Scanning electron microscopy (SEM)

SEM studies were undertaken in order to investigate sample morphology and approximate particle sizes of NiO crystallites. SEM studies over a magnification range of 15,000 to 60,000 were undertaken on all calcined samples. SEM images for all samples following calcination are shown in Figure 4.16 - Figure 4.20. The arrows displayed on SEM images indicate the possible presence of cubic NiO crystallites.

Figure 4.16: SEM image of NiO/Al2O3 (600) (magnification 30,000)

112 Figure 4.17: SEM image of NiO/Al2O3 (700) (magnification 30,000)

Figure 4.18: SEM image of NiO/Al2O3 (800) (magnification 30,000)

113 Figure 4.19: SEM image of NiO/Al2O3 (900) (magnification 30,000)

Figure 4.20: SEM image of NiO/Al2O3 (1000) (magnification 30,000)

114 The SEM images show that the support morphology is dramatically influenced by calcination temperature. At temperatures between 600 C and 900 C, the formation of uniform well defined support species is observed, becoming more pronounced as the temperature is increased. Small well distributed support particles observed in all samples, but most prominently in NiO/Al2O3 (600), conglomerate as the calcination temperature is increased forming larger species [22]. Uniform, spherical support species are observed as the temperature is increased to 900 C (Figure 4.19). However, the SEM image for NiO/Al2O3

(1000) (Figure 4.20) shows the formation of large non-uniform sheets with a less defined morphology, significantly different to that of NiO/Al2O3 (900). This indicates that temperature is affecting support morphology most dramatically, at the highest calcination temperatures.

Few cubic well distributed NiO crystallites are observed on the support in NiO/Al2O3

(1000), compared to NiO/Al2O3 (900). This suggests that following high temperature calcination at 1000 C, NiO is no longer highly dispersed on the support surface, but may be incorporated into the bulk structure.

The SEM images suggest that at calcination temperatures between 600 C and 900 C, NiO particle sizes remain relatively constant ranging from ca. 50 nm to 200 nm in all samples calcined up to 900 ºC. However, at 1000 C due to the amorphous nature of NiO/Al2O3

(1000), the NiO crystallite sizes appear to be significantly larger. This is consistent with crystallite data determined using XRD, indicating sintering of NiO at this temperature. As NiO is incorporated into the bulk structure at high calcination temperatures, the morphology of the sample is considerably altered, becoming less uniform and defined.

115 4.2.2.5 X-ray photoelectron spectroscopy (XPS)

SEM studies show that the structural morphology and surface properties of the samples are altered with increasing calcination temperature. XPS measurements of the calcined samples were carried out to further study the surface properties.

The Ni 2p and O 1s spectra of NiO/Al2O3 (600) are shown in Figure 4.21 and Figure 4.23, respectively. The Ni 2p and O 1s spectra of NiO/Al2O3 (900) and NiO/Al2O3 (1000) are shown in Figure 4.22 and Figure 4.24 respectively. Curve fitting of the spectra was carried out in an attempt to more easily assign the different surface species present in the Ni 2p and O 1s spectra. All spectra show the experimental data (black), peak fitted components (blue, green, yellow and purple) and peak fitted spectra curve (red).

Figure 4.21: Peak fitted Ni 2p XP spectrum of NiO/Al2O3 (600)

The Ni 2p peak fitted spectrum shows the presence of two primary peaks at binding energies of 857.1 and 858.7 eV and two satellite deconvoluted peaks at 864.3 and 867.2 eV (Figure 4.21). The two primary peaks and corresponding satellite peaks, are similar to those

116 unsupported standard NiO. The Ni 2p peaks are shifted to higher binding energies, than for unsupported NiO, which is attributed to the interaction of NiO with the support, an increase of 3.1 and 2.2 eV is observed for Ni²⁺ and Ni³⁺ respectively. in both the NiO and the NiAl2O4 phase have satellite peaks at a very similar binding energy,

58000

117 therefore, only one satellite peak is observed when both phases are present [13].

Additionally, the XP spectrum of NiO/Al2O3 (900) shows the presence of a peak at 857.3 eV, this is attributed to the presence of stoichiometric NiO.

As with NiO/Al2O3 (600), the peaks are shifted to higher binding energies due to nickel-support interactions. In NiO/Al2O3 (900), the presence of NiO, in addition to NiAl2O4, shifts the Ni²⁺ satellite peak by 0.2 eV to a slightly lower binding energy, compared to NiO/Al2O3

(1000).

Figure 4.23: Peak fitted O 1s XP spectrum of NiO/Al2O3 (600)

The O 1s peak fitted spectrum shows the presence of two peaks at 530.5 and 534.0 eV (Figure 4.23), which are attributed to oxygen associated with Ni and Al2O3 [23], respectively.

20000 25000 30000 35000 40000 45000 50000

526 528 530 532 534 536 538 540

Intensity / cps

Binding Energy / eV

118 Figure 4.24: Peak fitted O 1s XP spectra of A, NiO/Al2O3 (900) and B, NiO/Al2O3

(1000)

The O 1s peak fitted spectra of the samples calcined at 900 and 1000 C suggests the presence of three peaks (Figure 4.24). Peaks at 530.6 eV for Ni/Al2O3 (900) and 530.4 eV for Ni/Al2O3 (1000), are attributed to oxygen bonding to Ni [23]. The peak at 533.7 eV, in both spectra, is attributed to oxygen bonding with Al [23]. Both samples show the presence of an additional peak at ca. 536 eV, which is attributed to surface hydroxyl groups [18]. This peak is also observed in the O 1s spectrum of unsupported NiO (Figure 4.8). Hydroxyl groups are reported to have been introduced to the catalyst during the preparative stage and retained by alumina during preparation [24].

119 Analysis of the Ni 2p spectra suggests two different surface states of Ni²⁺ are present, corresponding to NiO and NiAl2O4. Both surface species are present following calcination at 900 C. However, only surface NiO is present following calcination at 600 C and only surface NiAl2O4 is present following calcination at 1000 C. Analysis of the O 1s spectra distinguishes the oxygen bonded to the nickel and Al2O3, as well as adsorbed hydroxyl groups in some samples. Hydroxyl groups are present as samples are stored in air at room temperature. The Al 2p spectra of all samples show peaks at ca. 76 eV, corresponding to Al bonding with O. The O 1s and Al 2p spectra were less useful in the identification of surface nickel phases present.

The XP spectra clearly shows that as the calcination temperature is increased from 900 C to 1000 C, surface NiO is no longer present or able to be detected, and the presence of surface NiAl2O4 increases. XPS data is consistent with XRD results indicating the presence of an additional phase in samples that undergo calcination at 1000 ºC. However, it is important to note that XPS clearly identifies the presence of surface NiAl2O4 following calcination at 900

C, whereas XRD analysis does not detect this phase.