Etapa 1: Conducta de entrada

XRD is considered to be the fingerprint of a crystal structure. This can be used for qualitative and quantitative phase analyses of NiO as discussed in Section 4.3.2.2. The X- ray diffractograms of γ-Al2O3 and oxygen carriers in calcined and reduced forms are presented in Figure 22 with the reference peaks from JCPDS. Diffraction peaks corresponding to crystalline species of either La2O3, La(OH)3, La2(CO)2O2 or LaAlO3 phases were not detected indicating a well-dispersed, amorphous Lanthanum phases on the surface of the support. In addition, no Co3O4, CoAl2O4 or NiCo2O4 peaks were also

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observed in the XRD patterns. This is understandably due to the low concentration of Co (1%) in the carrier material.

Figure 22: XRD spectra of γ-Al2O3, calcined and reduced oxygen carrier carrier.

Reference peaks for γ-Al2O3, NiO and Ni are from JCPDS 10-0425, 44-1159 and 04- 0850 respectively.

The major diffraction lines were observed at 2θ values of 37.3°, 43.3°, 63°, 75.5°, and 79.5°. These lines could be traced back to the bulk NiO crystals in all the three cases of calcined oxygen carriers. These lines were sharper and more intense for HMF and MF than for BF. This shows that larger nickel oxide crystals were formed under HMF and MF conditions. On the other hand, in case of BF, a broad peak corresponding to well- dispersed small crystals on a thin layer of amorphous nickel species was observed (Figure

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25). A similar observation was reported by Heracleous (Heracleous et al. 2005) when more than 15 wt% of Ni was present over γ-Al2O3. This suggests the saturation of the alumina surfaces by two-dimensional Ni species and the formation of multilayer/crystalline Ni phases. The monolayer dispersion capacity of NiO on Al2O3 has been reported to range from 13 to 20 wt% Ni loading. This has been determined by ion scattering spectroscopy (Wu et al. 1979), XRD (de Bokx et al. 1987) and XPS (Xiaoyong Wang et al. 1999).

The average Ni crystal size was calculated using Eq. (16). The broadening of Ni (111) diffraction lines for the reduced oxygen carrier was used to calculate the crystal size and results are listed in Table 12. The calculated crystal sizes of Ni particles were found to be less than the ones obtained using H2 pulse chemisorption. This deviation resulted from the incapability of XRD to detect crystals below 2-4 nm, whereas H2 chemisorption experiments were able to account for both the small crystal sizes and amorphous phase. However, the decreasing order of crystal sizes based on preparation methods complements the values observed in Section 5.4

Table 12: Crystal Size of Ni calculated from XRD spectra using Scherrer’s equation

Sample Crystal Size (nm)

HMF 20.3

MF 17.7

BF 5.2

Based on the discussion in Section 5.4, one could also expect visible diffraction lines for NiAl2O4 and δ/θ-phase of alumina. To examine the presence of NiAl2O4, all the three oxygen carriers were reduced under H2 flow at 750°C before the XRD experiments. It is apparent from the TPR results that no NiAl2O4 was reduced below this temperature. As a result, if there was NiAl2O4 in crystalline from, one should expect a diffraction line at 2θ ~ 37°. However, the complete disappearance of NiO (111) diffraction lines (Figure 22) in all three reduced samples suggests the absence of crystalline NiAl2O4.

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Other strong diffraction lines of NiAl2O4 may appear at 2θ 45°, 60° and 66°. These diffraction lines may also be present in case of all the three prepared oxygen carriers. However, γ-Al2O3 also provides diffraction lines in these 2θ values. Therefore, identification of such species is not an easy task using XRD analysis, whereas their presence is easily detectable using TPR experiments. Bolt (Bolt et al. 1998) reported that NiAl2O4 exhibits structural, spectroscopic, and chemical similarities with bulk spinels but are not detectable with XRD.

Similar observations were reported by several others (Kwak et al. 2011; Paglia et al. 2004; Zhang et al. 2006) while characterizing transition alumina (γ-, δ- or θ-Al2O3) surfaces by XRD analysis. This is due to the intrinsic properties of these phases namely low crystallinity and small particle size. Thus, XRD is rather ineffective for the detection of molecularly thin surface layers that might possess structural properties (geometric, electronic, coordination) significantly different from those of the bulk. On the other hand, chemical probes (i.e., surface reactions TPR/TPO, TPD, chemisorptions etc.) provide important information, valuable for the inference of the changes in the near-surface structural properties of solid materials. The information gained from such studies can also be used to directly correlate catalytic properties with surface structure (Kwak et al. 2011a).

Therefore, even after having conclusive evidence of phase transformation from γ-Al2O3 to δ-phase for MF and θ-phase for HMF using the TPR study, phase changes could not be confirmed using XRD studies. This phase transformation may have occurred at the interfaces between the Ni crystallites and alumina support surfaces, whereas the bulk structure of the support remained in the γ-phase. It appears that transformation from γ to γ′ (an intermediate phase between γ and δ-Al2O) occurs by dehydroxylation in the amorphous region rather than the crystal phase. This is accompanied by cation ordering by maintaining the ratio of tetrahedral and octahedral sites (Paglia et al. 2004). Kwak (Kwak et al. 2011) reported the possibilities of core-shell structure. The bulk of the alumina crystallites are phase-pure γ-Al2O3, whereas the surfaces of the particles have structures of the δ-Al2O3 or θ-Al2O3 phase. These new phases are formed through dehydroxylation of the (1 and 11 ) facets of the γ-Al2O3. In a combined UV/visible

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Raman, XRD, and high-resolution TEM study similar conclusions were reported for TiO2 phase transformation (Zhang et al. 2006).