Libro Segundo LA REBELIÓN MULATA
UN DESTELLO QUE SE APAGA
The total reduction of the transition metals in BSCFMo0.375-(Co/Fe=4) to metals with oxidation state of (0), was studied by thermogravimetric analysis by heating at 1100oC/10h in a mixture of 50ml/min 5% H2/N2 and 80ml/min N2 as the carrier gas. The sample was initially heated at 150oC/3h for the removal of moisture.
Figure 4.7: Thermogravimetric analysis (TGA) showing the weight loss of BSCFMo0.375- (Co/Fe=4) after heating at 1100oC for 10h in reducing conditions (in a mixture of 50ml/min 5% H2/N2 and 80ml/min N2 as the carrier gas)
After the annealing at the reduced atmosphere (Figure 4.7), there was 4.12 mg of weight loss for initial mass of mo= 54.88mg, corresponding to the reduction reaction of Ba0.5Sr0.5Co0.5Fe0.125Mo0.375O3-δ to Ba0.52+Sr0.52+Co0.5xFe0.125yMo0.375zOred2-, where [Ored] is
Page | 150 calculated according to charge balance. For total reduction of all transition metals to their metal state, i.e. x= y=z=0, the oxygen content of the reduced sample ([Ored]) is calculated to be 1. The oxygen content [Ο] of Ba0.5Sr0.5Co0.5Fe0.125Mo0.375O3-δ (BSCFMo0.375-(Co/Fe=4)) at room temperature in air is calculated according to Equation 4.1.
Equation 4.1
where [Ored] the oxygen content of the reduced sample Ba2+0.5Sr2+0.5Cox0.5Fe y 0.125Moz0.375Ored2- with x=y=z=0 for total reduction,
denotes the number of moles that are reduced normalised by the number of moles (
) of the reduced material.
However, according to the XRD analysis (Figure 4.8) of the post-reduction specimen, complete reduction of BSCFMo0.375-(Co/Fe=4) was not achieved.
20 30 40 50 60 70 80 P Fe Fe3O4 Co perovskite (B) (A) BaMoO4 BaMoO4 P R elat ive i ntensi ty Position [ o2 theta]
Figure 4.8: XRD pattern of BSCFMo0.375-(Co/Fe=4) sample (A) before and (B) after annealing at 1100oC for 10h in 5%H2/N2, showing the decomposition products.
Page | 151 The oxidation states of the transition metals vary, with cobalt mostly found as Co (0) and Fe in the form of Fe3O4 and Fe (0). The most intense reflection of a perovskite phase is still present but shifted indicating change in the lattice parameters due to compositional changes, whilst there is also some small amount of amorphous BaMoO4, which is the evidence of some amount of Mo6+. There is no evidence of the Ba, Sr binary oxides, suggesting that they are either in the form of amorphous hydroxides as the alkalines in BSCF have great tendency to absorb moisture[69], or that they occupy the A-site of the survived perovskite phase. The remaining amount of Mo is likely to form the B-site of the perovskite phase with oxidation state of Mo4+, since BaMo(IV)O3 and Sr(IV)O3 are reported to be very stable compounds in reduced atmospheres[229] .
According to thermodynamic analysis, the reduction of the transition metals, occurs only if ΔGreaction< O, where ΔG reaction is the calculated free Gibbs energy for the reduction reactions by oxidization of H2 (Table 4.4). The data for free energy of reduction of the binary oxides were derived from Ellingham diagrams [230]at the temperature of the reduction experiment (1100oC). Table 4.4: Calculation of the free Energy Gibbs (ΔG reaction) for the reduction reactions of
the transition metal oxides in BSCFMo0.375-(Co/Fe=4), in the form of binary oxides, by H2
(2 H2 + O2 ↔ 2 H2O, ΔGf = 342.37 kJ) at 1100oC and 0.21 atm. The thermodynamic data
were derived from Ellingham diagrams[230].
Reduction
Reaction
ΔG
reaction(kJ/mol)
Co
3+/4+ Co
2+2 Co
3O
4+ 2 H
2↔ 6 CoO + 2 H
2O
-382.57
Co
2+ Co (0)
2 CoO + 2 H
2↔ 2 Co + 2 H
2O
-68.77
Co
3+/4+ Co (0)
Co
3O
4+ 4 H
2↔ 3 Co + 4 H
2O
-588.88
Fe
3+ Fe
2+/3+6 Fe
2O
3+ 2 H
2↔ 4 Fe
3O
4+ 2 H
2O
-229.74
Fe
2+/3+ Fe
2+2 Fe
3O
4+ 2 H
2↔ 6 FeO + 2 H
2O
-61.48
Fe
3+ Fe
2+2 Fe
2O
3+ 2 H
2↔ 4 FeO + 2 H
2O
-352.7
Fe
2+ Fe(0)
2 FeO + 2 H
2↔ 2 Fe + 2 H
2O
+5.1
Fe
3+ Fe(0)
Fe
2O
3+ 6 H
2↔ 6 Fe + 6 H
2O
-322.1
Mo
6+ Mo
4+2 MoO
3+ 2 H
2↔ 2 MoO
2+ 2 H
2O
-242.57
Mo
4+ Mo(0)
MoO
2+ 2 H
2↔ Mo + 2 H
2O
+6.14
Mo
6+ Mo(0)
2 MoO
3+ 6 H
2↔ 2 Mo + 6 H
2O
-230.4
The reduction of Co3+/4+ to Co (0) is thermodynamically favourable (ΔG = -588.88 kJ/mol), with the intermediate step to Co2+ also favored under the conditions of the experiment. The total
Page | 152 reduction of Fe3+to Fe(0), with intermediate reductions to Fe2+/3+ and Fe2+ , also gives negative value of ΔG reaction (ΔG = -322.1 kJ/mol) and explains the fact that Fe(0) is observed in the PXRD pattern of BSCFMo0.375-(Co/Fe=4) after the reduction experiment. The relatively low energy barrier for the thermodynamically disfavored reduction of Fe2+ to Fe (0) (ΔG =+5.1 kJ/mol> O) in the case of binary oxides can differ in the case of perovskite oxides and become even favorable. Another possible explanation for the observed Fe(O) content in the XRD pattern of the reduced material could be that the highly favorable reaction of Fe3+ to Fe2+ (ΔG = -352.7 kJ) promotes the reduction of Fe2+ to Fe (0).
One might expect that the total reduction of Mo6+ to Mo (0) (ΔG = -230.4 kJ/mol) would also occur similarly to the Fe case, by surpassing the energy barrier for the slightly thermodynamically disfavored reduction of Mo4+to Mo(0) (ΔG = +6.14 kJ). However, reduction to Mo(0) was not observed, suggesting that the Mo4+-containing perovskite phase formed at the end of the reduction has a very high lattice energy and the energy released by the reduction to Mo4+ is not sufficient to overcome this energy barrier and promote the reduction to Mo(0). Whilst inthe diffraction pattern there are also reflections indexed to (Ba,Sr)MoO4, where Mo is in its hexavalent state, the relative intensities suggest that amount of this phase is small and hence Mo is considered to exist mostly as Mo4+ in the perovskite phase surviving after the reduction experiment. Finally, the fact that both Fe2+/3+3O4and Fe (0) are indexed in the PXRD pattern, with no evidence of the intermediate Fe2+O, suggests that the highly stable Mo containing perovskite phase might also consist of some amount of Fe2+/3+3O4 which cannot obtain the required energy for reduction to Fe(0). The oxygen content of BSCFMo0.375-(Co/Fe=4) was then calculated (Equation 4.1), based on the identified oxidation states by XRD and confirmed by the thermodynamic considerations, of Co(0), Mo4+ and a mix valence of Fe between Fe2O3 and Fe(0).
Page | 153 Table 4.5: Oxygen content ([O]) based on the reduction experiment of BSCFMo0.375- (Co/Fe=4) in 5%H2/N2 at 1100oC for 10h, using oxidation states of Co(0), Mo4+ and considering
different cases for the Fe charge, as coming out from the concordant outcomes of XRD analysis of the post-reduction sample and thermodynamic considerations.
Case Co(x) Fe(y) Mo(z) [O]
1 0 2+/3+ 4+ 2.998
2 0 0 4+ 2.830
3 0 2+/3+
0
4+ 2.914
Three different cases of the oxidation state of Fe (y) were considered, Fe2+/3+ (case 1), Fe (0) (case 2) and a 1:1 mixture of Fe2+/3+ andFe (0) (case 3), giving oxygen contents of 2.998, 2.830 and 2.914 respectively. Case 3 is more consistent with the observations from the diffraction pattern as both Fe2+/3+3O4andFe (0) were identified and gives an oxygen content of 2.914.