2-Theta-Scale
4.3. Spent catalyst characterisation
4.2.1. PXRD
51 With higher cobalt loadings, some aggregation of the cobalt species exists as indicated by the encircled regions in Figure 4.13.
52 Figure 4.15: Diffractogram of the spent 20Co/TiO2 catalyst after the ODH reaction, showing the presence of the cobalt oxide phase and a new peak for the cobalt titanate phase
4.2.2. BET
Table 4.3: BET Surface area and pore volume results obtained on the spent catalysts
Catalyst Fresh catalyst Spent catalyst
Surface area Pore volume Surface area Pore volume
TiO2 99 0.31 42 0.16
5 CoTiO2dehydrog 68 0.23 56 0.21
5 CoTiO2(8C:2O) 68 0.23 50 0.19
5 CoTiO2(8C:4O) 68 0.23 35 0.15
10 CoTiO2(8C:4O) 65 0.22 25 0.13
20 CoTiO2(8C:2O) 61 0.18 53 0.17
The most significant decrease in surface area and pore volume was observed in the titania run as a catalyst by itself, the 5Co/TiO2 catalyst and the 10Co/TiO2 catalyst, run under oxygen rich conditions. This may be as a result of the higher conversions over these catalysts and greater deformation of the surface structure from repeated reduction/oxidation cycles.
4.2.3. TGA
Different types of coke appear on the spent catalysts. Coking can exist in the form of soft coking with non-crystalline amorphous carbon or hard coking comprised of polymeric or graphitic type carbonaceous material [5]. Coking leads to blockage of active sites and causes deactivation of the catalyst, thereby compromising the catalyst lifetime.
CoTiO3 Co3O4
Co3O4
Co3O4 Co3O4
53 Figure 4.16: The TGA weight loss profile of the 5Co/TiO2 spent catalyst after the catalytic testing under anaerobic conditions.
The weight loss profile in Figure 4.16 of the spent catalyst from the dehydrogenation reaction, displayed a significantly higher mass loss of ca. 8 % following thermal treatment compared to the fresh catalyst. This mass loss occurred under 400 °C, indicative of soft coking on the catalyst post reaction. This spent catalyst’s appearance was distinctly different from the spent catalyst run under aerobic conditions. The former was black, while the latter was green. Brik et al. [1] noted the same colour in their spent catalysts from ethane ODH and attributed this colour change to possible change in co-ordination and/or oxidation state of the cobalt cations. The 5Co/TiO2 spent catalyst after the aerobic catalytic testing, showed a weight loss (Figure 4.17) of ca. 2 % at temperatures below 200 °C, which is due to adsorbed water. This catalyst had the same weight loss profile as the fresh catalyst with a weight loss of <3 %, thus indicating the absence of coke on the catalyst.
Figure 4.17: The TGA weight loss profile of the 5Co/TiO2 spent catalyst after the catalytic testing under aerobic conditions with an 8C:4O ratio.
54 4.2.4. Laser Raman spectroscopy
Upon comparison of the laser Raman spectra for the 5Co/TiO2 fresh catalyst in Figure 4.18 (a) and the catalysts run under oxygen richer (8C:4O) conditions in Figure 4.18 (b) the latter only showed the bands due to anatase with the absence of any bands representing carbonaceous species. Generally the distinct bands at 1582 cm-1 and 2700 cm-1 in laser Raman spectra (when using a 514.5 nm laser source) are indicative of the G and G´ band which represents polyaromatic hydrocarbons (PAH’s) or graphitic material with sp2 carbons.
This supports the argument that the oxidising environment for the oxidative dehydrogenation reaction suppresses coke formation and inhibits catalyst deactivation. Most of the carbon forms carbon oxides as opposed to forming a layer over the catalyst surface, which is typical of ODH catalysts. The green colour of the catalyst post reaction also provided a visual indication of the absence of coke.
Figure 4.18: Laser Raman spectra of the 5Co/TiO2 (a) fresh catalyst and (b) spent catalyst after the catalytic testing under aerobic conditions with an 8C:4O ratio.
Upon comparison of the laser Raman spectra from the fresh 5Co/TiO2 catalysts in Figure 4.18 (a), and the spent 5Co/TiO2 catalysts in Figure 4.18 (b) and Figure 4.19 (a), it is clear that the catalyst run under anaerobic conditions has additional bands at 788, 1552 and 2330 cm-1. These bands at 1552 and 2330 cm-1 are possibly indicative of sp2 carbon species. This is consistent with deposition or accumulation on the catalyst surface of aromatics like benzene, ethylbenzene and styrene, which were the product profile from the catalytic testing. The spent catalyst’s black appearance also provided a visual indication of coke.
The bands at 788 and 798 cm-1 in Figure 4.19 may be as a result of the formation of other cobalt species, as observed by Brik et al. [1] on the spent cobalt titania catalysts in ethane ODH. The strong band observed at 483 and the weaker band at 687 cm-1 in Figure 4.18 (b), is indicative of Co3O4 [3] species in the spent 20Co/TiO2 catalyst, thereby showing that following reduction, the cobalt was reoxidised. This spent catalyst, showed the absence of any carbonaceous species, from the Raman analysis.
-150 50 250 450 650 850
200 300 400 500 600 700 800
Intensity
Raman shift (cm -1) 392
686
509
633 a
480
-250 250 750 1250 1750 2250
0 500 1000 1500
Intensity
Raman shift (cm-1)
633
509
392
b
55
Figure 4.19: Laser Raman spectra of the (a) 5Co/TiO2 spent catalyst after the catalytic testing under anaerobic conditions and (b) 20 Co/ TiO2 after aerobic (8C: 2O) catalytic testing Titania amongst the other oxide supports is reported to be excellent in inhibiting or suppressing carbon deposition according to research by Takanabe et al. [16]. Evidence of this was obtained by the absence of carbon species deposited on the catalysts during the ODH reactions, as was found for the spent catalysts characterised by the above techniques.
4.2.5. TEM
The TEM micrographs in Figure 4.20 (a) reveal that the spent titania support maintained crystalline structure post reaction, while the 10 Co/TiO2 catalyst shows deformation of the original shape as observed in Figure 4.20 (b) and a fused appearance of the catalyst particles, which may be attributed to sintering and agglomerating of individual particles.
Figure 4.20: TEM micrographs of the spent catalysts: (a) Titania support (b) 10Co/TiO2
catalyst after the (8C: 4O) ODH reaction
The spent catalyst from the dehydrogenation reaction in Figure 4.21 clearly reveals the presence of metallic cobalt particles, (the larger of which is encircled and measures ca.
40 nm) as a result of the reduction of the Co3O4, with n-octane acting as the reductant, as well as the presence of the hydrogen generated in the dehydrogenation reaction.
-100 0 100 200 300 400 500 600 700 800
-500 500 1500 2500 3500
Intensity
Raman shift (cm -1)
1552
788
2330 a
-500 0 500 1000 1500 2000 2500
0 500 1000 1500
Intensity
Raman shift (cm -1) 483
687 798
627
b
50nm 50nm
a b
56 Figure 4.21: TEM micrographs of the spent 5 Co/TiO2 catalysts after the dehydrogenation reaction with the encircled region showing the presence of the metallic cobalt particle
4.2.6. SEM
SEM images in Figure 4.22 reflect a significant change in the morphology of the catalyst, upon comparison of the fresh and spent catalysts. These images show the complete transformation of the surface structure with fragmentation and disintegration clearly visible, after cooling down back to room temperature, post reaction.
Figure 4.22: SEM micrographs of the spent catalysts: (a) Titania support and the 5 Co/TiO2
catalyst after (b) dehydrogenation, (c) (8C: 4O) ODH reaction and (d) the 10 Co/TiO2
catalyst after the (8C: 4O) ODH reaction.
50nm
a
c d
b
57 Several factors may be directly linked to the deformation of the catalyst: repeated reduction and re-oxidation of the surface, products such as water in the product stream, heating effects from the exothermicity of the reactions, as well as ‘hotspots’ within the catalyst bed.
EDX mapping images of the spent catalysts in Figure 4.23 and Figure 4.24 show the presence of carbon deposits in the encircled regions, which were not observed in the fresh catalysts.
This may be from traces of the products from catalytic testing that remained adsorbed on the catalyst. A higher concentration of carbon, appearing in more clumped deposits, was visible on the dehydrogenation reaction catalyst.
Figure 4.23: SEM-EDX imaging of the 5Co/TiO2 spent catalyst from the dehydrogenation reaction displaying accumulated carbon deposits in the encircled regions
The carbon deposits appear evenly distributed over the surface of the catalyst after the ODH reactions. Carbon deposition has been minimised, since most of the carbon formed carbon oxides in the ODH reactions.
Figure 4.24: SEM-EDX imaging of the 5Co/TiO2 spent catalyst from the ODH reaction with carbon deposits evenly spread throughout the catalyst surface
58
4.3. Results: Catalytic testing