2-Theta-Scale
4.1.5. Laser Raman Spectroscopy
The results in Figure 4.5 (a) obtained from laser Raman analysis of the support clearly show three distinct strong bands at 396, 517 and 639 cm-1. These bands confirm the presence of the anatase phase of titania. The spectrum in Figure 4.5 (b) for the 25Co/TiO2 catalyst displays a very strong band indicative of cobalt oxide at 687 cm-1. These results are in good agreement with the findings of Ozkan et al., Xiao et al. and Che et al. [3, 7, 11] in their research involving cobalt titania catalysts.
a
d c
b
45 Figure 4.5: Laser Raman spectra of the (a) titania support and (b) 25Co/TiO2 catalyst
4.1.6. TPR
Thermal treatment and its impact on the reducibility of a catalyst is important. Ease of reduction of a catalyst often closely correlates to the catalyst activity [3], since these catalysts function on a redox cycle. The reducibility of the catalysts was investigated by means of TPR analysis. Hydrogen consumption was measured as a function of temperature up to 950 °C.
Figure 4.6: Combined TPR results for titania supported cobalt catalysts
The onset of reduction occurred at 300 °C for the 5-20 wt % cobalt loaded catalysts and at 250 °C for the 25 wt % catalyst. From the results obtained, it is evident that the reduction occurs in a stepwise manner in two stages. Looking at the hydrogen consumption and the peak shape, two distinct peaks are apparent which correspond to the reduction of Co3O4
species to metallic cobalt via CoO. The smaller peak, consuming less hydrogen, is indicative of the presence of Co3O4 and its reduction to CoO, which occurred in the lower temperature
-5000 0 5000 10000 15000 20000 25000 30000
100 600
Intensity
Raman shift (cm -1) 517
639
a
396
-100 400 900 1400 1900 2400 2900 3400 3900
200 400 600 800 1000
Intensity
Raman shift (cm -1) 687
b
480 517
394 630
46 range of 300 °C - 500 °C for all the catalysts, as displayed in Figure 4.6. The higher temperature reduction peak corresponds to that of CoO to Co0. Furthermore, the reduction occurred at progressively lower temperatures as the cobalt loadings increased. Results by Yung et al. [3] on titania supported cobalt catalysts for the oxidation of nitric oxide showed the same trends in reduction behaviour and TPR experiments on the bare titania showed no reduction.
Overall the catalysts displayed very similar reduction behaviour as is evident in Figure 4.6.
Reduction was complete by a temperature of 750 °C, but with increasing hydrogen consumption as the metal loading increased, indicative of an increasing number of reducible species with increase in metal loading. The results for the 15% Co/TiO2 catalyst do not follow the trend observed for the other catalysts and requires further investigation.
Figure 4.7: TPR profile showing deconvolution of the peaks for the 25 % Co/TiO2 catalyst with temperature maxima at 411 ° C, 524 ° C and 615 ° C.
From the TPR analysis of the 25% Co/TiO2 catalyst, in Figure 4.7, the results reflect three consecutive merged peaks for the two-step reduction of Co3O4 to metallic cobalt via CoO.
The first step occurs at 524 °C, while the second step occurs at 615 °C. The second step higher temperature reduction is attributed to species that have stronger degrees of interaction with the titania support e.g. cobalt titanate species or cobalt oxide species within the support pore structure. It has been reported in literature that under reducing conditions, strong support-metal interactions occur, forming less easily reduced species. Titania specifically is known to encapsulate the metal particles [4, 7], thereby hindering interaction with the gas phase. These results are consistent with work carried out on titania and other supports by Vob et al. [8], Backman [12] and Li et al. [13]. The stepwise reduction of the cobalt species also indicates that the majority of the trivalent species are completely reduced to the divalent species, before subsequent reduction to the metallic state begins, as is clearly seen from the deconvolution of the peaks.
47 4.1.7. TGA
Thermogravimetric analysis provides a means of assessing the thermal decomposition behaviour of materials. From TGA analysis of the uncalcined support material, inferences were made regarding suitable calcination temperatures and hence 500 °C was chosen for this research effort. From the results in Figure 4.8, it is evident that the mass loss on heating was constant and minimal (ca. 5 %) over the temperature range up to 1000 °C under air. A yield of > 95 % for all catalysts was observed after thermal treatment up to 1000 °C. A conclusion may be drawn that these catalysts are thermally stable under air in the temperature range investigated. Thermal stability is one of the most sought after criteria in catalytic material, in order to withstand rigorous reaction conditions over extended periods of time. The derivative of the TGA curve displays the maximum mass loss temperature (Tmax) to be ± 80 °C. This mass loss is attributed to loosely bound/adsorbed water molecules. At higher temperatures, lattice expansion is known to occur, resulting in sputtering of the anatase particles and the violent release of water molecules ie waters of hydration. The mass loss evident at a higher temperature, ± 680 °C, can also possibly result from the anatase being consumed in another process e.g. in the formation of cobalt titanate (CoTiO3) species during the phase transformation from anatase to the rutile stage, with some amount of the anatase not being transformed to rutile. This is in good agreement with work done by Rayner [6] on the thermal treatment and particle size evaluation of titania supported cobalt catalysts, as well as studies by Yung et al. [3] and Iglesia et al. [14] showing the relationship between cobalt titanate species and the anatase to rutile phase transformation in titania supported cobalt catalysts. They reiterate that phase transformation is accompanied by reduction in surface area as well as volume contraction. Their results reflect that cobalt titanate species form from temperatures in excess of 650 °C.
Figure 4.8: Graph depicting the weight loss profile of the 10 Co/TiO2 catalyst, following thermal treatment under air
48 4.1.8. TEM
From the TEM micrographs in Figure 4.9 (a) and (b), the TiO2 support and cobalt catalyst material comprise of elongated, spherical and other irregular shaped particles of varying sizes between 8-12 nm and 15-20 nm for the support and Co/TiO2 catalysts respectively. From the micrographs it is further evident that the catalyst material is crystalline. It is difficult to distinguish between the cobalt and titania particles. This may be as a result of insignificant mass contrast owing to the small difference in mass of the two components as well as the orientation of the particles.
Figure 4.9: TEM micrographs of the (a) TiO2 support and (b) 20% Co/ TiO2 catalyst at 600K magnification.
4.1.9. SEM
The function of the support is to allow good dispersion of the catalytically active phase. In this regard medium to high surface area materials are preferred over lower surface area materials. Good dispersion of the active phase is a pre-requisite for commercial catalysts and is a challenge during synthesis. SEM micrographs show that the support and catalyst material comprise predominantly of spherically shaped particles which appear to have aggregated and clustered to form small ‘florettes’.
Figure 4.10: SEM images of the TiO2 support revealing clusters of particles at (a) 71.44 K and (b) 110.56 K magnification.
Upon close perusal and comparison of the images in Figure 4.11 of the titania supported catalysts, distinct ‘patches’ or white spots are visible - as indicated by the circles - of the
a b
a
50 nm 50 nm
a b
49 cobalt oxide strewn on the surface of the support. This was also observed in material synthesised by Jongsomjit et al. [15].
Figure 4.11: SEM images of the (a) 20% Co/ TiO2 and (b) 25% Co/ TiO2 catalyst at 38.27 K and 30.76 K magnification respectively, with encircled regions denoting the presence of cobalt oxide.
It is apparent from the images that the material is very coarse textured and appears to be multiple-layered, revealing stepped ridges at the edges. This is particularly noticeable in Figure 4.12
Figure 4.12: SEM image showing coarse textured layered material with patches of cobalt oxide strewn on the surface of the titania
a b
50 EDX elemental mapping was able to approximate the surface composition of the catalysts, the breakdown of which appears in Table 4.2. These results correlate very closely with results obtained from ICP analysis.
Table 4.2: EDX elemental mapping of cobalt and carbon composition in catalysts Catalyst Nominal wt %
cobalt
EDX % cobalt
Sample A Sample B
1 5 4.77 4.32
2 10 10.37 10.59
3 15 14.97 14.73
4 20 18.81 18.65
5 25 22.85 24.97
From the elemental mapping, it is evident that the cobalt is well dispersed on the titania support, when lower cobalt loadings were used e.g. in the 5Co/TiO2 catalyst in Figure 4.13 (a) and in the 10Co/TiO2 catalyst.
Figure 4.13: SEM-EDX elemental mapping indicated good dispersion of cobalt (green) on titania (red) in the (a) 5Co/TiO2 catalyst and with aggregation in the (b) 15Co/TiO2, (c) 20Co/TiO2 and (d) 25Co/TiO2 catalysts
a b
c d
51 With higher cobalt loadings, some aggregation of the cobalt species exists as indicated by the encircled regions in Figure 4.13.