed fractions: 3.1Al2O3, 2.9CaO, 0.64K2O, 0.66SiO2, and 1.0 of other components). Oxygen in iron oxides was 6.0 wt %. Apart from that, the samples contained ca. 1.7 wt % of volatile substances (e.g., water) removed during drying in nitrogen. According to commonly known models of iron catalyst,8,9,27 textural promoters (Al2O3, CaO) aid in the maintenance of stable structure of small (ca. 20 nm) iron nanocrystallites. The porosity of such structure is ε = 0.5. The porous structure of the catalyst can be proved by microscopic observations (scanning (SEM) or transmission electron microscopy (TEM)). K2O, as a chemical promoter, increases catalytic activity. Commercial iron catalysts are supplied in oxidized or prereduced form. In our case the prereduced form was applied; thus, conditions of reduction process given below are sufficient to preform complete reduction of iron oxides. Details on the preparation, activation and morphology are available in comprehensive review by Schlögl.9 Mean size of the iron nanocrystallites (dm = 20 nm) was determined by X-ray diffraction measurements (X-ray diffractometer Philips X’Pert PRO, Cu Kα, Almelo, The Netherlands) using the Scherrer method and the one developed by Rietveld. Specific surface area of catalyst (S = 12 m2 g−1) was determined using BET method (QuadraSorb SI apparatus, Quantachrome Instruments, Boynton Beach, USA). Iron nanocrystallites size distribution was determined by measuring the nitriding reaction rate of the catalyst as described in the work.28 Fraction 1.0−1.2 mm of the catalyst particles was selected. A sample of 1 g was placed in a form of single layer of grains in a platinum basket, suspended on the arm of thermo-balance. Based on earlier studies,19 it was found that the nitriding process was carried out in the area where the chemical reaction

αNH3 =

XH2 1, 5 − XH2

(1)

where XH2 is the molar concentration of hydrogen in the reactor, mol mol−1. Molar concentrations of ammonia, XNH3, in the stationary states were calculated making use of results of measurements of hydrogen concentrations in a gaseous reaction mixture using the mass balance of the reactor XNH3 =

1 − αNH3 1 + αNH3

(2)

The concentration of nitrogen in a gas phase was calculated when concentrations of ammonia and hydrogen were known. The rate of catalytic decomposition of ammonia, r, was determined from the following equation: r=

0 αNH3FNH3 nFe

(3)

where nFe is the number of moles of the iron contained in the catalyst sample. 6179



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Further considerations were performed for the results of measurements corresponding to the stationary states (samples’ mass reached a constant value, the reaction of the catalytic decomposition of ammonia occurred at a constant rate) setting up after each change in ammonia concentration in hydrogen, i.e., at specific nitriding potentials (Figure 3b). Thermogravimetric measurement results and values of the rate of catalytic decomposition of ammonia determined on the basis of eq 3 and corresponding to each nitriding potential are shown in Figure 4. In the range P = 4.5 × 10−5 to 5.5 × 10−4 Pa−0.5 catalyst weight gains were observed within 2.5 − 3.0 mg. It is related to the adsorption of ammonia on the catalyst surface and saturation of α-Fe phase with nitrogen. This step is described in more detail in the work.12 Then, in the range up to P = 3.0 × 10−3 Pa−0.5 iron nitriding reaction occurs and the mass of the solid increases with increasing concentration of ammonia in hydrogen. For P > 3.0 × 10−3 Pa−0.5, despite the relatively large change in the nitriding potential, under the measurement conditions (measuring range ±100 mg, resolution 0.1 mg, sample weight of 1 g) change in weight of the catalyst was not observed. Based on the results of XRD studies it was concluded that, in the range P = 4.5 × 10−5 to 2.0 × 10−2 Pa−0.5, there are three areas with different phase composition of the catalyst. In case of the smallest value of P, the catalytically active component is a α-Fe(N) solution. After initiation of the nitriding, with average values of P, the solid is a mixture of αiron and γ′-iron nitride nanocrystallites. After fixing the weight at the maximum value, when the iron present in the sample had reacted to the γ′-nitride, catalytic ammonia decomposition occurs only on the phase γ′-FexN; x ≅ 4. From the above figure, it can also be concluded that the dependence of the rate of catalytic decomposition onto the catalyst surface on nitriding potential cannot be described by the Temkin-Pyzhev equation. Therefore, a new model, explained below, describing the observed phenomena is proposed. Further interpretation of the obtained results becomes easier when changes in the rate of ammonia decomposition reaction are presented as a function of ln P (Figure 5). These rates can then be described by the general equation r = A ln P + B (A and B are constants; A > 0 or A < 0). Based on the figure above also empirical equations describing the surface reaction rate: rFe = 9.0 × 10−6 ln P + 1.0 × 10−4 (line no. 1 ammonia decomposition on α-iron) and rFexN = −7.0 × 10−6 ln P + 2.0 × 10−5 (line no. 2 ammonia decomposition on γ′-nitride) have been proposed.

Changes in the partial pressures caused changes in the nitriding potential, P, defined as follows: p P = NH3 3/2 pH2 (4) where pNH3 is the partial pressure of ammonia and pH2 is the partial pressure of hydrogen. Nitriding process and reduction of the obtained nitrides were carried out many times using the same sample of the catalyst. Analysis of the structure and phase composition of the catalyst were performed by XRD (Figure 1). Every time similar results were obtained, what implies the stability of catalyst structure during the investigated nitriding process.

Figure 1. XRD pattern of the catalyst sample after nitriding reaction at 475 °C. Various nitriding degrees resulting from different values of nitriding potential of a gas phase.

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RESULTS Exemplary results of the thermogravimetric measurements for the reduction of passive layer present in the catalyst are shown in Figure 2.

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DISCUSSION α-Iron Phase. Catalytic ammonia decomposition occurs at different rates depending both on the phase composition of the iron catalyst and the nitriding potential. For α-iron phase, the reaction rate of catalytic decomposition of ammonia increases with increasing ammonia concentration in the mixture NH3/ H2/N2. This reaction was carried out so that the rate-limiting step of the process was the rate of chemical reaction on iron surface. In the work,29 it was found that recombination of two nitrogen atoms on solid surface is the rate limiting step. Under the conditions of the experiment, hydrogen concentration on solid surface is negligible due to almost immediate desorption. Surface reaction rate depends on coverage degree of the active species. Modeling of the ammonia decomposition reaction was

Figure 2. Changes of sample’s mass and process temperature during reduction of passive layer present in the catalyst samples (hydrogen flow 9 dm3 h−1 g−1).

For every sample practically identical TG curves, as presented above, were obtained. After reaching a constant weight of the solid sample, it was annealed at 500 °C under hydrogen flow for ca. 3 h. Despite the presence of a reducing atmosphere, there was no additional mass change observed. Both the results of thermogravimetric measurements and the changes in gas phase chemical composition observed during iron catalyst nitriding process with ammonia are presented in Figure 3a. 6180



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Figure 3. (a) Direct experimental data: nitriding degree and concentration hydrogen during nitriding of iron catalyst with ammonia/hydrogen gas mixtures of different nitriding potentials at temperature 475 °C. (b) Values of the respective concentrations and nitriding degrees in the stationary states.

degree, θ; neighboring adsorbed molecules do not interact with each other) was analyzed θ = kL1P /(kL1P + kL2)

( 6)

where kL1 and kL2 are constants. The following combination was considered: rH = kHS(kL1P /(kL1P + kL2))2 = k*P 2/(kL1P + kL2)2 (7)

Summary of data for modeling together with the obtained coefficient of determination (R2) for fitting the model curve to experimental data (the method of least-squares) for the combination are listed in Table 1. Figure 4. Nitriding degree, α, and catalytic ammonia decomposition reaction rate, r, as a function of nitriding potential, P (475 °C).

Table 1. Values of the Constants Applied for Modeling and the Coefficient R2 kL1 97000

kL2 1

R2

kHS −5

7.2 × 10

0.9876

Comparison of the results of the modeling and experimental data are shown in Figure 6.

Figure 5. Nitriding degree, α, and catalytic ammonia decomposition reaction rate, r, as a function of ln P (475 °C).

performed taking into account the Langmuir−Hinshelwood model: rH = kHSθ 2

(5) Figure 6. Experimental and model results for the reaction rate of catalytic decomposition of ammonia on the nanocrystalline α-iron phase (475 °C).

where rH is the reaction rate, kH is the reaction rate constant, S is the active surface area, and θ is the coverage degree with nitrogen. When the rate of adsorption/desorption is high compared to the rate of surface reaction, these processes are in equilibrium, and the dependence of the surface coverage with adsorbate on pressure of this component in the gas phase describes the adsorption isotherm. It was assumed that the nitriding potential of NH3/N2/H2 mixture is an equivalent of nitrogen partial pressure. Adsorption isotherm by Langmuir (assuming that heat of adsorption does not change with increase in coverage

Area of the γ′ Nitride Phase Saturation. Surprisingly, for γ′-nitride phase, the reaction rate of catalytic decomposition of ammonia decreases with increasing ammonia concentration in the mixture NH3/H2/N2, according to rFexN = −7.0 × 10−6 ln P + 2.0 × 10−5 (line 2 in Figure 5). As the catalytic ammonia decomposition reaction rate is related to the degree of surface coverage of iron with chemisorbed nitrogen, a decrease in the 6181



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for the spherical crystallites of size distribution shown above. The maximum concentration of nitrogen in the iron is 0.001 mol mol−1, while in γ′ nitride 0.2 mol mol−1. Enthalpy of nitrogen segregation, ΔGseg, from iron was taken after30,31 as −90 kJ mol−1. Values of θ for iron corresponding to the measured nitriding potentials were determined from eq 6. Values of θ for a nitride were selected in such a way that the reaction rates calculated from eq 5 for both phases, taking into account the size distribution of nanocrystallites, correspond to the values of reaction rates measured during the experiment. Then, the enthalpy of nitrogen segregation from γ′ nitride was determined on the basis of McLean−Langmuir equation32 that involves the above-mentioned parameters:

reaction rate, despite an increase in the nitriding potential of the gas phase, may therefore indicate a reduction in the coverage degree of γ′ phase with nitrogen. The decrease in the reaction rate on a single phase with a minimal increase in volume concentration (saturation process of γ′ phase) indicates that ΔGseg strongly depends on the bulk concentration of nitrogen (see further in Figures 8 and 10). Thus, the decomposition rate within a single solid phase is influenced and regulated by changes in segregation enthalpy and bulk concentration of nitrogen. Changes in the structure of the solid is not so important then. Transition Region (α and γ′ Phase Mixture). After exceeding the minimum nitriding potential, P0, and after beginning of nitriding, i.e., on a mixture of α and γ′ phases, the rate of ammonia catalytic decomposition, contrary to expectations, decreases with increasing nitriding potential (starting from point “A” in Figure 5). Moreover, the ammonia decomposition reaction rate on a mixture of α + γ′ phases decreases nonlinearly with respect to ln P. The sections marked by the letters A and B in Figure 5 correspond to the experimentally measured values of ammonia decomposition reaction rate, which are higher compared to what would be observed in this area (calculated from the equations shown at the end of the Results section), if there would be only one phase present in the catalyst, namely γ′ nitride (line 2 in Figure 5). The reaction rate of catalytic ammonia decomposition on the catalyst surface is undoubtedly related to the degree of surface coverage of iron with chemisorbed nitrogen, as was mentioned above. A decrease in the reaction rate, despite an increase in the nitriding potential of the gas phase, results from a reduction in the coverage degree of γ′ phase with nitrogen. The decrease in the surface coverage degree of a newly formed crystallographic structure (γ′), while a significant increase in the concentration of nitrogen in the volume of the crystallites is involved, can be explained by a significant change in enthalpy of segregation, ΔGseg (eq 8). In order to explain the observed phenomenon in more detail, at first, measurements of the nitriding process rate of the catalyst were carried out. On the basis of these results the size distribution of nanocrystallites (GSD) was determined by the method described in the work.28 The results are shown in Figure 7. Such a shape of size distribution was also verified by XRD and TEM methods. This was followed by model calculations

Xb −ΔGseg(Xb)/ RT θ = e 1−θ 1 − Xb

(8)

where Xb is the bulk concentration of nitrogen in a volume of nanocrystallite, R is the gas constant, and T is the temperature. The modeling problem in question involves an influence of the particles’ size on the phase transition process. Such an influence for iron-containing systems (the change in the thermodynamic parameters of the phase transition of α-Fe to γFe along with the change in particle size) was already studied.33 The observed phenomena were a result of the Gibbs− Thomson effect.34 This effect can not be neglected in the case of the crystallites with nanometric sizes. The theoretical findings on the Gibbs−Thomson effect led to the conclusion that the critical concentration of one component required to initiate the phase transition in binary systems depends on the size of the particles. Considering the system α-Fe/γ′-Fe4N, the nitrogen concentration in the iron crystallites must be high enough for the phase transition to take place. It was found and confirmed in great detail33 that the phase transitions will occur at different concentrations of nitrogen for the particles of different sizes. On the other hand, the concentration of nitrogen in iron during the nitriding process depends on the gas phase composition. It also was observed33 that in the new stationary states, setting up with increasing conversion degree and nitriding potentials (and thus increasing Xb), smaller and smaller crystallites of iron undergo the phase transition. Therefore, in this work it also is assumed that the phase transition of crystallites with larger diameter takes place at a lower nitriding potential. Finally, the validity of this and any other model assumptions is confirmed when the good compatibility between experimental and model data was obtained (see Figure 11). The calculations were performed for each crystallite separately. The exemplary results of the calculations for two individual crystallites with diameters of 25 and 45 nm are shown in Figure 8. After reaching the value of P at which the critical nitrogen concentration in a given crystallite is achieved a phase transformation of this crystallite occurs. The smaller crystallite, the greater “bulk” concentration, Xb, must occur for the phase transition to be observed (Gibbs−Thompson effect). This is accompanied by a stepwise change in value of segregation enthalpy. As a result, the weight of the crystallite increases abruptly due to dissolution of nitrogen in the newly formed structure of γ′ phase. New chemical equilibrium state is reached, at which almost complete saturation of the γ′ phase with nitrogen occurs (Xb ≅ 0.2 mol mol−1). Further increase in the nitriding potential causes complete saturation of the γ′

Figure 7. Distribution (GSD) of sizes of iron nanocrystallites (determined by the method described in ref 28). 6182



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Figure 8. Changes in the concentration of nitrogen in the volume of iron nanocrystallites, Xb, coverage degree, θ, and the enthalpy of segregation, ΔGseg, depending on the value of ln P for crystallite with different sizes (thick solid line −25 nm; thin dashed line −45 nm).

Figure 10. Segregation enthalpy, ΔG, changes depending on the bulk concentration, Xb, of the substance dissolved in a volume of crystallite; spherical crystallite with diameter of 25 nm.

phase with nitrogen (Xb = 0.2 mol mol−1). Changes in enthalpy of segregation cause the “bulk” concentration of nitrogen to remain approximately constant (although nitriding potential of a gas phase is changing). The phase transition also causes a change in the surface coverage of iron with nitrogen. Figure 9 is an enlarged fragment of Figure 8. It presents the change in the degree of coverage, θ, depending on the value of ln P for spherical crystallites with diameters of 25 and 45 nm.

coverage of the iron surface with nitrogen decreases with an increase of the nitriding potential and the concentration of nitrogen in the volume of the crystallites. This in turn leads to a decrease in the rate of the decomposition reaction of ammonia with an increase in the nitriding potential. This may be a reflection of energetic nonequivalence of adsorption sites present on the surface of nanocrystallites. Changes in segregation enthalpy as a function of the concentration of the substance dissolved in a volume of crystallite should correspond to changes in heat of adsorption of gaseous substrate. The results of calculations for the ammonia decomposition rate at a single crystallites, ri, were summed, taking into account the size distribution (from Figure 7), using the following equation: r = Σri =

∑ xikiSiθi

(9)

where xi is the fraction of a specified crystallite size, ki is the ammonia decomposition reaction rate constant for a given crystallite (iron or nitride), and Si is the surface area of the ith crystallite with a specified size. During the summation it also was taken into account that as the conversion degree and nitriding potential increase then smaller and smaller iron crystallites undergo the phase transition. The resulting model curve is shown in Figure 11 (solid line no. 3).

Figure 9. Changes in the degree of coverage, θ, depending on the value of ln P; spherical crystallites with diameters of 25 and 45 nm.

Up to the phase transition (marked with an asterisk (*) for d = 45 nm) the coverage degree gradually increases with increasing nitriding potential to a maximum value θ = 0.983 at ln P = −7.4. The phase transition is accompanied by a rapid decrease in the coverage degree. The coverage degree is reduced (to 0.850 at the phase transition of crystallite with d = 45 nm; from the value of 0.993 to 0.940 at the phase transition marked with two asterisks (**) of crystallite with d = 25 nm) with an increase in the nitriding potential. Figure 10 shows the changes in enthalpy of segregation for the γ′ phase as a function of bulk concentration of the substance dissolved in a volume of crystallite (d = 25 nm). As the concentration of nitrogen in the crystallite increases the enthalpy of nitrogen segregation to the surface of iron nanocrystallite saturating with nitrogen is changing. These changes, however, do not inhibit the increase in concentration in the volume of the crystallites. As a result, the degree of

Figure 11. Rate of catalytic decomposition of ammonia (475 °C): comparison of experimental (dots) and modeling data (solid lines). 6183



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(5) Durbin, D. J. D.; Malardier-Jugroot, C. Density Functional Theory Analysis of Metal/Graphene Systems As a Filter Membrane to Prevent CO Poisoning in Hydrogen Fuel Cells. J. Phys. Chem. C 2011, 115, 808−815. (6) Arabczyk, W.; Pelka, R. Studies of the Kinetics of Two Parallel Reactions: Ammonia Decomposition and Nitriding of Iron Catalyst. J. Phys. Chem. A 2009, 113, 411−416. (7) Ertl, G. Reactions at solid surfaces; Wiley: Hoboken, NJ, 2009. (8) Farrauto, R. J.; Bartholomew, C. H. Fundamentals of industrial catalytic processes; Chapman & Hall: London, 1997. (9) Jennings, J. R., Ed.; Catalytic ammonia synthesis: fundamentals and practice; Plenum Press: New York, 1991. (10) Lendzion-Bieluń, Z.; Pelka, R.; Arabczyk, W. Study of the Kinetics of Ammonia Synthesis and Decomposition on Iron and Cobalt Catalysts. Catal. Lett. 2009, 129, 119−123. (11) Nielsen, A. An Investigation on Promoted Iron Catalysts for Synthesis of Ammonia; Jul. Gjellerups: Forlag, 1968. (12) Pelka, R.; Arabczyk, W. Studies of the Kinetics of Reaction Between Iron Catalysts and Ammonia-Nitriding of Nanocrystalline Iron With Parallel. Catalytic Ammonia Decomposition. Top. Catal. 2009, 52, 1506−1516. (13) Pelka, R.; Kiełbasa, K.; Arabczyk, W. The Effect of Iron Nanocrystallites’ Size in Catalysts for Ammonia Synthesis on Nitriding Reaction and Catalytic Ammonia Decomposition. Cent. Eur. J. Chem. 2011, 9, 240−244. (14) Pelka, R.; Moszyńska, I.; Arabczyk, W. Catalytic Ammonia Decomposition over Fe/Fe4N. Catal. Lett. 2009, 128, 72−76. (15) Schlögl, R. In Handbook of heterogeneous catalysis; Ertl, G., Knözinger, H., Schüth, F., Weitkamp, J., Eds.; Wiley-VCH Verlag: Weinheim, Germany, 2008. (16) Slack, A. V., James, G. R., Eds.; Ammonia; Marcel Dekker, Inc.: New York, 1973. (17) McKay, H. L.; Jenkins, S. J.; Wales, D. J. Theory of NHx±H Reactions on Fe{211}. J. Phys. Chem. C 2009, 113, 15274−15287. (18) Panczyk, T. Comparative Analysis of Nitrogen Adsorption Kinetics on Fe(100) and Fe(111) Based on Applying the Statistical Rate Theory. J. Phys. Chem. C 2007, 111, 3175−3184. (19) Pelka, R.; Glinka, P.; Arabczyk, W. The Influence of Iron Nanocrystallite Size on a Nitriding Process Rate. Mater. Sci.-Poland 2008, 26, 349−356. (20) Tian, N.; Zhou, Z.-Y.; Sun, S.-G. Platinum Metal Catalysts of High-Index Surfaces: From Single-Crystal Planes to Electrochemically Shape-Controlled Nanoparticles. J. Phys. Chem. C 2008, 112, 19801− 19817. (21) Kiełbasa, K.; Pelka, R.; Arabczyk, W. Studies of the Kinetics of Ammonia Decomposition on Promoted Nanocrystalline Iron Using Gas Phases of Different Nitriding Degree. J. Phys. Chem. A 2010, 114, 4531−4534. (22) Alexander, A.-M.; Hargreaves, J. S. J.; Mitchell, C. The Denitridation of Nitrides of Iron, Cobalt and Rhenium Under Hydrogen. Top. Catal. 2013, 56, 1963−1969. (23) Langmuir, I. The Adsorption of Gases on Plane Surfaces of Glass, Mica and Platinum. J. Am. Chem. Soc. 1918, 40, 1361−1403. (24) Freundlich, H. Colloid and Capillary Chemistry; Methuen: London, 1926. (25) Nielsen, A., Ed.; Ammonia. Catalysis and manufacture; SpringerVerlag: Berlin, 1995. (26) Marczewski, A. W.; Jaroniec, M. A New Isotherm Equation for Single-Solute Adsorption from Dilute Solutions on Energetically Heterogeneous Solids. Mh. Chem. 1983, 114, 711−715. (27) Tompkins, F. C. Chemisorption of Gases on Metals; Academic Press: London, 1978. (28) Pelka, R.; Arabczyk, W. A New Method for Determining the Nanocrystallite Size Distribution in Systems Where Chemical Reaction Between Solid and a Gas Phase Occurs. J. Nanomater. 2013, vol. 2013, Article ID 645050, 6 pages. (29) Kowalczyk, Z.; Sentek, J.; Jodzis, S.; Muhler, M.; Hinrichsen, O. Effect of Potassium on the Kinetics of Ammonia Synthesis and

Model results very well correspond to the obtained experimental data. Using the results of calculations one can determine the values of parameters such as Xb, θ, ΔG = f(Xb), which are difficult or impossible to determine in other way. In this paper, a new model of a chemical reaction, which is accompanied by a phase transition, was proposed and verified. It was explained how the phase transition affects the chemical reaction, how different process parameters are changed during the whole process.

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CONCLUSIONS During investigations of the nitriding process of nanocrystalline iron the unexpected phenomenon was observed. Namely, rate of ammonia decomposition reaction decreased along with increasing nitriding potential of a gas phase. Moreover, the rate of that reaction on a mixture of α-iron + γ′-nitride phases decreases nonlinearly with respect to the natural logarithm of the nitriding potential. A new model describing a chemical reaction, which is accompanied by a phase transition, was proposed and model calculations were performed to explain in detail that effect. It was found that the decrease in decomposition reaction rate, related with a decrease in the iron surface coverage degree, resulted from changes in segregation enthalpy of nitrogen. Nonlinear character of the reaction rate−nitriding potential dependence is an effect of the nanocrystallite size distribution and the fact that two phases in a solid are present simultaneously. The model allowed the determination of such parameters as bulk concentration of nitrogen, enthalpy of segregation, surface coverage degree. It was explained how the phase transition affects the chemical reaction and showed how different process parameters are changed during the whole process.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +48 91 449 47 30. Fax: +48 91 449 46 86. E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The scientific work was financed from the budgetary resources for science in 2012−2014, Project No. IP 2011040771.

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REFERENCES

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Catalytic Ammonia Decomposition during Nanocrystalline Iron

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