Deposition Of Filamentous Carbon Research Articles

Stability of a NiAl2O4 Derived Catalyst in the Ethanol Steam Reforming in Reaction-Regeneration Cycles: Effect of Reduction Temperature

The catalyst regeneration is still a challenge to make the ethanol steam reforming (ESR) process feasible for sustainable H2 production. NiAl2O4 spinel derived catalysts are highly active and selective for ESR, but they require avoiding irreversible deactivation to ensure their regeneration. Their stability depends on the catalyst structure, and herein we report different Ni/Al2O3-NiAl2O4 catalysts obtained upon reduction of a NiAl2O4 spinel at 700, 750, or 850 °C. The catalysts were tested in ESR reaction-regeneration cycles, with reaction at 600 °C and regeneration by coke combustion at 850 °C followed by reduction at the corresponding temperature. The fresh, spent, and regenerated catalysts were characterized using X-ray diffraction, N2 physisorption, temperature programmed reduction and oxidation, and scanning electron microscopy. The irreversible deactivation is due to Ni volatilization and catalyst particle fragmentation. These phenomena are prompted by a high filamentous carbon deposition favored by the Al2O3 content in the catalyst. The reduction in the 700–750 °C range is optimum for controlling the Al2O3 content, increasing the NiAl2O4/Al2O3 ratio in the resulting catalyst. These catalysts show a period of partial reversible deactivation by coke with a change in the H2 formation mechanism reaching a pseudo-stable state with a H2 yield of 40% and a reproducible performance in successive reaction-regeneration cycles.

Ni/CaO-SiO2 catalysts for assessment in steam reforming reaction of acetol

The Aqueous impregnation method obtained catalysts with different CaO contents (0, 20, 40 and 100 wt%) as additive on Ni/SiO2. The materials were characterized by EDS, N2 physisorption, SEM, IR, XRD, TPR and XRD in situ. The addition of CaO on NiSi increased the Ni metallic dispersion, which was weakened by further CaO additions, probably due to a larger agglomeration of particles in the support, according to SEM. Catalytic tests showed acetol, in the absence of catalyst, was converted only to liquid products. However, in the presence of catalyst, the gas production was favored. The CaO addition and consequent increase in the metallic dispersion of Ni(II) interfered with the catalytic results, favoring H2 yield and reducing the deposition of filamentous carbon on the catalyst surface.

Effects of dealumination on the performance of Ni-containing BEA catalysts in bioethanol steam reforming

The effects of dealumination of BEA zeolite on the formation of nickel active sites and the performance of Ni-containing BEA zeolite catalysts in the steam reforming of ethanol have been studied. Ni-containing BEA zeolite catalysts were prepared by the impregnation of unmodified and dealuminated BEA zeolites with Ni(NO3)2 precursor. The properties of Ni10HAlBEA and Ni10SiBEA zeolite catalysts were studied by means of X-ray diffraction, 1H, 27Al and 29Si magic-angle spinning nuclear magnetic resonance, Fourier-transform infrared and Raman spectroscopy, transmission electron microscopy, temperature-programmed reduction, temperature-programmed ammonia and hydrogen desorption methods. High initial activity and selectivity of Ni10HAlBEA to hydrogen and carbon dioxide with unmodified BEA zeolite support in the steam reforming of ethanol reaction performed at 500 °C was observed. However, fast deactivation of Ni10HAlBEA catalyst, manifested in the decrease of water conversion, drop of selectivity to H2 and CO2, and increase in the selectivity to ethylene with the time-on-stream, was observed. In contrast, Ni10SiBEA zeolite catalyst showed lower initial activity but higher durability and resistance for carbon deposition. It was stated that dealumination of BEA zeolite led to the slight structural changes and simultaneously pronounced decrease of acidity. Formation of the large nickel crystallites was hindered on Ni10SiBEA zeolite catalyst. TEM and Raman spectroscopy studies indicated that deactivation of Ni10HAlBEA was related to formation of nickel mediated filamentous, graphitic and amorphous carbon deposits. Much smaller amounts of filamentous carbons were observed on the Ni10SiBEA zeolite catalyst prepared by the use of dealuminated zeolite support.

Performance of Fe/SiC catalysts for cracking of toluene under microwave irradiation

In this work, toluene as a model compound of biomass tar and inexpensive Fe supported on SiC powder as catalysts were used to investigate the influence of specific microwave power levels, Fe content, and space velocity on the microwave cracking of toluene. The results revealed that the microwave-aided cracking of toluene was efficient and that the composition of cracking gas was mainly H2. During the reaction, the instantaneous cracking efficiency and the instantaneous hydrogen yield increased initially and then decreased, with the highest values being recorded as 94.4% and 89.8% at 10–12 min of the reaction. For the whole reaction, the overall cracking efficiency and the overall hydrogen yield reached maximum (86.3% and 84.5%, respectively) when the tests were conducted under the optimum conditions. The catalysts before and after reaction were analyzed by scanning electron microscopy-energy dispersive spectrometer (SEM-EDS), thermogravimetry (TG), X-ray diffraction (XRD), and temperature programmed oxidation (TPO). The results confirmed that the deposited carbon included spherical and filamentous carbon and carbon deposition yield could reach 2.3%, which had a negative effect on the cracking of toluene.

Catalytic evaluation of nickel nanoparticles in methane steam reforming

Development of a highly efficient and coke-resistant, nickel nanoparticles (Ni.NPs) based catalyst in the steam reformation reaction of methane is reported. The catalyst was prepared by dispersing Ni-nanoparticles over silica functionalized alumina support. The synthesized samples were characterized by a combination of analytical techniques of X-ray diffraction (XRD), nitrogen physisorption (BET surface area), X-ray photoelectron spectroscopy (XPS), temperature programmed reduction (H2-TPR) and temperature programmed oxidation (TPO).Characterization results revealed that functionalization of alumina resulted in improved physicochemical properties of Ni-nanoparticles catalyst compared to the conventional nickel based catalysts. More importantly, a clear improvement on the catalytic hydrogen production and resistance to coke was observed. At all temperature studied Ni.NPs catalyst exhibited high methane conversions which reached to 100% at 750 °C. Also, the Ni.NPs catalysts exhibited high H2 selectivity and excellent thermal stability.The functionalization of alumina with silica improved the dispersion of Ni nanoparticles and prevented sintering and aggregation. Ni.NPs catalyst exhibited stable catalytic activities for a period of 48 h where no carbon deposition was evidenced neither by TPO nor XPS techniques. Contrary to Ni.NPs the conventional nickel catalyst suffered from severe deactivation due to deposition of filamentous carbon on the surface.

Oxidative and non-oxidative steam reforming of crude bio-ethanol for hydrogen production over Rh promoted Ni/CeO2-ZrO2 catalyst

The catalytic oxidative and non-oxidative steam reforming of crude bio-ethanol for hydrogen production was studied in a tubular fixed bed reaction system over 30wt.%Ni/CeO2-ZrO2 and 1wt.%Rh-30wt.%Ni/CeO2-ZrO2 catalysts. The catalysts were prepared by impregnation-co-precipitation method and characterized by BET, XRD, TPR, TGA, XPS, XANES and EXAFS techniques. Characterization results revealed that the addition of Rh promotes reducibility of NiO at lower temperature for the bimetallic catalysts. Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) analysis depicted that NiO with 6-fold local co-ordination is present in all the samples. Ethanol conversion, hydrogen yield and product selectivity were investigated at 600°C and space time of 9.17kgcath/kgmol[EtOH]) at atmospheric pressure. In steam reforming of crude bio-ethanol, 81% ethanol conversion was achieved with 59% hydrogen selectivity on Ni/CeO2-ZrO2 catalyst, whereas, higher ethanol conversion (86%) with 73% hydrogen selectivity was achieved using Rh-Ni/CeO2-ZrO2 catalysts. In oxidative steam reforming of crude bio-ethanol, hydrogen yield and selectivity reduced due to the partial oxidation of oxygenate compounds present in the feed. The used catalysts were also analysed by BET, TGA/DTA, TPR, TPO, Raman spectroscopy, SEM, and TEM techniques to identify the cause of catalyst deactivation. The results indicate that the catalyst deactivation occurred mainly due to amorphous and filamentous carbon deposition on the catalysts surface, due to the presence of the impurities in the feed.

Novel Ni–Mg–Al–Ca catalyst for enhanced hydrogen production for the pyrolysis–gasification of a biomass/plastic mixture

A Ni–Mg–Al–Ca catalyst was prepared by a co-precipitation method for hydrogen production from polymeric materials. The prepared catalyst was designed for both the steam cracking of hydrocarbons and for the in situ absorption of CO2 via enhancement of the water–gas shift reaction. The influence of Ca content in the catalyst and catalyst calcination temperature in relation to the pyrolysis–gasification of a wood sawdust/polypropylene mixture was investigated. The highest hydrogen yield of 39.6molH2/g Ni with H2/CO ratio of 1.90 was obtained in the presence of the Ca containing catalyst of molar ratio Ni:Mg:Al:Ca=1:1:1:4, calcined at 500°C. In addition, thermogravimetric and morphology analyses of the reacted catalysts revealed that Ca introduction into the Ni–Mg–Al catalyst prevented the deposition of filamentous carbon on the catalyst surface. Furthermore, all metals were well dispersed in the catalyst after the pyrolysis–gasification process with 20–30nm of NiO sized particles observed after the gasification without significant aggregation.

Preparation and characterization of a plasma treated NiMgSBA-15 catalyst for methane reforming with CO2 to produce syngas

A plasma-assisted method was employed to prepare a NiMgSBA-15 catalyst. The catalytic properties of this catalyst were tested in methane reforming with carbon dioxide. The plasma-treated catalyst exhibited a higher initial catalytic activity and a better stability than those of the untreated counterpart catalyst. To elucidate the effect of the plasma treatment, the catalysts were characterized using XRD, N2 physisorption, XPS, TPR, CO2-TPD, TEM, TGA, Raman and temperature-programmed hydrogenation (TPH) techniques. The results obtained confirmed that the plasma treatment of the Ni precursor had a significant impact on the surface characteristics of the active sites. The plasma treatment reduced the sizes of the Ni clusters and enhanced Ni–support interaction, which enhanced the catalytic performance. The stronger basic sites derived from plasma treatment favored the chemisorption of CO2 and drove the coke elimination. The sintering of the metallic Ni particles did not take place during tests due to the confinement effect of the pore channels of mesoporous material. In addition, the plasma-treated catalyst showed excellent resistance to the deposition of filamentous carbon and encapsulating carbon. This was in contrast to the calcined sample, whose surface was covered by large amounts of graphitic carbon species, resulting in activity loss.

Isooctane decomposition and carbon deposition over ceria–zirconia supported nickel catalysts

Catalytic decomposition of hydrocarbons is a major contributor to carbon formation during hydrocarbon reforming. This work investigates the effects of temperature and hydrocarbon decomposition intermediates on carbon deposition during catalytic isooctane decomposition over ceria–zirconia supported nickel. The carbon deposition rate and hydrocarbon concentrations were monitored in real-time via thermogravimetry and FT-IR spectroscopy. Carbon-deposited catalysts were characterized by temperature-programmed oxidation (TPO) and scanning electron microscopy (SEM). Carbon deposition rate, morphology, and quantity were strongly dependent on decomposition temperature ( T); three distinct decomposition temperature regimes were identified, i.e. T ≤ 575 °C, 575 °C < T < 725 °C, and T ≥ 725 °C. Gaussian deconvolution of TPO data and carefully designed oxidation experiments indicated the presence of different types of carbon: coating, filamentous, and pyrolytic carbon. Catalytic and homogeneous decomposition of isooctane, propene, and methane suggest that smaller hydrocarbon fragments, typically formed during reforming of large hydrocarbon molecules, play an important role in carbon deposition. Specifically, low-temperature decomposition of isooctane into olefins, such as propene or isobutene, leads to the formation of both coating and filamentous carbon deposits. At high temperatures, decomposition of acetylene and methane leads to the formation of significant quantities of coating and pyrolytic carbon deposits in addition to filamentous deposits.

The effect of space time on Co/CeO2 catalyst deactivation during oxidative steam reforming of ethanol

This paper studies the effect of space time on the deactivation of a Co/CeO2 catalyst during oxidative steam reforming of ethanol. Increasing space time increased the ethanol conversion and hydrogen selectivity whereas acetaldehyde formation decreased. Both thermogravimetric and scanning electron microscopy analyses of post-reaction samples indicate that higher space times favor the deposition of filamentous carbon. However, the amount of carbon formed passes through a maximum and then decreases at high space time. Higher space times appear to allow the support, based on its oxygen mobility property, enough contact time to supply O needed to clean the carbon.

Evaluation of the performance of Ni/La 2O 3 catalyst prepared from LaNiO 3 perovskite-type oxides for the production of hydrogen through steam reforming and oxidative steam reforming of ethanol

This paper studies the performance of LaNiO 3 perovskite-type oxide precursor as a catalyst for both steam reforming and oxidative steam reforming of ethanol. According to results of temperature-programmed desorption of adsorbed ethanol and by carrying out diffuse reflectance infrared Fourier transform spectroscopy analyses of ethanol steam reforming, ethanol decomposes to dehydrogenated species like acetaldehyde and acetyl, which at moderate temperatures, convert to acetate by the addition of hydroxyl groups. Demethanation of acetate occurs at higher temperatures, leading to a steady state coverage of carbonate. Catalyst deactivation occurs from the deposition of carbon on the surface of the catalyst. Both thermogravimetric and scanning electron microscopy analyses of postreaction samples indicate that lower reaction temperatures and lower H 2O/EtOH ratios favor the deposition of filamentous carbon. However, less carbon formation occurs when the H 2O/EtOH ratio is increased. Increasing reaction temperature or including O 2 in the feed suppresses filamentous carbon formation.

MCM-41 supported PdNi catalysts for dry reforming of methane

A series of bimetallic PdNi catalysts supported on mesoporous MCM-41 with different Ni content (Ni/Si ratio of 0.2–0.4) was synthesized. The effect of Pd addition to Ni-containing catalysts as well as the effect of the Ni content on the surface and catalytic properties of the catalysts was studied. The samples were characterized using various techniques, such as energy-dispersive X-ray spectroscopy, N2 adsorption–desorption isotherms, X-ray diffraction, thermogravimetric and differential analyses, X-ray photoelectron spectroscopy, high resolution transmission electron microscopy and temperature-programmed reduction. Reforming of methane with carbon dioxide was used as a test reaction. The results indicated that the addition of a small amount of Pd (0.5%) to Ni-containing catalysts leads to formation of small nano-sized, easy reducible NiO particles. Agglomeration of NiO as well as of metallic nickel phase over PdNi samples increased with increasing the Ni content. Formation of filamentous carbon over surface of spent monometallic Ni and bimetallic PdNi catalyst was observed. In spite of filamentous carbon deposition, the catalytic activity and stability of bimetallic PdNi catalysts are higher than those of monometallic Ni one. Within bimetallic system, the PdNi catalyst with Ni/Si ratio of 0.3 revealed the best performance and stability caused by presence of small nickel particles well dispersed on the catalyst surface.

N-Dodecane reforming over monolith-based Ni catalysts: SEM study of axial carbon distribution profile

The extent and nature of carbon deposition that occurs during catalytic reforming reactions of hydrocarbons depends strongly on the reaction conditions and the surface properties of the catalyst. In conventional packed bed reactors it is difficult to track the spatial distribution of carbon and link it to the location of individual catalyst particles. The present work uses monolith-based nickel catalysts to investigate the axial carbon gradient post n-dodecane reforming. The carbon, which was deposited during autothermal reforming (ATR), partial oxidation (POX), and steam reforming (SR) over Ni/cerium-zirconium oxide (CZO)/monolith and Ni/monolith catalysts, was characterized by a combination of temperature-programmed oxidation (TPO), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The amount, location, and type of carbon deposited strongly depended on the type of reforming reaction. The majority of carbon deposition was concentrated inside cordierite pores of the monolith. In general, the catalysts showed the expected trend of increased filamentous carbon deposition down the length of the monolith, reflecting the axial variation in temperature and oxygen concentration, except for POX over Ni/monolith catalyst where massive coating carbon deposition occurred in the upstream section of the monolith.

Surface State of a Silver Catalyst for Ethylene Glycol Oxidation

The surface state of electrolytic silver before and after treatment with a reaction mixture in the course of ethylene glycol oxidation to glyoxal was studied using X-ray photoelectron spectroscopy and scanning electron microscopy. It was found that electrophilic forms of adsorbed oxygen, which participate in the selective conversion of ethylene glycol, were formed on the surface of electrolytic silver crystals under exposure to oxygen under conditions similar to catalytic process conditions. The treatment of the catalyst with a reaction mixture resulted in the formation of filamentous carbon deposition products. A mechanism of formation of carbon-containing products was proposed.

Carbon deposition on stainless steel in oxidising environments

The formation of low-density carbon deposits onto metallic heat transfer surfaces in industrial environments of high carbon activity can have a deleterious effect on thermal efficiency. Often this deposition occurs under reducing conditions and involves carbide (typically of iron) formation as part of the process. It is possible, however, for filamentous carbon deposits to form at much higher oxygen potentials where carbide formation is not favoured thermodynamically. This study considers one such situation by examining the behaviour of a 20Cr25Ni austenitic steel used as fuel cladding in Advanced Gas-cooled nuclear reactors (AGRs). Carbon deposition has been produced in laboratory by exposure at 550°C to CO2 containing 1%CO and 1000 vppm C2H4; the dissociation of the latter additive generates carbon activities much greater than unity. The oxygen potential of this gas is sufficient to oxidise iron and chromium, of the major alloy constituents, but not nickel. The deposition takes place onto substrates that are prepared as electron microscope samples which are subsequently examined by SEM and HREM (high resolution electron microscopy) techniques. The deposits formed consist of solid carbon filaments, approximately 30 nm in diameter, with turbostratic atomic layering. Each filament contains a 10–20 nm metallic nickel particle near its tip. It is argued, and supported by HREM evidence, that such particles are produced as a direct result of the oxidation of the alloy and that prior oxidation is necessary to produce the observed carbon deposition. It is also shown that inhibition of carbon deposition occurs when the oxygen potential of the depositing gas is increased sufficiently to oxidise nickel.

In situ environmental scanning electron microscope observations of catalytic processes encountered in metal dusting corrosion on iron and nickel

High temperature corrosion of pure iron and nickel has been studied in situ with an environmental scanning electron microscope (ESEM). Metal samples were exposed to methanol vapour and hydrogen–carbon monoxide gas mixtures at temperatures from 773 to 1073 K. Carbon filaments were formed on the iron metal surface at 773 K, and the extent of corrosion was found to depend on the gaseous environment. The precipitation of surface carbides and the catalytic gasification of carbon filaments on iron were followed at high temperatures. Nickel was apparently catalytically inert at lower temperatures but showed heavy surface restructuring in the higher temperature range. Grain boundary grooving, grain growth, surface rearrangement, and the nucleation of embedded particles were observed in situ on nickel. The surface restructuring of nickel due to massive diffusional processes was seen to be affected by the adsorbed gaseous species. The formation of filamentous carbon deposits was found to be controlled by reaction kinetics rather than by thermodynamic restraints. The in situ observations show the metal surface fragmentation encountered in metal dusting corrosion which is caused by the precipitation of a secondary phase and accompanied by the formation of filamentous carbon.

Nickel catalysts supported on carbon nanofibers: structure and activity in methane decomposition

Structures of Ni catalysts supported on filamentous carbon (CFC) produced by methane decomposition over coprecipitated Ni and Ni-Cu/alumina catalysts were studied by EXAFS and TEM. Thermal pre-treatment in N2 at 350°C of samples impregnated by nickel nitrate precursor was found to produce either NiO or nickel carbide, Ni3C, phase. This was explained by different reducing properties of the carbon nanofibers which depend on the surface structure. High stability of the Ni/C catalysts in methane decomposition reaction at 550°C was found with those prepared from only nickel chloride precursor, due to the formation of large (30-70 nm) Ni particles further leading to new carbon filaments growth. Data implies a common mechanism of the filamentous carbon deposition in all Ni-based catalysts, independent of the support (silica, alumina, carbon) being used. However, accumulation of filamentous carbon is strongly influenced by morphology and texture of the support.

Carbon deposition over transition metal alloys—II. Kinetics of deposition over 〈FeNi〉 and 〈FeCo〉 alloy foils

The effects of 〈FeNi〉 and 〈FeCo〉 alloy composition on the deposition of filamentous carbon was investigated. This was accomplished by comparing the relative rates of carbon formation over pure Fe, Ni, and Co foils along with their respective carbide(s), to those over 〈FeNi〉 and 〈FeCo〉 alloys and the associated metal phases. It was observed that the heat treatment to which a foil is subjected prior to carbon-depositing conditions can affect the rate of carbon deposition. Heat treatment causes grain nucleation and growth, decreases internal stresses, as well as allowing dislocation migration. These changes within the foil inhibit subsequent carbide formation, decreasing the rate of surface break-up, leading to longer induction periods. Alloy foils ranging from pure Fe to either pure Ni or Co were exposed to a reaction gas having an a ̂ C of 10. This a ̂ C is sufficiently high to favor solid carbon deposition and, depending on the alloy concentration, ≤ 50 wt% Ni in 〈FeNi〉, or ≤ 35 wt% for Co in 〈FeCo〉 alloys, Fe 3C is thermodynamically favored to form at these conditions. For the 〈FeNi〉 system it was determined that when iron carbides are among the thermodynamically favored solid phases, the rate of carbon deposition was high (75 to 100 μg/cm 2/min). If iron carbides were not favored then the rate was low, <30 and <5 μg/cm 2/min for 〈FeNi〉 and Ni 3C, 〈FeNi〉, respectively. Also, 〈FeCo〉 alloys near 45 wt% Co were exposed to various gas mixtures with a ̂ Cs ranging from ≈ 10 to ≈35. It was determined that when iron carbides are among the thermodynamically favored solid phases, the rate of carbon deposition was high (80 to 100 /μg/cm 2/min). If iron carbides were not favored, then the rate was significantly lower, <30 and < 10 μg/cm 2 min for 〈FeCo〉 and Co 2C, 〈FeCo〉, respectively. If the 〈FeCo〉 alloy foils were preoxidized then the rate of carbon deposition increased. For the 〈FeCo〉 system the increased rates of carbon deposition are attributed to an increase in the surface area of the “catalyst” as Fe 3C forms and breaks up the foil's surface.

Effect of oxide scale on carbon deposition on Fe-Ni alloys in carburizing gas

The behavior of carbon deposition on preoxidized Fe-Ni alloys containing 0 to 57.0 mass pct Ni in 10 pct CH4-H2 mixture at 1203 K was studied by metallography and thermogravimetry. Nickel retarded carburization and carbon deposition by lowering the solubility limit of graphite in austenite and by reducing catalytic activity for the pyrolytic reaction of CH4. On oxidation in air, the addition of nickel to iron depressed the development of FeO and, thereby, caused a significant decrease in the thickness of the scale. The exposure of the alloys to 10 pct CH4-H2 mixture after the oxidation in air led to a sudden mass loss in the early stage and then a rapid mass gain. This mass change is primarily ascribed to mass loss by reduction of iron oxides and to mass gain by carbon deposition. The rapid mass gain by carbon deposition is probably due to the formation of active iron by reduction of iron oxides and to the increase in the reaction area by spallation of the scale; the active iron formed may promote filamentous carbon deposition through Fe3C formation and decomposition. Carbon deposition on the alloys containing 27.2 mass pct Ni or more was considerably retarded because of the formation of a thin oxide scale which consists of α-Fe2O3 and spinel (NixFe3−xO4) and the reduction of catalysis by enrichment of nickel in the subscale. However, the amounts of carbon deposition increased compared with those on the as-polished alloys, owing to the presence of reducible iron oxides.

Localized Coking on Austenitic Heat-resisting Steels in CO-H2 Mixtures at 923 K.

The coking behavior on the austenitic heat-resistant steels (SUS309S and SUS310S) and iron in CO-H2 mixtures at 923 K was studied. Two types of carbon, laminar carbon and filamentous carbon, deposited on iron in large amount, while only filamentous carbon slightly deposited on the high temperature steels because of the formation of noncatalytic chromium-rich carbides on the surface instead of laminar carbon. On the steels, filamentous carbon deposited at some defects such as pittings and cracks occurred on the carbide scale. Thus, the increase in such surface defects by a severe surface finishing caused the acceleration of filamentous carbon deposition. The deposition of filamentous carbon on the surface defects or the localized coking is probably due to the depletion of chromium or the enrichment of iron at the exposed subsurface and thereby to the formation of an more unstable carbide. The behavior of filamentous carbon deposition on the heat-resistant steels seems to be essentially the same as that on iron. That is, the formation and decomposition of unstable carbides such as Fe3C and (Fe, Cr)3C may be involved in the process of filamentous carbon deposition on both iron and the heat-resistant steels.