Deactivation of supported metal catalysts by carbon or coke formation is a problem of serious magnitude in steam reforming, methanation, and other important catalytic processes. Its causes are generally threefold: (1) fouling of the metal surface, (2) blockage of catalysts pores and voids, and/or (3) actual physical disintegration of the catalyst support. Since loss of catalytic activity and physical destruction of the catalyst by carbon deposits can occur rapidly (within hours or days) under unfavorable conditions, understanding and control of these effects are of major technological and economical importance.

Carbon Deposition in Steam Reforming and Methanation

The steam reforming process converts hydrocarbons into mixtures of hydrogen, carbon monoxide, carbon dioxide, and methane 

$$C_n H_m + nH_2 O \to nCO + \left( {n + \frac{m}{2}} \right)H_2 \left( { - \Delta H_{298}^0 < 0} \right) $$

(1)



$$ CO + H_2 O \rightleftarrows CO_2 + H_2 \left( { - \Delta H_{298}^0 = 41.2kJ\,mol^{ - 1} } \right) $$

(2)



$$ CO + 3H_2 \rightleftarrows CH_4 + H_2 O\left( { - \Delta H_{298}^0 = 206.2kJ\,mol^{ - 1} } \right) $$

(3)

The expression reforming is misleading since it is used also for the well-knowm process for improvement of the octane number of gasoline [1]. In the gas industry, reforming has generally been used for “the changing by heat treatment of a hydrocarbon with high heating value into a gaseous mixture of lower heating value” [2].

Catalytic Steam Reforming

Precise knowledge of the Fe-C-Si system has been the subject of numerous previous studies because of the importance of this system for steelmaking processes and cast iron foundry. How-ever, the most recent articles on this subject still reveal uncertainties or insufficient information. The purpose of this work was to assess this system and to give it a precise description, but one which was simple enough for practical applications. An evaluation of the parameters describing the thermodynamic properties of the phases involved in the binary Fe-Si system has been achieved, mainly based on values of the silicon activity in the body-centered cubic (bcc) phase and on knowledge of the phase diagram. The A2-B2 ordering reaction of the bcc phase has been included. It has been shown that this reaction is of paramount importance for the description of the thermodynamic properties of this phase and its field of stability. This preliminary work and previous similar studies of the Fe-C and Si-C systems were used as a base for the evaluation of the ternary stable and metastable Fe-C-Si systems. Optimization of the parameters describing the properties of the phases involved in both of these systems was achieved using experimental thermodynamic data and the critical assessment of the phase diagrams.

An Assessment of the Fe-C-Si System

Vanadium as an important alloying element in steels was initially associated with the properties achieved following tempering. Interest in the microstructure was stimulated by the advent of transmission electron microscopes with a resolution of ∼1 nm together with selected area electron diffraction techniques. A second timely development was that of controlled rolling, particularly of plate and sheet products. The scope of this review will include the historical background on quenched and tempered vanadium steels, precipitation during isothermal aging, conventional controlled rolling and during thin slab direct charging and the development of strength and toughness in vanadium microalloyed steels. The characterisation of microstructure, in particular the methods for the analysis of the chemical composition of precipitates has progressed since the availability of X-ray energy dispersive analysis in the 1970s, and the role played by electron energy loss spectroscopy in providing quantitative analysi...

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Processes, microstructure and properties of vanadium microalloyed steels

The thermodynamic stabilities of Cr carbides have been reviewed in the Cr-C, Fe-Cr-C, Ni-Cr-C, Fe-Cr-V-C and Fe-Cr-W-C systems considering the relative stabilities between Cr carbides and V carbide or W carbides. A new description of the Cr carbides was made, which can give satisfactory agreement between calculation and experimental data both in the Ni-Cr-C system and the Fe-Cr-V-C system. Taking into consideration the Ni-Cr-C and Fe-Cr-V-C systems, and the relative stabilities between Cr carbides themselves and between Cr carbides and V or W carbide, reassessments were made of the Ni-C, Ni-Cr, Cr-C, Ni-Cr-C and Fe-Cr-C systems. Comparisons between calculation and experimental data are presented for each system reassessed and also for the Fe-Cr-V-C and Fe-Cr-W-C quaternary systems.

On the stability of Cr carbides

The activity of carbon in the fcc solid solution of the Fe−Ni−C system has been measured at 800°, 1000°, and 1200°C by comparison with observed values in the Fe−C binary by equilibration with methane-hydrogen mixtures. Defining the lattice ratioz
 C≡n
 C/(n
 Fe+n
 Ni−n
 C), the activity coefficient ΨC≡a
 C/z
 C has been determined as a function of temperature and composition. At infinite dilution log ΨC goes through a maximum at about 70 pct Ni in agreement with Smith. The partial molar free energy of carbon in the dilute solution referred to graphite is not a linear function of the base alloy composition, but has a large deviation with maximum at about 60 pct Ni. Similar maxima occur in both ΔH
 C
 °
 and ΔS
 C
 °
 . Linear equations are derived for the activity coefficient of carbon in three composition ranges of Fe−Ni−C alloys; a simplified equation applicable to nickel steels is included. The solubility of graphite in nickel has been determined. The marked deviation from linearity is ascribed to the existence of iron atoms in two electronic states, γ1 and γ2 which differ in energy and are antiferromagnetic and ferromagnetic, respectively.

Thermodynamics of the Fcc Fe−Ni−C and Ni−C alloys

The activity of carbon in austenitic Fe-Mo-C, Fe-Cr-C, and Fe-V-C alloys has been studied by equilibration with controlled CH4-H2 atmospheres at temperatures in the range 850° to 1200°C. The observations included a number of compositions in the two-phase fields, γ + carbide. Equations are given for the activity coefficient of carbon as a function of temperature and composition in the austenite field and from these the other thermodynamic properties of the solution may be computed as desired. The phase boundaries γ/γ + carbide were determined by breaks in the isoactivity lines. This was supplemented in the case of Fe-Mo-C alloys by metallographic linear analysis of equilibrated samples. The results confirm certain published phase diagrams and discredit others.

Activity of carbon and solubility of carbides in the FCC Fe-Mo-C, Fe-Cr-C, and Fe-V-C alloys

The activity of carbon in austenitic Fe-Mn-C and Fe-Si-C alloys has been studied by equilibration with controlled CH4-H2 atmospheres at temperatures in the range 848° to 1147°C and for composition up to about 60 pct Mn and 7 pct Si. The activity coefficient of carbon is diminished by manganese and is increased by silicon. Activity coefficients and derived values of the partial molar free energy, enthalpy, and entropy of solution of graphite in the alloy are expressed in mathematical form. The heat of solution of graphite, which is positive in the Fe-C binary alloys, decreases with increasing manganese and increases with increasing silicon concentrations. The partial molar entropy is independent of manganese, but is decreased by silicon.

Tsuguyasu Wada

Papers

Thermodynamics of the Fcc Fe−Ni−C and Ni−C alloys

Activity of carbon and solubility of carbides in the FCC Fe-Mo-C, Fe-Cr-C, and Fe-V-C alloys

Thermodynamics of the fcc Fe-Mn-C and Fe-Si-C alloys