DECLARATION 2 CONFERENCE CONTRIBUTIONS
4. CHAPTER FOUR - CHARACTERISATION
4.2. Characterisation of barium promoted catalysts
Figure 4.15: TPD curve for the Mg-V-HTlc
The XRD patterns of the 0.2% and 0.7% barium promoted catalysts (Figure 4.16 and Figure 4.17 respectively) shows the presence of the hydrotalcite-like peaks observed for the Mg-V-HTlc (at ~ 17 2, ~ 27 2, ~ 43 2 and ~ 70 2). No additional phases were detected; here too the spike at
~ 30 2 is due to the XRD sample holder. The 1.4% barium promoted catalyst does, however, show the formation of phases (between 30 - 40 2 theta) other than those attributed to the hydrotalcite-like structure (Figure 4.18). These peaks have been tentatively (as the hydrotalcite-like peaks are possibly overlapping the peaks associated with the mixed metal phases) assigned to the formation of BaMg2 (JCPDS 6-0368), BaO2.8H2O (JCPDS 3-0306) and BaCO4 (JCPDS 3-0659).
As the barium loading was increased to 6%, these phases were more crystalline in nature, as inferred from the intensity of the peaks in Figure 4.19, and were coupled with the appearance of new peaks in the diffratogram not present in the XRD data of the other barium promoted compounds. The peaks associated with the hydrotalcite-like phase can still be discerned in the XRD diffractogram for the 6% barium promoted catalyst. In addition to the barium phases identified above, other phases that possibly co-exist with the hydrotalcite-like phase in the 6% BaMg-V-HTlc are Ba(OH)2 (JCPDS 1-0630), Ba(OH)2.8H2O (JCPDS 3-0306) and BaCO3 (JCPDS 5-0378). The distinct production of the additional phases was confirmed by literature, which suggests that barium is too big for octahedral coordination and can form other structures [5]. The values of the lattice parameters presented in Table 4.7. show that, with the exception of the 6% BaMg-V-HTlc, there are marginal increases in the a and c parameter as the barium loading increases. The increase in the a parameter is likely due to the increase in the barium content; Ba(II) has a crystal radius of 1.49 Å, much larger than that for Mg quoted earlier. The formal positive charge of the brucite-like layers is also likely to increase, albeit minimally, as the Ba(II) loading increases and this accounts for the small differences in the c parameter observed. The decreases in both the lattice parameters for the 6% BaMg-V-HTlc is likely due to the presence of additional phases in the material.
Table 4.7: Lattice parameters and Mg/V ratio of barium promoted catalysts
CATALYST c (Å) a (Å) Mg/V
0.2% BaMg-V-HTlc 23.48 3.10 1.8
0.7% BaMg-V-HTlc 23.54 3.11 1.9
1.4% BaMg-V-HTlc 23.96 3.12 1.9
6% BaMg-V-HTlc 23.50 3.07 1.8
Figure 4.16: XRD of 0.2% BaMg-V-HTlc
Figure 4.17: XRD of 0.7% BaMg-V-HTlc
Object 197
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Figure 4.18: XRD of 1.4% BaMg-V-HTlc
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Figure 4.19: XRD of 6% BaMg-V-HTlc
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TPR experiments showed a single, slightly asymmetric reduction peak for each of the catalysts (Figure 4.20) and these results suggest that it becomes increasingly difficult to reduce the catalyst as the barium loading increases. The maxima of the reduction peaks increased with increasing barium loading and were higher than that obtained for Mg-V-HTlc (Section 4.1). The oxidation state of vanadium in the reduced catalyst was found to change marginally with increasing barium loading viz. 0.2% < 0.7%, 0.7% = 1.4% and 1.4% < 6% (Table 4.8). The results suggest that the vanadium in the catalyst was not completely reduced to the +3 state, as also observed for the unpromoted catalyst. The peak shape of the 6% Ba promoted catalyst was found to be flatter than the shape observed in all other catalysts, possibly due to the overlapping of peaks.
Table 4.8: Vanadium oxidation state and % reducibility of Ba promoted catalysts CATALYST V OXIDATION STATE
(REDUCED CATALYST)
% REDUCIBILITY Peak Tmax
(C)
0.2% BaMg-V-HTlc 3.1 95 733
0.7% BaMg-V-HTlc 3.2 87 743
1.4% BaMg-V-HTlc 3.2 92 754
6% BaMg-V-HTlc 3.3 86 787
Figure 4.20: TPR curves for the barium promoted catalysts
TPD results (Figure 4.21 and Table 4.9) show that the acidity of the catalyst decreased with increasing Ba loading. All of the barium promoted samples had a lower total acidity than the Mg-V- HTlc discussed previously (Section 4.1) and showed the presence of weak and medium acid sites.
Barium is a basic promoter and an increased concentration in the sample is likely to increase basicity.
Figure 4.21: TPD curves for the Ba promoted catalysts
Table 4.9: Total acidity of Ba promoted catalysts
CATALYST TOTAL ACIDITY (µmol NH3/g catalyst) 0.2% BaMg-V-
HTlc
622.8 0.7% BaMg-V-
HTlc
397.7 1.4% BaMg-V-
HTlc
277.2
6% BaMg-V-HTlc 270.7
The IR spectra for each of the barium promoted catalysts clearly showed the presence of OH- stretching vibrations ( ~ 3350 cm-1 – 3400 cm-1), while the water bending vibrations were observed
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at ~ 1635 cm-1 (Figure 4.22). Bands attributed to the presence of carbonate ions at ~ 1480 cm-1, 1421 cm-1, ~880 cm-1 and 667 cm-1 were clearly seen in the three lowest Ba loaded catalysts. The 6%
BaMg-V-HTlc was found to have one clear band at ~1433 cm-1 and a small shoulder peak in the
~1480 cm-1 region and only one further band attributed to CO32- at ~ 878 cm-1. It is likely that the splitting of the carbonate into two bands is not as intense as for the lower loadings due to possible interaction between barium and interlayer water molecules. This reduces the water molecules available for interaction with carbonate ions – resulting in reduced CO3 bands in the region discussed.
Figure 4.22: IR spectra for Ba promoted catalysts
The surface areas of the catalysts was found to increase with increasing Ba loading, except for the 6% BaMg-V-HTlc which showed a lower surface area than all other Ba containing catalysts (Table 4.10). The increased surface area of the 0.2%, 0.7% and 1.4% catalysts is suggestive of a slightly less crystalline material. The 6% BaMg-V-HTlc is a mixture of phases – almost less hydrotalcite- like as determined from XRD and this is likely to have resulted in the low surface area obtained. In comparison to the unpromoted catalyst, barium was found to increase the surface area of the catalysts, except for the 6% BaMg-V-HTlc.
4000.0 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 600.0
cm-1
%T
0.7% BaHT
1.4% BaHT 6% BaHT 0.2% BaHT
3369.19
1630.04
1482.76 1423.86
1121.85
883.14 659.27
3353.00
2164.15
2120.26 1635.33
1567.82
1530.43
1433.07
1059.51 878.42
845.81 799.91
775.82 698.76
3395.90
2102.71
1634.87
1483.44 1422.63
1098.79
883.37 658.88
3404.97
1636.99 1480.89
1112.50 882.34
854.49 800.36
658.82
Table 4.10: Surface area measurements for the Ba promoted catalysts
The surface morphology of the catalysts was found to be significantly affected by the percent Ba loaded (Figure 4.23). The 0.2% BaMg-V-HTlc and 0.7% BaMg-V-HTlc displayed an irregular surface structure with a “spongy” morphology, similar to the Mg-V-HTlc shown previously.
However, the 1.4% BaMg-V-HTlc showed the presence of rod-like particles on the surface and these became more clustered with an increase in barium loading. The large crystal radius of the Ba ion makes it difficult for octahedral coordination. It is therefore likely that at the higher loadings, the excess barium remains on the surface of the HTlc thus causing a change in the expected morphology. The elemental mapping results for the selected areas of the imaged in Figure 4.23 are presented in Appendix B, Figure B5 – Figure B8. The 0.2% and 0.7% samples showed an even distribution of Mg and V. Barium was found to have a single spot of high concentration for the 0.2% loading while the 0.7% Ba loaded sample showed three spots due to clustering of barium. The 1.4% BaMg-V-HTlc also showed a concentration of barium on the surface, corresponding to the spongy surface morphology on the corresponding SEM image, while the V and Mg were fairly evenly distributed. The highest Ba loaded catalyst showed the largest number of spots in the Ba distribution map suggesting a much higher accumulation of Ba on certain regions of the catalyst.
The V and Mg distribution for the 6% BaMg-V-HTlc was found to be similar to that observed for all other Ba promoted catalysts.
Thermogravimetric analyses on the barium containing catalysts were similar to that of the Mg-V- HTlc in that each showed three weight loss steps accounting for a total weight loss of 28 % - 34 % (Figure 4.24 – Figure 4.27). The observed losses were due to a combination of dehydroxylation and decarboxylation, as discussed for the Mg-V-HTlc. The presence of the barium in the catalyst did not
CATALYST SURFACE AREA (m2/g) 0.2% BaMg-V-
HTlc
62.9 0.7% BaMg-V-
HTlc
67.1 1.4% BaMg-V-
HTlc
76.4
6% BaMg-V-HTlc 57.2
appear to significantly affect the thermal behaviour of the samples, when compared to the Mg-V- HTlc.
Figure 4.23: SEM images of a) 0.2% BaMg-V-HTlc b) 0.7% BaMg-V-HTlc c) 1.4% BaMg-V- HTlc d) 6% BaMg-V-HTlc
b
c a
d
14.21%
(0.8824mg)
123.47°C
2.208%
(0.1371mg) 280.97°C
14.65%
(0.9100mg) 398.61°C
-0.5 0.0 0.5 1.0 1.5 2.0
Deriv. Weight (%/min)
60 70 80 90 100 110
Weight (%)
0 100 200 300 400 500 600 700
Temperature (°C) Sample: 0.2% BaHT01M
Size: 6.2110 mg Method: 20 deg ramp
Comment: Run 1 - 10 deg cel. ramp.
DSC-TGA File: C:...\TGA\UKZN\Ba HT\0.2% BaHT 01M Operator: Alisa
Run Date: 29-Jul-2009 17:05 Instrument: SDT Q600 V20.9 Build 20
Universal V4.5A TA Instruments
Figure 4.24: TG and DTG curves for 0.25 BaMg-V-HTlc
Figure 4.25:TG and DTG curves for 0.7% BaMg-V-HTlc
Figure 4.26: TG
and DTG curves
for 1.4% BaMg-V-
HTlc
3.649%
(0.2568mg) 53.83°C
9.157%
(0.6443mg)
119.06°C
16.43%
(1.156mg)
393.13°C
-0.5 0.0 0.5 1.0 1.5 2.0
Deriv. Weight (%/min)
60 70 80 90 100 110
Weight (%)
0 100 200 300 400 500 600 700
Temperature (°C) Sample: 0.7 % BaHT 01 M
Size: 7.0360 mg Method: 20 deg ramp Comment: Run 1
DSC-TGA File: C:...\TGA\UKZN\Ba HT\0.7 % BaHt 01 M Operator: Alisa
Run Date: 12-Aug-2009 16:10 Instrument: SDT Q600 V20.9 Build 20
Universal V4.5A TA Instruments
14.99%
(0.7007mg)
124.87°C
19.19%
(0.8975mg) 386.00°C
-1 0 1 2 3 4
Deriv. Weight (%/min)
60 70 80 90 100
Weight (%)
0 100 200 300 400 500 600 700
Temperature (°C) Sample: 1.4 % BaHT 01 M
Size: 4.6760 mg Method: 20 deg ramp Comment: Run 1
DSC-TGA File: C:...\TGA\UKZN\Ba HT\1.4 % BaHt 01 M Operator: Alisa
Run Date: 12-Aug-2009 12:05 Instrument: SDT Q600 V20.9 Build 20
Universal V4.5A TA Instruments
Figure 4.27: TG and DTG curves for 6% BaMg-V-HTlc