Thermal Analysis

The thermogravimetric (mass loss, TG), derivative thermogravimetric (rate of mass loss, DTG) and differential thermal analysis (DTA) curves of 50 mg of uncalcined samples, over the range from room temperature up to 900°C in the flow of an atmosphere of dry air, are shown in Figs. 18, 19, 20, 21 and 22.

3.3.1. TG and DTA of Mo/Al2O3

The TG-DTG and DTA curves recorded in argon flow for pure (NH4)6Mo7O24.4H2O (AHMT) at a scanning speed of 5°C/min up to 600°C are shown in Fig. 17. The TG-DTG exhibit three decomposition peaks with the maximum located at 127, 220 and 307°C respectively. The TG shows that AHMT loses weight with heating over three steps.

The first endothermic peak at 100-127°C was followed by 7.3% mass loss corresponding to the loss of three H2O molecules and two molecules of NH3, which is consistent with the theoretical value (according to the literature, the theoretical weight loss, assuming release of three water molecules and two NH3 molecules is 7.1%) [160–162]. The second endothermic peak at 200-225°C which was accompanied by a mass loss of 4.4% is probably related to the elimination of (NH4)2O (i.e. 2NH3 + H2O) which is in agreement with the theoretical value (4.2%). The third endothermic peak takes place within 267-327°C was associated with 7.4%

weight loss (theoretical value 7.1%) and corresponds to the evolution of two NH3 molecules together with three molecules of H2O. Moreover, the exothermic peak of maxima at 327°C confirms the crystallization of new phases (probably MoO3). Therefore, the overall mass loss was 19.1%. These three stages can be explained to proceed according to the following equations:

(NH4)6Mo7O24.4H2O ⎯100⎯⎯−127⎯⎯°⎯C→ (NH

→

→

4)4Mo7O23.2H2O + 3H2O + 2NH3 (Eq-24) (NH4)4Mo7O23.2H2O ⎯200⎯⎯−225⎯⎯°⎯C (NH

4)2Mo7O22.2H2O + H2O + 2NH3 (Eq-25) (NH4)2Mo7O22.2H2O ⎯267⎯⎯−327⎯⎯°⎯C 7MoO3 + 3H2O + 2NH3 (Eq-26)

Fig. 17. TG-DTG and DTA curves for AHMT in argon atmosphere

The TG-DTG and DTA curves recorded in dry air flow for Mo/Al2O3 exhibit a large mass loss (37.8%) up to 300°C with an endothermic peak of maxima at 83°C in the DTA thermogram (Fig. 18). This is corresponding to the loss of structural and intercalated water and to the decomposition of (NH4)6 Mo7O24.4H2O [160, 161].

The DTA thermogram of this sample displays a very big exothermic peak of a maximum at around 300°C that could be due to the process of the formation of new phases probably to the crystallization of MoO3 species [162-165].

However, the three decomposition steps of (NH4)6Mo7O24.4H2O are missed on the TG-DTA curves of Mo/Al2O3 due to several factors such as: first, Mo/Al2O3 is impregnated and dried, second, the heating atmosphere (dry air) and heating rate (10°C/min) are different from those (argon flow, 5°C/min) applied for thermal analysis of pure (AHMT).

Fig. 18. TG-DTG and DTA curves of Mo/Al2O3

3.3.2. TG and DTA of Ce-Mo/Al2O3 and Mo/CeO2

TG-DTG and DTA curves recorded for Ce-Mo/Al2O3 (Fig. 19) indicate that the weight loss (20.5%) under 300°C is less than that of Mo/Al2O3. This mass loss is due to the removal of water and to the decomposition of Ce(NO3)3 and (NH4)6 Mo7O24.4H2O producing the corresponding oxides according to the equation:

(NH4)6 Mo7O24.4H2O + Ce(NO3)3 + 2O2 → 7MoO3 + CeO2 + 3NO2 + 6NH3 + 12H2O (Eq-27)

This indicates that the weight loss is highly affected by the mode of the preparation that in turn affects the interaction mode of CeO2–MoO3 compounds. The latter affects by its turn the thermal stabilities of the produced compounds. In the meanwhile, the position of the endothermic peak shifts to a higher value with a maximum at 101°C, whereas the big exothermic peak remained with the maximum at around 300°C comparatively.

Of particular interest, the TG and DTA of Ce-Mo/Al2O3 display a large mass loss (23%) extended between 700 and 900°C with an endothermic peak of maxima at 758°C indicating that the material undergoes morphological and structural modifications. This may be due to the high oxygen storage and release of ceria resulting in lattice defects and thus enhancing the mobility of Mo and Ce ions. Thus, the crystalline structure is progressively modified comparatively favoring the possible mutual diffusion of the Mo and Ce ions. The last mass loss in this material may characterize the sublimation of some molybdena produced [65, 80].

The thermal decomposition course presented for Mo/CeO2 is shown to be of multistep and represented by four endothermic peaks of maxima at 61, 98, 173 and 737°C in the DTA curve (Fig. 20) besides a big exothermic peak of a maximum at 308°C. As can be seen, this material contributes a total weight loss around 34.7%. On the other hand, it is worth mentioning that the decomposition temperature of either Mo or Ce precursor salts in the synthesized materials are not similar revealing that the interactions between them are varied according to altering the preparation methods and their content, which indeed affect the mobility of Mo species.

Fig. 19. TG-DTG and DTA curves of Ce-Mo/Al2O3

Fig. 20. TG-DTG and DTA curves of Mo/CeO2

3.3.3. TG and DTA of Sn-Mo/Al2O3 and Mo/SnO2

On the TG curve of Sn-Mo/Al2O3 a large weight loss step is marked (38.3%) quite similar to that of Mo/Al2O3 (37.8%) comparatively implying that the addition of SnO2 did not affect the thermal behaviour of Mo/Al2O3. This step with 38.3% mass loss of the original mass corresponds to dehydration and decomposition of ammonium heptamolybdate (Fig. 21).

The DTA curve of Sn-Mo/Al2O3 shows three endothermic peaks at 69, 103 and 211°C indicating the presence of weakly bounded, strongly bonded and structural water species.

The TG curve of Mo/SnO2 is definitely different (Fig. 22). Two mass loss steps are observed.

The rate in the first stage of its mass loss between 70 and 300°C is relatively low (11.4%), while the mass loss in the second stage is higher (41.7%). The overall weight loss measured up to 900°C is 53.1%. The DTA curve of Mo/SnO2 shows first an endothermic dehydration occurring between 70 and 200°C (temperature peaks marked at 71 and 197°C). It is followed by the exothermic effect starting from 300°C (peaks marked at 327 and 396°C) indicating the process of crystallization in this sample (probably formation of molybdenum oxide).

Nevertheless, the DTA peaks centred at 797 and 846°C explicitly are two strongly endothermic heat effects of the main degradation presumably due to the release of some volatile tin and molybdena species and/or due to surface and structure modifications of the contact between phases leading to lattice defects, and thus enhancing the mobility of the two ions and mutual interaction between them. These changes indicate that Mo can migrate, some Mo and Sn species can be destroyed, while others are created or increased. It is, however, hard to estimate if new segregated phases are formed upon thermal treatment above 750°C that can be followed more certainly by TG-MS studies [83].

Fig. 21. TG-DTG and DTA curves of Sn-Mo/Al2O3

Fig. 22. TG-DTG and DTA curves of Mo/SnO2