Studies on the Kinetics of Carbon Deposit Formation on

Article pubs.acs.org/JPCC

Studies on the Kinetics of Carbon Deposit Formation on Nanocrystalline Iron Stabilized with Structural Promoters Rafal J. Wrobel,* Agnieszka Hełminiak, Walerian Arabczyk, and Urszula Narkiewicz West Pomeranian University of Technology Szczecin, Institute of Chemical and Environment Engineering, Pulaskiego 10, 70-322 Szczecin, Poland ABSTRACT: The carburization of nanocrystalline iron was studied. The process was conducted at a constant temperature (620 °C) under varied partial pressures of methane and hydrogen (pCH4 = 0.67, 0.75, 1.00 bar) and also at pCH4 = 1.00 bar for various temperatures (T = 580, 620, 650 °C) to achieve various carburization degrees (nC/nFe). The combined thermogravimetric analysis, X-ray diffraction, and scanning electron microscopy data were used to propose a model of carbon deposit formation, containing kinetic equations of the process. The kinetic equations can be used to determine the rate of deposit formation over Fe3C not covered with a carbon deposit, which is impossible in direct measurement.

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INTRODUCTION The process of dehydrogenation of hydrocarbons over metal surfaces has many applications ranging from the catalytic industry to the production of sooth and carbon nanotubes (CNTs).1−6 In the case of iron, the process is very complex due to the formation of iron carbide, carbon deposit, and CNTs.7,8 The nanocrystalline iron doped with structural promoters has a well-developed surface area. This enables investigation of the process of carburization where adsorption is the rate-limiting step.9 Thus, one can investigate phenomena inaccessible to direct observation in the case of coarse grain iron. A phase diagram of the system consisting of iron−methane− hydrogen in the temperature range of 300−900 °C was published in 1957 by R. Schenck.10 The temperature range was next extended by Pilipenko and Veselov11 to 200−1700 °C. According to the equilibrium diagram, there are three areas corresponding to α-Fe(C), saturated solution α-Fe(C) + C, and Fe3C + C. Studies of hydrocarbon decomposition on doped nanocrystalline iron demonstrated some deviations of the system behavior compared to Schenck’s diagram. We observed a hysteresis phenomenon in the carburization/decarburization process. A similar behavior was previously observed for the process of nitriding of the same material.12 Buyanov and Chesnokov13 studied iron carbide formation. They described three characteristic areas of temperatures. In the first one, below 500 °C, the rate of iron carbide formation was higher than the rate of its decomposition. In the temperature range of 500−725 °C, the rate of iron carbide formation was initially higher than that of its decomposition, but when the iron carbide phase was starting to form, the ratio of corresponding rates changed and iron carbide gradually transformed into iron. In the third area, above 725 °C, the rate of iron carbide decomposition was higher than that of its © 2014 American Chemical Society

formation; then, from the beginning of the process, the metallic phase was stable. Ermakova14 et al. observed that, at 680 °C, a decomposition of iron carbide occurs and a phase of iron is formed, which enables hydrocarbon decomposition, probably according to the so-called carbide cycle. This schema involved the formation of cementite on free surface areas of the iron catalyst particles and decomposition to graphitic carbon close to the surface covered by graphite. An equilibrium is established when graphite covers a part of the surface of a particle. As a result, a gradient of iron carbide concentration is the driving force of carbon diffusion through a particle iron−iron carbide. Despite numerous papers on the formation of carbon nanotubes and nanofibers, there are few papers on the kinetics of the process. The kinetics of iron carbide and carbon deposit formation was described elsewhere.15−17 Nevertheless, there are still some unclear points to be explained, mainly concerning the kinetics of the process, and in the present paper, a more indepth analysis of carbon deposit formation is presented. The aim of the present paper is an extended study of the kinetics of carbon deposit formation on nanocrystalline iron doped with structural promoters (Al2O3, CaO, K2O).

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EXPERIMENTAL SECTION Under carburization (580−650 °C), pure nanocrystalline iron quickly sinters, forming a microcrystalline material. An iron doped with calcium, aluminum, and potassium oxides does not undergo sintering, which enables a study of the process kinetics of the nanocrystalline material. Promoter oxides do not dissolve either in iron or in iron carbide; therefore, they do not Received: November 4, 2013 Revised: June 23, 2014 Published: June 23, 2014 15434

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Figure 1. SEM images of nanocrystalline iron doped with promoter oxides after carburization (nC/nFe = (a) 1.2, (b) 8, (c) 19, (d) 29). Inset in (a) shows iron particle covered with thin carbon layer. Inset in (c) shows carbon nanotube (CNT) grown on iron carbide particle.

performed under normal pressure. A gas for the analysis was taken from the vicinity of the sample. A time period needed to reach a stable level of partial pressures of methane and hydrogen in the reactor is equal to 200 s. In that time, the mass increase of the sample corresponds to a carbon-to-iron ratio of nC/nFe = 0.05 [mol/mol]. After this time, at the constant methane pressure, the hydrogen partial pressure is very low and can be neglected. Before carburization experiments, it was verified that a carbon deposit had not formed under the reaction conditions with an empty sample holder (without the iron sample). Next, still before the carburization experiments, the sample of nanocrystalline iron was heated under hydrogen at the carburization temperature. During this step, the iron oxides formed during passivation were removed. Next, carburization was conducted at a constant temperature (620 °C) under a varied concentration of methane in the reaction methane−hydrogen (pCH4 = 0.67, 0.75, 1.00 bar) and also at pCH4 = 1.00 bar for various temperatures (T = 580, 620, 650 °C) to achieve various carburization degrees (nC/nFe). An increase of the mass of the sample is caused by formation of iron carbide, carbon deposit, and, in a negligible amount, by formation of carbon solution in an iron matrix. XRD analysis enables determination of the iron-to-iron carbide ratio and thus iron conversion α. A combination of TGA with XRD enables determination of how the total amount of carbon is split up into graphitic carbon and carbon present in Fe3C. Samples with different carburization degrees were studied using the scanning electron microscope (Hitachi SU8020 with cold field emission) equipped with elemental microanalysis

influence thermodynamic properties of iron and iron carbide. They form bridges between iron nanocrystallites and wet their surface.18 Nanocrystalline iron doped with oxides was obtained through a melting of the mixture of iron, aluminum, calcium, and potassium oxides, followed by the reduction of the obtained lava under a hydrogen atmosphere. Because of the phyrophoric properties, after reduction process, iron was passivated first. The mixture containing 0.1 wt % of oxygen in nitrogen in a temperature lower than 50 °C was used during passivation (the maximum increase of the sample weight caused by iron oxide formation amounted to 1% referring to the mass of iron). The content of structural dopants determined with AES-ICP (Optima 5300DV Spectrometer; PerkinElmer) was as follows: Al2O, 3.3 wt %; CaO, 2.8 wt %; and K2O, 0.65 wt %. The specific surface area of nanocrystalline iron was determined using a thermal desorption method (AutoChem II 2920 Apparatus; Micromeritics Company). The porosity coefficient ε amounts to 0.5. The phase analysis was performed and the mean crystallite size of iron was measured (25 nm) using X-ray diffraction and Scherrer’s equation19 (XRD Empyrean diffractometer, Cu Kα; PANalytical). These values depend on the maximum reduction temperature of iron oxides. A catalytic decomposition of methane over obtained nanocrystalline iron doped with oxides was carried out in a differential tube reactor with thermogravimetric mass measurement (TGA; homemade). A single layer of iron grains was located in the basket made of platinum wire. The flow of gases and thus the composition of the gas mixture were fixed using and controlled by electronic mass flow meters. All experiments were 15435

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Figure 2. Model of carbon deposit formation over nanocrystalline iron: (a) dissociative adsorption of methane, (b) after phase transition adsorption occurs further, (c) nucleus of graphite layer, (d) formation of carbon concentration in the crystallite, (e) growth of carbon layer, (f) occlusion of particle with carbon, followed by iron carbide decomposition to iron, (g−i) CNT formation.

(Thermo Scientific EDS NSS 312, resolution 135 eV). The 15 kV accelerating voltage of electrons and 5 nA current were used to obtain SEM images.

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crystallites (kinetic area of chemical reaction) is the ratelimiting step. As a result, a chemical equilibrium exists between carbon adsorbed on the iron surface and carbon dissolved in iron.9,20 There is a maximal available concentration of carbon in α-Fe. Exceeding this concentration leads to rearrangement of iron atoms, i.e., phase transition of a crystallite. As a result of phase transition, the carbon concentration in the volume of crystallite can rapidly increase. Finally, the carbon concentration corresponds to Fe3C (Figure 2b). Further adsorption process leads to saturation of iron carbide with carbon and formation of a graphite phase nucleus on one of the crystallographic areas of Fe3C (Figure 2c). The growth of a nucleus creates a 3D carbon layer (Figure 2d). The presence of carbon over the metal surface inhibits adsorption (Figure 2d−f).21 Further dissociative methane adsorption only takes place on the surface not covered with a three-dimensional structure of carbon (Figure 2d,e). In places where the crystallite is covered with the threedimensional carbon structure, thermal decomposition of iron carbide occurs to form a two-phase region: three-dimensional structure of carbon and Fe3+xC lean in carbon (Figure 2d,e). In the crystallite between the surface coated with the threedimensional structure of carbon and the surface available for dissociative methane adsorption, the concentration gradient of carbon is formed (Figure 2d,e). The consequence of this state is diffusion of carbon adsorbed on the surface of iron carbide, which is not coated with carbon. Finally, particles can either be totally covered by carbon (Figure 2f) or produce CNT (Figure 2h−j). The results of thermogravimetric measurements of the ratio nC/nFe as a function of time are shown in Figure 3. As was

RESULTS AND DISCUSSION

Samples of different carburization degrees (nC/nFe) were obtained in a reaction with methane at 650 °C. Figure 1 shows SEM images of the samples. The difference in the elemental contrast allows observation of carbon layer over iron particles (Figure 1a, inset). The carbon layer is semitransparent for electrons, and thus, iron cores can be observed. In Figure 1c, formation of a carbon nanotube (CNT) can be observed. It grows over an iron carbide particle. From one side of the particle, methane adsorption is allowed, and on the other side covered by CNT, iron carbide decomposition occurs, which leads to CNT growth. The process takes place only on nanometric metallic particles, such as those in the inset of Figure 1c. Larger particles are covered with a carbon deposit totally. Therefore, one can observe only few CNTs after significant carburization (Figure 1d). The observed phenomena can be interpreted on the basis of the model of carbon deposit formation on iron carbide. In Figure 2, the model of carbon occluded particles as well as a model of CNT growth is presented. In carburization of iron with methane, as a result of catalytic decomposition, the iron surface is coated with chemisorbed carbon (Figure 2a). The other product of decomposition, i.e., hydrogen, desorbs into the gas phase. In the first stage of carburization, a solution α-Fe(C) is formed (Figure 2a). Disassociative methane adsorption on the surface of iron 15436

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Figure 3. Thermogravimetric curves of carburization of nanocrystalline iron doped with oxides (a) for various temperatures and (b) for various methane partial pressures.

mentioned above, only in the initial step of the process (first 200 s), gas composition changes; after that, it is stable. Carburization was studied at various temperatures (580, 620, and 650 °C) and for a fixed constant methane pressure pCH4 = 1 bar (Figure 3a). It was also studied for a fixed temperature (650 °C) and for various partial pressures of methane (0.67, 0.75, 1.00 bar) (Figure 3b). The conditions were chosen in such a way as to reach the state in which carburization starts. The course of the thermogravimetric curves is not monotonic. The inflection points indicate complex kinetics of carbon deposit formation. On the basis of the TG data presented in Figure 3 by taking the time derivative, the dependencies of the reaction rates on degree of sample carburization were determined and are shown in Figure 4. The right axis (Figure 4a) shows iron conversion, which corresponds to relative iron carbide content. Analysis of carburization can be divided into four ranges denoted with numbers (Figure 4). A model of the whole process is depicted in Figure 5. Range I. At the first step of carburization, iron is saturated with carbon, which leads to phase transition and formation of iron carbide. The rate-limiting step is the rate of dissociative adsorption of methane over the surface of iron crystallites and iron carbide (Figure 5a−c). Range II. At the next step, in pores between the particles of iron carbide, a carbon deposit is formed, and the reaction rate decreases as a result of reducing the volume of the pores (Figure 5d). It is a transition from diffusion to the kinetic region of the chemical reaction. Range III. Grains of the material disintegrate into individual crystallites of iron carbide connected by bridges of carbon

Figure 4. Dependence of the carburization rate of nanocrystalline iron on the carburization degree (a) for various temperatures and (b) for various methane partial pressures. Right axis denotes iron conversion.

deposit (Figure 5e). As a result of this disintegration, the uncovered surface with carbon deposit increases and thus intensifies the total reaction rate. Range IV. Further growth of the carbon deposit reduces the iron carbide surface, on which dissociative methane adsorption can occur, and thus, the rate of carburization decreases. At the final step, thermal decomposition of iron carbide occurs in nanocrystallites entirely occluded with carbon (Figure 5e,f). The composite consists of occluded nanocrystalline iron in a carbon matrix. In the fourth range, the dependence of the rate of carbon deposit formation over the nanocrystalline iron carbide on carburization degree r(nC/nFe) can be given by r(nC /nFe) = k·SFe3C·pCH

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r(nC /nFe) nC / nFe = 0.33 G = SFe − SFe = SFe3C 3C 3C k·pCH 4

where k is the reaction rate constant of dissociative adsorption of methane over the Fe3C surface, SFe3C is the surface area of iron carbide free from the carbon deposit, pCH4 is the methane partial pressure, and SGFe3C is the surface area of iron carbide covered with a carbon deposit (G-graphite). If the process proceeds at the constant methane partial pressure and at the constant temperature (k = const.), the 15437

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Figure 5. Model of carbon deposit formation over iron carbide.

reaction rate depends proportionally on the uncovered surface of iron carbide. The correlation between the content of iron carbide (determined by XRD) and the reaction rate was found. It is noteworthy that the relative iron carbide content is proportional to the iron conversion α. In Figure 4a, the dependencies of process rate and iron conversion α are presented. Assuming that the coverage of the iron carbide surface is constant at this step, the dependence was evaluated and is presented in Figure 6.

nC/nFe > 6) is higher under a methane partial pressure of pCH4 = 0.75 bar than that under a partial pressure of pCH4 = 1 bar.

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CONCLUSIONS Carburization conducted at 620 °C under a varied concentration of methane in the reaction methane−hydrogen (pCH4 = 0.67, 0.75, 1.00 bar) reveals that the rate of carbon deposition can be higher for a lower partial pressure of methane. This is due to the influence of the reaction product (carbon deposit) on the catalyst, i.e., uncovered surface of iron carbide. The carbon deposit can either build 3D structures and/or cover the surface of iron carbide. This leads to a nonmonotonic rate of the process. The proposed kinetic equations enable determination of the reaction rate over the iron carbide surface without the blocking effect of the carbon deposit. Iron carbide particles totally covered by a carbon deposit undergo decomposition to iron.

nC / nFe = 0.33 r = f (α) = a ·SFe 3C

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +48 91 449 41 32. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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Figure 6. Dependence of deposit formation rate on iron conversion, i.e., relative Fe3C content. Horizontal line denotes maximum rate for Fe3C not covered with carbon.

REFERENCES

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Via extrapolation of this dependence to full conversion, one can obtain the rate of deposit formation running over the surface of iron carbide (about 6 × 10−4 [(nC/nFe)/s]). From the measurements of the reaction rate, the relative change of the noncovered iron carbide surface, on which the reaction occurs, can be found. Taking into account the above-mentioned phenomena, one can explain the dependence of carburization rate on methane partial pressure from Figures 3b and 4b. Under higher methane partial pressure (pCH4 = 1 bar), the carburization rate in the initial stage is higher. However, in comparison with a lower methane partial pressure (pCH4 = 0.75 bar), this results in faster occlusion of the Fe3C surface with carbon. As a result, the carburization rate measured in range IV (carburization degree 15438

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