DECLARATION 2 CONFERENCE CONTRIBUTIONS
4. CHAPTER FOUR - CHARACTERISATION
4.3. Characterisation of cesium promoted catalysts
Figure 4.27: TG and DTG curves for 6% BaMg-V-HTlc
parameter relative to that calculated for the Mg-V-HTlc (Table 4.1) is possibly due to the presence of cesium, which has an ionic radius of 1.67 Å [15].
Figure 4.28: Diffractorgram of 0.4% CsMg-V-HTlc
Figure 4.29: Diffractogram of 0.9 % CsMg-V-HTlc
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Figure 4.30: Diffractogram of 1.9 % CsMg-V-HTlc
Table 4.11: Lattice parameters for Cs promoted catalysts
CATALYST c (Å) a (Å)
0.4% CsMg-V-HTlc 23.3 3.1
0.9% CsMg-V-HTlc 23.2 3.1
1.9% CsMg-V-HTlc 23.2 3.1
The surface area measurements of the different Cs loaded catalysts (Table 4.12) suggests that the presence of Cs in different concentrations does not affect the surface area of the catalyst to a great extent, as there are negligible increases in surface area with increasing cesium loading. All three catalysts are, however, slightly higher in area than the Mg-V-HTlc.
Table 4.12: Effect of Cs loading on surface area
CATALYST SURFACE AREA (m2/g)
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0.4% CsMg-V-HTlc 100.1
0.9% CsMg-V-HTlc 100.4
1.9% CsMg-V-HTlc 101.3
The IR spectra of each of the promoted catalysts presented in Figure 4.31, clearly show a broad band around ~ 3400 cm-1 due to OH- stretching vibrations, while those due to water bending vibrations are observed around 1630 cm-1.These peaks are common to the Mg-V-HTlc and Ba promoted catalysts as well. The splitting of the CO3 bands results in two sharp peaks between 1420 cm-1 and 1485 cm-1 in each promoted catalyst. Additional bands associated with the presence of CO3
and due to a change in symmetry of the molecule appear around 880 cm -1. All the bands associated with carbonate ions discussed here were also present in the IR spectrum of the unpromoted catalyst (Figure 4.5).
Figure 4.31: IR spectra of Cs promoted catalysts
TPR studies on the Cs promoted catalysts (Figure 4.32) showed a single deep reduction peak in each case, attributed to the reduction of the vanadium ions in the sample. The peak maxima were found to increase marginally with increasing Cs loading: 734 C for 0.4% Cs loading, 745 C for 0.9% Cs loading and 766 C for 1.9% Cs loading, likely due to the interaction between Cs and V, which makes it difficult to reduce vanadium. The oxidation state of the vanadium in the reduced
4000.0 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 600.0
cm-1
%T
0.3% Cs-HTlc
0.9% Cs-HTlc 1.9% Cs-HTlc
3410.21
2051.67 2001.22
1742.19 1631.46
1483.58 1422.98
1115.40 1056.95
878.57 853.27
3378.55
2076.04
1636.32
1485.59 1425.05
1109.87
886.04 651.80
3416.01
2051.08 2001.19
1632.62
1483.93 1423.10
1114.20
878.65 853.94
catalyst was calculated based on the amount (mol) of hydrogen consumed (Table 4.13) during the experiment. The data suggests that the vanadium was not completely reduced to the +3 oxidation state; some vanadium species are likely to exist in the +4 oxidation state as well.
Table 4.13: TPR results for the Cs containing Mg-V-HTlc’s CATALYST H2 CONSUMED
(mol)
V OXIDATION STATE IN REDUCED CATALYST
%
REDUCIBILITY
0.4% CsMg-V-HTlc 0.076 3.2 90.1
0.9% CsMg-V-HTlc 0.064 3.3 86.3
1.9% CsMg-V-HTlc 0.065 3.4 85.1
Figure 4.32: TPR curves for Cs promoted catalysts
The Cs promoted catalysts showed weak (100 C – 200 C) and medium acid sites (350 C – 500 C) and the total acidity of the catalysts was found to decrease with increasing Cs loading (Table 4.14), with all catalysts having a lower acidity than the unpromoted Mg-V-HTlc. The observed behaviour may be attributed to the basic properties of cesium.
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Figure 4.33: TPD curves for the Cs promoted catalysts Table 4.14: Total acidity of the Cs promoted catalysts
CATALYST TOTAL ACIDITY (mol NH3 / g catalyst)
0.4% CsMg-V-HTlc 571.9
0.9% CsMg-V-HTlc 561.7
1.9% CsMg-V-HTlc 446.2
The surface morphology of the promoted catalysts was found to be quite similar to that of the Mg-V-HTlc, such that it was found to have an irregular structure (Figure 4.34). Increased metal loadings did not significantly change the morphology. Elemental mapping results for each of the selected areas shown in Figure 4.34 (Refer to Appendix B, Figure B9 – Figure B11) suggest that cesium, magnesium and vanadium are evenly distributed along the surface of the catalyst.
b a
c
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Figure 4.34: SEM images of a) 0.4% CsMg-V-HTlc b) 0.9% CsMg-V-HTlc and c) 1.9% CsMg-V-HTlc
The TG and DTG results show that each catalyst undergoes a loss of physisorbed water at ~ 113 C.
The second weight loss step observed for the Mg-V-HTlc is not present in the 0.9% CsMg-V-HTlc and is observed at a lower temperature in the 0.4% CsMg-V-HTlc and 1.9% CsMg-V-HTlc’s. In hydrotalcite-like catalysts, this peak is usually due to the loss of interlayer water [6]. XRD analyses on the Cs promoted catalysts shows that the HT-like structure is maintained, irrespective of the loading – interlayer water is therefore still present. It is therefore likely that cesium could possibly interact with interlayer water, therefore making it much more difficult to remove under the experimental conditions. The final weight loss step (due to decarboxylation and dehydroxylation) is present in all the promoted samples and occurs at approximately 400 C. The total weight loss observed for the 0.4%, 0.9% and 1.9% Cs promoted catalysts was determined to be 28.5%, 27.2%
and 26.4% respectively.
12.05%
(0.5703mg)
112.35°C
13.07%
(0.6184mg) 396.08°C 1.309%
(0.06197mg) 216.36°C
-2 -1 0 1 2
Deriv. Weight (%/min)
60 70 80 90 100 110
Weight (%)
0 100 200 300 400 500 600 700
Temperature (°C) Sample: 0.4%Cs HT
Size: 4.7330 mg Method: 20 deg ramp Comment: 10 deg cel + nitrogen
DSC-TGA File: C:...\TGA\UKZN\CsHT\0.4 % Cs HT Operator: Alisa
Run Date: 13-Oct-2009 13:31 Instrument: SDT Q600 V20.9 Build 20
Universal V4.5A TA Instruments
Figure 4.35: TG and DTG curves for the 0.4% CsMg-V-HTlc
Figure 4.36: TG and DTG curves for the 0.9% CsMg-V-HTlc
11.10%
(0.5328mg) 113.44°C
15.10%
(0.7249mg)
407.30°C
-0.5 0.0 0.5 1.0 1.5
Deriv. Weight (%/min)
70 80 90 100 110
Weight (%)
0 100 200 300 400 500 600 700
Temperature (°C) Sample: 0.9% CsHT01Q
Size: 4.8000 mg Method: 20 deg ramp
Comment: Run 1 - 10 deg cel. ramp.
DSC-TGA File: C:...\TGA\UKZN\CsHT\0.9% CsHT01Q Operator: Alisa
Run Date: 24-Jul-2009 12:36 Instrument: SDT Q600 V20.9 Build 20
Universal V4.5A TA Instruments
Figure 4.37: TG and DTG curves for the 1.9% CsMg-V-HTlc