**3. The system V – O – Cl**

Vanadium is a transition metal that can form a variety of oxides. At ambient temperature and oxygen potential, the form V2O5 is the most stable. It is a solid stoichiometric oxide, where vanadium occupies the +5 oxidation state. By lowering the partial pressure of O2, the valence of vanadium varies considerably, making it is possible to produce a family of stoichiometric oxides: V2O4, V3O5, V4O7, VO, VO2 and V2O3. Recently, it has been discovered that vanadium can also form a variety of non-stoichiometric oxygenated compounds (Brewer Ebinghaus, 1988), however, to simplify the treatment of the present chapter, these

A simplified version of the predominance diagram of Figure (11) can be achieved through considering each possible gaseous chloride as a pure substance. In this case, the field representing the gas phase will be divided into sub-regions, each one representative of the stability of each gaseous chlorinated compound. By considering, that, besides MCl5 and MCl4, gaseous MOCl3 can also be formed, a diagram similar to the one presented on Figure

The diagram of Figure (12) is associated with a temperature value where gaseous MCl5 can not be present in equilibrium for any suitable value of *P*(Cl2) and *P*(O2) chosen. It is interesting to note, that in this sort of diagram, there is a direct relation between the inclination of a line representative of the equilibrium between a gaseous chloride or oxychloride and an oxide, with the stoichiometric coefficients of the chemical reaction

According to Eq. (32), the inclination of the line associated with the equilibrium between MOCl3 and MO2 should be lower than the one associated with the equilibrium between MOCl3 and M2O5. On the other hand, in the case of the equilibrium between MO and MOCl3, the line is horizontal (does not depend on *P*(O2)), as the same number of oxygen atoms is present in the reactant and products, so O2 does not participate in the reaction.

22 2

*P P KT*

Cl O MO

2 2 2 5

Where, MO2 *<sup>K</sup>* , OM <sup>52</sup> *<sup>K</sup>* and *K*MO represent respectively the equilibrium constants for the

2 2 32 25 2 3 2 2 3

MO 1.5Cl MOCl 0.5O M O 3Cl 2MOCl 1.5O

 

The diagrams of Figures (11) and (12) depict a behavior, where no condensed chlorinated phases are present. For many oxides, however, there is a tendency of formation of solid or liquid chlorides and or oxychlorides, which must appear in the predominance diagram as fields between the pure oxides and the gas phase regions. Such a behavior can be observed

Vanadium is a transition metal that can form a variety of oxides. At ambient temperature and oxygen potential, the form V2O5 is the most stable. It is a solid stoichiometric oxide, where vanadium occupies the +5 oxidation state. By lowering the partial pressure of O2, the valence of vanadium varies considerably, making it is possible to produce a family of stoichiometric oxides: V2O4, V3O5, V4O7, VO, VO2 and V2O3. Recently, it has been discovered that vanadium can also form a variety of non-stoichiometric oxygenated compounds (Brewer Ebinghaus, 1988), however, to simplify the treatment of the present chapter, these

*P P KT*

Cl O M O

(32)

(33)

(12) would represent possible stability limits found in equilibrium.

2

formation of MOCl3 from MO2, M2O5 and MO (Eq. 33).

in the equilibrium states accessible to the system V – O – Cl.

**3. The system V – O – Cl** 

<sup>2</sup> ln ln

Cl MO

*P KT*

3

MO 1.5Cl MOCl

1 2 ln ln ln 3 3

1 1 ln ln ln 2 3

behind the transformation.

phases will not be included in the data-base used for the following computations. Additionally, it was considered that the concentration of the oxides in gas phase is low enough to be neglected. Further, on what touches the computations that follows, the software *Thermocalc* was used in all cases, and it will always be assumed that equilibrium is achieved, or in other words, kinetic effects can be neglected.

The relative stability of the possible vanadium oxides can be assessed through construction of a predominance diagram in the space *T* – *P*(O2) (see Figure 13). As thermodynamic constraints we have *n*(V) (number of moles of vanadium metal – it will be supposed that *n*(V) =1), *T*, *P* and *P*(O2). The reaction temperature will be varied in the range between 1073 K and 1500 K and the partial pressure of O2 in the range between 8.2.10-40atm and 1atm.

Fig. 13. Predominance diagram for the system V – O

The total pressure was fixed at 1atm. It can be seen that for the temperature range considered and a partial pressure of O2 in the neighborhood of 1atm, V2O5 is formed in the liquid state. Through lowering the oxygen potential, crystalline vanadium oxides precipitate, VO2 being formed first, followed by V2O3, VO, and finally V. The horizontal line between fields "5" and "6" indicates the melting of V1O2, which according to classical thermodynamics must occur at a fixed temperature. Next it will be considered the species formed by vanadium, chlorine and oxygen.

### **3.1 Vanadium oxides and chlorides**

The already identified species formed between vanadium, chlorine and oxygen are: VCl, VCl2, VCl3, VCl4, VOCl, VOCl2, VOCl3, VO2Cl.

On Table (1) it was included information regarding the physical states at ambient conditions and some references related to phase equilibrium studies conducted on samples of specific vanadium chlorinated compounds.

Only a few studies were published in literature in relation to the thermodynamics of vanadium chlorinated phases. On Table (1) some references are given for earlier

On the Chlorination Thermodynamics 805

Finally, by starting in a state inside a field representing the formation of VCl4 or VCl3 and by making *P*(O2) progressively higher, a value is reached, after which VOCl3(g) appears. So, the mol fraction of VCl4 and VCl3 in gas should reduce when *P*(O2) achieves higher values. This

is again consistent with the speciation computations developed on topic (3.1.3.2).

Fig. 14. Predominance diagram for the system V – O – Cl at 1073 K

Fig. 15. Predominance diagram for the system V – O – Cl at 1273 K

investigations associated with measurements of the vapor pressure for the sublimation of VCl2 and VCl3, and the boiling of VOCl3 and VCl4. There are also evidences for the occurrence of specific thermal decomposition reactions (Eq. 34), such as those of VCl3, VOCl2 and VO2Cl (Oppermann, 1967).


Table 1. Physical nature and phase equilibrium data for vanadium chlorinated compounds

$$\begin{aligned} \text{2VCl}\_3(\text{s}) &\rightarrow \text{VCl}\_2(\text{s}) + \text{VCl}\_4(\text{g})\\ \text{2VO}\_2\text{Cl}(\text{s}) &\rightarrow \text{VOCl}\_3(\text{g}) + \text{V}\_2\text{O}\_5(\text{s})\\ \text{2VOCl}\_2(\text{s}) &\rightarrow \text{VOCl}\_3(\text{g}) + \text{VOCl}(\text{s}) \end{aligned} \tag{34}$$

Chromatographic measurements conducted recently confirmed the possible formation of VCl, VCl2, VCl3, and VCl4 in the gas phase (Hildenbrand et al., 1988). In this study the molar Gibbs energy models for the mentioned chlorides were revised, and new functions proposed. In the case vanadium oxychlorides, models for the molar Gibbs energies of gaseous VOCl, VOCl3, and VOCl2 have already been published (Hackert et al., 1996).

For gaseous VO2Cl, on the other hand, no thermodynamic model exists, indicating the low tendency of this oxychloride to be stabilized in the gaseous state.
