3.1 Characterizations
The thermogravimetric analysis (TG) of the LaFeO3 precursor indicates the temperature at which the oxide structure remains stable (Fig. 1). The initial mass loss of around 40% is related to volatile compounds’ elimination and decomposition of nitrate and citrate (Pino et al. 2011). Below 200°C, it releases weakly adsorbed water and volatile gases. From 200°C to 400°C, there is the decomposition of nitrates and free citrates, and between 400°C to 650°C, the decomposition of nitrites and carbonates. The thermo-differential (DTA) profile indicates an exothermic decomposition of the citrate at 360°C suggesting structural changes. Above 660°C, the decomposition or phase transformation was not observed, suggesting stable oxides formations. Thus, 650°C was the temperature used for the calcination step.
The LF perovskite catalytic support and NFL catalyst diffractogram indicate a highly crystalline structure through the intense peaks observed (Fig. 2). After nickel impregnation (Fig. 2b), the orthorhombic form of the perovskite-type oxide was not modified. NiO peaks were identified at 37.2°, 43.2°, 62.7°, and 75.4° (ICDD 01-073-1519). The cell parameters calculated by the Rietveld refinement method indicate an orthorhombic structure and suggest that the deformation or distortion from the ideal cubic structure occurred during the synthesis (Table 1). The average crystallite size was calculated from the full width at half maximum (FWHM) at 2θ = 32.2°, using Scherrer’s equation, and was 28 nm. The mean crystallite size of the peak at 37.2° was 23 nm (Kumar et al. 2019).
Table 1
XRD cell parameters and crystallite sizes of the perovskite (LF) and Ni catalyst (NLF).
Sample
|
Plane (hkl)
|
2θ (°)
|
FWHM
|
Crystallite size (nm)
|
LF
|
(121)
|
32.22
|
0.29708
|
27.9
|
NLF
|
LF (121)
|
32.22
|
0.3105
|
26.7
|
NiO (111)
|
37.26
|
0.35894
|
23.1
|
(200)
|
43.35
|
0.53324
|
15.5
|
Perovskite LaFeO3 is like the A+ 3B+ 3O3 structure and belongs to the spatial group of compounds with Pbnm structure. In this group, a subgroup (7Ag + 5B1g + 7B2g + 3B3g) presents an orthorhombic structure giving 24 active vibrational and can be observed in Raman spectra (Galasso 1969; Rosseau et al. 1981). In our study, LF shows the bands relative to the inorganic compounds located below 500 cm− 1, at 145 cm− 1 and 428 cm− 1 (Socrates 2004). The bands with high intensity 145, 168, and 263 cm− 1 are attributed to the Fe2O3, while the bands at 494 and 627 cm− 1 to LaO2. On the other hand, the bands located at 1300 cm− 1 are attributed to carbon-containing compounds, which may be residuals of organic compounds used in the synthesis.
Nickel particles are non-uniformly distributed over the perovskite support (Fig. 5a). Transmission Electron Microscopy (TEM) allowed observing the interaction between the catalyst particles and estimating the particle sizes of 27.5 and 25 nm to LF and NiO particles, respectively (Fig. 5b). This result agreed with crystallite sizes obtained by XRD (28 and 23 nm), as presented in Table 1. The interplanar distance (Fig. 5c) indicates the plane’s position related to the lanthanum and iron oxide phases and the nickel oxide phase (Fig. 5d). It may be noted that the NiO is scattered throughout the structure. According to the XRD data, the NiO phase corresponds to the planes (200) and (111) and the others to the lanthanum and iron phases, as presented in Table 2.
Table 2
Crystalline phases and particle sizes of the samples.
Sample
|
Crystal size (nm)
|
Particle size (nm)
|
Crystal planes (hkl)
|
Phases
|
XRD
|
TEM
|
Ni/LaFeO3
|
28
|
27.5
|
(121)
|
LaFeO3
|
23
|
25
|
(111)
|
NiO
|
To better understand the specific role of nickel oxide on the catalytic activity under LF support, TPR studies were conducted on NLF catalysts. The hydrogen pre-treatment involves different levels of reduction in each metal element. In the case of nickel, the total reduction of nickel oxide NiO to metal nickel Ni0 can be suggested, as shown in Fig. 6. The first region has a shoulder around 350°C, followed by a broad peak with a maximum of 560°C in the second region and a complete reduction at 700°C. For comparison, the reduction of bulk NiO is shown separately in Fig. 6, indicating that the maximum reduction peak of the bulk NiO occurs at 527°C (Khan and Smirniotis 2008), close to the top peak NiO supported on LF. Thus, the shoulder around 350°C can be attributed to the reduction of larger NiO particles. The NiO reduction between 350°C and 700°C also be described by Eq. (3). Consequently, the calculation of the reduction of NiO to metallic Ni0 was 26%.
\({\text{N}\text{i}\text{O} +\text{H}}_{2} \to {\text{N}\text{i}}^{0} + {\text{H}}_{2}\text{O}\)
|
(3)
|
\(3{\text{F}\text{e}}_{2}{\text{O}}_{3}{ +\text{H}}_{2} \to {2\text{F}\text{e}}_{3}{\text{O}}_{4} + {\text{H}}_{2}\text{O}\)
|
(4)
|
In the case of LF support, the role of iron was considered, mainly in the partial reduction of hematite (Fe2O3) to magnetite (Fe3O4). Observing the TPR profiles, the first reduction region begins at around 300°C, corresponding to the transitions of Fe2O3 to Fe3O4 (Eq. 4) and provoking the shift from 527°C to 566°C. TPR curves of hematite showed a peak at 510°C related to magnetite formation, and another around 770°C was attributed to the metallic iron formation (Khan and Smirniotis 2008).
The interaction between catalyst particles shown in the TEM image turns the light on what occurs with them during the reduction process. TPR profile of NLF showed a reduction at temperatures higher than 600°C, suggesting metal-support solid interaction (Fig. 5b). The broad peak can be associated with the metal-support interactions (MSI), which were identified as, weak interactions (WMSI) for temperatures less than 500°C; medium interactions (MMSI) between 500°C and 600°C, and strong interactions (SMSI) for temperatures higher than 600°C. Furthermore, the broad peak between 600°C and 800°C can be attributed to the loss of adsorbed oxygen or the formation of vacancies at sub-stoichiometric perovskite. In summarizing, the reduction of Fe2O3 → Fe3O4 was observed at around 350°C, followed by the reduction of Ni2+ to Ni0 at 566°C. Thus, catalytic tests were performed up to a temperature of 600°C.
3.2 Catalytic activity
The activity of the NLF catalysts was compared between TRM and POM processes under identical pre-treatment conditions, i.e., they were reduced under H2 flow (50 cm3/min) at 650°C for 1 h before the reactions. The principal aim was to determine the catalyst efficiency to produce hydrogen for further application in fuel cells. For 700°C, the catalyst achieved 35% CH4 conversion in the TRM, while in the POM, it was 50% (Fig. 7). Note that methane conversion for the perovskite support LaFeO3 decreased by half, about 26% at 700°C. Thus, these last results prove that Ni’s addition doubled the activity for 7 h. Thinking of improving the outcomes of TRM, we increase the reaction temperature from 700°C to 800°C. However, the tests showed a fast deactivation due to the carbon amount being too higher for times longer than 150 min, clogging the reactor.
Both reaction conversions resulted in high selectivity to hydrogen, especially TRM (Fig. 8). The hydrogen selectivity was approximately 78%, while the CO selectivity was 21% at 700°C. This result suggests that the TRM would be very suitable to operate at this temperature, with low carbon deposition and high hydrogen yield for a space velocity of 1.06x106 cm3/g.min. Even increasing the temperature from 700°C to 750°C in the POM, the WHSV = 2x105 cm3/g.min, and the highest selectivity of 60% are minor than TRM. LaFeO3 perovskite support presented only 30% H2 selectivity, but the 57% CO2 selectivity was unexpected. The CO2 selectivity of the NLF catalyst decreased three times compared to the LaFeO3 perovskite support for the same temperature. Concerning TRM, these values were insignificant. Table 3 shows the CH4 and CO2 conversions and selectivity for both reactions.
Table 3
Conversion and selectivity at 700°C and 800°C for the TRM and POM reactions.
Reaction
|
Temperature (°C)
|
Conversion (%)
|
Selectivity (%)
|
Ratio
|
CH4
|
CO2
|
H2
|
CO
|
H2/CO
|
TRM
|
700
|
33.4
|
2.9
|
77.31
|
22.67
|
3.4
|
800
|
43.1
|
18.2
|
71.32
|
28.64
|
2.5
|
POM
|
700
|
55.0
|
-
|
41.0
|
37.0
|
1.2
|
750
|
65.1
|
-
|
58.3
|
52.1
|
1.2
|
3.3 Discussion
During TRM and POM processes, there is a complex network of reactions. The nickel added to the perovskite increased the syngas formation. Consequently, the produced hydrogen reduces the metal oxide to metallic nickel. However, the decrease of active metals and the increase of O2 on the surface during the reaction demonstrate that the reaction’s limit is fixed at the surface by oxygen (García-Vargas et al. 2015; Zhao et al. 2017). The lattice oxygen is transferred from the sub-surface to the surface during the reaction and participates in the response. We believe that the H2 formed is responsible for reducing Ni ions into metallic Ni0.
On the other hand, CH4 is dissociated and reacts with the oxygen molecule, increasing the hydrogen selectivity (Boudouard reaction). This carbon reacts with oxygen and produces carbon monoxide and carbon dioxide. Total and partial oxidation produces carbon and water, promoting the water-gas shift reaction and producing more CO2. Consequently, it favors the dry reforming reaction.
In the TRM, the steam reforming process combines with the structure breaking the OH bond of the vapor. The O atom derived from H2O can provide oxygen in vacancies immediately. Meanwhile, the two H atoms generated from the OH binding break to form H2 (Zhao et al. 2014; García-Vargas et al. 2015). Notably, the H2 selectivity for the TRM reaction at 700°C was higher than for the POM reaction. This suggests the water-gas shift reaction and dry reforming preference in the tri-reforming process, preventing carbon formation (García-Vargas et al. 2015). Also, the NLF catalyst stability can be attributed to vacancies in the perovskite structure provoked by Fe+ 3 and Ni+ 3 ions and oxygen mobility (redox) to the surface (Zhao et al. 2017). These factors favor the carbon oxidation and the total methane oxidation during the TRM.
To better understand the carbon deposition, we analyzed the samples of used NFL catalyst in POM after the tests through Raman spectroscopy (Landon et al. 2012). The sample showed the active vibrational modes of the Raman spectra of FeO3 (155 and 283 cm− 1) and La2O3 (490 and 698 cm− 1) and the presence of carbon at the 1313 cm− 1 band (D band). These carbons can be amorphous, defective filamentous carbon, considering that the D band is associated with impaired and disordered structures. The band at 1313 cm− 1 reveals the presence of carbon defects D and the total absence of graphitic carbon around band 1600 cm− 1. It is important to stress that the band around 698 cm− 1, which could be assigned to the presence of Fe3O4, corresponds to the magnetite, shifted to the lower band 667 cm− 1 (Hai et al. 2008). The bands 494 and 629 cm− 1 are attributed to Fe2O3 and La2O3.
The G band appearing at 1515 cm− 1 is related to the Raman phonon mode E2g and involves graphene structures. This vibration mode corresponds to the presence of ordered carbon and can provide the electronic properties of carbon filamentous information. The original sample in Fig. 3 displayed the principal bands corresponding to the LaFeO3 at 145 and 428 cm− 1; the band intensities at 145, 169, and 263 cm− 1 are attributed to the Fe2O3, while the bands at 494 and 629 cm− 1 to La2O3. The Raman spectra of Fig. 9 show new bands, and it is noteworthy that the band of the original LaFeO3 at 145 and 429 cm− 1 disappeared after the reaction. New bands appeared with higher intensity.
After the reaction, the Raman spectrum (532 nm) shows nickel oxide in nano-powders for grain sizes between 100 nm and 1500 nm. Peaks due to one-phonon (1P), two-phonon (2P), and two-magnon (2M) scattering were identified (Mironova-Ulmane et al. 2007). The Raman spectrum at RT shows a single-crystal NiO band above 400 cm− 1. The characteristic band of NiO corresponds to one-phonon (1P) TO and LO modes at 570 cm− 1. The disorder-induced 1P band at 570 cm− 1 has minimal intensity, indicating good single-crystal quality (Lazarević et al. 2013). The intense band at 788 and 968 cm− 1 are attributed to Fe3O4 nanoparticles (Soler and Qu 2012). It suggests that the LaFeO3 was decomposed during the reaction in Fe2O3 nanoparticles and after reduction to Fe3O4.
After the reaction, the thermo-analysis showed a weight loss of the Ni/LaFeO3 catalysts. Figure S1 shows the profile of the catalysts used at 700°C, and Figure S2 shows the Raman images of spent Ni/LaFeO3 catalyst tested at 700°C (a) and 750°C (b). At 700°C, no significant weight loss was observed, which indicated low carbon formation. These results agree with those shown in the Raman spectra, where the D band correspondent to the filamentous carbon was not observed.
The following surface species are proposed for these processes over the catalyst, equations (5)–(12). Note worth is the presence of NiO is reduced to metallic Ni0, besides lattice oxygen and Fe2O3 at the surface, or
2LaFeO3 → La2O3 + Fe2O3
|
Decomposition (5)
|
3Fe2O3 + H2 → 2Fe3O4 + H2O
|
Reduction (6)
|
NiO + H2 → Ni0 + H2O
|
Reduction (7)
|
H2O → H+ + (OH−)
|
Dehydroxilation (8)
|
O2(g) → 2[O−]lat
|
Lattice oxygen (9)
|
O2(g) → 2[O*]
|
Adsorption of oxygen (10)
|
Ni0 + [O−]lat → NiO
|
Lattice oxygen (oxidation) (11)
|
Ni0 + 1/2O2(g) → NiO
|
Oxidation (12)
|
one can speculate the following reactions: Equations (13)–(16) occur over the surface sites due to the adsorption of methane, oxygen, and CO2 on the surface sites.
CH4a + O* → CO + 2H2
|
(13)
|
CH4a + [O-]lat → CO + 2H2
|
(14)
|
CH4a + H2O + CO2a → 2CO + 3H2
|
(15)
|
2CO → C + CO2
|
(16)
|
where O* adsorption and an index adsorbed species. |
Figure 10 displays the scheme of reactions at the surface, where methane and oxygen adsorb preferentially on metallic Ni0 sites and CO2 on Fe2O3 sites of the decomposed perovskite structure. Carbon is also present due to the Boudouard reaction.